7708
Langmuir 1999, 15, 7708-7713
Langmuir and Langmuir-Blodgett Films of a Partially Esterified Carboxymethyl Pullulan Ste´phane Alexandre,* Ve´ronique De´rue, Sergio Gomes-Ferreira, Jovanka Huguet, and Jean-Marc Valleton UMR 6522 CNRS/Universite´ de Rouen, Universite´ de Rouen, 76821 Mont-Saint-Aignan, France Received February 19, 1999. In Final Form: June 28, 1999 A carboxymethyl pullulan with 1.4 carboxymethyl functions per anhydroglucose unit has been esterified with bromododecane. The number of dodecyl chains per anydroglucose unit calculated from elemental analysis is 1.15 ( 0.09. The partially esterified carboxymethyl pullulan is able to form Langmuir films. Pressure-area isotherms are reproducible, and the film is characterized by a very high stability. IR spectroscopy analysis of Langmuir-Blodgett films prepared on subphases with various pH confirms that the esterification was not complete. Scanning force microscopy images of a monolayer transferred onto muscovite show the great homogeneity of the monolayer. While the monolayer transferred on muscovite is fragile, multilayers transferred on CaF2 slides are resistant to mechanical and chemical treatments.
1. Introduction Langmuir-Blodgett (LB) technique consists of transferring Langmuir monolayers from the air-water interface onto a solid support. In general, the LB films in the more condensed states are characterized by a molecular order similar to that found in crystals.1 Because of these properties, LB films have been proposed for different kinds of applications such as molecular electronics, nonlinear optics, or sensors.2,3LB films are also of great interest for mimicking biological structures). However, LB films involving small amphiphilic molecules are usually not chemically or physically resistant enough for most applications. To overcome these problems, the use of amphiphilic polymers has been proposed to build LB films. One way to prepare LB films of polymers is to use small amphiphilic molecules which can be polymerized under certain conditions. The polymerization of these molecules in Langmuir or LB films allows the formation of LB films of polymer.4 However, in many cases the polymerization leads to phase separation or structural changes. This prevented the formation of a well-organized film of polymer. Another way is the use of amphiphilic polymers to build LB films.5-7 Films of amphiphilic polymers are usually very stable at the air-water interface. However, difficulties occur when transferring Langmuir films of high molecular weight polymer. In other cases, the high flexibility of the polymers leads to a bad organization of the LB films. Recently, a new class of amphiphilic polymers has been used for building LB films: polysaccharides, which are natural polymers consisting of sugar units, have been modified by grafting long alkyl chains onto the polymer backbone in order to render them amphiphilic. Most of (1) Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990. (2) Petty, M. C. Thin Solid Films 1992, 210/211, 417. (3) Nicolini, C. Thin Solid Films 1996, 284/285, 1. (4) Tieke, B. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, 1992; pp 105-181. (5) Ulman, A. In An Introduction to Ultrathin Organic Films - from Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: New York, 1991; pp 191-203 (6) Miyashita, T. Prog. Polym. Sci. 1993, 18, 263. (7) Wegner, G. Thin Solid Films 1992, 216, 105.
