abstract
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Linear polymers, copolymers, microgels and macrogels based on N -isopropylacrylamide were prepared and the properties of these materials at the air/water interface were investigated. Dynamic surface tensions were measured by pendent drop experiments in which either the drop volumes were constant or were oscillated. The ability of colloidal microgels to lower surface tension is unusual and, for the first time, Environmental Scanning Electron Microscopy provided images showing an ordered array of 500 nm diameter microgels adsorbed at the air/water interface. The surface tension of poly(N -isopropylacrylamide) homopolymers and microgels was about 43 mJ/m2 at 25°C, and surface tension decreased slightly with increasing temperature between 25∼40°C. The temperature insensitivity of surface tension is remarkable because in solution the linear polymer phase separated and the microgel were dehydrated when the temperature was raised from 25 to near 40°C. The introduction of hydrophilic acrylamide moieties increased surface tension and the results were fit to a one-parameter model analogous to the Margules model for the fire energy of mixing of solutions. Kinetic models were developed and fitted to the dynamic surface tension data. A key challenge was relating surface tension to the amount of adsorbed material at the interface. Results are shown for calculations based on both published data and statistical mechanical models of adsorbed polymer. The inability of mass transport models to predict the kinetics suggested that rearrangement of gels or linear polymer at the interface contributed to the kinetics. Macroscopic water swollen gels like the linear polymers and the microgels displayed a low surface energy due to the accumulation of isopropyl groups at the interface. However, unlike the microgels and linear polymers, the surface characteristics reflected in contact angle measurements was sensitive to temperature in the critical range 35 to 40°C.