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On the mechanism of collapse of monolayers of macromolecular substances. I. Polymethylmethacrylate
On the Mechanism of Collapse of Monolayers of Macromolecular Substances i. Polymethylmethacrylate
Introduction The results obtained in a previous work (1) on the collapse of monolayers of a carboxylic fatty acid showed that the kinetic studies are useful to obtain information on the mechanism of separation of a bulk phase from a two-dimensional one. Such a mechanism could be correlated with the interaction energies prevailing in the condensed monolayer. In this paper the extension of these studies to monolayers of macromolecular substances is reported. The aim was to clarify the mechanism of collapse of the monolayers of polymeric substances and to start the search for a possible correlation between such a mechanism and the energies connected with it and the distribution of the macromolecules in the condensed phase monolayers. On one hand this appeared interesting because of the importance that the phenomenon of separation of three-dimensional phases from the corresponding two-dimensional ones has from both theoretical and practical point of view; on the other hand it is interesting because it is difficult to obtain the distribution of the macromolecules in two-dimensional condensed phases, e.g., even by comparison between the virial coefficients of the two-dimensional equation of state and the theoretically deduced corresponding ones'. Indeed, it has been shown (2) that the latest theoretical equations agree with the experimental results only for noncondensed films. Moreover very few papers, concerning this field of research and polymeric substances in particular, have appeared. Polymethylmethacrylate (PMMA) was chosen as the first substance to investigate because it gives stable two-dimensional condensed films and also because its distribution at the W/A interphase is known from previous work (2, 3). PMMA is therefore to be considered a model macromolecular compound particularly useful to begin an attempt of correlation between the mechanism of collapse and the conformation of the macromolecules in the two-dimensional state. For the above study surface pressure and ellipsometric measurements were used.
Experimental PMMA (MW = 250,000; "0 = 0.66) was supplied by Montedison S.p.A. High-purity Riedel de Haen benzene was employed as spreading solvent. Bidistilled
water purified from colloidal impurities by active carbon was used as support. The Wilhelmy method was used for the measurements utilizing a Cahn Model 7550 electrobalance equipped with microvoltmeter and modified for surface pressure measurements. As the pressure collapses, the corresponding area and chiefly the mechanism of collapse depend upon the rate and the rnodality of compression; first of all, the compression conditions were determined in order to obtain reproducible results. These were as follows: initial area 1.3 m 2 mg -t, discontinuous compression-with interruptions lasting 2 rain for every 0.06 m 2 r a g - l - - t o a pressure value of 17.74 dyn cm -1. At 20°C this was the lowest value for which, using the previously described experimental techniques, it was possible to detect an appreciable lowering of the surface pressure with time and it was therefore selected to get the first kinetics. As soon as the evolution of the system at the area corresponding to the lowest pressure terminated, the compression was res u m e d - i n steps of 0.06 m z mg -1 and waiting after every step for a time comparable to the duration of the previously determined kinetics--to reach the pressures (27.1 and 30.8 dyn cm -I) at which new kinetics were to be followed. The values of the area corresponding to the selected pressures may be obtained from the previously reported spreading isotherm (3) and are respectively 0.90, 0.80, and 0.72 m z mg 1. The above experimentally determined requirements were rigorously followed for every experiment at every temperature and they were chosen as (i) they ensured reproducible surface pressure variations as function of time; (ii) they avoided the superposition of different kinetics; and (iii) they allowed the maximum approach of the compression process to a succession of states of equilibrium. The temperatures chosen were 15, 20, and 25°C. The ellipsometric measurements were made using the Officine Galileo horizontal ellipsometer previously described (1). This PMMA films to be studied by ellipsometry were transferred--from the liquid support to a thin glass plate coated with a vacuum-evaporated chromium l a y e r - - b y the Langmuir-Blodgett method (4) using an oleic acid piston and an extraction rate of 5 cm rain-l; all the measurements were made at 20°C.
185 0021-9797/78/0641-0185 $02.00/0 Journal of Colloid and Interface Science, Vol. 64, No. 1, March 15, 1978
Copyright © 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.
