Polyethylene Raman spectroscopy, crystal

JAHM Moonen, WAC Roovers, RJ Meier, BJ Kip. Crystal and molecular deformation in strained high-performance polyethylene fibers studied by wide-angle X-ray scattering and Raman spectroscopy. J Polym Sci B Polym Phys 30 361-372, 1992. 

Over the IS years since the original Raman deformation studies upon polydiacetylene single crystals, the technique has been developed and refined to involve the study of a wide range of different high-performance polymers and other materials. These have included rigid-rod polymer fibres [19-21], carbon fibres [22-24] and ceramic fibres [2S-27]. This present chapter will concentrate upon recent research concerning the use of Raman spectroscopy to follow the deformation of aramid fibres and gel-spun polyethylene fibres and the possibility of the extension of the technique to isotropic polymers, and also the important and developing application of the method to the study of the deformation of fibres within composites. 

One of the main uses of Raman spectroscopy in morphological analysis is the measurement of the longitudinal acoustic mode (LAM) in polyethylene which goes back to relatively early work (1949) on paraffins. The LAM frequency (u(LAM- )) can be converted to all-tmns chain length or crystal thickness (Lc) according to the following formula ... 

In solution-crystallized polyethylene fractions, Raman spectra have demonstrated that crystalline structure is invariant with molecular mass and that crystallinity is far from complete. The interfadal region is relatively small, as expected from theoretical considerations. Densities of solution-grown crystals of linear polyethylene show that the crystals are 8(C90% crystalline [145 147]. This conclusion is supported by measurements of the enthalpy of fusion, infrared and Raman spectroscopy, and other physical properties [148]. Consequently there is a small but appreciable... 

Online Raman spectroscopy was used in this study to measure the orientation development during blown film extrusion of low-density polyethylene (LDPE). The analysis is valid for uniaxial symmetry of the structural units with the chain axis. The trans C-C stretching vibration of PE at 1130 cm whose Raman tensor is coincident with c-axis of the orthorhombic crystal, was used to solve a set of five intensity ratio equations. Crystalline orientation (P2) was found to increase along the axial distance in the film line. The P2 values also showed an increasing trend with crystalline evolution during extrusion... 

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]. 

Fig. 10.2 shows the infrared, Raman and INS spectra of polyethylene in the 0-1600 cm" region. The infrared and Raman spectra show no coincidences since the crystal is centrosymmetric. The INS spectrum is very different the absence of selection rules means that the features present in both optical spectra are apparent, but the major difference is that the regions between the infirared and Raman bands are filled-in in the INS spectrum. This is because INS spectroscopy gives information at all values of k, not just those at zero as for the optical spectroscopies. 

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