Flexible and rigid polyurethane (PU) foam nanocomposites were synthesized using unmodified and organically modified montmorillonite clays. X-ray diffraction and transmission electron microscopy studies showed that, while the unmodified clays were intercalated, the modified clays were exfoliated in the foams produced. The cell morphology of the foams was investigated by environmental scanning electron microscopy. The fraction of open cells (defined as the cells in which the foam lamellae are all ruptured) in the foam is an important parameter governing many properties, such as the degree of softness in the case of flexible foams and dimensional stability for both rigid and flexible foams. The open cell content in PU-clay foamed nanocomposites was investigated. It was found that clays act as efficient cell openers in both rigid and flexible foams and a greater fraction of open cells was obtained with increasing clay concentration. Modified clays were found to be more efficient cell openers than the unmodified clay. The degree of softness of flexible foam was increased and the dimensional stability of both rigid and flexible foams was improved, with clay addition. The thermal conductivity and compressive strength of the rigid foams were not significantly affected by clay addition. The kinetics of the foaming process with different clays was investigated. The polymerization and the foaming reactions were found to be affected in different ways for the modified and unmodified clays. Comparison with chemical cell openers in the case of flexible foams indicated that clays could be a good alternative.

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Foam stability and segmented polymeric phase morphology of polyurethane foams synthesized partially and completely from castor oil are investigated. Preliminary analysis of the impact of alterations in the polymeric phase on macroscopic stress dissipation in foams is also carried out. The stability and morphology show unique trends depending on the concentration of castor oil used in foam synthesis. While low and intermediate concentrations of castor oil does not significantly affect the foaming process; at high concentrations, the volumetrically expanding liquid matrix remains in a nonequilibrium state during the entire foaming period, resulting in significant foam decay from top. This increases the final foam cell density and decreases the plateau border thickness at bottom. In the polymeric phase of castor oil based foams, the fraction of monodentate urea increases at the cost of non-hydrogen bonded urea. These monodentate urea domains undergo flocculation in foams synthesized completely from castor oil, thus prominently modifying the segmented morphology. The glass transition temperature of soft segments of partially substituted foams shows moderate increase, with indications of phase mixing between the polyether and castor oil generated urethane domains. Foams synthesized entirely from castor oil have significant sol fraction due to unreacted oligomers. The microscopic alterations in polymeric phase reduce the elastic recovery of partially substituted castor oil foams compared to its viscous dissipation under an applied stress. V C 2014Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40668.

Nanocomposites of rigid polyurethane foam with unmodified vermiculite clay are synthesized. The clay is dispersed either in polyol or isocyanate before blending. The viscosity of the polyol is found to increase slightly on the addition of clay up to 5 pphp (parts per hundred parts of polyol by weight). The gel time and rise time are significantly reduced by the addition of clay, indicating that the clay acts as a heterogeneous catalyst for the foaming and polymerization reactions. X-ray diffraction and transmission electron microscopy of the polyurethane composite foams indicate that the clay is partially exfoliated in the polymer matrix. The clay is found to induce gas bubble nucleation resulting in smaller cells with a narrower size distribution in the cured foam. The closed cell content of the clay nanocomposite foams increases slightly with clay concentration. The mechanical properties are found to be the best at 2.3 wt% of clay when the clay is dispersed in the isocyanate; the compressive strength and modulus normalized to a density of 40 kg/m 3 are 40% and 34% higher than the foam without clay, respectively. The thermal conductivity is found to be 10% lower than the foam without clay.

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.