Intelligent Dual-Responsive Cellulose Surfaces via Surface-Initiated

Jul 18, 2008 - With changes in pH and temperature, these “intelligent” surfaces ..... from BiMaC (Biofibre Materials Centre), BioMime (Swedish Cen...
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Biomacromolecules 2008, 9, 2139–2145

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Intelligent Dual-Responsive Cellulose Surfaces via Surface-Initiated ATRP ¨ stmark, Per Antoni, Anna Carlmark, Josefina Lindqvist,† Daniel Nystro¨m,† Emma O Mats Johansson, Anders Hult, and Eva Malmstro¨m* Royal Institute of Technology, KTH School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56-58, SE-100 44, Stockholm, Sweden Received February 20, 2008; Revised Manuscript Received May 9, 2008

Novel thermo-responsive cellulose (filter paper) surfaces of N-isopropylacrylamide (NIPAAm) and pH-responsive cellulose surfaces of 4-vinylpyridine (4VP) have been achieved via surface-initiated ATRP. Dual-responsive (pH and temperature) cellulose surfaces were also obtained through the synthesis of block-copolymer brushes of PNIPAAm and P4VP. With changes in pH and temperature, these “intelligent” surfaces showed a reversible response to both individual triggers, as indicated by the changes in wettability from highly hydrophilic to highly hydrophobic observed by water contact angle measurements. Adjusting the composition of the grafted blockcopolymer brushes allowed for further tuning of the wettability of these “intelligent” cellulose surfaces.

Introduction In recent years, a large number of publications has focused on the ability to covalently modify surfaces by polymeric chains to tailor the interfacial properties.1,2 This modification can be achieved using “grafting-to” and “grafting-from” techniques, respectively. In the “grafting-to” method, preformed, endfunctionalized polymer chains are attached to the surface using appropriate conditions. However, this technique suffers from a low grafting density, because already grafted chains to a large extent maintain their coil-shaped conformation and, hence, hinder the diffusion of additional chains to the reactive sites at the surface.3 Higher grafting densities and thicker polymer layers can be obtained using the “grafting-from” approach, where the polymer brushes are formed by in situ surface-initiated polymerization from a layer of immobilized initiators.3 Part of the research in our group is focused on surface modification of cellulose, in terms of both insoluble cellulose fiber surfaces and soluble cellulose derivates, using atom transfer radical polymerization (ATRP).4–9 ATRP is a controlled/“living” radical polymerization technique, useful for the synthesis of functional macromolecules with controlled and complex architectures.10–16 Surface-initiated ATRP has been used for the grafting of well-defined polymer brushes from a variety of substrates.17–24 The great benefit of using ATRP, compared to conventional free radical polymerization, is the uniform growth of brushes, and the ability to control the length of the grafted polymer chains and, thereby, tailor the properties of the surface. Also, synthesis of block copolymer grafts can be accomplished via reactivation of the dormant halide groups present at the chain ends. Other controlled radical polymerization techniques have also been employed for the modification of cellulose. For example, Perrier and co-workers reported the grafting of styrene from filter paper using reversible addition fragmentation chain transfer (RAFT).25 Stenzel and co-workers modified soluble hydroxypropyl cellulose using RAFT.26 Shen et al. reported the modification of ethyl cellulose using ATRP,27 and Daly et al. * To whom correspondence should be addressed. Tel.: +46 8 790 8273. Fax: +46 8 790 8283. E-mail: [email protected]. † These authors contributed equally to the work.

