Grafting Through - ACS Publications - American Chemical Society

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Surface Reconstruction by a “Grafting Through” Approach: Polyacrylamide Grafted onto Chitosan Film Elza Chu and Alexander Sidorenko* Department of Chemistry & Biochemistry, University of the Sciences in Philadelphia, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Grafted polymers and polymer brushes in particular have attracted significant attention in the last 2 decades as a way to alter and control interfacial properties. In the case of polymer brushes on solid substrates, a high grafting density of polymer chains results in stretching of the polymer coils normal to the substrate surface due to the effect of excluded volume. In this study, polyacrylamide is grafted to the surface of relatively soft thin films of chitosan. The “grafting through” approach is used by introducing double CC bonds to amino groups of chitosan. The acquired kinetic data of grafting along with AFM and ellipsometry characterization suggest that the chitosan substrate undergoes surface reconstruction during the grafting of PAAm and simultaneously induces PAAm growth inside the soft substrate. As a result, much higher amounts of grafted polymer are achieved in comparison to traditional hard substrates like silicon or glass. Additionally, selective plasma etching of PAAm reveals filament-like structures oriented normal to the surface.

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of vinyl (−CH 2 CH) or acryl [CH 2 CHC(O)O−] functions onto a surface; they act as both chemical anchors and propagation sites during polymerization. Initiation occurs in the bulk, and propagating macroradicals generated in close proximity to the surface form grafted chains.14,15 This technique provides relatively high grafting density along with high stability and homogeneity. Although the aforementioned modifications on hard substrates are relatively straightforward and highly efficient, there are inherent thermodynamic limitations due to an impenetrable interface and finite surface area. According to recent estimations, the free energy of a polymer coil in the stretched regime may reach the value of a covalent bond tethering the chain to the substrate.16,17 In many cases this limitation can be surmounted by a complex architecture of grafted polymer or multiple-site chemisorbed polymeric initiator.18,19 More specifically, in ref 19 the authors obtained polymer brushes with very high grafting density due to distribution of grafting sites in the depth of an initiating polymer layer. Recently, we have combined the multiple-site chemisorption platform with the “grafting through” approach to obtain polymer brushes of polyacrylamide.15 The same approach was successfully employed to generate an electrically switchable copolymer brush of poly(acrylamide-co-acrylic acid).20 This has led us to delve into the less explored area

INTRODUCTION Grafted polymers are comprised of polymer chains covalently attached at one or more sites of a surface, typically grown on solid substrates; silicon wafer or glass slides serve as model substrates in most cases. One of the characteristics of grafted polymer systems is grafting density.1 If a high grafting density is reached, the effect of excluded volume causes the polymer coils to stretch in the direction normal to the substrate plane to avoid overlapping.1 This regime with the extended conformation of polymer coilsso-called polymer brushhas attracted much interest for its strong application potential. Polymer brushes, which are a type of grafted polymers, can significantly affect surface properties, such as hydro- and oleophobicity, adhesion, reflectance, and degradation.2−7 Grafted polymers are often used to alter surface properties of underlying substrates without affecting their bulk properties. Most commonly, grafted polymer layers both in and out of the brush regime are generated by introducing an anchoring layer to a solid substrate, allowing polymers for “grafting to”, “grafting from”, or “grafting through” the substrate. In the former grafting to approach, end-functionalized polymers covalently bond to a self-assembled monolayer or adsorbed functional polymers on the substrate.8−10 This method provides rather low grafting density and thickness of up to 10 nm. The grafting from approach requires the attachment of an initiator to the substrate’s surface prior to in situ polymerization initiated from the surface.11−13 It provides significantly higher grafting density capable of producing more than 100 nm thick brushes. The latter grafting through approach consists of incorporation © 2013 American Chemical Society

Received: July 15, 2013 Revised: September 5, 2013 Published: September 11, 2013 12585

dx.doi.org/10.1021/la402609w | Langmuir 2013, 29, 12585−12592

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Figure 1. Grafting through of the chitosan-graf t-PAAm system by cross-linking chitosan substrate via the formation of a secondary amine by nucleophilic substitution reactions with 1,4-dibromobutane vapors (i), −CH2CH grafting sites were grafted to the soft substrate by exposure to GMA solution (ii), and PAAm is grafted through chitosan via photoinitiation of the monomer solution (iii).

