11 research outputs found
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<sub>d</sub>), 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<sub>d</sub> 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<sub>d</sub> 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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>), nonprotonated hydrogen (H)-bonded water (H<sub>2</sub>O···H<sub>2</sub>O), 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><sub><i>i</i> (<i>i</i>=each deconvoluted band)</sub> 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 CH<sub>2</sub> 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><sub>0</sub>(<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>) – ρ<sub>0</sub>(<b>x</b>) = ρ<sub>X</sub>(<b>x</b>) – ρ<sub>N</sub>(<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><sub>0</sub>(<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>) – ρ<sub>0</sub>(<b>x</b>) = ρ<sub>X</sub>(<b>x</b>) – ρ<sub>N</sub>(<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