Langmuir 1996,11,3953-3958
3953
Enhanced Reactivity of the Epoxy Oligomers in Organized Monolayers at the Air-Water Interface V. V. Arslanov,* L. S. Sheinina, R. A. Bulgakova, and A. V. Belomestnykh Institute of Physical Chemistry of the Russian Academy of Sciences, Leninsky prospect 31, 117915 Moscow, Russia Received July 22, 1994. I n Final Form: March 15, 1995@ The behaviors of monolayers of novolac and diglycidyl ether of bisphenol A type epoxy oligomers on the surface of water were studied. Using a surface balance technique and FT-IR spectroscopy, it was found that the time evolution of surface pressure and surface area of the monolayers is controlled by the chemcial reactions of these oligomers with water and carbon dioxide. The specific reactivity of epoxy oligomers in a one-layer organized system at the air-water interface (in bulk similar reactions may proceed only under severe conditions) is enabled by the increased concentration and the mobility of protons in a thin layer of water adjacent t o the monolayer and by the basicity of the epoxy groups of the oligomers. Surface reactions of epoxy oligomers are shown to be topochemical in nature; the kinetics of these reactions may be described by the Avrami-Erofeev equation. The magnitudes of the parameters of this equation which were determined for the monolayers of each of the oligomers lead to the conclusion that, generally, the dynamics of epoxy oligomer monolayers is controlled by chemical transformations in and rearrangement of the surface layer.
Introduction Two stages can be distinguished in the chronology of investigations on thin organized films of polymers. Having begun with the studies by Devauxl in 1903, the early and longest stage involved mainly the investigation of monolayers of macromolecules on the surface of water. In the past 20 years more attention was devoted to LangmuirBlodgett films (LBF) in connection with the development of research in the field of molecular architecture. Since LBF are formed by transferring monolayers from the surface of a liquid and reproduce to some extent the structure of these monolayers, the significance of both stages of the research in problems of molecular design becomes evident. Investigation of the specific features of the behavior of organized assemblies of macromolecules on fluidfluid or fluidsolid interfaces will make it possible not only to realize in practice the already recognized advantages of polymer LBF (thermal and mechanical stability, resistance to aggressive media effects) but also to design a new generation of superstructures with allowances for the chain structure ofthe po1yme1-s.~~~ These aspects are most markedly displayed by network structures; however, they have not been investigated for monolayers and LB films. In particular, this fact holds true for epoxy oligomers and derived polymers, which are widely used in polymer chemistry. The only study dealing with monolayers of common epoxy oligomers based on diglycidyl ether of bisphenol Awas related to their use in polymer adhesive^.^ The little attention that these interesting objects have attracted so far is apparently explained by the instability of monolayers of epoxy oligomers on the surface ofwater. This instability is manifested in that the surface pressure and limiting area of the monolayer depend on the age of the monolayer. The aim of this study, which stands within the framework of a more general study of two-dimensional organic networks, was, first, to disclose what causes the instability
Table 1. Molecular Characteristics of Epoxy Oligomers
no. of epoxy oligomer
mol wt 642
GY-1180 GT-6097 GT-6610
epoxide index, equivkg 5.58 0.56 0.29
residues in the chain, n 1.6 12.0 23.0
3580 6780
of monolayers of epoxy oligomers of different structures and molecular weights at the air-water interface. The second task of this study was to identify the specific features of two-dimensional reactions involving epoxy oligomers in one-layer ensembles on the surface of water. These tasks were dealt with using the surface balance technique and FT-IR spectroscopyto examine the behavior of the monolayers of different epoxy oligomers on the surface of water.
Experimental Section Monolayers of phenol-novolac oligomer (GY-1180,Ciba-Geigy) and diglycidylether ofbisphenolA (GT-6097 and GT-6610,CibaGeigy)were examined in this study. The chemical formulas of these oligomers are shown.
GY-1180
Jn
L
Abstract published in Advance A C S Abstracts, September 1, 1995. (1)Devaux, H. C. R . Hebd. Seances Acad. Sci. 19S5,201,109. (2) Arslanov, V. V. Russ. Chem. Rev. 1991,60, 584. (3)Arslanov,V. V. Russ. Chem. Rev. 1994,63,1. (4)Glazer, J. J . Polym. Sci. 1954,13, 355.
