WO1993015134A2 - Method for enhancing irradiation sterilization stability of optical polymers - Google Patents

Method for enhancing irradiation sterilization stability of optical polymers Download PDF

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Publication number
WO1993015134A2
WO1993015134A2 PCT/US1993/000015 US9300015W WO9315134A2 WO 1993015134 A2 WO1993015134 A2 WO 1993015134A2 US 9300015 W US9300015 W US 9300015W WO 9315134 A2 WO9315134 A2 WO 9315134A2
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Prior art keywords
polymer
modified
glass transition
temperature
polymers
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PCT/US1993/000015
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French (fr)
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WO1993015134A3 (en
Inventor
Julianne P. Harmon
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University Of Florida
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Publication of WO1993015134A3 publication Critical patent/WO1993015134A3/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C71/00—After-treatment of articles without altering their shape; Apparatus therefor
    • B29C71/04—After-treatment of articles without altering their shape; Apparatus therefor by wave energy or particle radiation, e.g. for curing or vulcanising preformed articles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C35/00—Heating, cooling or curing, e.g. crosslinking or vulcanising; Apparatus therefor
    • B29C35/02—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould
    • B29C35/08—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould by wave energy or particle radiation
    • B29C35/0866—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould by wave energy or particle radiation using particle radiation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2025/00—Use of polymers of vinyl-aromatic compounds or derivatives thereof as moulding material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2033/00—Use of polymers of unsaturated acids or derivatives thereof as moulding material
    • B29K2033/04—Polymers of esters
    • B29K2033/12—Polymers of methacrylic acid esters, e.g. PMMA, i.e. polymethylmethacrylate

Definitions

  • the present invention relates to optically transparent polymers.
  • Optical polymers are often employed in environ ⁇ ments where they are exposed to ionizing radiation.
  • plastics contain ⁇ ing fluorescent dyes which act as high energy scintilla- tion particle detectors in nuclear particle accelerator facilities and in space flights.
  • Plastics doped with fluorescent dyes function as absorbers of light of one wavelength and emit light at a different wavelength.
  • these plastic wavelength shifters are used as solar light concentrators.
  • Such plastics are often used as the core of optical fibers which transmit light for the purpose of communicating information.
  • the transmit ⁇ ted light can be in the form of unique pulses (digital) of light or of light with a time varying amplitude (ana- log) •
  • These optical fibers are used in particle acceler ⁇ ators, nuclear reactors, fusion machines, space flights, local area networks, automotive communications, military systems and display systems.
  • These types of plastics are also used in biomedical or other devices which must be sterilized by the application of ionizing radiation.
  • Optical polymers are exposed to ionizing radia- tion when biomedical plastic members are sterilized by gamma radiation or when used as scintillating or wave ⁇ length shifter plastics in high energy radiation detec ⁇ tors.
  • optical radiation hard linear and cross-linked polymers with glass transitions below the end use temperature have been identified. It will be highly desirable to be able to produce optical thermo ⁇ plastics which recover from ionizing radiation-induced optical damage at accelerated rates.
  • Such polymers should, optimally, be self-supporting, melt processed by conventional techniques and have glass transition temper ⁇ atures above room temperature.
  • thermoplastic poly ⁇ mer structures could be designed that recover at acceler ⁇ ated rates when incubated at temperatures below the glass transition temperatures and, in the best case, that re- cover during or shortly after irradiation without the need to incubate at elevated temperatures.
  • Conventional thermoplastic optical polymers can warp at temperatures needed to induce recovery in a practical time frame. It has been proposed in the past to modify certain polymer systems to reduce radiation-induced color center formation.
  • U.S. Patent No. 4,939,186 describes the incorporation of certain alkenes in polycarbonates to color-stabilize the latter against irradiation-induced coloration.
  • the alkene is added at levels of 0.01 - 5%, based on the weight of the polymer.
  • the utilization of such small amounts of alkene additive is indicative that the mechanism of color stabilization is dependent upon the chemical properties of the additive, i.e., its poten- tial for chemically reacting with the system in which it is incorporated.
  • U.S. Patent No. 4,624,972 relates to the use of , esters of polycarboxylic acids with glycol ethers for
  • U.S. Patent No. 4,904,710 refers to the use of 15 polyetherpolyols or alkyl ethers thereof to stabilize polycarbonates against the discolorization effects of irradiation.
  • the patent discloses the use of such low concentrations, i.e., up to only 5%, that the mechanism of color stabilization must depend upon the chemical 20 reactivity of the additives during irradiation.
  • the present invention comprises a 30 method of enhancing the ionizing irradiation steriliza ⁇ tion stability of a substantially optically transparent, rigid, heat processible thermoplastic polymer comprising modifying the polymer to decrease the glass transition temperature (Tg) to a value such that: a) the glass transition temperature of the modified polymer is above the desired end-use temperature thereof; b) the modified polymer remains substantially optically transparent, rigid and heat-processible; and c) the modified polymer recovers from the discoloration effects produced by exposure to ionizing sterilizing irradiation at a temperature below the glass transition temperature thereof.
