Stratified Temperature-Responsive Multilayer ... - ACS Publications

Sep 12, 2016 - Birmingham, Alabama 35294, United States. §. Spallation ... architecture on the temperature-responsive behavior and surface morphology...
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Stratified Temperature-Responsive Multilayer Hydrogels of Poly(N‑vinylpyrrolidone) and Poly(N‑vinylcaprolactam): Effect of Hydrogel Architecture on Properties William Higgins,† Veronika Kozlovskaya,† Aaron Alford,† John Ankner,*,§ and Eugenia Kharlampieva*,†,‡ †

Department of Chemistry and ‡Center for Nanoscale Materials and Biointegration, University of Alabama at Birmingham, Birmingham, Alabama 35294, United States § Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: We report on the effects of hydrophilicity and architecture on the temperature-responsive behavior and surface morphology of nonionic double-stack hydrogels prepared from cross-linked hydrogen-bonded layer-by-layer films. A hydrophilic poly(N-vinylpyrrolidone) (PVPON)n multilayer hydrogel is integrated with a relatively hydrophobic temperature-sensitive poly(N-vinylcaprolactam) (PVCL)m network as either a top or bottom stratum, where n and m represent numbers of layers for each individual stratum. Neutron reflectometry revealed that all double-stack films in the dry state are well stratified with two distinct (PVPON) and (PVCL) strata of higher and lower scattering density, respectively, unlike highly mixed alternating (PVCL/PVPON) hydrogels. We have found that the order of stacking and stack thickness significantly influence hydration of the (PVPON)n(PVCL)m and (PVCL)m(PVPON)n networks at ambient temperature and above the LCST of PVCL. The hydration of the hydrogels consistently increases with PVPON amount within the network, resulting in suppressed temperature response. This effect is more pronounced for (PVPON)n(PVCL)m as compared to its mirror counterpart as explained by the two adjacent aqueous interfaces in which the (PVCL)m stack is sandwiched between the hydrophilic (PVPON)n stack below and the bulk of water above it. Our results yield new insights into controlling the temperature response and surface properties of nanostructured polymer networks, which is relevant to both fundamental and applied research where the dynamics of hydration, thickness, and control of surface hydrophobicity are important.

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INTRODUCTION Stimuli-responsive hydrogels are capable of changing their chemical, mechanical, and biological properties in a defined and controlled manner, leading to numerous potential applications in sensing, drug and cell delivery, tissue engineering, and microfluidic systems.1−9 Among those, temperature-responsive hydrogels have risen with considerable interest in their temperature-dependent reversible changes in hydrophilicity in aqueous media, resulting in reversible volume phase transitions from swollen to collapsed states; these have been used for cell culture substrates,10,11 drug delivery vehicles,12−15 absorbents,16,17 actuators,18,19 and microfluidic devices.20,21 In temperature-sensitive networks, thermosensitive polymer segments undergo a coil-to-globule phase transition when heated above their lower critical solution temperature (LCST) and can recover their original state when cooled in return. Cross-link density,20 chain hydrophobicity/hydrophilicity,15,22 polymer composition,12,23 and incorporation of metal nanoparticles24 have been varied to manipulate the thermal response and morphology of thermosensitive hydrogels. However, the physical influence exerted through variations in architecture onto the temperature-induced hydrogel collapse, chain hydration, and surface properties has been largely overlooked © XXXX American Chemical Society

as a method to direct and modulate the properties of nonhomogeneous hydrogels. Yet, network architecture has been recently found to greatly affect the behavior of “bulk” hydrogels,25−28 which is essential for applications in tissue regeneration and medical devices.29−32 Hydrogel internal structure has been shown to be crucial for regulating hydration and mechanical properties of cellulosebased macrogels.29 Controlling microstructures of 3D sol−gel transitional hydrogels has been used for modulating cellular behavior,30 while native 3D cell matrices have been simulated with a multilayer organization of hyaluronic acid−based hydrogel, and having been proved useful for coculturing various cancer cell lines.31 Recently introduced multilayer hydrogels produced using the layer-by-layer (LbL) approach offer thin films ( LCST (f).

