Chain symmetry

Polymer Chain Axis (A) Chain Symmetry Space Group References... 

With regard to orientation, consider a repeating side-group originating at atom the first atom of the side-group being BL. For certain chain symmetries (helical, for instance) the bond vectors b(A. B )... 

Table 1. Chain symmetry of some crystalline polymers [9]...

Glass Transition Temperature Increases with the presence of bulky pendant groups stiffening groups as 1,4-phenylene chain symmetry polar groups cross-linking... 

Sheth JP, Klinedinst DB, Wilkes GL, Yilgor E, Yilgor I. Role of chain symmetry and hydrogen bonding in segmented copolymers with monodisperse hard segments. Polymer 2005 46 7317-7322. 

Rietveld (g.c.) analysis of the neutron diffraction data on isotactic polypropylene is still in progress. It has afforded the interesting result, already discussed, that the profiles are better approximated by Cauchy than by Gaussian functions. The structural analysis is now restricted to the fourth model (P2 /c, Immirzi), which gives an excellent agreement between observation and calculation, but with the fraction of reversed helices close to 50% instead of 25% and with less chain symmetry. The other models will be tested for a more complete comparison with x-ray results. We cannot exclude, however, the possibility that the two samples used, which have different chemical, thermal and mechanical history, can really have different structures. 

If, however, the phenylene rings are para-oriented, the chains retain their axial symmetry and can crystallize more readily. Similarly, double bonds in trans configuration maintain the chain symmetry thus allowing for crystallite formation. This is highlighted by a comparison of the amorphous elastomeric ciy-polyisoprene (Tm = 28°C) with highly crystalline trans-polyisoprene (Tm — 74°C) which is a non-elastomeric rigid polymer, or c -l,4-polybutadiene (Tm = - 11°C) with ram-l,4-polybutadiene (Tm = 148°C). 

PSCL(nc,nb) = side chain symmetry properties (Proprietes de Symetries des Chaines Laterales). PSCL(nc,nb) = V. r 11/cr = Yffn=1 U(np,nc,nb) where np = main chain length. 

Another aspect of high chain symmetry is the possibility of molecular motion within the crystal lattice contributing to higher T. For example, polyethylene and polytetrafluoroethylene are both sufficiently symmetrical to be considered as smooth, stiff cylindrical rods. In the crystal, these rods tend to roll over each other and change position when thermally agitated. This motion within the crystal lattice, called premelting, effectively stabilizes the lattice. Consequently, more thermal energy is required to break down... 

Introduction of lateral substitnents to dismpt the chain symmetry... 

We have already seen, in earlier chapters, how chain symmetry, flexibility, and tacticity can influence the individual values of both T and T. Thus, a highly flexible chain has a low T, which increases as the rigidity of the chain becomes greater. Similarly, strong intermolecular forces tend to raise Tg and also increase crystallinity. 

We have already mentioned the importance of and and know that these can be alfectedby chain symmetry, stiffness, and intermolecular bonding. The tensile strength of a fiber is observed to increase with crystallinity consequently, this is a... 

Figure 8.42 Projection diagram of the hydrogen-bonded squarate-thiourea chain in 17. Each squarate ion cross-links two thiourea molecules protruding from two adjacent puckered thiourea layers to form a wide platform, while another two pairs of thiourea molecules bridge the squarate dianions and inter-link parallel squarate-thiourea chains. Symmetry...

A visual inspection of the perturbed structure showed that the rings were only slightly distorted, but they appeared to be rather much tilted out as a whole off their normal position in the lattice. Bridges with trigonal chain symmetry have been formed between neighbouring rings which had opened slightly. 

Increasing the triol/diol ratio raises the transparency as shown in Fig. 12.7, although hardness and tensile strength fell due to the disruption of chain symmetry. Not surprisingly compression set fell as the level of crosslinking was increased. 

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