NANO LETTERS
Reversible Stimuli-Responsive Nanostructures Assembled from Amphiphilic Block Copolymers
2006 Vol. 6, No. 2 282-287
Chen Xu,† Xuefeng Fu,‡ Michael Fryd,‡ Song Xu,§ Bradford B. Wayland,‡ Karen I. Winey,† and Russell J. Composto*,† Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6272, Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323, and Molecular Imaging, Tempe, Arizona 85282-6731 Received November 25, 2005; Revised Manuscript Received January 9, 2006
ABSTRACT We present a novel route to assemble perpendicular cylinders by converting an asymmetric diblock copolymer from poly(styrene-b-tert-butyl acrylate) (PS-b-PtBA) to poly(styrene-b-acrylic acid) (PS-b-PAA) using an autocatalytic reaction. Upon exposure of the films of PS-b-PAA to water, PAA cylinders constrained by the continuous, glassy PS phase protrude 10 nm above the surface and swell laterally to form mushroom caps, rendering the entire surface hydrophilic. Upon annealing, the original nanostructures re-form demonstrating reversibility of swelling. Because of their stimuli-responsive behavior, these nanoscale materials are excellent candidates for sensors and microfluidic applications.
Stimuli-responsive materials are attractive as sensors, membranes, microactuators, microfluidic devices, and other advanced nanodevices.1-5 Polymeric materials, in particular, can respond to external stimuli (such as solvent, temperature, pH, light, etc.) by cooperative conformational changes on the nanometer length scale.4 As a result, smart responsive polymer films can spontaneously alter or change their surface properties, such as wettability, reactivity, or biocompatibility. These types of materials have recently been employed to tune film stability, wettability, adhesion, and permeability and to regulate the interaction of cells and proteins with biomaterials.1,3,4 For example, self-assembled monolayers have been programmed to absorb or release proteins under a thermal stimulus.1 Similarly, macromolecules such as stimuli-responsive polymer brushes provide versatile surfaces that can respond to changes in temperature, solvent, pH, and other stimuli.2,4-9 Polymer brushes prepared from weak polyelectrolytes, such as poly(acrylic acid), were found to undergo substantial changes in swelling, upon varying pH or ionic strength, which suggest that polyelectrolytes are potential “active” components in nanostructured smart surfaces.10-12 * Corresponding author: phone, (215) 898-4451; fax, (215) 573-2128; e-mail,
[email protected]. † Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania. ‡ Department of Chemistry, University of Pennsylvania. § Molecular Imaging. 10.1021/nl052332d CCC: $33.50 Published on Web 01/21/2006
© 2006 American Chemical Society
Upon microphase separation, block copolymers can selfassemble into ordered, periodic nanostructures, such as spheres, cylinders, and lamellae with typical dimensions of 5-50 nm. The morphology and dimensions can be tailored by manipulating the volume fraction of each block, molecular weight, monomer-monomer interactions, and temperature.13,14 Nanostructures based on amphiphilic block copolymers are particularly attractive because each block can be designed to be selectively responsive to different stimuli such as aqueous and organic solvents.15,16 However, because of the large repulsive segmental interactions and solubility differences between blocks, amphiphilic block copolymer molecules tend to self-associate in aqueous solution and form micelles.17 Once these micelles form and solvent is removed, the microstructure is trapped and unable to achieve the equilibrium morphology that occurs in the melt. As a result, solvent casting of amphiphilic block copolymers is a problematic route to prepare controlled nanostructures.15 Recently, block copolymers containing tert-butyl acrylate or tert-butyl methacrylate blocks were spun cast and then converted by thermal or acid-catalyzed deprotection to a hydrophilic block.15,18 Building upon these studies, a novel approach is developed here to create an amphiphilic block copolymer that assembles into a nanostructure containing hydrophilic cylinders oriented perpendicular to the surface. Upon exposure to water, the hydrophilic cylinders are constrained from swelling laterally by the glassy continuous
