Langmuir 1995,11, 3119-3129
3119
Polymerized Monomolecular Films: Microscopic Structure, Viscosity, and Photopolymerization Kinetics R. Rolandi,*lt S. Dante,? A. Gussoni,? S. Leporatti,? L. Maga,' and P. Tundog Department of Physics, University of Genoa, 16146 Genoa, Italy, Department of Chemical Engineering, University of Genoa, Genoa, Italy, and Department of Environmental Science, University of Venice, Venice, Italy Received January 2, 1995. I n Final Form: May 9, 1995@ The photopolymerization of monolayers prepared at the gas-water interface with two styrene functionalized surfactants is studied and peculiar features of the film structures are described. When these films are irradiated with U V light, at constant surface pressure, their areas decrease monotonically as a function of the irradiation time. Phase separation and microscopic morphological changes induced by the polymerization process are investigated by using fluorescence microscopy and scanning force microscopy. During irradiation, domains of higher density and thickness are formed. The shapes and sizes of these domains, which are evidently the polymericphase, depend on surface pressure and irradiation time. The film viscosities are measured before and after polymerization and are related to the irradiation time and the mean sizes of the polymeric domains. Film surface area measurements and image analysis are used to obtain the area occupied by a bound monomer in the polymeric phase. A kinetics model of the photopolymerization, taking into account the peculiar features of the almost two-dimensional system, is used to fit the experimental results of the surface area variation with the irradiation time.
Introduction Polymerization is a promising method to confer mechanical resistance to thin molecular films used in surface modification and membrane preparation. Ultrafiltration, drug delivery, lubrication, wetting, adhesion, and construction of chemical, physical and biological sensing devices are some of the technological fields where polymeric molecular films offer real advantages. Furthermore, polymeric films occur in biological structures such as the cell walls of bacteria and the basal lamina of mammals. For these reasons the morphology and the physical properties of thin polymeric films formed by molecular layers are of interest to scientists involved in different research fields. Different methods, which generate films with different thicknesses and different microscopic and macroscopic structures, are described in literature. Polymerization of bilayers has been achieved in ve~iclesl-~ and planar membranes.5@Polymer layers anchored on gold have been obtained by using self-assembly methods.' Polymeric mono- and multilayers have been prepared with the Langmuir-Blodgett method using both amphiphilic polymers and monomers.8-11 Department of Physics, University of Genoa.
* Department of Chemical Engineering, University of Genoa.
Department of Environmental Science, University of Venice. Abstract published in Advance A C S Abstracts, July 1, 1995. (1) Gros, L.; Ringsdorf, H.; Scupp, H. Angew. Chem., Int. Ed. Engl. 8
@
1981,20,305. (2)Kunitake, T.; Nakashima, N.; Takarabe, K.; Nagai, M.; Tsuge, A.;Yanagy, H. J . A m . Chem. SOC.1981,103,5945. (3)Nome, F.; Reed, W.; Politi, P.; Tundo, P.; Fendler, J. H. J . A m . Chem. SOC.1984,106,8086. (4) Fuhrhop, J. H.; Mathieu, J. Angew. Chem., Int. Ed. Engl. 1984, 23, 100. ( 5 ) Benz. R.: Prass. W.: Rinesdorf. H. Angew. - Chem., Int. Ed. E n-d . 1982,21,368.' (6)Rolandi, R.; Flom, S. R.; Dillon, I.; Fendler, J. H. Prog. Colloid Polym. Sci. 1987,73, 134. (7)Lenk, T. J.;Hallmark, V. M.; Rabolt, J. F.;Erdelen, G.;Hiiussling, L.; Ringsdorf, H. S. Sixth International Conference on Organized Molecular Films, 1993,Tho-PM 11.1. (8) Hodge, P.; Davis, F.;Tredgold,R. H. Philos. Trans.R. Soc. London 1993,A330, 153-166. (9)Dubault, A.;Casagrande, C.; VeyssiB, M. J . Phys. Chem. 1976, 79,2254. ,
,
I
.
Monomeric Langmuir films can be polymerized by irradiation, most commonly with ultraviolet light. Depending on the monomer structure, polymerization can contract the film area, changing the molecular arrangement of the film and varying macroscopic parameters such as surface potential and v i s c ~ s i t y . ~ J ~ Fluorescence microscopy and scanning force microscopy (SFM) are capable of revealing the phase separation and the microscopic texture of polymerized monolayers.12,13 As described in this paper, we use these methods, and the measurements of some macroscopicparameters, to obtain better insight of the mechanism of the two-dimensional polymerization of surfactants functionalized with styrene groups. Polymerization kinetics is studied and compared with the classical model of radical polymerization. Two surfactants functionalized with styrene groups, bis[2-(n-hexadecanoyloxy~ethyll methyl((o/m/p)-vinylbenzy1)ammonium chloride (1)and bis(n-octadecyl)methyl((o/m/ p)-vinylbenzy1)ammonium chloride (21, have been used to prepare polymerizable monolayers at the gas-water interface. Photopolymerization of vesicles prepared from these surfactantspulled together some 10-20 aryl groups, thereby creating surface cleft^.^ An analogous result was produced by the photopolymerization of BLMs prepared from 1. Decreased transmembrane resistances were observed as a consequence of reducing the average area occupied by each surfactant molecule.6 The cleft formation in vesicles as well as the transmembrane resistance decrease are clues of nonuniform surface structure. In this paper we show that polymerization leads to two-dimensional phase separations.
