WO2020201533A1

WO2020201533A1 - A composite comprising a shape-memory polymeric material (smp) which is switchable between an optically scattering state and an optically transparent state

Info

Publication number: WO2020201533A1
Application number: PCT/EP2020/059626
Authority: WO
Inventors: Koen NICKMANS; Danielle Anna Catharina VAN DER HEIJDEN; Albertus Petrus Hendrikus Johannes Schenning; Yari FOELEN
Original assignee: Technische Universiteit Eindhoven
Priority date: 2019-04-03
Filing date: 2020-04-03
Publication date: 2020-10-08

Abstract

The present invention relates to composite comprising a substrate provided with at least an upper layer having an outer surface, wherein the upper layer comprises a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state, the SMP having a glass transition temperature Tg, SMP, characterized in that the outer surface of the upper layer is at least partially a rough surface having an arithmetic average roughness Ra of at least 0.1 µm.

Description

Title: A composite comprising a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state.

FIELD OF THE INVENTION

The present invention relates to a composite comprising a substrate provided with at least an upper layer having an outer surface, wherein the upper layer comprises a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state, the SMP having a glass transition temperature Tg, SMP.

BACKGROUND OF THE INVENTION

Shape memory polymers (SMPs) which can be deformed into a temporary shape and subsequently recovered into a permanent memorized shape via external stimuli are receiving a lot of attention. Shape memory polymers (SMPs) are a class of “smart” materials that can switch between two shapes on command, from a fixed (temporary) shape to a pre-determined permanent shape upon the application of an external stimulus such as heat. Such polymers can also be used as coatings capable of changing their surface topography. These smart surfaces have been fabricated for example by thermomechanical programming at biologically relevant temperatures for dynamic control of adhesion, wetting of liquids, and cell-topography interactions.

To circumvent the cumbersome top-down nanoforming steps to generate the structural color, photonic SMP coatings generated by bottom-up self-assembly have also employed. For example, shape memory photonic films have been produced from core- interlayer-shell polymer microspheres that form opal structures. Alternatively porous inverse- opal SMPs have been templated by silica colloids. Capillary pressure-induced “cold” programming of these materials results in a disordered temporary state consisting of collapsed pores and an arbitrarily roughened surface, which can be recovered by pressure, heat, organic vapors, solvents, and microwave radiation. Unfortunately, the fabrication of shape memory coatings that change both reflectivity and topography is hampered by the lack of facile methods and materials.

WO 2010/084010 relates to a multifunctional optical sensor, having at least two areas which independently react to different input parameters, the sensor comprising a substrate and a polymeric layer comprising polymerized liquid crystal monomers having an ordered morphology, wherein the color, the reflectivity or the birefringence of the sensor changes due to a change of the morphology, wherein the change of the morphology is caused by physical contact with a chemical agent such as a gas or liquid a change of temperature, or passage of time. WO 2010/084010 also relates to a process for the preparation of such a sensor comprising the steps of providing a substrate or a substrate having an alignment layer, applying a film of a coating composition on the substrate, forming a cholesteric or birefringent liquid crystalline structure within the film, applying a dose of electromagnetic radiation to the film to at least partly cure the film, wherein the coating composition comprises at least one liquid crystalline material having at least one polymerizable group, optionally a chiral compound and a photo-initiator.

US 2017/297258 relates to a method of preparing a functionally graded shape memory polymer, the method comprising the steps of: providing a shape memory polymer comprising a first end and a second end; photocuring the shape memory polymer by use of a radiation source through a gradient photomask, wherein the gradient photomask allows an increasing amount of radiation to reach the shape memory polymer from the first end to the second end, wherein the photocuring produces a corresponding increasing gradient in crosslink density and glass transition temperatures (Tg's) to the shape memory polymer from the first end to the second end.

Top-down techniques, such as compression molding and nanoimprint lithography have been used in combination with SMPs to generate surface features that generate specific optical properties such as structural colors or scattering. Deformation of the surface features using for example a hot press, allows for the optical effect to be temporarily disabled. Upon heating, the surface features are restored, thereby resulting in an optical contrast. The use of top down methods to generate the intricate surface topographies is however relatively slow and cumbersome.

