Article pubs.acs.org/Langmuir
Tunable Swelling and Rolling of Microgel Membranes Ling Zhang, Mark William Spears, Jr., and L. Andrew Lyon* School of Chemistry and Biochemistry, Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
ABSTRACT: The tunable swelling and rolling of films assembled via layer-by-layer (LbL) methods from poly(Nisopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and poly(ethylenimine) (PEI) have been systematically studied. Microgel/PEI films assembled at pH 7.4 display a high degree of in-plane swelling at low pH that dramatically increases the film area and drives self-delamination from the substrate to form a free-standing film. The degree of film swelling can be controlled by the size of microgels used in film fabrication. Taking advantage of this feature, self-rolled scrolls can be easily obtained from microgel/PEI films prepared from microgels of two different sizes. The rolling direction can be controlled by the assembly of different size microgels in different film strata, and the final shape of the scrolls can be controlled by scratching the desired film edges. The present work contributes to a deeper understanding of microgel/PEI film swelling properties and introduces a facile and novel method to prepare free-standing films and self-rolled scrolls.
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INTRODUCTION The swelling of polymers with solvent plays an important role in a number of chemical and physical processes. For instance, taking advantage of swelling, polymeric films can be used as drug loading and releasing systems,1−3 self-healing films,4−6 separation interfaces,7−9 and surfaces for controlled cell adhesion.10 Typically, film swelling on a rigid substrate proceeds in the z dimension, the direction perpendicular to the plane of the film,11,12 because swelling in the plane is primarily constrained by the rigid substrate. When the extent of film swelling is sufficiently large, patterned surfaces can be formed because of the buckling or folding instabilities during the swelling process. 13−15 However, it is much more challenging to design a system that can swell sufficiently to drive complete delamination from a substrate to form a freestanding film, even when the interaction between the film and the underlying substrate is weak. In the past few decades, great effort has gone into the development of free-standing films, which in and of themselves are promising materials for flexible electronics,16 sensors,17−19 separation membranes,20,21 and biomedical applications.22,23 Free-standing films are no longer constrained by substrate dimensions and, thus, exhibit much more freedom of motion. If a free-standing film is composed of two or more materials with different swelling capacities, a force imbalance may be created during swelling, which can greatly affect the shape of the film. This imbalance can drive the transition of the film from two© 2014 American Chemical Society
dimensional (2D) to three-dimensional (3D), which enables its use in complex applications with specific geometric requirements.24−29 While the concept of creating 3D structures based on differential swelling capacities is straightforward, manipulation of this process can be limited by a lack of control over the swelling capacity of a material. To achieve superior control over the properties of swelling films, it generally requires that polymeric films have a tunable swelling property. For example, photopatterning allows for control over not just the shape of the pattern but the amount of cross-linking, material composition, and swelling ability.30−32 Swelling control has also been illustrated by changing the Mw of poly(ethylene glycol) covalently bound to hydrogel bilayers. Another example of tunable swelling is that of a “microwalking” device, in which swelling of the films controlled by external humidity can drive the material to walk on certain substrates.27 Films formed by layer-by-layer (LbL) assembly of poly(Nisopropylacrylamide) (pNIPAm)- or poly(N-isopropylmethacrylamide) (pNIPMAm)-based microgels are materials that can be tuned in terms of microgel composition, properties, and assembly architecture.33,34 These materials are distinct from hydrogel sheets in that they are made of discrete microgel building blocks rather than a continuous polymer network Received: March 4, 2014 Revised: June 11, 2014 Published: June 13, 2014 7628
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water. The reagent solutions were filtered through 0.2 μm syringe filters and subsequently transferred to 250 mL three-neck roundbottom flasks. The solutions were heated to ∼70 °C while being purged with N2 gas under vigorous stirring for ∼1 h. After that, AAc was added to the flask. After thermal stabilization for 15 min at 70 °C, polymerization was initiated by the addition of 1 mL of a 1 mM APS solution. The reactions were allowed to proceed at 70 °C for another 4 h under a N2 blanket. After the microgels were cooled to room temperature, they were purified by repeated centrifugation and resuspension in deionized water to remove unreacted monomer, oligomers, and surfactant. The purified microgel solutions were lyophilized prior to storage. Film Preparation. Cover glass (25 × 25 mm, VWR International) was used as a substrate for film deposition. We note that simple cover glass can be somewhat rougher and have more complex ion exchange properties than more ideal substrates, such