Experimental Details Materials. Bis[2-(n-hexadecanoyloxy)ethyllmethyl((o/m/p)vinylbenzy1)ammonium chloride [n-C15H3iCOz(CHz)zlzN'[CH31(10)Miyano, K.; VeyssiB, M. J. Chem. Phys. 1987,87,3153. (11)Rolandi, R.; Paradiso, R.; Xu,S. Q.; Palmer, C.; Fendler, J. H. J . A m . Chem. SOC.1989,111,5233. (12) Rolandi, R.; Gussoni, A,; Maga, L.; Robello, M.; Tundo, P. Thin Solid Films 1992,2101211,412. Rolandi, R.; Dante, S.; Maga, L.; Robello, M. Prog. Colloid Polym. Sci. 1991,84, 273-274. (13)Leporatti, S.;Cavalleri, 0.;Rolandi, R.; Tundo, P. Langmuir 1994,10,1334-1336.
0743-7463/95/2411-3119$09.00/00 1995 American Chemical Society
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[CH&6H&H=CH2]Cl- (l),was prepared from bis[2-(hexa50 decanoy1oxy)ethyllmethylamine according to the method described in ref 14. The preparation of [CH~(CH~)~&O~CHZCHZ]ZNCH~HCI is 40 h described in ref 15. The whole synthesis of bis(n-octadecy1)E methyl((o/m/p)-vinylbenzy1)ammonium chloride (2) is reported 2 in ref 3. E v All the compounds and solvents were reagent grade and were 30 used without further purification. The fluorescence probe, E N-(lissamine rodhamine B sulfony1)diacylphosphatidylethanolVI VI amine (RPE), was purchased from Avanti Polar Lipid (BirmingEQ ham, AL)and used without additional purification. 20 0, Film Preparation. The monolayerswere formed in a circular 0 Teflon trough of a RCM2-T monofilmmeter (Mayer Feintechnik, L Gottingen, Germany). Usually, during compression, the film 3 10 area changes from 340 to 40 cm2. The Wilhemy plate, a rectangular piece offilter paper (-1.5 x 1cm2),was placed about 1cm away from the still Teflon barrier and 3 cm away from the trough rims. To prepare the films, the monomeric surfactant 0 and 0.3% (molar ratio) of the fluorescent probe were dissolved 0.50 1 .oo 1.50 2.00 in spectroscopicgrade chloroform (BDH Limited Poole, U.K.) at 2 a concentration of 2 mg/mL.. A microsyringewas used to carefully area (nm /monomer) inject appropriate amounts (5-2OpL) ofthis spreading solution 50 on water purified by a Millipore Milli-Q filter system (specific resistivity > 18 MB cm). The cleaning and control procedures have already been described elsewhere.ll The Teflon trough was thermostated at 40 E 23 f 1 "C. Surface pressure-area isotherms were obtained \ z through continuous compression at a rate of -1 cm2/s. The trough E was put in a laminar air flow hood (Steril Milan, Italy) with a v 30 filter for class 10 air quality. 2 The films were polymerized on the water surface in a nitrogen VI VI atmosphere by using a UVG-I1 Minerallight lamp, which L 0, generates light at 254 f 3 nm. The uncertainty is the half20 width, at the half-height, ofthe emission peak. The light intensity V on the film surface was approximately uniform and equal to about 0 c15 pWlcm2. During irradiation, the film surface pressure was 3 kept constant by a feedback system moving a Teflon barrier. VI 10 After irradiation, the film was deposited onto a 22 x 22 x 0.1 mm3 glass slide vertically drawing out the dipped slide at the speed of 3 mm/min and maintaining a constant surface pressure. 0 The glass slides were cleaned with chromic-sulfuric acid solution 0.50 100 150 2.00 and chloroform and thoroughly rinsed with purified water. The 2 ratio of deposition was sufficiently close to 1 in all the trials. area (nm /monomer) Microscopy. Fluorescence microscopy and imaging were Figure 1. Surface pressure-surface area isotherms of 1 and performed using an Olympus BHS microscope connected with a 2 monolayers spread at the nitrogen-water interface (2' = 23 C2400-08 SIT TV Hamamatsu camera. Images were acquired f 1"0.The reported value are the means of the results of five by a PC-AT personal computer via a PIP-1024B Matrox board measurements and the error bars represent the 90%confidence and processed using Eidoips software (EIDOSOFTMilan, Italy). intervals of the Student t distribution: (a) 1 monolayers, (e) SFM images were obtained in air by using Nanoscope I1 and monomer, (0)monomeric 1 0.3% ofRPE, (v)1 0.3%ofRPE I11 (Digital Instruments, CA) equipped with a 87-pm AFM scan after 120 s of UV irradiation at a constant pressure of 35 mN/ head (type J). The same samples prepared for fluorescence m; (b) 2 monolayers, (e)monomer, (0)monomeric 2 + 0.3% of microscopy were used. The slides were cut to fit into the W E ,(7) 2 0.3% of RPE after 180 s of W irradiation at 35 Nanoscope sample holder. Tips microlithographed on silicon mN/m. In this case the results of only one measurement are reported. nitride cantilevers with a force constant of 0.58 N/m (Digital Instruments) were used. Tip checking and XY scanning calibrareduction as a function of the time, t . The flow through the canal tion were routinely performed imaging a diffraction grating. 2 was recorded as calibration in the micrometer range was achieved by measuring the diameter of Sephadex beads. SFM images were analyzed by Q = AA/At using the Nanoscope Software and the STWAFM Graphics the viscositywas calculated from the flow using Joly's equation.16 Processor purchased from Advanced Surface Microscopy, Inc. Numerical CalculationMethod. The mathematical model Surface Viscosity Measurements. The surface viscosity of the polymerization kinetics consists of a system of nonlinear for the film was measured using the "canal method". One of the differential equations in which the chemical kinetics constants barriers of the RCM-2 monofilmmeter was provided with a 1mm are adjustable parameters (vide infra). wide and 2 cm long slit. After reachinga chosen surface pressure The kinetics constants have been calculated by fitting the on compression, the slit was opened and the film was allowed to numerical solution of the system to the experimental data. The flow through the slit into the other part of the trough, where the DAFNE17 software package was used. Following the method surface pressure remained negligible (n~ 0 . mN/m). 1 With the proposed by Bardl8 for standard dynamic models, the objective film surface pressure constant, the film area A undergoes a function which, in our case, is the sum of squares of the residues
-
-
t i +
+
+
(14)Reed, W.; Guterman, L.; Tundo, P.; Fendler, J. H. J . A m . Chem. SOC. 1984,106, 1897-1907. (15)Kippenberger,D.; Rosenquist, K.; Odberg,L.;Tundo,P.;Fendler, J. H.J . Am. Chem. SOC.1983,105,1129-1135.
