Experimental criterion for the crystallization regime in polymer crystals grown from dilute solution: possible limitation due to fractionation
Experimental criterion for the crystallization regime in polymer crystals grown from dilute solution: possible limitation due to fractionation
复制标题
DOI：
10.1002/polb.1986.090240212
复制
发表时间：
1986-02
期刊：
影响因子：
--
通讯作者：
J. Point;M. Colet;M. Dosière
中科院分区：
文献类型：
--
作者：
J. Point;M. Colet;M. Dosière
通过对Frank导出的方程的积分，证明了聚合物晶体的生长速率与晶体的尺寸有关，只要晶体的持续长度Lp或动力学长度Lk =（2g/i）~（1/2）明显大于初生核。一种新的方法装饰的折叠表面（isoelectron装饰）允许测量的准瞬时生长速率的非常小的晶体从稀释的二甲苯溶液中获得的尖锐的聚乙烯部分的中等分子量（Mw = 17,000，Mw/Mn = 1.11）。尽管理论预测，生长速率随着晶体的尺寸而增加，只要其尺寸小于持续长度和/或动力学长度，但在目前使用的尖锐PE部分的情况下，在实验上没有观察到这种增加。因此，看来，动力学长度和（假设的）持久性长度都超出了电子显微镜的分辨率极限，并且结晶发生在多核化制度。一个上限获得的速率g，在该速率下，一个局部的新层在两个方向上的基板上传播。这一比率低于普遍接受的理论所估计的比率。这些理论也导致一个异常高的值的横向表面自由能。观察到的生长速率曲线的初始线性的可能性，可能会导致从平衡的相反的效果（一方面，增加与晶体的大小，减少与降低浓度和其他可能的分馏）进行彻底检查和排除。事实上，必须强调的是，在结晶的早期开始，样品的结晶部分可以忽略不计，只有在结晶结束时，这些影响才会出现。结晶结束时生长速率的下降既不是由于溶液本身的逐渐耗尽，也不是由于低分子量PE的这一锐分数的扩散引起的浓度耗尽。主要影响来自分馏。这种分离的各种分子量的基础上预测的一个简单的模型，并通过凝胶渗透色谱法（GPC）验证。事实上，在如此尖锐的部分中发生显着的分馏，排除了对过冷和实际结晶聚合物浓度的任何准确确定。分子量分布的细微差异可能导致生长速率的显著变化。总之，由于在本工作的第一部分中使用的数据是在只有小百分比的溶解聚合物样品结晶的情况下获得的，所观察到的生长速率的恒定性不是由相反效应的相互补偿引起的，并且我们关于结晶状态、动力学和持续长度的数量级的结论，和次生核侧向扩展率的值是有根据的。
By integration of equations previously derived by Frank, the growth rate of polymer crystals is shown to be dependent on their size, provided that the persistence length Lp or the kinetic length Lk = (2g/i)1/2 are significantly larger than the primary nucleus. A new method of decorating the fold surface (isochronous decoration) allows the measurement of the quasi-instantaneous growth rate of very small crystals obtained from dilute xylene solution of a sharp polyethylene fraction of moderate molecular weight (Mw = 17,000, Mw/Mn = 1.11). Although the theory predicts that the growth rate increases with the size of the crystals as long as its dimension is smaller than the persistence length and/or the kinetic length, such an increase is not observed experimentally with the sharp PE fraction presently used. Therefore it appears that both the kinetic length and the (hypothetical) persistence length are beyond the resolution limit of electron microscopy and that crystallization occurs in the polynucleation regime. An upper bound is obtained for the rate g at which a locally new layer spreads in two directions on the substrate. The rate is lower than is estimated by the commonly Accepted theories. These theories lead also to an abnormally high value for the lateral surface free energy. The possibility that the observed initial linearity of the growth-rate curve may results from a balance of opposite effects (an increase with the size of the crystals on the one hand, a decrease with decreasing concentration and possible fractionation on the other) is thoroughly examined and ruled out. In fact, it must be stressed that at the early beginning of crystallization, negligible parts of the sample are crystallized and it is only at the end of crystallization that these effects appear. The fall in the growth rate as crystallization ends is due neither to progressive exhaustion of the solution alone nor to a depletion of the concentration by diffusion for this sharp fraction of low-molecular-weight PE. The major effect comes from fractionation. This segregation of the various molecular weights is predicted on the basis of a simple model and is verified by gel permeation chromatography (GPC). The fact that in such a sharp fraction significant fractionation occurs precludes any accurate determination of the supercooling and of the concentration of the polymer actually crystallizing. Subtle differences in the molecular weight distributions may result in significant variation of the growth rate. In conclusion, as the data used in the first part of this work were obtained with only a small percentage of the dissolved polymer sample crystallized, the observed constancy of the growth rate does not result from mutual compensation of opposite effects, and our conclusions about crystallization regime, order of magnitude of the kinetic and persistence lengths, and value of the rate of lateral spreading of a secondary nucleus are well founded.