Degradation, thermal poly

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

It has already been shown (e.g. Chapters 20 and 21) that the insertion of a p-phenylene into the main chain of a linear polymer increased the chain stiffness and raised the heat distortion temperature. In many instances it also improved the resistance to thermal degradation. One of the first polymers to exploit this concept commercially was poly(ethylene terephthalate) but it was developed more with the polycarbonates, polysulphone, poly(phenylene sulphides) and aromatic polyketones. 

McNeill and Basan [148] have studied the thermal degradation of blends of PVC with poly (dimethylsilox-ane) (PDMS). Structure of PDMS is given in Eq. (39). 

A number of polymers are capable of fulfilling these demanding requirements. Typically negative photoresists are based on cyclised poly(l,4-isoprene). These polymers are prepared by dissolving poly(l,4-isoprene) in an appropriate solvent and subjecting it to thermal degradation. This is followed by treatment with acid to produce the cyclised material (see Reaction 8.8). 

Even though poly(ortho esters) contain hydrolytically labile Linkages, they are highly hydrophobic materiads and for this reason are very stable and can be stored without careful exclusion of moisture. However, the ortho ester linkage in the polymer is inherently thermally unstable and at elevated temperatures is believed to dissociate into an alcohol and a ketene acetal (33). A possible mechanism for the thermal degradation is shown below. This thermal degradation is similar to that observed with polyurethanes (34). 

The chemistry of the solid-state polycondensation process is the same as that of melt-phase poly condensation. Most important are the transesterification/glycolysis and esterification/hydrolysis reactions, particularly, if the polymer has a high water concentration. Due to the low content of hydroxyl end groups, only minor amounts of DEG are formed and the thermal degradation of polymer chains is insignificant at the low temperatures of the SSP process. 

Kashiwagi T, Grulke E, Hilding J, Harris R, Awad W, Douglas J (2002). Thermal degradation and flammability properties of poly(propylene)/carbon nanotube composites. Macromol. Rapid Commun. 23 761-765. 

Wakeman, LB. and Johnson, H.R., Vinyl Chloride Eormation from the Thermal Degradation of Poly(Vinyl Chloride), Polym. Eng. Set, 18, 404 (1978)... 

Peng S, An Y, Chen C, Fei B, Zhuang Y, Dong L (2003) Thermal degradation kinetics of uncapped and end-capped poly(propylene carbonate). Polym Degrad Stab 80 141-147... 

Liu Q, Zou Y, Bei Y, Qi G, Meng Y (2008) Mechanic properties and thermal degradation kinetics of terpolymer poly (propylene cyclohexene carbonates). Mater Lett 62 3294—3296... 

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