Controlling Dispersion and Migration of Particulate Additives with

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Langmuir 2006, 22, 10251-10257

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Controlling Dispersion and Migration of Particulate Additives with Block Copolymers and Diels-Alder Chemistry Philip J. Costanzo, J. Derek Demaree, and Frederick L. Beyer* Army Research Laboratory, Materials DiVision, Aberdeen ProVing Ground, Maryland 21005-5069 ReceiVed August 18, 2006 Reversible Diels-Alder chemistry was exploited to develop thermo-responsive polymer films. Here, low molecular weight poly(styrene) (PS) and poly(ethylene glycol) (PEG) were prepared with furyl and maleimido chain ends, respectively. These polymers were then tethered together to form a thiol-terminated PEG-b-PS diblock copolymer ligand via a Diels-Alder linkage and were employed to randomly disperse 10 nm diameter Au nanoparticles within a matrix of PEG. Thermal treatment caused the Diels-Alder linkages between the polymer blocks to be severed, resulting in controllable surface functionalization due to phase separation. Migration of the Au nanoparticles to the surface of the films was characterized by Rutherford backscattering spectroscopy, small-angle X-ray scattering, contact angle measurements, and atomic force microscopy.

Introduction The development of stimuli responsive films and surfaces has been explored and studied for many applications including tissue engineering,1,2 permeable membranes,3 and packaging materials.4 These materials usually undergo some significant change in their physical properties due to a global stimulus, such as temperature, pH, or light. Through deliberate materials design and engineering, these changes in physical properties can be used to engineer a specific response to a specific trigger, allowing the design of devices or systems which respond in a useful manner to changes in their environments, without monitoring or activation. A wide range of techniques have been utilized to study and prepare phase segregated polymer surfaces.5-17 One compelling approach utilizes the microphase separation of block copolymers, driven by the immiscibility of the component blocks. Of particular relevance is the work of Koberstein et al., who have not only prepared a variety of systems focusing on the effects of variations in polymer architecture and composition, but also provided * Corresponding author. E-mail: [email protected]. (1) Xu, F. J.; Zhong, S. P.; Yung, L. Y. L.; Neoh, K. G. Biomacromolecules 2004, 5, 2392-2403. (2) Okajima, S.; Sakai, Y.; Yamaguchi, T. Langmuir 2005, 21, 4043-4049. (3) Csetneki, I.; Filipcsei, G.; Zrinyi, M. Macromolecules 2006, 39, 19391942. (4) Mcelhanon, H. R.; Russick, E. M.; Wheeler, D. R.; Low, D. A.; Aubert, J. H. J. Appl. Polym. Sci. 2002, 85, 1496-1502. (5) Elman, J. F.; Johs, B. D.; Long, T. E.; Koberstein, J. T. Macromolecules 1994, 27, 5341. (6) Bhatia, Q. S.; Pan, D. H.; Koberstein, J. T. Macromolecules 1988, 21, 2166-2175. (7) Pan, D. H.; Prest, W. M. J. J. Appl. Phys. 1985, 58, 2861. (8) Schmitt, R.; Gardella, J. A.; Magill, J. H.; Salvat, L. Macromolecules 1985, 18, 2675-2679. (9) Wu, S. Polymer Interfaces and Adhesion; Marcel Dekker: New York, 1982; Vol. 1. (10) Gaines, G. L. J. Chem. Phys. 1969, 73, 3143. (11) Hunt, M. O. J.; Belu, A. M.; Linton, R. W.; DeSimone, J. M. Macromolecules 1993, 26, 4845-4853. (12) Affrossman, S.; Hartshorne, M.; Kiff, T.; Pethrick, R. A.; Richards, R. W. Macromolecules 1994, 27, 1588-1591. (13) Hopkinson, I.; Kiff, F. T.; Richards, R. W.; Bucknall, D. G.; Clough, A. Polymer 1997, 38, 87. (14) Schaub, T. F.; Kellogg, G. J.; Mayes, A. M.; Kulasekere, R.; Anker, J. F.; Kaiser, H. Macromolecules 1996, 29, 3982-3990. (15) Yuan, C.; Ouyang, M.; Koberstein, J. T. Macromolecules 1999, 32, 23292333. (16) Mason, R.; Jalbert, C. J.; O’Rourke-Muisener, P. A. V.; Koberstein, J. T. AdV. Colloid Interface Sci. 2001, 94, 1. (17) Jalbert, C. A.; Koberstein, J. T.; Hariharan, A.; Kumar, S. Macromolecules 1997, 30, 4481-4490.

