Thursday, March 19, 2009

Macromolecules, v42, issue 6

Dipolar and Bond Vector Correlation Function of Linear Polymers Revealed by Field Cycling 1H NMR: Crossover from Rouse to Entanglement Regime

A. Herrmann, V. N. Novikov and E. A. Rssler*
Macromolecules, 2009, 42 (6), pp 2063–2068

Abstract: We apply field cycling NMR to study segmental reorientation dynamics in melts of linear 1,4-polybutadiene (PB) in the entanglement regime (M ≥ Me). Dispersion data of the spin−lattice relaxation time T1(ω) are transformed to the susceptibility representation χ′′(ω) = ω/T1(ω), and using frequency temperature superposition master curves χ′′(ωτs) are constructed which reflect spectral contributions from glassy as well as polymer specific dynamics. The correlation time τs is determined by glassy dynamics. Transforming χ′′(ωτs) into the time domain and studying the crossover from Rouse to entanglement regime, the full dipolar or segmental reorientational correlation function F2(t/τs) is presented covering six decades in amplitude and 8 decades in time. Assuming F2(t) ub(t)ub(0)2 the bond vector correlation function b(t) = ub(t)ub(0) is obtained. Reaching Z = M/Me ≤ 9, comparison with theoretical predictions by the tube-reptation model as well as renormalized Rouse theory reveals significant discrepancies whereas good agreement is found with simulations. The crossover to entanglement dynamics appears to be very protracted.

1H HRMAS NMR Study on Phase Transition of Poly(N-isopropylacrylamide) Gels with and without Grafted Comb-Type Chains

Geying Ru, Nian Wang, Shaohua Huang and Jiwen Feng*
Macromolecules, 2009, 42 (6), pp 2074–2078

Abstract: Phase transition occurring in three different types of poly(N-isopropylacrylamide) gelsnormal cross-linked gel, comb-type grafted gel, and comb-type grafted gel with styrene-modified comb chainshas been investigated by variable-temperature measurements of 1H NMR spectra and spin−spin relaxation time. Three different gels exhibit distinct collapse behaviors in response to increasing temperature. For the normal gel, remarkable network shrinking occurs in a relatively narrow temperature range from 32 to 35 °C. For the styrene-modified comb-type gel, overall chain shrinkage appears in a very broad temperature range from 22 to 35 °C in which the styrene-modified comb chains shrink at lower temperatures (22−32 °C) than the backbone networks (32−35 °C). In the comb-type gel without styrene modification, however, the backbone networks shrink first (at 32−35 °C) on heating, followed by collapsing of comb chains (at 35−36 °C). During shrinkage of backbone networks the comb chains are expulsed from the main gel networks which is revealed by abnormal T2 increase of comb chains. T2 measurements also reveal that the styrene-modified comb-type gel in the equilibrium swelling state has more rigid network structure than both conventional gel and comb-type gel without styrene modification.

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