Biodegradable polymers and their layered silicate nanocomposites: In greening the 21st century materials world

Domain aspects of multi-ferroics are reviewed, i.e. of materials, in which two or all three of the properties ‘ferroelectricity.’ ‘ferromagnetism’ and ‘ferroelasticity’ occur simultaneously in the same phase, and in which the magnetic point group has been reliably established by magnetoelectric, optical, dielectric, magnetic and related studies on single crystals and single domains. Nearly only members of the boracite crystal family are concerned, whereas for the perovskite family and other classes of material there is great paucity of data on single crystals. Polarized light microscopy is shown to be an indispensable tool for the study of multi-ferroics, in particular when ferroelasticity is involved. Potential multi-ferroic magnetoelectrics, so far only known in ceramic form, are excluded from the survey.

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Multi-ferroic magnetoelectrics

Poly(lactic acid) crystallization

REVIEWS OF TOPICAL PROBLEMS: Ferroelectromagnets

Recently, we reported the isothermal crystallization behaviors of poly(l-lactic acid) (PLLA) from the melt and glassy states, respectively [J. Phys. Chem. B 2004, 108, 11514; Macromolecules 2004, 37, 6433]. Surprisingly, the quite different infrared (IR) spectral evolutions occur in the two crystallization processes at different temperatures in which the same crystal modification is expected to be formed. To clarify this unusual phenomenon, the crystal modifications and thermal behavior of PLLA samples prepared under different crystallization temperatures are investigated in detail by TEM, WAXD, and FTIR techniques. On the basis of the WAXD and IR data, a new crystal modification named the α form is proposed for the crystal structure of PLLA samples annealed at temperature below 120 °C. Such crystal modification with loose 103 helical chain packing is less thermally stable than the standard α form of PLLA. This assignment can explain all the experiment observations well. Other possible mechanisms for the ...

Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy

Fibrous and crystal structures of a helical polymer, poly‐L‐lactic acid (PLLA), were analyzed by using x‐ray diffraction experiments. It was confirmed that the molecular residues were arranged on a nonintegral 10/3 helix as De Santis and Kovacs [Biopolymers 6, 299 (1968)] reported. The atomic positions in a monomeric unit, which were proposed by Hoogsteen, Postema, Pennings, ten Brinke, and Zugenmaier [Macromolecules 23, 634 (1990)], were validated. However, the previous reports on the positions of the two helical chains were found to be in error. The correct positions were determined. The second helical chain shifts from the base center by 0.45, 0.25, and 0.61 A along a, b, and c axes. Besides, the second chain rotates by 2.46° with respect to the first. Distribution function of the crystallites in various drawn fibers were determined as a function of spiral angle. Optical gyrations of PLLA and poly‐D‐lactic acid fibers were successfully measured by using high accuracy universal polarimeter, as functions of temperature and drawing ratio. By using x‐ray data of the change of the fibrous structure by drawing treatments, the gyration tensor components of PLLA could be calculated. It is of great interest that gyration tensor component g33 along the helical axis is extremely large, ∼(3.85±0.69)×10−2, which corresponds to a rotatory power of (9.2±1.7)×103°/mm, about two orders of magnitude larger than those of ordinary crystals. This is the first experimental evidence that helical polymers will produce enormous optical activity in the solid state. Helical polymers will be important for the elucidation of gyro‐optical properties of solids and promising for new optical applications utilizing their large optical activity.

Structural and optical properties of poly lactic acids

Structural and dielectric studies have been performed on CsH2PO4 for scrutinizing the validity of the previously reported structure. It is found that the symmetry in the paraelectric state is not orthorhombic as previously assumed but monoclinic with the space group of C. The net configuration of hydrogen bond and the sequence of Cs and P are found to differ substantially from those of KH2PO4. Structural evidences combined with dielectric measurements indicate that the ferroelectricity of CsH2PO4 takes place through different atomic structure from that of KH2PO4 type ferroelectrics.



L'etude structurale et dielectrique de CsH2PO4 a ete effectuee en vue de scruter la structure anterieurement rapportee. Nous avons trouve que la symetrie a la phase paraelectrique n'est pas orthorhombique comme prealablement connue mais monoclinique au groupe spatial C. La conformation de la liaison hydrogenee et la sequence de Cs et P different solidement de celles de KH2PO4. Ces faits structural combines avec les mesures dielectrique indiquent que la ferroelectricite de CsH2PO4 a lieu sous la structure atomique differente de celle des ferroelectriques au type de KH2PO4.

