Studies of cyclic and linear poly(dimethylsiloxanes): 19. Glass transition temperatures and crystallization behaviour

Differential scanning calorimetry (d.s.c.) was used to investigate the thermal behaviour of cyclic and linear poly(dimethylsiloxanes) over the temperature range 103–298 K. Fractions of the polymers studied had number-average molar masses in the range 160 < Mn < 25 500 g mol−1 and heterogeneity indices MwMn < 1.1 in most cases. D.s.c. was applied to measure the glass transition temperatures Tg cold crystallization temperatures Tc and polymer crystalline melting temperatures Tm of the oligomer and polymer fractions. Cyclic siloxanes [(CH3)2SiO]x with number-average numbers of skeletal bonds nn in the range 24 ≦ nn ≦ 79 and linear siloxanes (CH3)SiO[(CH3)2SiO]ySi(CH3)3 with nn in the range 10 ≦ nn ≦ 40 were found not to crystallize. The Tg values of the linear siloxanes were found to be in agreement with values in the literature and they increased with increasing Mn. By contrast, the Tg values of the cyclics were found to decrease with increasing Mn.