Second virial coefficient of a series of polysiloxanes: the effect of side groups and molecular weight

Abstract

This study reports the results of analyzing the second virial coefficient (A$_{2})$ in a good solvent for a series of polysiloxane chains in which the pairs of substituents connected to the silicon are CH$_{3}$, CH$_{3; }$C$_{2}$H$_{5}$, C$_{2}$H$_{5}$; CH$_{3}$, C$_{6}$H$_{13; }$CH$_{3}$, C$_{16}$H$_{33; }$and CH$_{3}$, C$_{6}$H$_{5. }$The effect of the side group and molecular weight on A$_{2}$ was investigated using the Helical Wormlike (HW) chain model. A theoretical analysis of the interpenetration function ($\psi )$ was carried out using a Two-Parameter (TP) scheme. The theoretical-experimental behaviour of A$_{2 }$in a series of polysiloxanes was investigated considering the effects of the type of substituent on the main chain, molecular conformation, and molecular weight. The results obtained were strongly dependent on the flexibility of the side groups and the molecular weight of the polymer. Within the HW model and TP scheme, the A$_{2}$ value for poly(dimethylsiloxane) (PDMS1 and PDMS2), poly(diethylsiloxane) and poly(methylhexylsiloxane) agrees with the experimental results obtained through the coupled system (Gel Permeation Chromatography/Light Scattering: GPC/LS). However, the A$_{2}$ calculated for PDMS3, poly(methylhexadecylsiloxane) (PMHDS) and poly(methylphenylsiloxane) (PMPS) showed a significant difference from the experimental value. On the basis of the theory for the HW model, the deviation in PDMS3 may be due to the effect of the end chains. For PMHDS and PMPS, the model used is outside of the coil limit (validity of TP model). In PMHDS the determination of the (RMSradius)$^{2}$ value from the GPC/LS system is strongly influenced by the interaction between the bulky side group and the main chain, which affects the calculation of A$_{2}$.