Thermally Reversible Aggregation of Gold Nanoparticles in PolymerNanocomposites through Hydrogen BondingKyuyoung Heo, Caroline Miesch, Todd Emrick,* and Ryan C. Hayward*

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States

*S Supporting Information

ABSTRACT: The ability to tune the state of dispersion or aggregationof nanoparticles within polymer-based nanocomposites, throughvariations in the chemical and physical interactions with the polymermatrix, is desirable for the design of materials with switchable properties.In this study, we introduce a simple and effective means of reversiblycontrolling the association state of nanoparticles based on the thermalsensitivity of hydrogen bonds between the nanoparticle ligands and thematrix. Strong hydrogen bonding interactions provide excellentdispersion of gold nanoparticles functionalized with poly(styrene-r-2-vinylpyridine) [P(S-r-2VP)] ligands in a poly(styrene-r-4-vinyl phenol) [P(S-r-4VPh)] matrix. However, annealing at highertemperatures diminishes the strength of these hydrogen bonds, driving the nanoparticles to aggregate. This behavior is largelyreversible upon annealing at reduced temperature with redispersion occurring on a time-scale of ∼30 min for samples annealed50 °C above the glass transition temperature of the matrix. Using ultraviolet−visible absorption spectroscopy (UV−vis) andtransmission electron microscopy (TEM), we have established the reversibility of aggregation and redispersion through multiplecycles of heating and cooling.

KEYWORDS: Thermally switchable aggregation-dispersion, polymer nanocomposites, gold nanoparticles,poly(styrene-r-2-vinylpyridine), poly(styrene-r-4-vinyl phenol), hydrogen bond

Controlling the state of dispersion or aggregation ofinorganic nanoparticles within polymer matrices is a

critical factor in efforts to tune the mechanical, thermal,electrical, or optical properties of polymer nanocomposites.1−3

Most commonly, dispersion of nanoparticles is achieved bypromoting favorable interactions (i.e., a nearly zero, or negative,Flory−Huggins interaction parameter) between the matrixpolymer and the nanoparticle surface, or ligand, function-ality.4−8 For the case of nanoparticles with polymer ligands,dispersion in polymer matrices is also highly dependent on theratio of molecular weights of ligand and matrix polymers, alongwith the graft density of ligands.7−9 In other cases, however, theability to drive controlled aggregation or percolation ofnanoparticles is of interest for kinetically quenching thedemixing of polymer blends,10 providing electrical conductiv-ity,11,12 or tailoring properties of sensors13 and catalysts.14,15

Furthermore, strategies have been explored to selectivelyincorporate nanoparticles into specific domains, or to segregateto domain interfaces, in block copolymers16,17 and polymerblends.18

The possibility to reversibly modulate the dispersion,aggregation, and segregation of nanoparticles within nano-composites opens new possibilities for the design of switchablematerials. As recently reviewed,19 a wide variety of approacheshave been developed to achieve reversible aggregation anddispersion of nanoparticle assemblies in solvated environments.By contrast, work on switchable control of nanoparticledispersion in polymer nanocomposites has been more

limited,20−22 at least in part due to the challenges associatedwith achieving initial dispersion of nanoparticles in highmolecular weight polymer matrices.One promising approach to switchable organization of

nanoparticles in polymer nanocomposites is through hydro-gen-bonding interactions, which are straightforward toincorporate into many material systems, and yield highlyreversible supramolecular assemblies23 with inherent sensitivitytoward external stimuli including temperature24 and pH.25

Indeed, hydrogen bonding between particles and polymermatrices is an effective strategy to obtain dispersion of fillerssuch as carbon black, silica, and carbon nanotubes inpolyamides and polyesters26,27 and has been employed todrive self-assembly28 and selective placement21,29 of nano-particles in block copolymers, as well as hydration-sensitiveinteractions between cellulose fillers and polymer matrices.30−32

In one example, gold nanoparticles were functionalized withmixtures of dodecane-thiol and 11-mercapto-1-undecanolligands to facilitate dispersion in poly(styrene-b-4-vinylpyr-idine) (PS-b-P4VP) diblock copolymers.21 Annealing atelevated temperatures was found to drive redistribution ofparticles within the microdomains, although this transition wasnot reversible and may have been caused in part by theaccompanying coarsening of particles that occurred due to the