of grafting polymers on “soft” substrates that can potentially deform to reveal new grafting site locations. Here we use the term “soft” to stress on the stretchable and plastic nature of the soft substrate’s surface as opposed to the traditionally used rigid substrates such as silicon or glass.21−23 In this work, chitosan, the deacetylated form of chitin, was used as the soft substrate for the grafting of polyacrylamide. Chitosan was selected for several reasons. It contains a large number of free amino and hydroxy groups that are available for further modification. Chitosan has a much lower Young’s modulus (3.8 kPa) in comparison to glass and silicon {100}, 77 and 130 GPa, respectively, which renders it as a possible candidate for deformation as stated above.24−27 It is also biodegradable, nontoxic, and soluble in acidic aqueous mediums, which allows for aqueous reactions.28,29 So far, extensive research has been done on chitosan modifications in bulk including molecular grafting.28−30 An interesting approach has been explored by Wang et al., who used an adsorbed layer of chitosan for surface modification of soft PDMS substrates by subsequent “grafting to” of poly(ethylene glycol).31 To the best of our knowledge, however, grafting of polymers onto the surface of chitosan films has never been studied. In this paper we investigate the synthesis and kinetics of grafting polyacrylamide (PAAm) on to chitosan thin films via grafting through. We will demonstrate that this method allows chemical grafting of PAAm onto surface of chitosan films, as

well as formation of the chitosan-graf t-PAAm compositions with unique morphology. The role of soft substrate will be deciphered by comparing the results with data obtained using the same approach on solid silicon substrate reported in ref 15. We will also show that this method can be used for convenient and reproducible fabrication of nanoscopic filament-like structures of chitosan.

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EXPERIMENTAL SECTION

Materials. Chitosan powder (190−310 kDa molecular weight, 75− 85% deacetylated), glacial acetic acid, ammonium hydroxide, 1,4dibromobutane, poly(glycidylmethacrylate) (PGMA), acrylamide (AAm), and 2-hydroxy-4′-(2-hydroxyethoxy)2-methylpropiophenone (photoinitiator or PI) were all purchased from Sigma-Aldrich Chemicals. All solvents were also obtained from Sigma-Aldrich and were used without further purification. Silicon wafers obtained from Addison Engineering were of {100} orientation, and glass slides were cut (about 20 × 20 mm) and successively cleaned in ultrasonic baths of dichloromethane, methanol, and deionized water for 15 min each. The samples were then immersed in an “alkali piranha” bath composed of 25% H2O2 and 25% NH4OH in deionized water at 82 °C for 40 min and thoroughly rinsed with water and dried with argon prior to use. Synthesis. Clean silicon wafers were immersed into a 1% solution of PGMA in chloroform for 30 min for chemisorption. More details for substrate modification by PGMA can be found elsewhere.15,32 All sample series used a solution of 2% chitosan in 2% aqueous acetic acid, unless stated otherwise. The sample of chitosan in a powdered form was dissolved in a 2% acetic acid solution overnight and filtered prior 12586


