Polymer Crystallization with Configurable Birefringence in Double Emulsion Droplets

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.