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July 04, 2015

C 2015 American Chemical Society

Film-Stabilizing Attributes ofPolymeric Core�Shell NanoparticlesXiao-Jing Cai,† Hao-Miao Yuan,†,^ Anton Blencowe,‡,§ Greg G. Qiao,‡ Jan Genzer,*,† and

Richard J. Spontak*,†, )

†Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States, ‡Department ofChemical & Biomolecular Engineering, University of Melbourne, Parkville, Victoria 3010, Australia, §Mawson Institute, Division of ITEE, The University of SouthAustralia, Mawson Lakes, South Australia 5095, Australia, and )Department of Materials Science & Engineering, North Carolina State University, Raleigh,North Carolina 27695-7907, United States. ^Present address: Department of Polymer Science & Engineering, University of Massachusetts, Amherst, MA 01003.

Afundamentally interesting and tech-nologically enticing aspect of con-temporary nanoscience is the devel-

opment of novel structures that form dueto nanoparticle (NP) self-assembly, spanmultiple length scales and exhibit uniquemacroscopic properties. Glotzer, Kotov andco-workers1�5 are indisputable pioneers inthis field by repeatedly demonstrating thatinorganic nanoscale colloids can be used toconstruct macroscale wires, ribbons andsheets, as well as a variety of unique assem-blies, by understanding and applying thecoupled effects of surface interactions (whichcan be altered through the addition ofstabilizers), composition and shape. Of parti-cular interest here are their free-floatingNP sheets,2,3 which constitute examples of

extended assemblies by mimicking surface-layer proteins. In their study of amino-stabilized CdTe NPs measuring 3.4 nm indiameter, thedriving forcepromotingmono-layer formation via self-assembly reliesmainly on a combination of different inter-actions, such as a dipolemoment and hydro-phobic attraction.2 We return to this excitingobservation later in the context of syntheti-cally derived organic NPs. Unlike their in-organic counterparts, organic NPs consistprimarily of polymeric building blocks andare therefore larger, broadly ranging from10 nm to 1 μm across. Because they containmolecular chains that remain semiflexible atthese dimensions,6 such organic NPs can beeffectively designed as nanospheres or nano-capsules. Polymeric nanospheres possess a

* Address correspondence toJan_Genzer@ncsu.edu,Rich_Spontak@ncsu.edu.

Received for review January 12, 2015and accepted July 4, 2015.

Published online10.1021/acsnano.5b00237

ABSTRACT Self-organization of nanoparticles into stable, molecularly thin films provides an

insightful paradigm for manipulating the manner in which materials interact at nanoscale dimensions

to generate unique material assemblies at macroscopic length scales. While prior studies in this vein

have focused largely on examining the performance of inorganic or organic/inorganic hybrid

nanoparticles (NPs), the present work examines the stabilizing attributes of fully organic core�shellmicrogel (CSMG) NPs composed of a cross-linked poly(ethylene glycol dimethacrylate) (PEGDMA) core

and a shell of densely grafted, but relatively short-chain, polystyrene (PS) arms. Although PS

homopolymer thin films measuring from a few to many nanometers in thickness, depending on the

molecular weight, typically dewet rapidly from silica supports at elevated temperatures, spin-coated

CSMG NP films measuring as thin as 10 nm remain stable under identical conditions for at least 72 h.

Through the use of self-assembled monolayers (SAMs) to alter the surface of a flat silica-based support, we demonstrate that such stabilization is not

attributable to hydrogen bonding between the acrylic core and silica. We also document that thin NP films consisting of three or less layers (10 nm) and

deposited onto SAMs can be fully dissolved even after extensive thermal treatment, whereas slightly thicker films (40 nm) on Si wafer become only

partially soluble during solvent rinsing with and without sonication. Taken together, these observations indicate that the present CSMG NP films are

stabilized primarily by multidirectional penetration of relatively short, unentangled NP arms caused by NP layering, rather than by chain entanglement

as in linear homopolymer thin films. This nanoscale “velcro”-like mechanism permits such NP films, unlike their homopolymer counterparts of

comparable chain length and thickness, to remain intact as stable, free-floating sheets on water, and thus provides a viable alternative to ultrathin

organic coating strategies.

