Читать книгу Polymer Nanocomposite Materials - Группа авторов - Страница 32

Melt blending is processed by using kneading machine, molding machine, internal mixer or double screw extruder, etc. [31, 32] to evenly mix the polymer matrix and conductive fillers with the processing temperature above the melting point of polymer. Thus, high temperature and high shear force are needed in melt blending process to ensure the homogeneous dispersion of the conductive nanofiller in the melting polymer matrix. During the melting blending, the nanofillers are forced to disperse by the mechanical shear force and at the same time prevented from re-aggregation by the viscous polymer matrix. After the masterbatch is obtained, the final CPCs can be prepared by using different polymer processing technologies like spinning, hot press, and injection molding [33–35]. Melt blending is an environmental-friendly process method and is feasible for large-scale industrial production of the CPCs. Many recent studies have investigated the effect of fillers introduction, dispersion state, and processing factors on the physical and electrical properties of the CPCs prepared by the melt blending [36].

Kim [37] studied the influence of the CNTs concentration and dispersion state on thermal, rheological, and mechanical properties of the polybutylene terephthalate (PBT) nanocomposites. It was found that the storage and loss moduli of the composite increased with the increase of the CNTs content. The interaction between nanotube–nanotube and polymer–nanotube was regarded as the main cause for the nonterminal behavior of the PBT nanocomposites. Besides, it can be found that the heat distortion temperature and the thermal stability of the composite were substantially enhanced at a low CNTs concentration. During the melt blending, the interaction between conductive fillers and polymer matrix greatly influences the dispersion state of the filler. For example, due to the interaction of π–π stacking between graphene and polystyrene (PS), the graphene could be easily dispersed in PS matrix during the melt blending [38]. It can be seen in Figure 2.1a that the suspension of PS/graphene without melting blending is transparent, suggesting the absence of graphene in the suspension. However, the solubility of graphene in toluene is greatly enhanced by prolong the melt blending time (5–60 minutes). This dark-colored suspension keeps stable and homogeneous even after three months or longer. The forming of π–π stacking between PS and graphene sheet in melt blending process is schematically demonstrated in Figure 2.1b. In melt blending, the PS chains were stretched by shear forces, forming some closely aligned aromatic rings parallel to the graphene sheet. Therefore, the interaction between PS and graphene was enhanced.

Figure 2.1 (a) Photographs of the PS/graphene/toluene suspension prepared by centrifuged at 8000 rpm for 30 minutes. (b) Schematic illustration for the formation of π–π stacking between graphene and PS in melt blending process. Source: (a)–(b) Reproduced with permission. [38] Copyright 2011, American Chemical Society. Microscopic morphology of (c) 1% pristine MWCNTs/PP; (d) 1% pristine MWCNTs/PP-g-MA/PP. (e) Electrical conductivity of various PP composites. Source: (c)–(e) Reproduced with permission. [39] Copyright 2009, Elsevier Ltd.

But mostly graphene and CNTs tend to aggregate in the polymer matrix [polymethyl methacrylate (PMMA), polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS), polypropylene (PP)] [40, 41]. To solve this problem, compatibilizers, chemical modification, or surfactants have often been used to improve the interaction between the filler and the nonpolar polymer matrix [42–44]. Pan et al. [39] studied the correlation between the dispersion state of multiwalled carbon nanotubes (MWCNTs) in PP and electrical conductivity of its composite. They adopted two methods to achieve the uniform dispersion of MWCNTs in PP matrix. One is chemical modification of MWCNTs and another one is incorporation of a master batch [polypropylene-grafted-maleic anhydride (PP-g-MA)] as a compatibilizer followed by simple melt blending. Through comparing the optical microscopic images of MWCNTs dispersion in PP in Figure 2.1c,d, we can get that the addition of compatibilizer could largely enhance the uniform dispersion of MWCNTs in PP matrix. Although the interfacial interaction between MWCNTs and PP could enhance the dispersion of MWCNTs, it may cause the reduction of the electrical conductivity of the composites (Figure 2.1e). Meanwhile, post-heat treatment can improve the connection of MWCNTs in the composite, leading to the increase of the electrical conductivity. Herein, a balance between uniform dispersion of CNTs and construction of conductive networks is vital to the enhancement of electrical conductivity for composites.

In short, melt blending is a simple method to fabricate CPCs. The electrical properties of the CPCs are strongly dependent on the processing parameters like mixing time, shear stress, and temperature as well as the surface modification and introduction of compatibilizers.