MOLECULAR-WEIGHT AND COMPARATIVE STUDIES OF POLY-3-BCMU AND POLY-4-BCMU MONOLAYERS AND MULTILAYERS

The molecular weights of three poly-4-BCMU samples were determined with a film balance technique by assuming near ideal behavior at low film concentrations. For the high, medium, and low molecular weight samples the number-average molecular weights were 410 000, 300 000, and 66 000 g/mol, respectively. All three samples showed a monolayer/bilayer, yellow coil/red rod conformational change similar to those we previously reported;1 however, the low molecular weight sample initiated the compres-sional transition at a significantly lower area/residue than did either the medium or high molecular weight samples. This was interpreted to mean that the shorter chains in the low molecular weight sample had an increased number of semisoluble end groups. The visible absorption spectra indicated that while the low molecular weight sample underwent a yellow coil/red rod transition more readily and to a greater extent, the resultant red rod form was somewhat less organized. We have also made further comparisons of poly-3- and poly-4-BCMU. A réévaluation of the thermodynamics of the monolayer/bilayer transition, as indicated by surface pressure/area per residue isotherms, using a modified Clapeyron equation, showed slight exothermicity and increasing order accompanying multilayer formation of red rod poly-4-BCMU. Resonance-enhanced, laser Raman spectroscopy clearly reveals that the upper layer of the condensed bilayer consists of the red rod form but that the residual lower layer remains in the yellow coil form. Transmission electron microscopy of both poly-3-BCMU and poly-4-BCMU reveals a transition from a smooth surface in the monolayer region to one exhibiting numerous rodlike features in the transition region until in the condensed phase a more uniform but textured appearance is achieved. The effects of both solid and aqueous substrates were also examined. Deposition of monolayers of both polymers on hydrophobic solid substrates gave clear visible spectroscopic evidence of a small, partially induced coil to rod transition, in contrast to similar monolayers deposited on hydrophilic solid surfaces. Changing the pH and the addition of both sodium and calcium ions to an aqueous substrate also had significant effects. For poly-4-BCMU high pH (12.5) values resulted in an overall expansion for both expanded and condensed isotherm segments. For poly-3-BCMU the condensed state was expanded but the expanded (monolayer) state was condensed. A combination of ionization and hydrolysis was invoked for both polymers with ionization predominating for poly-4-BCMU and hydrolysis playing a more significant role for poly-3-BCMU. Calcium ions were shown to greatly condense ionized films while high concentrations of sodium ions produced expansion. The latter resulted in nearly identical compressional isotherms for poly-3-BCMU and poly-4-BCMU, except for a shift of the highly sensitive transition region. © 1990, American Chemical Society. All rights reserved.