Effect of Shear Flow on the Crystallization Morphology of Isotactic Polypropylene

The study of polymer melt crystallization stimulated by flow has attracted much interest because it implies the possibility of controlling the final morphology, and consequently quality and properties, of semi-crystalline polymers. The renewed interest in the flow induced crystallization is due to the availability of new experimental tools that can yield insight into molecular characteristics that control the crystallization pathways adopted by a stresses polymer melt.
In this work, the effects of a shear flow applied during crystallization on the morphology evolution and on the kinetics of isothermal crystallization of two iPP have been studied experimentally.
The group of Material Technology of the Department of Mechanical Engineering of the University of Eindhoven - TU/e, The Netherlands (Prof. G.W.M. Peters) has a Multi Pass Rheometer (MPR) equipped with a in house designed slit flow cell that can be used to perform flow experiments at processing conditions, i.e. high pressures, high shear rates, high cooling rates (but also isothermally) and (multiple times) reversed flow. This device is, as a result, an experimental setup to study in-situ and ex-situ structure and morphology development of polymers with a control over the processing conditions and shear history.
The aim of this thesis is to contribute to the understanding of the relation between processing condition and final morphology of polymers using this powerful tool and investigating the flow-induced crystallization in melts.
The use of this new shear cell which can be operated up to shear rates of 104 s-1, in some occasion in combination with a modulated laser, made it possible to present flow induced crystallization in two iPP samples with different MW and MWD.
A series of isothermal crystallization tests at three temperatures were conducted by imposing shear rates, ranging from 0 (quiescent conditions) to 800 s-1 and varying the shear time.
The resulting micro-structure of the samples has been analyzed by in situ measurements like turbidity and birefringence measurements and by optical microscopy (OM) and FT-IR measurements (ex-situ measurements).
Shearing the polymer for a duration much shorter than the quiescent crystallization time at a crystallization temperature lower than the melting temperature always led to a significant change into crystallization morphology as compared to quiescent.
It was found that the effects of flow on morphology were mainly a rise of the nucleation density than the quiescent condition and the development of an evident “fine-grained layer”. These effects were stronger with increasing shear times and shear rates.
However, when shearing above a critical stress was continued for a time greater than a critical duration, the transition to the formation of oriented structures occurs. An oriented, crystalline structure is formed during shear when the wall shear stress is above some critical value and is applied for a critical shearing time. The formation of this layer correlates with the subsequent formation of a “skin-core morphology”.
The temperature influences the critical stress for the formation of the oriented layer, because the temperature controls the relaxation time of the orientation and the stretch experienced by the material.
Orientation determined by FT-IR measurements over the thickness of the sample confirmed what it was possible to observed in OM pictures.
Final morphological characteristics of samples were compared with van Meerveld et al.’ s rheological classification (van Meerveld, Peters, & Hutter, 2004). It was a good starting point, but its weak spot is the lack of the introduction of the shear time as quantitative parameter in the classification.
A Maxwell model (Pantani, Speranza, Sorrentino, & Titomanlio, 2002) was adopted to describe the evolution of molecular orientation in the sheared material. In most cases, the model explained the obtained morphology.