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Block copolymers at solid surfaces through physisorbed monolayers of macroinitiators
Materials Science and Engineering C 8–9 Ž1999. 225–229 www.elsevier.comrlocatermsec
Block copolymers at solid surfaces through physisorbed monolayers of macroinitiators T. Stohr ¨ a, J. Heinz b, J. Ruhe ¨
a,)
a
b
Max-Planck-Institute for Polymer Research, P.O. Box 3148, 55021 Mainz, Germany Schott Glas, Business Group Pharmaceutical Packaging, P.O. Box 2480, 55014 Mainz, Germany
Abstract We report on the radical chain polymerization of n-butyl methacrylate using monolayers of azo-type polyŽ ´-caprolactone. macroinitiators physisorbed to the surface of planar silicon oxide substrates. In this process block copolymers consisting of polyŽ ´-caprolactone. and polyŽ n-butyl methacrylate. ŽPnBMA. are formed in situ at the surface of the silicon oxide substrate. The preparation of the attached layers and the characterization by surface plasmon spectroscopy ŽSPS., infrared techniques and water contact angle measurements are described. This ‘‘grafting-from’’ technique can be used to prepare hydrophobic layers on hydrophilic substrates. Following this macroinitiator route some major limitations of the block copolymer physisorption process, such as solubility problems of the block copolymer and intrinsic limitations of the layer thickness can be avoided. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Block copolymer; Surface; Physisorption; Macroinitiator
1. Introduction Due to the simplicity of the process physisorption of block copolymers from solution is a frequently used procedure to modify the surface of solid substrates. Such systems, usually A–B block copolymers, consist of one block Žthe ‘‘anchor’’. which allows attachment to the surface of the substrate and a second block Žthe ‘‘buoy’’. which does not strongly interact with the surface and carries the desired surface properties w1x. However, two problems are inherent with such an approach. A major problem is that a common solvent for the blocks A and B has to be found despite the fact that the blocks should have very different chemical properties. If the anchor block is in a bad solvent environment the block copolymer will form micelles in solution and possibly even on the surface. In addition, extremely long equilibration times can be observed and in some cases equilibrium never may be reached. Especially if the polarity of the blocks is very different from each other no solvent for the block copolymer will be found. Due to that the number of ) Corresponding author. Tel.: q49-6131-379-162; fax: q49-6131379-100; E-mail: [email protected]
systems studied so far is relatively small Že.g., polystyrene-block-polyŽethylene oxide. w2,3x, polystyreneblock-polyŽ2-vinyl-pyridine. w4x.. A second problem is that monolayers prepared by this technique are inherently very thin with film thicknesses typically between 3 and 5 nm. The reason for this is a kinetic hindrance for the attachment of polymer chains due to a diffusion barrier created by the already attached molecules. In recent years, radical polymerization using chemisorbed azo-type initiators has been established w5–7x and a first approach using a physisorbed macroinitiator is described w8x. In these cases, the polymer is grown directly at the surface of the substrate Ž‘‘grafting-from’’ polymerization.. In this work, we present a macroinitiator system that allows to create hydrophobic layers on hydrophilic substrate surfaces. Here an hydrophilic anchor block bearing initiator groups, denoted as I–I, is physisorbed from solution to a hydrophilic surface. The hydrophobic buoy block is polymerized in situ resulting in a mixture of di- and triblock copolymers ŽFig. 1.. PolyŽ ´-caprolacton. is known to physisorb strongly to hydrophilic surfaces w9,10x. Thus, an oligomeric polyŽ ´-
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 9 9 . 0 0 0 6 8 - 5
226
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
methacrylate 2 ŽPolysciences. was chromatographically purified over basic alumina oxide, distilled in vacuum from copperŽI. chloride and stored under dry argon at y188C. Dry decaline Žp.a.. and cyclohexane Žp.a.. were used as obtained. 2.2. Macroinitiator physisorption After drying the substrates in vacuo at 10y2 mbar for 2 h, a solution of the polyŽ ´-caprolactone. macroinitiator 1 in dry toluene was added. The physisorption experiments were performed using varying concentrations from 0.0001 to 1.5 molrl Žreferring to ´-caprolactone repeat units.. After 16 h the substrates were thoroughly rinsed with toluene. To remove weakly physisorbed macroinitiator the substrates were extracted in dry toluene at 158C for 16 h using a Soxhlet extractor. 2.3. ‘‘Grafting-from’’ polymerization
Fig. 1. Macroinitiator concept. The anchor block bearing initiator groups, denoted as I–I, is physisorbed from solution to a solid surface. The buoy block is subsequently polymerized in situ.
