热致感应型形状记忆高分子材料及其研究进展

讨论了热致感就型形状记忆高分子材料的记忆原理和对材料的基本要求厅求,介绍了目前已遥产品品种及制备方法,并进一步评述阳了近的研究进展。     

Thermally Stimulated Polarization and Depolarization in Halogen-Containing Lead Silicate Glasses

Thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements were made on lead halide silicate glasses having compositions (65 − x )PbO· x PbX₂·35SiO₂ where x = 0, 0.1, 2, 10 and X = F, CI, Br, and I. The addition of halogen ions to lead silicate glasses gives rise to a new high-temperature TSDC peak in the vicinity of the peak previously observed in binary lead silicate glasses. The integrated area of the new peak is dependent on the amount and type of halogen ion present in the glass and does not saturate in the temperature range of our measurements. This new peak is attributed to space charge polarization of halogen ions.     

Physical aging effects on high temperature relaxations of poly(DTH carbonate) and poly(DTO carbonate) monitored by TSPC and TSDC

Thermal stimulated polarization and depolarization experiments with blocking electrodes are performed on two tyrosine-derived polycarbonates: poly(DTH carbonate) and poly(DTO carbonate). Aging effects are studied in a high temperature range that involved the contribution of the glass transition relaxation peak and the charge redistribution peak originated by the conduction through the sample and the presence of blocking electrodes. A reliable new models are used to described these high temperature relaxations and the results are presented as a function of aging times, polarization conditions and changing structure. For each sample the glass transition temperature, the parameter related with the shape of the glass transition peak and the contribution between the two modeled peaks, are found to be constant and independent of aging time. The variations in shape and temperature position of the maximum of the charge redistribution peak show logarithmic time dependence with aging time. These results are compared to the ones previously reported on a particular tyrosine-derived polyarylate, and the analysis suggests that structural differences in mobility and flexibility of the polymeric chains originate the observed variations. The examination of the experimental data indicates that charge redistribution and glass transition peaks share the same origin, and that the conduction aging effects are triggered by the onset of the glass transition.     

Thermally stimulated depolarization effect in thiourea-formaldehyde condensate

A thiourea-formaldehyde condensate was synthesized in an acid medium and characterized by elemental analysis, infra-red spectroscopy, and thermal methods of analysis. The thermally stimulated depolarization effect was studied in samples polarized under different conditions. The results indicate two distinct transitions in the temperature ranges 94°–100°C and 122–126°C. The polarization-depolarization phenomena were correlated with the physicochemical changes occurring in the matrix. Depolarization kinetics data such as activation energy and relaxation times of the electrets are reported.     

Thermotropic liquid crystalline main-chain polyamides containing diaza-18-crown-6-ether units

Polymers incorporating conventional mesogens and diazo-18-crown-6-ether were prepared. The crown ether unit seems to be too flexible and only weak liquid crystal character can be observed by optical microscopy except when the mesogen is terphenyl. These liquid crystalline characteristics disappeared when the polymers were complexed with potassium ions.     

Molecular motions in poly(vinyl acetate) revisited. A thermally stimulated current study

The dipolar relaxation mechanisms in poly(vinyl acetate) have been studied in detail using the technique of thermally stimulated currents. The papers published in the literature about this subject are very contradictory, particularly with respect to the assignment of the observed discharges to the corresponding motions at the molecular level. This work aims at clarifying these problems. We detected and characterised three different relaxation mechanisms: (1) a low temperature one (around ?140°C) which was attributed to local internal rotations in the acetate side-groups; (2) a relaxation whose maximum occurs at 42°C, which corresponds to the glass transition relaxation, and shows a compensation behaviour; (3) an upper glass transition relaxation whose maximum appears at 87°C and was attributed to a liquid-liquid transition. These assignments have been made on the basis of the analysis of the behaviour of the samples when submitted to different thermal and electrical treatments.     

Dielectric Properties of Thermotropic Polymer Liquid Crystals

Abstract

Dielectric relaxation spectrometry is a useful tool to study molecular dynamics of macromolecular systems. Theoretical principles as well as experimental techniques are analyzed from the point of view of their capabilities. Results are reported for PET/xPHB, where PET = poly(ethylene terephthalate), PHB = p-bydroxybenzoic acid (the liquid crystalline component) and x is the mole fraction of PHB in the copolymers.     