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to use. Chitosan was deposited on PGMA-modified Si wafers by spincoating initially at 600 rpm, followed by 2700 rpm for complete drying. This resulted in homogeneous films with thicknesses of 200 ± 50 nm. The resulting films were comprised of chitosan acetate soluble in water; to convert chitosan into its nonionic form the acetate ions were removed by immersing the samples in a 2% ammonia solution for 2 min. The chitosan films were then exposed to 1,4-dibromobutane vapors for 15 min to form cross-links. The films were further dried in a vacuum oven overnight at 50 °C. Next, methacrylic groups bearing CC bonds were introduced to the chitosan surface by immersing them in a 10% solution of GMA in chloroform for 15 min. The excess GMA was removed by rinsing with chloroform. The grafting of PAAm onto the modified chitosan film was performed via photoinitiated polymerization by the grafting through approach. In a typical experiment, several droplets of aqueous 20% solution of AAm with 1% of PI were deposited on the surface of the chitosan sample, covered with a glass coverslip, and exposed to ultraviolet light (mercury “BlakRay” lamp) for 30 min from the distance of 50 cm. We used a grafting time of 30 min for all series to ensure saturation was reached. Next, samples were soaked in deionized water for at least 24 h to completely dissolve unattached polymer. Samples were additionally rinsed multiple times with Nanopure water and dried under an argon flux. FTIR measurement samples were prepared by spin-coating a ∼150 nm chitosan film using a 2% chitosan in 2% acetic acid solution on a PGMA-modified ATR ZnSe crystal accessory. The accessory disk was manually attached to the spin-coater stage. Sample FTIR and background spectra were scanned for each modification (hydrolysis, cross-linking, GMA modification, and grafting of PAAm) as the chitosan-graf t-PAAm system was built successively. The ATR crystal was cleaned using 2% acetic acid solution and dried with argon gas after each modified sample FTIR scan in order to run a fresh background scan for every modification. The chitosan-graft-PAAm system was prepared step-by-step after every background scan to incorporate the next modification. SEM samples were prepared as stated above. After etching the samples, the filaments were sputter-coated with ∼50 nm of gold and characterized either by AFM or SEM. Characterization. The thickness of the polymer films was a routinely measured parameter, as the gain in chitosan film thickness upon polymerization reflects the amount of grafted PAAm. We assume the additive model for the entire thickness of the composite film and the density (ρ) of both chitosan and PAAm to be 1.0 g cm−3. Then the amount of grafted PAAm, Γ (mg/m2), can be calculated from the gained thickness d (nm) according to a simple formula: Γ = ρd. Thickness measurements were done using an AFM and an Angstrom Advanced Inc. PHE102 spectroscopic ellipsometer. The psi and delta profiles for the ellipsometric scans were produced in the 320−850 nm wavelength range at a 70° incident angle. Atomic force microscopy (AFM) images were obtained on a diInnova (Veeco Metrology) scanning probe microscope in tapping mode. The AFM probes (Budget Sensors) had a resonant frequency of 160−180 kHz and a spring constant of 48 N/m. The AFM images were treated and analyzed using the WSxM software by Nanotec Electronica. Sessile droplet contact angle measurements were taken using a custom-made assembly to examine the evolution of hydrophobicity/philicity at different steps of the process. Successful grafting of PAAm onto chitosan substrates was confirmed by FTIR measurements performed using a Thermo Nicolet Avatar 370 spectrometer. The spectra were taken using an automated VeeMAX accessory (PIKE Technologies).The samples were plasma etched using a Desk V HP (Denton Vacuum). SEM images were produced using a JEOL JSM 6300 scanning electron microscope to visualize the morphology of the structures produced after etching the chitosan-graf t-PAAm thin films. The length of the filaments was measured by SEM and AFM using contact mode as described in the Supporting Information.

Article

EXPERIMENTAL RESULTS We study grafting through of PAAm onto a soft substrate of chitosan film. Recently, we have explored photoinitiated grafting through of PAAm on chemisorbed PGMA modified by introduction of methacrylic CH2C(CH3)C(O)O− functions with a double carbon−carbon bond.15,33,34 Similarly, in the present study we introduce acrylic groups to chitosan by reaction between GMA and primarily amino groups of chitosan (Figure 1). Although chemical reaction between glycidyls and hydroxyls is also possible, amino groups are more efficient in this reaction.35 The approach of introduction of GMA sites into the matrix for further polymer tethering by radical polymerization was adopted from recent works by Aizenberg et al.36 The resulting surface is capable for grafting through, as we observe from the increase in thickness of the polymer film. Both ellipsometry and AFM scratch tests are consistent in the amount of polymer film formed upon grafting. The concept of “soft substrate” is the keystone of this work; therefore, careful characterization of successive modifications for the chitosan film is crucial to correctly interpret results of these modifications. The contact angle (CA) measurements offer qualitative information on the evolution and nature of the top layer of a film. Significantly different CAs demonstrate that each modification was carried out successfully during the surface preparation and grafting (Table 1). The chitosan “as Table 1. Evolution of Contact Angles of Chitosan Thin Films upon Modifications chitosan modifications unmodified hydrolyzed cross-linked dried in oven grafted PAAm plasma etched

