Nucleation of Poly(lactide) on the Surface of Different Fibers

Abstract

The nucleation process of poly(lactide) (PLA) on a series of fibers was studied by means of in-situ Polarized Optical Microscope (POM) during crystallization. Several synthetic and natural fibers (PLLA stereocomplex fibers (SC), PET, carbon, Kevlar, glass, hemp, linen and cellulose) were employed, and compared to custom-spun fiber of stereocomplex enantiomeric PLA blend. Meaningful differences in the nucleating ability towards PLA could be found for all the considered fibers. Stereocomplex PLA fibers display extremely high nucleating efficiency, with the development of a continuous transcrystalline morphology on their surface, up to high crystallization temperatures. Quantitative measurement of the nucleation rate allowed a comparison of the different fiber substrates in the light of classical heterogeneous nucleation theory, by considering the interfacial free energy difference parameter, Δσ, directly related to the nucleation barrier. The topography of the fibers surface was investigated by atomic force microscopy (AFM), and tentatively related to the measured nucleation ability. While a general effect of surface roughness on lowering the heterogeneous nucleation energy barrier can be deduced, deviations can be observed, in particular for carbon and stereocomplex PLA fibers. The different fiber wettability by PLA melt suggests that chemical interactions between the substrate and the crystallizing polymer also play a meaningful role in promoting the nucleation, although this aspect is generally disregarded in the literature - in favor of surface roughness. Moreover, the specific surface topography is shown to largely affect the density of available nucleation sites along the fiber.