as Si/SiOx, which might provide different film properties. However, glass is a widely used substrate for LbL assembly; we have chosen the approach described below to allow for comparison of our work to other approaches in the field. The glass substrates were first placed in a ceramic glass slide holder and ultrasonically cleaned in dilute soapy water and deionized water for 30 min each, respectively. After being dried under N2, the glass substrates were immersed in a lightly boiled piranha solution (3:7 mixture of 30% H2O2 and 98% H2SO4) for ∼30 min. Caution: Piranha solution reacts violently with organic materials and should be handled carefully. Afterward, the substrates were rinsed with copious volumes of water and dried under flowing N2. For PDADMAC modification, the cleaned substrates were immersed in 2 mg/mL PDADMAC phosphate-buffered saline (PBS) (pH 7.4 phosphate buffer containing 24 mM ionic strength) solution for 1 h to obtain a positively charged surface. For APTMS modification, the dry and cleaned substrates were incubated with APTMS/absolute ethanol (1 wt %) solution for 2 h to obtain a −NH2 functionalized surface. All buffer solutions used for film fabrication in this paper are pH 7.4 PBS containing 24 mM ionic strength. LbL-assembled microgel/PEI films were prepared manually by immersing PDADMAC-modified glass substrates first in 0.5 mg/mL microgel PBS solution for 20 min. After that, the substrates were washed with deionized water twice for 1 min each, followed by drying under flowing N2. The substrates were then transferred to 2 mg/mL PEI PBS solution for another 20 min, followed by rinsing with water twice for 1 min each and drying with N2 flow. Multilayered (microgel/PEI)*n films (n refers to the number of deposition cycles) can be fabricated by repeating these steps in a cyclic fashion until the desired number of layers is reached. In a control experiment, microgel/PEI films prepared at pH 4.5 were fabricated in the same way as those at pH 7.4, except using pH 4.5 potassium hydrogen phthalate (KHP) buffer instead of PBS buffer to dissolve microgels and PEI. During the film fabrication process, KHP buffer was used to wash the films after each layer deposition instead of water. For EDC/NHS or glutaraldehyde cross-linked microgel films, as prepared microgel/PEI films were dipped into 2 mM EDC and 5 mM NHS mixed solution (in 10 mM MES buffer at pH 6) or into 2 wt % glutaraldehyde solution for 2 h. The films were then rinsed with water and dried by N2 gas. For microgel/PEI films on APTMS-modified substrates, after the deposition of the first layer of microgels, the substrates were immersed in MES buffer containing 2 mM EDC and 5 mM NHS for 2 h to covalently cross-link microgels with the substrates. After that, the microgel/PEI films were prepared in the same way as on PDADMACfunctionalized substrates until the desired number of layers was deposited. For better visualization, as-prepared microgel films were dipped into 5 mg/mL acid orange PBS solution for 2 min to load a small amount of dye within the films, followed by a water rinse and drying by N2 gas. For amine blocking, the film was immersed for 2 h at 37 °C in a 0.01% (w/v) solution of TNBSA in 0.1 M sodium bicarbonate buffer at pH 8.5. The film was then rinsed with water and dried with N2 before further treatment. Preparation of Free-Standing Films/Scrolls. The edges of the microgel films were scratched with a clean razor blade to separate the
structure. Microgel properties, such as size, porosity, transition temperature, and charge, are easily changed on the basis of synthesis conditions.35 For example, incorporation of acrylic acid (AAc) into microgels allows for LbL assembly with polycations to fabricate microgel films.34 Film parameters, such as layer number, polycation characteristics, thickness, and modulus, can be changed on the basis of how the assembly is carried out.36−38 In our previous work, the LbL assembly and the responsive behavior of microgel films to temperature and pH had been systematically studied.39,40 In this paper, we investigate the swelling mechanism of LbL-assembled microgel/poly(ethylenimine) (PEI) films. The results demonstrate that pNIPAm/PEI-based microgel films display extensive swelling in acid, which can drive delamination from the substrate to form large-area, free-standing films. More importantly, the swelling of microgel/PEI films can be finely controlled by the size of microgels used for film fabrication. Self-rolled, scroll-type structures can be conveniently prepared by taking advantage of differential swelling in bilayered microgel/PEI films composed of the same kind of microgels but with different microgel sizes in each stratum. The work described below not only provides insight into the mechanism that controls the swelling properties of microgel/PEI films but also introduces a facile and novel method to prepare freestanding films and self-rolled scrolls. In addition to providing mechanistic insight, this contribution represents the first example of 3D folding of microgel-based polymer membranes/thin films. The microgel composition and dimensions and the details of the self-assembly conditions dictate the resultant film swelling, delamination, and rolling; this level of design sophistication has not previously been demonstrated for microgel-based materials and suggests new routes to even greater control over such responsive, nanostructured materials.
