A. Maus, K. Saalwächter.
Crystallization Kinetics of Poly(dimethylsiloxane) Molecular-Weight Blends —
Correlation with Segmental Orientations in the Melt?
Macromol. Chem. Phys. 208, 2066-2075 (2007).
The crystallization of bimodal PDMS molecular-weight blends consisting of long
(115 kda) and short (5.3 kda) chains is investigated using DSC and proton NMR.
In agreement with earlier studies, it is found that the pure long-chain sample
crystallizes more rapidly than the short-chain sample. Furthermore, the rate
passes through a maximum for the 60:40 blend. These observations hold true for
non-isothermal cooling at 5 K/min and isothermal crystallization at 203 K (both
DSC) as well as at 218 K (NMR). The samples with 40% long chains or less
crystallize to about 75–80% under all conditions, and the crystalline fractions
derived from DSC and NMR agree very well, thus also confirming the literature
value of 61.3 J/g as the perfect heat of fusion for PDMS, and disproving some
earlier studies. A relation of the crystallization kinetics with
entanglement-induced segmental orientation correlations in the melt is
postulated. The local chain order is characterized by proton multiple-quantum
NMR, and an attempt is made to correlate this observable with a reduced
crystallization rate, corrected for effects of chain transport.