reported the modification of cellulose by nitroxide mediated polymerization (NMP).28 Various parameters, such as wettability,29 dispersibility,13 and biocompatibility30 can be obtained or improved via surface modification. The wettability, or hydrophobicity, of a surface is of utmost importance for some advanced applications. Intelligent, or stimuli-responsive, surfaces that changes wettability in response to external triggers, such as pH,31 temperature,32 or radiation,33 have attracted great interest in biotechnological applications as drug carriers and sensors.34 Such surfaces can be prepared via grafting of stimuli-responsive polymers. One of the most widely studied responsive polymers is poly(N-isopropylacrylamide) (PNIPAAm), which is a thermoresponsive polymer with a lower critical solution temperature (LCST) around 32 °C in aqueous solution.32 PNIPAAm is hydrophilic below LCST due to efficient hydrogen bonding with water; however, above the LCST, intramolecular hydrogen bonds are formed and precipitation of the polymer occurs due to its increased hydrophobicity.35 PNIPAAm brushes have previously been grafted from different substrates using ATRP and employed for the fabrication of functional surfaces capable of a reversible transition between hydrophilic and hydrophobic character when subjected to changes in temperature.30,36–43 Other polymers, such as poly(acrylic acid) (PAA) and poly(4vinylpyridine) (P4VP) respond to changes in pH. PAA, which is hydrophobic in its protonated state (pH < 4) becomes hydrophilic after deprotonation.44–46 P4VP, on the other hand, is hydrophobic in its deprotonated state (pH > 5) and becomes water-soluble in its protonated state (pH < 5).47,48 Brittain and co-workers reported the grafting of block copolymers of 4VP and tert-butyl acrylate from silicon wafers using ATRP and subsequent deprotection by pyrolysis of the tert-butyl groups to obtain polymer brushes that respond to changes in pH.48 Jiang and co-workers reported on the preparation of a dual responsive (temperature and pH) silicon surface by grafting of PNIPAAm and PAA brushes obtained via ATRP.49 Li and co-workers reported the formation of another pH and temperature responsive silicone surface consisting of PNIPAAm and postfunctionalized PHEMA polymer brushes synthesized by ATRP.50 The hydroxyl

10.1021/bm800193n CCC: $40.75  2008 American Chemical Society Published on Web 07/18/2008

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groups in the PHEMA block were esterified with succinic anhydride to give the corresponding pH responsive carboxylic acid. The use of renewable, inexpensive, and biodegradable polymers, such as cellulose, in different applications has attracted great interest due to increasing environmental concern. Because of its many hydroxyl groups that readily can function as chemical handles, cellulose has great potential to be chemically modified to suit new application areas. For example, previous work in our group has demonstrated that superhydrophobic and self-cleaning filter paper surfaces can be fabricated via ATRP and postfunctionalization reactions.29 The present study describes the grafting of P4VP, PNIPAAm, P4VP-b-PNIPAAm, and PNIPAAm-b-P4VP, respectively, from a cellulose fiber substrate (filter paper) using surface-initiated ATRP. P4VP is a coordinative reagent for transition metals,51–53 meaning that biofiber surfaces modified with P4VP for instance could be used as membranes for extraction of heavy metals from wastewater. P4VP modified biofiber surfaces are also interesting in biotechnological applications because N-alkylated pyridinium polymers show antimicrobial properties.54,55 Thermo-responsive cellulose has previously been achieved by ceric(IV) ion-initiated graft-modification with NIPAAm as reported by Gupta et al.56 PNIPAAm is a biocompatible polymer and PNIPAAm-modified biofibers could be used as scaffolds in the fabrication of surfaces useful in biomedical applications for drug release or adhesion/ detachment of cells without the use of enzymes, analogous to what has previously been reported for PNIPAAm-grafted silica surfaces.30 Combining these two stimuli-responsive polymer brushes, yielding thermo and pH responsive surfaces, will increase the possibility to tailor surface properties. To the authors’ best knowledge this is the first report of a dual responsive surface prepared by grafting block copolymers of PNIPAAm and P4VP. This is also the first report describing surface initiated grafting of “intelligent” dual responsive polymer brushes from cellulose surfaces.

