The structure of filled and unfilled thermotropic liquid crystalline polymer injection moldings

Studies of the microstructure and orientation in thermotropic liquid crystalline polymer injection moldings have been carried out using a variety of techniques to reveal the complex hierarchical structure. The effect of particle filler on the level of molecular orientation in the flow direction appears relatively weak, but at high filler contents, there is a marked disruption of coarse structure in the matrix. © 1993 John Wiley & Sons, Inc.     

The short- and long-term mechanical properties of filled and unfilled thermotropic liquid crystalline polymer injection moldings

The basic mechanical properties of a series of injection moldings of the thermotropic liquid crystalline polymer Vectra have been investigated to determine the effect of fillers on the short- and long-term mechanical properties of these materials. Mineral-filler addition results in a slight decrease in strength and an increase in stiffness in the flow direction, but has relatively little effect on the anisotropy. Glass fiber addition, on the other hand, may substantially lower the anisotropy depending on the degree of fiber orientation transverse to the flow direction. The glass fiber-filled grade investigated also showed improved creep properties at high temperature compared with both the unfilled and mineral-filled grades. © 1993 John Wiley & Sons, Inc.     

Surface fracture in injection molding of filled polymers

In injection molding certain polymers, fracture of the polymer stream sometimes occurs at the mold surface. This phenomenon has been found to be a tearing apart of the polymer surface layer accompanied by downstream slip of the flowing melt at the polymer/mold interface. Fracture occurs early in mold filling and is initiated usually at the gate to the mold cavity. Analysis of the fracture mechanism indicates that fracture is caused by: (1) high shearing stress in the melt as it fills the mold; (2) poor polymer/mold adhesion; and (3) low polymer surface cohesive strength.     

A preliminary investigation of flash formation during injection molding of polyphenylene sulfide and liquid crystalline polymer blends

Flashing in pure polyphenylene sulfide (PPS) and a blend containing PPS and liquid crystalline polymer (LCP) during injection molding was investigated by differential scanning calorimetry and scanning electron microscopy. The shape of the flash was observed by use of a projector. Flashing was detected in pure PPS and 90/10 PPS/LCP blend but was not found in other compositions, including pure LCP. The DSC thermograms of the flash revealed both exothermic and endothermic peaks at around 120° and 285°C. The first peak, known as crystallization temperature on heating, occurred as a result of early crystallization of PPS. The observed double peaks indicated that the degree of crystallinity was lower in the flash than in the molded part. The morphological studies revealed the presence of LCP fibrils in the skin region and droplets in the core region of 90/10 PPS blend. The absence of flash was attributed to the diameters of the fibrils and droplets, which were found to increase with increasing LCP component.     

Measurement of molecular orientation in thermotropic liquid crystalline polymers

Procedures for obtaining molecular orientational parameters from wide angle X-ray scattering patterns of samples of thermotropic liquid crystalline polymers are presented. The methods described are applied to an extrusion-aligned sample of a random copolyester of poly(ethylene terephthalate) (PET) and p-acetoxybenzoic acid. Values of the orientational parameters are obtained from both the interchain and intrachain maxima in the scattering pattern. The differences in the values so derived suggest some level of local rotational correlation     

In situ composites: Blends of isotropic polymers and thermotropic liquid crystalline polymers

Melt blending of a variety of conventional isotropic polymers (both crystalline and amorphous) has shown that considerable reinforcement is obtained from the inclusion of thermotropic liquid crystalline polymers (LCP). When the LCP is a minor component it forms highly elongated domains parallel to the flow direction. The intrinsically high strength and stiffness of the LCP improve the mechanical properties of the resulting blend. This approach is distinguished from the common practice of filling polymers with chopped glass and carbon fibers by the fact that the reinforcing component comes into existence during processing (molding or extrusion). Many of the problems associated with fibrous fillers are avoided. For example, viscosity of the “in situ composite” is actually lower than that of the base polymer and wear on the compounding and processing equipment is reduced.     