the studies have been acheived so far with modified celluloses. Long-chain cellulose esters have been notably investigated.8-11 In most cases the transfer of the films was difficult and multilayers could be prepared only by using the horizontal lifting method. Cellulose was also modified by another procedure to obtain aliphatic cellulose ethers.12,13 In those studies, the cellulose was modified by etherification of all the free hydroxyl groups. The area per monomer was found to increase when increasing the length of the alkyl chains. This suggested that the alkyl chains were not perpendicular to the interface. The interactions between two cellulose backbones are then limited.13 A carbazole ether cellulose derivative exhibiting excimer formation is also under study.14 One of the interesting properties of polysaccharides is their ability to interact with proteins. Because of this property, it should be possible to form mixed proteins/ amphiphilic polysaccharides LB films. Such films could be used for building biosensors. Actually, a biosensor made by adsorption of glucose oxidase onto an LB film of cellulose acetate propionate has recently been described.15 Difficulties in transferring Langmuir films have been encountered with some cellulose derivatives. This could be due to the rigidity of the cellulose backbone, since it is constituted of anhydroglucose units linked in β-(1f4). Pullulan is a linear water-soluble polysaccharide consisting of maltotriose units connected by R-(1f6) links. The maltotriose units consist of three R-(1f4) linked anhydroglucose units. The presence of R-(1f6) links gives to the pullulan some flexibility absent from the cellulose. (8) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (9) Fukuda, K.; Miyagawa, M.; Kawai, H.; Yagi, N.; Kimura, O.; Ohta, T. Polym. J. 1987, 19, 785. (10) Matsumoto, M.; Itoh, T.; Miyamoto, T. In Cellulose and its Utilization; Inagaki, H., Phillips, G. O., Eds.; Elsevier: Amsterdam, 1989; pp 151-160. (11) Kusano, H.; Kimura, S.-I.; Kitawaga, M.; Kobayashi, H. Thin Solid Films 1997, 295, 53. (12) Shaub, M.; Fakirov, C.; Schmidt, A.; Lieser, G.; Wenz, G.; Wegner, G.; Albouy, P.-A.; Wu, H.; Foster, M. D.; Marjrkzak, C.; Satija, S. Macromolecules 1995, 28, 1221. (13) Basque, P.; De Gunzbourg, A.; Rondeau, P.; Ritcey, A. M. Langmuir 1996, 12, 5614. (14) Mao, L.; Ritcey, A. M. Thin Solid Films 1996, 284-285, 618. (15) Guiomar, A. J.; Evans, S. D.; Guthrie, J. T. Supramol. Sci. 1997, 4, 279.
10.1021/la990175q CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999
Langmuir Films of a Modified Pullulan
Langmuir, Vol. 15, No. 22, 1999 7709
Figure 1. Reaction scheme of the esterification of carboxymethyl pullulan. To simplify only one anhydroglucose unit is shown.
In this paper, we will report the ability of a partially esterified carboxymethyl pullulan (PE-CMP) to form Langmuir films which could be easily transferred. The pullulan derivative was obtained by partial esterification of carboxymethyl pullulan (CMP) with bromododecane. The presence of residual hydroxyl and carboxylic acid groups shall play a role for making mixed LB films of proteins and modified pullulan. 2. Materials and Methods Chemical Modification of CMP. The partial esterification of the sodium salt of CMP was performed according to a procedure already described for the preparation of hyaluronic acid esters.16 The reaction scheme is described in Figure 1. A CMP sodium salt with a DS of 1.4 (DS is the degree of substitution defined as the number of carboxylic acid groups per anhydroglucose unit) was first converted to its acidic form. Then, it was neutralized by tetrabutylammonium hydroxide to yield the CMP organic ammonium salt. The esterification is carried out by adding an esterifying agent such as an halogenoalcan in an aprotic solvent. Three grams of CMP sodium salt (10.9 mmol), prepared according to Bataille et al.,17 was dissolved in 80 mL of MilliQ water and acidified by percolating on an ion exchange-resin (form H+ Amberlite IRN77). The acidic polymer solution was neutralized to pH 7.4 by a 40% tetrabutylammonium hydroxide aqueous solution (Aldrich), frozen, and freeze-dried to yield about 4 g (6.9 mmol) of CMP tetrabutylammonium salt. The solid sample was placed into a two-neck flask and dried under vacuum at 35 °C for a night. Forty milliliters of dimethyl sulfoxide (ACROS) was added, and the mixture was stirred at 30 °C until complete solubilization of the polymer. A 2.4 mL (10 mmol) portion of bromododecane (ACROS) was added, and the resulting solution was kept under magnetic stirring for 24 h at 30 °C. A solution of 3 g of sodium chloride in 20 mL of water was added to convert to the PE-CMP sodium salt. The resulting mixture was poured into 400 mL of ethanol under constant agitation. We obtained a precipitate which was filtered, washed three times with 200 mL of 3:1 ethanol/water and three times with acetone, and finally vacuum-dried for 12 h at 30 °C. We then obtained 1.9 g of PE-CMP (Figure 2). PE-CMP is found to be quite insoluble in water. Because of its very poor solubility in polar organic solvents, it was not possible to have access to the number of alkyl chains content by NMR analysis. Elemental analysis was performed. This gave a carbon (16) Della Valle, F.; Romeo, A. U.S. Patent 4,965,353, 1990. (17) Bataille, I.; Huguet, J.; Muller, G.; Mocanu, G.; Carpov, A. Int. J. Biol. Macromol. 1997, 20, 179.