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EXPERIMENTAL SECTION
Materials. All chemicals were purchased from Sigma-Aldrich unless otherwise specified. The monomers NIPAm and NIPMAm were recrystallized from hexanes (VWR International, West Chester, PA) and dried under vacuum before use. Reagents N,N′-methylenebis(acrylamide) (BIS), AAc, sodium dodecyl sulfate (SDS), and ammonium persulfate (APS) were all used as received. Polycations poly(diallyldimethylammonium chloride) (PDADMAC, Mw = 400 000−500 000) and branched PEI (Mw ∼ 750 000) were used as received. Covalent coupling reagents glutaraldehyde (Alfa Aesar), N[3-(dimethylamino)propyl]-N′-ethylcarbodiimide hydrochloride (EDC), and N-hydroxysuccinimide (NHS) were used as received. The surface modification reagent (3-aminopropyl)trimethoxysilane (APTMS, TCI America) was used as received. The dye acid orange was used as received. 2,4,6-Trinitrobenzenesulfonic acid (TNBSA) was purchased as a 5% (w/v) solution in methanol. Buffer chemicals sodium dihydrogen phosphate monohydrate, 2-(N-morpholino)ethanesulfonic acid (MES), and sodium hydroxide were used as received. Water used for all of the experiments was house-distilled, deionized to a resistance of at least 18 MΩ (Barnstead Thermolyne EPure system). Microgel Synthesis. Microgels were synthesized using aqueous free-radical precipitation polymerization as previously described.34,39 The molar composition of all microgels was 68% NIPAm (or 68% NIPMAm), 2% BIS, and 30% AAc. For p(NIPAm-co-AAc) (microgel 1) and large p(NIPMAm-co-AAc) (microgel 2) microgels, the total monomer concentration was 100 mM with a total reaction volume of 100 mL. Small p(NIPMAm-co-AAc) (microgel 3) microgels were synthesized with a total monomer concentration of 70 mM in a reaction volume of 100 mL using 1 mL of a 3 mM APS solution to initiate the polymerization. Reactions were performed by first dissolving NIPAm (or NIPMAm), BIS, and 1 mM SDS in deionized 7629
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Table 1. Microgel Synthesis Conditions and Propertiesa monomer % NIPAm (NIPMAm)/AAc/BIS
RH pH 3.0 (nm)
RH pH 7.4 (nm)
RS deposited at pH 7.4 (nm)
ζ potential pH 7.4 (mV)
68:30:2 68:30:2
189 ± 4 313 ± 6
381 ± 18 523 ± 28
341 ± 54 444 ± 43
−33 ± 1 −27 ± 1
68:30:2
181 ± 3
373 ± 14
323 ± 49
−31 ± 1
microgel 1 p(NIPAm-co-AAc) microgel 2 p(NIPMAm-co-AAc) microgel 3 p(NIPMAm-co-AAc) a
Reported error is ±1 standard deviation (SD) with n = 4.
