Mechanical Orientation Behavior

When LCEs are synthesized in the absence of external fields, so-called polydomain LC elastomers are obtained, which show macroscopically isotropic properties similar to polycrystalline materials. This resembles, e.g., bulk material of a low molar mass liquid crystal, where thermal fluctuations prevent a uniform director orientation over the whole sample. For nematic elastomers the overall isotropic behavior also indicates an overall isotropic conformation of the polymer chains, which is the consequence of the maximization of the chain entropy. 

However, it is well known that a mechanical deformation of a conventional, isotropic polymer network causes anisotropy. Under deformation the chain segments become oriented according to the symmetry of the external field and the state of order of the network can be characterized by an order parameter similar to that of nematic liquid crystals. Very early mechanical experiments on nematic polydomain elastomers actually demonstrate that a uniaxial deformation of a nematic elastomer converts the polydomain structure into a macroscopically xmi-formly ordered monodomain network [44]. This is shown in Fig. 2, where the opaque polydomain becomes optically transparent and converts into a monodomain 

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In this chapter we outline the basic aspects to be considered when working with liquid crystal elastomers (LCEs), including techniques for their synthesis and characterization. For readers new to the field - who may not have a strong backgrotmd in macromolecular chemistry - we shall introduce strategies for a successful approach. We start with an introduction to the synthesis of LC polymer networks and their basic characterizatiOTi (Sect 2). Subsequently their mechanical orientation behavior will be discussed (Sect 3). Techniques to prepare elastomers that are permanently oriented to form a single crystal are the subject of Sect 4. Finally a brief outlook is given in Sect. 5. 

Based on the results of two amorphous compatible blend systems of 50/50 NC/PCL and 75/25 PVC/PCL, the following conclusion can be drawn. Compatible amorphous blend constituents show identical segmental orientation behavior, indicating good mixing at the molecular level. Thus, an amorphous compatible blend can exhibit the characteristics of a single homopolymer not only in its glass-transition behavior and mechanical properties but also in the uniform way in which the polymer chains orient. 

Infrared dichroism is one of numerous methods used to characterize molecular orientation. The degree of anisotropy of the strained pol3rmers may also be accurately characterized by other techniques such as X-ray diffraction, birefringence, sonic modulus, polarized fluorescence and polarized Raman spectroscopy [2]. These techniques directly probe the orientational behavior of macromolecular chains at a molecular level, in contrast to the macroscopic information provided by mechanical measurements. 

Deformation such as drawing, compression, annealing, strain, creep and stress relaxation of polymers including fibers may produce quite different orientational behavior, the results of which can be examined with solid-state NMR from both the static and dynamic viewpoints. The accurate model produced on the basis of atomic resolution of the local structure and the local dynamics can be built up in order to interpret the mechanical properties of polymers and the deformation mechanisms. 

We begin in Section 9.2 with the morphology in binary blends of iPP and various rubbery olefin copolymers where we remark the interrelation between the miscibility and dynamic mechanical properties. Section 9.3 describes the molecular orientation behavior under tensile deformation of iPP-based blends, and we compare the differences in deformation behavior between miscible and immiscible blends. Section 9.4 contains the solidification process in iPP-based blends where the effects of miscibility in the molten state on the crystallization of iPP matrix are discussed. 

Rheo-optical techniques (46 8) afford information on the strain dependence not only of stress but also of optical quantities associated directly with the structure or molecular morphology. The techniques were developed extensively for crystalline polymers to investigate the molecular deformation mechanism underlying the tensile elongation. In this part, the chain orientation behavior is characterized by infrared dichroism measured simultaneously with tensile deformation at a constant rate of elongation. 

X-ray investigations were used to study the orientation behavior of a sidechain SmC elastomer under mechanical stress [83]. If a... 

Nishikawa E, Finkelmann H. 1997. Orientation behavior of smectic polymer networks by uniaxial mechanical fields. Macromol Chem Phys 198 2531 2549. 

The response of spherulites to a deformation appeared to result from a number of mechanisms including slip, tilt (rotation in the plane of orientation) and twinning of the crystalline lamellae. Besides this complexity, which is inherent in all spherulitic polymers, the plastic deformation of PP is affected by the early occurrence of a necking process. Ultrahigh molecular weight PP has also been studied and its colddrawing orientational behavior was observed to be very different from that of conventional PP. 

