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 mo lecular 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 miniemulsifi cation technique. Compression molding of the cleaned and dried latex p owder 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 late x films was carried out using a custom-built dental burr grinding inst rument 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 s cission energy and the uncoiling energy (due to rubber elasticity) wer e calculated. Then, by using an energy balance approach, the viscoelas tic 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)) w ere 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 sam ple via GPC, while the low molecular weight latex films did not show a ny apparent change in the molecular weight on grinding. Under fully an nealed conditions, the contribution to the total energy from chain sci ssion was about 40% for the medium molecular weight and about 0% for t he low molecular weight film. Present data are compared with high mole cular 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 negligib le. Molecular frictional coefficient values obtained using Prentice's model indicate that the temperature for the chain pullout process is a bout 150-250 degrees C.