Crystallization, glass transition, and molecular dynamics in PDMS of low molecular weights: A calorimetric and dielectric study

In this work linear polydimethylsiloxane (PDMS) of five low molecular weights, W-m similar to 2-8 kg/mol, i.e. mainly below the entanglement threshold, is studied employing differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), a special dielectric technique in the temperature domain, and broadband dielectric spectroscopy (BDS). It is shown mainly via raw data that the crystallization and glass transition for the larger W(m)s follow expected trends, however, for W-m similar to 2 k (corresponding to similar to 22 monomers/chain) crystallization is severely suppressed and slower. Moreover, it is demonstrated that upon isothermal annealing the crystalline fraction (CF) is maximized for all W(m)s with the exception of the lower one. For the latter, results indicate that during the annealing only the first stages of crystallization (nucleation, ordering of the polymer around the nuclei) take place, while the main crystallization event (formation of crystals) is recorded only upon heating, via the cold crystallization process. The suppressed crystallizability originates from the retarded nucleation. In particular for the lower W-m, the presence of large numbers of chain ends should be at the origins of hindered short range ordering of the polymer chains, as well as in relation to the difficult formation of physical crosslinks. At the same time, in TSDC and BDS uniquely for W-m similar to 2 k upon annealing and CF similar to 0, next to the relaxation peaks related with the glass transition of the bulk-like polymer (alpha and alpha(c)), an additional quite strong peak (alpha(n)(ew)) is recorded at higher temperatures. The overall characteristics of alpha(n)(ew) relaxation resemble those of PDMS chains constrained and ordered in the interfacial layer in amorphous silica/PDMS nanocomposites (alpha(int) relaxation). Thus, combing the overall results with results from the literature, it is proposed that alpha(n)(ew) is a direct recoding of retarded dynamics of the polymer chains being ordered around the crystallization nuclei or around extremely small crystals.