Directed Polymer Crystallization at Liquid Interfaces
Authors:   , Qi Hao , Li Christopher Y. 
Published: July 2021

Liquid-liquid and liquid-air interfaces are crucial to study amphiphile and colloidal self-assembly. Crystallization of polymers can also be regarded as a self-assembly process. In this dissertation, liquid interfaces are employed to direct polymer crystallization. Due to the particular interaction between the polymer chain and the interface, polymer crystallizations are confined in certain manners. According to the shape of the interface, this dissertation is divided into two parts, flat liquid-liquid interface and curved liquid-liquid interface. A flat water/pentyl acetate interface was created as a model system to study evaporative crystallization of poly ([epsilon]-caprolactone) which has carboxylic acid groups on both ends. A millimeter scale uniform polymer single crystal (PSC) film can form at the water/pentyl acetate interface. Due to the asymmetric nature of the liquid-liquid interface, the PSC film exhibit Janus property - a hydrophobic side and a hydrophilic side. Moreover, the effect of polymer chain conformation prior to on crystallization is studied, using PCL with different hydrophobic/hydrophilic ends. By using atomic force microscopy and transmission electron microscopy, terminal structures and temporal evolution of the crystalline films were explored. Different crystallization routes were revealed as a result of different polymer chain conformation. We furthermore proposed a phase diagram to explain different crystallization routes and demonstrated the phase diagram can be regulated via tuning polymer chain conformation at the liquid-liquid interface. Next, we demonstrated evaporative crystallization of PCL on a curved template will result in a curved lamella. The curved template was from PCL-b-PAA block copolymer Janus plates which can undergo a pH-triggered 2D to 3D shape transformation, due to the controlled degree of PAA ionization. As evidenced by selected area electron diffraction experiments, PCL crystalline domains arranged themselves into tortoise shell-like domain packing to accommodate the curvature. Curved space is intrinsically incommensurate with 3D translational symmetry. However, soft materials, including colloids, amphiphiles and block copolymers can form structures depicting curved surface/interfaces through self-assembly or directed assembly. It is therefore intriguing to study how polymer crystallization takes place in a curved space. Curve liquid-liquid interface was created through oil in water emulsion. Amphiphilic PLLA-b-PEG block copolymer was used as both surfactant to stabilize the emulsion and crystallization material. We demonstrated emulsion crystallization method can effectively confine and direct PLLA crystallization at curved emulsion droplet interface. Temporal evolution revealed there is only one nucleus per droplet and nucleation occurred at the interface because the crystalline block is confined at the liquid/liquid interface prior to crystallization. With the purpose to apply the obtained block copolymer crystalsomes (BCCs) for in-vivo study, we fabricated fully enclosed BCCs within 200 nm using PLLA-b-PEG/toluene and water emulsion system. Detailed structural characterization demonstrated the emulsion solution crystallization method is efficient to guide polymer chain folding into uniform crystalline packing at curved liquid-liquid interface to form single-crystal-like structure. As a new type of block copolymer ensemble, BCCs exhibit superior mechanical stability and extremely long circulation performance in vivo.