186
NOTES
Results and Discussion (1) Surface pressure measurements. To study the collapse phenomenon (5) we followed the variation of the surface pressure with time keeping constant the surface area at the three selected values. It is possible to verify from the spreading isotherms previously reported (3) that these three values fall within the field in which the pressure-area relation is linear and therefore it is possible to consider the values 7r0-~" obtained for the decrease of the surface pressure (Tr and ~'0 are the surface pressures at times t and t = 0, respectively) as if they were the result of a normal chemical reaction for which the decrease in concentration of the reagents is similar to the decrease of substance in the monolayer. As we already pointed out for the monolayers of arachidic acid (1), the order eventually deduced for the kinetics is to be considered only formal because the process is not a true chemical reaction and we do not measure changes of concentration as function of time directly. At the three selected areas the values for the decrease of the surface pressure resulted almost independent of temperature. Many authors (including ourselves) (1, 5, 6) assimilate the collapse phenomenon to the process of formation of a new condensed massive phase; therefore it appeared plausible to make use of the known theories on nucleation and growth of new phases for the interpretation of the experimental data. Of the several relationships tested (7, 8, 9) none succeeded in giving good fit to the pressure data over the whole transformation. However, satisfactory alignment of the experimental points was obtained using a n = 2 power law for the initial portion of the reaction followed by a n = 2 Avrami-Erofeev equation; the fitting obtained by the use of the latter equation being particularly good for the high-pressure experiments. It is also to be noted that, particularly for the high values of the surface pressure, there exists a time interval of a few minutes in which neither of the kinetic laws used gives good fit to the experimental data. These results cannot be used as a definitive proof for a particular mechanism of collapse; however, they tend to indicate, at least for the system under investigation, that a nucleation and growth process is t a k i n g place in which (initial power law portion) nuclei originate from a combination of active intermediates formed at constant rate. This appears in good agreement with the "buckling" process proposed by Yin and Wu (9). On the other hand the AvramiErofeev equation gives an indication that nuclei interfere during growth and this agrees with the efiipsometric results particularly for what concerns high-pressure exPeriments.- ~ .. Finally it is worth notiiig that the kinetic processes studied are independent of temperature. This means that the activation energy is negligible and a temperaJournal of Colloid and Interface Science, Vol. 64, No. 1, March i5, 1978
ture-independent preexponential entropic factor accounts for the whole rate of the process. This is in good agreement with a nucleation mechanism and shows that, at variance with the case of arachidic acid (1) in which high cohesive energies among, hydrophobic chains are present, the separation takes place without any sudden breaking of the monolayer. The different distribution and the flexibility of the macromolecular substance are therefore conditioning the mechanism of collapse and the energies connected with it. (2) Ellipsometric measurements. Ellipsometric measurements on PMMA films, transferred by the previously described technique from the liquid to a solid support, were done for two values of the surface pressure held rigorously constant during the extraction process. At the pressure of 13.0 dyn cm -1 (a value lower than the 17.1 dyn cm -1 one corresponding to the lowest pressure at which the collapse takes place) a layer thickness of about 14 /~ was obtained confirming for PMMA the "horizontal" disposition of the two-dimensional quasi-rigid spirals already found by different techniques (2, 3). At the pressure of 27.1 dyn cm -a (at which we determined the second kinetics) the values obtained for the layer thickness--having an order of magnitude of 50 A - were less reproducible and differed from zone to zone of the layer. This lack of reproducibility and homogeneity may confirm the presence of a nucleation process. The value of the thickness, far higher than bimolecular, and the inhomogeneity of the layer do not satisfactorily agree with a mechanism of successive superpositions of rigid layers as suggested for monolayers of polypeptidic substances (11); on the contrary, an irregular growth of the collapsed massive phase might account for them.
Conclusions The above results allow us to conclude that, for monolayers of PMMA at the W/A interphase, the collapse process can be formally represented by a nonactivated nucleation and growth mechanism. While this behavior shows several general similarities with the behavior of arachidic acid, there are nevertheless also significant points of difference. These differences may be readily ascribed to the disposition and the energies of the macromolecules at the interphase. Accordingly it seems worthwhile to extend the present approach to elucidate the properties of related systems. This may then provide further information about the distribution of polymer molecules in the two-dimensional condensed phase. ACKNOWLEDGMENTS The authors feel deeply indebted to prof. E. Ferroni for helpful discussions and to C.N.R. for financial support.