software. Contact angle measurements at pH 3, 7, and 9 were conducted by first immersing the modified cellulose surfaces in the pH solution for 30 min, followed by removal from the pH solution and drying at 50 °C for 20 min, where after the contact angles were measured using 5 µL droplets of the pH solution. Imaging of the surfaces was performed using tapping-mode AFM on a NanoScope IIIa Multimode AFM, using silicon tips (supplied by Veeco) with resonance frequencies of 267-348 kHz (according to the manufacturer). Immobilization of Initiator on Filter Paper. The procedure for immobilization of initiator on the cellulose surface was adopted from Carlmark and Malmstro¨m.58 The filter paper (2 × 3 cm) was rinsed in acetone and tetrahydrofuran (THF) and subsequently ultrasonicated in both solvents prior to immobilization of initiator. The accessible hydroxyl groups on the surface of the cellulose substrate were thereafter converted into ATRP initiators by reaction with 2-chloropropionyl chloride or 2-bromoisobutyryl bromide. The reaction was performed by immersing the substrate in a solution containing TEA (148 mg, 1.46 mmol) and a catalytic amount of DMAP in THF (20 mL) followed by addition of CPC (169 mg, 1.33 mmol) or BiB (305 mg, 1.33 mmol). The reaction was allowed to proceed overnight at room temperature on a shaking device. The filter paper was thereafter thoroughly rinsed in EtOH and THF successively to remove residual reagents and byproduct. The filter paper was finally dried under vacuum at 50 °C overnight.

Experimental Section

Grafting of 4-Vinylpyridine from Initiator-Functionalized Filter Paper. The CPC-modified filter paper was immersed into a flask containing 2-propanol (3 mL), 4-VP (3.50 g, 33.3 mmol), Cu(I)Cl (17.0 mg, 172 µmol), Cu(II)Cl2 (3.0 mg, 22 µmol), and Me6-TREN (44.0 mg, 191 µmol). The flask was sealed with a rubber septum, cooled in an ice bath, and thereafter evacuated and back-filled with argon three times. Grafting of 4VP from the initiator-modified filter paper was conducted at 50 °C, and the reaction was allowed to proceed for 120-360 min. After the polymerization was completed, the filter paper was thoroughly washed in THF, CH2Cl2, THF/H2O, and MeOH successively to remove residual monomer and the catalyst complex. The filter paper was finally dried under vacuum at 50 °C overnight.

Materials. Copper(I) chloride (Cu(I)Cl, 99+%), copper(II) bromide (Cu(II)Br2, 99%), copper(II) chloride (Cu(II)Cl2, 97%), N,N,N′,N′′,N′′pentamethyldiethylenetriamine (PMDETA, 99%), 2-bromisobutyryl bromide (BiB, 98%), 2-chloropropionyl chloride (CPC, 97%), and Whatman 1 filter paper were used as received from Aldrich. Buffer concentrates for pH 3, 7, and 9 were acquired from Aldrich. NIsopropylacrylamide (NIPAAm, 97%, Aldrich) was recrystallized twice from hexane and dried under vacuum prior to use. 4-Vinylpyridine (4VP, 95%, Aldrich) was passed through a column of basic aluminum oxide prior to use to remove the inhibitor. Tris(2-(dimethylamino)ethyl)amine (Me6-TREN) was prepared similar to the procedure by Ciampolini and Nardi from tris(2-aminoethyl)amine (98% Aldrich).57 4-(Dimethylamino)pyridine (DMAP, 99%) and triethylamine (TEA, 99%) were used as received from Acros. Glycidyl metchacrylate (GMA, 97%, Acros) was passed through a column of neutral aluminum oxide prior to use to remove the inhibitor. Instrumentation. Infrared spectra were recorded on a Perkin-Elmer Spectrum 2000 FT-IR equipped with a MKII Golden Gate, Single Reflection ATR System from Specac Ltd., London, U.K. The ATRcrystal was a MKII heated Diamond 45° ATR Top Plate. The obtained spectra were normalized against a region unaffected by the surface modification at around 2400 cm-1 to make comparisons possible. Static contact angle measurements were conducted on a KSV instruments CAM 200 equipped with a Basler A602f camera, using 5 µL droplets of MilliQ water and standards of pH 3, 7, and 9. The measurements were conducted at ambient temperature and at about 50 °C, respectively. The contact angles were determined using the CAM

