- Email: [email protected]
The role of adsorption layers of surface active substances on the formation of polyepoxides
Polymer Science Vol. 33, No. 5, pp. 967-973, 1991 Printed in Great Britain.
0032-3950/91 $15.00 + .00 © 1992Pergamon Press Ltd
THE ROLE OF ADSORPTION LAYERS OF SURFACE ACTIVE SUBSTANCES ON THE FORMATION OF POLYEPOXIDES* L. S. S H E I N I N A , SH. G. V E N G E R O V S K A Y A , R. A. VESELOVSKII and V. V. DAVIDENKO Institute for the Chemistry of High Molecular Compounds, Academy of Sciences of the U.S.S.R.
(Received 25 June 1990)
The thermodynamics of adsorption layers of siloxane surfactants in simple binary systems simulating real epoxide compositions are studied. The microstructure, liability to swelling, and adhesionai strength of polyepoxides cured in the presence of a number of different surfactant concentrations are indicated. All the polymer characteristics studied are essentially dependent on the quantity of surfactant introduced, but these relations are non-monotonic in character. The observed relations can be explained in terms of the different structures of the adsorption layers, formed in reaction systems at the polymer formation stage.
SOLUTIONSof surfactants in heterogeneous oligomers show both a typical change in properties of the binary system at the separation interface [1, 2], and also a number of special features. The ability to react chemically with or to be indifferent [3, 4] to the oligomer-solvent is a specific feature of the behaviour of surfactants on introduction into reactive oligomers. As is shown, in the first case it is possible to obtain a stable increase in a number of properties of polymers obtained in the presence of surfactants, including the adhesional strength, while in the second case the relation between these properties and the surfactant concentration is extremal in character [3]. Furthermore, as the oligomer is consumed and the MM of the product increases the polarity of the medium [3] and the solubility of the surfactant in the reaction mixture [3] are changed, which can lead to a change in its role as a structure former [5]. Thus, it is known that for polyurethane systems the mechanism of the structuring action of indifferent surfactants is due to the formation of adsorption layers in the bulk of the reaction mixture and at the interface with the air, and is determined by the structure of the latter, which depends on the concentration of the surfactant additive [2, 6]. This phenomenon is a consequence of the microheterogeneity of the reaction mixtures [7] arising during hardening of the cross-linked polyurethanes. It is known from published information that the rheokinetics of the formation of cross-linked epoxide polymers can be described adequately only by using as a starting point the concept that local cross-linked formations, i.e. microgels [8], are formed in the reaction system. In such systems, which have a developed internal separation interface the role of surfactants is very important, but this aspect of the effect of a substance having high surface activity has been very little studied. In view of the above, this work is concerned with establishing the effect and role of surfactants in the formation of the structure of cross-linked polyepoxides. The materials used were ED-20 epoxide resin (M -- 350, epoxide number 22.9, evacuated under a pressure of 133 Pa at 365 K), polyepoxides cured by the amine method, and also a phenylglycidyl ether (PGE), a chemically pure low viscosity compound, having the chemical structure of half of the ED-20 molecule, i.e. convenient as a model [9]. The PGE was prepared by vacuum distillation (375 K/19 Pa) after holding above Call2 over one month. The residual moisture content of the epoxide *Vysokomoi. soyed. A33: No. 5, 1055-1061, 1991.
967
968
L.S. SHEININAet aL
compounds was <0.02%. In order to study the structure forming role of the surfactant, oligomeric surfactants were used, i.e. block-copolymers of dimethylsiloxane and alkyleneglycol, chemically indifferent to the components of the system under study. These are effective surfactants, operating at very low concentrations, which gives reality to studying the structure of their adsorption layers and provides a path to a solution of the problem posed.
--Si--O--
(CHs)s--Si--O--
--[--Si--O--/---Si--(CHa)a
Lc .
