Behaviour Of Liquid Crystalline Polymers Research Articles

Warpage analysis of a micro-molded parts prepared with liquid crystalline polymer based composites

The purpose of this study is to understand the warpage behavior of a micro-injection molded part induced by anisotropy of the material. Microstructural anisotropy was characterized to understand warpage behavior of liquid crystalline polymer based composites. A new definition of the microstructure was introduced along the thickness of a molded part, i.e., “skin–shear–core” layer was proposed depending on the orientation of LCP molecules and fillers. The microstructural anisotropy of LCP composites was verified by using WAXD, CT, and SEM analyses. The anisotropic elastic modulus and coefficient of thermal expansion (CTE) of micro-injection molded parts were investigated by using a new numerical method with the modified Mori–Tanaka (M–T) model. The interaction coefficient, CI, and coefficient of thermal expansion (CTE) were the most important factors determining the warpage of a micro-injection molded part made of an anisotropic material. The control of warpage due to the orientation of glass fibers was investigated by simulation.

Kinetic Structure Simulations of Nematic Polymers in Plane Couette Cells. I: The Algorithm and Benchmarks

The Doi--Hess theory coupled with an anisotropic Marrucci--Greco distortional elasticity potential provides a kinetic, mean field description of the coupling between hydrodynamics, molecular orientation by excluded volume, and elastic distortions of flowing nematic liquid crystalline polymers (LCPs). In this paper we provide the first numerical algorithm and implementation of kinetic-scale models for structure formation in confined, planar Couette cells. The model and algorithm extend kinetic simulations of Larson and Ottinger [Macromolecules, 24 (1991), pp. 6270--6282], Faraoni et al. [J. Rheol., 43 (1999), pp. 829--843], Grosso et al. [Phys. Rev. Lett., 86 (2001), pp. 3184--3187], and Forest et al. [Rheol. Acta, 43 (2004), pp. 17--37; Rheol. Acta, 44 (2004), pp. 80--93] for homogeneous nematic polymers in imposed shear flows. The focus here is one-dimensional flow and LCP structures that form between oppositely moving, parallel plates, a classical model problem that has been studied in detail with continuum models [P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Clarendon Press, Oxford, 1993; G. Marrucci, Macromolecules, 24 (1991), pp. 4176--4182; G. Marrucci and F. Greco, Flow behavior of liquid crystalline polymers, in Advances in Chemical Physics, Vol. 86, Wiley, New York, pp. 331--404] and a variety of mesoscopic, second-moment orientation tensor models [T. Tsuji and A. D. Rey, J. Non-Newt. Fluid Mech., 73 (1997), pp. 127--152; R. Kupfermann, M. Kawaguchi, and M. M. Denn, J. Non-Newt. Fluid Mech., 91 (2000), pp. 255--271; G. Sgalari, G. L. Leal, and J. Feng, J. Non-Newt. Fluid Mech., 102 (2002), pp. 361--382; D. Grecov and A. D. Rey, Phys. Rev. E, 68 (2003), 061704; M. G. Forest et al., J. Rheol., 48 (2004), pp. 175--192; Z. Cui, M. G. Forest, and Q. Wang, On weak plane Couette and Poiseuille flows of nematic polymers, SIAM J. Appl. Math., submitted]. The model consists of a Smoluchowski equation for the space-time evolution of the orientational probability distribution function, coupled with a momentum flow balance equation, a constitutive equation for the extra stress, and the continuity equation. The Smoluchowski equation is first reduced to a finite set of partial differential equations in time and space for spherical harmonic amplitudes. Then we discretize the spatial variables (by the method of lines) using high-order finite differences, which reduces the full system to a large set of ordinary differential equations. Adaptive grid generation techniques are implemented. To provide an accurate and stable time integration, we employ the newly developed spectral deferred correction algorithm. We close with an application of the kinetic theory and numerical code to explore steady state flow-molecular structures in slow planar Couette experiments (low Deborah number) and low Ericksen number, where distortional elasticity dominates short-range excluded volume effects.

Transient shear flow properties of carbon fiber-filled liquid crystalline polymer

Unsteady flow of filled polymer systems can cause some difficulties in their processing, such as injection or extrusion molding, in which flow in the molten state is involved. In order to determine the optimum conditions and thus improve the performance of the end products, it is necessary to understand the transient/unsteady flow behavior of these systems. In a previous work, we found remarkable non-Newtonian properties and shear thickening flow behavior of liquid crystalline polymers (LCP) and their filled systems. This article deals with the dependence of transient flow properties, including shear thickening, of pure and carbon fiber-filled LCP upon the shear rate and shear strain history; the effect of the filler content is also discussed. The results indicate that the abnormal flow behavior of the tested materials may be caused by gradual disintegration of the domain structure formed in LCP (materials exhibit apparent yield stress) and then continual orientation of molecules under shear flow. The shear thickening behavior of the materials seems to disappear with increasing fiber content. This behavior suggests that it is necessary to measure the flow properties under a sufficient ramp point delay time and equilibration time in order to obtain reliable data under stable conditions. POLYM. COMPOS., 26:470–476, 2005. © 2005 Society of Plastics Engineers.

