Hierarchical structure of thermotropic liquid crystalline polymer formed in blends jointly by dynamic and thermodynamic driving forces
Introduction
Thermotropic liquid crystalline polymer (TLCP) is an important group of engineering resins because of its excellent processability, good chemical resistance, unique mechanical properties, and high thermal stability. These excellent performances come from its special hierarchical structure units in favor of stress transfer [1], [2]. In the study of the structure of thermotropic copolyesters, Sawyer et al. [1] observed three distinct fibrillar species in the spun, extrudated and molded samples of the neat resin. These species were microfibrils of the diameter at about 50 nm, fibrils about 500 nm, and macrofibrils about 5 μm. The formation of such hierarchical structure strongly depends on the processing history of polymer melts. For pure TLCP, the formation of this hierarchical structure is only controlled by the dynamic driving force in the melt flow. Weng et al. [2] also revealed this hierarchical structure of thermotropic liquid crystalline copolyesters in injection-molded plaques of the pure resin.
When a TLCP was dispersed in a resin matrix, no such hierarchical structure could be formed. The droplets with a size of 101–10−1 μm might be in the shape of sphere, ellipsoid, rod and fiber in binary polymer/TLCP blends [3], [4], [5], [6], [7], [8], [9], [10], [11]. For example, Isayev et al. [3] found the size of TLCP fibrils in polyether ether ketone (PEEK)/TLCP blends was 10–15 and 5 μm at 5 and 10 wt% of TLCP, respectively. He et al. [5] found the diameter of TLCP fibrils in polyethersulfone (PES)/TLCP blends was 0.3 μm at 10 wt% of TLCP. The size of TLCP in ternary polymer/TLCP/carbon fiber blends was in the range of 100–10−1 μm [12], [13]. The morphology of TLCP in blends depends on polymer matrix, viscosity ratio of blend components, shear and elongation field, and shape of fillers [14], [15], too. In general, several factors can promote the fibrillation of TLCP in the blends, for example, the content of TLCP in the blend and the external forces exerted on the blend melt. Saengsuwan et al. [16] observed the long TLCP fibers with bigger diameters about 13 μm in polypropylene/TLCP specimens when the content of TLCP was up to 30 wt%. Meng et al. [17], [18] observed the formation of TLCP fibrils only in maleic anhydride compatibilized polyamide 6 and the content of TLCP was more than 20 wt%. However, for certain systems with low contents of TLCP (lower than or equal to 10%), for example, polyamide (PA) 6/TLCP system, it is difficult to promote TLCP to deform into fibrils. The morphology of TLCP in binary PA6/TLCP blends is always spherical or ellipsoidal [19], [20].
In the present study, the hierarchical structure of TLCP, especially its microfibrils of 30 nm, has been formed in a ternary blend system by dynamic and thermodynamic driving forces jointly. For the formation of this structure, a ternary system, PA6/TLCP/Glass bead (PA6/TLCP/GB) blends, was designed. By utilizing the long relaxation time of TLCP rigid chains, the fine microstructure was frozen easily by cooling the melt for further morphology observation. Then the mechanism for the formation of TLCP hierarchical structure in the ternary blend was analyzed dynamically and thermodynamically.
Section snippets
Materials
The polymer matrix used was a polyamide 6 (PA6), commercially known as F-x9025 (DSM, the Netherlands). Its relative viscosity and density were 3.2 and 1.14 g/cm3, respectively. The TLCP was commercial thermotropic liquid crystalline copolyester (Vectra A950, Hoechst Celanese, USA), comprising of 73 mol% hydroxybenzoic acid and 27 mol% hydroxynaphthoic acid, hereafter referred to as TLCP. Its density was 1.4 g/cm3. One grade of hollow glass bead (cenosphere) was provided by Shenzhen Microspace
TLCP microfibrils formed under strong extensional action
Fig. 2 shows a SEM micrograph of fracture surface of a PA6/TLCP/GB ternary blend under lower magnification. It shows that TLCP droplets disperse uniformly in PA6 matrix and deform into microfibrils and fibrils of the hierarchical structure. Some beads are clearly seen embedded in the matrix. Fig. 3 shows a locally magnified morphology of a PA6/TLCP/GB ternary blend. Microfibrils having diameters at the order of magnitude of 101 nm were being pulled out from the circumference of a melt droplet of
Conclusions
The hierarchical structure of TLCP in PA6/TLCP/GB ternary blends has been obtained by capillary flows. The hierarchical structure of TLCP, especially the microfibrils in PA6/TLCP/GB ternary blends is similar to that in the spun fibers of pure TLCP. Under the strong extensional action of the micro-rollers of the glass bead, TLCP microfibrils with an average diameter of 30 nm are formed. The conditions responsible for the formation are thermodynamic and dynamic driving forces. In PA6/TLCP/GB
Acknowledgements
This work was supported by the National Nature Science Foundation of China, Grant No. 50233010.
References (37)
- et al.
Polymer
(1991) - et al.
Polymer
(1995) - et al.
Polymer
(1997) - et al.
Comp Sci Technol
(2000) - et al.
Polymer
(2003) - et al.
Polymer
(1998) - et al.
Polymer
(1990) - et al.
Curr Opin Colloid Interface Sci
(2001) - et al.
Polymer
(2000) - et al.
J Mater Sci
(1986)
J Mater Sci
Polym Eng Sci
Polym Eng Sci
Polym Eng Sci
Polym Compos
Polym Eng Sci
Polym Eng Sci
Polym Eng Sci
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