Crystallization-Induced Fluid Flow in Polymer Melts Undergoing Solidification

Abstract

The influence of processing conditions on the morphology of solidified semicrystalline polymer materials formed under dynamic melt processing conditions has been of concern for decades.1 Although there has been much recent progress in modeling the kinetics and origin of polymer spherulitic crystallization,2,3 the influence of fluid flow and the viscous properties of the undercooled polymer melt on the crystallization morphology have received relatively little attention due to the great computational problems involved in modeling and the experimental difficulties in observing this phenomenon. It is often argued that the macromolecules of the undercooled melt can diffuse to the crystal growth front during crystallization,4 and fluid flow processes are simply ignored. While this assumption may be a reasonable approximation under low undercooling conditions, one may well wonder whether this situation remains true under the conditions of rapid solidification that normally arise during polymer material processing. Flow in the undercooled polymer melts has not been studied much experimentally previously because of the lack of methods of sufficient temporal and spatial resolution to discriminate between the crystalline and amorphous phases during the course of the crystallization process. It is well-known, however, that the formation of commercial semicrystalline polymeric products by injection moldings, film extrusions, film blowing, etc, can be accompanied by the formation of unwanted defects (voids, pores, and other imperfections) and that the ultimate properties of these materials can be appreciably affected by processing conditions.5 The present study indicates that rapid fluid flow can occur in a model undercooled polymer melt (a common commercial polymer, isotactic polypropylene) during the course of crystallization under confinement conditions (simply crystallizing polymer film between cover glasses). We have also observed a similar behavior (not yet published) in crystallizing poly(ethylene oxide) (PEO) melts, indicating some degree of generality for the phenomenon. The isotactic polypropylene (iPP) polymer employed in this work was a commercial grade polymer obtained from the Αldrich Chemical Co., and we show typical results for an isothermal crystallization temperature of 138 °C. The “weight” and “number” molecular masses Mw and Mn were about 340 000 and 97 000, respectively, so this polymer was rather polydisperse, typical of commercial material. The carbon black (CB) relative mass of the probe particles to the polymer matrix was relatively low, 0.5%. The iPP and CB were mixed by meltpressing repeatedly (five times) on a hot plate at 200 °C. Samples for the crystallization and flow measurements were prepared by pressing the melt mixtures between two cover glasses into thin films having a thickness of about 30 μm, thus providing a model confinement environment for the polymer melt crystallization process. Our optical microscope (Carl Zeiss JENA, made in Germany), equipped with a CCD camera (HV1301UC, made by the Da Heng Co. in Beijing), was used to image flows in the undercooled melts by following the motion of CB particles during isothermal crystallization. The resolution of the CCD camera in the x and y directions was about 0.2 μm. A homemade dual-temperature microscope hot stage provided a temperature control, with a temperature uncertainty of ( 0.1 °C. The iPP samples were first melted at 200 °C for 10 min to melt the crystallized structures formed in the course of the previous sample history and then were rapidly transferred to a crystallization temperature below the melting temperature (160 °C). Figure 1 shows a serial of micrographs of the iPP sample during isothermal crystallization at 138 °C. The present work not only considers the standard problem of characterizing the spherulite growth, but also considers how spherulite growth induces flow in the surrounding polymer melt by observing the motion trajectories of CB particles in the undercooled iPP “melt”. The basic concept of our measurement can be understood from the following nautical analogy: The undercooled melt can be viewed as a sea and the growing spherulites as islands that grow up out of this sea. The CB particles are convected by the fluids, providing information about the prevailing local “currents” in the undercooled polymer melt.