Single-helix crystal in atactic polypropylene

The conformational arrangement of a locally stereoregular sequence of atactic polypropylene (aPP) was investigated via X-ray diffraction, thermal analysis, and infrared spectroscopy. Since the aPP used was an isotactic-rich sample, the X-ray diffraction pattern showed the typical profile of an α-iPP (isotactic polypropylene) crystal, indicating that the crystal is constructed from locally stereoregular sequences. Even for a sample with low stereoregularity (meso pentad = 16%), small peaks due to α-iPP crystals were observed. The melting temperature of aPP was reduced compared with an α-iPP crystal. This corresponds to a shortening of the lamellar thickness. Helical units from chloroform insoluble and soluble fractions having meso pentads of 0.64 and 0.16, respectively, were composed of 3.5 and 1.2 repeating units, respectively. Such short conformational order was also confirmed by infrared spectroscopy. In general, a conformationally sensitive band is observed over a critical regular sequence (n). For the chloroform soluble fraction, a 977 cm−1 band for n = 5 was observed, but the typical 998 and 841 cm−1 bands for n = 11–12 and 13–15, respectively, were not observed. This signals that the regular conformation consists of 1–2 repeating units. It is concluded that a short conformational order (about 1–2 helical units) can construct the crystalline unit for aPP.