Polymethyl(3,3,3-trifluoropropyl)Siloxanes:the Kinetics Of Polymerization And Rheological Properties

Polymethyl(3,3,3-trifluoropropyl)siloxanes (PMTFPS) can be synthesized by the ring-opening polymerization from 1,3,5-tris(trifluoropropylmethyl)cyclotisiloxane (D3F). However, some key scientific problems in the preparation of PMTFPS, such as equilibration reaction, initiator activities, molecular weight and molecular weight distribution and the kinetics of polymerization need be further studied. In addition, the rheological properties of PMTFPS are not investigated in detail. In the present work, we will focus on the study of initiators, polymerization kinetics in the polymerization of PMTFPS and its rheological properties.A series of linear polymethyl(3,3,3-trifluoropropyl)siloxanes with various molecular weights (PMTFPSs) in the range of 1×104 g/mol and 2.17×105 g/mol were synthesized by anionic bulk ring-opening polymerization. The initiator in the polymerization of D3F was LiOH silanolate. The synthesized PMTFPSs were characterized by FTIR,1H NMR,29Si NMR, GPC and Ubbelohde viscometer. The conversations of monomer were in-situ monitored by GPC in the ring-opening polymerization of D3F. It was found that the consumption rate of monomer was followed well with first-order kinetics equation. The equilibration reaction was weakened effectively in the initiating system of LiOH silanolate, which was help to synthesized PMTFPS with stable molecular weight. The organic solvents as promoters can increase activity of initiators reduce temperature and time of polymerization reaction.The rheological and thermal properties of PMTFPSs were further studied. The rheological properties of PMTFPSs including viscoelasticity, viscosity, fluid type, creep-recovery response, transient shear stress response and thixotropic behavior were measured by a rotational rheometer. The PMTFPSs with lower molecular weights exhibited a Newtonian fluid behavior. However, the PMTFPSs with higher molecular weights exhibited pseudoplastic behavior, the fluids were dominated by an elastic behavior. The calculated flow activation energy of PMTFPSs were about 34 kJ/mol which was greater than that of polysiloxanes. The strain of PMTFPSs were linear related to the time in the process of creeping, and were not recovered when the external force were removed. The transient shear stress response and thixotropic behavior became stronger with the increasing of molecular weights. The DSC results showed that PMTFPS exhibited one glass transition temperature. No crystallization was found for PMTFPSs. The glass transition temperature was changed from-75℃ to -65℃ with the increasing of molecular weights, when the molecular weight was above 3×104 g/mol, the glass transition temperature kept constant, about 65℃. The TGA results showed that the temperature of 5% weight loss (Td5) of PMTFPSs were over 290℃, and the thermal stability of PMTFPSs was improved as the molecular weights increased.