extensive modeling of their systems to explain the observed phenomena.18,19 They demonstrated the mobilization, also referred to as “blooming”, of chain-end fluorinated PS to the surface of a poly(dimethylsiloxane) (PDMS) spin-coated film. It was found that the key driving force for blooming is the reduction of surface energy. While useful, one limitation of this approach is that the blooming of molecules occurs immediately upon processing. For some applications, it may be more preferable to control the timing with which the surface of a material is modified. DielsAlder (DA) chemistry offers the characteristics required to control the blooming of polymers to the surface.20,21 The combination of DA chemistry and block copolymers has been used to prepare organic-inorganic polymer hybrids,22 thermoplastic elastomers,23,24 polyurethanes, and foams.4 The goal of the current study is the fabrication of a film that is sensitive to a global, external stimulus, such as temperature. As illustrated in Figure 1, DA chemistry will be used to create appropriate block copolymer ligands that can be used to disperse nanoparticles in a polymer matrix. The temperature-dependent reversibility of the DA linkage between the two blocks of the ligand diblock copolymer should allow the controlled dissociation of the two blocks of the ligand, leaving the particles modified with a ligand that is immiscible in the polymer matrix. The combination of immiscibility and mobility at elevated temperatures should allow the functional particles to bloom to the surface of the polymer film, completing the material response to the external stimulus.

Results and Discussion A detailed description of the synthesis of the block copolymerfunctionalized Au nanoparticles (1-DA-3 Au), with a nominal (18) O’Rourke-Muisener, P. A. V.; Koberstein, J. T.; Kumar, S. Macromolecules 2003, 36, 771-781. (19) O’Rourke-Muisener, P. A. V.; Jalber, C. A.; Yuan, C.; Baetzold, J.; Mason, R.; Wong, D.; Kim, Y. J.; Koberstein, J. T. Macromolecules 2003, 36, 29562966. (20) Fringuelli, F.; Taticchi, A. Dienes in the Diels-Alder Reaction; John Wiley & Sons: New York, 1990. (21) Carruthers, W. Cycloaddition Reactions in Organic Synthesis; Oxford, UK, 1990. (22) Imai, Y.; Itoh, H.; Naka, K.; Chujo, Y. Macromolecules 2000, 33, 43434346. (23) Gheneim, R.; Perez-Berumen, C.; Gandini, A. Macromolecules 2002, 35, 7246-7253. (24) Chen, X.; Wudl, F.; Mal, A. K.; H., S.; Nutt, S. R. Macromolecules 2003, 36, 1802-1807.

10.1021/la0624541 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/19/2006

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Figure 1. (Top) Schematic representation of thermo-responsive films. (A) Additive dispersed within film at room temperature. (B) Thermal treatment results in cleavage of compatibilizing shell from core. (C) Migration of additive due to phase separation. (Bottom) Proposed model system based upon Diels-Alder chemistry.

Figure 2. Contact angle measurement data for films comprised of 2000 and 5000 g/mol PEG containing various wt % of 1-DA-3 Au additive after annealing at 60 and 90 °C. O, Samples after annealing at 60 °C; b, 2000 g/mol sample after annealing at 90 °C for 24 h; 9, 5000 g/mol sample after annealing at 90 °C for 24 h.

