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A Reversibly Switching Block Copolymer Surface Karsten Reihs*,† and Matthias Voetz‡ SuNyx GmbH, Stolberger Strasse 370, 50933 Ko¨ ln, Germany, and Bayer Technology Services GmbH, 51368 Leverkusen, Germany Received June 13, 2005. In Final Form: August 16, 2005 We present linear (AB)n multiblock copolymers that exhibit a thermally induced reversible alteration of the surface composition at a sharply defined transition temperature Ts of 120-170 °C depending on the polymer structure. At temperatures below Ts the surface consists of block A, a 4,4′-methylenediphenyl diisocyanate (4,4′-MDI) type polyurea, whereas above Ts the hydrophobic block B, a poly(ricinoleic acid hexanediol ester) dominates the surface composition. The ratio of surface concentrations cA/cB changes by a factor of at least 1000 within an analyzed depth of approximately 10 Å. The full A-B surface transition is obtained within minutes. A mechanism is proposed where microphase crystallization of block A in the bulk effectively locks surface segregation of the hydrophobic block B, yielding an A-rich surface. The topology of the copolymers imposes sufficient restrictions for the lateral separation of the connected constituents such that surface segregation is largely reduced. Only above the transition temperature Ts of microphase crystallization of block A can block B segregate to the surface, yielding a B-rich surface. Such a scheme of competing self-organizing processes in copolymers may potentially be used to reversibly switch surface properties such as adhesion and wetting in various applications.
Introduction Many applications require surfaces with well-defined surface properties such as adhesion, wetting, selective adsorption, or catalysis. These properties are defined by the molecular surface structures as well as the dynamic phenomena resulting from the structure, for example, chemical reactivity and diffusion. Very often individual chemical groups at the surface are primarily responsible for the surface properties. The structure of the uppermost monolayer at surfaces is of particular importance for many applications. In particular, polymers are important materials that can provide a large variety of different chemical groups at materials surfaces. The chemical structures of technical surfaces as well as the resulting properties are usually fairly static or at least require extensive provisions for their selective timedependent alteration. In this context, the design of a polymer system that allows reversibly switching chemical functionalities at the surface by use of only an external control parameter such as temperature presents a remarkable challenge. By use of a liquid crystalline polymer,1 the temperature-controlled modification of both tackiness and dewetting dynamics has been demonstrated and is discussed as a result of some change of surface characteristics but more importantly to modifications of bulk properties. Another group2 reports a reversibly switching surface based on conformational transitions between a hydrophilic and moderately hydrophobic state in surfaceconfined single-layered molecules in response to an electrical potential yielding dynamic changes in interfacial properties such as wettability. In another system the oxidized surface of cross-linked 1,4 polybutadiene3 and cis-1,4-polyisoprene4 provided a * Corresponding author: phone + 49 221 485 2453; fax +49 221 485 2479; e-mail
[email protected]. † SuNyx GmbH, Stolberger Str. 370, 50933 Ko ¨ ln, Germany, and ‡ Bayer Technology Services GmbH, 51368 Leverkusen, Germany (1) De Crevoisier, G.; Fabre, P.; Corpart, J.-M.; Leibler, L. Science 1999, 285, 1246. (2) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S., Somorjai, G. A.; Langer, R. Science 2003, 299, 371.
hydrophilic surface in contact with water at room temperature that reconstructed against hot water at 80 °C to become more hydrophobic. The reversible transitions are suggested to arise from competition between the enthalpically favorable solvation of hydrophilic surface functional groups at the polymer-water interface and entropically unfavorable chain extension. Repeated cycling led to damping of the temperature-induced oscillations that is suggested to arise from alignment of extended interfacial chains and to a decay of the restoring force on the interfacial chains under extension.5 The restoring force due to entropic loss associated with chain extension was also shown to compete against enthalpically favorable chemical interactions at an interface of oxidized 1,4polybutadiene and aluminum oxide.6 Besides such twostate oscillations, coupled consecutive reconstructions at a polymer/air interface were observed in plasma-oxidized cross-linked polyisoprene.7 Here, the polymer surface initially became more hydrophilic when cooled from an elevated temperature and then proceeded to a more hydrophobic state. To achieve switching of chemical functionalities at polymer surfaces, we here proposed a mechanism with a linear (AB)n multiblock copolymer. Surface segregation of the lower surface energy block B is efficiently locked by an ordered bulk structure of block A of the polymer. The topology of the copolymer with relatively short chains in block B imposes sufficient restrictions in the lateral separation that efficiently prevents surface segregation of block B to a large extent. Surface segregation is only possible after a phase transition in the bulk has taken place, yielding a disordered state. The “unlocked” surface segregation then generates the corresponding alteration of the chemical structure of the polymer surface. (3) Khongtong, S.; Ferguson, G. S. J. Am. Chem. Soc. 2001, 123, 3588. (4) Khongtong, S.; Ferguson, G. S. Macromolecules 2002, 35, 4023. (5) Khongtong, S.; Ferguson, G. S. Langmuir 2004, 20, 9992. (6) Khongtong, S.; Ferguson, G. S. J. Am. Chem. Soc. 2002, 124, 7254. (7) Grunzinger, S. J.; Ferguson, G. S. J. Am. Chem. Soc. 2001, 123, 12927.