Figure 5. NR data (left panels) and corresponding SLD profiles (right panels) for spin-assisted dry alternating (PVCL/PVPON)10 (a, b) and double-stack (PVCL)10(PVPON)10 (c, d), (PVCL)20(PVPON)10 (e, f), and (PVPON)10(PVCL)20 (g, h) hydrogels. Open symbols and solid lines show NR data and fit, respectively.

Neutron reflectivity R is plotted versus wavevector transfer Q (Q = 4π sin θ/λ, where λ is the neutron wavelength and θ the incident angle). Scattering parameters for the NR fits are listed in the Supporting Information. The scattering length density (SLD) profiles obtained from fitting NR data show the density distribution along the direction normal to the film surface (Figure 5, right panels). The SLD profile for the alternating (PVCL/PVPON)10 hydrogel does not show any internal structure. There are no defined sequences of the PVCL and PVPON layers within the total dry hydrogel thickness of 26.4 nm (Figures 5a,b and Table 2). Apparently, the lack of contrast is explained by the very close scattering densities for PVCL and PVPON in addition to some intermixing of the individual PVCL and PVPON layers

Figure 4. (a) Growth of double-stack hydrogen-bonded multilayer: the (PVPON-NH2-5/PMAA) stack is followed by the (PVCL-NH2-4/ PMAA). (b) Thicknesses of single-stack (PVCL)20 (squares) and double-stack (PVCL)20(PVPON)n hydrogels (n = 10, 20, 30) (circles) measured in dry (filled) or wet (open) states.

respectively, with an average PVCL layer thickness of 1.1 ± 0.3 nm (Table 1). When instead of being at the bottom, the PVCL stack was deposited on top of the (PVPON)10, the thickness of the (PVPON)10(PVCL)m hydrogel also increased linearly with increasing number of PVCL layers from m = 10 to m = 20 to m = 30, resulting in corresponding hydrogel thicknesses of 27.2 ± 0.6, 30.6 ± 0.2, and 38.0 ± 0.7 nm with an average PVCL layer thickness of 1.1 ± 0.4 nm (Table 1). Similarly, when a E

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where the experiments were conducted changing the baseline thicknesses of the films (Table 2).

Table 2. Thicknesses of (PVCL), (PVPON), (PVCL/ PVPON), (PVCL)(PVPON), and (PVPON)(PVCL) Hydrogels Measured by Neutron Reflectivity in Dry State, t(Dry), and in D2O at 25 °C, t(D2O), with Calculated D2O Content ( f) inside the Hydrogels and Hydrogel Hydration in D2O at 25 °C, W25(D2O) thickness (nm) no. 1 2 3 4

5

6

7

sample (PVCL)20 (PVPON)20 (PVCL/PVPON)10 (PVCL)10(PVPON)10 PVCLa PVPONa (PVCL)20(PVPON)10 PVCLa PVPONa (PVPON)10(PVCL)10 PVPONa PVCLa (PVPON)10(PVCL)20 PVPONa PVCLa

t(dry)

t(D2O)

27.9 22.8 26.4 27.5 15.4 12.1 38.6 21.7 17.5 22.3 10.95 11.3 34.4 11.1 23.2

81.9 104.2 106.7 81.8 31.4 50.4 116.2 63.7 53.9 69.5 46.6 22.9 96.6 43.8 52.5

D2 O content, f 0.66 0.80 0.753

W25(D2O) 0.66 0.78 0.753 0.67

0.49 0.78

Figure 6. Dry thicknesses of hydrogel films obtained via ellipsometry and neutron reflectivity (NR).

0.66 0.55 0.79

Hydration of Hydrogels Tested with in Situ Ellipsometry and Neutron Reflectometry. To determine the swelling of the single- and double-stack hydrogels, in situ ellipsometry was first applied to the surface-anchored hydrogel films in DI water at 25 °C. We found that 20 nm thick single-stack (PVCL)20 swelled 3-fold with its dry thickness changing from 21 to 58.5 ± 0.1 nm in dry and hydrated states, respectively (Figures 6 and 7). The alternating (PVCL/PVPON)10 and a

0.65 0.77 0.51 0.64 0.75 0.57

a

A single hydrogel stratum within the double-stack multilayer hydrogel.