(16)Joly, M. J . Phys. Rad. 1938, 9,345-351. (17) Dovi, V. G. Proceedings of SIM0'88, Tolosa 1988, pp 70-71. (18) Bard,Y.Non linearparameterestimation; AcademicPress: New
York, 1974; pp 221-230.
Polymerized Monomolecular Films
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Figure 2. Photomicrographs of 2 monolayers a t various irradiation times and polymerization surface pressures. The irradiation times are 90,300, and 1200 s, increasing from left to right. Polymerization surface pressures are 5,15, and 35 mN/m, increasing from bottom to top. The magnification is the same for all the images and the bar (bottom right) corresponds to 10 pm. of the surface area reduction, was obtained as a function of adjustable parameters using sensitivityequations. The objective function is then minimized by the Marquardt algorithm which uses the derivatives of the objective function with respect to the parameters. To guarantee high computational accuracy, the aforementioned derivatives are computed using symbolic differentiation.
Results Surface Pressure-Area and Fluorescence Microscopy. As already reported, the compounds l and 2 form Langmuir films which can be polymerized by W irradiation." .Figure 1shows isotherms of monomeric 1 and 2 at T = 23 f 1 "C with and without 0.3% (molar ratio) RPE. The presence of the fluorescenceprobe slightly affects the isotherms and for a great part of the curves the variation is within the range of experimental error. Polymerization is accompanied by the reduction of the film areas and such a reduction is not affected by the presence of the fluorescence probe. When recorded immediately after the irradiation has stopped, the isotherms of the polymerized films tend to shift toward larger areas with repeated cycles of compression and decompression until stable values are reached. For this reason, the isotherms of the polymerized films, shown in Figure 1,have been recorded after the films were allowed to relax for a few minutes a t zero surface pressure. Monomeric films of 2 show collapses at about 45 mN/m and molecular areas of 0.55 nm2/monomercorrespond well with what is reported in ref 11. A few differences, with respect to what is reported in ref 11,have been observed in the behavior of 1 films. The real collapse could not be observed since loss of the film over the trough rims occurred at a surface pressure of -40 d / m . In ref 11collapse was reported at 67 d / m . The isotherms obtained on a water subphase are similar to those on the 5 mM NaCl solution
reported in ref 11and no strong dependence on the ionic strength was observed. The spectroscopic and chromatographic analysis of the sample used do not reveal any difference from the sample used in ref 11. Impurities in the chemicals used for the preparation could be responsible for these differences. Qualitatively the effect of U V irradiation on films formed with 1 and 2 is very similar. Keeping the surface pressure constant and the irradiation uniform, the areas occupied by the films decrease monotonically with a quasiexponential behavior. If irradiation is stopped the decrease ceases. On the micrometer scale, the morphology changes of monolayers undergoing polymerization are provided by fluorescence microscopic images. Figure 2 shows photomicrographs of two films, obtained with the fluorescence microscope, at different pressures and differentirradiation times. Similar images have been obtained also for surfactant 1.12 The images of films at the air-water interface, obtained in control inspections, have the same features. Such images strongly resemble those of monomeric monolayers undergoingphase separations and can be interpreted in the same way. The fluorescence probe partitions in the liquid phase and the domains of the more condensed phase, which lack in fluorescence probes, appear darker. The correlation of the growth of the dark domains with the irradiation time and the shFinking of the film area (vide infra) identify the dark domains as a phase rich in polymeric molecules. The appearanceofbright edges at high surface pressures is due to accumulation of the fluorescent molecules on the borders of the two phases. At a high rate of polymerization, the rate at which the probe molecules are extruded from the polymeric clusters is faster than their diffusion rate in the monomeric phase and they accumulate on the
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I
Figure 3. Micrographs of 1 monolayer. Kept in the dark for 0,1800, and 3600 s at n = 35 rnN/m and T = 23 f 1 "C. Bar is as in Figure 2. Times increase fiom top to bottom. borders of the two phases. Separated phases are present in unpolymerized films at a surface pressure of more than 10mN/m (Figures 3 and 4). At the surface pressure of 10 and 15 mN/m almost no dark domains are visible immediately after the monolayer formation. Eventually, dark domains grow very slowly to cover a few percents of the monolayer area after 90 min. Instead, at higher pressures, there are dark domains immediately after the monolayer formation but the evolution of their sizes is very slow, as shown in Figure 3. A quantitative analysis of the fluorescence images is reported in Figure 4. In Figure 4a-cy the increase of the dark area with the irradiation time is shown for three different surface pressures. Note that a t surfacepressures of 15 and 35 mN/m there are dark areas at t = 0 and that only a few minutes of irradiation is necessary for the dark areas to cover a large part of the film surface. The qualitative indication of Figure 2 that larger domains are easily formed a t high surface pressures is quantitatively confirmed by the histograms of the distributions of the dark domain areas (Figure 4d). The histograms shown in Figure 4e indicate that at the beginning of the polymerization process a great number of small polymeric domains is formed. Subsequently, a lower number of larger domains are formed. This indica-
