Polydimethylsiloxane molecular weight distributions

Because the interfering amine functionality was replaced with an imine, it was possible to analyze each fraction for molecular weight distribution by GPC using either polystyrene or specially synthesized polydimethylsiloxane calibration standards. The low molecular weight cyclic content of the early fractions was determined by HPLC and the higher cyclics (Dg, D7, Dg) were identified by GC-MS. The absence of amine functionality in the early fractions, which is indicative of the presence of cyclics, was also confirmed by FT-IR. It is apparent, then, that complete characterization of the fractionated polymer was no small task, especially in light of the fact that even though supercritical fluid fractionation can process initial sample sizes of 20-25 g, some of the fractions were only 0.5-1.0g in size, and therefore the workup required substantial effort and care. 

Tabie 9.26 Molecular Weight Distribution in Parent Polydimethylsiloxane and in Fractions Separated by Supercritical CO2... 

Llorens, I, Rude, E. and Marcos, R. M., Prediction of polymer molecular weight distribution from rheology polydimethylsiloxane blends. Mater. Sci Forum, 480-481,281-286 (2005). 

Materials. Both paraffin and fluorocarbon wax surfaces were studied. The paraffin was a household wax distributed by Esso, Inc. Two fluorocarbon waxes, telomers of tetrafluoroethylene (TFE), were used. One, a methanol telomer,had a crystalline melting point of 278°C. and an approximate molecular weight of 2000 [5] the other, a telomer of polydimethylsiloxane (Dow Corning DC 200 silicone oil), had a crystalline melting point of 275°C. [13]. 

The silicone pol)oners used are generally linear, unbranched polydimethylsiloxanes with vinyl groups (Fig. 1) and a molecular weight between 10 (XK) und 100 000 g/mol, since these are well suited to LIM processing. Polymers with a heterogeneous vinyl group distribution improve tear strength and are therefore also used [5]. 

Thomas et al. [16] used CSLM to observe in situ the formation of polyamide membranes and the measurements were used to study polymer precipitation kinetics. Turner and Cheng [17] applied CSLM and hydrophilic fluorescent probes of varying molecular weights to image the size distribution of poly(methacrylic acid) (PMAA) hydrogel domains in polydimethylsiloxane (PDMS)-PMAA interpenetrating polymer networks. The combination of CSLM with AFM, SEM and X-ray spectroscopy allowed characterization of the structure of stimuli-responsive polymeric composite membranes [18]. 

---