(CrI,),I I
J~
(O~aH6)n - OC4H9 (a~3-4, m~t3, n~t7, x~50-55). l- CHa I
1
-
(CHa)a--Si--O-- ] --Si-'O--/--
L
I
-I CHs
--~i--O--
_1,
I
L
H
-
--Si--(CHa) s (n = 9--i0)
I
_17
The surfactant adsorption layers were studied thermodynamically on the basis of data on the surface tension of their solutions in PGE, obtained by the Wilhelm method. In view of the long time required to establish the equilibrium state of the surface layers of high molecular surfactants [10] their solutions were held for two weeks before determining the values of 7, which were measured in thermostatted glass cells, using the method described in [11]. The surface tension was measured during the curing action by the suspended droplet method [12]. The dynamic viscosity of the compositions was measured with a Rheostat-2 setup in a cylinder-cylinder measuring unit at a constant shear rate and a temperature of 298 + 0.1 K. The equilibrium swelling of the cross-linked polyepoxides was studied by a volume method in Zarczynski-type dilatometers [13] at 298 + 0.1 K. The degree of equilibrium swelling was defined as the ratio of the volume of absorbed solvent to the volume of polymer up to swelling. The microstructure of the cured polymer samples was studied with carbon replicas of the surfaces of ground off samples under a JEM-100C electron microscope. The strength on normal peeling off was determined in accordance with GOST 14760-69 for polymers cured over 7 days at 298 + 2 K. The introduction of either surfactant (I or II) into ED-20 leads to a significant decrease in 3' (limiting values 27.1 and 38.8 mN/m respectively). As can be seen, in the case of I this effect is significant, and in using a model compound with 3'20 = 40.9 mN/m it is advisable to study solutions of compound I in it. The effects of temperature (283-313 K) and concentration [0.001-0.1%, within the range over which the critical micelle concentration (CMC) is not attained] on the surface tension of its solutions in PGE and correspondingly on the surface pressure r is calculated in [14]. From the plots of the surface pressure p against surfactant concentration in Fig. 1, for boundary values of the temperature it can be seen that the maximum change of surface pressure is observed at a low temperature in the region of concentration of the additive I<0.01%. At higher surfactant concentrations the increase in surfactant concentration has very little effect on the increase in surface pressure.
Adsorption layers of surface active substances "q, Pa s
969
~mN/m
• ,,&.
Z ,~
5
c~x 102, %
I5
18
FIG. 1. Plots of the dynamic viscosityof ED-20 resin (1) and the product from the interaction ED-20 resin and PGE (85 : 15 by weight) with polyethylenepolyamine (4) at 298°C and the surface pressure of solutions of I in PGE at 283 (3) and 308 K (3) against concentration. Starting from the values of p at each of the temperatures studied and at all concentrations, the values of the surface entropies ASs and enthalpies AHs characterizing the thermodynamics of formation of an adsorption surfactant layer under corresponding conditions (Fig. 2) are calculated. The plots of ASs and AH, against concentration bring out the extremal character of the changes in the thermodynamic functions over the range 298-313 K (Fig. 2, plots 4, 4'--6, 6') and their approximations to the minimum value of lower temperatures (Fig. 2, plots 1, 1'-3, 3'). The decrease in temperature leads to broadening of the maximum and displacement of its position in the region of higher concentrations (0.01-0.075%). The decrease in both the thermodynamic functions to the minimum values is an indication of the formation of a more ordered structure of the surface layer
[15].
I88
x 7o~ %
#fl
:£ 0
-50 Fro. 2. Plots of the thermodynamicfunctions AS (1-6) and AH (1'-6') of surface layers of solutions of I in PGE against surfactant concentration at 283 (1, 1'), 288 (2, 2'), 293 (3, 3'), 298 (4, 4'), 308 (5, 5'), and 311 K (6, 6'). PS 3 3 : 5 - I
970
L.S. SHEI/qlblAet al.
According to the ideas developed in [15], on compression of a PDMS monolayer reorganization of its structure occurs, from molecules convoluted into the form of fiat disks [16], through consolidation of the two-dimensional formations, to a subsequent change in conformation of the molecules into a helix-like form. Such restructuring is accompanied by a transition of the values of the function AS into the region of negative values [15]. Consolidation of a siloxane surfactant monolayer because of the growth of the number of molecules required per unit surface obeys the same laws, irrespective of its causes: compressive force [15] or growth in the concentration of surfactant in the system, as in our case. Making the same analogy, and starting from the observed change in the thermodynamic functions (Fig. 2), it can be concluded that the functions ASs and AHs in the region of negative values describe a process of packing lyophobic molecular fragments of helix-like conformation up to a state of maximum order (region of the extremum). With further increase in the concentration of compound I there is a gradual disordering of the adsorption layer [15], which is accompanied by an increase in the values of AS and AH (Fig. 2, curves 4, 4'-6, 6'). Accordingly, it follows from the given thermodynamic study of solutions of I in PGE that the plots of ASs and AH~ against concentration enable the values of Csur to be determined at which a surface layer is formed having the maximum packing density of the additive molecules introduced. These concentrations lie in a region of values well known to be below the critical and depend on the temperature. Further study shows that the relations obtained for the thermodynamic functions can be applied to real systems. Figure 1 (plots I and 4) shows relations for the viscosity of ED-30 and an epoxide composition, which are the product of the curing of a mixture