tion is confirmed by the decrease of the mean distance of the domain centers of gravity (result not shown). All the images, shown up to now, belong to monolayers deposited immediately after irradiation has stopped, at the same pressure at which they have been polymerized. There is a slow evolution to roundish shapes if the monolayers are kept at the gas-liquid interface (Figure 5). The higher the surface pressure, the slower the evolution. It is accelerated by repeated compression and decompression of the film. When deposited on glass slides the monolayers are very stable and the shapes of the domains remain unchanged even after several months. In preliminary experiments it has been observed that the domains of monolayers deposited on mica for SFM inspection change their shape over the time. Scanning Force Microscopy. Scanning force microscopy investigations of the polymerized monolayers provide images resembling those obtained by fluorescence microscopy. The samples of films irradiated at surface pressures of 5,10,and 35 mN/m, for 5 and 10 min, show domains with shapes and sizes that are very similar to those shownby the fluorescencemicroscopy images (Figure 6). For the sake of comparison an image is shown of a film deposited at the same conditions but without RPE (Figure 6d). In SFM images the polymeric domains appear light owing to the gray scale which makes lighter tones correspondto zones of a higher level. Sections, as reported in Figure 6e, show that there are two distinct levels: one corresponding to the polymeric domains, the other corresponding to the monomeric areas. SFM images reveal the presence of light spots, with diameters smaller than -0.1 pm, dispersed in the background. The fact that the height of the spots is similar to the level of the polymerized phases suggests that they are clusters of polymeric molecules. The presence of two distinct levels is also confirmed by the statistical analysis of the heights measured with respect to an arbitrary reference level. Bimodal distributions, as shown in Figure 7,are obtained for all the images comprising both polymeric and monomeric phases. It is extremely important to point out that this kind of analysis does not discriminate between large domains and small clusters: both contribute to form the peak of the higher levels. In most images the two peaks of the height distribution are rather well separated, and we have assumed that the distribution is the superposition of two Gaussian distributions. The difference of the modas has been considered the difference of the mean levels of the polymerized and unpolymerized parts of the film. These level differences do not vary significantly with the polymerization conditions and the mean value obtained from 40 images is 0.93 f 0.05 nm. The roughnesses of the large polymeric domains and background were separately measured by analyzing similar areas in different samples. The surfaceroughness is the root mean square of the heights with respect to a reference level. For the comparison the mean values of 15 images are considered, while the 95% confidence intervals of the Student t distribution are taken as uncertainties. The roughness of the large polymeric domains seems to be independent of the polymerization condition. For instance, it is 0.29 f 0.07 nm in samples irradiated for 5 min at the surface pressure of 5 mN/m and 0.28 f 0.01 mn in samples irradiated for 15 min a t 35 mN/m. The small polymeric domains enhance the background roughness, which is 0.5 f 0.1 nm in samples irradiated for 5 min a t 5 mN/m and it decreases to 0.29 f 0.02nm in samples irradiated for 15 min at 35 mN/m. This decrease confirms the visual impression that the
Langmuir, Vol. 11, No. 8, 1995 3123
Polymerized Monomolecular Films 100
I
1
ao
,
I
,
I
I
~ f =SmN/m n=+lOmN/m n=35mN/m
60
40
20
-0
.n
n
500
1000
1500
2000
2500
S (Nm')
t= 90 s t= 300 s t=1200 s
i
Q)
2
50
40 30
20 10
0
400
800
1200
0
0
500
1000
1530
ZCGG
2500
3000
t (sec.) S (/*ma) Figure 4. Analysis of the fluorescenceimages. (a,b, and c) Growth of the dark area, in 1 (0)and 2 (0)monolayers. The percentage of the dark area on the total area of the image is reported as a function of the irradiation time, at three different polymerization surface pressures: 5,15,and 35 mN/m. The points are the mean values on 10 images. For the sake of clarity the error bars are not reported. The 90% confidence intervals of the Student t statistics correspond to uncertainties of 20-30% of the percentages of the dark areas. (d and e) Distributions of the areas of the dark domains of 2 monolayers. (d)Area distributions at three different polymerization surface pressures at 600 s of UV irradiation. (e)Area distributions at three different irradiation times at a surface pressure of 35 mN/m. Similar distributions are obtained for 1 monolayers. number ofthe small domains decreases as polymerization and condensation continue. Higher magnification images show the small domains and the particulate structures ofthe large domain surface, which is evident next to the domain border (Figure 8a). An area with high density of small polymeric domains is shown in Figure 8b. Viscosity. Monolayers at the gas -liquid interface are resistant to applied shear stress in the plane ofthe surface and exhibit a two-dimensional surface viscosity. The canal method, which is the two-dimensional analog of the Ostwald viscometer for bulk liquids, was used to measure the viscosity of the unpolymerized and polymerized films. The monolayer moving under the effect of a surface pressure gradient carries some of the underlying water as a consequence of the lack of slippage between monolayer molecules and the subphase. The following equation, introduced by Joly,lGwas used to consider this effect:
pressure difference, 1 is the canal length, A is the bulk viscosity ofthe water subphase (0.01poise), D is the canal width, 7 is the surface viscosity, andA is a semiempirical parameter, which is assumed to be independent of the substance forming the film, but it is a function of the channel width. From the physical point of view A can be considered as the reciprocal of the thickness of the water layer dragged by the film. If the subphase drag is neglected, eq 1 yields the two-dimensional analog of the Poiseuille equation:
where Q is the flow through the canal, An is the surface
(19) Relini, A.; Ciuchi, F.;Rolandi, R. J. Phys. ZZ 1995,5, 1-10,
Q = AzD3/12~1
(2)
Furthermore, the Joly equation approximates the Poiseuille equation in the limit of very narrow channels. A was calculated to be 18 f 3 cm-l for subphases at the pH and temperature conditions used in this work (pH = 5.5, T = 23 "C)and for a canal 1 mm wide. The calculation was carried out by measuring the flows of two different surfactants and two different channel widths (1 and 2 mm) and numerically solving the four-equation system obtained from eq l . 1 9
3124 Langmuir, Vol. 11, No. 8,1995
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tion of the area must be also introduced and the resulting kinetic equations are:
.. .
dM-MdA -----
A dt
dt
I
'
I M - k ,MAR
-_-m dt - DACdt I A + ( l-@)IM -_-dR -
CIA
(3) (4)
+ @IM - k ,R2r
dt A dt where M is the monomer number, A is the monolayer area, Iis the initiation constant depending on the radiation energy adsorbed by the monomer, k, is the propagation rate constant, D is the dead monomer number, 4 is the fraction of the irradiated monomers transformed in radicals, R is the total number of polymer radicals, and kt is the termination rate constant. Assumingthat every double-bonded monomer occupies an area aM larger than the area a, occupied by the saturated monomer, the following equation is true:
A(t)= (uM - u,)(M
+ D ) + Mourn
(6) MOis the double-bonded monomer number at t = 0. Mass conservation provides
M
+ D + m = Mo
where m is the saturated monomer number. The values of a M can be inferred directly from the isotherms of the monomeric films, while a, has been measured by comparing film areas with the areas of the dark domains obtained from fluorescence images. Assuming that, in a fluorescence image, the ratio between the dark area and the total area is equal to the ratio between the area of the polymeric phase and the total area of the film, the following relation may be used
Figure 5. Micrographs of a 2 monolayer irradiated for 180 s a t n = 20 mN/m and then kept in the dark for 0,1800,and 3600 min. Times increase from top to bottom. Bar is as in Figure n
In Figure 9 surface viscosities of the two monomeric compounds are reported as a function of the surface pressure. In Figure 10a the surface viscosities of 1 and 2 monolayers polymerized a t 35 mN/m are reported as a function of the irradiation time. The viscosities were measured at 4 mN/m. In Figure 10bthe surface viscosities are reported as a function of the mean surface of the dark domains. For both compounds the viscosities remain almost constant until critical values of the domain areas, which are 2000 ,um2for 2 and 3300 pm2 for 1, and then they increase very quickly. Monolayer Photopolymerization Kinetics. The classical kinetics of the free radical polymerization is described in terms of initiation, propagation, and termination.20 Since fluorescence images show that residual unpolymerized domains remain after long irradiation time, we have introduced a side reaction producing a dead monomer (D)parallel to the initiation. Owing to the considerable reduction in area, the time-dependent varia~
where Ai is the area of the image, A, is the total area of the dark domains in the image, and A0 is the film area at t = 0. When the experimental values of AdAi are plotted as a function of (A(t)- Ao)/A(t),slope a, can be obtained from the regression line, provided thataM is known (Figure 11). The fit of the experimental area reductions as a h c t i o n of time, using eqs 1, 2, 3, and 4 with I, 4, k,, and kt as adjustable parameters, does not produce a satisfactory result. The propagation and termination rate constant k, and kt are indeterminated. Reed et al.14 suggest that in the two-dimensional polymerization, occurring in vesicles prepared with 1, the propagation process could be described with a first order reaction due to the low mobility of the radicals in a two-dimensional lattice. Furthermore, Budde and Wulkov20suggested that the termination coefficient ktcan be modeled as proportional to the square root of the reciprocal of the viscosity coefficient. Since the polymeric monolayer is a very viscous mean, kt might be small. A more efficient termination process should be the reaction of the radical with oxygen, solvent, and/or any impurity present. This process can be better described by a first-order reaction. The propagation and termination terms are then modified in
&pR and &&
(8)
~~
(20) Budde, U.;Wulkow, M.Chem. Eng. Sei. 1991,46,497-508.