Alternatively bottom-up techniques have been used to self-assemble SMPs that inherently exhibit structural color. Deformation of such coatings results in a disappearance of the color, which is restored upon heating. These materials are, however, also relatively difficult to produce since they often rely on templating or complex processing steps. An object of the present invention is to generate optical contrast using common shape memory materials (or photonic materials such as liquid crystal polymers), without the need for first thermoforming intricate surface features or other complex processing steps.

Another object of the present invention is to provide a method that can generate optical contrast in SMP coatings based on programmable and thermally reversible surface scattering.

Another object of the present invention is to provide a SMP coatings that can be used as reconfigurable nanophotonic devices for optical data storage, photovoltaics, optical sensors, as well as smart adhesives, bio-surfaces, and battery-free optical time- analyte indicators.

The present invention thus relates to a composite comprising a substrate provided with at least an upper layer having an outer surface, wherein the upper layer comprises a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state, the SMP having a glass transition temperature Tg, SMP, wherein the outer surface of the upper layer is at least partially a rough surface having an arithmetic average roughness R_a of at least 0.1 pm, the arithmetic average roughness R_a being measured according to the method disclosed in the description.

The present inventors found shape memory polymeric coatings on a substrate (composites) which can switch between an optically scattering state and an optically transparent state. Initially a smooth coating is produced which is transparent. The surface of the coating is roughened by pressing into a rough stamp at elevated temperature (above the glass transition temperature of the polymeric coating), which results in surface scattering and thereby an opaque appearance. Upon heating, the shape memory coating returns to its original smooth state, which is optically transparent.

In an embodiment of the present composite the arithmetic average roughness R_a is at least 0.2 pm, more preferably at least 0.4 pm. The method for determining the arithmetic average roughness R_a is disclosed in the present description.

In an embodiment of the present composite the thickness of the upper layer is at most 20.0 pm, preferably at most 10.0 pm, more preferably at most 6.0 pm, even more preferably of at most 4.0 pm, and most preferably of at most 2.5 pm. In an embodiment of the present composite the Tg.SMP is below 70 °C, preferably below 50 °C, more preferably below 30 °C, and most preferably below 20 °C.

In an embodiment of the present composite the substrate comprises one or more sheets.

In an embodiment of the present composite one or more sheets are selected from the group of a glass sheet, a polymeric material sheet, a paper sheet, a paperboard sheet, a metal sheet, a mineral sheet and a sheet made of ink, or a combination thereof.

In an embodiment of the present composite at least one sheet is transparent.

In an embodiment of the present composite at least one sheet is coloured, such as white and black, preferably coloured black.

In an embodiment of the present composite shape-memory polymeric material (SMP) comprises a cholesteric liquid crystalline (CLC) polymeric material.

In an embodiment of the present composite the CLC polymeric material comprises a difunctional crosslinker, monofunctional acrylates, a chiral dopant, a photo initiator and a di-functional thiol.

In an embodiment of the present composite the upper layer is a sheet comprising CLC polymeric material.

In an embodiment of the present composite the CLC polymeric material has multiple colours and/or multiple transition temperatures programmed in the polymerization process through a difference or pattern in applied UV-intensity and/or temperature.

In an embodiment of the present composite the CLC polymeric material is configured as particles.

The present invention also relates to a method of manufacturing a composite as discussed above the method comprising the following steps:

a. providing a substrate layer,

b. applying an upper layer having an outer surface onto the substrate layer of a), the upper layer comprising a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state, the SMP having a glass transition temperature Tg,SMP,

c. heating the construction of b) to a temperature above the Tg, SMP, d. contacting the upper layer with a stamp having a rough surface, e. cooling down the construction of d) to a temperature below the Tg, SMP, while the stamp is in contact with the upper layer,

f. removing the stamp thereby obtaining a composite in which the outer surface of the upper layer is at least partially a rough surface having an arithmetic average

roughness R_a of at least 0.1 pm.

