Cicolella, Alessandra (2023) Polymer nanostructures of tunable periodicity from hierarchical self-assembly and crystallization of crystalline block-copolymers. [Tesi di dottorato]

	Testo Cicolella_Alessandra_35_completa.pdf Visibile a [TBR] Amministratori dell'archivio Download (23MB) | Richiedi una copia
Anteprima	Testo Cicolella_Alessandra_35_parziale.pdf Download (7MB) | Anteprima

Abstract

Aim of this Ph.D. project was exploring the properties of a novel class of semicrystalline block copolymers (BCPs) composed of crystallizable blocks of stereoregular polyolefins. These innovative materials have been synthesized only recently thanks to the development of organometallic catalysts able to promote stereoselective and living olefin polymerization. The interest towards these systems is driven by the possibility of creating thermally resistant nanostructured materials with improved mechanical properties thanks to the presence of high temperature melting crystalline domains. Moreover, the orientation of such domains can be controlled through the control of the crystallization process. However, in semi-crystalline BCPs the solid-state morphology is more complex than in classic amorphous BCPs and is the result of the competition between the phase separation of incompatible blocks and the crystallization. To address the complexity of these systems, research activities have been focused on the systematic characterization of BCPs containing crystallizable isotactic or syndiotactic polypropylene linked to amorphous blocks of random propene-ethylene copolymers or to crystalline PE and LLDPE blocks. The influence of the different compositions and block lengths on the crystallization behavior and morphology on different length scales of these systems has been examined. The thermal characterization of the crystalline-crystalline samples generally showed an overlap of the crystallization processes of the two blocks, but the optical microscopy (POM) analysis allowed clarifying the sequence of crystallization from the melt. The phase-separated structure eventually present in the melt has been revealed by TEM analysis of samples crystallized by fast quenching from the melt. It was found that the fast quenching of thin films of BCPs and of their blends can freeze the eventual nanostructure existent in the melt, allowing to image it at room temperature with TEM. Phase-separated morphologies were observed in iPP-b-PE, iPP-b-LLDPE and PE-b-sPP copolymers due to the incompatibility of the constituting blocks. The segregation was partially retained even when the thin films were crystallized by slow cooling from the melt, however in this case thin crystalline lamellae randomly oriented and homogeneously distributed were also observed. The phase-separated morphologies displayed by blends of BCPs sharing a common block of PE or EPR rapidly cooled from the melt confirm the hypothetical formation of a tri-block copolymer with a central PE or EPR block linking iPP and sPP blocks. The presence of crystallizable components in the BCPs was exploited to induce crystal orientation in thin polymer films by epitaxial crystallization on suitable crystalline substrates. An extensive study of isothermal crystallization kinetics of the BCPs was performed with the aim to unveil how the different blocks mutually influence their crystallization rates. It was found that the presence of a linked amorphous EPR block or of a molten LLDPE block drastically reduces the overall crystallization rate of the iPP block, through dilution effect. Similarly, in iPP-b-PE copolymers the iPP block, which remains in the melt state or crystallizes partially, slows down the crystallization kinetics of PE, in contrast to what happens in a iPP/PE blend, where the crystallization kinetics of PE is not affected by the presence of iPP. Self-Nucleation experiments allowed to unveil the self-nucleation behavior of iPP, PE and LLDPE blocks in the BCPs, and revealed that when iPP block is linked to an amorphous block (EPR) or to a block that crystallizes at lower temperatures (LLDPE), higher temperatures with respect to the homopolymer are necessary to obtain an isotropic melt. On the other hand, when iPP is linked to a PE block, which crystallizes before iPP and acts as a nucleating agent, lower temperatures are needed to allow the self-nucleation of iPP block, which is drastically prevented. Finally, the copolymers have been thermally fractionated with experiments of Successive Self-Nucleation and Annealing. This thesis work contributes to expand the knowledge in the field of semi-crystalline block copolymers, illustrating the influence of the nature and the length the blocks over the phase-separation, the morphology, and the crystallization behavior of polyolefin-based BCPs.

Downloads

Actions (login required)

Modifica documento