Received: July 28, 2013Revised: September 14, 2013Published: October 28, 2013

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low molecular weight of the ligands. Reversible changes in thelocation of CdSe nanoparticles within block copolymermicrodomains have been achieved using the temperaturedependence of hydrogen bonding interactions between PS-b-P4VP block copolymers and 3-n-pentadecylphenol (PDP).22 Inthis case, nanoparticles were shown to reversibly migrate fromthe P4VP microdomains to the PS/P4VP interfaces uponbreaking of the P4VP-PDP associations due to the affinitybetween the alkyl nanoparticle ligands and PDP. However,despite the essential role played by hydrogen bonds in manypolymer nanocomposite systems, their behavior in drivingthermally triggered reversible aggregation and dispersion ofnanoparticles in polymer matrices remains largely unexplored.In the present study, we demonstrate that hydrogen bonding

interactions between random copolymer ligands on nano-particle surfaces and complementary random copolymermatrices provide a simple means to achieve reversible thermaltransitions between aggregated and dispersed states of goldnanoparticles. As shown in Scheme 1, we use copolymers based

on PS, but containing 2-vinylpyridine (2VP) and 4-vinylphenol(4VPh) comonomers in the ligands and matrix, respectively,providing a hydrogen bonding pair that has previously beenused to disperse particles in block copolymer microdomains,17

and to form reversible supramolecular gels.33 The favorableinteractions between matrix and ligand polymers are sufficientto overcome entropic effects that should otherwise yieldparticle aggregation (due to the lower molecular weight of theligands), thus leading to good dispersion at low temperatures.However, above ∼150 °C, breaking of hydrogen bonds leads toaggregation of nanoparticles, a reversible process over multipleheating/cooling cycles.Results and Discussion. Gold nanoparticles with average

diameters of 4.5 ± 0.4 nm (as measured by TEM, SupportingInformation Figure S1), functionalized with poly(styrene-r-2-vinylpyridine) ligands [P(S-r-2VP), Mn = 2.8 kg/mol, 2-vinylpyridine content = 64 mol %, PDI = 1.07, and graft density∼ 1.7 chains/nm2 from thermogravimetric analysis (TGA)],were used to form nanocomposites by mixing with apoly(styrene-r-4-vinylphenol) matrix [P(S-r-4VPh), Mn = 24.4kg/mol, PDI = 1.09, 4-vinylphenol content =5.3 mol %, Tg =

103 °C]. Upon casting from chloroform at room temperature(RT), the nanoparticles dispersed cleanly in the polymermatrix, as shown by the transmission electron microscopy(TEM) images in Figure 1a for a loading of 4.1 vol % particles,and in Supporting Information Figure S2 for a loading of 14.2vol %. Such clean dispersion is due to the presence of hydrogenbonding interactions between the phenol and pyridine groups.As the strengths of hydrogen bonding interactions are well-

known to diminish with increasing temperature, we expect thenanoparticles to aggregate upon heating. Indeed, as shown inFigure 1b for the sample with 4.1 vol % particles, annealing at200 °C for 24 h yielded clear aggregation of particles into sub-100 nm sized clusters. While a distribution of aggregate sizescan clearly be seen, the level of aggregation shown in theimages in Figure 1 was found to be fairly uniform across allareas of the samples. In addition to the breaking of hydrogenbonds, which may raise the effective interaction parameterbetween matrix and ligand chains χeff to above zero, we notethat the nanoparticles also experience entropic driving forcesfor aggregation, since dewetting of the much higher molecularweight matrix chains from the short ligand brushes isanticipated.7

