Molecular Dynamics And Phase Separation Of Polymer Blends With Dynamic Asymmetry

Dynamic asymmetry exsits in polymer blends with big difference in components'glass transition temperature.In miscible polymer blends with dynamic asymmetry,each component has its intrinsic molecular mobility,hence micro-heterogenity at nanoscale is found.The theory of molecular dynamics in compatible system has important significance in understanding liquid-liquid phase separation and crystallization processes.Phase separation kinetics is related to the whole chain diffusion,the crystallization process involves the diffusion of the whole chain and also the folding of chain segment.It becomes very complex when the two phase transitions happened simultaneously.However,to now,the quantitative relation between molecular mobility and phase transitions is still unclear.In the literatures,the dynamic heterogeneity behaviors at chain segmental scale,eg.the failiure of time temperature superposition and two distinct glass transitions in miscible blends,can be well understood by combining the self concentration model and the thermal concentration fluctuation model.Whereas,the mechanism research about the dynamic heterogeneity at the whole chain scale is still in the exploration stage,and its relationship with chain segmental scale is still in debate.So far,most published works are limited in the model athermal systems,the understanding in polymer blends with interaction is rather limited.Therefore,it is highly necessary to extend the current work through emphasizing on molecular dynamics research on the whole chain scale and in systems with special interactions.Based on above background,this thesis will focus on the research of molecular dynamics at both segment and whole chain scales of dynamic asymmetry miscible blends,on the research of phase separation kinetics and its effect on the following crystallization kinetics of dynamic asymmetry partial compatible blends.The main contents and conclusions are as follows:1.The molecular dynamics at segment and whole chain length scales of dynamic asymmetry polystyrene acrylonitrile/poly caprolactone?SAN/PCL the difference of their glass transition temperature is about 170??system were investigated by broadband dielectric spectroscopy and rheology,it was found that they are different from relaxation begaviors in dynamic symmetry system.The relaxation time at both the chain segment and the whole chain scales are higher than predicted values according to self-concentration model,it is assumed to be related to the interaction between two components which hindered molecule motions.In addition,the width parameter of the whole chain relaxation is wider than that of the chain segment,both increased by the same proportion with the decrease of temperature,which indicates they are linearly correlated.The interaction contribution is evaluated from the relaxation time at effective glass transition temperature of each blend and then introduced to TDD-DR-SC model,which had integrated the double reptation model with time dependent diffusion and the self-concentration model.The new model can accurately predict linear viscoelastic properties of blend at any temperature and composition,in other words,it can quantitatively analyze the origin of dynamic heteriogenety of this system.2.The effect of nanoparticles on molecular dynamics in compatible SAN/PCL system was studied.The dispersity and affinity of two kind nanosilicas(hydrophobic SiO₂-106 and hydrophillic SiO_2-380)were investigated by means of transmission electron microscopy?TEM?and contact angle experiments,respectively.The dielectric sepctrum results show that only at temperature lower than Tg+50? nanosilica can affect segment dynamics.The influence of nanosilica species and its content on the terminal relaxation were quntified by the width parameter,the results show that only at temperature high than Tg+100? nanosilica can affect the whole chain dynamics.The terminal relaxation was influenced by both of nanoparticle dispersion and its affinity to polymer chains.3.The nonlinear rheological behavior of the compatible SAN/PCL blends and its nanosilica filling system were investigated,the nonlinearity parameter I_1/3 and Q???increased with the content of SAN in binary blends,in the presence of nanosilica,the increment of the nonlinear response is more significant in SAN-50% than in SAN-70%,which is attributed to the larger agglomeration of nanosilica in SAN-50%and ultimately reduced the composition asymmetry.In addition,the dynamic modulus?Fourier-transform?and nonlinearity Q?w?T? were investigated at two different strain amplitude?5%and 100%?by frequency sweep.Both the third harmonic modulus and Q?w?T? indicate the degree of nonlinearity.The temperature dependence of tTS shift factors showns that Q?w?T?is better in evaluating nonlinearity.4.A quantitative analysis method is first time to be proposed to analyze the amplitude and the wavelength of concentration fluctuation during liquid-liquid phase separation of viscoelastic asymmetric blends.The phase separation of poly(methyl methacrylate/polystyrene acrylonitrile?PMMA/SAN?were investigated by rheometer through the cycle frequency sweep,the recored evolution of dynamic moduli were decoupled into composition and interface contributions,corresponding to the amplitude and the wavelenth of concentration fluctuation,respectively.Up obove the critical frequency w_r,the composition contribution is dominant,the separated phases'composition were inferred from the modulus based on the established relation between modulus with blend composition in miscible state,morover,the steady values of phases'composition were deemed as quasi-equilibrium binodal points of phase diagram.On the other side,below the critical frequency w_r,the interface contribution is dominant,the characteristic wavelength of concentration fluctuation is calculated through YZZ model.The stage of phase seperation was clarified from the evolution of separated phases'concentration and wavelength length.5.The theory of molecular dynamics in miscible blend was first time to be considered into the description of crystallization kinetics,and quantitatively described the free energy of chain folding caused by concentration fluctuation assisted nucleation.New methods were designed to investigate the miscibility of dynamic asymmetry PLA/PEG blends,their very close refraction index means that the optical method is unavailable here.Liquid–liquid phase separation?LLPS?was confirmed by the appearance of two glass transitions after the samples have been annealed at low temperatures?95–125??.Moreover,according to a fluctuation-assisted nucleation mechanism,the accelerated crystallization rate after samples been annealled at high temperatures?140–160??also indicated the existence of LLPS.An upper critical solution temperature?UCST?type of phase diagram was determined by the combination of DSC and rheological methods.Furthermore,the effects of LLPS on the kinetics of the subsequent crystallization of PLA were investigated.We proposed a new model that integrates the self-concentration model into the Lauritzen–Hoffman theory to accout for the change of chain mobility on the free energy of chain folding.