Radiation degradation of α-substituted acrylate polymers and copolymers

Radiation degradation is observed in poly(methyl α-chloroacrylate), poly(methyl α-cyanoacrylate), and poly(α-chloroacrylonitrile) homopolymers and their respective MMA copolymers when γ-irradiated in vacuo. Polymer degradation susceptibilities are quantified in terms of G(scission radicals) and G(scission) ? G(crosslinks), measured by EPR and membrane osmometry, respectively, values by these two methods are compared. Higher G(rads) values ranging from 2 to 6 and [G(s) ? G(x)] values ranging from 2 to 11 are obtained for the substituted polymers and copolymers relative to the values for PMMA (1.6; 1.9), a standard e-beam positive resist, which suggests that these modified polymers are more sensitive e-beam resists than PMMA.     

Studies of methyl methacrylate-methyl α-chloroacrylate,copolymers and poly(methyl α-chloroactylate) as electron sensitive positive resists

Poly(methyl α-chloroacrylate) (PMCA) and the copolymers of methyl methacrylate and methyl α-chloroacrylate (poly(MMA-co-MCA)) have been reported recently to be more susceptible to radiation degradation than poly(methyl methacrylate) (PMMA). In this paper we report our studies of PMCA and poly(MMA-co-MCA) as electron-sensitive positive resists. It has been found that both PMCA and the copolymers are more sensitive than PMMA. Using mixtures of dimethylformamide and 2-propanol as developers, the sensitivities of PMCA and poly(MMA-co-MCA) (38 mole percent MCA) have been found to be 1 × 10^?5 and 6 × 10^?6 coulomb/cm², respectively. It has also been found that crosslinking predominates in PMCA when the electron dose exceeds 6 × 10^?4 coulomb/cm².     

The thermal stability of some poly(di-aryl itaconates)

Summary The non-oxidative thermal stability of poly(di-aryl itaconates) was established by thermogravimetric analysis. The polymers investigated were poly(diphenyl itaconate) (PDPhI), poly(di-ortho-, meta- and para-tolyl itaconates) (PDoTI, PDmTI, PDpTI) and poly(dibenzyl itaconate) (PDBzI). The shapes of the DTG curves indicated that at least two reactions were involved in the degradation mechanisms. Carbonaceous residues were registered in all polymers. Preliminary thermal degradation experiments showed that the polymers decomposed mainly by depolymerisation with side chain scission registered in the cases of PDoTI and PDBzI. All polymers except PDBzI crosslinked slightly at lower degradation temperatures.     

Dry etching durability of positive electron resists

The effects of positive electron resist structures and G values for scission on dry etching durability have been studied with regard to plasma etching, reactive sputter etching, and plasma ashing. Polymers of aromatic methacrylates, especially poly(α-methylstyrene), all of which contain a benzene ring in the side chain, exhibit superior durability for each etching system. On the other hand, polymers of fluorinated and chlorinated methacrylates show low durability compared with the nonsubstituted polymethacrylates. In addition, they vary in durability with respect to the etching gas used. The sputtering effect which is induced by ion bombardment in flat bed-type reactors produces a significant loss in thickness of positive resists. Rough correlations between dry etching rates and Gs(scission) as the indication of sensitivity were obtained. The large dispersion of the correlations implies the possibility of more useful positive electron resists of high durability and high sensitivity.     

Scission of the poly(methyl methacrylate‐co‐methacrylic acid) chain by excited‐state acridine

A photochemical reaction between acridine and poly(methyl methacrylate‐co‐methacrylic acid) (PMCA) was studied in benzene to build a recyclable polymer photodegradation system. The illumination of acridine in the presence of PMCA with 365‐nm light induced the bleaching of acridine and the degradation of PMCA. The average molecular weight of the degraded polymer decreased rapidly for the first 30 min of the photolysis. A nonvolatile product of this reaction was found to have a 2‐methyl‐2‐propenyl end group. The efficiency of the PMCA scission by this method was 30 times as large as that of poly(methyl methacrylate). These results suggest that an efficient photochemical polymer decomposition system can be built by adding the mixing process of a little methacrylic acid into the synthetic processes of general vinyl polymers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1209–1212, 2005     