which can result in a homogeneous multilayer hydrogel for the alternating (PVCL/PVPON)10 hydrogel. In striking contrast, the SLD profile of the (PVCL)10(PVPON)10 hydrogel reveals the presence of two distinct layers of (PVCL) and (PVPON) hydrogel strata in (Figures 5c,d). In this case, the bottom PVCL layer with lower SLD level is at the silicon−PGMA interface and a PVPON top layer with higher SLD is at the polymer−air interface. The total thickness of this double-stack hydrogel is 27.5 nm with PVCL and PVPON strata of 15.4 and 12.1 nm thickness (Table 2). In both cases, the total thicknesses of the (PVCL/PVPON)10 and (PVCL)10(PVPON)10 hydrogels measured by NR are in good agreement with those obtained by ellipsometry (Table 1). When the number of PVCL layers was increased from n = 10 to n = 20 in (PVCL)20(PVPON)10, the SLD profile of the resultant hydrogel was similar to that of (PVCL)10(PVPON)10 showing two distinct hydrogel strata of PVCL and PVPON, with the thickness of the (PVCL)20(PVPON)10 hydrogel and the PVCL hydrogel layer increased to 38.6 and to 21.7 nm, respectively (Figures 5e,f and Table 2). When the bottom PVCL stratum was switched to PVPON as in (PVPON)10(PVCL)20, the SLD profile shows the presence of two regions of different scattering density with the first PVPON layer of higher SLD now neighboring the silicon−PGMA interface, followed by the lower SLD level from the PVCL layer (Figures 5g,h). The total (PVPON)10(PVCL)20 hydrogel thickness measured by NR was 34.4 nm with thicknesses of PVPON and PVCL strata of 11.2 and 23.2 nm, respectively, which were similar to those measured by NR in the (PVCL)20(PVPON)10 with 21.7 and 17.5 nm for PVCL and PVPON layers, respectively (Table 2). In general, the dry hydrogel thicknesses measured by NR were slightly thicker than those measured with ellipsometry (Figure 6), which may be due to variations of ambient humidity between the environments

Figure 7. Swelling of the single-stack (PVCL)20 and (PVPON)20 and the alternating (PVCL/PVPON)10 multilayer hydrogels as measured using in situ ellipsometry in DI water at 25 °C.

single-stack (PVPON)20 hydrogels, both 20 nm in thickness, swell up to 80 and 109 nm undergoing 4- and 5-fold swelling, respectively, under those conditions. The results indicate that hydration consistently increases with PVPON amount within the hydrogel. Indeed, exchanging half of the PVCL layers with PVPON resulted in almost 40% increased swelling as compared to that of pure (PVCL), which we attribute to a more hydrophilic nature of PVPON in contrast to PVCL (Figure 7). Moreover, the hydrogel swelling increased almost 2-fold more than that of (PVCL) when all PVCL layers were changed to PVPON in the (PVPON)20 hydrogel. This resulted in a swollen thickness of 109 ± 1 nm despite the similar dry thicknesses of the (PVCL)20 and (PVPON)20 hydrogels (Figure 7 and Table 1). Similar results were obtained for the hydrogel swelling measured with neutron reflectivity, yielding thicknesses of 81.9, 104.2, and 106.7 nm for (PVCL)20, (PVPON)20, and (PVCL/ PVPON)10, respectively, exposed to D2O at 25 °C (Table 2). The shrinkage of the double-stack hydrogel films fitted with single- or double-stack models resulted in similar values, for example, hydrogel shrinkage S (%) calculated as S = 1 − (t50/ t25), where t50 and t25 are the hydrogel film wet thicknesses at T F

