Amorphous polymers, halo

When the diffractogram of the pure amorphous polymer is not available at room temperature, the shape of the halo can be deduced either by peak fitting or estimating the halo pattern from the corresponding shape of the molten material. The uncertainties associated with these methodologies arise from the need to give a... 

Polymers with little structural symmetry and with bulky pendant groups, such as atactic polystyrene (PS), are usually amorphous. Amorphous polymers have no long-range order, and their x-ray diffraction patterns are diffuse halos rather than the sharp peaks which are characteristic of crystalline polymers. 

On the other hand, WAXS measurements of PE melt clearly indicate a range of intermolecular distance correlations of about 25 A [3]. Together with the relatively high density of polymer melts, the fact that the first interchain halo in WAXS patterns of oriented amorphous polymers tends to lie in the equatorial direction and the relatively high WAXS intensity of the interchain halo support the idea of parallel chain segments on the short range scale. 

Table 25406 shows the results of X-ray diffraction analysis of polyelectrolyte components and their complexes88. PMAA shows only a halo ring at about 2 = 20° which demonstrates that it is a completely amorphous polymer. In contrast, the polycation is a partially crystalline polymer, i.e. many diffraction rings are observed. Moreover, the following conclusions have been drawn ... 

X-ray diffraction studies were carried out for cyclolinear copolymers and it was shown, that they are amorphous systems. Independently of the length of single-unit dimethylsiloxane chain, diffraction patterns display two amorphous halos. It is known that the first amorphous halo, d, characterizes the average interchain distance in amorphous polymer [52], whereas d2 is more complicated and corresponds to both intrachain and interchain or interatomic distances. 

In the literature there are reports of radial distribution function analyses performed with polystyrene,6 7 polycarbonate of bisphenol-A,7 8 and a number of other amorphous polymers.9 As an illustration, we present results obtained with atactic polystyrene. Figure 4.2 shows the x-ray scattering intensity data obtained with CuKa radiation. The strong peak at 26 around 20° represents the so-called amorphous halo, whereas the smaller peak at around 10° is called the polymerization peak by some and has attracted interest with regard to its structural origin. The experimentally measured intensity is first corrected for background, polarization, absorption, etc.,... 

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

The major areas of application of ion beam techniques to polymer surface and interface problems concern (i) segregation at polymer surfaces and interfaces (ii) polymer-polymer interfaces and diffusion and (Hi) transport of non-polymeric materials through thin polymer films. A prototypical case of the latter application is swelling of an amorphous polymer by a small molecule solvent. Kramer et al. [223,224] probed the diffusion front in swelling of PS, PMMA and other polymers by halo-genated solvents. Adhesion problems, glass/polymer interfaces, and polymer surfaces have been studied. 

On flat detection films, wide angle X-ray diffraction (WAXD) from crystalline polymers yields circle-shaped patterns, the circles being related to reflections from different lattice planes, as can be seen in Figure 6.8. By contrast, amorphous polymers yield diffuse halos. 

As a rule, the wide angle X-ray halos of amorphous polymers have declinations from the ideal shape (asymmetry, indistinctly expressed maximum and so on) that allow [3] the availability of superposition of several simpler by shape scattering curves to be supposed. One expects that using strictly monochromatic scattering, purified from constituent, will raise the capability of solvable X-ray diffractometry and as... 

There is a number of experimental facts in favour of the approach considered above. For example, in paper [11] reduction in amorphous halo intensity I for nitrocellulose with increase in its ageing temperature was noted. Since the ageing process of amorphous polymers is associated with their ordering degree growth [12], then the latter can be connected with I reduction. The decrease in I at formation of clusters was also noted in paper [13]. It is possible that this process mechanism is similar to... 

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