Adsorption at Interfaces

10 Monolayer Studies V . Styrene-Acrylic Acid Films

Downloaded by NORTH CAROLINA STATE UNIV on November 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch010

on Aqueous Substrates ERWIN SHEPPARD and NOUBAR TCHEUREKDJIAN Chemical Research Department, S. C. Johnson and Son, Inc., Racine, Wis. 54303

Introduction Low molecular weight styrene-acrylic acid copolymers of varying degrees of polymerization were spread as monolayers on water and various salt solutions at neutral and alkaline pH's. The monolayer properties of high molecular weight styreneacrylic acid systems were studied by Müller (1), who could not attain complete spreading and consequently monolayers were not formed. Monomolecular film behavior of other high molecular weight copolymers have been studied successfully. For example, Fowkes et al. (2) have reported on C8-C18 α-olefins-vinyl acetate systems, Labbauf and Zack (3) have reported on methyl methacrylate-styrene systems, and Hironaka et al. (4) have discussed films of methyl acrylate and n-butyl acrylate copoly mers. Glazer (5) has reported on the spreading properties of ethoxylin resins with molecular weights from 400-2500 prepared by the reaction of epichlorohydrin and 4,4-dihydroxydiphenylpropane. The adhesive properties of these resins were corre lated with their interfacial spreading properties. Experimental Table I lists the styrene-acrylic acid, S/AA, copolymers studied, the number average molecular weights, Mn, and the acid numbers or neutralization values defined as milli-equivalents of base needed to neutralize one gram of the copolymer. The theoretical neutralization value was 4.5 meq. The starting monomer mole ratio for the polymerization was 0.41 for acrylic acid and 0.59 for styrene. The initiator used was benzoyl peroxide. The molecular weights were determined by vapor phase osmometry using ethanol as the solvent. Pressure-area isotherms at 22°C were determined by the Wilhelmy platinum hanging plate method with automatic recording of both the film pressure and the area (6). Minor modifications were incorporated in the design of Mauer's (7) analytical 157 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


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Table i . Sample 1 2 3 4 5 β 7

Description of Styrene/Acrylic Acid Copolymers Initiator Level (mole %) 0.5 1 2 3 5 6 7

Meq. of base/g of copolymer 4.89 4.32 4.18 4.12 3.96 3.96 3.80

M

n

2540 2400 2250 1750 1500 1280

In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


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balance for recording changes in weight keeping the sensing plate nearly stationary. International Rectifier Type CS120V6 photocells were used in the Wheatstone bridge c i r c u i t , miniatur ized equivalent tubes in the amplifier c i r c u i t , and a 75 V D.C. power supply. The film trough and barriers used to clean the substrate surface and compress the monolayers were FEP Teflon-coated tool aluminum. The trough was rinsed with deionized water before each determination. Deionized d i s t i l l e d water and materials of highest purity available were used throughout. The mono layers were spread from 10% ethanol and 90% benzene (w/w) solu tions and were formed on deionized d i s t i l l e d water, on sub strates adjusted to pH=9, on dilute solutions containing various cations and on their hydroxides. About 15 minutes were allowed for the spread films to equilibrate before compression was started at a constant rate of 150 cm /mg/min. Gaines (8) has summarized most of the pertinent experi mental techniques and precautions to be used in measuring mono layer properties. 2

Results and Discussion Terminology. Monolayers of styrene/acrylic acid low molec ular weight copolymers were studied i n order to determine the interfacial properties among the several samples of different molecular weights and acid values and to determine the effect of pH and of various cations on their spreading behavior at air/aqueous solution interfaces. The specific area, Ao> at zero film pressure was obtained by extrapolating the linear portion of pressure-area isotherm to the area axis as shown in Figure IB. In this study atypical 7Γ-Α curves as represented i n Figure 1A were obtained for the S/AA films under certain conditions. The pressures at which the isotherms deviated from linearity in the direction of higher compressibilities, - A " (dA/d7r)-r, were designated as 7 T , collapse pressure for the expanded region, and TTçç, collapse pressure for the condensed region. The former represented the i n i t i a l collapse point and the maximum film pressure in going from the higher areas to the transition region while the latter represented the f i n a l collapse pressure and the highest pressure attained prior to f i n a l collapse of the spread f i l m . The designations for the various regions of the isotherm were made purely for convenience i n this discussion and to describe the high and low area regions, and the transition area regions (areas between AQE and AQC)· Here AQE represents the extrapolated limiting value of the high area region and AQC represents the extrapolated limiting value of the low area region. 1

CE

Spreading of S/AA Copolymers on Water. Hydrocarbon homopolymers such as polyethylene and polystyrene do not spread



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completely on water and water soluble polymers such as polyacrylic acid give unstable films on neutral substrates (8). Thus a proper balance of water soluble and water insoluble groups i s required in order to obtain stable, completely spread monolayers of copolymers. In Figure 2 are shown three typical pressurearea isotherms for the S/AA copolymers spread as monolayers at the air-water interface. Each of the seven copolymers exhibited the three distinct monolayer regions which are shown schematical ly i n Figure 1A. They have a highly compressible intermediate transition region as characterized by the A A Q = A O E - A O C value which i s the magnitude of the transition region. Monolayer film areas, collapse pressures, and A A Q ' S for the copolymers spread on water are recorded in Table II. A plot of A A Q VS. copolymer acid number i s given in Figure 3. When the linear region of the A A Q vs. copolymer acid number curve was extrapolated t o A A g ^ O , the corresponding acid number was found to be about 4.5. In other words, a copolymer of this series with an acid number of at least 4.5 i s required i n order to enhance spreading without intermediate collapse pressure and transition regions. Copolymers with acid numbers less than 4.5 would have their characteristic p a r t i a l l y collapsed transition regions which divide the high and low film area regions of the isotherms. It i s fortuitous that the polymerization process u t i l i z e d a starting monomer composition with a theoretical neu tralization value of 4.5 meq. which i s numerically identical to the extrapolated limiting value of 4.5 meq. for the spread films when ΔΑο=0. In spite of the hydrophilic nature of the S/AA copolymers, collapse of the monolayers started at r e l a t i v e l y high film areas. The region over which collapse occurred i s controlled by the copolymer composition as stated above and shown in Figure 3. Therefore, the interfacial film properties of the copolymers may be altered at w i l l to give any desired surface intermolecular cohesion and film-substrate adhesional properties. The transition region, A A Q , extended over larger monolayer areas when the copolymer acid number was decreased. The lower molecular weight S/AA copolymers were relatively more hydrophobic when compared to the higher molecular weight samples as indicated in Table I. As the surface film pressure i s increased the nonfilm forming, hydrophobic portions of the copolymers are assumed to be squeezed out of the film carrying along some of their film forming neighbors. Depending upon the structure of the copoly mers, i t appears that more of a polymer chain could be squeezed out of the surface film with increasing polystyrene concentra tion i n the copolymer. At high pressures, the composition and structure of the copolymers appeared to dominate the AQC values while at lower pressures the strongly hydrophilic nature of the acrylic acid groups appeared to control the spreading character i s t i c s of the films without regard to the polystyrene content as indicated by the almost constant AQE values except for Sample 1 which appears to be a special case. These data are shown in


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Adsorption at Interfaces

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