NOTES REFERENCES 1. Gabrielli, G., Guarini, G. G. T., and Ferroni, E., J. Colloid Interface Sci. 54, 424 (1976). 2. Gabrielli, G., Ferroni, E., and Huggins, M. L., Progr. Colloid Polym. Sci. 58, 201 (1975). 3. Gabrielli, G., Puggelli, M., and Faccioli, R., J. Colloid Interface Sci. 37, 213 (1971); 41, 63 (1972). 4. Blodgett, K. B., J. Amer. Chem. Soc. 57, 1007 (1935); Blodgett, K. B., and Langmuir, I., Phys. Rev. 51, 964 (1937). 5. Gaines, G. L., Jr., "Insoluble Monolayers at Liquid Gas Interfaces," pp. 144-151. Interscience, New York, 1966. 6. Joos, P., Bull. Soc. Chim. Belges 76, 591 (1967); Demel, R. A., and Joos, P., Chem. Phys. Lipids 2, 35 (1968); Joos, P., and Demel, R. A., Biochim. Biophys. Acta 183, 447 (1969); Phillips, M. C., and Joos, P., Colloid Z. Z. Polym. 238, 499 (1969).
187
7. Garner, W. E., "Chemistry of the Solid State." Butterworths, London, 1955. 8. Young, D. A., "Decomposition of Solids." Pergamon, Oxford, 1966. 9. Avrami, M., J. Chem. Phys. 7, 1103 (1939); 8, 212 (1940); 9, 177 (1941). 10. Yin, T. P., and Wu, Souheng, J. Polym. Sci. C 34, 265 (1971). 11. Malcom, B. R., Polymer 7, 595 (1966); Proc. Roy. Soc. London A 305, 363 (1968); J. Polym. Sci. C 34, 87 (1971); Progr. Surface Membrane Sci. 7, 183 (1973). GABRIELLA GABRIELLI GIULIO G. T. GUARINI Institute of Physical Chemistry University of Florence Via Gino Capponi 9 Florence 50121 Italy Received November 17, 1976; accepted September 22, 1977
Journal of Colloid and Interface Science. Vol. 64, No. 1. March 15, 1978
Introduction The results obtained in a previous work (1) on the collapse of monolayers of a carboxylic fatty acid showed that the kinetic studies are useful to obtain information on the mechanism of separation of a bulk phase from a two-dimensional one. Such a mechanism could be correlated with the interaction energies prevailing in the condensed monolayer. In this paper the extension of these studies to monolayers of macromolecular substances is reported. The aim was to clarify the mechanism of collapse of the monolayers of polymeric substances and to start the search for a possible correlation between such a mechanism and the energies connected with it and the distribution of the macromolecules in the condensed phase monolayers. On one hand this appeared interesting because of the importance that the phenomenon of separation of three-dimensional phases from the corresponding two-dimensional ones has from both theoretical and practical point of view; on the other hand it is interesting because it is difficult to obtain the distribution of the macromolecules in two-dimensional condensed phases, e.g., even by comparison between the virial coefficients of the two-dimensional equation of state and the theoretically deduced corresponding ones'. Indeed, it has been shown (2) that the latest theoretical equations agree with the experimental results only for noncondensed films. Moreover very few papers, concerning this field of research and polymeric substances in particular, have appeared. Polymethylmethacrylate (PMMA) was chosen as the first substance to investigate because it gives stable two-dimensional condensed films and also because its distribution at the W/A interphase is known from previous work (2, 3). PMMA is therefore to be considered a model macromolecular compound particularly useful to begin an attempt of correlation between the mechanism of collapse and the conformation of the macromolecules in the two-dimensional state. For the above study surface pressure and ellipsometric measurements were used.