GT-6097
@
CH3
0743-7463/95/2411-3953$09.00/0 0 1995 American Chemical Society
U
3954 Langmuir, Vol. 11, No. 10, 1995
Arslanou et al.
Some of their characteristics are listed in Table 1. To prepare the monolayers,epoxy oligomerswere dissolvedin twice-distilled chloroform (0.26 g/L) and applied with a microsyringe onto a surface of bidistilled water (pH 5.5), taking care to distributethe droplets uniformlyoverthe surface ofthe subphase. The pH 7 of the subphase was adjusted by addition of phosphate buffer: mixture of KHzPOl (0.018 M) and KzHPO4 (0.022 M); ionic strength of buffer was 0.084. The surface pressure l7 versus area A per molecule (or per residue) isotherms were measured with a specially designed automated Langmuir trough equipped with step motors which recorded both parameters automatically. The surface pressure was measured using the Wilhelmy method, employing a glass plate with polished edges and an inductive transducer with a sensitivity of 0.06-0.30 mN/mV. The accuracyofmeasurements was 0.1 mN/m. The studies were conducted at 20 1 "C in a Teflon trough with a mobile Teflon barrier. The compression rate was about 3.5 A 2 residue-' min-l, allowing us to minimize the surface pressure gradient overthe surface ofthe mon0layer.~,6 The purity of the water surface and the purity of the solvent were controlled by measuringthe surface pressure for pure water and for water after applying and evaporating the solvent. Transmission IR spectra were registered with a Perkin-Elmer 1720 Fourier spectrometer. In order to obtain reliable transmission spectra that would allow their processing, up to 20 monolayers of GY-1180 oligomer were collected on the surface of a KRS-5 plate. Each of the collected monolayers was preliminary exposed on the surface ofwater for a specifiedperiod of time. Then, the monolayer was compressed in both longitudinal and lateral directions to occupy an area of 1 cm2;it was then collected from the surface with a Teflon spatula and transferred onto a KRS-5 plate. In these experiments, the area per moleculeof GY-1180oligomerwas 40 A2. To remove residual moisture from the sample, a KRS-5 plate with applied monolayers was kept in air at 20 "C for various waiting times. It was found that both the intensity of the band at 3300-3500 cm-l and the weight of sample did not change on exposure time of 20 h (at 20 " C ) . This treatment was used for all samples including those of bulk oligomerapplied onto a KRS-5 plate in the form of thin film. When the transmission spectra were recorded, the number of scans was 25. Quantitative analysis of IR spectra involved calculation of the apparent optical density D of characteristic band in the spectra of oligomer. The average of the optical densities of the bands at 1507 and 1610 cm-l, which are characteristic of the substituted benzene ring,' was used as an internal standard. The accuracyofmeasuringthe optical density was 15-20%. The optical densities calculated in this way for each ofthe analyzed systemswere compared to the corresponding values obtained from the spectrum of bulk oligomer GY-1180 coated onto a KRS-5 plate, as a thin film.
*
Results and Discussion For monolayers of three epoxy oligomers, Figures 1 and 2 show the compression isotherms recorded at different waiting times, t, between the application of the oligomer onto the surface of water and recording of the isotherms. A comparison of the obtained curves reveals that the behavior of the monolayers of different epoxy oligomers shows both common tendencies and significant differences. The common features involve a n increase in the limiting area (A,) of the monolayer and the surface pressure (n,) corresponding to collapse of the monolayer with a n increase in t. The expansion of the monolayer is controlled by the type of oligomer. For bisphenol-based oligomers (Figure 2), a n increase in the limiting area is quite significant (16-18 A2 per residue, which amounts to 2630%), whereas for a novolac oligomer (Figure l), the increase inA, is only 2%,which is within the experimental error. The absolute magnitudes ofthe increase in collapse ( 5 ) Malcolm, B. R. J. Colloid Interface Sci. 1986,104, 520. ( 6 ) Bohanon, T. M.; Lee, A. M.; Ketterson, J. B.; Dutta, P. Langmuir 1992.8. 2497.
(7j Simadzaki, A.; Kozima, M. J. Chem. SOC.Jpn., Ind. Chem. Sect. 1963,55,1610.
?C.mN/m
E
5 .O'
4.0.