10.1021/la051579n CCC: $30.25 © 2005 American Chemical Society Published on Web 09/24/2005
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Figure 1. Repeat unit of the linear (AB)n multiblock copolymers. The composition of the different polymers range from p + q ) 3 ... 7 and k ) 24 ... 222 with molecular ratios k/(p + q) ) 10 ... 30.
The surface composition in multicomponent systems such as a mixture of homopolymers is usually different from the bulk.8-10 The driving forces of surface segregation are the difference in surface energies of the components and the interaction between the polymer segments.11-13 In copolymers the topology with connected constituents imposes restrictions on the lateral separation of the components, creating a large diversity of complex selforganized structures14-16 both in the bulk17,18 and at the surface19 with both existing and unrealized materials applications. The competition of processes of molecular self-organization is of particular importance in the structural formation of polymeric materials.14 In our mechanism we use competing interactions of molecular self-organization that allows for external control of surface segregation in copolymers. Experimental Section Polymer Synthesis and Sample Preparation. A prepolymer was synthesized from neat components by polymerization of cis-12-hydroxy-9-octadecenoic acid (ricinoleic acid) that was started with hexanediol before 4,4′-methylenediphenyl diisocyanate (4,4′-MDI, Desmodur 44V10L) was added to yield a clear product. All compounds were obtained from Bayer and used without further purification. Prior to analysis the prepolymers were always treated as follows. The unreacted isocyanate was allowed to polymerize to yield urea linkages with water from ambient humidity (approximately 50% relative humidity) at room temperature for more than 4 weeks. The reaction was alternatively carried out by annealing fresh polymer films at 200 °C for 1 h in an oven under atmospheric conditions. The reaction was monitored by the reduction of the 2280 cm-1 absorption band of NCO groups in IR. The switching properties reported in this study were completely absent when fresh prepolymer films were investigated. The chemical structure of the block copolymers obtained is shown in Figure 1. Block A is a polyurea from 4,4′-methylenediphenyl diisocyanate (4,4′-MDI) linked to block B, a poly(ricinoleic acid hexanediol ester), via an urethane group. The aliphatic structure of block B was chosen because its low surface energy leads to a pronounced propensity for surface segregation. (8) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989, 62, 280. (9) Bhatia, Q. S.; Pan, D. H.; Koberstein, J. T. Macromolecules 1988, 21, 2166. (10) Schmidt, J. J.; Gardella, J. A., Jr.; Salvati, L., Jr. Macromolecules 1989, 22, 4489. (11) Jones, R. A. L.; Kramer, E. J. Polymer 1993, 34, 115. (12) Genzer, J.; Faldi, A.; Composto, R. J. Phys. Rev. E 1994, 50, 2373. (13) Norton, L. J.; Kramer, E. J.; Bates, F. S.; Gehlsen, M. D.; Jones, R. A. L.; Karim, A.; Felcher, G. P.; Kleb, R. Macromolecules 1995, 28, 8621. (14) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (15) Bates, F. S. Science 1991, 251, 899. (16) Hajduk, D. A.; Ho, R. M.; Hillmyer, M. A.; Bates, F. S. J. Phys. Chem. B 1998, 102, 1356. (17) Lohse, D. J.; Hadjichristidis, N. Curr. Opin. Colloid Interface Sci. 1997, 2, 171. (18) Spontak, R. J.; Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1999, 4, 149. (19) Binder, K. Acta Polym. 1995, 46, 204.
