Quiescent and flow-induced crystallization in polyamide 12/cellulose nanocrystal composites

Understanding the crystallization kinetics and microstructure that result after an imposed shear flow in an additive-containing polymeric system is imperative for the development of robust polymer composites suitable for advanced engineering applications. Both nucleating agents and flow accelerate the crystallization kinetics as well as alter the ultimate polymer microstructure. During melt processing, polymers are subject to shear flow prior to the solidification of the melt. As an unsheared baseline in this study, the addition of 5 wt % cellulose nanocrystals (CNC) into quiescent polyamide 12 (PA 12) revealed that CNCs act as a natural nucleating agent to the quiescent PA 12 during slow cooling and at high temperatures, as measured by standard differential scanning calorimetry. To evaluate the role of shear work in promoting flow-induced crystallization, both neat and PA 12/CNC composite were subjected to known amounts of shear work. Then fast scanning calorimetry was used to differentiate the nucleation activity from both flow-induced precursors and CNC particles during isothermal crystallization across a wide temperature range. It was found that the addition of the CNC accelerated crystallization in the heterogeneous nucleation regime (T > 100 °C) in the quiescent material. With the addition of shear, the neat system displayed a reduced crystallization peak time with increasing shear history. In the nanocomposite system, the CNCs are an extremely efficient nucleating agent, achieving a saturating limit for nucleation of crystallization, such that shear was only a factor at low supercooling, specifically at temperatures greater than 140 °C.