Grafting of N-Isopropylacrylamide from Initiator-Functionalized Filter Paper. The CPC-modified filter paper was immersed into a flask containing a mixture of MeOH/H2O (2:1 v/v, 3 mL), NIPAAm (1.50 g, 13.3 mmol), Cu(I)Cl (18.3 mg, 185 µmol), Cu(II)Cl2 (10.7 mg, 79.6 µmol), and Me6-TREN (60.6 mg, 263 µmol). The flask was sealed with a rubber septum, cooled in an ice bath, and thereafter evacuated and back-filled with argon three times. Grafting of NIPAAm from the initiator-modified filter paper was conducted at ambient temperature and the reaction was allowed to proceed from 15-60 min. After stopping the polymerization, the filter paper was thoroughly washed in THF, dichloromethane (CH2Cl2), THF/H2O, and MeOH successively to remove residual monomer and the catalyst complex. The filter paper was finally dried under vacuum at 50 °C overnight.

Grafting of Glycidyl Methacrylate from Initiator-Functionalized Filter Paper. The BiB-modified filter paper was immersed into a flask containing toluene (4 mL), GMA (4.30 g, 30.2 mmol), Cu(I)Cl (14.0 mg, 141 µmol), Cu(II)Br2 (4.0 mg, 18 µmol), and PMDETA (25.0 mg, 140 µmol). The flask was sealed with a rubber septum, cooled in an ice bath, and thereafter evacuated and back-filled with argon three times. Grafting of GMA from the initiator-modified filter paper was conducted at 30 °C, and the reaction was allowed to proceed for 60 min. After the polymerization was completed, the filter paper was thoroughly washed in THF, CH2Cl2, THF/H2O, and MeOH successively to remove residual monomer and the catalyst complex. The filter paper was finally dried under vacuum at 50 °C overnight. Hydrolysis of Epoxide Group in PGMA Brushes. The oxirane groups in the PGMA-grafted filter paper were hydrolyzed in a solution of THF (10 mL), 12 drops of H2O, and 12 drops of concd HCl. The reaction was let to proceed for 60 min; the paper was thereafter rinsed

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Figure 1. Grafting of PNIPAAm, P4VP, P4VP-b-PNIPAAm, and PNIPAAm-b-P4VP from cellulose surfaces.

in THF, H2O, EtOH, and MeOH successively and finally dried under vacuum at 50 °C overnight.

Results and Discussion Stimuli-responsive polymer brushes of PNIPAAm, P4VP, P4VP-b-PNIPAAm, and PNIPPAm-b-P4VP were grafted from Whatman 1 filter paper, as schematically outlined in Figure 1. This substrate has high cellulose content and has proven useful for graft-modifications as reported in our previous work.5–7,29,58 To enable grafting via ATRP, the hydroxyl groups on the surface of the substrate were first esterified by reaction with 2-chloropropionyl chloride to yield covalently linked ATRP initiators on the surface. Grafting with 4-vinylpyridine (4VP) or Nisopropylacrylamide (NIPAAm) was conducted by immersing the initiator-modified cellulose substrates into a reaction mixture containing monomer, copper salt, ligand and solvent. Deactivating Cu(II)Cl2 was added to achieve control over the reaction by efficient deactivation of the propagating radicals on the surface.59 The graft length was controlled by varying the reaction time. Thermo-Responsive Cellulose Surfaces. The surface-initiated ATRP of NIPAAm to produce thermo-responsive polymer brushes from a cellulose substrate was performed in a solution of MeOH/H2O and catalyzed by Cu(I)Cl/Cu(II)Cl2/Me6TREN. The grafting reactions were conducted at ambient temperature, and the reaction time was varied between 15 and 60 min to vary the graft lengths. At longer reaction times, swelling of the cellulosic substrate was observed. This can be explained by the formation of hydrogen bonds between PNIPAAm and cellulose, breaking up the interfibrillar structure that is also held together by hydrogen bonding, causing the cellulose fibrils to separate from each other. FT-IR analysis of the PNIPAAm-grafted cellulose surfaces showed amide peaks (I and II) at 1650 cm-1 and 1540 cm-1, not seen for unmodified filter paper, thus confirming the grafting, Figure 2a,b. The area of the amide peaks (I and II) increased with increasing reaction time, thus indicating a continuous growth of the brushes. The PNIPAAm brushes caused variations in the wettability of the cellulose surface in response to changes in temperature, due to the LCST of the polymer at 32 °C, Figure 3. Below LCST, the brushes assume a stretched conformation as a result of intermolecular hydrogen bonding with water, resulting in a hydrophilic behavior.42 However, at LCST, the brushes undergo a phase transition, resulting in a collapsed brush conformation due to intramolecular hydrogen bonding and resulting in reduced wettability of the surface.42 The phase transition of the PNIPAAm brushes is not only characterized by a change in wettability; it is also reflected as a change in brush height as earlier reported by Li and co-workers.50 However, the height of the polymer layer is difficult to measure on filter paper due to its inherent high surface roughness. The change in brush conformation and wettability of the PNIPAAm-grafted cellulose surface was instead monitored by static water contact angle (CA)

