Influence of spinline stress on release properties of a coaxial controlled release device based on EVA polymers

Purpose. To investigate the influence of the extrusion parameters on the polymeric structure and release properties of polyethylene vinyl acetate (EVA) coaxial fibers, used for controlled release of steroids. Methods. Coaxial fibers were prepared under various extrusion conditions. Both spinline stress and release properties were determined. The polymeric structure of the membrane was investigated with wide angle X-ray scattering (WAXS). Results. Upon leaving the spinneret, the polymeric fiber exhibits a large die swell. As a consequence, it is necessary to apply a force to draw the fiber to its desired diameter. A larger drawing force is needed at lower extrusion temperature, at a smaller air gap, or at a higher spinning velocity. It was found that the release rate of a steroid from the coaxial fiber increases, when the fibers are prepared at a higher spinline stress. X-ray measurements reveal that at higher spinline stress, the crystalline volume fraction of the membrane decreases. As a result of a decreasing crystallinity, the permeability of the polymer increases. Conclusions. It is demonstrated that the extrusion parameters and spinline stress have a significant influence on the polymeric structure of the membrane and hence the release properties. Higher spinline stress results in a higher release rate.