Solid-state polymerization studies. I. 2-vinylnaphthalene post-polymerization initiated by 60Co γ-irradiation

⁶⁰Co γ-irradiated 2-vinylnaphthalene was obsreved to post-polymerize in the solid phase. Plots of conversion versus time indicated a 14% limiting conversion of monomer to polymer. The post-polymerization was found to be first-order in monomer with an Arrhenius activation energy of 19.0 kcal./mole.     

Modified amylose biodegradability by α-amylases

The site-selective syntheses of water-soluble (O-2/O-3)-substituted amylose derivatives containing butyl ester, carboxymethyl, succinyl or quaternary ammonium side groups was carried out. In addition, routine chemical methods which did not employ protection-deprotection steps provided water soluble (O-2/O-3/O-6)-substituted products. The biodegradation of these products as a function of site-of-substitution and degrees of substitution (ds) was studied by the α-amylases from Aspergillus oryzae and Bacillus subtilis. Consistent with the results reported previously for acetyl amylose (Ref. 1), the O-2/O-3 substituted butyl amylose derivatives degraded faster and to higher percentages than the corresponding O-2/O-3/O-6 substituted derivatives. Also, the overall rates and percentages of degradation for butyl amylose were much lower than acetyl amylose derivatives of comparable degree and site of substitution. Therefore, for the two enzymes studied, the introduction of longer chain length acyl side groups greatly reduced substrate degradability. For the charged derivatives, the rate and percentage of degradation decreased in the order carboxymethyl > succinyl > quaternary ammonium at comparable degree and site of substitution. We concluded that, as the length of the charged side chain substituent increased, the rate and extent of biodegradation decreased. Furthermore, in contrast to the results for neutral acetyl and butyl side groups, there was no significant difference in the rate and %-degradation for charged side chain substituent groups when comparing O-2/O-3 and O-2/O-3/O-6-substituted derivatives. Finally, the introduction of anionic or cationic side chain groups resulted in decreased susceptibility to α-amylase catalyzed chain cleavage when compared to similarly substituted acetyl and butyl amylose derivatives.     

Evaluation of crosslinking and scission yields in the degradation of polystyrene by γ-irradiation in air

A sample of polystyrene with narrow molecular weight distribution (M?_w/M?_n = 1.03) has been formed into a 3-mm sheet and subjected to γ-irradiation at 25°C in air. Simultaneous crosslinking and scission of polymer chains resulted. Analyses by velocity and equilibrium sedimentation, gelpermeation chromatography, and osmometry of the polystyrene subjected to doses in the range 0-90 Mrad, all within the pre-gel region, have yielded average values of G(S) = G(X) = 0.022 ± 0.002 for the complete sample. Combination of these various experimental techniques has been successful in yielding consistent values of G(S) and G(X) and should be applicable with advantage to other polymers, particularly when neither G(S) nor G(X) is zero. The wide variation in literature values of G(S) and G(X) for irradiation of polystyrene under vacuum precludes any unequivocal conclusion on the effect of air on the radiation yields.     

Swelling behavior of ampholytic gels prepared by γ-irradiation

Heterogeneous ampholytic gels were prepared by two-step γ-irradiation of aqueous solutions of poly(vinyl alcohol)s having sulfonate and quaternary ammonium groups, respectively. A reference gel was prepared by one-step irradiation. The heterogeneous gels turned out to swell to lesser degrees with increasing ionic strength of the immersing solution than the reference gel. However, the ion-specific swelling and deswelling behavior, which had been found for the reference gel, was also observed for the present heterogeneous gel.     

