Structural Characterization of Amphiphilic Homopolymer MicellesUsing Light Scattering, SANS, and Cryo-TEMJoseph P. Patterson,†,| Elizabeth G. Kelley,‡,| Ryan P. Murphy,‡ Adam O. Moughton,† Mathew P. Robin,†

Annhelen Lu,† Olivier Colombani,§ Christophe Chassenieux,§ David Cheung,†,⊥ Millicent O. Sullivan,‡

Thomas H. Epps, III,‡,* and Rachel K. O’Reilly†,*†Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, United Kingdom‡Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716,United States§Departement PCI, LUNAM Universite, Universite du Maine, IMMM UMR CNRS 6283, Avenue Olivier Messiaen, 72085 Le MansCedex 09, France

*S Supporting Information

ABSTRACT: We report the aqueous solution self-assembly of a series ofpoly(N-isopropylacrylamide) (PNIPAM) polymers end-functionalizedwith a hydrophobic sulfur−carbon−sulfur (SCS) pincer ligand. Althoughthe hydrophobic ligand accounted for <5 wt % of the overallhomopolymer mass, the polymers self-assembled into well-definedspherical micelles in aqueous solution, and these micelles are potentialprecursors to solution-assembled nanoreactors for small moleculecatalysis applications. The micelle structural details were investigatedusing light scattering, cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). Radial densityprofiles extracted from the cryo-TEM micrographs suggested that thePNIPAM chains formed a diffuse corona with a radially decreasing corona density profile and provided valuable a prioriinformation about the micelle structure for SANS data modeling. SANS analysis indicated a similar profile in which the coronasurrounded a small hydrophobic core containing the pincer ligand. The similarity between the SANS and cryo-TEM resultsdemonstrated that detailed information about the micelle density profile can be obtained directly from cryo-TEM andhighlighted the complementary use of scattering and cryo-TEM in the structural characterization of solution assemblies, such asthe SCS pincer-functionalized homopolymers described here.

■ INTRODUCTION

In recent decades, the solution self-assembly of amphiphilicblock copolymers (BCP)s has attracted significant attentiongiven the utility of BCPs in a variety of applications, includingdrug and gene delivery systems,1,2 nanoreactors in separationscience,3 and nanoelectronics.4 The solution self-assembly ofanother class of amphiphiles, so-called “associative polymers” or“amphiphilic homopolymers”, also has been studied exten-sively.5−26 Examples of these systems include homopolymers inwhich the monomer units contain both hydrophilic andhydrophobic moieties6−8 or homopolymers end-functionalizedwith ionic head groups12 or small hydrophobic groups, such asalkyl chains.8,10,11,14,16,17,23−25 Like amphiphilic diblock ortriblock copolymers, these amphiphilic homopolymers havebeen shown to self-assemble into well-defined nanostructures,making them promising materials for applications thatnecessitate aqueous solution-assembly.10,22

Recent advances in controlled radical polymerizationmethods, such as reversible addition−fragmentation chaintransfer (RAFT), have facilitated the incorporation of hydro-phobic end-groups onto polymer chains.10,11,22 Incorporating

hydrophobic groups on RAFT chain transfer agents (CTA)s isa convenient means of synthesizing amphiphilic homopolymerswith controlled molecular weights, low dispersities (Đ), andhigh end-group fidelity.27,28 Furthermore, careful design ofhydrophobic end-groups not only facilitates self-assembly butalso incorporates reactive centers for potential catalysisapplications. There is significant interest in developing aqueousnanoreactors given their potential to improve catalyst efficiencyas well as simplify product and catalyst recovery.29−31 Werecently incorporated a sulfur−carbon−sulfur (SCS) pincerligand into a RAFT agent, allowing for simple access to avariety of polymer structures for catalysis.32 We showed that byusing aqueous nanoreactors formed from poly(acrylic acid)hompolymers end-functionalized with a Pd pincer end-group,we were able to create hydrophobic nanopockets to increasethe activity of a Pd catalyzed coupling reaction.32 However,these amphiphilic homopolymers formed a mixture of spherical