Samples for IR, TDMS, SSIMS, and contact angle determination were prepared by spin-coating neat prepolymers on 300 µm thick polished silicon wafers and heating to 200 °C for 1 h. The films had a thickness of up to 1 mm; thin films of a few micrometers showed the same properties. Samples for TGA and DSC were spun or cast on Teflon sheets where they form lensshaped droplets that were taken off the sheet for analysis after heating to 200 °C for 1 h. Thermal Analysis. Thermogravimetric analysis (TGA) was carried out in a Perkin-Elmer TGS-2 instrument. The heating rate was 20 °C/min and sample size was 5-6 mg. Samples were purged with dry nitrogen and heated from room temperature to 600 °C in an open platinum crucible. Differential scanning calorimetry (DSC) data were obtained on a Perkin-Elmer DSC-2 instrument. The heating rate was 20 °C/min and sample size was 6-10 mg. Samples were purged with dry nitrogen when heated from -100 to 250 °C followed by fast cooling to -100 °C at 320 °C/min. Infrared Spectrometry. Infrared absorption spectrometry (IR) was carried out on a Nicolet Magna 550 instrument at room temperature or at 200 °C by use of a heated sample support. Sample spectra were obtained by subtracting the spectrum of a clean silicon wafer. Thermal Desorption Mass Spectrometry. Thermal desorption mass spectrometry (TDMS) was performed on a homebuilt standard ultrahigh vacuum (UHV) system consisting of an airlock, UHV preparation chamber, andUHV main chamber with a differentially pumped quadrupole mass spectrometer (Hiden HAL/3F 301C) equipped with a 70 eV electron impact ion source. The thin film samples on polished silicon wafers as substrates were heated at a rate of 6 °C/min from room temperature to 200 °C by use of an electrically heated sample support. Samples were mechanically attached to sample holders made from copper and stainless steel. Fragments emitted from the sample upon heating were ionized and monitored by their partial pressure on the quadrupole mass spectrometer. Static Secondary Ion Mass Spectrometry. Molecular surface characterization was performed by static secondary ion mass spectrometry (SSIMS) with a 10 keV Ar+ primary ion beam (1 pA raster-scanned over an area of 100 × 100 µm2) in a time of flight (TOF)-SIMS IV system (ION-TOF, Germany). The sample stage was equipped with a copper sample support that was thermally connected to a liquid nitrogen heat sink and was temperature-controlled with an electrical heater. The polished silicon sample substrates were mounted to the flat copper surface of the sample holder. Samples were heated with a linear temperature rate of up to 15 °C/min. Data were continuously accumulated and stored in sets of 1 spectrum/min. Within the maximum primary ion dose density for each experiment of 1014 cm-2 no ion beam-induced degradation of the polymer was detected at room temperature. Prior to all SSIMS experiments, samples were heated for 15 min at 50 °C at approximately 10-9 mbar in the sample chamber. Contact Angle Determination. Water contact angles were obtained from sessile drops of approximately 10 µL volume in a temperature-controlled high-pressure cell at 25 bar in a nitrogen atmosphere saturated with water vapor at temperatures up to 200 °C. Static contact angles were determined by contour shape analysis on a commercial contact angle goniometer (Kru¨ss, DSA 10). Temperature-dependent contact angles were obtained in separate experiments for each temperature with the same sample and a new water drop. In each experiment the droplets were applied at a temperature below 100 °C at ambient pressure before
A Reversibly Switching Block Copolymer Surface
Figure 2. DSC curve (second ramp) for a copolymer with p + q ) 5 and k ) 123 showing a broad peak from approximately 128 to 185 °C with a heat of transition of 1.8 J/g. the cell was sealed, pressurized, and heated to the desired temperature. The contour shape was analyzed 10-15 min after the pressure and temperature of the cell was constant. The experimental error of the contact angle determination is estimated to approximately (0.5°. Contact angles determined at 23.5 °C at 1 and 25 bar are the same within experimental error (74.8° and 75.1°, respectively). To minimize evaporation of the water drop upon heating, liquid water was supplied to the sample cell to saturate the nitrogen atmosphere. However, due to unavoidable slight evaporation of the water drop, the contact angles are likely to be regarded as receeding angles.
