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Role of polymer–clay interactions and nano-clay dispersion on the viscoelastic response of supercritical CO2 dispersed polyvinylmethylether (PVME)–Clay nanocomposites References and further reading may be available for this article. To view references and further reading you must purchase this article. Mihai Manitiua, Steven Horsch1, a, Esin Gulari2, a and Rangaramanujam M. Kannan, a, aWayne State University, Chemical Engineering and Materials Science, Detroit, MI 48202, USA Received 1 April 2009; revised 11 May 2009; accepted 19 May 2009. Available online 27 May 2009. Abstract Clay dispersion and polymer–clay interactions play a key role in producing property enhancements in nanocomposites; yet characterizing them in complex polymer–clay systems is often a challenge. Rheology can offer insights into clay dispersion and clay–polymer interactions. We have investigated the viscoelastic response for a series of supercritical CO2 (scCO2) processed polyvinylmethylether (PVME)/clay nanocomposites with varying polymer–clay interactions and nano-clay dispersion. PVME is used in this study because it is highly swellable in scCO2, thereby enabling processing of PVME/clay mixtures without the presence of a co-solvent. Since PVME and natural clay are water-soluble, highly dispersed PVME-clay nanocomposites were prepared using water, followed by lyophilization in the presence of polymer. In this ‘weakly interacting’, but highly dispersed systems, with clay loadings above the percolation threshold, terminal behavior was observed in the linear viscoelastic moduli (i.e. no low frequency plateau is observed). When the nanocomposites were processed in scCO2, with 15 wt% of 30B and I.30P, the WAXD patterns of the resultant nanocomposites were largely comparable, indicating partial dispersion, with intercalation peaks. However, the rheology of these two nanocomposites were significantly different despite similar inorganic volume loading (4 vol%). Even with less dispersion compared to the water-based system, the low-frequency moduli were significantly more enhanced, accompanied by a plateau, and a cross-over frequency shift. Neglecting the small differences in the actual clay content between these clays (4–5 vol% of inorganic matter), this suggests that rheology may be sensitive to strong interactions between the clay surfactant and the polymer. Therefore, polymer–clay interactions and clay–clay interactions may both be important in the ability to sustain a “so-called” percolated network, rather than just clay dispersion. Graphical abstract Full-size image (26K) Keywords: Supercritical CO2 processing; Dispersed polymer–clay nanocomposites; Rheology Article Outline 1. Introduction 2. Experimental methods 2.1. Materials 2.2. scCO2 Processing 2.3. Nanocomposite formation via scCO2 processing 2.4. Nanocomposite formation via solution cast freeze drying 2.5. Wide-angle X-ray diffraction (WAXD) 2.6. Rheology 2.7. Thermogravimetric analysis (TGA) 3. Results and discussion 3.1. Determination of clay mass fraction in organophilic nano-clays 3.2. Role of substantial nano-clay dispersion with ‘weak’ polymer–clay interactions 3.2.1. PVME versus PEO 3.2.2. PVME – highly dispersed versus scCO2-processed 3.3. Role of polymer–clay interactions with comparable levels of intercalation 3.3.1. PVME-30B nanocomposite 3.3.2. PVME-I.30P nanocomposite 4. Conclusions Acknowledgements Appendix. Appendix References Fig. 1. TGA analysis of pure nano-clays, Cloisite Na+, Cloisite 30B and Nanocor I.30P. The 6% reduction in weight of Cloisite Na+ is attributed to the evaporation of water. Cloisite 30B organophilic nano-clay contains 14 wt% ammonium salt. Nanocor I.30P organophilic nano-clay contains 31 wt% ammonium salt. View Within Article -------------------------------------------------------------------------------- Fig. 2. (a) WAXD of sample Cloisite Na+/PEO reveals the sample is intercalated as evident by the significant shifting of the d001 diffraction peak and the presence of higher order peaks (d002).(b) WAXD of PVME/Na+ samples 15-NA, 15NA-S, and as received Cloisite Na+. Sample 15-NA has a weak diffraction peak thought to be the result of a disordered intercalated structure and sample 15NA-S is delaminated as evident by the lack of a coherent diffraction pattern. View Within Article |
4楼2009-07-21 14:03:32