Au particle core diameter of 10 nm, can be found in the Supporting Information. Polymer films were then prepared by dissolving a PEG matrix (Mn 2000 or 5000 g/mol; polydispersity index (PDI) < 1.05) in THF and adding various amounts of the 1-DA-3 Au stock solution. Slow evaporation of solvent reduced drying defects and resulted in bulk polymer films with thicknesses ranging from 350 to 700 µm. The films were annealed at various temperatures and times, and subsequently characterized by contact angle measurements, atomic force microscopy, small-angle X-ray scattering (SAXS), and Rutherford backscattering (RBS) to explore the migration of the functionalized Au particles within the PEG matrix. Figure 2 provides contact angle data from films composed of 2000 and 5000 g/mol PEG containing 0.5-4 wt % of 1-DA-3 Au additive that were annealed at 60 and 90 °C for 24 h. Anneal temperatures were chosen to shift the DA equilibrium toward the adduct (60 °C) or individual components (90 °C). After annealing at 60 °C for 24 h, the resulting contact angle was independent of the weight percent of additive within the matrix or the molecular weight of the PEG matrix employed and was

within experimental error of pure PEG (30° ( 5°).25 Thus, the PEG shell effectively served to compatibilize the PS-Au core from the surrounding PEG matrix. After the films were annealed at 90 °C for 24 h, significant changes in the contact angle data were observed. The changes were dependent upon the wt % of additive present. At 90 °C, cleavage of the DA adduct is favored and the decreased viscosity allows migration to occur. Such an increase in the contact angle indicates a reduction of surface energy most likely caused by an increase in the presence of PS at the surface of the samples. Based on the contact angle data only, it is unclear if this change in contact angle results from the migration of the PS-functionalized nanoparticles, or the PS ligand alone. Interestingly, after a period of time (ranging from 30 s to 3 min depending upon the wt % of additive), the contact angle decreases until the water droplet is absorbed into the PEG matrix.26 The process of surface rearrangement is discussed in further detail below. Analysis of RBS data from the same samples provides conclusive evidence of the migration of the Au particles. RBS is a technique that indirectly measures the composition of a surface to a depth of approximately 2 µm. A beam of ions, typically He+, is targeted toward a surface. As the ions collide with the different atomic nuclei present in the film, some ions are absorbed while others backscatter. The energy of the backscattered ions is dependent upon the both the depth and the mass of the atomic nuclei. For example, if elements of high atomic mass, such as Au, are present as the surface of a film, the backscattered energy will be very large; as the atomic mass of the element from which the backscattered ions decreases, the backscattered energy is reduced. In addition, as the nuclei are placed deeper within the film relative to the surface, the measured backscattered energy will decrease. In the data presented here, channels 450-200, 260-100, 150-0, and 100-0 represent the distribution of Au, Si, O, and C atoms, respectively, from the surface of the film to approximately 2 µm in depth. Because of overlapping signals, (25) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 1071410721. (26) We would like to point out that the observed occurrence is not a “stickslip” phenomenon that is due to surface roughness of the film. The interfacial area between the water droplet and film surface remains contact, while the volume of the droplet decreases due to absorption into the polymer film.

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Figure 4. SAXS data for olylamine-modified Au nanoparticles in solution (O), and PEG-matrix samples containing 4 wt % of 1-DA3-Au additive. For a 2000 g/mol matrix, data are shown for a 60 °C anneal (blue b) and a 90 °C anneal (blue O). For the 5000 g/mol matrix, data are shown for a 60 °C anneal (red 9) and a 90 °C anneal (red 0). Note: Data have been offset for clarity. Additionally, not all data points are shown.

Figure 3. RBS data for films comprised of (A) 2k; (B) 5k PEG containing 4 wt % of 1-DA-3-Au additive after annealing at room temperature (solid line), 60 °C (dotted line), and 90 °C (dashed line). Expanded view of channels 475-375 has been inset to clearly view the increase in Au concentration at the surface of the films after annealing at 90 °C.

