Langmuir 1995,11, 1688-1692
1688
Adhesive Interaction in Aqueous Media between Polymer Surfaces Grafted with Anionic and Cationic Polymer Chains Junfeng Zhang, Emiko Uchida, Yoshikimi Uyama, and Yoshito Ikada" Research Center for Biomedical Engineering, Kyoto University, 53 Kawaharacho, Shogoin, Sakyoku, Kyoto, 606 Japan Received July 19, 1994. I n Final Form: February 2, 1995@ A polyester film surface was graft-polymerized with an anionic monomer, acrylic acid, and a cationic monomer, N,N-(dimethy1amino)ethyl methacrylate. Appreciable adhesive interaction was observed instantaneously in water when the oppositely charged film pair was brought into direct contact in the presence of water. The shear strength depended on the duration of loading between the two surfaces in water, but insignificantly depended on the graft density if the film pair was allowed to contact under low loading. The adhesive shear strength leveled off at 20-30 N/cm2, when the graft density approached 2 x pmol of monomer unit/cm2for both the films. The strong adhesive interaction could be attributed to the Coulombic attractive force between the grafted anionic and cationic polymer chains since salt concentrationand pH exhibited a remarkable influence on the adhesive interaction. The adhesion between the oppositely charged film pair in distilled water was drastically reduced or vanished when salt was added to the water at high concentrations. There was a difference by a factor of 102-103 in the surface density of graft chains between the observed and the estimated from shear strength.
Introduction Bonding of two objectives is practically accomplished in the atmosphere of air or vacuum using adhesives unless mechanical devices such as nails and bolt-nut units are employed. However, in the world ofbiology, two different objectives are bonded in aqueous media without any additional adhesives. Antigen-antibody conjugates, cellcell aggregates, and shell-ligament bonding of shellfishes are the typical examples. The bonding without adhesives in the presence of water may be achieved through molecular forces such as van der Waals' interaction, hydrogen bonding, hydrophobic interaction, and Coulombic interaction. Such molecular forces play a role also in the bonding with synthetic adhesives. Recently, molecular forces between polymer surfaces in aqueous media have been used in a n attempt to direct measure atomic force microscopy (AF'M) equipped with a cantilever or surface force measurement apparatus (SFA) provided with a pair ~ f m i c a . ' - ~ In the latter case, polymer molecules such as poly(ethy1ene oxide) are physically adsorbed on the smooth mica surface to study the molecular interaction between polymer chain^.^^^ However, it would occasionally happen that the physically adsorbed macromolecular chains desorb from the substrate surface if a drastic change is given to the environment, for instance, by the addition of organic solvents, surfactants, and ions. This desorption can be avoided by covalently fixing soluble polymer chains on a solid polymer surface through immobilization technologies such as surface graft polymerization of monomers or coupling reaction of existing polymer molecules onto the substrate polymer.6 A problem of this surface grafting technology
* To whom correspondence should be addressed. Abstract published in Advance A C S Abstracts, April 15,1995. (1)Israelachvili, J. N. Intermolecular and Surface Forces: with Applications to Colloidal and Biological Systems; Academic Press: London. 1985. (2) Israelachvili, J. N.; Adams, G. E. J . Chem. Soc., Faraday Trans. 1001.- - ~ -11987. - - - - ,74. -, ~ (3) Klein, J. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 99. (4)Mama, J.;Hair, M. L. J . Phys. Chem. 1988, 92, 6044. ( 5 ) Kurihara, K.; Kunitake, T.; Higashi, N.; Niwa, M.Langmuir 1992, -R , -2-n-w. . (6)Ikada, Y.Biomaterials 1994, 15 (101, 725. @