KEYWORDS: microgel particles . polymer thin film . nanolaminate . film stabilization . polymer coating . dewetting

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solid matrix, in which case their properties are chieflyregulated through surface modification.7�10 Alterna-tively, nanocapsules contain an empty, yet potentiallyfillable, core that is surrounded by a solid organicshell.11�13

In general, polymeric NPs are prepared by eitherphysical or chemical means and play increasinglyimportant roles in, for example, sensory,14 electronic,15

photonic,16 medical,17,18 and environmental19,20 appli-cations. Physical methods frequently used to generatepreformed polymeric NPs21 include solvent evapora-tion,22 salting-out,23 precipitation,24 dialysis25 andsupercritical fluid deposition.26 The chemical route bywhich to produce such NPs employs various syntheticapproaches, such as emulsion (including mini- andmicro-),27�29 interfacial30 and controlled/living radicalpolymerizations.31,32 This strategy can also be used togenerate hybrid NPs possessing an inorganic core andan organic shell33�36 or, ofmore direct relevance to thepresent study, microgel NPs wherein the chains arechemically cross-linked and, depending on the chemi-cal constitution, responsive to external stimuli (e.g.,temperature, pH, salt, or applied electric field).37�44

These hybrid and microgel NP designs can be com-bined to yield fully organic core�shell microgel(CSMG) NPs (also designated45 as “core cross-linkedstar polymers”) with a chemically cross-linked A coreand surface-grafted B chains often referred46,47 to as“arms”, which can interact freely with their environ-ment. While Wooley and co-workers48�51 have estab-lished that the reverse arrangement (i.e., shell cross-linked NPs) can likewise be of tremendous interest andpractical use, we only consider the CSMG NP designfurther. More specifically, the CSMG NP investigatedhere has been synthesized via the “arm-first” route byatom transfer radical polymerization (ATRP)45,52 andpreviously introduced in a study of autophobicity-drivensurface segregation in polymer nanolaminates.53

Prior endeavors aimed at utilizing CSMG NPs tostabilize molecularly thin polymer films have reliedexclusively on dispersing the NPs into a polymer thinfilm before the molten film destabilizes at elevatedtemperatures. For stabilization to occur in this scenario,the NPs must diffuse through the polymer matrix to apolymer interface, where they can alter the viscosityand/or chemical compatibility. Examples of nanoscaleadditives that have been reported to serve this functioninclude fullerenes54�56 and gold nanoparticles,57,58 aswell as latex nanoparticles59 and dendrimers.60 Failureto achieve satisfactory thin-film stabilization can beattributed to an insufficient number of arms on theCSMG61 or mismatch between film destabilization andNPdiffusion time scales. To explore the latter possibility,we have investigated the stability of nanoscale filmscomposed entirely of CSMG NPs (i.e., they are notdispersed in a homopolymer matrix) and surprisinglydiscovered several unique features, reported herein.

RESULTS AND DISCUSSION

The PEGDMA-PS CSMG NPs examined here wereprepared via the arm-first approach and ATRP usinga PS macroinitiator. The weight-average molecularweight (Mw) of the CSMG NPs is 438 kDa and theirgyration diameter in tetrahydrofuran (THF) is 29 nm, asdetermined by size exclusion chromatography equippedwith multiangle laser light scattering (SEC-MALLS).Independent atomic force microscopy (AFM) analysis62

of these NPs individually deposited onto Si wafer revealsthat the cross-linked core is less than 3 nm in diameter, infavorable agreement with previous studies63 of chemi-cally similar CSMGNPs. The graft density of PS arms (σPS)is subsequently determined by dividing the number ofarms per NP (32) by the surface area of the core, yieldinga value of 1.13 arms/nm2 on the basis of a 3 nm corediameter. A smaller core would produce a correspond-ingly larger value of σPS. Since σPS > 1, we presume thatthe arms are sufficiently packed to form a polymerbrush.64 This result, coupled with the relatively lowmolecular weight of the PS arms (Mw = 10.7 kDa), mustbe borne inmind during the following comparison withthe linear PShomopolymer analog. Thinfilms composedof low-molecular-weight homopolymers become un-stable and typically dewet from a solid substrate attemperatures above the glass transition temperature(Tg).