Again the substrates were dried in vacuo at 10y2 mbar for 2 h. Under dry argon n-butyl methacrylate 2 and decaline were added Ž1r2 vrv., degassed in vacuum through repeated freeze–thaw cycles and heated under vacuum to 60.0 " 0.18C. The polymerizations were stopped after selected periods of time by cooling to room temperature. To remove weakly physisorbed polyŽ n-butyl methacrylate. ŽPnBMA. homopolymer the substrates were extracted in cyclohexane at 158C for 16 h using a soxhlet extractor. 2.4. Characterization
caprolactone. macroinitiator 1 containing AIBN analogous azo-moieties ŽFig. 2. was adsorbed to silicon oxide surfaces and subsequently used for polymerization of n-butyl methacrylate 2 ŽFig. 2..
2. Experimental 2.1. Materials The substrates for surface plasmon spectroscopy ŽSPS. were glass slides ŽB270, Berliner Glas. onto which a ˚ thick Ag layer and a 300-A˚ thick SiO x Ž1 F =F 2. 500-A ˚ thick layer was evaporated. A glass slide having a 1000-A ˚ Ž . Au layer and a 100-A thick SiO x 1 F x F 2 layer on top was used for grazing incidence FTIR spectroscopy. A double side polished silicon wafer ŽAurel. with a natural ˚ was used for transmisSiO x layer of approximately 25 A sion FTIR spectroscopy. The azo-type oligomeric polyŽ ´caprolactone. macroinitiator 1 of Mn s 9250 grmol was synthesized and characterized as described elsewhere w11x. Toluene Žp.a.. was distilled under dry argon atmosphere from sodium using benzophenone as an indicator. N-butyl
SPS measurements were carried out in air using the Kretschmann w12x configuration with a He–Ne Laser Ž l s 632.8 nm.. A 908 prism ŽBK7, n D s 1.5151, Spindler & Hoyer. and index match fluid Ž n D s 1.5160, Cargill. were used. Assuming a refractive index of n D s 1.500 for polyŽ ´-caprolactone. macroinitiator 1 and using n D s 1.4830 for PnBMA w13x 2, the layer thickness was determined according to Fresnel equations. The grazing incidence FTIR spectrum was recorded using the Nicolet Magna-IR 850 Spectrometer Series II at an incident angle of 58. The transmission FTIR spectrum was recorded using
Fig. 2. Oligomeric initiator of Mn s9250 grmol and monomer used.
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
227
the Nicolet 730 FTIR Spectrometer. Here the film thickness was determined using ellipsometry ŽEL X-1, Dr. Riss Ellipsometerbau. with the same refractive indices as mentioned above. The bulk IR spectra of films cast onto NaCl plates were recorded using the Perkin-Elmer FTIR Spectrometer Paragon 1000. Water contact angle measurements were performed with a contact angle microscope ŽKruss ¨ . using ultrapure water ŽMilli-Q Plus 185, Millipore..
3. Results and discussion 3.1. Macroinitiator physisorption A basic requirement for successful physisorption is that the solvation energy of the polymer molecules is weaker than free energy of physisorption. Toluene was found to fulfil this requirement for the physisorption of the polyŽ ´caprolactone. macroinitiator 1 onto silicon oxide. Fig. 3 qualitatively proofs the presence of the macroinitiator monolayer. The grazing incidence FTIR spectrum ˚ polyŽ ´-caprolactone taken for a thin film of ; 18 A initiator., which was physisorbed from a 0.075 molrl solution Žreferring to ´-caprolactone repeat units., and the FTIR spectrum taken for a cast film are in good agreement. The remarkable resonance shift of the n ŽC5O. valence vibration is most likely due to the loss of the crystallinity of the macroinitiator at the silicon oxide surface. SPS reveals that some part of the macroinitiator layer is only weakly physisorbed and can be removed by extraction in toluene. Some other part of the monolayer is strongly physisorbed and remains attached to the silicon oxide surface even after prolonged extraction ŽFig. 4.. The initiator layer thickness is a function of the solution concentration from which it is physisorbed. At higher concentrations the layer thickness of the strongly physisorbed initiator reaches a plateau value. Here all the silanol ad-
All polymerization experiments were performed using macroinitiator monolayers adsorbed from a 0.075 molrl solution Žreferring to ´-caprolactone repeat units. and re˚ after extraction ŽFig. 4.. sulting in a thickness of ; 18 A The surface polymerization reactions of n-butyl methacrylate 1 using physisorbed azo-type polyŽ ´-caprolactone. macroinitiators 2 were performed in dry decaline in a volume ratio of 1:2 at 608C. This ratio was chosen because the initiator is soluble in pure monomer at 608C. Thus, decaline, which is a non-solvent for the initiator and a solvent for PnBMA at 608C, was added up to the above mentioned volume ratio to keep the initiator in a non-
Fig. 3. Grazing incidence and bulk IR spectra for the polyŽ ´-caprolactone. macroinitiator.
