Langmuir 1994,10, 1654-1656
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First-Order Transition in a Polymer Monolayer: Structural Analysis by Transmission Electronic Microscopy and Atomic Force Microscopy F. Boury,? A. Gulik,i J. C. Dedieu,t and J. E. Proust*>t Pharmacie Galbnique et Biophysique Pharmaceutique, Facultb de Pharmacie, 16 Boulevard Daviers, 49100 Angers, France, and Centre de gbnbtique molbculaire, CNRS, 91190 Gif sur Yvette, France Received February 2, 1994. In Final Form: March 30, 1994" Isotherm measurements and structural analysis of LB films have shown that a first-order transition appears on compression of a PLA50 monolayer. Transmission electronic microscopy and atomic force microscopy have revealed the coexistence of a condensed and an expanded phase. During the growth of the condensed phase, we observed the formation and the fusion of submicrometer domains with an overheight limited to about 17 A. This indicated that the expulsed lactic units associate in structured domains.
I. Introduction The isotherms obtained with the homopolymer of lactic acid (PLA50), a biodegradable polymer class, presented a large, well-defined plateau at high surface pressure (see Figure 1) which was attributed to a first-order phase transition.lI2 We have speculated that this transition was accompanied by the formation of microdomains. It was then interesting t o confirm the presence of t h e domains by a structural study of the monolayer. By coupling isotherm studies and structural analysis of LangmuirBlodgett (LB) sampling, it is possible t o have a better understanding on t h e modification of t h e film structure, on a microscopic scale, during the compression. Transmission electronic microscopy (TEM) appeared t o be a convenient technique t o precise the structural changes occurring in monolayers compressed at the collapse level.= R e c e n t l p i t has been shown by atomic force microscopy (AFM) the micromorphology of domains in the LC phase of stearic acid monolayer sampled by t h e LB method. I n this paper, we report the result obtained on LB films of PLA50 analyzed by TEM a n d by AFM. We discuss t h e micromorphology of t h e film during the transition phase. 11. Experimental Section Materials. The polymer was a pOly(D,L-laCtiC acid) stereocopolymer (PLA5O) obtained from CRBPA (URA CNRS 1465 Montpellier, France). According to the Vert classification,' it contained 50% L-repeating units. Its mean molecular weight (M,)determined by size exclusion chromatography was 41 600 and its polymolecularity index was kept in the range 1.6-1.9. The polymer was dissolved in dichloromethane at a concentration of 1 mg/mL. The water subphase was taken from a Millipore Milli-Q system. The experiments were carried out at 25 "C. Pharmacie Galenique et Biophysique Pharmaceutique. Centre de ghbtique moleculaire. *Abstract published in Advance ACS Abstracts, May 1, 1994. (1)Boury,F.; Olivier,E.;Proust, J. E.; Benoit, J. P.J. Colloidlnterface f f
Sci. 1993, 160, 1.
