Transient elongational flow behavior of thermotropic liquid crystalline polymers (original) (raw)
Related papers
Polymer Engineering and Science, 1992
The shear and elongational viscosities of a thermotropic liquid crystalline polymer (LCP), polycarbonate (PC), and their 2O%LCP/8O%PC blend, were studied using a capillary rheometer. The investigation focused on experimental studies using two sets of capillaries. The first set comprised capillaries having a converging entrance followed by a cylindrical section. The second set, "zero length" set, included capillaries having only the converging section. In the two sets various entrance angles were used. Experimental results have shown that shear viscosities and entrance pressures are practically independent of the entrance angles. The entrance pressure drop was small in the case of PC and reached 50% of the total pressure drop for LCP. The elongational viscosities of the LCP were found to be higher than those of the PC in the elongational-rate range studied, while shear viscosities of the LCP were higher in the lower shear rate region and lower in the higher shear rate region compared to those of PC. This was attributed to the orientability of LCP in elongational and shear flows.
Melt flow behaviour of liquid crystalline polymer
A. V. Shenoy, D. R. Saini, 1986
The method proposed earlier to estimate complete flow curves depicting the variation of melt viscosity over industrially relevant shear rate and temperature through the melt flow index has been extended to liquid crystalline polymer melts. The present work considers the study of the thermotropic system of the copolymer of poly(ethylene terephthalate) and p-hydorxybenzoic acid with different ratio of the components. A master curve is shown to exist for the system when modified viscosity function is plotted against modified shear rate. A new rheological model has been suggested to fit the master curve adequately as the curve is different from the conventional form.
The viscoelasticity of thermotropic liquid crystalline polymers: effects of the chemical composition
Rheologica Acta, 2006
In this work, the rheological properties of the wholly aromatic random copolyester HBA/HNA (60/40), the commercial copolyesteramide Vectra B950, and the semiflexible commercial copolyester Rodrun 3000, PET/HBA 40/60, were investigated. All the thermotropic liquid crystalline polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes. The strain sweep experiments have clearly indicated that the onset for non linearity significantly decreases
Thermotropic main-chain liquid crystalline polymers typically have very low melt viscosity with strong temperature dependence compared to other common thermoplastics. While this is beneficial in some processing applications, such as injection molding, it presents challenges for others, such as coextrusion. In this study, the rheological properties of a thermotropic main-chain liquid crystalline polymer (Vectra A950) were enhanced by melt-state reactive processing with triphenyl phosphite (TPP), which can react with up to three polymer chain-ends through their chain-end functionalities. The influence of processing time and TPP content on the shear viscosity and other important material properties were investigated. Optimal conditions, which increased the shear viscosity by nearly a factor of 20 over the neat polymer, were found to be 4 wt% TPP and 30 min of reaction time at 290 C. Further results from differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy confirmed that coupling with TPP did not affect the microstructure, melting/crystallization behavior or liquid crystallinity. The stability of TPPmodified samples was also studied at 80 C in air and following melt reprocessing at 290e300 C under N 2 or air. Samples were stable (as measured by shear viscosity) for more than one month at 80 C in air or when reprocessed in N 2 at 290 C for up to 10 min. However, when reprocessed at 300 C in air, the viscosity enhancement was partially reversed due to scission of PeO bonds that were formed during the initial reaction between the polymer chain-ends and TPP.
2000
Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (T NI). It was observed that, during both shear startup and flow reversal, the first normal stress difference N1 + (γ , t) exhibits a large overshoot followed by an oscillatory decay, while shear stress σ + (γ , t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1 + (γ , t) and σ + (γ , t), and the longer the persistence of oscillations in N1 + (γ , t). Similarity was found between the ratio N1 + (γ , t)/ σ + (γ , t) and flow birefringence ∆n + (γ , t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio σ + (t,γ)/σ scales with γ t but the ratio N1 + (γ , t) /N1 does not, where σ denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.
Rheologica Acta, 2005
The properties of liquid crystalline polymers (LCPs) are strongly influenced by flow-induced changes in the degree of molecular orientation during processing . Thus, it is not surprising that much research has been devoted to unraveling the interrelationships between rheology, defect texture, and molecular orientation states in LCPs. These efforts have steadily yielded improved understanding, particularly for lyotropic LCP solutions . Progress in thermotropic LCP melts has come more slowly, but recent work on ''model'' mainchain thermotropic LCPs has led to new insights into the factors which control rheology and orientation development under flow in these materials . These model materials incorporate flexible spacers in the backbone, which leads to lower transition temperatures, accessible isotropic phases and improved thermal stability. These attributes facilitate detailed fundamental studies that have, to date, been impossible to conduct on commercial main-chain thermotropic LCPs, Unfortunately, it seems that the backbone flexibility which renders these materials attractive for research purposes strongly affects their dynamic behavior . As a result, scientific understanding still falls short for the one class of LCP materials where it
Crystallinity and linear rheological properties of polymers
2007
The crystallization of a polymer melt, taking place during transformation processes, has a great impact on the process itself, mainly because it causes a large increase in the viscosity (hardening). Knowledge of the hardening kinetics is important for modeling and controlling the transformation processes. In this work, first an overview is given of the experimental and modeling work on the hardening of crystallizing polymers. Next, we present isothermal crystallization experiments using differential scanning calorimetry (DSC) and rotational rheometry to measure the dynamic viscosity. The evolution of the relative crystallinity and normalized complex viscosity are correlated by a novel technique which allows simultaneous analysis of several runs, even if they are not carried out at same temperatures; the main requirement with the traditional technique. The technique, described in detail in this paper, provides an experimental relationship between the crystallinity and the hardening, i. e. the increase in the viscosity. Moreover, by measuring the dynamic viscosity at different frequencies, surprisingly, a master curve is obtained which combines the effects of shear rate, temperature and the level of crystallinity.
Orientation dynamics in commercial thermotropic liquid crystalline polymers in transient shear flows
Rheologica Acta, 2007
In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a highspeed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.
Flow���induced structure in a thermotropic liquid crystalline polymer as studied by SANS
1998
Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers.
Effect of fillers on the steady state rheological behaviour of liquid crystalline polymers
Rheologica Acta, 1998
The effect of fillers on the flow curves of polymeric liquid crystals is investigated. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose (HPC) in water are used. By reducing the HPC concentration an isotropic solution can be prepared. It serves as a reference to isolate the effect of the isotropic/anisotropic structure of the suspending medium on the rheological behaviour. Suspensions in the isotropic solution behave as expected for filled viscoelastic matrices in general. In the anisotropic medium the shear rate rather than the shear stress seems to govern the changes in the relative viscosity. This behaviour is clearly different from isotro-pic viscoelastic media. The most dramatic effect however is that even small amounts of particles eliminate or drastically shift the region of negative normal stress differences. As far as the structure is concerned, microscopic observations show that particles align in anisotropic as well as in isotropic media. At rest or at relatively low shear stresses the liquid crystalline structure is, in the present case, hardly affected by the presence of the particles. If anything, it becomes more homogeneous.