Figure 2. FT-IR spectra of cellulose substrate: (a) unmodified, (b) PNIPAAm-grafted, (c) P4VP-grafted, and (d) P4VP-b-PNIPAAmgrafted.

Figure 3. Variations in wettability of the PNIPAAm-grafted cellulose surface due to conformation changes of the brushes below LCST (left) and above LCST (right).42

measurements at temperatures above and below LCST. Below LCST, the PNIPAAm-modified cellulose surface was hydrophilic, and the applied water droplet was adsorbed into the surface as expected, Figure 3. At temperatures above LCST, the cellulose surface was hydrophobic and the CA to water was 110°, Figure 3. The stability of the CA over time decreased with increasing graft length, indicating that the grafting of NIPAAm does break up the interfibrillar structure leading to exposure of unmodified areas, as previously discussed. To limit the swelling of the cellulose surface arising from the PNIPAAm-modification and to obtain a stable CA also at increased amounts of grafted PNIPAAm, a branched “graft-ongraft” architecture of the polymer brushes was pursued. This was achieved by the graft-copolymerization of NIPAAm from poly(glycidyl methacrylate) (PGMA) brushes on a cellulose surface. GMA was grafted from the cellulose surface using a procedure previously reported by our group.29 To obtain ATRP initiators along the PGMA brushes, the epoxide groups in the side chain were first hydrolyzed using hydrochloric acid in a THF/H2O mixture to obtain a high density of hydroxyl groups

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Figure 5. Conformation of P4VP brushes below (left) and above (right) the pKa (∼5.2) for the pyridyl group. Figure 4. FT-IR spectra of cellulose surface grafted with (a) PNIPAAm and (b) PGMA-g-PNIPAAm brushes.

along the polymer backbone.29 The subsequent attachment of 2-chloropropionyl chloride to the formed hydroxyl groups resulted in a multitude of ATRP initiators along the PGMA brushes. The grafting of NIPAAm from these initiating sites yielded a branched “graft-on-graft” architecture of the brushes, and it was evident from FT-IR analysis that significantly larger amounts of PNIPAAm (amide peaks I and II) had been grafted with this method compared to the linear brush architecture (Figure 4). The PGMA-g-PNIPAAm cellulose surface was still hydrophilic below LCST; however, above LCST, the modified surface was hydrophobic with a stable CA of 130°, thus indicating a less affected interfibrillar structure from the NIPAAm grafting due to the protecting PGMA layer. More importantly, the PGMA layer does not affect the thermoresponsivity of the surface. The heating and cooling of the thermo-responsive surface was repeated several times, without affecting the difference in CA below and above LCST, suggesting that the switching between hydrophilic (CA 0°) and highly hydrophobic nature (CA 130°) of the modified cellulose surface is of reversible character. pH-Responsive Cellulose Surfaces. The ATRP of 4VP has shown to be problematic due to the coordination of the copper halide to the pyridine ring present in both monomer and polymer. This problem can, however, be overcome by using a more active ligand, such as Me6-TREN, that forms a strong complex with the alkyl halide.60 The use of chlorine-based mediating systems with 4VP has shown to give rise to less side reactions, such as hydrolysis and methanolysis, compared to bromine-containing systems.61 In addition, the formation of branched structures formed via nucleophilic substitution of the halide end-group with the pyridine groups in monomer and/or polymer can also be avoided using chlorine-containing systems.60,61 The grafting of 4VP from the initiator-modified cellulose surface to produce pH-responsive brushes was therefore performed in 2-propanol and catalyzed by Cu(I)Cl/Cu(II)Cl2/Me6-TREN to minimize the aforementioned side reactions. These conditions are similar to the ones reported by Du Prez and co-workers for the controlled polymerization of P4VP.53 The grafting reactions were conducted at 50 °C and the reaction time was varied between 2 and 6 h to vary the graft lengths. FT-IR analysis confirmed the success of the grafting of P4VP from cellulose

