Structures of Two Crystalline Forms of Poly(butylene terephthalate) and Reversible Transition between Them by Mechanical Deformation

In situ study of fiber structure development of poly(butylene terephthalate) in a continuous laser-heated drawing process

“…However, in this study, no obvious quasi-smectic fibrillar structure was observed during the laser-heated drawing of PBT, likely because the transformation from the PBT mesophase to the crystalline structure occurs too rapidly to be observed with this measurement's time resolution of 0.3 ms. Although all the measured d-spacings were larger than the calculated values for the reported unit cells, 1,6,7 it is reasonable to conclude that the samples are in the b-crystal orientation, except for the 'drawn' sample. The differences should be caused by molecular packing in the equatorial directions and can also be caused by the applied stress in the meridional direction.…”

“…The dspacing and crystallite size D were calculated using Bragg's equation (3) and Scherer's equation (4), respectively, with a constant k of 0.918. The crystal orientation factor f was estimated by fitting the azimuthal intensity profiles of equatorial (010), (100) and ( À104) reflections to the Pearson VII function (equation (5)) and then using the fitted profiles in equation (6).…”

“…The crystallite sizes of the (010), (100) and ( À104) planes increased sharply up to 2 ms after necking and then remained nearly unchanged with further elapsed time. This indicates that bphase crystallites were formed and increased in size within 2 ms after the crystal-crystal transition and then stopped growing, which seems β [6] β [7] α [1] β [6] β [6,7] β [7] α [1] α[1] Figure 5 Interplanar spacings of (a) (010), (b) (100) and (c) ( À104) planes. The calculated d-spacings of the reported unit cells 1,6,7 are also shown.…”

“…[1][2][3][4][5][6][7] The a-form crystalline structure, in which the chain consists of four methylene groups with a gauche-trans-gauche conformation, is found in a relaxed state and is the most stable structure. The a-crystal is transformed into the b-crystal when the polymer is held under strain, and the polymer reversibly transforms to the a-crystal upon removal of the strain.…”

“…Mencik 1 was the first to index the unit cell of the a-crystal, which is triclinic and has lattice parameters of a ¼ 0.483 nm, b ¼ 0.596 nm, c ¼ 1.162 nm, a ¼ 99.91, b ¼ 115.21 and g ¼ 113.81. Yokouchi et al 6 proposed that the a-crystal exhibits ca. 681 internal rotation of the methylene bonds, that the terephthalate segment is nonplanar with the carbonyl bonds, and that the b-phase structure has lattice parameters of a ¼ 0.473 nm, b ¼ 0.575 nm, c ¼ 1.311 nm, a ¼ 104.21, b ¼ 120.81 and g ¼ 100.91.…”