Polymer macro-conformations

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

Rg. 5.11 A schematic representation of the macro-conformations of polymer chains. The vertices indicate the limiting cases. A, amorphous B, chain-folded C, chain-extended. The area indicates intermediate structures D, fringed micelle. (Reproduced by permission of Academic Press.)... 

Finally, as in macro-Raman experiments, orientation-insensitive spectra can also be calculated for spectromicroscopy. A method has been developed recently for uniaxially oriented systems and successfully tested on high-density PE rods stretched to a draw ratio of 13 and on Bombyx mori cocoon silk fibers [65]. This method has been theoretically expanded to biaxial samples using the K2 Raman invariant and has proved to be useful to determine the molecular conformation in various polymer thin films [58]. 

The reactions of intramolecular cross-linking is a rather poorly investigated area in the field of macro-molecular reactions. However, the problems of regularities of such processes are related to such important problems of polymer chemistry as chemical modification of polymers, networks formation, sorption of low molecular reagents by polymers, intramolecular catalysis, conformational transitions and so on. In spite of the great importance of the study of regularities of cross-linking reactions, the experimental and theoretical analysis of such processes is complicated by many difficulties. ... 

Professor Ubbelohde s suggestions are certainly worthwhile. But there are many directions that we are trying to pursue more detailed studies of the conformation of the polymers, introduction of new functional groups, macro molecular characterization by hydrodynamic methods, and so on. If I were a professor at Imperial College, I could assign all these problems, and those you suggest, to members of the staff but I am constrained by a smaller research group. 

The conformational structures of polysilane main chains at the macro-and microscopic levels are controllable by suitable choice of the side chain structures. Similarly, it is also the side chain which controls the optoelectronic properties by effecting the optical band gap. In the case of phenyl-substituted polysilanes, electronic interaction between the delocalized Si chain cr-bonding orbitals and the it-orbitals of the aryl groups causes a dramatic modification of both the band gap and conformational properties [61,83]. These aryl-containing polysilanes may be potential candidates for applications in a molecular-based chiroptical switch and memory in the UV/visible region. On the other hand, the precise control of helical polymers is now a subject of great interest and importance, due to the tech-... 

Since its introduction in the early 1970s, the small angle neutron scattering (SANS) method has had a substantial impact on polymer research. When used with partially deuterated polymers, SANS permits a close monitoring of macro-molecular conformations in polymer solutions, melts, and blend mixtures. This advantage has made it a unique tool for the understanding of the morphology of polymer materials and of the relationship between their structures and physical properties. 

We described the basic aspects of NOESY in Section 10.1 as an introductory example of a 2D experiment. NOESY is very widely used in measuring macro-molecular conformation, as we see in Chapter 13. However, as shown in Fig. 8.4, the H— H nuclear Overhauser enhancement 17 varies from its value of +0.5 in small molecules to a limiting value of — 1 in large polymers with very long Tc, and at intermediate values of rc the NOE may vanish. An alternative is to use the NOE measured in the rotating frame, as this quantity is always positive. By analogy to NOESY, this technique has the acronym ROESY (rotating frame Overhauser enhancement spectroscopy),... 

The theoretical analysis of the relationship between small-scale relaxation processes in polymers PL reveals just these processes) and the intra- or intermolecular inter-actions shows that these processes are sensitive to various changes in the number (probability of formation) of intra- or interchain contacts in polymer solutions. The experimental data reveal that the intramolecular mobility (IMM) of the polymer in solution is highly susceptible to various conformational transformations of macro-molecules and to changes in the intermolecular interactions in complex multicom-... 

The nearly constant value observed for for a variety of macro-molecules suggests that all of these materials have essentially the same conformation during flow. More instances of deviation of X from a "universal value may be found as polymers that are not completely disordered, that have "stiff chains, for example, are studied. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

MAT Matsuo, K., Stockmayer, W.H., and Bangerter, F., Conformational properties of poly(l-octadecene/maleic anhydride) in solution, Macro/wo/ecw/ei, 18, 1346, 1985. 1985MIN Minsker, K.S., Panchesnikova, R.B., Monakov, Yu.B., and Zaikov, G.E., Thermodynamic and hydrodynamic properties of chlorine-containing carbochain polymers in solutions, Eur. Polym. J., 21, 981, 1985. 

As discussed in [2], in a condis crystal cooperative motion between various conformational isomers is permitted, whereas in the CD-glass this motion is frozen, but the conformationally disordered structure remains. In the case of polymers, a condition for formation of condis crystals is that the macro-molecules exist in conformational isomers of low energy, which leave the macromolecules largely in extended conformations so that the parallelism of chain axes is maintained. 

Some kinds of disorder are as follows First of all, polymers (as well as oligomers, in general) are materials consisting of molecules of non-uniform molecular mass [74]. At variance with biological macromolecules, as nucleic acids and polypeptides, which are known to be uniform at least with respect to the molecular structure of the core (that is, sequence arrangement of constitutional units, stereoregularity and overall conformation), the macro-molecular chains in a given polymer may be similar, but, certainly, not all alike [74,75]. 

Polymers organized in this unusual state behave as hexagonal mesophases similar to those observed in the bulk at suitable temperature/pressure conditions and adopt the extended chain conformation. Polymers included in nanochannels were discovered a few decades ago [33,34], but the mesomorphic properties and the stabilizing interactions were established much later by advanced spectroscopic techniques [35-41]. The preparation of novel macro-molecular adducts, melting at temperatures as high as 350 °C and sustained by CH- 71 intermolecular interactions, has been a success of supramolecular chemistry in fabricating high performance nanostructured materials [42]. 

---