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25 Jun 2015 - the swollen PNIPAAM brush led to long waiting times and a high probability of readsorption, which resulted in short flight lengths, whil...
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Dynamic Molecular Behavior on Thermoresponsive Polymer Brushes Huai-Ying Chin, Dapeng Wang, and Daniel K. Schwartz* Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: The surface dynamics of individual surfactant and polymer molecules on thermally responsive polymer brushes (poly(N-isopropylacrylamide), PNIPAAM) were studied using high throughput single molecule tracking microscopy. The probe molecules universally exhibited intermittent hopping motion, in which the diffusion switched between mobility and confinement with a broad distribution of waiting times; this was analyzed in the context of a continuous time random walk (CTRW) model described using “waiting time” and “flight length” distributions. We found that the surface mobility, which was affected by waiting times and flight lengths, of both probe molecules increased abruptly with temperature above the 32 °C lower critical solution temperature (LCST) transition of the PNIPAAM brush. In particular, above the LCST, where the polymer brush collapsed into a more hydrophobic dense polymer film, the effective diffusion coefficients and mobile fraction of probe molecule increased, suggesting that mobility was inhibited by penetration into the brush at lower temperatures. Waiting times at lower temperature were twice as long as at higher temperatures, and the longest flight length increased from 0.9 to 1.8 μm. Moreover, we found that the high density of strong binding sites available on the swollen PNIPAAM brush led to long waiting times and a high probability of readsorption, which resulted in short flight lengths, while the absence of strong binding sites on collapsed PNIPAAM films led to short waiting times and long flights.

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INTRODUCTION Stimulus-responsive polymer brushes have attracted significant attention because of their applications in biology and medicine.1−5 Thermally responsive poly(N-isopropylacrylamide) (PNIPAAM) brushes are of considerable interest for biomaterials and controlled-release applications because they exhibit a structural transition at a lower critical solution temperature (LCST) near 32 °C.6−9 Below the LCST, the PNIPAAM brush is relatively hydrophilic and readily swollen by aqueous solution. Above the LCST, however, the PNIPAAM brush collapses into a dense polymer film that is considerably more hydrophobic.7,10 Because of the similarity between the LCST of PNIPAAM and human body temperature, PNIPAAM has been of significant interest as a controlled-release system for drug delivery.3,4,6,9,11−13 Accordingly, many scientific studies have been performed to resolve and understand the structure of thermoresponsive PNIPAAM using e.g. atomic force microscopy,14−17 neutron reflectometry,15,18 contact angle goniometry,7 etc. However, the dynamic behavior of molecules interacting with PNIPAAM brushes, which are functionally related to many proposed applications, remains poorly understood. The traditional view of interfacial polymer motion involves two-dimensional (2D) Brownian motion of a flexible chain in a train−loop−tail configuration.19 Using single-molecule tracking methods, however, it has recently been demonstrated in several independent laboratories that polymers (and other molecules) exhibit an intermittent transport mechanism at various liquid− solid interfaces.20−22 This behavior was inconsistent with conventional Brownian motion but was successfully interpreted © XXXX American Chemical Society

using a continuous time random walk (CTRW) model, which describes motion using “waiting-time” and “flight-length” distributions.20,23,24 These observations, which were uniquely enabled by single-molecule tracking, provided a more nuanced view of surface dynamics, compared to the traditional view of interfacial flexible chain dynamics,25,26 in which surface diffusion is mainly attributed to the polymer conformation and the interaction between the surface and each segment. Therefore, although the surface dynamics of small molecules and macromolecules have been explored using ensembleaveraging approaches,25−27 it is appealing to revisit the dynamics of probe molecules on stimuli-responsive polymer brushes using single molecular tracking where individual trajectories are resolved. Elliott et al.28,29 showed that single molecule tracking could be used to identify (and characterize) the confinement of small molecules within PNIPAAM brushes. Here we adopt a related experimental approach to develop a comprehensive picture of interfacial transport in the context of CTRW motion. In this work, we systematically explored the surface dynamics of a fluorescently labeled small molecule surfactant (dodecanoic acid) and a fluorescently labeled macromolecule (dextran, average MW = 10 kDa, ranging from 9 to 11 kDa) on and within thermally responsive PNIPAAM brushes of various thicknesses over a range of temperatures spanning the LCST. We also studied the same probe molecules on ethylsilaneReceived: April 8, 2015 Revised: May 26, 2015