Results and Discussion Thermal Analysis. Upon heating to 200 °C at 20 °C/ min under nitrogen, the mass loss is 0.03% by thermogravimetric analysis (TGA). Polymer degradation only starts at T ≈ 250 °C as observed by TGA. To monitor the stability over longer times, samples were heated to 200 °C for 25 h. While IR gives no indication of sample decomposition, TGA of the polymer p + q ) 3.3, k ) 37 shows a detectable mass loss of around 0.1% after 1 h that reaches 4.8% at 6 h, 5.5% at 12.5 h, and 6.2% at 25 h. Since there is a stabilization of mass loss, the process most likely consists of removal of residues from synthesis or unreacted low molecular weight material. These phenomena were not investigated in further detail. In our studies of temperature-dependent surface compositions we did not treat samples longer than about 1 h at 200 °C. Differential scanning calorimetry (DSC) was performed repeatedly between -100 and 250 °C at 20 °C/min, yielding a broad endotherm starting at approximately 80 °C only in the first ramp, which is absent in subsequent ramps. This may be melting of a separate phase that does not recrystallize upon subsequent rapid cooling. Hard segment glass transitions were observed between 214 and 221 °C (see Figure 2) for the different polymers, with no significant dependence on the polymer structure. These glass transitions, however, are accompanied by thermal decomposition at a temperature around 250 °C, which makes the determination of the glass transition temperature difficult. Thermal decomposition effects are also observed by the appearance of an endotherm at around -65 °C in a subsequent DSC ramp (see Figure 2). When the polymer samples are only heated to 150 °C in the first ramp, this endotherm is absent in the second ramp. Figure 2 shows a DSC curve of a second ramp for the polymer with p + q ) 5 and k ) 123. A broad weak endotherm from approximately 128 to 185 °C with a heat of transition of 1.8 J/g can be seen. This endotherm will be associated with the onset of partial intersegmental mixing of “noncrystalline” hard and soft microphases that
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accompany the microphase separation transition (MST) from an ordered to an unordered homogeneous mixed phase and will be discussed later. Thermal Desorption Mass Spectrometry. Thermal desorption mass spectrometry (TDMS) was performed to investigate to what extent thermal desorption processes are responsible for surface concentration alterations of the polymer constituents during heating of the polymer films. A copolymer film with p + q ) 3.3 and k ) 24.4 was heated from room temperature to 200 °C at 6 °C/min. The intensities of low mass fragment ions m/z ) 15, 16, 17, 26, and 27 increase by a factor of up to 10 upon heating to 200 °C. While the temperature of the copolymer film was held at 200 °C, the fragment intensities quickly decrease to values that are about equal to those obtained at 200 °C with the sample holder only. This desorption process is absent in subsequent heating cycles, indicating that only outgassing and/or removal of adsorbates of the sample take place. Possible sources for these processes include residuals from synthesis or unreacted low molecular weight material. The temperature-dependent surface concentrations that are detected repeatedly in many consecutive heating cycles by SSIMS therefore do not originate from desorption and/or outgassing processes. In line with the observation of desorption/outgassing in TDMS during initial heating of fresh samples are timedependent SSIMS signals when fresh samples are examined. At 30 °C the intensity IA (for the definition, see section Static Secondary Ion Mass Spectrometry) increases by a factor of 6 before it stabilizes after approximately 70 min at 30 °C. Upon heating to 50 °C, the signal stabilizes within approximately 15 min. This behavior during initial heating suggests desorption of adsorbates that most likely include low molecular weight material such as residues from synthesis of the sample. To avoid these effects completely, the temperature cycles during SSIMS experiments always included an initial treatment as follows. Samples were ramped from 20 to 50 °C at 6 °C/min, held at 50 °C for 15 min, ramped to 20 °C at 6 °C/min, and held at 20 °C for 5 min. SSIMS spectra were recorded and checked for signal stabilization during this initial temperature cycle of 30 min in total. Static Secondary Ion Mass Spectrometry. Changes in surface composition of the copolymer were monitored by the intensities of sets of unique fragment ions originating from blocks A and B within a sampling depth of static secondary ion mass spectrometry (SSIMS) of approximately 10 Å.20 Figure 3 panels a and b show identically scaled negative ion SSIMS spectra of the copolymer surface with p + q ) 5 and k ) 54 recorded at 200 and 20 °C, respectively. The spectra were recorded 8096 s (200 °C) and 12 650 s (20 °C) after starting temperature cycles between 20 and 200 °C with a ramp of 6 °C/min. The spectra differ significantly over the entire mass range. Figure 3a (200 °C) shows typical fragment ions from block B as obtained by comparison of spectra from pure poly(ricinoleic acid hexanediol ester) on a silicon wafer (not shown) and from known fragment ions of similar molecules.21,22 Practically every series of ions (CnH2n+2COO-, CnH2nCOO-, and CnH2n-2COO-) is present for n ) 4-17 (e.g., from C4H6COO- at m/z ) 97 to C17H31COOat m/z ) 279). The experimentally observed intensity (20) Hearn, M. J.; Briggs, D.; Yoon, S. C.; Ratner, B. D. Surf. Interface Anal. 1987, 10, 384. (21) Vickerman, J. C.; Briggs, D.; Legget, G. J.; Hagenhoff, B.; Chilkoti, A.; Bryan, S. R.; McKeown, P. J. Wiley Static SIMS Library; Wiley: Chichester, U.K., 1996. (22) Hearn, M. J.; Ratner, B. D.; Briggs, D. Macromolecules 1988, 21, 2950.
