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Publications - 2010


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Solubilisation of model adjuvants by pluronic block copolymers.

Melissa A. Sharp, Clive Washington, and Terence Cosgrove

Journal of Colloid and Interface Science (2010) 344, 438-446
DOI: 10.1016/j.jcis.2010.01.005

Abstract

The effect of two model adjuvants (benzyl benzoate and benzyl alcohol) on the structure and dynamics of three Pluronic triblock copolymers (P85, P105 and F127) was studied using small-angle neutron scattering and pulsed-field gradient NMR. The two adjuvants studied have different aqueous solubilities. It was found that both adjuvants promoted the micellisation of the Pluronic block copolymers. In addition they lead to a swelling of the micelles, as shown by small-angle neutron scattering. From the pulsed-field gradient NMR results it was possible to determine the amount of adjuvant bound to the micelles.


A critical size ratio for viscosity reduction in poly(dimethylsiloxane)-polysilicate nanocomposites.

Randall G. Schmidt, Glenn V. Gordon, Cecile A. Dreiss, Terence Cosgrove, Val J. Krukonis, Kara Williams, and Paula M. Wetmore

Macrmolecules (2010) 43, 10143-10151
DOI: 10.1021/ma1004919

Abstract

The zero-shear-rate viscosity ?0 of polymer?nanocomposites (PNCs) derived from entangled poly(dimethylsiloxane)s and rigid polysilicate nanoparticles were investigated as a function of molecular size and concentration. Narrow molecular weight fractions of polymer and nanoparticle were obtained by supercritical fluid extraction. Molecular weight properties were analyzed by size exclusion chromatography and the nanoparticle radius of gyration Rg was characterized by small-angle neutron scattering. All seven polysilicate fractions were smaller (0.75 ? Rg, nm ? 2.1) than the five polymers (3.0 ? Rg, nm ? 12). Relative to the polymer ?0 the PNC ?0 exhibited either plasticization (viscosity reduction) or reinforcement (viscosity increase). Only reinforcement was observed in PNCs based on the polymer below Mc-the critical molecular weight for chain entanglement effects to start influencing ?0-which included an increase in the PNC ?0 by over 3 orders of magnitude using 0.30 volume fraction of the largest nanoparticle. For polymers above Mc, the crossover from plasticization to reinforcement behavior could be described by a critical molecular-size ratio based on the unperturbed Rg of the nanoparticle and of the polymer. Viscosity reductions of up to 52% were achieved, and were more significant at the higher of the two nanoparticle concentrations studied. The critical nanoparticle-to-polymer Rg ratio at 298 K was 0.18 0.006 and 0.13 0.003 for a nanoparticle volume fraction of 0.17 and 0.30, respectively. A generalized form for the concentration dependent crossover ratio is proposed to account for perturbations in the molecular size of the PNC components that can be the basis for future studies. The effects of particle size polydispersity and temperature are discussed.


Impact of polymer molecular weight on the dynamics of poly(dimethylsiloxane)- polysilicate nanocomposites.

Glenn V. Gordon, Randall G. Schmidt, Marlitt Quintero, Natalie J. Benton, Terence Cosgrove, Val J. Krukonis, Kara Williams, and Paula M. Wetmore.

Macromolecules (2010) 43, 10132-10142
DOI: 10.1021/ma100490c

Abstract

Introducing hard spherical polysilicate nanoparticles up to 0.30 volume fraction into narrow molecular weight (M) fractions of poly(dimethylsiloxane) (PDMS) that spanned its unentangled and entangled polymer chain regimes produced transparent and colorless polymer nanocomposites (PNCs) with zero-shear-rate viscosities ?0 either greater or less than the polymer matrix. Below the PDMS critical molecular weight for chain entanglements to start influencing ?0 (Mc), nanoparticle inclusion increased the PNC ?0 consistent with the reinforcement mechanism from traditional fillers of much larger particle sizes. Conversely, PNCs using entangled PDMS (M > Mc) exhibited a reduction in ?0 within a certain concentration range of nanoparticles. The reduction in ?0 was proposed to be primarily due to the dilution of the entanglement density of polymer chains as evidenced by a shift to higher Mc that was a function of the volume fraction of polymer chains, 1?0.56, and an increase in the translational motion manifested through an increase in the polymer self-diffusion coefficient. In contrast to the 1?1 dependence of Mc in concentrated polymer solutions, the dilution effect of the polysilicate nanoparticle on the polymer chain entanglement density was moderated by polymer adsorption, hydrodynamic effects and to some extent by free volume. Above Mc, the PNC ?0 scaled as 2M?w3.5 where M?w is the polymer weight-average molecular weight. Nuclear magnetic resonance T2 spin?spin relaxation measurements found that the increase in polymer mobility due to the nanoparticles became evident only above 2Mc. In contrast to the ?0 results, the transition to a stronger M?w dependence by 1/T2 shifted to lower M with increasing nanoparticle concentration.


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