by the appearance of peaks at 1597 cm-1 and 1556 cm-1, corresponding to the pyridyl ring-stretching (III and IV), Figure 2c. The area of the pyridyl peaks increased with increasing reaction time, thus indicating a continuous growth of the brushes. The P4VP-grafted cellulose surfaces responded to changes in pH via protonation and deprotonation of the pyridyl group, Figure 5. In the protonated state, the brushes exhibit an extended conformation due to electrostatic repulsion of the charged pyridinium segments.48 When pH is increased to above 5, however, a collapse of the brushes occurs with deprotonation of the pyridyl group (pKa ∼5.2).48 This transition is also reflected by changes in the wettability of the surface. The P4VPmodified cellulose surfaces were characterized by static CA measurements at pH 3, 7, and 9, respectively. The graft-modified surfaces were saturated in a pH buffer for 30 min and dried on a hot plate at 50 °C prior to the characterization. The CA was thereafter measured using a droplet of the appropriate pH buffer. The protonation of the pyridyl group, below pKa, was confirmed by FT-IR measurements and resulted in a shift of the peaks corresponding to the pyridyl group to 1638 cm-1 and 1605 cm-1, respectively, as earlier reported by Ruokolainen et al. and Li and co-workers.62,63 At pH 3, the protonated, P4VPmodified cellulose surface is hydrophilic (CA 0°), and the applied water droplet is rapidly adsorbed into the surface within a couple of seconds, Figure 5. The collapse of the polymer brushes due to the deprotonation of the pyridine rings greatly enhances the hydrophobic character of the cellulose surface. At pH 9, well above the pKa for P4VP, the cellulose surface is highly hydrophobic with a maximum CA of 125°, Figure 5. The (unexpectedly) high CA is explained by the inherent surface roughness of filter paper, which is an important parameter for the hydrophobicity as described by Cassie and Baxter.64 It is suggested that the cavities on the filter paper surface are filled with air, thus meaning that the applied water droplet mainly is situated on air-pockets and a high contact angle is obtained. At pH 7, the P4VP-grafted cellulose surfaces, overall exhibit lower CAs (90-100°) than at pH 9. The CAs were furthermore not constant at this pH, and different values were obtained at different measurements for the same sample. This suggests that the deprotonation of the pyridyl group is incomplete at this pH, resulting in regions of more hydrophilic and hydrophobic character, respectively. Cycling between pH 3 and pH 9 several times, measuring the CA for each step showed a repeatable pH-responsive

Intelligent Dual-Responsive Cellulose Surfaces

Figure 6. CAs at different pH for a P4VP-modified cellulose surface: half-cycles, pH 3; and integral cycles, pH 9.

