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SYNOPSISIn this paper, we present some new results of our work in a novel polymerization process (called the free-radical retrograde precipitation polymerization, or FRRPP, process) that occurs at temperatures above the lower critical solution temperature. Our polymerization experiments basically involve the methacrylic acid-poly(methacry1ic acid)-water system. Experimental results indicate a gradual increase in conversion with time after what seemingly is the onset of phase separation. In an equivalent solution polymerization system, conversion of methacrylic acid reaches almost 100% at a much shorter time than in the FRRPP system. Molecular weights of poly(methacry1ic acid) at different times for the FRRPP system are not dramatically different from those obtained in the solution system. However, the FRRPP system yields a relatively narrow molecular weight distribution at a wide range of conversion compared to that obtained in the equivalent solution system. The unique characteristics of the FRRPP process is shown in the asymptotic time behavior of the free-radical concentration compared to the decay behavior in other polymerization systems. 0 1996 John Wiley & Sons, Inc. I NTRO DUCT1 0 NWithin the past several years, we have been studying the physico-chemical and product material aspects of a free-radical precipitation polymerization reaction systems wherein phase separation occurs above the lower critical solution temperature (LCST).l In contrast with conventional precipitation polymerization (CPP) p r o c e s~~-~ that involves phase separation below the upper critical solution temperature (UCST). We call this new process a free-radical retrograde precipitation polymerization (FRRPP) process (see Fig. 1). The added complication of retrograde precipitation could be offset by tighter control of reactor operating conditions and polymer molecular properties. Also, the relatively low operating pressures typically needed are not of great economic disadvantage inasmuch as commercial implemen-

The strength of diamond braided composites is examined and the experimentally observed trends from the braid architectures are simulated using the micromechanics models. The effect of continuity of yarns at the edge of specimen is also studied. Standard tensile tests performed on both types of specimens showed a significant difference in failure strength and failure strain along with the failure mode between the specimens with continuous and discontinuous yarn reinforcement. A micromechanical model is developed to predict the failure strength of the above mentioned composites under uniaxial tensile loading. The model predicts with fair degree of accuracy the stress–strain response for the specimens with yarn discontinuity at the edges.

Yttria-tetragonal zirconia polycrystal (Y-TZP) is a difficult substrate to bond to due to the absence of a glass phase and the material's chemical inertness. This study evaluated the effect of two monomers for metal, MDP (10-methacryloyloxydecyl dihydrogen phosphate) and VBATDT (6-(4-vinylbenzyl-n-propyl)amino-1,3,5-trizaine-2,4-dithiol) on bond strength to Y-TZP. Seven combinations with different concentrations of MDP and VBATDT-monomers (0.0, 0.1, 0.5, or 1.0 wt %) in acetone solution were developed and applied to the surface of Y-TZP slabs, which were bonded to composite resin substrates using a resin cement under standard loading. Non-primed samples were used as controls. Bonded specimens were cut for microtensile testing and tested after either 48 h or 180 days in water storage at room temperature. All samples from control group (no primer) and MV5 group (0% MDP/0.5% VBATDT) debonded spontaneously. Two-way ANOVA showed that the primer had a significant effect (p < 0.001) on bond strength to zirconia, whilst storage time did not (p = 0.203). Tukey HSD (honest significant difference) test indicated that groups with at least 0.5% of each monomer resulted in higher initial bond strength values. Although chemical bonding to zirconia is credited to MDP, a correct balance between MDP and VBATDT may imply in better bond strength results. The minimum concentration of each monomer should not be lower than 0.5 wt %.

An analytical model based upon repeat unit cell approach is proposed to predict the in-plane elastic constants of two dimensional diamond braided composites. A new geometrical model is developed which considers the yarn undulation and inter-yarn gap in a diamond braided fabric reinforced composite. The actual state of yarns in composites is modeled by the use of sinusoidal shape functions, which employ parameters such as braid construction, yarn geometry, inter-yarn gap and overall volume fraction to quantify the geometry of braided fabric. Stiffness is predicted by calculating the engineering constants for each subcell followed by averaging it over the repeat unit cell's volume using a combination of iso-stress and iso-strain conditions. Composites using four different fabric architectures are fabricated and tested in tensile mode to verify the model. The experimental results indicate reasonable correlation with predicted results. Parametric study is conducted over a range of structurally possible geometrical parameters to study the effects of braiding angle, yarn aspect ratio and gap on the in-plane elastic properties of the textile composite.