TTTT conformation

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

All four five-membered proline ring constraints in the CTP are, however, essential for stabilizing any m-amide-bond conformers (132). A single substitution of N-methyl-L-alanine, or the six-membered homolog, L-pipecolic acid, destabilizes all the cm-amide conformers of c[Pro-pro-Pro-pro] and only the all-trans-amide tttt conformers were populated (see Fig. 9). 

Fig. 10.9 Illustration of all-trans TTTT conformation of polyethylene and TGTG 3/1 helix conformation of polypropylene...

Since two resonance lines at 39.0 and 47.7 ppm that correspond to those observed in the ttgg form and a resonance line at 49.0 ppm that corresponds to that in the tttt form are recognized in the gel spectrum, a coexistence of these two forms in the gel might be supposed. In an attempt to determine the possibility of the coexistence of the two forms in the gel, we measured the IR spectrum that is sensitive to the molecular conformation. The number of normal vibrational modes depends sensitively on the molecular conformation based on the selection rule of the symmetry species. Kobayashi et al. confirmed the vibrational modes assignable to the ttgg conformation in the IR spectrum for the gel from a sPP/carbon disulfide system [117]. However, since we used o-dichlorobenzene as solvent, we examined whether the gel structure depends on the solvent. 

The CC2 calculation indicates that the emission maximum, corresponding to the vertical transition from the tttt minimum to the electronic ground state, lies at longer wavelength for conformer B (—440 nm) than for conformer A ( 400 nm) [24], This would lead to a dual fluorescence with the picosecond (30 ps) component having longer-wavelength emission relative to the nanosecond (12 ns) component. Consistent with this prediction, the time-resolved fluorescence spectra of PdG, taken at short times (0-100 ps) has emission peak at about 440 nm, whereas that taken at... 

Extensive mapping of the conformational surface of c[Pro-pro-Pro-pro] with DFT calculations by Che and Marshall (124) showed that a set of eight distinct amide-bond conformers and their interconversions (Fig. 9) based on the number and location of cis- or tmns-amide bonds tttt, or all trans cccc, or all cis tctc ctct, tccc, cttt, ttcc, and cctt) were accessible. Individual conformers could be selectively stabilized by use of azaproline in which the alpha carbon of proline was replaced by nitrogen and other proline analogs. 

It would be difficult by inspection of Figure 14 to decide which pair of conformers are the most similar. For example, one may think that TTTT and TTTC, or TTCT and TCTC may be the most similar, but the computation shows the most similar to be the pair TTTC, TTCT, which are at the distance 0.136. Next come the pair TCTC, CTTC (0.233) TTCC, TCCT (0.237) TTCT, TCTC (0.245) TTCC, CTCC (0.256) and so on. The pair TTTT, TTTC, with the distance measure of 0.305, is clearly not among the most similar conformers, illustrating the difficulty of estimating similarity by visual perception. 

PVDF has at least five crystalline phases (a-, P-, y-, 5-, and s-phase), arrd the a- and P-phases are the most common. Because of the all trans planar zigzag conformation (TTTT), the P-phase has the largest spontaneous polarization, and many applications take advantage of the properties of this polar phase. Therefore obtaining the polar P-phase and suppressing the non-polar a-phase (thermodynamically favored) is the major goal of PVDF processing. ... 

In some cases, vibration bands are also conformation-sensitive. In particular, when a polymer exists in several crystal forms, different chain conformations may exist in these crystal modifications one example is poly(vinyhdene fluoride) (PVDF). At least four types of crystal modifications are known forms I, II, Hpoiar, and III (26). Form I has nearly a zig-zag chain conformation. Forms II and Ilpoiar exhibit the glide-type TGTG conformation, while form HI has a TTTGTTTG conformation. The ir spectra of these crystal modifications are shown in Fig. 9 (27). For example, the intensive bands observed in the spectra of form I are 1275, 840, and 510 cm and others correspond to the vibration modes intrinsic to long trans sequences of. .. TTTT. The bands at 800, 766, 610, and 530 cm and others in the spectra of form II are attributed to the conformation sequence of TG. On the other hand, in form III, the TG and TT bands coexist in addition to the bands characteristic of the TTTG sequence (27). 

The anion has two isomers of C2 and symmetry in [C4mim][NTf2] [22]. Two isomers were found for the 1-hexyl-3methylimidazolium cation, [Cgmim]. They have the TTTT form (all-trans conformation) and the GTTT form (gauche conformation around the C7—C8 bond and... 

---