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Macromolecules = 50 °C and T = 25 °C, respectively, was 15% for the (PVCL)10(PVPON)10 hydrogel using both single- and doublestack fitting models (see Experimental Section), and S was 7% and 5% for (PVPON)10(PVCL)10 as fitted by the single- and double-stack models, respectively (see Table S1 in Supporting Information). Also, the refractive index for the PVCL hydrogel stratum in (PVCL)10(PVPON)10 only slightly increased from 1.38 to 1.39 when the temperature was changed from 25 to 50 °C. In the case of the (PVPON)10(PVCL)10 hydrogel film the fitted refractive index for the (PVCL) stratum was 1.36 at T = 25 °C, which was lower than that for the reverse hydrogel configuration, suggesting a more hydrophilic interior than that in the case of (PVCL)10(PVPON)10. The refractive index value of the (PVCL)10 in (PVPON)10(PVCL)10 slightly increased to 1.37 at T = 50 °C upon PVCL stack collapse. We have checked the hydrogel films after swelling/drying using optical microscopy and SEM, and no sign of surface buckling instabilities was observed (Figures S5−S7). As reported by Hayward et al., there is a minimum thickness required for surface wrinkling of the surface-attached films, and the minimum thickness for wrinkling increases with a higher degree of cross-linking due to swelling, as shown for surfaceattached films of cross-linked block copolymers on silicon surfaces.75 The minimum dry thickness for the onset of wrinkling was 110 nm at 3.5% cross-linking for 2-vinylpyridine block copolymer films and 130 nm with 6% cross-linking for the homopolymer films. Thinner films did not show wrinkling at the same degree of cross-linking. In our work, hydrogel films with dry thicknesses ranging from 20 to 40 nm did not show wrinkling or buckling when hydrated and/or dried. Since measuring the swollen hydrogel thickness using ellipsometry requires fitting the complex refractive index, we relied on the neutron reflectivity measurements to calculate hydrogel hydration since these measurements depend only on detecting D2O presence within the hydrogel. Figure 8 shows

The SLD data were used to calculate the hydrogel hydration representing the water content absorbed by the hydrogel network. The hydration at a given temperature, WT, was calculated as WT = (tT − tdry)/tT, where tT is the thickness of the hydrogel at a given temperature and tdry is the thickness of a dry hydrogel film. Based on that, the (PVPON)20 hydrogel contained 78% water, while (PVCL)20 only 66% water (Table 2). Increasing the (PVCL)n thickness from n = 20 to n = 30 resulted in only a slight increase in the hydrogel water uptake to 69% as measured by ellipsometry in liquid (Table 1). Importantly, the degree of swelling for the hydrogels measured by ellipsometry agrees well with that measured by neutron reflectivity (Table S2). Importantly, in situ NR allows for discerning the swollen thicknesses of individual (PVCL) and (PVPON) stacks within the overall hydrogel film. Figure 9 shows the neutron

Figure 9. NR data and corresponding SLD profiles for spin-assisted alternating (PVCL/PVPON) 1 0 (a, b) and double-stack (PVCL)10(PVPON)10 (c, d) and (PVPON)10(PVCL)10 (e, f) hydrogels measured in D2O at 25 °C. Open symbols and solid lines show experiments NR data and fit, respectively.

reflectivity curves and corresponding SLD profiles for alternating (PVCL/PVPON) 10 as well as double-stack (PVCL)10(PVPON)10 and (PVPON)10(PVCL)10 hydrogels measured in D2O at 25 °C. As seen in the SLD profile for the alternating (PVCL/PVPON)10 hydrogel (Figure 9b), a silicon interfacial layer remains unchanged and is attributed to precursor PGMA nonswollen layer, which is followed by a partially swollen precursor PVCL layer and the highly swollen (PVCL/PVPON)10 hydrogel. The farthest from the silicon surface SLD region with the highest scattering density corresponds to pure D2O. In contrast to the alternating (PVCL/PVPON)10 network, the SLD profiles for double-stack (PVCL)10(PVPON)10 and (PVPON)10(PVCL)10 hydrogels (Figures 9d and 9f, respectively) immersed in D2O display two distinct regions of swollen PVCL and PVPON strata. The thicknesses of the individual hydrogel strata from the SLD profiles were used to calculate the hydrogel hydration, W25, and D2O content, f, presented in Table 2 and Figure 10.

Figure 8. SLD profiles for spin-assisted single-stack (PVCL)20 (a) and (PVPON)20 (b) hydrogels in dry state and in D2O at 25 °C.