65 The illustrations displayed in Figure 1 depict filmswith a thickness of 40 nm spin-coated onto silica: lin-ear PS homopolymer (Figure 1a) and the CSMG NP(Figure 1b). The molecular weight of the PS homopoly-mer (13.5 kDa) is similar to that of the PS armson theNPsand is below the critical molecular weight of entangle-ment for PS (Me,PS ≈ 18 kDa, according to Ferry66). Asevidenced by the optical image in Figure 1c, the PShomopolymer film clearly dewets via nucleation andgrowth (NG) after only 2min of annealing at 180 �C. Theholes apparent in this image enlarge with time at adewetting rate of 21 μm/min and ultimately impinge.

Figure 1. Schematic depictions of (a) PS and (b) CSMG NPthin films (thickness: 40 nm) spin-coated onto Si wafer. Themolecular weight of the PS homopolymer (13.5 kDa) iscomparable to that of the PS arms on the NPs (10.7 kDa).Optical images acquired from the corresponding thin filmsannealed at 180 �C are displayed for (c) 2 min and (d) 72 h.An AFMheight image is included for the annealed CSMGNPthin film in (d).

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In sharp contrast, the thin film composed of CSMGNPs remains stable under identical conditions even after72 h at 180 �C (cf. Figure 1d). The AFM image includedin the inset of Figure 1d further demonstrates that thefilm topography remains featureless even at nanoscopiclength scales. Values of the root-mean-square (rms)roughness measured from the film surface before andafter heat treatment are 0.94 and 0.49 nm, respectively,thereby confirming that such NP-based thin films arenot affected by temperature in the same fashion as PShomopolymer thin films. In general, the stability of thinpolymer coatings can be improved by increasing (i) thetime scale associated with polymer chain mobility, (ii)the distance through which a rupture proceeds, and (iii)the physicochemical affinity between the coating andsubstrate. The first approach is achieved readily byincreasing the polymer molecular weight, which pro-motes a corresponding increase in melt viscosity.67 Inthis spirit, we have also examined two grades of PShomopolymer with molecular weights above (900 kDa)and below (216 kDa) the total molecular weight of theCSMG NPs (438 kDa). As anticipated, exposure of thesehigh-molecular-weight PS films, spin-coated to a thick-ness of 10nmon silica, to the sameannealing conditionsused to generate Figure 1d reveals that they do notdisplay evidence of dewetting (cf. the SupportingInformation) and are therefore deemed stable. Thesecond strategy relates to the coating thickness, sincea reduction in film thickness promotes a marked in-crease in dewetting rate (and possibly a transition to adifferent dewetting mechanism).65,68,69 We explore theimportance of this design consideration in a later sec-tion. Lastly, the affinity between the coating and sub-strate can be a system-specific consideration or it canalternatively be engineered through the use of surfacetemplating.70,71 In the present case, the rapid dewettingobserved in Figure 1c indicates that low-molecular-weight PS possesses little affinity for silica. The CSMGNPs, however, consists of another chemical constituent,a cross-linked PEGDMA core, and the carbonyl group inacrylic polymers such as poly(methyl methacrylate)(PMMA) is known to undergo strong hydrogen bondingwith the surface-bound hydroxyl groups on silica.72

To discern if the PEGDMA core in the NPs is capable ofinteracting with the silica substrate through the PS armsand therefore responsible for the enhanced film stabilitydemonstrated in Figure 1d, we compare the stability ofPMMA and CSMG NP films, each with a thickness of10 nm, at 180 �C. Deposition of PMMAdirectly onto silica(illustrated in Figure 2a) results in a featureless, stablefilmeven after 72 h of annealing (cf. Figure 2d), thus verifyingthe expectation that hydrogen bonding between PMMAand the silica substrate serves to stabilize the thin films.72

