Controllable Growth of Gradient Structures for Biomedical Applications

Co-continuous phase structures of immiscible polymers can be developed under appropriate melt-blending conditions. Because of the presence of interfacial tension, such co-continuous structures start to coarsen when heated to a temperature higher than the melting/softening temperature of both phases. In this article, a systemic study of controllable growth of gradient porous structures utilizing variable coarsening rates in either a gradient temperature field or a gradient shear field is presented. Based on experimental results, the gradient of shear viscosity is identified as the mechanism for generating variable coarsening rates inside a co-continuous blend. By controllable variation of the shear viscosity distribution in a blend, a spatially varied and controllable gradient in phase structure is created. After dissolution of one of the two phases, the desired porous structure of the remaining polymer is obtained. A poly (lactic acid) (PLA)/polystyrene (PS) 50/50 wt% blend was used as a model system. By designing proper thermal and/or dynamic boundary conditions and introducing different thermal/shear rate gradients during annealing, several gradient porous structures of PLA were created.