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reflectance infrared Fourier transform spectroscopy (Thermo Scientific Nicolet 6700) was performed to obtain vibrational spectra of the deposited layers. The detailed results are shown in the Supporting Information, including a spectrum in Figure S10, which shows clear signatures of PNIPAAM. Single-Molecule Tracking. Imaging experiments were performed using a prism-based total internal reflection fluorescence microscope (TIRFM), Nikon Eclipse TE2000.31 The CCD camera operated at −95 °C and recorded the movies of the sparsely adsorbed dodecanoic acid (labeled with BODIPY FL C12, Life Technologies) or dextran (MW = 10 000, labeled with Alexa Fluor 647, Life Technologies), which randomly adsorbed and diffused at the interface of water and the modified surface. Since the BODIPY label represents a significant perturbation to the unlabeled fatty acid probe molecule (dodecanoic acid), it is likely that the detailed behavior of the fluorescent fatty acid analogue is not identical to that of an unlabeled fatty acid. However, both the hydrocarbon chain and BODIPY are hydrophobic, and in previous work, which followed the behavior of BODIPY fatty acids as a function of chain length, we found that the BODIPY had a relatively modest effect.34 Alexa Fluor 647 was illuminated by a 647 nm laser (Crystal Laser Direct Series), and the emission was filtered using a 692 ± 40 nm band-pass filter (Semrock). BODIPY FL was excited by a 491 nm DPSS laser (Cobalt Calypso) with emission filtered by a 542 nm long pass filter (Semrock). In each experiment, a sequence of 1000 frames, with time intervals of 100 ms, was recorded. Particle identification, trajectory tracking, and adsorption/desorption events were analyzed using a previously described Mathematica program.35 Data Analysis. Cumulative squared displacement distributions (CSDD) were used to determine the characteristic diffusion coefficients of adsorbed probe molecules.33,34 The cumulative distribution is a function of squared displacement and lag time, C(R2,Δt). The displacements were determined by measuring the locations of molecules in consecutive frames, and the lag time was the image acquisition time, Δt = 0.1 s. C(R2,Δt) represents the probability that a molecule moves a distance greater than R in a time interval Δt. To account for the fact that measured CSDD data may represent multiple modes of diffusion, a Gaussian mixture model was used to fit the CSDD

modified surfaces for comparison. While the detailed interactions between the distinct probe molecules and the PNIPAAM brush were clearly very different (i.e., the fatty acid and dextran interacting primarily via hydrophobic and hydrogen-bonding interactions, respectively), we found that the phenomenological behaviors were very similar and could be described using the same kinetic model. We found that the probe molecules exhibited intermittent hopping on PNIPAAM brushes with temperature-dependent flight-length and waitingtime distributions that varied systematically with PNIPAAM film thickness. These results were interpreted in terms of distinctive molecular interactions with swollen brushes vs dense polymer films, in the context of single molecule measurements, which permitted us to understand the influence of dynamic and spatial heterogeneity.

