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Opposite Trends in Glass Transition and Vogel Temperatures in Long-Chain Branched Poly(l-Lactic Acid)
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DOI：10.1002/pola.70240.png)
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Abstract
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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 (Tg) measured by differential scanning calorimetry (DSC). Understanding the glass transition dynamics is therefore important for optimizing the material performance. In this study, the Tg 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 (Tv) and the dynamic fragility. The results reveal that Tg, as determined by conventional DSC, increases from 326 K to 329 K while Tv 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 Tg and Tv are accompanied by a significant reduction in dynamic fragility, with PLLA becoming a stronger glass former as the degree of long-chain branching increases.
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