The present invention also relates to an optical sensor comprising a composite material as discussed above, wherein the optical sensor is located on one or more goods chosen from the group of food, medicine, chemicals, and temperature sensitive perishable goods.

The present invention also relates to an optical sensor comprising a composite material obtained according to the present method, wherein the optical sensor is located on one or more goods chosen from the group of food, medicine, chemicals, and temperature sensitive perishable goods.

The present SMP coatings that change both topography and color are appealing for a range of reconfigurable nanophotonic devices for optical data storage, photovoltaics and optical sensors, as well as smart adhesives, biosurfaces and battery-free optical time-analyte indicators.

SUMMARY OF THE INVENTION

The present inventors found that shape memory photonic coatings can be fabricated by high-speed flexographic printing and UV-curing in air of a chiral nematic liquid crystal ink. Formable polymeric films with a red reflection band and a smooth surface topography are obtained which can be thermally programmed above room temperature by using a rough stamp. This thermomechanical programming results in a temporary rough surface topography change leading to a grey color below room temperature. By heating the coatings, a shape recovery to the permanent state is observed, thereby restoring the smooth surface topography and the iridescent red reflection color. The present inventors found that this recovery is highly temperature dependent, which allows for a fast and distinct optical and topography change upon exceeding room temperature. These thermo-responsive photonic crystal coatings have a great potential as low-cost optical sensors, smart adhesives and adaptive biosurfaces. Therefore, the coating switches from a scattering state to a transparent state when it is above the trigger temperature and this effect can be used to conceal a background message or underlying pigment layer. The glass transition temperature may be modified to be dependent on other variables such as humidity (e.g. Nylon), light (photo softening), or pH. Incorporating these materials into the shape memory layer would allow the sensing of these variables.

The present inventors found that the use of photonic material such as a cholesteric liquid crystal (CLC) polymer offers the additional advantage of a coloured state In such a case, a red reflection was obtained by the use of CLC material. Upon deformation, the surface is roughened, resulting in a flattened reflection band. Upon heating, the smooth surface and red reflection are restored.

The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings in which:

Fig 1 is an example of a reactive liquid crystal mixture.

Fig 2 is a schematic illustration showing the preparation of polymeric photonic coatings using flexographic printing.

Fig 3 is a schematic illustration showing the compression of the CLC coating into a rubberized metal sheet using a custom hot-stamp tool.

Advantages of the invention are illustrated by the following Example. However, the particular materials and amounts thereof recited in these examples, as well as other conditions and details, are to be interpreted to apply broadly in the art and should not be construed to unduly restrict or limit the invention in any way.

DETAILED DESCRIPTION

For the fabrication of shape memory CLC coatings, the present inventors designed a reactive liquid crystal mixture that is composed of a difunctional crosslinker 1 , monofunctional acrylates 2 and 3, chiral dopant 4, photo-initiator 5, and a di-functional thiol 6 (see Figure 1). Please note that the reference numbers used in Figure 1 only apply for the chemical components shown here. The thiol acts as an oxygen inhibitor thereby allowing photopolymerization in air, and as a chain extender via radical thiol-acrylate polymerization, thereby reducing the cross-linking density and lowering the Tg to room temperature (vide infra).

The composition of the mixture was chosen such that a red reflective, deformable coating was obtained, but in principle any color is possible by varying the chiral dopant concentration. Thermal characterization of the mixture by polarized optical microscopy (POM) and differential scanning calorimetry (DSC) showed a room temperature chiral nematic phase, characterized by a typical Schlieren defect texture. The chiral nematic to isotropic (N*-l) transition was situated around 50 °C upon heating, but upon cooling suppressed to around 25 °C and less well-defined, likely due to the relatively high amount of non-mesogenic material present. The mixtures did not crystallize on the timescale of the DSC measurement (> 1 hour). Therefore, the mixtures can be printed and annealed at room temperature, without risk of crystallization prior to photopolymerization.