Figure 2. Chemical structure of the partially esterified carboxymethyl pullulan. content of 61.7 ( 0.2% and a hydrogen content of 9.5 ( 0.2%. No nitrogen was found, showing the absence of any tetrabutylammonium salt or derivative from the prepared compound. In addition to the elemental analysis, a thermogravimetric analysis of PE-CMP was performed in order to get the percentage of water in the prepared compound. The percentage of water was found to be 0.8 ( 0.2% in weight. Taking into account this percentage of water and the results of the elemental analysis for carbon, the number of dodecyl chains is 1.15 ( 0.09. From the hydrogen content, the number of dodecyl chain is found to be 1.2 ( 0.2. Both results are in agreement. Using the more reliable value (1.15 ( 0.09 dodecyl chains), we found that the percentage of esterified function is 82 ( 8%. The molecular weight of PE-CMP per monomer is 437 ( 16 g/mol (taking into account a water content of 0.8 ( 0.2%). The solubility of PE-CMP in organic solvants is very low. However, its solubility in a 80:20 mixture of dichloromethane/ ethanol is sufficient to make a solution for preparing Langmuir films. Langmuir-Blodgett Experiments. An LB trough (Atemeta, Paris, France) was used for the monolayer formation and its transfer. The dimensions of the trough are 50 cm × 6.5 cm, and the volume of the subphase is 250 cm3. This system uses a mobile barrier for compressing the molecules, and a Wilhelmy balance for measuring the interfacial pressure. The water used was purified with a Millipore system (Milli RO and MilliQ units) involving reverse osmosis, deionization, an active charcoal cartridge, and filtration. The subphase was water (pH 5.6), diluted solutions of hydrochloric acid (pH 2.5), or potassium hydroxide (pH 9.5). The solvent used for preparing the PE-CMP solution was a 80:20 mixture of dichloromethane/ethanol. The concentration of the solution was 1.37 mg/mL (3.1 ( 0.2 mM). The Langmuir
7710
Langmuir, Vol. 15, No. 22, 1999
Alexandre et al.
Figure 3. Pressure-area isotherms of the partially esterified carboxymethyl pullulan with various pH values of the subphase: (a) pH ) 2.5; (b) pH ) 5.6; (c) pH ) 9.5. films were prepared by spreading 27 µL of this solution with a capillary micropipet (Nichyrio, Japan). Surface pressure-area isotherms were recorded at 21 ( 1 °C by compressing the film at a constant barrier speed of 0.5 cm/ min. The stability of the film was studied by compressing the film at various pressures and by following the evolution of the film area while keeping the surface pressure constant (feedback loop on). Samples were prepared by transferring the PE-CMP films at 30 mN/m on two types of hydrophilic supports. CaF2 slides (Sorem, Uzos, France) were used for the IR characterization of the film. Before the transfer, the CaF2 slides were cleaned twice in an ultrasonic bath with an octanoate solution (1 g/L) for 10 min. The same procedure was repeated with acetone and then with dichloromethane (twice each). Then they were dried for several hours under vacuum. Muscovite slides (Me´tafix, Montdidier, France) were used for the scanning force microscopy (SFM) characterization. They were freshly cleaved before the transfer. Transmission IR spectra were recorded on a Nicolet 510M FT-IR spectrophotometer with a DTGS detector with a KBr window. Three hundred scans at a resolution of 4 cm-1 were collected and averaged out. The spectra are expressed in absorbance units. The IR spectrum of the CaF2 slide was recorded before the transfer of the PE-CMP film. After the transfer of the film on the CaF2 slide, the sample was placed during 24 h in a desiccator. The sample spectrum (PE-CMP LB film deposited on the CaF2 slide) was then recorded. The IR spectrum of the PECMP film was obtained by subtracting the spectrum of the respective CaF2 slide to the sample spectrum. Scanning force microscopy (SFM) experiments were performed with a Nanoscope II from Digital Instruments (Santa Barbara, CA) in the contact mode, with a 140 µm scanner. The cantilevers used were characterized by a low spring constant of about 0.06 N/m. All the measurements were performed in the air with the feedback loop on (constant force ) 10-9 to 10-8 N). The PE-CMP monolayers deposited on freshly cleaved muscovite slides were placed during 24 h in a desiccator prior to the SFM experiments.