Effect of short-chain polymer on scission of long-chain polymer in solution by high-speed stirring

The effect of short-chain poly(methyl methacrylate) (PMMA) (P?_v = 950) on scission of longchain PMMA (P?_v = 6150) in solution by high-speed stirring was investigated by measuring changes in [η] and GPC, stirring at 30,000 ± 500 rpm at 30° ± 5°C benzene solutions containing the above two polymers at several concentrations and at various mixing ratios. It was found that the scission of long-chain PMMA was small or a little suppressed by addition of short-chain PMMA. The scission-suppressing effect of the coexisting polymers depended on the chain length. The longer the chain, the larger the effect. It was also found that the rate constant of scission, k, in Ovenall's equation had a certain significance in polymer systems with the same MWD but no significance in those with different MWDs.     

Poly(methyl methacrylate): influence of tacticity on its use as an electron resist

The effects of tacticity on the molecular weight distribution, the sensitivity and nature of products after exposure to electron beams are reported. Significant differences in the sensitivity are observed, the isotactic polymer being more sensitive than either the syndiotactic or atactic polymers. The differences in the degradation product can be explained in terms of two competing processes; the first associated with chain scission leading to the expected reduction in molecular weight, and secondly chain extension with a consequent increase in molecular weight. Measurements of the contact angle also indicate changes with the tacticity, indicating differences in the surface energy; however, these data are the opposite of the adhesion strength determined by tape tests. The implications of these observations on the performance of poly(methyl methacrylate) as an electron resist are discussed.     

Thermal and photolytic degradation of plates of poly(methyl methacrylate) containing monomer

Specimens of 1.5 mm thick absorber-free poly(methyl methacrylate) (PMMA) containing ～0.6% monomer but no absorber have been photolytically degraded in air at 50°, 85° and 115°C and thermally degraded in air at 115° and 125°C. Specimens were exposed to a simulated solar spectral range. Degradation was followed by gel permeation chromatographic determinations of molecular weight as a function of depth in the specimens. The results show increased photodegradation at the plate faces (back and front) over that occurring in the centres, and a rapidly attained constant amount of degradation for thermal degradation. Degradation mechanisms are proposed. The thermal degradation is ascribed to weak links and unspent initiator. Photolytic initiation is ascribed, at least in part, to degradation of the ester group by wavelengths in the range 300 to 330 nm. The effect of oxygen is to convert alkyl radicals into peroxyl radicals, some of which form alkoxyl radicals which tne undergo β-scission to give in-chain ruptures. Where the oxygen concentration is low, monomer changes non-tertiary alkyl radical sinto tertiary radicals by addition to the monomer double bond. After their peroxidation by molecular oxygen, tertiary radicals react with one another to give alkoxyl radicals and subsequent chain scission rather than undergoing the Russell termination reaction with no chain scission characteristic of non-tertiary peroxyl radicals. The effect of temperature is mostly to decrease the importance of the cage effect and to allow the initial radicals formed to diffuse away from one another. The products of photo-oxidation absorb the shorter (300 to 330 nm) radiation significantly and progressively shield the remainder of the plate as degradation proceeds.     

Sensitized photodegradation of polyisobutylene film by addition of tris(α-thiopicolinanilide)–cobalt(III)

The photodegradation of polyisobutylene (PIB) film in air at a temperature where volatile formation is negligible was studied by means of light scattering, chemical actiometry, and spectrophotometric techniques. The degradation is accelerated by addition of tris(α-thiopicolinanilide)—cobalt(III) (TPAC). The sensitization and the course of the degradation were determined by weight-average molecular weight, energy of activation, and quantum yield of the photolysis of the polymer film with 254-nm light. The plots of molecular weight, weight-average chain scission, and degree of degradation vs. irradiation time are linear and confirm the random nature of chain scission of the polymer. The unsaturation produced is proportional to the time of irradiation. Ultraviolet and infrared absorption spectra have been employed to substantiate a mechanism of the degradation process which does not involve hydrogen abstraction from the polymer, but direct cleage of the polymer backbone and addition of initiating radicals of TPAC at the sites of scission.     

