P(PO-r

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Viscosity Transitions Driven by Thermoresponsive Self-Assembly in PHOS‑g‑P(PO‑r‑EO) Brush Copolymer Aristeidis Papagiannopoulos,*,† Junpeng Zhao,‡ Guangzhao Zhang,‡ Stergios Pispas,† and Charl J. Jafta§ †

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece ‡ Faculty of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China § Institute of Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn Meitner Platz 1, 14109 Berlin, Germany S Supporting Information *

ABSTRACT: We report on a remarkable viscosity transition of poly(p-hydroxystyrene)-graf t- poly(propylene oxide-ran-ethylene oxide) (PHOS-g-P(PO-r-EO)) bottle-brush copolymers and their underlying spatial solution arrangement in aqueous solutions. The changes of morphology are measured by small-angle neutron scattering, and the rheological alterations are characterized by viscometry and video particle tracking microrheology. At room temperature core−shell micelles are formed, and the dynamics of the solutions have softcolloidal nature. The viscosity passes through a pronounced maximum as temperature increases, and at temperatures higher than the transition regime the solutions end up as thin and milky-white dispersions of particulate clusters. The double temperature-thickening/ temperature-thinning transition is related to the interactions and associations of the formed clusters. These findings are very useful for the design and application of thermoresponsive brush copolymers in a variety of fields.

1. INTRODUCTION

Thermoresponsive copolymers of comblike and starlike architectures are proved very promising for future developments. PNIPAM-grafted gelatin has been suggested for a cell-adhesive matrix in tissue engineering devices;11 polymers with a chitosan backbone and PNIPAM-COOH side chains were evaluated as injectable scaffolds for the culture of articular chondrocytes and meniscus cells,12 and PNIPAM-grafted hyaluronic acid was evaluated for controlled drug and protein release.13 Other PNIPAM-based comb copolymers have shown interesting thermothickening behavior opening possibilities for oil-industry applications.14,15 Star triblock copolymers containing a hydrophobic poly(ε-caprolactone) segment, a hydrophilic poly(oligo(ethylene oxide) methacrylate) segment, and a thermoresponsive poly(di(ethylene oxide) methyl ether methacrylate) segment reveal different properties depending on the position of the thermoresponsive block. When this block is at the periphery of the formed micelles in solution, the system undergoes a sol−gel transition between room and body temperature.16 The sequence of hydrophilic and thermoresponsive blocks has also played a critical role in the temperature-sensitive aggregation of thermoresponsive-ionic four-armed star block copolymers.17

Thermoresponsive copolymers are unequivocally an important class of macromolecules1 as they belong in the broader category of the stimuli-responsive materials.2 Temperature triggers affect both their solution properties, i.e., mechanical and morphological, and their interactions with other molecular components. A great amount of work has been published on their rheological temperature response and possibilities for drug delivery applications.3,4 The mechanisms of morphological change may have several origins. In poly(ε-caprolactone)−poly(ethylene glycol)−poly(ε-caprolactone) gel-forming aqueous solutions a sol−gel transition at body temperatures was caused by bridging connections between micelles.5 For that system in vitro release of hydrophobic, hydrophilic, and protein drugs was also successfully tested. Poly(n-butyl acrylate)-b-poly(N-isopropylacrylamide)−carboxylic acid (PnBA-b-PNIPAM-COOH) micelles revealed a length-scale-dependent temperature response.6 Aqueous solutions of polystyrene-b-poly(N-isopropylacrylamide)-b-polystyrene (PS-b-PNIPAM-b-PS) triblock copolymers demonstrate hierarchical aggregation of flowerlike micelles in aggregates and clusters above7,8 or below and above9 the LCST of PNIPAM. In temperature-sensitive polysaccharides as gelatin and carrageenan helix−helix associations create interchain crosslinks upon cooling.10 © XXXX American Chemical Society

