On the Determination of Gel Points

TL;DR

This paper investigates the gel point of cross-linked polysiloxanes, comparing experimental data with gelation theories, and finds that assumptions about molecular weight distribution significantly affect theoretical agreement.

Contribution

It reexamines the gelation behavior of polysiloxanes, highlighting the impact of molecular weight distribution assumptions on theoretical predictions.

Findings

Theories are nearly consistent with experimental data under certain assumptions.

Using mean molecular weight yields exact theoretical agreement, implying monodispersity.

Experimental validation with monodisperse monomers is recommended for further clarity.

Abstract

A critical composition of cross-linked polysiloxanes observed by Scanlan and Winter is reinvestigated in comparison with the theory of gelation. We assume, based on the Scott findings, the geometric distribution for one of the monomers, divinyl-terminated poly(dimethylsiloxane). Calculation results show that the two theories are in near-consistency, supporting the Scanlan-Winter estimation based on the linear viscoelastic theory. On the other hand, there is a disturbing result that calculation using the mean molecular weight, $M_{n}$, leads to exact agreement between the two theories, suggesting that the distribution is in effect monodisperse, contrary to the assumed geometric one and also the observed polydispersity, $M_w/M_n=2.1$. Further experimental studies employing monodisperse monomers would be highly valuable to consolidate the bridge between these two fundamental theories.