Donald G. Baird

Review of the entry flow problem: Experimental and numerical

Abstract This paper is concerned with a review of both experimental and numerical studies of axisymmetric and planar entry flows which have been considered as test problems for the numerical simulation of viscoelastic fluids. The test of the method is usually based upon whether the numerical model predicts vortices in the entry corners. However, it is not clear as to whether one should observe vortices for all viscoelastic fluids. Polyacrylamide solutions and Boger fluids exhibit vortices in axisymmetric flow and the size of the vortex does increase with fluid elasticity. However, the vortex is nearly suppressed in planar entry flow. On the other hand, not all polymer melts are found to exhibit vortices in either axisymmetric or planar entry flow. It is our belief that the origin of vortices is not related to the elasticity based on shear flow propertes but to the behavior of the transient extensional viscosity. Certain polymer melts such as low density polyethylene exhibit vortices in both planar and axisymmetric flow along with unbounded stress growth at the start up of extensional flow. It is believed that the constitutive equations used in the numerical simulation must reflect this extensional behavior if vortices are to be predicted. A review of the numerical simulations concerned with entry flow shows that there is considerable doubt about the accuracy of the predictions for most of the studies. Even for those where the numerical solution is thought to be accurate, the magnitude of the stream function associated with the vortices is usually very low. None of the differential models used to date predicts strain hardening extensional viscosity, but those which are thought to predict vortices do rise more rapidly to the steady-state extensional viscosity values with time. It is recommended that the search of test fluids be widened beyond polymer solutions as there may already exist a number of polymer melts which behave similarly to the predictions of existing constitutive equations.

Impact toughening of polycarbonate by microcellular foaming

Abstract The microcellular foams of polycarbonate (PC) were studied to gain an understanding of the effect that small bubbles have on the impact properties. The PC matrix was selected based on its intrinsic ductility. Sharply and bluntly notched samples were tested in Charpy impact configuration, so that the effects of the sharpness of the notch could also be studied. Sharply notched microcellular PC exhibited the most promising behaviour. The maximum load of foamed PC was substantially improved over that of unfoamed PC. Furthermore, the impact toughness (total energy absorbed) per unit thickness improved dramatically over that of the unfoamed PC. This effect is explained by the fact that the presence of small bubbles induces a transition from brittle to ductile behaviour by relieving the triaxial stress condition in front of the crack tip.

The effect of compatibilization on blends of polypropylene with a liquid-crystalline polymer

Abstract Compatibilization of blends of polypropylene (PP) with a thermotropic liquid-crystalline polymer (LCP) was considered in an attempt to improve the mechanical properties obtained by blending alone. The effect of compatibilization was investigated on injection-moulded plaques and tensile bars. Significant improvements were seen in the values of tensile modulus and strength. Substantial improvements were also obtained for torsion modulus and compliance. The surface finish of the blends was very smooth and homogeneous. The morphological investigations showed that the key to property enhancements lay in improved interfacial adhesion and a less phase-separated and more uniform dispersed-phase (LCP) distribution. The properties of the compatibilized PP/Vectra B blends compared favourably with those of the glass-filled PP.

Compatibilization of thermoplastic composites based on blends of polypropylene with two liquid crystalline polymers

Abstract In an earlier paper we considered the effect of polypropylene modified with maleic anhydride (MAGPP) on the properties of in situ composites based on polypropylene (PP) and a thermotropic copolyesteramide (Vectra B). In this paper we consider the effect of MAGPP on the properties of in situ composites based on PP and two different thermotropic copolyesters. It was observed that the MAGPP was nearly as effective in improving the properties for the copolyester systems as it was for the copolyesteramide system. The addition of MAGPP leads to finer and better dispersed fibrils of liquid crystalline polymer relative to the blends with no MAGPP added. These results were somewhat unexpected based on earlier reports which suggested that compatibility would be detrimental to the formation of fibrils.

Additional observations on the surface melt fracture behavior of linear low-density polyethylene

Abstract An experimental investigation of the surface melt fracture behavior of linear low-density polyethylene (LLDPE) is reported. The results of capillary experiments with different L/D ratios and slit-die experiments with various regions coated with fluoro-elastomer are presented; the role of the entry, land and exit regions in the surface melt fracture behavior is examined. In agreement with the mechanism proposed by Kuritz the results seem to suggest that the surface melt fracture arises at the die with the aid of pre-stressing conditions upstream of the exit.