Fig. 5. Transmission and bulk IR spectra for PnBMA.
Fig. 4. PolyŽ ´-caprolactone. macroinitiator layer thickness in dependence of solution concentration directly after physisorption from toluene for 16 h ŽI. and after extraction in toluene for 16 h Ž`..
sorption sites of the substrate have built up hydrogen bonds to the carbonyl groups of the macroinitiator. 3.2. ‘‘Grafting-from’’ polymerization
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
228
Fig. 7. Time dependence of layer thickness ŽI. for the polymerization of n-butyl methacrylate in decaline Ž1r2 vrv. at 608C including 16 h extraction in cyclohexane. The solid line describes a kinetic of first order using the AIBN decompostion rate. The dashed line reflects a reference without initiator after 48 h of exposition.
Fig. 6. SPS reflectivity curves of selected PnBMA layer thicknesses. Solid lines are fitted reflectivity curves according to Fresnel equations.
Herein, G PnBMA is a time-dependent parameter and can be described by a first order kinetics.
G PnBMA s 2 Gazo f Ž 1 y exp Ž yk D t . . solvent and to prevent dissolution of the layer system during polymerization. An additional amount of non-bonded polymer can result from transfer reactions of a grafted growing polymer chain to solvent or monomer molecules andror thermal polymerization in solution. Therefore, the PnBMA covered substrates were carefully extracted for 16 h in cyclohexane which again is a non-solvent for the initiator and a good solvent for PnBMA. Further extraction revealed no additional decrease in the PnBMA thickness. Thus, Fig. 5 proofs the surface coverage by a good agreement between the transmission FTIR spectrum taken ˚ PnBMA, as determined by for a thin film of 2 = 225 A ellipsometry, and the FTIR spectrum recorded for a cast film. Samples were withdrawn after selected periods of time and SPS reflectivity curves recorded. Selected reflectivity curves are given in Fig. 6. The plot of the dry PnBMA layer thickness d PnBMA vs. polymerization time t ŽFig. 7. ˚ can be realized. Furtherreveals that values up to 1000 A more, it shows that with increasing polymerization time eventually a maximum layer thickness is approached. This can be explained if d PnBMA is considered to be a product of the PnBMA graft density G PnBMA and the number average molecular weight Mn of the surface-attached PnBMA blocks, which is time independent due to a practically constant monomer concentration during the surface polymerization process w14x, divided by the PnBMA density r : d PnBMA s
G PnBMA Mn r
.
Ž 1.
Ž 2.
where Gazo is the surface density of azo-groups within the macroinitiator monolayer, f is the radical efficiency factor and k D is the azo decomposition rate. The factor 2 is due to the fact that each successful initiation process results in the growth of two surface attached PnBMA chains. Now the layer thickness at infinite time d PnBMA,` can be derived using Eqs. Ž1. and Ž2.: d PnBMA s d PnBMA ,`Ž 1 y exp Ž yk D t . .
Ž 3.
where: d PnBMA ,` s
2 Gazo fMn
r
.
Ž 4.
All parameters in Eq. Ž4. were determined separately w11,13x. The good agreement between the measured thickness values in Fig. 7 and the kinetic model applied to the system shows that the growth of the polymer layer can be easily controlled. The experimental thickness dependence in Fig. 7 is described using Eq. Ž3. with k D s 9.2 = 10y6 sy1 for AIBN at 608C and d PnBMA,` s ˚ 1180 A.
