Phase Transition Mechanism of Poly(l‑lactic acid) among the α, δ, and β Forms on the Basis of the Reinvestigated Crystal Structure of the β Form

The crystal structure of poly­(l-lactic acid) β form has been reinvestigated on the basis of the 2-dimensional X-ray diffraction diagram measured for the sample obtained by stretching the highly crystalline α form at a high temperature. The six helical chains of (3/1) conformation are packed in a rectangular unit cell of the space group <i>P</i>1 with the complicated but systematic packing mode of the upward and downward chains. This structural model is different from the previously reported trigonal model [Puiggali Polymer 2000, 41, 8921]. The structural phase transition mechanism from the α form to the β form via the δ form has been proposed by assuming the cooperative displacements of the upward and downward helical chains as well as the conformational change. To support this mechanism, the two types of experiments were performed: (i) The highly oriented regular α form was stretched at about 165 °C to the various drawing ratios and cooled to the room temperature with the sample length fixed constantly. The X-ray diffraction data of these samples revealed the transition from the α to the mechanically deformd α (α_d), to the δ form, and then to the β form depending on the drawing ratio. (ii) The α sample was suspended vertically with a constant load in the heating process, and the X-ray diffraction pattern was measured as a function of time (temperature). The original α form was found to melt at about 200 °C via the transition to the α_d form and then to the δ form, followed by the recrystallization into the highly oriented β form. From these two experiments, the tension-induced structural transition from the α to the β form was found to occur via the disordering of the α form to the α_d and to δ form under tension. The structural change process was derived on the basis of all of the knowledge collected from the X-ray structure analysis and the <i>ex-situ</i> and <i>in-situ</i> X-ray diffraction measurements: the molecular chains experience the cooperative translational slippage along the diagonal directions in the <i>ab</i>-plane to give the complicatedly mixed packing of the upward and downward chains to give the characteristic structure of the β form. At the same time, the chain conformational changes from the regular (10/3) to the disordered (10/3) and then to the (3/1) form during the disordering in the chain packing mode

Reinvestigation of the β‑to‑α Crystal Phase Transition of Poly(butylene adipate) by the Time-Resolved X‑ray Scattering and FTIR Spectral Measurements in the Temperature-Jump Process

Poly­(butylene adipate) (PBA) exhibits the two types of crystal modification, the α and β forms, depending on the sample preparation conditions. They show the different degree of biodegradability. A majority of papers published so far reported that the phase transition from the β-form to the α-form occurs as the direct solid-to-solid process when the sample is heated up to the high temperature of around 55 °C. We have reinvestigated this β-to-α phase transition by performing the temperature-jump time-resolved measurement of the FTIR, WAXD, and SAXS measurements. This transition has been found to be not a solid-to-solid phase transition but the combined phenomena of the melting of the β-phase followed by the recrystallization to the high-temperature α-phase

Observation of Water-Stimulated Supercontraction of Uniaxially Oriented Poly(vinyl alcohol) and the Related Hierarchical Structure Change Revealed by the Time-Resolved WAXD/SAXS Measurements

A uniaxially oriented poly­(vinyl alcohol) (PVA) film was found for the first time to respond to the humidity change in the two different modes under the restrained condition, which are essentially the same as the modes of the supercontraction and cyclic contraction observed in spider dragline silk. Once the atmospheric humidity started to increase, the PVA film showed at first the irreversible stress generation (supercontraction stress), followed by the reversible stress generation synchronizing with the cyclic change of humidity. These irreversible and reversible stress changes have been connected to the changes of higher-order structure caused by the cyclic change of wet/dry atmosphere as revealed by the detailed <i>in situ</i> measurements of the 2-dimensional wide-angle and small-angle X-ray scatterings during these processes. On the basis of a simple mechanical model and the thus-collected information on the higher-order structural change, it was concluded that the irreversible and reversible stress generations are governed mainly by the irreversible hydration-induced stress relaxation in the highly tensioned amorphous region and the reversible swelling in the normal amorphous region, respectively

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion–water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H⁺(H₂O)_<i>n</i>), nonprotonated hydrogen (H)-bonded water (H₂O···H₂O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O–H bending band, it was found that the volume fraction <i>f</i>_{<i>i</i> (<i>i</i>=each deconvoluted band)} of H-bonded water for elevated temperatures (>80–100 °C) increased remarkably higher than for 60 °C

Clarification of Cross-Linkage Structure in Boric Acid Doped Poly(vinyl alcohol) and Its Model Compound As Studied by an Organized Combination of X‑ray Single-Crystal Structure Analysis, Raman Spectroscopy, and Density Functional Theoretical Calculation

When boric acid (BA) is added to poly­(vinyl alcohol) (PVA), a chemical reaction occurs to form the cross-linkages between the amorphous PVA chains. The local structural change caused by this reaction has been clarified concretely from the microscopic level on the basis of the X-ray-analyzed crystal structure, Raman spectra, and <i>ab initio</i> density functional theory using a model compound produced by the reaction between pentanediol (PENT) and boric acid (PENT–BA). The PENT–BA compound was found to take the TT and TG conformations in the methylene segmental parts depending on the stereoregularity of the PENT molecule itself, <i>meso</i> and <i>racemo</i> configurations, respectively. These two conformations give the Raman bands at the different positions. By comparison of the Raman spectra between the PVA–BA and PENT–BA model compounds, the local structures of PVA chains connected to BA molecules have been derived concretely: the syndiotactic PVA parts in the amorphous region form the TG-type ring structure with the 3-coordinate boron atom, where T and G are trans and gauche conformers, respectively. On the other hand, the isotactic PVA part takes the TT conformation when it forms a ring with boron atom. The thus-created rings are hydrogen-bonded to form a dimer, which plays a role as cross-linkage between the neighboring PVA chain segments in the amorphous region

