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44149-AC7
Impact of Clay Nanoparticles on Structure and Relaxation of a Polar Polymer

Peggy Cebe, Tufts University

In this research work, we investigated the properties of polymer-based nanocomposites which have been electrospun into nanofibers.  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 structure and properties of polymer-clay nanofibers as a function of the amount of clay.  The selected polymer is a polar polymer, poly(vinylidene fluoride), PVDF.  The clay is Lucentite^TM, an organically modified silicate (OMS).  During the second year, we concentrated on the creation of nanofibers and measurements of the structural properties, including the phase of the host polymer.

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.  Since beta phase is also favored by solution casting from certain solvents, this year we created composite nanofibers by electrospinning from solutions of PVDF/OMS in these solvents. 

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 STM^TM, obtained from Zen-Noh Unico, America as fine powder.  Lucentite is an organically modified layered silicate (similar to hectrite) prepared by the supplier by ion-exchanging the Na ions in a synthetic smectite clay (Lucentite SWN, with a cation exchange capacity of approximately 0.65 mequiv/g) for trioctyl methyl ammonium cations.

To prepare the composite nanofibers, PVDF was dissolved, and OMS was separately dispersed, in dimethylformamide and acetone mixture at volume ratio of 4:1, at 25°C with stirring for about 2 days.  They are mixed with desired weigh ratio of OMS to PVDF for making nanocomposites with different PVDF/OMS composition.  Mixed solutions were placed into an inclined pipette containing a metal needle electrode connected to a 20kV power supply. About 10 cm away from the pipette is the grounded collector plate covered by aluminum foil. The static electric force between the pipette tip and plate electrosprayed the PVDF/OMS solution, and a nanofiber mat formed on the collector foil.

Fourier transform infrared spectroscopy was used to identify the crystallographic phase of PVDF crystals in PVDF/OMS nanocomposites

Scanning electron micrographs of PVDF/OMS composite nanofibers with different nanoclay content were taken for 0 wt% (pure PVDF), up to 10 wt%, with fibers prepared at electrospining voltage of 20 kV, and 10cm tip to collector distance.  As more nanoclay was added, the diameters of the nanofibers became more uniform and the number and size of the beads became smaller.  The result shows that the addition of nanoclay has the effect to eliminate the beads and irregularity in fiber diameter. The hectrite has negative charge while the organically modifier has positive charge attached, so the addition will increase the conductivity of the solution, which will decrease the beads.

FTIR absorbance vs. frequency scans were taken for all the electrospun fiber of PVDF with OMS Lucentite STN. The vibrational bands at 614 cm^-1, 764 cm^-1 and 976cm^-1 correspond to the TG conformation and alpha phase, while the bands at 834 cm^-1 and 1274 cm^-1 are considered to appear when the chain has a longer trans sequence than TT.  From the graph, we can see the alpha phase disappears when 1% or more STN was added, which means the STN has the effect to impede the growth of alpha phase.

PVDF has the lowest absorbance intensity at 1274cm^-1 and 834cm^-1 and composite nanofiber with 0.2% OMS shows stronger peak intensity. As the OMS content increases to 1%, 5% and 10% these two peaks achieve a constant intensity.  Based on the literature, the 834cm^-1 peak is the indicator of trans chains longer than TT, while the 1274cm^-1 peak stands for much longer trans sequences. The longer trans sequences in the composite nanofibers increased as more STN was added, which means the beta conformation was increased by the addition of STN.  However, at larger contents of OMS, more aggregation of the clay occurred, so that the 5% and 10% nanoclay ended up having have almost the same effect as 1% nanoclay addition.

We conclude that composite nanofibers of PVDF/OMS have alpha phase at low OMS content, and beta phase at high OMS content, equal to or above 1% OMS.  The nanofiber quality is very consistent and the mats are capable of use as fibrous membranes.

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