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EXPERIMENTAL METHODS

Surface Preparation. PNIPAAM brushes were prepared using a previously reported approach,30 known as activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). Fused silica (FS) wafers were cleaned using a piranha solution (H2SO4:H2O2 v/v = 7/3) for 3 h at 70 °C, followed by thorough rinsing with Millipore water (18.2 MΩ·cm). FS wafers were further treated with UV-ozone (Boekel UV Clean model 135500) for 60 min.31 After this, wafers were immediately immersed in a solution composed of 0.5 mL of N-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPTES, Gelest) and 200 mL of toluene for 45 min, followed by rinsing with toluene and Millipore water. The ATRP initiator groups were prepared using a solution of 2-bromo-2methylpropionic acid (Acros Organics, 534 mg) and 4(dimethylamino)pyridine (TCI America, 96 mg) in 180 mL of dichloromethane at 0 °C, to which a solution of N,N′-dicyclohexylcarbodiimide (Acros Organics, 826 mg) in 20 mL of dichloromethane was added.30 The solution was warmed to room temperature and reacted with FS wafers overnight. After the initiator groups were deposited, wafers were rinsed with ethanol and water. Polymer brushes were prepared by ARGET ATRP.32 N-Isopropylacrylamide (NIPAAM, TCI America) monomer (0.88−1.30 M), ascorbic acid (Sigma-Aldrich, 1.10 g), and 1.40 mL of 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA, Acros Organics) were dissolved in a mixture of methanol and water (v/v = 1/1). The solution was degassed by two freeze−thaw cycles. The solution was added into a degassed reactor containing copper bromide (320 mg) and the premodified wafers. The reaction was carried on under a stream of nitrogen for 60−120 min. The ethyltrimethoxysilane (Gelest) selfassembly monolayer (SAM) was prepared by vapor deposition overnight, as described previously.33 In order to characterize the polymerization process in solution, a small amount of the initiator (2bromo-2-methylpropionic acid) was added to the reactor with the premodified wafers. The free polymer chains in solution were separated using dialysis and analyzed using gel permeation chromatography following previously reported methods (GPC).26 Surface Characterization. The thickness of polymer brushes in a dry state was characterized by a variable-angle spectroscopic ellipsometry (J.A. Woollam VASE-VB 250). Oxidized silicon wafers (WRS Materials), which have similar chemical properties to the FS wafers, were used in these measurements. The refractive index of PNIPAAM used in the analysis was 1.47.7 The spectroscopic scan spanned the range 500−900 nm, and the incident angles ranged from 60° to 80° in 5° increments. Contact angle measurements were performed using a custom-built goniometer with a heating stage. A droplet of approximately 1 μL of Millipore water was placed on the polymer brush surface, and the contact angle was measured at temperatures ranging from 25 to 45 °C. For each sample at each temperature, contact angles were measured at five independent locations; the reported values and uncertainties were based on the mean and standard deviation of these measurements. To further confirm the presence of PNIPAAM on oxidized silicon wafers, diffuse

n

C(R2 , Δt ) =

⎛ − R2 ⎞ ⎟ ⎝ 4DiΔt ⎠

∑ fi exp⎜ i=1

(1)

where the fitting parameters f i and Di represented the fraction and the corresponding diffusion coefficient for diffusive mode i. Given the experimental resolution of the tracking method (∼80 nm),36 an apparent diffusion coefficient smaller than 0.025 μm2/s, for Δt = 0.1 s, was indistinguishable from immobility, and such steps were assigned to an apparent immobile fraction. For modes with diffusion coefficients greater than 0.025 μm2/s, the mean diffusion coefficient was calculated using a weighted average, D̅ = ∑f iDi/f, where f is the mobile fraction. Two-dimensional adsorption maps were prepared using a previously reported super-resolution imaging approach.37,38 The positions of adsorption events were placed in a pseudo-super-resolution image and were blurred by 2D Gaussian functions. Distinct adsorption sites were therefore identified by grouping the locations of contiguous adsorption events, and the adsorption count of each site was taken as the maximum local value.37 The distribution of adsorption counts over the whole surface was fitted by a Poisson mixture model37 2

fads (x) =

∑ pi i=1

λ i x e −λ i x! (1 − e−λi)

(2)

where pi and λi represented the fractions (with ∑pi = 1) and the mean number of adsorption events per site, respectively, for population i. The average number of adsorption events per site was calculated using the weighted average λ̅ = ∑piλi. For an ideal homogeneous surface, molecules adsorb on every surface site with an equal probability, so the ideal number of surface adsorption sites on the surface can be estimated as A/d, where A is the surface area and d is the cross-sectional area of the adsorbed molecule. B

