Self-Assembly of Mixtures of Telechelic and Monofunctional

Sep 13, 2017 - We attribute this to the balance between the hydrophobic enthalpic terms that favor anchoring of both chain-ends to the core and the en...
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Self-Assembly of Mixtures of Telechelic and Monofunctional Amphiphilic Polymers in Water: From Clusters to Flowerlike Micelles Thomas Zinn,† Lutz Willner,*,‡ Kenneth D. Knudsen,§ and Reidar Lund*,† †

Department of Chemistry, University of Oslo, Postboks 1033 Blindern, 0315 Oslo, Norway Jülich Centre for Neutron Science JCNS and Institute for Complex Systems ICS, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany § Department of Physics, Institute for Energy Technology, Postboks 40, 2027 Kjeller, Norway ‡

S Supporting Information *

ABSTRACT: We study the self-assembly of mixtures of nalkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the selfassembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike micelles. This was confirmed both by DLS, which is very sensitive to formation of larger clusters, as well as with SANS, which also showed a clear transition from attractive to repulsive intermicellar interactions upon increasing n-alkyl length. We attribute this to the balance between the hydrophobic enthalpic terms that favor anchoring of both chain-ends to the core and the entropic cost associated with the bending of the polymer chains. For short n-alkyls, exposure of the chain-ends in the corona structure leads to net dominance of the attractive interactions while for longer hydrophobic chains it leads to a stabilization of loops and consequently flowerlike micellar morphology. Using contrast-variation SANS, the contribution of mono- and difunctional chains could be separated, confirming the flowerlike micellar structure.

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INTRODUCTION Associative polymers consist of a polymer backbone carrying segments or blocks that can self-assemble in selective solvents. An important subclass is polymers, so-called telechelic polymers, with a water-soluble backbone and hydrophobic pendant groups distributed along the backbone and/or at the terminal position. In aqueous solution these polymers can form a variety of structures ranging from single “flowerlike” micelles, where the two chain-ends are connected to the same micelle, to clusters of interconnected micelles, where the chain-ends span (“bridges”) two different micelles forming physically crosslinked networks of micelles (hydrogels), depending on concentration and temperature.1 However, given their strong tendency to self-assemble and form intermicellar associations, stable flowerlike micelles are generally not found in experiments. Instead, most studies have been focused on higher concentrations and their intriguing rheological properties, e.g., linear viscoelasticity governed by the lifetime of the bridging chains2 and their “shear-thinning” non-Newtonian behavior.3−7 Telechelic polymers are also used to modify interactions in micellar8−11 or microemulsion systems.12−14 This is further © XXXX American Chemical Society

motivated by the importance of these materials as viscosity modifiers in many technical applications. Physically cross-linked associative polymers can also be used as scaffolds for tissue engineering15 and in biomedical applications, e.g., as drug delivery systems.16,17 n-Alkyl difunctionalized poly(ethylene oxide) (PEO) polymers are archetypical telechelic polymers where the n-alkyl groups are located in α and ω (telechelic) positions at the PEO-chain ends, Cn-PEOx-Cn. Here n denotes the number of carbons of the n-alkyl-group and x the molecular weight of PEO in kg/mol. Because of the complexity of the association behavior of telechelic PEOs in water, a comprehensive structural characterization is more challenging than for simple micellar systems formed by corresponding monofunctionalized polymers, Cn-PEOx. Many of the previous studies found in the literature were done using fluorescence techniques,18−23 nuclear magnetic resonance (1H NMR),24 or static and Received: July 17, 2017 Revised: August 24, 2017

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Figure 1. Schematic representation of micellar aggregates formed by n-alkyl mono- and difunctionalized PEO polymers and mixtures in water dependent on polymer concentration and composition; f tel = fraction of difunctional (telechelic) polymer.

