Chain conformation and packing

Poly(e-Caprolactone). Two separate x-ray studies (17,18) have been reported for the crystal structure of poly(e-caprolac-tone). Both studies involved excellent fiber diffraction data but the final chain conformations and packing differed somewhat. The two structures are compared in Fig. 3. 

Molecular modeling of chain conformation and packing was done using the software packages LALS (101 and MOLEDITOR (111. Bond lengths and angles derived from the crystal structures of suitable model compounds (12-151 were used in the models. 

Wide-angle X-ray scattering techniques can provide direct information on key features such as crystallinity, preferred orientation, phase identification and compositional analysis.. zfs jjoj-g detailed analysis can yield details of local chain conformations and packing arrangements in both crystalline and disordered polymers. ... 

Farmer, B. L., Hopfinger, A. J., and Lando. J. B., Polymorphism of poly(vinylidene fluoride) potential energy calculations of the effects of head-to-head units on the chain conformation and packing of poly(vinylidene fluoride), J. Appl. Phys., 43, 4293-4303 (1972). 

A model for the chain conformation and packing of diphenyl methane 4,4 -di-isocyanate butane diol (M) hard segments in polyurethane elastomers" has been made based on the single crystal structure of the monomer unit, Me-M-Me in which... 

Multidimensional NMR 2D NMR experiments can be used for connectivity studies, can detect interactions between polymer chains and determine chain conformation and packing. 

Most properties of the substituted polyacetylenes (PA), such as permeability to penetrants, solubility, and thermal stability are strongly related to the number and/or steric hindrance of die substituents 1-4). However, these properties cannot be related in a strai tforward fashion to the kind of substituents or to the mono- or doublesubstitution along the main chain. In fact, the configuration of the repeat units and, consequently, the chain conformation and packing have to be taken into account and might be responsible for the properties of Afferent polymers. For instance, it has been shown that structural differences originating from different cis/trans tactic ratio in... 

There is evidence that the structure of crystalline regions (crystallites) is far from perfect. The chain conformation and packing often differ from those obtained for specimens crystallized under optimum conditions. In some fibres the situation is further complicated by polymorphism moreover, stable and metastable crystal modifications may occur simultaneously. " Similarly, the noncrystalline regions are far from being completely disordered indeed, there is a qualitative difference between their structure and that of an amorphous polymer. 

Although the diffraction techniques are unique in providing detailed information on the structural organization at the molecular level in the different crystalline forms, there are other characterization techniques which are sensitive to the chain conformation and in some cases to the chain packing, which can be used advantageously (and in some case more efficiently than diffraction techniques) in the recognition and quantification of the different polymorphs in polymeric materials. 

Amylose complexes (wet precipitates) were prepared with fluoro-benzene, 1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, bromo-form, and ferf-butyl alcohol. The conformation and packing of the amylose chains complexed with halogen-substituted hydrocarbons are the same as found in the complex with tert-butyl alcohol, namely,... 

Structural studies of amylose have, in turn, revealed a wide range of crystalline polymorphy, both in chain conformation and in crystalline packing. An example is the group of V-amyloses that exist as complexes with small organic molecules, water, or iodine. The latter complex is particularly interesting because it displays an intense blue color. The V-amyloses can be prepared by precipitation or drying from solution, and they crystallize readily. Consequently, their crystal structures are of interest in connection with any regenerated form of starch material. 

Moreover, studies on the structures of cellulose polymorphs are still being pursued extensively this is evident from the numerous entries on cellulose in this and the previous articles1,2 of this series. In addition to the conformation of the isolated, cellulose chain, the packing of the chains in the lattice presented formidable difficulties in the past. From the modest, theoretical attempts by D. W. Jones,3 Ramachandran,4 and Sundararajan5 to analyze the chain conformation and the... 

X-Ray analysis of hyaluronic acid revealed a broad spectrum of conformations and packing modes. For the monovalent-salt form, containing traces of divalent cation, two threefold, helical chains pass through the unit cell, with a = b = 1.7 nm, c = 2.85 nm, and y = 120°. For calcium hyaluronate, six threefold, helical chains are contained in the unit cell, with a = b = 2.09 nm, c = 2.83 nm, and y = 120°. Upon he-miprotonation of the potassium salt form, the chain had a sixfold screw symmetry, and six such chains pack in the unit cell with a = b =2.12 nm, c = 0.547 nm, and y = 120°. In the sodium salt form, the chains have a fourfold symmetry. The unit cell is tetragonal, with a = b = 0.99 nm, c = 3.39 nm, and y = 90°, and contains two chains. 

Let s summarize what we have just learned. Polymer crystallization is incomplete (i.e., polymers are semi-crystalline ). X-ray diffraction experiments tell us the chain conformation and the mode of packing. 

Polyamides are an important example of polymers which do not contain pseu-doasymmetric atoms in their main chains. The chain conformation and crystal structure of such polymers is influenced by the hydrogen bonds between the carbonyls and NH groups of neighboring chains. Polyamides crystallize in the form of sheets, with the macromolecules themselves packed in planar zigzag conformations. 

It has been reported extensively that fats solidify in more than one crystalline type (2-23). Triglycerides exhibit three main crystal types—ot, p, and p—with increasing degrees of stability and melting point. The molecular conformations and packings in the crystal of each polymorph have been reported. In the ot form, the fatty acid chain axes of the triglyceride are randomly oriented and the ot form reveals a freedom of molecular motion with the most loosely packed hexagonal subcell structure. 

The linker between helix and strand (H-L3-E) has one major sub-cluster. The first residue is mostly Gly and the second is hydrophobic with significant population of Ala residues, liie side chain of the centi linker residue points outwards but the carbonyl oxygen of L2 points inside the space between the packed helices (Figure 7), nu g this different from those observed in H-L3-H or E-L3-E linkers. The linker E-L3-H that links strands and helices has two major clusters and descriptions of backbone conformations are listed in Table n. The third linker residue L3 of the first sub-cluster has x, of g conformation. For the second sub-cluster, L2 has main chain conformation and g side chain conformationand L3 has extended backbone conformation and g side chain conformation. 

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