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Figure 1. Representative images of sessile drop contact angle measurements of water on a 29.9 nm thick PNIPAAM layer at (a) 25 °C and (b) 40 °C and on a C2-modified surface at (c) 25 °C and (d) 40 °C. (e) Contact angle measurements of water PNIPAAM layers (thickness as annotated) and on a C2-modified surface as annotated. Given the sparse data in real experiments, the number of adsorption sites may be estimated by N/λ̅, where N is the total number of trajectories and λ̅ is the average number of adsorption events per site. One measure of the degree of surface heterogeneity (defined as the heterogeneity index h) is given by the ideal number of adsorption sites divided by the actual number of adsorption:39

h=

A /d N /λ ̅

determine approximate molecular weights of the polymer chains using GPC.26 More details about this approach, including a more nuanced discussion about its limitations, are included in the Supporting Information. The three conditions employed to prepare grafted PNIPAAM layers yielded films with ellipsometric thicknesses (in the dry state) of 7.65 ± 0.15, 17.8 ± 0.5, and 29.9 ± 0.9 nm, respectively. Under the same conditions that the three films were prepared, the polymer chains formed in solution were determined (using GPC) to have mass-averaged mean molecular weights (Mw) of 44.3, 50.3, and 85.3 kg/mol, respectively. The polydispersity indices, PDI = Mw/Mn (where Mn is the number-averaged mean molecular weight), were in the range 2.5−2.7. Additional detail is provided in the Supporting Information (Table S5). Extrapolating from the results of previous work,42 the mean radii of gyration of these PNIPAAM chains in aqueous solution were 7.8, 8.4, and 11.4 nm, respectively. If one makes the assumption that the grafted chains have the same molecular weight properties as those formed in solution under identical conditions (and using the density of dry PNIPAAM of 1.1 g/ cm3), one can calculate the approximate grafting density, giving mean graft separations of 1.8, 1.3, and 1.3 nm, respectively, for the three conditions. In all cases, the graft separation was much smaller than the radius of gyration, consistent with extended brush conformations. This focus on mean properties obviously neglects the significant degree of heterogeneity stemming from the polydispersity of the grafted chains. As illustrated in Figure 1, a distinct change in the contact angle of water was observed with temperature for the PNIPAAM films. Two representative pictures of water droplets on a 29.9 nm thick PNIPAAM film are shown in Figures 1a and 1b for T = 25 °C and T = 40 °C, respectively, where the contact angle increased from 59° (25 °C) to 83° (40 °C), signifying a significant increase in hydrophobicity with increasing temperature. Similar behavior was observed for all PNIPAAM brushes, as shown in Figure 1e. In contrast, the contact angle of water on the C2-modified surface was insensitive to temperature (Figure 1c,d). Detailed information about the contact angle of water on all surfaces is shown in Figure 1e. Taken together, these results suggest that the changes in contact angle on the PNIPAAM brushes were related to the brush collapse associated with the LCST transition. Apparent Diffusion Coefficients. The interfacial dynamics of probe molecules were analyzed quantitatively using cumulative squared displacement distributions (CSDD), as detailed in the Experimental Methods section. Representative

(3)

For heterogeneity calculations, the estimated molecular cross-sectional areas of fluorescently labeled dextran and dodecanoic acid were calculated to be 34.2 and 1.03 nm2, respectively.40 The area of the field of view was A = 3364 μm2, and the number of trajectories was N = 2 × 105. Thus, the ideal numbers of adsorption site on the surface were 9.83 × 107 (dextran) and 5.24 × 108 (dodecanoic acid).