Received: April 11, 2013Revised: July 1, 2013Published: July 18, 2013

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and cylindrical micelles, making detailed analysis of thenanostructures (through small angle scattering and electronmicroscopy methods) difficult.Cryogenic transmission electron microscopy (cryo-TEM) is

a powerful tool for characterizing solution-assembled nano-structures and has provided unique insights into nanoscalemorphologies as well as self-assembly processes and phasetransition behavior.33−35 While cryo-TEM is used throughoutliterature to study solution assembled structures,33−35 fewerreports have demonstrated the utility of this technique foranalyzing the radial density distribution of polymeric nano-particles.36−38 The detailed density distribution of nanostruc-tures is investigated more often using small angle neutron or X-ray scattering (SANS or SAXS) experiments.36,39−42 However,as highlighted by Ballauff et al., both cryo-TEM and small anglescattering methods are sensitive to the local density distributionof the sample.36 By using both SAXS and cryo-TEMmicrograph analysis, Ballauff et al. showed that quantitativestructural information can be determined from cryo-TEMmicrographs of colloidal particles.36 Herein, we use a similarapproach and exploit both cryo-TEM micrograph analysis andSANS experiments to study the solution-assembled structure ofamphiphilic homopolymers.We report the preparation of a series of poly(N-

isopropylacrylamide) homopolymers functionalized with anSCS pincer ligand using RAFT and the detailed study of thesolution assembly behavior of these homopolymer amphiphilesusing a combination of scattering techniques and cryo-TEM.Importantly, these pincer-functionalized amphiphilic homopol-ymers are potential precursors to solution-assembled nano-reactors exhibiting catalytic activity. The current work focuseson characterizing the aqueous solution assembly of pincer-functionalized homopolymers with different molecular weights,as the ability to understand and tailor the self-assembly of thesematerials will allow for more detailed studies of the effects ofnanoreactor structure on catalytic properties.

■ EXPERIMENTAL SECTIONMaterials. All chemicals were used as received from Aldrich, Fluka,

or Acros unless otherwise stated. AIBN (azobis(isobutyronitrile)) wasrecrystallized twice from methanol43 and stored in the dark at 5 °C.SCS pincer CTA was synthesized as reported previously.32

General Procedure of RAFT Polymerization of N-Isopropylacry-lamide (NIPAM). SCS pincer CTA (0.050 g, 57 μmol), NIPAM (1.3 g,12 mmol), AIBN (2.8 mg, 17 μmol), and dimethylformamide (DMF)(2.7 mL) were added to a clean, dry ampule under N2 (g). Thesolution was degassed via three freeze−pump−thaw cycles and heatedto 65 °C for 5 h under N2 (g). The viscous crude reaction medium wasdissolved in the minimum amount of tetrahydrofuran (THF), and thepolymer was precipitated into diethyl ether and filtered. Theprecipitation process was repeated, and 1.35 g of yellow polymerwas recovered. Mn

NMR (21.3 kDa), MnGPC (20.2 kDa), Mw/Mn

GPC

(1.22). 1H NMR 400 MHz (CDCl3): δ (ppm) 7.19 (s, 1H, Ar−H),7.14 (s, 2H, Ar−H), 5.70−5.50 (br, NH, polymer), 5.05 (s, 2H, Ar−CH2O), 4.00 (s, br, N(CH3)2CH, 180H, polymer), 3.68 (s, 4H,SCH2−Ar), 3.33 (t, J = 7.6 Hz, 2H, SCSCH2C), 2.39 (t, J = 7.4 Hz,4H, CH2SCH2CH2), 1.2−2.5 (br, CH, CH2, CH3 polymer).General Procedure for End Group Removal. PNIPAM (1.1 g, 72