RBS is most effective at detecting elements of large atomic mass at the surface of a film. As indicated by the results in Figure 3, little to no Au is present at the surface of the film, which indicates a random distribution of Au particles within the matrix. A more pronounced signal in the highest channels after annealing for 24 h at 60 °C indicates that some migration of Au nanoparticles to the surface of the film occurred, and much stronger evidence for migration is observed when the films are annealed for the same time at 90 °C. At 60 °C, the formation of the DA adduct is favored, but exists in equilibrium with the back reaction. When the DA adduct is severed, polymer 3 is able to diffuse away into the PEG matrix. The high bulk viscosity at 60 °C retards both polymer 3 and particle migration, allowing minor phase separation. The reduced viscosity present at 90 °C reduces the likelihood of reformation of the DA adduct and also facilitates the migration of the Au

particles, resulting in a much greater presence of Au at the surface of the material. SAXS analysis of the samples also indicates a change in the morphology of the films. SAXS data for gold particles dispersed in 2000 g/mol PEG and gold particles dispersed in 5000 g/mol PEG are presented in Figure 4. The SAXS data collected for gold nanoparticles modified with oleylamine and dispersed in toluene are also presented for comparison. Annealing either sample at 60 °C for 24 h results in a morphology in which the particles are not only not scattering independently, but may be giving rise to Bragg-like diffraction peaks. In particular, the 2000 g/mol matrix material shows two clear peaks, one at q ) 0.040 Å-1, and one at q ) 0.1 (Å-1). The relationship between the features is not readily apparent, but fitting form factor scattering model data to the experimental data clearly shows the higher-angle reflection is not a higher-order reflection from the Au particle form factor. The 5000 g/mol matrix material does not show a second order reflection, but the primary peak is virtually identical to that for the 2000 g/mol matrix sample. It may be possible that the minor phase separation that occurs at 60 °C induces longrange order, giving rise to Bragg-like diffraction peaks. The additional ordering within the 2000 g/mol matrix is attributed to the increased mobility, which allows for a more ordered structure to form. Cross-sectional transmission electron microscopy would provide insight into the bulk morphology; however, the poor mechanical properties of low molecular weight PEG matrixes inhibited the preparation of samples via cryo-microtoming, regardless of the temperature employed (-135 °C to room temperature). SAXS analysis after annealing the films for 24 h at 90 °C shows a clear change in bulk morphology. Not only does any evidence of higher-order Bragg reflections disappear, but the shape of the primary peak changes to that which one would expect from dispersed scattering centers with minor interparticle scattering effects,27 that is, the formation of large aggregates without long-range order. It is unknown if all functionalized particles migrate to the surface. It is most probable that migration occurs to both the sample/air and the sample/ substrate interfaces, as the main driving force is the exclusion of the PS-functionalized particles from the PEG matrix. Ad(27) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-ray and Neutron Scattering; New York, 1987.

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Figure 6. Contact angle measurement data for films comprised of 2000 g/mol (b) and 5000 g/mol (9) PEG containing 8 wt % of 1-DA-3-Au additive after annealing at 90 °C.

Figure 5. RBS data for films comprised of 5000 g/mol PEG containing 5 wt % of (A) PEG-Au; (B) PEG-b-PS-Au after annealing at 60 °C (O) and 90 °C (b), demonstrating the uniform distribution of Au throughout the film. Note: Free R-mercapto-ω-methoxy PSb-PEG block copolymer creates an S signal at channel 250 in Figure 3B.

ditionally, it may be possible that large aggregates form within the film; however, longer annealing times would drive toward complete phase separation and will be discussed in further detail below. Ultra-SAXS (USAXS) experiments were performed to investigate this possibility, and while no indication of aggregation was observed, strong multiple-scattering effects were evident, clouding interpretation of the USAXS data. Control experiments were conducted to verify that the temperature-sensitive Diels-Alder linkage is required for the observed particle migration effect. First, 5 wt % of R-mercaptoω-furyl PS (polymer 3) was dispersed within a 5000 g/mol PEG matrix. Contact angle measurements on the corresponding films after annealing at 60 and 90 °C resulted in identical results of 58° ( 2°, indicating that for both annealing temperatures, the

Figure 7. RBS data for films comprised of 5k PEG containing 8 wt % of 1-DA-3-Au additive after annealing at 90 °C for 0 h (O), 6 h (2), 16 h (9), and 24 h (b), displaying an increase in the concentration of Au at the surface of the films.

