A selective surface degradation study of polysiloxane homopolymers and random-block copolymers with hydrofluoric acid solution

Abstract

Solution-grown crystals of homopolymer fractions of poly(tetramethyl-p-silphenylene siloxane) (pTMPS) and block copolymers of tetramethyl-p-silphenylene siloxane (TMPS) and dimethyl siloxane (DMS) have been selectively degraded using 48% HF solution at 30 °C. The HF preferentially attacks the Si-O bonds on the crystal surfaces. Changes in the degradation kinetics were monitored primarily by following the change in sample weight with etching time. Observation on changes in molecular weight, molecular distribution (by GPC), small-angle x-ray spacing, heat of fusion (by DSC), and degree of crystallinity were all employed to probe changes in crystal morphology with etching time within these polymeric systems. In the case of the homopolymer the correlated experimental results favored a chain-grown folded-crystal model for the TMPS homopolymer solution crystals and a substantially disordered surface topography in the crystals of the TMPS-DMS block copolymers. The size of the disordered interface expanded as the weight fraction of noncrystallizable DMS component was increased. In the copolymer series the dimension of the crystalline core in the chain direction exhibited a constant thickness on selective chemical etching, irrespective of the weight percent of DMS present in the TMPS-DMS system.