Amorphous polymers conformation

B. Bom, H. W. Spiess, Ah Initio Calculations of Conformational Effects on NMR Spectra of Amorphous Polymers Springer-Verlag, New York (1997). 

A suitable approach to the equilibration of an amorphous polymer system at bulk density becomes much more likely when the fully atomistic model in continuous space is replaced by an equivalent coarse-grained model on a lattice with sufficient conformational flexibility. Different strategies, which seek results at different levels of detail, can be employed to create an appropriate coarse-grained model. Section 4 (Doruker, Mattice) describes an approach which attempts to retain a connection with the covalent bonds in the polymer. The rotational isomeric state (RIS) [35,36] model for the chain is mapped into... 

The transition between crystalline and amorphous polymers is characterized by the so-called glass transition temperature, Tg. This important quantity is defined as the temperature above which the polymer chains have acquired sufficient thermal energy for rotational or torsional oscillations to occur about the majority of bonds in the chain. Below 7"g, the polymer chain has a more or less fixed conformation. On heating through the temperature Tg, there is an abrupt change of the coefficient of thermal expansion (or), compressibility, specific heat, diffusion coefficient, solubility of gases, refractive index, and many other properties including the chemical reactivity. 

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]. 

On a molecular level, partially crystalline to amorphous polymers are normally used. As the material is heated, Brownian motion occurs resulting in a more random chain arrangement. When a unidirectional force is applied to a resting polymer melt, the chains tend to move away from the applied force. If the applied force is slow enough to allow the Brownian movement to continue to keep the polymers in a somewhat random conformation, the movement of the polymer melt is proportional to the applied stress, i.e., the flow is Newtonian. 

The extended chain is another conformation which has been discussed often and proved to exist—e.g., in linear polyethylene crystallized at high pressures. It has also been assumed for amorphous polymers in the form of strands of parallel molecules [kink model (57)]. The maximum length L (x-ray) of a sequence of vinyl monomers of basic molecular weight M0 is 2.52 A. Straight strands of molecules can be... 

If the amorphous polymer is not submitted to a strain the macromolecule take a random coil conformation but when is submitted to a strain a rearrangement take place and, a new structural order of the macromolecules with a stretch conformation... 

Several general requirements must be met for a polymer to be elastic—that is, to stretch under the application of force but return to its original shape when the force is released. The polymer should be predominantly amorphous so that its Tg is below room temperature. The individual molecules of an amorphous polymer are not in fully extended, anti conformations instead, they have random, coiled conformations. When a force that pulls on opposite ends of the molecules is applied, the molecules assume an anti conformation about more bonds and thus they become longer that is, they stretch. Although the stretching tends to arrange the molecules in extended, zigzag conformations that are favorable for crystallization, the overall shapes of the elastomer molecules are such that crystallization does not readily occur. Furthermore, most elastomers Eire nonpolar, so only weak attractive forces exist between chains. Therefore, when the force is removed, the molecules tend to return to their initial random conformations because these random shapes are favored by entropy (disorder). 

Amorphous polymer—A polymer with random chain arrangements or conformations a polymer can be entirely amorphous or it can contain some crystalline regions. 

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