Effect of matrix/particle interface on mechanical properties of polypropylene nanocomposites

Abstract

The effect of the matrix/particle interface on the mechanical properties of polypropylene (PP) nanocomposites is studied. In the present study, grafting silica nanoparticles with polypropylene chains (PP-grafted silica, PGS) is proven to be a promising strategy to strengthen or weaken the interfacial binding between PP matrix and PGS. The extents of strengthening or weakening effect are correlated to the grafting chain length of PGS. When the molecular weight of a graft is above the critical molecular weight (M_c) of PP, the T_g of the PP/PGS nanocomposite shows higher than the neat PP, indicating a strong interfacial interaction, and vice versa. Entanglement interaction between the grafting chain and the matrix is considered to be the key parameter governing interfacial strength. The change in the matrix/particle interfacial strength leads to a transition in the yield stress of nanocomposites. Compared with the unfilled PP, the yield stress of the PP/PGS nanocomposites is decreased in the case of unfavorable interaction and can be enhanced in the case of a strong interaction. In addition, benefiting from good dispersion, the PP/PGS nanocomposites with a strong matrix/particle interface not only exhibit increased Young’s modulus and yield stress, but also the strain at break remains in line with the unfilled PP, which is in contrast to the conventional wisdom that the gain in modulus and strength must be at the expense of the lower break strain. Due to the weak matrix/filler interface, the great improvement in impact toughness of PP filled with stearic acid modified CaCO₃ (SA-CaCO₃) nanoparticles has been reported. In the present study, the ligament strength relating to entanglement density was proven to be one of factors to influence the toughening effect by comparing the impact strength of the PP/SA-CaCO₃ nanocomposites before and after annealing. Considering PGS can be applied to strengthen the PP ligament, a ternary nanocomposite (PP/PGS/SA-CaCO₃) with multiple matrix/particle interfaces was prepared and studied. Young’s modulus and yield stress of the ternary nanocomposites performs as expected. However, the impact strength is comparable to that of neat PP. The toughening effect of SA-CaCO₃ in the PP/ SA-CaCO₃ system is ineffective in the ternary nanocomposites, which is attributed to the following fact: Because the attraction between PGS and SA-CaCO₃ is higher than that between the PP matrix and inorganic particles, the aggregates of two different particles form, where the weak interface of CaCO₃ acts as a sharp crack, leading to an insignificant change of impact resistance.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology