Specific volume of polymers : influence of the thermomechanical history Citation for published version (APA): van der Beek, M. H. E. (2005). Specific volume of polymers : influence of the thermomechanical history. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR590887 DOI: 10.6100/IR590887 Document status and date: Published: 01/01/2005 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Proefschrift. – ISBN 90-386-2567-7 NUR 971 Subject headings: isotactic polypropylene / semi-crystalline polymers / specific volume / PVT behavior / cooling rate / pressure dependence / flow induced crystallization / dilatometry Printed by Universiteitsdrukkerij TU Eindhoven, Eindhoven, The Netherlands. Specific volume of polymers Influence of the thermomechanical history PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 14 juni 2005 om 16.00 uur door Maurice Hubertus Elisabeth van der Beek geboren te Roermond Dit proefschrift is goedgekeurd door de promotoren: prof.dr.ir. H.E.H. Meijer en prof.dr.ir. J.M.J. den Toonder Copromotor: dr.ir. G.W.M. Peters Veur mien maedje Contents 1 Introduction 1 1.1 Context . 1 1.2 Background . 1 1.3 Scope . 4 1.4 Outline . 4 References . 6 2 Concentric cylinder dilatometer: design and testing 9 2.1 Introduction . 9 2.2 Design and instrumentation . 12 2.3 Experimental . 19 Sample preparation . 20 Procedure . 21 2.4 Comparison with confining fluid based dilatometer . 22 2.5 Example: isotactic polypropylene . 26 2.6 Conclusions . 27 References . 28 2.A Appendix: material properties . 30 3 The influence of cooling rate on specific volume 33 3.1 Introduction . 33 3.2 Experimental part . 35 Materials . 35 Dilatometer experiments . 36 X-ray analysis . 36 Density measurements . 37 3.3 Results and discussion . 37 Specific volume . 37 Crystalline morphology . 43 Modelling aspects . 47 3.4 Conclusions . 55 References . 56 vii viii CONTENTS 3.A Appendix: specific volume of the melt . 58 4 The influence of shear flow on specific volume 61 4.1 Introduction . 61 4.2 Experimental part . 62 Materials . 62 Dilatometry . 64 Density gradient column . 64 X-ray analysis . 65 Scanning electron microscopy . 65 4.3 Results and discussion . 65 Specific volume . 65 Crystalline morphology . 74 4.4 Conclusions . 79 References . 80 5 Classification of the influence of flow on specific volume: The Deborah number. 85 5.1 Introduction . 85 5.2 Methods . 87 Deborah number . 87 Dimensionless transition temperature . 88 Dimensionless transition rate . 88 5.3 Experimental part . 89 Materials . 89 Experimental techniques . 90 5.4 Results and discussion . 90 Crystalline morphology . 90 Specific volume . 94 5.5 Conclusions . 97 References . 98 6 Conclusions and recommendations 101 6.1 Main conclusions . 101 6.2 Recommendations . 103 References . 106 Samenvatting 107 Dankwoord 111 Curriculum Vitae 113 Summary Nowadays, semi-crystalline polymers are widely used in many product applica- tions that display high dimensional accuracy and stability. However, the relation- ship 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 cho- sen processing conditions. In this thesis, the dependence of the specific volume of crystallizing polymers on the thermomechanical history as experienced during pro- cessing is investigated. Emphasis is placed on selecting and reaching those process- ing conditions that are relevant for industrial processing operations such as injection molding and extrusion. To extent the interpretation of the results obtained on the de- velopment of specific volume, structure properties of the resulting crystalline mor- phology 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 quanti- tatively 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 er- ror 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 ¡ 0.2%. However, especially for higher cooling rates, this part of the measured specific volume curve should be taken as qualitative rather than quantitative. ix x SUMMARY 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. Addi- tionally, an increasing cooling rate causes the final specific volume to increase, which agrees with a decrease in the degree of crystallinity determined from WAXD anal- ysis. 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 experi- mental 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 pre- diction during fast cooling, G(T, p) and N(T, p) should be characterized for a suf- ficiently large temperature range, including temperatures typically lower than the temperature where the maximum in G(T, p) occurs. Deviations in predicted tran- sition 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 dur- ing 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.