Abstract
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^−1^ 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^−1^ 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.
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