Constraints in Semicrystalline Polymers During the Transition from the Liquid to the Solid State

Tufts University, Medford MA

Investigators

Abstract

TECHNICAL SUMMARY: The confining effect of crystals, during the transformation from liquid to semicrystalline solid of semicrystalline polymers, will be investigated in this research program. The research will answer the following specific questions about structure and relaxation dynamics vis--vis confinement. 1. Does the length, xCRR, of cooperatively rearranging regions, CRRs, at the glass transition correlate with the development of the solid fraction, the crystal fraction, or both? 2. Do the alpha relaxation (glass transition) dynamics vary in a manner correlated with the development of these same fractions? and, 3. What is the structure of the resultant nanophases during solidification? The answers to these questions will provide a fundamental level of understanding about the relationship between the crystals and the non-crystalline portions of the semicrystalline polymer, as confinement occurs during the transition from the liquid to the solid state. A major, and novel, aspect of this research will be the use of quasi-isothermal measurements of the reversing heat capacity to study the time development of formation of the crystal constraints. The time development of the solid fraction, and the crystalline fraction, respectively, will be assessed from the earliest stages of crystallization, through spherulite impingement, and finally to completion of the crystallization process. The alpha relaxation (glass transition process) will be studied using dielectric relaxation spectroscopy, and lamellar structure will be determined using small angle X-ray scattering and atomic force microscopy. NON-TECHNICAL SUMMARY: Plastics are ubiquitous materials in everyday life. The majority of commercially useful plastics are processed using heat and pressure to form them into desired shapes. An important class of plastics comprises those that also have the ability to crystallize during processing. The crystals have very complicated shapes, and tend to form in clusters. But in a very simplified picture, the crystals act like miniature staples, holding the plastic together on the nanometer length scale, and preventing the constituent molecules from shifting around. To improve the ultimate properties of the plastics, it is important to: understand when and how the staples form; characterize the actual crystal shape, size, and organization; and determine how crystals restrict the motion of the other molecules. This research will lead to new products and processing strategies for plastics, improving our global competitiveness. The program also serves to educate the next generation of scientists, with a special focus and outreach to students who are deaf or hard of hearing.