Co60 γ-radiation-induced copolymerization of ethylene and carbon monoxide

The γ-radiation-induced free-radical copolymerization of ethylene and CO has been investigated over a wide range of pressure, initial gas composition, radiation intensity, and temperature. At 20°C., concentrations of CO up to 1% retard the polymerization of ethylene. Above this concentration the rate reaches a maximum between 27.5 and 39.2% CO and then decreases. The copolymer composition increases only from 40 to 50% CO when the gas mixture is varied from 5 to 90% CO. A relatively constant reactivity ratio is obtained at 20°C., indicating that CO adds 23.6 times as fast as an ethylene monomer to an ethylene free-radical chain end. For a 50% CO gas mixture, the above value of 23.6 and the copolymerization rate decrease with increasing temperature to 200°C. The kinetic data indicate a temperature-dependent depropagation reaction. Infrared examination of copolymers indicates a polyketone structure containing ? CH₂? CH₂? and ? CO? units. The crystalline melting point increases rapidly from 111 to 242°C., as the CO concentration in the copolymer increases from 27 to 50%. Molecular weight of copolymer formed at 20°C. increased with increasing CO, indicating M?_n values >20,000. Increasing reaction temperature results in decreasing molecular weight. Onset of decomposition for a 50% CO copolymer was measured at ≈250°C.     

Evidence of crosslinking and chain scission in the degradation of poly(tert-butyl crotonate) by γ-irradiation

The effect of γ-irradiation on poly(tert-butyl crotonate) has been investigated by velocity sedimentation and solubility studies. Evidence of significant crosslinking as well as of chain scission has been obtained, the estimates of G(X) and G(S) being 0.66 and 0.59, respectively.     

γ-irradiation of polypropylene in vacuum and in air

Polypropylene films were irradiated with ⁶⁰Co γ-rays in vacuum or in air and stored in air. Just after irradiation, the concentration of carbonyl group of the sample irradiated in air only increased with dose. The concentrations of both samples increased with storage time. The more the absorbed doses, the higher the increasing rates. The increasing rate of the concentration of carbonyl group during irradiation in air was higher than those during storage in air. Just after irradiation, the tensile strengths and the elongations of the both samples somewhat increased with dose at the doses less than 5 Mrad, but decreased at doses more than 13 Mrad. The tensile strength and the elongation of the sample irradiated in air decreased with storage time. Those of the sample irradiated in vacuum also decreased with storage time but the decreasing rates were much smaller than those in the sample irradiated in air. The gel fractions of the samples irradiated in vacuum and annealed in vacuum were somewhat higher than those irradiated in vacuum and not annealed. To elucidate high oxidation rate in the sample irradiated in air during and after irradiation, reaction mechanisms were discussed. To clarify the difference of mechanical properties between the samples irradiated in vacuum and in air, the effect of crosslink was considered, together with the oxidation.     

High photosensitivity in cis-polyphenylacetylene films irradiated with 60Co γ-ray

This paper reports that ⁶⁰Co γ-ray irradiation can convert cis-polyphenylacetylene (cis-PPA) films prepared with rare-earth coordination catalysts to highly photosensitive materials. The dependence of the photosensitivity on irradiation dose, preparation methods, and microstructure of the PPA films has been investigated by means of a potential discharge technique. The photosensitivity was enhanced with increasing irradiation dose. The critical dose to produce a light response was 5 × 10³ Gy. The maximum surface potential discharge rate was 618 V/s, and the dark decay was approximately 2 V/s for cis-PPA films irradiated with ⁶⁰Co γ-ray (dose: 2 × 10⁵ Gy). The cis-transoidal-PPA and an electrophotographic photoreceptor device incorporating cis-PPA showed a higher irradiation effect. The structure and properties of ⁶⁰Co γ-ray irradiated rare-earth PPA films are similar to the unirradiated films.     

An ESR study of polyisobutylene radicals produced by γ-irradiation at 77 K

An electron spin resonance (ESR) study has been made of radicals trapped in polyisobutylene irradiated at 77 K. It is concluded that only the ? C(Me)₂? CH? radical remains after heat treatment above 213 K. This radical shows a very sharp doublet ESR spectrum with hyperfine splitting of about 2.2 mT. A broad doublet ESR spectrum observed without warming, which was previously assigned as ? C(Me)₂? CH? is explained as a mixture of a doublet and a triplet due to the ? C(Me)₂? CH? and radicals. γ-irradiation at 77 K produces these major radical components and other carbon radicals as minor species. It has also been found that a conversion of other radicals into ? C(Me)₂? CH? occurs without decay on annealing below 183 K. Heating in the range 183–213 K causes both conversion and decay of the radicals. On annealing above 213 K, only decay occurs. The ESR spectra of polyisobutylene radicals are easily saturated by microwave power even at 1.6 μW. The mechanism of radiolysis for polyisobutylene is briefly discussed.     

