Effect of temperature on the frictional forces between polystyrene brushes

Normal and shear forces between opposing polystyrene (PS) brushes made from preferentially assembled PS-polyvinylpyridine diblock copolymers were measured in toluene and in near-theta cyclohexane at 32, 40, and 50 degrees C, using a modified surface forces apparatus. In cyclohexane, over the temperature window probed, the normal forces of interaction are repulsive and the range of those force profiles changes only slightly; however, for both of the PS brushes studied, the onset of shear forces in neartheta cyclohexane is strongly influenced by changes in temperature: As the temperature is increased, the onset of the frictional interactions between the brushes in cyclohexane shifts to smaller distances, approaching the distances where frictional forces are observed for brushes in the good solvent toluene. The pattern of behavior seen in the frictional response between the limits of good and theta condition is attributed to composition fluctuations, which increase near the theta condition because of the decrease in excluded volume interactions. These fluctuations may give rise to increased drag during shearing motion because of interfacial roughness or interchain coupling across the brush-brush interface. (c) 2006 Wiley Periodicals, Inc.