3. Results and Discussion Experiments at the Air/Water Interface. Pressurearea isotherms of PE-CMP were made using three different pH values of the subphase (2.5, 5.6, and 9.5). For each pH value, the isotherms were found reproducible: the differences in molecular areas were smaller than 2 Å2/ anhydroglucose at a given pressure. This good reproducibility is an indication of the absence of solubilization of the polymer in the subphase during the spreading. It also shows the good ability of the molecules to form monolayers at the air-water interface.
The isotherms obtained with the different pH values of the subphase are very similar (Figure 3). In all cases, the isotherms are characteristic of a film in a single liquid phase as expected with such a polymer. We observed a slight decrease of the slope when increasing the pH; at an interfacial pressure of 10 mN/m, the area per anhydroglucose unit is 41.2 ( 0.5 Å2/anhydroglucose, 39.5 ( 0.5 Å2/anhydroglucose, and 39.5 ( 0.5 Å2/anhydroglucose for pH 2.5, 5.6, and 9.5, respectively, while at 30 mN/m, the area is 32.3 ( 0.5 Å2/anhydroglucose, 32.0 ( 0.5 Å2/ anhydroglucose, and 31.1 ( 0.5 Å2/anhydroglucose, respectively. The main differences are in the decreasing collapse pressure when increasing the pH (48, 45, and 42 mN/m for pH 2.5, 5.6, and 9.5, respectively). After the beginning of the collapse, the evolutions of the pressure when decreasing the film area are also different. These differences are due to the fact that the esterification of the carboxylic acid from the carboxymethyl pullulan is not complete and that some carboxylic acid groups are still present. The Langmuir film of PE-CMP is characterized by a very good stability. After the spreading, the film was compressed to 30 mN/m. The pressure was then maintained, and the evolution of the film area was recorded. For each pH value of the subphase, no change in the area of the film was observed during 3 h. Similar experiments were carried out on a subphase with a pH value of 5.6 at a constant pressure of 30 and 40 mN/m during 24 h. Again, no change at all in the area of the film was observed. FT-IR Spectroscopy Characterization. The PECMP films prepared on subphase of various pH values (2.5, 5.6, and 9.5) were transferred by the classical LB technique on CaF2 slides for characterization by FT-IR spectroscopy. Up to 17 layers were transferred. In every case, the transfer of the film on CaF2 slides was perfect without any accident. The transfer ratio was found to be equal to 1 regardless of the number of monolayers transferred (3-17 layers) or the pH value of the subphase (pH ) 2.5, 5.6, or 9.5). IR spectra of the LB films of PE-CMP are shown in Figure 4. The different peaks were attributed by comparison with IR spectra of pullulan and CMP (acid form and sodium salt) deposited on a CaF2 slide (Table 1). The main peaks are a broad peak around 3430 cm-1 attributed to the OH groups, sharp peaks at 2855 cm-1 (νs (CH2)),
Langmuir Films of a Modified Pullulan
Langmuir, Vol. 15, No. 22, 1999 7711
Figure 4. FT-IR spectra of LB films of partially esterified carboxymethyl pullulan transferred on CaF2 slides. The numbers of layers transferred for each spectra were 15, 17, and 13 for pH values 2.5, 5.6, and 9.5, respectively. The intensity of the spectra has been divided by the number of transferred layers, and the spectra then correspond to one monolayer of partially esterified carboxymethyl pullulan. Table 1. Transmission IR Absorption Bands for Pullulan, CMP (Acid Form and Sodium Salt) and PE-CMP (pH 2.5 and 9.5)a CMP group OH CH3 νas CH2 νas CH, CH2 CH2 νs CdO (COOH et COOR) νs OH CdO (COO-) CH2 CH(-OH) (CH-)OH C-C-O (COOR) C-O (COOR) C-O-C a
PE-CMP
pullulan
acid form
sodium salt
pH ) 2.5
pH ) 9.5
3375
3430 (3230 sh)
3375 (3230 sh)
3430 (3230 sh) 2958 2925
3430 (3230 sh) 2958 2925
2855 1748 1740 (1650 sh)
2855 1748 1740 (1650 sh) 1594 1466 1430 1355
2929 2890
2929 2890
2929 2890
1732 1645 (1455 sh) 1425 1355
1150
1645 (1455 sh) 1430 1355 (1330 sh) 1145
(1650 sh) 1597 (1455 sh) 1425 (1350 sh) 1325 1155
1466 1430 1355 1275 1213 1144
1275 1213 1144
The numbers in parentheses correspond to a shoulder from the closest absorption band.
