Healing of interfaces of high- and ultra-high-molecular-weight polystyrene below the bulk Tg

Abstarct ous bulk samples of high-molecular-weight (HMW) polystyrene (PS) with a weight-average molecular weight Mw of 102.5 kg/mol and a number-average molecular weight Mn of 97 kg/mol and of ultra-high-molecular-weight PS (UHMWPS) with Mw=1110.5 kg/mol and Mn=965.6 kg/mol were brought into contact to themselves below the glass transition temperature Tg of the bulk Tg-bulk, in a lap-shear joint geometry, at a constant healing temperature Th for a healing time th of 10 min to 24 h. The lap-shear strength σ of the symmetric HMWPS–HMWPS and UHMWPS–UHMWPS interfaces has been measured at the ambient temperature. Larger values of σ have always been measured for the HMWPS–HMWPS interface after healing at the same conditions. However, the difference between the values of σ for the HMWPS–HMWPS and UHMWPS–UHMWPS interfaces observed at Th=Tg-bulk−33 °C and Th=Tg-bulk−23 °C became negligible at Th=Tg-bulk−13 °C at long times. The kinetics of strength evolution at the two interfaces involved at Th=Tg-bulk−33 °C and Th=Tg-bulk−23 °C was found to follow the form σ∝(th)1/4(Mn)−1/4, in accordance with the minor chain reptation model, suggesting that the build-up of the interface structure develops via the diffusion of chain segments above Tg of the interface layer Tg-interface.