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Multifunctional properties of polymer foams and composites

Özkutlu Demirel, Merve
Polymer foams are low-density materials that consist of solid and gas phases and are used for insulation and transportation applications. This study aims to produce poly(methyl methacrylate) (PMMA) foams with low density and improved thermal and mechanical properties using syntactic foam production and supercritical CO2 (scCO2) foaming methods. Syntactic foams were produced dispersing 5, 10, and 15 wt.% hollow glass microspheres (HGMs) in the PMMA matrix. Using different HGM types based on their density, surface coating of the HGMs, and the hybrid syntactic foam approach by dispersing polyhedral oligomeric silsesquioxane (POSS) nanoparticles as a reinforcement material were considered to improve the morphology, density, thermal and mechanical properties. PMMA foams were obtained with scCO2 processing in which CO2 diffuses and dissolves in the polymer matrix with the aid of high pressure and the releasing of the CO2 from the matrix forms the voids in the polymer matrix. The effects of foaming parameters: time, temperature, pressure, and venting rate as well as the addition of a CO2-philic POSS nanoparticles on foam morphology in terms of pore size and pore density, thermal properties, and density of the foams were investigated. In syntactic foam studies, the density of the polymer was reduced by 10% with the addition of 15 wt.% high-density HGMs. Higher mechanical properties were also obtained using these HGMs. Surface coating of the HGMs and the hybrid syntactic foam approach by dispersing POSS nanoparticles as a reinforcement material improved the flexural strength by 12% and 15%, respectively. In scCO2 foaming studies, the average pore diameter of PMMA foam, which was processed at 35⁰C, 32 MPa for 24 h and depressurized with fast venting rate, decreased from 1.1 to 0.3 µm with the addition of 5 wt.% POSS nanoparticles. PMMA syntactic foams containing 5 wt.% HGM and 5 wt.% POSS, processed with scCO2 at 35⁰C, 11 MPa for 24 h and depressurized with fast venting rate, had bimodal pore structure with the ratio of the diameters of the larger pores to smaller pores of 154