Carbon Nanotube Grafted Poly(l‑lactide)-block-poly(d‑lactide) and Its Stereocomplexation with Poly(lactide)s: The Nucleation Effect of Carbon Nanotubes
journal contributionposted on 05.04.2016, 00:00 by Zhi-Qiang Cao, Xiao-Rong Sun, Rui-Ying Bao, Wei Yang, Bang-Hu Xie, Ming-Bo Yang
Multiwalled carbon nanotube (CNT) grafted poly(l-lactide)-block-poly(d-lactide) copolymer (CNT-L-D) was synthesized by ring opening polymerization of d-lactide on the end hydroxyls of poly(l-lactide) (PLLA) prepolymers grafted on the surface of CNT. The stereocomplexation of solution cast and melt crystallized CNT-L-D, PLLA/CNT-L-D, and PDLA/CNT-L-D blends were investigated by differential scanning calorimetry, wide-angle X-ray diffraction, and optical microscopic observation. Solution cast and melt crystallized CNT-L-D copolymer form exclusive stereocomplex (sc) crystallites. The melting temperature, normalized crystallinity, and the fraction of sc crystallites for both solution cast and melt crystallized PDLA/CNT-L-D blends are higher than those of PLLA/CNT-L-D blends, indicating distinct stereocomplexation mechanisms in both blends. The heterogeneous nucleating effect of CNT makes the PLLA block grafted directly on CNT always participate in sc crystallite formation. So, for PDLA/CNT-L-D blends, the stereocomplexation mainly occurs between PLLA blocks and free PDLA molecules, and for PLLA/CNT-L-D blends, the stereocomplexation mainly occurs between PLLA blocks and PDLA blocks. The nucleating effect of CNT enhances the sc formation of free PDLA molecules during solution casting and melt crystallization by the introduction of CNT-L-D, and the sc crystallites can reform after complete melting. However, the dilution effect of free PLLA molecules to PDLA blocks lowered down the stereocomplex efficiency of PLLA/CNT-L-D blends compared with that of PDLA/CNT-L-D blends, especially during melt crystallization, and the morphology transition of sc crystallites occurs from original well-rounded Maltese-cross spherulites for PDLA/CNT-L-D blends to dendritic form in PLLA/CNT-L-D blends.