Degradation of ultra-high molecular weight poly(methyl methacrylate-co-butyl acrylate-co-acrylic acid) under ultra violet irradiation

Abstract

A new acrylic terpolymer, poly(methyl methacrylate-co-butyl acrylate-co-acrylic acid) [P(MMA-co-BA-co-AA)] of ultra-high molecular weight (UHMW) was synthesized via seeded emulsion polymerization. This polyacrylic showed good film properties; high transparency, water resistance and mechanical flexibility that may suitable for many environmental based applications such as coating, packaging, label sensors etc. In order to access the photo-stability of this material for environmental application, studies were conducted under UV illumination of a short-wavelength (λ = 254 nm) in air. The responses were collected at different irradiation times by using several characterization techniques: infrared/UV-visible spectroscopy (FTIR/UV-Vis), gel permeation chromatography (GPC), atomic force microscopy (AFM) and thermogravimetric analysis (TGA). Two distinguishable structures, cross-linked and fragmented chains, were formed under photo-irradiation at different times of exposure. The formation of cross-linked structures at short irradiation times (t < 60 min) increases the chain length as validated from the increase in average-molecular weight (M_w), whilst at longer irradiation time the fragmentation causes a decrease in the chain length (decrease in M_w). Only the chain scission at longer irradiation time (t > 60 min) causes the copolymer to degrade. The centre of reaction was identified at the pendent group and no effect of main chain destabilization was observed throughout the experimental condition. The occurrence of chain cleavage during photo-degradation causes chain–chain separation, as visually seen under the imaging technique and this coincides with the observed drop in thermal stability. Photo-oxidation was also proposed to occur simultaneously with photo-degradation as the irradiation was performed in air.