films on front and back sides of the glass substrate from each other. The microgel films were then dipped into pH 2 HCl solutions to detach the films from glass substrates and form free-standing films or scrolls. Characterization. Dynamic light scattering (DLS, Wyatt Technology) was used to characterize the average hydrodynamic radius (Rh) and polydispersity of the microgels at pH 3.0 and 7.4 at 20 °C. Data are an average of 30 measurements with 30 s acquisition times. The ζ-potential values were obtained by a Malvern Zetasizer Nano at 20 °C. The atomic force microscopy (AFM) images were taken using an Asylum Research MFP-3D AFM Instrument (Santa Barbara, CA) in air under ambient conditions. AFM was operated in the tapping mode with aluminum-coated Si cantilevers purchased from NanoWorld (force constant, 42 N/m; resonance frequency, 320 kHz). The microgel size was determined by imaging a monolayer and drawing a line profile across the middle of the microgel (RS). The root-meansquare (RMS) roughness of microgel films was calculated as an average of four scan areas. The film thickness was determined by AFM line profiles across a scratch in the film introduced by a clean razor blade. Reported thickness values were obtained by measuring five different scan areas on two films prepared in two batches. Scanning electron microscopy (SEM) images were obtained with a FEI (Hillsboro, Oregon) Nova scanning electron microscope. All samples for SEM imaging were lightly coated using a Hummer 6 gold/ palladium sputterer at 25 mA for 3 min.
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At pH 7.4, microgels containing AAc can be used as building blocks for LbL film fabrication using PEI as the polycation “glue”.44 Figure 1a shows an AFM image of a sub-monolayer of
Figure 1. (a and c) AFM height images of (a) microgel 1 and (c) microgel 2 (sub)monolayers. (b and d) AFM images of (b) (microgel 1/PEI)*20 and (d) (microgel 2/PEI)*20 films. Scale bars are 5 μm. The inset in panel b is an enlarged AFM image of a (microgel 1/ PEI)*20 film, with the scale bar being 1 μm.
RESULTS AND DISCUSSION
Three kinds of microgels were synthesized to study the fabrication and swelling of microgel films. All microgels were synthesized via precipitation polymerization with the same composition of 68% NIPAm or NIPMAm, 30% AAc, and 2% cross-linker BIS. The microgel sizes at different pH values, the dry size as determined by AFM, and the ζ potentials at pH 7.4 are shown in Table 1. We also note that the polycation hydrodynamic sizes and ζ potentials have been reported recently, although under slightly different solution conditions.41 We have determined the hydrodynamic radius of the PEI used in our experiments as 7 nm using DLS, but the hydrodynamic radius of PDADMAC used here could not be obtained using the light scattering equipment available to us, apparently because of a lack of instrument sensitivity. Microgels 1 and 3 are approximately the same radius, but pNIPMAm-based microgels contain extra methyl groups along the polymer backbone. Microgels 2 and 3 are the same composition, but they differ in radius. Microgel 2 (pNIPMAm-based) shows a relatively lower ζ potential than microgel 1 (pNIPAm-based), despite both microgels having the same AAc content. However, microgel 2 is larger, which will result in a lower surface charge density. All of the microgels exhibit smaller sizes at pH 3.0, demonstrating incorporation of AAc into the microgel. This difference in swelling is due to protonation of AAc groups at low pH (pKa ∼ 4.2), which results in counterion egress and deswelling. The feed composition for such microgels has previously been found to correspond well with the resultant microgel composition.42,43
microgel 1 deposited on a PDADMAC-modified glass substrate. After the first microgel layer is deposited, the substrate is immersed into a PEI solution that results in polycation adsorption and penetration into the microgel network, leading to charge reversal at the surface. Multilayered microgel/PEI films can be fabricated by alternate immersion into microgel and PEI solutions to the desired number of layers. The surface morphology of (microgel 1/PEI)*20 films, as determined by AFM, is shown in Figure 1b. In Figure 1b, individual microgels are not clearly distinguishable because of condensation and overlapping of microgels during the assembly. (Microgel 1/PEI)*20 films have a RMS roughness of 48 nm (scan area of 50 × 50 μm2) and a dry thickness of 651 ± 40 nm. Microgel 2 can also be used for LbL film deposition with PEI; panels c and d of Figure 1 show AFM images of a microgel 2 sub-monolayer and (microgel 2/PEI)*20 films, respectively. Microgel 2 appears significantly larger than microgel 1 in AFM images, which is consistent with light scattering data. Because of the larger size, individual microgels can be distinguished in Figure 1d. The (microgel 2/PEI)*20 film has a RMS roughness of 97 nm (scan area of 50 × 50 μm2) and a dry thickness of 739 ± 52 nm. Microgel/PEI films can rapidly and completely delaminate from the substrate in pH 2 HCl solutions to form free-standing films. Before being immersed in the acid solution, all of the 7630