γ-irradiation of siloxane polymers in air and vacuum at several temperatures

The effects of γ-irradiation on two polysiloxanes in environments of air and vacuum at several selected temperatures were studied. The physical property monitored during the γ-irradiation of the two polysiloxanes was the change in dynamic modulus which, in turn, is proportional to the change in the number of elastically effective chains in the polymer network. The measurements were accomplished by recording in situ the freeend response of the polymeric cantilever reeds in forced resonance at and near their fundamental frequency of resonance. The apparent energy of activation of crosslinking for the polydimethylsiloxane polymer was found to be 0.56 kcal./mole in vacuum over a limited temperature range, and 3.5 kcal./mole in air. For the polydimethyl-diphenylsiloxane polymer, the apparent energy of activation was found to be about 1.3 kcal./mole in vacuum, whereas in air the polymer response was complex, displaying both scissioning and crosslinking reactions so that a value could not be obtained. The ability of the pendant phenyl groups to reduce the effective crosslinking rate was found to be sensitive to both temperature and environment.     

The effect of γ-irradiation on slow crack growth in polyethylene

HDPE was γ-irradiated at room temperature. The resistance to slow crack growth (SCG) was measured in single edge notched tensile specimens under constant load as a function of the dose. The resistance to SCG initially decreased to a minimum value at a dose between 0.05 and 0.10 Mrd. The minimum value was 45% less than for the undosed state. For doses greater than 0.10 Mrd, the resistance to SCG increased up to a dose of 50 Mrd, where its value had increased by a factor of 10². The gel point occurred at 1–3 Mrd. MI and the crack opening displacement exhibited maximum values at a dose of 0.1 Mrd. The behaviors of SCG, MI and crack opening displacement were consistent with the explanation that chain scission dominated for doses less than 0.1 Mrd, and cross-linking dominated at the higher doses. For doses beyond 50 Mrd, the resin became so brittle that it cracked during the loading of the specimen. Beyond the gel point the density increased from 0.9694 to 0.9716 g/cm³ at a dose of 160 Mrd. ©1995 John Wiley & Sons, Inc.     

Comparison of the stable products formed by fast atom bombardment and γ-irradiation of glycerol

The major end-products formed by fast atom bombardment (FAB) and γ-irradiation of glycerol have been identified using capillary gas chromatography/mass spectrometry. The product distributions differ dramatically. Products resulting from the recombination of either carbon-centered or oxygen-centered radicals are evident for both sources of irradiation. However, the major FAB-generated products are formed by the recombination of carbon-centered [G – H]^· radicals with other carbon-centered radicals while the major γ-radiolysis products result from the recombination of oxygen-centered [G–H]^· radicals with lower-weight carbon-centered species. γ-Radiolysis of an N₂O-saturated aqueous solution of glycerol, experimental conditions that strongly favor the formation of carbon-centered [G – H]^· radicals at the expense of the oxygen-centered species, yielded some products that were identical to those produced by FAB of neat glycerol. Finally, the free radicals produced by γ-radiolysis of glycerol were investigated by electron spin resonance spectroscopy and spin trapping with 5,5-dimethyl-1-pyrrolne-TV-oxide. The presence of only the carbon-centered free radical was established by this technique. The implications of these results are discussed.     

New Structure Formation on γ-irradiation of Poly(chlorotrifluoroethylene)

The radiation chemistry of PCTFE at different temperatures has been studied. The polymer was irradiated under vacuum to absorbed doses of up to 1500 kGy. Three irradiation temperatures were chosen. These included ambient temperature, a temperature well above the T_g and a temperature above the crystalline melting temperature. These were 298, 423 and 493 K, respectively. The formation of new structures was identified by solid-state FTIR and ¹⁹F NMR. No branching was observed below the melting temperature, but branches were observed above the melting temperature. G-values for chain-end formation were 1.5 and 2.4 at room temperature and 423 K, respectively and the G-value for the formation of double bonds was found to be <0.1. For the irradiations at 493 K, the G-values for the formation of chain ends, double bonds and branching points were 3.6, 0.2 and 0.5, respectively. The presence of long-chain branches within the polymer structure could not be proven for radiolysis at 493 K, but scission predominates and network formation does not occur upon irradiation. DSC studies of the polymers irradiated at ambient temperature were consistent with chain scission leading to an increase in the percentage crystallinity, as observed for other fluoropolymers.     