  • Tg glass transition temperature
  • a further embodiment of the invention comprises a method of restoring optical transparency to an optical ⁇ ly transparent, thermoplastic polymer modified according to the above method which has become discolored due to exposure thereof to ionizing irradiation comprising rais ⁇ ing the temperature of the discolored thermoplastic poly ⁇ mer to an effective decoloration temperature below the glass transition temperature thereof for a time suffi- cient to effect decoloration thereof.
  • Another embodiment of the invention comprises an article of manufacture constructed at least in part from the modified polymer produced according to the above-described method.
  • a final embodiment of the invention comprises an article of manufacture described above which has been subjected to ionizing irradiation sufficient to discolor the modified polymer and subsequently heated to a temper ⁇ ature below its glass transition temperature and for a time sufficient to decolorize the polymer.
  • Figs. 1-6 depict discolored volume boundary velocities in various polymers at various temperatures.
  • radiation hard polymers which are optically transparent; recover rapidly from yellowing due to expo ⁇ sure to ionizing radiation; are rigid, self-supporting materials; and are heat processible by such methods as 20 extrusion, injection molding, thermoforming, blow mold ⁇ ing, compression molding, calendaring, etc.
  • the goal of the invention is to design polymers with the above characteristics. It is normally very difficult to maximize these characteristics simultaneous- 25 ly.
  • the goal is achieved by decreasing the Tg of optical polymers while keeping the Tg sufficiently above the use temperature of the plastic part to enable its employment in intended applications.
  • glassy optical polymers When glassy optical polymers are exposed to 30 ionizing radiation, they discolor. Examples of such polymers are poly(methyl methacrylate) , polystyrene, polyvinyltoluene, polyvinylmethylpentane, etc.
  • the dis ⁇ colored portion often fluoresces. This discoloration is r recoverable. That is, a sharp recovery boundary moves from the outside toward the center of the sample leaving clear material behind. The boundary moves at a rate that is linear with time and is controlled by polymer relax ⁇ ations. The process is thermally activated and exhibits Arrhenius behavior. Recovery is instantaneous when the plastics are heated above the glass transition tempera ⁇ ture.
  • Heating above the glass transition is a disadvan ⁇ tageous method for inducing recovery in rigid thermoplas ⁇ tic parts since they flow and lose their shape.
  • polymers above the glass transition are used for structural members, while they may not discolor upon exposure to ionizing radiation, they must be cross-linked or encapsulated with a rigid media so that they form self-supporting structural members.
  • Cross-linked plas- tics cannot be heat processed by conventional methods.
  • the present invention provides a process for altering the structure of optical polymers to impart accelerated recovery from radiation-induced discoloration while still keeping the advantageous properties of heat processibility and rigidity. Specifically, the Tg of the polymer molecules is decreased, but not to the extent that the glass transition is below the temperature of use. This results in accelerated recovering during radi ⁇ ation or shortly thereafter without the need for extended annealing at a temperature above the use temperature. This may be accomplished in one of the following ways: a) appending side chains that decrease the glass transition temperature.
  • These may be composed of chain, branched or cyclized groups containing aliphatic, ether, carbonyl groups or groups containing other hetero- ato s such as nitrogen; b) adding flexible substituents such as ether groups to the polymer backbone; c) copolymerizing monomers of optical poly ⁇ mers with monomers of lower glass transition transparent polymers.
  • the copolymers may be of the random, alternat ⁇ ing, graft or block type; d) blending low and high glass transition polymers to yield a polymer with a glass transition above the use temperature; e) plasticizing the polymer with low molecu ⁇ lar weight compounds.
  • plasticizers may be in free dissolved form or bound to the polymer on a side chain, end or in the polymer backbone. These plasticizers may range from small molecules to oligomers. They decrease the glass transition temperature by increasing the free volume in the polymer matrix; or f) deforming the polymer molecules. Polymers often contain residual stresses and areas of increased free volume due to processing techniques such as injec ⁇ tion molding and spinning in initial stages of drawing. The deformation process may be such that initial stages of deformation increase free volume. This is observed in the initial stages of drawing of polymers where the free volume increases due to structural rearrangements which precede high orientation at high deformation levels. Local areas of increased free volume result in a de- creased glass transition temperature.
  • the advantage of this invention is the flexi ⁇ bility attained from the ability to select from a wide variety of polymer classes.
  • One embodiment of the present invention is a process for altering the structure of optically transpar ⁇ ent, rigid, heat processible thermoplastic polymers to impart them with optical radiation hardness. More specifically, the invention comprises decreasing the Tg of the modified polymers to the extent that the Tg is above the temperature at which they are employed in envi ⁇ ronments, whereby they are exposed to ionizing radiation. This results in a decrease of polymer discoloration with associated time dependence. Decreasing the Tg of the polymers may be accom ⁇ plished by one of the following methods.
  • the polymers or copolymers may be modified to decrease the Tg thereof by appending flexible sub- stituents to at least one of the monomers employed to prepare the polymers or copolymers.
  • the substituent is a flexible lin ⁇ ear or branched hydrocarbon chain, optionally interrupted by at least one heteroatom, e.g., 0, S, N, etc., or ringed structures such as cyclohexyl or phenyl groups attached to flexible chains.
  • Modification of the polymers or copolymers to decrease the Tg thereof may be effected by the copolymer- ization of the monomer(s) employed to prepare the poly- mers or copolymers with at least one additional comono- mer.
