Piotr Wojciechowski

Dielectric Properties and Characterisation of Titanium Dioxide Obtained by Different Chemistry Methods

This work was financially supported by the National Science Center (Poland) grant awarded by Decision no. DEC-2011/03/D/ST5/06074. The authors are grateful to Professor Adam Tracz from the Polish Academy of Science in Lodz for his help in performing SEM investigations.

Thermally stable optically anisotropic polymer networks obtained from mesogenic LC cellulose derivatives

The birefringence of anisotropic polymer composites obtained by photopolymerisation of lyotropic liquid crystalline systems composed of (hydroxypropyl)cellulose derivatives (R-PC) and photopolymerisable organic acids that are also lyotropic solvents is studied. The orientation of the systems before photopolymerisation was induced by shearing. The mechanically oriented lyotropic solutions after cessation of shearing form a band texture visible under crossed polarisers. This band texture shows relaxation phenomena and the grain texture observed before shearing recovers with time. The decay time of the relaxation of the band texture at room temperature is much longer than the time of band texture formation after cessation of shearing. The photopolymerisation of acrylic acid (or methacrylic acid) in the oriented R-PC/acid lyotropic solutions leads to birefringent polymer composites. These composites show reversible anisotropic optical properties as examined by thermo-optical analysis (In contrast with the samples before photopolymerisation); therefore they can be classified as reversible anisotropic polymer networks. It is demonstrated that the thermal stability of the birefringence of the R-PC/poly(acid) composites is mainly determined by the molecular mobility of the poly(acid) connected with its glass transition temperature.

α-Relaxation processes in the composites of LC-cellulose derivatives

Molecular relaxation processes in the liquid crystalline cellulose derivatives (CDs) and in their composites were studied by dielectric spectroscopy. The composites, which are anisotropic polymer networks, were prepared by photopolymerization of acrylic acid in the oriented lyotropic solutions of cellulose derivatives. The investigated composites comprise poly(acrylic acid) and hydroxy-, propionyloxy-, hexanoyloxy-, or cyanoethyl-propyl cellulose. In these systems different types of intermolecular interactions are possible: hydrogen bonds, dispersive, or dipolar interactions. The temperature dependences of the α-relaxations of CDs can be fitted by the WLF equation, while the β-relaxation of poly(acrylic acid) is described by an Arrhenius function. The molecular relaxations in the composites are affected in different way by different intermolecular interactions, depending on the chemical structure of CDs.

Mesomorphism and photocuring processes of (2-hydroxypropyl)cellulose cinnamate

Mesomorphism and photocuring processes of (2- hydroxypropyl)cellulose cinnamate (CYPC) were investigated. The CYPC was synthesized in the course of estrification of (2-hydroxypropyl)cellulose (HPC) by cinnamoyl chloride. The chemical structure of CYPC was confirmed by IR and Raman spectroscopy. The substitution degree of HPC by cinnamoyl ester groups was calculated from 1H-NMR studies. The thermotropic mesomorphism of CYPC was confirmed by DSC, thermo-optical and X-ray investigations. The thermo-optical behaviors of CYPC tablet obtained under pressure and results of the dielectric spectroscopy investigations were interpreted in terms of the strong hydrophobic effect of the cinnamoyl ester groups. The cycloaddition of the cinnamoyl groups is initiated either by UV radiation or temperature, inside of the hydrophobic aggregates of the cinnamoyl ester groups. The chemical modification of the hydrophobic aggregates in the course of thermo- or photoaddition of the cinnamoyl ester groups slightly changes a molecular movement of the poly(saccharide) main chains of CYPC. However, these crossed aggregates can recover macromolecular orientation of CYPC from the isotropic state via the cooling of sample. These suggest that the hydrophobic aggregates after thermal or photochemical modification shows an anisotropic behavior.

Anisotropic mesogenic liquid crystalline cellulose derivatives as a basis for functional materials

Liquid crystalline cellulose derivatives are promising materials which can be used for the formation of novel functional systems. Microcrystallites were obtained from cellulose fibers which were complexed with Fe(III) according to known methods. Dispersions of these crystallites were mixed with liquid crystalline (hydroxypropyl)cellulose and fixed in an oriented state by photopolymerization of polymerizable lyotropic solvent. The solid anisotropic polymeric networks exhibit high stability of orientation. These systems containing an organized iron complex of liquid crystalline cellulose may have new functional properties expected for dispersed metals or metal oxides in polymeric matrices.

Liquid crystalline cellulose derivatives for thermally stable or reversible anisotropic polymer film

New aspects of cellulosic material science and application are related to the lyotropic liquid crystalline properties of this classical natural polymer. In this work the preparation of anisotropic polymer networks based on (hydroxypropyl) cellulose and its derivatives as well as their properties are discussed. Cellulose derivatives form liquid crystalline solutions also in polymerizable solvents, e.g. acrylic acid thus anisotropic polymeric networks can be fixed by photopolymerization. These films have been investigated with regard to their orientation birefringence and thermal stability. Some possible applications of these interesting systems are briefly shown.

