A Polymer Visualization System with Accurate Heating and Cooling Control and High-Speed Imaging
A visualization system to observe crystal and bubble formation in polymers under high temperature and pressure has been developed. Using this system, polymer can be subjected to a programmable thermal treatment to simulate the process in high pressure differential scanning calorimetry (HPDSC). With a high-temperature/high-pressure view-cell unit, this system enables in situ observation of crystal formation in semi-crystalline polymers to complement thermal analyses with HPDSC. The high-speed recording capability of the camera not only allows detailed recording of crystal formation, it also enables in situ capture of plastic foaming processes with a high temporal resolution. To demonstrate the system’s capability, crystal formation and foaming processes of polypropylene/carbon dioxide systems were examined. It was observed that crystals nucleated and grew into spherulites, and they grew at faster rates as temperature decreased. This observation agrees with the crystallinity measurement obtained with the HPDSC. Cell nucleation first occurred at crystals’ boundaries due to CO2 exclusion from crystal growth fronts. Subsequently, cells were nucleated around the existing ones due to tensile stresses generated in the constrained amorphous regions between networks of crystals.
A batch foaming visualization system with extensional stress-inducing ability
In typical plastic foaming processes, plastics are subjected to various amount of stress/strain, which can induce cell nucleation and affect the final cell morphology of plastic foams. Despite many valuable insights in previous work on stress-induced foaming, further studies are needed to clarify the subject. While in-line observations of foaming processes provide useful cell growth and interaction data, a controllable stress/strain field cannot be easily induced in a plastic sample to evaluate its effect. This study presents a novel batch foaming in-situ visualization system that was developed to capture the foaming process of a plastic specimen under controllable extensional strain. Using the in-situ visualization system, this study explored the effects of extensional stress/strain on the foaming behaviors of polystyrene blown by carbon dioxide under various processing conditions. It was observed that cell density increased with the application of extensional strain and that this effect was more apparent at low processing temperatures and with the addition of talc.
Zur Netzwerktheorie von Polymer-Schmelzen
ZusammenfassungDie vonLodge vorgeschlagene Theorie der „gummiartigen“ Flüssigkeit stellt bei kleinen Deformationen eine korrekte Beschreibung des rheologischen Verhaltens polymerer Lösungen und Schmelzen dar. Bei größeren Deformationen versagt sie jedoch zunehmend, da sie nicht in der Lage ist, strukturviskoses Verhalten, d. h. die Abnahme der Scherviskosität und der 1. Normalspannungsfunktion mit zunehmender Schergeschwindigkeit, zu erklären. Auch bei uniaxialer Dehnung verhalten sich Polymer-Schmelzen vom Standpunkt der Lodgeschen Gleichung aus betrachtetstrukturviskos. Aus dem Vergleich des experimentell ermittelten und des für eine gummiartige Flüssigkeit vorausgesagten Spannungsverlaufs bei Relaxationsversuchen und bei Spannversuchen kann der Schluß gezogen werden, daß die Struktur des temporären Netzwerkes einer Polymer-Schmelze bei größeren Deformationen zunehmend zerstört wird. Die Zahl der das Netzwerk aufbauenden Haftstellen nimmt mit zunehmender Deformation ab. Für den Fall der einfachen Scherung und der uniaxialen Dehnung einer wohldefinierten Polyäthylen-Schmelze wird die Deformationsabhängigkeit der Haftstellenzahl angegeben und verglichen.SummaryLodge's rubberlike-liquid theory is a valid description of material behaviour at small strains. Unfortunately, it fails in the nonlinear region, i.e. at higher strains. In shear flows, the phenomenon of shear thinning, which is a fundamental property of nearly all polymer fluids, is not explained. From a point of view of Lodge's equation the deviation of material behaviour from theoretical prediction in uniaxial elongational flow is one of strainthinning, too, and not of strain thickening. By comparison of experimental stress growth and stress relaxation data with theoretical predictions of the rubberlike liquid constitutive equation the conclusion can be drawn that the structure of the temporary network of a polymer melt is destroyed increasingly at higher strains. The number of entanglements decreases with increasing strain. For simple shear and uniaxial extension of a well-defined polyethylene melt the strain dependence of the number of entanglements is given and compared.