contact angles (deg) 67.0 44.2 71.1 95.1 28.6 48.4

± ± ± ± ± ±

1.0 1.0 3.0 0.50 1.5 2.0

deposited” films reveal a relatively hydrophilic surface with θ = 67° because chitosan may absorb some amount of water in the presence of acetate salts, thus giving a smaller CA compared to 92° ± 5° of native chitosan.37 We hydrolyzed the samples using a 2% ammonium hydroxide solution to remove the acetate ions, which form salts with the amines of chitosan. It prevents the films from swelling and dissolving in the successive steps. Upon hydrolysis we have observed a further decrease in CA that we associate with the slightly porous nature of the film.28,38,39 Further cross-linking and especially drying result in hydrophobization, as revealed by an increase of the CA up to 95°. The grafting of PAAm has a dramatic effect on the wetting properties of the films. The CAs have been measured upon excessive wash with nanopure water; it decreases by about 70° (θ = 28.6 ± 1.5°). Naturally, we associate such hydrophilicity with the presence of grafted PAAm on top of chitosan films. The kinetics of grafting of PAAm onto chitosan substrate has been investigated as a gain in thickness measured by AFM and/ or ellipsometry of the dried polymer film upon removal of ungrafted PAAm. The amount of grafted PAAm was calculated from the thickness gained upon polymerization assuming a PAAm density of 1.0 g cm−3. The results are shown in Figure 2. In accordance with the findings of Enright et al., there are three kinetic regimes starting with a short period of inhibition due to impeding contaminations like oxygen.15 After 2−3 min, a sharp increase in thickness of grafted PAAm is observed that then 12587


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shown in Figure 4. The observed grafting amount of PAAm depends on the degree of cross-linking of the chitosan

Figure 2. Effect of varying grafting time on amount of grafted PAAm. Initial thickness of chitosan film was 250 ± 11 nm, 20% w/v AAm, 1% PI. Figure 4. Effect of varying cross-linking time on amount of grafted PAAm (mg/m2).

reaches saturation at about 6−10 min, depending on the polymerization compositions (concentration of the monomer, PI, etc.). The maximal gain in thickness reaches about 180 nm, which is about 40 times thicker than PAAm brush obtained under the identical conditions (4.5 nm in thickness using 20% AAm, 1% PI, same light intensity) and about 6 times thicker than the maximal thickness produced at the highest monomer concentrations (27 nm in thickness using 45% AAm solution) as recorded in the case of rigid Si substrate in ref 15. We also have studied the effect of variations in monomer concentrations on the amount of grafted polymer. Figure 3

substrate. When the chitosan film is exposed to the crosslinker vapors for longer than 30 min, the number of amino groups available for further transformation into grafting sites CH2C(CH3)C(O)O− significantly decreases, which results in poor grafting density. Upon cross-linking for 1 h, no PAAm is grafted. This fact is easy to explain by deactivation (SN2 nucleophilic substitution) of the amino groups used for further modification with GMA. In Figure 5 we show how varied chitosan substrate thickness affects the grafting PAAm amount. It linearly increases with

Figure 3. Effect of varying monomer concentration (%) on grafted PAAm (mg/m2). The line is a guide for the eye.

Figure 5. Effect of chitosan substrate thickness on the gained amount of grafted PAAm (mg/m2).

reveals a sharp increase in grafting amount with respect to monomer concentration till 15% w/v AAm before reaching a plateau. Observed behavior clearly deviates from the first-order kinetic scheme, where the amount of grafted polymer is directly proportional to monomer concentration. This indicates the complex nature of the process where surface reconstruction of the substrate is involved. Some decrease at 40% w/v AAm reflects the instability of polymerization when the polymerization solution is saturated by the monomer. A priori the properties of the chitosan substrate are important for grafting onto it. Therefore, we have probed several vital parameters of the substrate. One of them is the degree of cross-linking. Thin polymer films are potentially unstable. Hence, in order to provide better morphological stability of the substrate, we have cross-linked chitosan segments by exposing them to vapors of 1,4-dibromobutane. We have varied the degree of cross-linking via exposure time as

respect to chitosan substrate thickness in the range up to 320 nm. Although the amount of grafting sites on chitosan is limited by the surface, the grafting amount depends on the thickness of the chitosan film. This indicates that PAAm is formed both inside and outside of the soft substrate. The extrapolation of the amount of PAAm grafted on the zeroth thickness of the substrate (intercept of the linear regression) may formally be associated with the thickness of PAAm grown solely on the surface (“exterior”) of the chitosan film; it is approximately 3 mg/m2. We suggest that a very small amount of PAAm is grafted on the surface rather than the actual stated amount. It infers that the majority of the grafted polymer resides within the substrate (“interior”). Therefore, as opposed to the traditional grafting on hard substrates, the majority of PAAm is grown in confinement inside the soft substrate. We speculate that polymerization occurring in the interior is complicated by 12588