Figure 7. AFM amplitude images (2 × 2 µm) of (a) unmodified and (b) P4VP-b-PNIPAAm-grafted cellulose surfaces.

character of the P4VP-grafted surfaces switching between hydrophilic and highly hydrophobic character of the cellulose, Figure 6. Dual-Responsive Cellulose Surfaces. To create novel, “intelligent”, cellulose surfaces that respond to two different separate triggers (pH and temperature), PNIPAAm-b-P4VP and P4VP-b-PNIPAAm block-copolymer brushes were grafted from the cellulose substrates. The appearance of the amide stretch peaks (I and II) at 1650 cm-1 and 1540 cm-1 and the pyridyl stretch peaks (III and IV) at 1597 cm-1 and 1556 cm-1 from FT-IR analysis confirmed the formation of block-copolymer brushes, Figure 2d. The polymerization of a second block using the graft-modified cellulose surfaces as macro-initiators indicate the presence of living chain ends in the first block. AFM imaging of a P4VP-b-PNIPAAm grafted cellulose substrate showed a much smoother surface structure compared to the unmodified cellulose surface, as can be seen in Figure 7. The fibrillar structure is hardly visible in the graft-modified substrate and appears to be covered by the polymer. The block copolymer modified cellulose surfaces were characterized by CA measurements at pH 3 and 9 (below and above the pKa value of P4VP) and at 20 and 50 °C (below and above the LCST of PNIPAAm). The results from the CA measurements of the modified cellulose at different pH-values and temperatures are presented in Table 1. The behavior and the resulting CA for the modified cellulose surfaces can be divided in four regions, as illustrated in Figure 8. In region I, pH 3 (below pKa) and 20 °C (below LCST), the pyridine segments in the P4VP block are protonated and the PNIPAAm block forms intermolecular hydrogen bonds with water, meaning that both of the blocks

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exhibits extended conformations. Under these conditions, irrespectively of block lengths and block order, all the surfaces are hydrophilic (CA 0°), as expected, and the applied droplet is rapidly adsorbed into the substrate. In region IV, pH 9 (above pKa) and 50 °C (above LCST), both blocks are in their collapsed state and the highest hydrophobicity was observed for all the modified cellulose surfaces, as expected. The CA ranged between 115-120° and it is shown that the combination of high pH and temperature results in a reduced wettability of the cellulose surfaces (Table 1). However, the PNIPAAm-b-P4VP modified samples showed less stable CAs than for the surfaces with P4VP as the first grafted block. This is probably an effect of the disruption of the interfibrillar structure caused by the PNIPAAm grafting as discussed earlier. In region II, pH 9 (above pKa) and 20 °C (below LCST), the P4VP block is deprotonated, resulting in a collapsed hydrophobic conformation, whereas the PNIPAAm block still is in a hydrophilic extended chain conformation. A complex and competitive situation between hydrophilicity and hydrophobicity occurs between the two different blocks in the grafted polymer brushes. The surfaces grafted with PNIPAAm-b-P4VP were found to be hydrophilic (CA 0°) in region II when the PNIPAAm block is long (tR1a 30 and 60 min) and the P4VP block is short (tR2b 120 min), Table 1. The hydrophilic character of these surfaces may be explained by the dominating effect of the long hydrophilic PNIPAAm block compared to the relatively short collapsed hydrophobic P4VP block. Alternatively, the hydrophilic character of the surface may be caused by rearrangement of the brushes to expose the hydrophilic PNIPAAm block instead of the hydrophobic P4VP block. For the surface modified with a short PNIPAAm block (tR1a 15 min) followed by a long P4VP block (tR2b 360 min), a hydrophobic character was observed (CA 100°). The reverse equivalent to the latter, where the grafting of a long P4VP block (tR1a 360 min) is followed by a short PNIPAAm block (tR2b 15 min), was observed to exhibit a similar hydrophobicity with a CA of about 100°. These results indicate that the hydrophobic character of the P4VP block is dominating, resulting in similar wettability of the equivalent cellulose surfaces regardless of the position of the blocks (Table 1). In region III, pH 3 (below pKa) and 50 °C (above LCST), the P4VP block is protonated resulting in an extended brush conformation, whereas the PNIPAAm block is collapsed due to intramolecular hydrogen bonding. A competitive situation between hydrophilicity and hydrophobicity occurs in a similar way as in region II. For the surfaces grafted with a long PNIPAAm block (tR1a 30 and 60 min) followed by a short P4VP block (tR2b 120 min), the hydrophobic character of the PNIPAAm block is dominating, resulting in CAs of 65-85° for the PNIPAAm-b-P4VP grafted surfaces (Table 1). For the surface instead modified with a short PNIPAAm block (tR1a 15 min), followed by a long hydrophilic P4VP block (tR2b 360 min), the applied water droplet was observed to adsorb into the PNIPAAmb-P4VP grafted surface, suggesting that the hydrophilic character of the P4VP block is dominating in this case. A hydrophilic surface was also observed when P4VP was grafted as the first block from the substrate (P4VP-b-PNIPAAm), indicating that the long P4VP block has the dominant effect on surface wettability regardless of the position of the blocks, as could also be observed in region II. The reversibile switching between hydrophilic and hydrophobic character for the P4VP-b-PNIPAAm modified surfaces was demonstrated by cycling between pH 3 below LCST (region

