Study On Ultralow Temperature Resistance Silicone Rubber

Silicone rubber is a kind of important silicone products.By using linear polysiloxanes as its base gum,the elastic vulcanizate can be obtained through vulcanizing under certain conditions.The polysiloxane has flexible Si-O main chain and organic side groups,which provides silicone rubber with a lot of unique and excellent properties such as temperature resistance,radiation resistance,dielectric properties,ozone resistance,weatherability,physiological inertia,and so on.Silicone rubber can be used at special environment.But the common silicone rubber will gradually lose elasticity at rather low temperatures because of weakness of molecular thermal motion and crystallization of molecular chains.In order to solve these problems,it is necessary to synthesis the polysiloxanes with more excellent low temperature properties to prepare new silicone rubber based on them.In this paper,we synthesized a series of poly(dimethyl-methyltrifluoropropyl-methylvinyl)siloxanes(PFMVS) with various contents of trifluoropropyl groups,polydiethylsiloxane (PDES),poly(diethyl-dimethyl)siloxane(PEMS) with various contents of ethyl groups and copolysiloxane(PEmMEPmS) by multi-copolymerization. And their properties such as low temperature performances were measured and analyzed.The poly(dimethyl-methyltrifluoropropyl-methylvinyl)siloxanes (PFMVS) with various contents of trifluoropropyl groups as base gum of fluorosilicone rubber were synthesized.Their structures were characterized,and then molecular weight and intrinsic viscosity were measured.Through thermogravimetric analysis(TGA) and differential scanning calorimetry analysis(DSC),their thermal properties and the kinetics of thermal degradation were analyzed.The reasults show that compared with polydimethylsiloxane,though the introduction of trifluoropropyl side group makes the values of Tg increase because of the larger steric hindrance and strong polarity of trifluoropropyl side group, the crystallization of molecular chains at low temperature can be inhibited.The properties of the vulcanizates such as temperature resistance and solvent resistance were measured.It is found the vulcanizates have good solvent resistance and hydrophobicity.Taking potassium hydroxide as the catalyst,the polymerization conditions and polymerization kinetics of hexaethylcyclotrisiloxane(D₃^Et) were studied by means of column separation,intrinsic viscosity and GPC methods.The feasible polymerization temperature is 150～156℃and polymerization time is more than 8h.The activation energy was calculated to be 122.68kJ/mol.Then a series of poly(diethyl-dimethyl)-siloxanes (PEMS) with various contents of ethyl groups were synthesized by using octamethylcyclotetrasiloxane(D₄) and hexaethylcyclotrisiloxane (D₃^Et).The DSC analysis show that the introduction of ethyl groups decreases the T_g of the polysiloxane,and T_g gradually decreases with the content of ethyl groups increasing.When the diethylsiloxane units reaches 40mol%,crystallization of molecular chains could be farthest inhibited at low temperatures.Five different cyclic siloxane monomers were used to synthesize copolysiloxanes.Their structures were characterized by IR and ¹H-NMR,and then thermal properties were primarily studied by thermal gravimetric analysis(TG) and differential scanning calorimetry analysis (DSC).The results show that the polymer is thermostable below 300℃, and the weight loss is less than 5%at this temperature.Its glass transition temperature(T_g) is-117℃.