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seems likely that the swelling of the microgel/PEI film plays an important role in its delamination. A proposed swelling mechanism is illustrated Figure 2g. Microgel/PEI films prepared at pH 7.4 will contain highly (negatively) charged microgels, while the weak polyelectrolyte PEI is only partly charged. The respective pKa values for acid groups of the microgel and amines on PEI (Mw = 750 000 Da) are 4.2 and ∼10.0.44,45 Therefore, many free (non-microgelassociated) amine groups will be contained within microgel/ PEI films at pH 7.4. When such a film is immersed in a pH 2 solution, the microgel carboxylate groups become protonated, which markedly decreases Coulombic attraction between the microgels and PEI. More importantly, the amine groups of PEI within the films become fully charged (−NH3+). Consequently, the Coulombic repulsion between the positive charges along with solvated counterion ingress results in a swelling of the whole film. This swelling of the LbL microgel films in acid leads to interfacial stress at the film/substrate interface. Finally, we note that the electrostatic interaction between the first layer of microgels with the PDADMAC-modified substrate is also weakened because of the protonation of microgel AAc units. Thus, the microgel films detach from the substrate via a combination of swelling and decreased interfacial adhesion. AFM was used to study the substrate after film detachment (data not shown), and no evidence of remnant film was observed. To further demonstrate the mechanism of film delamination, the following control experiments were performed. First, to study the importance of free PEI amine groups in the delamination process, we cross-linked a prepared film with EDC/NHS that conjugates the free amines of PEI and caroboxyl groups of the microgels. This film was able to separate from the glass substrate, but delamination took approximately 10 min, which was approximately double the time required for delamination of an uncross-linked film. Next, we used a 2% glutaraldehyde solution to cross-link amine groups of PEI. After this treatment, few free amine groups remain to drive film swelling. The film was not able to break free from the substrate even after 2 days of immersion, because of the lack of increase in interfacial tension. When prepared at pH 4.5, (microgel 1/PEI)*20 films are also unable to separate from the substrate in pH 2 solution even after 2 days. At pH 4.5, PEI is in a fully protonated state; therefore, no significant additional protonation occurs upon dropping the pH to 2. In another experiment, primary amines were reacted with TNBSA before the film was immersed in acid. TNBSA is typically used as an assay reagent for determination of free amine groups in proteins, but in this case, it serves to block amines from impacting film swelling. The blocked film did not separate from the substrate in pH 2 HCl solution, even after 2 days of immersion. The interaction between the substrate and microgel/PEI film is also of importance for delamination. When a microgel monolayer was covalently cross-linked to an APTMS-functionalized substrate using EDC/NHS chemistry, the microgel/PEI films assembled on top of that monolayer could not detach at low pH. The above results demonstrate that both the swelling of microgel films and disruption of electrostatic interactions between the film and substrate are critical for film delamination. Unlike LbL-assembled weak polyelectrolyte films that tend to quickly dissolve or become porous when the interaction between two building blocks is decreased,46,47 delaminated microgel/PEI films are stable in pH 2 solution for 1−2 days;
edges of the films were scratched by a clean razor blade to ensure that any film deposited on the edges of the coverslip was separated from the main portion of the film. A small amount of the dye acid orange was loaded within the film for ease of film visualization. Panels of a−d Figure 2 exhibit the detaching
Figure 2. (a−e) Delamination process of a dye-loaded (microgel 1/ PEI)*20 film in pH 2.0 acid solution: (a) 0 min, (b) 0.5 min, (c) 2 min, (d) 5 min, and (e) floating film after detachment. (f) AFM image of (microgel 1/PEI)*20 free-standing film after transfer to a clean glass substrate. Scan area = 20 × 20 μm2. (g) Scheme of the delamination mechanism of microgel/PEI films in acid solution. The detached (microgel 1/PEI)*20 film in panel e has an area of 4.5 × 4.5 cm2.