dynamic light scattering.11,21,22,25,26 These techniques mainly yield information on the global properties of the formed aggregates but do not give any details of the internal structure. Only a few studies have been done using more suitable techniques with an appropriate spatial resolution, i.e., smallangle X-ray (SAXS) and small-angle neutron scattering (SANS).8,11,27−29 However, most of these studies are lacking systematics for instance in the variation of hydrophobic chainends which are crucial determinants for their self-assembly. Hence, the structural picture in aqueous solution still remains incomplete; in particular, a direct structural verification of the presence of flowerlike micelles in such systems has not been presented so far. The characterization of aggregates formed by telechelic PEOs is often hampered by macroscopic phase separation at low to moderate polymer volume fractions into a dense gel phase of micelles and a dilute phase consisting of individual small clusters, micellar entities, and telechelic chains. This was theoretically predicted by Semenov et al.30 and experimentally verified by François et al.31 and Pham et al.32 In a work of Laflèche and co-workers,21 it was reported that phase separation does not occur when reducing the number of bridging chains by simply diluting with monofunctionalized PEO chains of half the length, Cn-PEO(x/2), i.e., the same hydrophilic/lipophilic balance. The absence of phase separation in earlier studies may be explained by the use of ill-defined materials contaminated with mono- and unfunctionalized polymers. These contaminations may be due to incomplete end-group functionalization, since many of the studied polymers have been prepared by attaching the hydrophobic tails to broadly distributed commercial poly(ethylene glycol) by using e.g. classical isocyanate/alcohol linking reactions. A summary of the possible structural scenario of mixtures of telechelic/monofunctional polymers, Cn-PEOx-Cn/Cn-PEO(x/ 2), is schematically illustrated in Figure 1 as a function of overall polymer concentration and composition. At low polymer concentrations monofunctional Cn-PEO(x/2) polymers aggregates into core−shell micelles with starlike geometry while at higher concentrations above the overlap concentration, mesoscopic crystalline phases are formed.33−36 For dilute solutions of pure telechelic polymers, the Cn-PEOx-Cn chains

can principally assume two conformations: looped chains forming “flowerlike” micelles or starlike micelles with free dangling ends as illustrated on the right-hand side of Figure 1. By increasing the concentration, i.e., reducing the distance between micelles, the chains start to build bridges between them and for sufficiently strong attractive interactions the system macroscopically phase separates. The gel structure and its stability can be modulated by the addition of monofunctional polymers reducing the attractive interactions. In order to systematically characterize the association behavior of telechelic polymers, well-defined model materials are required without uncertainties in their degree of end-group functionalization. Furthermore, mixtures of difunctional and monofunctional materials are needed to circumvent phase separation in order to be able to unfold the structural details of such polymers in aqueous solution. In this work we present a comprehensive structural study of Cn-PEOx-Cn/Cn-PEO(x/2) mixtures in dilute aqueous solution by SANS and dynamic light scattering (DLS). Model polymers were obtained by living anionic polymerization of ethylene oxide using corresponding n-alkyl alcohols as initiator, followed by a coupling reaction of two of the parent Cn-PEO(x/2), with x=10 polymers by a condensation reaction of terminal OH groups. The applied synthetic pathway guarantees end-group functionalization close to 100% on narrowly distributed PEO chains. By systematically varying the hydrophobic chain length from n = 14 to n = 28 and the composition of telechelic/ monofunctional polymers, f tel, we show that clusters are favored for short hydrophobic blocks (n = 14−22) while for the longest C28-PEO10-C28 polymers stable flowerlike micelles with purely repulsive intermicellar interactions are found. We attribute this to the anchoring of chains at sufficiently long hydrophobic chains where the enthalpic gain of loop formation dominates the entropic cost of the PEO chain bending. Moreover, in light of recent time-resolved SANS experiments of the exchange kinetics, short n-alkyl chains are expected to exchange rapidly,37−39 and thus terminal hydrophobic groups will be exposed to the micellar surface leading to effective attractive interactions. B

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the difunctional copolymer and a smaller signal at higher times corresponding to the residual uncoupled monofunctional PEO. For purification and enrichment of the desired difunctional polymer, the crude material was fractionated with chloroform/heptane. Details including SEC data of the different fractions can be found in the Supporting Information Figure 2 shows SEC data of the final product

