Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Ferroelectric properties of vinylidene fluoride copolymers

Crystallization behavior of nylon 6 nanocomposites

Effects of the crystallization temperature on the crystal structure and its melting behavior of poly (l-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the α form when the crystallization temperature Tc was higher than 120 °C, while significant change in lattice parameters was seen for Tc's below 120 °C. The ratio of the a- and b-axis lengths begins to decrease with Tc below 120 °C and is 31/2 below 90 °C, which suggests a new crystalline form with hexagonal packing, namely, the α‘ form. The possible reason for α‘ formation is discussed. High-temperature WAXS and SAXS measurements showed that α‘ crystal transforms into ordered a form during heating. The transition takes place at 150 °C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.

Crystallization and Melting Behavior of Poly (l-lactic Acid)

Non-isothermal crystallization of polymers

The state-of-the-art polymer dielectrics have been limited to nonpolar polymers with relatively low energy density but ultralow dielectric losses for the past decades. With the fast development of power electronics in pulsed power and power conditioning applications, there is a need for next-generation dielectric capacitors in areas of high energy density/low loss and/or high temperature/low loss polymer dielectrics. Given limitations in further enhancing atomic and electronic polarizations for polymers, this Perspective focuses on a fundamental question: Can orientational polarization in polar polymers be utilized for high energy density and low loss dielectrics? Existing experimental and theoretical results have suggested the following perspectives. For amorphous polar polymers, high energy density and low loss can be achieved below their glass transition temperatures. For liquid crystalline side-chain polymers, dipole mobility is so high that they saturate at relatively low electric fields, and only li...

Novel Ferroelectric Polymers for High Energy Density and Low Loss Dielectrics

Glassy isotactic propylene (PP) films of thickness up to 0.3 mm were obtained by an ultraquenching technique. The structure and properties of the as-quenched and subsequently crystallized samples were characterized by various techniques. Electron microscopy indicates the glass has no structure larger than 25 A. X-ray diffraction shows PP crystallizes from the glass into a smectic structure at ca. −20°C and then transforms to monoclinic microcrystals at ca. 40°C; a nodular structure (80 to 100 A in diameter) was observed on the surface. The transformation temperature increases with the film thickness. Annealing above the α-relaxation temperature results in an increase in the nodule size. A correspondence was found between the diameter of the nodules observed on the surface and long spacings obtained by small-angle X-ray scattering from the bulk. Dynamic mechanical spectra show the presence of two relaxation-like peaks at ca. −10°C and 10°C for the as-ultraquenched samples. X-ray scattering, differential scanning calorimetry (DSC), and torsion pendulum measurements show PP crystallizes from the glass at a temperature, depending on the rate of heating, that corresponds to the lower relaxation peak temperature.

Structure and properties of polypropylene crystallized from the glassy state

The structure, phase diagram and compression behavior of polyethylene are studied by X-ray diffraction up to 11 kbar and 300°C. Distinct diffuse scattering on the non-equatorial line is first observed for the hexagonal phase of oriented polyethylene, revealing conformational disorder of the molecule. The phase diagram determined shows that the hexagonal phase exists in a temperature range which increases with increasing pressure up to 11 kbar. It is found that the compressibility of the hexagonal phase takes higher values on the lower pressure side, and approaches the value for the orthorhombic phase with increasing pressure. A simple relation between the lattice parameters of the orthorhombic phase, ao/bo=1.57-1.58, is found to hold at the transition temperature to the hexagonal phase irrespective of pressure.

https://iopscience.iop.org/article/10.1143/JJAP.16.1891/pdf

Structure and Properties of High Pressure Phase of Polyethylene

The size distribution of nodules on the surface of polypropylene films crystallized from the glass and subsequently annealed was studied. The origin of the nodule was the crystallite produced by the growth of many nuclei which were generated on crystallization from the glass and survived on annealing. The surviving nuclei grow until they collide with the neighbors. In this case, the size distribution of nodules is represented by that of Voronoi polygons. The agreement between observed and calculated ones is good.

Nodular Structure of Polypropylene

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δe is analyzed on the basis of the two-site model. A linear relation between Δe/χξ and cos2θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

X‐ray integrated intensities of meridional Bragg reflections due to paraffin molecules in urea adducts are measured from 173 to 351 K. The B factor in the Debye–Waller factor, exp[−2B(T)(sin θ/λ)2], becomes extraordinarily large with increasing temperature (11.4 nm2 at 351 K for the second‐order meridional reflection) and depends on the reflection index l for the meridional reflections; this l dependence cannot be explained by the harmonic approximation. The experimental results are analyzed with a model where a paraffin molecule in urea adducts vibrates in an anharmonic potential field with cubic and quartic terms in addition to a harmonic one. The origin of the anharmonic potential field is discussed.

Kenjiro Asai

Papers

Structure and properties of polypropylene crystallized from the glassy state

Structure and Properties of High Pressure Phase of Polyethylene

Nodular Structure of Polypropylene

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

Anharmonic vibration of n-paraffin molecules in urea adducts