On the molecular orientation and viscoelastic behaviour of liquid crystalline polymers: the influence of macromolecular architecture

The influence of macromolecular architecture on the flow‐induced orientation of main‐chain thermotropic liquid crystalline polymers (LCPs) is investigated using in situ wide‐angle X‐ray scattering. In order to get more insight into the interrelationship between microscopic and rheological behaviour, the viscoelastic properties of the nematic melts were also studied. The LCPs studied are wholly aromatics , composed only of mesogenic units, and semiflexibles , which consist of mesogenic units separated by alkyl spacers. Rheo‐X‐ray scattering evidenced the detrimental influence of the flexible spacers on the orientation process. The wholly aromatic LCPs readily orient along the flow direction, even under modest shear flows ( γ = 0.1 s‐1). The semiflexible LCPs, however, display lower levels of orientation than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterization shows that, like conventional flexible‐chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measurements, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behaviour is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associated with a molecular network formed by the less‐than‐rigid molecular chains. Furthermore, the steady shear viscosity is always smaller than the dynamic viscosity, i.e. the Cox‐Merz rule does not hold. This is due to the fact that steady shear induces molecular orientation, whereas oscillatory shear does not. The relaxation of orientation after steady shear showed that the molecular orientation relaxes after hundreds of seconds. The shear stress, however, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.

Light-Controllable Chiral Photochromic Multifunctional Liquid Crystal Polymers

ABSTRACTA new family of light-controllable chiral photochromic multifunctional liquid crystalline (LC) side-chain acrylic polymers consisting of nematogenic, chiral and photochromic monomer units was obtained. The new principle of photoregulation of the helical supramolecular structure and optical properties of the binary and ternary chiral photochromic LC polymers based on the change of helical twisting power of the chiral photochromic monomer units was developed. The features of photooptical behaviour of LC polymers possessing the dual photochromism and the peculiarities of optical data recording based on the photochemical spectral gap burning were presented and discussed. The synthesized polymers are shown to be promising candidates for colour data recording and storage.

Modelling of shear flow in liquid crystalline materials

The flow behaviour of liquid crystalline polymers (LCPs) is quite complex and these materials exhibit varied and complicated textural patterns when subject to a flow field. The complexity arises from two general factors, the first that they are long chained and thus have long relaxation times, and second that they are liquid crystalline, and thus there is co-operative motion of the molecules. In both thermotropic and lyotropic LCPs subject to low shear flows, it is known that defects and disclinations influence the microstructure and rheology, but it is not clear by what mechanisms these distortions shrink or multiply during flow. In this work, a model is developed to examine the behaviour of defects in shear flows. The simulations based on the model show a spectrum of microstrucural development as a function of applied shear rate: reorientation of domains of different alignment associated with disclinations at low shear strains; the multiplication of wall type defects and the orientation of these normal to the shear gradient axis at intermediate shear rates, and the tendency towards disclination annihilation; the generation of a flow-aligned monodomain at higher shear rates.

Syntheses and characterization of poly(ethylene terephthalate) modified withp-acetoxybenzoic acid

Though the structure and properties of a copolyester of 40 mole % of polyethylene terephthalate (PET) and 60 mole % p-hydroxybenzoic acid (PHB) (PET/60PHB) and their blends have been well documented, no work has been reported in an open literature on the systematic investigation of the PET copolymers modified with broad range of p-acetoxybenzoic acid (PABA) composition as yet. In this study, several PETA-x copolyesters having various PABA compositions from 10 to 70 mole % were prepared by the melt reaction of PABA and PET without a catalyst, where x denotes the mole % of PABA. And the modified polyesters obtained were characterized by 1H-NMR spectrophotometry, X-ray diffraction pattern, polarizing microscopy, thermal analysis, and rheometry. The anisotropic phase appeared when x is above 50 mole % of PABA, and especially for the x's of 60 and 70 mole %, the nematic liquid crystalline texture appeared clearly on the whole matrix. As the mole % of PABA increased, melting temperature, heat of fusion, crystalline temperature, degree of crystallinity, and the glass transition temperature of the modified PET were decreased, but the thermal stabilities of those copolyesters were increased. The dependence of melt viscosity on the shear rate for PETA-50 ∼ 70 followed the typical rheological behavior of liquid crystalline polymers. Finally, it was concluded that the PETA-x copolyesters having compositions of higher than 50 mole % of PABA exhibit the behavior of thermotropic liquid crystalline polymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1707–1719, 1999

Corona poling of liquid crystalline polymers with nitroazobenzene side chains

The orientational behavior of liquid crystalline polymers with para-nitro azobenzene as side chains under electric field was studied by UV-visible spectroscopy. The results showed that λmax of the poled polymer films was around 394nm, compared to that of the unpoled films, the absorption decreased due to poling. The orientational parameters increased linearly with the increase of the electric field. The temporal stability of the poled polymer film is good at room temperature. This kind of materials showed promise application as nonlinear optical component in photorefractive polymers

Effect of fillers on the steady state rheological behaviour of liquid crystalline polymers

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 isotropic 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.

