Stress-induced orientation

The uniaxial contribution to the stress-induced orientation (the second term in Equation 15.4) has been attributed mainly to orientational interactions between chain segments. When guest molecules are introduced in a rubber matrix, these interactions take place between network chain segments and guest molecules as well. This effect has been recognised earlier, and may be used to study indirectly the behaviour of the matrix. Two types of guest molecules have been used for this purpose. 

It was shown that the stress-induced orientational order is larger in a filled network than in an unfilled one [78]. Two effects explain this observation first, adsorption of network chains on filler particles leads to an increase of the effective crosslink density, and secondly, the microscopic deformation ratio differs from the macroscopic one, since part of the volume is occupied by solid filler particles. An important question for understanding the elastic properties of filled elastomeric systems, is to know to what extent the adsorption layer is affected by an external stress. Tong-time elastic relaxation and/or non-linearity in the elastic behaviour (Mullins effect, Payne effect) may be related to this question [79]. Just above the melting temperature Tm, it has been shown that local chain mobility in the adsorption layer decreases under stress, which may allow some elastic energy to be dissipated, (i.e., to relax). This may provide a mechanism for the reinforcement of filled PDMS networks [78]. 

Kanda K Matsuda T (1994) Mechanical stress-induced orientation and ultrastruc-tural change of smooth muscle cells cultured in three-dimensional collagen lattices. Cell Transplantation 3 481-492. 

Zentel R, Benalia M. 1987. Stress induced orientation in lightly crosslinked liquid crystal line side group pol5mers. Makromol Chem 188 665 674. 

Because of the complications caused by the stress-induced orientation of clay platelets resulting in different rheological responses, the studies of CPNC flow focus on smaU-amplitude oscillatory shear flow (SAGS). As the discussion on the steady-state flow indicates, there is a great diversity of structures within the CPNC family. Whereas some nanocomposites form strong three-dimensional structures, others do not thus while nonlinear viscoelastic behavior is observed for most CPNCs, some systems can be smdied within the linear regime. 

Dichromatic dyes included in the liquid crystalline elastomers can be used to investigate the LC orientation behavior. The stress-induced orientational... 

Zentel, R. Benalia, M., Stress-Induced Orientation in Lightly Crosslinked Liquid-Crystalline Side-Group Polymers. Makromol. Chem. 1987,188,... 

Dichromatic dyes when introduced in LC elastomers can be used to investigate the LC orientation behavior. The stress-induced orientational elastomers could be developed to form a color polarizer. It can be designed by chemically attaching dye molecules in side chains of LC elastomer [54]. Also, dichromatic dye-containing liquid crystalline elastomers can be synthesized by reacting a terpolymer, as a precursor, with hexamethylene diisocyanate. The elastomers, even with 10 mol% of cross-linkage, remain nematic in their liquid crystalline nature [55]. 

The general idea [211] that the polymer undergoes stress-induced orientation (SIO) and crystallization (SIC) has been used for a long time to guide the conventional fiber formation process. However, it seems insufficient to further improve the process. 

The orientational order generated in a polymer network by an uniaxial stress is probed with deuterium NMR. The experiments are performed either on polydimethylsiloxane (PDMS) network chains or on PDMS probe chains dissolved in the network. The stress-induced orientation observed on both kind of chains is explained by short-range, orientational correlations between chain segments. 

Interestingly, IR dichroism studies on liquid crystalline polymers can reveal some insight into the mechanism of mechanical stress-induced orientation processes. For example, in studies with polyurethanes carrying mesogenic side chains (Chart 2.7), it was found that an initially stress-induced orientation system would be broken up and reoriented when the strain surpassed a critical value [77]. 

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