EP4683980A1 - Method of preparing an aligned liquid crystal elastomer - Google Patents

Method of preparing an aligned liquid crystal elastomer

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Publication number
EP4683980A1
EP4683980A1 EP24714989.1A EP24714989A EP4683980A1 EP 4683980 A1 EP4683980 A1 EP 4683980A1 EP 24714989 A EP24714989 A EP 24714989A EP 4683980 A1 EP4683980 A1 EP 4683980A1
Authority
EP
European Patent Office
Prior art keywords
liquid crystal
crystal elastomer
strain
auxetic
elastomer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP24714989.1A
Other languages
German (de)
French (fr)
Inventor
Helen Frances Gleeson
Peter John Hine
Mariam HUSSAIN
Richard MANDLE
Thomas RAISTRICK
Zhaopeng Zhang
Devesh Arvind MISTRY
Ethan Isaak Luke JULL
Matthew Reynolds
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
University of Leeds
University of Leeds Innovations Ltd
Original Assignee
University of Leeds
University of Leeds Innovations Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by University of Leeds, University of Leeds Innovations Ltd filed Critical University of Leeds
Publication of EP4683980A1 publication Critical patent/EP4683980A1/en
Pending legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/04—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit
    • C09K19/38—Polymers
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/04—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit
    • C09K2019/0444—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit characterized by a linking chain between rings or ring systems, a bridging chain between extensive mesogenic moieties or an end chain group
    • C09K2019/0448—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit characterized by a linking chain between rings or ring systems, a bridging chain between extensive mesogenic moieties or an end chain group the end chain group being a polymerizable end group, e.g. -Sp-P or acrylate
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/04—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit
    • C09K19/06—Non-steroidal liquid crystal compounds
    • C09K19/08—Non-steroidal liquid crystal compounds containing at least two non-condensed rings
    • C09K19/10—Non-steroidal liquid crystal compounds containing at least two non-condensed rings containing at least two benzene rings
    • C09K19/12—Non-steroidal liquid crystal compounds containing at least two non-condensed rings containing at least two benzene rings at least two benzene rings directly linked, e.g. biphenyls
    • C09K2019/121—Compounds containing phenylene-1,4-diyl (-Ph-)
    • C09K2019/122—Ph-Ph

Definitions

  • the present invention relates to a method of reducing an auxetic response threshold in a liquid crystal elastomer.
  • An auxetic material has a negative Poisson’s ratio, where the Poisson’s ratio is described as the negative ratio of the proportional decrease in a lateral measurement to the proportional increase in length in a sample of material that is elastically stretched. On stretching, auxetic materials become thicker in one or both of the directions perpendicular to the applied deformation.
  • auxetic materials are of particular interest because of their desirable and enhanced mechanical, and potentially meta-acoustic properties.
  • Such auxetic materials have been used in applications including sportswear due to their improved shock absorbance and shear-resistance performance.
  • auxetic liquid crystal elastomers have significant advantage over previous synthetic auxetic structures including: transparency; no effective lower size limit on materials/devices; improved strength (no porosity); accessibility of a wide variety of manufacturing methods.
  • the auxetic liquid crystal elastomers can also be chemically tuned, offering significant advantages over existing technology.
  • Molecular liquid crystal elastomers are soft materials, with better potential for matching biological tissue and hence likely to give improved performance in biomedical applications.
  • auxetic response threshold A factor that limits the potential applications of molecular auxetic liquid crystal elastomer materials is that the negative Poisson’s ratio is only observed after a specific threshold of deformation, hereinafter referred to as the auxetic response threshold.
  • a method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying a pre-strain condition to a liquid crystal elastomer.
  • Reducing the auxetic response threshold by applying a pre-strain condition may expand the application potential of auxetic liquid crystal elastomers. It has been identified that the auxetic response threshold is strain-rate dependent, with an auxetic threshold that decreases as the rate that strain is applied decreases. When quasistatic strain (at a strain-rate of approximately 10 -5 s -1 ) is applied to the liquid crystal elastomer, it has been identified that the auxetic response threshold approaches its formulation-dependent minimum.
  • Applying the pre-strain condition may comprise straining the liquid crystal elastomer.
  • Straining the liquid crystal elastomer may comprise straining the liquid crystal elastomer to a predetermined strain.
  • the predetermined strain may be at least 0.1 , such as at least 0.15, such as at least 0.2, such as at least 0.25, such as at least 0.3, such as at least 0.35, such as at least 0.4, such as at least 0.45, such as at least 0.5, such as at least 0.55, such as at least 0.6, such as at least 0.65, such as at least 0.7, such as at least 0.75, such as at least 0.8, or even at least 0.82.
  • the predetermined strain may be up to 1 .9, such as up to 1 .85, such as up to 1 .8, such as up to 1 .75, such as up to 1 .7, such as up to 1 .65, such as up to 1 .6, such as up to 1 .55, such as up to 1 .5, such as up to 1 .45, such as up to 1 .4, such as up to 1 .35, such as up to 1 .3, such as up to 1 .25, such as up to 1 .2, such as up to 1.15, such as up to 1 .1 , such as up to 1 .05, such as up to 1 , or even up to 0.98.
  • up to 1 .9 such as up to 1 .85, such as up to 1 .8, such as up to 1 .75, such as up to 1 .7, such as up to 1 .65, such as up to 1 .6, such as up to 1 .55, such as up to 1 .5, such as up to
  • the predetermined strain may be from 0.1 to 1.9, such as from 0.15 to 1.9, such as from 0.2 to 1 .9, such as from 0.25 to 1 .9, such as from 0.3 to 1 .9, such as from 0.35 to 1 .9, such as from 0.4 to 1 .9, such as from 0.45 to 1 .9, such as from 0.5 to 1 .9, such as from 0.55 to 1 .9, such as from 0.6 to 1 .9, such as from 0.65 to 1 .9, such as from 0.7 to 1 .9, such as from 0.75 to 1 .9, such from 0.8 to 1 .9, or even from 0.82 to 1 .9.
  • the predetermined strain may be from 0.1 to 1.85, such as from 0.15 to 1.85, such as from 0.2 to 1.85, such as from 0.25 to 1.85, such as from 0.3 to 1.85, such as from 0.35 to 1.85, such as from 0.4 to 1.85, such as from 0.45 to 1 .85, such as from 0.5 to 1 .85, such as from 0.55 to 1 .85, such as from 0.6 to 1 .85, such as from 0.65 to 1 .85, such as from 0.7 to 1 .85, such as from 0.75 to 1 .85, such from 0.8 to 1.85, or even from 0.82 to 1.85.
  • the predetermined strain may be from 0.1 to 1.8, such as from 0.15 to 1 .8, such as from 0.2 to 1 .8, such as from 0.25 to 1 .8, such as from 0.3 to 1 .8, such as from 0.35 to 1 .8, such as from 0.4 to 1 .8, such as from 0.45 to 1 .8, such as from 0.5 to 1 .8, such as from 0.55 to 1 .8, such as from 0.6 to 1 .8, such as from 0.65 to 1 .8, such as from 0.7 to 1.8, such as from 0.75 to 1.8, such from 0.8 to 1.8, or even from 0.82 to 1.8.
  • the predetermined strain may be from 0.1 to 1.75, such as from 0.15 to 1.75, such as from 0.2 to 1 .75, such as from 0.25 to 1 .75, such as from 0.3 to 1 .75, such as from 0.35 to 1 .75, such as from 0.4 to 1.75, such as from 0.45 to 1.75, such as from 0.5 to 1.75, such as from 0.55 to 1.75, such as from 0.6 to 1.75, such as from 0.65 to 1.75, such as from 0.7 to 1.75, such as from 0.75 to 1.75, such from 0.8 to 1.75, or even from 0.82 to 1.75.
  • the predetermined strain may be from 0.1 to 1 .7, such as from 0.15 to 1 .7, such as from 0.2 to 1 .7, such as from 0.25 to 1.7, such as from 0.3 to 1.7, such as from 0.35 to 1.7, such as from 0.4 to 1.7, such as from 0.45 to 1 .7, such as from 0.5 to 1 .7, such as from 0.55 to 1 .7, such as from 0.6 to 1 .7, such as from 0.65 to 1.7, such as from 0.7 to 1.7, such as from 0.75 to 1.7, such from 0.8 to 1.7, or even from 0.82 to 1 .7.
