Polymer orientation, characterization methods

The purpose of this section is not only to confirm the identification, but also to characterize certain polymers and polymer types in detail. Although methods to determine microstructures and impurities, such as chemical inversions, modifications, and multiple bond formations, are different from polymer to polymer and are discussed separately, the methods used for the determination of density and crystallinity, as well as polymer orientation, are common to most polymers. Thus, the determination of crystallinity and density will be covered in this section, in Sec. 3.1, and likewise, the orientation of the polymer chain will be described in Sec. 3.2. The use of absorption coefficients to calculate properties, such as crystallinity, doublebond content, chain branching, and monomer ratios, is described in reference texts [14,15]. Today most work is performed by Fourier transform infrared (FTIR), and so an attempt has been made to feature coefficients from the latest reference sources, which include data acquired by FTIR. 

The polymer is subjected to a biaxial stretching, which creates orientation both in the crystalline and in the amorphous phases. As in fiber spinning, many studies have been dedicated to orientation characterization, especially in the crystalline phase, but biaxial orientation requires more sophisticated methods. More recently, much effort has been devoted to the description of the morphology at the lamellar scale, for example, by electron microscopy. 

Table 2.4 provides an abbreviated list of methods used for characterization of polymer blends, starting with determination of molecular structure and interactions and ending with determination of the crystallinity and flow-imposed orientation. The methods are general - their adaptation to HTPBs may require modification of equipment (e.g., high temperature cell for FT-IR or special heating oven for rheology of PEEK). 

AppHcations of soHd-state nmr include measuring degrees of crystallinity, estimates of domain sizes and compatibiHty in mixed systems from relaxation time studies in the rotating frame, preferred orientation in Hquid crystalline domains, as weU as the opportunity to characterize samples for which suitable solvents are not available. This method is a primary tool in the study of high polymers, zeoHtes (see Molecular sieves), and other insoluble materials. 

Birefringence is one of the simplest methods for the characterization of molecular orientation in polymers. The polarizability of a structural unit is usually not equivalent in all directions, leading to three independent refractive indices along its principal axes. In an isotropic sample, a single averaged macroscopic refractive index is observed whereas birefringence or trirefringence is observed... 

In order to calculate polymer/filler interaction, or more exactly the reversible work of adhesion characterizing it, the surface tension of the polymer must also be known. This quantity is usually determined by contact angle measurements or occasionally the pendant drop method is used. The former method is based on the Young, Dupre and Eowkes equations (Eqs. 21,8, and 10), but the result is influenced by the surface quality of the substrate. Moreover, the surface (structure, orientation, density) of polymers usually differs from the bulk, which might bias the results. Accuracy of the technique maybe increased by using two or more liquids for the measurements. The use of the pendant drop method is limited due to technical problems (long time to reach equilibrium, stability of the polymer, evaluation problems etc.). Occasionally IGC is also used for the characterization of polymers [30]. 

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