  • Modification of the polymer to decrease the Tg thereof may be effected by incorporating flexible linkag- es such as ether linkages into the polymer backbone.
  • Modification of the polymer or copolymer to decrease the Tg thereof may also take the form of inti ⁇ mately blending the polymers or copolymers with a second polymer or copolymer having a glass transition tempera- ture Tg substantially lower than that of the polymer or copolymer, but above the end-use temperature.
  • Modification of the polymer or copolymer to increase the free volume thereof can also be achieved by the plasticization of the polymer or copolymer with a low molecular weight compound, ranging from one unit to an oligomer.
  • Plasticizer in the conventional sense means any organic molecule which is soluble in the polymer in question over the appropriate concentration range and which has a glass transition temperature lower than that of the polymer.
  • the plasticizer may consist of one or more repeat units.
  • the molecular weights of plasticizers with multiple repeat units may extend to 9,000.
  • Plasticizers may be classified by chemical structure as follows: a) Phthalic acid esters, i.e., dibutyl phthalate, octyl phthalate, decyl phthalate, etc.
  • Phosphoric acid esters i.e., trioctyl phosphate, diphenyl 2-ethyl-hexyl phosphate, etc.
  • Polyfunctional fatty acid esters i.e., dioctyl sebacate, dibutyl sebacate, stearyl citrate, etc.
  • Polymeric plasticizers of molecular weights up to 9,000 including poly(ethylene glycol), polyesters, polystyrenes, polyepoxides, etc.
  • Low molecular weight phenyl containing molecules such as euphenyl, bibenzyl, naphthalene and diphenylmethane.
  • Modification of the polymer or copolymer may also be carried out by deformation of the polymer or copolymer to effect structural rearrangements therein which result in local areas with an increase in the free volume and a decrease in the glass transition tempera ⁇ ture. It will be understood by those skilled in the art that the above list of methods is only exemplary and is not intended to be limitative of the methods described in the appended claims for suppressing the glass transi ⁇ tion temperature of optical polymers or copolymers. It is also understood that substituents, monomers or plasti ⁇ cizers that form non-annealable color centers upon expo ⁇ sure to radiation are not the subject of this invention.
  • the phrase "increasing the free volume" of the polymer, as used herein, is intended to mean the glass transition temperature and below at the point at which free or unoccupied volume reaches a constant value.
  • the free volume remains constant or frozen below Tg.
  • the free volume is relatively constant for all polymers.
  • Simha et al [R. Simha and R.F. Boyer, J. Chem. Phys. , Vol. 37, p. 1003 (1962) estimate this critical fractional free volume to be 0.113. Expansion in the glassy state occurs at constant free volume.
  • the coefficient of ther ⁇ mal expansion increases at Tg, allowing for a regular increase in free volume with temperatures above Tg.
  • the mobility of polymer chains is proportional to the free volume.
  • Factors that increase free volume decrease the glass transition temperature.
  • flexible side chains force the main chains of the polymer apart, increasing the free volume and decreasing the Tg. This means that the polymer melts must be cooled to a lower temperature to reduce the free volume to the critical value of 0.113.
  • plasti ⁇ cizers or flexible backbone units are incorporated into polymers or when polymers are copolymerized with a low Tg polymeric unit.
  • Non-polar polymers appear to recover at faster rates than polar polymers. Therefore, if more rapid recovery is required, a non-polar optical polymer should be the subject of the procedure to decrease the Tg of the polymer. In addition, as the irradiation dose decreases, recovery time decreases. At low doses, a polar polymer may suffice.
  • the choice of polymers is flexible enough that the structures may be tailored to the use. There are also great advantages in the ability to heat process these materials and to use them for self- supporting articles.
  • cross-linkers may be added to increase strength or rigidity. While these structures cannot be heat processed, they can be cast or processed by more exotic means such as reaction injection molding. The glass transition temperature is still greater than room temperature.
  • the polymers of this invention can be used alone or in combination with fillers or dyes. In biomed- ical applications, it may be desirable to use fillers or pigments for mechanical or aesthetic purposes if these additives are optically radiation hard. These additives may cause the polymers to lose transparency. In fact, it may be desirable to produce non-transparent structures. In this instance, the radiation will not discolor, for example, a white filled part. Radiation hard scintillat ⁇ ing or wavelength shifting fluors may be added to the polymers to produce scintillators or wavelength shifting plastics for use in high energy radiation detectors.
  • the advantages of the invention lie in the fact that the polymers are transparent, rigid, heat pro- cessible and recover during the irradiation or shortly thereafter. If the proper compositions are used, no heat treatment is required.
  • Poly(methylmethacrylate) [PMMA] , poly(ethyl- methacrylate) [PEMA] and poly(propylmethacrylate) [PPMA] were compression molded into 1 cm thick specimens. The samples were annealed at 20°C below the respective Tg of the polymers; PPMA was not annealed as its Tg is 35°C.
  • Fig. 1 shows border movement versus time for PMMA and PEMA.
  • PEMA has a room temperature velocity of 0.85 x 10 "5 meters/hour versus that of 0.01 meter/hour for PMMA.
  • Figs. 2 and 3 show PEMA recovery at various tem ⁇ peratures and the activation energy for recovery, 11.2 Kcal/mole.
  • PPMA with a Tg of 35°C showed no evidence of a radiation-induced recoverable boundary. This indicates recovery occurred within the radiation time.