  • the predetermined strain may be from 0.1 to 1 .65, such as from 0.15 to 1.65, such as from 0.2 to 1.65, such as from 0.25 to 1.65, such as from 0.3 to 1.65, such as from 0.35 to 1 .65, such as from 0.4 to 1 .65, such as from 0.45 to 1 .65, such as from 0.5 to 1 .65, such as from 0.55 to 1 .65, such as from 0.6 to 1 .65, such as from 0.65 to 1 .65, such as from 0.7 to 1.65, such as from 0.75 to 1.65, such from 0.8 to 1.65, or even from 0.82 to 1.65.
  • the predetermined strain may be from 0.1 to 1 .6, such as from 0.15 to 1 .6, such as from 0.2 to 1 .6, such as from 0.25 to 1 .6, such as from 0.3 to 1 .6, such as from 0.35 to 1 .6, such as from 0.4 to 1 .6, such as from 0.45 to 1 .6, such as from 0.5 to 1 .6, such as from 0.55 to 1 .6, such as from 0.6 to 1 .6, such as from 0.65 to 1 .6, such as from 0.7 to 1 .6, such as from 0.75 to 1 .6, such from 0.8 to 1.6, or even from 0.82 to 1.6.
  • the predetermined strain may be from 0.1 to 1 .55, such as from 0.15 to 1 .55, such as from 0.2 to 1 .55, such as from 0.25 to 1 .55, such as from 0.3 to 1 .55, such as from 0.35 to 1 .55, such as from 0.4 to 1 .55, such as from 0.45 to 1.55, such as from 0.5 to 1.55, such as from 0.55 to 1.55, such as from 0.6 to 1.55, such as from 0.65 to 1.55, such as from 0.7 to 1.55, such as from 0.75 to 1.55, such from 0.8 to 1.55, or even from 0.82 to 1 .55.
  • the predetermined strain may be from 0.1 to 1 .5, such as from 0.15 to 1 .5, such as from 0.2 to 1 .5, such as from 0.25 to 1 .5, such as from 0.3 to 1 .5, such as from 0.35 to 1 .5, such as from 0.4 to 1 .5, such as from 0.45 to 1 .5, such as from 0.5 to 1 .5, such as from 0.55 to 1 .5, such as from 0.6 to 1 .5, such as from 0.65 to 1 .5, such as from 0.7 to 1 .5, such as from 0.75 to 1 .5, such from 0.8 to 1 .5, or even from 0.82 to 1 .5.
  • the predetermined strain may be from 0.1 to 1.45, such as from 0.15 to 1.45, such as from 0.2 to 1.45, such as from 0.25 to 1 .45, such as from 0.3 to 1 .45, such as from 0.35 to 1 .45, such as from 0.4 to
  • 1 .45 such as from 0.45 to 1 .45, such as from 0.5 to 1 .45, such as from 0.55 to 1 .45, such as from 0.6 to 1 .45, such as from 0.65 to 1 .45, such as from 0.7 to 1 .45, such as from 0.75 to 1.45, such from 0.8 to 1.45, or even from 0.82 to 1.45.
  • the predetermined strain may be from 0.1 to 1 .4, such as from 0.15 to 1 .4, such as from 0.2 to 1 .4, such as from 0.25 to 1 .4, such as from 0.3 to 1 .4, such as from 0.35 to 1 .4, such as from 0.4 to 1 .4, such as from 0.45 to 1 .4, such as from 0.5 to 1 .4, such as from 0.55 to 1 .4, such as from 0.6 to 1 .4, such as from 0.65 to 1 .4, such as from 0.7 to 1 .4, such as from 0.75 to 1 .4, such from 0.8 to 1 .4, or even from 0.82 to 1.4.
  • the predetermined strain may be from 0.1 to 1.35, such as from 0.15 to 1.35, such as from 0.2 to 1 .35, such as from 0.25 to 1 .35, such as from 0.3 to 1 .35, such as from 0.35 to 1.35, such as from 0.4 to 1.35, such as from 0.45 to 1.35, such as from 0.5 to 1.35, such as from 0.55 to 1 .35, such as from 0.6 to 1 .35, such as from 0.65 to 1 .35, such as from 0.7 to 1.35, such as from 0.75 to 1.35, such from 0.8 to 1.35, or even from 0.82 to 1.35.
  • the predetermined strain may be from 0.1 to 1.3, such as from 0.15 to 1.3, such as from 0.2 to 1.3, such as from 0.25 to 1 .3, such as from 0.3 to 1 .3, such as from 0.35 to 1 .3, such as from 0.4 to 1 .3, such as from 0.45 to 1 .3, such as from 0.5 to 1 .3, such as from 0.55 to 1 .3, such as from 0.6 to 1.3, such as from 0.65 to 1.3, such as from 0.7 to 1.3, such as from 0.75 to 1.3, such from 0.8 to 1.3, or even from 0.82 to 1.3.
  • the predetermined strain may be from 0.1 to 1.25, such as from 0.15 to 1.25, such as from 0.2 to 1.25, such as from 0.25 to 1.25, such as from 0.3 to 1.25, such as from 0.35 to 1.25, such as from 0.4 to 1.25, such as from 0.45 to 1.25, such as from 0.5 to 1 .25, such as from 0.55 to 1 .25, such as from 0.6 to 1 .25, such as from 0.65 to 1 .25, such as from 0.7 to 1 .25, such as from 0.75 to 1 .25, such from 0.8 to 1 .25, or even from 0.82 to 1.25.
  • the predetermined strain may be from 0.1 to 1.2, such as from 0.15 to 1 .2, such as from 0.2 to 1 .2, such as from 0.25 to 1 .2, such as from 0.3 to 1 .2, such as from 0.35 to 1 .2, such as from 0.4 to 1 .2, such as from 0.45 to 1 .2, such as from 0.5 to 1 .2, such as from 0.55 to 1 .2, such as from 0.6 to 1 .2, such as from 0.65 to 1 .2, such as from 0.7 to 1 .2, such as from 0.75 to 1 .2, such from 0.8 to 1 .2, or even from 0.82 to 1 .2.
  • the predetermined strain may be from 0.1 to 1.15, such as from 0.15 to 1.15, such as from 0.2 to 1.15, such as from 0.25 to 1.15, such as from 0.3 to 1.15, such as from 0.35 to 1.15, such as from 0.4 to 1.15, such as from 0.45 to 1.15, such as from 0.5 to 1.15, such as from 0.55 to 1.15, such as from 0.6 to 1.15, such as from 0.65 to 1.15, such as from 0.7 to 1.15, such as from 0.75 to 1.15, such from 0.8 to 1.15, or even from 0.82 to 1.15.
  • the predetermined strain may be from 0.1 to 1 .1 , such as from 0.15 to 1 .1 , such as from 0.2 to 1 .1 , such as from 0.25 to 1 .1 , such as from 0.3 to 1.1 , such as from 0.35 to 1.1 , such as from 0.4 to 1.1 , such as from 0.45 to 1.1 , such as from 0.5 to 1 .1 , such as from 0.55 to 1 .1 , such as from 0.6 to 1 .1 , such as from 0.65 to 1.1 , such as from 0.7 to 1 .1 , such as from 0.75 to 1.1 , such from 0.8 to 1.1 , or even from 0.82 to 1.1.
  • the predetermined strain may be from 0.1 to 1.05, such as from 0.15 to 1.05, such as from 0.2 to 1 .05, such as from 0.25 to 1 .05, such as from 0.3 to 1 .05, such as from 0.35 to 1.05, such as from 0.4 to 1.05, such as from 0.45 to 1.05, such as from 0.5 to 1.05, such as from 0.55 to 1 .05, such as from 0.6 to 1 .05, such as from 0.65 to 1 .05, such as from 0.7 to 1.05, such as from 0.75 to 1.05, such from 0.8 to 1.05, or even from 0.82 to 1.05.
  • the predetermined strain may be from 0.1 to 1 , such as from 0.15 to 1 , such as from 0.2 to 1 , such as from 0.25 to 1 , such as from 0.3 to 1 , such as from 0.35 to 1 , such as from 0.4 to 1 , such as from 0.45 to 1 , such as from 0.5 to 1 , such as from 0.55 to 1 , such as from 0.6 to 1 , such as from 0.65 to 1 , such as from 0.7 to 1 , such as from 0.75 to 1 , such from 0.8 to 1 , or even from 0.82 to 1.
