Autoadhesion of High-Molecular-Weight Monodisperse Glassy Polystyrene at unexpected low temperatures

Healing of symmetric interfaces of amorphous anionically polymerized high- and ultra-high-molecular weight (HMW and UHMW, respectively) polystyrene (PS) in a range of the weight-average molecular weight M-w from 102.5 (M-w/M-n = 1.05) to 1110 kg/ mol (M-w/M-n = 1.15) was followed at a constant healing temperature, T-h, well below the glass transition temperature of the polymer bulk [Tg-bulk = 105 - 106degreesC as measured by differential scanning calorimeter (DSC)]. The bonded interfaces were shear fractured in tension on an Instron tester at ambient temperature. Autoadhesion at symmetric HMW PS-HMW PS and UHMW PS-UHMW PS interfaces was detected mechanically after healing at T-h = 38degreesC for 107 hr, and even at 24degreesC (for longer healing times). The occurrence of autoadhesion between the surfaces of the UHMW PS with M-w = 1110 kg/mol at 24degreesC implies that the glass transition temperature at the interface, Tg-interface, of this polymer was a least lower: by 82degreesC than its DSC Tg-bulk, by 30-40degreesC than the Vogel temperature, T-infinity-the lowest theoretical value of a kinetic Tg-bulk at infinite long time-and by 20degreesC than T-2 (a "true" thermodynamic Tg-bulk corresponding to a second-order phase transition temperature). To our knowledge, this is the first observation of such nature, which gives further evidence of the lowering of the T-g at polymeric surfaces and the persistence of this effect at early stages of healing of polymer-polymer interfaces.