The screening distance afforded by the existence of PSarms on the CSMG NPs is emulated here by modifyingthe silica surface with an n-octyltrichlorosilane (OTS) self-assembled monolayer (SAM) measuring 1.2 nm thickaccording to variable-angle spectroscopic ellipsometry(VASE) measurements. [The gyration diameter of anunperturbed PS arm is ≈6 nm, but the arms in a brushwith σPS ≈ 1.13 will be slightly extended.] Since OTS ishydrophobic (with a water contact angle, WCA, of 105�),PMMA films are first spin-coated onto glass, floated ondeionized water (DIW) and then transferred to the OTSSAM, as schematically depicted in Figure 2b. After 15minof annealing at 180 �C, the PMMA thinfilm shows signs ofdewetting by NG in Figure 2e. This result confirms that, ifthe carbonyl groups of PMMA are sufficiently distant(1.2 nm in this case) from the hydroxyl groups on silica,hydrogen bonding is thwarted and the thin film desta-bilizes in similar fashion as PS. If PMMA cannot anchorto the underlying silica substrate in the presence of theOTS monolayer, it is reasonable to expect that otheracrylates (such as PEGDMA) would likewise be unableto do so. For comparison, CSMGNP films of comparablethickness have been prepared in an identical fashion asthe PMMA films and likewise floated onto the OTS SAM(cf. Figure 2c). Surprisingly and unlike the PMMA films,the CSMG NP films are observed to remain stable after72 h of annealing at 180 �C (cf. Figure 2f). This findingunequivocally establishes that the remarkable stabilityof the organic NP coatings examined here cannotbe attributed to physical interactions between thePEGDMA NP core and the silica substrate.To ascertain if the stability of the 10 nm CSMG NP

thin films on the OTS SAM is due to the NP molecular

Figure 2. Illustrations of (a) a PMMA thin film spin-coated onto Si wafer, (b) a PMMA thin film floated onto anOTS SAM and (c)a CSMG NP thin film floated onto an OTS SAM. The molecular weight of the PMMA homopolymer (112 kDa) is comparable tothat of the PEGDMA core of each NP (96 kDa). In all cases, the film thickness is 10 nm, and each SAM extends 1.2 nm from thesurface of the silica layer located on top of a Si wafer. Optical images collected from the corresponding thin films annealed at180 �C are displayed for (d) 72 h, (e) 15 min, and (f) 72 h.

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weight, we have also examined the stability of 10 nmfilms of the two high-molecular-weight PS grades onOTS SAMs after exposure to the same annealing con-ditions used to generate Figure 2f. Unlike the stableresults observed for analogous PS films spin-coated onsilica substrates, the films deposited on OTS substratesappear fully dewetted, as evidenced by the imagesincluded in the Supporting Information. For compar-ison, the stability of 10 nm films of a high-molecular-weight poly(styrene-ran-methylmethacrylate) (PS-ran-PMMA) copolymer (558 kDa) on silica and OTS sub-strates has been explored after 72 h at 180 �C andyields similar results as the high-molecular-weight PShomopolymers: the films on silica remain stable,whereas the films on OTS fully dewet. These observa-tions unequivocally indicate that the CSMG NP filmspossess unique thermal stability attributes. In an effortto destabilize the CSMG NP thin films, the polarity ofthe silica substrate is modified through the deposi-tion of a N,N-dimethylaminopropyl trimethoxysilane(DMAPTMS) SAM with a thickness of 0.8 nm accordingto VASE. Because DMAPTMS is moderately hydrophilic(WCA ≈ 60�), thin films measuring 10 nm thick of theCSMG NPs and PS homopolymer (13.5 kDa) can bespin-coated directly from toluene solutions on the SAM,as illustrated in Figure 3, panels a and b, respectively.Optical images of the two systems collected at 180 �Creveal that the NP-based film again remains stable afterbeing annealed for 72 h (Figure 3c), while the PS filmstarts todewet after just 15 s (Figure 3d). Taken together,the results presented in Figures 2 and 3 demonstratethat the stability of the CSMGNP films is not sensitive tosubstrate chemistry/polarity or film thickness over therange of thicknesses examined. With these two effectsdismissed, the only reason left to explain the unusualstability of CSMG NP films pertains to impeded molec-ularmobility. Since the PS armson theNPs are belowMe,PS and since the linear PS homopolymer of comparablemolecular weight readily destabilizes under the condi-tions investigated here, impeded flow in the case of theCSMG NP films does not appear to be a result of chainentanglement as in conventional homopolymer thin

