Structure - Glass transition temperature relationship for non-polymeric molecules: The concept of internal plasticizing effect

Despite years of intense investigation, calculating glass transition temperature, Tg, of non-polymeric molecules remains challenging due to many factors affecting this physical quantity. In this article, we have synthesized and thoroughly analyzed three aromatic esters of 1,2-bis(2-hydroxyethylthio)-4-methylbenzene: 1,2-bis(2-acetoxyethylthio)-4-methylbenzene, 1,2-bis(2-propionyloxyethylthio)-4-methylbenzene, and 1,2-bis(2-butyroxyethylthio)-4-methylbenzene. Utilizing differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS), we reveal that Tg of these compounds diminishes gradually as the length of the ester group increases from the two-carbon acetic to four-carbon butyric moiety, contradicting the so-far established structure - Tg relationships. We indicate that the primary origin of this unexpected trend lies in the increasing flexibility of the aromatic ring side chains. Consequently, we examine the concept of the internal plasticizing effect among low-molecular-weight glass-formers, defining it as a phenomenon, wherein the presence of a flexible substituent (particularly when attached to a rigid moiety) leads to a decrease in Tg of a non-polymeric organic compound due to hindered intermolecular packing and enhanced intramolecular dynamics. We demonstrate the universality of this theory, which goes beyond the studied set of aromatic esters, being applicable to CHN-, CHON-, CHOS-, CHNS-, and CHO-based compounds encompassing both simple van der Waals liquids and H-bonded systems. Finally, we highlight the importance of this effect for models parametrizing the structure-Tg relationship of nonpolymeric glass-formers.