3,O-
2,o-
1 .o
-
0.0
Figure 1. Surface pressure (Ill-area (A)isotherms of novolac epoxy oligomer GY-1180 on the surface of water at various waiting periods: A, 3; B, 10; C, 60; D, 90; E, 1080 min. X,"/
10
8
4
6
4
2
0
AD 50 60 7 0 80 90 100
A,A2/residue
Figure 2. Surface pressure (Ill-area (A) isotherms ofdiglycidyl ether of bisphenol A epoxy type oligomers GT-6097 (A, C ) and GT-6610 (B) on the surface of water at the following waiting periods: A, 3 min; B, C, 1440 min.
pressure are close for both types of oligomers, but the relative increase is 22-26% for the bisphenol-based oligomer and amounts to 165%for the novolac oligomer. However, these changes, although resulting, in all cases, in a slightly reduced compressibility, do not alter the liquid condensed phase state of the monolayers. Note that when the dynamics oftransformations in these monolayers are assessed, the increase in A, observed for bisphenol-based oligomers and the growth of II,for both types of epoxides proceed over a rather large period of time, extending to tens or even hundreds of minutes. Expansion of the monolayers formed by reactive compounds was reported to result from reactions with compounds dissolved in the subphase which led to increased molecular sizes; it was observed during the oxidationof unsaturated compounds and in polymerization under specific conditions.*-ll In all cases, the growth of (8)Rabe, J. B.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. J. Chem. Phys. 1986,84, 4096.
Langmuir, Vol. 11, No. 10, 1995 3955
Epoxy Oligomers in Organized Monolayers the limiting area did not exceed several Azper molecule of monomer. Incomplete spreading of oligomers and occurrence of polymerization could not cause the observed expansion of the monolayers because the rigidity of the monolayers increased only slightly and the compression-expansion isotherms showed no hysteresis a t different exposure times. The observed variation of characteristics of the monolayers is similar to the behavior of two-dimensional mixtures and may be formally explained within the framework of the rule suggested by Crisp,12according to which the increase in surface pressure accompanying collapse of mixed monolayers suggests complete compatibility of the components within the entire range of compositions. This approach allows the conclusion that a new component is formed in the monolayer, resulting in the formation of a mixed monolayer containing the initial oligomer and products of its reactions with the components of the bulk phases. This conclusion is nontrivial because reactions of epoxy compounds in bulk with water and carbon dioxide (i.e., with the components of phases in contact with the monolayer) occur only under severe conditions that may involve catalysis, irradiation, or heating.13J4 Otherwise, the epoxy groups are stable. This fact is evidenced by chromatographic analysis of the epoxide modeling diglycidyl ether of bisphenol A. This model epoxide was exposed for 130 days to 60 "C a t 96% relative humidity.15 A DSC study of a novolac epoxy oligomer that was exposed for 6 days to aqueous environment a t 50 "C (the water uptake reached 39.9%) corroborates this inference.16 In order to clarify the feasibility of chemical transformations in organized two-dimensional systems on the surface ofwater, IR spectroscopic studies of the monolayers of epoxy oligomerwith high epoxide index (GY-1180)were conducted. In these studies, the epoxy oligomerwas aged at the air-water interface for different periods of time. Figure 3 shows the fragments of IR spectra revealing the most notable changes in the monolayers that occur during the 10- and 30-min aging on a water subphase. The spectrum of the initial epoxy oligomer is also shown in this figure. As is seen, as a result of contact of the epoxy oligomer with water, the optical densities of the band a t 915 cm-l and shoulder at 870 cm-' (asymmetric stretching vibrations of the epoxy rings in trans- and cis-positions, respectively) decrease, as do the intensities of the bands a t 3050, 3040, and 3000 cm-l (CHZ- and CH-groups of terminal epoxy rings). The band a t 3490 cm-l, which is due to the stretching vibrations of hydroxyl group^,^^,^^ dramatically increases in intensity, broadens, and shifts to the low frequency region by 130 cm-l. These changes (9) Day, D.; Ringsdorf, H. J . Polym. Sci., Polym. Lett. Ed. 1979,16, 205. (10) Beredjick, N.;Burlant, W. J. J.Polym. Sci. Part A-1 1970,8, 2807. (11)Letts, S. A,; Fort, T.; Lando, J. B. J.CoZloidInterface Sci. 1976, 56,64. (12) Crisp, D. J. Res. Suppl. Surf. Chem. (London) 1947,17,23. (13)Sidjakin, P. V.; Karpov, B. L.; Egorov, B. N.; Egorova, Z. S. Vysokomol. Soedin. (Russ.) 1971,A13, 2195. (14) Shapiro, A. L.; Lubovsky, I. S.; Romanova, V. I.; Levin, S. Z. J. Org. Chem. (RussJ 1970,6,1366. (15)Pearce, P. J.;Davidson, R. G.; Norris, C. E. M. J.Appl. Polym. Sci. 1983,28,283. (16) Stark, E. B.; Ibrahim, A. M.; Munns, T. E.; Seferis,J. C. J.Appl. Polym. Sci. 1986,30, 1717. (17)Nakanishi, K. Infrared Absorption Spectroscopy. Practical; Holden-Day,Inc.: San Francisco, and Nancodo Company, Ltd.: Tokyo, 1962. (18)Hallam, H. E. InInfra-RedSpectroscopyandmolecuZarstructure; Davies, M., Ed.; Elsevier Publishing Company: Amsterdam, London, New York, 1963.
