F. Szöcs

Study of rotational mobility of stable nitroxide radicals in solid polymers

Abstract The rotational mobility of stable nitroxide radicals in PS, PMMA, PVC, PP and PE has been studied over a wide temperature range by the e.s.r. method. At temperatures T lower than T g , spin probes act as kinetically independent particles, the rotational frequency of which is deter mined mainly by the micropore dimensions of the polymer and depends indirectly on the mobility of segments or side groups.

Free-radical decay in a polymer blend PMMA/PEO studied by the EPR method

Abstract The EPR method was used for studying decay of free radicals in a blend of poly(methyl methacrylate) (PMMA) with poly(ethylene oxide) (PEO). The influence of the composition of the blend (up to 70 vol% PEO) and of the time of mixing on the transfer of a radical centre was determined. The time of mixing for a blend affects molecular motions as a result of specific interactions between dissimilar PMMA and PEO chains. These molecular motions control the transfer of the radical centre so that free radicals decay more rapidly in the blend mixed for 6 min than in that mixed for 12 min. A slower decrease in the concentration of free radicals in a sample of a PMMA/PEO blend containing 50 vol% PEO compared with samples with 10, 30 and 70 vol% is also observed.

Study of the influence of physical ageing and rejuvenation on macroradical reactivity in polycarbonate

Abstract Amorphous polycarbonate with various mechanical and/or thermal histories was investigated via macroradical decay in the temperature interval between −30°C and 100°C by e.p.r. spectroscopy. The decay of end macroradicals was found to take place at low temperatures more rapidly in anisotropic drawn samples than in isotropic materials. The rate constants of both samples show breaks at ∼20°C and 70°C and coincide at higher temperatures. After heating in the region below the glass transition temperature, the order of the rate constants is maintained but the decay is more rapid in aged samples compared to unaged materials. The concentration and kinetic characteristics are correlated with previous small-angle X-ray scattering results and the dynamic-mechanical relaxation results presented here and interpreted in terms of changes in free volume characteristics after cold deformation and/or annealing of polycarbonate.

High pressure effect on polyethylene crosslinking initiated by benzoyl peroxide

Abstract High pressure effects (300, 600, and 800 MPa) on polyethylene crosslinking initiated by benzoyl peroxide have been studied at 120°C. An increase in pressure leads to a decrease in yield of the insoluble gel; the yields at 300, 600, and 800 MPa were respectively 96, 86, and 79%. With increasing pressure the rate of polyethylene crosslinking decreases and the rate of benzoyl peroxide decomposition at 300, 600, 800 MPa can be anticipated from the time of crosslinking. The activation volume ΔV ‡ =9.1 cm 3 mol −1 was determined for benzoyl peroxide decomposition on polyethylene at 120°C.

Study of radical products of mechanically destructed poly(N-vinylene carbazole)

Abstract Free radicals generated by mechanical destruction of poly(N-vinylene carbazole) have been studied. Two types of chain-end radicals have been found in the system at 100K in accord with the polymer fracture theory. At 273K these radicals started to decay and new, more stable main-chain radicals have been observed. When O2 was introduced into the system, the chain-end radicals disappeared rapidly giving rise to ROO· radicals, which decayed at 200K generating nitrogen centred cation radicals.

Reactivity of 2(2-hydroxy-5-vinylphenyl)-2h-benzotriazole-methyl methacrylate copolymer towards alkyl and peroxy radicals in the solid state

Abstract The reactivity of 2(2-hydroxy-5-vinyl)2H-benzotriazole (2H5V) structural units in a copolymer with methyl methacrylate (MMA) with respect to alkyl or peroxy radicals in the solid phase was studied. Alkyl radicals were generated through mechanical degradation of the copolymer and peroxy radicals were obtained by oxidation of alkyl radicals. Formation of oxy radicals expected in 2H5V-MMA copolymer was not observed even in the presence of alkyl or peroxy radicals. The resistance of 2H5V structural units of a copolymer to both types of radicals is attributed to the stability of the structure of 2H5V which is probably assisted by hydrogen bonding.

E.s.r. study of free radicals in poly(vinyl carbazole)

Abstract Poly(N-vinylcarbazole) was γ-irradiated at 77 K. The analysis of e.s.r. spectra showed that cation radicals centred at N were produced in the system. Hyperfine splitting constants are: a N =0.57 mT and a H β =0.68 mT. The dependence of the rate constant of free radical decay on pressure (1–800 MPa) was studied at various temperatures (90–170°C). Activation volumes were determined for two characteristic pressure and temperature ranges. The results of e.s.r. measurements are compared with those obtained from dynamic-mechanical measurements.

Free radicals observed in the grafting of maleic anhydride on poly(methyl methacrylate)

Abstract The ESR method was used for investigating free radicals formed during thermal decomposition of dibenzoyl peroxide in the poly(methyl methacrylate) (PMMA)-maleic anhydride (MAN) system at 400 MPa. No primary product from the addition of a polymer radical to maleic anhydride was observed, but instead a polyene radical was formed within the polymer matrix. Possibilities of the formation of polyene radicals as well as the effect of temperature and reaction time on the concentration of free radicals are discussed.

E.s.r. study of radical polymerization of pentaerythrityl monoacetal dimethacrylate in polystyrene matrix at high pressure

Abstract The e.s.r. spectrum of free radicals formed during polymerization of a bifunctional monomer of pentaerythrityl monoacetal dimethacrylate by thermal decomposition of benzoyl peroxide in a polystyrene matrix at 700 MPa is studied. Formation of propagating radicals of cyclopolymerization and crosslinking polymerization is discussed.

Pressure dependence of free radical decay in γ-irradiated main chain liquid crystalline polymers

Abstract A decay of free radicals in γ-irradiated rigid main chain thermotropic polymer BRY 856 was studied. The dependence of the rate constant for free radical decay over the pressure region between 1 MPa and 800 MPa was determined in the temperature range between 80°C and 230°C. Activation volumes of free radical decay were established. A decrease in activation volume at temperatures above the glass transition temperature of the polymer was observed and attributed to rotational structural order along the long axis of the macromolecules.