of ED-20 and PGE (weight ratio 85 : 15), and I wt% ethylenediamine as a function of the concentration of additive I. As can be seen, these relations indicate a decrease in viscosity with a minimum value at 0.05% surfactant, which is in good agreement with the position of the extremum at the corresponding temperature on the plots of ASs and AHs against concentration (Fig. 1, plots 4, 4'). It follows from all this data that on introducing compound I the viscosity of ED-20 in the reaction mixture is decreased, the extent of this decrease being directly proportional to the density of packing of the surfactant molecules in the adsorption layer. Since in this case it is a bulk parameter which is measured, i.e. the viscosity, we are speaking of the effect of two interfaces: the solution-air interface, and the interface determined by the microheterogeneous structure of the epoxide oligomer itself [17] or the reaction mixture. The latter is confirmed by the data on studying the change in surface tension in the system without a surfactant and containing 0.05% of compound I. Over 1.5 h (time to the beginning of the passage of the reaction mixture into the solid state) the system devoid of surfactant during curing has an almost constant value of 3'~45.5 mN/m, and in the presence of surfactant there is a constant increase in this value from 31 to 36 mN/m. Consequently, the increase in 3' observed in the presence of surfactant can be a consequence of the appearance and quantitative growth of 3' in the system of microgel formations with corresponding redistribution of surfactant between the reaction mass-air interface and the interface between the liquid part of the reaction mass and the microgel. The gel formation process in the formation of cross-linked epoxide polymers by the amine curing method begins fairly rapidly and it is to be expected that the change in properties of the polyepoxide in the presence of various quantities of surfactant must also obey the relations in which there is an extremum at concentrations found on studying the thermodynamic functions. In order to test this proposition, equilibrium swelling in acetone of polymer samples in the form of identical tablets and the strength in normal breaking of an adhesive joint, obtained from cross-linked polymers with different contents of each of the surfactants were studied. In all these studies surfactants I and II were used, which differ in the structure of the side chains and, as was shown above, are able to decrease the surface tension of the epoxide resin.
Adsorption layers of surface active substances
TABLE 1.
971
C O M P A R A T I V E C H A R A C T E R I S T I C S OF E Q U I L I B R I U M SWELL OF
P O L Y E P O X I D E IN T H E PRESENCE OF V A R I O U S Q U A N T I T I E S OF S U R F A C T A N T
Surfactant concentration I
0.0 0.003 0.03 0.05 0.07 0.1
II 0.0
Equilibrium swelling in acetone
Maximum strength MPa
1.9
24
--
2.3
20
--
1.5
28
--
1.4
30
--
1.6
27
--
2.2
20
0.0003 0.003 0.005 0.007 0.01
2.5 1.6 1.4 1.5 2.0
18 29 32 30 21
Table 1 shows these characteristics as a function of temperature for cured polyepoxide. As can be seen, with both surfactants they are extremal in character and are in good agreement with each other. The positions of the strength maximum and the density of cross-linking of the polymers are 0.05% for I and 0.005% for II. Accordingly, the existence of densely packed surfactant layers in the cured system promotes the formation of a cross-linked polyepoxide of high cross-linking density and higher adhesional characteristics for compounds I and II. It follows from Table 1 that in the concentration region 0.003% (I) and 0.003% (II) the presence of surfactact results in the formation of a polymer with a defective structure and correspondingly low adhesion strength. A similar change in the adhesion strength was observed previously [4] in the region of small additions of surfactant. However, this experimental fact has received no satisfactory explanation. Starting from the existing role of surfactants in the formation of the structure and properties of cured polyepoxides, it seemed important to evaluate the effect of the additives I and II on the supermolecular organization of the polymers. In view of this the morphological structure of polyepoxides formed in the presence of different amounts of surfactant was studied. It can be seen from the electron micrographs that the cross-linked polymer without additives (Fig. 3a) has a globular type structure with globules of size 50 and 900 nm. Moreover, individual supermolecular formations are grouped in folded structures of width 50 to 1000 nm. A study of polymers with an increasing content of surfactant has shown that the introduction of surfactants of this class promotes a decrease in the mean size of the globular formations in polyepoxides and the formation in them of a more homogeneous and ordered supermolecular structure. Thus, for example, at II (Fig. 3d) and I (Fig. 3c) concentrations of 0.005 and 0.05 respectively, the polymer structure shows characteristic clear restricted globular formations of diameter 50 to 80 nm, which in their turn form structures of irregular form of maximum extent 250 nm, i.e. both surfactants produce the same changes in structure of the polyepoxide, at concentrations differing by an order of magnitude. The polymer containing 0.003% I is a special case. As can be seen from Fig. 3b, the microstructure of this sample differs sharply from all the remaining, and is characterized by a low density of packing of the random globular formations of different size. Consequently, it can be seen from these morphological data that the surfactants have the same type of effect on the different structural parameters of polyepoxides cured in their presence. Accordingly, the introduction of surface active block-copolymers of dimethylsiloxane and
972
L . S . SHEININA et al.
