Crystallinity and orientation in silicone rubber. II. Physical measurements

Density data for a silica‐filled dimethylpolysiloxane rubber define incipient crystallization conditions and indicate the forms of crystalline growth for the unstressed rubber and for a sample at 200% elongation. Temperature—tension data for the same silicafilled rubber show incipient crystallinity temperatures as a function of extension. While stress relaxed filled silicone rubber samples show a tension decrease with falling temperatures, they are far from ideal rubbers. The change in polymer filler interaction links with temperature may account for this nonideality. Both the filled and unfilled rubbers show a thermoelastic inversion. The comparison with x‐ray data yields good evidence that crystallization is responsible for the tension rise at low temperatures. This rise may stem from the tendency of silicone rubber to crystallize without altering the orientation of chain segments. The ability to crystallize chain elements in all states of orientation may be founded upon a much greater chain segment mobility for silicone rubber than found with natural rubber. Normal stress—strain curves taken at low temperatures qualitatively show the effects of crystallization.