Effect of die geometry on the structural development of a thermotropic liquid crystalline polymer in a thermoplastic elastomer matrix

Abstract

This study investigates deformation of a thermotropic liquid crystalline polymer (TLCP) in different die geometries. Blends of a TLCP with a thermoplastic elastomer of EPDM were made in a twin-screw extruder. Morphological observation of the extruded blends demonstrates the complimentary effect of shear in the die exit on dispersed phase deformation and fibril formation. Shear strain can affect fibril formation for a relatively large dispersed phase in the region close to the die wall. However, the main role of shearing is in breaking up the larger particles and initial polydomain structure. A strong elongational deformation on the blended melt after the die exit is required, and fine microfibrils normally observed in in situ compos ites were not easily formed by shear deformation only in the die.

In Situ Reinforcing Elastomer Composite Based on Polyolefinic Thermoplastic Elastomer and Thermotropic Liquid Crystalline Polymer

“…It was found that, for the blends possessing the same composition, no significant difference in morphology of TLCP domains at core and edge regions is observed (SEM micrographs of the edge region are not shown here). The slightly deformed TLCP domains at core region was observed for Vectra B950/EPDM extruded strand reported by Soe et al (17) They also found that the TLCP domains in the strand, prepared using a twin-screw extruder equipped with 3 mm-diameter straight die and the screw speed of 10 rpm, appeared as large particle in the core region. Furthermore, more elongated Vectra B950 domains was observed at the edge region which has encountered mostly shear flow.…”

Effect of the Molecular Structure Change of a Matrix Polymer (Nylon 6) on the Deformation of Dispersed Phase (a Thermotropic Liquid Crystalline Polymer) Droplets in Shear Flow

“…Thermotropic liquid crystal polymers (TLCPs) have been extensively studied as blend additives because they can satisfy the rheological (low melt viscosity) and mechanical (high modulus and strength) properties required for various applications. − One of the requirements is the ability to form in situ composites (blends of TLCPs and thermoplastics), which are useful because they can solve some of the problems commonly encountered in the conventional processing of fiber-reinforced composites and also have the potential to provide a sustainable route to fiber-reinforced composites. − These useful properties are attributed to the spontaneous molecular orientation of TLCPs during the processing to form a fibril morphology with a high degree of orientation. − The morphological development of the TLCP phase in thermoplastic matrices is critical for the performance of the resultant in situ composites. − Although numerous studies on the TLCP blends and in situ composites prepared with commercially available thermoplastics have been reported, there were few studies on the fabrication of in situ composites in shear flow. − Once an elongational force is applied to the two-phase systems of TLCPs, deformation and fibrillation of the TLCP phase can readily occur because the elongational flow field stabilizes elongated droplets, independent of the viscosity ratio. − In processes where the shear flow dominates, such as injection molding or extrusion, TLCP particle deformation is not as straightforward. − , …”

“…Fibrils of the TLCP phase are usually formed in the extruder near the die exit with an orientation of the flow direction because of the extensional forces . However, in pure shear flow (a weak flow), not all immiscible thermoplastic/TLCP blend pairs can generate in situ composites.…”

“…Several critical factors affect the deformation of the dispersed TLCP phase under the shear flow such as the applied shear stress, viscosity ratio, and the interfacial tension. − Numerous studies have shown that these rheological factors influence the deformation of the dispersed phase, particularly the ratio of the viscosities of the dispersed phase and the matrix polymer as well as the elasticity effect. ,− Kirchberger and Münstedt have recently surveyed previous studies of droplet deformation . Over the past 2 decades, we have continuously sought for a feasible method for easily deforming a TLCP phase dispersed in a matrix with a viscosity lower than that of the TLCP phase. − In principle, such a deformation is possible if the interfacial tension is sufficiently low that the capillary number (i.e., a ratio of the shear stress to the interfacial tension) becomes larger than the critical capillary number (see the discussion of Figure ).…”

“…Several critical factors affect the deformation of the dispersed TLCP phase under the shear flow such as the applied shear stress, viscosity ratio, and the interfacial tension. − Numerous studies have shown that these rheological factors influence the deformation of the dispersed phase, particularly the ratio of the viscosities of the dispersed phase and the matrix polymer as well as the elasticity effect. ,− Kirchberger and Münstedt have recently surveyed previous studies of droplet deformation . Over the past 2 decades, we have continuously sought for a feasible method for easily deforming a TLCP phase dispersed in a matrix with a viscosity lower than that of the TLCP phase. − In principle, such a deformation is possible if the interfacial tension is sufficiently low that the capillary number (i.e., a ratio of the shear stress to the interfacial tension) becomes larger than the critical capillary number (see the discussion of Figure ). − We have studied the behaviors of blends of nylons with some immiscible TLCPs and have found that when an extensional flow is applied during the processing, the TLCPs can be elongated into a fibrous shape oriented in the flow direction. − , In the absence of this extensional component, however, only spherical domains were observed. − , Although strong drawing after the die exit can somewhat deform the dispersed droplets, this approach is not readily applicable to nylon blends because of their drawing instability. − These difficulties also arise with other polymers synthesized via polycondensation, such as polyamides or polyesters, which exhibit peculiar rheological properties that differ dramatically from those of other polymers synthesized via free-radical polymerizations, i.e., they have a broad Newtonian viscosity plateau over a wide range of shear rates as well as a rather low melt viscosity .…”