Bimodal, orientation

Polypropylene exhibits a distinctive bimodal orientation of the unit cells when crystallized from melts undergoing extension. There are crystals whose c-axis is oriented parallel with the fiber axis and a smaller population of crystals whose a-axis is oriented parallel with the fiber axis. For the H-0660 filaments at high stress, Nadella et al. [79] estimate that about 10-20% of the crystals have an n-axis orientation. Anderson and Carr [192] and Clark and Spruiell [198] concluded that the a-axis-oriented crystals were very small and were distributed throughout the sample and perhaps grew epitaxially to the c-axis-oriented crystals. 

Martin [710] provided evidence of intermolecular twist defects in extended chain fibers, such as PBO, by use of molecular modeling. Spatially resolved electron diffraction also revealed the hierarchical structure in as spun LCP fibers at a resolution of 100-200nm with bimodal orientation, whereas the heat treated fibers were more uniform in orientation [713]. These authors also used low dose HREM diffraction to calculate local orientation order parameters from the same TLCP fibers. 

Resins with a bimodal molecular weight distribution, as illustrated in Fig. 5.3 b), are more readily processed than those with a most probable molecular weight distribution, but are more likely to be oriented in the solid state and have a somewhat reduced impact resistance. 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

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. 

Figure 8.28 shows the pore size distributions for the PS formed on highly doped / -Si at different current densities." For the PS formed on heavily doped silicon, the pores at a given HF concentration have a narrower distribution at lower current at a given current density, the distribution is narrower at lower HF concentrations. A bimodal distribution of pore diameter is gena-ally associated with two-layer PS on lowly doped / -Si and iUmninated n-Si. The PS formed on p-Si has also been found to have a bimodal distribution of particles sizes small spherically shaped particles with a diameter of a few nanometers and large cylindrically shaped particles oriented with their axis perpendicular to the surface.The distribution of pore size with multiple peaks may be attributed to the fact that PS may have a surface micropore layer and smaller branched pores. Due to the hierarchical structure of the branched pores, the distribution of pore diameters for highly branched PS is found to be fractal-like. ° ... 

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