Crystallization and hydrolytic degradation behaviors of poly(L-lactide) induced by carbon nanofibers with different surface modifications

Interfacial interaction exhibits great role in determining the microstructure evolution and performance of the polymer composites. In most cases, it is expected to enhance the degree of the interfacial interaction as much as possible. In this work, carbon nanofibers (CNFs) were modified through two different methods, i.e. carboxyl functionalization in compounded H2SO4/HNO3 (3:1, vol/vol) to obtain the f-CNFs and then further reaction with poly (ethylene glycol) (PEG) to obtain the PEG-grafted CNFs (g-CNFs). The results showed that the grafting ratio of functional groups on f-CNFs was about 13.09 wt%, while after being grafted by PEG, the grafting ratio was further enhanced up to 50.95 wt%. Largely enhanced dispersion stability in aqueous was achieved for f-CNFs and g-CNFs. Surface modification also improved the dispersion states of CNFs in the poly (clactic acid) (PLLA) composites, and stronger interfacial adhesion was achieved in the PLLA/f-CNF and PLLA/g-CNF composites. Studying on crystallization behaviors of the composites showed that raw CNFs (r-CNFs) and f-CNFs exhibited good nucleation effects on PLLA crystallization with nucleation activation energies of -12.49 and -11.81 kJ/mot, respectively, while the nucleation effect of the g-CNFs was very weak and the nucleation activation energy was -5.45 kJ/mot. However, compared with the PLLA/f-CNF composite, the PLLA/g-CNF composite exhibited largely enhanced growth rate of spherulites during the isothermal crystallization process. Researches about the hydrolytic degradation behaviors of the PLLA composites showed that incorporating f-CNEs accelerated the hydrolytic degradation while it was greatly suppressed by g-CNFs. For example, the PLLA/g-CNF-2 sample showed much lower hydrolytic degradation rate (0.0134%/h) compared with the PLLA/r-CNF-2 (0.0521%/h) and PLLA/f-CNF-2 (0.086%/h) samples. This work confirms that appropriate interfacial interaction is more favorable for the realization of the nucleation effect of CNFs and the acceleration of the hydrolytic degradation of the PLLA matrix, while strong interfacial interaction achieved through grafting polymer chains not only suppresses the nucleation effect but also enhances the hydrolytic degradation resistance of the PLLA matrix. (C) 2019 Elsevier Ltd. All rights reserved.