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of an increase in carbonyl stretching, as well as, symmetric stretching of methyl groups present in PAAm.40 We also observed a strong increase of the peak at 1660 cm−1 due to an increase in CO groups upon grafting of PAAm. The series of three peaks centered at 1074 cm−1 is characteristic of the C−O stretching vibrations in chitosan.40 Their intensity significantly decreases upon grafting of PAAm, as chitosan is “diluted” by the PAAm chains in the film. The evolution of the FTIR spectra was also measured for every modification of the soft substrate to ensure effective chemical changes within chitosan, as shown in the Supporting Information (Figure S3). The AFM images shown in Figure 7a and b depict the flat surface of the films. In an independent experiment, we found that plasma etching is kinetically selective with respect to chitosan and PAAm, where PAAm etches 3 times faster than chitosan. Thus, we used atmospheric plasma etching to reveal the internal morphology of the chitosan-graf t-PAAm, as shown in Figure 7c. The surface of the etched film appears as a densely packed layer (“rug”) of normally oriented filaments with the average neighbor-to-neighbor distance of approximately 50 nm. The limitation of an AFM tip to fully penetrate into the dense array of the filaments prevents direct measurement of the filaments’ length. Therefore, SEM has been used. The SEM study of chitosan-graf t-PAAm films confirms such peculiar shaped moieties on the etched sample. Also, they demonstrate the effect of varying etching time on filament heights. The SEM images show the evolution of the film with exposure to plasma. A relatively flat film of chitosan-graf tPAAm (Figure 8a) turns into an array of low-profile dots (Figure 8b). Further etching results in clearly seen elongated filaments (Figure 8c) until the filaments are fully exposed (Figure 8d). The height of the filaments is approximately 100 nm. Further etching leads to disintegration of the film. We were inspired by Watson et al. to develop a method for measuring filament heights using the AFM in contact mode by shaving off filaments from the bulk chitosan substrate as shown in the Supporting Information (S4).42 Filament heights were

the Trommsdorff effect and foresee some deviation of properties of grafted PAAm, e.g., higher molecular weight. We also explored the role of chitosan substrate exposure to AAm monomer solution for extended periods of time prior to grafting PAAm. As shown in the Supporting Information (Figure S1), any swelling that chitosan may have undergone when exposed to AAm solution does not have any significant effect on grafted polymer thickness. In order to ensure successful grafting of PAAm to chitosan soft substrate, we measured the FTIR spectra of the chitosangraf t-PAAm as shown in Figure 6. More technical details of the FTIR acquisition are given in the Supporting Information.

Figure 6. FTIR spectra for chitosan-graft-PAAm (a) and chitosan substrate (b).

As supported by analysis of the FTIR spectrum of chitosan given in Saifuddin et al., the broad peak at 3366 cm−1 is due to hydroxyl group stretching vibrations overlapping with N−H extension vibrations.40 The same peak in the chitosan-graf tPAAm is even broader due to the additional hydrogen bonding between the carboxylic oxygen in PAAm and the amine groups in chitosan.40,41 The FTIR spectrum of the chitosan-graf tPAAm sample showed a weak shoulder at 2950 cm−1 because

Figure 7. AFM images of 1 μm × 1 μm scan of neat chitosan and height profile (a, d), 2 μm × 2 μm scan of unetched chitosan-graft-PAAm surface and height profile (b, e), and 2 μm × 2 μm scan of etched chitosan-graf t-PAAm surface and height profile (c, f). 12589


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Figure 8. SEM images of unetched chitosan-graft-PAAm sample (a) and filament-like structures on the surface after etching for 15 min (b), 30 min (c), and 1 h (d), respectively.

