The crystal structure of poly(l-lactic acid) (PLLA) α form has been analyzed in detail by utilizing the 2-dimensional wide-angle X-ray (WAXD) and neutron diffraction (WAND) data measured for the ultradrawn sample. The WAXD data were collected using a synchrotron-sourced high-energy X-ray beam of wavelength 0.328 Å at SPring-8, Japan and the WAND data were measured using a neutron beam of wavelength 1.510 Å with a cylindrical imaging-plate camera of BIX-3 system at Japan Atomic Energy Agency. The initial crystal structure model was extracted successfully by a direct method under the assumption of the space group P212121 using about 700 X-ray reflections observed at −150 °C, the number of which was overwhelmingly large compared with the data reported by the previous other researchers and allowed us to perform more precise structural analysis. The crystal structure model obtained by the direct method was refined so that the best agreement between the observed and calculated integrated intensities was obtained or the reliability factor (R) became minimal: R was 18.2% at −150 °C and 23.2% at 25 °C. The thus-obtained chain conformation took the distorted (10/3) helical form with 21 helical symmetry along the chain axis. However, the symmetrically forbidden reflections 003, 007, 009 etc. were detected in a series of the 00L reflections, requiring us to erase the 21 screw symmetry along the molecular chain. By assuming the space group symmetry P1211, the structural refinement was made furthermore and the finally obtained R factor was 19.3% at −150 °C and 19.4% at 25 °C. Although the structural deviation from the 21 screw symmetry was only slightly, this refined model was found to reproduce the observed reflection profiles well for all the layer lines. The thus X-ray-analyzed crystal structure was transferred to the WAND data analysis to determine the hydrogen atomic positions. The R factor was 23.0% for the 92 observed reflections at 25 °C. The agreement between the observed and calculated layer line profiles was good. In this way the crystal structure of PLLA α form has been established on the basis of both the X-ray and neutron diffraction analyses.
A film of melt-crystallized poly(L-lactic acid) (PLLA) consisting of R-form crystals was uniaxially drawn by solid-state coextrusion at 110-170 °C using split billets of different polymers. The effects of extrusion variables, including the extrusion draw ratio (EDR), temperature (Text), and pressure (Pext), on the crystal transformation from the initial R-form to the oriented β-form crystals were studied. The crystal transformation proceeded rapidly with EDR. It was found that when coextrusion was made at a constant P ext and EDR but at different Text's, there was a Text (130 °C) where the crystal transformation proceeded most efficiently. Furtheremore, when coextrusion was made at a constant Text and EDR but at different Pext's, the transformation proceeded more efficiently at a higher Pext. These results show that crystal transformation proceeds with the EDR most efficiently for the coextrusion at a Text of 130 °C and a higher Pext. As a result of the specific effect for each of the extrusion variables on the crystal transformation and ductility, a highly oriented film consisting of β crystals alone was obtained by coextrusion to the highest EDR of ∼14 achieved at a high Text of 170 °C, near the Tm, and a high Pext of ∼50 MPa.
Melt-crystallized, low molecular weight poly(L-lactic acid) (PLLA) consisting of ␣ crystals was uniaxially drawn by solid-state extrusion at an extrusion temperature (T ext ) of 130 -170°C. A series of extrusion-drawn samples were prepared at an optimum T ext value of 170°C, slightly below the melting temperature (T m ) of ␣ crystals (ϳ180°C). The drawn products were characterized by deformation flow profiles, differential scanning calorimetry (DSC) melting thermograms, wide-angle X-ray scattering (WAXD), and small-angle X-ray scattering as a function of the extrusion draw ratio (EDR). The deformation mode in the solid-state extrusion of semicrystalline PLLA was more variable and complex than that in the extensional deformation expected in tensile drawing, which generally gave a mixture of ␣ and  crystals. The deformation profile was extensional at a low EDR and transformed to a parabolic shear pattern at a higher EDR. At a given EDR, the central portion of an extrudate showed extensional deformation and the shear component became progressively more significant, moving from the center to the surface region. The WAXD intensities of the (0010) ␣ and (003)  reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial ␣ form to the oriented  form proceeded rapidly with increasing EDR at an EDR greater than 4. Furthermore, WAXD showed that the crystal transformation proceeded slightly more rapidly at the sheath region than at the core region. This fact, combined with the deformation profiles (shear at the sheath and extensional at the core), indicated that the crystal transformation was promoted by shear deformation under a high pressure rather than by extensional deformation. Thus, a highly oriented rod consisting of only  crystals was obtained by solid-state extrusion of melt-crystallized, low molecular weight PLLA slightly below T m . The structure and properties of the ␣and -form crystals were also studied.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.