  • the effect of structure on radia ⁇ tion recovery boundary is seen in Fig. 4, PS > PEMA > PMMA. Note the following table with PS also irradiated to 10 Mrad.
  • Poly(isobutyl methacrylate) was mixed with 0.1% by weight LUPERSOL 231, a free radical catalyst.
  • the sample was polymerized in a capped vial at 85°C for 6 hours and post-cured in an uncapped vial at 100°C for 6 hours.
  • the sample was placed between glass disks in a piston mold and molded under pressure at 110°C for 40 minutes. Transmission spectra were recorded before and immediately after a gamma radiation dose of 10 MRad in air. The results are shown in Fig. 5. There is very little discoloration due to radiation. In addition, no recovery border is noted.
  • the Tg of this sample as re- corded prior to radiation is 54°C. This is less than that of PEMA, but greater than that of PPMA.
  • Polystyrene is a non-polar molecule, whereas the methacrylate polymers contain the polar group:
  • Fig. 6 shows the effect of dose on recovery time. The lower the dose, the faster the recovery.
  • the normal dose for sterilization is 2.5 Mrad. Border move ⁇ ment was measured with a reticule and the aid of a fluo ⁇ rescent light.
  • This invention shows that by controlling Tg and dose, transparent, rigid, heat processible polymers can be designed for use in high energy radiation environ ⁇ ments.

Abstract

A method of enhancing the ionizing irradiation sterilization stability of an optically transparent, rigid, heat processible thermoplastic polymer comprising modifying the polymer to decrease the glass transition temperature thereof to a value such that: a) the glass transition temperature of the modified polymer is above the desired end-use temperature; b) the modified polymer remains optically transparent, rigid and heat processible; and c) the modified polymer recovers from the discolorization effects produced by exposure to ionizing sterilizing irradiation at a temperature below the glass transition temperature thereof.

Description

METHOD FOR ENHANCING IRRADIATION STERILIZATION STABILITY OP OPTICAL POLYMERS
BACKGROUND OF THE INVENTION
Research leading to completion of the invention was supported, in part, by Grant No. DE-FG05-90ER40547 issued by the Department of Energy. The United States Government has certain rights to the invention described herein.
Field of the Invention
The present invention relates to optically transparent polymers.
Description of the Prior Art
Optical polymers are often employed in environ¬ ments where they are exposed to ionizing radiation.
One such application involves plastics contain¬ ing fluorescent dyes which act as high energy scintilla- tion particle detectors in nuclear particle accelerator facilities and in space flights. Plastics doped with fluorescent dyes function as absorbers of light of one wavelength and emit light at a different wavelength. In addition, these plastic wavelength shifters are used as solar light concentrators. Such plastics are often used as the core of optical fibers which transmit light for the purpose of communicating information. The transmit¬ ted light can be in the form of unique pulses (digital) of light or of light with a time varying amplitude (ana- log) • These optical fibers are used in particle acceler¬ ators, nuclear reactors, fusion machines, space flights, local area networks, automotive communications, military systems and display systems. These types of plastics are also used in biomedical or other devices which must be sterilized by the application of ionizing radiation.
-ι- In the past, many such optical polymers discol¬ or or otherwise lose a significant degree of their opti¬ cal transparency upon exposure to ionizing radiation.
Optical polymers are exposed to ionizing radia- tion when biomedical plastic members are sterilized by gamma radiation or when used as scintillating or wave¬ length shifter plastics in high energy radiation detec¬ tors. In the past, optical radiation hard linear and cross-linked polymers with glass transitions below the end use temperature have been identified. It will be highly desirable to be able to produce optical thermo¬ plastics which recover from ionizing radiation-induced optical damage at accelerated rates. Such polymers should, optimally, be self-supporting, melt processed by conventional techniques and have glass transition temper¬ atures above room temperature. Thus, thermoplastic poly¬ mer structures could be designed that recover at acceler¬ ated rates when incubated at temperatures below the glass transition temperatures and, in the best case, that re- cover during or shortly after irradiation without the need to incubate at elevated temperatures. Conventional thermoplastic optical polymers can warp at temperatures needed to induce recovery in a practical time frame. It has been proposed in the past to modify certain polymer systems to reduce radiation-induced color center formation.
Thus, U.S. Patent No. 4,939,186 describes the incorporation of certain alkenes in polycarbonates to color-stabilize the latter against irradiation-induced coloration. The alkene is added at levels of 0.01 - 5%, based on the weight of the polymer. The utilization of such small amounts of alkene additive is indicative that the mechanism of color stabilization is dependent upon the chemical properties of the additive, i.e., its poten- tial for chemically reacting with the system in which it is incorporated.
U.S. Patent No. 4,624,972 relates to the use of , esters of polycarboxylic acids with glycol ethers for
5 color stabilization of polycarbonates against radiation- , induced color center formation. The patent states that alkyl esters are inoperable for the stated purpose, whereas the glycol ether esters are operable. This, coupled with the fact that the patent discloses the in- 10 elusion of no more than 10% of the ester and amounts as low as 0.1%, demonstrates that the crux of the patented invention resides in some sort of chemical reaction mech¬ anism as the basis of color stabilization.