Notably, this aggregation is reversible, with nanoparticlesregaining a similar level of dispersion as in the as-cast filmsupon annealing the nanocomposites at 120 °C (Figure 1c).This temperature was chosen as it is sufficiently above the glasstransition temperature of the matrix to allow nanoparticlemobility, but low enough that hydrogen bonding interactionscan be maintained. While TEM provided clear evidence forreversal aggregation and dispersion of particles, ultraviolet−visible absorption spectroscopy (UV−vis) was employed toquantify the extent of aggregation through four heating/coolingcycles (Figure 1g,h). For each cycle, the sample was firstannealed at 200 °C for 24 h and subsequently annealed at 120°C for 24 h. Aggregation of nanoparticles leads to a slight redshift of the peak absorbance wavelength (λmax) due to thesurface plasmon resonance, and a broadening of the peak, ascharacterized by the full-width at half-maximum (FWHM).While a small increase in both λmax and FWHM can be seenbetween the as-cast sample and the first cycle of annealing at120 °C (which apparently arises from a small amount ofcoarsening of nanoparticles as shown in Supporting Informa-tion Figure S4), subsequent cycles of annealing at 200 and 120°C yield nearly complete recovery of the optical properties,suggesting a high degree of reversibility for the redispersion.Interestingly, the spectra after annealing at 200 °C are slightlydifferent from cycle to cycle, suggesting a greater degree ofstructural variability in the aggregated state. As shown inSupporting Information Figure S3, we also compared the valuesof λmax and FWHM determined from UV−vis to the averageaggregate size from TEM, for all samples studied here exceptthe samples annealed in PS matrices, where aggregation was tooextensive to allow for unambiguous measurement of aggregatesize. Both optical characteristics were found to be wellcorrelated with, and to show a nearly linear dependence on,aggregate size.To clearly establish the role of hydrogen bonding

interactions in the observed behavior, we performed controlexperiments using a similar molecular weight polystyrenehomopolymer matrix (PS, Mn = 25.0 kg/mol, PDI = 1.04). TheTEM image in Figure 1d reveals significant nanoparticleaggregation in this case, even for the as-cast sample. Withouthydrogen bonds, we expect a substantial enthalpic driving force

Scheme 1. Schematic Illustration of Thermally ReversibleAggregation and Dispersion of P(S-r-2VP)-FunctionalizedGold Nanoparticles in P(S-r-4VPh) Based on HydrogenBonding between Pyridine and Phenol Groups

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for aggregation due to the immiscibility of the P2VP-richligands and the PS matrix as well as the entropic driving forcedue to the much lower molecular weight of the ligands. As seenin Figure 1e,f, heating to 200 °C leads to large and highlyanisotropic aggregates that do not redisperse upon annealing at120 °C. Similarly, UV−vis reveals values of λmax and FWHMthat only increase with each annealing step before reachingplateau values (Figure 1h and Supporting Information FigureS5), consistent with irreversible aggregation. These aggregateshave a branched structure, generally several particles in width,suggesting a process of diffusion-limited aggregation.34,35

We next consider the Fourier transform infrared spectrosco-py (FTIR) data presented in Figure 2, which verify the role ofhydrogen bonding in driving reversible aggregation andredispersion. Three characteristic IR bands were monitored:free hydroxyl stretching around 3540 cm−1,36 pyridine flexion at1597 cm−1,37 and pyridine stretching at 993 cm−1.37 Thehydroxyl stretching region is broad (from 3000 to 3600 cm−1),but with a clear peak at 3540 cm−1 assigned to free hydroxyls

from non-hydrogen bonded 4VPh units (Figure 2a). Thisfeature grows in relative intensity between the as-cast sampleand that annealed at 200 °C but then returns to its initialintensity upon annealing at 120 °C, consistent with breakingand reforming of hydrogen bonds. Similarly, the peak at 1597cm−1 due to pyridine ring flexion was observed only in thesample at 200 °C (Figure 2b). This peak is known to shift toslightly higher wavenumber upon hydrogen bonding with aphenol, and in principle could allow for quantitative measure-ments of the fraction of free pyridine groups.37,38 Here,however, this analysis is complicated due to the overlap of thispeak in the hydrogen bonded state with the CC stretch ofthe aromatic rings in PS and P(S-r-4VPh), which appearsaround 1600 cm−1. Finally, in Figure 2c the pyridine stretch at993 cm−1 increases substantially in intensity for the 200 °Csample relative to the other two, indicating a greater proportionof free pyridines. Thus, all three characteristic peaks show clearqualitative changes consistent with the breaking of pyridine-phenol hydrogen bonds at high temperature, and their