SLD profiles for the single-stack (PVCL)20 and (PVPON)20 multilayer hydrogels dry and swollen in D2O at 25 °C. As seen from this figure, the (PVPON)20 film exhibits a drastic expansion with a swelling ratio (SR) of 4.6, where SR = t(D2O)/t(dry), after immersion in D2O, while the swelling ratio of the (PVCL)20 hydrogel was 2.9 with the uniform SLD profiles. The lower SLD at the silicon interface near z = 5 nm is attributed to a nonswollen PGMA−PVCL (Figure 8a) or PGMA−PVPON (Figure 8b) first layer due to its high hydrophobicity. This sharp contrast in hydration for singlestack (PVCL)20 and (PVPON)20 hydrogels is in excellent agreement with the ellipsometry data for these systems (Table 1). G

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stack were very similar, with f(PVPON) = 0.79 and f(PVPON) = 0.75, respectively (Table 2 and Figure 10c). Effect of Architecture on Hydrogel Temperature Response. We have shown previously that PVCL multilayer hydrogel films feature distinct and highly reversible thermoresponsive behavior demonstrating various degrees of shrinkage, defined as the ratio of wet thicknesses at 25 to 50 °C, depending on cross-link density.22 Herein, we explore whether this temperature response can be regulated by hydrogel composition and architecture. Hydration and shrinkage of single-stack, double-stack, and alternating hydrogels in response to increased temperature were investigated by measuring the network thicknesses at T = 25 °C and T = 50 °C using in situ ellipsometry (Table 1). We found that both single-stack (PVPON)10 and (PVPON)20 hydrogels showed no shrinkage, defined as S = 1 − (t50/t25), where t25 and t50 are the hydrogel thicknesses at 25 and 50 °C, respectively, and maintained the same thicknesses at both temperatures (Figure S3 and Table 1). Conversely, the single-stack (PVCL)n hydrogels shrank by 37 and 53% when transferred to T = 50 °C with n = 20 and n = 30, respectively (Table 1). In contrast, the alternating (PVCL/ PVPON)10 hydrogel demonstrated a shrinkage of 10%, while that for (PVPON)10(PVCL)10 was only 7%. However, when the PVCL stratum is adjacent to the silicon surface, the shrinkage of (PVCL)10(PVPON)10 is increased by a factor of 2−15%. Apparently, the shrinkage in the alternating and double-stack hydrogels is due to PVCL layers. The lower shrinkage in the (PVPON)10(PVCL)10 configuration versus its mirror counterpart can be rationalized by taking into account the aqueous environment adjacent to the (PVCL)10 stack. Indeed, the ‘freestanding’ (PVCL)10 stack is sandwiched between the hydrophilic (PVPON)10 stack below and the bulk of water above it. This double hydrophilic exposure results in trapped hydrophilic PVPON beneath the PVCL stack, suppressing its collapse at T = 50 °C. The PVCL stack at the silicon surface is hydrated only from the PVPON stack above it and is exposed to a less hydrophilic environment. Thickness- and architecture-dependent swelling of nanoscale films has been observed in previous studies.76,39,40 For example, the linear swelling ratio of a nanothin cross-linked dimethylacrylamide film was almost 4 times larger than that for the unconstrained free network.76 Also, well-stratified 40 nm thick anionic PMAA hydrogels made via spin-assisted LbL exhibited a dramatic 10-fold increase in thickness when transitioned between pH = 5 and 7.5, unlike the 2-fold swelling observed in less-organized PMAA hydrogels obtained via dipping LbL.39,40 Increasing the thickness of the bottom PVCL stack in the (PVCL)n(PVPON)10 hydrogel from n = 10 to n = 20 resulted in an increased temperature response of the (PVCL)20(PVPON)10 hydrogel with its shrinkage reaching 46% at T = 50 °C (Table 1). Increasing the thickness of the hydrophilic PVPON top stack to n = 20 and n = 30 only slightly decreased the temperature response of the hydrogel, resulting in corresponding shrinkage values of 37 and 31%, respectively (Table 1). We also found that the suppressing effect of the underlying PVPON stack on the temperature response in the (PVPON)10(PVCL)10 hydrogel could be counteracted by growing a thicker (PVCL)n stratum. Thus, for example, shrinkage of the hydrogel increased from 7 to 25 to 34%, when a PVCL stack with n = 10, n = 20, and n = 30, respectively, was constructed instead (Table 1). The latter results demonstrated that having a (PVPON)10 stack beneath

Figure 10. Hydration of the multilayer hydrogels in D2O (a, b) and D2O content, f (c), as analyzed by neutron reflectivity.

