Abstract
The glass transition temperature (T(g)) is undoubtedly one of the most important characteristics of polymers, and investigating its dependence on their structure and composition is crucial from both fundamental and application points of view. This study deals with the unexpected relationship between T(g) and the average molecular weight between crosslinking points (M(c)) in nanophase-separated polystyrene-l-poly(dimethylsiloxane) (PSt-l-PDMS) and polystyrene-l-poly(dimethylsiloxane)/divinylbenzene (PSt-l-PDMS/DVB) polymer conetworks. In order to reveal the correlation between the T(g) and M(c), a library of PSt-l-PDMS and PSt-l-PDMS/DVB conetworks was synthesized, and their compositions and T(g)s were determined. Instead of the expected increase of T(g) with decreasing M(c), a reverse correlation was found. Namely, the T(g) decreases with decreasing M(c) in these conetworks. Correlation analyses showed that the T(g) linearly depends on 1/M(c), similar to the Fox-Flory relationship for homopolymers with their M(n), that is, T(g) = T(g,ꝏ) -K/M(c) for the investigated conetworks, independent of the absence or presence of relatively low amounts of DVB as an additional small molecular weight crosslinker. This means that the PDMS macrocrosslinker acts like scissors by interrupting the mobility of the crosslinked PSt chains in the conetworks, and the T(g) of the PSt segments will be close to that of PSt homopolymers with the same M(n) as M(c), as found by comparison. Consistent with previous findings with other conetworks, the presence of the scissors effect of the macromolecular crosslinker in the PSt-l-PDMS and PSt-l-PDMS/DVB conetworks indicates that the scissors effect is a general phenomenon for polymer conetworks formed by crosslinking with a macromolecular crosslinker. The observed unusual T(g) versus M(c) relationship in the conetworks can be utilized in designing such novel materials with predetermined T(g)s required for targeted applications.