| The self-similar mixing microstructures generated in chaotic mixing of prepolymer and chain extender were utilized to augment conversion and molecular weight in the synthesis of thermoplastic polyurethanes (TPU) and polyurethane nanocomposites. The values of time scales of mixing and chemical reaction were varied so as to obtain the best possible product.; The chaotic mixing parameter (theta), catalyst concentration, and chemical structures of polyols and diisocyanates were used as factors to exert influence on the speed of conversion, molecular weight, and phase separation during synthesis. The time to reach maximum torque during mixing and tensile properties of the products were evaluated. In addition, the barrier to diffusion of water vapor through composites of reactive and nonreactive clay particles and pristine polyurethanes was determined. The time scales of reaction and mixing were obtained respectively from the values of kinetic rate constant of the reaction system and Liapunov exponents, the latter characterizes fluid element separation. As a general observation, it was found that best polyurethane products were obtained when the time scale of mixing and the time scale of reaction between -NCO and -OH groups were close to each other. It was found that a value of theta = 1440° with sinusoidal rotor speed variation provided the best mixing condition and the time scale of reaction was gradually shortened using higher concentration of catalysts. In addition, the chaotic mixing protocol with cylindrical rotors produced better products than a mixer formed by a set of commercial neutral kneading discs. The effective shear rate was kept the same in both cases for fair comparison. It was also found that phase separation of hard segments in systems with 38 wt% hard segments limited the maximum value of conversion even though the time scales of mixing and chemical reactions were matched. This indicates that the time scale of phase separation must also be considered in the analysis.; Composites of polyurethanes with reactive nanoclay contained micrometer size clay particles. However, they provided significant reduction in water permeation due to alignment of clay particles by chaotic mixing flow. It was found that very little clay-polymer tethering reaction occurred due to strong imbalance in reactivity---the reactivity between prepolymer and butanediol were found to be 25 times faster than those between prepolymer and reactive clay. The knowledge gleaned from the study can be extended to other reactive systems and to designing of continuous chaotic mixing devices. |
