Effects Of Chain Condensed Structure On Molecular Dynamics And Phase Separation Behavior Of Polymer Blends

It is well established that the structure-property relationship of polymer blends strongly depends on preparation and processing conditions. Heretofore, most of polymer blends could be prepared via solution mixing (including solution casting and solution coprecipitation) and melt mixing methods in present. The structure and properties of solution-cast polymer blends are likely to be strongly dependent on the solvation process, and the materials that have undergone processing are far removed from their thermodynamic equilibrium. Besides, it is accepted that the processing or preparation route may influence the macromolecular structure of the polymer, and in turn influence its performances. Hence, it is of academic and practical importance to establish the relationship among processing conditions, molecular condensed structure, molecular dynamics and phase separation behavior of polymer blends. Such a fundamental understanding is also of great significance for optimizing the structure and resultant properties, and is helpful for us to give a reasonable interpretation at the segment level for the material macroscopic performance.In this dissertation, a binary blend composed of poly(methyl methacrylate)/poly (styrene-co-maleic anhydride)(PMMA/SMA) exhibiting lower critical solution temperature (LCST) behavior is selected as a model system. The nonlinear phase separation behavior of PMMA/SMA blends was studied by means of time-resolved small-angle laser light scattering (SALLS). The effects of solvent casting parameters, annealing, and preparation route on chain entanglement and molecular dynamics of PMMA/SMA blends were probed via a combination of dynamic rheological measurement and broadband dielectric spectroscopy. The contribution of chain entanglement to thermodynamics and kinetics of liquid-liquid phase separation (LLPS) behavior in PMMA/SMA blends was also discussed.The phase separation behavior of PMMA/SMA blends presents obvious nonlinear characteristic. It is found that during non-isothermal process the dependence of cloud point (Tc) on heating rate deviates from the linearity obviously. Hence, the equilibrium phase separation temperature of PMMA/SMA blends could be hardly obtained through the linear extrapolation of heating rate to zero. The nonlinear relationship between Tc and heating rate could be described as the logarithm function. While the parameter, reflecting the heating rate dependence, is much dinstict for different compositions due to phase-separation rate and activation energy difference. For isothermal phase separation process, an Arrhenius-like equation can be successfully applied to describe the temperature dependences of apparent diffusion coefficient (Dapp) and the relaxation time (τ) of the early stage as well as the late stage of spinodal decomposition (SD) of PMMA/SMA blends. Hence, it is possible to predict the temperature dependence of phase separation behavior of binary polymer mixtures during isothermal annealing over a range of100℃above glass transition temperature (Tg) using Arrhenius-like equation.Both casting solvent and solution concentration have a pronounced effect on the segmental motion of PMMA/SMA blend films, including relaxation transition temperatures and segmental dynamics. In films cast using methyl ethyl ketone (MEK) as solvent with various concentrations, Tg and relaxation time (τmax) increase with increasing solution concentration due to an increased entanglement density, decreased molecular mobility and entanglement recovery. No obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. In the case of films cast using chloroform, MEK and tetrahydrofuran (THF) as solvent, Tg and τmax of the resultant films are hardly affected, while Tg and τmax of films cast using N, N-Dimethylformamide (DMF) as solvent are much higher than the other three solvent due to a higher entanglement degree and strong interaction contributions. Moreover, the poor dissolving capacity of DMF may result in more heterogeneous dynamics and subsequently a larger dc conductivity process and broader and more symmetric α-relaxation spectra. Neither the dynamics nor the distribution width of the subglass relaxation (β and γ-relaxation) processes is affected by the casting solvent or solution concentration, indicating little change in the local environment of the segments.Chain entanglement density increases when annealing temperature and/or time increases, resulting from the increased efficiency of chain packing and entanglement recovery. The results of the annealing without cooling reveal that the increase of entanglement density occur during the annealing process instead of the subsequent cooling procedure. Annealing above Tg exerts a profound effect on segmental motion including transition temperature and dynamics. Namely, Tg shifts to higher temperatures and τmax increases due to the increased entanglement density and decreased molecular mobility. Either Tg or τmax approaches an equilibrium value gradually, corresponding to the equilibrium entanglement density which might be obtained through the theoretical predictions. However, no obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. Furthermore, side group rotational motion could be freely achieved without overcoming the chain entanglement resistance. Hence, neither the dynamics nor the distribution width of the subglass relaxation processes is affected by chain entanglement resulting from annealing, indicating that the local environment of the segments is unchanged.PMMA/SMA blends with different compositions were prepared through solution casting and melt mixing. It is found that the chain entanglement density of the solution-cast samples are lower than those of corresponding melt-mixed samples. The difference in chain entanglement density of the samples prepared from different routes has a pronounced effect on the segment motion. Tgs of the solution-cast samples are lower than melt-mixed ones due to the enhanced mobility of segments. Tgs of both samples shift to higher temperatures, approaching the equilibrium entanglement density after annealing at a temperature higher than Ts. These phenomena are thought to be resulted from gradual interpenetration and re-entanglement of the isolated single and few-chain coils through thermal diffusion during annealing. τmax of the a-relaxation process for solution-cast samples is shorter than that for melt-mixed samples with higher chain entanglement density due to the mobility of segments being increasingly hindered as inter-and intrachain entanglements increase. Neither the dynamics nor the distribution width of the β-relaxation process is affected by the blending and entanglement state, indicating that the local environment of the segments is not changed, regardless of the blend composition or the chain entanglement density. The effects of chain entanglement on thermodynamic and kinetics of LLPS behavior of PMMA/SMA blends were also examined in detail. The melt-mixed blends with a higher chain entanglement density present a lower Tc and a shorter delay time, but lower phase separation rate at the given temperature than those of solution-cast ones, suggesting that for the polymer blends with different condensed state structure, thermodynamically more facilitation to phase separation (lower Tc) is not necessarily equivalent to faster kinetics (decomposition rate). The experimental results indicate that the lower Tc of melt-mixed sample is ascribed to smaller concentration fluctuation wavelength (Am) induced by a higher entanglement degree, while a higher entanglement degree in melt-mixed sample leads to a confined segmental dynamics and consequently a slower kinetics (decomposition rate) dominated by macromolecular diffusion at a comparable quench depth.