Assuming that the surface of filaments is chitosan, we tried the hydrophobization of the “rug” with (heptadecafluoro1,1′,2,2′-tetrahydrodecyl)trimethooxysilane (FDS) using the available hydroxy and amino groups of chitosan. The modification was carried out with 1% FDS in toluene for 15 min. The fluorination was successful, as the surface became hydrophobic with the contact angle increasing to 114°. Besides the demonstrated opportunity to modify the surface properties of the filaments, successful silanization/hydrophobization also showed that grafted PAAm was completely etched out by plasma. Otherwise, the grafted PAAm would screen out hydrophobic fluoroalkyls and the surface would remain hydrophilic. Unfortunately, the results of the contact angles can be used only qualitatively in the case of ruglike surfaces, as the effect of roughness will result in strong deviations in contact angles according to Wenzel.43 We have considered two scenarios for the formation of filaments: either by pulling off chitosan filaments or polymerization inside the chitosan soft substrate. The former could be caused by excluded volume effects applied to the chitosan, which lead to pulling up the potential sites capable for new grafting of the substrate by already grafted PAAm chains in extended conformation.44 Thus, exposing new grafting sites on the stretched segments of the soft substrate would result in thicker grafted polymer in comparison to traditional hard substrates.15 When plasma is applied, the PAAm part is etched away and the pulled chitosan filaments are revealed. However, this hypothesis cannot explain the relatively uniform shape and density of the filaments. The second scenario suggests polymerization within the layer of chitosan. It takes into consideration the curious feature of many polysaccharides to form crystallites. Polymer chains rich in hydrogen bonds and other weak interactions, such as chitosan, are capable of packing in nanoscopic needlelike crystallites of 20−30 nm in diameter.45 A well-known example is microcrystalline cellulose. We speculate that cross-linking of the substrate produces filaments by preserving the pseudocrystallite nature of chitosan. As seen in Figure 4, in the absence of

measured for two different series, namely, varying PAAm grafting times and plasma-etching times, as shown in Figure S2 (Supporting Information). The series demonstrated filament heights ranging between 5−35 and 10−50 nm, respectively. The underestimation of heights in comparison to the SEM images could be due to incomplete shaving of filaments, as explained in the Supporting Information. The extrapolation of the filament height data indicated that at least 3 min of grafting PAAm is required to procure filament structures, and a minimum of 3 min of etching is needed to expose the ruglike morphology of the films.

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DISCUSSION Figure 4 had revealed that cross-linking of chitosan does not affect the amount of grafted PAAm prior to 30 min of 1,4dibromobutane exposure. If no cross-linking is applied at all, the films of chitosan-graf t-PAAm are homogeneous both before and after plasma etch. We therefore deduced that cross-linking forms a well-defined chitosan matrix for the grafted PAAm to propagate through it; it ultimately results in the self-assembly of filaments. In order to suggest the most probable scenario of the formation of the ruglike surface composed of nanoscopic filaments, several additional experiments were performed. First of all, we have tested the ability of etched samples (filaments) to withstand soaking in water. While water with neutral pH does not affect their morphology, slightly acidic solutions (e.g., 1% acetic acid) dissolve the films immediately. These observations lead us to conclude that both the filaments and the underlayment are composed of chitosan and not of PAAm. Additionally, in a separate series of experiments we have compared the etching rates of chitosan and PAAm films; the latter is etching about 3 times faster. This confirms our suggestion that the features revealed by plasma etching are chitosan. The relatively low water contact angle of about 48° can be explained by surface modification of chitosan exposed to atmospheric plasma. Also, surface roughness may contribute to a further decrease of the observed contact angle.43 12590


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Figure 9. Schematic of cross-linked chitosan substrate modified with GMA (a), chitosan-graf t-PAAm (b), and chitosan nanocarpet exposed via plasma etching (c).

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the cross-linking step, a similar amount of grafted PAAm in comparison to 5−30 min of cross-linked chitosan is observed, but it does not reveal any filaments upon etching; this infers that cross-linking is necessary to shape the filaments. On the other hand, excessive exposure to the cross-linker results in a very minute amount of grafted PAAm. The latter can be explained by excessive cross-linking of the entire matrix, which restricts the rearrangement of the filaments and thus prevents formation of a new surface available for grafting. During the first step of polymerization in our experiments, the grafting occurs at the surface of chitosan, resulting in “reactive swelling” (i.e., reaction results in hydrophilization and successive swelling) of underlying strata of cross-linked chitosan and so forth, as shown in Figure 9. The Trommsdorff effect may play a crucial role because polymerization takes place in a very viscous gel-like medium and in nanoscopic confinement. Subsequent polymerization of the laterally confined chitosan-graf t-PAAm film causes an irreversible “swelling” of the film and, thus, results in vertical orientation of the rigid nanoscopic crystallites. Upon plasma etching, the PAAm domains are etched off along with surrounding chitosan, revealing the ruglike morphology of the chitosan-graf t-PAAm film. The latter scenario is not free of internal contradictions, in particular movement of crystallites in highly viscous medium. Therefore, we plan to further investigate filament formation. We plan to discuss these and other issues in a separate publication.

