I.C. McNeill

Polymer degradation and characterization by thermal volatilization analysis with differential condensation of products

Abstract The amount of information which may be obtained in studies of polymer degradation, or in polymer characterization, using thermal volatilization analysis, is very considerably increased by using a modified apparatus permitting differential condensation of volatile degradation products. Comparison of the responses of Pirani gauges in four parallel vacuum lines, each preceded by a trap at a different temperature and followed by a trap at a common temperature (—196°), and also that of a gauge placed just beyond the —196° trap, permits conclusions to be drawn regarding the nature and rates of transfer of substances passing through the system. The design and operation of the apparatus, and the interpretation of results, are discussed in detail. Studies of the following polymers are reported: polyethylene, polypropylene, polyisobutene, polystyrene, poly(α-methylstyrene), poly(p-methylstyrene), poly(methacrylic acid), poly(methyl vinyl ketone), poly(vinyl alcohol), poly(methyl acrylate), poly(methyl methacrylate), poly(ethyl methacrylate).

Degradation of polymer mixtures—III.: Poly(vinyl chloride)/poly(methyl methacrylate) mixtures, studied by thermal volatilization analysis and other techniques. The nature of the reaction products and the mechanism of interaction of the polymers

Abstract Mixtures of PVC and PMMA yield only hydrogen chloride and methyl methacrylate as major products of degradation. Minor products include carbon dioxide, methyl chloride and benzene. The changes which occur in the PMMA in the course of the degradation were studied by extracting this polymer with toluene from the partly-degraded blend. It was found that chain scission had occurred, that anhydride structures had been formed, and that the polymer showed differences in degradation behaviour compared with the original PMMA sample. All these features, and the previously reported interaction effects in the blends, can be satisfactorily explained in terms of two processes, occurring simultaneously during the degradation of the blend. The first is attack on PMMA by chlorine radicals produced during the dehydrochlorination of the PVC; the second is the reaction between methacrylate ester groups and the hydrogen chloride. The effect on the subsequent degradation behaviour of PMMA of heating the polymer in the presence of hydrogen chloride has also been examined.

Degradation of polymer mixtures—II: Poly(vinyl chloride)-poly(methyl methacrylate) mixtures, studied by thermal volatilization analysis and other techniques. The effect of mixing on the thermal stability of each polymer and the general nature of their interaction

Abstract In a preliminary survey of the degradation of a number of mixed polymer systems, evidence has been found for interaction between the polymers during degradation. A feature common to all the mixtures examined containing PVC is some initial retardation of the PVC dehydrochlorination. The PVC-PMMA interaction has been studied in detail for a number of specimens of each polymer, using film and powder samples. For the mixed polymers as a film, essentially the same features are found in all cases. The PVC is slightly more stable in the mixture, but the PMMA is initially less stable and gives monomer at temperatures corresponding to PVC dehydrochlorination. Subsequent breakdown of the remaining PMMA, under programmed heating conditions, is shifted to higher temperatures. The interaction is explained in terms of attack on PMMA by chlorine atoms from the degrading PVC. The mixed powder samples and the mixed films show some differences in behaviour.

Thermal degradation of vinyl chloride-vinyl acetate copolymers — I. Bulk degradation studies by thermal volatilization analysis

Abstract The rates of production of volatile material from PVA, PVC and vinyl acetate-vinyl chloride copolymers, covering the entire composition range, bave been compared using thermal volatilization analysis. It is found that, at each extreme of the composition range, incorporation of the co-monomer unit results in a copolymer less stable than the homopolymer. Minimum stability occurs for compositions of approximately 40–50 per cent VA. The proportions of acetic acid and hydrogen chloride produced from the copolymers appear to remain constant during degradation, indicating that neither is evolved preferentially, once reaction has begun.

The thermal degradation of copolymers of vinyl acetate with methyl methacrylate and other monomers

Abstract The thermal degradation of copolymers of vinyl acetate with methyl methacrylate, styrene and ethylene has been investigated using thermal volatilization analysis and thermogravimetry, together with analysis of volatile and involatile degradation products. All three copolymer systems show some of the features characteristic of the homopolymers of the monomers concerned. There is evidence, however, for an intramolecular lactonization process in VA—MMA copolymers, involving reaction of adjacent VA and MMA units with elimination of methyl acetate. This reaction occurs less readily than the analogous process in vinyl chloride—MMA copolymers. Mechanisms of the various degradation reactions are discussed.