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Figure 3. Negative ion static secondary ion mass spectra of a polymer sample (p + q ) 5, k ) 54) (a) at 200 °C and (b) at 20 °C. Both spectra are scaled identically. Some important negative fragment ions (see Table 1) for block A are marked with solid arrows, and for block B, with open arrows (see text). Table 1. Some Characteristic Negative Fragment Ions in SSIMS from Polymer Blocks A and B m/z 26 42 127 129 155 181 183 195 197 209 211 255 279 281 283
proposed fragment ion structure Block A CNOCN-, H3C-CHdNBlock B C6H11-COOC6H13-COOC8H15-COOC10H17-COOC10H19-COOC11H19-COOC11H21-COOC12H21-COOC12H23-COOC15H31-COOC17H31-COOC17H33-COOC17H35-COO-
patterns of this series and the intensity distribution among the group members (CnH2n+2COO-, CnH2nCOO-, and CnH2n-2COO-) can be regarded as a signature of the specific compound and its molecular environment. Table 1 lists some characteristic fragment ions. At 20 °C (see Figure 3b) all characteristic fragments of block B are absent. This can clearly be seen from the mass range of 150-300, where none of the ions are detected. Instead, the intensity of those fragment ions is increased that can be attributed to block A. The most intense ions are m/z ) 26 and 42 (see Table 1). The intensity of some other ions such as m/z ) 50 and 60 also increases but their intensity is much smaller. However, m/z ) 26, 42, 50, and 60 are also detected at 200 °C in small yields (see Figure 3a). At least partially this is likely due to hydrocarbon
fragment ion formation from block B (e.g. C2H2- at m/z ) 26 and C3H6- at m/z ) 42). Fragment ions with high yields that are more specific for the molecular structure of block A but that are not emitted from block B cannot be found. Likewise fragment m/z ) 58 is also emitted at 20 °C in small yields (see Figure 3b). This, however, is largely due to emission from block A since the intensity ratio I(200 °C)/I(20 °C) is much higher for all other molecular fragments of block B. The relative concentrations of blocks A and B are characterized by the sum of negative fragment ion intensities IA and IB of the following nominal masses: block A (m/z ) 26, 42, 50) and block B (m/z ) 127, 129, 155, 181, 183, 195, 197, 209, 211, 255, 279, 281, 283). These ions are selected to obtain a high molecular specificity for detection of block A or B at maximum secondary ion yield. Figure 4 shows the steplike alteration of the surface composition of a copolymer film with p + q ) 3.3 and k ) 37. Upon increasing the temperature with a linear ramp of 6 °C/min from 90 to 200 °C, the signal IA of block A decreases while IB increases accordingly. The transition temperature TS at half-maximum of the step functions is TS ) 165 °C. The transition time from 20% to 80% of the maximum amplitude of the A f B transition is tS ) 3 min. The intensities IA and IB are not calibrated. An estimation of the surface concentrations of blocks A and B before and after the transition is obtained as follows. The intensity profiles of blocks A and B appear symmetric, thus indicating that the intensities IA and IB are likely to be proportional to relative molar surface concentrations cA and cB. Such proportional relationships between fragment ion yields and the surface concentration of the corresponding molecular precursor structures are well
A Reversibly Switching Block Copolymer Surface
Figure 4. Temperature-dependent copolymer surface composition for a copolymer with p + q ) 3.3 and k ) 37.