μmol), AIBN (5.0 mg, 29 μmol), 1-ethylpiperidine hypophosphite(EPHP) (0.065 g, 0.36 mmol), and toluene (ca. 10 mL) were added toa clean, dry ampule under N2 (g). The reaction vessel was degassed via5 freeze−pump−thaw cycles. The ampule was filled with N2 (g) andheated to ∼100 °C for 12 h. All volatiles were removed in vacuo, thewhite solid was dissolved in the minimum volume of THF, and thepolymer was precipitated into hexanes. Thus 0.79 g of white polymerwas recovered. For additional purification, the polymer was dialyzed

against deionized water. MnNMR (21.0 kDa), Mn

GPC (20.3 kDa), Mw/Mn

GPC (1.22). 1H NMR 400 MHz (CDCl3): δ (ppm) 7.19 (s, 1H, Ar-H), 7.14 (s, 2H, Ar-H), 5.70−5.50 (br, NH, polymer), 5.05 (s, 2H, Ar-CH2O), 4.00 (N(CH3)2CH, 180H, polymer), 3.68 (s, 4H, SCH2Ar),2.39 (t, J = 7.4 Hz, 4H, CH2SCH2CH2), 1.2−2.5 (br, CH, CH2, CH3polymer).

Polymer Characterization. Proton Nuclear Magnetic Reso-nance Spectroscopy (1H NMR). 1H NMR spectra were recorded on aBruker DPX-400 spectrometer in CDCl3. Chemical shifts are given inppm downfield from tetramethylsilane (TMS). The degree ofpolymerization (NPNIPAM) was determined by comparing theintegration of the end group peaks (δ 5.05, 3.68 and 3.33) to theCH peak (δ 4.00) of the polymer backbone. The number-averagemolecular weight from NMR was calculated according to Mn =(NPNIPAM × Mo + Mpincer), in which Mo is the repeat unit molecularweight of PNIPAM, and Mpincer is the end-group molecular weight.

Size Exclusion Chromatography (SEC). SEC measurements wereconducted on a system comprised of a Varian 390-LC-Multi detectorsuite fitted with differential refractive index (DRI) and ultraviolet(UV) detectors and equipped with a guard column (Varian PolymerLaboratories PLGel 5 μM, 50 × 7.5 mm) and two mixed D columns(Varian Polymer Laboratories PLGel 5 μM, 300 × 7.5 mm). Themobile phase was THF with 5 vol % triethylamine at a flow rate of 1.0mL min−1, and samples were calibrated against Varian PolymerLaboratories Easi-Vials linear poly(methyl methacrylate) standardsusing Cirrus v3.3 software.

Micelle Preparation. Micelle solutions were prepared by addingH2O to the dried polymer powder and stirring overnight. Theresulting solutions were filtered through a 0.45 μm nylon filter.

Micelle Characterization. Light Scattering (LS). Static lightscattering (SLS) and dynamic light scattering (DLS) measurementswere performed on a ALV CGS3 spectrometer operating at λ = 632.8nm. All LS data were collected at 25 °C.

SLS and DLS data were recorded simultaneously for each system.Measurements were made at 4 different concentrations ranging from1.0 to 10 mg mL−1 and 5 different angles (θ) ranging from 30° to140°. The scattering vector was defined as q = 4πn/λ[sin (θ/2)], inwhich n is the refractive index of the solvent. The scattered intensitywas measured over a period of 100 s to determine both the intensityauto correlation function, g2(t), from DLS and the mean scatteredintensity, I, from SLS. For DLS data, the measured g2(t) was related tothe electric field auto correlation function, g1(t), using the Siegertrelation.44 The resulting functions were analyzed using the REPESroutine45 assuming a continuous distribution of relaxation times,A(log(τ)), according to eq 1. The difference between the measuredand calculated baseline for the DLS correlation functions was less than0.1%.

