Trans-gauche transformation

A cross-linked polymer has a density of 0.94 g cm" at 25°C and a molecular weight between crosslinks of 28,000. The conformation of one bond in the middle of the molecule changes from trans to gauche, and the molecule opens up by 120°. In w-butane, the trans to gauche transformation requires about 3.3 kJ mol". Estimate a value for AH of stretching based on this model, and use the law of cosines to estimate the magnitude of the opening up that results. 

In addition, the hairpin activation energy Uh can be calculated from the Arrhenius fit to the data in Figure 1.10, from which we obtain a value Uh = 3.6 kJ/mol. Noting that hairpin formation in the decamethylene spacer requires two trans-to-gauche transformations, each of which involves an energy expenditure of about 1 kJ/mol [Morrison and Boyd, 1983], it appears that the experimental result is of the correct order of magnitude. 

In some polymers a reversible transformation from one conformation to another takes place under tension. Thus, upon application of stress the crystal structure of poly(tetramethylene terephthalate) (PTMT) changes from a crumpled near gauche-trans-gauche (g-t-g) conformation of the aliphatic chain segments (a-form) to an extended form with an all-trans (t-t-t) sequence (p-form) (Fig. 22a) This... 

Hydroxylamine exists in cis- trans- and various gauche- transformations. H-.VI H-... 

Boyd [19] discusses a motion related to the three-bond mechanism that can accomplish the appropriate shape change without encountering problems associated with free volume. It involves the conformational sequence GTG occurring in an otherwise all-trans chain (G and G represent alternative gauche transformations). From Figure 10.15 it can be seen... 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

In several cases the transitions under tension involve the transformation of gauche-bonds to trans-bonds for instance, for PBT the transition is from a form with dihedral angles, relative to the bonds of the tetramethylene group, of the kind GGTGG toward a form with the same dihedral angles of the kind TTTTT. Analogously for PVDF, under tension, there is a transition from the a form (TGTG chains) toward the 3 form (TTTT chains). 

Such a theoretical model has been quite successfully checked by Fourier transform infra-red spectroscopy (FTIR) on polystyrene [11,12]. Indeed, the FTIR spectrum of polystyrene contains vibrational lines associated with out-of-plane modes of the phenyl ring, whose position depends on the gauche or trans conformation of the main-chain C - C bonds. By recording FTIR spectra during sample stretching, it is possible to determine the amount of gauche or trans conformation as a function of strain. 

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