studied in numerous cases.23-30 They have been used for the analysis of surface concentrations of isolated compounds as well as for the quantitative determination of particular intramolecular structures in polymers at surfaces.31-33 However, quantitative determinations of surface concentrations by SSIMS require reference components of known concentrations and similar molecular structure, yet leading to differentiable fragments, such as internal standards or reference samples prepared and analyzed under identical conditions. To estimate the surface concentration of blocks A and B we use pure ricinoleic acid hexanediol ester and MDI as reference samples that were prepared and analyzed under identical conditions as the block copolymers under investigation. We use molecular specific fragments for block A (m/z ) 26 and m/z ) 42) and block B (m/z ) 255, m/z ) 283) for the determination of surface concentrations, assuming a linear dependence of fragment intensity and their relative molar surface concentration. Their specificity appears to be higher than about 99%, that is, less than 1% nonspecific yield of the fragment is detected from the reference polymer sample that does not contain the corresponding structure. The pure hydrocarbon fragment ions m/z ) 13 (CH-) and m/z ) 51 (C4H3-) do not change their absolute intensity during a temperature cycling experiment like the one shown in Figure 4. Therefore, their ion yields can be used for referencing, thus making the determinations of the ratios of ion yields Y for block A Y26/Y13, Y26/Y51, Y42/Y13, Y42/Y51 and for block B Y255/ Y13, Y255/Y51, Y283/Y13, Y283/Y51 much less dependent on the absolute ion yields. Fragment ion yields were obtains from peak integrals of the corresponding fragments. The nonspecific yield ratios were subtracted accordingly where appropriate. For the (23) Reed, N. M.; Vickerman, J. C. In Surface Characterization of Adcanced Polymers; Sabbatini, L., Zambonin, P. G., Eds.; VCH: Weinheim, Germany, 1993. (24) Vickerman, J. C., Briggs, D., Eds. Surface Analysis by Mass Spectrometry; Surface Spectra and IM Publications: Manchester and Chichester, U.K., 2001. (25) Belu, A. M.; Hunt. M. O., Jr.; De Simone, J. M.; Linton, R. W. Macromolecules 1994, 27, 1905. (26) Muddiman, D. C.; Gusev, A. I.; Procter, A.; Hercules, D. M.; Venkataramanan, R.; Diven, D. Anal. Chem. 1994, 66, 2362. (27) Li, J.-X.; Gardella, J. A., Jr. Anal. Chem. 1994, 66, 1032. (28) Castner, D. G.; Ratner, B. D. Surf. Interface Anal. 1990, 15, 479. (29) Vanden Eynde, X.; Reihs, K.; Bertrand, P. Macromolecules 1999, 32, 2925. (30) Reihs, K. Thin Solid Films 1995, 264, 135. (31) Vanden Eynde, X.; Reihs, K.; Bertrand, P. Macromolecules 2001, 34, 5073. (32) Reihs, K.; Voetz, M.; Kruft, M.; Wolany, D.; Benninghoven, A. Fresenius’ J. Anal. Chem. 1997, 358, 93. (33) Reihs, K.; Aguiar Colom, R.; Gleditzsch, S.; Demel, M.; Hagenhoff, B.; Benninghoven, A. Appl. Surf. Sci. 1995, 84, 107.
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block copolymer sample with p + q ) 5 and k ) 54 we estimate 180 °C to complete wetting at around 200 °C. This result matches with the observed alteration of the surface composition to a block B-rich surface starting at T >150 °C. The more hydrophobic block B-rich surface leads to higher contact angles indicated by the dashed line than the block A-type surface given by the solid line. At 200 °C, where the surface consists mostly of block B, the contact angle is 40° whereas complete wetting is expected for a block A-rich surface of the same composition as for a temperature below 150 °C. The low surface tension of water at 200 °C of 38 mN/m (see Figure 5a) compared to 72 mN/m at 25 °C yields much lower water contact angles than known from water on hydrophobic surfaces at room temperature, for example, 94° as the advancing contact angle on melt crystallized polyethylene35 at 20 °C. The difference in wettability of the two surfaces accounts for a difference in cos θ at 200 °C of ∆cos θ ) 0.23. Discussion of the Switching Mechanism. The MDIbased polyurethane-ureas in this study are examples for a class of segmented block copolymers, consisting of alternating hard segments (urethane or urea) and soft segments (polyester or polyether).36 The segments are chemically incompatible, that is, they have a positive heat of mixing. Thus, there is a propensity toward phase separation of the two components. However, the topology (35) Zisman, W. A. Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43, p 1. (36) Lamba, N. M. K.; Woodhouse, K. A.; Cooper, S. L. Polyurethanes in Biomedical Applications; CRC Press: Boca Raton, FL, 1998.