The NR analysis of hydrogel hydration in D2O (W25) is shown in Figures 10a,b. When the top portion of the PVCL layers in (PVCL)20 are exchanged for their hydrophilic homologue PVPON as in (PVCL)10(PVPON)10, the hydrogel D2O uptake increases from 66% to 67% (Figure 10a). When it is the bottom layers that become hydrophilic as in (PVPON)10(PVCL)10, the overall effect on hydrogel hydration is similar, yielding 65% D2O uptake (Figure 10a). Introducing hydrophilic PVPON by placing the layers individually between PVCL layers, as in the alternating (PVCL/PVPON)10 hydrogel, results in a hydrogel hydration of 75%, which is very close to that of the highly hydrophilic (PVPON)20 hydrogel with W25(D2O) = 78% (Figure 10a). Increasing the thickness of the bottom PVCL stack from n = 10 to n = 20 and changing its placement to the top stack has little effect on the overall (PVCL)n(PVPON)10 and (PVPON)10(PVCL)n hydrogel hydration, which remains in the range from 67 to 66% and from 65 to 64%, respectively (Figure 10b). The D2O content, f, for hydrogel films and the individual hydrogel strata within a double-stack hydrogel was calculated as follows: f = [(Nb)M − (Nb)p]/[(Nb)w − (Nb)p], where (Nb)p is the scattering density of dry polymer film, (Nb)w is the scattering density of D2O, and (Nb)M is the scattering density of polymer film in situ. The f values allow us to evaluate the liquid content within the individual PVCL and PVPON strata within the hydrogel. We found that within the (PVCL)10(PVPON)10 hydrogel with approximately equally thick PVCL and PVPON strata of 15.4 and 12.1 nm, respectively, the PVCL stack contained 1.6-fold less D2O with f(PVCL) = 0.49, in contrast to f(PVPON) = 0.78 (Table 2). Bringing the PVCL stratum closer to the D2O interface did not have a significant effect on the liquid content within the (PVPON)10(PVCL)10 hydrogel with the corresponding values of f(PVPON) = 0.77 and f(PVCL) = 0.51 (Table 2 and Figure 10c). Increasing the PVCL thickness from n = 10 to n = 20 regardless of whether that stratum was at the silicon or D2O interface allowed only a slight increase in D2O content within the PVCL stack, to f(PVCL) = 0.55 and f(PVCL) = 0.57 in the case of (PVCL)20(PVPON)10 and (PVPON)10(PVCL)20, respectively, while the corresponding values for the PVPON H

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Macromolecules the (PVCL)20 or (PVCL)30 hydrogel strata decreases their temperature response by 1.5-fold compared to that of the corresponding single-stack (PVCL)20 and (PVCL)30 hydrogels. We also found that the temperature response of the doublestack (PVCL)10(PVPON)10 hydrogel can be adjusted to that of the corresponding single-stack (PVCL) multilayer hydrogel films by increasing the thickness of the hydrophobic (PVCL) layer. Thus, for example, the shrinkage of (PVCL)n(PVPON)10 was increased from 15 to 46 to 53% with n = 10, 20, and 30, respectively, which is similar to the shrinkage of single-stack (PVCL)20 and (PVCL)30 hydrogels at T = 50 °C in DI water. These data support our suggestion that having a PVCL hydrogel stack at the interface with the template results in a more hydrophobic film than when PVCL is at the water interface. Surface Properties of Hybrid Multilayer Hydrogels. Wettability of dry hydrogel surfaces of the single-stack (PVPON)10 and (PVCL)10 was measured using advancing contact angle. The data demonstrated that the (PVPON) hydrogel surface was more hydrophilic than that of (PVCL) with corresponding contact angle values of θ(PVPON) = 63 ± 3° and θ(PVCL) = 74 ± 4° (Table 3 and Figure 11). AFM Table 3. Contact Angle Values (deg) and Roughness (nm) of Single-Stack (PVPON)10 and (PVCL)10 and Double-Stack (PVPON)10(PVCL)5 and (PVCL)10(PVPON)5 Dry Hydrogels sample (PVCL)10 (PVPON)10 (PVPON)10(PVCL)5 (PVCL)10(PVPON)5

contact angle, θ (deg)

RMS (nm)

± ± ± ±

1.07 0.64 1.2 1.08

74 63 82 53

4 3 3 4

Figure 11. AFM topography images for (PVPON)10 (a), (PVCL)10 (b), (PVPON)10(PVCL)5 (c), and (PVCL)10(PVPON)5 (d) hydrogels with the corresponding optical images of contact angle measurements. Scan size of the AFM images is 1 μm, and z-scale is 17 nm.