Crystal structure and ferroelectricity of cesium dihydrogen phosphate CsH2PO4

A phenomenological theory has been developed for explaining the anomalous dielectric and electro-mechanical properties of some improper ferroelectrics. An ordering parameter 8, which plays the role of a trigger for the onset of the spontaneous polarization P is introduced into the thermodynamical potential function. The peculiar dielectric properties of (NH4)2SO4- and Gd2(MoO4)3-group crystals and the anomalous behaviour of the lattice strains of Fe-I-boracite can be well explained on the basis of this theory. A graphical investigation in a proper θ–P plane shows how the strength of the coupling between 8 and P characterizes the transition mechanisms of the improper ferroelectrics. As a result, it has become clear that the common origin of the peculiar physical properties of both crystal groups lies in the extreme weakness of the couplings. Experimental methods to disclose improper ferroelectric mechanisms are discussed.



Eine phanomenologische Theorie wurde entwickelt, die die anomalen dielektrischen und elektromechanischen Eigenschaften einiger nichtidealer Ferroelektrika erklart. Ein Ordnungsparameter θ, der die Rolle eines Triggers fur den Einsatz der spontanen Polarisation P spielt, wird in die thermodynamische Potentialfunktion eingefuhrt. Die eigen-artigen dielektrischen Eigenschaften von (NH4)2SO4- und Gd2(MoO4)3-Kristallen und das anomale Verhalten der Gitterdeformation der Fe–I-Borazite lassen sich auf der Grundlage dieser Theorie gut erklaren. Eine graphische Untersuchung in einer geeigneten θ–P-Ebene zeigt, wie die Kopplungsstarke zwischen θ und P die Ubergangsmechanismen nichtidealer Ferroelektrika charakterisiert. Als Ergebnis wird klar, das der gemeinsame Ursprung der besonderen physikalischen Eigenschaften der beiden Kristallgruppen in der extrem geringen Kopplung liegt. Es werden experimentelle Methoden zur Aufdeckung der Mechanismen nichtidealer Ferroelektrika diskutiert.

A phenomenological theory of dielectric and mechanical properties of improper ferroelectric crystals

Measurements of the elastic compliance and electrostrictive constant of a (NH4)2SO4 crystal, together with the dielectric constant and spontaneous polarization, have been made. It is observed that there are anomalies in the elastic compliances and that the electrostrictive constants are of the ordinary order of magnitude. A phenomenological theory has been developed to analyse the experimental results, where an order parameter is introduced and its couplings with polarization and stress are taken into account. The anomalous temperature dependence observed in the dielectric constant, spontaneous polarization, spontaneous strain, and elastic compliance is explained in terms of the temperature dependence of the order parameter. The only exception is the shear component for which a further investigation might be necessary.



Messungen des Elastizitatskoeffizienten und der Elektrostriktionskonstante des Ammoniumsulfates, (NH4)2SO4, zusammen mit der Dielektrizitatskonstante und der spontanen Polarisation, wurden durchgefuhrt. Es wurden Anomalien in den Elastizitatskoeffizienten und ein Wert der Elektrostriktionskonstante von normaler Grosenordnung gefunden. Eine phanomenologische Theorie wurde entwickelt, um das experimentelle Ergebnis zu analysieren, wobei ein Ordnungsparameter eingefuhrt wird und seine Kopplungen mit der Polarisation und der elastischen Spannung berucksichtigt werden. Die beobachtete anomale Temperaturanderung der Dielektrizitatskonstante, der spontanen Polarisation, der spontanen Deformation und des Elastizitatskoeffizienten last sich gut mit der Temperaturabhangigkeit des Ordnungsparameters erklaren. Nur die Scherungskomponenten bilden eine Ausnahme und machen eine weitere Untersuchung notwendig.

Jinzo Kobayashi

Papers

Structural and optical properties of poly lactic acids

Crystal structure and ferroelectricity of cesium dihydrogen phosphate CsH2PO4

A phenomenological theory of dielectric and mechanical properties of improper ferroelectric crystals

Elastic anomaly in (NH4)2SO4

Origin of the Visibility of the Antiparallel 180° Domains in Barium Titanate