Yuri A. Chekanov

Preparation of Functionally Gradient Materials via Frontal Polymerization

Ascending frontal polymerization in a body with a moving boundary was accomplished experimentally. This process was shown to be a steady-state process within a certain range of the parameters. Temperature profiles of the front were recorded. This new method gives an excellent opportunity to prepare functionally gradient materials because composition of a monomer feedstream can be varied in a programmable manner. Polymer samples with hyperbolic gradients of optical dye concentration were manufactured at ambient pressure and temperature.

Frontal Curing of Epoxy Resins: Comparison of Mechanical and Thermal Properties to Batch-Cured Materials

The epoxy resin diglycidyl ether of bisphenol F (DGEBF) was cured by the aliphatic amine curing agent Epicure 3371 in a stoichiometric ratio both frontally and in a batch-cure schedule. Glass transition temperatures (Tg) were determined using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DMA also was used for studying the storage modulus (E′) and tan delta (tan δ) of the cured samples. Tensile properties of epoxy samples were tested according to ASTM D638M-93. The properties of the frontally cured epoxy resin were found to be very close to that of batch-cured epoxy resin. Velocity of cure-front propagation was measured for both neat and filled epoxy. Rubber particles (ground tires) were used as a filler. The maximum percentage of filler in the epoxy resin allowing propagation was 30%. Because of convection, only descending fronts would propagate. Advantages and disadvantages of frontal curing of epoxy resins are discussed.

Gas‐free initiators for high‐temperature free‐radical polymerization

Quaternary ammonium persulfates as free-radical initiators for high-tem- perature polymerization are synthesized and their shelf-life stability investigated. These initiators do not have gaseous byproducts and are therefore ideal for frontal polymerization, a process in which polymeric materials are produced via a thermal front that propagates through the unreacted monomer/initiator solution. Quaternary ammonium persulfate initiators offer additional qualities such as high solubility in organic media and low volatility, which are desirable for frontal polymerization. The initiators are synthesized using two procedures, and the initiating efficacy of the respective products is compared to a peroxide initiator in the frontal polymerization of 1,6-hexanediol diacrylate. Of all the quartenary ammonium persulfates synthesized, tricaprylmethylammonium (Aliquat) persulfate (APSO) is the best initiator for frontal polymerization because it is soluble in organic media, is very reactive, and does not produce volatile byproducts under decomposition. A study of the decomposition kinetics of APSO is performed, and frontal polymerization is proposed as a quicker analytical technique to assay the purity.

Effective interfacial tension induced convection (EITIC) in miscible fluids

Using spinning drop tensiometry, we confirmed that an effective interfacial tension (EIT) exists between miscible fluids, which is necessarily transient. We studied glycerin and water and found that the value depends on how the drop of water is introduced into the glycerin. We developed a procedure in which the less dense fluid was introduced into the more dense fluid and then the capillary was rapidly spun up to its working value. We also studied dodecyl acrylate in poly(dodecyl acrylate) at high temperature. Using the concept of the Korteweg stress induced by concentration and temperature gradients, we show with numerical simulations that convection should occur in miscible systems with sharp but nonuniform concentration and/or temperature gradients, analogous to surface-tension induced convection in immiscible fluid layers. The parameter for the stress was estimated by comparing the Cahn-Hilliard formula for the interfacial tension to experimental data from the spinning drop tensiometer. Steady-state simulations with a variation in the width of the concentration gradient were compared to simulations for a standard immiscible interface in which the variation of interfacial tension was equivalent to the variation in EIT. The maximum velocity was within 20%. Because concentration and temperature gradients also cause buoyancy-induced convection, experiments must be performed in weightlessness. We attempted to observe convection in low g caused by a temperature gradient imposed along the interface between glycerin and water but because of the short time of low g available in our free-floating experiment, our results were inconclusive. Therefore, long term, high quality weightlessness is required to test the predictions of the modeling. Introduction We consider the problem of two miscible fluid in contact. If we impose temperature and/or concentration gradients parallel the transition zone, can convection occur in the absence of buoyancy? We propose that it can in a process analogous to surface-tension-induced convection. We review the thermodynamic background, evidence for an interfacial tension between miscible fluids and show with our simulations that such flows could occur but would require microgravity to observe.