Nematic Electroclinic Effect in Mixtures

Abstract

Origin of the nematic electroclinic effect (NEE) in monomer liquid crystals (MLCs) and in polymer liquid crystals (PLCs) is discussed. Results for two different compounds, one forming a smectic A and another forming a smectic C phase are reported, along with results for the mixtures of these two compounds. Different explanations of NEE are analyzed on the basis of the experimental results and the most plausible explanation is pointed out.     

Dynamic interfacial properties between a flexible‐chain polymer and a thermotropic liquid crystalline polymer investigated by an ellipsoidal drop retraction method

In this study, the dynamic interfacial properties between an isotropic polymer and a thermotropic liquid crystalline polymer (TLCP) were investigated by measuring the time‐dependent interfacial tension between them. As a TLCP drop retracts in a flexible polymer matrix, the evolution of its shape is recorded by microscopy. By fitting the ellipsoidal model of Maffettone and Minale, the model of Marrucci and Santo, and large deformation ellipsoidal models by Jackson–Tucker and Yu–Bousmina, the interfacial tension could then be determined. It was found that the retraction of a TLCP ellipsoidal drop in a flexible polymer cannot be described by these models as accurately as in Newtonian systems. The apparent interfacial tension obtained from these models evolves with time; the evolution is ascribed to the slow relaxation of domain orientation within the TLCP drop. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94:1404–1410, 2004     

Electrooptical and Photooptical Properties of Polymer Liquid Crystals

Abstract

Effects of electric fields on monomer liquid crystals (MLCs) and polymer liquid crystals (PLCs) are analyzed, dealing separately with nematic and cholesteric PLCs. Electro-hydrodynamic instabilities affect the behavior of these systems. Thermooptical recording depends on the magnitude of the thermooptical effects. Information recording can be alternatively performed on the basis of photoorientation and the photoinduced birefringence. Capabilities and limitations of these techniques are pointed out.     

Effects of Magnetic Fields on Polymer Liquid Crystals

Abstract

The effects of magnetic fields on polymer liquid crystals (PLCs) are analyzed, dealing in turn with isotropic solutions, lyotropic LCs (including polypeptides, polyribonucleotides and DNA) and thermotropic PLCs (including polyesters and polypeptides). Looking for a common denominator in LC behavior, living systems are also analyzed, including viruses, biological membranes, sperm nuclei, rhodopsin and fibrinogen. Magnetropism is discussed, as are applications of PLC oriented in magnetic fields for the purpose of producing ultra-high modulus materials.     

Single Molecule Studies on Dynamics in Liquid Crystals

Single molecule (SM) methods are able to resolve structure related dynamics of guest molecules in liquid crystals (LC). Highly diluted small dye molecules on the one hand explore structure formation and LC dynamics, on the other hand they report about a distortion caused by the guest molecules. The anisotropic structure of LC materials is used to retrieve specific conformation related properties of larger guest molecules like conjugated polymers. This in particular sheds light on organization mechanisms within biological cells, where large molecules are found in nematic LC surroundings. This review gives a short overview related to the application of highly sensitive SM detection schemes in LC.     

Optical Storage

Abstract

Digital and holographic optical storage systems are compared. Requirements for the improvement of current storage systems are defined. Techniques to study storage properties of polymer liquid crystals (PLCs) are described; they include diffraction grating, polarization holography, spectroscopic methods, refractive index determination (interferometry, birefringence), methods of surface characterization, and testing reversibility and the number of write-read cycles. Results are reported for several PLCs and interpreted in terms of the effects of the choice of the main chain, the spacer, the chromophore, the LC components, the molecular weight, photoorientation and the surface morphology.     

A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites

Carbon materials particularly in the form of sparkling diamonds have held mankind spellbound for centuries, and in its other forms, like coal and coke continue to serve mankind as a fuel material, like carbon black, carbon fibers, carbon nanofibers and carbon nanotubes meet requirements of reinforcing filler in several applications. All these various forms of carbon are possible because of the element's unique hybridization ability. Graphene (a single two-dimensional layer of carbon atoms bonded together in the hexagonal graphite lattice), the basic building block of graphite, is at the epicenter of present-day materials research because of its high values of Young's modulus, fracture strength, thermal conductivity, specific surface area and fascinating transport phenomena leading to its use in multifarious applications like energy storage materials, liquid crystal devices, mechanical resonators and polymer composites. In this review, we focus on graphite and describe its various modifications for use as modified fillers in polymer matrices for creating polymer-carbon nanocomposites.