Internal stress,molecular orientation,and distortion in injection moldings: Polypropylene and glass-fiber filled polypropylene

Residual stress measurements and distortion analyses have been conducted on injection molded plaques made from polypropylene (PP) and a short glass-fiber filled polypropylene (GFPP). The residual stress analyses include measurements both parallel and perpendicular to the direction of flow during mold filling. Residual stresses are very anisotropic in GFPP, but not in PP. The residual stress levels in PP fall on aging at room temperature, whereas in GFPP the proportion of stress relaxation is smaller, and significant stresses remain even after heating to elevated temperatures. A significant contribution to distortion has been linked to the ejection process, and the long- and short-term distortion of moldings is discussed within the framework of the properties of the materials measured here.     

Effect of complex flow kinematics on the molecular orientation distribution in injection molding of liquid crystalline copolyesters

Wide-angle X-ray scattering (WAXS) is used to probe the molecular orientation in steady isothermal complex channel flows (in situ) and in injection molded plaques (ex situ) of a new, low-cost aromatic copolyester based on the mesogen 4,4′-dihydroxy-α-methylstilbene (DHαMS). Complex orientation states arise from the competition of inhomogeneous mixed shear and extension in isothermal flows. Slit-contraction flows lead to a significant but temporary increase in the average degree of molecular orientation, suggesting that this polymer is of the ‘shear-tumbling’ type. Conversely, bimodal orientation states are observed in slit-expansion flows, where transverse extension leads to a strong reduction in the average degree of molecular orientation along the flow direction. Similar bimodal orientation states are observed in injection molded plaques, suggesting that these kinematic concepts translate rather directly to the more complex transient non-isothermal case of injection molding. Variations in orientation state induced by changes in plaque thickness may be rationalized by systematic changes in the relative importance of shear and extension. These results suggest a complementary perspective on ‘skin-core’ morphologies in liquid crystalline polymer moldings, and provide a clear conceptual link between more fundamental studies in isothermal flows and structure development during processing.     

Blends of two thermotropic liquid crystalline polymers: The influence of reactions during injection molding on the phase structure and the mechanical behavior

Blends of two different thermotropic liquid crystalline polymers, poly(p‐hydroxy benzoic acid‐co‐2, 6‐hydroxy naphthoic acid), VA, and poly(p‐hydroxybenzoic acid‐co‐terephthalic acid‐co‐aminophenol), VB, were obtained by direct injection molding throughout the full composition range. The blends were almost fully immiscible apart from a low degree of reaction. This, as well as the nature of the blends, probably influenced their mechanical properties. Enhanced properties were obtained both in the modulus of elasticity and in the tensile strength, at all the compositions studied, with absolute maxima at VB contents between 50% and 75%. These synergisms were mainly due to a greater orientation of the components in the blends than in the pure components.     

Mechanical properties of in situ composites based on partially miscible blends of polyetherimide and liquid crystalline polymers

Results related to the mechanical properties of in situ composites based on partially miscible blends of polyetherimide (Ultem) and liquid crystalline polymers (HX1000 and HX4000) are discussed. It is observed that, at least in terms of the tensile and flexural modulus, there is a positive deviation from the law of mixtures. These results are analyzed and an attempt is made to explain the origin of this synergistic behavior based upon morphological data. The behavior of these partially miscible systems is compared to that of an immiscible system (Ultem/Vectra) and also that of glass-reinforced Ultem. Using blends subjected to two passes through a single screw extruder, and thereby increasing the mixing history of the Ultem/HX4000 blend, led to improved dispersion but did not lead to any measurable improvement, within experimental error, in the tensile modulus or ultimate strength (though the toughness was enhanced). The dynamic creep compliance was measured and the creep behavior with composition and temperature is discussed. The data seem to suggest that a number of factors (including partial miscibility, the properties of the specific LCP chosen as the reinforcement and the final blend morphology) all interact in a complex and as yet undetermined manner to produce the enhanced mechanical properties.     