Radical annihilation of γ‐ray‐irradiated contact lens blanks made of a 2‐hydroxyethyl methacrylate copolymer at elevated temperatures

The annihilation of the radicals in irradiated 2‐hydroxyethyl methacrylate copolymer was analyzed by the use of electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra were deconvoluted into three radicals: a quartet (Ra), a triplet (Rb), and a broad singlet (Rc). Radical Ra was attributed to coupling with a methyl radical and/or a doublet or triplet with about the same hyperfine coupling due to a methylene radical. Radical Rb was due to a methylene radical produced by main‐chain scission. Radical Rc was attributed to various free radicals without coupling to protons. By comparing the EPR spectra of radicals Ra, Rb, and Rc with the spectrum of a 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) standard with a known spin number, we calculated the spin numbers of the radicals, which decreased with time in the temperature range 25–45°C, regardless of the irradiation dose. The annealing of Ra and Rb and the annealing of Rc at longer times followed second‐order kinetics; these were different from the kinetics for the color formation and defect‐controlled hardening of polymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Degradation of poly (methyl methacrylate) by flash irradiation

Degradation of poly(methyl methacrylate) (PMMA) caused by irradiation of flash light from discharged xenon lamp was investigated in a vacuum and in the presence of nitrogen. Degradation products at a sequence of flashes were analysed by a gas chromatograph with hydrogen flame ionization detector after each flash. Among the degradation products of PMMA in vacuum by regular flash light, an amount of monomer was detected with the significant production of methanol and gaseous mixture. The molecular weight of the residual polymer decreased markedly by the first several flashes, showing that chain scission occurred. By filtering the u.v. component of flash light the monomer formation and molecular weight decrease were not observed. From these facts the flash degradation of PMMA in vacuum was concluded to be conducted by a photodegradation mechanism. The absorption of u.v. light by ester groups forms active radicals, which initiate chain scissions and depolymerization of main chains following methanol and monomer formation. At the flash degradation in nitrogen at atmospheric pressure PMMA degraded in the different manner. At the initial several flashes the photodegradation mechanism is also conducted in the polymer matrix and decomposed secondarily at the subsequent flashes to form a significant amount of carbon particles. Thereafter, carbon particles acted as a photoabsorber, and the thermal degradation of the polymer occurred predominantly around the heated carbon particles.     

Influence of γ‐irradiation on poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) was γ‐irradiated (5–20 kGy) by a ¹³⁷Cs source at room temperature in air. The changes in the molecular structure attributed to γ‐irradiation were studied by mechanical testing (flexure and hardness), size‐exclusion chromatography, differential scanning calorimetry, thermal gravimetric analysis, and both Fourier transform infrared and solution ¹³C‐NMR spectroscopy. Scanning electron microscopy was used to investigate the influence of the dose of γ rays on the fracture behavior of PMMA. The experimental results confirm that the PMMA degradation process involves chain scission. It was also observed that PMMA presents a brittle fracture mechanism and modifications in the color, becoming yellowish. The mechanical property curves show a similar pattern when the γ‐radiation dose increases. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 886–895, 2002     

Electron beam‐induced piezoelectric phase in poly(vinylidene fluoride) nanohybrid: effect at the molecular level

