Electric Field Alignment of Cellulose Based-Polymer Nanocomposites

Cellulose whiskers (CWs) obtained from naturally occuring cellulose are nano-inclusions which show a lot of promise as mechanical reinforcements in polymers. Typically, a relatively high content is added to realize improvement in effective mechanical behavior. This enhancement in modulus is usually followed by a modest increase in strength but generally the ductility and toughness decrease. Our approach is to use small concentrations of CWs so as not to detrimentally affect processability, toughness and ductility. By aligning the small concentrations, we target the same kind of improvement in modulus and strength as reported in the literature, but at much smaller volume contents.

In this work, we investigate the effect of AC electric field on the alignment of dispersed nanoscale CW in a polymer. Polyvinyl acetate (PVAc) is used as the model polymer because of the good interaction between CWs and PVAc. A low concentration of 0.4wt% was used for the study. Two dispersion methods, namely basic and modified, were developed. The basic method led to micron scale dispersion. Using the modified method, CWs were individually dispersed in PVAc with average lengths and diameters of 260 nm and 8 nm respectively yielding an aspect ratio of approximately 30. The behavior of CWs (alignment and chain formation) under an applied electric field was found to be a function of applied electric field magnitude, frequency and duration. Following alignment, the CW/PVAc nanocomposites are thermally dried in the presence of electric field to maintain the aligned microstructure. Improvements in dielectric constant and mechanical properties were observed for the aligned cases as compared to random case and pure PVAc. The optimal electric field magnitude, frequency and duration for the alignment and chain formation were found to be 200Vpp/mm, 50 KHz for duration of 20 minutes for the microcomposite and 250Vpp/mm, 10KHz for a duration of 1hr for the nanocomposite. At 0.4wt% concentration, 21% increase in dielectric constant for the optimal nanocomposite case. Above Tg, a 680% improvement in elastic modulus at 0.4wt% concentration for the optimal nanocomposite case. The reason for the significant reinforcement is attributed to alignment (rotation and chain formation) and chain-chain interaction (3D network formation and hydrogen bonding).