Non-isothermal crystallization behaviors of poly(lactic acid)/cellulose nanofiber composites in the presence of CO2

• Crystal nucleation density was significantly enhanced by cellulose nanofibers.
• The crystallization rate increased with the cooling rates.
• Dissolved CO2 increased PLA’s degree of crystallinity.
• Tc, Tm, and Tg decreased with an increase in CO2 pressure.
• Cellulose nanofibers and CO2 pressure did not induce a new crystalline structure.

The effects of cellulose nanofiber (CNF) on the non-isothermal crystallization behaviors of poly(lactic acid) (PLA) at atmospheric pressure and at various CO2 pressures were investigated using a regular differential scanning calorimeter (DSC) and a high-pressure DSC at different cooling rates of 1, 2, 3, and 5 °C/min. The POM images revealed that the CNFs acted as crystal nucleating agents, increasing the number of crystals and decreasing the crystal sizes. The non-isothermal crystallization showed that PLA’s crystallization rate increased with cooling rates and the incorporation of CNFs accelerated the overall crystallization kinetics by providing more nuclei, thereby decreasing the crystallization half-time. The degree of crystallinity was not proportional to the CO2 pressure. The highest crystallinity was obtained at a higher pressure with increased cooling rates. The activation energy analysis showed that the incorporation of CNFs restricted the movement of PLA molecular chains, thereby hindering crystallization. By comparing the Avrami analysis, Mo analysis, and activation energy results, it was speculated that heterogeneous crystal nucleation with the presence of CNFs might be the dominant factor in determining the overall non-isothermal crystallization rate of the PLA/CNF composites. Wide angle X-ray diffraction (WAXD) diffractograms showed that CNFs and CO2 pressure had no influence on the crystalline structure of PLA. The effects of CNF content, CO2 pressure, and cooling rate on Tc, Tm, and Tg were also investigated.

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