The thermal degradation of copolymers of methyl methacrylate with methacrylic acid

Abstract The degradation has been studied using thermogravimetry and thermal volatilization analysis; product analysis has been carried out by GLC and spectroscopy. The copolymers yield water and methanol below 300°; at higher temperatures, the products also include methyl methacrylate, carbon monoxide, carbon dioxide and methane. Quantitative comparison of the yields of methanol and methyl methacrylate has been made with predicted yields based upon sequence distribution calculations. Methanol is believed to result by two routes (i) intramolecular cyclization of adjacent ester and acid chain units at low temperatures, and (ii) fragmentation of ester units, in competition with depolymerization, at higher temperatures. Methyl methacrylate yields are substantially lower than the MMA content of the copolymer, as a result of these processes; some MMA units also appear in the product fraction volatile at degradation temperatures but not at ambient temperature. The partially-degraded copolymers develop anhydride ring structures in the chain as a result both of dehydration and of methanol production. The mechanisms of the various reactions are discussed.

Thermal degradation of graft copolymers of PVC prepared by mastication with styrene and methyl methacrylate, and of further PVC mixtures and vinyl chloride copolymers

Abstract Graft copolymers prepared by mastication of PVC in the presence of styrene or of a styrene/ methyl methacrylate mixture, have been studied by thermogravimetry, estimation of hydrogen chloride, thermal volatilization analysis, and flash pyrolysis/g.l.c. The degradation behaviour of PVC/ polystyrene mixtures, vinyl chloride/styrene random copolymers, a random copolymer of methyl methacrylate and styrene, and PVC/poly-α-methylstyrene mixtures has also been studied. The graft copolymers resemble the PVC/methacrylate graft copolymers previously studied in showing retardation of the dehydrochlorination reaction, but contrast with them in yielding chain fragments but no monomer during HCl production. Some stabilization of the second component at higher temperatures is also found. PVC/polystyrene mixtures behave in the same way as the corresponding graft copolymers, but vinyl chloride/styrene copolymers show reduced stability towards both dehydrochlorination and monomer production compared with the homopolymers. PVC/poly-α-methylstyrene mixtures yield some monomer concurrently with HCl loss, and display marked retardation of the latter reaction. Stabilization of the second polymer at higher temperatures is again observed. Many of these results add further strong support to the view that chlorine atoms are involved as chain carriers in the thermal dehydrochlorination of PVC.

The thermal degradation of polychloroprene-I thermal analysis studies of the stability of polychloroprene samples, and measurements of the kinetics of degradation

Abstract Previous studies of polychloroprene degradation in air or nitrogen have been reviewed. The stability of various samples of the polymer under vacuum has been studied by thermal volatilization analysis, and under nitrogen by thermogravimetry and differential thermal analysis. Samples prepared in air show lower stability than those prepared in the absence of air. The main stage of dehydrochlorination occurs at higher temperatures than for PVC, under programmed heating conditions; it is not so clearly separated in temperature range from the subsequent carbonization as in the case of PVC. About 90 per cent of the available chlorine is lost as hydrogen chloride, which accounts for nearly all of the material lost in the main stage of breakdown of polychloroprene. The kinetics of degradation in the absence of air have been studied under isothermal conditions by thermal volatilization analysis, thermogravimetry, and estimation of hydrogen chloride, and kinetic parameters have been evaluated. The energy of activation is significantly higher than for PVC degradation.

The thermal degradation of polymethacrylamide and copolymers of methacrylamide and methyl methacrylate

Abstract Thermal analysis demonstrates that the thermal degradation of polymethacrylamide (PMAM) occurs in two well defined steps. The only volatile products formed in the first step, below 340, are ammonia and water while imide replaces amide absorption in the infra-red spectrum of the residue. Above 340° the major product consists of chain fragments (approximately 50%) in which a high proportion of the amide groups have been converted to cyclic imides. Copolymers with methyl methacrylate (MMA) comprising more than 35% methacrylamide (MAM) also degrade in two similar steps but the overall behaviour becomes progressively more like that of polymethylmethacrylate as the MAM content is decreased below 10%. In the first stage of the reaction in the copolymer, MMA and methanol are important products in addition to ammonia and water. Chain fragments remain the major product in the second stage but a number of minor, but very significant, products are also formed. All these products and the structural features of the chain fragments and residue have been accounted for mechanistically.

The thermal degradation of polychloroprene-III degradation of polychloroprene/poly(methyl methacrylate) mixtures

Abstract The mechanism of dehydrochlorination has been studied by examining the degradation of polychloroprene/poly(methyl methacrylate) blends, using thermal volatilization analysis and infrared spectroscopy; the behaviour has been compared with that previously found for PVC/PMMA blends. Unlike the latter system, the polychloroprene blends did not show any increased production of methyl methacrylate monomer in the early stages of breakdown. The stabilization effect on PMMA due to reaction of ester groups with hydrogen chloride, on the other hand, is much more evident in the case of polychloroprene blends than for PVC, PVC dehydrochlorination is retarded by the presence of PMMA, but evolution of hydrogen chloride from polychloroprene is unaffected to any significant extent. It is concluded that the dehydrochlorination of polychloroprene is not a radical chain process. A unimolecular mechanism is suggested.