phase and forced to swell above the surface resulting in mushroomlike caps that render the entire surface hydrophilic. Upon partial drying in air, the mushroom collapses as stretched hydrophilic chains relax, which results in nanometer depression in the center of each cap. The nanostructures revert to their initial dimensions and area fractions upon annealing, which demonstrate that the sequential swelling and shrinking of the nanostructures is entirely reversible. The diblock copolymer in this study is poly(styrene-btert-butyl acrylate) (PS-b-PtBA) where the Mn of the blocks are 66 200 and 32 000 g/mol, respectively, and PDI is 1.05 (Polymer Source Inc.). The PtBA block has a thermally labile ester linkage that can be cleaved by thermal or acid-catalyzed deprotection.18 Wallraff et al.19 studied the kinetics of thermal and acid-catalyzed deprotection of poly(tert-butyl methacrylate) (PtBMA) and demonstrated that the thermal cleavage of the tert-butyl ester linkage occurs after a slow induction period followed by a fast autocatalytic process in which the deprotected groups catalyze further deprotection. They also showed that acid-catalyzed deprotection occurs at a substantially reduced temperature and proceeds to completion rapidly. This acid-catalyzed approach has been employed to hydrolyze coatings and Langmuir-Blodgett films of PtBMA and PtBA using a hydrochloric acid catalyst.15,20,21 Extending this idea of an acid catalyst, films of PS-b-PtBA were cast on silicon substrates with an oxide layer. Although their acidity is low, the surface hydroxyl (-OH) groups can act as a catalyst to cleave tert-butyl groups at 130 °C (Scheme 1).18 To the best of our knowledge, this route has not yet been explored to controllably transform PtBA to PAA. Scheme 1. Chemical Transformation of PS-b-PtBA to PS-b-PAA by Thermal Deprotection of the tert-Butyl Groups Catalyzed by Surface Hydroxyl Groups
Prior to film deposition, silicon wafers were treated to produce surface hydroxyl groups. Silicon wafers were cleaned with a piranha solution (98% H2SO4/30% H2O2, volume ratio ) 3:1) at 80 °C for 30 min. After being cooled for 30 min at room temperature, the wafers were rinsed and then immersed in ultrapure water (Millipore Direct-Q, 18 MΩ cm resistivity) for 1 day. After being dried with a flow of nitrogen gas, the substrates were exposed to UV-ozone for 10 min.22 Thin films were prepared by spin casting PSb-PtBA from a toluene solution onto cleaned silicon substrates and then dried in a vacuum at room temperature for 1 day. Using a Rudolph Research AutoEL-II Null ellipsometer at a fixed incident angle of 70° and helium-neon laser source (λ ) 632.8 nm), the measured thicknesses of the ascast films ranged from 20 to 80 nm. Samples were annealed at 130 °C in a vacuum for 2 days and then quenched to room temperature. The films thicknesses were then measured by ellipsometer immediately after annealing. The chemical transformation from PS-b-PtBA to PS-bPAA was analyzed by attenuated total reflection Fourier Nano Lett., Vol. 6, No. 2, 2006
Figure 1. (a) FTIR-ATR spectra before (I) and after (II) surfacecatalyzed hydrolysis of a 40 nm thick film of PS-b-PtBA to PSb-PAA. (b) The percentage change in film thickness for initial thickness values from 20 to 80 nm. The solid line at 16.8% denotes complete conversion of PtBA to PAA.
transform infrared (FTIR-ATR) spectroscopy (Nicolet Nexus 470 FT-IR ESP spectrophotometer configured with a Nicolet Smart DuraSampl/R ATR module). The initial film thickness (40 nm) of PS-b-PtBA was similar to the thickness used for the atomic force microscopy studies. The spectra were recorded before (I) and after annealing at 130 °C for 2 days (II), as shown in Figure 1a. Before the sample was annealed, the spectrum shows absorption bands characteristic of Cd O stretching at 1725 cm-1, tert-butyl (tBu) double absorption at 1392 and 1368 cm-1,21 and the aromatic group at 1491 and 1450 cm-1.18,21 After the sample was annealed, the Cd O stretching vibration broadens and shifts to 1717 cm-1, which is consistent with the formation of carboxylic acid. Second, the broad band from ∼3300 to ∼3600 cm-1 is attributed to the O-H vibration in -COOH, although the presence of water is possible. Third, disappearance of the two characteristic tert-butyl peaks at 1392 and 1368 cm-1 provides strong evidence for complete conversion to carboxylic acid.15,21 To complement FTIR-ATR studies, the conversion of PSb-PtBA to PS-b-PAA was determined by measuring the decrease in film thickness due to the evolution of the tertbutyl groups after annealing at 130 °C for 2 days.12 Because lateral dimensions of each sample are fixed, the percent change in thickness coincides with the volume change. Table 1 shows the molecular characteristics of PS, PtBA, and PAA, 283
Table 1. Molecular Characteristics of PS, PtBA, and PAA
Mn (g/mol) F (g/cm3)a v (cm3/mol) a
PS
PtBA
PAA
66 200 1.05 63 048
32 000 1.06 30 189
18 000 1.24 14 516
Density values from Polymer-Design Tools (DTW Associates, Inc.).