Phenomenological transient shear behavior of liquid crystalline polymers after the start‐up of shear flow

AbstractTransient stresses of liquid crystalline polymers appear as damped oscillatory patterns after the start‐up of shear flow. They are examined by using the constitutive equations which were modified to include the idea of an initial domain state in the Larson‐Doi polydomain equation set. In order to be consistent with the phenomenological transient shear stress after the start‐up in the region of low shear rates, the Hinch‐Leal closure approximations were adopted for the domain‐averaged (mesoscopic) fourth‐order tensors of the director field. It is predicted as an important result that the amplitude of the first overshoot in the transient stress increases with an increase in the shear rate imposed at the start‐up, which was found to be largely in agreement with phenomenology. In addition, the effects of parameters such as molecular concentration, initial domain size, and intensities of viscous and elastic contributions on the transient stress under the start‐up process were also evaluated.

Transient rheological behaviour of poly-para-henylenetherephthalamide (PpPTA) in sulphuric acid

Nearly all the available information on the transient flow behaviour of liquid crystalline polymers has been obtained on model systems, especially on solutions of polybenzylglutamate (PBG) and hydroxypropylcellulose (HPC). The assessment of rheological models has been based almost entirely on these model systems. It is not clear how much of the available theoretical and experimental knowledge can be applied to systems of industrial relevance, which have quite different molecular structures. Here, an industrial lyotropic system, poly(p-phenylenetherephthalamide) (PpPTA) in sulphuric acid (TWARON from AKZO), is investigated. Various techniques to study transient behaviour are used, these include measurements of transient shear and normal stresses after sudden changes in shear rate, dynamic moduli and stress relaxation after cessation of flow and elastic recoil. At all shear rates studied the PpPTA solution is shear thinning, and the first normal stress difference remains positive. For the stress transients a strain scaling applies reasonably well as it did in model systems. The moduli increase with time upon cessation of flow, indicating that the molecules become less oriented in the previous flow direction. This particular behaviour is similar to that of HPC. Transients also resemble more closely those of HPC rather than those of PBG. This latter difference might be attributed to the higher flexibility of HPC and PpPTA chains as compared with PBG molecules.

Shear Flow Rheology of Liquid Crystalline Polymers

Abstract The rheological behavior of liquid crystalline polymers in shear flows is reviewed with the aim of pointing out the most important features which characterize tumbling. The latter expression refers to the fact that, in some nematics, the director (i.e., the average molecular orientation) is not stable in shear flows unless some other stabilizing field prevails. Many liquid crystalline polymers seem to be tumbling nematics, so that the molecular orientation achieved in a shear flow may be difficultto control.

Isotropic-nematic interface of liquid-crystalline polymers.

The behavior of liquid crystalline polymers in the interfacial region between the isotropic and nematic phases is investigated based on an inhomogeneous free-energy functional. A mean-field approximation is used for the system of semiflexible polymers obeying the Saito-Takahashi-Yunoki description and interacting via the Onsager-type repulsive interaction. The density distribution of the polymers crossing the interface is computed by using a spherical-harmonics expansion. The calculated interfacial tension is consistent with the results of the scaling argument which is also presented in this paper.

The peculiarity of side-chain liquid crystalline polymers with mesogenic units attached directly in a side-on mode

The peculiarity of a series of side-chain liquid crystalline polymers with mesogenic units attached directly in a side-on mode was studied by using nuclear magnetic resonance, wide-angle X-ray diffraction, polarizing microscopy and molecular simulation techniques. It was found that in special cases molecules of side-chain liquid crystalline polymers may become rather rigid as a whole and take the more extended conformation, like a rod with the main chain approximately parallel to the rod axis. The side-chain mesogens are packed around the main chain with their axes almost perpendicular to the main chain. These side-chain polymers may exhibit behaviour similar to that of main-chain polymers. The effect of interaction between main-chain and side-chain mesogens on the mesophase formation of side-chain polymers and the influence of mesogen location in molecules on the behaviour of liquid crystalline polymers are discussed.