∫ τ ττ

τ= −∞

⎜ ⎟⎛⎝

⎞⎠g t A

t(log( )) ( ) exp d[log ]1 0 (1)

For many of the samples, the resulting distribution of relaxationtimes was bimodal (Figure S2, Supporting Information). Both the fast(τfast) and slow (τslow) relaxation times were q2-dependent (Figure S3,Supporting Information), indicating that diffusive motions wereprobed. The apparent diffusion coefficient (D) was calculated usingthe relation Di = (q2τi)

−1, in which i denotes fast or slow. The diffusioncoefficient was extrapolated to zero concentration (Figure S4,Supporting Information) and was used to calculate the hydrodynamicradii (RH) for the fast relaxation mode according to the Stokes−Einstein equation.

For SLS experiments, both the slow and fast modes of relaxationcontributed to the total scattered intensity. The Rayleigh ratio for thefast mode (Rθ, fast) of relaxation was calculated according to eq 2

θ θθ θ

θ= =

−θ θR A R A

I I

IR( ) ( )

( ) ( )

( )fast fast fastsample solvent

referencereference,

(2)

in which Afast(θ) is the scattered intensity contribution from the fastmode of relaxation at a given scattering angle determined by DLS (eq

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1), Isample, Isolvent, and Ireference are the scattered intensities at angle θ, bythe sample, solvent, and reference liquid, respectively, and Rreference isthe Rayleigh ratio for the reference liquid. Toluene was used as thereference.Assuming the concentration of species contributing to the fast mode

of relaxation was equal to the polymer concentration in solution (i.e.,negligible concentration of larger species, see text for discussion), theweight-average molecular weight (Mw), second virial coefficient (A2),and radius of gyration (Rg) were estimated according to,

= + +θ

⎛⎝⎜⎜

⎞⎠⎟⎟Kc

R M

q RA

11

32 c

fast

g

, w

2 2

2(3)

in which c is the solution concentration and K is a constant, given byeq 4.

π

λ=K

n n c

N

4 ( d /d )ref2 2 2

4A (4)

In eq 4, nref is the refractive index of the reference liquid, dn/dc is therefractive index increment of the polymer, and NA is Avogadro’snumber. The dn/dc values were 0.12 (for each polymer) at 25 °C, asdetermined using a refractometer (Bischoff RI detector) with a laserwavelength of 532 nm.Kc/Rθ ,fast was independent of q2 (Figure S5, Supporting

Information), therefore Kc/Rθ,fast values were averaged for all anglesand plotted versus concentration to determine the Mw and A2 (FigureS6, Supporting Information). The reported error for all light scatteringresults was 10% of the average value.Cryogenic Transmission Electron Microscopy (Cryo-TEM). Micelle

solutions for cryo-TEM experiments were prepared at concentrationsranging from 2.0 to 5.0 mg mL−1. Samples for cryo-TEM wereprepared at 25 °C in a constant humidity environment using a FEI 110Vitrobot. A 2−10 μL droplet of micelle solution was applied to a holeycarbon-coated copper grid, and the grid was blotted to remove excesssolution. Subsequently, the sample was vitrified by plunging the gridinto liquid ethane. Grids were transferred to a Gatan cryo stage andimaged using a Tecnai G2 12 Twin TEM at an accelerating voltage of120 kV. The temperature of the cryo stage was maintained below−170 °C.The radial gray values, G(r), from the cryo-TEM micrographs were

determined in ImageJ,46 and the profiles were fit according to

ρ= − −G rG

K r R r( )

exp( 2 ( ) )corona0

2 2

(5)

in which G0 is the background gray value, K is a fitting constant,ρcorona(r) is the density distribution of polymer in the micelle corona, Ris the micelle radius, and r is the distance from the center of themicelle.36 The density distribution, ρcorona(r), was modeled as the linearcombination of 2 b splines.41,42 Additional details pertaining to the

cryo-TEM micrograph analysis are provided in the SupportingInformation.

Small Angle Neutron Scattering (SANS). Solutions for SANSexperiments were prepared at a concentration of 2.0 mg mL−1 bydirect dissolution of the polymer powder in D2O and then filteredusing a 0.2 μm nylon filter. All micelle solutions were dried todetermine the exact concentration after completion of the SANSexperiments.

