Ttgg crystal

Molecular Conformation of sPP gel. Figure 27 shows the DD/MAS 13C NMR spectrum of sPP gel. This spectrum was obtained by a single-pulse sequence (tt /2—FIDdd-tt)ii with the repetition time ty more than 5 times the longitudinal relaxation time Tic. Hence, this spectrum reflects the thermal equilibrium state of the gel. For comparison, the spectrum of the bulk ttgg crystal of this sample... 

Fig. 28. Molecular structure of the ttgg crystal form of sPP. a and b show the relationship between the methylene and methine carbons separated by three bonds, in trans (a) and gauche (b). c shows the side view and the view from the c axis of the molecular sequence in the ttgg crystal form. The closed and open circles in the main chain show the methylene and methine carbons, respectively. The methylene carbons marked as A and B are the methylene carbon with two gauche effects (the gt-gt carbon) and that without gauche effect (the tg-gt carbon), respectively. See the text...

Fig. 27. Equilibrium DD/MAS 13C NMR spectrum of sPP/o-dichlorobenzene gel (13.6 wt%), obtained by a n/2 single pulse sequence with a repetition time of 300 s. In the lower part the equilibrium spectrum of the bulk sPP crystal in ttgg form is shown for reference. The arrows indicate the resonance of the amorphous component of each carbon...

Fig. 29. IR spectra of the tttt and ttgg bulk crystals and sPP/o-dichlorobenzene gel (13.6 wt%). The spectrum of the gel represents only the contribution from the sPP polymer that was obtained by subtracting the contribution from the solvent...

Syndiotactic polypropylene exists in various crystal modifications. The most stable chain conformation is helical and involves a succession of the type TTGG or TTG G . The helix conformation is very nearly a rectangular stair-... 

SPS takes two different conformations in its crystal, TT and TTGG conformations, depending on the crystallization conditions, and exhibits a very complex polymorphic structure [6-14]. One is the TT conformation which appears when SPS crystallizes from the melt. Two different transcrystals are reported when SPS takes the TT conformation in the crystal, a-form [15-19] and 3-form. The identity period is 5.06 A (Figure 18.3) in both crystal forms. The 3-form is more thermally stable than the a-form. Figure 18.4 shows the detailed structure of the 3-form [6,10]. 

SPS takes the TTGG conformation when it crystallizes in the presence of organic solvents. SPS takes the TTGG conformation in -y-form and 8-form crystals. The 8-form is a complex of SPS and solvent [20-24], Figure 18.5 shows the complex of SPS and toluene. 

Figure 3 shows the infrared spectra of the phenyl out-of-plane mode of sPS and sPPMS, before and after soaking in phenol solution. The peaks characteristic of the ttgg conformation at 572 cm i and 566 cm i for sPS and sPPMS are well-known to be very sensitive to the conformation. These peaks increased in intensity for both cast-crystaUized and melt-quenched films after soaking in phenol solution. This is direct evidence that the crystal with a ttgg conformation was induced by the presence of phenol. 

Syndiotactic polypropene crystallizes mainly in a conformation (TTGG)2 which corresponds to a 2i helix. 

Forms I and II of Syndiotactic Polypropylene sPP exists in different crystal modifications. Two of these are of interest in the present context. They are both based on a ttgg chain conformation, which gener-... 

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