ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Relationships between Thermal, Photothermal and Photolytic Degradation Processes in Methacrylate/Acrylate Copolymers

N. Grassie, A. Scotney, R. Jenkins, and T. I. Davis

Department of Chemistry, University of Glasgow, Glasgow, W. 2

Abstract: Degradation reactions in acrylate/methacrylate copolymers have been studied under four sets of conditions, namely, photolysis reactions in film and in solution at ordinary temperatures, photothermal reactions, that is photolysis reactions in molten polymer below the thermal degradation threshold, and thermal degradations. The paper seeks to demonstrate some of the relationships which exist between these reactions. Methyl methacrylate (MMA)/methyl acrylate (MA) copolymer films have been irradiated with 2537 Á radiation and the rates of scission and crosslink - ing obtained from molecular weight measurements and sol-gel analysis. Crosslinking decreases with methyl acrylate content to zero when 45 mole % of acrylate is present in the copolymer. It is concluded that crosslinking is associated with groups of adjacent methyl acrylate units rather than single units. Rates of chain scission are comparable in pure poly(methyl acrylate) (PMA) and poly(methyl methacrylate) (PMMA) but pass through a minimum at 40 mole % MMA. Crosslinking is inhibited in solution. The rate of chain scission again passes through a minimum at 30 — 40% MMA in the copolymer and although PMA scissions at comparable rates in film and in solution, the rate of scission of PMMA is greatly accelerated in solution. Under photothermal degradation conditions (160—170°C) the primarily formed methacrylate radicals can unzip to form monomer but in copolymers, unzipping is inhibited by acrylate units and a new series of acrylate decomposition products appears including (from a butyl acrylate copolymer) n-butyl methacrylate, n-butyraldehyde and n-butanol. These and other products have been accounted for qualitatively. At the higher temperature at which thermal degradation occurs (220°C) a similar spectrum of products appears to that in the photothermal reaction but the higher temperature often induces new kinds of reactions which may yield new products and also interfere with, and thus modify, the radical reactions and thus the products typical of photothermolysis. Examples are discussed.

Full paper in Portable Document Format: 263a208.pdf

Chemical Papers 26 (3) 208–216 (1972)