EXPERIMENTAL SECTION

Synthesis and Characterization of Functionalized PEO. Monofunctionalized PEO. The synthesis of poly(ethylene oxide) mono n-alkyl ethers, Cn-PEO5, has already been presented in earlier publications.40,41 In brief, they were prepared by living anionic polymerization of ethylene oxide. As initiator we have used an 80:20 mixture consisting of an 1-alcohol (CnH2n+1OH, n = 14, 16, 22, 28) and the corresponding potassium 1-alkoxide (CnH2n+1O−K+). The polymerizations were carried out in toluene for 24 h at 95 °C. The living polymers were terminated with acetic acid, leading to a terminal hydroxyl group. As discussed in an earlier publication,40 spurious amounts of water lead to α,ω-dihydroxy PEOs as byproducts. These contaminations were almost completely removed by fractionation from chloroform/heptane as solvent/nonsolvent pair for PEO. Finally, purified polymers were obtained by filtration, precipitation in cold acetone, centrifugation, and freeze-drying from benzene. Additionally, a C28-dh-PEO5 polymer consisting of a random distribution of d- and h-EO repeat units has been prepared from a mixture of 18% h- and 82% d-EO monomer for contrast variation experiments. All polymers were targeted to have a molar mass of Mn ∼ 5 kg/mol. Exact molar masses and molar mass distributions, Mw/Mn, were determined by size exclusion chromatography and 1H NMR as described in refs 40 and 41. Mw/Mn values were found to be smaller than 1.03. The important polymer mass characteristics are summarized in Table 1.

Figure 2. SEC chromatograms of C 22 -PEO5 (red) and C22-PEO10-C22 (black) containing 10% of C22-PEO5.

Table 1. Molar Mass Characteristics of Cn-PEO5 Diblock Copolymers n

M(CnH2n+1)a

Mn(PEO)b

NPEOc

Mn(Cn-PEO)

14 16 22 28 28d

197 225 310 394 394

4.76 4.97 4.87 4.60 5.06

108 113 111 104 107

4.96 5.2 5.18 5 5.45

of C22-PEO10-C22 after fractionation containing 10 area % remaining diblock C22-PEO5. For comparison also the chromatogram of pure C22-PEO5 is depicted. Mw/Mn values of the difunctional polymer were estimated to be smaller than 1.02. For the present study we have not tried to increase the amount of difunctional polymer as mixtures of difunctional and monofunctional PEO will be used. Moreover, further fractionations would significantly reduce the yield of the product. The polymers were additionally characterized by 1H NMR in deuteriochloroform. The spectra of monofunctional and difunctional PEO are shown in the Supporting Information including peak assignments. Mn(PEO) values were calculated by taking the signals of the n-alkyl groups as internal reference. The number-average molecular weight of the triblock/diblock mixture differs slightly from twice the Mn of the precursor diblock because of the fractionation with chloroform/ heptane. Molar mass characteristics of the difunctional polymers and amount of residual monofunctional PEO are shown in Table 2.

a

M: molar mass of the n-alkyls in g/mol. bMn: number-average molar mass in kg/mol calculated from 1H NMR measurements. cNPEO = Mn(PEO)/M(EO): number of EO repeat units. dC28-dh-PEO5.