(2) Boury,F.; Olivier,E.; Proust, J. E.; Benoit,J. P. J.Colloidlnterface Sci. 1994, 163, 37. (3) Gulik, A.; Tchoreloff, P.; Proust, J. E. Chem. Phys. Lipids 1990, .55. - -, 341 - - -. (4) Tchoreloff, P.; Gulik, A.; Denizot, B.; Proust, J. E.; Puisieux, F. Chem. Phys. Lipids 1991,59, 151. (5)Kato, T.; Iriyama, K.; Araki, T. Thin Solid Films 1992,210/211,
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Figure 1. P A curves of a PLA50 monolayer spread at the air/ water interface. The arrows indicate the position of the film transfers studied by TEM (bold arrow) and by AFM (light arrow). Surface pressure was kept constant during the sampling. Isotherm Measurements and LB Technique. Surface pressure (+area (A) isotherms were performed by using a thermostated Langmuir film balance (Lauda FW2, Germany). The polymer was spread from dichloromethane solution (40 pL) on the maximum available area of the aqueous subphase (927 cm2)with an Exmire microsyringe. Isotherms were recorded at a constant rate (150 cm2/min). All details of the experiments are described in a previous paper.' The LB transfer was carried out on a freshly cleaved mica plate. The mica plate was firstly immersed into the aqueous subphase; after spreading the polymer (80 pL of the organic solution), the film was compressed until the desired surface pressure and then deposited at a constant surface pressure on the mica. The speed of the transfer was 5 mm/min. Transmission Electron Microscopy (TEM). The mica plates were mounted at room temperature on the stage of a Balzers BAF 301 freezeetch unit (used here only for shadowing).sp' Unidirectional shadowing was performed using platinum-carbon at an angle of 15". The replicas were washed and then observed in a Philips 301 electron microscope. The step thickness was determined from the shadowing length measured on the images multiplied by tan 15". Atomic Force Microscopy. We carried out AFM imaging in the noncontact mode with a commercial instrument (Nanoscope 111,Digital Instruments). The experiments were done with a cantilever 125 pm long and a monocristalline silicium tip. The angle of the cone was 20" with a final curvature radius of 10 nm. The linear rate of scanning was 1.8 Hz.
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(6)Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993,259, 213. (7) Vert, M.; Christel, P.; Chabot, F.; Leray, J. Macromolecular Biomaten'ak; Hasting,G.W.,Ducheyne,P.,Ed.;CRCPress: BocaRaton,
FL, 1984; p 119.
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111. Results 111.1. Isotherm Measurements. The general shape of t h e isotherm of PLA50 (Figure 1) is in favor of a 0 1994 American Chemical Society
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Figure 2. TEM images obtained on the LB films of PLA50 (a) monolayer transferred a t 15.2 mN/m (beginning of the plateau); (b) monolayer transferred at 15.6 mN/m (end of the plateau); bar = 0.2 pm.
condensed type monolayer as described by Gaines.8 For the largest areas, one observes a continuous increase of the surface pressure by compression. For an area of 20 A2 per lactic unit, an inflection point is observed. This is followed by a large, flat plateau occurring at high surface pressure (15-16 mN/m). Such a plateau is not frequently observed in surface pressure-area curves obtained for polymer monolayer~.~J~ It has been recently shown1',' that a copolymer of lactic acid and glycolic acid presented a less horizontal plateau with a smaller surface pressure. For PLA50, we explained the presence of the flat plateau by a first-order transition with formation of microdomains.192 A poorly compressible state appears on further compression, leading to the abrupt increase of the surface pressure. The apparent area of a lactic unit was estimated to be 2.8 A2. Hysteresis appeared only when the film was compressed after the end of the plateau.' 111.2. TEM and AFM Study. Electron micrograph of the PLA50 monolayer sampled by the LB technique are shown in Figures 2 and 3. The transfers were (8) Gaines,G.L.Insoluble Monolayersat Gas-LiquidInterfaces;WileyInterscience: New York, 1969; p 172. (9) Adamson, A. W. Physical Chemistry of Surfaces,3rd ed.; J. Wiley & Sons: New York, 1976; p 154. (10) Nakamae, K.; Takeya, T.; Fujimara, Y.;Sakai, I.; Matsumoto, T. J. Macromol. Sci. 1982, B21, 157. (11)Minones, J.; Iribarnegaray, E.;Varela, C.; Vila, N.; Conde, 0.; Cid, L.; Casas, M.Langmuir 1992,8,2781.
