Morphology of fractured polymer-polymer interfaces bonded at ambient temperature as studied by atomic force microscopy

The amorphous polymer surfaces of polystyrene (PS, M-n = 200 kg/mol, M-w/M-n = 1.05) and poly(methyl methacrylate) (PMMA, M-n = 51.9 kg/mol, M-w/M-n <= 1.07) were brought into contact at 21 degrees C to form PS-PS (for 54 days) and PMMA-PMMA auto-adhesive joints (for 11 days). After contact at that temperature corresponding to Tg-bulk -81 degrees C for PS and to Tg-bulk-88 degrees C for PMMA, where Tg-bulk is the calorimetric glass transition temperature of the bulk sample, the bonded interfaces were fractured and their surfaces were analyzed by atomic force microscopy (AFM). The surface roughness, R-q, of the fractured interfaces was larger by a factor of 3-4 than was that of the free PS and PMMA surfaces aged for the same period of time. A similar increase in Rq was found by comparison of the free PS surface aged at Tg-bulk + 15 degrees C for 1 h and of the surface of the PS-PS interface fractured after healing at Tg-bulk + 15 degrees C for 1 h. These observations, indicative of the deformation of the fractured interfaces, suggest the occurrence of some mass transfer across the interface even below Tg-bulk -80 degrees C.