Stress induced chain orientation

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

Very frequent are the cases of stress-induced crystallizations. A typical case is that of slightly vulcanized natural rubber (1,4-m-polyisoprene) which, under tension producing a sufficient chain orientation, is able to crystallize, while it reverts to its original amorphous phase by relaxation [75],... 

Elastomers exhibit this behavior due to their unique, crosslinked structure (cf. Section 1.3.2.2). It has been found that as the temperatme of an elastomer increases, so does the elastic modulus. The elastic modulus is simply a measme of the resistance to the uncoiling of randomly oriented chains in an elastomer sample under stress. Application of a stress eventually tends to untangle the chains and align them in the direction of the stress, but an increase in temperatme will increase the thermal motion of the chains and make it harder to induce orientation. This leads to a higher elastic modulus. Under a constant force, some chain orientation will take place, but an increase in temperatme will stimulate a reversion to a randomly coiled conformation and the elastomer will contract. 

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]. 

For PDMS 17, chain orientation phenomena (especially for free and pendant chains) induced by the interfacial shear stress during friction are important due to the greater length of the chains [8]. This orientation and alignment of chains at the interface can act like a self-lubrication layer, hindering the direct contact between the glass substrate and the bulk network, and therefore decreasing the bulk dissipation and the friction resistance. 

The very last portion of the stress-strain curve indicates strain hardening, induced mainly by further chain orientation. In semi-crystalline polymers, one detects an increase in crystallinity and orientation—both leading to an increase of stress. In these cases, Sb > Sy. 

Injection molded plaques or bars of PLC have a skin—core structure [33]. The molecular chains in the skin regions are largely aligned in the mold fill direction while the chain orientation in the core is more or less random. The high molecular alignment in the skin layer is induced by the elongational stress in the fountain flow and is immediately frozen upon contact with the mold surface. 

Undeformed NR forms spherulitic crystallites below 0°C. The tempera-ture/time induced crystallites generally form as folded chain lamellae, whereas the morphology of the strain/stress induced crystallites has been reported to be various fibrils, fibrils and folded lamellae, and shish-kebabs. " Strain-induced crystallites and their orientation have been indicated as the key reasons for the sharp increase in modulus that accompanies strain-induced crystallization. 

At low spinning speeds, usually 1000-2000 m/min, stress-induced crystallization is absent. Although an increasing level of spinning stress promotes polymer chain orientation, low spinning speeds do not have any significant effect on crystallization. 

Ziabicki and Jarecki have studied the effect of thermodynamic and hydro-dynamic parameters on the orientation distribution of the nuclei, to explain oriented fibrillar growth. Wu has also taken a thermodynamic approach to stress induced crystallization of cross-linked rubbers and its effect on morphology. He considers that extended chain crystals are only formed above the zero-strained melting point, Tva, and folded crystals are formed below Tm if the molecules are stretched less than some critical amount. 

Higher shear rate and stress are indeed able to induce a chain orientation at the PDMS surface [24]. Such an orientation will probably modify the rheological behavior of the polymer interface, and the anisotropy of the confined interfacial layer is able to induce a lower shear resistance. Moreover, it is also necessary to interpret the speed dependence of the friction for both substrates, with an increase of the friction coefficient with speed for the hydrophobic wafer and a decrease for the hydrophilic one. 

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]. 

---