Table 1 Advancing, sessile and receeding contact angles on silicon, after physisorption of the polyŽ ´-caprolactone. macroinitiator and after polymerization of PnBMA Surface
Advancing
Sessile
Receeding
Silicon oxide PolyŽ ´-caprolatone. macroiniator PnBMA
41 53 94
36 45 89
13 21 72
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
Water contact angle measurements finally underline the decrease in surface polarity ŽTable 1.. Starting with an advancing contact angle of 418 for silicon oxide, the angle increases from 538 after physisorption of the polyŽ ´caprolactone. macroinitiator to 948 after coverage with PnBMA.
229
Acknowledgements Financial support under the BMBF grant ‘‘Innovative Methoden der Polymercharakterisierung fur ¨ die Praxis’’ ŽNumber 03N6010. is gratefully acknowledged. References
4. Conclusions The macroinitiator concept allows to create block copolymers of different polarities at solid surfaces. Here block copolymers consisting of a hydrophillic anchor block ŽpolyŽ ´-caprolactone. and a hydrophobic buoy block ŽPnBMA. are generated at hydrophillic surfaces Žsilicon oxide.. The major limitations of the block copolymer physisorption process are avoided. I.e., the solubility problems of block copolymers of different polarities can be overcome and the obtainable layer thicknesses are well ˚ even values of about 1000 A˚ can be obtained. above 50 A, Using this approach reproducible macroinitiator layers can be prepared and the thickness of the polymerized layers can be controlled. By attaching these block copolymers the surface polarity can be changed drastically.
w1x G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, B. Vincent, Polymers at Interfaces, Chapman & Hall, London, 1993. w2x M. Motschmann, M. Stamm, C. Toprakcioglu, Macromolecules 24 Ž1991. 3681. w3x D.A. Guzonas, D. Boils, C.P. Tripp, M.L. Hair, Macromolecules 25 Ž1992. 2434. w4x E. Parsonage, M. Tirrell, H. Watanabe, R.G. Nuzzo, Macromolecules 24 Ž1991. 1987. w5x G. Boven, M.L.C.M. Oosterling, G. Challa, A.J. Schouten, Polymer 31 Ž1990. 2377. w6x O. Prucker, J. Ruhe, ¨ Macromolecules 31 Ž1998. 592. w7x O. Prucker, J. Ruhe, ¨ Macromolecules 31 Ž1998. 602. w8x F. Kempkes, B. Adams, S. Serreau, J. Ruhe, ¨ to be published. w9x M. Korn, E. Killmann, J. Colloid Interface Sci. 76 Ž1980. 19. w10x R.E. Felter, J. Polym. Sci., Polym. Lett. Ed. 12 Ž1974. 147. w11x T. Stohr, ¨ J. Ruhe, ¨ to be published. w12x E. Kretschmann, Opt. Commun. 6 Ž1972. 185. w13x J. Brandrup, E.H. Immergut ŽEds.., Polymer Handbook, 3rd edn., Wiley, New York, 1989. w14x G. Odian, Principles of Polymerization, 3rd edn., Wiley, New York, 1991.
Block copolymers at solid surfaces through physisorbed monolayers of macroinitiators T. Stohr ¨ a, J. Heinz b, J. Ruhe ¨
a,)
a
b
Max-Planck-Institute for Polymer Research, P.O. Box 3148, 55021 Mainz, Germany Schott Glas, Business Group Pharmaceutical Packaging, P.O. Box 2480, 55014 Mainz, Germany
Abstract We report on the radical chain polymerization of n-butyl methacrylate using monolayers of azo-type polyŽ ´-caprolactone. macroinitiators physisorbed to the surface of planar silicon oxide substrates. In this process block copolymers consisting of polyŽ ´-caprolactone. and polyŽ n-butyl methacrylate. ŽPnBMA. are formed in situ at the surface of the silicon oxide substrate. The preparation of the attached layers and the characterization by surface plasmon spectroscopy ŽSPS., infrared techniques and water contact angle measurements are described. This ‘‘grafting-from’’ technique can be used to prepare hydrophobic layers on hydrophilic substrates. Following this macroinitiator route some major limitations of the block copolymer physisorption process, such as solubility problems of the block copolymer and intrinsic limitations of the layer thickness can be avoided. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Block copolymer; Surface; Physisorption; Macroinitiator