Real Time X-Ray Rheology of Polymers

The ability to prcbe the structure of a polymer during deformation is of considerable technological interest as it lends an insight into the mechanisms involved in polymer processing. The behaviour of Liquid Crystalline Polymers (LCP's) during shear flow has generated much experimental interest by virtue of their unusual behaviour compared to that of simpler polymeric melts. The theory developed by Doi attempts to explain the observed phenomena in terms of considering interactions of rigid-rods Exact solutions to the Doi theory have been compared to experimental procedures using lyotropic LCP systems. Although much of the available data relates to mechanical measurements, the comparisons with quantitative structural data can provide useful insight.

Transient normal stresses in lyotropic liquid crystalline polymers

Extensive experimental information is available about the rheological behaviour of liquid crystalline polymers, especially the stationary-flow behaviour. Only recently models that describe some of the transient-flow properties of the materials under consideration also have been suggested. It turns out that the various features are always more pronounced in the normal stresses than in the shear stresses. Therefore, the time evolution of the normal stresses could be very useful for evaluating existing models and in guiding further modelling. Yet, no systematic data of this nature are available in the literature. Here, data are presented on the time-dependent response of shear stresses and first normal stress differences to stepwise changes in shear rate. Flow reversal, stepwise increase in shear rate and stepwise decrease in shear rate are considered for two lyotropic systems: a solution of polybenzylglutamate and one of hydroxypropylcellulose. Scaling laws for the stress transients are investigated. Depending on the type of transient flow, two types of scaling behaviour are observed for both shear and normal stresses; they are compared with theoretical arguments. For each type of transient experiment, a characteristic interrelation between shear stress and normal stress is obtained which should prove very useful in model evaluation.

Effects of Orientation on the Properties of Liquid Crystalline Polymers

Abstract Mechanical properties and thermal behavior of liquid crystalline polymers with mesogene groups in the main chain were studied as functions of jet draw ratio. It is shown that tensile strength F and Young's modulus E increase with draw ratio and the maximum E depends mainly on the chemical structure of chain and F is strongly affected by molecular packing that may be improved by heat treatment. The differential thermal analysis, thermal mechanical analysis and thermal acoustic analysis were used to find the spinning conditions for achieving maximum orientation and annealing conditions for macromolecular reconstructions without disorientation.

SAXS investigations on chiral liquid crystalline polymers: antiferroelectric and ferrielectric structures?

AbstractThe structure and phase behavior of liquid crystalline polymers (LCPs) having a common chiral side chain mesogen but different main chain structures have been investigated using small‐angle X‐ray scattering (SAXS). While the low molecular weight chiral side chain mesogen by itself exhibits ferroelectricity, the SAXS data of the side chain LCP with a flexible polyacrylate backbone contains a bilayered superstructure peak that is indicative of antiferroelectric order. The combined LCP with a nonpolar main chain mesogen also has a bilayered superstructure, but has a different structural organization in the proposed antiferroelectric phase compared to the side chain LCP. Further changes in the phase behavior and structural organization occur when a polar group is introduced into the main chain mesogen. A ferrielectric phase has been proposed to explain the observation of a trilayered superstructure in the corresponding SAXS data. The influence of the chemical structure and connectivity on the phase behavior and superstructure formation in the chiral LCPs is discussed.

Rheology of liquid-crystalline polymers. Theory and experiments

The rheological behaviour of liquid crystalline polymers differs considerably from that of ordinary polymers as well as of low-molecular-weight liquid crystals. The only theoretical approach which has reached a significant development is based on the rigid rodlike model of the polymer molecule. In spite of the crudity of the model, the theoretical predictions based on it prove able to qualitatively describe the most peculiar features of the observed rheological behaviour. A comparison between the relevant experimental peculiarities and the qualitative predictions of the theory is presented.

Effect of Metal Complexation on the Behavior of Liquid Crystalline Polymer

Abstract Liquid crystalline side-chain polymers (3b-3f) crosslinked by a trans Pt(II) complex were synthesized by a ligand exchange reaction with dichlorobis(benzonitrile)platinum(II) and poly(methylsiloxane) (3a) bearing a p-cyanobiphenyloxy mesogen unit attached to the polymer backbone by a flexible spacer group. The amount (x) of p-cyanobiphenyloxy mesogen coordinating to the Pt(II) ion in polymers 3b, 3c, 3d, 3e, and 3f was 4.9, 9.7, 14.8, 20.2, and 49.6 mol%, respectively. The effect of Pt(II) coordination on the thermotropic liquid crystalline behavior of 3a was examined by DSC, x-ray diffraction, and polarizing microphotography. Polymers 3a-3d exhibited a nematic mesophase between Tg and Tc. On the other hand, 3e-3f, which have a large amount of Pt(II)-coordinating mesogen, did not show a definite nematic mesophase, but exhibited an ambiguous one. The introduction of Pt(II) ion into 3a resulted in a crosslinked structure and consequently prevented the formation of the ordered nematic mesophase.