The modified equations better fit the experimental data
Langmuir, Vol. 11, No. 8,1995 3125
Polymerized Monomolecular Films
Figure6. Scanning force microscope images of 2 monolayers transferred on glass slides after having been irradiatedfor different times ( t )at different surface pressures (n):(a) n = 5 mN/m, t = 900 s; (b) n = 10 mN/m, t = 900 s; (c) n = 35 mN/m, t = 900 s; (d) n = 10 mN/m, t = 300 s but without RPE;(e) n = 10 mN/m, t = 300 s; the vertical section along the traced line is shown.
1.0 -
0.8 0.6 0.4 .
0.2
.
0.0
'
I
I
1
I
I
0
1
2
3
I
Depth (nm)
Figure 7. Histogram of the depths with respect to the highest of a 2 monolayer level within the considered area (0.6 x 0.6pm2) irradiated for 900 s at n = 5 mN/m. The P portion belongs to the polymerizeddomains and the B portiontothe unpolymerized background. and yield the numerical value reported in Table 1. Figure 12 provides examples of the fitsof the experimental data for compound 1 at three different polymerization surface pressures. Figure 13 reports the behavior of the number of the double-bonded monomers, M,the saturated monomers, D,and the radicals,R, as a function of the irradiation time. As expected, the decrease of M is parallel to the area reduction (see Figure 12). There is a fast formation and a slow reduction of the radicals. Note that R is 1or 2 orders of magnitude smaller than M.
Discussion Microscopy Inspection. The main qualitative result, provided by both fluorescence and scanning force microscopy, is that film irradiation leads to phase separation. Sincecontrol experimentshave shown separated phases also in nonirradiated 1 and 2 films, it is necessary to point out the differencesbetween the pressure-induced and Winduced phase separations. In both kinds of films, spots of a condensed phase, visible with fluorescence and scanning force microscopy, are formed at surface pressures higher than 210mN/m-see Figure 3and the initial values of the plots in Figure 4b,c. This phase, which is more easily formed in 1 monolayers, occupies larger areas at higher pressures, but it never reaches the extension obtained in the irradiated films. Furthermore, in the dark, at constant pressure, it evolves very slowly, and its increase is negligible with respect to what is achieved during W irradiation. At this stage of our observation, it is not possible to say if the condensed phase, observed in nonirradiated films, is monomeric or polymeric, since a small degree of polymerization could be caused by ambient light, thermal activation, surface pressure, and, in fluorescence microscopy, exciting light. The evidence that the UV light causes the phase separation is the following. In the presence of W irradiation, the increase of the condensed phase is much faster and phase separation occurs also at pressures lower than 10 mN/m (Figure 2). The condensed phase increase clearly depends on the irradiation time (Figure 4) and there is a well-proved correlation between the area reduction, caused by irradiation, and the increase of the condensed phase (Figure 11). Furthermore, in fluorescence microscope inspections of films on the aqueous subphase, it was observed that the domains, formed at constant area by irradiation, kept growing also after the surface pressure had been decreased to about 0 mN/m.
Rolandi et al.
3126 Langmuir, Vol. 11, No. 8, 1995
Figure 8. High magnification images of 2 monolayers irradiated for 5 min at n = 5 mN/m: (a)border zone of a large domain; (b) clustering zone of small domains. Table 1 (a) Parameters of the Kinetics of 1 Monolayers Polymerization nP (mN/m) 5 10 15 20 35
aM
(nm2/mol)
1.17 f 8 1.00 f 5 0.88f 4 0.78 f 4 0.54 f 5
am
(nm2/mol) 0.56 f 13 0.37 f 6 0.46 f 6 0.37 f 4 0.26 f 4
I (10-5s-1) 142 f 16 118 f 9 117 f 14 82.0 f 0.3 280 f 40
kt
&P
@
(10-48-11 120 f 60 250 f 70 300 f 10 1806 f 6 undeter
0.39 f 0.04 0.38 f 0.03 0.68 f 0.06 0.35 f 0.01 0.66 f 0.06
x2
(10-48-1) 80 f 30 140 f 20 260 f 70 279 f 11 undeter
5.3 x 4.1 x 2.3 x 2.8 3.5 x
10-4 10-3 10-3 10-3
(b)Parameters of the Kinetics of 2 Monolayers Polmerization
5 10 15 20 35
1.21 f 4 1.06 f 2 0.96 f 4 0.89 f 4 0.73 f 6
0.42 f 5 0.30 f 3 0.24 f 6 0.27 f 8 0.18 f 3
144 f 3 94f 1 84.2 f 0.2 100.3 f 0.3 64.7 f 0.4
The general features of the SFM images confirm the results of the fluorescence microscopy and prove that the two techniques reveal the same phenomenon. Furthermore, SFM images indicate that the condensed phase domains are thicker than the background phase. This is an expected result, since, previously, the increase of the mean thickness of the polymerized monolayer was deduced from elipsometric measurements. The thickning is caused by the reorganization of the film two-dimensional order. The monomers, being bound in the polymerization process, either assume a more vertical orientation or partially overlap. Though the existence of a thickness difference between the monomeric and the polymeric phase is very probable, the measured difference could be larger than the real value. In fact, the pressure of the SFM tip acting on the film might also be involved. In the air, the pressure of a tip, with a curvature radius of 20 nm, is around lo5N/m2. With these pressures there is some degree of penetration of the tip in organic samples like Langmuir-Blodgett films.21 If the tip penetrates more deeply in the softer monomeric phase, a higher difference in level between the two phases is recorded. (21)Weisenhorn, A. L.;Egger, M.;Ohnesorge, F.; Gould, S. A. C.; Heyn, S. P.; Hansma, H. G.; Sinsheimer,R. L.; Gaub, H. E.; Hansma, P. K. Langmuir 1991, 7,8-14.
