Peter J. Pomery

Copolymer hydrogels of 2-hydroxyethyl methacrylate with n-butyl methacrylate and cyclohexyl methacrylate: synthesis, characterization and uptake of water

The bulk free radical copolymerizations of 2-hydroxyethyl methacrylate (HEMA) with n-butyl methacrylate (BMA) or cyclohexyl methacrylate (CHMA) were studied over the composition mole fraction interval of 0-1 for HEMA in the monomer feed. The C-13 NMR (125 MHz) spectra of the copolymers were analysed to determine the copolymer composition and the stereochemical configuration of the copolymers. The terminal model reactivity ratios of HEMA and BMA were found to be r(HEMA) = 1.73 and r(BMA) = 0.65 and for HEMA and CHMA, r(HEMA) = 1.26 and r(CHMA) = 0.31. The BMA and CHMA homopolymers were found to be predominantly syndiotactic with isotacticity parameters of theta(BB) = 0.18 and theta(CC) = 0.19, respectively. The copolymers were also found to be predominantly syndiotactic, indicating a strong tendency for racemic additions of the monomers in the formation of the copolymers. The diffusion of water into cylinders of poly(HEMA-co-BMA) and poly(HEMA-co-CHMA) was studied over a range of copolymer compositions and was found to be Fickian. The diffusion coefficients of water at 37 degrees C were determined from swelling measurements and were found to vary from 1.72 x 10(-11) m(2) s(-1) for polyHEMA to 0.97 x 10(-11) m(2) s(-1) for poly(HEMA-co-BMA) having a mole fraction F-HEMA = 0.80 and to 0.91 x 10(-11) m(2) s(-1) for a poly(HEMA-co-CHMA) also having F-HEMA = 0.80. The mass of water absorbed at equilibrium relative to the mass of dry polymer varied from 58.8 for polyHEMA to 27.2% for poly(HEMA-co-BMA) having F-HEMA = 0.85 and to 21.3% for poly(HEMA-co-CHMA) having F-HEMA = 0.80

An electron spin resonance study on γ-irradiated poly(l-lactic acid) and poly(d,l-lactic acid)

The effect of gamma irradiation on poly(l-lactic acid) (l-PLA) and poly(d,l-lactic acid) (d,l-PLA), has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G values for radical formation of both polylactic acids have been calculated at 77 and 300 K; G(R) = 2.0 at 77 K and G(R) = 1.5 at 300 K for l-PLA and G(R) = 2.4 at 77 K and G(R) = 1.2 at 300 K for d,l-PLA. The ESR spectrum at 300 K for the polymers was assigned to one radical, resulting from H atom abstraction from the quaternary carbon atom. The G values for crosslinking and scission have also been determined for the polymers at 300 K; G(S) = 2.3 and G(X) = 0.0 for d,l-PLA, G(S) = 2.4 and G(X) = 0.28 for l-PLA.

Intramolecular cyclization in hyperbranched polyesters

The effect of monomer structure and catalyst on the synthesis of hyperbranched polyesters based on 4,4-(4′-hydroxyphenyl)pentanoic acid has been examined. The nature of the ester group and the catalyst have a significant effect on the molecular weight of the hyperbranched polyester but do not effect the degree of branching for these materials. The fate of the single ester group at the focal point of these hyperbranched macromolecules is probed by the synthesis and polymerization of 13C labeled methyl 4,4-(4′-hydroxyphenyl)pentanoate. Comparison of the molecular weights determined by 1H- or 13C-NMR spectra with those determined by osmometry suggest that intramolecular cyclization does not occur to a significant extent in these systems.

Radical polymerization of phenyl acrylate as studied by ESR spectroscopy: Concurrence of propagating and mid-chain radicals

The electron spin resonance (ESR) spectrum of the propagating radical of phenyl acrylate (PhA) was successfully recorded in benzene as a non-polar solvent. The hyperfine coupling constants for the α and β-protons were evaluated on the basis of the spectra of the propagating radicals of PhA and phenyl acrylate-α-d. The simulated spectrum satisfactorily fits those observed during polymerization, and the spectrum of the poly(PhA) radical obtained at low conversion ( 15%) indicated the presence of two types of radical species, a propagating radical and a mid-chain radical produced by abstraction of the α-hydrogen of the monomeric unit. The content of branching in the polymers as a result of the formation of a mid-chain radical was found to be 1–3% by 13C NMR spectroscopy. The absolute rate constants for propagation (kp) and termination (kt) of PhA at low conversions were determined based on the quantification of the propagating radical by ESR spectroscopy at 60°C: kp = 3 580 dm3·mol–1·s–1 and kt = 6,8×106 dm3·mol–1·s–1. However, these seemed to be apparent values because the propagating radical is expected to be converted to the mid-chain radical by intra- and intermolecular hydrogen abstraction before the loss of its activity by bimolecular termination. Conversion from the propagating radical to the mid-chain radical followed by reinitiation was estimated to occur more than twenty times during the lifetime of each polymer chain.