is a lack of analytical methods effective for the determination of the grafted surface structure. The graft density will be as low as 10-1 ,ug/cm2 if the surface of substrate is immobilizedin amonolayer state a t the nearest neighbor distance of 10 nm with polymer chains of the average molecular weight of lo5. However, surface graft polymerization takes place not only onto the outermost surface of the substrate polymer but in the subsurface region even for monomers that apparently cannot penetrate into the substrate polymer matrix, yielding high graft density. In previous papers, we have reported adhesion between two films having surface-grafted polymer chain^.^,^ The grafting was carried out by graft polymerization of watersoluble monomers onto the surface of hydrophobic films, and the adhesion was estimated by measuring the shear strength of two films lapped in the presence of water, followed by spontaneous drying of the water present a t the interface between the two films under air. It was found that significant adhesion appeared between the two films grafted with nonionic polymers through the polymer chains entanglement caused by water evaporation from the interface, whereas appreciable adhesion instantaneously occurred upon contact of two films in water although lots of water molecules were still present in the graft layer, if the films carried oppositely charged ionic groups on their surface. The purpose of the present work is to measure the adhesive force appearing between two films, one ofwhich possesses anionically charged graft chains and the other of which has cationically charged graft chains, the two films being always kept in aqueous environment without drying both before and after film contact. The substrate polymer used is a thin film of poly(ethy1ene terephthalate) (PET) because PET is a crystalline, chemically inert polymer with a high glass transition temperature and yields a very thin tough film with high smooth surface in comparison with other conventional polymers. The monomers used for surface graft polymerization are acrylic acid (7) Chen, K.-S.; Uyama, Y.; Ikada, Y. J . Adhes. Sci. Technol. 1992, 6 (91, 1023. (8)Chen, K . 3 ; Uyama, Y.; Ikada, Y. Langmuir 1994, 10, 1319.
0743-746319512411-1688$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11,No. 5, 1995 1689
Adhesive Interaction in Aqueous Media (AAc) as anionic and NJV-(dimethy1amino)ethyl methacrylate (DMAEMA) as cationic monomer.
Experimental Section Materials. PET film with a thickness of 40 pm was kindly suppliedby Teijin Co., Ltd., Tokyo, Japan, and purified by Soxhlet extraction with methanol for 20 h before use. The surface was macroscopically smooth when inspected by a surface texture measuring instrument made by Tokyo Seimitsu Co., Ltd., Tokyo, Japan, and a scanning electron microscope at a magnification of 5000x. The average roughness (R,) was 0.5 pm. AAc and DMAEMA were both of chemical grade and were purified by conventionaldistillation. Other chemicalswere of chemical grade and were used as received without further purification. Graft Polymerization. Graft polymerization of the monomers onto the PET film was performed according to the method described el~ewhere.~ Briefly, the PET film of 1.5 x 7.0 cm2was placed in a Pyrex test tube together with 10%aqueous monomer solution to which sodium periodate (NaI04) was added by 1.0 x M. To effect graft polymerization, the mixture was kept at 30 "C without degassing and was irradiated with W from a high-pressure mercury lamp (1kW, Riko-1000 HL type) a t a distance of 10 cm from the tube for predetermined periods of time, ranging from 0.5 to 3 h in order to yield film surfaces of different graft densities. Care was taken for the film to be homogeneously irradiated with W. NaI04 was added to the monomer solution because NaI04 photochemicallyconsumes the oxygen dissolvedin the monomer solution upon W i r r a d i a t i ~ n . ~ Otherwise,oxygen would inhibit graft polymerizationthat should start from the polymer radicals formed on and in the film by W irradiation. The homopolymer formed was removed by washing the grafted film with running tap water and then immersing in distilled water a t 70 "C for 20 h under continuous stirring. The water for homopolymer removal was repeatedly renewed until it contained no trace of homopolymer. The AAc and DMAEMA polymers grafted onto the film were quantitatively determined by the dyeing method described elsewhere,1° and the surface density was recorded in a unit of pmole of repeating monomer units per square centimeter of film surface under the assumption of complete surface smoothness. In the present study, an AAcgrafted film with a graft density of 6.9 x pmol/cm2 and a DMAEMA-grafted film with a