The Molecular Basis of Fracture in Poly(methyl methacrylate) Latex Films

Abstract

Latex films of poly(methyl methacrylate) (PMMA) (Mn = 485,000 g/mol); (polydispersity index, PDI=1.85), were prepared by molding the dry latex powder. Two different conditions of molding and annealing temperatures (T) were used : (a) low conditions (mold T = 130 and anneal T = 140°C), and (b) high conditions (mold T=150 and anneal T=180°C). Fracture studies on the annealed latex films were carried out using a dental burr grinding instrument (DGBI) at a burr rotational frequency of 16 Hz. The DGBI cuts the films at a depth of 500 nm per pass. In the latex films, the fracture energy increased with annealing time for both sets of processing conditions. For the lower annealing temperature, however, a maximum in the fracture energy was observed around 30 min, apparently related to the reptation time. For the higher annealing temperature, no peak was observed, and the fracture energy approached a plateau value after 24 hr of annealing. Also, the molecular weight dependence of the fracture energy in fully annealed PMMA latex films was studied in the number-average molecular weight range 2–48 × 104 g/mol. The experimental chain pullout energy for fully annealed PMMA was 211×106 J/m3, close to that predicted by Mark, but somewhat larger than that predicted by Evans. Mer frictional coefficients suggest that the actual temperature of a chain being pulled out is in the range of 220°C, above the glass transition temperature of the polymer.