Molecular engineering of liquid crystalline polymers by living polymerization

Summary This paper describes the synthesis and characterization of poly{3-[4-cyano-4-biphenyl)oxy]propyl vinyl ether} [poly(6-3)] macromonomers obtained by the functionalization of the growing chain end of the corresponding living polymer obtained by the initiation of the cationic polymerization of 3-[4-cyano-4-biphenyl)oxy]propyl vinyl ether with CF₃SO₃H/S(CH₃)₂, with 2-hydroxy ethyl methacrylate [poly(6-3)-I], 2-[2-(2-allyloxyethoxy)ethoxy]ethanol [poly(6-3)-II] and 10-undecen-1-ol [poly(6-3)-III].Part 20: V. Percec and M. Lee, J. Mater. Chem., submitted     

Extrusion blow molding of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion blow molding of bottles from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated in elongational flow fields that occur in polymer processing operations and to determine if TLCP reinforced bottles could be produced by extrusion blow molding of microcomposites. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of TLCP concentration and in situ composite strand properties on the mechanical properties and anisotropy of bottles made from microcomposites. Specifically, strands having three different draw ratios are used to produce bottles at 10 and 20 wt% TLCP. Increasing the in situ composite strand modulus is shown to cause an increase in both the machine and transverse direction moduli of the composite bottles. The mechanical properties of the bottles increase with increasing TLCP composition. Finally, the machine and transverse direction properties are not balanced in the composite bottles produced in this study (degrees of anisotropy ranging from 1.5 to 1.8). The mechanical anisotropy is probably the result of a low blow up ratio (2) in the bottles and the TLCP fibers being oriented primarily in the machine direction due to the shear flow in the die.     

A novel microstream injection molding method for thermotropic liquid crystalline polymers to promote mechanical isotropy: A matrixing microbeam X-ray study

The effects of flow-altering inserts and mold cavity geometry on the mechanical properties of an injection molded liquid crystalline polymer were studied to produce parts with properties approaching macroscopically isotropic state. By inserting fine metal mesh barriers to the gates of the mold cavities, a large number of highly oriented microstreams are produced. After their creation these highly oriented streams of differing flow vectors intertwine and this texture remains reasonably intact even after substantial shearing and extension history imparted on them during ensuing flow into the cavity. This method is effective in the interior away from the skin regions formed under the shearing flow during injection. The local molecular orientation was determined using a matrixing microbeam WAXS technique that allows precision movement of the sample in the microbeam X-ray. Samples produced with the 1.0 mm² mesh showed large variations in the local symmetry axis with respect to the machine as measured by microbeam X-ray diffraction incrementally from the edge to the core of the parts. In comparison, samples with no mesh insert showed only gradual changes in the tilt angle (angle between local symmetry axis and flow direction). The modulus and tensile strengths of all samples with the 1.0 mm² mesh inserts were found to approach virtual global mechanical isotropy.     

Mechanical and morphological anisotropy in injection molding of thermotropic liquid crystalline copolyesters

Relationships among mechanical properties, degree of molecular orientation and molding conditions are investigated in injection molded plaques fabricated from a 4,4′-dihydroxy-α-methylstilbene (DHαMS)-based thermotropic liquid crystalline copolyester. Wide-angle X-ray scattering (WAXS) patterns reveal bimodal orientation states at most locations in the plaques. One population aligns roughly along the anticipated flow direction while a separate population is generated as a result of transverse stretching associated with diverging streamlines during mold filling. Micro-tensile bars are cut from the plaques both parallel and perpendicular to the filling direction to assess anisotropy in properties. Enhanced molecular orientation and properties ‘in-shear’ are observed for thinner plaques fabricated at relatively low mold temperatures and melt temperatures slightly above the nominal melting point of the polymer. Injection fill speed is not found to have a significant effect on anisotropy in tensile strength/stiffness. Mechanical properties such as tensile modulus and fracture stress are found to obey a ‘universal’ correlation with X-ray measurements of molecular orientation projected onto the axis of the testing specimens. These results suggest that even in the presence of complex, spatially heterogeneous orientation states, simple average measures of orientation can provide a robust means of anticipating macroscopic properties.     

注射成型中聚合物剪切诱导结晶行为的三维模拟

在考虑剪切导致分子链取向并升高其平衡熔点的基础上,建立了基于Nakamura方程的剪切诱导结晶动力学模型。在WLF-Cross黏度模型中引入结晶对黏度系数的影响,构建了考虑结晶的注射成型过程模型。采用改进的有限体积法对聚合物剪切诱导结晶行为进行了三维数值模拟,模拟中耦合了流动场、熔体压力、温度、诱导时间与结晶度。结果表明,本方法可清晰模拟出注射成型过程中聚合物的三维“喷泉”流动行为以及3层“皮-芯”结晶结构,同时,诱导结晶时间指数与相对结晶度的模拟结果与理论及实验结果吻合。     

