Viscoelasticity liquid crystalline polymers

On a global scale, the linear viscoelastic behavior of the polymer chains in the nanocomposites, as detected by conventional rheometry, is dramatically altered when the chains are tethered to the surface of the silicate or are in close proximity to the silicate layers as in intercalated nanocomposites. Some of these systems show close analogies to other intrinsically anisotropic materials such as block copolymers and smectic liquid crystalline polymers and provide model systems to understand the dynamics of polymer brushes. Finally, the polymer melt-brushes exhibit intriguing non-linear viscoelastic behavior, which shows strainhardening with a characteric critical strain amplitude that is only a function of the interlayer distance. These results provide complementary information to that obtained for solution brushes using the SFA, and are attributed to chain stretching associated with the space-filling requirements of a melt brush. 

Finally, there are complex fluids that are intermediate between solid and liquid in more than one of the ways listed above. Liquid crystalline polymers (LCPs) are both viscoelastic and liquid crystalline. Ordered block copolymers are viscoelastic and anisotropic. Glassy polymers possess long viscoelastic time scales both because they are glassy and because they are polymeric. Filled polymer melts possess the properties of both polymer melts and suspensions. 

E. Shiva Kumar, C. Das, K. Banik, and G. Mennig. Viscoelastic properties of in situ composite based on ethylene acrylic elastomer (AEM) and liquid crystalline polymer (LCP) blend. Compos. Sci. Tech., 67(6) 1202-1209, May 2007. 

Shivakumar E, Das C, Segal E, Narkis M. Viscoelastic properties of ternary in situ elastomer composites based on fluorocarbon, acrylic elastomers and thermotropic liquid crystalline polymer blends. Polymer 2(X)5 46(10) 3363-71. 

Liquid crystalline polymers (LCPs) exhibit ordered molecular arrangements in the melt state. The polymer molecules of LCPs are generally semi-rigid or contain rigid units. The rigid units are either along the backbone or in the side branches. LCPs are viscoelastic and anisotropic. The interested reader can learn details about LCPs from Feng and Leal (1997), Larson (1999), Fan et al. (2003), and Klein et al. (2008). 

Pang JN, Owens RG, Tacher L, Parriaux A (2006) A numerical study of the SPH method for simulating transient viscoelastic free surface flow. J Non-Newtonian Fluid Mech 139 68-84 Feng J, Leal LG (1997) Simulating complex flows of liquid crystalline polymer using the Doi theory. J Rheol 41 1317—1335... 

Kietzmann C, Van der Walt JP, Morsi YS (1998) A free-front tracking algorithm for a control-volume-based Hele-Shaw method. Int J Numer Methods Eng 41 253-269 Kim SH, Kim CH, Oh H, Choi CH, Kim BY, Youn JR (2007) Residual stresses and viscoelastic deformation of an injection molded automotive part. Korea-Australia Rheol J 19 183-190 Klein DH, Leal LG, Garcfa-Cervera CJ, Ceniceros HD (2008) Three-dimensional shear-driven dynamics of polydomain textures and disclination loops in liquid crystalline polymers. J Rheol 52 837-863... 

New mathematical techniques [22] revealed the structure of the theory and were helpful in several derivations to present the theory in a simple form. The assumption of small transient (elastic) strains and transient relative rotations, employed in the theory, seems to be appropriate for most LCPs, which usually display a small macromolecular flexibility. This assumption has been used in Ref [23] to simplify the theory to symmetric type of anisotropic fluid mechanical constitutive equations for describing the molecular elasticity effects in flows of LCPs. Along with viscoelastic and nematic kinematics, the theory nontrivially combines the de Gennes general form of weakly elastic thermodynamic potential and LEP dissipative type of constitutive equations for viscous nematic liquids, while ignoring inertia effects and the Frank elasticity in liquid crystalline polymers. It should be mentioned that this theory is suitable only for monodomain molecular nematics. Nevertheless, effects of Frank (orientation) elasticity could also be included in the viscoelastic nematody-namic theory to describe the multidomain effects in flows of LCPs near equilibrium. 

Marrucci G, Maffettone PL (1993) Liquid crystalline polymers. Pergamon, New York Mead DW (1994) Determination of molecular weight distributions of linear flexible polymers from linear viscoelastic material functions. J Rheol 38 1797-1827 Moldenaers P, Yanase H, Mewis J (1990) Effect of shear history on the theological behavior of lyotropic liquid crystals. Liq Cryst Polym 24(1990) 370-380 Muir MC, Porter RS (1989) Processing rheology of liquid taystal polymers a review. Mol Cryst Liq Cryst 169 83-95... 

Singh S (2000) Phase transitions in liquid crystals. Phys Rep 324 107-269 Susanne EM, Gleissle W, Mckinley GH, Buggisch H (2002) The normal stress behaviour of suspensions with viscoelastic matrix fluids. Rheol Acta 41 61-67 Tai-Shung C (1986) The recent developments of thermotropic liquid crystalline polymers. Polym EngSci26(13) 901-919... 

Non-covalent interactions can result in the formation of supramolecular polymers even from small organic molecules. Typically multiple, strong directional interactions are involved and interesting materials with liquid crystalline, viscoelastic or gel-type behaviour are observed. 

When drawn into fibers from the melt, these polymers are observed to have two preferred orientations, with the director of one mesophase parallel to the flow direction and that of the other mesophase perpendicular to the flow direction. Also presented are the first linear viscoelastic data of polymers which form liquid crystalline glasses. 

The rheology of dimolybdenum and dicopper octanoates was studied in their liquid-crystalline state and was found to be similar to that of conventional viscoelastic polymers such as polyethylene or polypropylene. This strongly supports the observation that these metallomesogens form polymeric chains in their columnar mesophases, and thus can be processed in a way similar to that of conventional polymers to form films and fibers. Note, however, that the chains are believed to be dynamic, constantly being formed and re-formed due to the weak nature of the axial, intermolecular M "0 interactions. 

Thermoplasts are linear or weakly branched polymers. Their application temperature lies below the melting temperature in the case of crystalline polymers and below the glass transition temperature in the case of amorphous polymers. They are converted to an easily deformable plastic state on heating above these characteristic temperatures. This plastic state can be termed liquid with respect to the molecular order, or viscoelastic with respect to the rheological behavior. On cooling below the characteristic temperatures,... 

Materials with molecular networks, such as cross-linked elastomers and crystalline polymers, do not flow and so are classified as viscoelastic solids. Shear stresses do not decay to zero with time, ie, equilibrium stresses can be supported. Above Tg, such amorphous materials are still classified as solids, but most of their physical properties such as thermal expansivity, thermal conductivity, and heat capacity are liquid-like. 

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