1. Introduction Due to the simplicity of the process physisorption of block copolymers from solution is a frequently used procedure to modify the surface of solid substrates. Such systems, usually A–B block copolymers, consist of one block Žthe ‘‘anchor’’. which allows attachment to the surface of the substrate and a second block Žthe ‘‘buoy’’. which does not strongly interact with the surface and carries the desired surface properties w1x. However, two problems are inherent with such an approach. A major problem is that a common solvent for the blocks A and B has to be found despite the fact that the blocks should have very different chemical properties. If the anchor block is in a bad solvent environment the block copolymer will form micelles in solution and possibly even on the surface. In addition, extremely long equilibration times can be observed and in some cases equilibrium never may be reached. Especially if the polarity of the blocks is very different from each other no solvent for the block copolymer will be found. Due to that the number of ) Corresponding author. Tel.: q49-6131-379-162; fax: q49-6131379-100; E-mail: [email protected]
systems studied so far is relatively small Že.g., polystyrene-block-polyŽethylene oxide. w2,3x, polystyreneblock-polyŽ2-vinyl-pyridine. w4x.. A second problem is that monolayers prepared by this technique are inherently very thin with film thicknesses typically between 3 and 5 nm. The reason for this is a kinetic hindrance for the attachment of polymer chains due to a diffusion barrier created by the already attached molecules. In recent years, radical polymerization using chemisorbed azo-type initiators has been established w5–7x and a first approach using a physisorbed macroinitiator is described w8x. In these cases, the polymer is grown directly at the surface of the substrate Ž‘‘grafting-from’’ polymerization.. In this work, we present a macroinitiator system that allows to create hydrophobic layers on hydrophilic substrate surfaces. Here an hydrophilic anchor block bearing initiator groups, denoted as I–I, is physisorbed from solution to a hydrophilic surface. The hydrophobic buoy block is polymerized in situ resulting in a mixture of di- and triblock copolymers ŽFig. 1.. PolyŽ ´-caprolacton. is known to physisorb strongly to hydrophilic surfaces w9,10x. Thus, an oligomeric polyŽ ´-
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 9 9 . 0 0 0 6 8 - 5
226
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
methacrylate 2 ŽPolysciences. was chromatographically purified over basic alumina oxide, distilled in vacuum from copperŽI. chloride and stored under dry argon at y188C. Dry decaline Žp.a.. and cyclohexane Žp.a.. were used as obtained. 2.2. Macroinitiator physisorption After drying the substrates in vacuo at 10y2 mbar for 2 h, a solution of the polyŽ ´-caprolactone. macroinitiator 1 in dry toluene was added. The physisorption experiments were performed using varying concentrations from 0.0001 to 1.5 molrl Žreferring to ´-caprolactone repeat units.. After 16 h the substrates were thoroughly rinsed with toluene. To remove weakly physisorbed macroinitiator the substrates were extracted in dry toluene at 158C for 16 h using a Soxhlet extractor. 2.3. ‘‘Grafting-from’’ polymerization
Fig. 1. Macroinitiator concept. The anchor block bearing initiator groups, denoted as I–I, is physisorbed from solution to a solid surface. The buoy block is subsequently polymerized in situ.
Again the substrates were dried in vacuo at 10y2 mbar for 2 h. Under dry argon n-butyl methacrylate 2 and decaline were added Ž1r2 vrv., degassed in vacuum through repeated freeze–thaw cycles and heated under vacuum to 60.0 " 0.18C. The polymerizations were stopped after selected periods of time by cooling to room temperature. To remove weakly physisorbed polyŽ n-butyl methacrylate. ŽPnBMA. homopolymer the substrates were extracted in cyclohexane at 158C for 16 h using a soxhlet extractor. 2.4. Characterization
caprolactone. macroinitiator 1 containing AIBN analogous azo-moieties ŽFig. 2. was adsorbed to silicon oxide surfaces and subsequently used for polymerization of n-butyl methacrylate 2 ŽFig. 2..
