Orientation development liquid crystalline polymers

In Chapter 3, Mitchell and Windle give a critical account of the assessment of orientation in liquid crystalline polymers, a rapidly developing new field in which they have played a leading part. 

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. 

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. 

Larson, R.G. Mead, D.W. Development of orientation and texture during shearing of liquid-crystalline polymers. Liq. Cryst. 1992,12, 751-768. 

The following examples describe two different approaches to developing liquid crystalline elastomers. The hrst two examples, developed at Reading, utilize the acrylate-based polymer described in Protocol 4 the hnal technique, invented in Freiburg utilizes siloxane-based polymers. This latter process is particularly useful where high levels of orientation are required (and of course room temperature liquid crystalline phases). 

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

Assume that the degree of the ordering of liquid crystalline polymers is high and the orientational distribution function is simply Gaussian, Odijk (1986) developed the analytical formulae for elastic constants... 

Recently, we have developed a real time pulsed NMR(RTPNMR) system(2) controlled by a microcomputer as shown in Figure 3 to study dynamics of structure formation in polymer systems. It has been successfully applied to study crystallization process (3), gelation process(4) and so on. In this paper, application of RTPNMR for characterization of curing process of epoxy resins, polymerization crosslinking process of diacetylenes, and orientation process of liquid crystalline polymers(LCPs) under magnetic field are described. Papers related to the detailed analysis of these data will appear in the near future. EXPERIMENTAL... 

What we have attempted to do here is to present rheological tests for identifying the development and relaxation of orientation and structure in liquid crystalline polymers. Because these fluids are typically quite turbid, it is difficult to use rheo-optical techniques. The interpretation of the rheological tests must then come partly from studies on quenched solid specimens. In summary, it is believed that a detailed set of rheological tests based on the transient response of LCP can be used to evaluate various liquid crystalline polymers and identify processing conditions which will lead to the optimum physical properties. 

The second or alternative mechanism is used by the Maier-Saupe mean field theory in which the stability of the nematic phase is derived from an anisotropic potential. Picken has developed a theory for the nematic phase formation of liquid crystalline polymers, which is based on the Maier-Saupe mean field theory [37, 38], A molecule in a nematic domain, with its axis at an angle

average orientation axis of the domain, is assumed to feel the influence of the surrounding medium only in terms of an anisotropy potential... 

Liquid crystals (or plastic crystals as they are sometimes called) are low molecular mass materials that show molecular alignment in one direction but not three-dimensional crystalline order. During the last 20 years, liquid crystalline polymers have been developed where the polymer chains are so straight and rigid that small regions of almost uniform orientation (domains) separated by distinet boundaries are produeed. In the ease where these domains occur in solution, polymers are... 

Lin, D. Y. and D. C. Martin (2006). Orientation development in electrospun liquid-crystalline polymer nanofibers. In Polymeric Nanofibers. ACS Symposium Series 918. Edited by D. H. Reneker and H. Fong. Oxford University Press (USA). 

Doi developed a constitutive theory for liquid crystalline polymers that takes into account the rotational diffusion of the large rodUke molecules and reduces to the LesUe-Ericksen theory for slow deformation rates, and this has been generalized by Marrucci and Greco and others to incorporate a nematic potential that accounts for gradients in the orientation tensor. The theory has a structure that is... 

A third, recently developed group of polymers, is the liquid-crystalline polymers showing orientational order but not positional order. They are thus intermediates between the amorphous and the crystalline polymers. A detailed discussion of liquid crystalline polymers is given in Chapter 6. 

A relatively new class of polymers, the liquid-crystalline polymers, exhibits orientational order, i.e. alignment of molecules along a common director in the molten state. Liquid-crystalline polymers are used, after solidification, as strong and stiff engineering plastics and fibres. Functional liquid-crystalline polymers with unique electrical and optical properties are currently under development. The fundamental physical and rheological aspects of liquid-crystalline polymers are the third subject of this chapter (section 6.5). 

The first commercially important liquid-crystalline polymer was Kevlar. Kwolek wrote in 1965 about anisotropic solutions of wholly aromatic polyamides in alkylamide and alkylurea solvents. This development led to Kevlar, i.e. ultra-oriented fibres of poly(p-phenylene terephthalamide) (Fig. 6.17). The solution of the polymer in concentrated sulphuric acid is nematic (the term nematic will be explained in section 6.5.3) and fibres of high modulus and high strength can be spun from the solution. 

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