graft density of 3.2 x pmoll cm2were used, unless otherwise stated. The molecular weight of grafted AAc and DMAEMA chains was estimated by measuring the homopolymer elution time of gel filtration chromatography using TSK-gel G6000PW~and G3000 P W n columns (ToyoSoda Co. Ltd., Tokyo, Japan). Polymer was eluted with 1 M NaCl aqueous solution at a flow rate of 0.9 mumin. The molecular weight was calculated referring to the calibration curve obtained from poly(ethy1ene glycol) standard with molecular weights of 300-500 000. Surface Analysis. The static contact angle of grafted films against water was measured at 25 "C and 65%RH with a sessile drop method using a 3-pL water droplet and a telescopic goniometer (M2010A-611 type, Elma Inc., Tokyo, Japan). The magnification power of the telescope was 40 x and was equipped with a protractor of 1" graduation. At least 10 readings on different surface locations were made to obtain the averaged contact angle. The (-potential of grafted film surfaces was measured at 25 "C, pH 5.8, and anionic strength of 1.0 x 10-3(KCl)by a streaming potential method using the cell unit described by Van Wagenen and Andrade.ll X-ray photoelectron spectroscopy (XPS)spectra were recorded with Shimadzu 850 photoelectron spectrometer using Mg K a l , ~ as the X-ray source. Surface Observation. An opticalphotograph was taken aRer staining the cross-section of DMAEMA-grafted surface with CI Acid Red 26, which was not able to stain the hydrophobicsubstrate polymer but only the grafted hydrophilicregion. The AAc-grafted (9) Uchida, E.; Uyama, Y.; Ikada, Y. J.Appl. Polym. Sci. 1990,41, 677. (10) Uchida, E.; Uyama, Y.; Ikada, Y. Langmuir 1993,9,1121. (11)Van Wagenen, R. V.; Andrade, J. D. J. Colloid Interface Sci. 1980,76,305.
Table 1. Water Contact Angle and Potential of Surface-ModifiedPET Films contact 5 graft density6 surface angle (deg) potentiala (mV) (umoVcm2) -40 f 3 0 virgin 64 f 2 6.9 x AAc-grafied 39 f 2 -58 f 2 3.2 x +39 f 4 DMAEMA-grafted 78 f 3 ~~
a
Ionic strength,
Based on repeated monomer units.
(a)
Figure 1. Cross-section of AAc- and DMAEMA-grafted PET films. (a) Virgin, (b) AAc-grafted, and (c) DMAEMA-grafted. PET surface was stained with methylene blue, and the photograph of the cross-section was taken in a similar manner as above. Adhesive Force Measurement. The adhesive force between a film pair was determined by measuring the shear strength with the aid of a tensile testing machine, after lapping the two strips together in aqueous media and bringing into close contact under a constant load of 4.9 N or using fingers. The pressure given by the fingers was approximately 1.0 N. The tensile machine was manufactured by Shimadzu Inc., Kyoto, Japan. The contact area of films for the shear strength measurement was fured to 0.5 x 0.5 cm2. The shear strength was measured at a crosshead speed of 10 m d m i n , always keeping the film specimen in the aqueous environment at 25 "C.
Results Surface Grafting. The PET film surface became hydrophilic upon graft polymerization ofAAc. The contact angle of the surface-modified films is presented in Table 1, together with the graft density and the observed 5 potential. The water contact angle of PET film was reduced from 64" of the virgin to 39" by graft polymerization of AAc even though the graft density was as low as 6.9 x ,umollcm2,whereas graft polymerization of DMAEMA yielded more hydrophobic surface than the virgin. Although poly(DMAEMA) is soluble in water in 25 "C, its water solubility is low because it contains both hydrophilic and hydrophobic moieties. The lower critical solution temperature (LCST)of the polymer is around 55 "C, as demonstrated previously.12 Figure 1 shows the optical microscopic photograph of the cross-section of the^ AAc- and DMAEMA-graftedPET films after staining. As can be seen, the graft layer is localized at the surface region for both the films. This is in good accordance with our previous result^.^ Since there is no effective method to evaluate the molecular weight of graft chains, we determined only the average molecular weight of homopolymers formed in the outer solution of the graft polymerization. They were found to be 4.0 x lo5for AAc and 1.5 x lo5 for DMAEMA graft polymerization. (12) Uchida, E.; Uyama, Y.; Ikada, Y. Langmuir 1994,10,1193.
Zhang et al.
