Langmuir 1991, 7, 1749-1754
1749
Surface-Pressure Gradients in Monolayers of Poly(methy1 methacrylate) J. B. Peng and G. T. Barnes* Department of Chemistry, University of Queensland, Brisbane, Queensland 4072, Australia Received September 4, 1990 Compressionof poly(methy1methacrylate) (PMMA)monolayersgeneratesgradients in surface pressure similar to but smaller than those previously reported for monolayers of poly(viny1stearate) (PVS). The gradient increases sharply with compression of the condensed film and is dependent on the molar mass of the polymer: the higher the molar mass, the steeper the gradient formed. Both the spreading and compressiontechniques affect,to some extent, the formation of the gradient, but a seriousgradient implying inhomogeneityin the monolayer is alwaysproduced during compression. When the compression is stopped, the gradient may decay to a limited extent in the first 10 min, but it then remains relatively constant. The collapse pressure of PMMA monolayers suggested by these experiments is 17-20 mN m-1. None of the isotherm measurements for PMMA or PVS monolayers can satisfy the requirements of Huggins' theory on polymer monolayers. Some discrepancies in the published isotherms of PMMA monolayers can now be attributed to the inhomogeneity of the film.
Introduction In a previous paper1 we reported that a serious surface pressure gradient is formed during compression of poly(vinyl stearate) (PVS) monolayers, indicating that an inhomogeneous monolayer is produced. The formation of this surface pressure gradient was attributed to high viscosity and low molecular diffusion coefficient in the condensed monolayer state.l Consequently, since the viscosity of bulk polymers is related to both their molar mass2 and their molecular morphology,3 it is desirable to investigate whether these factors affect the surface pressure gradients in polymer monolayers. For this work we selected poly(methy1 methacrylate) (PMMA) as a suitable linear polymer to study, as a contrast with the comblike structure of PVS. Indeed, monolayers of PMMA (and ita derivatives) have been investigated but there are since 1928,"* and are still being some discrepancies in the published results. For example, the isotherms and, in particular, the collapse pressures reported are quite different from one author to another. Some reported the collapse pressure to be 10-15 mN m-l,6 some found it to be around 15-17 mN m-l,12J3while others gave a higher value, even up to 34 mN m-1.7 Furthrmore, Gabrielli et al.loJ4have applied the theories of Huggins (1) Peng, J. B.; Barnes, G. T. Lclngmuir 1990,6,578. (2) Freeman, R. Kinetics of Nonhomogeneow Processes; John Wiley & Sone: New York, 1987. (3) Cheremieinoff, N. P. Handbook of Polymer Science and Technology: Marcel Dekker: New York, 1989; Vol. 2, p 472. (4) Katz, J. R.;Samwel, P. J. P. Naturwissenschaften 1928,16, 592. (5) Adam, N. K. The Physics and Chembtry of Surfaces;Clarendon Preer: 1930. . __ - -. (6) Crisp, D. J. J..Cplloid Sci. 1946, 1, 49, 161. (7) Stroeve. P.; srinivaean, M.P.; Higains. -- . B. Thin Solid F i l m 1987. 146,209. (8) Baglioni, P.; Dei, L.; Puggelli, M. Colloid Polym. Sci. 1985, 263, 266.
(9) Puggelli, M.; Gabrielli, G.Colloid Polym. Sci. 1987, 265, 432. (10) Caminati, G.;Gabrielli, G.; Ferroni, E. Colloid Polym. Sci. 1988, 266,776. (11) Naito, K. J. Colloid Interface Sci. 1989, 131, 218. (12) Gabrielli, G.; Puggelli, M.; Baglioni, P. J . Colloid Interface Sci. 1982,86,485. (13) Gabrielli, G.; Guarini, G.G. T. J . Colloid Interface Sci. 1978,64, 185. (14) Gabrielli, G.;Pug elli, M.; Faccioli, R. J. Colloid Interface Sei. 1972, 41, 63. Gabrielli, Ferroni, E.; Huggins, M. L. Prog. Colloid Polym. Sci. 1976, 58, 201.
8,;
and others1"17 to the monolayers of PMMA and other polymers and calculated spreading entropy, enthalpy, and a set of parameters based on their measurements. However, these theories were developed for characterizing monolayers of polymers in thermodynamic equilibrium and require allthe monomer units of the polymer molecules to be sited in equivalent positions on the subphase with a close-packed arrangement (two-dimensional solution model). It is therefore necessary to determine whether actual monolayers of linear polymers satisfy all these requirements. Our results show that there is a considerable surface pressure gradient formed during compression of PMMA monolayers, but it is markedly smaller than that formed in PVS monolayers. This gradient depends strongly on the molar mass of the sample. The formation of the gradient also depends to some extent on both the spreading and the compression techniques. Once the gradient has been formed, interruption of the compression is followed by a relatively limited decay of the gradient within the first 10 min and then it remains almost constant. The slopes of isotherms of PMMA monolayers startto decrease at about 17-20 mN m-1, indicating the beginning of collapse.
Experimental Section PMMA samples used in this work were produced by Polyscience, Inc., Warrington, PA, as standards for calibrating GPC measurements. Five different samples were used with certified molar masses of 27,40, 74, 126, and 461.5 kg mol-', and molar mass distributions less than 1.1. All of the polymer samples were atactic. The samples were kept in a vacuum desiccator (