films. Rather, we hypothesize that the dense brushesof PS arms on neighboring NPs interpenetrate in alldirections to immobilize the NPs and thus maintain theintegrity of thin NP films.Although the OTS SAM employed here appears to

be sufficiently thick to minimize, if not eliminatealtogether, hydrogen bonding between the CSMG NPfilm deposited on top and the silica substrate below,the possibility that the CSMG NPs are somehow ad-ditionally stabilized by mixing with the OTS moleculescannot be discounted. This situation is particularlyimportant when the film thickness approaches mono-layer coverage (according to an independent AFManalysis,62 a monolayer would measure ≈3 nm thick).To ascertain if interlayermixing occurs, we have floateda NP layer measuring 10 nm thick on top of theOTS SAM, dried the bilayered construct overnight atambient temperature, annealed the bilayer for 72 h at180 �C, and finally subjected the system to a toluenerinse. The results of this sequence of events are dis-played in Figure 4 and reveal two important outcomes.First, the thicknesses of the two layers (1.2 nm from theOTS monolayer and 10 nm from the CSMG NP film) areadditive (to 11.2 nm) after the initial deposition of the toplayer, as well as after the extended heat treatment,verifying that they do not mix to a discernible extent.Second, the NPs are not chemically cross-linked sincethey readily redissolve in toluene, leavingbehind only theOTS SAM at its original thickness. These observations areconsistent with our proposed hypothesis that the NPsremain stable as thin films due to a physical processinvolvingmultidirectional interpenetration of the denselygrafted PS arms comprising the shell of the CSMG NPs.The data shown in Figures 2�4 have been collected

from CSMGNP thin films that measure 10 nm thick andthus approximate a trilayer. Recalling from Figure 4 thatsuch films deposited onto SAMs can be redissolvedin toluene even after prolonged high-temperaturetreatment, we now return to explore the dissolutioncharacteristics of a thicker (40 nm) film spin-coated

Figure 3. Schematic diagrams of (a) CSMG NP and (b) PSthin films (10 nm) spin-coated on DMAPTMS SAMs measur-ing 0.8 nm thick. Corresponding optical images of the thinfilms annealed at 180 �C aredisplayed for (c) 72 h and (d) 15 s.

Figure 4. Ellipsometric thickness measurements and asso-ciated illustrations of OTS SAMs (thickness: 1.2 nm)with andwithout a CSMG NP film (thickness: 10 nm). The sequence(from left to right) portrays the SAM, the thin film floatedon the SAM, the thin film on the SAM after 72 h at 180 �C,and the remaining SAM after the thin film is subsequentlyredissolved in toluene.

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directly onto Si wafer. According to the VASE resultsdisplayed in Figure 5a, the thickness of the film de-creases slightly (to 38 nm) after annealing at 180 �C for24 h, suggesting that the film densifies as the NPs packmore efficiently. This observation may be related tothermal jamming,73 a process by which polymer chainssurface-grafted to solid inorganic nanoparticles seek tointerpenetrate to fill loosely packed interstitial space.[We discount here the likelihood of densification dueto solvent evaporation on the basis of toluene diffusionconsiderations74,75 through glassy PS films measuring40 nm in thickness.] Rinsing this annealed film withtoluene promotes a further reduction to 30 nm,which�interestingly�corresponds to a loss of the top 8 nm(nearly a trilayer of NPs). This result is likewise attainedwhen the rinsing time (with intermittent agitation) isextended to 24 h, suggesting that the film is remarkablyrobust and may be an excellent candidate for coatingapplications. Despite the addition of significantacoustic energy, sonication applied for 2 min duringrinsing has little effect on film thickness beyond rinsingalone, whereas sonication conducted for 60 min intoluene removes all but the bottom 5 nm (a little morethan an expected monolayer) of the CSMG NP film.One noticeable change in the films after sonication is

