An Investigation into the Thermal Degradation of Several Chlorinated Polymer Systems and Poly-(Ethylene Oxide): Salt Blends

Millan, Edith Jane (1990) An Investigation into the Thermal Degradation of Several Chlorinated Polymer Systems and Poly-(Ethylene Oxide): Salt Blends. PhD thesis, University of Glasgow.

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Polymer degradation is a complex branch of chemistry. A general outline of the major types of process which can occur during thermal decomposition is given in Chapter 1. In addition, a brief summary of the degradation of polymers in the presence of additives along with a more detailed acount of the decomposition of several chlorine-containing polymers are presented. The chapter concludes with a short section on the importance of polymer degradation. The thermal analysis techniques employed in this research are described in depth in Chapter 2 together with the additional analytical methods used in identifying and quantifying degradation products. The experimental work is described in Chapters 3, 4 and 5. Chapter 3 is concerned with the synthesis and thermal degradation of chlorinated poly(ethylene oxide). As a basis the decomposition of poly(ethylene oxide) (PEO) is first investigated and it is observed that the polymer thermally degrades in a random fashion, initiated by C-O and C-C bond scission, which yields a variety of degradation products. The synthesis of chlorinated poly (ethylene oxide) in both air and in an atmosphere of nitrogen are described. Chlorinated polymers can be obtained with chlorine contents 60.96% and 71.75% which correspond to the substitution of approximately 2 and 3 chlorine atoms respectively per ethylene oxide repeat unit. Degradation results show that chlorination destabilises poly(ethylene oxide) to heating. The mechanism of decomposition is similar to that for the unchlorinated polymer in that it is a random process. Chlorine-containing compounds are produced including hydrogen chloride, phosgene and chlorinated acetyl chlorides. Quantitative analysis indicates that in trichlorinated poly(ethylene oxide) approximately one molecule of HCl is evolved per four monomer units and this figure is increased to roughly one molecule of HCl produced per three monomer units in dichlorinated poly(ethylene oxide). Chain fragments in both cases amount to about 60% by weight of products evolved. Chapter 4 details the degradation of several chlorine-containing polystyrenes, in which the chlorine substituent is either on the ring or in the polymer backbone, and compares the results with those obtained for polystyrene. The position of chlorine within the macromolecule has a pronounced effect on the breakdown mechanism of the polymer. Poly-p-chlorostyrene and poly-o- chlorostyrene show similar thermal properties to polystyrene with depolymerisation being the major process occurring during degradation. In ring-chlorinated polystyrene however, the polymer is thermally destabilised and displays a two stage decomposition. These observations are attributed to a small amount of chain-chlorination which occurs simultaneously with ring-chlorination. Thus the initial degradation is due to the elimination of HCl which is followed subsequently by depolymerisation and chain transfer processes, which account for the bulk of the decomposition products. A destabilised two stage degradation is also observed with chain-chlorinated polystyrene (30.19% Cl). The two stage decomposition arises from dehydrochlorination, in which all available chlorine is eliminated, and chain fragmentation. It has also been possible to prepare chain-chlorinated polystyrenes with chlorine contents which correspond to an approximately di- and trisubstituted styrene unit (41.56% Cl and 49.07% Cl respectively). In these highly chlorinated polymers a two stage degradation is less easily distinguished. However, increased yields of HCl (31.78%) and residual char (10.48%) are produced on decomposition. To account for these observations, a mechanism is proposed in which, following HCl elimination from the polymer main-chain, dehydrochlorination continues via a cyclisation reaction involving the chlorinated polyene backbone and its phenyl side groups. The second dehydrochlorination is favoured as it restores aromaticity. The thermal degradation of several poly(ethylene oxide)-salt blends of low (10:1 EO:salt) and high (2:1 EO:salt) salt concentrations are discussed in Chapter 5. The decomposition of PEO in the presence of some transition metal, alkaline earth and alkali metal salts is detailed. The final chapter, Chapter 6, gives a summary of conclusions reached and mechanisms proposed during this research and ends with some suggestions for future work.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Polymer chemistry
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78102
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:40
Last Modified: 30 Jan 2020 15:40

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