process of a dye-loaded (microgel 1/PEI)*20 film in acid. As soon as the microgel film was immersed in a pH 2 HCl solution, small wrinkles formed on the edges of the film (panels a and b of Figure 2) that gradually expanded from the edges to the middle part of the film and simultaneously combined together to form large undulations (Figure 2c). Gradual film detachment was evident throughout the wrinkling and detachment process. When immersion time reached 5 min (Figure 2d), the solution was slightly agitated and the film completely detached to form a flat, floating film with increased dimensions (Figure 2e). The detached (microgel 1/PEI)*20 film has an area of 4.5 × 4.5 cm2, which is 3.2 times its original area (2.5 × 2.5 cm2). On the basis of these observations, it 7631
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(microgel 1/PEI)*20 free-standing films. To illustrate this, microgel 3 (having the same composition as microgel 2 and similar size to microgel 1) was used in LbL deposition with PEI. Figure 3b shows the free-standing film of (microgel 3/PEI)*20 delaminated from a 2.5 × 2.5 cm2 glass substrate in pH 2 acid solution. After delamination, the dimension of the (microgel 3/ PEI)*20 film is 4.7 × 4.7 cm2, which is very similar to the area of the delaminated (microgel 1/PEI)*20 film. This further suggests that microgel size (and perhaps charge density) plays an important role in the fabrication and swelling of microgel/ PEI films. In addition, it is also worth noting that the swelling ability of microgel/PEI films appears to be independent of the film thickness and area. The differential degrees of swelling in the experiments above allowed us to create microgel/PEI films that autonomously form rolled structures in solution. When a stratified film constructed from (microgel 1/PEI)*10 and (microgel 2/ PEI)*10 was dipped into an acid solution, the film delaminated from the substrate and then rolled into a free-standing scroll (Figure 4). Moreover, the rolling direction in the z dimension
after being transferred to distilled, deionized water, these films can be kept for 1 week without visible degradation. We hypothesize that the network structure of microgels plays an important role in the stability of microgel/PEI films, because even though the electrostatic interactions between microgels and PEI are decreased in acid, it is difficult for branched PEI to completely diffuse out of the microgel network when compared to LbL films composed entirely of linear polyelectrolytes.46,47 Finally, if the whole film is subjected to EDC/NHS coupling before being immersed in acid solution, delaminated microgel films are stable in acid or distilled water for months. Delaminated microgel/PEI films can be easily transferred to various substrates, such as Si, glass, metal, and plastic, which could provide an easy method for control of the interfacial properties of those materials.48 Figure 2f shows an AFM image of a (microgel 1/PEI)*20 film after being transferred from the air/water interface to a glass substrate. In contrast to the asprepared (microgel 1/PEI)*20 film (Figure 1b), individual microgels are easily discernible after transfer (Figure 2f) because both the size of microgels and the intermicrogel distance increases during the swelling process. Note that this swelling behavior observed at low pH seems to contradict the data in Table 1, which shows the size of microgels decreasing in acidic media. However, after being assembled into films, the microgel size is largely determined by the degree of condensation induced by polycation binding.49 In addition, (microgel 1/PEI)*20 free-standing films have a lower surface roughness (RMS roughness ∼ 10 nm) than the as-prepared film. After delamination, the dry thickness of (microgel 1/ PEI)*20 films decreased to 220 ± 6 nm as a result of the film occupying a larger surface area. In the same way, (microgel 2/PEI)*20 films can also detach completely from the substrate in pH 2 HCl solution by taking advantage of swelling. After delamination, the area of a (microgel 2/PEI)*20 film has increased by 1.5× (Figure 3a).
Figure 3. Photographs of (a) (microgel 2/PEI)*20 and (b) (microgel 3/PEI)*20 free-standing films in acid solutions. After delamination from a 2.5 × 2.5 cm2 glass substrate, the area of the microgel films increases to (a) 4.0 × 4.0 cm2 and (b) 4.7 × 4.7 cm2. Figure 4. Photographs of the self-rolling process of a bilayered (a) (microgel 1/PEI)*10 + (microgel 2/PEI)*10 film and (b) (microgel 2/PEI)*10 + (microgel 1/PEI)*10 film in acid solutions.