Polymeric photonic coatings were prepared using flexographic printing (Figure 2). A printing ink was prepared by adding a compatible solvent (cyclopentanone) and a small amount of surfactant to promote planar alignment at the coating-air interface. The ink 1 was deposited on an anilox cylinder 3, spread evenly using a doctor blade 2, transferred to a printing cylinder 5 containing 2 x 2 cm fields (full-tone), and subsequently transferred to the substrate 4 thereby producing the coating. A black biaxially-oriented polyethylene terephthalate (PET) substrate was selected to facilitate viewing of the structural color. After printing, the solvent 6 was quickly evaporated at elevated temperature (70 °C), and the coating was briefly held at room temperature, which led to development of the CLC phase, and the appearance of a red reflection. The coatings were then photopolymerized by passing through a UV dryer 7 in air, resulting in non-sticky and highly flexible polymeric PC coatings with a bright iridescent color. The step of deform is indicated with reference number 8. Upon normal (perpendicular) viewing, the coatings display a red reflection which is marked by a reflection band centered at 620 nm. The surface roughness and thickness of the coatings were approximately 0.02 pm and 3 pm respectively, as measured by profilometry. The mid point of the Tg was determined by DSC to be 18 °C, close to room temperature.

Thermomechanical programming was performed by compressing the CLC coating 11 into a rubberized metal sheet using a custom hot-stamp tool 10 (Figure 3). The rubberized sheet had an arithmetic roughness (Ra) of « 1 pm. The temperature was set at 35 °C, which was chosen to be sufficiently above the Tg of the coating to induce the shape memory effect, but not so much as to cause irreversible plastic deformation. Compression below the Tg had no noticeable effect. After compression, the stamp was quickly removed thereby quenching the coating to room temperature, and the coating was peeled off (reference number 12) of the rubberized sheet. Upon inspection the deformed coating revealed a uniform matte grey appearance across the coating dimensions (2 x 2 cm) which was marked by a contrasting reflectivity spectrum (Figure 2b). The reflection at 620 nm is reduced, and the reflection at initially non-reflective wavelengths is significantly increased when compared with the initial state. As such, the reflection at all visible wavelengths is approximately equal, in agreement with the grey visual appearance of the coating as a result of the deformation. Investigation of the surface topography by profilometry revealed that the surface height profile is significantly altered from relatively smooth (R_a = 0.02 pm) to rough (R_a = 0.3 pm) as a result of the compression by the rough stamp. Surface scattering is therefore identified as the primary source of the optical contrast. In fact, application of the same deformation protocol to a CLC coating in which the reflection band was shifted out of the visible range induced a similar optical effect. Interestingly, the presence of a particle contamination had no effect on the deformation.

Briefly heating the deformed coating above the Tg (30 s, 40 °C) restored the initial red appearance of the coating, and completely returned the reflection spectrum back to the initial state. At the same time, the surface topography was restored completely to its smooth initial state (R_a = 0.02 pm), indicating that the perceived optical change of the shape memory photonic coating is caused by the shape recovery from the deformed geometry to the initial geometry upon heating above Tg. The present inventors found that the low cross-linking density of the chiral nematic polymer network (having 10% difunctional monomers) was critical toward achieving the desired combination of mechanical deformation across the entirety of the coating thickness, and the remarkable ability to nevertheless make a complete recovery from the deformed state.

The present inventors found that the rate of the optical (and surface topographical) response of the deformed coating was highly temperature dependent. When stored at 10 °C (T < Tg, onset), no color change was observed to occur in the space of several months. When stored at 30 °C (T > Tg, end), a complete recovery was observed within approximately 3 minutes (Figure 3a), while at 25 °C (T = Tg, end), the recovery took approximately 1 hour. This finding further demonstrates that the kinetics of the photonic response are related to the physical shape recovery dependent on polymer chain mobility around the Tg.

The present inventors investigated the temperature dependence of the photonic response near Tg by measuring the color change under isothermal conditions using reflective colorimetry (see experimental). The onset of color over time is described by the normalized sum of the chromacity coordinates“a” and“b” in the CIELab colour space. In each case, the optical response was fitted using an exponential decay function, according to: norm (a + b) = 1— e^~kt where k is the rate constant and t the time. By plotting the rate constants over the measured range in an Arrhenius plot, the activation energy was estimated to be approximately 381 kJ moM .