Experimental PMMA (MW = 250,000; "0 = 0.66) was supplied by Montedison S.p.A. High-purity Riedel de Haen benzene was employed as spreading solvent. Bidistilled
water purified from colloidal impurities by active carbon was used as support. The Wilhelmy method was used for the measurements utilizing a Cahn Model 7550 electrobalance equipped with microvoltmeter and modified for surface pressure measurements. As the pressure collapses, the corresponding area and chiefly the mechanism of collapse depend upon the rate and the rnodality of compression; first of all, the compression conditions were determined in order to obtain reproducible results. These were as follows: initial area 1.3 m 2 mg -t, discontinuous compression-with interruptions lasting 2 rain for every 0.06 m 2 r a g - l - - t o a pressure value of 17.74 dyn cm -1. At 20°C this was the lowest value for which, using the previously described experimental techniques, it was possible to detect an appreciable lowering of the surface pressure with time and it was therefore selected to get the first kinetics. As soon as the evolution of the system at the area corresponding to the lowest pressure terminated, the compression was res u m e d - i n steps of 0.06 m z mg -1 and waiting after every step for a time comparable to the duration of the previously determined kinetics--to reach the pressures (27.1 and 30.8 dyn cm -I) at which new kinetics were to be followed. The values of the area corresponding to the selected pressures may be obtained from the previously reported spreading isotherm (3) and are respectively 0.90, 0.80, and 0.72 m z mg 1. The above experimentally determined requirements were rigorously followed for every experiment at every temperature and they were chosen as (i) they ensured reproducible surface pressure variations as function of time; (ii) they avoided the superposition of different kinetics; and (iii) they allowed the maximum approach of the compression process to a succession of states of equilibrium. The temperatures chosen were 15, 20, and 25°C. The ellipsometric measurements were made using the Officine Galileo horizontal ellipsometer previously described (1). This PMMA films to be studied by ellipsometry were transferred--from the liquid support to a thin glass plate coated with a vacuum-evaporated chromium l a y e r - - b y the Langmuir-Blodgett method (4) using an oleic acid piston and an extraction rate of 5 cm rain-l; all the measurements were made at 20°C.
185 0021-9797/78/0641-0185 $02.00/0 Journal of Colloid and Interface Science, Vol. 64, No. 1, March 15, 1978
Copyright © 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.
186
NOTES
Results and Discussion (1) Surface pressure measurements. To study the collapse phenomenon (5) we followed the variation of the surface pressure with time keeping constant the surface area at the three selected values. It is possible to verify from the spreading isotherms previously reported (3) that these three values fall within the field in which the pressure-area relation is linear and therefore it is possible to consider the values 7r0-~" obtained for the decrease of the surface pressure (Tr and ~'0 are the surface pressures at times t and t = 0, respectively) as if they were the result of a normal chemical reaction for which the decrease in concentration of the reagents is similar to the decrease of substance in the monolayer. As we already pointed out for the monolayers of arachidic acid (1), the order eventually deduced for the kinetics is to be considered only formal because the process is not a true chemical reaction and we do not measure changes of concentration as function of time directly. At the three selected areas the values for the decrease of the surface pressure resulted almost independent of temperature. Many authors (including ourselves) (1, 5, 6) assimilate the collapse phenomenon to the process of formation of a new condensed massive phase; therefore it appeared plausible to make use of the known theories on nucleation and growth of new phases for the interpretation of the experimental data. Of the several relationships tested (7, 8, 9) none succeeded in giving good fit to the pressure data over the whole transformation. However, satisfactory alignment of the experimental points was obtained using a n = 2 power law for the initial portion of the reaction followed by a n = 2 Avrami-Erofeev equation; the fitting obtained by the use of the latter equation being particularly good for the high-pressure experiments. It is also to be noted that, particularly for the high values of the surface pressure, there exists a time interval of a few minutes in which neither of the kinetic laws used gives good fit to the experimental data. These results cannot be used as a definitive proof for a particular mechanism of collapse; however, they tend to indicate, at least for the system under investigation, that a nucleation and growth process is t a k i n g place in which (initial power law portion) nuclei originate from a combination of active intermediates formed at constant rate. This appears in good agreement with the "buckling" process proposed by Yin and Wu (9). On the other hand the AvramiErofeev equation gives an indication that nuclei interfere during growth and this agrees with the efiipsometric results particularly for what concerns high-pressure exPeriments.