7/~m
FIG. 3. Electron micrographs of the surfaces of polyepoxidesamples cured in the absence of surfactants (a) and in the presence of different amounts of surfactant I (b, c) and II (d). Surfactant concentrations: 0.003 (b), 0.05 (c), 0.005 (d). (a) magnification x20000 (b--g) magnification xl0000.
alkylene glycols into curing reactive epoxide systems determines their structure both in the liquid and in the cured state. Such structural continuity confirms the microheterogeneous character of the formation of cross-linked polyepoxides, and as a consequence the increased role of of surface phenomena. This confirms the important role of small additions of surfactants, the character of which depends on the conformation, the packing density, and the location in relation to the surface of separation of the surfactant molecules in the surface layer.
Translated by N. STANDEN
REFERENCES 1. V. I. MYSHKO, R. A. VESELOVSKII and Yu. S. LIPATOV, Sintez i fizikokhimiya polimerov (Synthesis and Physical Chemistry of Polymers). No. 10, 77, Kiev, 1972, 2. T. Ye. LIPATOVA, Sh. G. VENGEROVSKAJA, A. Ye. FEINERMAN and L. S. SHEININA, J. Polymer Sci. Polymer Chem. Ed. 21: 2085, 1983. 3. R. A. VESELOVSKH, Fizikokhimiya mnogokomponentnykh polimernykh sistem (Physical Chemistry of Multi-component Polymer Systems) (Edited by Yu. Lipatov). Vol. 1, Kiev, 1986. 4. R. A. VESELOVSKII, Regulirovanie adgezionnoi prochnosti polimerov (Control of the Adhesion Strength of Polymers). Kiev, 1988. 5. R. A. VESELOVSKH, G. V. VYSOTSKAYA and Yu. S. LIPATOV, Dokl. Akad. Nauk SSSR 248: 915, 1979.
Adsorption layers of surface active substances
973
6. L. S. SHEININA, T. Ye. LIPATOV, Sh. G. VENGEROVSKAYA, A. Ye. NESTEROV and Ye. V. LEBEDEV, Vysokomol. soyed. A23: 1358, 1981 (translated in Polymer Sci. U.S.S.R. 23: 6, 1504, 1981). 7. T. Ye. LIPATOV, Pure and Applied Chem. 43: No. 1/2, 27, 1975. 8. S. G. KULICHIKIIlN, P. A. ASTAKHOV, Yu. P. CHERNOV, V. A. KOZHINA, L. I. GOLUBENKOVA and A. Ya. MALKIN, Vysokomol. soyed. A28: 2115, 1986 (translated in Polymer Sci. U.S.S.R. 28: 10, 2350, 1986). 9. Kh. LU and K. NEVILL, Spravochnoe rukovodstvo po epoksidnym smolam (Handbook on epoxide resins). Moscow, 1974. 10. P. P. PUGACHEVICH, Ye. M. BEGLYAROV and L A. LAVYGIN, Poverkhnostnye yavlenia v polimerakh (Surface Phenomena in Polymers). Moscow, 1982. 11. A. Ye. FAINERMAN and Yu. S. LIPATOV, Informatsionnoe pismo (Informational letter). No. 6, Inst. Chem. High Molecular Compounds, Academy of Sciences of the Ukrainian SSR, 1969. 12. A. Yu. KOSHEVNIK, M. M. KUSAKOV and N. M. LUBMAN, Zh. Fiz. khimiya 27: 1887, 1953. 13. A. ZARCZYNSKI, Polymery 13: 156, 1968. 14. J. J. JASPER and B. L. HOUSEMAN, J. Phys. Chem. 69: 310, 1965. 15. V. A. OGAREV, V. V. ARSLANOV and A. A. TRAPEZNIKOV, Kolloid. Zh. 34: 372, 1972. 16. A. A. TRAPEZNIKOV, T. I. ZATSEPINA, T. A. GRACHEVA, R. N. SHCHERBAKOVA and V. A. OGAREV, Dokl. Akad. Nauk SSSR 160: 174, 1965. 17. M. A. MARKEVICH, B. L. RYTOV, L. V. VLADIMIROV, D. P. SHASHKIN, P. A. SHIRYAYEV and A. G. SOLOVEV, Vysokomol. soyed. A28: 1595, 1986 (translated in Polymer Sci. U.S.S.R. 28: 8, 1773, 1986).