1690 Langmuir, Vol. 11, No. 5, 1995
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L
io-2 io-’ i o o [KCII (M) Figure3. Influence of addedionon the shear strength between AAc- and DMAEMA-grafted PET films. Graft density: [AAc] = 6.9 x ymol/cm2and [DMAEMA]= 3.2 x 10-2pmol/cm2. 10 I 0
10,000
20,000
n
Time(s) Figure2. Dependence of shear strength in DDW between AAcand DMAEMA-grafted PET films on the contacting time (the inset is the semi-logarithmicplot. Graft density: [AAcl = 6.9 x pmol/cm2 and [DMAEMA]= 3.2 x umol/cm2.
Shear Strength. The shear strength in doubly distilled water (DDW)between the AAc-grafted and the DMAEMAgrafted film after lapping them under a load of 4.9 N is shown in Figure 2 as a function of duration of contact time. Apparently, the two films instantaneously attracted each other, exhibiting a shear strength of 28 N/cm2. After that, the shear strength gradually increased. Once the films were separated by stress application, they exhibited no significant adhesion any more even when again brought into close contact by loading in DDW. As can be seen from the inset semi-logarithmic plot of the shear strength against the loading time, the plateau strength was reached around 28 N/cm2 within 100 s, but the shear strength steeply increased again after loading for about 10 000 s. As is well known, ionic interaction in aqueous media is greatly influenced by salt ions. To examine the salt effect, shear strength was measured in KC1 aqueous solution of different concentrations after lapping the film pair in the solution under 1.0 N. The result is shown in Figure 3. It is evident that the shear strength remarkably decreased with the increasing KC1 concentration. Also in this case, the shear strength in 1.0 x M KC1 increased again when a load of 4.9 N was given for 2 x lo4 s, similar to Figure 2 (data not shown). The delayed increase in strength may be ascribed to enhanced entanglement of the graft chains, as will be discussed later. In the following experiments, shear strength was measured without adding the load of 4.9 N on the film pair by simply pressing the fingers in order to avoid such a n additional complicated factor as the entanglement of graft chains. If the Coulombic attractive force is operative between the two oppositely charged surfaces, adhesion will be influenced not only by the ionic strength of aqueous medium but also by pH, because PAAc is an acid with a pKa of 4.75 and PDMAEMA is a weak base with a pKa of 7.2.12 Figure 4 shows the dependence ofthe shear strength of the film pair on pH of the medium, which was adjusted
16 L
m Q) r v)
12
I
\1
t
\
I
I d
2
4
6
8 1 0 1 2
PH Figure 4. Influence of pH on the shear strength between AAcand DMAEMA-grafted PET films without added ion. Graft density: [AAc] = 6.9 x pmoVcm2and [DMAEMA] = 3.2 x ymoVcm2. by adding HC1 and NaOH to DDW. Obviously, the shear strength became maximum around pH 7, where the extent of ionization seems to be optimal for the couple of PAAc and PDMAEMA. Figure 5 gives the change of the shear strength between the AAc- and DMAEMA-grafted PET films with time when they were lapped in DDW and then kept in aqueous KC1 media of different KC1 concentrations. The shear strength remained unvaried when the lapped film pair was kept in DDW for 30 h, but it decreased markedly in KC1 solutions. The rate of the strength reduction in 1.0 N KC1 solution was greater than that in 0.1 N KC1. The film pair separated by immersion in 1.0 N KC1 exhibited adhesion again when brought into contact in DDW. To study the dependence of the graft density of film on the shear strength, grafted film pairs with different graft densities were subjected to the strength measurement. Figure 6 shows the shear strength between a n AAc-grafted film with a graft density of 2.1 x ymol/cm2 and DMAEMA-grafted films with different graft densities, while the shear strength observed between a DMAEMAgrafted film with a constant graft density of 6.4 x pmol/cm2 and AAc-grafted films with various graft densities is shown in Figure 7. As can be seen clearly from Figures 6 and 7, the shear strength is maintained in the
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Adhesive Interaction in Aqueous Media 30
Langmuir, Vol. 11, No. 5, 1995 1691
YE
p
I d
N n
201
Z
v
5 CI) t Q)
m
f; L
Q
r 0
20
10
30
lo/
Time (hr) Figure 5. Reduction of shear strength between AAc- and DMAEMA-graftedPET films upon standingin different media. Graft density: [AAcl = 6.9 x pmol/cm2and [DMAEMA] = 3.2 x pmol/cm2. (0) in DDW, ( 0 )in 0.1 M KC1, (A) in 1.0 M KC1.