α,ω-Difunctionalized PEO. The synthesis of the poly(ethylene oxide) di-n-alkyl ethers, Cn-PEO10-Cn, was accomplished by intermolecular coupling reaction of the Cn-PEO5 diblocks via the terminal hydroxyl groups. Here we followed a route which has already been successfully applied for the synthesis of cyclic PEO where an ether linkage was formed by reaction with tosyl chloride (TsCl) in the presence of solid potassium hydroxide.42−44 In detail, 4.5 g of the freeze-dried Cn-PEO5 was dissolved in 50 mL of dry THF. Tosyl chloride (Fluka Analytical) was used as received and separately dissolved in dry THF (2.1 g/40 mL). KOH was dried at 475 °C and finely ground in an argon box before use. The coupling reactions were carried out inside the glovebox. For the reaction, at first, 500 mg of KOH was dispersed in the Cn-PEO5 solution followed by dropwise addition of the stoichiometric amount of the TsCl (1.6 mL of THF solution). The reaction mixture was stirred for 2.5 h at room temperature. To prove the yield of triblock, a small amount of the reaction product was removed and analyzed by size exclusion chromatography (SEC). SEC data were obtained with THF/N,Ndimethylacetamide (85:15) as eluent at 50 °C using three Agilent PlusPore GPC columns with a continuous pore size distribution at a flux rate of 1 mL/min. The chromatograms revealed a maximal yield of 88% of coupling product. In some cases the yield was lower but could be increased by repeatedly adding KOH and TsCl solution. After stirring overnight, the THF was removed by rotary evaporation. The residual solid was dispersed in deionized water, neutralized with hydrochloric acid, and extracted twice with 150 mL of chloroform. The combined chloroform phases were dried with Na2SO4 and filtered. The solvent was then removed by rotary evaporation. The composition of the crude product was again determined by SEC. The elution curve shows two narrow signals: a major peak indicating

Table 2. Molar Masses of Cn-PEO10-Cn Difunctional PEO and Remaining Fraction of C22-PEO5 Monofunctional PEO

a

n

M(CnH2n+1)

Mn(PEO)

NPEO

Mn(Cn-PEO-Cn)

diblock fractiona

14 16 22 28 28b

197 225 310 394 394

9.34 9.69 9.96 9.65 10.1

212 220 226 220 213

9.8 10.2 10.6 10.5 10.9

0.09 0.05 0.10 0.13 0.02

Calculated from SEC peak areas. bC28-dh-PEO10-C28.

Sample Preparation. The polymer blends of mono- and difunctional PEOs with different composition, f tel, were prepared by weight, taking into account the amount of remaining monofunctionalized PEO in the coupled product. As the overall density of both polymers is assumed to be the same, f tel denotes the volume or weight fraction of difunctionalized PEO in the blend. The polymers were homogeneously mixed by dissolving in chloroform followed by thoroughly drying in vacuum. A small amount (0.1%) of (+)-sodium- L -ascorbate was added as antioxidant. The final composition was verified by the peak areas of the size exclusion chromatograms. Aqueous solutions were prepared with a common polymer volume fraction of ϕ = 0.5% and for some selected samples also with ϕ = 1% and 2.5%. The solutions were obtained by dissolving the blends or the C

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absolute units (cm−1) vs Q scattering profiles where Q = 4πλ−1 sin(ϑ/ 2). A more detailed expression is given in the Supporting Information. Theoretical Modeling. The system was first modeled as a linear combination of single micelles and clusters.

