Theories of Polymer Crystallization Challenged by Molecular Simulations
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distribution. These characteristics of carbon led to better filling of carbon into the zeolite nanochannels, which are shown by nuclear magnetic resonance analysis. Since the highly ordered structure suggested the absence of mesoporosity, the researchers concluded that one possible carbon structure may be “a curved graphene structure...accommodated to the curved inner surface of the zeolite nanochannels.” KINSON C. KAM
Theories of Polymer Crystallization Challenged by Molecular Simulations A team of researchers at the University of Massachusetts has demonstrated their modeling results on polymer crystallization from solution in which entropic barriers control the initial lamellar thickness by initiation of crystal nuclei. The nuclei then grow by chain absorption at the crystalline interface, and the lamellae thicken in a cooperative process requiring mobility of all chains in the crystal. These results challenge the conventional Lauritzen–Hoffman (LH) theory and its generalizations. In contrast to the crystallization of small molecules, the crystallization of polymers from solution is still poorly understood. Polymer molecules can participate in different initial nuclei, which leads to entropic frustration and incomplete crystallization, in which the polymer chains fold back and forth to form crystalline lamellae. So far, thermodynamics estimates have failed to predict the initial lamellar thickness, and all estimates are about 2 orders of magnitude higher than the 10 nm observed. There is also some controversy about the growth mechanism of the lamellae. As reported in the November 19, 2001, issue of Physical Review Letters, the group used Langevin dynamics simulations to reinvestigate these problems. The results of the dynamics simulations show that the initial crystallization from solution does not occur by spinodal dynamics, but through a nucleation and growth mechanism, in which the initial lamellar thickness is dictated by freeenergy barriers. The initial thickness is spontaneously selected, and the chains thicken by negotiating free-energy barriers before asymptotically approaching a thickness that is much smaller than the thermodynamically predicted extended chain limit. Chain growth occurs by simultaneous adsorption and crystallographic registry of diffusing chains at the growth front. In contrast to the LH model, this step is not hindered by an energy barrier. The chains then rearrange to form stems that are commensurate with the MRS BULLETIN/FEBRUARY 2002
crystal thickness at the growth front. M. Muthukumar, Barrett Professor in the Department of Polymer Science and Engineering, said, “Our results suggest that the definitions of quench depth and equilibrium melting temperature, which depend on the equilibrium lamellar thickness, need to be redefined. This opens up 40 years of accumulated data for re-analysis.” CORA LIND
Hypersensitization Improves Performance of Rare-Earth-Doped Active Waveguides The photosensitivity of optical waveguides has been enhanced in the past with the addition of hydrogen. The
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