Structure development during flow of polyblends containing liquid crystalline polymers

The microstructure development during capillary flow of polyblends containing Liquid Crystalline Polymers (LCPs) was studied. In the present investigation the wholly aromatic LCP constituent was the minor phase suspended in polycarbonate (PC), poly(butyleneterephthalate) (PBT) or Nylon 6 (N-6), in addition to previously studied amorphous nylon matrix Experimental results showed that the viscous forces acting at the components' interface are predominating the elongational deformation and the resulting structure development of the LCP phase. In cases where the viscosity of the suspending matrix was higher than the LCP one (PC, amorphous nylon) scanning electron micrographs indicated that fibrillar structure developed. In cases where the viscosity of the matrix polymers was lower than the LCP suspended phase, fibrous structures developed only at very high shear rates. Due to velocity rearrangement effects at the capillary exit a skin core morphology was observed. Since the polymers viscosity depends both on shear rate and temperature, the in situ composite structure development depends on the specific processing methods and conditions that the LCP containing polyblends experience.     

Random orientation of in situ composites based on liquid‐crystalline polymers by compression moulding

In this study, randomly oriented in situ composites based on liquid‐crystalline polymers (LCPs) were prepared by thermal compression moulding. The LCP employed was a semi‐flexible liquid‐crystalline copolyesteramide with 30 mol% of p‐aminobenzoic acid (ABA) and 70 mol% of poly(ethylene terephthalate) (PET). The matrices were poly(butylene terephthalate) (PBT) and polyamide 66 (PA66). The rheological properties, compatibility and morphological structures of these in situ composites were investigated. The results showed that PA66‐LCP and PBT–LCP component pairs of the composites are miscible in the molten state, but partially compatible in the solid state. The ratios of viscosity, λ₁ = η_LCP/η_PA66 and λ₂ = η_LCP/η_PBT, are all greater than 1.0. However, the melt viscosity of the LCP/PBT and LCP/PA66 blend is much lower than that of PBT and PA66, and it decreases markedly with increasing LCP content. When the LCP/PA66 or LCP/PBT blends are compression moulded, the LCP/PA66 or LCP/PBT melts and flows easily due to their low viscosity, and the LCP phases in the melts deform easily along the flow directions, which are random. It leads to uniformly dispersed LCP micro‐fibres randomly orientation in the thermoplastic matrix due to the compatibility between the blending components. © 2003 Society of Chemical Industry     

Polydomain model predictions of molecular orientation in isothermal channel flows of thermotropic liquid crystalline polymers

We report process simulations of molecular orientation of liquid crystalline polymers for isothermal channel flows in extrusion. The simulations use a “polydomain” model due to Larson and Doi (Larson and Doi, J. Rheol., 35, 539 (1991)), which is implemented by exploiting a nearly exact analogy with a fiber orientation model that is widely used for analysis of composites processing. Simulation results are compared to experimental data previously obtained using a customized X‐ray capable extrusion die that allows a wide range of channel flow geometries to be explored. Competition between inhomogeneous shear and extension in these kinematically complex flows has a profound effect on the resulting molecular orientation distributions. We find that the Larson–Doi model successfully predicts most aspects of the experimental observations, demonstrating its utility for process modeling. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Investigation of the amorphization process of partially crystalline polymers by hydrostatic weighing in an inert liquid

The potential of hydrostatic weighing of partially crystalline polymers in an inert liquid during the study of their amorphization and crystallization processes over a broad temperature range was demonstrated using the low-density polyethylene—polymethylsiloxane system as an example.     

Molecular structure effect of thermotropic liquid crystalline polymers on interfacial adhesion with polycarbonate

The effect of molecular structure of thermotropic liquid crystalline polymers (TLCP) on the interfacial adhesion with polycarbonate (PC) was investigated by varying the flexible spacer length in the main chain of the TLCP. The interfacial adhesion was estimated by two methods: (1) the debonding stress from the melt-contacted TLCP/PC plates was measured by a modified pin-pull test; (2) the mechanical response of the melt-mixed blend was analyzed by the composite theory. Both methods indicated that the interfacial adhesion of the TLCPs with flexible spacer was improved over that of a rigid TLCP (i.e., Vectra A950), and that it was further enhanced by an increase in the length of a flexible unit.