Fibrillation of thermotropic liquid crystalline polymer enhanced by nano-clay in nylon-6 matrix
Introduction
Since the emergence of aromatic liquid crystalline co-polyesters [1], polymer blends containing thermotropic liquid crystalline polymers (TLCPs) have received significant attention from the academic and industrial fields. Extensive studies on this topic have been carried out, and some review articles concerning various aspects of this kind of blends are available in the literature [2], [3], [4], [5].
It is worth noticing that, even at the beginning of the studies on this kind of blends, researchers had tried to introduce inorganic reinforcements into such blend systems [6], [7]. The addition of inorganic fibers could not only enhance the mechanical properties of the blends but also reduce the anisotropy of resulted materials [8]. Rheological testing revealed that TLCPs could reduce the melt viscosity of glass-filled thermoplastics [7], [9], [10]. When the TLCP component was deformed into fibrils in situ during the melt processing of ternary composites containing a solid reinforcement, the composites were named as in situ hybrid composites [11], [12]. Much work has been published regarding the studies on TLCP-containing systems with different inorganic solid reinforcements, such as glass fibers [13], [14], carbon black [15], [16], whisker [17], [18] and silica [19], [20]. Most of above researches paid attention mainly to the balance between mechanical properties and processability of such filled TLCP blends, though other properties were also in consideration (e.g. the conductivity of the system, in Ref. [16]).
In order to obtain high strength, high modulus and heat-resistant materials, a high loading (up to 30–50 wt%) of conventional solid reinforcement is needed in such a hybrid composite. Therefore, the lightweight merit of in situ composites is lost because of the relatively high density of conventional solid reinforcement (ca. 2.5 g/cm3) compared to that of TLCPs' (ca. 1.4 g/cm3). On the other hand, the dispersion of silicate platelet at nanometer scale thickness in a polymer matrix can lead to higher reinforcing efficiency and good thermal-resistant properties even at very low loading levels (typically <10 wt%), as a result of their high specific surface area and aspect ratio [21], [22]. But in order to obtain the composite with super mechanical properties, the matrix with high viscosity is frequently needed to promote the exfoliation of silicate platelets [23]. A masterbatch preparing process has been developed to balance the exfoliation degree and the melt viscosity. However, the lowing of viscosity was realized by trading-off the modulus of the composites [24]. Based on the roles that the TLCP plays in the reinforced polymer blends [12], [13], [17], blending the nanocomposite of high viscosity with a TLCP is the feasible method to improve its processability while do not harm its mechanical properties. Therefore, the combination of TLCP and silicate platelet reinforcement is a promising way to develop in situ hybrid composites that inherit almost all the advantages of the in situ composites and the filler-reinforced composites, with good processability, low density and high mechanical properties at the same time.
Moreover, the interactions between the dispersed TLCP droplets and silicate platelets with one of its dimensions in nanometer scale in a polymer blend have not been reported yet, although the role of organoclay as a compatibilizer in immiscible polymer blends has been the focus of some researches in recent years [25], [26], [27], [28], [29], [30], [31]. A distinct appearance for the compatibilization effect is the dramatic decrease of the dispersed domain size with the addition of organoclay. Though the physical mechanism behind the compatibilizing effect of the clay in immiscible polymer blends is still not very clear, it is commonly accepted that the effect originates from both thermodynamic and kinetic factors [28]. The thermodynamic factor may be induced by the interaction of clay with both polymer components at the interphase as that a block polymer compatibilizer does. In addition, the thermodynamic effect may appear as the thermal property change of the components, such as the shift of Tgs towards each other [25]. In a given polymer blend, it is possible that only the kinetic factor contributes to the compatibilization effect. As the situation in the nylon-6/EPR system, Khatua suggested that the steric hindrance effect from the exfoliated clay plates prevented the coalescence of the dispersed phase [30]. A concise review on the topic of clay filled immiscible polymer blends can be found in a recent publication of Ray et al. [31].
If the minor component in a polymer blend changes from a flexible-chain polymer to a rigid one as the TLCP, what effects will be brought by the organoclay to the dispersed phase? Does the compatibilizing effect of the organoclay also exist in a TLCP-containing blend? If the compatibilizing effect exists in such a blend, what could be manifested besides the decrease of dispersed domain size?
To answer these questions, clay filled polymer blends with a TLCP as the minor phase were prepared in this study. Nylon-6 was chosen as the thermoplastic matrix because of the easy exfoliation of organic silicate layers in it by using melt-blending method [32], [33], [34]. Furthermore, some interesting interactions of fillers (i.e. glass fiber, glass bead) with the dispersed TLCP in Nylon-6 matrix and consequential influences on the morphology and rheological properties of hybrid systems were reported from our laboratory [35], [36]. Nylon-6 nanocomposites with well-dispersed silicate platelets were prepared by melt extrusion using a twin-screw extruder. Then nylon-6 nanocopmosites with different clay loadings were melting blended with a commercial TLCP (Vectra A950 from Hoechst Celanese) at temperatures above its melting point. The morphological observation showed that the introduction of silicate platelets had significant effects on the morphology of the TLCP phase. The fibrillation of TLCP phase was enhanced when the content of organoclay was 5 and 7%, relatively low compared to 30% of the solid-filler content in Refs. [35], [36].
Section snippets
Materials
The matrix polymer used in this work was nylon-6 (N6) with the trademark of Akulon F-x 9025, kindly supplied by DSM, The Netherlands. The thermotropic liquid crystalline polymer used was Vectra A950 (VA), wholly aromatic co-polyester of 73 mol% hydroxybenzoic acid (HBA) and 27 mol% hydroxynaphthoic acid (HNA) manufactured by Hoechst Celanese. Organophilic montmorillonite (OMMT) used in this study was prepared by using standard ion-exchange procedure and suitable for use with polyamides. The
Dispersion of clay platelets in the composites
The dispersion state and nanostructure of clay platelets in a polymer layered silicate nanocomposite (PLSN) have dramatic influences on its mechanical, thermal and rheological performances. In this study, a good dispersion of clay platelets is basic for studying the interactions between the clay and the deformable TLCP phase.
Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) are the most commonly used techniques to elucidate the nanostructure of PLSNs [38].
Concluding remarks
The morphology of minor TLCP phase was influenced by well-dispersed nano-clay in nylon-6 matrix. As an immiscible pair, the blend of VA and N6 had interfacial slip between the dispersed VA droplets and the N6 matrix. The inefficient shear transfer hindered the formation of VA fibrils in the binary N6/VA blend. While in the ternary N6/OMMT/VA blends, the size of VA droplets decreased with increasing clay content when the clay content was ≤3 wt%, and VA droplets were deformed into fibrils with
Acknowledgements
The work is financially supported by the National Natural Science Foundation of China (Grant No. 50233010) and the Outstanding Overseas Chinese Scholars Fund of Chinese Academy of Sciences. The authors would like to express their appreciation to DSM Research, The Netherlands, for their supplying of nylon 6.
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