M. Klimová

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.

Spin Probe Study of Drawn UHMW PE Fibres

The ESR method has been used to investigate the structure and properties of highly drawn ultra-high molecular weight polyethylene fibres. The values of T 50G , rotational correlation times and the free radical decay curves for fibres of various draw ratios have been obtained from ESR spectra. The model of soft domain formation in fibres at high draw ratios is used to explain the experimental results.

EPR study of free radical decay in low density polyethylene filled with silica

Samples of polyethylene and crosslinked polyethylene filled with different amounts of silica were prepared. Free radicals were generated by gamma radiation and measured by an EPR method. The rate constant for the decay of free radicals depends on the amount of the filler, as well as on the degree of polymer crosslinking. The crosslinks and the interaction of polyethylene with inorganic filler slows down the transport of free radicals. Free radicals act as molecular probes and inform about the changes of molecular movement in main chains of the polymers.

Influence of high pressure annealing on free volume in polycarbonate studied by positron annihilation

Abstract The method of positron annihilation was used to study changes of free volume in polycarbonate. Annealing at 180 °C and pressure 800 MPa causes significant changes of free volume (~23%) compared to nonannealed polycarbonate. The free volume compressibility was estimated to be ~3.10 −5 MPa −1 . The distribution of free volume has changed. This work discusses the influence of free volume on molecular dynamics and the transportation mechanism of free radical decay.

An ESR study of the influence of fatigue on the decay of free radicals in gamma irradiated polycarbonate

Polycarbonate was subjected to long-term stress by transverse bending. Stressed, as well as non-fatigued, samples were irradiated by gamma radiation in evacuated ampoules. The concentration of free radicals was measured by ESR in the course of gradual heating of the samples. It appeared that the free radicals in the samples subjected to long-term stress were more stable than the free radicals in the non-fatigued samples. This suggests that the arrangement of polymer chains changes during fatigue and the free volume in the proximity of radical centres decreases so that the transfer of these centres to adjoining chains is retarded.

Concept of mobile voids in the interpretation of decay of free radicals in solid polymers

Abstract An alternative idea of decay of free radicals in solid polymers based upon a concept of voids existing in each polymer with the mobility induced by the motion of respective polymer segments and vice versa has been put forward taking into account formal reaction kinetics. It has been shown that this approach enables the qualitative interpretation of second-order decay, stepwise decay of free radicals initiated by a stepwise increase of temperature as well as the effect of high pressure on the overall rate constant of the decay process.

Influence of pressure on a free radical decay in a rigid main chain thermotropic polymer

Abstract Free radical decay has been studied in a gamma-irradiated rigid rod-like liquid crystalline copolymer over wide ranges of pressure (1–900 MPa) and temperature (70–140°). Rate constants and activation volumes for the decay were determined. A remarkable decrease of activation volume at temperatures above T g of the polymer was observed and attributed to rotational structural order along the long axis of the rod-life molecules.

Fatigue-effect on free radical decay in irradiated polymethyl methacrylate

The influence of material fatigue on molecular motion in polymethyl methacrylate (PMMA) was investigated by the ESR method. Free radicals served as molecular labels informing on changes in molecular dynamics. It has been found that the fatigue of PMMA accelerates the decay of free radicals. In the course of material fatigue new degrees of freedom of motion of polymer chains come into existence. An increase in free volume as well as its redistribution sets in, owing to cyclic stress of the polymer.

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.

Spin Probe Study of IPN-like Systems based on Polyethylene and Polymethacrylates

A system similar to an interpenetrating polymer network (IPN), based on polyethylene/poly(methylmethacrylate-co-dodecyl methacrylate), was studied by the spin probe method with a nitroxide spin probe, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). The influence of changes in the composition of the copolymer (methyl methacrylate-co-dodecyl methacrylate) on the molecular mobility and composition of the IPNs is characterized by changes of the empirical spectral parameters T50G and τR. The molecular mobility was shown to diminish with rising content of methyl methacrylate in the copolymer.