Investigation On The Dynamic Mechanical Properties And Molecular Simulation Of High Phenyl Silicone Rubber

As the most widely used viscoelastic damping material,rubbers can reduce vibration and noise by absorbing and dissipating external energy.The loss factor(tanδ)and damping temperature range(ΔT)are commonly used to evaluate the damping capacity.Phenyl silicone rubbers have outstanding performance such as radiation resistance,high and low temperature resistance and oxidation resistance,etc..And the properties can be tuned by adjusting the structure and content of phenyl siloxane units to meet various applications,which makes phenyl silicone rubbers indispensable in the fields of industrial production,aerospace,military equipment,etc..High phenyl silicone rubbers(the ratio of phenyl to silicon>30%)possess superior damping performance,mechanical properties and thermal stability.However,due to the challenging of preparation,high phenyl silicone rubber mainly relies on imports from other countries,which severely restricts the development of the related equipment and technology.Based on the significant application of the wide temperature range and high damping silicone rubber for aerospace,this thesis systematically investigated the relationship between the microstructure and macroscopic performance of phenyl silicone rubber by combining experiments and molecular dynamics simulation,which shed light on the relaxation behavior and damping mechanism at the molecular level.Two types of damping additives were designed and prepared based on the π-πinteraction and B-O dynamic bond,which were compounded with high phenyl silicone rubber to increase the loss factor and expand the damping temperature range,while retaining the excellent low-temperature performance of the silicone rubber.This paper provides a new strategy of combining theory and experiment for the research and development of high performance damping silicone rubber.The main works and conclusions are as follows:(1)The relationship between the composition,sequence structure of silicone gums and the dynamic mechanical properties,photophysical properties of silicone rubbers was established by experiments and molecular dynamics simulation.The sequence structure of phenyl silicone rubber was quantitatively analyzed by 29Si NMR.The results showed that phenyl siloxane units were randomly distributed on the polymer chains.With the increase content of phenyl groups,the content of consecutive phenyl siloxane units significantly increased and the sequence length of dimethyl units gradually decreased.Moreover,the unique fluorescent performance of high phenyl silicone rubber was speculated and confirmed based on the characteristics of sequence distribution.The influence of the structure and content of the phenyl siloxane units on the damping properties of the silicone rubber was studied in detail through the dynamic mechanical analyzer(DMA)and the dielectric relaxation spectrum.The results showed that with the increase of phenyl contents,the relaxation behavior of different motion units overlapped with each other,and the intramolecular friction of segmental motion increased,which improved the damping performance of the silicone rubber.Meanwhile,the introduction of diphenyl siloxane unit effectively reduced the temperature dependence of damping performance.In addition,molecular dynamics(MD)simulation was used to quantitatively analyze the microstructure parameters such as bond angle,radial distribution function,dihedral angle and self diffusion coefficient.The effects of phenyl siloxane units on the conformational transition,interaction mechanism and segmental relaxation behavior were studied.The results showed that the π-π interaction between phenyl groups remarkably enhanced the intra-and inter-molecular forces,which strengthened the intramolecular friction and the energy dissipation,increasing the loss factor.On the other hand,it hindered the relaxation of long polymer chains and increasd the relaxation time distribution,which broadened the damping temperature range of the silicone rubber.(2)Based on sequence structure of silicone rubber and π-π interaction,a series of damping additives were designed and prepared,which were compounded with high phenyl silicone rubber.The effects of additives and curing agents on the relaxation time and crosslinking density of composites were investigated by the low field nuclear magnetic resonance.The results showed that the longer molecular chains and the more phenyl content of damping additives,the shorter T2 relaxation time and the higher crosslinking density.Compared with DBPMH vulcanization system,the DCBP vulcanization system had higher contents of the dangling chain and free chain,and thus lower crosslinking density.The effects of structure and content of additives,curing agent and test frequency on the dynamic mechanical properties of composites were investigated by DMA.The results indicated that the loss factor was improved and the effective damping temperature range(tanδ≥0.3)was widened without sacrificing the low temperature properties of the silicone rubber by adjusting the molecular structure and molecular weight of damping additives.At 1Hz,the effective damping temperature range of the optimal formula was 49℃ and the maximum loss factor was 0.85,which was 250%and 64%higher than that of the blank one,respectively.At 100Hz,the effective damping temperature range of the optimal formula was 82℃ and the maximum loss factor was 0.95,which was 257%and 70%higher than that of the blank one,respectively.Therefore,it can be concluded that the introduction of miscible polysiloxane oligomers as additional relaxation components can effectively improve damping properties of the silicone rubber.(3)Polyborosiloxanes(PBS)with certain structures were prepared by end group condensation from boric acid(BA)and hydroxyl silicone oil(PDMS-OH).The model of chemical structure and physical interaction in PBS was proposed,and the effects of molecular structure on the thermodynamic properties,dynamic rheological performance and thermal stability of PBS were studied.FTIR and DSC results showed that with the decrease of molecular weight of PDMS-OH,the boronized cross-linking density in PBS increased gradually.The Si-O-B cross-linking structure and B-O dynamic bond effectively limited the movement of segmental chains and reduced the crystallization temperature.The shear thickening mechanism of PBS was analyzed from the perspective of molecular structure,and the viscoelasticity of PBS could be effectively controlled by adjusting the molecular weight of PDMS-OH.Moreover,compared with the PDMS-OH,the initial and fastest degradation temperatures of PBS were obviously increased.The mechanism of thermal degradation of PBS was furtherly investigated by TG-IR.The results indicated that it was mainly the "backbiting" degradation of terminal hydroxyl group and the"decoupling" degradation of Si-O-Si backbone at low degradation temperature,leading to the production of cyclic oligomers.Methane and oligomers were formed at high degradation temperature,since the "decoupling" degradation and the free radical degradation simultaneously involved,and the thermal degradation rate was the fastest.(4)The dual network structures were designed and prepared by compounding PBSs with high phenyl silicone rubber,in which contained chemically cross-linked phenyl silicone rubber network and dynamically cross-linked polyborosiloxane network.The effects of the structure and content of PBS on the cross-linking network,dynamic mechanical properties,mechanical strength and surface properties of composites were systematically studied.The analysis of the relaxation time and cross-linking density of composites indicated that B-O dynamic bond and the entanglement of molecular chains were the key factors affecting the segmental relaxation and network structure.The B-O dynamic bond effectively increased the energy dissipation of composites through the fracture and reconstruction,and had a significant frequency dependence.The internal friction mainly originated from the contribution of B-O reversible dynamic bond rather than the relaxation of the main chain,reducing the temperature dependence of damping properties.The composites with highest boron content had a tanδ>0.2 platform in the range of 30℃-150℃,which significantly improved the damping performance of silicone rubber at high temperatures.The effects of the structure and content of PBs on the morphology and surface properties of the composite system were analyzed by SEM,surface contact angle and roughness test.The results showed that the dual network of PBS and phenyl matrix was formed through the synergistic effect of covalent bond and B-O weak bond.Due to the difference of solubility parameters between PBS and phenyl matrix,the microphase separation occurred to a certain extent,resulting in the increase of surface hydrophobicity and roughness.Furthermore,the non-bond energy,free volume fraction,and segmental diffusion of composites at vary temperatures were quantitatively studied through molecular dynamics simulations.The results showed that the B-O dynamic bonds restricted the movement of segments and effectively reduced the sensitivity to temperatures;meanwhile,the internal friction of the segmental movement was increased,improving the damping performance of composites.