Macromolecules 1990,23, 816-823
816
(15) F b , p. J.; Mark, J. E.;Abe, A. J. Am. Chem. Soc. 1966,88, 639. (16) Mark, J. E. J . Chem. Phys. 1968,49, 1398.
(17)
Stern, S. A.; Shah, V. M. and Hardy, B. J. J. Polym. Sci., Polym. Phys. Ed. 1987, 25, 1263.
Registry No. CO,, 124-38-9; CH,, 74-82-8; C,H,, 74-98-6.
Molecular Weight and Comparative Studies of Poly-3- and Poly-4-BCMU Monolayers and Multilayers J. E. Biegajski, R. Burzynski, D. A. Cadenhead,*and P. N. Prasad’ Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214. Received January 10, 1989; Revised Manuscript Received July 13, 1989 ABSTRACT 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 OOO, 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;’ however, the low molecular weight sample initiated the compressional 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 poly3- and poly-4-BCMU. A reevaluation 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.
Introduction
ular hydrogen bonding to take place between adjacent side groups of the same polymer chain. Previously we have demonstrated that monolayers of In this paper we report and compare the monomolecboth poly-4-BCMU1 [poly(dibutyl4,19-dioxo-5,18-dioxaular film behavior of high and low molecular weight (MW) 3,20-diaza-10,12-docosadiynedioate)] and poly-3poly-4-BCMU samples. We will see that these show simBCMU’ [poly(dibutyl 4,17-dioxo-5,16-dioxa-3,18-diaza- ilar but somewhat different behavior from that of a medium 9,ll-eicosadiynedioate)]have surface pressure (*)-area MW poly-4-BCMU1 through both a - A isotherms at the per residue ( A ) isotherms exhibiting a phase transition air/water interface and visible absorption spectra of Langthat incorporates an intramolecular conformational tranmuir-Blodgett (LB) transferred films. sition from an expanded monomolecular amphipathic yelThe number-average molecular weights of high, low, low coil form to a condensed multimolecular nonamphiMW samples of poly-4and previously studied medium pathic red or blue rod form. The conformational transiBCMU were determined by using a film balance and a tion, from coil to rod, has been previously reported, both modified ideal gas law type equation, which applies at in solution and solid-state films by others3-’ and has been low surface pressures. A - A isotherms for medium MW interpreted as representing an increased effective A poly-4-BCMU along with visible absorption spectra of electron conjugation length along the polymer backLB transferred films are presented. Resonance Raman bone. The transition is brought about either by changspectra of medium MW poly-4-BCMU transferred films ing the solvent toward a more nonpolar composition or provide complimentary evidence of the coil to rod conby decreasing the temperature. This allows intramolecformational transition. The relaxation observed between a continuous compression and a stepwise, “equilibrium” isotherm of poly* To whom correspondence should be addressed. 0024-9297/90/2223-0816$02.50/0
0 1990 American Chemical Society
Macromolecules, Vol. 23,No. 3, 1990 4-BCMU was also studied. Transmission electron microscopy of LB transferred films of both poly-4-BCMU and poly-3-BCMU reveal rodlike domains that exist in both the transition region and condensed multimolecular state, while the expanded monomolecular state film shows only a flat featureless topography lacking such domains. The effect of a low-energy hydrophobic solid surface on the conformation of horizontally lifted poly-4-BCMU and poly-3-BCMU monolayers was studied by visible absorption spectroscopy. The hydrophobic surface was apparently able to induce increased order in films transferred in the expanded state and, to a lesser degree, the condensed state of both poly-4-BCMU and poly-3BCMU. The role of subphase pH and cation composition (Le., Na+, Ca2+)in determining the monolayer behavior of poly4-BCMU and poly-3-BCMU were also studied. Both polymers showed monolayer behavior consistent with an ionized state and/or hydrolysis at elevated pH (pH 12.5). Addition of Ca2+ to the subphase (lo-, M CaC1,) was followed by a marked condensation. In contrast an overall expansion of both poly-4-BCMU and poly-3-BCMU was obtained when films of these substances were spread on a 3.0 M NaCl subphase. On such a subphase the films of both showed near coincident a-A isotherms for all areas except within the transition region, where different pressures at the compressional initiation of the transition (a,)reflected residual film differences.
Experimental Section Surface pressure ( T ) versus area per residue ( A ) isotherms were obtained by usin a Langmuir film balance, which has been described previously.1 Continuous compressional isotherms were obtained at a compressional speed of 10 A2 residue-' min-'. Stepwise isotherms were determined through a series of short compressions followed by a waiting period (typically 10-15 min) until all relaxation had apparently ceased. Surface pressure was measured by the Wilhelmy plate method using a hydrogen fluoride, vaporetched, glass coverslip and an automatic-nullingR.G. Cahn electrobalance and an analog chart recorder. Langmuir-Blodgett vertical film transfer was carried out by using a motorized screwdriven lift mechanism while a second, servo-driven motor adjusted a Teflon barrier in order to maintain a constant surface pressure ( f O . l dyn/cm). Horizontally lifted films were transferred by lowering a hydrophobic substrate parallel to the filmcovered interface until contact was achieved, at which time any remaining film surrounding the slide was suctioned off by a pipet connected to an aspirator. The slide with the adherent monolayer film was then carefully removed from the interface. All glass slides used as substrateswere cleaned by first immersing them in hot chromic acid for 12 h. This was followed by a thorough rinsing of the slides with singly distilled water and then with quadruply distilled water. The slides to be used for vertical transfers (with an original hydrophilic surface)were then treated for approximately 5 min in an argon plasma prior to use. Those slides used for horizontal transfers (with a hydrophobic surface) were first cleaned as above and then alkylated by immersing them in a dilute solution of octadecyltrichlorosilane (OTS) in chloroform (0.1 mL of OTS in 50 mL of CHCl,) for approximately 10 min, after which they were rinsed with chloroform and quadruply distilled water and then finally dried in a stream of nitrogen. Subphase water was first distilled and then deionized to give a resistivity of at least 15.0 M Q cm. This water was then quadruply distilled, first from an alkaline stage, then from an acidic stage, and finally twice from an all-quartz still. Both NaOH and CaCl, were used as supplied without further purification for isotherm determination since neither gave an indication of surface-activeimpurities (