the introduction of discrete holes, which are discussedin more detail below. [Due to the presence of holes,however, the refractive index of each film must becorrected in the film thickness calculation included inFigure 5. This correction is provided in the SupportingInformation.] Similar responses of the NP films tosolvent exposure are likewise observed at other filmthicknesses, as evidenced by the results presented inFigure 5b. The corresponding surface topologies of theCSMG NP films after deposition, annealing and rinsingin Figure 5a are visible in the AFM height images pro-vided in Figure 6, panels a, b, and c, respectively. Whilethe low-magnification images show little difference, thehigh-magnification images reveal that the NPs initiallyform sharply defined features (including individual NPsmeasuring 15.7 ( 1.4 nm in diameter) that comprisehills and valleys varying from �2 to þ2 nm in height(with an rms roughness of 0.94 nm) in Figure 6a. Thesefeatures are much less pronounced after annealing inFigure 6b (the rms roughness decreases to 0.49 nm),but they are clearly recovered after the toluene rinse inFigure 6c (the rms roughness increases to 0.87 nm). The

Figure 5. In (a), ellipsometric thickness measurements ofa CSMG NP film spin-coated to 40 nm onto Si wafer afterdifferent stages (labeled): deposited (at ambient tempera-ture), annealed (for 24 h at 180 �C), rinsed (quiescently intoluene), and sonicated (also in toluene). The error barsrepresent the standard error in the data. In (b), remainingfilm thickness as a function of initial CSMGNP film thicknessafter annealing (b), rinsing (O) and sonication (4). Thedashed line corresponds to as-deposited films (where theinitial and remaining thicknesses are equal), and the solidline is a linear regression to the data obtained by rinsing.The ratio of their slopes reveals that the annealed filmthickness after rinsing is consistently about 73% of theinitial thickness.

Figure 6. AFM height images acquired from the CSMG NPfilms discussed in relation to Figure 5a: (a) as-depositedfrom toluene (thickness: 40 nm), (b) annealed at 180 �C for24 h (thickness: 38 nm), and (c) rinsed in toluene (thickness:30 nm). Enlargements (shade-coded as dark at�2.0 nm andlight as þ2.0 nm) are included in each part to facilitateexamination of individual NPs.

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accompanying NP diameters measured from imagessuch as these are 13.6 ( 1.0 and 16.7 ( 1.2 nm, res-pectively. It must be remembered that, due to theerosion evident in Figure 5a, the film showcased inFigure 6c is thinner than that in Figure 6a,b.As mentioned earlier, the use of sonication to pro-

mote NP dissolution from 40 nm films during solventrinsing produces discrete holes, which are visiblethroughout the AFM image displayed in Figure 7a.These holes range from 200 nm to 2 μm in diameter.According to the height profile shown in Figure 7b(which corresponds to the horizontal trace seen inFigure 7a), at least some of the holes are capable ofextending all the way through the entire film thickness,which is just under 30 nm according to Figure 5a. Thus,even though the thickness of the film subjected to soni-cation is not very different from that without sonication,the added energy successfully excavates additional NPsas ill-defined columnar pores extending through thefilm.Included in Figure 7 are AFM height images acquiredfrom the two identified regions in Figure 7a illustratingno macroscopic features (Figure 7c) and a single hole(Figure 7d). The image presented in Figure 7c closelyresembles those obtained from the film after initialdeposition (Figure 6a) and solvent rinsing (Figure 6c),and possesses a comparable rms roughness (0.87 nm).In marked contrast, the hole evident in Figure 7d is verydifferent in shape and topology from that generated byclassical NG dewetting. [Holes generated by NG aretypically circular and possess a well-defined rim.] Closeexamination of the enlargement of the noncircular,rimless hole in Figure 7e reveals the uneven packing

of individual NPs due to erosion along the periphery.Analysis of the corresponding thresholded image, gen-erated with the ImageJ software, yields the size andshape metrics listed in the caption of Figure 7.Previous studies reported by Tang et al.2 have

established that free-floating monolayer films of inor-ganic NPs measuring a mere 3.4 nm thick can beachieved by carefully exploiting the various forces in-volved. In the case of organic NPs, an equally importantconsideration in the production of free-floating homo-polymer films is the critical molecular weight of entan-glement, sincephysical chain entanglements are neededto prevent the surface energy ofwater frombreaking thefilm apart. Since the entanglement molecular weight oflinear PS is ≈18 kDa,66 a stable, free-floating thin filmcannot be prepared from PS homopolymer possessinga molecular weight of 13.5 kDa. This is not the case,however, for the CSMGNPs composed of PS armswhosemolecular weight is

