Amorphous state chain conformation

As explained earlier (Sect. 1.3.1), macromolecules in a low-molecular-weight solvent prefer a coiled chain conformation (random coil). Under special conditions (theta state) the macromolecule finds itself in a force-free state and its coil assumes the unpertubed dimensions. This is also exactly the case for polymers in an amorphous melt or in the glassy state their segments cannot decide whether neighboring chain segments (which replace all the solvent molecules in the bulk phase) belong to its own chain or to another macromolecule (having an identical constitution, of course). Therefore, here too, it assumes the unperturbed ) dimensions. 

A polymer chain has an enormous number of chemical bonds. For this, in the solution and amorphous states the NMR chemical shifts of polymers are often the averaged values for all of the possible conformations because of rapid interconversion by rotation about chemical bonds. In solids, however, chemical shifts are often characteristic of specific conformations because of strongly restricted rotation about the bonds. The NMR chemical shift is affected by a change of the electronic structure through the structural change(2). Solid state NMR chemical... 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

As discussed earlier, solid polymers can be distinguished into amorphous and the semicrystalline categories. Amorphous solid polymers are either in the glassy state, or - with chain cross linking - in the rubbery state. The usual model of the macromolecule in the amorphous state is the "random coil". Also in polymer melts the "random coil" is the usual model. The fact, however, that melts of semi-crystalline molecules, although very viscous, show rapid crystallisation when cooled, might be an indication that the conformation of a polymer molecule in such a melt is more nearly an irregularly folded molecule than it is a completely random coil. 

Macromolecular conformations describe the positions of the atoms that occur due to rotation about the single bonds in the main chain.2 Polymer chains in solution, melt, or amorphous state exist in what is termed a random coil. The chains may take up a number of different conformations, varying with time. Figure 15.4 shows one possible conformation for a single polymer chain. In order to describe the chain, polymer scientists utilize the root mean square end-to-end distance ((r2)m), which is the average over many conformations. This end-to-end distance is a function of the bond lengths, the number of bonds, and a characteristic ratio, C, for the specific polymer. 

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],... 

The conformations of rigid chains will not be much different in the amorphous state near Tm than they are in the crystal lattice. This means that the melting process confers relatively little additional disorder on the system A5m is low and Tm is increased correspondingly. For example, ether units in polyfethylene oxide) (1-42) make this structure more flexible than polyethylene, and Tm of high-molecular-weight versions of the former species is only 66"C. By contrast, poly(/ -xylene) (11-1) is composed of stiff chains and its crystal melting point is 375°C. 

Figure 15b shows a solid-state spectrum recorded under conditions such that only the mobile portions of the solid PDHS sample are observed. In this polymer (as previously indicated), the mobile portion of the sample consists of the locally disordered phase II and any amorphous material to the extent that it exists. The chemical-shift pattern for the carbons agrees very well with the solution spectrum (Figure 15a). Because carbon resonances are very sensitive to bond conformation (22), this result demonstrates that the phase II portion of the sample has the same average chain conformation as the polymer chains in solution. Although these NMR data permit a comparison of local bond conformations, they do not provide an indication of the more global chain dimensions. Figure 15b shows increased line widths for the carbons near the silicon backbone, with the C-1 resonance almost broadened into the baseline. This broadening reflects the severe restriction of motion near the backbone.

The disordered state of a statistical coil is what is displayed by polymers in the molten and amorphous states and also in solution. To describe the conformation of a macromolecule consisting of a main chain N +l atoms, the positions of all them have to be determined. Using vectorial... 

Solid polymers can occur in the amorphous or crystalline state. Polymers in the amorphous state are characterized by a disordered arrangement of the macromolecular chains, which adopt conformations corresponding to statistical coils. The crystalline state is characterized by a long-range three-dimensional order (order extending to distances of hundreds or thousands... 

The unordered (amorphous) state of aggregation in which the polymer chains also assume random conformations represents one extreme in the physical state of the polymer. This is the state that exists in such amorphous states as solution, melts, or some solids, the randomness being induced by thermal fluctuations. The other extreme is the case where the molecules are able to pack closely in perfect parallel alignment as is found in those polymers that exhibit fibrous behavior— that is, in those possessing a high degree of crystallinity and crystal orientation. In between these two extremes of amorphous and crystalline polymers there is a wide spectrum of polymeric materials with different degrees of crystallinity and amorphous character. These are called semicrystalline. 

The development of the random coil by H. F. Mark and many further developments by P. J. Flory led to a description of the conformation of chains in the bulk amorphous state. Neutron scattering studies revealed that the conformation in the bulk is close to that found in solution in 0-solvent (see Chapter 3), thus strengthening the random coil model. On the other hand, some workers suggested that the chains have various degrees of either local or long range order. 

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