SANS experiments were performed at the National Institute ofStandards and Technology (NIST), Center for Neutron Research(NCNR, Gaithersburg, MD) on the NG-7 30 m SANS beamline. Anincident wavelength of 6.0 Å with a wavelength spread (Δλ/λ) of 0.12was used with sample to detector distances of 1.0, 4.0, and 13.5 m toaccess a scattering vector (q) range of 0.004 Å−1 < q < 0.6 Å−1. Here,the scattering vector is defined as q = 4π/λ sin (θ/2), in which θ is thescattering angle. All measurements were performed at ambienttemperature (20 ± 1 °C). SANS data were reduced using standardprocedures provided by NIST,47 and background scattering from D2Owas subtracted from the data. SANS data were fit with a form factor forspherical micelles,39−42 and details of this model are provided in theSupporting Information. The reported errors for the SANS datamodeling results were due to the uncertainty in solutionconcentration.

The corona profiles obtained from the SANS data modeling wererescaled using eq 6

∫ πρ ν =r r r N4 ( ) dcorona agg corona2

(6)

in which ρcorona(r) is the rescaled corona profile and represents thevolume fraction of the corona chains, r is the distance from the centerof the micelles, Nagg is the aggregation number, and νcorona is thevolume of the PNIPAM chain.39,40 The micelle radius was defined asthe radius at which the volume fraction of PNIPAM in the coronaprofile was less than 0.02.40,41

■ RESULTS

The pincer CTA 2 was designed to facilitate end-functionaliza-tion of polymer chains with reactive centers for potentialcatalytic applications (see Scheme 1).32 As shown in Scheme 1,RAFT agents can be readily coupled to the primary alcohol onpincer ligand 1, allowing for the polymerization of a widevariety of monomers.32,48 In this work, we chose the RAFTagent S-dodecyl-S′-(α′,α′-dimethyl-α″-acetic acid) (DDMAT)for its ability to polymerize acrylamide monomers.27 RAFTpolymerization of NIPAM using CTA 2 produced a series ofend-functionalized PNIPAM homopolymers, 3, with controlledmolecular weights and molecular weight distributions. Thehydrophobic RAFT end group was removed from 3 to affordpolymers functionalized only at the α-end by the pincer ligand,

Scheme 1. Synthesis of SCS-Pincer Functionalized RAFT Agenta and End-Functionalized PNIPAM Polymersb

a(Top) Coupling of a pincer alcohol, 1, with DDMAT to form an SCS-pincer functionalized RAFT agent, 2. b(Bottom) Polymerization of NIPAMto form a telechelic polymer, 3, followed by RAFT end-group removal to generate a monofunctionalized homopolymer, 4.

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4.28 The polymers examined in this study are summarized inTable 1.

Peaks associated with the hydrophobic pincer end-groupwere seen clearly in the 1H NMR spectra (at δ = 3.7, 1.2, and0.8 ppm, relative to TMS) in chloroform, a good solvent forboth the end-group and the polymer (Figure 1). However in

D2O, a selective solvent for PNIPAM, the end-group peakswere attenuated. The attenuation of these peaks indicated thatthe end-group was confined to a less mobile environment andimplied that the polymers self-assembled in D2O.The aqueous solution assembly of these polymers was

investigated further by DLS and SLS. All scattering experimentswere performed at concentrations much greater than the criticalmicelle concentration (CMC ≈ 10−2 mg mL−1); therefore theconcentration of unimers was negligible (see Figure S1,Supporting Information). At all concentrations, the DLS resultsshowed a major population of small micelles with hydro-dynamic radii of 10−20 nm, and a minor population of largeraggregates, RH ∼ 200 nm (Figure S2 inset, (SupportingInformation). The RH values for the micelles (Table 2) werecalculated according to the Stokes−Einstein equation, using thediffusion coefficient for the fast mode of relaxation extrapolatedto zero concentration. As expected, the micelle radii increased

with PNIPAM molecular weight.49−52 The larger aggregateswere not easily removed by filtration, changes in concentration,or by a variety of different preparation methods. However, theweight concentration of these large aggregates was negligiblegiven that the scattered light intensity is proportional to theparticle molecular weight and has a power law dependence onparticle size (where the exponent is related to the shape of thescatterers).53 Similar populations of large aggregates werereported for poly(ethylene oxide) (PEO) end-capped withdodecyl groups and were identified as spurious aggregates.53