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droplet on the NP film immediately induces a well-defined hole, as evidenced by the photograph includedin Figure 8c and the corresponding thresholded imageprovided in the inset. This result is consistent with thefindings associated with Figure 4 and implies that asmall number of multilayers may not possess sufficientmultidirectional armpenetration to remain stable in thepresence of an applied liquid (even if it is not a solvent).

CONCLUSIONS

In this study, we have explored the stabilizationattributes of CSMG NP thin films alone and in conjunc-tion with surface-modifying SAMs deposited on silicasubstrates. Annealing PS homopolymer of approxi-mately the same chain length as the PS arms comprisingthe shell of the NPs above Tg results in rapid destabiliza-tion byNGdewetting. Contrariwise, equivalent CSMGNPfilms remain stable, displaying no evidence of anydewetting mechanism, for more than 72 h under iden-tical experimental conditions. By introducing a SAMbetween these films and the silica substrate, we unequi-vocally establish that hydrogen bonding between theacrylic core of the NPs and silica is not responsible forthe observed thin-film stability. Moreover, by examiningthe thicknesses of NP films and SAMs before and afterdeposition/dissolution of the films, we discount penetra-tion of the PS arms into the SAM as the origin of suchremarkable stability. Relatively thin NP films containingabout 3 or less layers and deposited onto SAMs can befully redissolved even after long-term thermal treatmentat 180 �C. Thicker films spin-coated onto Si wafers,however, become only partially soluble under similarconditions, suggesting that the relatively short PS armsinterpenetrate from multiple directions within the filmsto lock-in the positions of the NPs and, in so doing,prevent dissolution even when sonicated. Sonicationdoes introduce holes, which differ substantially in theircharacteristics from holes generated by NG dewetting,at locations where NPs are vertically excavated from thefilms. Due to the mechanism by which these NP thinfilms remain stable, they are able to form free-floatingsheets onwater at thicknesses of only 10 nm. Since theseCSMG NP films do not depend on chain entanglementfor their stability, they provide an attractive and viablealternative to organic thin-film coating technologies.

METHODSMaterials. Styrene monomer (S, >99% pure), ethylene glycol

dimethacrylate (EGDMA, >98% pure), tetrahydrofuran (THF),hydroquinone (HQ), monomethyl ether hydroquinone (MEHQ),1-bromoethylbenzene, copper bromide (CuBr), N,N,N0 ,N00 ,N00-pentamethyldiethylenetriamine (PMDETA), methanol, anhydrousanisole, toluene, and N,N-dimethylaminopropyl trimethoxysilane(DMAPTMS) were all purchased fromSigma-Aldrich (St. Louis,MO).TheSmonomerwaspurified throughbasic alumina twice,whereasthe EGDMA monomer was purified through a sinter bed contain-ing HQ and MEHQ on top of basic alumina. One low-molecular-weight polystyrene (PS) (Mw=13.5 kDa, polydispersity index, PDI, =1.06) and two high-molecular-weight PS homopolymers (Mw =216 kDa, PDI = 1.06, andMw = 900 kDa, PDI = 1.10) were obtainedfromPressure Chemical, Inc. (Pittsburgh, PA), whereas poly(methylmethacrylate) (PMMA) homopolymer (Mw = 112 kDa, PDI = 1.09)and a poly(styrene-ran-methyl methacrylate) (PS-ran-PMMA) co-polymer (Mw = 558 kDa, PDI = 1.83) were procured from PolymerSource, Inc. (Dorval, Canada). n-Octyltrichlorosilane (OTS) wasacquired from Gelest, Inc. (Morrisville, PA) and used as-received.Deionized water (resistivity >15 MΩ-cm) was produced using aMillipore water purification system.

