Leo A. Wall

Degradation of Polymers

Abstract Recent literature pertaining to both the theoretical aspects and experimental results of the degradation of polymers by heat and radiation is reviewed and critically evaluated. Theories of random and chain thermal degradation of vinyl polymers and co-polymers are reviewed. The degradation of polymethacrylates, polyacrylates, poly-α-methylstyrene, polyolefins, polystyrene, other vinyl polymers, cellulose, polyesters, polyamides, dienes, natural rubber, and copolymers are discussed in the light of these theories. The thermodynamics and energetics of the degradation of these polymers is also reviewed. Chain scission, crosslinking, and gel formation and the kinetic mechanisms of these processes which take place during degradation of polymers by ionizing radiation and ultraviolet light are included. Degradative, rather than synthetic, effects are emphasized in the discussion.

Gamma irradiation of fluorocarbon polymers

Several fluorocarbon polymers were irradiated with Co60 gamma radiation at doses up to 1022 ev/g. The polymers studied included polytetrafluoroethylene, polytrifluoroethylene, polychlorotrifluoroethylene, a copolymer of tetrafluoroethylene with hexafluoropropylene, and several rubbery vinylidene fluoride copolymers. G-values were measured for volatile products, for free radicals detected by electron spin resonance, and, in the case of polychlorotrifluoroethylene, for scissions. The course of degradation or crosslinking was followed by zero-strength-time and tensile-strength measurements. It was found that for polytetrafluoroethylene and its hexafluoropropylene copolymer the presence of air-accelerated scission drastically. The mechanism of the radiation-induced changes is discussed in terms of free-radical intermediates.

Reaction of Deuterated Polystyrenes with Hydrogen and Deuterium Atoms

Investigation of the electron spin resonance spectra shown by finely divided polystyrene and selectively deuterated polystyrenes, upon exposure either to hydrogen atoms or to deuterium atoms, demonstrates that the predominant reaction is the addition of a hydrogen atom to the phenyl ring, thus producing a cyclohexadienyl‐type radical. The specific rate of this reaction at room temperature is estimated to be 4×103 liters mole—1·sec—1; the specific rate at which hydrogen atoms react with the cyclohexadienyl‐type radical is estimated to be 106 liters mole—1·sec—1. The rate at which iodine vapor reacts with the cyclohexadienyl radicals is probably about the same value. Qualitatively, the results produce an insight into the mechanism of the radiolysis of polystyrene. It seems evident that although the mechanism is complex, both the evolved hydrogen and the crosslinking seen in the irradiation of polystyrene occur by means of radical precursors.

Electron Spin Resonance Spectra of Free‐Radical Intermediates Formed by Reaction of Polystyrene with Atoms of Hydrogen and Deuterium

A method for investigating the reaction of hydrogen and deuteriunn atoms with polystyrene is described, and the spectra obtained is discussed. By freeze- drying benzene solutions of polystyrene and other polymers, extremely finely divided fibrous fluffs are formed. The technique permits direct observation of free-radical intermediates during a reaction. The differences between the two spectra give evidence that the observed polynner radicals are not produced by abstraction reactions alone. (P.C.H.)

Pyrolysis of Polymers

The nature of their pyrolysis products, monomer, oligomers and carbonaceous residues enables one to group polymers into three classes, those that decompose by net main chain scission; by stripping of the main chain, for example the thermal dehydrochlorination of polyvinyl chloride; and by crosslinking of the main chain followed by some production of volatiles. Highly unsaturated or aromatic chains tend to follow the latter course. At the present time a theoretical framework exists which permits, provided adequate experiments are performed, the elucidation of the decompositions of the first type. For most of the well-known polymers in this class, this framework of knowledge gives very acceptable mechanistic explanations or interpretations of the decomposition process based on observations of rate of weight loss, of molecular weight changes and composition of volatile products. Knowledge of the decomposition of the second class of polymers is at an intermediate state development. However, an important practical research objective would be the acquisition of methods for converting the mechanisms of decomposition of class one substances to that for class two type. For the third class of materials, knowledge of their pyrolysis mechanisms is nonexistent. This is largely due to the fact that methods for quantitatively following solid-phase processes of decomposition are relatively difficult and unsatisfactory.

GAMMA IRRADIATION OF HEXAFLUOROBENZENE

Mixtures of hexafluorobenzene and benzene were irradiated in liquid phase by means of a Co60 gamma source at 20° and at 218° C. Perfluoroheptane and various binary mixtures involving perfluoroheptane, hexafluorobenzene, benzene, and cyclohexane were also irradiated at 20° C. Hexafluorobenzene resembled benzene very closely in its behavior upon radiolysis. Generally the fluorocarbon-hydrocarbon mixtures evolved much more SiF4 (indicating the formation of HF, which reacts with the glass vessel) than the pure fluorocarbon components. The polymer from hexafluorobenzene-benzene mixtures was probably rich in cyclohexadiene and cyclohexene units, resembling that from pure benzene, and its composition ratio exhibited a strong “alternating” tendency. The results are discussed in terms of free-radical and excited-state mechanisms. At 218° C hexafluorobenzene and also its mixtures with benzene showed qualitative differences from their behavior at 20° C, although the G values for SiF4 and polymer remained moderate.

High pressure polymerization of perfluorostyrene

Abstract Perfluorostyrene was prepared by reaction between pentafluorophenyllithium and tetrafluoroethylene and its physical behavior and thermal polymerization were studied in the temperature and pressure ranges 17-155° and 6800-20 000 atm respectively. When highly superpressed the monomer becomes viscous and often glassy. The polymerization rates range from 10 -3 to more than 10 2 per cent h -1 . They generally increase with temperature and pressure. Polymer intrinsic viscosities range from 0.07 to 0.41 dlg -1 . In liquid phase polymerizations they increase with pressure and decrease with temperature. In the glassy phase, polymer of lower intrinsic viscosity is formed; in the crystalline phase, higher polymer is formed.