RUI: Metastability of Crystals in Double Crystalline PEO-b-PCL Films and Their Role in Transport Properties

Lafayette College, Easton PA

Investigators

Abstract

NON-TECHNICAL ABSTRACT: Polymer films are an integral part of technologies in the 21st century. The film’s ability to provide functionality is tied to its physical structure, or arrangement of molecules. For example, degradation of the film is slower in denser areas. Polymers contain some crystals that are denser than the uncrystallized, or amorphous, regions. Controlling the amount and location of the crystalline areas is important for preparing the right material for each application. This research aims to develop structure-property relationships based on the crystallization of the polymer in the film. The amount and size of polymer crystalline aggregates will be controlled by preparing films using different solvents and thermal treatments including preparation temperature or melt-cool cycles. In addition, the crystallinity of the film may change over time; therefore, films will be aged at different temperatures to evaluate whether crystalline content increases or decreases. The crystalline aggregates will be analyzed post-production using spectroscopic and microscopic techniques to learn how the preparation method influenced the crystal development. Once a better understanding of the crystallization behavior is achieved, the film properties, degradation rate, for example, will be tested to see the effect of the tailored changes in crystallization. In addition to scientific discovery, major goals of this research include the training of future scientists and engineers as well as promoting STEM education for underrepresented groups. Undergraduate students will participate in all aspects of the project from lab experiments to public presentation of the results. Performing high-level research helps prepare them for success in graduate school and industrial careers through strong independent thinking and problem solving. Outreach activities will include hosting local high-school students in research and engagement in a K-6 community outreach program that brings students to campus for engineering activities. TECHNICAL ABSTRACT: Poly(ethylene oxide)-block-poly(caprolactone) (PEO-b-PCL) copolymer films are important crystalline polymers for a number of applications. These copolymers serve as model crystalline-crystalline (CC) diblock copolymers with similar melting temperatures. In these copolymers, the crystallization mechanism is complicated, and final crystallinity can be manipulated using solvent interactions and thermal treatment. The manipulation of the crystal structure creates a new thermodynamically stable state within the film. The metastability can be altered with minor changes in state variables, such as temperature and concentration. Additionally, due to the mobile nature of polymer crystal stems, annealing or aging of the films results in a shift from one metastable state to another. It is important to evaluate and understand these changes in metastability to develop reliable structure-property relationships. In PEO-b-PCL, these metastable states are relatively similar, and the interplay of double crystallinity should result in competition between the most stable state for PEO compared to PCL. The goal of this research is to study the metastability of PEO and PCL crystals when the minority block crystallizes first, how aging affects the metastability of each crystal type, and initial correlations between physical structure and macroscopic transport properties like water uptake, degradation, and small-molecule release as models for e.g. drug delivery. By manipulating the physical structure in new ways, changes in the film’s transport properties may open up new avenues for PEO-b-PCL where these characteristics could be tailored to specific applications. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.