It is worth noting that, under the same conditions, (microgel 1/ PEI)*20 and (microgel 2/PEI)*20 films exhibit different degrees of swelling following delamination (Figures 2e and 3a). We hypothesize that it is the size of microgels that regulates the degree of swelling of microgel/PEI films. As demonstrated above, the lower charge density of microgel 2 results in a lower deposition density in each layer than that of microgel 1. Consequently, relatively less PEI was needed to induce charge reversal during fabrication of microgel 2/PEI films, resulting in fewer free −NH2 groups being present in (microgel 2/PEI)*20 films. This likely results in a lower swelling proportion of (microgel 2/PEI)*20 when compared to
can be controlled; if a stratified film is fabricated with microgel 1/PEI films at the bottom (substrate interface) and microgel 2/ PEI films on the top (solution interface), the film rolls from the bottom to the top as a result of the greater swelling of the bottom layers (Figure 4a). Conversely, the bilayered film rolls from the top to the substrate when microgel 1/PEI is on the top and microgel 2/PEI is at the bottom (Figure 4b). 7632
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Langmuir The rolling processes of stratified microgel films along the x and y axes are hard to predict because the detaching process of the films is influenced by both the diffusion of acid into the films and the adhesion forces between the films and the substrate. However, control over these axes can be attained by pre-scratching only select edges of the film before immersing in acid solution. As shown in Figure 4a, if two opposite edges (indicated by arrows) were pre-scratched by a blade, the film rolled and folded gradually into two scrolls from the two scratched edges to the middle part of the film. This observation can be attributed to the faster diffusion of acid from the lateral surfaces of the films. In the same way, when only one edge of the film is pre-scratched, the bilayered microgel film rolls from the scratched edge to its opposite edge (Figure 4b). The cross-sectional morphology of free-standing microgel/ PEI scrolls in the dry state was studied by SEM. The microgel/ PEI scroll distorts during the drying process because of its softness; however, the cylinder shape can still be identified under vacuum using SEM (Figure 5a). From the images in
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CONCLUSION
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AUTHOR INFORMATION
Article
In conclusion, microgel films prepared by LbL deposition of pNIPAm-based microgels and PEI have unique swelling properties that enable complete detachment from a substrate in acidic conditions. By swelling at low pH, the area of the microgel film increases dramatically and the Coulombic forces between the film and the substrate are decreased, driving separation from the substrate and formation of a free-standing film. The large degree of film swelling is a consequence of electrostatic repulsion of a large number of extra −NH3+ groups within microgel/PEI films in acid. At the same time, the network structure of microgels and film connectivity also plays an important role in the stability of microgel/PEI films during swelling. Furthermore, the swelling of microgel/PEI films can be tuned by controlling the size of microgels used for film fabrication. As a result, self-rolled microgel scrolls can be obtained from bilayered microgel/PEI films prepared from microgels of two different sizes. Swelling-induced, selfdelamination methods not only provide a convenient way to fabricate free-standing films and self-rolled scrolls, but the large volumetric response of microgel/PEI films to pH changes also suggests potential for their use in the fabrication of sensors and actuators. These charged films could also be used for loading of charged drug species, and the self-rolling ability could be used for biocompatible encapsulation of nanoparticles and cells.50,51
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for Mark William Spears, Jr. was provided by the Georgia Tech Center for Drug Design, Development, and Delivery Graduate Assistance in Areas of National Need (GAANN) Fellowship.
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REFERENCES
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Figure 5. SEM images of (microgel 1/PEI)*10 + (microgel 2/ PEI)*10 microgel scrolls: (a) scroll that has rolled up off-center with the edge of each layer visible and (b) scroll that has been cut so that the cross-section of the structure is visible. Each layer appears as a ring, and the top layer extends off the back of the rings. Scale bars are 20 μm.
Figure 5, one can observe that the film has clearly rolled up on itself to form a scroll structure. Figure 5a shows a film that rolled up slightly off-center, so that each layer is visible and protrudes from the edge further than the layer above. Figure 5b shows the cross-section of a scroll, with the exposed end showing a structure similar to tree rings. In this image, part of the film extends off the back of the rolled structure, illustrating the thickness of the dried film. 7633
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