d
-
1 '
6.8 x 1.8 x 4.2 x 8.4 x 10-4 3.5 10-3
B'PP
Q,
c
2
69 f 5 887.4 f 1.4 222.2 f 0.5 5020 f 60 6950 f 130
63 f 6 133 f 2 550 f 1 118 f 1 1800 f 40
0.37 f 0.01 0.3880 f 0.0003 0.234 f 0.0003 0.7215 f 0.0001 0.650 f 0.002
0
Omo
0 '
0
I
I
I
I
I
I
1
5
10
15
20
25
30
35
-
l
40
surface pressure (mN/m)
Figure 9. Surface viscosities of 1 (0)and 2 (0)monomeric film as a knction of the surface pressure. The bars are the 90% confidence range of the Student t distribution on 5- 10 values.
Owing to the styrene group, the minimum area per monomer, for both the compounds, should be about 0.50
Langmuir, Vol. 11, No. 8, 1995 3127
Polymerized Monomolecular Films 0.5
7
I
I
I
I
I
I
Ap
51*
.-
Ai b e
0.4
l
i
0.3
0.2
I Oi 0
I
I
I
I
I
10
20
30
40
50
0.1
J
0.0 -1.2
60
irradiation time (min) 7, A
.'
I
I
-0.8 - 0 . 6 -0.4
-0.2
0.0 0.2
A(t)-A(O) 1
I
A(t)
Figure 11. Ratio of the surfaces of the dark area (A,) and the total area of a fluorescencemicroscopy image (Ai) as a function of the surfaceof a 2 film duringUV irradiation. Polymerization pressure = 10 mN/m. A similar plot has been obtained at the various polymerization pressures for both compounds 1 and 2. The continuous line is the linear regression line of the experimental data.
'5 5 c
H i: .->
P
0 1 2
P
a
0
-0
0 n= 15mN/m A n = 35mN/m
1
-0 2
p
0 1 0
-1.0
I
I
I
1000
2000
3000
1
4000
d
-01
-04
2
polymeric domain mean area (pm ) Figure 10. (a) Surface viscosities of 1 (0) and 2 (0)films as a function of the irradiation time. The bars are the 90% confidence range of the Student t distribution on five values. (b) Surface viscosities of 1 ( 0 )and 2 (0)films as a function of the mean areas of the dark domains.
nm2/mol-the collapse area for 2 is about 0.55 nm2/mol. Since the areas occupied by the bound monomers are smaller than these limiting areas for a surface pressure larger than 10 mN/m, some overlapping of the monomers should occur during polymerization. An indication of this fact is that the roughness (root mean square) of the monomeric film, deposited on a glass slide, is 0.17 f 0.01 nm while that of the large domains is within the range 0.24-0.32for different irradiation times and polymerization pressures. The SFM images of monolayers not containing the fluorescence probe (Figure 6d) are similar to those of monolayers which do contain the probe. Differences seem to occur in the structure ofthe borders which appear more ragged. But these differences were not thoroughly studied because of their poor reproducibility. Two other important facts are revealed by the SFM higher resolution: the presence of small domains of the condensed phase, with diameters smaller than 0.1 pm, and the particulate structure of the polymeric domains. Small domains, similar to those shown in Figure 8, have been observed by Ringsdorf and collaborators in films of stearic acid on a subphase containingpoly(ethylenimine).22
-3 5
-0 6
'OOG
2500
3000
4000
5OCO
time ( s )
Figure 12. Examples of the fits of the experimental data (symbols)related to compound 2with the solutionof the kinetics equations (lines). "he surface reductions @(t)- Ao)/Ao of 2 films plotted as a function of the irradiation time at three different polymerization pressures: V, R = 5 mN/m; 0, n = 15 mN/m; A, R = 35 mN/m. For the sake of clarity, the fits of single experiments for each polymerization pressure are presented. Usually three experiments for each pressure were performed and all the results fitted. In the inset the data are reported on an expanded scale. As these authors do, we cannot exclude that these small
domains are formed during the deposition process, but there are clues that they contribute to form the largest domains. In a few images their density seems to decrease near the borders of the largest domains, while the particulate structure of the largest domains, more evident at the borders, seems to suggest that they are formed by the clustering of the small ones. Also, the smallest domains observed are very probably clusters of oligomers. In fact, the mean number of monomers per macromolecule (22) Chi, L. F.;Anders, M.; Fuchs,H.; Johnston,R. R.; Ringsdorf, H. Science 1993,259,213-216.
3128 Langmuir, Vol. 11, No. 8, 1995
Rolandi et al. pressures, the films may assume a non-Newtonian behavior, resulting in a decrease of the apparent viscos-
C
1000
2000
3000
4000
I
1
time (s)
80
I
1
1
i 0 0
1000
2000
3000
4000
time ( s )
Figure 13. Behaviors of the amount of the chemical species during the polymerization process. Compound 2, n = 15 mN/ m. (a)M = monomer, D = inactive monomer, R = radicals. (b) The curve of the radical amount is also reported on an enlarged scale.