In situ reinforcement of polypropylene with liquid-crystalline polymers: effect of maleic anhydride-grafted polypropylene

Abstract The effect of maleic anhydride-grafted polypropylene (MAP) on the mechanical properties of three polypropylene/liquid-crystalline polymer (PP/LCP) blends was investigated. Three LCPs were used in the study. One LCP was a poly(ester amide) and the other two LCPs were copolyesters. The tensile properties of the PP/liquid-crystal (LC) poly(ester amide) blend were affected by the addiction of MAP to a greater extent than were the tensile properties of the two PP/LC copolyester blends. The tensile strength of the former blend increased without reaching a plateau as the level of MAP was increased, whereas the tensile strength of the latter blends showed that a plateau or maximum in the strength existed. The tensile modulus of the PP/LC poly(ester amide) blend showed a 19% increase as the amount of MAP was increased above 5 wt%. Only small changes occurred in the modulus of the PP/LC copolyester blends. The effect of MAP on the mechanical properties of PP/LCP blends indicated that MAP interacted with the amide group preferentially over the ester group. An excess of MAP was also shown to change the morphology from larger elongated structures to smaller structures that can lead to reduced tensile moduli. Measurement of the contact angles of liquids on PP,PP/MAP and the three LCPs indicated that MAP lowered the interfacial tension for all three PP/LCP blends. The reduced interfacial tension and increased adhesion indicated that MAP compatibilized the PP/LCP blends. While in most polymer blends compatibilization leads to finely dispersed drops of the minor components, LCP fibrils were obtained over a large range of compatibilizer concentrations. Finally, isolation of the PP/MAP phase from the LCP phase indicated that MAP did not react with the LCP. Instead, an interaction such as hydrogen bonding was believed to be responsible for the compatibilizing effect of MAP on PP/LCP blends.

Miscibility and mechanical properties of poly(ether imide)/poly(ether ether ketone)/liquid crystalline polymer ternary blends

Abstract This paper is concerned with a novel ternary blend composed of poly(ether imide) (PEI), poly(ether ether ketone) (PEEK) and a liquid crystalline polymer (LCP; HX4000, Du Pont). Different compositions were prepared by extrusion and injection moulding. Dynamic mechanical thermal analysis and the observation of the fracture surfaces, before and after annealing, allowed determination of the cold crystallization temperatures and miscibility behaviour of these systems. PEEK/PEI blends are known from previous studies to be miscible at all compositions. In this case it was observed that the PEEK/HX4000 blend was miscible up to 50 wt% HX4000 but partially miscible above this value. The PEI/HX4000 blends were found to be partially miscible in the whole concentration range. As a result, some ternary blend compositions exhibited only one phase, while others exhibited two phases. The measurement of the tensile properties showed that ternary blends with high modulus can be obtained at high LCP loadings, while compositions with high ultimate tensile strength can be obtained with high loadings of PEI or PEEK.

Rheological Properties of Copolyester Liquid Crystalline Melts. I. Capillary Rheometry

Rheological properties of two copolyesters which exhibit liquid crystalline behavior in the melt state were determined using an Instron capillary rheometer. The two polymer melts with nematic liquid crystalline order consisted of copolymers of polyethylene terephthalate (PET) and 60 and 80 mole % of p‐hydroxybenzoic acid (PHB). Data was also obtained on PET which was used as a control. Measurements included the temperature and shear rate dependence of viscosity, entrance pressure losses (ΔPent), and die swell (Dj/D). The viscosity of the liquid crystalline melts are as much as two orders of magnitude less than those of PET at temperatures at which they are ordinarily processed. The extrudate actually contracts at the lower end of processing temperatures but does increase to values greater than one with increasing temperature for the liquid crystalline polymers. Although the die swell data indicate that there is negligible elastic recovery at the capillary exit, values of the ratio of ΔPent to wall shear s...

Processing and Associated Properties of In Situ Composites Based on Thermotropic Liquid Crystalline Polymers and Thermoplastics

Abstract The concept of generating liquid crystalline polymer (LCP) reinforcing fibrils on an in situ basis has a number of potentially attractive advantages over the use of the more commonly used glass-reinforced composites. The potential advantages include a wider range of processing options, improved surface appearance, recyclability, and lower processing energy requirements. Many of these advantages have been discussed in previous reviews such as those written by Dutta et al. [1] and La Mantia and Valenza [2]. However, to successfully compete with glass-reinforced thermoplastics, in situ composites based on thermoplastics reinforced with thermotropic LCPs (TLCPs) should at least exhibit similar properties to injection-molded glass-reinforced thermoplastics. One purpose of this review is to address the advantages and disadvantages of in situ composites, and the other is to provide some standards for comparison against more commonly used composites.

Studies on the Transient Shear Flow Behavior of Liquid Crystalline Polymers

An understanding of the flow behavior of liquid crystalline polymers (LCPs) is of immense practical value because of the potential to form high modulus materials from these polymers. These fluids exhibit a high degree of structure even in the quiescent state, as evidenced by their ability to transmit polarized light. In an effort to understand how the structure changes during flow, we have carried out a study on the transient shear flow properties of two thermotropic copolyesters of 60‐ and 80‐mole % para‐hydroxybenzoic acid (PHB) and polyethyleneterephthalate (PET) and a lyotropic system of poly‐p‐phenyleneterephthalamide (PPT) in 100% sulfuric acid. In one of the first theories concerned with the flow behavior of liquid crystalline fluids, which was proposed by Ericksen, the transient flow properties are all predicted to be due to changes in orientation of a director which describes the orientation of packets of rod‐like molecules. Stress growth, interrupted stress growth, and stress relaxation experim...