  • the predetermined strain may be from 0.1 to 0.98, such as from 0.15 to 0.98, such as from 0.2 to 0.98, such as from 0.25 to 0.98, such as from 0.3 to 0.98, such as from 0.35 to 0.98, such as from 0.4 to 0.98, such as from 0.45 to 0.98, such as from 0.5 to 0.98, such as from 0.55 to 0.98, such as from 0.6 to 0.98, such as from 0.65 to 0.98, such as from 0.7 to 0.98, such as from 0.75 to 0.98, such from 0.8 to 0.98, or even from 0.82 to 0.98.
  • 0.15 to 0.98 such as from 0.2 to 0.98, such as from 0.25 to 0.98, such as from 0.3 to 0.98, such as from 0.35 to 0.98, such as from 0.4 to 0.98, such as from 0.45 to 0.98, such as from 0.5 to 0.98, such as from 0.55 to 0.98, such as from 0.6 to 0.98, such as from 0.65 to 0.98, such as from 0.7 to 0.98, such
  • the predetermined strain may be from 0.2 to 1 .8, such as from 0.3 to 1 .7, such as from 0.35 to 1 .66, or even from 0.82 to 1.66.
  • Applying the pre-strain condition may comprise allowing the liquid crystal to stress- relax. Allowing the liquid crystal to stress-relax may comprise allowing the liquid crystal to relax whilst being maintained in the strained condition.
  • a suitable time for the stress-relaxation may depend on the formulation of the liquid crystal elastomer and the temperature relative to the glass transition temperature, T g , of the liquid crystal elastomer. For example, the liquid crystal elastomer may be allowed to relax for approximately 30-120 minutes at a temperature of approximately 8°C above T g of the liquid crystal elastomer. At increased temperatures above Tg, the time at which the liquid crystal elastomer is left to stress-relax may be reduced.
  • the liquid crystal elastomer may be a film.
  • the film may have a large x/y aspect ratio, preferably greater than 4, to avoid inhomogeneous strain profiles.
  • the aspect ratio may be greater or lower than 4.
  • Allowing the liquid crystal elastomer film to stress-relax may equilibrate the liquid crystal elastomer film.
  • the liquid crystal elastomer film may thereafter have a reduced auxetic response threshold.
  • the liquid crystal elastomer film may be at the auxetic response threshold in equilibrium, such that when further strain is applied to the liquid crystal elastomer film, the liquid crystal elastomer film will display auxetic behaviour.
  • the liquid crystal elastomer film may have an auxetic response threshold that is above zero when in equilibrium, but is reduced relative to the auxetic response threshold of the film prior to the pre-strain condition being applied, such that a reduced amount of deformation is required for the liquid crystal elastomer film to display auxetic behaviour.
  • Straining the liquid crystal elastomer may comprise at least one of straining the liquid crystal elastomer mechanically, swelling or deswelling the liquid crystal elastomer with solvent, or heating or cooling the liquid crystal elastomer.
  • Straining the liquid crystal elastomer mechanically may comprise applying strain in a direction perpendicular to the director of the liquid crystal elastomer.
  • the strain rate may be non-quasistatic.
  • the applied strain rate may be determined based on the temperature of the liquid crystal elastomer relative to T g .
  • the liquid crystal elastomer may be strained continuously up to the predetermined strain.
  • Straining the liquid crystal elastomer mechanically may comprise straining the liquid crystal elastomer at a rate of 0.05 per minute or faster.
  • the liquid crystal elastomer may be strained at a rate of at least 0.1 per minute, such as at least 0.15 per minute, such as at least 0.2 per minute, such as at least 0.25 per minute, such as at least 0.3 per minute, such as at least 0.35 per minute, such as at least 0.4 per minute, such as at least 0.45 per minute, such as at least 0.5 per minute, such as at least 0.55 per minute, such as at least 0.6 per minute, such as at least 0.65 per minute, such as at least 0.7 per minute, such as at least 0.75 per minute, such as at least 0.8 per minute, such as at least 0.85 per minute, such as at least 0.9 per minute, such as at least 0.95 per minute, even at least 1 per minute.
  • the liquid crystal elastomer is strained at a rate of 0.01 per second or faster.
  • the predetermined strain may be the strain at which the liquid crystal elastomer reaches the auxetic response threshold when being strained at a quasistatic rate.
  • the plurality of different rates may be 10 -4 s -1 , 10 -3 s -1 , 10 -2 s -1 , and 10 -1 s -1 .
  • a method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying strain to the liquid crystal elastomer in a series of steps and allowing the liquid crystal to stress-relax between steps.
  • the amount of strain applied in each step may be determined based on the temperature of the liquid crystal elastomer above T g .
  • the liquid crystal elastomer may be strained by at least 0.02in each step, such as at least 0.025, such as at least 0.03, such as at least 0.035, such as at least 0.04, such as at least 0.045, even at least 0.05.
  • the liquid crystal elastomer may be strained by up to 0.1 , such as up to 0.095, such as up to 0.09, such as up to 0.085, such as up to 0.08, such as 0.075, such as up to 0.07, such as up to 0.065, even up to 0.06.
  • the applied strain in each step may be from 0.02 to 0.1 , such as from 0.025 to 0.1 , such from 0.03 to 0.1 , such as from 0.035 to 0.1 , such as from 0.04 to 0.1 , such from 0.045 to 0.1 , even from 0.05 to 0.1 .
  • the applied strain in each step may be from 0.02 to 0.095, such as from 0.025 to 0.095, such from 0.03 to 0.095, such as from 0.035 to 0.095, such as from 0.04 to 0.095, such from 0.045 to 0.095, even from 0.05 to 0.095.
  • the applied strain in each step may be from 0.02 to 0.09, such as from 0.025 to 0.09, such from 0.03 to 0.09, such as from 0.035 to 0.09, such as from 0.04 to 0.09, such from 0.045 to 0.09, even from 0.05 to 0.09.
  • the applied strain in each step may be from 0.02 to 0.085, such as from 0.025 to 0.085, such from 0.03 to 0.085, such as from 0.035 to 0.085, such as from 0.04 to 0.085, such from 0.045 to 0.085, even from 0.05 to 0.085.
  • the applied strain in each step may be from 0.02 to 0.08, such as from 0.025 to 0.08, such from 0.03 to 0.08, such as from 0.035 to 0.08, such as from 0.04 to 0.08, such from 0.045 to 0.08, even from 0.05 to 0.08.
  • the applied strain in each step may be from 0.02 to 0.075, such as from 0.025 to 0.075, such from 0.03 to 0.075, such as from 0.035 to 0.075, such as from 0.04 to 0.075, such from 0.045 to 0.075, even from 0.05 to 0.075.
  • the applied strain in each step may be from 0.02 to 0.07, such as from 0.025 to 0.07, such from 0.03 to 0.07, such as from 0.035 to 0.07, such as from 0.04 to 0.07, such from 0.045 to 0.07, even from 0.05 to 0.07.
  • the applied strain in each step may be from 0.02 to 0.065, such as from 0.025 to 0.065, such from 0.03 to 0.065, such as from 0.035 to 0.065, such as from 0.04 to 0.065, such from 0.045 to 0.065, even from 0.05 to 0.065.
  • the applied strain in each step may be from 0.02 to 0.06, such as from 0.025 to 0.06, such from 0.03 to 0.06, such as from 0.035 to 0.06, such as from 0.04 to 0.06, such from 0.045 to 0.06, even from 0.05 to 0.06.
  • the liquid crystal elastomer may be allowed to stress-relax in between each step.
  • a suitable time for the stress-relaxation may depend on the formulation of the liquid crystal elastomer and the temperature relative to T g of the liquid crystal elastomer.
  • the liquid crystal elastomer may be allowed to relax for 5 to 120 minutes at a temperature of approximately 8°C above T g of the liquid crystal elastomer.
  • the liquid crystal elastomer may be allowed to stress relax at a temperature above T g .
  • T g the temperature above T g at which the liquid crystal elastomer undergoes the strain steps and relaxation.
  • Figure 1 shows an example process of applying a pre-strain condition
  • Figure 2 shows an example liquid crystal elastomer prior to being strained mechanically
  • Figure 3 shows an example process of determining a predetermined strain
  • Figure 4 shows an example process of reducing the auxetic response threshold
  • Figure 5A is a graph of Poisson’s ratio as strain is varied
  • Figure 5B is a graph of true stress as true strain is varied
  • Figure 6 is a graph of the reduction in true strain threshold with applied true pre-strain.