Influence of γ-irradiation on the transition temperatures of crystalline polytetrafluorethylene

The influence of γ-irradiation on the melting and two solid-solid transitions, occurring near 19° and 30°C, of polytetrafluorethylene was studied by differential scanning calorimetry. A continuous depression of all three transition temperatures, with increasing dose was observed in a first scan of highly crystalline samples at a heating rate of 20°C/min. Additional information was obtained about the accompanying heats of transition. Values for the number of CF₂ units excluded from the crystal lattice per 100 eV energy deposition were calculated by using an equation for the depression of the melting point by chemical impurities. The value obtained of G(–units) = 3.3 ± 0.3 is in agreement with a value of G(chemically damaged units) = 3.0 ± 0.1 previously reported on the basis of scavenging techniques. It is concluded that chemical radiation-induced damage in this polymer may be estimated by reference to changes in the melting temperature. A value of G(–units) = 1.6 ± 0.2 was obtained by reference to the 19°C transition.     

Pyrolysis of vinyl and vinylidene fluoride polymers: Influence of prior γ-irradiation

Quantitative comparisons were made between the rates of thermal volatilization of several fluoropolymers before and after exposure to γ-radiation. The effects of γ-irradiation on poly(vinyl fluoride) and poly(vinylidene fluoride) were also investigated by swelling and sol-gel ratios. With both polymers as well as with polytrifluoroethylene, crosslinks occur predominantly, though there is an appreciable number of scissions. The rates of volatilization and char formation were enhanced by γ-radiation, whereas the previously studied polytrifluoroethylene did not produce more char upon irradiation, although radiation did accelerate its volatilization. It is believed that in polytrifluoroethylene the enhanced rates of volatilization occur by a different mechanism than in the case of the vinyl and vinylidene fluoride polymers.     

Synthesis of γγγ-trifluorocarbonyl compounds

γγγ-Trifluorocarbonyl compounds are easily obtained in a good yield by introduction of the 1,1,1-trifluoroethyl moiety (CF₃-CH₂-) on the -methylene group of a ketone.     

Effects of γ-irradiation on the morphology and slow crack growth in high-density polyethylene

The effects of γ-irradiation were measured in a HDPE and in the resin after it was recrystallized. The fracture mode of the initial material transformed from crazing to complete brittle failure at a critical dose. The failure mode of the recrystallized material transformed from crazing to shear deformation, which produced an extremely long failure time, and finally, at a higher dose, its fracture became brittle. The relationship between morphology and slow crack growth is presented where crosslinking was the major factor. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1211–1218, 1998     

Synthesis of β3‐Peptides and Mixed α/β3‐Peptides by Thioligation

Five β‐peptide thioesters ( 1 – 5 , containing 3, 4, 10 residues) were prepared by manual solid‐phase synthesis and purified by reverse‐phase preparative HPLC. A β‐undecapeptide ( 6 ) and an α‐undecapeptide ( 7 ) with N‐terminal β³‐HCys and Cys residues were prepared by manual and machine synthesis, respectively. Coupling of the thioesters with the cysteine derivatives in the presence of PhSH (Scheme and Fig. 1) in aqueous solution occurred smoothly and quantitatively. Pentadeca‐ and heneicosapeptides ( 8 – 10 ) were isolated, after preparative RP‐HPLC purification, in yields of up to 60%. Thus, the so‐called native chemical ligation works well with β‐peptides, producing larger β³‐ and α/β³‐mixed peptides. Compounds 1 – 10 were characterized by high‐resolution mass spectrometry (HR‐MS) and by CD spectroscopy, including temperature and concentration dependence. β‐Peptide 9 with 21 residues shows an intense negative Cotton effect near 210 nm but no zero‐crossing above 190 nm, (Figs. 2–4), which is characteristic of β‐peptidic 3₁₄‐helical structures. Comparison of the CD spectra of the mixed α/β‐pentadecapeptide ( 10 ) and a helical α‐peptide (Fig. 5) indicate the presence of an α‐peptidic 3.6₁₃ helix.