Segment-oriented prediction

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

Segmental orientation in model networks of PDMS in uniaxial tension is measured by infrared dlchroism, Measurements are made for four tetrafunctlonal end-linked networks. Results of experiments are compared with predictions of calculations based in (i) the widely used Kuhn expression and (ii) the RIS formalism. The Kuhn expression is found to considerably overestimate the segmental orientation. The RIS approach leads to values of segmental orientation that fall between predictions of the affine and phantom network models. This indicates that the nematic-like Intermolecular contributions to orientation are not significant. 

Also consistent with theoretical predictions was the observation that the ratio An/fp was essentially constant, independent of extension and reaction time. Here = (/)—l)/(Z)-l-2) is the polyene segment orientation function which follows from eqn. (11), noting that the absorbance is a maximum in the direction of the segment axis and that the small perpendicular component of absorbance is negligible. Assuming that the constant vklue A /f0 = may also be applied to the pure PVC, we may write. 

The molecular theory of extensional viscosity of polymer melts is again based oti the standard tube model. It considers the linear viscoelastic factors such as reptation, tube length fluctuations, and thermal constraint release, as well as the nonlinear viscoelastic factors such as segment orientations, elastic contractimi along the tube, and convective constraint release (Marrucci and lannirubertok 2004). Thus, it predicts the extensional stress-strain curve of monodispersed linear polymers, as illustrated in Fig. 7.12. At the first stage, the extensional viscosity of polymer melts exhibits the Newtonian-fluid behavior, following Trouton s ratio... 

As a preliminary, the recently proposed theory of segmental orientation of real networks with constraints on junctions is reviewed. Basic principles of fluorescence polarization technique and its application to investigation of molecular orientation of mobile, anisotropic networks are then described. Illustrative data on elasticity and orientation behaviors of cis-polyisoprene networks are consistent with theoretical predictions. 

The rubberlike liquid model is able to predict, qualitatively, certain nonlinear viscoelastic phenomena. In particular, some effects arising from the finite orientation of chain segments are predicted, for example a nonzero first normal stress difference. However, it fails to describe many other nonlinear effects. For example, it predicts that the viscosity is constant with shear rate and the second normal stress difference is zero. In fact, all its predictions for the shear stress in simple shear are the same as those of the Boltzmann superposition principle. We can gain some insight into the origins of nonlinearity by examining the features of the rubberlike liquid model that limit its predictive ability. 

For example, if many chains are perfectly aligned, e.g. in the z-direction, the probability of the test chain also placing its segments in the z-direction becomes strongly enhanced. The oriented molecules leave an open corridor in the direction of the director. In terms of the vacancy probability, it is an event that it is enhanced with respect to the classical mean-field prediction. As a result, the test... 

Orientation of molecules can normally be detected using polarized light in either transmission or scattering experiments. The prediction of the observed effect (birefringence or dichroism, for example) requires a description of the basic light-matter interaction combined with a calculation of the distribution of orientations associated with the individual molecules or segments of molecules. 

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