A Fresh Perspective on the Internal Plasticizing Effect in Non-Polymeric Glass-Formers

The internal plasticizing effect occurs when a flexible substituent, especially one linked to a rigid moiety, lowers the glass transition temperature (<i>T</i>_g) of an organic compound by disrupting molecular packing and increasing the internal movement. While this concept has long been recognized in polymers, its potential application to low-molecular-weight, nonpolymeric compounds has gained attention only recently. In this study, we provide direct evidence supporting this concept among low-molecular-weight glass-formers by comparing the thermal and dynamic properties of structurally related 1,2-bis(2-halogenethylthio)-4-methylbenzene and ester derivatives of 1,2-bis(2-hydroxyethylthio)-4-methylbenzene. We show that increasing molar mass through a simple atom substitution without altering intermolecular interactions, such as exchanging chlorine for bromine, decelerates molecular dynamics in the liquid phase and raises <i>T</i>_g according to the typical dependence <i>T</i>_g ∼ <i>M</i>^α. This behavior arises from nearly identical conformational transformations in these compounds, where comparable energy barriers and intramolecular dynamics render molar mass the primary determinant of structural relaxation times. In contrast, similar increase in molar mass achieved by elongating the flexible substituent lowers the <i>T</i>_g value and enhances both the intra- and intermolecular dynamics, underscoring internal molecular flexibility and conformational diversity as a crucial factor controlling the glass transition and near-glass-transition phenomena. Our findings, supported by differential scanning calorimetry, broadband dielectric spectroscopy, and density functional theory calculations, indicate internal plasticization as a fundamental mechanism shaping the properties of molecular glass-formers.