Tailoring properties of silicone Elastomers via Chemical and Physical Modification

Elisa Ogliani

Research output: Book/ReportPh.D. thesisResearch

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Silicone polymers and elastomers are materials of high value due to the unique combination of structural features resulting in diverse favorable macroscopic properties, such as flexibility, hydrophobicity, thermal stability, and electrical resistance, just to mention a few. These properties are, in turn, accountable for the wide range of applications that silicones can fit, spanning from well-established to more inspiring, advanced applications, for instance components for stretchable electronics, soft robotics, and electronic skin with biomimetic properties.
There is a continuous demand for high-performance silicones due to their widespread use, hence, developing silicones with improved properties or functional tailor-made properties are challenges addressed by both the academic and the industrial research. This project targets three specific properties with the overall aim of expanding both the functionality and the lifetime of silicone elastomers. Particularly, the objective is the improvement of inherent properties of silicone elastomers – thermal stability – and the development of smart, custom-made properties – selfhealing properties and sensing properties. This goal is pursued by designing experimental methods for chemical and physical modification of silicone elastomers, and each of the designed methods of modification with its respective result on the targeted property is illustrated across the chapters.
First, a new way to improve readily the inherent thermal stability was designed for silicone elastomers intended for use in high temperature environments. The correlation between the structure of the silicone network and the thermal behavior thereof was examined by formulating silicone elastomers with different stoichiometric ratios and comparing their thermal degradation performances by thermogravimetric analysis. It was shown how the degradation pattern is strongly
influenced by the fraction of elastic network, dangling chains, and sol fraction in the network. These findings can be used to tune the stoichiometric ratio used to synthesize the silicone elastomers and, consequently, to optimize their thermal stability in high temperature applications.
Second, a thermoplastic silicone elastomer with smart self-healing properties was synthesized via free radical polymerization of monomethacryloxypropyl terminated polydimethylsiloxane (PDMSMA) and 2-ureido-4[1H]-pyrimidinone methacrylate (UPyMA) monomers. The material owes its thermoreversible nature to the UPy functionalities that dimerize via four cooperative hydrogen bonds. Healing of the material after a damage was achieved in response to both direct
and indirect heat stimuli, the latter generated by physical incorporation of 20 wt% magnetic particles in the elastomer and subsequent exposure to an alternating magnetic field. Direct heating of the material for 1 hour at 70 °C led to a full recovery of the pristine mechanical properties, while indirect magnetic field-generated heating resulted in over 70% of recovery of the mechanical properties. In addition to thermoplasticity and self-healing capability, recyclability of the silicone elastomer was also demonstrated.
Lastly, a colorimetric sensor was developed by physical incorporation of 0.03 wt% 2,2-diphenyl- 1-picrylhydrazyl (DPPH) radicals in condensation cure silicone elastomers. The sensor was employed for the assessment of antioxidant activity as a rapid, cost-effective and facile alternative to the traditional solution-based DPPH analytical assay. The response of the sensor was tested successfully towards various reference antioxidant compounds (vitamin E, vitamin C, butylated hydroxytoluene, and quercetin) and food/beverage samples (e.g., olive oil, green and black tea, and black coffee). The colorimetric sensor showed a remarkable versatility, and was prepared in different formats that allowed for both a quick screening and quantitative evaluation of the antioxidant activity of the tested compounds by means of spectrophotometric techniques.
An additional study describes the coupling of a PDMS polymer with the antioxidant astaxanthin and its use as a stabilizing additive in organic solar cells. The introduction of this compound in the active layer of organic solar cells aims at achieving a twofold stabilization effect against photochemical and mechanical degradation of the devices. The astaxanthin-containing PDMS showed promising singlet oxygen quenching properties and induced an increase in the lifetime of the cells. Both of these outcomes suggested a positive effect against photooxidative degradation and, concurrently, the second step of the study involving the investigation of the mechanical properties of the cells with and without additive is ongoing. A mechanical stabilization of the devices owing to the PDMS units of the additive is expected to complement the proven increase in the photochemical stability, resulting in flexible organic solar cell devices provided with an overall superior stability.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages104
Publication statusPublished - 2019


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