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RESULTS AND DISCUSSION Polymer Film Characterization. While the ATRP “grafting-from” approach applied here is technologically important and has several advantages (including the potential to achieve high grafting densities),7,41 it results in polymer films that are significantly more heterogeneous (e.g., the polydispersity of the grafted chains) than films that are prepared in a “grafting to” approach using purified polymer precursors. Moreover, this heterogeneity presents a challenge in terms of characterization, since in general the distribution of grafted chain lengths is not known a priori. In special cases, chains can be selectively cleaved from the surface and characterized using analytical chromatography; however, this is not possible for PNIPAAM in particular. Nevertheless, we attempted to characterize the structural properties of the films within these limitations. The overall thickness of PNIPAAM layers was controlled by changing both the monomer concentrations and polymerization times as detailed in the Experimental Methods section. The thicknesses of the brushes studied, in their dry state, were measured using ellipsometry. An ethyltrimethoxysilane-modified (C2) surface, which did not exhibit thermal-responsive behavior, was used as a control. The overall thickness of a grafted polymer film is a combined consequence of chain length and grafting density, and ideally it would be desirable to characterize each of these properties accurately and independently. As mentioned above, however, in practice this is very difficult for PNIPAAM films formed using a grafting-from approach. While it is not a perfect substitute, it is common to use chains that are polymerized in solution (under identical conditions as brush formation) as a proxy, in order to C

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Figure 2. Cumulative squared displacement distributions (CSDD) of dextran on (a) a 29.9 nm PNIPAAM layer and (b) a C2-modified surface. Black squares (25 °C), red circles (30 °C), green-up triangles (35 °C), and blue-down triangles (40 °C) represent the data, and the solid lines represent the fitting function associated with a Gaussian mixture model.

Table 1. Fitting Values of Diffusion Coefficients from CSDD at Low Temperature (25 °C) and High Temperature (40 °C)a D1 (μm2/s) dextran

C2-modified

dodecanoic acid

7.65 nm PNIPAAM 17.8 nm PNIPAAM 29.9 nm PNIPAAM C2-modified 7.65 nm PNIPAAM 17.8 nm PNIPAAM 29.9 nm PNIPAAM

D2 (μm2/s)

Tlow

Thigh

1.10 (0.08) 1.16 (0.10) 0.82 (0.12) 0.72 (0.11) 0.96 (0.06) 0.84 (0.15) 0.75 (0.18) 0.74 (0.08)

1.15 (0.12) 2.19 (0.31) 1.81 (0.21) 2.17 (0.41) 0.98 (0.04) 2.61 (0.32) 2.23 (0.28) 2.33 (0.21)

Tlow

D3 (μm2/s) Thigh

0.06 (0.01) 0.05 (0.007) 0.05 (0.01)

0.07 (0.02) 0.06 (0.01) 0.05 (0.02)

0.08 (0.03) 0.05 (0.01) 0.17 (0.01)

0.08 (0.02) 0.07 (0.02) 0.35 (0.02)

Tlow

Thigh

0.007 (0.001) 0.006 (0.003) 0.007 (0.003) 0.009 (0.003) 0.01 (0.006) 0.01 (0.002) 0.01 (0.003) 0.01 (0.002)

0.006 (0.001) 0.006 (0.005) 0.007 (0.002) 0.008 (0.001) 0.01 (0.008) 0.008 (0.001) 0.01 (0.002) 0.01 (0.002)

The numbers in parentheses represent experimental uncertainties. All fitting parameters, including the fractions associated with the various modes, are given in the Supporting Information Table S1. a

Figure 3. Ensemble mean diffusion coefficients of (a) dextran and (b) dodecanoic acid on 7.65 nm (black triangle), 17.8 nm (blue circle), and 29.9 nm (red square) PNIPAAM layers and a C2-modified surface (green diamond) as a function of temperature.

tions were successfully fitted using a Gaussian mixture model, as shown in eq 1, with three components. Some salient fitting parameters are listed in Table 1 for 25 °C and 40 °C. We calculated the ensemble-average mean diffusion coefficients D̅ of dextran and dodecanoic acid on PNIPAAM and the C2-modified surface using the expression D̅ = ∑f iDi, where ∑f i =1, as shown in Figures 3a and 3b. The mean diffusion coefficients of both probe molecules on the C2-modified surface

CSDD plots for dextran on the 29.9 nm PNIPAAM brush and the C2 control surface are shown in Figure 2 (all CSDD data are shown in Figures S1 and S2 of the Supporting Information). The CSDDs of dextran on PNIPAAM brushes at low temperatures (25 and 30 °C) were distinctly different from these at high temperature (35 and 40 °C). In contrast, on C2modified surfaces, the CSDD data evolved gradually and systematically upward as temperature increased. The distribuD

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Figure 4. Average diffusion coefficients of mobile modes for (a) dextran and (b) dodecanoic acid on 7.65 nm (black triangles), 17.8 nm (blue circles), and 29.9 nm (red squares) PNIPAAM layers.