A nanohybrid has been synthesized by incorporating organically modified layered silicate in a poly(vinylidene fluoride) (PVDF) matrix. Molecular‐level phenomena have been explored after exposing PVDF and its nanohybrid to an electron beam of varying doses. The electron beam interacts with polymer chains and thereby generates different free radicals, the number of which is quite high in nanohybrid as compared to pure PVDF. The stability of free radicals has been confirmed through density functional theory energy minimization, predicting stable β‐phase free radicals in the nanohybrid. Quantitative analyses of chain scission, crosslinking and double bond formation are reported and compared after irradiation for both PVDF and its nanohybrid using UV‐visible and Fourier transform infrared spectroscopies, sol–gel analyses and gel permeation chromatography, revealing both chain scission and crosslinking phenomena in irradiated PVDF and its nanohybrid, but at higher dose (>90 Mrad) crosslinking dominates in the nanohybrid due to more free radicals and proximity of radical chains on top of templated system in the nanohybrid as compared to pure PVDF. The enhanced crosslinking alters the nanostructure causing disappearance of the peak at 2θ ≈ 3°. Moreover, the electron beam induces significant piezoelectric β‐phase in the nanohybrid against only α‐phase in pure PVDF at a similar dose and raises the possibility for the use of electron‐irradiated nanohybrid as an electromechanical device. β‐Phase formation is also supported through solid‐state NMR, scanning electron microscopy and differential scanning calorimetry studies. The thermal properties in terms of heat of fusion and degradation temperature have been verified indicating steady decrease of melting point and heat of fusion for pure PVDF while considerably less effect is observed for the nanohybrid. The combined effect of chain scission and crosslinking makes both PVDF and its nanohybrid brittle, but with greater stiffness with respect to unirradiated specimens. © 2014 Society of Chemical Industry     

Reactive compatibilization of polymer blends by γ‐irradiation: Influence of the order of processing steps

Results concerning γ‐irradiation of polymer blends such as HDPE/ground tire rubber (GTR) and PP/HDPE are reported in this article with a special emphasis on the order of processing steps. Irradiation dose varied in the range 0–100 kGy. The two first polymers (HDPE and rubber) are preferentially crosslinked under γ irradiation while PP undergoes chain scission. Mechanical tests and differential scanning calorimetry (DSC) analysis show that the efficiency of the reactive compatibilization by γ irradiation depends greatly on the chronology of γ‐irradiation and injection‐molding steps. Electron spin resonance (ESR) results reveal that numerous radicals remain trapped in the materials after γ‐irradiation even after a long time. Then the effect of irradiation on material properties is different if polymers are melted after irradiation or not. Crosslinking and chain scission are not affected in an equivalent way by the order of processing steps. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

On the thermal degradation of poly(vinyl chloride). II. The effect of atmosphere

The early stages of the thermal degradation of PVC were studied. Two commercial, suspension-polymerized resins were thermally treated at different temperatures and oxygen contents. Dehydrochlorination kinetics were followed by conductometric measurements and the formation of polyene sequences by ultraviolet-visible spectroscopy. Crosslinking and chain scission were followed by gel chromatography (GPC) and viscometry. No chain scission was observed in nitrogen atmosphere and no crosslinking in oxygen. Degradation in air proceeded by both reactions. The rate of dehydrochlorination for one of the investigated polymers increased linearly with the logarithm of the oxygen pressure. In nitrogen, an increasing degradation temperature was found to give both an increasing crosslinking and less discoloration. In oxygen, chain scission reactions showed a slight temperature dependence. The temperature effect on the discolorations was similar to that in nitrogen. The main difference between the investigated resins was the amount of internal doubled bonds in the original polymers, the ratio being 2:1. The higher amount resulted in a higher rate of dehydrochlorination, a larger extent of chain scission in oxygen, and a lower extent of crosslinking in nitrogen. Both in oxygen and nitrogen, the obtained results are consistent with allylic mechanisms. In nitrogen, the polyene formation supposedly proceeds by a unimolecular reaction and crosslinking by an intermolecular nonradical dehydrochlorination. In oxygen, radical reactions are superposed and may lead to chain rupture via β-scissions of alkoxy radicals.     