2. Experimental 2.1. Materials The substrates for surface plasmon spectroscopy ŽSPS. were glass slides ŽB270, Berliner Glas. onto which a ˚ thick Ag layer and a 300-A˚ thick SiO x Ž1 F =F 2. 500-A ˚ thick layer was evaporated. A glass slide having a 1000-A ˚ Ž . Au layer and a 100-A thick SiO x 1 F x F 2 layer on top was used for grazing incidence FTIR spectroscopy. A double side polished silicon wafer ŽAurel. with a natural ˚ was used for transmisSiO x layer of approximately 25 A sion FTIR spectroscopy. The azo-type oligomeric polyŽ ´caprolactone. macroinitiator 1 of Mn s 9250 grmol was synthesized and characterized as described elsewhere w11x. Toluene Žp.a.. was distilled under dry argon atmosphere from sodium using benzophenone as an indicator. N-butyl
SPS measurements were carried out in air using the Kretschmann w12x configuration with a He–Ne Laser Ž l s 632.8 nm.. A 908 prism ŽBK7, n D s 1.5151, Spindler & Hoyer. and index match fluid Ž n D s 1.5160, Cargill. were used. Assuming a refractive index of n D s 1.500 for polyŽ ´-caprolactone. macroinitiator 1 and using n D s 1.4830 for PnBMA w13x 2, the layer thickness was determined according to Fresnel equations. The grazing incidence FTIR spectrum was recorded using the Nicolet Magna-IR 850 Spectrometer Series II at an incident angle of 58. The transmission FTIR spectrum was recorded using
Fig. 2. Oligomeric initiator of Mn s9250 grmol and monomer used.
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
227
the Nicolet 730 FTIR Spectrometer. Here the film thickness was determined using ellipsometry ŽEL X-1, Dr. Riss Ellipsometerbau. with the same refractive indices as mentioned above. The bulk IR spectra of films cast onto NaCl plates were recorded using the Perkin-Elmer FTIR Spectrometer Paragon 1000. Water contact angle measurements were performed with a contact angle microscope ŽKruss ¨ . using ultrapure water ŽMilli-Q Plus 185, Millipore..
3. Results and discussion 3.1. Macroinitiator physisorption A basic requirement for successful physisorption is that the solvation energy of the polymer molecules is weaker than free energy of physisorption. Toluene was found to fulfil this requirement for the physisorption of the polyŽ ´caprolactone. macroinitiator 1 onto silicon oxide. Fig. 3 qualitatively proofs the presence of the macroinitiator monolayer. The grazing incidence FTIR spectrum ˚ polyŽ ´-caprolactone taken for a thin film of ; 18 A initiator., which was physisorbed from a 0.075 molrl solution Žreferring to ´-caprolactone repeat units., and the FTIR spectrum taken for a cast film are in good agreement. The remarkable resonance shift of the n ŽC5O. valence vibration is most likely due to the loss of the crystallinity of the macroinitiator at the silicon oxide surface. SPS reveals that some part of the macroinitiator layer is only weakly physisorbed and can be removed by extraction in toluene. Some other part of the monolayer is strongly physisorbed and remains attached to the silicon oxide surface even after prolonged extraction ŽFig. 4.. The initiator layer thickness is a function of the solution concentration from which it is physisorbed. At higher concentrations the layer thickness of the strongly physisorbed initiator reaches a plateau value. Here all the silanol ad-
All polymerization experiments were performed using macroinitiator monolayers adsorbed from a 0.075 molrl solution Žreferring to ´-caprolactone repeat units. and re˚ after extraction ŽFig. 4.. sulting in a thickness of ; 18 A The surface polymerization reactions of n-butyl methacrylate 1 using physisorbed azo-type polyŽ ´-caprolactone. macroinitiators 2 were performed in dry decaline in a volume ratio of 1:2 at 608C. This ratio was chosen because the initiator is soluble in pure monomer at 608C. Thus, decaline, which is a non-solvent for the initiator and a solvent for PnBMA at 608C, was added up to the above mentioned volume ratio to keep the initiator in a non-
Fig. 3. Grazing incidence and bulk IR spectra for the polyŽ ´-caprolactone. macroinitiator.
Fig. 5. Transmission and bulk IR spectra for PnBMA.
Fig. 4. PolyŽ ´-caprolactone. macroinitiator layer thickness in dependence of solution concentration directly after physisorption from toluene for 16 h ŽI. and after extraction in toluene for 16 h Ž`..
sorption sites of the substrate have built up hydrogen bonds to the carbonyl groups of the macroinitiator. 3.2. ‘‘Grafting-from’’ polymerization
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
228
Fig. 7. Time dependence of layer thickness ŽI. for the polymerization of n-butyl methacrylate in decaline Ž1r2 vrv. at 608C including 16 h extraction in cyclohexane. The solid line describes a kinetic of first order using the AIBN decompostion rate. The dashed line reflects a reference without initiator after 48 h of exposition.