Figure 5. (a) Representative trajectories of dextran on a 29.9 nm PNIPAAM layer at 40 °C. (b) Representative lateral positions of dextran molecules on a PNIPAAM layer as a function of time.

in Figure 5a. Some trajectories were apparently completely immobile (or confined to a small domain) on the brush layer, and others exhibited single or multiple hops alternating with periods of apparent immobility. Figure 5b shows the xcoordinate of several representative trajectories versus time, again demonstrating that some molecules (e.g., trajectory 2) were immobile during their entire residence time on the surface, while others exhibited intermittent hopping with a distribution of waiting times. Qualitatively similar behavior was observed for both dextran and dodecanoic acid on all surfaces and at all temperatures. This diffusion behavior is clearly inconsistent with simple Brownian motion (i.e., Gaussian statistics) but qualitatively consistent with CTRW, where periods of mobility alternate with periods of immobility that exhibit a distribution of “waiting times”.20,23,24 We hypothesized that the phenomenology described above, i.e., the changes in the average diffusion coefficient as a function of temperature and brush thickness, could be explained in the context of the CTRW model, which is described by two distributionsa “waiting-time” distribution and a “flight-length” distribution. This analysis is described in detail below. Spatial Heterogeneity. In previous work, we found that interfacial dynamics can be strongly influenced by spatial heterogeneity, in particular by the presence of relatively rare, but anomalously strong binding sites.37,39 Therefore, as described in the Experimental Methods section, we explored the spatial heterogeneity of molecular adsorption using a superresolution imaging technique permitting the identification of individual binding sites.37 Representative adsorption site maps are shown in Figures 6a and 6b. These maps clearly show the presence of distinct binding sites with anomalously large numbers of adsorption events at low temperatures (Figure 6a);

increased slightly as temperature increased. On PNIPAAM layers, the mean diffusion coefficients exhibited an abrupt discontinuous increase between 30 and 35 °C. This distinct change occurred near the LCST transition, suggesting that this structural transition directly influenced the surface dynamics of adsorbed molecular species. These ensemble-average mean diffusion coefficients comprised all trajectories, including those that were apparently immobile throughout their entire surface residence time. They also included steps taken during apparent time intervals of immobility as discussed further below. As described in the Experimental Methods section, the experimental CSDD can be decomposed into multiple components using eq 1. Given the approximate experimental resolution of the particle tracking method (60−100 nm),36 and the acquisition time of 0.1 s, a characteristic diffusion coefficient smaller than 0.025 μm2/s represented apparent immobility. As shown in Table 1, each CSDD included a diffusive mode, D3, whose value was smaller than 0.025 μm2/s. We used this as a criterion to define f 3 as an immobile fraction of steps. The fractions f1 and f 2 therefore corresponded to mobile modes. We calculated the diffusion coefficients of the mobile modes (i.e., the apparent diffusion coefficient during periods of mobility, Dmobile) by weight-averaging D1 and D2 as shown in Figures 4a and 4b. Interestingly, Dmobile exhibited a qualitatively similar dependence on temperature as the overall average diffusion coefficient, D̅ , suggesting that the abrupt increase in mobility at the LCST was not solely due to an increase in the mobile “fraction”. Moreover, a general trend of decreasing mobility with increasing brush thickness was apparent. Intermittent Hopping. A representative group of dextran trajectories on a 29.9 nm PNIPAAM surface at 40 °C is shown E