To account for the presence of the larger aggregates in theSLS experiments, DLS and SLS data were collectedsimultaneously.53−55 The relative scattered intensity contribu-tions from the micelles (fast mode of relaxation) and largerspurious aggregates (slow mode of relaxation) were determinedfrom the DLS data, which allowed for the calculation of theRayleigh ratio for the micelles only (Rθ,fast) (see ExperimentalSection for additional details). Because the micelle Rg was lessthan 20 nm, Kc/Rθ,fast was independent of q (Figure S5,Supporting Information), and an accurate Rg could not bedetermined using light scattering methods. However, the Kc/Rθ,fast values were averaged across the different q-values andplotted versus concentration to determine the micellemolecular weight (Mw) and second virial coefficient (A2)(Figure S6, Supporting Information). The micelle molecularweights were used to determine the aggregation numberaccording to Nagg = Mw,micelle/Mw,polymer (Table 2). Theaggregation number decreased with increasing PNIPAMmolecular weight, consistent with previous experimentalliterature and scaling theories for block copolymer assem-blies.49−52 Values for MwA2 ranged from −9.5 × 10−4 to +9.3 ×10−4 mL g−1, which were several orders of magnitude smallerthan typical values for polymeric micelles (∼100 to 102 mLg−1).56−58 Interpreting MwA2 values for self-assembled systemsoften is complicated.58,59 Here, the small MwA2 values likelysuggest minimal interactions between the micelles.The solution assembly of sample 4c also was investigated

using cryo-TEM. While polymeric coronas often are not visiblein cryo-TEM due to their hydrated nature; the directvisualization of micelle coronas (including PNIPAM) hasbeen reported previously.36,38,56,60−62 As seen in Figure 2, cryo-TEM further supported that the amphiphilic homopolymerformed spherical micelles in aqueous solutions. Moreover,assuming mass−thickness contrast dominates (i.e., smallcontribution from the phase contrast),36,63 the gray scaleprofile extracted from cryo-TEM micrographs should be relatedto the electron density profile of the micelles. The profile wasfit assuming a radially decreasing density profile that wasmodeled as a linear combination of 2b splines. This functionalform for the corona profile gave a good fit to the gray scaleprofile and suggested that the micelle radius was ∼20 nm,which was consistent with the light scattering results.To further investigate the structural profile of the micelles,

SANS experiments were performed on micelle solutionsprepared in D2O. The SANS data were fit with a form factormodel for spherical polymer micelles with a homogeneous coreand radially decreasing corona density profile, also modeled as alinear combination of 2b splines.39,40,42,64 Accounting for thecorona density profile resulted in good fits to the SANS data,shown in Figure 3a. A slight upturn in the low-q data for sample4a deviated from the model fit, suggesting there may beaggregates in solution as possibly indicated by the lightscattering measurements. Similarly, Winnik et al. reported an

Table 1. Polymer Characterization Data

SampleMn

a

(kDa)Mn

b

(kDa)Mw

c

(kDa) NPNIPAMd Đa whydrophobic

e

4a 11.8 14.2 15.9 120 1.12 0.044b 20.3 21.0 25.4 180 1.21 0.034c 23.8 31.2 38.6 270 1.24 0.02

aFrom SEC based on poly(methyl methacrylate) standards. bOn thebasis of end-group analysis from 1H NMR. cCalculated from 1H NMRand SEC according to Mw = (NPNIPAMMo + Mpincer) × Đ. dDegree ofpolymerization of PNIPAM block from 1H NMR end-group analysis.eHydrophobic weight fraction calculated from 1H NMR.