In conclusion, simple symmetry considerations allow for a successful orientation of poly domain elastomers using mechanical fields. In principle knowledge is only needed of the local chain conformation of the LC polymer on which the elastomer is based, and the consistent mechanical deformation must be applied. Nevertheless, the chemical constitution of the whole polymer network has to be considered. Often, the orientational behavior is strongly influenced by the crosslinking topology. As a mle of thumb, prolate chain conformations are increasingly preferred when the crosslinker concentration is increased and when the crosslinker molecules are more rod-shaped [90, 91]. 

Three basic types of physical phenomenon are responsible for electroopti-cal behavior of a macromolecule in solution dipole moment, diffusion coefficients, and extinction coefficients. Amplitudes and time constants depend on both the properties of the macromolecules and experimental conditions. The sum of relaxation amplitudes is related to the linear dichroism of the solution at saturation, and depends on both the electric and optical properties of the molecule under investigation. The saturating behavior of linear dichroism calculated for a pure permanent moment, a pure induced moment or a mixed orientational mechanism is traditionally used in determining electrical responses and optical anisotropy by fitting the experimental results to a theoretical curve.Pqj. molecules with effective cylindrical symmetry (regarding their orientational behavior), the optical signal observed in the experiment can be represented as a product of orientational factor, < )(j, and a limiting reduced dichroism at infinite field. 

The center of diffusion is a unique point, because when the molecular coordinate system is centered at CD, the coupling tensor is symmetric. Moreover, for molecules with elements of symmetry (e.g., spheres, cylinders, ellipsoids), Dj. CD vanishes, and it is clear that for such bodies the dipole moment referred to CD is the quantity to be compared to the experimental dipole moment. Similar situations can be expected for globular proteins because their shape is close to a sphere or an ellipsoid. Brownian dynamics simulations of electrooptical relaxation experiment indicated that even for a large molecule such as tRNA, the difference is negligible between transient dichroism and derived dipole moments with and without inclusion of the coupling tensor when the coordinate system is centered at CD. Therefore it can be expected that calculated dipole moments of a molecule relative to its center of diffusion should closely correspond to those derived from orientational behavior. Some possibility for a bias of experimental dipole moments exists, however, since none of the orientation mechanisms considered in Eqs. [96] and [97] includes a possible contribution due to the translation-rotation coupling diffusion tensor. 

As mentioned frequently the mechanical and optical response of molecules — and of their crystallites — is highly anisotropic. Depending on the property under consideration the carriers of the molecular anisotropy are the bond vectors (infrared dichroism), chain segments (optical and mechanical anisotropy), or the end-to-end vectors of chains (rubber elastic properties). For the representation of the ensuing macroscopic anisotropies one has to recognize, therefore, the molecular anisotropy and the orientation distribution of the anisotropic molecular units (Fig. 1.9.). Since these are essentially one-dimensional elements their distribution and orientation behavior can be treated as that of rods such a model had been used successfully to explain the optical anisotropy [78], and the anisotropies of thermal conductivity [79], thermal expansion or linear compressibility [80], and Young s modulus [59,... 

Crash-simulation in general is performed with explicit solvers like LS-Dyna, Abaqus/Explicit or PamCrash to include mass inertia into simulation. For thermoplastics it has to be based on an exhaustive, plastics oriented material description that includes elastic and plastic deformation as well as failure. The most important influencing factors on the mechanical material behavior of non-reinforced thermoplastics under high deformation speeds are the strain-rate dependency and the non-linear behavior during plastic deformation. A failure criterion has to be applied in order to reflect the amount of energy the material can absorb through deformation until it fails. 

Noncrystalline domains in fibers are not stmctureless, but the stmctural organization of the polymer chains or chain segments is difficult to evaluate, just as it is difficult to evaluate the stmcture of Hquids. No direct methods are available, but various combinations of physicochemical methods such as x-ray diffraction, birefringence, density, mechanical response, and thermal behavior, have been used to deduce physical quantities that can be used to describe the stmcture of the noncrystalline domains. Among these quantities are the amorphous orientation function and the amorphous density, which can be related to some of the important physical properties of fibers. 

Mechanical Properties. The principal mechanical properties are Hsted in Table 1. The features of HDPE that have the strongest influence on its mechanical behavior are molecular weight, MWD, orientation, morphology, and the degree of branching, which determines resin crystallinity and density. 

Because the fibers generally are anisotropic, they tend to be deposited on the wire in layers under shear. There is Htde tendency for fibers to be oriented in an out-of-plane direction, except for small undulations where one fiber crosses or passes beneath another. The layered stmcture results in the different properties measured in the thickness direction as compared to those measured in the in-plane direction. The orthotropic behavior of paper is observed in most paper properties and especially in the electrical and mechanical properties. 

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