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SEM images were kindly provided by Dr. Igor Tokarev from Clarkson University, Potsdam, NY. We would also like to thank Timothy P. Enright (Clarkson University) for the fruitful discussions and Rahul Samudralwar (USciences) for technical assistance.

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REFERENCES

(1) Milner, S. T. Polymer Brushes. Science 1991, 251 (4496), 905− 914. (2) Blum, M. M.; Ovaert, T. C. A Novel Polyvinyl Alcohol Hydrogel Functionalized with Organic Boundary Lubricant for Use as LowFriction Cartilage Substitute: Synthesis, Physical/Chemical, Mechanical and Friction Characterization. J. Biomed. Mater. Res., Part B 2012, 100 (7), 1755−1763. (3) Luzinov, I.; Minko, S.; Tsukruk, V. V. Adaptive and Responsive Surfaces through Controlled Reorganization of Interfacial Polymer Layers. Prog. Polym. Sci. 2004, 29 (7), 635. (4) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Minko, S. Stimuli-Responsive Command Polymer Surface for Generation of Protein Gradients. Biointerphases 2009, 4 (2), FA45−FA49. (5) Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Mueller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M. Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9 (2), 101−113. (6) Tokarev, I.; Motornov, M.; Minko, S. Molecular-Engineered Stimuli-Responsive Thin Polymer Film: A Platform for the Development of Integrated Multifunctional Intelligent Materials. J. Mater. Chem. 2009, 19 (38), 6932−6948. (7) Ionov, L.; Minko, S. Mixed Brushes with Locking Switching. ACS Appl. Mater. Interfaces 2012, 4 (1), 483−489. (8) Luzinov, I.; Julthongpiput, D.; Tsukruk, V. V. Thermoplastic Elastomer Monolayers Grafted to a Functionalized Silicon Surface. Macromolecules 2000, 33 (20), 7629−7638. (9) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Luzinov, I.; Minko, S. Inverse and Reversible Switching Gradient Surfaces from Mixed Polyelectrolyte Brushes. Langmuir 2004, 20 (23), 9916−9919. (10) Iyer, K. S.; Zdyrko, B.; Malz, H.; Pionteck, J.; Luzinov, I. Polystyrene Layers Grafted to Macromolecular Anchoring Layer. Macromolecules 2003, 36 (17), 6519−6526. (11) Minko, S.; Gafijchuk, G.; Sydorenko, O.; Voronov, S. Radical Polymerization Initiated from a Solid Substrate. 1. Theoretical Background. Macromolecules 1999, 32 (14), 4525−4531. (12) Ruhe, J.; Knoll, W. Functional Polymer Brushes. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; pp 565−613. (13) Edmondson, S.; Armes, S. P. Synthesis of Surface-Initiated Polymer Brushes Using Macro-Initiators. Polym. Int. 2009, 58 (3), 307−316. (14) Bialk, M.; Prucker, O.; Ruhe, J. Grafting of Polymers to Solid Surfaces ty Using Immobilized Methacrylates. Colloids Surf., A 2002, 198−200, 543−549.

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CONCLUSION The grafting through approach was successfully implemented to thin film chitosan soft substrates. Substrate morphology and preparation strongly affect both the grafting amount and structure of the resulting films. The amount of grafted PAAm linearly depends on the thickness of the chitosan film in the range of up to 300 nm. It may reach up to 150 mg/m2. Crosslinking of chitosan with 1,4-dibromobutane (vapor exposed) is required to preserve the crystallites of chitosan. Upon plasma etching, these films reveal densely packed arrays of vertically aligned crystallites (∼30 nm in diameter) homogeneously distributed over the whole sample. Such systems may find applications in different fields of engineering, such as biomedical surfaces or energy storage devices.42,46

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

* Supporting Information S

Additional information regarding FTIR sample preparation and filament height analysis methodology using the AFM are supplied as Supporting Information. Results concerning effects of extended exposure to monomer solution and varying grafting and plasma etching time on filaments are also provided. This material is available free of charge via the Internet at http:// pubs.acs.org. 12591


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