【作者】 林宇；

【导师】 郑强； 上官勇刚；

【作者基本信息】 浙江大学 ， 高分子化学与物理， 2013， 博士

【摘要】 高分子共混材料的宏观性能强烈依赖于加工制备过程及其所形成的凝聚态结构。现有的高分子共混物制备方法主要有溶液共混法和熔融共混法。其中溶液共混法制备的高分子共混材料结构与性能强烈依赖于溶剂化过程,而熔融共混过程中因拉伸和剪切作用导致的分子链结构变化使材料远离热力学平衡态。显然,不同制备方法得到的高分子共混材料分子链聚集态结构存在差异,直接影响其最终性能。建立高分子共混材料加工制备条件、分子链聚集态结构、分子动力学与相分离行为之间的关联,不仅能为高分子共混材料的凝聚态结构与宏观相行为等提供分子水平的合理解释,也能为材料结构的调控与性能优化提供理论指导。本文选择具下临界共溶温度(LCST)高分子共混体系—聚甲基丙烯酸甲酯/苯乙烯-马来酸酐共聚物(PMMA/SMA)共混体系为研究对象,结合小角激光光散射(SALLS)、动态流变及宽频介电谱等手段系统研究该共混体系的非线性相分离行为,考察溶剂化效应(浇铸溶液浓度和溶剂性质)、热处理条件及不同制样方法(溶液浇铸和熔融共混法)对该共混体系链缠结密度和分子动力学的影响,阐述分子链缠结对该共混体系相分离行为(热力学和动力学)的影响规律。PMMA/SMA共混体系的升温和恒温相分离行为表明,该共混体系相分离行为符合旋节线相分离(SD)机理,且呈典型的非线性特征。升温相分离过程中,体系的浊点强烈依赖于升温速率,明显偏离线性,无法采用将浊点与升温速率关系曲线线性外推至零获得该共混体系的平衡浊点,但可通过对数函数定量描述浊点对升温速率的依赖性,且与组成相关。恒温相分离过程中,可通过类Arrhenius方程描述该共混体系整个SD相分离行为(包括初期和后期)对温度的依赖性,即可用类Arrhenius方程描述其表观扩散系数DaPP(T)、归一化光强IN的松弛时间τ(IN)、以及SD相分离后期的松弛时间τ(qm)和τ(Im)对温度的依赖性。溶剂化效应(浇铸溶液浓度和溶剂性质)显著影响PMMA/SMA共混物薄膜链缠结密度和链段动力学。当溶剂均为丁酮(MEK)时,随浇铸溶液浓度增大,共混物薄膜链缠结密度增大,玻璃化转变温度(Tg)升高,链段松弛时间(τmax)延长,材料脆性增加,但几乎不影响a松弛分布。当浇铸溶液浓度一定,通过不同溶剂浇铸薄膜时,氯仿、MEK和四氢呋喃(THF)溶液浇铸薄膜的Tg和τmax基本相同。由于二甲基甲酰胺(DMF)为不良溶剂及其与PMMA的强相互作用,致使DMF浇铸薄膜的Tg升高,Tmax延长,脆性增加,离子电导过程增强,且α松弛显著加宽且更对称,说明链段松弛单元不均一性增大。对PMMA/SMA共混物的热处理显著影响体系的链缠结密度和链段运动(包括转变温度和动力学)。随热处理温度升高和(或)热处理时间延长,高分子链逐渐相互贯穿和再缠结,呈现紧密堆积的链构象,使体系有效自由体积减小,共混物薄膜链缠结密度增大。共混体系链缠结密度增大发生在热处理过程中,而非冷却过程,且存在一平衡缠结密度。体系链段运动能力随热处理下降,Tg升高,τmax延长,材料脆性增加,但α松弛分布几乎不变。通过溶液浇铸和熔融共混法制备的PMMA/SMA共混物呈现明显不同的链缠结密度。与熔融共混样品相比较,溶液浇铸样品链缠结密度较低、Tg较低,热处理使两者Tg均升高,向平衡缠结态趋近。熔融共混样品链缠结密度较高,链段运动能力下降,α松弛时间延长,材料脆性增大。共混物组成影响体系a松弛分布,但链缠结不影响其α松弛分布。然而,溶剂化效应、热处理和制样方法均不影响PMMA/SMA共混体系的次级松弛动力学和分布,说明链缠结既不影响共混体系次级松弛动力学,也不影响其分布,意味着链段的局部环境并未随之改变。链缠结程度可显著影响PMMA/SMA共混物的相分离行为。由于链缠结密度的差异,与溶液浇铸样品相比较,熔融共混样品呈现更小的临界浓度涨落和涨落周期,故更容易发生相分离,浊点较低,迟滞时间较短；由于链缠结密度较高和链段运动能力较弱,熔融共混样品相分离速率反而较慢。链缠结程度对该共混体系相分离热力学和动力学的影响不具一致性,即相分离温度低,迟滞时间短,并不意味着相分离速率快。这是由于两者主导因素不同所导致。相分离热力学由临界浓度涨落所决定,而相分离动力学取决于链段运动能力主导的扩散过程。更多还原

【Abstract】 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.更多还原