  • Figure 7A is a graph of z-strain as x-strain is varied with different stress-relaxation times between strain steps;
  • Figure 7B is a graph of the Poisson’s ratio as strain is varied for different relaxation times between strain steps
  • Figure 8A is a graph of z-strain as x-strain is varied at different temperatures
  • Figure 8B is a graph of the Poisson’s ratio as strain is varied at different temperatures.
  • an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold is prepared by applying a pre-strain condition to the liquid crystal elastomer.
  • An example process 10 of applying a pre-strain condition is shown in in figure 1 .
  • an aligned nematic liquid crystal elastomer film is strained to a predetermined strain.
  • the film is strained in a direction that is perpendicular to the director of the liquid crystal elastomer
  • the strain is applied mechanically by clamping the film at either end in a longitudinal direction and straining the film in the longitudinal direction, perpendicular to the director.
  • Figure 2 shows the film 20 clamped at ends 22, 24 in the x-direction (longitudinal direction), in which arrow n indicates the direction of liquid crystal director.
  • the strain may be applied by swelling or deswelling the liquid crystal elastomer with solvent or heating or cooling the liquid crystal elastomer.
  • the strained film is left to stress-relax whilst the strained condition is maintained.
  • the time period for which the strained film is left to stress-relax may be determined based on the temperature of the film relative to T g .
  • the predetermined strain is determined according to the process 30 shown in figure 3.
  • a plurality of auxetic response thresholds for the liquid crystal elastomer are determined when the liquid crystal elastomer film is strained at a plurality of different rates.
  • a first auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10 -4 s -1
  • a second auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10 -3 s -1
  • a third auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10 -2 s -1
  • a fourth auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10 -1 s -1 .
  • the strain in the z-direction decreases as the applied strain in the x-direction increases.
  • the strain in the z-direction increases as the applied strain in the x-direction increases.
  • the auxetic response threshold can thus be determined based on the calculated strain in the z-direction.
  • the auxetic response threshold when the straining the liquid crystal elastomer at a quasistatic rate is determined by extrapolating from the determined auxetic response thresholds to a rate of 10 -5 s -1 .
  • the determined auxetic response threshold when straining the elastomer at a quasistatic rate is determined to be the predetermined strain.
  • Figure 4 shows another example method 40 of reducing the auxetic response threshold of an aligned nematic liquid crystal elastomer.
  • the method comprises, at step 42, applying strain to the liquid crystal elastomer in strain steps.
  • the liquid crystal elastomer is strained mechanically in a direction x that is perpendicular to the director of the liquid crystal elastomer, as indicated in figure 2.
  • the method further comprises, at step 44, allowing the liquid crystal to stress-relax between steps.
  • Steps 42 and 44 are repeated, with stress-relaxation between sequential steps.
  • Auxetic aligned nematic liquid crystal elastomer films for use according to the invention were synthesised as follows using the following materials:
  • Films of liquid crystal elastomer were produced by polymerising the monomer mixture inside thin film moulds of approximately 100 pm thickness, a width of approximately 15 mm and a length of approximately 60 mm. In other examples, film moulds of different thicknesses, widths and lengths may be used. The films were cured in the nematic phase at room temperature, with planar alignment, where the surface of the mould was coated with a thin layer of PVA alignment layer and the rubbing direction was anti-parallel. The films were polymerised for two hours. After polymerisation, the films were removed from the moulds and kept in a methanol/DCM solvent mixture overnight in order to wash out the 6OCB.
  • a pre-strain condition was applied to a first film, according to the method of figure 1 .
  • the strain condition was applied at a rate of approximately 0.01 s -1 and the film was allowed to equilibrate under the strain for between 0.5-1 hours.
  • a second film, as a comparative example, was formed by the same method, but without a pre-strain condition applied.
  • Figure 5A shows a graph of the Poisson’s ratio for the first film and for the comparative example second film.
  • the Poisson’s ratio, v xz is determined using on Equation 2, wherein the true strain is determined based on the engineering strain using Equation 3.
  • the strain value at which the Poisson ratio becomes negative is lower for the first film to which the pre-strain condition is applied (data-points labelled 52) than for the comparative second film to which the pre-strain condition is not applied (data-points labelled 54).
  • Figure 5B shows a graph of the true stress as the true strain increases for the first film to which the pre-strain condition is applied (data-points labelled 56) and the comparative second film to which the pre-strain condition is not applied (data-points labelled 58).
  • the graph in Figure 6 shows the reduction in the auxetic response threshold achieved by applying the method of figure 1 to the liquid crystal elastomer films, for various predetermined strain values, when the strain was applied at a rate of approximately 0.01 s -1 and was allowed to equilibrate under the strain for between 0.5-1 hours.
  • a reduction in the auxetic response threshold can be achieved when applying a pre-strain condition.
  • the method of figure 4 was applied to a plurality of the films, wherein the films were strained in steps of 0.05 and the films were allowed to equilibrate between strain steps at different temperatures and for different amounts of time.
  • the T g of the films was approximately 15°C.
  • Figure 7A shows a graph of strain in the z-direction as the applied strain in the x- direction is increased in steps, with stress-relaxation times between strain steps of 5 minutes (data-points 71), 10 minutes (data-points 72), 60 minutes (data-points 73) and 120 minutes (data points 74). The measurements were taken with the liquid crystal elastomer at room temperature (23°C), which is 8°C above T g .
  • Figure 7B shows a graph of the Poisson’s ratio as the applied strain in the x-direction is increased in steps, with stress-relaxation times between strain steps of 5 minutes (line 75), 10 minutes (line 76), 60 minutes (line 77) and 120 minutes (line 78). The Poisson’s ratio is calculated using equation 2.
  • the auxetic response threshold (at which the strain in the z-direction begins to increase in figure 7A and at which the Poisson’s ratio becomes negative in figure 7B) decreases as the time-period of stress-relaxation between strain steps increases.
  • Figure 8A shows a graph of strain in the z-direction as the applied strain in the x- direction is increased in steps, with stress-relaxation times between strain steps of 10 minutes, when the liquid crystal elastomer is at 8°C above T g (data-points 81) and when the liquid crystal elastomer is heated to 15°C above T g (data-points 82) and 20°C above T g (data-points 83).
  • Figure 8B shows a graph of the Poisson’s ratio as the applied strain in the x-direction is increased in steps, with stress-relaxation times between strain steps of 10 minutes when the liquid crystal elastomer is at 8°C above T g (line 84) and when the liquid crystal elastomer is heated to 15°C above T g (line 85) and 20°C above T g (line 86).
  • the Poisson’s ratio is calculated using equation 2.
  • the auxetic response threshold (at which the strain in the z-direction begins to increase in figure 8A and at which the Poisson’s ratio becomes negative in figure 8B) decreases as the temperature of the liquid crystal elastomer during the strain steps and stress-relaxation increases.

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Abstract

A method of preparing method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying a pre-strain condition to a liquid crystal elastomer.

Description

Method of preparing an aligned liquid crystal elastomer
[0001] The present invention relates to a method of reducing an auxetic response threshold in a liquid crystal elastomer.
[0002] An auxetic material has a negative Poisson’s ratio, where the Poisson’s ratio is described as the negative ratio of the proportional decrease in a lateral measurement to the proportional increase in length in a sample of material that is elastically stretched. On stretching, auxetic materials become thicker in one or both of the directions perpendicular to the applied deformation.
[0003] Auxetic materials are of particular interest because of their desirable and enhanced mechanical, and potentially meta-acoustic properties. Some auxetic materials exist in nature and other first synthetic auxetic materials, are carefully engineered structures that exhibit their auxetic behaviour because of their bulk, honeycomb-like structures. Such auxetic materials have been used in applications including sportswear due to their improved shock absorbance and shear-resistance performance.
[0004] An aligned nematic liquid crystal elastomer having auxetic properties has recently been developed, as described in WO2019077361 A1 . The auxetic liquid crystal elastomers have significant advantage over previous synthetic auxetic structures including: transparency; no effective lower size limit on materials/devices; improved strength (no porosity); accessibility of a wide variety of manufacturing methods. The auxetic liquid crystal elastomers can also be chemically tuned, offering significant advantages over existing technology. Molecular liquid crystal elastomers are soft materials, with better potential for matching biological tissue and hence likely to give improved performance in biomedical applications.
