The process of seeded growth of fibrillar polyethylene crystals has been studied in a tubular flow geometry for 0.01‐wt % solutions of a high‐molecular‐weight polyethylene in xylene. The transformation sequence has been followed visually by using polarized‐light illumination in conjunction with a video camera. Data are presented to show that transformation is initiated by the formation of a concentrated, unoriented, amorphous precursor fiber within which oriented birefringent crystals subsequently grow in consequence of the stresses transmitted by the flowing solution. Time constants for the precursor formation, birefringence initiation, and completion of birefringence were measured as functions of temperature and flow rate over a range of growth conditions. Wide‐angle x‐ray diffraction, overall birefringence, and optical hot‐stage melting data were also obtained on the grown fibers. The net result of these observations is to conclude that fibrillar crystal growth during flow is always preceded by the formation of a liquidlike phase transformation which produces the concentrated, unoriented precursor. Subsequent orientation is in consequence of stress‐induced crystallization with overall fiber orientation showing an increase with solution flow rate at a fixed temperature and a decrease with temperature at a fixed flow rate. At higher temperatures and lower flow rates, birefringence develops in an oscillatory fashion, indicating a remelting process possibly due to slippage of trapped chain entanglements formed by flow. A discussion is given of the implications of these observations for the understanding of flow‐induced structure development, phase transformation, and oriented crystallization; this is expanded upon in a companion paper, Part II.
The accurate and precise measurement of process stream temperatures during injection molding can be difficult, since the cyclic operation results in spatial and temporal variations of the stream temperature. This paper examines the application of a spectral infrared (IR) pyrometer to monitor the cooling of a polymer melt within the mold cavity during a typical injection molding cycle. An outline for interpreting the radiation signal collected with the IR pyrometer is presented. The discussion includes theoretical aspects as well as experimental results. The theoretical approach accounts for the polymer transparency (attenuation behavior) at the spectral wavelength of the pyrometer and also for the temperature gradient within the polymer, thereby establishing the concept of a critical depth for a given pyrometer/polymer combination. The final analysis reveals good agreement between the predicted and measured results for the transient cooling conditions of the polymer within the mold cavity. Depending on the degrees of polymer transparency used in the theoretical prediction, the deviation between the measured and predicted transient bulk temperatures after mold filling (during the mold cooling stage) varies from ±2°K to ±9°K.
This is a continuation of the preceeding paper, Part I, and presents a discussion of the nature of the precursor structure formation process observed in the flow‐induced crystallization experiments described in I. A discussion of stress‐induced crystallization theory as applied to these experiments is also given and a first‐order analysis of crystal nucleation rates is presented. Conclusions regarding the nature of flow‐induced crystallization and our current ability to quantitatively model the overall process are also presented.
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