- ~ .. Finally it is worth notiiig that the kinetic processes studied are independent of temperature. This means that the activation energy is negligible and a temperaJournal of Colloid and Interface Science, Vol. 64, No. 1, March i5, 1978
ture-independent preexponential entropic factor accounts for the whole rate of the process. This is in good agreement with a nucleation mechanism and shows that, at variance with the case of arachidic acid (1) in which high cohesive energies among, hydrophobic chains are present, the separation takes place without any sudden breaking of the monolayer. The different distribution and the flexibility of the macromolecular substance are therefore conditioning the mechanism of collapse and the energies connected with it. (2) Ellipsometric measurements. Ellipsometric measurements on PMMA films, transferred by the previously described technique from the liquid to a solid support, were done for two values of the surface pressure held rigorously constant during the extraction process. At the pressure of 13.0 dyn cm -1 (a value lower than the 17.1 dyn cm -1 one corresponding to the lowest pressure at which the collapse takes place) a layer thickness of about 14 /~ was obtained confirming for PMMA the "horizontal" disposition of the two-dimensional quasi-rigid spirals already found by different techniques (2, 3). At the pressure of 27.1 dyn cm -a (at which we determined the second kinetics) the values obtained for the layer thickness--having an order of magnitude of 50 A - were less reproducible and differed from zone to zone of the layer. This lack of reproducibility and homogeneity may confirm the presence of a nucleation process. The value of the thickness, far higher than bimolecular, and the inhomogeneity of the layer do not satisfactorily agree with a mechanism of successive superpositions of rigid layers as suggested for monolayers of polypeptidic substances (11); on the contrary, an irregular growth of the collapsed massive phase might account for them.
Conclusions The above results allow us to conclude that, for monolayers of PMMA at the W/A interphase, the collapse process can be formally represented by a nonactivated nucleation and growth mechanism. While this behavior shows several general similarities with the behavior of arachidic acid, there are nevertheless also significant points of difference. These differences may be readily ascribed to the disposition and the energies of the macromolecules at the interphase. Accordingly it seems worthwhile to extend the present approach to elucidate the properties of related systems. This may then provide further information about the distribution of polymer molecules in the two-dimensional condensed phase. ACKNOWLEDGMENTS The authors feel deeply indebted to prof. E. Ferroni for helpful discussions and to C.N.R. for financial support.
NOTES REFERENCES 1. Gabrielli, G., Guarini, G. G. T., and Ferroni, E., J. Colloid Interface Sci. 54, 424 (1976). 2. Gabrielli, G., Ferroni, E., and Huggins, M. L., Progr. Colloid Polym. Sci. 58, 201 (1975). 3. Gabrielli, G., Puggelli, M., and Faccioli, R., J. Colloid Interface Sci. 37, 213 (1971); 41, 63 (1972). 4. Blodgett, K. B., J. Amer. Chem. Soc. 57, 1007 (1935); Blodgett, K. B., and Langmuir, I., Phys. Rev. 51, 964 (1937). 5. Gaines, G. L., Jr., "Insoluble Monolayers at Liquid Gas Interfaces," pp. 144-151. Interscience, New York, 1966. 6. Joos, P., Bull. Soc. Chim. Belges 76, 591 (1967); Demel, R. A., and Joos, P., Chem. Phys. Lipids 2, 35 (1968); Joos, P., and Demel, R. A., Biochim. Biophys. Acta 183, 447 (1969); Phillips, M. C., and Joos, P., Colloid Z. Z. Polym. 238, 499 (1969).
187
7. Garner, W. E., "Chemistry of the Solid State." Butterworths, London, 1955. 8. Young, D. A., "Decomposition of Solids." Pergamon, Oxford, 1966. 9. Avrami, M., J. Chem. Phys. 7, 1103 (1939); 8, 212 (1940); 9, 177 (1941). 10. Yin, T. P., and Wu, Souheng, J. Polym. Sci. C 34, 265 (1971). 11. Malcom, B. R., Polymer 7, 595 (1966); Proc. Roy. Soc. London A 305, 363 (1968); J. Polym. Sci. C 34, 87 (1971); Progr. Surface Membrane Sci. 7, 183 (1973). GABRIELLA GABRIELLI GIULIO G. T. GUARINI Institute of Physical Chemistry University of Florence Via Gino Capponi 9 Florence 50121 Italy Received November 17, 1976; accepted September 22, 1977
Journal of Colloid and Interface Science. Vol. 64, No. 1. March 15, 1978