Thursday, January 08, 2009

Macromolecules August 2008 (Volume 41, Issue 16) to January 2009 (Volume 42, Issue 1)

Segmental Dynamics of Bulk Poly(vinyl acetate)-d3 by Solid-State 2H NMR: Effect of Small Molecule Plasticizer

Rakesh R. Nambiar and Frank D. Blum*
Macromolecules, 2008, 41 (24), pp 9837–9845

Abstract: The effect of dipropyleneglycol dibenzoate, a plasticizer, on the glass-transition temperature (Tg) of poly(vinyl acetate) was studied using deuterium solid-state NMR and modulated differential scanning calorimetry (MDSC) from 0 to 20% plasticizer content. Quadrupole-echo 2H NMR spectra were obtained for methyl deuterated PVAc-d3 samples with different plasticized amounts. The Tgʼs of different plasticized samples were determined from NMR as the temperatures at which the deuterium powder patterns collapsed. It was found that the Tgʼs decreased by approximately 6 °C for every 5% increment in the plasticizer content and that the trends in the NMR-determined Tgʼs, that is, Tg(NMR), were consistent with those determined by modulated differential scanning calorimetry (MDSC). The Tg(NMR) values were about 36 °C above those of the Tg(DSC) values. This difference in the Tgʼs was due to the different time scales of the two experiments which could be accounted for on the basis of time−temperature superposition principles. The experimental NMR line shapes were fitted using a set of simulated spectra generated from the MXQET simulation program. The spectra were based on a model of nearest-neighbor jumps on a truncated icosahedron (soccer ball). The resulting average correlation times were also found to fit a time−temperature superposition with the same parameter. While the Tg was decreased by the amount of plasticizer, it was found that the breadth of the transitions from either the NMR line shapes or the MDSC thermograms did not seem to change much with the amount of added plasticizer.

Segmental Dynamics of Poly(ethylene oxide) Chains in a Model Polymer/Clay Intercalated Phase: Solid-State NMR Investigation

Cédric Lorthioir*, Françoise Lauprêtre, Jérémie Soulestin and Jean-Marc Lefebvre

Macromolecules, 2009, 42 (1), pp 218–230
DOI: 10.1021/ma801909s
Abstract: A model poly(ethylene oxide) (PEO)/laponite hybrid material, characterized by a high silicate content, was used to probe the dynamical behavior of polymer chains at the surface with clay platelets. Such a system mimics the intercalated phases that may occur in polymer/clay nanocomposites with usual silicate amounts of 5 wt %. The segmental motions underlying the α-relaxation of fully amorphous PEO chains confined within the nanometer-thick laponite galleries were monitored over the tens of microseconds time scale by means of 13C and 1H solid-state NMR. A significant slowing down of these motions was mostly observed, as compared to the local dynamics in the amorphous phase of neat PEO. Strong dynamical heterogeneities among the intercalated PEO monomer units remain even at room temperature, i.e., more than 50 K above the temperature at which the frequency of the segmental motions displayed by a significant part of the PEO chain segments gets above 52 kHz. Such heterogeneities are related to a pronounced extension of the α-relaxation process toward the low-frequency side. The slowing down of the PEO segmental motions was assigned to ion-dipole interactions between the PEO oxygen atoms and the Na+ counterions located in the laponite galleries. The domains formed by PEO monomer units characterized by a reduced segmental mobility were found to display rather long lifetime, about 13 ms at room temperature.

2H NMR Studies of Polymer Multilayer Capsules, Films, and Complexes

Blythe Fortier-McGill and Linda Reven

Macromolecules, 2009, 42 (1), pp 247–254
DOI: 10.1021/ma801929g
Abstract: The chain dynamics of aqueous suspensions of polyelectrolyte complexes, supported multilayers deposited on submicron silica colloids, and hollow capsules were characterized by wide-line 2H NMR spectroscopy (DNMR) as a function of layer number, temperature, and ionic strength. The strong polyelectrolytes, poly(diallyldimethyl ammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) were employed with selective deuteration of the PDADMAC methyl group. DNMR line-shape analyses showed that there is enhanced chain mobility in the systems with excess positive monomer units, that is, supported multilayers and capsules capped with PDADMAC. Selective deuteration of the first, fifth, and ninth layers confirms that the alternation in chain mobility with the capping layer is a through-film effect. Differential scanning calorimetry (DSC)-detected phase transitions were found to occur between 32 and 45 °C for the PSS/PDADMAC complexes, supported multilayers, and capsules. Whereas the glass transitions of bulk-state polymers are detected by DNMR typically 30 to 40° above the DSC-detected Tg, the onset of fast chain motion for the water-saturated polyelectrolyte complexes and supported multilayers coincides with calorimetric transitions.

Anisotropic Diffusion and Morphology in Perfluorosulfonate Ionomers Investigated by NMR

Jing Li, Kyle G. Wilmsmeyer and Louis A. Madsen

Macromolecules, 2009, 42 (1), pp 255–262
DOI: 10.1021/ma802106g

Abstract:Anisotropy in ionomer membranes represents a powerful interaction for modulating properties such as mechanical moduli, thermal expansions, and small molecule transport, all tunable via controlled processing. We observe uniform hydrophilic channel alignment in three perfluorosulfonate ionomer membrane types, quantified by 2H NMR spectroscopy of absorbed D2O molecules. Our measurements show biaxial or uniaxial in-plane alignment for extruded membranes, but uniaxial through-plane alignment for dispersion-cast membranes, and further demonstrate affine swelling with both water uptake and thermal expansion. In order to correlate alignment data with a quantity relevant to proton transport, we measure the anisotropy of water self-diffusion using pulsed-field-gradient NMR along different membrane directions. Extruded membranes with stronger alignment exhibit 18% faster in-plane diffusion than through-plane diffusion, while diffusion anisotropy is minimal for weakly aligned membranes. These results should lead to a more quantitative understanding of and control over membrane properties via manipulation of molecular order.

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