The molecular basis of fracture in polystyrene films: Role of molecular weight

Abstract

AbstractThe molecular basis for fracture was examined using a custom‐built Dental Burr Grinding Instrument, which cuts at a depth of 500 nm per pass. A direct miniemulsification method was used to form uniform‐sized latex particles from narrow molecular weight distribution, anionically synthesized polystyrenes. Several polystyrenes were examined as a function of molecular weight, and blends were made of high and low molecular weight polystyrenes. In addition, a broad molecular weight polystyrene was included for comparison. These latexes were dried and cleaned, and molded under mild conditions, followed by annealing for various lengths of time at 144°C. The Dental Burr Grinding Instrument measures the total energy required to fracture the sample. The total number of chains undergoing scission per unit volume was determined via GPC before and after the fracture process. Using an energy balance approach, the total number of chains undergoing pullout (from either side of the fracture surfaces) was estimated. In order to obtain a broader picture of the process, data collected by Mohammadi et al., and by Sambasivam et al., were integrated into the analysis. Basically, at very low molecular weights, ca. 32,000 g/mol, substantially 100% pullout occurs. At the midmolecular weight range, about 150,000 to 180,000 g/mol, chain scission and chain pullout contributions to the total energy are approximately equal. For very high molecular weights, the chain scission contribution is about 90%. A scaling relationship is proposed between the molecular weight of the polymer and the fraction of chains undergoing scission. © 1995 John Wiley & Sons, Inc.