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Figure 3. TEM images obtained on the LB films of PLA50 (a) monolayer transferred a t 10 mN/m; (b) monolayer transferred a t 20 mN/m; bar = 0.2 pm. Table 1. Surface Pressures and Areas per Lactic Unit Corresponding to the LB Samples Studied by TEM surface pressure (mN/m) 10.2 15.2 15.6 20 18 10.8 4.8 2.8 apparent area (A2/lactic unit)
performed in four different stages of compression (Figure 1 and Table 1). Until the end of the plateau the area transferred at the solid/air interface was equal to the variation of area at the air/water interface. At the beginning of the plateau (Figure 2a), two types of objects with different height and size are observed (the surfaceoccupied by these objectswas estimated to be about 20% of the total surface). The smallest ones are smaller than 100 A. The biggest ones exist in large, flat domains with an apparent diameter of 0.3-1 pm; the step thickness was estimated to be about 17 A. Similar results were obtained by AFM (Figure 4). For the sampling at the end of the plateau (Figure 2b) the domains became interconnected. Their surface was about 50% of the total surface of the LB film. The step thickness was estimated to be about 17 A. The smallest domains increase in number. For the samplingat 10mN/m, before the plateau (Figure 3a), one observes a number of structures with a smaller thickness and with less defined outlines than the structures observed in the plateau. Figure 3b corresponds to the LB film sampled at 20 mN/m. The background appears homogeneous and is obtained very likely by the fusion of all the domains. It
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1656 Langmuir, Vol. 10, No. 6,1994 6 . 8 nu
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Figure 4. AFM images obtained on the LB films of PLAM transferred at 15.2 mN/m (beginning of the plateau).
appears to have some foldings and structures with a thickness higher than 100 A. These structures appear oriented parallel to the compression barrier.
IV. Discussion and Conclusion The results obtained by TEM and AFM give a direct visualization of the structural behavior of a PLA50 monolayer during its condensation. From isotherms measurements, the surface area measured at the inflection point before the plateau (20 A2/ lactic unit) corresponds probably to the limiting area occupied by one lactic unit. At the end of the plateau, it can be assumed that the apparent areas occupied by one lactic unit (2.8 A2)indicates that the compressed layer is formed by the stacking of five or six lactic units. The height of the objects measured in Figures 2 and 4 indicates the expulsion of polymer segments (probably a few lactic units). The condensed phase (small and bigger objects) is probably the result of polymer aggregation and could coexist with a more expanded phase (corresponding to the background of the images). The coexistenceof the two phases at the beginning of the plateau and the growth
of the condensed phase during the compression in the plateau (Figure 2b) are characteristic of a first-order transition in the PLA50 monolayer. The overheight of the condensed phase measured in Figure 2 is constant (17 A) as the proportion of the condensed phase changesfrom 20% to 50%. Such observations have been described during the compression of long chain fatty acids.6 The height difference between the LE and the LC phase is explained in such cases, by differencesof rigidity. In the case of the PLA50, the expulsion of polymer segments has to be considered. The interesting point is that this expulsion seems to concern a finite number of lactic units which associate in structured domains. The resultsobtained by samplingthe PLA5oat 10mN/m (Figure 3a) shows that the aggregation between the polymer segments occurs before the plateau. A t this pressure, the area occupied by a lactic unit (18 A2/lactic unit) is smaller than the limiting area; this explains the appearance of the domains. The smallest objects observed in Figures 2, 3a, and 4 could be nucleation sites for the growth of the biggest domains. For the highest pressure (Figure 3B),the background which became homogeneous,indicatesthat the monolayer is constituted of only condensed phase. The formation of tridimensional surstructures explains probably the thermodynamical instability of the system. It is probable that the rate of compression plays a role on the size and the shape of the domains. Relaxation measurements of the compressed monolayer are now in progress. Measurement (observation at the Brewster angle12)and X-ray reflectivity13or ellipsometry14could also provide informationon the thickness and the structure of the polymer monolayer. Acknowledgment. We thank E. Lepletut (Instrumat) for its technical assistance on the use of the Nanoscope 111. (12) Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991,62,936. (13) Bblorgey,0.;Tchoreloff,P.;Benattar,J. J.; Proust,J. E. J. Colloid Interface Sci. 1991,146,373. (14) Mann,E. K.; Langevin, D. Langmuir 1991, 7,1112.