Figure 1. 1H NMR spectra of 4b in CDCl3 and D2O, in which boxedregions highlight the peaks associated with hydrophobic pincer end-group. The end-group peaks were attenuated in D2O due to polymerself-assembly.

Table 2. Summary of Micelle Characterization Data

Nagg

sampleRH (nm)DLS

R (nm)SANS

Rg,corona (nm)SANS SLS SANS

4a 11 ± 1 15 ± 1 4.5 ± 0.9 54 ± 5 48 ± 24b 15 ± 2 17 ± 1 5.1 ± 0.4 40 ± 4 43 ± 24c 22 ± 2 18 ± 1 6.2 ± 0.5 36 ± 4 34 ± 3

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increase in scattered intensity at low-q upon heating theirtelechelic−PNIPAM flower micelles, which they fit with amodel for micellar aggregates.11 Here, accounting for thescattering contributions from micellar aggregates did notsignificantly affect the fit results for the individual micellesand therefore was not included while modeling the SANS data.The results of the SANS data modeling are summarized in

Table 2 and Figure 3b. The analysis suggested that the micellecore radius was between 1 and 2 nm, which was consistent withthe length of a fully extended C12 chain.

65,66 However, the fitswere not sensitive to values within this range due to the smallcontribution of the core block to the overall scattered intensity.The micelle cores were surrounded by a diffuse, hydratedcorona characteristic of star-like micelles, as seen in the coronaprofiles in Figure 3b. The micelle structure determined fromthe SANS data modeling also was qualitatively consistent withmolecular simulations using dissipative particle dynamics (seeSupporting Information for additional details). As expected, the(Rg) of the corona chains and overall micelle size increased withPNIPAM molecular weight (Table 2). Additionally, the SANSdata suggested that the aggregation number decreased withincreasing PNIPAM molecular weight, and this trend wasconsistent with the SLS results.

■ DISCUSSIONThe results presented here clearly indicate that the SCS pincerfunctionalized polymers self-assembled into well-definedmicelles. Additionally, the amphiphilic homopolymers studiedhere followed trends similar to those reported for blockcopolymers. As seen in Table 2, all of the characterizationresults were in good agreement and indicated that the micelleradius increased with increasing NPNIPAM. The overall size ofstar-like micelles, particularly those with small cores, dependson the dimensions of the hydrophilic block. Accordingly, themicelle radius, R, should scale with the degree of polymer-ization of the hydrophilic block, Nphilic, as R ∼ Nphilic

δ.5,52,67

Fitting the data in Table 2 suggests that the NPNIPAM scalingexponent for micelle radius is within the range of literaturevalues for δ, with a value between 0.5 and 1.49,67 However;additional data points would be needed to definitively assignthe scaling dependence. Nagg weakly decreased with increasingNPNIPAM (Table 2), consistent with experimental studies of BCPmicelles in which Nagg ∼ Nphilic

−β and β ranged from 0.0 to0.51.49 Scaling theories for star-like BCP micelles also predictedthat Nagg should logarithmically decrease with increasingNphilic.

52,68

Comparing the corona thickness to the root-mean-squareend-to-end distance of PNIPAM in solution69 suggests that thecorona chains are moderately stretched in the micelles studiedhere (Table S3, Supporting Information). Stretching of thecorona chains has been reported for both amphiphilichomopolymer66 and BCP micelles49 and is attributed to the

Figure 2. (a) Cryo-TEM micrograph and (b) corresponding gray scaleprofile from cryo-TEM micrograph analysis and fit for sample 4c,supporting the PNIPAM chains form a diffuse, hydrated corona.Contrast shown in part a was enhanced by 5% in ImageJ for clarity; thecontrast was not adjusted for the profile analysis. Scale bar is 100 nm.The gray scale profile in part b was averaged over 50 micelles.

Figure 3. (a) SANS data (symbols) and fits with a spherical micelleform factor (lines) for 4a, 4b, and 4c in D2O and (b) micelle coronaprofiles from the SANS data fits.

