Novel aspects in the crystallization of polybutene-1
Creators: name: WANG, WEI
Publisher: name: Universit� degli studi di Genova
Year: 2022

description: Semicrystalline polymers cover over two thirds of commercially produced polymeric materials, and have been widely applied to many areas of the modern society, including building and construction, electronics, packaging, etc. Understanding the behavior of polymer crystallization is of critical importance due to the significant impact of the crystallization process on the properties of materials. Polymorphism is the ability of a polymer, in analogy with low molecular mass substances, to crystallize in different modifications, characterized by different crystal structures (polymorphic forms). Much effort has been made to find proper methods to develop different crystal modifications for polymorphic polymers. However, it is still a challenge to control the polymorphic outcome of the crystallization process. Furthermore, semicrystalline polymers are composed of stacked crystalline lamellae and entangled amorphous polymeric chain segments in between them. The interaction between amorphous and crystalline phases plays an important role in determining final mechanical and transport properties. Despite this importance, the effect of polymorphism on the amorphous phase is not well clarified because of its complexity. In this thesis, one typical polymorphic polymer, polybutene-1 (PB-1), was selected for a detailed crystallization study. Polybutene-1 is one of the most investigated polymorphic polymers. It has applications in pipes and films with a service life up to 50~100 years due to its excellent mechanical properties. In practice, among all crystalline polymorphisms within PB-1, Forms I and II are the most relevant modifications from the processing perspective. With the goal to establish a comprehensive understanding of the heterogeneous nucleation between these two modifications (cross-nucleation), we monitor the crystallization process of Form II induced by Form I crystals with different type of substrate (spherulitic, hedritic, fiber-like) using a direct investigation technique of optical microscopy. The different cross-nucleation efficiencies of Form II are tentatively attributed to differences in the Form I lamellar thickness, on the basis of an epitaxial crystallization and secondary nucleation mechanism. A quantitative analysis of the induction time for nucleation determined the cross-nucleation energy barrier, which could be reasonably described by classical models. The results revealed that the rate determining step for nucleation is the growth of the nucleus to critical sizes. Furthermore, the hypothesized epitaxy in PB-1 Form II-on-Form I cross-nucleation is probed by employing in-situ nanofocused synchrotron X-ray diffraction. Comparing the two-dimensional diffraction patterns at the interface between the two modifications, a preferred mutual orientation of the two structure, with the (200)II plane aligned ~8.5� apart from the (110)I plane, is revealed. This demonstrates a parallel (110) plane between the two polymorphs. Then, both mismatches between the inter-chain distances and along the chain axes within (110) plane were considered, and found to lay well-below the accepted mismatch criterium for epitaxy. This confirms that the cross-nucleation of Form II on Form I occurs at the (110) contact planes through epitaxial nucleation. Next, an in-depth study of fiber-induced nucleation ability and crystalline morphology in polybutene-1/single fiber composites is presented. Using different fibers as substrates, we could unveil the difference of Form II crystalline morphology: a transcrystalline layer (TCL) induced by PB-1 Form I fiber and hybrid shish?calabash structure (HSC) induced by other fibers, namely carbon, glass, PP, PLLA homocrystal and stereocomplex. Based on a quantitative analysis of the nucleation kinetics, it was found that the nucleation free energy barrier is affected both by surface roughness and surface chemistry or specific surface-polymer interactions (such as epitaxy). In view of the number of nucleation sites correlating with the fiber surface roughness, it was demonstrated that transcrystallinity can be obtained only when a sufficient amount of nucleation sites is available, notwithstanding the height of the nucleation barrier. Besides the phenomenon of heterogeneous nucleation, the three-phase structure is also influenced by crystal polymorphism. Therefore, in the last part of the thesis, we focused on the study of the rigid amorphous fraction in both polymorphs, i.e., the part of amorphous chains constrained by direct coupling to the crystalline lamellae. Isochronous aging experiments with differential scanning calorimetry (DSC) on both crystalline phases are performed, in a wide temperature range between the glass transition of the mobile (bulk) amorphous fraction and the onset of crystal melting. An endothermic peak above the aging temperature is typically observed. The trend of the enthalpy of this annealing peak with temperature can be described by a bell-shaped curve, approaching zero recovered enthalpy at temperatures of 100-110 �C, and 40-50 �C for Form I and Form II, respectively. These temperatures are thus identified as the upper limit of the glass transition of rigid amorphous fraction for the two polymorphs. Overall, our results demonstrate that for PB-1, at least within the investigated temperature range, the annealing peaks can be related to the RAF strive to attain thermodynamic equilibrium in its glassy state.