DOI: 10.1002/pola.70240 ISSN: 2642-4150
Opposite Trends in Glass Transition and Vogel Temperatures in Long‐Chain Branched Poly(
l
‐Lactic Acid)
Rui Zhang, Mengxue Du, Bao Wang, Yue Yu, Rene Sattler, Paul‐Maximilian Runge, Pengju Pan, Mario Beiner, Holm Altenbach, Katalee Jariyavidyanont, René Androsch, Christoph Schick 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.