0
2
----
4000
3500
3000
1800
1600
1200
1000
800 600 v , om-'
Figure 3. Infrared spectra ofnovolac epoxy oligomer GY-1180 in the bulk (A) and in the monolayers (B, C, D) on the surface of water (B, C) and on the surface of phosphate buffer (D) at the following waiting periods: B, 10 min; C, D, 30 min.
suggest a significant increase in the content of hydroxyl groups in the sample and their association with subsequent hydrogen bonding. The content of ether bonds (1110-1080 cm-l) also increases. Very important events observedin the spectra of monolayers of the GY-1180epoxy oligomer involve the appearance of intense bands a t 1737 and 1725 cm-l, a group of bands near 1660 cm-l, and the bands a t 1560 and 1540 cm-l. These bands are characteristic of different carbonyl groups, double bonds, and carboxylate ions.lg As can be inferred from the comparison of spectral data (Figure 3, curves B and C), increasing the time during which the monolayer of oligomer is aged on the surface of water results in a n overall decrease in the intensity ofboth bands (associated with vibrations of epoxy groups) in the low-frequency region of the spectrum by 1.5 times and in a similar increase in the intensity of the bands due to carbonyl groups and associated OH groups. Furthermore, the number of unsaturated bonds increases by more than 2 times, whereas the increase in the number of carboxylate ions amounts to 5-6 times. Because the published data cited above suggest that epoxy oligomers are not hydrolyzed when in bulk, the chemical transformations in monolayers a t the air-water interface (the occurrence is evidenced by the changes in IR spectra) may be related t o the specific state of the molecules in an organized structure. For epoxy oligomer in this state, the changes in the IR spectrum ofthe oxirane rings correspond to transformations, initiated by heating or irradiation or promoted by catalysts. Analyzing the data of IR spectroscopy and relating it to the welldocumented reactions of epoxides under various conditions,13J4one may suggest the following major routes for chemical transformations of epoxy oligomers: ROCH2CH-CH
\o/
H+
H20
ROCHPCH-CHz
I
OH
I
(1)
OH
The first reaction which occurs under common conditions and results in opening of the rings is initiated by a proton. The second reaction, namely, the formation of ring ester of carbonic acid from epoxy and COZ, occurs in (19) Brand, J. C. D.; Eglinton, G. Applications of spectroscopy t o organic chemistry; Oldbourne Press: London, 1965.
Arslanov et al.
3956 Langmuir, Vol. 11, No. 10, 1995
CH2-yHCH20R
o.c,o II 0
the bulk only a t elevated temperatures in the presence of catalysts (quaternary ammonium salts). The formation of ether bonds may occur in the bulk a t a n elevated temperature and may also be initiated by a proton: PROCHpCH -CH2
\ /
H+
ROCH2CHCH20CH2CH2CH2OR
I
0
(111)
OH
y-Irradiation induces the isomerization of epoxides and the formation ofan aldehyde and ketone.13 Aldehyde may also be obtained from diol (reaction I):
ROCHzCH-CH
I
OH
I
H+
40
ROCHZCH~C,
H
+ H20
(VI)
OH
In the presence of a proton, the aldehyde may undergo aldolic condensation, which may further lead to a,@unsaturated aldehydez0 OH
R'CH2CH=CR'C