Fig. 6. SPS reflectivity curves of selected PnBMA layer thicknesses. Solid lines are fitted reflectivity curves according to Fresnel equations.
Herein, G PnBMA is a time-dependent parameter and can be described by a first order kinetics.
G PnBMA s 2 Gazo f Ž 1 y exp Ž yk D t . . solvent and to prevent dissolution of the layer system during polymerization. An additional amount of non-bonded polymer can result from transfer reactions of a grafted growing polymer chain to solvent or monomer molecules andror thermal polymerization in solution. Therefore, the PnBMA covered substrates were carefully extracted for 16 h in cyclohexane which again is a non-solvent for the initiator and a good solvent for PnBMA. Further extraction revealed no additional decrease in the PnBMA thickness. Thus, Fig. 5 proofs the surface coverage by a good agreement between the transmission FTIR spectrum taken ˚ PnBMA, as determined by for a thin film of 2 = 225 A ellipsometry, and the FTIR spectrum recorded for a cast film. Samples were withdrawn after selected periods of time and SPS reflectivity curves recorded. Selected reflectivity curves are given in Fig. 6. The plot of the dry PnBMA layer thickness d PnBMA vs. polymerization time t ŽFig. 7. ˚ can be realized. Furtherreveals that values up to 1000 A more, it shows that with increasing polymerization time eventually a maximum layer thickness is approached. This can be explained if d PnBMA is considered to be a product of the PnBMA graft density G PnBMA and the number average molecular weight Mn of the surface-attached PnBMA blocks, which is time independent due to a practically constant monomer concentration during the surface polymerization process w14x, divided by the PnBMA density r : d PnBMA s
G PnBMA Mn r
.
Ž 1.
Ž 2.
where Gazo is the surface density of azo-groups within the macroinitiator monolayer, f is the radical efficiency factor and k D is the azo decomposition rate. The factor 2 is due to the fact that each successful initiation process results in the growth of two surface attached PnBMA chains. Now the layer thickness at infinite time d PnBMA,` can be derived using Eqs. Ž1. and Ž2.: d PnBMA s d PnBMA ,`Ž 1 y exp Ž yk D t . .
Ž 3.
where: d PnBMA ,` s
2 Gazo fMn
r
.
Ž 4.
All parameters in Eq. Ž4. were determined separately w11,13x. The good agreement between the measured thickness values in Fig. 7 and the kinetic model applied to the system shows that the growth of the polymer layer can be easily controlled. The experimental thickness dependence in Fig. 7 is described using Eq. Ž3. with k D s 9.2 = 10y6 sy1 for AIBN at 608C and d PnBMA,` s ˚ 1180 A.
Table 1 Advancing, sessile and receeding contact angles on silicon, after physisorption of the polyŽ ´-caprolactone. macroinitiator and after polymerization of PnBMA Surface
Advancing
Sessile
Receeding
Silicon oxide PolyŽ ´-caprolatone. macroiniator PnBMA
41 53 94
36 45 89
13 21 72
T. Stohr ¨ et al.r Materials Science and Engineering C 8–9 (1999) 225–229
Water contact angle measurements finally underline the decrease in surface polarity ŽTable 1.. Starting with an advancing contact angle of 418 for silicon oxide, the angle increases from 538 after physisorption of the polyŽ ´caprolactone. macroinitiator to 948 after coverage with PnBMA.
229
Acknowledgements Financial support under the BMBF grant ‘‘Innovative Methoden der Polymercharakterisierung fur ¨ die Praxis’’ ŽNumber 03N6010. is gratefully acknowledged. References
4. Conclusions The macroinitiator concept allows to create block copolymers of different polarities at solid surfaces. Here block copolymers consisting of a hydrophillic anchor block ŽpolyŽ ´-caprolactone. and a hydrophobic buoy block ŽPnBMA. are generated at hydrophillic surfaces Žsilicon oxide.. The major limitations of the block copolymer physisorption process are avoided. I.e., the solubility problems of block copolymers of different polarities can be overcome and the obtainable layer thicknesses are well ˚ even values of about 1000 A˚ can be obtained. above 50 A, Using this approach reproducible macroinitiator layers can be prepared and the thickness of the polymerized layers can be controlled. By attaching these block copolymers the surface polarity can be changed drastically.
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