Kinetic parameters for polymerization of methyl methacrylate at 60°C

Abstract Fourier transform infrared spectroscopy (FTi.r.) and electron spin resonance spectroscopy (e.s.r.) have been used to follow the kinetics of the polymerization of methyl methacrylate to high conversion at 60°C. The FTi.r. absorbance at 6152 cm−1 was used to monitor the time dependence of the concentration of double bonds, and the concentration of the polymer-chain propagation radicals was monitored using the e.s.r. absorption spectrum. These data were analysed to obtain instantaneous estimates of the kinetic rate parameters for propagation and termination across the range of conversion, and of the initiator efficiency at high conversion. The kinetic parameters were found to be consistent with values obtained by other methods and with the predictions of recent theories.

Development of wear-resistant thermoplastic polyurethanes by blending with poly(dimethyl siloxane). II: a packing model

It has been shown in a previous article that melt blending of low levels of commercial poly(dimethyl siloxane) (PDMS) fluid with commercial thermoplastic polyurethanes has a significant positive impact on the coefficient of friction (CoF) and on the mechanical and wear properties of the polyurethanes. The improvements in CoF and wear resistance were expected due to surface modification of the polymer; however, the improvements in the mechanical properties were much more significant than expected. Evidence presented in the earlier publication suggests that the changes in the wear and mechanical properties are not due to surface modification alone, but are largely due to modification of the bulk by PDMS. In this article a model is presented that accounts for the observed relationship between PDMS content and the properties of the blends. It is proposed that the addition of PDMS facilitates an improved packing efficiency (antiplasticization) in the polyurethane soft domain, leading to improved material performance. Beyond an optimum PDMS concentration of 1.5–2.0%, phase separation of PDMS becomes significant, plasticization sets in, and mechanical properties then begin to diminish rapidly. This model has been rigorously investigated and has proven to be highly robust.

An electron spin resonance study of the radiation chemistry of poly(hydroxybutyrate)

Abstract The spectra of the radical species resulting from gamma irradiation of poly(hydroxybutyrate) and its copolymers with hydroxyvalerate have been obtained after radiolysis at 77 and 303 K, and the corresponding radiation chemical yields have been calculated. Photobleaching and annealing experiments have allowed the major radical species to be identified and their reactivities to be assessed. Radical anions, as well as neutral radicals, were observed to be present at 77 K, but the radical anions were found to decay to neutral radicals at temperatures around 140K. Above about 250 K a multi-line component of the spectrum was lost, but the radical associated with this spectral change has not been unequivocally assigned, though it is believed to be a radical resulting from chain scission. Annealing over the temperature range 300–350 K resulted in the loss of a triplet, which accounts for most of the radicals present. This triplet has been assigned to a radical located on the carbon atom adjacent to the carbonyl group. This radical is believed to exist in two possible conformations. All of the radicals decayed at temperatures above 400 K.

Thermal degradation of polymethacrylonitrile

The thermal degradation properties of polymethacrylonitrile (PMAN) have been studied by isothermal heating and thermogravimetric analysis. There are two initiation processes for weight loss from PMAN degraded in nitrogen, namely chain-end and random scission initiation. There is also an internal cyclization reaction which forms a thermally stable residue during the thermal degradation process. The activation energies of the weight loss and formation of stable residues have been calculated. X-ray photoelectron spectroscopy has been used to investigate the structure of the stable residue and thus to confirm the degradation mechanism.

The effects of γ-radiation on polyacrylonitrile

The effect of γ-radiation on polyacrylonitrile has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G-values for radical formation of polyacrylonitrile have been calculated at 77 and 300K; G(R·) ≅ 2·8 at 77K and G(R·) ≅ 4·2 at 300K. The ESR spectrum at 300K was assigned primarily to two radicals; one resulting from H abstraction from the methylene group, to form a chain radical, and the other from radical addition to the nitrile group, to form a polyimine radical. These radicals are consistent with a dominant cross-linking reaction. The G-values for cross-linking and scission have been determined at 300K; G(S) ≅ 0·0 and G(X) ≅ 0·59. The cross-linking possibly results from addition of backbone radicals to nitrile groups on adjacent chains.

Polymerisation kinetics of allyl monomers at low conversions

Free radical polymerisation kinetics of several mono- and di-allyl monomers were studied using FT-NIR, ESR and GPC techniques to measure the monomer conversion, nature and the concentration of the radicals and the polymer molecular weight, respectively. By manipulating the modulation amplitude of the ESR, the allyl radical and the propagating radical concentrations were estimated and these values were used to calculate the propagation, transfer, re-initiation and termination rate constants. The activation energy for the allyl radical termination reaction was found to be higher than that for the termination of other vinyl monomers. The percentage cyclization for the di-allyl monomers was estimated and for CR39 the value was found to be as high as 25%. A gel with a higher modulus was found to form in CR39 when the reaction rate was kept low.