Finite concentration fibers

Since IGC is able to generate adsorption isotherms and to evaluate acid/base interactions for specified adsorbate-adsorbent pairs, it follows that the technique is able to develop a detailed picture of surface properties for non-volatile stationary phases. This is illustrated, again for carbon fibers, by Vukov and Gray (48). They combine IGC information at essentially zero coverage of the injected probes with finite concentration data to obtain heat of adsorption values ranging from zero to multi-layer coverage. Their meticulous study shows the effects of thermal pretreatment on fiber surface characteristics, and underscores the convenience and power of IGC to generate information otherwise far more difficult to obtain. 

Zero coverage. In order to eliminate physically adsorbed species, fibers were cleaned by heating at 160°C In a N2 (Linde, ultra high purity, with C02 content less than 1 ppm) atmosphere until constant retention volumes were obtained (100 to 120 h). Using finite concentration IGC and n-alkanes as sorbates, the surface area of these fibers was determined to be 0.40 m g"1 and 0.59 m g 1 for T-300 and P-55, respectively. The n-alkanes octane to trldecane (analytical grade) were obtained from Polyscience Corporation (Quantklt). Retention data were measured with a Hewlett-... 

The carbon fiber surface areas were previously determined by BET krypton adsorption to be 0.62 0.01 m g-1 and 0.74 0.01 n g-1 for T-300 and P-55, respectively. The molecular area of krypton was taken as 0.195 nm2. Prior to these measurements, the fibers were degassed at 300°C for 15 h. The elution of a characteristic point method of finite concentration IGC was used to determine the Isotherms for a series of n-alkanes. Approximately 15 to 20 Injections were used for each Isotherm. The hand-drawn curve through the peak maxima was digitized for Integration and subsequent data handling. 

To act as UV stabilizers, UV absorbers must be present at a finite concentration in order to fully absorb harmful UV light. At practically used concentrations, this means that a finite absorbing thickness is required. In the case of thin films [poly(ethylene) for agricultural use] or fibers (polypropylene fibers) this thickness criterion is not met and UV absorbers offer inadequate UV light protection. 

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

In the preparation of these concentrated PANI-AMPSA/DCAA solutions according to the procedures reported by Monkman and coworkers [59-61], it was noticed that the high-molecular-weight EB powders, which had been dried in the vacuum oven (water content <0.5 wt%), had difficulty wetting upon addition to the DCAA solvent. This problem could be overcome by the addition of a trace amount of water to the EB powder before it was dissolved in the DCAA solvent. However, there was a finite water limit. Above 30 wt% water in EB powder, the nascent fiber flattened and fell apart in the coagulation bath. Furthermore, it was found that fiber with good mechanical properties could only be spun if the water content in the EB powders was between 2 and 10 wt%. Thus, with appropriate mixing conditions, PANI-AMPSA/DCAA dope solutions with a total solids content between 4.5 and 14 wt% may be prepared... 

Several methods have been proposed for predicting the stress state at the interface, which can then be used to estimate the bond strength. The shear lag method has received extensive treatment by several investigators. This method determines the interface shear stress concentration at the end of the fiber as well as shear stress variation along the fiber. Additional methods include the Lame solution for a shrink fit, classical elasticity boundary value problems, and finite-element analysis. 

The remarkable properties of electrospun CNTs nanocomposites continue to draw attention in the development of multifunctional properties of nanostructures for many applications.. Multiscale model for calculation macroscopic mechanical properties for fibrous sheet is developed. Effective properties of the fiber at microscale determined by homogenization using modified shear-lag model, while on the second stage the point-bonded stochastic fibrous network at macroscale replaced by multilevel finite beam element net. Elastic modulus and Poisson s ratio dependence on CNT volume concentration are calculated. Effective properties fibrous sheet as random stochastic network determined numerically. We conclude that an addition of CNTs into the polymer solution results in significant improvement of rheological and structural properties. 

Fibers concentration is finite but not so large to have direct contact between them. 

---