Thursday, February 05, 2009

Macromolecules, Volume 42, Issues 2 and 3

Dynamics of a Polyphosphazene Melt Studied by Solid-State 2H NMR

Barbara Koch
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30, 48149 Münster, Germany
Michael Vogel
Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstr. 6, 64829 Darmstadt, Germany
Macromolecules, 2009, 42 (2), pp 531–536
DOI: 10.1021/ma802421e

Abstract: Poly[bis(methoxy)phosphazene] (PBMP) is used as a model to investigate the backbone dynamics of polyphosphazenes approaching their glass transitions. Specifically, we study PBMP featuring deuterated methyl groups so that, as a consequence of fast rotation of the methyl groups, 2H NMR probes the reorientation of their 3-fold symmetry axes and, thus, of the inorganic backbone. Combining 2H NMR spin−lattice relaxation, line-shape, and stimulated-echo analyses, we follow the slowdown of the segmental motion upon cooling over a broad temperature/time range. Comparison of present and previous results provides no evidence that polymers featuring inorganic and organic backbones, respectively, show fundamentally different dynamical behaviors during vitrification. In particular, typical of glass-forming polymer melts, we find for the α-process of PBMP that its temperature dependence deviates from an Arrhenius law and its time dependence differs from a single-exponential function. 2H NMR three-time correlation functions indicate that both homogeneous and heterogeneous dynamics contribute to the nonexponential relaxation. In addition, 2H NMR spin−lattice relaxation and line-shape analyses reveal the existence of some large-angle anisotropic precursor motion in the moderately viscous melt, which may be a peculiarity of polyphosphazene.

Polyolefin Blend Miscibility: Polarization Transfer versus Direct Excitation Exchange NMR
Marcin Wachowicz, Lance Gill and Jeffery L. White
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Macromolecules, 2009, 42 (2), pp 553–555
DOI: 10.1021/ma802531f
Publication Date (Web): December 29, 2008
Copyright © 2008 American Chemical Society
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Relatively simple chemical constituents in polyolefin macromolecules belie the fact that their phase behavior in mixtures can be complex.(1) Polyolefins are obviously important economically as commodity polymers, but many specialty applications require unique formulations of multiple polyolefins with slightly different chemical structures. Predicting the details of the ultimate phase mixing is difficult and nonintuitive, and experimental verification of chain level behavior is challenging due to similar chemical and physical properties among varying polyolefin chain structures. Many investigators have approached this problem through multiple theoretical and experimental avenues in recent years.(2-7) We have recently described aspects of binary polyolefin blend phase behavior, relying extensively on advanced solid-state NMR methods to show that configurational entropy is an important thermodynamic parameter in controlling miscibility between different polyolefin structures.(8-10) Important general conclusions based on these advanced NMR experiments are often complicated by complex pulse sequences that employ an initial polarization transfer step. CODEX NMR experiments have proven particularly powerful for direct chain level interrogation of mixing and dynamics in amorphous polyolefin blends, but to date, all work has involved polarization transfer from protons to carbons (cross-polarization or CP) to generate the initial signal in the CODEX experiment.(11, 12) Concerns about nonrepresentative sampling of a subset of polymer chains by polarization transfer steps can arise in cross-polarization solid-state NMR methods; differential polymer chain dynamics may lead to nonuniform polarization transfer efficiency in that step, often preferentially emphasizing the more spatially constrained or rigid regions of the sample which preserve larger heteronuclear dipolar couplings.(13, 14) To address this in the context of amorphous polyolefin blend miscibility, we have devised a modified version of the experiment employing only direct carbon polarization as the initial step in the experiment. On the basis of quantitative comparisons of the modified direct polarization versus CP-based CODEX results over a wide temperature range (including Tg) for atactic polypropylene (aPP), we demonstrate that results representative of all polymer chains in the sample are obtained here as well as in previously published polarization-transfer-based results. Our work shows that CODEX-based exchange methods can provide chain-level information representative of the bulk mixing and miscibility in amorphous polyolefin blends.

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