高聚物在有限体积元中的等温结晶动力学过程研究

【作者】 孙辉；

【导师】 张志英； 吴世臻；

【作者基本信息】 天津工业大学 ， 材料学， 2004， 硕士

【摘要】 本文采用Monte Carlo方法研究了高聚物在有限体积元中的等温结晶动力学过程，分析了在一定条件下，样品体积收缩、晶体线生长速率变化、样品厚度变化和晶核数目变化这四种因素对高聚物在有限体积元中的等温结晶过程的影响。利用模拟结果对Avrami方程和Price方程的有效性进行了评价，得到了上述四种因素对高聚物结晶动力学参数影响的定量数据。随着研究的深入，发现当样品厚度降到一定值时，在结晶过程开始后不久就会出现一个初期转折点。同时，通过热台偏光显微镜、DSC等实验验证了部分计算机模拟实验得到的结果。 计算机模拟实验首先建立了有限体积元模型，然后选定了PET为样品，一般模拟其在预先成核条件下、170℃时的等温结晶过程。结果表明，有限的体积使Avrami指数n值偏小。样品体积收缩，导致Avrami指数n值减小；结晶速度v_c值增加；表示实验值偏离理论预测直线时的相对结晶度α_s值减小；与Avrami方程相比，Price方程仍不能较准确地描述高聚物等温后期结晶行为。晶体线生长速率G减小，对Avrami指数n值和α_s值基本没有影响：而G减小，v_c值也会减小；热台偏光显微镜实验以PEO为样品，也证实了这些结果。当单位体积内晶核数目不变时，样品厚度h减小，n减小；v_c和α_s值基本不变：当h降到小于等于体系内球晶的平均直径时，在结晶最初期，会出现一个初期转折点。DSC实验以PEO为样品，验证了此结果。在体系晶核数目M变化的实验中，考虑了两种成核情况。在预先成核条件下，M的减小，n值减小；v_c值减小；α_s值增加。在散现成核条件下，考虑了晶核数目M随时间增加的四种不同情况，随着初始晶核数目M_o的增加，Avrami指数n值由接近于4逐渐减小到接近于3；v_c值增加。M随时间的增长方式对α_s值的影响程度不同。并且，晶核数目指数增加比线性增加对结晶过程影响明显。更多还原

【Abstract】 In this paper, the course of isothermal crystallization kinetics of polymer in limited volume unit is simulated by use of the method of Monte Carlo. Four factors influenting on the course of polymer in the limited volume unite isothermal crystallization are analyzed under the given conditions. The four factors are sample volume shrinkage, the change of the linear growth rate of entities G, the change of sample thickness and the change of the number of nuclei. Then the validity of Avrami equation and Price equation is evaluated. And the quantitative data are obtained that the four factors influent on the parameters of polymer crystallization kinetics. Moreover, the turning point that isn’t yet reported by relative literature before is found in the earlier stage of crystallization when the sample thickness reduce to the desired value. Some conclusions from simulating experiments are validated though the experiments of the hot stage polarizing microscopy(HSPOM) and differential scanning calorimetry(DSC).Firstly, the computer simulating model of the limited volume unit is established. Then PET is chosen as sample. Generally, under predetermined nucleation condition, the course of PET isothermally crystallized at 170 C in limited volume unit is simulated. It is shown that the value of the Avrami exponent n decreases because of the limited volume. Volume shrinkage of sample results in the increasing of the value of the crystallization rate vc and the decreasing the values of the Avrami exponent n in the primary crystallization and s, defined as the relative crystallinity degree when experiment data deviate theoretical predicting straight line. Compared with the Avrami equation, the Price equation hardly makes progress on describing the behavior of polymer later stage crystallization. When the linear growth rate of entities G reduces, the value of vc reduces too. However, the values of Avrami exponent and s are hardly affected by the value of G. The conclusions are approved by the data fromHSPOM experiment. The sample thickness declining, provided the number of nuclei per unit volume fix, the value of the Avrami exponent n falls, but the values of vc and s almost don’t change. When the value of thickness reduce to the average diameter of entities, the turning point turn up after the course of crystallization has begun soon. The conclusion is testified by the DSC experiment. In the experiment of the number of nuclei change, there are two nucleation conditions considered. Under predetermined nucleation condition, with the number of nuclei decreasing, the values n and vc drop and the value of s ascends. When being sporadic nucleation condition, four cases are considered that the number of nuclei increase with time by different ways. In every case, if the initial number of nuclei go up, the value of the Avrami exponent n goes down approximately from 4 to 3, on the contrary, the value of vc moves up. The different ways of the number of nuclei increasing with time vary the influence extent of the value of s. And the exponential growth of the number of nuclei has sharper effect on the course of crystallization than the linear growth of its.更多还原