Tube Dilation in Polymer Composite Melts by Oligomer Addition*

Dispersion of attractive nanoparticles in polymer melts is critically important for enhancing mechanical properties of nanocomposites. Adding oligomers into nanocomposites enables their easy processability, particularly at high particle loadings. Here, we prepared a series of plasticized poly(methyl acrylate) (PMA)-SiO2 (15 nm in diameter) nanocomposites at various oligomer volume fractions at the confined and unconfined particle limits. We observed that as oligomer amount increased in composite at the unconfined state, plateau modulus and terminal relaxation time decreased, whereas entanglement relaxation increased only slightly, which led to a decrease in the apparent entanglement density. At the confined state (with 13.8 vol% loading), the nanoparticle entanglement at terminal region predominated with oligomer addition. The dilation scaling exponent of GN is found to be 2.3 and 4 for loadings at 13.8 vol% and 6 vol%, respectively. In summary, we demonstrate that adding nanoparticles into polymer melt shrinks the host chain tube diameter by nanoparticle induced geometrical confinement; and oligomer inclusion increases the entanglement molecular weight of the confined host chains, hence, enlarges the reptation tube diameter. By combining the tube shrinkage and dilation effects of nanoparticles and oligomer, the plasticized polymer composites at confined states with enhanced fluidity is achieved.