Humidity and solvent effects in spin-coated polythiophene-polystyrene blends

Film blends of poly(3-butyltiophene-2,5diyl) (PT) and polystyrene (PS; 1 : I w/w) were spin-coated onto silicon wafers from chloroform, tetrahydrofuran, and cyclohexanone at a controlled relative humidity between 4 and 86%. The film morphologies were determined with atomic and lateral force microscopy and mapping and depth profiling modes of dynamic secondary-ion mass spectroscopy. Independently, white light interferometry was used to examine the expansion and response time (tau) of pure polymer layers exposed to solvent vapors and moisture. The higher PS solubility, in comparison with the PT solubility, in chloroform and tetrahydrofuran resulted in PS/PT//Si bilayers, which were the final structures for coatings from chloroform [with much larger tau(PS)//tau(PT) ratios]. For tetrahydrofuran, these bilayers were destroyed, most likely by surface and interface instabilities, yielding hierarchic lateral structures. For cyclohexanone (with the largest tau values), a large-scale component of the lateral structures was absent, and this suggested the leveling of surface instabilities. The humidity changed the structural scales and thickness of the films cast from tetrahydrofuran (because it had the best solubility with water). The humidity effects of chloroform and cyclohexanone [reported earlier for polyaniline and poly(vinyl pyridine) blends] were practically absent. Moisture was not easily absorbed by PT and PS [in contrast to polyaniline and poly(vinyl pyridine)] and probably adsorbed merely at the surfaces of blend films rich in tetrahydrofuran. (C) 2007 Wiley Periodicals, Inc.