pure monofunctionalized PEO in water at 60 °C for 3 h. This temperature is well above the melting temperature of the n-alkanes inside micellar cores leading to rapid chain exchange between micelles which guarantees thermodynamic equilibrium within a short period of time. Subsequently, the solutions were allowed to slowly cool down to room temperature while shaking. For SANS experiments D2O with a deuteration degree of 99.9% (Eurisotop) has been used. Millipore water was taken to prepare solutions for DLS experiments. The latter were additionally passed through 5 μm filters in a dust-free glovebox prior to the measurements. Polymer solutions containing difunctional polymer ( f tel > 0) were visually examined for phase separation in the temperature range between 20 and 60 °C. All samples used for further investigation were homogeneous and transparent and did not show any sign of phase separation. It is known that the critical point is shifted toward higher fractions of difunctionalized PEO when decreasing the size of the hydrophobic sticker.26 Hence, stable and optically clear solutions are obtained for blends with f tel ≤ 0.7 for polymers with alkyl length of n = 14 and f tel ≤ 0.5 for solutions prepared from n = 16 and n = 22, whereas for n = 28 stable solutions are only obtained for f tel ≤ 0.4. Dynamic Light Scattering (DLS). DLS experiments were conducted with an ALV multidetection compact goniometer platform (ALV-GmbH, Germany). The light source was a HeNe laser (22 mW) operating at a wavelength λ = 632.8 nm. The scattering vector is expressed as Q = 4πnλ−1 sin(ϑ/2) where n is the refractive index of the solvent (n = 1.33 for water at 25 °C) and ϑ the scattering angle. A high temperature quartz container filled with cis-decalin ensured a precise temperature control (±0.1 K) of the sample cell. The experimentally obtained intensity autocorrelation function, g(2)(t), is related to the normalized electric field autocorrelation function g(1)(Q,t) by the Siegert relation: g(2)(Q,t) = 1 + B|g(1)(Q,t)|2 where B is a factor correlated to the coherence and laser optics (≤1) . Assuming a solution containing only free and clustered micelles, g(1)(Q,t) can be expressed by a sum of a single-exponential mode with a characteristic time τf and a slow stretched exponential mode with τs of the presumably polydisperse clusters: β⎤ ⎡ ⎛ ⎡ t ⎤ t ⎞ ⎥ g(1)(Q , t ) = A f exp⎢ − ⎥ + A s exp⎢ − ⎜ ⎟ ⎢ ⎝ τs(Q ) ⎠ ⎥ ⎣ τf (Q ) ⎦ ⎣ ⎦

ϕ dΣ (Q ) = [f NcluS Nclu(Q ) + (1 − fclu )]Pmic(Q ) dΩ Vmic clu

with ϕ the polymer volume fraction, Pmic(Q) the form factor of an individual micelle, and fclu the fraction of clusters. The structure factor, SNclu(Q), of Nclu micelles connected randomly is given by S Nclu(x) =

kBT 6πηDapp, i

2[(1 − (1 − (sin x /x))Nclu ] 2 (sin x /x) −1− 1 − (sin x /x) Nclu(1 − (sin x /x)2 )

(4) here x = QD and D is the average distance between the neighboring micellar centers.45−47 For sake of simplicity, we assume that D is twice the overall micellar radius Rm. Nclu should have only integer values, and therefore for any noninteger values Nclu is weighted by taking a linear combination of ⌊Nclu ⌋ and ⌊Nclu ⌋ + 1.47 With p = Nclu − ⌊Nclu ⌋, SNclu(Q) can be expressed by

S Nclu(x) = (1 − p)S⌊Nclu⌋(x) + pS⌊Nclu⌋+ 1(x)

(5)

here ⌊·⌋ is the largest integer not greater than Nclu. The individual micellar units were considered to consist of spherical mixed micelles with total of Nagg chains where a fraction f tel is difunctional while 1 − f tel is monofunctional. Note that in this particular case for the modeling f tel was converted to the corresponding mole fraction. Hence, the total molar micellar volume is given by

Vmic = Nagg[ftel VCn‐PEO10‐Cn + (1 − ftel )VCn‐PEO5]

(6)

with VCn‑PEO10‑Cn and VCn‑PEO5 being the molar volumes of the two polymers. We note that VCn‑PEO10‑Cn = 2VCn‑PEO5 since difunctional polymer was prepared by linking two monofunctional chains. We further assume that the two polymers are distributed in the micelles identical to the composition f tel of the blend. In order to calculate the form factor of the micelles, Pmic(Q), we consider a core− shell model where the n-alkanes form the core and PEO chains the corona. However, it should be kept in mind that a fraction of n-alkane end-groups of the telechelics can also associate with two different micelles (”bridge”) or remain free with one end in the corona (”dangling end”). We assume that the fraction of free chains in solution is vanishingly small (cmc ≪ ϕ). We further introduce a parameter f loop which denotes the fraction of difunctional polymers forming either loops or bridges to another micelle. It is assumed that fraction of bridges is homogeneously distributed between the micelles such that the volume “lost” upon forming a bridge to a micelles is on average compensated by a gain of a bridge from another micelle. The form factor of the micelles, Pmic(Q), consists of four contributions: the core scattering Ac(Q), the corona scattering Ash(Q), the cross-term between the core and corona, and the interferences of different polymer chains in the corona B(Q)

