ON THE CRITICAL MOLECULAR CHAIN-LENGTH OF POLYDIMETHYLSILOXANE

Because a study of the results reported for the chain dimensions of polydimethylsiloxane (PDMS), critical for the onset of this polymer's non-Newtonian flow behavior, obtained from the viscosity-molecular weight relationships available from the literature, clearly revealed that considerable differences exist between the reported data, a detailed analysis of these data was performed together with an additional examination of 10 new PDMS samples that were selected so as to have molecular weights that would fill the gaps observed in the polymer viscosity-chain length relationship constructed from the accepted literature data. The results obtained were analyzed by using several different procedures integrated into a recently described comparative method that could allow for determination of what is called the most realistic critical value, Z(wc). The latter was determined as 930 PDMS main-chain atoms, which corresponds to this polymer's degree of polymerization of 464.5 and the weight-average molecular weight of 34,500. It is not only shown that after elimination of some clearly erroneous data points from the previously reported relationships the obtained critical chain-length values could very well fit the earlier relationships, but also that appropriate ''master'' relationships were constructed including 48 pairs of the old and 10 pairs of the new data points. It is suggested that this relationship be accepted as the best-fit viscosity-polymer chain-length dependence for PDMS, and it is pointed out that the obtained PDMS critical chain-length value ranks this polymer's macromolecules as the most flexible of the corresponding long-chain molecules presently known. (C) 1993 John Wiley & Sons, Inc.