topography images of the hydrogels reveal smooth surfaces of (PVPON)10 hydrogel of low roughness (RMS = 0.64 nm), in contrast to rougher (PVCL)10 hydrogel surfaces with an RMS value of 1.07 nm, which is in good agreement with contact angle measurements (Figures 11a,b). When the (PVPON)10 hydrogel was constructed with a (PVCL)5 stratum on top of it, the corresponding contact angle and the RMS roughness are increased from θ = 63 ± 3° to θ = 82 ± 3° and from 0.64 to 1.17 nm, respectively, confirming the increased hydrophobicity of the surface due to PVCL modification (Figure 11c). Conversely, when (PVCL)10 hydrogel had a (PVPON)5 stratum above it, the hydrogel surface became less hydrophobic compared to the single-stack (PVCL)10 with the contact angle decreasing from θ = 74 ± 4° to θ = 53 ± 4° (Figure 11d and Table 3). The roughness of the (PVCL)10(PVPON)5 hydrogel, however, remained similar to that of the pure (PVCL)10 hydrogel with RMS = 1.08 nm, most probably since the small number of PVPON layers was not enough to counteract the roughness of the underlying surface of the PVCL stratum.

stratified with two distinct (PVPON) and (PVCL) strata of higher and lower scattering density, respectively. In contrast, the alternating (PVCL/PVPON)10 hydrogel did not show any internal structure due to significant layer intermixing. Second, as shown with in situ ellipsometry and neutron reflectometry, the hydration of the hydrogels consistently increases with PVPON amount within a network due to a more hydrophilic nature of PVPON, in contrast to PVCL. The water content increased from 66 to 67% for single-stack (PVCL)20 and double-stack (PVCL)10(PVPON)10, respectively, while alternating (PVCL/PVPON)10 and single-stack (PVPON)20 hydrogels had similar hydrations of 78%. Third, heating hydrogels above PVCL’s LCST resulted in a thickness decrease which was regulated by strata location, with PVCL on the bottom showing a greater decrease in thickness than if stacked atop of PVPON. The suppressed temperature response of (PVPON)10(PVCL)10 versus its mirror counterpart is explained by the two adjacent aqueous interfaces in which the (PVCL)10 stack is sandwiched between the hydrophilic (PVPON)10 stack below and the bulk of water above it. Fourth, those effects were modulated by the PVCL stack thickness; lowering overall hydration and increasing magnitude of temperature-based shrinking occur with PVCL thickness increase. Finally, the wettability of (PVCL)n surface increased with the thickness of the hydrophilic (PVPON) stratum above it. Our results illustrate control over the temperature response and surface properties of nanometer thick films, which is relevant to both fundamental and applied research where the dynamics of hydration, thickness, and control of surface hydrophobicity are important.

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CONCLUSIONS We have produced a new type of highly stratified temperatureresponsive surface network by integration of nonionic (PVCL)n and (PVPON)n LbL-derived hydrogels as either top or bottom strata. We have demonstrated that temperature-responsive properties and surface morphology of these hybrid hydrogels were strongly correlated with the network architecture, thickness, and hydrophobicity. First, neutron reflectometry revealed that all double-stack films in the dry state are well I

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00964. Neutron reflectivity data for spin-assisted single-stack (PVCL)20 and (PVPON)20 hydrogels in dry and wet (D2O, 25 °C) states; temperature-independent thickness of (PVPON)20 multilayer hydrogel measured by in situ ellipsometry in DI water; combined SLD profiles for spin-assisted double-stack (PVCL)10(PVPON)10 and (PVCL) 20 (PVPON) 10 , and alternating (PVCL/ PVPON)10 hydrogels in dry state and in D2O at 25 °C and model parameters for the multilayer hydrogels; SEM images of the hydrogels; swelling ratios of the hydrogels at room temperature as measured by ellipsometry and neutron reflectivity (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.F.A.). *E-mail: [email protected] (E.K.). Author Contributions

W.H. and V.K. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF Career Award #1350370 (E.K.) and by EPSCoR DOE/JINS Travel Fellowship. ORNL is managed by UT-Battelle, LLC, for the US Department of Energy (DOE) under Contract DE-AC05-00OR22725.

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