Stress-Induced Reversible Phase Transition of Poly(tetramethylene naphthalate)

Abstract

Crystalline modifications R and β of uniaxially oriented poly(tetramethylene naphthalate) [PTMN] sample have been found to show a reversible phase transition, which is induced by an application of a tensile force along the draw axis, as clarified by the measurement of polarized infrared spectra under tension. The transition behavior was found to be essentially the same with that observed for uniaxially oriented poly(tetramethylene terephthalte) [PTMT] sample. The β phase fraction, evaluated by quantitative analysis of the infrared spectra, was found to be linearly proportional to the strain and increase dramatically beyond a critical stress. The stress-strain curve measured for the bulk PTMN sample showed a plateau region, in which the R-β transition occurred. The plateau length became longer by raising the temperature of heat treatment of the samples or with increasing the degree of crystallinity. All these characteristic features are similar to those observed for PTMT. But, in the PTMT case, the plateau stress was almost constant irrespective of the annealing temperature. The stress-strain curve of the PTMT sample could be interpreted reasonably on the basis of a mechanical model in which R crystal and amorphous region are connected in a series mode. On the other hand, the plateau region in the stressstrain curve of the PTMN sample was found to shift apparently toward higher stress side and became more difficult to detect for the sample annealed at lower temperature. This was due to higher residual strain or β fraction remained in the sample. High-temperature annealing accelerated a stress relaxation, causing a recovery of the original R form and making the plateau part clearer. All these behaviors could be interpreted reasonably by employing a mechanical model in which a serially connected part of R crystal and amorphous region is combined in a parallel mode with a serial part of β crystal and amorphous region. The contribution of the β crystal is higher for the sample prepared at a lower temperature.

Stress induced reversible crystal transition in polymers

“…It is clear that the chain in the β form is more stretched and elongated along the c axis. Similar results were observed in poly(butylene naphthalate) (PBN) …”

“…Similar results were observed in poly(butylene naphthalate) (PBN). 29 Ichikawa and coworkers 17 reported a new phase in stretched PBS fibers. Determined by X-ray diffraction, the phase of PBS was found to adopt an all trans planner zigzag conformation, different from the conventional TGTGT conformation of the butylene unit in form.…”

“…At room temperature the value was ca 74.8 MPa. Replacing the benzene ring by a naphthalene ring, PBN required a smaller stress ( ca 60 MPa) for crystal transition compared with PBT . According to Ichikawa and coworkers, PBS had a critical stress of 140 MPa at room temperature.…”

“…The Takayanagi model was applied by Ichikawa et al to PBS, in which the serial crystalline and amorphous phases were paralleled by a pure crystalline phase or amorphous phase, depending on the crystallinity of the system. To describe the mechanical behavior of PBN, Tashiro and coworkers modified the serial model to explain the vague plateau in the stress–strain curves …”

Crystal structure identification of poly(trimethylene 2,6-naphthalate) β-form crystal by X-ray diffraction and molecular modeling

“…In this study, the crystal structure of PTN b-form crystal, developed at high crystallization temperatures, is successfully (8) 100.85 b (8) 88.78 g (8) 120.63 Repeating units/unit cell 2 Calculated density (g cm 23 ) 1.393 Observed density (g cm 23 ) 1.37 identified by using the X-ray powder diffraction combined with the molecular modeling technique. The unit cell of PTN b-form crystal was found to be triclinic (P 1 space group) with dimensions of a ¼ 0:4665 nm; b ¼ 0:7014 nm; c ¼ 2:2177 nm; a ¼ 100:858; b ¼ 88:788; and g ¼ 120:638: The observed crystal density of 1.37 g cm 23 and the determined dimensions of unit cell indicate that the unit cell contains one polymer chain with two repeating units.…”