Figure 1. (a−c) TEM images showing thermally reversible nanoparticle aggregation and dispersion of a sample containing 4.1 vol % of P(S-r-2VP)-functionalized gold nanoparticles in P(S-r-4VPh): (a) as-cast samples show good dispersion, while (b) annealing at 200 °C for 24 h leads toaggregation, and (c) subsequent annealing at 120 °C for 24 h leads to redispersion. (d−f) Corresponding TEM images showing irreversibleaggregation for a sample containing 4.1 vol % of nanoparticles in PS homopolymer: (d) as-cast samples show significant aggregation due to theabsence of hydrogen bond donors in the matrix, while (e) annealing at 200 °C for 24 h leads to extensive aggregation that is not reversible upon (f)subsequent annealing at 120 °C for 24 h. Scale bars are 50 nm in all images. (g) UV−vis spectra of 4.1 vol % of P(S-r-2VP)-functionalized goldnanoparticles in P(S-r-4VPh) during four cycles of annealing; RT, as-cast sample; solid lines, following annealing at 200 °C for 24 h; dashed lines,following annealing at 120 °C for 24 h. (h) The full-width at half-maximum values from the spectra in (g) indicate excellent reversibility for the P(S-r-2VP) matrix and progressive aggregation for the PS matrix.

Figure 2. FTIR spectra of 4.1 vol % of P(S-r-2VP)-functionalized gold nanoparticles in P(S-r-4VPh): (a) free hydroxyl stretching around 3540 cm−1;(b) pyridinic flexion at 1597 cm−1; (c) pyridine stretching at 993 cm−1. Samples were annealed at 200 °C for 24 h, and where indicated, subsequentlyannealed at 120 °C for 24 h. Changes in each region after annealing at 200 °C reveal a substantial decrease in the number of hydrogen bonds, whilethe close match between the as-cast (RT) and 120 °C spectra indicates essentially full recovery upon annealing at 120 °C.

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essentially complete re-establishment upon subsequent anneal-ing at reduced temperature.The temperature dependence of nanoparticle aggregation

over the range of 120−200 °C can be assessed from the UV−vis and TEM data presented in Figure 3. For a sample heateddirectly to 120 °C and annealed, λmax does not changecompared to its value in the as-cast state. Even at an annealingtemperature of 150 °C, however, a clear shift in both quantitiesis observed, and TEM images reveal the presence of smallaggregates (Figure 3c). Annealing at 190 °C yields largeraggregates (Figure 3d) and further redshifting and broadeningof the plasmon peak (Figure 3a,b), although the extent ofaggregation is less than at 200 °C. This behavior suggests aprogressive clustering of particles, rather than a sharp transition

to a dense liquid-like or crystalline phase of nanoparticles.39−41

The finding that particles aggregate over this temperature rangeis consistent with a prior study showing that hydrogen bondingbetween 4-vinylpyridine and phenol groups is largely disruptedby ∼190 °C.24 We may expect slightly weaker interactions inthe current system due to the poorer steric accessibility ofhydrogen bond accepting sites in 2-vinylpyridine,42 however,the possibility for multiple hydrogen bonds to form betweenligand and matrix chains may provide enhanced stability againstaggregation.We next consider the kinetics of nanoparticle redispersion, as

characterized by the evolution of the FWHM of the plasmonpeak as a function of annealing time. For samples prepared with4.1 vol % nanoparticles (Figure 4a and Supporting InformationFigure S6a), an annealing temperature of 120 °C yields anapparent equilibrium level of dispersion within ∼1400 min,while a further increase to 150 °C (50 °C above the matrix Tg)yields an equivalent level of redispersion within ∼30 min. Usingliterature data for polystyrene melt viscosity,43,44 a simpleestimate of the nanoparticle diffusion coefficient based on theStokes−Einstein equation yields ∼0.1 and 30 nm2/s, atrespective temperatures of 120 to 150 °C. These valueswould correspond to characteristic time-scales of 300 and 1 minto equalize particle concentration over the typical distancesbetween clusters (∼100 nm). While not in close agreementwith the experimental time-scales, they are at least of similarmagnitudes. Interestingly, however, samples prepared with 14.2vol % nanoparticles take substantially longer to reach maximumrecovery (Figure 4b and Supporting Information Figure S6b),∼ 3000 min at 120 °C and ∼600 min at 150 °C, which alsoreveals a much weaker dependence of redispersion time-scaleon temperature than for the lower particle loading, suggestingthat the redispersion kinetics are more complex than a simplediffusion-limited process. We note that polymer chains bridgingbetween neighboring particles,41,45 and/or local increases inmatrix Tg in the vicinity of the nanoparticles1 are possiblemechanisms for slowing redispersion kinetics. Notably, inaddition to a higher density of particles, the change from 4.1 to14.2 vol % also corresponds to a change in the totalstoichiometry of hydrogen bond donors to acceptors from1.9:1 to 0.5:1. The relative scarcity of donors in the latter casemay lead to stronger interparticle interactions due to thepossibility for bridging of chains between nanoparticles and

