Polymer Crystallization with Configurable Birefringence in Double Emulsion Droplets
Baihui Li, Yue Zhao, Xiaotong Chen, Zhiqi Wang, Jun Xu, Weichao Shi
Abstract
Biodegradable semicrystalline polymers are broadly used in microfluidic techniques to create microdroplets for various applications. However, it remains a fundamental challenge to modulate polymer crystallization and optical properties in microdroplets which have complicated spatial geometries. Here we investigate the crystallization of double emulsion droplets formed by poly(1,4-butylene adipate) (PBA), polycaprolactone (PCL), and their blends. Crystallization of these polymers drives the spherical liquid droplets to anisotropic solid microcapsules. Crystal growth shows a linear relationship with time, irrespective of flat-on, edge-on, or continuous twisting lamellar arrangement in the shell. We further reveal that the anisotropic shape and lamellar arrangement impart the crystallized microcapsule with orientation-sensitive birefringence. A representative feature is that all anisotropic microcapsules show two types of Maltese cross, depending on the microcapsule orientation. The PBA shell with flat-on lamellar arrangement always shows positive birefringence, while the PCL shell with edge-on lamellar growth may present negative or positive birefringence at specific orientation angles. Microcapsules of PCL/PBA blends are regulated by the composition of two polymers (fPCL), realizing the transition from PBA-dominated shell to PCL-dominated shell across fPCL = 0.5. We emphasize the peculiarities of crystallization behavior in double emulsion droplets by comparing with crystallization on plane substrate. This work may serve as a fundamental reference for understanding polymer crystallization and further expanding potential applications of semicrystalline polymers for optical microdevices.