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Title: Controlled degradation of methacrylonitrile polymers and copolymers
Author: Mahmood, Tarique
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1993
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This research is concerned with the preparation, characterisation and thermal degradation of methacrylonitrile based polymers and copolymers. Various methacrylonitrile homopolymers including those containing reactive chain ends and methacrylonitrile copolymers with other comonomers (e.g. methacrylic acid, styrene and maleic anhydride) were prepared, characterised and their thermal behaviour was also studied under programmed and isothermal heating experiments using TVA, TG and DTA techniques. Polymer degradation is a complex branch of chemistry. A general description of the major types of degradation processes which may occur during thermal decomposition is given in Chapter One. This chapter also includes a brief review of the significance of polymer structure in polymer degradation. Chapter Two details the experimental procedure of the several thermal analysis techniques employed in the present work, with special emphasis on thermal volatilisation analysis as a more versatile technique. A brief description of additional analytical methods used in identifying and quantifying degradation products is also included. Chapter Three describes the preparation, isolation and characterisation of various homo- and copolymers of methacrylonitrile used in the present investigation. This also includes a brief description of the calculation determination of monomer feed for the preparation of copolymers of any required composition. The experimental work regarding the thermal aspects of various methacrylonitrile polymers and copolymers is described in Chapters Four, Five, Six, Seven and Eight. Chapter Four is concerned, as a basis, with the thermal behaviour of high and low molecular weight methacrylonitrile polymers made using AIBN and benzoyl peroxide as initiators. It is observed that polymer simply depolymerises to monomer (as it is expected from a polymer with l:l-disubstituted structure) in a two stage decomposition. The first stage of decomposition is attributed to the depolymerisation initiated from unsaturated chain ends and the second (and main stage in high molecular weight polymers) to unzipping after random chain scission. This chapter also contains a literature review on the thermal degradation of polymethacrylonitrile The programmed thermal degradation of methacrylonitrile polymers and copolymers capable of colouration is detailed in Chapters Five and Six, respectively. The thermal degradation of methacrylonitrile polymers with reactive chain-ends is the subject of Chapter Five. Programmed heating experiments were employed to investigate the effect of colouration initiated from the polymer chain ends on the thermal degradation of polymethacrylonitrile, and on the products of degradadon. It has been observed, in these studies, that the end-initiated methacrylonitrile polymer samples did not simply depolymerise to monomer as has been expected1 and observed2 in case of a polymer made from 1:1 disubstituted monomer. Monomer yields were significantly decreased with increasing reactive chain ends. A large amount of residue, cold ring fraction, volatile liquid products (mainly monomer) and various gases (condensable and non-condensable) were found amongst the degradation products. Oligomers with various naphthyridine types of structure were suggested to be present in CRF. Formation of these other degradation products indicateded towards some degradative routes other than depolymerisation. The thermal degradation of methacrylonitrile copolymers containing small amounts of methacrylic acid is considered in Chapter Six in order to asses, the effect of thermal colouration initiated from inside the polymer chain on thermal behaviour and products of degradation. It has been observed that with increasing concentration of MAA units in the copolymer chain, there is an increase in the relative amounts of residue and cold ring fraction (formed as a result of nitrile group cyclisation4) together with a corresponding decrease in the monomer yield. The degradation products from MAN/MAA copolymers were almost the same as has been identified in case of end-initiated methacrylonitrile polymers. A mechanism for the degradation products from MAN polymers with reactive chain ends and MAN/MAA copolymers has been put forward. In Chapter Seven, the thermal stability and degradation behaviour of a set of methacrylonitrile/styrene copolymers is discussed. It was found that the thermal behaviour of these copolymer samples can be related to that of polymethacrylonitrile and polystyrene. It was concluded that the mechanism of decomposition below and above 300°C depends upon the com position of the copolymer. A well separated two stage decomposition tends to merge into a sharp single stage degradation, with increasing ratios of styrene monomer in the copolymer. An increase in the Tonset and Tmax were also observed with increasing ratios of styrene units in the copolymer. A mechanistic route has been proposed for the formation of various degradation products from these MAN/Sty copolymers. Chapter Seven also includes the thermal studies and proposed m echanism of form ation of degradation products for a methacrylonitrile/maleic anhydride copolymer. Chapter Eight details , ' the isothermal colouration of MAN polymers. A set of methac;ylonitrile homopolymers with reactive end structures and three methacrylonitrile copolymers containing small amounts of methacrylic acid comonomer units were used to investigate the colouration initiated from chain-ends and from inside the polymer chain. These polymer and copolymer samples were isothermally degraded in two different environments (as KBr discs and cast films) to study the effect of environment (ionic in case of KBr disc) on the extent of colouration. In the present investigations, it has been confirmed that colouration in the methacrylonitrile polymer can only be initiated from some well defined impurity (present inside the polymer chain or at the chain-end), and the colouration is directly proportional to the number of initiating centres. It has also been observed that when the initiating centres (methacrylic acid units) are present in abundance, a side reaction (dehydration of adjacent methacrylic acid units in the chain) also occurs in the degradation of film samples, resulting in the formation of anhydride structures. A mechanism involving initial formation of an imide has been advanced, which satisfactorily explains the observed infrared spectral changes during thermal colouration under reduced pressure. Interpretation and tentative assignments of the observed infrared data are also given. A review of earlier work on thermal colouration of nitrile polymers is also included in Chapter Eight.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available