Fiber surface-induced nucleation of polylactide

B. Wang,1 T. Wen,2 X. Zhang,3 D. Wang,4 D. Cavallo1

1 Department of Chemistry and Industrial Chemistry, Genova (Italy)
2 Department of Chemical Engineering, Hsinchu (Taiwan)
3 Beijing Institute of Fashion Technology, Beijing (China)
4 Institute of Chemistry Chinese Academy of Sciences, Beijing (China)

Fiber-reinforced semicrystalline polymer composites are largely employed for their improved strength with respect to the pure polymer matrix. The adhesion between the polymer and the fiber is known to play a key role in determining the overall mechanical behavior. When semicrystalline polymers are employed, the heterogeneous nucleation on the surface of the solid fiber is an efficient way to improve the adhesion and speeding up the composite production rate. However, fiber induced nucleation studies are still scarce and mainly limited to polyolefins, despite the increasing importance of bio-based polymers and composites.
In the work, the nucleation process of polylactide (PLA) on several fibers was studied in-situ by means of hot-stage polarized optical microscope. Several commercially available fibers (i.e., PET, Kevlar and glass fibers) are employed and compared to stereocomplex enantiomeric PLA blend and annealed homochiral PLA fibers. The nucleating efficiency of the various heterogeneous substrates is quantitatively compared on the basis of the derived free energy barrier for critical nucleus formation, ΔG*.
It results clear that the PLA stereocomplex fibers has higher nucleating ability, due to the identical surface chemistry between the substrate and PLA homocrystal, although the crystalline structure of stereocomplex and homochiral crystals is not the same. On the other hand, the nucleation kinetics of PLA homocrystal on fibers of the very same crystal is even more efficient, and simply follows the secondary nucleation process: the induction times for the birth of the nucleus display the same temperature dependence of crystal growth.