Strain induced orientation

The compatibility, mechanical properties, and segmental orientation characteristics of poly-e-caprolactone (PCL) blended with poly (vinyl chloride) (PVC) and nitrocellulose (NC) are described in this study. In PVC blends, the amorphous components were compatible from 0-100% PCL concentration, while in the NC system compatibility teas achieved in the range 50-100% PCL. Above 50% PCL concentration, PCL crystallinity was present in both blend systems. Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in the blends. It was found for amorphous compatible blends that the PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndio-tactic PVC segments showed higher orientation functions, implying a microcrystalline PVC phase. 

Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in PVC/PCL and NC/PCL blends. It was found for amorphous compatible blends that PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndiotactic PVC segments showed much higher orientation functions, which implied the existence of a microcrystalline PVC phase. 

Ma L, Wang J, Ding F (2012) Strain-induced orientation-selective cutting of graphene into graphene nanoiibbons on oxidation. Angew Chem Int Ed 51 1161-1164... 

The first term represents the elastic component that is related to Poisson s effect by the equation = (1 — 2vei) 3 introduced above, where 3 is the elastic component of axial tme strain. The second term, P, corresponding to plastic shear, is usually considered to be zero in metals, but we showed elsewhere (32) that it can be slightly negative in some polymers, due to the compaction of macromolecular chains subjected to strain-induced orientation. The last term , measures the contribution of cavitation and/or crazing to the macroscopic volume change of the tensile specimen (33). 

Klimov, E. Hoffmann, G. G. Gumenny, A. Seisler, H. W., Low-Temperature FT-NIR Spectroscopy of Strain-Induced Orientation and Crystallization in a Poly(dimethylsiloxane) Network. Macromol. Rapid Commun. 2005,26, 1093-1098. 

General considerations on strain induced orientation and crystallization developments... 

The strain induced orientation of the PUs has been studied extensively by using a variety of experimental techniques [5,21, 161,162.169,170,174,348-350]. 

Oriented In-Plane Texture. In this kind of film the properties (H and in the various in-plane directions (texture and nontexture directions) are different. The texture of the film can be supported by the texture of the substrate and the crystal lattice can be smaller in the texture direction than in the transverse direction. This can be the source for strain-induced magnetic anisotropy (magnetostriction). It is also found that the crystal is aligned in the texture direction (92). 

When a rubbery polymer, such as natural rubber, is stretched the molecules become aligned. This orientation leads to crystallisation. The effect of this so-called strain-induced crystallisation is to make the extended polymer stiffer than the unstrained polymer. Such crystallisation is not permanent but disappears when the sample is allowed to retract and regain its original dimensions. 

An example of a relevant optical property is the birefringence of a deformed polymer network [246]. This strain-induced birefringence can be used to characterize segmental orientation, both Gaussian and non-Gaussian elasticity, and to obtain new insights into the network chain orientation (see Chapter 8) necessary for strain-induced crystallization [4,16,85,247,248]. 

Knitted nets are produced from blown film or specialized extmsion lines with corotating dies. The film rolls are slit into small tapes of 2-5 mm width. Then the tapes are stretched on heated rolls (galettes) in several steps to achieve and maintain a high level of orientation. Typical stretching factors are three to five times. Thus, the materiel undergoes a strain-induced crystallization, which maximizes the crystallinity level in the tape. 

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. 

The piezoelectricity of polymer films and its inverse effect, strain induced by applied voltage, have been observed for as-cast films, oriented films, and films which has been polarized under a static field. It is at present believed that all kinds of polymer films exhibit more or less piezoelectricity. 

An additional bonus exists if the chains in the bimodal network readily undergo strain-induced crystallization.92,274,278,282 It has been observed that the extent to which the bimodal networks are superior to their unimodal counterparts is larger at lower temperatures. This indicates that the bimodal character of the chain length distribution facilitates strain-induced crystallization. Apparently the short chains increase the orientation of the long chains, and this facilitates the crystallization process. 

Equation 15.6 shows that the splitting is indeed proportional to the strain function (A -Ar1). Thus, the induced orientational order parameter (see Equation 15.1) may be characterised by the slope P of the doublet splitting A versus (A -Ar1). Experimentally, in PDMS model networks, u amounts to a few 10"1 [23, 25]. 

As discussed below, the quality of the alignment (and even its direction in the case of lamellar morphology), is influenced by temperature, as well as the frequency and strain amplitude of the aligning shear field. No general theory for the alignment of block-copolymer phases has yet been developed. However, studies of a number of different block-copolymer systems show that in ordered states with cylindrical domains, shear orients the cylinders parallel to the flow, while for lamallar microdomains, two different shear-induced orientations are commonly found, depending on alignment conditions in both of these orientations, the flow direction lies in the plane of the lamellae. 

The fibrils (as seen in Fig. 4) are the remains of blunting at the notch tip. The formation of fibrils is also an indication of work hardening due to orientation. Strain induced crystallization may even further enhance this work hardening. 

Cross-linked elastomers (the other main class of polymers studied by ESR) can be pre-strained above Tg to any desired extent (up to fracture) and their orientation stabilised by cooling below Tg before testing. At high pre-strains, strain-induced crystallization may occur providing a morphology essentially similar to that of synthetic fibres Cross-linked polymers may therefore be used to explore in a systematic manner, the role of strain and orientation in rmlecular fracture. 

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