Figure 3. Temperature dependence of the aggregation behavior ofsamples containing 4.1 vol % of P(S-r-2VP)-functionalized goldnanoparticles in P(S-r-4VPh). Hydrogen bonds weaken above 150 °C,causing (a) the peak absorption wavelength to shift from 527 to 537nm−1 and (b) the full-width at half-maximum to increase as annealingtemperature increases. (c,d) TEM images of nanocomposite filmsannealed at 150 and 190 °C, respectively, also show progressivelygreater aggregation (scale bars: 20 nm). All films were annealed for 24h.

Figure 4. Redispersion kinetics of nanoparticles in P(S-r-4VPh) matrices upon annealing at 120 and 150 °C following an initial anneal for 24 h at200 °C, as monitored by the full-width at half-maximum values from UV−vis spectra, with particle loadings of (a) 4.1 vol %; (b) 14.2 vol %. Thehigher annealing temperature yields significantly faster redispersion kinetics.

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may also make the effective interaction parameter betweenmatrix and ligand chains less favorable.In addition to slower kinetics, samples containing 14.2 vol %

nanoparticles also exhibit only partially reversible dispersion onexperimental time-scales, as shown in Figure 5. Compared tosamples with 4.1 vol % loading, the more highly loaded samplesshowed larger aggregates after annealing at 200 °C (Figure 5b).While UV−vis measurements (Figure 5a) and TEM images(Figure 5c) do reveal substantial redispersion upon annealing at150 °C, after 3 cycles of heating and cooling very littleredispersion was obtained. Whether this represents trulyirreversible aggregation, or simply a kinetic limitation remainsunder study. However, we anticipate that further optimizationof the concentrations of hydrogen bonding donors andacceptors, as well as potentially the matrix and ligand molecularweights, will allow for fully reversible redispersion even at highparticle loadings.Conclusions. We have shown that reversible aggregation

and dispersion of gold nanoparticles in a polymer matrix can bedriven via the temperature sensitivity of hydrogen bondsbetween random copolymer ligands and matrix chains bearingcomplementary functionalities. These interactions lead toexcellent dispersion of nanoparticles at low temperatures,despite a much greater molecular weight of the matrixpolymers, and good reversibility over multiple cycles. Thekinetics of redispersion are highly dependent on bothnanoparticle loading and annealing temperature, but the timescale can be reduced dramatically under appropriate conditions.This strategy of controlling aggregation and dispersion byhydrogen bonding is a simple and powerful method that weexpect can be extended to other inorganic/polymer hybridmaterial systems, and may lead to new applications ofswitchable polymer nanocomposites.

■ ASSOCIATED CONTENT

*S Supporting InformationMaterials and methods, determination of nanoparticle size andgraft density, dispersion of samples containing 14.2 vol %nanoparticles, coarsening of nanoparticles after annealing, andadditional UV−vis results. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*(R.C.H.) E-mail: rhayward@mail.pse.umass.edu. Tel: 413-577-1317. Fax: 413-545-0082.*(T.E.) E-mail: tsemrick@mail.pse.umass.edu. Tel: 413-577-1613. Fax: 413-545-0082.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported as part of Polymer-Based Materialsfor Harvesting Solar Energy, an Energy Frontier ResearchCenter funded by the U.S. Department of Energy, Office ofScience, Basic Energy Sciences under Award #DE-SC0001087(assembly studies) and the National Science Foundationthrough Grant CHE-1152360 and the Center for HierarchicalManufacturing CMMI-1025020 (nanoparticle and ligandsyntheses). The work also made use of facilities supported bythe National Science Foundation MRSEC at UMass (DMR-0820506). We thank Jimmy Lawrence for preparation of theRAFT agent.

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Figure 5. Reversibility of 14.2 vol % of P(S-r-2VP)-functionalized goldnanoparticles in P(S-r-4VPh): (a) the peak absorption wavelength andfull-width at half-maximum values from UV−vis spectra during heatingand cooling cycles, here, samples were annealed at 200 °C for 24 h andsubsequently annealed at 150 °C for 24 h; (b,c) TEM images ofnanocomposite films annealed at 200 °C for 24 h and subsequentannealing at 150 °C for 12 h, respectively (scale scale bars: 20 nm).

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