Crystallization and second-order transitions in silicone rubbers

In t he course of an investigation to determine which rubbers might be suitable fo r usc at low temperatures, interferometric m eas urem e nts of the length-temp erature relat ionships of silicone rubbers have been made . C rystall ization was found between -60 a nd -67 C in Dow-Corning SiJastic X -6160 and in Ge neral E lectric 9979G sili co ne ru bber, thc lattcr of wh ich co nta in s no fi llcr. Crystallizat ion between -75 and -85 C \\"as found in Silast ic 250. ~1elting occurred ovcr a range of tempcrat ure above t he temperaturc of c rystalli zation . Th e volume change on crystalli zation vari cd from 2.0 to 7.8 pcrcent. Xo crystall izatio n PI' melt in g ph enom cn a were obscrvcd in Sil a st ic X-6073 between -180 and + 100 C .