U.S. Patent No. 4,904,710 refers to the use of 15 polyetherpolyols or alkyl ethers thereof to stabilize polycarbonates against the discolorization effects of irradiation. The patent discloses the use of such low concentrations, i.e., up to only 5%, that the mechanism of color stabilization must depend upon the chemical 20 reactivity of the additives during irradiation.
It is an object of the present invention to provide novel methods for enhancing the radiation hard¬ ness and optical transparency of a polymer or copolymer. It is a further object of the present invention 25 to provide novel optical polymers or copolymers with enhanced radiation hardness and/or optical transparency.
SUMMARY OF THE INVENTION
The above and other objects are realized by the present invention, one embodiment of which comprises a 30 method of enhancing the ionizing irradiation steriliza¬ tion stability of a substantially optically transparent, rigid, heat processible thermoplastic polymer comprising modifying the polymer to decrease the glass transition temperature (Tg) to a value such that: a) the glass transition temperature of the modified polymer is above the desired end-use temperature thereof; b) the modified polymer remains substantially optically transparent, rigid and heat-processible; and c) the modified polymer recovers from the discoloration effects produced by exposure to ionizing sterilizing irradiation at a temperature below the glass transition temperature thereof.
A further embodiment of the invention comprises a method of restoring optical transparency to an optical¬ ly transparent, thermoplastic polymer modified according to the above method which has become discolored due to exposure thereof to ionizing irradiation comprising rais¬ ing the temperature of the discolored thermoplastic poly¬ mer to an effective decoloration temperature below the glass transition temperature thereof for a time suffi- cient to effect decoloration thereof.
Another embodiment of the invention comprises an article of manufacture constructed at least in part from the modified polymer produced according to the above-described method. A final embodiment of the invention comprises an article of manufacture described above which has been subjected to ionizing irradiation sufficient to discolor the modified polymer and subsequently heated to a temper¬ ature below its glass transition temperature and for a time sufficient to decolorize the polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1-6 depict discolored volume boundary velocities in various polymers at various temperatures. DETAILED DESCRIPTION OF THE INVENTION
When glassy polymers such as poly(methylmethacrylate) , polystyrene or polyvinyltoluene r are exposed to ionizing radiation, they discolor. The 5 major part of the discoloration is annealable; i.e., a sharp boundary within which the discoloration exists moves from the outside faces of the polymer to the center of the polymer. This boundary moves at a uniform rate in an air environment. The rate of boundary movement de- 10 pends on the polymer and the temperature, but is about
0.05 mm/month for poly ethylmethacrylate at room temper¬ ature. It is desirable to avoid this time dependence since it has been found that the optical properties of such systems are not constant. 15 In accordance with the present invention, there are provided radiation hard polymers which are optically transparent; recover rapidly from yellowing due to expo¬ sure to ionizing radiation; are rigid, self-supporting materials; and are heat processible by such methods as 20 extrusion, injection molding, thermoforming, blow mold¬ ing, compression molding, calendaring, etc.
The goal of the invention is to design polymers with the above characteristics. It is normally very difficult to maximize these characteristics simultaneous- 25 ly. The goal is achieved by decreasing the Tg of optical polymers while keeping the Tg sufficiently above the use temperature of the plastic part to enable its employment in intended applications.
When glassy optical polymers are exposed to 30 ionizing radiation, they discolor. Examples of such polymers are poly(methyl methacrylate) , polystyrene, polyvinyltoluene, polyvinylmethylpentane, etc. The dis¬ colored portion often fluoresces. This discoloration is r recoverable. That is, a sharp recovery boundary moves from the outside toward the center of the sample leaving clear material behind. The boundary moves at a rate that is linear with time and is controlled by polymer relax¬ ations. The process is thermally activated and exhibits Arrhenius behavior. Recovery is instantaneous when the plastics are heated above the glass transition tempera¬ ture. Heating above the glass transition is a disadvan¬ tageous method for inducing recovery in rigid thermoplas¬ tic parts since they flow and lose their shape. In addi- tion, if polymers above the glass transition are used for structural members, while they may not discolor upon exposure to ionizing radiation, they must be cross-linked or encapsulated with a rigid media so that they form self-supporting structural members. Cross-linked plas- tics cannot be heat processed by conventional methods.
The present invention provides a process for altering the structure of optical polymers to impart accelerated recovery from radiation-induced discoloration while still keeping the advantageous properties of heat processibility and rigidity. Specifically, the Tg of the polymer molecules is decreased, but not to the extent that the glass transition is below the temperature of use. This results in accelerated recovering during radi¬ ation or shortly thereafter without the need for extended annealing at a temperature above the use temperature. This may be accomplished in one of the following ways: a) appending side chains that decrease the glass transition temperature. These may be composed of chain, branched or cyclized groups containing aliphatic, ether, carbonyl groups or groups containing other hetero- ato s such as nitrogen; b) adding flexible substituents such as ether groups to the polymer backbone; c) copolymerizing monomers of optical poly¬ mers with monomers of lower glass transition transparent polymers. The copolymers may be of the random, alternat¬ ing, graft or block type; d) blending low and high glass transition polymers to yield a polymer with a glass transition above the use temperature; e) plasticizing the polymer with low molecu¬ lar weight compounds. These plasticizers may be in free dissolved form or bound to the polymer on a side chain, end or in the polymer backbone. These plasticizers may range from small molecules to oligomers. They decrease the glass transition temperature by increasing the free volume in the polymer matrix; or f) deforming the polymer molecules. Polymers often contain residual stresses and areas of increased free volume due to processing techniques such as injec¬ tion molding and spinning in initial stages of drawing. The deformation process may be such that initial stages of deformation increase free volume. This is observed in the initial stages of drawing of polymers where the free volume increases due to structural rearrangements which precede high orientation at high deformation levels. Local areas of increased free volume result in a de- creased glass transition temperature.
