Opposite Trends in Glass Transition and Vogel Temperatures in Long‐Chain Branched Poly( <scp>l</scp> ‐Lactic Acid)

ABSTRACT Poly( l ‐lactic acid) (PLLA) is widely used as both a commodity thermoplastic and for engineering applications, for example, in 3D printing. Typically, the material is used at temperatures near room temperature, approximately 30 K below the glass transition temperature ( T g ) measured by differential scanning calorimetry (DSC). Understanding the glass transition dynamics is therefore important for optimizing the material performance. In this study, the T g of linear and long‐chain branched PLLA were systematically investigated over a broad range of cooling rates spanning five orders of magnitude, using fast scanning calorimetry (FSC) and DSC, employing Tool's method. The obtained limiting fictive temperatures were fitted using a Vogel–Fulcher–Tammann–Hesse (VFTH)‐like equation, allowing determination of both the Vogel temperature ( T v ) and the dynamic fragility. The results reveal that T g , as determined by conventional DSC, increases from 326 K to 329 K while T v decreases from 271 K to 255 K as the number of branch points per molecule increases from 0 to 3.3. These opposing trends in T g and T v are accompanied by a significant reduction in dynamic fragility, with PLLA becoming a stronger glass former as the degree of long‐chain branching increases.