Crystalline polymers chain conformation

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

We have only dealt with the main chain nematic networks so far. Actually many liquid crystalline networks are formed by crosslinking the backbones of side chain liquid crystalline polymers. The side chain nematic polymers have three nematic phases and their backbones have either prolate or oblate conformations, depending on their phase. It is expected that the rubber elasticity of a side chain nematic polymer network is more complex. For instance, the stress-induced Ni-Nm phase transition is predicted as the network shape transforms from oblate to prolate. Liquid crystalline networks have a bright potential in industry. 

FT-IR is a powerful and reliable technique for description of chemical characterization of graphene and also study of the structure and properties relationships in PVDF/PMMA/graphene polymer blend nanocomposite. The interactions between these three components including PVDF, PMMA and graphene sheets can be revealed and described by using this technique. As well, exploration of PVDF chains conformations (crystalline structures) are affected by presence of graphene sheets and PMMA chains can be done which is very important in order to design the new material with special properties. 

Commercial ETFE is an equimolar copolymer of ethylene and tetrafluoroethylene (1 1 ratio) and is isomeric with polyvinylidene fluoride. ETFE has a higher melting point than PVDF and a lower dissipation fac-torl l because of its special chain conformation. Crystalline density was 1.9 g/cm for a polymer containing 12% head-to-head defect, The unit cell of the crystal is expected to be orthorhombic or monoclinic with cell dimensions of a = 0.96 nm, b = 0.925 nm, c = 0.50 nm and 7= 96°. 

In concluding this discussion, it is important to point out that crystalline polymers can be polymorphic because of slight differences in the conformation of the helical disposition of stereoregular polymer chains the polymorphism is attributable to differences in the weak intermolecular bonds. This abstruse phenomenon (which does not have the same centrality in polymer science as it does in inorganic materials science) is treated by Lotz and Wittmann (1993). 

As usual, this can be due both to thermodynamic and kinetic reasons. In fact, the presence of comonomeric units increases, in general, the energy content of all the crystalline forms, but, since the extent of increase may be different, it may destabilize some chain conformation or some kind of packing more than other ones. On the other hand, the influence of the comonomeric units on the polymorphic behavior of a polymer can be due to a change in the crystallization rates of the various forms. 

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

In addition to quantitative crystallinity data, IR and Raman have been proven valuable tools to extract information on chain conformation in the three major phases [112-114], local order in amorphous polymers [115,116] high throughput characterization [117] and structural and polymorphic changes on heating and cooling semi-crystalline polymers [118-120]. 

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene (PTFE), /ra .s-1,4-poly (1,3-butadiene), and cis-1,4-poly(isoprcnc). 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

Boulanger, P., Pireaux, J. J., Verbist, J. J. and Delhalle, J., XPS study of polymer chain conformation in amorphous and crystalline poly(ethylene terephthalate) samples, J. Electron Spectrosc. Rel. Phenom., 63, 53-73 (1993). 

---