Nanoparticle Synthesis and Characterization. The chemical protocolemployed for synthesizing poly(ethylene glycol dimethacrylate)core-polystyrene shell (PEGDMA-PS) CSMG NPs by the arm-first approach and ATRP was similar to procedures previouslyreported by some of us.77�79 Briefly, a linear PS macroinitiator(i.e., the “arm”) was first prepared via ATRP of styrene using1-bromoethylbenzene as the initiator and CuBr/PMDETA as thecatalyst complex. To generate CSMG NPs, the PS macroinitiator(Mw = 10.7 kDa, PDI = 1.10) was linked together using EGDMA as across-linker in the presence of CuBr/PMDETA as the catalystcomplex. The crude CSMGwas purified via fractional precipitationto remove unreacted macroinitiator. Analysis of the isolatedPEGDMA-PS CSMG NPs via SEC-MALLS yielded Mw = 438 kDaand PDI = 1.16, and allowed calculation of the number of armsper NP (32), as well as the molecular weight contributionof the PEGDMA microgel core (96 kDa). Full details describingthe synthesis of the CSMG NPs are provided in the SupportingInformation.

Film Preparation and Characterization. Thin films of both homo-polymers and the NPs were prepared at ambient temperatureby spin-coating solutions onto Si wafer with the surface silicalayer intact. The linear PS, linear PMMA and CSMG NPs wereindividually dissolved in toluene at concentrations ranging from

Figure 8. In (a), a cross-sectional illustration of a CSMG NPfilm (thickness: ca. 10 nm) spin-coated onto glass andfloated on the surface of DIW. An optical image confirmingthe stability of such a free-floating NP sheet is provided in(b), whereas the same film after a DIW droplet produces amacroscopic hole is included in (c). The inset in (c) shows thehole in a high-contrast grayscale image.

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0.2 to 2.0 wt %, and the rotation speed was varied from 800 to2000 rpm to obtain the desired film thicknesses. Self-assembledmonolayers (SAMs) of OTS and DMAPTMS were deposited ontoSi wafers according to an established procedure.80 Briefly, theSi wafers were cleaned via ultraviolet/ozone treatment andthen rinsed with toluene. After drying the surface with nitrogen,each wafer was suspended upside down on the center of the lidof a Petri dish over a solution of OTS (or DMAPTMS) andmineraloil for 10 min. Film and SAM thicknesses were measuredby VASE on a J.A. Woollam Co. instrument. Ellipsometric datawere collected at wavelengths ranging from 400 to 1100 nm in10 nm increments at incidence angles of 70� and 75� relative tothe surface normal for the polymer films and SAMs, respectively.Specimen thicknesses were calculated using a Cauchy layermodel with the following indices of refraction: 1.45 for theOTS SAM, 1.42 for the DMAPTMS SAM and 1.59 for all otherorganic compounds.81 Water contact angle experiments wereperformed at ambient temperature on a Ramé�Hart Model100-00 contact-angle goniometer equippedwith a CCD camera.Static WCAs were discerned by depositing a 4 μL droplet of DIWat three different positions on a surface and then averaging themeasurements. Images of polymer thin films undergoing de-stabilization by dewettingwere acquiredwith anOlympus BX60optical microscope equipped with a computer-interfacedCCD camera and operated in reflection mode. Stability studieswere performed at 180 �C in a Mettler-Toledo hotstage undera circulating nitrogen gas blanket. The surface topographyof NP thin films was examined in air with an Asylum ResearchAFM instrument operated in the alternating-current tappingmode.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. This studywas supportedatNorthCarolinaStateUniversity by theNational Science Foundation and theOfficeof Naval Research (Grant No. N000141210642). Funding by theAustralian Research Council was provided through a DiscoveryProject Grant (DP0986271).

Supporting Information Available: The Supporting Informa-tion provides additional information regarding the synthesis ofthe CSMGNPs, a detailed description of the ellipsometric analysis,and stability results obtained fromhigh-molecular-weight homo/copolymer thin films deposited on different substrates for furthercomparisonwithCSMGNP thinfilms. The Supporting Informationis available free of charge on the ACS Publications website atDOI: 10.1021/acsnano.5b00237.

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