Determination of the molecular orientation of poly(propylene terephthalate) fibers using polarized raman spectroscopy: A comparison of methods
Determination of the molecular orientation of poly(propylene terephthalate) fibers using polarized raman spectroscopy: A comparison of methods
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DOI：
10.1366/000370204322886618
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发表时间：
2004-03-01
影响因子：
3.5
通讯作者：
Rabolt, JF
中科院分区：
文献类型：
--
作者：
Frisk, S;Ikeda, RM;Rabolt, JF
首次比较了通过偏振拉曼光谱测量确定分子取向程度的四种不同方法。分子取向对聚合物性能的巨大影响推动了多种实验技术和程序的发展。本研究基于聚对苯二甲酸丙二醇酯(PPT)在1614 cm(-1)处的C-1-C-4环伸缩振动。结果表明，简单地将平行于和垂直于样品的唯一轴的偏振获得的带强度进行比率提供了一种良好的定性方法来观察一系列相似样品中取向的演变。为了定量比较取向程度，需要使用一种更复杂的方法来产生取向分布函数的二阶和四阶参数（分别为 P-2 和 P-4）。迄今为止，大多数研究都是基于柱对称极化张量的假设。结果表明，尽管这种方法在过去已经被相当成功地使用，但这种假设是非常值得怀疑的。该方法产生的方向参数与通过两个更复杂的程序获得的方向参数明显不同。无论是理论上还是实验上，最复杂的方法都需要对每个样品进行最多的测量。在尝试计算所需参数时出现了主要问题，特别是对于具有高双折射的样品。这些问题与当样品相对于入射光的偏振方向倾斜时测量时出现的实验复杂性有关。在第三种方法中，这些测量被简单的去极化比测定所取代。该方法假设去极化率与分子取向和结构的变化无关。发现这个假设是不正确的。因此，最复杂的方法是选择定量确定取向分布函数的二阶和四阶参数的方法，除非了解所研究的每个样本的去偏振比。这些知识允许使用实验上更简单的方法来获得所需的参数。
For the first time, four different methods to determine the degree of molecular orientation from polarized Raman spectroscopy measurements are compared. The great influence of molecular orientation on the properties of polymers has driven the development of multiple experimental techniques and procedures. This study is based on the C-1-C-4 ring stretching vibration of poly(propylene terephthalate) (PPT) at 1614 cm(-1). It is shown that simply ratioing the band intensity obtained with the polarization parallel and perpendicular to the unique axis of the sample provides a good qualitative method to observe the evolution of orientation in a series of similar samples. To quantitatively compare the degree of orientation one needs to utilize a more complex method yielding the second- and fourth-order parameters of the orientation distribution function (P-2 and P-4, respectively). To date, most studies have been based on the assumption of a cylindrically symmetric polarizibility tensor. It is shown that this assumption is highly questionable although this method has been used fairly successfully in the past. This method results in orientation parameters that are clearly different front those obtained with the two more complex procedures. The most complex method, both theoretically and experimentally, requires the most measurements per sample. Major problems have occurred when trying to calculate the desired parameters, in particular for samples with high birefringence. These problems are related to experimental complexities occurring for measurements when the samples are tilted with respect to the polarization direction of the incident light. These measurements are replaced by a simple determination of depolarization ratio in the third method. This method assumes that the depolarization ratio is independent of changes in molecular orientation and structure. It was found that this assumption is not correct. Thus, the most complex method is the method of choice to quantitatively determine the second- and fourth-order parameters of the orientation distribution function, unless one has knowledge of the depolarization ratio of each sample being studied. That knowledge permits the use of an experimentally simpler method to obtain the desired parameters.