Specific volume of polymers : influence of the thermomechanical history

Abstract

Nowadays, semi-crystalline polymers are widely used in many product applications that display high dimensional accuracy and stability. However, the relationship between processing conditions and the main property determining macroscopic shrinkage, i.e. specific volume, is still not understood in sufficient detail to predict the resulting dimensions of a product dependent on the selected material and chosen processing conditions. In this thesis, the dependence of the specific volume of crystallizing polymers on the thermomechanical history as experienced during processing is investigated. Emphasis is placed on selecting and reaching those processing conditions that are relevant for industrial processing operations such as injection molding and extrusion. To extent the interpretation of the results obtained on the development of specific volume, structure properties of the resulting crystalline morphology are investigated using wide angle X-ray diffraction (WAXD) in combination with scanning electron microscopy (ESEM). A custom designed dilatometer is presented in chapter 2, which is used to quantitatively analyze the dependence of specific volume on temperature (up to 260 ±C), cooling rate (up to 100 oC/s), pressure (up to 100 MPa), and shear rate (up to 80 1/s). The dilatometer is based on the principle of confined compression, using annular shaped samples with a radial thickness of 0.5 mm. To quantify the measurement error arising from friction forces between the solidifying sample and dilatometer walls, a comparison is made with measurements performed on a dilatometer based on the principle of confining fluid (Gnomix). Measurements performed in the absence of flow, at isobaric conditions, and at a relatively low cooling rate of about 4-5 oC/min agree quite well with respect to the specific volume in the melt, temperature at which the transition to the semi-crystalline state starts, and the specific volume of the solid state. Detailed analysis shows a relative difference in specific volume of the melt of 0.1 - 0.4 %. An identical relative difference is assumed for specific volume measured during the first part of crystallization, since the ratio of shear and bulk modulus is still small and the influence of friction forces and loss of hydrostatic pressure can be neglected. The relative difference in the specific volume of the solid state ranges from 0.1 i 0.2%. However, especially for higher cooling rates, this part of the measured specific volume curve should be taken as qualitative rather than quantitative. The influence of cooling rate on the evolution of specific volume and the resulting crystalline morphology of an isotactic polypropylene is investigated in chapter 3. Experiments performed at cooling rates ranging from 0.1 to 35 oC/s, and elevated pressures ranging from 20 to 60 MPa show a profound influence of cooling rate on the transition temperature, i.e. the temperature at which the transition from the melt to the semi-crystalline state starts, and on the rate of transition. With increasing cooling rate and constant pressure, the transition temperature shifts towards lower temperatures and the transition itself is less distinct and more wide spread. Additionally, an increasing cooling rate causes the final specific volume to increase, which agrees with a decrease in the degree of crystallinity determined from WAXD analysis. For the relatively small pressure range that was experimentally accessible, a combined influence of pressure and cooling rate on the specific volume or crystalline morphology was not found. Experimental validation of numerical predictions of the evolution of specific volume showed at first large deviations in the calculated start and rate of the transition. These deviations increase with increasing cooling rate. Deviations in the rate of transition could partly be explained from small variations in model parameters, and can be justified from possible inaccuracies in the experimental characterization of important input parameters, i.e. the spherulitic growth rate G(T, p) and the number of nuclei per unit volume N(T, p), or from determining model parameters to describe these quantities numerically. Especially in the prediction during fast cooling, G(T, p) and N(T, p) should be characterized for a sufficiently large temperature range, including temperatures typically lower than the temperature where the maximum in G(T, p) occurs. Deviations in predicted transition temperature are however quite unexplained and could only be improved by introducing an unrealistic larger number of nuclei than determined experimentally at relatively high temperatures. This is subject to future investigation. The influence of shear flow on the evolution of the specific volume is investigated in chapter 4. The combined influence of shear rate, pressure and temperature during flow is investigated at non-isothermal conditions using two grades of isotactic polypropylene with different weight averaged molar mass (Mw). In general, shear flow has a pronounced effect on the evolution of specific volume. The temperature marking the transition in specific volume and the rate of transition are affected. The influence of flow increases with increasing shear rate, increasing pressure, decreasing temperature at which flow is applied, and higher Mw. Although the degree of orientation and the overall structure of the resulting crystalline morphology are greatly affected by the flow, the resulting specific volume and degree of crystallinity are only marginally affected by the processing conditions employed. If shear flow is applied at a temperature near the material’s equilibrium melting temperature T0m , i.e. at low undercooling, dependent on material and applied shear rate remelting of flow induced crystalline structures and relaxation of molecular orientation is able to fully erase the effect of flow. With increasing Mw, the effect of flow applied at low undercooling is prevailed longer. Although not investigated in this study, we think that an increased cooling rate (i.e. less time to remelt flow induced structures) would also enlarge the resulting effect on the evolution of specific volume when applied at low undercooling. In chapter 5, the use of the dimensionless Deborah number is investigated to analyze and classify the influence of shear flow on the specific volume and resulting crystalline morphology. Classification of the influence of flow on the orientation of the resulting crystalline morphology as visualized byWAXD could be performed if flow was applied at relatively large undercooling. With increasing Deborah number, the orientation of crystals increases and the classification of the flow strength resulting in a spherulitic, row nucleated, or shish-kebab morphology is possible. However, in case flow was applied at low undercooling, the influence of remelting and relaxation of molecular orientation yields the Deborah number of little use. The influence of flow could be erased totally, even when strong flow is applied, i.e. high Deborah numbers. For large undercooling, remelting and relaxation has little effect on the development of the flow-induced crystalline morphology as was already observed by others. These conclusions also hold for the classification of flow on the evolution of specific volume. If flow is applied at large undercooling, Deborah numbers Des (based on the process of chain retraction) or Derep (based on the process of reptation of chains) can equally well be used to classify the influence of flow on the evolution of specific volume, e.g. characterized by the dimensionless transition temperature µc and dimensionless rate of transition ¸. Even relatively large differences in cooling rate have little effect on the classification of the influence of flow on the evolution of specific volume, when applied at large undercooling. Finally, in chapter 6 the main conclusions of this thesis are outlined together with recommendations for future research.