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Nucleation and crystallisation of isotactic polypropylene by layered nucleating agents
thesisposted on 2017-02-21, 23:54 authored by Bhatia, Amita
It is well established that the microstructure of thermoplastic semi-crystalline polymers depends on the processing route leading to the final article, and that in processes such as injection moulding the microstructure shows large structural gradients within a particular moulding, particularly in the form of layers through the thickness of the articles. Generally, nucleating agents are used to enhance the polymer crystallisation, in order to speed up the processing and alter the morphology and the final properties of the product. This project focuses on the crystallisation of semi-crystalline polymers when layered nucleating particles were added. When the flow intensity is sufficient, a highly oriented thread-like core is formed (shish), which later offers a template for the formation and growth of oriented chain folded crystalline lamellae (kebabs). This whole structure “shish-kebab” is a model for understanding morphology development under flow conditions. To understand the structure-property relationship, layered nucleating agents like zinc monoglycerolate (ZMG), cobalt monoglycerolate (CoMG), bismuth monoglycerolate (BiMG), calcium monoglycerolate (CaMG) and boron nitride (BN) with 0, 0.02, 0.05, 0.1 and 1.0 wt% of loadings in isotactic polypropylene (iPP) have been used. The main focus of this study was to understand the mechanism instigating the morphological development in iPP due to the presence of these layered nucleating agents. This thesis develops an understanding of how the properties of a polymer depend on microstructure, taking into account orientation and crystallinity. Processing histories were controlled and studied using a shear cell. Differential scanning calorimetry (DSC) has been used to study the crystallisation kinetics during the crystallisation of the samples. Optical microscopy has also been used to investigate the in-situ time-resolved microstructural changes during the subsequent crystallisation for CoMG/iPP samples. Scanning electron microscopy (SEM) technique was also employed to characterise the microstructure for various samples. The influence of flow-induced crystallisation due to the presence of ZMG in iPP was investigated by in-situ time-resolved small- and wide-angle x-ray scattering (SAXS and WAXS respectively). An increase in the (Tc) of iPP was observed in the presence of all different nucleating agents, expect for BiMG. Even a loading as low as 0.02 wt% of ZMG and CoMG in iPP was found to be highly effective with regards to nucleation. ZMG and CoMG promoted crystallisation of the α-phase of iPP, although small amounts of the β-phase were also observed in CaMG/iPP. Mechanical properties, such as tensile strength and flexural modulus improved, but impact strength and elongation at break remained unchanged. A mechanism of nucleation of iPP by CoMG has been proposed. Nucleation appears to occur epitaxially on the edges of the platelets, where a small number of layers of the nucleating agent gives a strong lattice dimensional alignment with iPP along the (040) direction. Matching epitaxial configurations occur along all three spatial directions emanating from the edges of both ZMG and CoMG. Such multiple lattice matchings explain why ZMG and CoMG are such highly effective nucleating agents. It is proposed that at a given CoMG edge/side, the helices of iPP were oriented parallel to the flat surface. To provide the epitaxial relationship required for nucleation, it was inferred that two layers of CoMG align with three layers α-iPP. Molecular modelling indicated that the edges of nucleating platelets are structurally matched to the α-iPP crystallites along multiple directions, each with a lattice mismatch of less than 4%. Nucleation of iPP by ZMG was found to result from simultaneous heterogeneous and heterogeneous pathways. The kinetics for heterogeneous nucleation of ZMG/iPP samples under isothermal conditions could be described by modification of the traditional Avrami model. With the addition of ZMG, nucleation parameters (Kg) and surface free energy (σe) did not change, suggesting that the crystallisation was mostly governed by the crystal growth instead of the development of the nuclei. For non-isothermal crystallisation processes, heterogeneous nucleation becomes dominant, with the kinetics at the initial stages of crystallisation being adequately described by existing models. Deviations from these models were observed after some 65% initial crystallisations, as a result of the sporadic nucleation occurring under non-isothermal conditions and as a result of the non-radial crystallisation occurred from the ZMG crystallite when incorporated in iPP. The influence of two types of ZMG as nucleating agents for polypropylene crystallisation was evaluated also. The structural characteristics of these two types of ZMG (M42 and M44) are the same, the only difference being their particle size. Enhancement in Tc for iPP in the presence of M44 demonstrated that M44 is a better nucleating agent than M42 as the particle size of M44 is around one-third of M44, as the epitaxial crystallisation of a helical chain polymer on a fine powdered M44 substrate is most favourable. Mechanical properties like flexural moduli, tensile strength and Young’s modulus were improved with the loading of both the types of ZMG as compared to iPP(control) due to the much smaller domain size of PP. However, M44 proved to produce better results than M42 because the smaller the particle size, the better interfacial adhesion with the matrix, thus displayed superior performance. It was concluded as well that ZMG of smaller crystallite size in iPP seems to be the better indicator of nucleating ability, rather than the smaller particle size alone. XRD experiments showed that as result of the ability for ZMG to fragment and disperse, ZMG showed superior nucleating ability to most other nucleating agents. Therefore, in order to enhance Tc, care must be taken to reduce not only the particle size, but more importantly the crystallite size. Examination of the nucleation and crystal growth of isotactic polypropylene (iPP) by in-situ synchrotron wide-angle x-ray scattering (WAXS) and small-angle x-ray scattering (SAXS) techniques showed that the crystallisation half-time is substantially decreased in the presence of zinc monoglycerolate (ZMG). Under shear conditions, the rate of crystallisation is further enhanced when compared with quiescent conditions. SAXS data indicated the formation of additional ordered structure between the crystalline and amorphous regions during crystallisation as the ZMG levels increased, accompanying the branch formation of the characteristic “shish-kebab” structure of iPP. Parallel stacks of branched lamellae acted as nuclei for further crystallisation, growing predominates perpendicular to those branches. The polymer microstructure studied by reflected optical microscopy also showed a further decrease in spherulite size of iPP in the presence of ZMG under shear, when compared with non-nucleated iPP. Previous studies on the importance of the substrate during heterogeneous polymer nucleation took into account the importance of the similarity of crystallographic unit cell between polymer and substrate for epitaxial phenomenon. As a part of this work, newly developed layered nucleating agents like BN, CaMG and BiMG were also investigated to check their nucleating ability in iPP. Increase in Tc indicates good nucleating effect of BN, CaMG in iPP.