Examining the Impact of Squaric Acid as a Crosslinking Agent on the Properties of Chitosan-Based Films

Hydrogels based on chitosan are very versatile materials which can be used for tissue engineering as well as in controlled drug delivery systems. One of the methods for obtaining a chitosan-based hydrogel is crosslinking by applying different components. The objective of the present study was to obtain a series of new crosslinked chitosan-based films by means of solvent casting method. Squaric acid—3,4-dihydroxy-3-cyclobutene-1,2-dione—was used as a safe crosslinking agent. The effect of the squaric acid on the structural, mechanical, thermal, and swelling properties of the formed films was determined. It was established that the addition of the squaric acid significantly improved Young’s modulus, tensile strength, and thermal stability of the obtained materials. Moreover, it should be stressed that the samples consisting of chitosan and squaric acid were characterized by a higher swelling than pure chitosan. The detailed characterization proved that squaric acid could be used as a new effective crosslinking agent.     

Nonisothermal crystallization of highly‐filled polyolefin/calcium carbonate composites

Nonisothermal crystallization kinetics of highly‐filled polyolefin composites was studied by means of differential scanning calorimetry (DSC). Two types of commercial calcium carbonate based fillers (modified with stearic acid and nonmodified one) were used for our investigations. In order to evaluate the crystallization kinetics changes of composites, the Avrami theory modified by Jeziorny was used. Validity of mineral fillers modification with stearic acid has been proved by thermal analysis. Because of the suppression of the heterogeneous nucleation effect resulting from calcium carbonate with stearic acid modification, an increase in the processability of highly‐filled polyolefin cast films might occur. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41201.     

KIRCHHOFF'S TYPE FORMULA IN THERMOELASTICITY WITH FINITE WAVE SPEEDS

Kirchhoff's type formula for a solution of a central system of field equations of thermoelasticity with finite wave speeds is established in the paper. The solution of the system (φ, θ), where φ and θ denote the thermoelastic displacement potential and temperature respectively, is expressed by surface integrals over the boundary of a thermoelastic solid whose kernels have the form of infinite series involving Bessel functions. Application of the formula to reduce the solution of an initial-boundary problem to that of a vector Fredholm integral equation of the first kind is outlined.     

A method for the prediction of microwave properties of thick film conductors from physical measurements and d.c. conductivity

It is theoretically possible to calculate electrical losses in conductors at high frequencies using standard microstrip equations. To do this it is necessary to know the length, width and surface roughness of sample tracks. The sheet resistance at d.c. is replaced with a new term, surface resistance which is the bulk resistivity divided by the skin depth at the frequency under consideration. The paper looks at the relationship between these predictions and measured results. A wide range of commercial thick film conductors has had its physical properties measured and a comparison made with the figures for idealized structures. The paper contains a discussion on the differences between real and ideal materials. It was seen that the predictions were close to the measured results at lower frequencies, but that there was divergence as the frequency approached 15–20 GHz. For low-cost applications it is generally considered that economically viable conductors will be unsuitable for microwave applications. However, the paper discusses whether some of the lesser used conductors, such as fritted silver, could have an application in low-cost microwave devices, for example alarm systems. © 2001 Kluwer Academic Publishers     

KIRCHHOFF'S FORMULA FOR THERMOELASTIC SOLID

A generalization of Kirchhoff's theorem for the classical wave equation to the case of a central system of field equations of coupled thermoelasticity is established in this paper. The theorem asserts that a pair (Φ, θ), where Φ and θ denote the thermoelastic displacement potential and temperature, respectively, can be expressed by surface integrals over the boundary of a thermoelastic solid whose kernels have the form of an infinite series satisfying the wave-like and heat-like equations occurring in the decomposition theorem for the central system of equations ([3]).     

Investigations on printed elastic resistors containing carbon nanotubes

This paper presents the results of authors investigations on elaboration of a new thick film composition filled with carbon nanotubes (CNTs). The polymer composition consists of polymer vehicle, which is the solution of organic resin in certain combination of solvents, and functional phase—carbon nanotubes well dispersed in the vehicle. The pastes were applied with screen-printing on several substrates and temperature cured. The properties of obtained layers were characterized. Series of samples were prepared with different amount of CNTs to evaluate electrical properties. Changes in resistance were investigated during periodic mechanical and temperature stresses, realized through cyclical bending and rapid temperature change. Tensometric effect was also investigated. Investigations have proved that polymer composites based on carbon nanotubes exhibit high resilience to stress factors. Resistance change in function of temperature was also investigated to evaluate temperature coefficient of resistance (TCR). All this aspects are important for elastic resistors fabrication in printed electronics microcircuits. Resistance and noise measurements in cryostats have also been involved. 1/f type noise has been observed. Noise intensity, calculated in decade frequency bands, rises significantly with increasing temperature. Activation energies of thermally activated noise sources (TANS) have been revealed using low-frequency noise spectroscopy. Relatively large value of negative TCR has been obtained from resistance versus temperature curve. Calculated dimensionless sensitivity is similar to that observed in cryogenic temperature sensors. However, bulk noise intensity of resistive layer is larger than obtained for lead containing RuO₂ based resistive layers.     

Low cost microwave thick film structures

This paper examines the potential use of low cost materials and processes to produce thick film structures at low microwave frequencies. It examines empirically the upper frequency limits for the use of simple design techniques as a function of substrate thickness. It concludes that satisfactory results can be obtained using low cost methods up to about 12 GHz for 1 mm 96% alumina substrates and 16 GHz for 0.63 mm substrates. These are the most commonly available thick film substrate thicknesses. A low permittivity protective layer is desirable to maintain reliability.