| A detailed analysis of the crystallization behaviour of poly(dimethylsiloxane) (PDMS) was undertaken. Determination of the equilibrium melting temperature showed a six degree difference between the equilibrium melting of a hydroxy terminated polymer (PDMS-OH, Tmo = 266 K, M w = 92.9 kg mol−1) over that of a trimethylsilyl terminated polymer (PDMS-CH3, Tmo = 260 K, Mw = 100 kg mol−1). The inherent flexibility of the Si-O linkage of these polymers caused crystallization to be first detected at Tmo − 35 K, and was recorded down to 177 K, i.e., Tg + 30 K. Both systems followed well behaved regime III surface nucleation kinetics. The average fold interfacial surface free energy, σe, was calculated to be 17.6 erg cm −2 and resulted in a very low value for the work of chain folding, q, calculated to be 14.6 kJ mol−1. This is in accord with the small rotational barrier of the siloxane backbone. In addition, PDMS-OH crystallized with slower kinetics than PDMS-CH3, verified by chemical conversion from the hydroxy to trimethylsilyl end group.; The first reported detection of two different PDMS solid state morphologies is presented. A small population of very low birefringent (Δn ) spherulites was observed to nucleate simultaneously with a majority of bright positive Δn spherulites throughout the accessible crystallization temperature range. The sign of Δn of these darker spherulites varied with growth, and this growth rate was depressed from that of the bright spherulites. Calorimetric annealing experiments indicate that the PDMS system attains very high percent crystallinities very early after nucleation. Coupled with the results of small angle laser light scattering (SALLS) during slow (1 deg/min) melting, secondary crystallization processes were observed to be non-negligible very soon after primary crystallization.; SALLS of the crystallizing PDMS system was sensitive to the concurrent effects of (i) the presence of dark spherulites, (ii) secondary crystallization, and (iii) the distribution of spherulite sizes. These combined effects generated significant non-ideality within V v and Hv scattering envelopes. The effects of (i) and (ii) caused both the Vv pattern to remain circularly symmetric throughout the entire crystallization timeframe, and the 4-leaf clover pattern of the Hv mode scattering to contain multiple intensity maxima not restricted to an azimuthal angle of 45°. The effect of (iii) was to lower the calculated SALLS average radial growth rate relative to that observed optically. |