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of the polymer with connected constituents imposes restrictions on segregation, leading to microphase separation that has been directly observed by transmission electron microscopy.37,38 At room temperature the MDIcontaining hard segments form glassy or semicrystalline domains, whereas the soft segments form an amorphous or semicrystalline matrix in which the hard segments are dispersed at a low-to-moderate content. Phase separation of polyurethane-ureas is accompanied by extensive hydrogen bonding via the donor N-H group.39,40 In polyureas the presence of three-dimensional interurea hydrogen bonds of one urea carbonyl with two NH groups has been detected41 that leads to an unusually strong hard domain cohesion. Hard segments typically melt at temperatures above 200 °C. The endotherms observed in DSC at 214-220 °C in this study are attributed to hard segment melting. A lower-temperature broad and much weaker endotherm is observed around 160 °C (see Figure 2). The existence of such multiple endotherms for segmented polyurethaneureas has been documented in several studies.42-44 Koberstein and Russell45 made a similar observation of multiple endotherms for polypropylene oxide-based polyether polyurethanes with MDI-butanediol hard segments. They show by simultaneous differential scanning calorimetry and small-angle X-ray scattering (DSC-SAXS) that an intermediate-temperature endotherm is associated with the onset of partial intersegmental mixing of “noncrystalline” hard and soft microphases. The intersegmental mixing accompanies the microphase separation transition (MST) from an ordered to an unordered homogeneous mixed phase. The temperature of the MST in ref 45 is observed in the range of 140-210 °C depending on the annealing temperature, with a higher annealing temperature of the polymer yielding a higher temperature of the MST. A mechanism for temperature-induced switching of our copolymer surfaces can be proposed on the basis of the interpretation of the endotherms at temperatures below the hard segment glass transition by intersegmental mixing. At low temperature, polyurethane-urea hard segment domains dominate the polymer structure. For the copolymer with p + q ) 5 and k ) 54, the surface concentration of block A is >98% (91% in the bulk). The hard segment domain structure obviously leads to a suppression of surface segregation of block B. Only TS, where surface segregation of block B dominates.
Such structure seems to be unfavorable due to the relatively short segments B that impose sufficient sterical restrictions for the specific orientation of segments A that is required for the formation of three-dimensional interurea bonds of one carbonyl with two NH groups.41 Such a structure seems improbable also because of the varying length of segments A. The supposed structure therefore consists of “flat” domains with A segments parallel to the surface. At high temperature, surface segregation of block B takes place as seen from SSIMS and contact angle measurements. The surface concentration of block B changes from 96% (10% in the bulk). The onset of segregation of segment B corresponds to the temperature of an endotherm in DCS that is attributed to a microphase separation transition (MST) from an ordered to an unordered homogeneous mixed phase. Above this temperature TMST ) TS segregation of block B is possible due to a lack of crystalline microphases. The proposed structure is schematically illustrated in Figure 6b. Surface segregation of poly(ether urethanes) with perfluoro chain extenders as similar compounds has been investigated in ref 46. The authors find that the lower surface energy polyether compound migrates to the surface region upon annealing. The effect of the casting solvent on the surface and bulk structure was studied in further detail. The authors find a remarkable linear correlation between the fluorine surface composition by XPS and the IR band ratios of hydrogen-bonded to nonbonded urethane carbonyls. The higher the fluorine content from surface segregation is, the lower is the fraction of hydrogenbonded carbonyl. This correlation provides direct evidence for the suggestion that the surface structure is significantly controlled by the bulk structure, namely, by the extent of hard domain formation as seen from hydrogen bonding. By casting films with a very strong bulkstructure-perturbing solvent, even the hard segment with a high surface energy can be enriched at the surface. However, no reversible segregation was observed in the study. This may be due to the specific topology of the polymers used. (46) Yoon, S. C.; Ratner, B. D.; Macromolecules 1988, 21, 2401. (47) Wagner, W.; Kruse, A. Properties of Water and Steam; SpringerVerlag: Berlin, 1998. (48) Dooley, B. IAPWS Release on Surface Tension of Ordinary Water Substance 1994, International Association for the Properties of Water and Steam (IAPWS), Electric Power Research Institute: Palo Alto, CA, 1994.
