Polymer-consistent force field

As an illustration, we have reproduced the results for the diffusivity of methane in a silicalite by using the UA potentials (Leroy et al. 2004). We then used a better and a more detailed force field polymer consistent force field (PCFF) to compute the diffusivity of butane in silicalite. We see that the PCFF description of the problem yields results closer to experiments. 

Huige and Hezemans179 180 have performed extensive molecular mechanics calculations using the consistent force-field method on various oligo- and polyisocyanides. The hexadecamer of ferf-butyl isocyanide was calculated to have a helical middle section and disordered ends. The dihedral angle N=C—C=N in the middle section was found to be 78.6°, and the number of repeat units per helical turn was 3.75. The latter number is in agreement with circular dichroism calculations using Tinoco s exciton theory (3.6—4.6) and De Voe s polarizability theory (3.81). The molecular mechanics calculations further predicted that the less bulky polymers 56 and 57 form helical polymers as well, whereas a disordered structure was calculated for poly(methyl isocyanide) (55). 

In addition to the above effects, the intermolecular interaction may affect polymer dynamics through the thermodynamic force. This force makes chains align parallel with each other, and retards the chain rotational diffusion. This slowing down in the isotropic solution is referred to as the pretransition effect. The thermodynamic force also governs the unique rheological behavior of liquid-crystalline solutions as will be explained in Sect. 9. For rodlike polymer solutions, Doi [100] treated the thermodynamic force effects by adding a self-consistent mean field or a molecular field Vscf (a) to the external field potential h in Eq. (40b). Using the second virial approximation (cf. Sect. 2), he formulated Vscf(a), as follows [4] ... 

For polysilane, the corresponding C - oo values at 25°C are 4.1 and 3.9 based on the NR and FR calculations, respectively. The nearly identical values reflect the similarity in conformational characteristics obtained by the two different force fields for this chain. The relatively small value of Cj for polysilane is indicative of its rather low chain extensibility. This feature is consistent in turn with the chain s high overall conformational flexibility (i.e., no overwhelming preferences for any one particular conformational state), with its identifiable preferences for the more compact TG and states over the alternative and more chain-extending TT state and moreover, and with its allowance for G G" states whose occurrence typically leads to reversals in chain direction. For comparison, accepted values of at 25 C for two other flexible polymers. 

In the last years, the separation method of field-flow fractionation (FFF) comes more into the focus of size determination and separation of polymer mixtures consisting of different components, for example, aggregates, gels, or defined assemblies. Here, the separation takes place in a long and narrow chaimel, where a carrier liquid transports the molecules. Perpendicular to the main flow direction, a force field will be applied, which influences the sample molecules in order to separate thena. Caused by the channel architecture, the... 

Diatomic molecules The distribution function of the center of mass of Oxygen molecules in micro-structures of glassy PVC of ca. 40 A has been evaluated [58] assuming separability between the translational and rotational degrees of freedom of the O2 molecules [see Eq. (32)]. Two different diatomic force IMds for the O2 molecule have been used. In both cases, the same three modes of solute motion have been found and computed transport coefficients were consistent with experimental data. Nevertheless, we fed that tte use of tte explidt diatomic approach is impractical today In view of the inaccuracy which intrinsically pertains to the force-field approach, one can always conveniently optimize the parameters of a united- tom force-field to properly reproduce experimental transport coeffidents oS tte diatomic molecules in dense polymers. 

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