2925 cm-1 (νas (CH2)), and 2958 cm-1 (νas (CH3)) corresponding to the alkyl chain, a split peak at 1740 and 1748 cm-1 (νs (CdO)) attributed to the carbonyl functions, and the peak at 1466 cm-1 attributed to the CH2 groups. For the pH values of 5.6 and 9.5, an additional broad peak characteristic of ionized carboxylic groups appeared at 1594 cm-1. The appearance of this peak when increasing the pH confirms the results of the elemental analysis (some carboxylmethyl groups per anhydroglucose unit have not been esterified). At acidic pH values, the peak of the carboxylic function is probably part of the peak of the carbonyl functions. Actually, for the acid form of CMP, the peak of the nonionized carboxylic is located at a wavelength of 1732 cm-1. This value corresponds to a carboxylic acid in a monomeric configuration. This monomeric configuration
of the carboxylic group can be explained by the distance between two carboxylic functions in the polymer. In the case of PE-CMP, the carboxylic acid function shall absorb at about the same wavelength as CMP (1732 cm-1). The carbonyl peak for PE-CMP is split with two maxima at 1740 and 1748 cm-1. The maximum at 1740 cm-1 diminishes faster with the pH value than the one at 1748 cm-1. However it does not disappear completely. Then the maximum at 1740 cm-1 should not correspond to the carbonyl of the carboxylic acid functions. The carboxylic acid functions should be around 1730 cm-1 and then be a part of the carbonyl peak. This could explain the asymmetrical diminution of the carbonyl peak. The splitting of the peak may be due to two different populations of ester functions: the ester functions made with
7712
Langmuir, Vol. 15, No. 22, 1999
the carboxymethyl groups linked in the 6 position and the others with the carboxymethyl groups linked in the other positions. The sharpness of the peaks of the alkyl groups is characteristic of a fine order of the chain. For a given pH value of the subphase, the intensities of the peaks were found to be directly proportional to the number of layers deposited. This result could be expected because of the quality of the transfer. When the pH is increased, the intensity of most of the peaks decreases by about 25-30%. This phenomenon is difficult to explain. The concentrations per area unit (inversely proportional to the molecular area) at the air/ water interface for the different pH values of the subphase are very close. Because of the regularity of the transfer on the CaF2 slides, we would expect the concentrations per area unit to be the same in the Langmuir and in the LB films. Then only a slight change in the intensity of the peaks is expected. One hypothesis could be a reorganization of the film straight after the transfer. This could change the concentration per area in the LB films and then the intensity of the peaks. Besides this study, the LB films of PE-CMP transferred on CaF2 slides were found to be very resistant to mechanical and chemical constraints. After the IR study, the samples were cleaned using our classical cleaning procedure (using washing cycles in an octanoate solution, acetone, and dichloromethane; see Materials and Method). The IR spectra of the “cleaned” CaF2 slides were identical to the spectra of the LB films before the cleaning procedure showing that the LB films of PE-CMP were not disturbed by the cleaning procedure. The use of the 80:20 mixture of dichloromethane/ethanol was only partially successful in removing the LB films of PE-CMP from the CaF2 slides. SFM Characterization. Langmuir films of PE-CMP were transferred by the classical LB technique on muscovite slides for SFM characterization. The first layer is transferred during the withdrawal of the slide with a transfer ratio of 1. While the transfer of a single layer on muscovite slides happens without accident, it shall be noticed that we were not able to transfer multilayers on this type of substrate; the first layer separated from the muscovite surface when the muscovite is plunged a second time, leading to a backward movement of the barrier. SFM images of LB monolayers of PE-CMP prepared with various pH values of the subphase are similar showing that the structure of the film is not greatly influenced by the pH. At large scale (150 µm × 150 µm), the film appears very flat and homogeneous. No holes or cracks were observed. When zooming (5 µm × 5 µm), the film still appears homogeneous except for a few small bumps (Figure 5). The number of bumps slightly increased while decreasing the pH of the subphase. The bumps are characterized by a diameter of 200-300 nm and a height difference with the monolayer of 0.5-1 nm. We also observed that the interactions between the tip and the sample are higher on the bumps than in the other part of the film. Since the height difference is too small to be characteristic of a bilayer domain, it is difficult to determine the nature of these bumps. Actually they could be due to a heterogeneity in the number of alkyl chains added to the carboxymethyl pullulan. This would lead to differences in hydrophobicity which could explain the differences of interactions between the tip and the sample. The image on Figure 5 was obtained when using the lowest force possible between the tip and the sample. When the forces were even slightly higher, the samples appear quite fragile and the film is damaged (Figure 6). In the middle top part of the image, one can see greater damage
Alexandre et al.