It will be understood by those skilled in the art that method e) above, i^e., plasticizing the polymer with low molecular weight compounds, differs radically from the methods described in the above-discussed U.S. Patent Nos. 4,624,972; 4,939,186 and 4,904,710. In the latter patents, the additives are added in small amounts to provide a chemically reactive mechanism as the basis for color stabilization. According to the present inven¬ tion, the plasticizers are added in larger amounts to suppress the Tg sufficiently to enable the Tg reduction- free volume mechanism to operate. The amounts of addi¬ tive added in the above-described U.S. Patent Nos. 4,624,972; 4,939,186 and 4,904,710 are insufficient to have any appreciable effect on the Tg of the polymers to which they are added.
The advantage of this invention is the flexi¬ bility attained from the ability to select from a wide variety of polymer classes. One embodiment of the present invention is a process for altering the structure of optically transpar¬ ent, rigid, heat processible thermoplastic polymers to impart them with optical radiation hardness. More specifically, the invention comprises decreasing the Tg of the modified polymers to the extent that the Tg is above the temperature at which they are employed in envi¬ ronments, whereby they are exposed to ionizing radiation. This results in a decrease of polymer discoloration with associated time dependence. Decreasing the Tg of the polymers may be accom¬ plished by one of the following methods.
The polymers or copolymers may be modified to decrease the Tg thereof by appending flexible sub- stituents to at least one of the monomers employed to prepare the polymers or copolymers.
Preferably, the substituent is a flexible lin¬ ear or branched hydrocarbon chain, optionally interrupted by at least one heteroatom, e.g., 0, S, N, etc., or ringed structures such as cyclohexyl or phenyl groups attached to flexible chains.
Modification of the polymers or copolymers to decrease the Tg thereof may be effected by the copolymer- ization of the monomer(s) employed to prepare the poly- mers or copolymers with at least one additional comono- mer.
Modification of the polymer to decrease the Tg thereof may be effected by incorporating flexible linkag- es such as ether linkages into the polymer backbone.
Modification of the polymer or copolymer to decrease the Tg thereof may also take the form of inti¬ mately blending the polymers or copolymers with a second polymer or copolymer having a glass transition tempera- ture Tg substantially lower than that of the polymer or copolymer, but above the end-use temperature.
Modification of the polymer or copolymer to increase the free volume thereof can also be achieved by the plasticization of the polymer or copolymer with a low molecular weight compound, ranging from one unit to an oligomer. Plasticizer in the conventional sense means any organic molecule which is soluble in the polymer in question over the appropriate concentration range and which has a glass transition temperature lower than that of the polymer. The plasticizer may consist of one or more repeat units. The molecular weights of plasticizers with multiple repeat units may extend to 9,000. Plasticizers may be classified by chemical structure as follows: a) Phthalic acid esters, i.e., dibutyl phthalate, octyl phthalate, decyl phthalate, etc. b) Phosphoric acid esters, i.e., trioctyl phosphate, diphenyl 2-ethyl-hexyl phosphate, etc. c) Polyfunctional fatty acid esters, i.e., dioctyl sebacate, dibutyl sebacate, stearyl citrate, etc. d) Polymeric plasticizers of molecular weights up to 9,000 including poly(ethylene glycol), polyesters, polystyrenes, polyepoxides, etc. e) Low molecular weight phenyl containing molecules such as euphenyl, bibenzyl, naphthalene and diphenylmethane.
Modification of the polymer or copolymer may also be carried out by deformation of the polymer or copolymer to effect structural rearrangements therein which result in local areas with an increase in the free volume and a decrease in the glass transition tempera¬ ture. It will be understood by those skilled in the art that the above list of methods is only exemplary and is not intended to be limitative of the methods described in the appended claims for suppressing the glass transi¬ tion temperature of optical polymers or copolymers. It is also understood that substituents, monomers or plasti¬ cizers that form non-annealable color centers upon expo¬ sure to radiation are not the subject of this invention. The phrase "increasing the free volume" of the polymer, as used herein, is intended to mean the glass transition temperature and below at the point at which free or unoccupied volume reaches a constant value. The free volume remains constant or frozen below Tg. The free volume is relatively constant for all polymers. Simha et al [R. Simha and R.F. Boyer, J. Chem. Phys. , Vol. 37, p. 1003 (1962) estimate this critical fractional free volume to be 0.113. Expansion in the glassy state occurs at constant free volume. The coefficient of ther¬ mal expansion increases at Tg, allowing for a regular increase in free volume with temperatures above Tg. The mobility of polymer chains is proportional to the free volume. Factors that increase free volume decrease the glass transition temperature. For instance, flexible side chains force the main chains of the polymer apart, increasing the free volume and decreasing the Tg. This means that the polymer melts must be cooled to a lower temperature to reduce the free volume to the critical value of 0.113. Similar effects are noted when plasti¬ cizers or flexible backbone units are incorporated into polymers or when polymers are copolymerized with a low Tg polymeric unit.