0032-3950/91 $15.00 + .00 © 1992Pergamon Press Ltd
THE ROLE OF ADSORPTION LAYERS OF SURFACE ACTIVE SUBSTANCES ON THE FORMATION OF POLYEPOXIDES* L. S. S H E I N I N A , SH. G. V E N G E R O V S K A Y A , R. A. VESELOVSKII and V. V. DAVIDENKO Institute for the Chemistry of High Molecular Compounds, Academy of Sciences of the U.S.S.R.
(Received 25 June 1990)
The thermodynamics of adsorption layers of siloxane surfactants in simple binary systems simulating real epoxide compositions are studied. The microstructure, liability to swelling, and adhesionai strength of polyepoxides cured in the presence of a number of different surfactant concentrations are indicated. All the polymer characteristics studied are essentially dependent on the quantity of surfactant introduced, but these relations are non-monotonic in character. The observed relations can be explained in terms of the different structures of the adsorption layers, formed in reaction systems at the polymer formation stage.
SOLUTIONSof surfactants in heterogeneous oligomers show both a typical change in properties of the binary system at the separation interface [1, 2], and also a number of special features. The ability to react chemically with or to be indifferent [3, 4] to the oligomer-solvent is a specific feature of the behaviour of surfactants on introduction into reactive oligomers. As is shown, in the first case it is possible to obtain a stable increase in a number of properties of polymers obtained in the presence of surfactants, including the adhesional strength, while in the second case the relation between these properties and the surfactant concentration is extremal in character [3]. Furthermore, as the oligomer is consumed and the MM of the product increases the polarity of the medium [3] and the solubility of the surfactant in the reaction mixture [3] are changed, which can lead to a change in its role as a structure former [5]. Thus, it is known that for polyurethane systems the mechanism of the structuring action of indifferent surfactants is due to the formation of adsorption layers in the bulk of the reaction mixture and at the interface with the air, and is determined by the structure of the latter, which depends on the concentration of the surfactant additive [2, 6]. This phenomenon is a consequence of the microheterogeneity of the reaction mixtures [7] arising during hardening of the cross-linked polyurethanes. It is known from published information that the rheokinetics of the formation of cross-linked epoxide polymers can be described adequately only by using as a starting point the concept that local cross-linked formations, i.e. microgels [8], are formed in the reaction system. In such systems, which have a developed internal separation interface the role of surfactants is very important, but this aspect of the effect of a substance having high surface activity has been very little studied. In view of the above, this work is concerned with establishing the effect and role of surfactants in the formation of the structure of cross-linked polyepoxides. The materials used were ED-20 epoxide resin (M -- 350, epoxide number 22.9, evacuated under a pressure of 133 Pa at 365 K), polyepoxides cured by the amine method, and also a phenylglycidyl ether (PGE), a chemically pure low viscosity compound, having the chemical structure of half of the ED-20 molecule, i.e. convenient as a model [9]. The PGE was prepared by vacuum distillation (375 K/19 Pa) after holding above Call2 over one month. The residual moisture content of the epoxide *Vysokomoi. soyed. A33: No. 5, 1055-1061, 1991.
967
968
L.S. SHEININAet aL
compounds was <0.02%. In order to study the structure forming role of the surfactant, oligomeric surfactants were used, i.e. block-copolymers of dimethylsiloxane and alkyleneglycol, chemically indifferent to the components of the system under study. These are effective surfactants, operating at very low concentrations, which gives reality to studying the structure of their adsorption layers and provides a path to a solution of the problem posed.
--Si--O--
(CHs)s--Si--O--
--[--Si--O--/---Si--(CHa)a
Lc .