PS phase separates to the air interface to reduce surface energy, resulting in a larger contact angle measurement than that of pure PEG. Next, Au nanoparticles were functionalized with two different mercapto-terminated polymers, R-mercapto-ω-methoxy PEG (Mn 5000 g/mol; PDI < 1.05) and a R-mercapto-ω-methoxy PS-bPEG (Mn 40 000 g/mol; PDI 1.22; 88 wt % styrene), respectively. The functionalized Au nanoparticles were dispersed at 5 wt % in a 5000 g/mol PEG matrix. Contact angle measurements of the films after annealing at 60 and 90 °C produced results (28° ( 6°) that are nearly identical to those obtained for pure PEG (29° ( 4°). In Figure 5A, RBS experiments show no evidence of migration of Au to the surface, indicating that the PS is needed to drive the Au particles to the surface. Similarly, in Figure 5B, no migration of Au to the surface after annealing at 60 and 90 °C is seen, indicating that the thermally reversible DA linkage is required to expose the PS to the PEG matrix. Observation of a

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Figure 8. Pictographs following the absorption of a H2O droplet into a film comprised of 5k PEG containing 4 wt % of 1-DA-3-Au additive that had been annealed at 90 °C for 24 h. Note that lines have been added to denote the location of the edges of H2O droplet.

peak corresponding to sulfur nuclei (near channel 250) is due to free R-mercapto-ω-methoxy PS-b-PEG block copolymer. These control experiments indicate three concepts. First, PS will phase separate to the air interface when dispersed within a PEG matrix to reduce surface energy. Modifying PEG with polymer 3 clearly resulted in a rapid change in surface energy as the PS segregated to the sample surfaces. Second, PS is needed to induce migration of the Au particles. As the RBS data show, without PS, the gold particles do not accumulate at the surface after annealing. Third, the DA linkage between the blocks is necessary to expose the PS-Au core to the PEG matrix. Modifying gold particles with the R-mercapto-ω-methoxy PS-b-PEG block copolymer also does not result in any change in surface morphology with annealing. Having observed the temperature-induced surface segregation of gold particles modified with the Diels-Alder block copolymer, experiments were conducted to explore methods of further controlling the migration process. By changing the molecular weight of the PEG matrix, the speed at which the particles migrate to the surface can be altered. Annealing the 2000 g/mol PEG films containing 1-DA-3 Au additive for 24 h resulted in a layer of Au that is approximately 35 nm thick, whereas annealing 5000 g/mol PEG films containing 1-DA-3 Au additive resulted in a layer of Au approximately 15 nm thick.28 The lower molecular weight matrix provides fewer chain entanglements and lower bulk viscosity, thus allowing greater mobility of the functionalized nanoparticle additive through the matrix. To further demonstrate this point, an annealing time study was conducted to determine the rate at which functionalized particles migrated through the PEG matrix. Figure 6 displays contact angle measurements for films of 2000 and 5000 g/mol PEG containing 8 wt % 1-DA-3 Au additive after annealing at 90 °C for various lengths of time. Contact angle measurements after annealing at 60 °C are displayed at 0 h. In this case, a slightly higher loading of additive was used to (28) The thickness of the Au layer at the surface was calculated using the area under the curve, known scattering coefficients, and the RUMP program. See the Supporting Information for more details.

Figure 9. Contact angle measurement data for films comprised of 2000 and 5000 g/mol PEG containing 4 wt % of 1-DA-3-Au additive after annealing at 90 °C. Key: (blue O, dotted line), 2000 g/mol PEG, 6 h anneal; (blue b, dotted line), 2000 g/mol PEG, 72 h anneal; (red 0, dashed line), 5000 g/mol, 72 h anneal; (red 9, dashed line), 5000 g/mol, 72 h anneal.

shorten the overall experiment duration. As previously described, after annealing at 60 °C, initial contact angle measurements were higher than those for pure PEG, indicating some migration of functionalized Au particles to the surface. Although a lower loading of additive would have minimized this effect, a lower loading would also require a longer annealing time to see significant changes in the concentration of Au within the films. At first inspection, increasing the anneal time results in an increase in the contact angle and thus a decrease in surface energy, regardless of the molecular weight of PEG matrix. This observation is consistent with previous results and indicates that additional hydrophobically functionalized Au particles have migrated to the film surface. RBS analysis of the 5k film containing 8 wt % 1-DA-3 Au additive at various times also shows the increased concentration of Au at the surface with longer annealing time, Figure 7.