0
20
10
AAc Grafted (1 0-2pmol/cm2 ) Figure 7. Influence of graft density of AAc on the shear strength in DDW with the graft density of D M M A fixed to 6.4 x 10-2pmol/cm2.
30
Table 2. O/C and N/C Ratios of AAc-Grafted PET Surface with a Graft Density of 6.9 x pmoYcm2 Of c NIC
n
N
fi
v)
20
before adhesion after bonding fracture
\
z
0.48 0.30
0 0.025
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c
+J cn E
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c, v)
10
b.
m W c v)
I 1
0
10
20
DMAEMA Grafted (1 0-2Mmol/cm2) Figure 6. Influence of graft density of DMAEMA on the shear strength in DDW with the graft density of AAc fixed to 2.1 x
pmol/cm2. range of 20-30 N/cm2 so far as the graft density of both pmol/cm2. the films exceeds 2 x In order to study whether or not the adhered anionic and cationic polymer chains will be broken during the tensile measurement, the AAc-grafted PET film was subjected to XPS after the tensile measurement. The results of the O K and N/C ratios are presented in Table 2. The appearance of the N atom, which should be due to fragments of DMAEMA polymer chains remaining after bonding fracture on the AAc-grafted surface, gives evidence of chain scission. Another evidence is the decrease of the O/C ratio because PDMAEMA has a lower oxygen content than PAAc. Discussion When any film pair chosen from different grafted surfaces approaches each other in an aqueous environment, attractive or repulsive molecular forces will become operative between the grafted chains. However, the
molecular force is too difficult to directly measure, unless the surface is smooth enough at the molecular l e ~ e l . l ~ - ~ ~ The PET film is macroscopically smooth but seemingly rough a t the molecular level. It is, however, likely that the fraction of directly contacting area between two films significantly increases by each covering of the rough hard surface with a soft, water-swollen, graft thin layer. Throughout the strength measurement, we attempted to bring two films in the presence of water into contact as close as possible by giving a pressure on them through loading or pressing with fingers. It is obvious from Figure 2 that the rapid increase in shear strength takes place through two stages. The first increase happened almost instantaneously when a pair of cation-grafted and aniongrafted films was brought into contact each other, the shear strength approaching 28 N/cm2. On the other hand, the second increase in shear strength took place after contact for much longer times, resulting in a shear strength of about 37 N/cm2. This second increase in strength seems to be caused by physical entanglement between polymer chains grafted to the different film surfaces, because the strength increased to a similar high level that was observed upon removing the water existing between the two lapped films even for film pairs having ionic charges of similar sign, as described p r e v i ~ u s l y .When ~ two identical AAcgrafted surfaces were brought into contact in the presence of water, no appreciable shear strength was observed in the beginning and showed a certain value only when water was expelled from the interface by simple diffusion or (13)Israelachvili, J. N.; Tirrell, M.; Klein, J.; Almog, Y.Macromolecules 1984, 17, 204. (14)Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Macromolecules 1990,23, 571. (15) Ingersent, K.; Klein, J.; Pincus, P. Macromolecules 1990, 23, 548 (16) Hadziioannou, G.; Patel, S.;Granick, S.;Tirrell, M. J . A m . Chem. SOC.1986, 108, 2869. (17) Ohta, T. Polym. Eng. Sci. 1983,23 (131, 697.