Formation of anisotropic polymer blend by photopolymerization of lyotropic LC-phase

The structural characteristics are given for the polymer blend (PB) prepared by photopolymerization of the uniaxially oriented liquid crystal (LC)-phase of the hydroxypropylcellulose-acrylic acid (HPC-AA) system. The uniaxially oriented films of HPC- AA mesophase show a so-called banded structure under cross polarizers, characteristic for uniaxially oriented LC-polymer films. The state of order of HPC-AA mesophase was investigated by effect of dye orientation in LC-media. The influence of the length of the cholesteric pitch of HPC-AA on the order parameter of dye was found. The polymer blend as a product of the photopolymerization of AA in the lyotropic and uniaxially oriented medium HPC-AA is a birefringent solid system, was strongly affected by crosslinking processes. The PB has a LC-organization up to thermal decomposition at 230 degrees C, as shown by TOA and microscopic observations. The PB can be regarded as a new kind of the thermally resistant LC-polymer network.

Synthesis and Characterization of Low Loss Dielectric Ceramics Prepared from Composite of Titanate Nanosheets with Barium Ions

We report a strategy for preparing barium titanate precursor, being the composite of titanate nanosheets (TN) with barium ions (Ba-TN), which subjected to step sintering allows obtaining TiO2 rich barium titanate ceramics of stoichiometry BaTi4O9 or Ba2Ti9O20. These compounds are important in modern electronics due to their required dielectric properties and grains’ size that can be preserved in nanometric range. The morphology studies, structural characterization, and dielectric investigations were performed simultaneously in each step of Ba-TN calcinations in order to properly characterize type of obtained ceramic, its grains’ morphology, and dielectric properties. The Ba-TN precursor can be sintered at given temperatures, so that its dielectric permittivity can be tuned between 25 and 42 with controlled temperature coefficients that change from negative 32 ppm/°C for Ba-TN sintered at 900°C up to positive 37 ppm/°C after calcination at 1300°C. XRD analysis and Raman investigations performed for the Ba-TN in the temperature range of °C showed that below 1100°C we obtained as a main phase BaTi4O9, whereas the higher calcinations temperature transformed Ba-TN into Ba2Ti9O20. Taking into account trend of device miniaturization and nanoscopic size requirements, temperatures of 900°C and 1100°C seem to be an optimal condition for Ba-TN precursor calcinations that guarantee the satisfactory value of dielectric permittivity ( and 32) and ceramic grains with a mean size of ~180 nm and ~550 nm, respectively.

The water structure in the polymer network with LC-organisation

The water structure existing in (2-hydroxypropyl)cellulose-poly(acrylic acid)-water (HPC-PAA-H2O) hydrogel, exhibiting liquid crystalline (LC) organization of the polymer network (anisotropic hydrogel), was assessed by means of Raman spectroscopy. It was shown that the LC-network of HPC-PAA-H20 exerts significant influence on the structure of liquid water present in the hydration shells around polymer chains of the anisotropic hydrogel. Due to poor stability of LC-ordering of polymer network, observed in HPC-PAA-H2O) after its long-term water immersion, the process of crosslinking of the anisotropic hydrogel was considered. The results of Raman measurements revealed that, during immersion of HPC-PAA-H2O in calcium ions (Ca2+) solution, formation of a salt by poly(acrylic acid) of HPC-PAA-H2O and Ca2+ takes place. This results in crosslinking of the anisotropic hydrogel, leading -- in a consequence -- to significant improvement of its mechanical toughness, as well as to stability of LC-ordering of the investigated network in the swollen state. However, during crosslinking of the anisotropic hydrogel (by Ca2+), apart from formation of the salt by Ca2+ and PAA, the ions influence additionally the structure of the water existing in hydration shells round polymer chains of both polymer constituents (HPC and PAA) creating the anisotropic hydrogel network.

Polymer-stabilized liquid crystals system based on liquid crystalline cellulose derivatives

The microstructure of the polymer composite based on the LC- cellulose derivative and poly(acrylonitrile) was investigated. The polymer composite was obtained by photopolymerization of the mechanically oriented (shearing) lyotropic LC-solution of the (2-hydroxypropyl)cellulose (HPC) in the acrylonitrile (AN). The use of multiple techniques: dielectric, thermogravimetric, mass spectroscopies, and thermooptical analysis enabled to overcome resolution problem related to behavior of the non- mesogenic polymer network of poly(acrylonitrile) in the mesogenic environment of the HPC. The results of the investigation of the composite microstructure show that the polymer composite is composed of two subsystems: mesogenic system of cellulose derivative and oriented fibril-like polymer network which is formed by non-mesogenic poly(AN). The polymer network of poly(AN) influences the optical anisotropy relaxation of the mesogenic HPC in the composite.