Volume 111, Issue 1, 01 July 1999
Index of content:
- POLYMERS, BIOPOLYMERS, AND COMPLEX SYSTEMS
111(1999); http://dx.doi.org/10.1063/1.479282View Description Hide Description
Simulations are carried out to determine how polydispersity affects the potential energy surface of a colloidal system. Increases in polydispersity are shown to cause the disappearance of potential energy minima, such that energy minima corresponding to homogeneous highly ordered structures exist only for and energy minima corresponding to homogeneous partially ordered structures exist only for These results show that colloidal systems cannot form homogeneous crystals, even as metastable phases, above certain values of polydispersity.
Residual dipolar couplings by dipolar-encoded longitudinal magnetization, double- and triple-quantum nuclear magnetic resonance in cross-linked elastomers111(1999); http://dx.doi.org/10.1063/1.479291View Description Hide Description
The measurements of residual dipolar couplings in elastomer system is desirable, because they reflect the hindrance to molecular motions by the cross-linking, topological constraints and the external factors like mechanical stress. Dipolar-encoded longitudinal magnetization nuclear magnetic resonance(NMR) decay curves, double-quantum and triple-quantum NMR buildup intensities for measuring the residual dipolar couplings, and the associated dynamic order parameters are introduced. It is shown that in the short excitation time regime the effective dipolar network is simplified. In the limit of this model based on localized dipolar couplings, the spin response to two-dimensional pulse sequences used to record multiple-quantum (MQ) NMR coherences was evaluated for longitudinal magnetization, double-, and triple-quantum coherences of methylene, and methyl protons in synthetic 1,4-cis-polyisoprene. The dynamic order parameters can be evaluated from this NMR response using a classical scale-invariant polymermodel. These dynamic order parameters were measured for a cross-link series of synthetic polyisoprene and correlated with the cross-link density. The decay rates of the Hahn-echo amplitudes reflecting residual dipolar couplings as well as effects of molecular motion are also measured for the same cross-link series. The contribution of molecular motions to the transverse relaxation can be separated from the residual dipolar couplings using a train of magic echoes. The sensitivity of these transverse relaxation rates to the cross-link density is compared to that of residual dipolar couplings. The NMR time scale is shorter for the dipolar-encoded longitudinal magnetization and MQ experiments as compared to transverse relaxation experiments leading to an increased sensitivity to cross-link density of the former approaches.
An investigation of the shape and crossover scaling of flexible tangent hard-sphere polymer chains by Monte Carlo simulation111(1999); http://dx.doi.org/10.1063/1.479292View Description Hide Description
Results are presented from our simulation study of flexible, tangent hard-sphere polymer chains, using a Monte Carlo technique with a reptation algorithm. We examine the crossover region from the swollen (dilute) region to the semidilute regime using chains of degree up to The (average) chain end-to-end distance is analyzed as a function of the number of chain links, as is a correlation function for the internal separation of segments, expressed in terms of their connectivity. We study the structure of the polymer chains using scattering functions, which are presented in addition to the segment–segment distribution functions from which they are calculated. Not surprisingly, the Flory exponent ν changes gradually from the dilute value of ν≈0.59 to ≈ at high density. We find that for chains of 2000 segments, this transition begins at the very low molecular volume fraction of about 0.1%. An analysis of the correlation function for internal segments provides a similar but more detailed picture, in that the role of the screening length ξ becomes explicit. The pair distribution and scattering functions in the swollen, crossover, and melt regions are determined and the scaling behavior is reexamined in the context of these functions. The system considered will form a reference system for subsequent studies in which the model will be broadened to include a network and/or attractive interactions. In this paper we seek first of all to establish that reference, and to relate our work to previous studies in this area of interest. As an important technical point, we also note that the use of an inappropriate random number generator can cause the complete failure of simulations of this type.
Inelastic neutron scattering spectra of the longitudinal acoustic modes of the normal alkanes from pentane to pentacosane111(1999); http://dx.doi.org/10.1063/1.479293View Description Hide Description
The inelastic neutron scatteringspectra of the polycrystallinen-alkanes, have been obtained at ∼25 K. At this temperature the n-alkane molecules are extended in their all-trans configuration. The spectral region from 200 to 600 cm−1 is reported with emphasis on the higher frequency longitudinal acoustic modes (LAM). These modes are identified by their nominal translational symmetry eigenvector, κ. A complete assignment of experimental frequencies out to is presented for the first time. This assignment is consistent with the known optical data and ab initio Hartree–Fock and DFT calculations. On the basis of these theoretical results, the lower frequency, higher κ LAM modes are also assigned. The LAM sequences exhibit bandhead behavior, i.e., a maximum frequency value, at some value of κ. The maximum frequency is consistently found at [cf. polyethylene (PE), ] for all chain lengths but the value of the maximum frequency, increases roughly linearly with n for At about it reaches the value found for PE, 525 cm−1. This variation in frequency with chain length for the same nominal value of κ and also the full set of calculated DFT LAM results is compared with the expectations of a simple zig–zag bead bending model.