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Figure 10. Schematic of particle location before and after absorption of a H2O droplet of films annealed for different lengths of time. Black circles represent PS-functionalized Au nanoparticles.

Further inspection of Figure 6 allows one to qualitatively understand how particle migration rates are dependent upon Mn of the matrix. Knowing that bulk viscosity scales with Mn, it is expected that increasing the Mn of the matrix will increase bulk viscosity and slow particle migration. This effect is clearly observed in Figure 6, where samples comprised of 2000 g/mol PEG matrix exhibited higher contact angles than samples comprised of 5000 g/mol PEG matrix for the same wt % and annealing time. Furthermore, a maximum contact angle of 106° ( 4° is reached after 48 h anneal time when a 2000 g/mol PEG matrix is utilized, whereas 144 h is required to reach a similar contact angle when a 5000 g/mol matrix is employed. While investigating how different sample properties such as the matrix molecular weight or annealing procedure affected particle migration, an unusual rearrangement process was noted. During contact angle measurements, it was observed that the water droplet began to solvate the PEG matrix and was ultimately absorbed into the film. It was observed that the water droplet did not spread across the film, but was instead absorbed into the film, with the location of the edges of the droplet remaining constant while the resulting contact angle decreased. Interestingly, the rate of absorption seemed to depend on multiple parameters including the weight fraction of additive, molecular weight of the PEG matrix, and annealing temperature and time. Figure 8 shows several images following the absorption of the H2O droplet over a period of 7 min into a film of 5000 g/mol PEG containing 4 wt % additive that had been annealed at 90 °C for 24 h. One possible explanation for the dependence of this absorption rate on annealing time is the following: As the functionalized Au particles migrate to the surface, at some point a continuous

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film of PS will form. It is reasonable to assume that the homogeneity, and certainly thickness, of this film will be dependent upon the molecular weight of the PS and the number of functionalized Au particles at the surface. As the annealing time increases, more functionalized Au particles migrate to the surface, forming a semi-continuous surface layer until a minimum concentration of Au nanoparticles is able to form a continuous film. The further migration of Au particles to the film surface will thicken this layer, affecting the rate at which a water droplet can be absorbed into the bulk PEG matrix. When a droplet of water is added to determine the contact angle, any exposed PEG will begin to absorb the droplet and rearrange with the noncontinuous PS film. As the molecular weight of the PEG matrix decreases, polymer chain mobility increases, resulting in facile penetration through the discontinuous PS film. Any factor that enhances the concentration or number of Au particles and corresponding PS ligand at the film surface would increase the absorption time for the water droplet. To investigate this hypothesis, contact angle measurements were recorded over 7 min for the absorption of a water droplet into films composed of 2000 and 5000 g/mol PEG containing 4 wt % of 1-DA-3 Au additive that were annealed at 90 °C for 6 and 72 h. These data are presented in Figure 9. Regardless of the annealing time, films comprised of 2000 g/mol PEG exhibited a larger total change in the contact angle (45° and 36° for 6 and 72 h, respectively) than was observed for films comprised of 5000 g/mol PEG (27° and 20° at 6 and 72 h, respectively). The change in the contact angle is a measure of the rapidity with which the water droplet is absorbed, and thus is an indicator of the mobility of the matrix. Larger changes indicate higher chain mobility, and the data confirm the idea that the 2000 g/mol PEG matrix can more easily rearrange with the noncontinuous PS film than can the 5000 g/mol PEG matrix. A more interesting trend is observed when one compares the change in contact angle for films annealed for different lengths of time when a H2O droplet is allowed to sit on the film surface until absorbed. Films annealed for 6 h exhibited a larger change in the contact angle (45° and 27° for 2000 and 5000 g/mol PEG, respectively) than films of the same PEG matrixes when annealed for 72 h (36° and 20° for 2000 and 5000 g/mol PEG, respectively), indicating that a factor besides the molecular weight of the matrix has an effect on the absorption process. From RBS data, it is clear that longer anneal times result in more Au particle migration to the surface. Therefore, the concentration of functionalized Au particles is directly related to the rearrangement process. As the water droplet begins to solvate the PEG matrix, the PS functionalized Au particles will act as immovable posts and create