[0005] A factor that limits the potential applications of molecular auxetic liquid crystal elastomer materials is that the negative Poisson’s ratio is only observed after a specific threshold of deformation, hereinafter referred to as the auxetic response threshold.
[0006] According to the present invention there is provided methods as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.
[0007] According to an aspect of the invention, there is provided a method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying a pre-strain condition to a liquid crystal elastomer. [0008] Reducing the auxetic response threshold by applying a pre-strain condition may expand the application potential of auxetic liquid crystal elastomers. It has been identified that the auxetic response threshold is strain-rate dependent, with an auxetic threshold that decreases as the rate that strain is applied decreases. When quasistatic strain (at a strain-rate of approximately 10-5 s-1) is applied to the liquid crystal elastomer, it has been identified that the auxetic response threshold approaches its formulation-dependent minimum. However, applying quasistatic strains to reach the auxetic response threshold can be very time consuming, and it typically takes approximately 10 hours to reach the auxetic response threshold when straining the liquid crystal elastomer at a quasistatic rate. Applying the prestrain condition to the liquid crystal elastomer may produce a liquid crystal elastomer that has a reduced auxetic response threshold when in equilibrium, without the need to strain at a quasistatic rate. This may significantly reduce the time needed for the liquid crystal elastomer to enter the auxetic regime.
[0009] Applying the pre-strain condition may comprise straining the liquid crystal elastomer. Straining the liquid crystal elastomer may comprise straining the liquid crystal elastomer to a predetermined strain. The predetermined strain may be at least 0.1 , such as at least 0.15, such as at least 0.2, such as at least 0.25, such as at least 0.3, such as at least 0.35, such as at least 0.4, such as at least 0.45, such as at least 0.5, such as at least 0.55, such as at least 0.6, such as at least 0.65, such as at least 0.7, such as at least 0.75, such as at least 0.8, or even at least 0.82. The predetermined strain may be up to 1 .9, such as up to 1 .85, such as up to 1 .8, such as up to 1 .75, such as up to 1 .7, such as up to 1 .65, such as up to 1 .6, such as up to 1 .55, such as up to 1 .5, such as up to 1 .45, such as up to 1 .4, such as up to 1 .35, such as up to 1 .3, such as up to 1 .25, such as up to 1 .2, such as up to 1.15, such as up to 1 .1 , such as up to 1 .05, such as up to 1 , or even up to 0.98.
[0010] The predetermined strain may be from 0.1 to 1.9, such as from 0.15 to 1.9, such as from 0.2 to 1 .9, such as from 0.25 to 1 .9, such as from 0.3 to 1 .9, such as from 0.35 to 1 .9, such as from 0.4 to 1 .9, such as from 0.45 to 1 .9, such as from 0.5 to 1 .9, such as from 0.55 to 1 .9, such as from 0.6 to 1 .9, such as from 0.65 to 1 .9, such as from 0.7 to 1 .9, such as from 0.75 to 1 .9, such from 0.8 to 1 .9, or even from 0.82 to 1 .9. The predetermined strain may be from 0.1 to 1.85, such as from 0.15 to 1.85, such as from 0.2 to 1.85, such as from 0.25 to 1.85, such as from 0.3 to 1.85, such as from 0.35 to 1.85, such as from 0.4 to 1.85, such as from 0.45 to 1 .85, such as from 0.5 to 1 .85, such as from 0.55 to 1 .85, such as from 0.6 to 1 .85, such as from 0.65 to 1 .85, such as from 0.7 to 1 .85, such as from 0.75 to 1 .85, such from 0.8 to 1.85, or even from 0.82 to 1.85. The predetermined strain may be from 0.1 to 1.8, such as from 0.15 to 1 .8, such as from 0.2 to 1 .8, such as from 0.25 to 1 .8, such as from 0.3 to 1 .8, such as from 0.35 to 1 .8, such as from 0.4 to 1 .8, such as from 0.45 to 1 .8, such as from 0.5 to 1 .8, such as from 0.55 to 1 .8, such as from 0.6 to 1 .8, such as from 0.65 to 1 .8, such as from 0.7 to 1.8, such as from 0.75 to 1.8, such from 0.8 to 1.8, or even from 0.82 to 1.8. The predetermined strain may be from 0.1 to 1.75, such as from 0.15 to 1.75, such as from 0.2 to 1 .75, such as from 0.25 to 1 .75, such as from 0.3 to 1 .75, such as from 0.35 to 1 .75, such as from 0.4 to 1.75, such as from 0.45 to 1.75, such as from 0.5 to 1.75, such as from 0.55 to 1.75, such as from 0.6 to 1.75, such as from 0.65 to 1.75, such as from 0.7 to 1.75, such as from 0.75 to 1.75, such from 0.8 to 1.75, or even from 0.82 to 1.75. The predetermined strain may be from 0.1 to 1 .7, such as from 0.15 to 1 .7, such as from 0.2 to 1 .7, such as from 0.25 to 1.7, such as from 0.3 to 1.7, such as from 0.35 to 1.7, such as from 0.4 to 1.7, such as from 0.45 to 1 .7, such as from 0.5 to 1 .7, such as from 0.55 to 1 .7, such as from 0.6 to 1 .7, such as from 0.65 to 1.7, such as from 0.7 to 1.7, such as from 0.75 to 1.7, such from 0.8 to 1.7, or even from 0.82 to 1 .7. The predetermined strain may be from 0.1 to 1 .65, such as from 0.15 to 1.65, such as from 0.2 to 1.65, such as from 0.25 to 1.65, such as from 0.3 to 1.65, such as from 0.35 to 1 .65, such as from 0.4 to 1 .65, such as from 0.45 to 1 .65, such as from 0.5 to 1 .65, such as from 0.55 to 1 .65, such as from 0.6 to 1 .65, such as from 0.65 to 1 .65, such as from 0.7 to 1.65, such as from 0.75 to 1.65, such from 0.8 to 1.65, or even from 0.82 to 1.65. The predetermined strain may be from 0.1 to 1 .6, such as from 0.15 to 1 .6, such as from 0.2 to 1 .6, such as from 0.25 to 1 .6, such as from 0.3 to 1 .6, such as from 0.35 to 1 .6, such as from 0.4 to 1 .6, such as from 0.45 to 1 .6, such as from 0.5 to 1 .6, such as from 0.55 to 1 .6, such as from 0.6 to 1 .6, such as from 0.65 to 1 .6, such as from 0.7 to 1 .6, such as from 0.75 to 1 .6, such from 0.8 to 1.6, or even from 0.82 to 1.6. The predetermined strain may be from 0.1 to 1 .55, such as from 0.15 to 1 .55, such as from 0.2 to 1 .55, such as from 0.25 to 1 .55, such as from 0.3 to 1 .55, such as from 0.35 to 1 .55, such as from 0.4 to 1 .55, such as from 0.45 to 1.55, such as from 0.5 to 1.55, such as from 0.55 to 1.55, such as from 0.6 to 1.55, such as from 0.65 to 1.55, such as from 0.7 to 1.55, such as from 0.75 to 1.55, such from 0.8 to 1.55, or even from 0.82 to 1 .55. The predetermined strain may be from 0.1 to 1 .5, such as from 0.15 to 1 .5, such as from 0.2 to 1 .5, such as from 0.25 to 1 .5, such as from 0.3 to 1 .5, such as from 0.35 to 1 .5, such as from 0.4 to 1 .5, such as from 0.45 to 1 .5, such as from 0.5 to 1 .5, such as from 0.55 to 1 .5, such as from 0.6 to 1 .5, such as from 0.65 to 1 .5, such as from 0.7 to 1 .5, such as from 0.75 to 1 .5, such from 0.8 to 1 .5, or even from 0.82 to 1 .5. The predetermined strain may be from 0.1 to 1.45, such as from 0.15 to 1.45, such as from 0.2 to 1.45, such as from 0.25 to 1 .45, such as from 0.3 to 1 .45, such as from 0.35 to 1 .45, such as from 0.4 to