1692 Langmuir, Vol. 11, No. 5, 1995 evaporation. The results of ionic strength and pH effects given in Figures 3-5 clearly indicate that the attractive force between the two surfaces is Coulombic, provided that one is grafted with a n anionic polymer and the other with a cationic polymer. We observed no adhesion if a charged grafted surface and the virgin PET surface were brought into contact in water, similar to two ionic surfaces with the same charge sign. This result also reveals that the first increase in shear strength shown in Figure 2 is attributable to the Coulombic electrostatic force between the anionic and the cationic groups fixed on the AAc- and DMAEMA-grafted PET surface, respectively. It is expected that adhesion between two oppositely charged surfaces becomes stronger as the charge density of each surface increases. In fact, this was confirmed by the results of Figures 6 and 7, which were obtained for two films, one of which had a fixed charge density and the other of which had varying charge densities from 0 to 20 x pmollcm2. Figures 6 and 7 further demonstrate that the strength increment slows down when the charge density exceeds a critical value, which is approximately 2x pmollcm2for the DMAEMA-grafted surface and 6 x pmollcm2 for the AAc-grafted surface. As the counterpart of each surface has a fixed graft density of 2.1 x (AAc) and 6.4 x pmollcm2 (DMAEMA), one can say that the shear strength between two oppositely charged surfaces continues to increase until the graft density of the two films becomes similar. This result suggests that the excessive charge remaining after neutralization of the ionic charge with the same amount of the opposite charge, in other words by polyion complexation, does not contribute any more to the adhesion between the two films. The shear strength recorded in the present study is the force per square centimeter of the grafted surface required to separate the two adhered films from the equilibrated distance to the infinite separation in aqueous environment. In an ideal case, this force should be proportional to the number of tie molecules connecting the two opposing surfaces. A very simple model for the connection of two grafted surfaces by ionic force is schematically illustrated in Figure 8. For simplicity, each ofthe surfaces is depicted in this model to carry only one graft chain, which is represented by a cylinder. When shear stress is given to the film pair, the two cylinders attracted by Coulombic force with each other will be separated in the aqueous medium when salt is added, or some chemical bonds tethering the graft cylinders to the film surface will be broken in the absence of salt. It is likely that both the mechanisms are responsible for the film separation if the ionic interaction connecting polymer chains is weak. The remarkable reduction in the shear strength observed when two grafted films lapped in distilled water were placed in aqueous KC1 solution (see Figure 5) seems to support the mechanism of separation against the ionic attraction. As is well known, a high concentration of salt markedly reduces the Coulombic interaction between charged polymer chains. The scission of a chemical bond in a tie molecule upon stress application is supported by the finding that any apprgciable adhesion could not be observed any more for a grafted surface pair once it was separated in DDW after lapping together in DDW. Most simply, the shear strength is equal to (number of the paired cylinders/cm2) x (force to separate a paired
Zhang et al.
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'I Chain Scission
Contact
'
Slipping
Figure 8. Schematic representation of polyion complexation between anionic and cationic graft chains on a film surface in water and its separation upon stress application.
cylinder). It seems probable that the force required to separate a paired cylinder is equal to the force to scission a tie molecule if the ionic strength of the aqueous media is sufficiently low not to allow loosening of the ion complexation between the oppositely charged cylinders. Under the assumption that the force to scission a tie molecule is equal to the bond force of CH2-CH2 (6.1 x N),I7we can calculate the number of the paired cylinders/ cm2to be 8 x 10-gpmolfromthe leveling-offshear strength (28 N/cm2). In order to compare this surface density of cylinders with the graft density of the surfaces, we need the molecular weight of each graft chain. The assumption of no difference in molecular weight between the graft chain and the homopolymer allows us to estimate the surface density of graft chains to be 1.2 x pmol of pmol of chains/cm2 for chains/cm2for AAc and 3.2 x DMAEMA. A big differenceby a factor of lo2- lo3between these values and the surface density of graft chains estimated from the shear strength measurement suggests that only a very small fraction of charged polymer chains could participate in polyion complexation with the counterpart chains or did contribute to the shear strength, probably because of large steric hindrance among graft chains or the occurrence of graft polymerization into the subsurface of the film. Comparison of the theoretical with the observed adhesive force between two grafted surfaces carrying oppositely charged chains will be studied in more detail using well-characterized grafted surfaces and a surface force measurement apparatus in the near future. LA9405758