Figure 11. Three-dimensional atomic force microscopy images of films comprised of 2000 g/mol PEG containing 4 wt % of 1-DA-3-Au additive after annealing at 90 °C for 6 h (A) and 72 h (B).

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a tortuous path for the polymer chains to navigate. Increasing the concentration of PS-functionalized Au particles will create a more tortuous path and decrease the mobility of the PEG matrix and reduce the surface rearrangement process, Figure 10. In an attempt to support this hypothesis, atomic force microscopy (AFM) was employed to explore the surface morphology. Figure 11 displays typical three-dimensonal AFM images of films comprised of 2000 g/mol PEG containing 4 wt % of 1-DA-3 Au additive that were annealed at 90 °C for 6 and 72 h. Analysis of the film annealed for 6 h yielded a surface roughness of 367 nm with a maximum height of 2.14 µm, whereas the film annealed for 72 h had a surface roughness of 137 nm with a maximum height of 847 nm. Typically, an increase in surface roughness will increase the surface energy of the film and raise the corresponding contact angle measurement; however, these films exhibited the same contact angle regardless of the surface roughness. (See the first data point in Figure 9.) The main difference between these films is the rate at which a H2O droplet is absorbed into the film. Longer annealing times increase the concentration of PS-functionalized Au nanoparticles at the surface of the film, as supported by RBS analysis (Figure 7), and result in a more continuous film of PS, which decreases the surface roughness. The continuous PS surface impedes the penetration of H2O into the underlying PEG matrix and retards the surface rearrangement process. Future work involves use of higher molecular weight PEG matrixes to develop films with improved mechanical properties. Additionally, the use of fluorinated PS is being developed to increase the blooming rate of the additive within the matrixes.

Conclusion Poly(styrene) and poly(ethylene glycol) polymers with maleimido and furyl chain-end functionality were prepared in high

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yield, and then assembled using Diels-Alder reactions. Subsequently, Au nanoparticles were synthesized, functionalized with Diels-Alder assembled PS-b-PEG copolymers, and homogeneously dispersed within a PEG matrix. Thermal treatment of the films was found to cleave the diblock copolymer, rendering the PS-functionalized Au nanoparticles immiscible with the PEG matrixes. This caused migration of the PS-functionalized Au nanoparticles to the film surfaces as indicated by contact angle measurements, RBS, and control experiments. SAXS data indicate that the migration of the Au nanoparticles effectively destroys the weak long-range order observed prior to separation of the Diels-Alder bond. The mobility of the particles was found to be limited by the molecular weight of the PEG matrix. Longer annealing times resulted in migration of greater numbers of Au particles to the surface of the films. An interesting rearrangement process was discovered, where absorption of water into the film was dependent upon both the molecular weight of the PEG matrix and the concentration of additive at the surface of the film. Acknowledgment. Funding was provided by the Army Research Laboratory (ARL). This research was also supported in part by an appointment to the Research Participation Program at the U.S. Army Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARL. We thank Professor Timothy Patten for donating R-mercaptoω-methoxy PEG and R-mercapto-ω-methoxy PS-b-PEG polymer samples. Mr. Kris Stokes (MIT) and Dr. Josh Orlicki (ARL) are acknowledged for helpful discussions. Supporting Information Available: Experimental conditions and techniques. This material is available free of charge via the Internet at http://pubs.acs.org. LA0624541