including molecular weight (Mn), density (F), and molar volume (V). The molecular weight of PAA assumes complete conversion of PtBA. By use of the values from Table 1, the percent change in thickness can be calculated for complete conversion from PtBA to PAA ∆h ∆ν (νPS + νPtBA) - (νPS + νPAA) ) ) ) 16.8% (1) h0 ν0 (νPS + νPtBA) where ∆h (∆ν) is the thickness (volume) change upon annealing and h0 (ν0) is initial thickness (volume). Figure 1b shows that the experimental values of ∆h/h0 are independent of h0 between 20 and 80 nm and in excellent agreement with the theoretical value, 16.8%, assuming complete conversion of PtBA to PAA. Thus, the volume fraction of PS in the copolymer increases from 0.68 to 0.81. Both the ATR and thickness measurements indicate that the PtBA is completely converted to a PAA block after annealing PS-b-PtBA films at 130 °C for 2 days on a hydroxyl-rich silicon surface. Two additional observations support this conclusion. First, the absorption bands associated with aromatic vibration (1491 and 1450 cm-1) increase in intensity after annealing as shown in Figure 1a, consistent with an increase in PS volume fraction. Second, thickness measurements show that annealing results in an increase in the refractive index from 1.53 to 1.57, which is expected for the conversion of PtBA to PAA.23 The resulting amphiphilic block copolymer of PS-b-PAA, with a PAA volume fraction of 0.19, is an excellent candidate as a reversible, moisture responsive material because the minority PAA nanodomains impart functionality whereas the PS matrix provides the mechanical stability required for multiple cycles as shown below. Atomic force microscopy (AFM) images were obtained using a Molecular Imaging PicoPlus scanning force microscope. Figure 2 shows the height (left) and phase (right) images of an ordered PS-b-PAA copolymer film (thickness h ∼ 33 nm) on a silicon substrate. The images were obtained at ambient conditions in air immediately after annealing. The cylindrical nanodomains of PAA (bright) are oriented normal to the substrate and organize into nearly hexagonal packing. This is consistent with a hexagonally packed array of cylindrical PAA domains with a lattice parameter of 40.3 nm in the bulk, indicated by small-angle X-ray scattering (SAXS). Very similar ordering is observed for PS-b-PAA film thicknesses from 20 to 50 nm. All AFM images were analyzed using the Grain Analysis Module of SPIP software (Image Metrology, Inc.). The PAA cylinders have a diameter of 23.7 ( 2.7 nm. The average center-to-center distance 284
Figure 2. AFM images of a PS-b-PAA film (h ∼ 33 nm) after conversion of PS-b-PtBA by annealing at 130 °C for 2 days on a hydroxyl-rich surface. Height (left) and phase (right) images show PAA cylindrical domains aligned perpendicular to the substrate. Both images are 1 × 1 µm2. The height scale from dark to bright is ∆z ) 0-20 nm.
between adjacent cylinders is 52.0 nm, which is greater than the bulk value 40.3 nm.24 The area per cylinder is 446.3 ( 23.3 nm2, resulting in a PAA area fraction of 0.20, which is in outstanding agreement with the PAA volume fraction of 0.19. This consistency further supports the premise that the PAA cylinders align perpendicular to the substrate. The height difference between the cylinders and the matrix is about 2 nm and most likely results from the swelling of hygroscopic PAA cylinders even at ambient conditions.15 The functionality of the nanostructured film is dictated by the properties of the PAA nanocylinders. At low pH (pH < 4), PAA chains collapse in aqueous solutions because of the inability of the carboxylic acid groups to ionize. At high pH (pH > 8), PAA swells considerably in water because the carboxylic acid groups are converted to carboxylate ions.11,15 In our experiment, the swelling reagent, ultrapure water (Millipore Direct-Q, 18 MΩ cm resistivity) with a pH of 5-6, is expected to induce moderate swelling of PAA chains. The swelling of PS-b-PAA films was followed by in situ AFM. Parts a (height) and b (phase) of Figure 3 were obtained immediately after the sample was immersed in the fluid cell. Thus, the first images (Figure 3a,b) were taken after 10 min of immersion. Qualitatively, the domains are much larger compared to the dry state in Figure 2. Scans from the same region were taken every 5 min for a total of 120 min. Comparison of the images after 10 min (Figure 3a,b) and 70 min (Figure 3c,d) shows that the domain diameters are similar. However, the domain height has grown significantly as denoted by the greater contrast in Figure 3c relative to Figure 3a. Before a quantitative analysis of swelling behavior is presented, it is worth noting that in situ imaging is particularly challenging because of complications presented by the medium perturbing the oscillating tip and the possible indentation of the tip into the soft, swollen nanoscopic domains. Nevertheless, the reference square box inserted in parts a and c of Figure 3 demonstrates that the same region could be imaged for at least 1 h with limited drift. In contrast to the cylindrical PAA domains observed in dry films (cf. Figure 2), the morphology of wet films (e.g., Figure 3c) is characterized by mushroomlike domains, consisting of a PAA stem and swollen PAA cap. Although the initial domains are circular viewed from above (Figure Nano Lett., Vol. 6, No. 2, 2006
Figure 3. Nanostructure in films (dry thickness h ∼ 33 nm) of amphiphilic PS-b-PAA after swelling in water. AFM height and phase images after 10 min (a, b) and 70 min (c, d). In panels a and c, the reference box denotes the same area and the height scale is ∆z ) 0-20 nm. All images are 1 × 1 µm2. (e) Schematic illustration of mushroomlike nanostructure showing how PAA chains extend to form the water-swollen cap. (f) Histogram of diameters observed for cylinders (dry) and swollen mushrooms (wet). The average values are 23.7 ( 2.7 nm and 52.0 ( 6.8 nm, respectively.