(1)

The exponent β is related to the width of the cluster size distribution where β = 1 for a perfectly monodisperse sample. Af and As denote the amplitudes of fast (single micelles) and slow mode (clustered micelles) with Af + As = 1. The characteristic mean relaxation time of the slow mode is given by ⟨τs⟩ = τs/βΓ(1/β) where Γ is the gamma function. The Stokes−Einstein relation is used to calculate an apparent hydrodynamic radius Rh,i of the micelles (i = f) or aggregates (i = s)

R h, i =

(3)

(2)

where Dapp,i is the apparent translational self-diffusion coefficient for the respective mode, kB the Boltzmann constant, T the absolute temperature, and η the dynamic viscosity of the solvent (η = 0.89 mPas for water at 25 °C). SANS Measurements. SANS investigations were carried out at the SANS installation at the JEEP-II reactor at Kjeller, Norway. The wavelength was set with a velocity selector (Dornier), using a wavelength resolution, Δλ/λ, of 10%. The detector was a 3He-filled RISØ type, mounted on rails inside an evacuated detector chamber. Each complete scattering curve was composed of three independent measurements, using different wavelength−distance combinations (5.1 Å/1.0 m, 5.1 Å/3.4 m, and 10.2 Å/1.0 m). The resulting Q-range for the experiment was 0.006−0.3 Å−1. The solutions were filled in 2 mm Starna quartz cuvettes. The temperature was controlled by a water circulator, maintaining the set value within ±0.1 °C. In all of the SANS measurements, D2O was used as a solvent instead of H2O to obtain good contrast and low background. The scattering data were treated according to standard procedures. Finally, we obtain dΣ/dΩ in

Pmic(Q ) = Δϱc 2Nagg 2Vc 2Ac 2(Q ) + Δϱsh 2Nagg(Nagg − B(0)) × Vsh 2A sh 2 (Q ) + 2ΔϱcΔϱshNagg 2VcVshAc(Q )A sh (Q ) + Vsh 2Δϱsh 2B(Q )

(7)

where Δϱc/sh = ϱc/sh − ϱ0 are the excess scattering length densities of the core (c) and the shell (sh) with respect to the solvent ϱ0. Vc/sh are the molar volumes of core and shell blocks, respectively, which for the polymers used in this work are Vc = VCn and Vsh = VPEO, the molar volumes of the n-alkyl and PEO block, respectively. Hence, the individual contributions from the core and the shell can be written as

Vc = Nagg(1 + ftel floop )VCn D

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Figure 3. Normalized (g(2)(t) − 1)/B relaxation functions: (a) n = 14 ( f tel = 0, 0.5, and 0.7), (b) n = 16 ( f tel = 0 and 0.5), (c) n = 22 ( f tel = 0 and 0.5), and (d) n = 28 ( f tel = 0, 0.25, and 0.4) micellar solutions at 25 °C. The solid lines represent best fits obtained by eq 1. Inset layers show determined Γf,s (fast mode = full and slow mode = open symbols) as a function of Q2. Vsh = Nagg[(1 + ftel )VPEO + ftel (1 − floop )VCn]

(9)

B(Q ) =

The individual scattering amplitudes of the core Ac(Q) and the corona Ash(Q) are given by

Ac(Q ) =

3(sin(QR c) − QR c cos(QR c)) (QR c)3

1 C

∫R

∞ c

4πr 2n(r )sh

exp(− Q 2σint 2/2)

sin(Qr ) dr exp(− Q 2σint 2/2) Qr

(11)

ϕ dΣ (Q ) = Pmic(Q )Sapp(Q ) dΩ Vmic

with Rm and Rc the radius of the micelle and the core, respectively. The core radius Rc can be calculated by assuming a compact and homogeneous spherical core: ⎛ 3Nagg(1 + f f )VC ⎞ tel loop n ⎟ R c = ⎜⎜ ⎟ π 4 N Avo ⎝ ⎠