(CrI,),I I
J~
(O~aH6)n - OC4H9 (a~3-4, m~t3, n~t7, x~50-55). l- CHa I
1
-
(CHa)a--Si--O-- ] --Si-'O--/--
L
I
-I CHs
--~i--O--
_1,
I
L
H
-
--Si--(CHa) s (n = 9--i0)
I
_17
The surfactant adsorption layers were studied thermodynamically on the basis of data on the surface tension of their solutions in PGE, obtained by the Wilhelm method. In view of the long time required to establish the equilibrium state of the surface layers of high molecular surfactants [10] their solutions were held for two weeks before determining the values of 7, which were measured in thermostatted glass cells, using the method described in [11]. The surface tension was measured during the curing action by the suspended droplet method [12]. The dynamic viscosity of the compositions was measured with a Rheostat-2 setup in a cylinder-cylinder measuring unit at a constant shear rate and a temperature of 298 + 0.1 K. The equilibrium swelling of the cross-linked polyepoxides was studied by a volume method in Zarczynski-type dilatometers [13] at 298 + 0.1 K. The degree of equilibrium swelling was defined as the ratio of the volume of absorbed solvent to the volume of polymer up to swelling. The microstructure of the cured polymer samples was studied with carbon replicas of the surfaces of ground off samples under a JEM-100C electron microscope. The strength on normal peeling off was determined in accordance with GOST 14760-69 for polymers cured over 7 days at 298 + 2 K. The introduction of either surfactant (I or II) into ED-20 leads to a significant decrease in 3' (limiting values 27.1 and 38.8 mN/m respectively). As can be seen, in the case of I this effect is significant, and in using a model compound with 3'20 = 40.9 mN/m it is advisable to study solutions of compound I in it. The effects of temperature (283-313 K) and concentration [0.001-0.1%, within the range over which the critical micelle concentration (CMC) is not attained] on the surface tension of its solutions in PGE and correspondingly on the surface pressure r is calculated in [14]. From the plots of the surface pressure p against surfactant concentration in Fig. 1, for boundary values of the temperature it can be seen that the maximum change of surface pressure is observed at a low temperature in the region of concentration of the additive I<0.01%. At higher surfactant concentrations the increase in surfactant concentration has very little effect on the increase in surface pressure.
Adsorption layers of surface active substances "q, Pa s
969
~mN/m
• ,,&.
Z ,~
5
c~x 102, %
I5
18
FIG. 1. Plots of the dynamic viscosityof ED-20 resin (1) and the product from the interaction ED-20 resin and PGE (85 : 15 by weight) with polyethylenepolyamine (4) at 298°C and the surface pressure of solutions of I in PGE at 283 (3) and 308 K (3) against concentration. Starting from the values of p at each of the temperatures studied and at all concentrations, the values of the surface entropies ASs and enthalpies AHs characterizing the thermodynamics of formation of an adsorption surfactant layer under corresponding conditions (Fig. 2) are calculated. The plots of ASs and AH, against concentration bring out the extremal character of the changes in the thermodynamic functions over the range 298-313 K (Fig. 2, plots 4, 4'--6, 6') and their approximations to the minimum value of lower temperatures (Fig. 2, plots 1, 1'-3, 3'). The decrease in temperature leads to broadening of the maximum and displacement of its position in the region of higher concentrations (0.01-0.075%). The decrease in both the thermodynamic functions to the minimum values is an indication of the formation of a more ordered structure of the surface layer
[15].
I88
x 7o~ %
#fl
:£ 0
-50 Fro. 2. Plots of the thermodynamicfunctions AS (1-6) and AH (1'-6') of surface layers of solutions of I in PGE against surfactant concentration at 283 (1, 1'), 288 (2, 2'), 293 (3, 3'), 298 (4, 4'), 308 (5, 5'), and 311 K (6, 6'). PS 3 3 : 5 - I
970
L.S. SHEI/qlblAet al.
According to the ideas developed in [15], on compression of a PDMS monolayer reorganization of its structure occurs, from molecules convoluted into the form of fiat disks [16], through consolidation of the two-dimensional formations, to a subsequent change in conformation of the molecules into a helix-like form. Such restructuring is accompanied by a transition of the values of the function AS into the region of negative values [15]. Consolidation of a siloxane surfactant monolayer because of the growth of the number of molecules required per unit surface obeys the same laws, irrespective of its causes: compressive force [15] or growth in the concentration of surfactant in the system, as in our case. Making the same analogy, and starting from the observed change in the thermodynamic functions (Fig. 2), it can be concluded that the functions ASs and AHs in the region of negative values describe a process of packing lyophobic molecular fragments of helix-like conformation up to a state of maximum order (region of the extremum). With further increase in the concentration of compound I there is a gradual disordering of the adsorption layer [15], which is accompanied by an increase in the values of AS and AH (Fig. 2, curves 4, 4'-6, 6'). Accordingly, it follows from the given thermodynamic study of solutions of I in PGE that the plots of ASs and AH~ against concentration enable the values of Csur to be determined at which a surface layer is formed having the maximum packing density of the additive molecules introduced. These concentrations lie in a region of values well known to be below the critical and depend on the temperature. Further study shows that the relations obtained for the thermodynamic functions can be applied to real systems. Figure 1 (plots I and 4) shows relations for the viscosity of ED-30 and an epoxide composition, which are the product of the curing of a mixture