Experimental Section

Materials

2-methyl-1 ,4-phenylene bis(4-(((4-(acryloyloxy)butoxy)carbonyl) oxy)benzoate) 1 , and (3R,3aS,6R,6aS)-hexahydrofuro[3,2-b]furan-3,6-diyl-bis(4-((4-(((4-(acryloyloxy)- butoxy)carbonyl)oxy)benzoyl)oxy)benzoate) 4 were obtained from BASF.

4-((6-(acryloyloxy)hexyl)oxy)phenyl 4-methoxybenzoate 2 was purchased from Synthon Chemicals. 4-cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate 3 was obtained from Merck. 2,2'-(ethane-1 ,2-diylbis(oxy))bis(ethane-1 -thiol) 6 was purchased from Sigma Aldrich. The photo initiator 2-benzyl-2-(dimethylamino)-1-(4-morpholinophenyl)butan-1 -one 5 was obtained from IGM resins. Cyclopentanone was purchased from Acros.

Preparation of ink

The chiral nematic liquid crystal mixture contained 10% (w/w) of difunctional monomers, 84% of monofunctional monomers, 3% of dithiol, and 3% of photo- initiator. A small amount of surfactant BYK-361 N was also added. These components were dissolved in cyclopentanone (2:1 solids: solvent ratio), and the resulting solutions were filtered through a 0.2 pm PTFE syringe filter.

Preparation of coatings

Coatings were prepared on an IGT Printability Tester F1 from IGT Testing System Pte Ltd., operating in flexo mode at 0.3 m s ¹. Biaxially oriented PET (Tenolan OCN0003, 36 pm thickness) was used as substrate. After coating, the coatings were heated to 70 °C for 30 seconds to evaporate the solvent, and annealed for 60 seconds at room temperature to align the CLC phase. Photopolymerization was performed by passing the substrate through an IGT UV Dryer in air (Intensity: 155 mW/cm² (UVA); 134 mW/cm² (UVB), Dose: 88 mJ/cm² (UVA); 73 mJ/cm² (UVB)). Finally, the coatings were heated to 70 °C for 30 s to ensure complete conversion of reactive acrylates, which was confirmed using Fourier Transform Infrared Spectroscopy (FTIR).

Activation protocol

A modified hot-stamp machine from KBA-Metronic GmbH was used to compress the surface of the coating into a rubberized metal sheet (R_a ~ 1 pm) for 30 s at a pressure of 4.5 bar. The machine was fitted with a brass stamp heated to 35 °C. Following the compression the coating was quickly returned to room temperature, removed from the machine, and stored in the fridge to avoid a premature color change.

Characterization

Polarized optical microscopy (POM) was performed with crossed polarizers using a Leica DM6000M equipped with a DFC420C camera and a Linkam THMS600 hot-stage for temperature control. Phase transition temperatures of the CLC mixtures and polymeric coatings were determined using a TA Instrument Q1000 differential scanning calorimeter (DSC). 3-4 mg of material was hermitically sealed in aluminum pans. The heating and cooling rate was 10 °C min ¹. For the monomer mixtures, the second cooling curve was used to determine the transition temperature. For the polymers the second heating curve was used. Fourier transform infrared spectroscopy (FTIR) spectra were obtained using a FTS 6000 spectrometer from Bio-Rad equipped with Specac Golden gate diamond ATR and were signal-averaged over 50 scans at a resolution of 1 cm ¹. Profilometry experiments were performed using a Bruker DektakXT, set to measurement range 65.5 pm and stylus force 3 mg. The arithmetic average roughness R_a was calculated by the formula:

where n is the number of measurements, and y is the vertical deviation from the mean, according to DIN 4768/1. The arithmetic average roughness Ra as mentioned in the claims is thus measured according to the method disclosed in the description.