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of the grafted chains, potentially leading to a wide range of distinct steric environments within the brush layer. Moreover, there may be strong interactions between probe molecules that penetrate into the brush and defects in the silica surface (e.g., unreacted silanol sites); such interactions are believed to be a significant cause of heterogeneity in reverse phase chromatography separations, which were characterized carefully in previous work.37 Representative distributions of adsorption event counts are shown in Figure 6c. The adsorption process on a homogeneous surface is expected to follow a single Poisson distribution because molecules have the same probability of adsorbing on every site. However, the histogram in Figure 6c (particularly at low temperatures) exhibited a heavy-tailed distribution, again indicating the presence of anomalously strong binding sites. We used a superposition of two Poisson distributions, eq 3, to fit the histograms of the adsorption events and evaluated the mean number of adsorption events per site. The fitting parameters of the mixture Poisson model are shown in Table S2 of the Supporting Information. In general, weak sites had a mean number of adsorption events, λ, of 2 for the thickest brushes studied. We hypothesize that this trend is directly related to the ratios of surface heterogeneity parameter as shown in Figures 7c and 7d. In particular, the high density of strong binding sites available on the surface at low temperatures (in the presence of PNIPAAM brushes) led to long waiting times, while the absence of strong binding sites at high temperatures (on collapsed PNIPAAM films) led to short waiting times. Moreover, the presence of a high density of strong binding sites at low temperatures increased the probability of readsorption during desorption-mediated flights, resulting in truncated flight-length distributions as shown in Figure 9. Descriptions of desorption-mediated diffusion20,23,24,45 generally incorporate the notion that molecules are expected to have multiple contacts with the surface during each flight, where each contact has a finite probability for readsorption. Clearly, this readsorption probability is higher in the vicinity of strong binding sites, and therefore one expects to observe a narrower flight-length distribution on more heterogeneous surfaces, as seen here on PNIPAAM brushes at low temperatures. Taken together, the prevalence of strong binding sites (i.e., surface heterogeneity) at low temperatures (and particularly for thicker brushes) is expected to lead to truncated flight lengths and longer waiting times, both of which are experimentally observed, and provide a molecular level explanation for the abrupt increase of mobility upon brush collapse in the text of a CTRW model for interfacial transport.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the analysis of adsorption site, site residence time, waiting time, and additional figures and tables. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00729.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation (NSF CHE 1306108). We thank Chen Wang and Chih-Heng Lien for the assistance with GPC and FTIR. We also acknowledge Dr. Joshua Mabry, Dr. Nathan Nelson, and Dr. Blake Langdon for their invaluable advice and assistance.

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REFERENCES

(1) Tam, T. K.; Pita, M.; Trotsenko, O.; Motornov, M.; Tokarev, I.; Halamek, J.; Minko, S.; Katz, E. Reversible “closing” of an electrode interface functionalized with a polymer brush by an electrochemical signal. Langmuir 2010, 26 (6), 4506−4513. (2) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 2010, 9 (2), 101− 113. (3) Alarcon, C. D. H.; Pennadam, S.; Alexander, C. Stimuli responsive polymers for biomedical applications. Chem. Soc. Rev. 2005, 34 (3), 276−285. (4) Cabane, E.; Zhang, X. Y.; Langowska, K.; Palivan, C. G.; Meier, W. Stimuli-responsive polymers and their applications in nanomedicine. Biointerphases 2012, 7 (1−4). (5) Tam, T. K.; Ornatska, M.; Pita, M.; Minko, S.; Katz, E. Polymer brush-modified electrode with switchable and tunable redox activity for bioelectronic applications. J. Phys. Chem. C 2008, 112 (22), 8438− 8445. (6) Cao, L.; Man, T.; Zhuang, J. Q.; Kruk, M. Poly(Nisopropylacrylamide) and poly(2-(dimethylamino)ethyl methacrylate) grafted on an ordered mesoporous silica surface using atom transfer

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CONCLUSIONS Small surfactants and macromolecules on PNIPAAM layers exhibited intermittent motion, consistent with a CTRW model as observed previously for several dense solid−liquid interfaces.20,23 At low temperatures, where the PNIPAAM layer was in the form of a hydrophilic brush, molecules exhibited long waiting times between jumps and short flight lengths, leading to slower diffusion on average. At higher temperatures, where the PNIPAAM film collapsed into a dense I

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Macromolecules

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DOI: 10.1021/acs.macromol.5b00729 Macromolecules XXXX, XXX, XXX−XXX