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Table 1. Contact Angles for Graft-Modified Cellulose at Different pH and Temperature, Corresponding to Regions I-IV brush type

tR1a (min)

tR2b (min)

CA, region I,c pH 3, 20 °C

PNIPAAm-b-P4VP PNIPAAm-b-P4VP PNIPAAm-b-P4VP P4VP-b-PNIPAAm

15 30 60 360

360 120 120 15

0 0 0 0

a Reaction time for the polymerization of the first block. Figure 8.

b

CA, region II, pH 9, 20 °C

CA, region III, pH 3, 50 °C

100° 0 0 100°

0 85° 65° 0

Reaction time for the polymerization of the second block.

CA, region IV, pH 9, 50 °C 115° 120° 115° 115°

c

Regions I-IV illustrated in

acter below the LCST of PNIPAAm and the applied water droplet was instantly adsorbed. Above LCST the PNIPAAMgrafted cellulose surfaces showed a hydrophobic character, but the observed CAs were, however, not stable over time. Employing a branched “graft-on-graft” architecture of the brushes by grafting NIPAAm from initiator-modified PGMA brushes on the cellulose, the CA above LCST could be increased by 20° and the hydrophobic behavior was observed to be stable over time. The P4VP-grafted cellulose surfaces exhibited a pHresponsive behavior, with changes in surface wettability from hydrophilic at pH 3 to hydrophobic at pH 9. Dual-responsive cellulose surfaces were achieved via grafting of PNIPAAm-bP4VP and P4VP-b-PNIPAAm. These intelligent surfaces respond to changes both in temperature and pH. The lengths of the different blocks was found to play an important role in the resulting surface properties, and allows further tailoring of the responsivity of the material. The dual-responsive behavior was also found to be reversible. We believe that these intelligent surfaces could be obtained also by modification of other kinds of cellulose-based substrates, which would open up for possible biomedical applications as drug carriers or sensors.

Figure 8. Conformation of dual-responsive block-copolymer brushes into different regions depending on the different pH and temperature.

Acknowledgment. The authors acknowledge financial support from BiMaC (Biofibre Materials Centre), BioMime (Swedish Center for Biomimetic Fiber Engineering), The Swedish Research Council, Wilhem Beckers Jubileumsfond, and The Research Council of Norway within the NanoMat program, project “Dendritic nanoporous materials with multifunctionality” (No. 163529/S10). Niklas Nordgren is acknowledged for assistance with AFM analysis.

References and Notes

Figure 9. CAs at different temperature and pH for a P4VP-bPNIPAAm-modified cellulose surface: half-cycles, T < LCST, pH 3; and integral cycles, T > LCST, pH 9.

I) and pH 9 above LCST (region IV) several times, showing repeatable CA, Figure 9.

Conclusions Novel stimuli-responsive cellulose surfaces have been achieved via grafting of NIPAAm and 4VP from filter paper. The PNIPAAm-grafted cellulose surface showed hydrophilic char-

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