Modification of rheological properties of a thermotropic liquid crystalline polymer by melt-state reactive processing (original) (raw)
Related papers
Improvement of the processability of advanced polymers
Journal of Applied Polymer Science, 1990
The possibility of improving processability of engineering polymers was studied in this work. Low percentages of a thermotropic liquid crystalline polymer supplied by Bayer Co. were added to the polyetherimide Ultem. The thermal and morphological analysis revealed a two phase system. Rheological measurements in shear flow were carried out in the temperature range of the existence of the mesophase. A significant drop of -30% in the Ultem viscosity occurs even with 5% w/w TLCP. The influence of the addition of the thermotropic polymer on the mechanical properties of Ultem was also investigated. Moreover, the morphological analysis of the unfilled polymers and of the blends was performed and related to their rheological and mechanical behavior.
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.
2001
The phase behavior, rheology, and morphology of binary blends of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) were investigated. Specifically, binary blends consisting of poly[(phenylsulfonyl)-p-phenylene alkylene-bis(4-oxybenzoate)]s (PSHQn) having five methylene groups (PSHQ5) and 11 methylene groups (PSHQ11) were prepared by solvent casting. It was found from differential scanning calorimetry (DSC) that PSHQ5, PSHQ11, and their blends are glassy thermotropic polymers, exhibiting only glass-to-nematic and nematic-to-isotropic (N-I) transitions. Approximate phase diagrams were constructed for PSHQ5/PSHQ11 blends based on DSC data. Using a cone-and-plate rheometer, transient shear flow experiments were conducted for the PSHQ5/PSHQ11 blends (i) at 160°C in the biphasic region where PSHQ11 forms an isotropic phase and PSHQ5 forms a nematic phase and (ii) at 130°C in the nematic region where both PSHQ5 and PSHQ11 formed the nematic phase. It was found for such PSHQ5/PSHQ11 blends that the steady-state shear viscosity at 130°C (in the nematic region) is lower than that at 160°C (in the biphasic region). However, the first normal stress difference at 130°C exhibits a very large overshoot followed by an oscillatory decay until reaching a steady state, while it is virtually zero at 160°C. The time evolution of morphology for the PSHQ5/PSHQ11 blends, upon shear startup and also upon cessation of shear flow, was investigated using a specially designed optical microrheometer equipped with a polarizing optical microscope. Contrasting observations are reported for the case of nematic PSHQ5 in isotropic PSHQ11 when compared to the nematic PSHQ5/ nematic PSHQ11 blend. Shearing of a nematic/nematic blend induces a much larger birefringence change than does shearing a nematic/isotropic blend, and a shear-induced isotropic-to-nematic transition is observed from a mixture of isotropic phases containing two TLCPs.
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.
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
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
Polymer Engineering & Science, 2009
Four random, differently ended (À ÀCl, À ÀNH 2 , À ÀOH, and À ÀCOO 2 ), polyethersulfone/polyetherethersulfone (PES/ PEES) copolymers were studied to investigate the influence of chain ends on thermal and rheological behaviors. The number average molar mass (M n % 9500 gÁmol 21 ) and the PES/PEES ratio (40/60) of all copolymers investigated were checked by 1 H NMR spectra. Thermal degradations were carried out in the scanning mode and initial decomposition temperatures (T i ) and activation energy values of degradation (E a ) were obtained. Glass transition temperature (T g ) was determined by differential scanning calorimetry and complex viscosity (g*) by rheological measurements in isothermal heating conditions (T ¼ 2708C). All parameters determined were largely affected by copolymer chain ends and decreased according to the same order, À ÀOH > À ÀNH 2 > À ÀCl > À ÀCOO 2 . The results were discussed and interpreted.
Rheology and Processing of Polymers
Polymers
I am so glad to share with you our Special Issue entitled ‘Rheology and Processing of Polymers’, which covers the latest developments in the field of rheology and polymer processing, highlighting cutting-edge research focusing on the processing of advanced polymers and their composites [...]
Polymer, 2014
The conversion behavior of 2MEP4FS, a polymer with thermally reversible DielseAlder cross-links, is modeled. A processing method is developed to create small, homogeneous prepolymer samples. The glass transition temperature of the prepolymer is estimated using temperature modulated differential scanning calorimetry and equated with the conversion of the polymer. Comparing the measured energy with the literature and computational estimates, the fully cured polymer appears to have a large portion of its moieties unreacted. An autocatalytic model is considered to approximate the reaction rate of 2MEP4FS as a function of conversion and temperature. Outside of the fitted temperature range, the model underpredicts the reaction rates at room temperature and 100 C. Manual mixing of the monomers is determined to be inadequate to obtain a maximum level of cross-linking. Viscosity measurements made at room temperature and elevated temperatures are correlated with the conversion of the prepolymer.
Rheologica Acta, 2000
An immiscible blend of a thermotropic liquid crystalline polymer (TLCP) with a thermoplastic resulted in a ®berreinforced thermoplastic called an in situ composite, the term coined by . Such composites arise due to the formation of a ®brillar TLCP phase during extensional melt¯ow. They are interesting because they have several outstanding features. In addition to the improvement on physical and dimensional stability of the matrix, the incorporation of TLCP also enhances the ease of processing, resulting from the reduced overall melt viscosity of the system. Processing conditions including temperature and the shear and elongational forces strongly aect the molecular orientation of the TLCP phase as well as the ®ber aspect ratio which determines the ®nal physical properties of the composite.