No abstract

Polyglycolic acid (PGA), a biodegradable polyester with a simple molecular structure, shows an abnormally high melting point of 220 °C which is the highest among biopolyesters.

Cocrystallization phenomenon between the hydrogenous (H) and deuterated (D) species of a series of isotactic polypropylene blends with various D/H contents has been established for the first time on the basis of the detailed analysis of thermal and infrared spectral data for both α1 and α2 crystalline forms, where the α1 form is the crystal form of disordered chain packing mode but the α2 form has the ordered chain packing structure. The melting point has been found to shift continuously towards higher temperature side with an increase in the H component. This continuous change in the melting and crystallization point was reasonably interpreted by assuming the coexistence of D and H chain stems in the crystal lattice. The so-called correlation splitting was detected as doublets of the infrared bands for the pure D species sample at around 1066 cm−1. This correlation band splitting width changed systematically depending on the D/H ratio and became a singlet for the sample with only 5 mol % D component. This systematic change in the splitting width of the infrared bands can be interpreted also reasonably based on the concept of the random existence of D and H chain stems in the crystal lattice. Although the details of the thermal behavior and the infrared band splitting are different between the α1 and α2 forms, the observed phenomena are essentially the same each other. Both of thermal and infrared data indicate reasonably the cocrystallization phenomenon of the D and H chain stems in the crystal lattice of the α1 and α2 forms when cooled from the melt, suggesting a possibility of the random chain folding mode in the crystalline lamellae.

The melt isothermal crystallization behavior has been investigated for a series of blend samples of deuterated (D) and hydrogenated (H) species of isotactic polypropylene by means of time-resolved Fourier transform infrared (FTIR) spectral measurement and simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation source in SPring-8. The SAXS data collected were analyzed in the three different time regions. The Guinier plot in the early stage of crystallization gave the average size of isolated domains formed in the melt. In the middle stage, the correlation between these domains was analyzed using the Debye−Bueche equation. In the later stage of crystallization, the correlation function was calculated to estimate the degree of crystallinity and the long period of the stacked lamellae. The IR data analysis clarified the formation process of regular helical chains in the isothermal crystallization phenomenon. The combination of FTIR, WAXD, and SAXS data has revealed a concrete structural evolution process as follows: (i) In the early stage, the short helices start to be formed, and they gather together to create an isolated domain of about 330 Å radius of gyration. (ii) The domain size decreases gradually to about 300 Å, and (iii) these domains start to have some weak correlation to each other with ca. 300 Å correlation length. (iv) In these domains, the regular chain segments grow to longer helices, and the correlation distance becomes shorter to ca. 200 Å, and finally, (v) the stacked lamellae with long period of ca. 200 Å are formed and stabilized. The crystallization behavior of the D/H 50/50 blend sample was also investigated, and the origin of cocrystallization phenomenon has been discussed.