Influence of Crystal Size on Crystallization and Structure of Second Component in Diblock Copolymer Films

The use of copolymers to combine the attributes of multiple monomer components have been extensively studied for fields including energy and biomedical applications. The homopolymer polycaprolactone (PCL) displays biocompatibility, hydrophobicity, and the ability to hydrolytically degrade at its ester bond. Polyethylene-oxide (PEO), a hydrophilic biocompatible polymer with well defined mechanical properties displays rapid degradation when exposed to water. These two components may be bonded to form a diblock copolymer, PEO-b-PCL, which combines the catalytic breakdown of PCL along with the strength and water degradation of PEO, the latter of which is often valued in medical applications. Through the manipulation of these two blocks, the resulting material properties may be tailored for a specific use, such as coronary stent coatings or vascular grafts. A better understanding of the material’s phase behavior is necessary to properly tune the copolymer, and as such, this study aims to determine the effects of varying solvents, copolymer molecular weights, additive molecular weights, and polymer concentrations on the relative crystallization of each block. Sample formulation employed the solvents toluene and ethyl acetate, four separate molecular weights of the copolymer PEO-b-PCL, and three different molecular weights of additive homopolymer PEO. Fourier Transform Infrared Spectroscopy (FT-IR) was used to analyze the relative crystallization of PEO to PCL while Polarized Optical Microscopy (POM) permitted the study of film morphology and the verification of crystal order. Analysis of the resultant data revealed the solvent interactions exhibited by samples dissolved in ethyl acetate promoted greater relative PEO crystallization. With regards to PEO additives, low molecular weight PEO, roughly 400 g/mol, decreased the crystalline ratio as a result of its liquid-like state altering the order in which each block crystallizes. Higher molecular weight PEO (both 5400 g/mol and 36500 g/mol) added to solutions displayed consistent improvements in relative crystallization across all concentrations. Additionally, multiple concentration saturation points were discovered which provided insight into the effective maximization of crystal ratio for both copolymer in solution and additive in copolymer samples.