(15)

where Sapp(Q) acts as an apparent interparticle structure factor. Sapp(Q) was modeled as constituted by attractive contribution with a finite range as well as short-range repulsive interaction by using the ”sticky hard-sphere model” introduced by Baxter54 and modified by Menon et al.55 This model describes the micellar aggregates as a collection of (spherical) particles with radius RHS that interact via a finite range attractive square-well potential of the depth u0 ( 0).

is the same (same micellar volume) in both contrasts, the increase in micellar radius is solely due to a slight change in the corona structure. This suggests that the looped PEO chains contribute to an effectively larger micellar radius since the backfolding will give a higher density at the micellar surface. In fact, this should theoretically give rise to a density profile different from the starlike profile, n(r) ∼ r−4/3. However, trial fits did not reveal significant deviations from the n(r) ∼ r−4/3, and the change in density profile could be well-described by the small change in Rm. This correlates well with the observed larger hydrodynamic radius found by DLS. Hence, although the looped and single PEO chain in the corona displays very similar radial density distribution, SANS contrast variation analysis helps detecting an increased micellar radius that reflects a higher segmental density at the rim of the flowerlike micelles. It is interesting to note that the effective virial coefficient, ν, between the coronal PEO chains, which is proportional to the grafting density of polymer chains in the corona, decreases from 3.2 for the fully proteated micelles, where both mono-/ difunctionalized polymers are visible, to ν ≈ 0.7 for the partially contrast matched micelles. This is in line with the expected increased distance between the chains contributing to the scattered intensity at high Q.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01501. Figures S1 and S2; eqs S1−S8 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected] (L.W.). *E-mail: [email protected] (R.L.).

CONCLUDING REMARKS The self-assembly of a mixture of Cn-PEOx-Cn and Cn-PEO(x/ 2) polymers with a short hydrophobic sticker (n = 14−28) have been systematically studied using DLS, SANS, and theoretical modeling. By the combination of both techniques, DLS and SANS including data modeling, we found a distinct tendency for the formation of clusters for polymers with shorter n-alkyl length (n = 14, 16). The evaluation of SANS curves at different f tel indicates that the amount of clusters is increasing with the number of telechelic chains (see Figure 8a). The micellar subunits within the clusters do not change and remain identical to micelles formed by the pure monofunctionalized polymers. The trend to form clusters clearly decreases for polymers bearing C22 alkyls and disappears for C28 groups, which show an intriguing tendency to form looped, flowerlike micelles. This observation, somehow elusive in previous studies, could be further supported by studying the interactions at higher concentrations and through contrast variation. This clearly shows that stable flowerlike micelles with repulsive interactions could be observed only for the longest n-alkane chains. This observation was explained by the energetically favored formation of loops over dangling ends and bridging chains for larger n-alkyl groups following thermodynamic consid-

ORCID

Reidar Lund: 0000-0001-8017-6396 Present Address

T.Z.: ESRF - The European Synchrotron, 71 Avenue des Martyrs, Grenoble, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge grants from the Norwegian Research Council, under the SYNKNOYT program (Grants 218411 and 228573) and access to the IFE neutron facility in Kjeller, Norway.

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REFERENCES

(1) Winnik, M. A.; Yekta, A. Associative polymers in aqueous solution. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (2) Zinn, T.; Willner, L.; Lund, R. Telechelic Polymer Hydrogels: Relation between the Microscopic Dynamics and Macroscopic Viscoelastic Response. ACS Macro Lett. 2016, 5 (12), 1353−1356. (3) Chassenieux, C.; Nicolai, T.; Benyahia, L. Rheology of associative polymer solutions. Curr. Opin. Colloid Interface Sci. 2011, 16, 18.

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DOI: 10.1021/acs.macromol.7b01501 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01501 Macromolecules XXXX, XXX, XXX−XXX