of ED-20 and PGE (weight ratio 85 : 15), and I wt% ethylenediamine as a function of the concentration of additive I. As can be seen, these relations indicate a decrease in viscosity with a minimum value at 0.05% surfactant, which is in good agreement with the position of the extremum at the corresponding temperature on the plots of ASs and AHs against concentration (Fig. 1, plots 4, 4'). It follows from all this data that on introducing compound I the viscosity of ED-20 in the reaction mixture is decreased, the extent of this decrease being directly proportional to the density of packing of the surfactant molecules in the adsorption layer. Since in this case it is a bulk parameter which is measured, i.e. the viscosity, we are speaking of the effect of two interfaces: the solution-air interface, and the interface determined by the microheterogeneous structure of the epoxide oligomer itself [17] or the reaction mixture. The latter is confirmed by the data on studying the change in surface tension in the system without a surfactant and containing 0.05% of compound I. Over 1.5 h (time to the beginning of the passage of the reaction mixture into the solid state) the system devoid of surfactant during curing has an almost constant value of 3'~45.5 mN/m, and in the presence of surfactant there is a constant increase in this value from 31 to 36 mN/m. Consequently, the increase in 3' observed in the presence of surfactant can be a consequence of the appearance and quantitative growth of 3' in the system of microgel formations with corresponding redistribution of surfactant between the reaction mass-air interface and the interface between the liquid part of the reaction mass and the microgel. The gel formation process in the formation of cross-linked epoxide polymers by the amine curing method begins fairly rapidly and it is to be expected that the change in properties of the polyepoxide in the presence of various quantities of surfactant must also obey the relations in which there is an extremum at concentrations found on studying the thermodynamic functions. In order to test this proposition, equilibrium swelling in acetone of polymer samples in the form of identical tablets and the strength in normal breaking of an adhesive joint, obtained from cross-linked polymers with different contents of each of the surfactants were studied. In all these studies surfactants I and II were used, which differ in the structure of the side chains and, as was shown above, are able to decrease the surface tension of the epoxide resin.
Adsorption layers of surface active substances
TABLE 1.
971
C O M P A R A T I V E C H A R A C T E R I S T I C S OF E Q U I L I B R I U M SWELL OF
P O L Y E P O X I D E IN T H E PRESENCE OF V A R I O U S Q U A N T I T I E S OF S U R F A C T A N T
Surfactant concentration I
0.0 0.003 0.03 0.05 0.07 0.1
II 0.0
Equilibrium swelling in acetone
Maximum strength MPa
1.9
24
--
2.3
20
--
1.5
28
--
1.4
30
--
1.6
27
--
2.2
20
0.0003 0.003 0.005 0.007 0.01
2.5 1.6 1.4 1.5 2.0
18 29 32 30 21
Table 1 shows these characteristics as a function of temperature for cured polyepoxide. As can be seen, with both surfactants they are extremal in character and are in good agreement with each other. The positions of the strength maximum and the density of cross-linking of the polymers are 0.05% for I and 0.005% for II. Accordingly, the existence of densely packed surfactant layers in the cured system promotes the formation of a cross-linked polyepoxide of high cross-linking density and higher adhesional characteristics for compounds I and II. It follows from Table 1 that in the concentration region 0.003% (I) and 0.003% (II) the presence of surfactact results in the formation of a polymer with a defective structure and correspondingly low adhesion strength. A similar change in the adhesion strength was observed previously [4] in the region of small additions of surfactant. However, this experimental fact has received no satisfactory explanation. Starting from the existing role of surfactants in the formation of the structure and properties of cured polyepoxides, it seemed important to evaluate the effect of the additives I and II on the supermolecular organization of the polymers. In view of this the morphological structure of polyepoxides formed in the presence of different amounts of surfactant was studied. It can be seen from the electron micrographs that the cross-linked polymer without additives (Fig. 3a) has a globular type structure with globules of size 50 and 900 nm. Moreover, individual supermolecular formations are grouped in folded structures of width 50 to 1000 nm. A study of polymers with an increasing content of surfactant has shown that the introduction of surfactants of this class promotes a decrease in the mean size of the globular formations in polyepoxides and the formation in them of a more homogeneous and ordered supermolecular structure. Thus, for example, at II (Fig. 3d) and I (Fig. 3c) concentrations of 0.005 and 0.05 respectively, the polymer structure shows characteristic clear restricted globular formations of diameter 50 to 80 nm, which in their turn form structures of irregular form of maximum extent 250 nm, i.e. both surfactants produce the same changes in structure of the polyepoxide, at concentrations differing by an order of magnitude. The polymer containing 0.003% I is a special case. As can be seen from Fig. 3b, the microstructure of this sample differs sharply from all the remaining, and is characterized by a low density of packing of the random globular formations of different size. Consequently, it can be seen from these morphological data that the surfactants have the same type of effect on the different structural parameters of polyepoxides cured in their presence. Accordingly, the introduction of surface active block-copolymers of dimethylsiloxane and
972
L . S . SHEININA et al.
