THERMAL-DEGRADATION OF VINYL ACETATE-METHACRYLIC ACID COPOLYMER AND HOMOPOLYMERS .1. AN FTIR SPECTROSCOPIC INVESTIGATION OF STRUCTURAL-CHANGES IN THE DEGRADING MATERIAL

The thermal degradation in vacuo, up to 500 degrees C, of a vinyl acetate-methacrylic acid (VA-MAA) copolymer having 65% alternating units, and of the corresponding homopolymers, has been studied by reflection-absorption FTIR spectroscopy. Polyvinyl acetate (PVA) crosslinks form due to double bond formation through deacetylation. Crosslinking is due to some extent to aromatic rings formed by intermolecular Diels-Alder condensation, as shown by solubility tests correlated with FTIR spectra. PMAA generates little unsaturation, but crosslinks by intermolecular anhydride links. It decomposes above 350 degrees C when the anhydride breaks down with much chain scission. In VA-MAA copolymer, both types of crosslinking reactions are disturbed by lactone formation between alternating VA and MAA units. The lactone formation 'protects' the VA units from deacetylation and competes strongly with anhydride formation. As the temperature increases, the anhydride groups disappear, first with scission and, towards 400 degrees C, with aromatic ring formation, as in PMAA. Lactone groups are more thermally resistant and can be found in the charred polymer at 440 degrees C next to aromatic rings. At 480 degrees C the lactone decomposes, helping the 'fusion' of the aromatic rings in the strongly diminished char. It has been concluded that large-scale lactonisation in the VA-MAA copolymer due to its high content of alternating units diverts the degradation from the homopolymers.