2), the swollen caps display a planar interface because lateral growth is arrested by adjacent domains. Figure 3e shows a cartoon of a PAA mushroom with cap diameter (D) and cap height (H). Being confined by the continuous glassy PS matrix, the stem of PAA is unable to swell laterally and, thus, has a limited capacity to take up water. This interpretation is consistent with the invariance of the PAA domain separation and the hexagonal packing observed for the dry and wet films. Because of this matrix constraint, the PAA nanodomains can only absorb water by swelling vertically above the surface and horizontally along the surface, resulting in the mushroom-shaped domains described in Figure 3e. To form the cap, we propose that PAA chains rapidly (i.e., 1.0, diffusion is described as Fickian, anomalous, case II, or supercase II, respectively. The solid line in Figure 4 was fit to the data using n ) 2.64, suggesting that water diffusion into the PAA nanodomains follows supercase II. Physically, the accelerated swelling reflects an increase in diffusion rate due to plasticization of the PAA by water. Supercase II diffusion has been reported for PAA multilayers in aqueous solution, as well as homopolymer PS swollen with nhexane.25 At high volume fractions of water, swelling becomes dominated by molecular relaxations,25 and H ) 1 - B exp(-k2t) H∞
(3)
where B is a scaling constant and k2 reflects the rate of PAA stretching. The dotted line in Figure 4 was fit using k2 ) 0.001 s-1, corresponding to a characteristic PAA relaxation time of 1000 s. To test whether swelling was reversible, swollen films were first dried in air at room temperature for 1 day and then annealed in a vacuum oven at 130 °C for another day. Parts a (height) and b (phase) of Figure 5 show that the mushroom caps remain close packed upon drying in air. However, in the vertical direction, the cap flattens and the center region collapses resulting in a 1 nm depression. A possible explanation follows. During swelling, the PAA chains near the surface of the cylinders extend laterally across 286
the PS surface to form the base of the cap whereas PAA chains below the surface stretch toward the apex as shown in Figure 3e. On the basis of the values of D and H in the dry and wet states, PAA blocks would stretch by a factor of ∼2, which is consistent with literature values.10,12,26 Upon drying in air, the PAA chains around the periphery of the cap collapse onto the PS matrix; however, as water evaporates, the highly elongated PAA chains in the center retract to gain conformational entropy, resulting in depressions. Thus, the donut-shaped features in parts a and b of Figure 5 result from the collapse of the mushrooms in air. Similar features were also observed after further vacuum drying at room temperature for 2 days. This observation suggests that complete removal of water requires annealing above 100 °C to overcome the hydrogen bonding between water molecules and the -COOH groups of the PAA block.27 Thus, to reform the original nanostructure, annealing above 100 °C is required to both completely remove water and allow for molecule relaxation above the Tg (106 °C)23 of the PAA block. In an attempt to recover the original nanostructure, the same film was annealed at 130 °C for 1 day and the resulting morphology is shown in Figure 5c,d. A comparison of the annealed film and original film (cf. Figure 2) shows almost identical features demonstrating that the swelling process is highly reversible. In the former case, the cylinder diameter is 24.8 ( 3.1 nm, which is very similar to the original value of 23.7 ( 2.7 nm. Figure 6 summarizes the evolution of the molecular/ nanostructure in the dry, swollen, and partially swollen state. Initially, the cylindrical domains slightly protrude from the surface due to water adsorption under ambient conditions (Figure 6a). Upon exposure to water, the hydrophilic PAA nanodomains swell, while constrained by the glassy PS matrix. Thus, PAA can only swell vertically and laterally above the PS matrix resulting in mushroom caps that cover the entire surface as in Figure 6b. After a rapid lateral expansion (i.e.,
