Sigareva N. Thermophysical and mechanical properties of polymer composites filled with highly dispersed oxides and multilayer graphene

Thesis for the degree of candidate of physical and mathematical sciences in the specialty 01.04.18  Physics and chemistry of surface ̶ Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Kyiv, 2021. The thesis is devoted to the research of effect of titanium dioxide, titanium - silicon dioxide, highly dispersed silicon dioxide, oxidized and unoxidized graphene nanoparticles on the thermophysical and mechanical properties of their composites with polyester, epoxy and urea-formaldehyde resins. Investigations have been mainly performed by methods of thermoprogrammed desorption mass spectrometry, mass spectrometry with laser desorption ⁄ ionization loading to identify the products of thermal decomposition and study the thermal destruction of resin and its composites and determine the concentration limits of the effect of stabilizing the polymer structure. The method of static mechanical loads were used to compare the change in free volume with mechanical parameters such as compressive strength and modulus of elasticity. The temperature and concentration dependences of thermal destruction of polymers and their composites, as well as the filler effect on the elastic modulus and compressive strength of composites have been studied. It was shown that the growth in a heat resistance is accompanied by an increase in the elastic modulus and compressive strength in composites of epoxy resin with unoxidized graphene and a decrease of both parameters in polyester composites with nanosilica. It is established that filling with highly dispersed oxide particles leads to an increase of heat resistance of composites in the content range of 0.5 ≤ Сn ≤ 1.5 wt % and to its reduction with increasing filler content. In graphene composites the heat resistance improves in the content range of 1.0 ≤ Сn ≤ 5.0 wt %. Variations in the thermal stability of composites were accompanied by changes in the destruction activation energy and mechanical parameters. It is established that the growth in the heat resistance is caused by fixing of unbound fragments of polymer structure on the active surface centers of filler particles and by reduction in the thermal decomposition of polymer chains and crosslinks, whose desorption are correspondently observed at temperatures T ≤ 250 оС and in the temperature range of 250  450 oC. The thermal destruction of composites based on polyester resin at a content of 0.5; 2.5; and 5 wt % of highly dispersed silica under the action of a laser pulse was studied. The differences of degradation, which characterize the change in the structure of polyester resin when filled with highly dispersed silica, are shown, which consist in changing the composition of volatile products in the interval of 200 < m/z <800 and reducing the number of polymer chains with higher values of m/z. The Wigner-Polanyi formula was used to determine the effect of the filler content, filler type and its structure on the activation energy of thermodestruction Ed of polymer matrix of composites. It has been shown that the activation energy Ed for desorbed atomic fragments reduces upon filling in the composites on the base of polyester and epoxy resins. Besides, the Ed nonmonotonously decreases with slight maximum at Сn = 2 wt % in the graphene composites with increasing filling. Effect of electron subsystem of nanofillers on the thermal destruction in the epoxy composites with unoxidized and oxidized multilayer graphene was studied. The growth of thermal stability in the composites loading with unoxidized graphene was related to partial removal of heat flux energy at the interface in the electronic subsystem of graphene particles with subsequent lowering of vibrational energy of atoms at the interface. Model of thermal transport with participation of electron and phonon subsystems of graphene particles was developed. The influence of dielectric oxide layer on the thermal decomposition of composites with oxidized graphene is considered. Key words: thermal destruction, thermal stability, polymer composite, mechanical properties, destruction activation energy, oxidized graphene, unoxidized graphene, chemical resistance