No abstract

Melt crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 °C/min from 270 °C) was studied via a combination of wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (32/31) helical pairs below 220 °C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 °C; the intensity of this mesophase peak started to decrease at 150 °C, with concomitant emergence of WAXS- or DSC-discernible formation of stereocomplex (βc) crystals. Isothermal measurements at 200 vs 170 °C revealed the presence of low vs high populations of helical pairs; βc crystals were observed to develop only at 170 °C but not at 200 °C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of βc nuclei. The clear change in the melt structure well before the formation of incipient βc crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of βc crystals.

Isotactic copolymers of 1-propene and 1-hexene that contain over 10% and up to about 25% of hexane units in the chain have been shown to crystallize in an unusual crystal modification that differs significantly from the well-known structures of isotactic polypropylene (α, β, and γ phases) or of isotactic poly(1-hexene) (Macromolecules 2005, 38, 1232). The experimental data obtained by Poon et al. are reinterpreted and analyzed. The new crystal form has a trigonal unit-cell with parameters a = b = 1.717 nm, c = 0.65 nm, very similar to form I of isotactic poly(1-butene) (iPBu) (trigonal cell with parameters a = b = 1.770 nm, c = 0.65 nm). The chain conformation is the 3-fold helix of isotactic polypropylene. The side-chain material in the copolymer with the highest hexene content (25%) is about 12% less than in iPBu form I (all ethyl units). These copolymers of propene and hexene adopt a crystal structure isomorphous to that of a third polyolefin (polybutene) that, in terms of overall composition, is close to the average of the two “parent” olefins.