Amorphous polymers solid conformations

The secondary structure, such as a conformation, is studied mainly by solid-state NMR.2 In the solid state, NMR chemical shift is characteristic of specific conformations because the internal rotation around the chemical bonds is restricted. This shows that the NMR chemical shift can be used for elucidating the conformation of polymers in the solid state. In the amorphous phase, the conformation of the polymer chain is not fixed above Tg. Even in such a case, NMR chemical shift and the relaxation parameters can give us useful information such as the averaged conformation or the dynamics of the exchange. Solid-state NMR can also provide information about the crystalline structures, which are classified under the higher order structures through NMR chemical shift, since for most polymers, different crystalline structures accompany conformational changes which affect their NMR chemical shift. 

Determination of macromolecules conformations is one of the basic problems of science about polymers. Simultaneously with development of theory [4-6] the perfection and enrichment of experimental methods of determination of macromolecules conformations in various phase and aggregate states occurs. However the method of neutron scattering was almost the only one method allowing reliable determination of polymer chains conformation in solid amorphous state until now [7], Not long ago they begun to use with this aim also the method based on measurement of rate of electron excitement transfer between molecules of chromophores covalent bonded with polymer chain [8],... 

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]. 

At this point it is necessary to examine some basic concepts related to the polymer chain conformation that will certainly change in solution. In the solid state (amorphous or/and crystalline), the macromolecules contract and interpenetrate (entangle/co-crystallize) into the others, but once the solvent diffuses, they start to swell and eventually (high dilution) they disentangle to be finally dispersed in the solvent. In this process the polymer coils gradually expand reaching a conformational equilibrium dictated by thermodynamic laws. It was suggested that many properties of polymer solutions depend on the conformation of the chain, rather than on the nature of the chain atoms [45]. 

A noncrystalline polymeric material that has no definite order or crystallinity. A polymer in which the macromo-lecular chain has a random conformation in solid (glassy or rubbery) state. On the one hand, an amorphous polymer may show a short range order, while on the other, a crystalline polymer may be quenched to the amorphous state (viz., polyethylene terephthalate (PET)). The maximum value of a periodically varying function, e.g., used to describe the energy transmitted from the ultrasonic welding horn to the weld joint. 

A complete random arrangement (absence of longe-range molecular order) of polymer chains is given in an amorphous polymer . The macromolecules show a coil conformation in the solid state. 

Amorphous polymers (transparent in the solid state to be precise, it is not a solid but rather a supercooled liquid) are usually easy to dissolve in the good solvent. In contrast, crystalline and semicrystalline polymers (opaque in the solid state) are sometimes not easy to dissolve. Within a crystallite, polymer chains are folded into a regular, thermodynamically stable arrangement. It is not easy to unfold the chain from the self-locked state into a disordered state in solution even if the latter state is thermodynamically more stable. Heating may help the dissolution because it facilitates the unfolding. Once dissolved, polymer chains take a random-coil conformation unless the chain is rigid. 

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