1 .45, such as from 0.45 to 1 .45, such as from 0.5 to 1 .45, such as from 0.55 to 1 .45, such as from 0.6 to 1 .45, such as from 0.65 to 1 .45, such as from 0.7 to 1 .45, such as from 0.75 to 1.45, such from 0.8 to 1.45, or even from 0.82 to 1.45. The predetermined strain may be from 0.1 to 1 .4, such as from 0.15 to 1 .4, such as from 0.2 to 1 .4, such as from 0.25 to 1 .4, such as from 0.3 to 1 .4, such as from 0.35 to 1 .4, such as from 0.4 to 1 .4, such as from 0.45 to 1 .4, such as from 0.5 to 1 .4, such as from 0.55 to 1 .4, such as from 0.6 to 1 .4, such as from 0.65 to 1 .4, such as from 0.7 to 1 .4, such as from 0.75 to 1 .4, such from 0.8 to 1 .4, or even from 0.82 to 1.4. The predetermined strain may be from 0.1 to 1.35, such as from 0.15 to 1.35, such as from 0.2 to 1 .35, such as from 0.25 to 1 .35, such as from 0.3 to 1 .35, such as from 0.35 to 1.35, such as from 0.4 to 1.35, such as from 0.45 to 1.35, such as from 0.5 to 1.35, such as from 0.55 to 1 .35, such as from 0.6 to 1 .35, such as from 0.65 to 1 .35, such as from 0.7 to 1.35, such as from 0.75 to 1.35, such from 0.8 to 1.35, or even from 0.82 to 1.35. The predetermined strain may be from 0.1 to 1.3, such as from 0.15 to 1.3, such as from 0.2 to 1.3, such as from 0.25 to 1 .3, such as from 0.3 to 1 .3, such as from 0.35 to 1 .3, such as from 0.4 to 1 .3, such as from 0.45 to 1 .3, such as from 0.5 to 1 .3, such as from 0.55 to 1 .3, such as from 0.6 to 1.3, such as from 0.65 to 1.3, such as from 0.7 to 1.3, such as from 0.75 to 1.3, such from 0.8 to 1.3, or even from 0.82 to 1.3. The predetermined strain may be from 0.1 to 1.25, such as from 0.15 to 1.25, such as from 0.2 to 1.25, such as from 0.25 to 1.25, such as from 0.3 to 1.25, such as from 0.35 to 1.25, such as from 0.4 to 1.25, such as from 0.45 to 1.25, such as from 0.5 to 1 .25, such as from 0.55 to 1 .25, such as from 0.6 to 1 .25, such as from 0.65 to 1 .25, such as from 0.7 to 1 .25, such as from 0.75 to 1 .25, such from 0.8 to 1 .25, or even from 0.82 to 1.25. The predetermined strain may be from 0.1 to 1.2, such as from 0.15 to 1 .2, such as from 0.2 to 1 .2, such as from 0.25 to 1 .2, such as from 0.3 to 1 .2, such as from 0.35 to 1 .2, such as from 0.4 to 1 .2, such as from 0.45 to 1 .2, such as from 0.5 to 1 .2, such as from 0.55 to 1 .2, such as from 0.6 to 1 .2, such as from 0.65 to 1 .2, such as from 0.7 to 1 .2, such as from 0.75 to 1 .2, such from 0.8 to 1 .2, or even from 0.82 to 1 .2. The predetermined strain may be from 0.1 to 1.15, such as from 0.15 to 1.15, such as from 0.2 to 1.15, such as from 0.25 to 1.15, such as from 0.3 to 1.15, such as from 0.35 to 1.15, such as from 0.4 to 1.15, such as from 0.45 to 1.15, such as from 0.5 to 1.15, such as from 0.55 to 1.15, such as from 0.6 to 1.15, such as from 0.65 to 1.15, such as from 0.7 to 1.15, such as from 0.75 to 1.15, such from 0.8 to 1.15, or even from 0.82 to 1.15. The predetermined strain may be from 0.1 to 1 .1 , such as from 0.15 to 1 .1 , such as from 0.2 to 1 .1 , such as from 0.25 to 1 .1 , such as from 0.3 to 1.1 , such as from 0.35 to 1.1 , such as from 0.4 to 1.1 , such as from 0.45 to 1.1 , such as from 0.5 to 1 .1 , such as from 0.55 to 1 .1 , such as from 0.6 to 1 .1 , such as from 0.65 to 1.1 , such as from 0.7 to 1 .1 , such as from 0.75 to 1.1 , such from 0.8 to 1.1 , or even from 0.82 to 1.1. The predetermined strain may be from 0.1 to 1.05, such as from 0.15 to 1.05, such as from 0.2 to 1 .05, such as from 0.25 to 1 .05, such as from 0.3 to 1 .05, such as from 0.35 to 1.05, such as from 0.4 to 1.05, such as from 0.45 to 1.05, such as from 0.5 to 1.05, such as from 0.55 to 1 .05, such as from 0.6 to 1 .05, such as from 0.65 to 1 .05, such as from 0.7 to 1.05, such as from 0.75 to 1.05, such from 0.8 to 1.05, or even from 0.82 to 1.05. The predetermined strain may be from 0.1 to 1 , such as from 0.15 to 1 , such as from 0.2 to 1 , such as from 0.25 to 1 , such as from 0.3 to 1 , such as from 0.35 to 1 , such as from 0.4 to 1 , such as from 0.45 to 1 , such as from 0.5 to 1 , such as from 0.55 to 1 , such as from 0.6 to 1 , such as from 0.65 to 1 , such as from 0.7 to 1 , such as from 0.75 to 1 , such from 0.8 to 1 , or even from 0.82 to 1. The predetermined strain may be from 0.1 to 0.98, such as from 0.15 to 0.98, such as from 0.2 to 0.98, such as from 0.25 to 0.98, such as from 0.3 to 0.98, such as from 0.35 to 0.98, such as from 0.4 to 0.98, such as from 0.45 to 0.98, such as from 0.5 to 0.98, such as from 0.55 to 0.98, such as from 0.6 to 0.98, such as from 0.65 to 0.98, such as from 0.7 to 0.98, such as from 0.75 to 0.98, such from 0.8 to 0.98, or even from 0.82 to 0.98.
[0011] The predetermined strain may be from 0.2 to 1 .8, such as from 0.3 to 1 .7, such as from 0.35 to 1 .66, or even from 0.82 to 1.66.
[0012] Applying the pre-strain condition may comprise allowing the liquid crystal to stress- relax. Allowing the liquid crystal to stress-relax may comprise allowing the liquid crystal to relax whilst being maintained in the strained condition. A suitable time for the stress-relaxation may depend on the formulation of the liquid crystal elastomer and the temperature relative to the glass transition temperature, Tg, of the liquid crystal elastomer. For example, the liquid crystal elastomer may be allowed to relax for approximately 30-120 minutes at a temperature of approximately 8°C above Tg of the liquid crystal elastomer. At increased temperatures above Tg, the time at which the liquid crystal elastomer is left to stress-relax may be reduced.
[0013] The liquid crystal elastomer may be a film. In some examples, the film may have a large x/y aspect ratio, preferably greater than 4, to avoid inhomogeneous strain profiles. In other examples, the aspect ratio may be greater or lower than 4.
[0014] Allowing the liquid crystal elastomer film to stress-relax may equilibrate the liquid crystal elastomer film. The liquid crystal elastomer film may thereafter have a reduced auxetic response threshold. In some examples, the liquid crystal elastomer film may be at the auxetic response threshold in equilibrium, such that when further strain is applied to the liquid crystal elastomer film, the liquid crystal elastomer film will display auxetic behaviour. In other examples, the liquid crystal elastomer film may have an auxetic response threshold that is above zero when in equilibrium, but is reduced relative to the auxetic response threshold of the film prior to the pre-strain condition being applied, such that a reduced amount of deformation is required for the liquid crystal elastomer film to display auxetic behaviour.
[0015] Straining the liquid crystal elastomer may comprise at least one of straining the liquid crystal elastomer mechanically, swelling or deswelling the liquid crystal elastomer with solvent, or heating or cooling the liquid crystal elastomer.