Non-polar polymers appear to recover at faster rates than polar polymers. Therefore, if more rapid recovery is required, a non-polar optical polymer should be the subject of the procedure to decrease the Tg of the polymer. In addition, as the irradiation dose decreases, recovery time decreases. At low doses, a polar polymer may suffice. The choice of polymers is flexible enough that the structures may be tailored to the use. There are also great advantages in the ability to heat process these materials and to use them for self- supporting articles.
The invention does not, however, preclude the use of cross-linkers. In some instances, cross-linkers may be added to increase strength or rigidity. While these structures cannot be heat processed, they can be cast or processed by more exotic means such as reaction injection molding. The glass transition temperature is still greater than room temperature. The polymers of this invention can be used alone or in combination with fillers or dyes. In biomed- ical applications, it may be desirable to use fillers or pigments for mechanical or aesthetic purposes if these additives are optically radiation hard. These additives may cause the polymers to lose transparency. In fact, it may be desirable to produce non-transparent structures. In this instance, the radiation will not discolor, for example, a white filled part. Radiation hard scintillat¬ ing or wavelength shifting fluors may be added to the polymers to produce scintillators or wavelength shifting plastics for use in high energy radiation detectors.
The advantages of the invention lie in the fact that the polymers are transparent, rigid, heat pro- cessible and recover during the irradiation or shortly thereafter. If the proper compositions are used, no heat treatment is required.
The following examples are illustrative of the invention. •
EXAMPLE 1
Poly(methylmethacrylate) [PMMA] , poly(ethyl- methacrylate) [PEMA] and poly(propylmethacrylate) [PPMA] were compression molded into 1 cm thick specimens. The samples were annealed at 20°C below the respective Tg of the polymers; PPMA was not annealed as its Tg is 35°C.
Tα "C Polymer
105 PMMA
66 PEMA
35 PPMA Samples were exposed to a radiation dose of 10 Mrad in air. The dose rate was 1.6 x 105 rad/hour.
Fig. 1 shows border movement versus time for PMMA and PEMA. PEMA has a room temperature velocity of 0.85 x 10"5 meters/hour versus that of 0.01 meter/hour for PMMA. Figs. 2 and 3 show PEMA recovery at various tem¬ peratures and the activation energy for recovery, 11.2 Kcal/mole. PPMA with a Tg of 35°C showed no evidence of a radiation-induced recoverable boundary. This indicates recovery occurred within the radiation time. In addition, the effect of structure on radia¬ tion recovery boundary is seen in Fig. 4, PS > PEMA > PMMA. Note the following table with PS also irradiated to 10 Mrad.
Figure imgf000015_0001
EXAMPLE 2
Poly(isobutyl methacrylate) was mixed with 0.1% by weight LUPERSOL 231, a free radical catalyst. The sample was polymerized in a capped vial at 85°C for 6 hours and post-cured in an uncapped vial at 100°C for 6 hours. The sample was placed between glass disks in a piston mold and molded under pressure at 110°C for 40 minutes. Transmission spectra were recorded before and immediately after a gamma radiation dose of 10 MRad in air. The results are shown in Fig. 5. There is very little discoloration due to radiation. In addition, no recovery border is noted. The Tg of this sample as re- corded prior to radiation is 54°C. This is less than that of PEMA, but greater than that of PPMA.
EXAMPLE 3
Polystyrene is a non-polar molecule, whereas the methacrylate polymers contain the polar group:
0
II C-0 Thus, depressing the glass transition temperature of PS so that it is above room temperature but below 100°C will result in polymers that recover at accelerated rates. This may be accomplished, for example, by adding alkyl groups of 2 or more carbon atoms or alkoxy groups having 1 or more carbon atoms to the fourth position on the phenyl ring in PS.
Fig. 6 shows the effect of dose on recovery time. The lower the dose, the faster the recovery. The normal dose for sterilization is 2.5 Mrad. Border move¬ ment was measured with a reticule and the aid of a fluo¬ rescent light.
EXAMPLE 4 Styrene and styrene mixed with 20% by weight of dibutylphthalate plasticizer were polymerized with LUPER- SOL 231 as in Example 2. The samples were irradiated to
10 MRad in air with a Co 60 source. The unplasticized sample discolored and evinced a border which had not moved to the center of the sample after one month. The plasticized sample was significantly clearer and showed no evidence of a recovery border.
This invention shows that by controlling Tg and dose, transparent, rigid, heat processible polymers can be designed for use in high energy radiation environ¬ ments.

Claims

I CLAI I l. A method of enhancing the ionizing irradiation sterilization stability of a substantially optically transparent, rigid, heat processible thermoplastic polymer comprising modifying said polymer to de- crease the glass transition temperature thereof to a value such that: a) the glass transition temperature of the modified polymer is above the desired end-use tem- perature thereof; b) the modified polymer remains substantially optically transparent, rigid and heat processible; and c) the modified polymer recovers from the discolorization effects produced by exposure to ionizing sterilizing irradiation at a temperature below the glass transition temperature thereof.
