Molecular chain conformations spectroscopy

Changes in the conformation of the different forms of cellulose have been investigated157,158,195,196 by use of Raman spectroscopy. Celluloses I and II were found157 to have different, and distinct, molecular-chain conformations. No assignments of the frequencies were proposed, but the correlation between the spectra and the structure of celluloses was discussed. The major differences in the Raman spectra were observed below 800 cm-1, in the... 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

By their nature, and in contrast with microscopic and scattering techniques that are used to elucidate long-ranged structure, spectroscopic methods interrogate short-range structure such as interactions between fixed ions in side chains and counterions, main chain conformations and conformational dynamics, and the fundamental hopping events of water molecules. The most common methods involve infrared (mid-IR and to a much lesser extent near- and far-IR) and solid-state NMR spectroscopies, although other approaches, such as molecular probes, have been utilized. 

An important objective in materials science is the establishment of relationships between the microscopic structure or molecular dynamics and the resulting macroscopic properties. Once established, this knowledge then allows the design of improved materials. Thus, the availability of powerful analytical tools such as nuclear magnetic resonance (NMR) spectroscopy [1-6] is one of the key issues in polymer science. Its unique chemical selectivity and high flexibility allows one to study structure, chain conformation and molecular dynamics in much detail and depth. NMR in its different variants provides information from the molecular to the macroscopic length scale and on molecular motions from the 1 Hz to 1010 Hz. It can be applied to crystalline as well as to amorphous samples which is of particular importance for the study of polymers. Moreover, NMR can be conveniently applied to polymers since they contain predominantly nuclei that are NMR sensitive such as H and 13C. 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

The molecular chain in crystal lattice of the monoclinic a form has a helical conformation with three units per turn to avoid the steric hindrance of the bulky methyl groups and a large cross-section of 0.344 nm. The crystal modulus along molecular chains therefore is low, which is estimated to be 41.2 GPa by X-ray diffraction measurement or 88.2 GPa by Raman spectroscopy. The theoretical tensile strength is calculated to be 18.2 GPa. 

Infrared and Raman spectroscopy (considered together) Molecular identification determination of chemical functionality chain and sequence length quantitative analysis stereochemical configuration chain conformation. (d,e)... 

In order to understand the close picture of the organized phases, conformational properties of the molecular chains in the aggregates are fundamentally important [6-8]. No definitive experimental evidence of the conformational relevance to the organized phases has been reported, possibly because of the limited experimental techniques to provide precise information of the conformational state of the molecules in these systems. Vibrational spectroscopy, which implies infrared and Raman spectroscopy, is one of the most powerful techniques for this purpose, because the spectra exhibit a number of bands characteristic of particular conformational states of the molecular chain [9]. One of the practical advantages of this method over the others is its applicability to the molecular systems in any physical states of the substance. 

In this work, we utilized a newly developed technique of isolated C-D stretching vibrational spectroscopy. This method using a number of selectively monodeuterated species of the surfactant is capable of determining the conformational state of each of the specified sites of the molecular chain and eventually, after examining all possible monodeuterated species, the conformational state of... 

The molecular conformation in other C E -water systems has been studied by Gaufres et al. [16,17] by means of Raman spectroscopy. They have concluded on the CgEs-water and Ci2Es-water systems that there is no substantial discontinuity in the conformational state of the molecular chain at the Lj/Hi and Li/Ij phase transitions. This conclusion is consistent with the present results for... 

The improvements in technology in the last decade allowed NMR spectroscopy to emerge as one of the most important methods for polymer characterization. High resolution solution NMR, solid-state NMR, and the introduction of multidimensional NMR offer more detailed information, at higher resolution and in diverse conditions, on the polymer structure and polymerization mechanisms, chain conformation, and molecular dynamics of the polymers, blends, and multiphase polymer systems. 

Infrared spectroscopy also provides information on molecular microstructure, e.g. the repeat units resulting from addition polymerization of dienes. For example, polyisoprenes (Fig. 2.9) can be distinguished, based on differences in absorption between C-H out-of-plane bending vibrations. The infrared spectra of stereoregular polymers are also distinct from those of their less regular counterparts, but these differences do not arise directly from tacticity but indirectly due to its effect on chain conformation. 

It should be pointed out that the process illustrated in Figure 12.18a is difficult to be monitored. However, the preordering process, if it exists, can be easily identified by polarized infrared (IR) spectroscopy as it is very sensitive both to chain conformation and local molecular environment of a polymer [167]. To find out whether there is a preordering process of the PCL chain in the supercooled melt on an oriented PE substrate, one need only to select a temperature at which the crystallization of PCL is suppressed. Fourier transform infrared (FTIR) spectroscopy in polarization mode can then be used to check the orientation status of the PCL chain segments during the annealing process. 

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