[0016] Straining the liquid crystal elastomer mechanically may comprise applying strain in a direction perpendicular to the director of the liquid crystal elastomer. The strain rate may be non-quasistatic. The applied strain rate may be determined based on the temperature of the liquid crystal elastomer relative to Tg. The liquid crystal elastomer may be strained continuously up to the predetermined strain. Straining the liquid crystal elastomer mechanically may comprise straining the liquid crystal elastomer at a rate of 0.05 per minute or faster. The liquid crystal elastomer may be strained at a rate of at least 0.1 per minute, such as at least 0.15 per minute, such as at least 0.2 per minute, such as at least 0.25 per minute, such as at least 0.3 per minute, such as at least 0.35 per minute, such as at least 0.4 per minute, such as at least 0.45 per minute, such as at least 0.5 per minute, such as at least 0.55 per minute, such as at least 0.6 per minute, such as at least 0.65 per minute, such as at least 0.7 per minute, such as at least 0.75 per minute, such as at least 0.8 per minute, such as at least 0.85 per minute, such as at least 0.9 per minute, such as at least 0.95 per minute, even at least 1 per minute. Preferably, the liquid crystal elastomer is strained at a rate of 0.01 per second or faster.
[0017] The predetermined strain may be the strain at which the liquid crystal elastomer reaches the auxetic response threshold when being strained at a quasistatic rate.
[0018] The method may comprise determining the predetermined strain. Determining the predetermined strain may comprise determining the auxetic response threshold when the liquid crystal elastomer is strained at each of a plurality of different rates and determining the auxetic response threshold for the straining the liquid crystal elastomer at a quasistatic rate by extrapolating from the determined auxetic response thresholds. The plurality of different rates may be 10-4 s-1 , 10-3 s-1 , 10-2 s-1 , and 10-1 s-1.
[0019] According to another aspect of the invention, there is provided a method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying strain to the liquid crystal elastomer in a series of steps and allowing the liquid crystal to stress-relax between steps.
[0020] It has been found that applying the strain in steps and allowing the elastomer to relax between steps has the effect of reducing the auxetic response threshold. Preparing the liquid crystal elastomer may achieve a reduced auxetic threshold without requiring the liquid crystal elastomer to be strained at a quasistatic rate. This may reduce the time needed for the liquid crystal elastomer to enter the auxetic regime and thereby expand the application potential of auxetic liquid crystal elastomers
[0021] The amount of strain applied in each step may be determined based on the temperature of the liquid crystal elastomer above Tg. The liquid crystal elastomer may be strained by at least 0.02in each step, such as at least 0.025, such as at least 0.03, such as at least 0.035, such as at least 0.04, such as at least 0.045, even at least 0.05. The liquid crystal elastomer may be strained by up to 0.1 , such as up to 0.095, such as up to 0.09, such as up to 0.085, such as up to 0.08, such as 0.075, such as up to 0.07, such as up to 0.065, even up to 0.06.
[0022] The applied strain in each step may be from 0.02 to 0.1 , such as from 0.025 to 0.1 , such from 0.03 to 0.1 , such as from 0.035 to 0.1 , such as from 0.04 to 0.1 , such from 0.045 to 0.1 , even from 0.05 to 0.1 . The applied strain in each step may be from 0.02 to 0.095, such as from 0.025 to 0.095, such from 0.03 to 0.095, such as from 0.035 to 0.095, such as from 0.04 to 0.095, such from 0.045 to 0.095, even from 0.05 to 0.095. The applied strain in each step may be from 0.02 to 0.09, such as from 0.025 to 0.09, such from 0.03 to 0.09, such as from 0.035 to 0.09, such as from 0.04 to 0.09, such from 0.045 to 0.09, even from 0.05 to 0.09. The applied strain in each step may be from 0.02 to 0.085, such as from 0.025 to 0.085, such from 0.03 to 0.085, such as from 0.035 to 0.085, such as from 0.04 to 0.085, such from 0.045 to 0.085, even from 0.05 to 0.085. The applied strain in each step may be from 0.02 to 0.08, such as from 0.025 to 0.08, such from 0.03 to 0.08, such as from 0.035 to 0.08, such as from 0.04 to 0.08, such from 0.045 to 0.08, even from 0.05 to 0.08. The applied strain in each step may be from 0.02 to 0.075, such as from 0.025 to 0.075, such from 0.03 to 0.075, such as from 0.035 to 0.075, such as from 0.04 to 0.075, such from 0.045 to 0.075, even from 0.05 to 0.075. The applied strain in each step may be from 0.02 to 0.07, such as from 0.025 to 0.07, such from 0.03 to 0.07, such as from 0.035 to 0.07, such as from 0.04 to 0.07, such from 0.045 to 0.07, even from 0.05 to 0.07. The applied strain in each step may be from 0.02 to 0.065, such as from 0.025 to 0.065, such from 0.03 to 0.065, such as from 0.035 to 0.065, such as from 0.04 to 0.065, such from 0.045 to 0.065, even from 0.05 to 0.065. The applied strain in each step may be from 0.02 to 0.06, such as from 0.025 to 0.06, such from 0.03 to 0.06, such as from 0.035 to 0.06, such as from 0.04 to 0.06, such from 0.045 to 0.06, even from 0.05 to 0.06.
[0023] The liquid crystal elastomer may be allowed to stress-relax in between each step. A suitable time for the stress-relaxation may depend on the formulation of the liquid crystal elastomer and the temperature relative to Tg of the liquid crystal elastomer. For example, the liquid crystal elastomer may be allowed to relax for 5 to 120 minutes at a temperature of approximately 8°C above Tg of the liquid crystal elastomer. By increasing the amount of time for stress-relaxation between strain steps, it has been found that the auxetic response threshold can be reduced.
[0024] The liquid crystal elastomer may be allowed to stress relax at a temperature above Tg. By increasing the temperature above Tg at which the liquid crystal elastomer undergoes the strain steps and relaxation, it has been found that the auxetic response threshold can be reduced.
[0025] Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. [0026] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which:
[0027] Figure 1 shows an example process of applying a pre-strain condition;
[0028] Figure 2 shows an example liquid crystal elastomer prior to being strained mechanically;
[0029] Figure 3 shows an example process of determining a predetermined strain;
[0030] Figure 4 shows an example process of reducing the auxetic response threshold;
[0031] Figure 5A is a graph of Poisson’s ratio as strain is varied;
[0032] Figure 5B is a graph of true stress as true strain is varied;
[0033] Figure 6 is a graph of the reduction in true strain threshold with applied true pre-strain.
[0034] Figure 7A is a graph of z-strain as x-strain is varied with different stress-relaxation times between strain steps;
[0035] Figure 7B is a graph of the Poisson’s ratio as strain is varied for different relaxation times between strain steps;
[0036] Figure 8A is a graph of z-strain as x-strain is varied at different temperatures;
[0037] Figure 8B is a graph of the Poisson’s ratio as strain is varied at different temperatures.
[0038] According to an example method, an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold is prepared by applying a pre-strain condition to the liquid crystal elastomer. An example process 10 of applying a pre-strain condition is shown in in figure 1 .
[0039] At step 12, an aligned nematic liquid crystal elastomer film is strained to a predetermined strain. The film is strained in a direction that is perpendicular to the director of the liquid crystal elastomer The strain is applied mechanically by clamping the film at either end in a longitudinal direction and straining the film in the longitudinal direction, perpendicular to the director. Figure 2 shows the film 20 clamped at ends 22, 24 in the x-direction (longitudinal direction), in which arrow n indicates the direction of liquid crystal director. In other examples, the strain may be applied by swelling or deswelling the liquid crystal elastomer with solvent or heating or cooling the liquid crystal elastomer. [0040] At step 14, the strained film is left to stress-relax whilst the strained condition is maintained. The time period for which the strained film is left to stress-relax may be determined based on the temperature of the film relative to Tg.
[0041] According to an example, the predetermined strain is determined according to the process 30 shown in figure 3.
[0042] At step 32 of figure 3, a plurality of auxetic response thresholds for the liquid crystal elastomer are determined when the liquid crystal elastomer film is strained at a plurality of different rates. A first auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10-4 s-1, a second auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10-3 s-1, a third auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10-2 s-1, and a fourth auxetic response threshold is determined when straining the liquid crystal elastomer film at a rate of 10-1 s-1.
[0043] To determine the plurality of auxetic response thresholds, local strains in the x- and y- direction, sx and sy respectively, are tracked. The directions of these strains are illustrated in figure 2. The strain is applied in steps, allowing the liquid crystal elastomer to relax between strain steps. The strain in the z-direction (the auxetic response) is calculated based on the measured strains in the x- and y- direction, using Equation 1 , by assuming that the volume of the liquid crystal elastomer remains constant:
[0044] (Equation 1)
[0045] Below the auxetic response threshold, the strain in the z-direction decreases as the applied strain in the x-direction increases. Above the auxetic response threshold, the strain in the z-direction increases as the applied strain in the x-direction increases. The auxetic response threshold can thus be determined based on the calculated strain in the z-direction.