was estimated to be between a few units and a few hundreds, both for compound 1 and 2.3J1J4 Viscosity. As already observed in Langmuir films,1° polymerization effects the surface viscosity. Following other authors, in the analysis of the flow data, we have assumed that the films are two-dimensional Newtonian fluids. This fact is probably true for the unpolymerized films, since the viscosity behavior is similar to those of other monomeric surfactants, like oleic acid or phospholipids, forming liquid expanded films.22 In our experimental system, the surface pressure at the outlet of the canal is kept approximately at zero; therefore, the pressure gradient An is equal to the film pressure n. For this reason the linear dependence of the viscosity on the surface pressure (Figure 9) seems to be caused by the dependence of the state of the film on the surface pressure rather than the dependence of the viscosity on the stress rate. For the polymeric films these considerations are no longer valid and we have restricted the surface viscosity measurements to low surface pressure (5 mN/m) in a range where Newtonian behavior is more probable. Notwithstanding this precaution, the value of the polymeric films viscosities must be considered with caution. The apparent surface viscosities, measured at 4 mN/m, of films polymerized at 35 mN/m, increase with the irradiation time until a maximum, which is higher for 1 films, and then such viscosities decrease (Figure loa). For high polymerization degrees, and also at low surface
It is interesting to note the behavior of the surface viscosity as a function of the mean area of the polymeric domains. The viscosity remains almost constant until reaching a critical value of the mean area of the domains. At this value there is an abrupt rise in the viscosity (Figure lob). The abrupt rise of the viscosity-domain mean size curve can be explained making the reasonable hypothesis that the monomeric and polymeric parts of the film have very different surface viscosities. When there are only small polymeric domains, the low viscosity of the monomers prevails since the small polymeric domains are carried by the monomer flow. When the large domains occupy almost all the film surface, their viscosity prevails, since polymeric film itself must flow through the canal. This rise has no equivalent in the surface viscosityirradiation time curves. These results are difficult to explain because of the complexity of the system being studied. The partially polymerized film is made up of two different parts with different viscosities, one formed by the monomers and the other formed by polymers ofvarious molecular weights. The ratio between the two parts depends on the irradiation time. Furthermore, it is quite likely that the viscosity of the polymeric part depends on the distribution of the molecular weights, which also depends on the irradiation time. For a comprehensive description of the behavior of the viscosities as a function of the irradiation time and the polymeric domain mean area, the dependence of the film viscosity on the distributions of the domain size and molecular weight should be known. For want of exhaustive theoretical models a qualitative interpretation is tried as a stimulus for further work. The viscosity depends almost linearly on the irradiation time. See the increasing part of the plots in Figure loa. The domain mean area varies roughly exponentially with the irradiation time. The experimental plots are very similar to those of the dark area percentage of Figure 4. Therefore, the viscosity depends on the logarithm of the domain mean area, and more precisely depends on ln(A(t)- AJA,, whereA is the domain mean area at time t and A, is the asymptotic value ofA(t) fort -. The logarithm argument assumes values between 0 and 1 and the logarithm rapidly decreases in this range. This fact justifies the fast rise of the plots in Figure lob. Kinetics. In a previous study of the photopolymerization of 1 and 2 films, the area reduction as a function of the irradiation time was fitted with a monoexponential fhction, assuming that first-order kinetics could describe the polymerization process." This kind of fit works well at low surface pressures but it is inadequate at high surface pressures. Furthermore, it is not justified if the change of the film area is considered together with the experimental evidence that the monomers are not completely consumed. Considering the self-consistency of the values ofl, 4, k,, and kt for 1 and 2, the proposed model seems to be a suitable description of the polymerization kinetics up to surface pressures of 15 mN/m. In this pressure range only the value of kt at 10 mN/m for 2 seems to be too high (Table 1).At 20 and 35 mN/m the model provides unreasonably high values for kt for 2 and is not able to determine k p and kt for 1.
-
(23) Sacchetti, M.;Yu, H.; Zografo, G . Langmuir 1993,9,21682171. (24) Wilkinson, W.L. Non-newtonianfluids;Pergamon Press: 1960, London, 1960; pp 5 and 28.
Langmuir, Vol. 11, No. 8, 1995 3129
Polymerized Monomolecular Films At these pressures the monomer conversion and domain formation are very fast (Figure 4) and the film viscosity is very high (Figure 9) and it is further increased by the polymerization. Therefore it is possible that k, and kt, due to the fast change in the physical condition of the film, may also change with time.
Conclusions Surfactants functionalized with styrene groups form Langmuir films which can be polymerized by UV irradiation. During polymerization at constant surface pressure, the films shrink. The fluorescence microscopy and SFM inspections reveal that a phase separation occurs as a consequence of the UV irradiation. The polymeric phase is denser and thicker than the monomeric phase. The morphology of the domains of this phase resembles the morphologyof the liquid condensed domains of monomeric films. Scattered domains, with linear dimensions smaller than 0.1 pm, which are not visible with the fluorescence microscope, are shown by SFM images.
The areas per monomer in the polymeric phase are smaller than the cross-section area of the two alkylic chains. This fact suggests that the two-dimensional order of the monomeric film is lost during the polymerization and a puckered monolayer is formed. The apparent surface viscosities of the films increase with polymerization up to maxima values and then decrease. There are critical values of the polymeric domain mean sizes for which the viscosities increase rapidly. A comparison of the experimental data of the area reduction as a function ofthe UV irradiation time with the radical polymerization kinetics model highlights a few peculiar aspects of the polymerization process in an almost two-dimensional system.
Acknowledgment. This workwas partially supported by MURST (40% and 60%funds) and by CNR “Programma Finalizzato Chimica Fine”. The technical assistance of Mr. C. Fucilli has been particularly appreciated. LA9500011