2. The method of claim 1 wherein said polymer is modified to decrease the glass transition tempera- ture thereof by appending substituents on the mono- mer or monomers polymerized to produce said polymer.
3. The method of claim 1 wherein said polymer is modified to decrease the glass transition tempera- ture thereof by copolymerizing the monomer or mono- mers employed to prepare said polymer with at least one additional comonomer to produce a polymer having a glass transition temperature lower than said un- modified polymer, but above the end-use temperature.
4. The method of claim 1 wherein said polymer is modified to decrease the glass transition tempera- ture by inserting flexible substituents into the polymer backbone.
5. The method of claim 1 wherein said polymer is modified to decrease the glass transition tempera- ture thereof by blending therewith at least one polymer to produce a blend having a glass transition
5 temperature lower than the unmodified polymer, but
6 above the end-use temperature.
1 6. The method of claim 1 wherein said polymer is
2 modified to decrease the glass transition tempera-
3 ture thereof by admixing therewith a plasticizer to
4 produce a composition having a glass transition
5 temperature lower than the unmodified polymer, but 6 above the end-use temperature.
7. The method of claim 1 wherein said polymer is modified by deforming the polymer to effect struc- tural rearrangements which result in an increase in the free volume thereof.
8. The method of claim 1 wherein said polymer is a polymethylmethacrylate.
9. The method of claim 1 wherein said modified polymer is polyisobutylmethacrylate.
10. The method of claim 1 wherein said modified polymer is polyethylmethacrylate.
11. The method of claim 1 wherein said modified polymer is polypropylmethacrylate.
12. The method of claim 1 wherein said polymer is polystyrene.
13. The method of claim 1 wherein said modified polymer is a polycarbonate.
14. The method of claim 1 wherein said modified polymer is a poly(vinylmethylpentene) .
1 15. The method of claim 1 wherein said modified
2 polymer is a polyester.
*
1 16. The method of claim 1 wherein said modified
■ 2 polymer is poly(o, m or p-alkyl) styrene wherein
3 said alkyl group has 2 or more carbon atoms or
4 branched carbon chains wherein the glass transition
5 temperature of said polymer is greater than the end-
6 use temperature.
1 17. The method of claim 1 wherein said modified
2 polymer is poly(o, m or p-alkoxy) styrene wherein
3 said alkoxy group has one or more carbon atoms where
4 the glass transition temperature of said polymer is
5 greater than the end-use temperature.
1 18. A method of restoring optical transparency to
2 an optically transparent, thermoplastic polymer
3 modified according to claim 1 which has become dis-
4 colored due to exposure thereof to ionizing irradia-
5 tion comprising raising the temperature of said
6 discolored thermoplastic polymer to an effective
7 decoloration temperature below the glass transition
8 temperature thereof for a time sufficient to effect
9 decoloration thereof.
19. An article of manufacture constructed at least in part from the modified polymer produced according to the method of claim 1.
20. An article of manufacture according to claim 19 which has been subjected to ionizing irradiation sufficient to discolor said modified polymer and subsequently heated to a temperature below the glass transition temperature thereof and for a time suffi- cient to decolorize said polymer.
PCT/US1993/000015 1992-01-17 1993-01-08 Method for enhancing irradiation sterilization stability of optical polymers WO1993015134A2 (en)

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Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4612340A (en) * 1983-03-09 1986-09-16 Yoshinori Ohachi Medical device
US4624972A (en) * 1985-11-25 1986-11-25 The Dow Chemical Company Gamma radiation resistant carbonate polymer compositions
US4681585A (en) * 1984-04-11 1987-07-21 Intermedics Intraocular, Inc. Intraocular implant
US4804692A (en) * 1987-11-09 1989-02-14 Mobay Corporation Gamma-radiation resistant polycarbonate compositions
US4873271A (en) * 1989-04-25 1989-10-10 Mobay Corporation Gamma radiation rsistant polycarbonate compositions
US4904710A (en) * 1985-10-31 1990-02-27 The Dow Chemical Company Gamma radiation resistant carbonate polymer compositions
US4939186A (en) * 1984-02-10 1990-07-03 General Electric Company Enhancing color stability to sterilizing radiation of polymer compositions
US4963598A (en) * 1988-04-18 1990-10-16 Mobay Corporation Gamma radiation resistant polycarbonate compositions

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4612340A (en) * 1983-03-09 1986-09-16 Yoshinori Ohachi Medical device
US4939186A (en) * 1984-02-10 1990-07-03 General Electric Company Enhancing color stability to sterilizing radiation of polymer compositions
US4681585A (en) * 1984-04-11 1987-07-21 Intermedics Intraocular, Inc. Intraocular implant
US4904710A (en) * 1985-10-31 1990-02-27 The Dow Chemical Company Gamma radiation resistant carbonate polymer compositions
US4624972A (en) * 1985-11-25 1986-11-25 The Dow Chemical Company Gamma radiation resistant carbonate polymer compositions
US4804692A (en) * 1987-11-09 1989-02-14 Mobay Corporation Gamma-radiation resistant polycarbonate compositions
US4963598A (en) * 1988-04-18 1990-10-16 Mobay Corporation Gamma radiation resistant polycarbonate compositions
US4873271A (en) * 1989-04-25 1989-10-10 Mobay Corporation Gamma radiation rsistant polycarbonate compositions

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