[0046] At step 34 of figure 3, the auxetic response threshold when the straining the liquid crystal elastomer at a quasistatic rate is determined by extrapolating from the determined auxetic response thresholds to a rate of 10-5 s-1. The determined auxetic response threshold when straining the elastomer at a quasistatic rate is determined to be the predetermined strain.
[0047] Figure 4 shows another example method 40 of reducing the auxetic response threshold of an aligned nematic liquid crystal elastomer. The method comprises, at step 42, applying strain to the liquid crystal elastomer in strain steps. The liquid crystal elastomer is strained mechanically in a direction x that is perpendicular to the director of the liquid crystal elastomer, as indicated in figure 2. [0048] The method further comprises, at step 44, allowing the liquid crystal to stress-relax between steps.
[0049] Steps 42 and 44 are repeated, with stress-relaxation between sequential steps.
[0050] Examples
Auxetic aligned nematic liquid crystal elastomer films for use according to the invention were synthesised as follows using the following materials:
2-ethylhexyl acrylate (EHA),
6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate (A6OCB),
1 ,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82),
4-cyano-4’-hexyloxybiphenyl (6OCB) and methyl benzoylformate (MBF).
[0051] The films were prepared using the following starting monomer mixture:
[0052] Films of liquid crystal elastomer were produced by polymerising the monomer mixture inside thin film moulds of approximately 100 pm thickness, a width of approximately 15 mm and a length of approximately 60 mm. In other examples, film moulds of different thicknesses, widths and lengths may be used. The films were cured in the nematic phase at room temperature, with planar alignment, where the surface of the mould was coated with a thin layer of PVA alignment layer and the rubbing direction was anti-parallel. The films were polymerised for two hours. After polymerisation, the films were removed from the moulds and kept in a methanol/DCM solvent mixture overnight in order to wash out the 6OCB.
[0053] A pre-strain condition was applied to a first film, according to the method of figure 1 . The strain condition was applied at a rate of approximately 0.01 s-1 and the film was allowed to equilibrate under the strain for between 0.5-1 hours. A second film, as a comparative example, was formed by the same method, but without a pre-strain condition applied.
[0054] Figure 5A shows a graph of the Poisson’s ratio for the first film and for the comparative example second film. The Poisson’s ratio, vxz, is determined using on Equation 2, wherein the true strain is determined based on the engineering strain using Equation 3.
[0055] vxz = -d£z true/d£x true (Equation 2)
[0056] strue = ln(£eng + 1) (Equation 3)
[0057] As shown in figure 5A, the strain value at which the Poisson ratio becomes negative (auxetic response threshold) is lower for the first film to which the pre-strain condition is applied (data-points labelled 52) than for the comparative second film to which the pre-strain condition is not applied (data-points labelled 54).
[0058] Figure 5B shows a graph of the true stress as the true strain increases for the first film to which the pre-strain condition is applied (data-points labelled 56) and the comparative second film to which the pre-strain condition is not applied (data-points labelled 58).
[0059] The graph in Figure 6 shows the reduction in the auxetic response threshold achieved by applying the method of figure 1 to the liquid crystal elastomer films, for various predetermined strain values, when the strain was applied at a rate of approximately 0.01 s-1 and was allowed to equilibrate under the strain for between 0.5-1 hours. As shown in figure 6, a reduction in the auxetic response threshold can be achieved when applying a pre-strain condition.
[0060] The method of figure 4 was applied to a plurality of the films, wherein the films were strained in steps of 0.05 and the films were allowed to equilibrate between strain steps at different temperatures and for different amounts of time. The Tg of the films was approximately 15°C. Local strains in the x- and y- direction, sx and sy respectively, were tracked and the strain in the z-direction (the auxetic response) was calculated based on the measured strains in the x- and y- direction, using Equation 1 .
[0061] Figure 7A shows a graph of strain in the z-direction as the applied strain in the x- direction is increased in steps, with stress-relaxation times between strain steps of 5 minutes (data-points 71), 10 minutes (data-points 72), 60 minutes (data-points 73) and 120 minutes (data points 74). The measurements were taken with the liquid crystal elastomer at room temperature (23°C), which is 8°C above Tg. [0062] Figure 7B shows a graph of the Poisson’s ratio as the applied strain in the x-direction is increased in steps, with stress-relaxation times between strain steps of 5 minutes (line 75), 10 minutes (line 76), 60 minutes (line 77) and 120 minutes (line 78). The Poisson’s ratio is calculated using equation 2.
[0063] As shown in figures 7A and 7B, the auxetic response threshold (at which the strain in the z-direction begins to increase in figure 7A and at which the Poisson’s ratio becomes negative in figure 7B) decreases as the time-period of stress-relaxation between strain steps increases.
[0064] Figure 8A shows a graph of strain in the z-direction as the applied strain in the x- direction is increased in steps, with stress-relaxation times between strain steps of 10 minutes, when the liquid crystal elastomer is at 8°C above Tg (data-points 81) and when the liquid crystal elastomer is heated to 15°C above Tg (data-points 82) and 20°C above Tg (data-points 83).
[0065] Figure 8B shows a graph of the Poisson’s ratio as the applied strain in the x-direction is increased in steps, with stress-relaxation times between strain steps of 10 minutes when the liquid crystal elastomer is at 8°C above Tg (line 84) and when the liquid crystal elastomer is heated to 15°C above Tg (line 85) and 20°C above Tg (line 86). The Poisson’s ratio is calculated using equation 2.
[0066] As shown in figures 8A and 8B, the auxetic response threshold (at which the strain in the z-direction begins to increase in figure 8A and at which the Poisson’s ratio becomes negative in figure 8B) decreases as the temperature of the liquid crystal elastomer during the strain steps and stress-relaxation increases.
[0067] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. Also, the recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.
[0068] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. [0069] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. [0070] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. [0071] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1 . A method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying a pre-strain condition to a liquid crystal elastomer.
2. The method of claim 1 , wherein applying the pre-strain condition comprises straining the liquid crystal elastomer and allowing the liquid crystal elastomer to stress-relax while being maintained in the strained condition.
3. The method of claim 2, wherein allowing the liquid crystal elastomer to stress-relax comprises allowing the liquid crystal elastomer to stress-relax at a temperature higher than the glass transition temperature Tg of the liquid crystal elastomer.
4. The method of claim 2 or claim 3, wherein straining the liquid crystal elastomer comprises at least one of straining the liquid crystal elastomer mechanically, swelling or deswelling the liquid crystal elastomer with solvent, and adjusting the temperature of the liquid crystal elastomer
5. The method of claim 4, wherein straining the liquid crystal elastomer mechanically comprises applying strain in a direction perpendicular to the director of the liquid crystal elastomer.
6. The method of any of claims 2-5, wherein straining the liquid crystal elastomer comprises straining the liquid crystal elastomer to a predetermined strain.
7. The method of claim 6, wherein the predetermined strain is the strain at which the liquid crystal elastomer reaches the auxetic response threshold when being strained at a quasistatic rate.
8. The method of claim 6 or claim 7, wherein the method further comprises determining the predetermined strain, wherein determining the predetermined strain comprises: determining the auxetic response threshold when the liquid crystal elastomer is strained at each of a plurality of different rates and determining the auxetic response threshold for the straining the liquid crystal elastomer at a quasistatic rate by extrapolating from the determined auxetic response thresholds.
9. The method of claim 10, wherein the plurality of different rates are 10-4 s-1, 10-3 s-1, 10-2s-1, and 10-1 s-1.
10. A method of preparing an aligned nematic liquid crystal elastomer with a reduced auxetic response threshold comprising applying strain to the liquid crystal elastomer in a series of steps and allowing the liquid crystal to stress-relax between steps.
11. The method according to claim 10, wherein straining the liquid crystal elastomer comprises straining the liquid crystal by 0.02 to 0.1 in each step.
12. The method according to any of claims 10-11 , wherein allowing the liquid crystal elastomer to stress-relax comprises allowing the liquid crystal to stress-relax at a temperature higher than Tg of the liquid crystal elastomer.
EP24714989.1A 2023-03-21 2024-03-20 Method of preparing an aligned liquid crystal elastomer Pending EP4683980A1 (en)

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