Kinetic measurements were performed by placing three samples on an IKA RCT basic hotplate held isothermally. The temperature was calibrated using a Comark KM340 industrial thermometer. Reflectivity spectra and colorimetric data were obtained using an i1 Pro from X-Rite Inc. The color change was described according to:

(a + b)— (a + b)_mi„

norm (a + b)

(a + b)_max— (a + b)_min where norm (a + b) represents the normalized color of the coating between compressed and heated states, and a and b are the measured chromacity coordinates in the CIELab color space, and (' + ));78 and (' + ));3< are the minimum and maximum sum of the sum of the measured a and b parameters respectively.

Thermoresponsive photonic coatings were fabricated that change topography and color by exploiting the shape memory behavior of mechanically deformed chiral nematic polymer networks. Upon heating above room temperature, the coatings exhibit a distinct color change from grey to iridescent red with a high activation energy (381 kJ mol-1) and therefore show a strong temperature dependence. The present method allows for the creation of temporary, arbitrary surface topographies by using the appropriate imprinted polymer stamp. While currently plate-to-plate, the deformation protocol can also be envisioned in high speed roll-to-roll or roll-to-plate processes, for example using a rubberized metal roll. Therefore, these thermoresponsive photonic coatings are promising for a wide range of low cost smart adhesives, adaptive biosurface and optical sensor and devices.

Claims

1. A composite comprising a substrate provided with at least an upper layer having an outer surface, wherein the upper layer comprises a shape-memory polymeric material (SMP) which is switchable between an optically scattering state and an optically transparent state, the SMP having a glass transition temperature Tg, SMP, characterized in that the outer surface of the upper layer is at least partially a rough surface having an arithmetic average roughness R_a of at least 0.1 pm.

2. The composite according to claim 1 , characterized in that the arithmetic average roughness R_a is at least 0.2 pm, more preferably at least 0.4 pm.

3. The composite according to any one or more of the preceding claims, characterized in that thickness of the upper layer is at most 20.0 pm, preferably at most

10.0 pm, more preferably at most 6.0 pm, even more preferably of at most 4.0 pm, and most preferably of at most 2.5 pm.

4. The composite according to any one or more of the preceding claims, characterized in that the Tg,SMP is below 70 °C, preferably below 50 °C, more preferably below 30 °C, and most preferably below 20 °C.

5. The composite according to any one or more of the preceding claims, characterized in that the substrate comprises one or more sheets, wherein one or more sheets are preferably selected from the group of a glass sheet, a polymeric material sheet, a paper sheet, a paperboard sheet, a metal sheet, a mineral sheet and a sheet made of ink, or a combination thereof.

6. The composite according to claim 5, characterized in that at least one sheet is transparent.

7. The composite according to any one or more of claims 5-6, characterized in that at least one sheet is coloured, such as white and black, preferably coloured black.

8. The composite according to any one or more of the preceding claims, characterized in that shape-memory polymeric material (SMP) comprises a cholesteric liquid crystalline (CLC) polymeric material.

9. The composite material according to claim 8, characterized in that the CLC polymeric material comprises a difunctional crosslinker, monofunctional acrylates, a chiral dopant, a photo-initiator and a di-functional thiol.

10. The composite material according to any one or more of claims 8-9, characterized in that the upper layer is a sheet comprising CLC polymeric material.

1 1. The composite material according to any one or more of claims 8-10, characterized in that the CLC polymeric material has multiple colours and/or multiple transition temperatures programmed in the polymerization process through a difference or pattern in applied UV-intensity and/or temperature.

12. The composite according to any one or more of claims 8-1 1 , characterized in that the CLC polymeric material is configured as particles.

13. A method of manufacturing a composite according to any one or more of the preceding claims, the method comprising the following steps:

a. providing a substrate layer,

f. removing the stamp thereby obtaining a composite in which the outer surface of the upper layer is at least partially a rough surface having an arithmetic average roughness R_a of at least 0.1 pm.

14. An optical sensor comprising a composite material according to any one or more of the claims 1-12, the optical sensor being located on one or more goods chosen from the group of food, medicine, chemicals, and temperature sensitive perishable goods.

15. An optical sensor comprising a composite material obtained according to the method of claim 13, the optical sensor being located on one or more goods chosen from the group of food, medicine, chemicals, and temperature sensitive perishable goods.