Amorphous Bulk State

An amorphous bulk polymer contains chains that are arranged in less than a well-ordered, crystalline manner. Physically, amorphous polymers exhibit a Tg but not a T, and do not give a clear x-ray diffraction pattern. Amorphous polymer chains have been likened to spaghetti strands in a pot of spaghetti, but the true extent of disorder that results in an amorphous polymer is still not fully understood. 

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A radius of gyration in general is the distance from the center of mass of a body at which the whole mass could be concentrated without changing its moment of rotational inertia about an axis through the center of mass. For a polymer chain, this is also the root-mean-square distance of the segments of the molecule from its center of mass. The radius of gyration is one measure of the size of the random coil shape which many synthetic polymers adopt in solution or in the amorphous bulk state. (The radius of gyration and other measures of macromolecular size and shape are considered in more detail in Chapter 4.)... 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

Bulk crystalline or amorphous solid-state materials whose conductivity is intermediate between metals and insulators and whose resistance decreases with increasing temperature. The valance band of an undoped semiconductor is completely filled, whereas its conduction band is empty. The energy difference between the valence and conduction bands (the band-gap) defines a semiconductor (see Fig. 95). 

Theta conditions are of great theoretical interest because the diameter of the polymer chain random coil in solution is thenequal to the diameter it would have in the amorphous bulk polymer at the same temperature. The solvent neither expands nor contracts the macromolecule, which is said to be in its unperturbed state. The theta solution allows the experimenter to obtain polymer molecules which are unperturbed by solvent but separated from each other far enough not to be entangled. Theta solutions are not normally used for molecular weight measurements, because they are on the verge of precipitation. The excluded volume vanishes under theta conditions, along with the second virial coelTicient. 

In the bulk state each polymer molecule is surrounded by other polymer molecules of the same type. Expansion of a given chain to relieve long-range intramolecular steric interactions only serves to create an equal number of intermolecular steric interactions with neighboring chains. These opposing volume exclusion effects exactly counteract each other and so in a bulk amorphous polymer the polymer molecules adopt their unperturbed dimensions (i.e., a = 1). 

Because the y carbon state could be populated to an extent equivalent to 10 monolayers, it seems obvious that this state was a bulk state and was either bulk nickel carbide, IH3C, or some form of free carbon. Based on x-ray diffraction analysis, and the fact that this state was never populated at 773 K (NijC is known to decompose above 600 K (10)), the y carbon state was most likely Ni C. X-ray diffraction measurements of reduced 17-wt% Ni/A Oj always show metallic nickel lines even after deposition of large amounts of carbon at 773 K and higher temperatures. However, following long exposure to C2H at 573 K, the nickel x-ray diffraction lines showed a pronounced decrease in intensity. Diffraction lines for NigC were not observed probably because most of the nickel crystallites were transformed into amorphous M3C or crystallites of NijC with small domain size (10) ... 

It seems appropriate to discuss here the probabUity of interpenetration of polystyrene coils in the model networks. As already mentioned, according to the theoretical considerations of Flory [138] and De Gennes [139], polymeric coils in an amorphous solid state retain unperturbed dimensions. Since the volume fraction of the polymer in an unperturbed coil under -conditions is weU known to be very smaU, only about 2%, the transition from swoUen coils to solid state has to be accompanied by the replacement of aU solvent molecules with fragments of other polymeric molecules. In other words, theoretical notions predict extremely high mutual interpenetration of the polymeric chains in bulk state. Indeed, in order to maintain the coil dimension that is characteristic for a -solution, the coil must accommodate, on removing the solvent, a 50- to 100-fold amount of alien polymeric matter. In the 1970s this problem was discussed in fiiU [149-165], The authors of the tailor-made networks also took part in the discussion. 

In the earlier discussion of the properties of crystalline homopolymers in the bulk state (Chapter 1), it was shown that such polymers always contain some amorphous material, various segments of the same chain lying in crystalline or amorphous regimes (Krigbaum et al, 1964). If the amorphous portion is above its 7, i.e., is elastomeric, a direct analogy exists between such materials and polymer blends, blocks, and grafts, in which the formation of hard and soft domains is induced by the use of polymer components differing in chemical composition and inherent properties thus, as pointed... 

The existence of pure amorphous bulk polymers has been a controversial issue since the beginning of polymer science. Natural rubber yields an X-ray pattern tiiat contains only amorphous halos, typical of any liquid. Nevertheless, it was difficult for many scientists to believe that molecules with a polymeric chain structure could pack in a truly amorphous way. There are still papers that are submitted for publication that assert that amorphous rubbery polymers are actually composed primarily of microcrystalline domains. This issue has been clarified by the incisive theoretical and experimental work of Flory. It is now understood that there are polymers that exhibit liquid crystalline phases upon melting of the crystals. The nature of the noncrystalline state of pure bulk polymers depends on tiie detailed local structure of the chain and the ratio of die persistence lengtii of die chain to the diameter of the mer. Molecules that are conformationally dexible enough to have a small persistence length can exist in the amorphous liquid state. 

Polymers confined in the nanosized spaces of the PCP channels typically show properties that are distinctly different from those shown for the same materials in the bnlk state becanse of the formation of specific molecular assemblies and conformations [19, 20]. The inclusion of polymers within crystalline microporous hosts (pore size < 2 mn) with ordered and well-defined nanochannel sfiuctures has atfiacted considerable levels of attention because, in confiast to amorphous bulk polymer systems and polymers in solution, this approach can prevent the entanglement of polymer chains and provide extended chains in resfiicted spaces. 

In amorphous polymer-based nanocomposites this question is not very crucial because the polymer molecules can either be in the amorphous bulk matrix or be confined within the clay interlayers depending on the interactions between the clay surface and the matrix. In semicrystalline polymer matrices, however, the situation is much more complex, as three situations can be distinguished the molecules (1) remain in the crystalline phase of the polymer matrix, (2) stay in the amorphous state, or (3) reside confined within the... 

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