Villaccio, Rosa (2024) Innovative functional elastomers based on syndiotactic homo and copolymers. [Tesi di dottorato]

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Abstract

The development of functionalizable elastomers stands as a promising field of research in polymer science. Syndiotactic polypropylene (sPP) has emerged as a particularly intriguing candidate, due to its unique combination of properties, including high mechanical strength and stiffness, typical of crystalline polymers, along with a peculiar elastic behavior. Moreover, copolymerization with non-conjugated dienes, such as 1,5-hexadiene (1,5-HD) or 1,7-octadiene (1,7-OD) allows for the synthesis of sPP-based materials featuring reactive groups with terminal C=C bonds. The potential applications of sPP can be significantly broadened by employing this unsaturated units for functionalization or cross-linking purposes. This thesis focuses on synthesizing these novel classes of copolymers and investigating their structural, thermal, mechanical and elastic behavior through a variety of techniques. By examining the structure-property relationships, the work provides critical insights into how diene incorporation into sPP chains influences material performance. Microstructural analysis revealed that copolymerization of sPP with 1,5-HD and 1,7-OD, allows for the synthesis of copolymers featuring cyclic structures and/or functionalizable unsaturated units. The incorporation of diene co-units significantly affects the crystallization and thermal behavior of sPP, leading to a reduction in crystallinity, melting and glass transition temperatures. These changes depends on the presence of cyclic structures and/or long dangling vinyl groups, providing a versatile approach for tuning the physical properties of the materials by adjusting the type and concentration of comonomer units. The study of the mechanical properties showed that all copolymers exhibit remarkable tensile strength, with significant improvements in flexibility and ductility compared to sPP homopolymer. Furthermore, the copolymers demonstrate excellent elastic behavior with very small values of the residual deformation after elastic recovery and values of the percentage of the recovered deformation after breaking close to 100%. Elastic recovery in more crystalline copolymers was attributed to a reversible phase transition between trans-planar form III and the helical form II, whereas in less crystalline samples, elasticity was primarily entropic, similar to conventional elastomers. Finally, the reactive sites incorporated into sPP chains of the copolymers were effectively utilized for post-polymerization modifications. These modifications included cross-linking reactions and the transformation of double bonds into functional groups, leading to the development of new properties, such as shape memory behavior or enhanced molecular flexiblity. This study provides a significant advancement towards developing smart elastomers, where elastic and mechanical properties can be tuned by designing the molecular architecture choosing the appropriate synthetic strategy, with applications depending on the inserted functionalities.

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