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Figure 7. Temperature-dependent reversibly switching copolymer surface composition as seen from the logarithmic SSIMS intensity IA of block A for a copolymer with p + q ) 3.3 and k ) 37. Two experiments with heating cycles from 20 to 200 °C are shown: (a) two cycles at 6 K/min and (b) eight cycles at 15 K/min.
The reversibility of our process is shown in Figure 7. Decreasing the temperature from a B-rich surface at T ) 200 °C reverses the process (see Figure 7a). However, a slower transition time is observed, suggesting a reduced mobility of the polymer segments at lower temperatures. A second temperature cycle yields the same processes. A full cycle at 6 °C/min essentially allows the system to switch between an A-rich and B-rich surface as seen from Figure 7a. A higher frequency temperature oscillation with 15 °C/min ramps (see Figure 7b) does not allow the system to equilibrate due to the slower kinetics of the transitions, hence the amplitudes of the oscillations are nearly exponentially damped. In addition, the phase shift between the temperature and the intensity changes slightly, both demonstrating the behavior expected of temperatureinduced oscillations. Repeated alterations of the surface compositions after the first transition occur at slightly higher temperatures, yielding TS ≈ 185 °C for the copolymers with p + q ) 3.3 and k ) 24, 37, and 53. This also is in qualitative agreement with intersegmental mixing as reported in ref 45, where the authors observed higher temperatures TMST at higher annealing temperatures of their polymers. Our samples were annealed at 50 °C prior to the first heating cycle. Subsequent heating cycles account for annealing at higher temperatures. The temperature for the first transition changes from 170 ( 15 °C for polymers with p + q ) 3.3 (k ) 24-53) to 120 °C for p + q ) 5 and k ) 54. However, this effect has not been investigated in detail so far. Still, the influence of the transition temperature TS on the polymer structure is consistent with the proposed model. This dependence of the transition temperature on the structure also rules out the most simple and straightforward model for the switching mechanism by simply assuming temperature-dependent surface tensions with different temperature coefficients of the segments A and B. Here, we would not obtain different transition temperatures. In summary, the proposed mechanism of a competition of microphase crystallization of the hard segments A and surface segregation of the short hydrophobic segments B presents a consistent model of competing molecular processes that allow switching of the copolymer surface composition. Segregation-on-demand is possible when surface segregation is efficiently locked by a competing molecular process. Block copolymers are favorable materials for this scheme, since the two mechanisms, segregation and the “locking process”, can be implemented in two different chemical structures. The connected constituents in block copolymers are important for the proper lateral ac-
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cumulation of the materials. As an example, in the block copolymers investigated here, the competing process is microphase crystallization. Here, segregation is thermally unlocked at the phase transition temperature, thus yielding a sharply defined switching parameter. It must be noted that only minuscule amounts of material may be involved in the segregation mechanism because the detection depth of SSIMS involves only the outermost 10 Å of the polymer surface. The scheme can be potentially extended to other competing molecular processes with different unlocking mechanisms and control parameters depending on the properties of the copolymer selected. Properly constructed materials may offer the prospect of forming surfaces with switchable surface properties such as adhesion, wetting, friction, or selective adsorption. Conclusions The concentration of functional groups at a copolymer surface can be tremendously changed at a sharply defined temperature. The transition is completely reversible with
Reihs and Voetz
a transition time on the order of minutes. A proposed model for the effect is based on two competing processes. Surface segregation of one copolymer constituent is effectively hindered by microphase crystallization in the bulk of another copolymer constituent. Only at temperatures above the bulk phase transition temperature can surface segregation take place, generating a sharp increase of the concentration of the corresponding chemical functionalities at the surface. Acknowledgment. We gratefully acknowledge support from persons of the Bayer Corporation: F. Kempkes and B. Ko¨hler for helpful discussions and U. Itter, H.-W. Loyen, D. Ru¨hle, and U. Wolf for experimental support. Polymer samples were kindly provided by P. Heitka¨mper and M. Schmidt. Experimental support in temperaturedependent contact angle determinations by R. Eggers and P. Jaeger (Technical University Hamburg-Harburg, Germany) is gratefully acknowledged. LA051579N
