Liquid-crystalline polymers under flow

X>ray Scattering Measurements of Molecular Orientation in Thennotropic Liquid Crystalline Polymers under Flow... 

These data illustrate how in situ x-ray scattering allows detailed, quantitative measurements of the orientation state in thermotropic liquid crystalline polymers under flow. Unlike model lyotropic LCPs, the model thermotropic PSHQ-6,12 and... 

A rep < 1, Des < 1, the nucleation dynamics is stochastic in nature as a critical fluctuation in one, or more, order parameters is required for the development of a nucleus. For DeYep > 1, Des < 1 the chains become more uniformly oriented in the flow direction but the conformation remains unaffected. Hence a thermally activated fluctuation in the conformation can be sufficient for the development of a nucleus. For a number of polymers, for example PET and PEEK, the Kuhn length is larger than the distance between two entanglements. For this class of polymers, the nucleation dynamics is very similar to the phase transition observed in liquid crystalline polymers under quiescent [8], and flow conditions [21]. In fast flows, Derep > 1, Des > 1, A > A (T), one reaches the condition where the chains are fully oriented and the chain conformation becomes similar to that of the crystalline state. Critical fluctuations in the orientation and conformation of the chain are therefore no longer needed, as these requirements are fulfilled, in a more deterministic manner, by the applied flow field. Hence, an increase of the parameters Deiep, Des and A results into a shift of the nucleation dynamics from a stochastic to a more deterministic process, resulting into an increase of the nucleation rate. 

There are now numerous compositions of liquid crystalline polymers under consideration as fiber spinning and injection molding materials. However, the problems involved in processing these systems are similar. In particular, how can one process these polymers to yield desirable isotropic properties or at least have biaxial orientation how can one achieve the optimum properties from a given composition and how does the chemical composition and structure affect the properties In flexible chain systems one must quench in orientation in a time scale which is faster than the relaxation process of the molecules. Typically there is a distribution of relaxation times in which the longest relaxation time is a matter of a few seconds. This longest relaxation time also governs a number of other flow characteristics. 

Liquid Crystalline Polymers. One class of polymers that requires some special attention from a structural standpoint is liquid crystalline polymers, or LCPs. Liquid crystalline polymers are nonisotropic materials that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. Because they are neither completely liquids nor solids, LCPs are called mesophase (intermediate phase) materials. These mesophase materials have liquid-like properties, so that they can flow but under certain conditions, they also have long-range order and crystal structures. Because they are liquid-like, LCPs have a translational degree of freedom that most solid crystals we have described so far do not have. That is, crystals have three-dimensional order, whereas LCPs have only one- or two-dimensional order. Nevertheless, they are called crystals, and we shall treat them as such in this section. 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

New approaches and techniques have been proposed to characterize peculiar aspects of L.C. phases or amphiphilic-based systems. The director reorientation of a side-chain liquid crystalline polymer was observed under extensional flow using a 4-roll mill placed in the magnet of a NMR spectrometer. A steric obstruction model was proposed to predict charge-induced molecular... 

It is well known that many parameteis can affect the flow of liquid crystalline polymers such as polymer rigidity, polymer-solvent interactions, and polymer concentration. The results presented in diis article demonstrate that there exist other, less obvious, parameters which sdso can dramatically affect the alignment of LCP under shear flow. The current understanding of the reported results suggest that the defect structure of an LCP solution is one such parameter. 

A difference in the defect structure of PBLG in BA versus PBLG in m-cresol may prove to be the underlying cause of this alignment behavior. Regardless, the results exemplify the need for a broader choice of model liquid crystalline polymer solutions when examining the flow-induced structure in liquid crystalline polymer solutions. Moreover, a more complete understanding of the important parameters that affect the flow of LCP solutions is needed so that a more universal theory can be developed which can predict flow behavior of non-model LCP solutions. 

Liquid crystalline polymers (LCP) have excellent mechanical properties in addition to dimensional and chemical stability. These materials form in-situ composites during processing under elongational flow and are starting to replace traditional fiber reinforced systems [1, 2]. Combined with their ease of processing, LCPs are ideal for applications in aerospace, automobile, marine and other markets requiring high performance composites [2, 3]. 

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