Figure 5. SFM image of an LB monolayer of partially esterified carboxymethyl pullulan transferred on a muscovite slide. The monolayer was prepared on a subphase with a pH of 5.6.
Figure 6. SFM image of an LB monolayer of partially esterified carboxymethyl pullulan transferred on muscovite slide (pH of the subphase 5.6). The force was not minimized and the image shows the degradation of the LB monolayer due to the scanning. In the top center of the image, the rectangle in which a greater disorder is observed is due to an attempt to zoom on the image (the zoom image was not completed because of the tip destroying the monolayer).
due to the effect of zooming in this zone. Because of the fragility of the film deposited on muscovite under mechanical constraints, we were not able to get reliable images at the molecular resolution. 4. Conclusion A CMP with 1.4 carboxymethyl functions per anhydroglucose unit has been partially esterified with bromododecanol. The elemental analysis shows that the number of dodecyl chain per anydroglucose unit is 1.15 ( 0.09. This leaves 0.25 ( 0.09 carboxylic functions per anydroglucose unit. The molecular weight of PE-CMP per monomer is 437 ( 16 g/mol. PE-CMP are able to make Langmuir films at the air/ water interface. These films are characterized by a great stability. The limiting area per anhydroglucose unit is 36.1 ( 0.5 Å2/anhydroglucose, 36.5 ( 0.5 Å2/anhydroglucose, and 36.7 ( 0.5 Å2/anhydroglucose for the respective pH values of the subphase 9.5, 5.6, and 2.5. These values are rather small since the expected area for a single glucose
Langmuir Films of a Modified Pullulan
unit lying flat on the surface is around 60 Å2.13 The R-(1f6) links between two maltotriose units actually give a flexibility to the molecule which could explain the low area per anhydroglucose units. Moreover the R-(1f4) links between the three anhydroglucose units imply a twist; the third anhydroglucose unit is almost perpendicular to the first one as shown by molecular modeling. This prevents the possibility for all the anhydroglucose units to lie flat at the air-water interface. In addition it should be noticed that because of the small area per anhydroglucose unit the alkyl chains shall be close to each other. SFM images of the LB film show that it is well homogeneous with only a few bumps. In addition, the transfer of multilayers of the PE-CMP films on CaF2 slides occurred without any accident. The transfer of multilayers onto muscovite was however impossible since the first layer separates from the muscovite surface when it is plunged a second time into the subphase. While the LB monolayers deposited on muscovite are characterized by a bad resistance to mechanical treatments, the LB films deposited on CaF2 were found to be highly resistant to mechanical and chemical treatments.
Langmuir, Vol. 15, No. 22, 1999 7713
Differences are also observed during the transfer of the film. On muscovite only the first monolayer of PE-CMP could be transferred in good condition with a transfer ratio of 1. On CaF2 slides, up to 17 layers of PE-CMP have been transferred successfully without accident and with a transfer ratio of 1. Therefore PE-CMP films exhibit differences of behavior during the transfer and differences in resistance in the resulting LB films. This may be explained by differences of interactions between the PE-CMP films and the different substrates. The muscovite substrate is more hydrophilic than CaF2. Moreover muscovite is an ionic crystal, while in the case of the CaF2 crystals, the CaF bonds are covalent. PE-CMP exhibits very interesting properties as LB films in the prospect of applications. The Langmuir films are highly stable and the LB films obtained on the CaF2 substrate are very resistant. The next step would be to have a better control of the reaction in order to obtain pullulan with various contents of alkyl chains. LA990175Q
