Molecule±surface interactions with the first monolayer governing the orientation in thin films has been seen, as well, in other types of thin films. [14] For SPECH, the end of the chromophore with the pyridinium group near the backbone is relatively more hydrophilic than the opposite end of the chromophore. Our data indicates that this feature interacting with the hydrophobicity of the substrate is the dominant mechanism that determines the sense of orientation in the first layer and chromophores in subsequent layers adopt the same sense of orientation and the same order parameter of the previously deposited layers. It is well known that polyelectrolyte layers are interspersed [15] so it is not surprising that chromophores in the process of being deposited are influenced with previously deposited layers.
These results demonstrate that the chromophore orientation in the first layer ªdirectsº the chromophore orientation throughout the v (2) NLO APD films and suggests new strategies for improvement of polar order in these films. Experiments are underway to test this hypothesis. The practical implications of these results include the possibility to pattern a substrate with hydrophobic and hydrophilic regions to control the sense of chromophore orientation in each region. This might be done to produce a push±pull Mach±Zehnder interferometer, for example.
Experimental
The v (2) NLO APD films were fabricated at room temperature in ambient air using a computer-interfaced Zeiss HMS programmable slide stainer that was kept in a Class 100 clean room under ultraviolet (UV)/blue blocked fluorescent light. The SPECH/PSS films were deposited on hydrophobicized glass slides and the SPECH/PAA films on both hydrophobic and base-washed glass slides. The 1319 nm line of a diode-pumped Nd:YAG laser was used as the fundamental for the SHG measurements.
SPECH/PSS films were deposited on 1,1,1,3,3,3-hexamethyldisilazane (HMDS)-treated glass substrates at approximately 25 C using the following parameters: solution and rinse pH 5.5; concentration of PSS = 10 ±4 mol of repeat units/L, SPECH concentration = 10 ±4 mol of chromophoric repeat unit/L; SPECH deposition time = 50 min, PSS deposition time = 20 min. The SPECH/ PAA films were deposited on HMDS-treated glass substrates at approximately 25 C using the following parameters: solution and rinse pH 7.0; SPECH concentration = 10 ±4 mol of chromophoric repeat unit/L, PAA = 10 ±4 mol of repeat units/L; SPECH deposition time = 50 min, PAA deposition time = 20 min. The SPECH/PAA films were deposited on base washed glass substrates at approximately 25 C using the following parameters: the pH of the solution and rinse baths was adjusted with dilute NaOH to pH 7.0; SPECH concentration = 10 ±4 mol of chromophoric repeat unit/L, PAA = 10 ±4 mol of repeat units/L; SPECH deposition time = 6 min, PAA deposition time = 5 min. The water drop contact angle of the substrates measured under air was 0 and 89 for the basewashed glass and HMDS-treated glass, respectively.
The PAA was purchased from Polysciences (catalog no. 00 627, Lot # 457 416) as a 25 % solution in H 2 O, molecular weight (MW) = 50 000, and diluted to the final deposition solution concentration with ultrapure water. The PSS was purchased from Aldrich (cat no. 26 244-7, Lot # 00 829JK) as a 20 % solution in H 2 O, MW approx. 70 000, and diluted to the final solution concentration with ultrapure water. The stilbazolium substituted polyepichlorohydrin was synthesized in-house by attaching the chromophore (degree of substitution was 90 %) to polyepichlorohydrin with MW = 1000 g/mol. SPECH solutions were made by dissolving in ultrapure water. All polymer solutions were filtered through 0.2 lm pore size membrane filters.