ENERGY-CONSUMING MICROMECHANISMS IN THE FRACTURE OF GLASSY-POLYMERS .2. EFFECT OF MOLECULAR-WEIGHT ON THE FRACTURE OF POLYSTYRENE

Narrow molecular weight distribution polystyrene latex films of low molecular weight (M(n) = 32 000; PDI = 1.04) and medium molecular weight (M(n) = 151 000; PDI = 1.02) were made by using a direct miniemulsification technique. Compression molding of the cleaned and dried latex powder was carried out at 110 degrees C and 10 MPa for 20 min, followed by annealing at 144 degrees C for various times. Fracture of the latex films was carried out using a custom-built dental burr grinding instrument from which the total fracture energy was determined. Molecular weights before and after fracture were determined using gel permeation chromatography (GPC). From the number of chain scissions, the chain scission energy and the uncoiling energy (due to rubber elasticity) were calculated. Then, by using an energy balance approach, the viscoelastic energy for pullout was calculated. Total fracture energies of 174 x 10(6) J/m(3) (or 17 J/m(2)) and 460 x 10(6) J/m(3) (or 230 J/m(2)) were obtained for fully annealed, low and medium molecular weight latex films, respectively. About 1 x 10(24) scissions/m(3) (or 7 x 10(17)/m(2)) were obtained for the fully annealed, medium molecular weight sample via GPC, while the low molecular weight latex films did not show any apparent change in the molecular weight on grinding. Under fully annealed conditions, the contribution to the total energy from chain scission was about 40% for the medium molecular weight and about 0% for the low molecular weight film. Present data are compared with high molecular weight polystyrene (M(n) = 420 000; PDI = 1.19), where about 90% chain scission and 10% pullout were reported at long annealing times. In all cases, the contribution from the uncoiling energy was negligible. Molecular frictional coefficient values obtained using Prentice's model indicate that the temperature for the chain pullout process is about 150-250 degrees C.