Unraveling Sequence Effect on Glass Transition Temperatures of Discrete Unconjugated Oligomers.

Sequence plays a critical role in enabling unique properties and functions of natural biomolecules, which has promoted the rapid advancement of synthetic sequence-defined polymers in recent decades. Particularly, investigation of short chain sequence-defined oligomers (also called discrete oligomers) on their properties has become a hot topic. However, most studies have focused on discrete oligomers with conjugated structures. In contrast, unconjugated oligomers remain relatively underexplored. In this study, three pairs of discrete oligomers with the same composition but different sequence for each pair are employed for investigating their glass transition temperatures (Tg s). The resultant Tg s of sequenced oligomers in each pair are found to be significantly different (up to 11.6 °C), attributable to variations in molecular packing as demonstrated by molecular dynamics and density function theory simulations. Intermolecular interaction is demonstrated to have less impact on Tg s than intramolecular interaction. The mechanistic investigation into two model dimers suggests that monomer sequence caused the difference in intramolecular rotational flexibility of the sequenced oligomers. In addition, despite having different monomer sequence and Tg s, the oligomers have very similar solubility parameters, which supports their potential use as effective oligomeric plasticizers to tune the Tg s of bulk polymer materials.

Williamson on Counterpossibles.

A counterpossible conditional is a counterfactual with an impossible antecedent. Common sense delivers the view that some such conditionals are true, and some are false. In recent publications, Timothy Williamson has defended the view that all are true. In this paper we defend the common sense view against Williamson's objections.