Amorphous polymers local order

Polycarbonates exhibit no crystalline structure when used in common manufacturing processes. At the molecular level, there is some evidence of localized ordering along a chain. It is believed that the polycarbonate repeat units can fold back onto the chain in a structure that resembles the letter Z. These individual units do not associate on a large enough scale to create a regular crystalline material. Under specific conditions, such as forcing the polymer to cool very slowly, we can create small crystalline domains. Since these conditions are not met in commercial processes, it is safe to say that the polycarbonates that we encounter are universally amorphous. 

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]. 

A given type of local motion in the glassy state must be responsible for a characteristic transition. This transition will be arbitrarily called (3, y, 8, etc., in the order of decreasing temperatures or increasing frequencies a is reserved in amorphous polymers to the transition between glass (local motions) and rubbery (cooperative motions) domains. 

To construct the model, it has been assumed that the amorphous polymer regions posses an approximately paracrystalline order with chain bundles locally parallel. A penetrant molecule may diffuse through the matrix of the polymer by two modes of motion, Fig. 5-2. 

III. The third step of direct longitudinal transmission of strain onto connected crystalline blocks leads to a perfect stretching of these fibrils. Because of the alignment of the molecules the fibers in this condition should possess a strength about 1 to 2 orders of magnitude higher than the yield stress of randomly distributed folded polycrystals. As the fibrils are able to stabilize the enhanced micro-void volume between them, a lateral coalescence of these voids finally provides a local deformation zone in the shape of a craze as known from amorphous polymers. 

While amorphous polymers constitute a significant fraction of the polymer sector, they are much less studied by vibrational spectroscopy. It is often easier to obtain the spectra than for crystalline systems but the interpretation is much more difficult. This is because the lack of local and long range order means that the group theoretical tools that are the backbone of vibrational analysis are largely inapplicable. Computational study is also difficult because the flexible nature of the polymer chains means that there are often many conformations with similar energy. In... 

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

In an un-cross-linked amorphous polymer, above its glass temperature, the molecular chains are continuously wriggling from one conformation to another. If a mechanical stress is imposed on such a system of wriggling chains, it can respond in three distinct ways instantaneous elastic response retarded (conformational) elastic response or viscous flow. Actually, in order to fit experimental data adequately, the retarded elastic element must be expanded into a whole series of such elements, some with shorter and some with longer response times. The local "kinkiness" of the chains can be straightened out (by stress) more rapidly than the... 

Summarizing, thermochromism only occurs above the glass temperature in amorphous polymer solids, and above the melting temperature for crystalline polymers [45], The crystallization of side chains, which may or may not occur, is not a necessary condition for the thermochromic transition. A more crucial requirement for thermochromism is that of the necessary level of regioregularity of the polymer. This will help define the local geometry of the polymer, which is crucial for the optical properties. Long-range order, like that found in crystalline phases, is not necessary for expression of the chromatic transitions. 

It is worth mentioning that the influence of the regions of local order, i.e., clusters, on the orientation behaviour of amorphous polymers has been discussed previously [14,15]. 

The cluster model assumes availability in the amorphous polymers structure of local order domains (clusters), consisting of several densely packed collinear segments of different macromolecules (amorphous analog of crystallite with stretched chains). These clusters are connected between themselves by tie chains, forming by virtue of this physical entanglements network, and are surrounded by loosely packed matrix, in which all fluctuation free volume is concentrated [107],... 

Polymers mechanical properties are some from the most important, since even for polynners of different special purpose functions this properties certain level is required [199], However, polymiers structure complexity and due to this such structure quantitative model absence make it difficult to predict polymiers mechanical properties on the whole diagram stress-strain (o-e) length—fi-om elasticity section up to failure. Nevertheless, the development in the last years of fractal analysis methods in respect to polymeric materials [200] and the cluster model of polymers amorphous state structure [106, 107], operating by the local order notion, allows one to solve this problem with precision, sufficient for practical applications [201]. 

For amorphous polymers value is accepted equal to relative total length of polymer segments, included in local order domains (clusters) and estimated as follows [204] ... 

X-ray diffraction studies have shown that native cellulose is a two phase system one is amorphous, with lower order and compactness, and is localized at the elemental fibril surface the other one is highly ordered and compact (crystallites), where the polymer chains are well organized (crystalline structure) and strongly bound by hydrogen bonds. 

Boyer, R. R, Evidence from Tll and related phenomena for local structure in the amorphous state of polymers, in Order in the Amorphous State, Miller, R. L., and Rieke, J. K., Eds., Plenum Press, New York, 1987. 

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