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crowding and associated stretching of the chains near themicelle core. As indicated in Figure 3b, sample 4a has thehighest polymer volume fraction near the core, correspondingto the most crowded and therefore the most stretched chains.Likewise, the degree of corona chain stretching increases withaggregation number (i.e., 4a > 4b > 4c), which is in agreementwith trends reported for BCP micelles.49 The extent of coronachain stretching for the amphiphilic homopolymer micelles iscomparable to literature results for BCP micelles with similaraggregation numbers.49

Modeling the corona profile as a linear combination of 2bsplines resulted in good fits to both the cryo-TEM gray scaleprofile and the SANS data (Figures 2 and 3). Similarly, previousscattering studies of telechelic-PNIPAM11 and hydrophobicallymodified PEO66,70 also reported a radially decreasing coronaprofile. Though the corona profile extracted from cryo-TEMsuggests that the corona chains extend to a smaller r than theprofiles from SANS, the relative shape of the corona profileswas very similar, as illustrated by the normalized profiles inFigure 4. These results showed that corona profiles can be

extracted from cryo-TEM micrographs, and highlighted thecomplementary use of scattering and cryo-TEM in thestructural characterization of solution assemblies.

■ CONCLUSIONSAn SCS-pincer functionalized RAFT agent was used tosynthesize a series of hydrophobically end-functionalizedPNIPAM polymers that are promising precursors to solution-assembled nanoreactors. These amphiphilic homopolymersself-assembled into well-defined spherical micelles, in which themicelle radius, aggregation number, and corona density profilewere dependent on the degree of polymerization of thePNIPAM block. To facilitate future investigations into theeffects of nanoreactor structure on catalytic performance, themicelle structures were characterized using DLS, SLS, cryo-TEM, and SANS. Importantly, detailed information about themicelle density profiles was extracted from the cryo-TEMmicrographs and was comparable to the SANS result,highlighting the immense potential for the complementary

use of these two techniques to characterize nanoscale solutionassemblies.

■ ASSOCIATED CONTENT*S Supporting InformationCMC fluorescence data (Figure S1), additional light scatteringdata (Figures S2−S6), additional cryo-TEM images (FigureS7), detailed explanation of cryo-TEM micrograph analysis,SANS data model, details of molecular simulations (Table S1,Table S2, and Figure S8), and calculated corona chainstretching (Table S3). This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: (T.H.E.) thepps@udel.edu; (R.K.O.) r.k.o-reilly@warwick.ac.uk.Present Address⊥Department of Pure and Applied Chemistry, University ofStrathclyde, Glasgow, G1 1XL, U.K.Author Contributions|J.P.P. and E.G.K. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe EPSRC and University of Warwick are thanked forfunding. Some equipment used in this research was funded byBirmingham Science City, with support from Advantage WestMidlands and part funded by the European RegionalDevelopment Fund. T.H.E., M.O.S., R.P.M., and E.G.K. thankan Institutional Development Award (IDeA) from the NationalInstitute of General Medical Sciences of the National Institutesof Health (NIH), Grant P20GM103541, for financial support.The statements herein do not reflect the views of the NIH.E.G.K. also acknowledges support from a Department ofDefense, Air Force Office of Scientific Research, NationalDefense Science and Engineering Graduate (NDSEG) Fellow-ship, 32 CFR 168a. We acknowledge support of the NationalInstitute of Standards and Technology (NIST), U.S. Depart-ment of Commerce, for providing the neutron facilities used inthis work. University of Delaware Center for Neutron Science(CNS) exploratory beam time was supported by NIST, U.S.Department of Commerce (No. 70NANB7H6178). Weacknowledge the Keck Microscopy Facility at the Universityof Delaware for use of their TEM and Vitrobot.

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Figure 4. Comparison of normalized corona profiles for sample 4cfrom cryo-TEM micrograph analysis and SANS data modeling. Shadedarea represents the range of dimensionless volume fraction profilesthat gave similar fits to the gray scale profile from cryo-TEM. Solid lineis the dimensionless profile from the SANS data modeling. R is theradius at which the respective corona profile decreased to 0.

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