Flow-induced Crystallization

Polarized optical micrograph of "skin-core" morphology Our goal is to understand the molecular level physics controlling the earliest crystallization events that can be detected in semi-crystalline polymers during the imposition of shear and how these relate to structure formation after cessation of shear. Semicrystalline polymers offer the highly desirable combination of strength conferred by crystalline material and toughness provided by finely dispersed non-crystalline regions among the crystallites. The morphology and orientation distribution dictate the ultimate material properties that can be achieved from a given crystallizable polymer. The semi-crystalline morphology observed during quiescent crystallization is well-documented; however, processing flows are known to accelerate crystallization kinetics by orders of magnitude and can strongly alter the orientation distribution of the crystallites. These phenomena provide the key to producing strong sheets by film blowing and strong fibers by spinning. On the other hand, in injection molding these effects can be the source of undesired characteristics such as stress whitening, warpage, and skin delamination. Qualitatively, it is known that successful application of these processes depends on having suitable melt rheological properties. Yet, the molecular level processes that give rise to the effects of flow on the kinetics and anisotropy of nucleation from a distorted melt remain elusive. Optical experiments indicate oriented precursor structures develop rapidly during shear. Further, these structures can persist beyond cessation of shearing even at high temperatures near the nominal melting point. The nature of these structures is the source of much debate in the literature, and it is yet to be proven whether or not they are crystalline.

2-D WAXD image of isotactic polypropylene We have developed experimental capabilities based on a “short-term shearing” protocol that not only permit dissecting the effects of flow and thermal histories, but also separate the effects of nucleation and crystal growth. Our novel apparatus is capable of attaining the high levels of stress often encountered in industry. In addition, it only requires a very small amount of polymer (~5g) to carry out several experiments, which enables research on well-characterized model samples that are typically available in very small quantities. The design of our unique experimental device allows in situ structural characterization during and after shearing using multiple techniques (turbidity, birefringence, SALS, SAXS, WAXD, and IR dichroism) appropriate for probing a variety of length scales. Birefringence and turbidity provide information on millisecond and longer timescales regarding the inception of crystallization and the transient orientation distribution of the polymers. To date, our synchrotron x-ray measurements are useful on timescales of a few seconds and longer: WAXD has helped identify the specific crystal morph that forms, the earliest time at which crystallites are detectable, the time evolution of the degree of crystallinity, and the orientation distribution of the crystallites; SAXS reveals the nanostructural features of the crystal/non-crystal lamellae, as well as providing additional measures of the degree of crystallinity and the orientation distribution.

Publications on Shear-enhanced crystallization from our group are referenced here.

Associated with this project:
• Derek Thurman
• Lucia Fernandez-Ballester