Reorganization and improvement of bulk polymers by processing with their cyclodextrin inclusion compounds

The formation of polymer-cyclodextrin inclusion compounds of polycarbonate (PC), poly(methylmethacrylate) (PMMA) and poly(vinylacetate) (PVAc) guests with host γ-cyclodextrin (γ-CD) have been successfully achieved. Coalesced bulk polymer samples were obtained by removal of γ-CD from their inclusion compounds (ICs). The chemical and crystalline structures of ICs and coalesced PC, PMMA and PVAc were studied by Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (WAXD). The thermal transitions, thermal stability, and degradation mechanisms of the samples were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and direct insertion probe pyrolysis mass spectrometry (DIP-MS). FTIR findings indicated that the chain conformations of the bulk polymers were altered when they were included inside the CD channels and extended chain conformations were retained when coalesced from their ICs. Significant improvements were observed in the thermal transitions observed for the coalesced polymers, with glass transitions shifted to higher temperatures. The TGA results reveal that the thermal stabilities of coalesced polymers increased slightly compared to the corresponding as-received polymers. The DIP-MS observations indicated that the thermal stability and degradation products of the polymers are affected once the polymers chains are included inside the γ-CD-IC cavities.     

Poly(methyl α-trifluoromethylacrylate) as a positive electron beam resist

A mechanistic hypothesis is presented which explains the radiation chemistry of poly(methyl α-haloacrylate)s. In order to test the hypothesis poly(methyl α-trifluoromethylacrylate) PMTFMA was synthesized together with copolymers of methyl methacrylate (MMA), and the α-trifluoromethyl analog. The mechanistic hypothesis predicts that PMTFMA should have a higher G scission than PMMA and that it should have no crosslinking propensity. This prediction was verified by experiment. Imaging of PMTFMA as a positive e-beam resist is also presented. The new material is a more sensitive resist than PMMA.     

Effects of solvent and concentration on scission of polymers with high-speed stirring

The effects of solvent and concentration on scission of polymers such as poly(methyl methacrylate) (PMMA) and polystyrene (PSt) in solution by high-speed stirring were investigated. Solvents were chloroform (good), benzene (intermediate), and ethyl acetate (poor) for PMMA and methyl ethyl ketone (good), toluene (intermediate) and dioxane (poor) for PSt, respectively. Concentration was varied from 0.04 to 2% w/v. The rate of scission of polymer chains was higher and the final molecular weight was lower in a good solvent than in a poor solvent at a low concentration for both polymers, but vice versa at a high concentration except for PSt in methyl ethyl ketone. Concentration dependence of the scission was large in a good solvent but small in a poor one. Polymer chains were ruptured to lower molecular weights with decreasing concentration, regardless of kind of polymer and solvent, showing that they were more easily broken in the isolated state.     

Variation of lexan polycarbonate properties by electron beam

The modifications in microstructural, optical, and photoluminescence properties of the Lexan polycarbonate (bisphenol‐A‐polycarbonate) films exposed to different electron doses have been studied using UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), positron annihilation lifetime spectroscopy, photoluminescence spectroscopy, and scanning electron microscopy (SEM). The obtained UV–vis spectroscopy results showed decrease in optical energy gap, optical activation energy, and increase in number of carbon atoms per cluster with increase in electron dose. The chemical changes in electron irradiated polymers due to chain scission and reconstruction have been observed from FTIR spectroscopy. The correlation of positron lifetime study with optical measurement is obtained, and electron irradiation‐induced microstructural modifications within the polymer is understood. SEM result shows the degradation of Lexan polymer after electron irradiation. The mechanical properties and average molecular weight of Lexan decrease after irradiation, whereas average number of chain scissions per original polymer molecule increases. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Electron irradiation of poly(olefin sulfones). Application to electron beam resists

All the poly(olefin sulfones) examined degraded rapidly under electron irradiation. The dose required to effect a molecular weight distribution completely separated from the original distribution as required for fractional solution development was similar for all polymers, viz., 1–2 × 10^?6 coulomb/cm². This indicates that they all have similar values for G(scission). The film thickness of the exposed area decreased at a rate dependent on olefin structure and temperature. This process, termed vapor development, has been attributed to concurrent chain scission and depolymerization. Factors determining the rate of depropagation are discussed.