7/~m
FIG. 3. Electron micrographs of the surfaces of polyepoxidesamples cured in the absence of surfactants (a) and in the presence of different amounts of surfactant I (b, c) and II (d). Surfactant concentrations: 0.003 (b), 0.05 (c), 0.005 (d). (a) magnification x20000 (b--g) magnification xl0000.
alkylene glycols into curing reactive epoxide systems determines their structure both in the liquid and in the cured state. Such structural continuity confirms the microheterogeneous character of the formation of cross-linked polyepoxides, and as a consequence the increased role of of surface phenomena. This confirms the important role of small additions of surfactants, the character of which depends on the conformation, the packing density, and the location in relation to the surface of separation of the surfactant molecules in the surface layer.
Translated by N. STANDEN
REFERENCES 1. V. I. MYSHKO, R. A. VESELOVSKII and Yu. S. LIPATOV, Sintez i fizikokhimiya polimerov (Synthesis and Physical Chemistry of Polymers). No. 10, 77, Kiev, 1972, 2. T. Ye. LIPATOVA, Sh. G. VENGEROVSKAJA, A. Ye. FEINERMAN and L. S. SHEININA, J. Polymer Sci. Polymer Chem. Ed. 21: 2085, 1983. 3. R. A. VESELOVSKH, Fizikokhimiya mnogokomponentnykh polimernykh sistem (Physical Chemistry of Multi-component Polymer Systems) (Edited by Yu. Lipatov). Vol. 1, Kiev, 1986. 4. R. A. VESELOVSKII, Regulirovanie adgezionnoi prochnosti polimerov (Control of the Adhesion Strength of Polymers). Kiev, 1988. 5. R. A. VESELOVSKH, G. V. VYSOTSKAYA and Yu. S. LIPATOV, Dokl. Akad. Nauk SSSR 248: 915, 1979.
Adsorption layers of surface active substances
973
6. L. S. SHEININA, T. Ye. LIPATOV, Sh. G. VENGEROVSKAYA, A. Ye. NESTEROV and Ye. V. LEBEDEV, Vysokomol. soyed. A23: 1358, 1981 (translated in Polymer Sci. U.S.S.R. 23: 6, 1504, 1981). 7. T. Ye. LIPATOV, Pure and Applied Chem. 43: No. 1/2, 27, 1975. 8. S. G. KULICHIKIIlN, P. A. ASTAKHOV, Yu. P. CHERNOV, V. A. KOZHINA, L. I. GOLUBENKOVA and A. Ya. MALKIN, Vysokomol. soyed. A28: 2115, 1986 (translated in Polymer Sci. U.S.S.R. 28: 10, 2350, 1986). 9. Kh. LU and K. NEVILL, Spravochnoe rukovodstvo po epoksidnym smolam (Handbook on epoxide resins). Moscow, 1974. 10. P. P. PUGACHEVICH, Ye. M. BEGLYAROV and L A. LAVYGIN, Poverkhnostnye yavlenia v polimerakh (Surface Phenomena in Polymers). Moscow, 1982. 11. A. Ye. FAINERMAN and Yu. S. LIPATOV, Informatsionnoe pismo (Informational letter). No. 6, Inst. Chem. High Molecular Compounds, Academy of Sciences of the Ukrainian SSR, 1969. 12. A. Yu. KOSHEVNIK, M. M. KUSAKOV and N. M. LUBMAN, Zh. Fiz. khimiya 27: 1887, 1953. 13. A. ZARCZYNSKI, Polymery 13: 156, 1968. 14. J. J. JASPER and B. L. HOUSEMAN, J. Phys. Chem. 69: 310, 1965. 15. V. A. OGAREV, V. V. ARSLANOV and A. A. TRAPEZNIKOV, Kolloid. Zh. 34: 372, 1972. 16. A. A. TRAPEZNIKOV, T. I. ZATSEPINA, T. A. GRACHEVA, R. N. SHCHERBAKOVA and V. A. OGAREV, Dokl. Akad. Nauk SSSR 160: 174, 1965. 17. M. A. MARKEVICH, B. L. RYTOV, L. V. VLADIMIROV, D. P. SHASHKIN, P. A. SHIRYAYEV and A. G. SOLOVEV, Vysokomol. soyed. A28: 1595, 1986 (translated in Polymer Sci. U.S.S.R. 28: 8, 1773, 1986).