Transformation of Coiled α‑Helices into <i>Cross</i>-β-Sheets Superstructure

The fibrous silk produced by bees, wasps, ants, or hornets is known to form a four-strand α-helical coiled coil superstructure. We have succeeded in showing the formation of this coiled coil structure not only in natural fibers, but also in artificial films made of regenerated silk of the hornet <i>Vespa simillima xanthoptera</i> using wide- and small-angle X-ray scatterings and polarized Fourier transform infrared spectroscopy. On the basis of time-resolved simultaneous synchrotron X-ray scattering observations for in situ monitoring of the structural changes in regenerated silk material during tensile deformation, we have shown that the application of tensile force under appropriate conditions induces a transition from the coiled α-helices to a <i>cross</i>-β-sheet superstructure. The four-stranded tertiary superstructure remains unchanged during this process. It has also been shown that the amorphous protein chains in the regenerated silk material are transformed into conventional β-sheet arrangements with varying orientation

Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials

The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH₂ sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morphology is enabled by the precise halogen placement in the chain and appears to be driven by the selection of the nucleus stem length in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows models for generating new materials based on nanostructures at the lamellar and sublamellar level not feasible in classical branched polyethylenes

Hierarchical Structural Change in the Stress-Induced Phase Transition of Poly(tetramethylene terephthalate) As Studied by the Simultaneous Measurement of FTIR Spectra and 2D Synchrotron Undulator WAXD/SAXS Data

The simultaneous measurement of Fourier transform infrared (FTIR) transmission spectra and 2-dimensional wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) patterns has been performed successfully to investigate the hierarchical structure changes occurring in the stress-induced phase transition phenomenon of uniaxially oriented poly­(tetramethylene terephthalate) film. The molar fraction of the β-crystal form, evaluated from the IR and WAXD data analyses, increased steeply in the plateru region of the stress–strain curve as already known well. The 2D SAXS data have revealed the remarkable and reversible change in the stacked lamellar structure just after the α-to-β phase transition was completed, where the tilting angle of the stacked lamellae measured from the draw axis of the oriented sample became zero, and the lamellar thickness increased due to the inclusion of amorphous region located in the boundary part of the crystalline lamellae. In parallel, the X-ray reflection spots in a wider diffraction angle region became diffuse in the observed WAXD pattern of the β form, indicating the packing disorder of the mechanically stressed chains. In this way, the simultaneous combination of the 3 different types of equipments has allowed us to deduce the detailed structural change from the various levels: the stress-induced α–β transition was found to occur not only with the remarkable changes in the molecular chain conformation and chain packing mode in the crystal lattice, but also with the large and reversible change in the lamellar stacking structure. The stress-induced changes in lamellar thickness and long period were simulated using a mechanical model with these hierarchical structure changes taken into account, giving relatively good reproduction of the observed data

Structure Analysis and Derivation of Deformed Electron Density Distribution of Polydiacetylene Giant Single Crystal by the Combination of X‑ray and Neutron Diffraction Data

The crystal structure of polydiacetylene giant single crystal has been analyzed on the basis of the two different methods of wide-angle neutron diffraction and X-ray diffraction. The X-ray result gives us the total electron density distribution [<b>ρ</b>(<b>x</b>)] of polymer chain. The neutron result tells the positions of atomic nuclei, which can allow us to speculate the electron density distributions [<b>ρ</b>₀(<b>x</b>)] around the nonbonded isolated atoms. As a result, the so-called bonded (or deformed) electron density Δρ­(<b>x</b>) [≡ ρ­(<b>x</b>) – ρ₀(<b>x</b>) = ρ_X(<b>x</b>) – ρ_N(<b>x</b>)], i.e., the electron density distribution due to the conjugation among the covalently bonded atoms along the polymer chain, can be estimated using the two information obtained by the X-ray and neutron data analyses (the so-called X-ray–neutron subtraction (X<i>–</i>N) method). The present report is the first example of the application of X–N method to the synthetic polymer species. The Δρ­(<b>x</b>) derived for polydiacetylene was found similar to that of the low-molecular-weight model compound having the similar electronically conjugated chemical formula. The Δρ­(<b>x</b>) was calculated by the density functional theory, which was in a good agreement with the experimental result qualitatively

Structure Analysis and Derivation of Deformed Electron Density Distribution of Polydiacetylene Giant Single Crystal by the Combination of X‑ray and Neutron Diffraction Data

The crystal structure of polydiacetylene giant single crystal has been analyzed on the basis of the two different methods of wide-angle neutron diffraction and X-ray diffraction. The X-ray result gives us the total electron density distribution [<b>ρ</b>(<b>x</b>)] of polymer chain. The neutron result tells the positions of atomic nuclei, which can allow us to speculate the electron density distributions [<b>ρ</b>₀(<b>x</b>)] around the nonbonded isolated atoms. As a result, the so-called bonded (or deformed) electron density Δρ­(<b>x</b>) [≡ ρ­(<b>x</b>) – ρ₀(<b>x</b>) = ρ_X(<b>x</b>) – ρ_N(<b>x</b>)], i.e., the electron density distribution due to the conjugation among the covalently bonded atoms along the polymer chain, can be estimated using the two information obtained by the X-ray and neutron data analyses (the so-called X-ray–neutron subtraction (X<i>–</i>N) method). The present report is the first example of the application of X–N method to the synthetic polymer species. The Δρ­(<b>x</b>) derived for polydiacetylene was found similar to that of the low-molecular-weight model compound having the similar electronically conjugated chemical formula. The Δρ­(<b>x</b>) was calculated by the density functional theory, which was in a good agreement with the experimental result qualitatively