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44149-AC7
Impact of Clay Nanoparticles on Structure and Relaxation of a Polar Polymer
In this research work, we investigate the properties of polymer-based nanocomposites. When polymers are mixed with clay, the clay nanoparticles often enhance the properties of the host polymer. Clay acts as a two-dimensional reinforcement, improving the mechanical, thermal, and electrical behavior of polymers. We are studying the effects of the addition of clay on polar polymers with applications as electrical transducers and sensors. The objective of our research is to measure the dynamic electrical and mechanical response of polymer-clay nanocomposites as a function of temperature and amount of clay. The selected polymer is a polar polymer, poly(vinylidene fluoride), PVDF. The clay is Lucentite^TM, a synthetic silicate, organically modified to enhance its interaction with the polymer host. During the first year, we concentrated on the electrical, thermal, and structural properties. Recent work under the Petroleum Research Grant showed that PVDF/OMS nanocomposites exhibit alpha and/or beta crystallographic forms depending upon OMS content and thermal history. Also, as OMS content increases, the amount of non-polar alpha phase PVDF systematically decreases. This observation suggests that OMS nanoparticles restrict formation of the alpha phase, and enhance formation of the polar beta phase. Degree of quenching of the molten material has an impact on crystallographic form of the host polymer. If the material is slowly quenched alpha phase may dominate; rapid quenching leads to formation of greater amounts of beta phase. The PVDF used in this study was a Kynar based resin, obtained from Elf Autochem as grade 740, in pellet form. The OMS is Lucentite^TM, obtained from Zen-Noh Unico, America as fine powder. Lucentite is an organically modified layered silicate prepared by the supplier by ion-exchanging the Na ions in a synthetic smectite clay (with a cation exchange capacity of approximately 0.65 mequiv/g) for trioctyl methyl ammonium cations. To prepare the nanocomposites, PVDF was dissolved, and OMS was separately dispersed, in dimethylacetamide (DMAc) at 25°C with stirring for about 2-3 days until they form transparent solvent. They are mixed with desired weigh ratio of OMS to PVDF for making nanocomposites with different PVDF/OMS composition. The mixture solutions were stirred and then ultra-sonicated for 30min. before being poured into Petri dishes. We heated the solutions to ~50°C to make the solvent evaporate more quickly. Finally, we get a thin, tough film that is dried in a vacuum oven at 25°C for 1-2 weeks. Films were compression molded and quenched into cold water then isothermally cold crystallized at 145°C in a Mettler hotstage for 1hr. Wide angle X-ray scattering (WAXS) was used to identify the crystallographic phase of PVDF crystals in PVDF/OMS nanocomposites. Dielectric measurements were determined as a function of frequency (f = 200–10^6Hz) in the temperature range from –60°C to 170°C using parallel plate sample geometry. Data collection and temperature control were handled by a TA instruments ARES; and impedance measurements were made using an Agilent 4284A precision LCR meter. The real and imaginary parts of the complex dielectric function are determined. Figure 1a shows WAXS results for materials cold crystallized at 145°C. Homopolymer PVDF (red curve, 0.0% OMS) forms only the alpha phase. As OMS increases to 0.1% (green curve), the amount of polar beta increases. At OMS of 4.0% (blue curve), only the beta crystallographic phase is formed. Figure 1b shows the corresponding dielectric loss spectra as a function of frequency at a temperature of -20°C. The relaxation peak shown is the polymer glass transition, marked as Tg on the lowest plot. With addition of OMS, there is a tendency for Tg to increase, and the relaxation strength (area under the loss curve) to increase. We conclude that addition of OMS shifts the PVDF phase from alpha to beta domination. The positive shift in Tg signals an improvement in thermal stability imparted by the OMS addition. Mechanical relaxation studies are underway, and we are investigating other compositions and different types of OMS.
