Femina, Giuseppe (2024) Structural and Dynamic Complexity of Advanced Elastomers. [Tesi di dottorato]

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Abstract

The aim of this PhD work is to investigate the structure-properties relationships of Hydrogenated Nitrile Butadiene Rubbers (HNBRs). Samples with different chemical composition (ACryloNitrile, ACN, and initial Residual Double Bond, iRDB, content) and different crosslink density were analyzed. The study is focused on the analysis of conformation and dynamics of the chains in both crystalline and amorphous states and the mechanisms inducing Strain-Induced Crystallization (SIC) at room temperature. Additionally, it aims at understanding how SIC is influenced by the degree of crosslinking and the ACN content. The objectives of this project are significant for both the fundamental understanding of polymer physics and rubber elasticity in particular and practical aspects related to rubber processing. Currently, there are few studies on the behavior of molecular chain orientation during the stretching of HNBR and its relationship with SIC in the literature. Another characteristic of these rubbers is the thermal behavior that some amorphous samples show, when subject to prolonged annealing at temperatures close to the glass transition Tg. In particular, endothermic peaks are generated in DSC heating thermograms at temperatures near Tg. These phenomena are influenced by various factors, such as ACN content, degree of unsaturation, and crosslink density. In the literature, these phenomena are attributed to thermally induced crystallization, but studies related to this aspect are still limited. This behaviour is similar to the behaviour of polystyrene which upon annealing at temperatures below Tg gives endothermic peaks with no melting involved, seems it is atactic. After a general introduction on rubber elasticity, and the state of the art related to the structure and properties of HNBRs (Chapter I), in the Chapter II the theoretical and experimental background behind the different techniques adopted for the characterization of the samples are illustrated. In the successive chapters, the results obtained from the different characterization techniques are described and discussed in depth. In particular, the results achieved in this PhD work are divided into three parts. Part 1 focuses on the characterization of chemical, physical, and mechanical properties of vulcanized HNBR samples with sulfur curative packages, along with their non-vulcanized counterparts. Studies have been conducted on the elastomeric network structure, SIC, chain orientation during stretching, and how these aspects depend on the crosslink density for samples with the same chemical composition. The analysis is focused on samples with ACN content of 43 and 44 wt%, with iRDB content of 5.5 and 9 mol%, respectively, and varying crosslink density. Vulcanized samples are amorphous at room temperature, with Tg values that increase as the crosslink density increases. The values of the Young's modulus of vulcanized samples are three times higher than that of the non-vulcanized counterparts. Furthermore, as the crosslink density increases, the tensile strength increases, while the strain at break decreases. SIC is observed in non-vulcanized and weakly to medium crosslinked samples, above a critical strain, while highly crosslinked samples break close to the SIC onset. Crystallinity arises from the presence of a non-negligible population of alternated tetramethylene/ACN (TMAC) sequences, crystallizing in the so-called Form II. The degree of orientation of the amorphous phase gradually increases as the strain increases, reaching a plateau at crystallization onset. The tensile strength of crystallizing HNBRs at high strains is attributed to the alignment of crystals formed by SIC in the stretching direction. The analysis is extended to sulfur vulcanized HNBR samples containing 34 and 36 wt% of ACN. It is shown that these samples are amorphous at room temperature. The glass transition temperature increases as the ACN content and crosslink density increases. They do not exhibit SIC at room temperature. Stretching induces orientation of the amorphous segments along the stretching direction, improving the tensile strength, and toughness while maintaining excellent elastic recovery. These results are detailed in the Chapter III. Part 2 is focused on the chemical, physical, and mechanical characterization of samples with different ACN content (21, 34, 39, 43, and 50 wt%) and iRDB ≤ 0.9 mol%, as well as on samples with 34 and 43 wt% of ACN and iRDB = 5.5 mol%. Analyses were carried out on non-vulcanized and peroxide vulcanized samples. The vulcanized samples contained also 30 phr of carbon black as a reinforcing filler. The structure of elastomeric network is found to be homogeneous regardless of ACN content. All samples are amorphous at room temperature, except for the non-vulcanized samples with 43 and 50 wt% ACN and iRDB ≤ 0.9 mol%, which are already crystalline at room temperature and in the undeformed state. Vulcanized samples exhibit similar, ACN-independent, mechanical properties, while the non-vulcanized samples show poor mechanical properties. Exceptions occur for the crystalline samples, with 43 and 50 wt% ACN. The possible increase in crystallinity level induced by strain, along with the effect of strain on the orientation of the amorphous chains during stretching of the crystalline samples, was studied in detail. This study highlights that these sample do not exhibit SIC behavior, as the increment of crystallinity level achieved by stretching is due to the nucleation effect exerted by the pre-existing crystals. During release of the tension, the newly formed crystals are partially retained. Furthermore, the crystalline phase and the pinned amorphous chains, in the sample with 50 wt% of ACN, remain partially oriented, probably because the crystals and the amorphous chains remain entrapped in the elastomeric network by effect of local tensions. On the contrary, for the sample with 43 wt% of ACN, the newly formed crystals and the amorphous phase completely lose the orientation. These results are detailed in the Chapter IV: Part 3 delves into the study of the DSC endothermic peaks generated by annealing treatments at temperatures near Tg, of some amorphous HNBR samples. This study is conducted through thermal analysis and dielectric spectroscopy on a peroxide vulcanized HNBR samples with 21 wt% of ACN and iRDB ≤ 0.9 mol% and on non-vulcanized, sulfur vulcanized and peroxide vulcanized HNBR samples with 34 wt% of ACN, different iRDB content and different crosslink density. From the analysis it emerges that the DSC endothermic peaks are not due to the formation of thermally induced crystals but are more likely related to physical aging phenomena that generate endothermic relaxation peaks at temperatures above Tg, during DSC heating scans, coupled with occurrence of microphase separation of segments richer of tetramethylene sequences from sequence richer in ACN units. These results are detailed in the Chapter V. In the conclusion the main results of the PhD thesis are outlined, emphasizing the importance of using complementary techniques and theoretical support to understand the material properties at the molecular level.

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