Received: December 21, 2017 Revised: February 3, 2018

A

DOI: 10.1021/acs.macromol.7b02711 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

an accuracy of 0.1 °C, and the samples were allowed to equilibrate for longer than 30 min at the set temperature. Video Particle Tracking Microrheology (VPTMR). An upright bright-field Olympus BH2 microscope equipped with a 100× oil immersion lens was used to monitor the thermal motion of the PS probe microspheres. Videos were captured by a Hitachi KP-M1A CCD camera at 25 frames/s and a shutter exposure time at 0.02 s. Sample solutions were loaded inside the cavity of a glass slide and covered by a thin glass coverslip carefully glued around its edges. The lens was focused approximately 15 μm underneath the coverslip so that any wall effects were negligible. The temperature of the glass slide was controlled by a Linkam TMS 94 hot stage. The immersion oil creates contact between the lens and the coverslip generating a substantial temperature gradient in the area of the contact. In order to avoid it, the objective lens was thermally equilibrated at the same temperature with the stage by a Warner Instruments TC-124A objective warmer. The accuracy of the temperature control of the VPTMR system was 0.4 °C. The tracks of the fluctuating particles (r(t)) were extracted by IDL tracking software based on standard image analysis algorithms. The mean-squared-displacement (MSD) over their ensemble (N) and initial times (t) as a function of lag-time (τ) is given in eq 1.

Finally, the thermally controlled release of doxorubicin from drug carriers made from star copolymers containing εcaprolactone and exhibiting an LCST behavior18 has been reported. In this work we investigate the thermal response of a brush copolymer with poly(propylene oxide-ran-ethylene oxide) side chains (P(PO-r-EO)) densely grafted on poly(p-hydroxystyrene) (PHOS). The study of such a system is motivated by the tuneability of its LCST in water depending on its composition19 and its potential for bio-related applications.20,21 We used smallangle neutron scattering, particle tracking microrheology, and viscometry to connect the morphology and rheological properties of the proposed system in semidilute aqueous solutions. The PHOS-g-P(PO-r-EO) copolymers form spherical micelles at room temperature that transform into clusters of spherical subparticles at high temperatures. This investigation elucidates the details of a fascinating transition in morphological and rheological properties that may find applications in many fields as in molecular drug delivery, protein separation, tissue engineering, viscosity modification, and oil industry. On a fundamental point of view it extends the studies in soft colloids made of starlike macromolecules to comblike architectures with intrinsically thermoresponsive groups in aqueous media. Overall, these graft macromolecules are equipped with a two-level hierarchical complexity in terms of tunable hydrophobicity (side chains) and architecture (graft) on top of the steric interactions expected in simpler molecules of the same architecture. The resulting double transition of viscosity within a relatively sharp temperature range manifests the complex and remarkable rheological behavior of the system.

⟨Δr 2(τ )⟩ = ⟨[rN (t + τ ) − rN (t )]2 ⟩t , N

(1)

2. MATERIALS AND METHODS

A detailed account on the particle tracking procedure and analysis of particle trajectories can be found elsewhere.26,27 Bulk Viscometry. Experiments were performed on a Brookfield DVI PRIME cone-and-plate digital viscometer. The desired sample amount was placed on the plate, and the cone was gently lowered during contact. The experiment was run from low to high shear rates, and reproducibility was checked by testing several shear rates (low and high) repeatedly. The limitations of the instrument in minimum and maximum measurable torque resulted in the presented range of measurable shear rates. Temperature was set by a circulating water bath with an accuracy of 0.3 °C.

Materials and Sample Preparation. The thermoresponsive brush copolymer PHOS-g-P(PO-r-EO) with poly(p-hydroxystyrene) as main chain and poly(propylene oxide-ran-ethylene oxide) as side chains was synthesized by a “grafting from” technique as described elsewhere.19 Its molar mass is 1.1 × 106 g/mol with Mw/Mn = 1.16. One PHOS-g-P(POr-EO) macromolecule contains 146 grafted chains (one on every phydroxystyrene monomeric units) with an average of 84 propylene oxide and 76 ethylene oxide monomers randomly distributed. The copolymer is in liquid state at room temperature, and it can be spontaneously dissolved in water. Sample solutions were prepared by mixing the proper amounts of distilled H2O with PHOS-g-P(PO-r-EO). For small-angle neutron scattering D2O (Euriso-top, 99.90% D) was used instead of H2O. The samples were left to equilibrate for more than 48 h at 4 °C before any test. For video particle tracking microrheology polystyrene (PS) microspheres 0.528 μm in diameter were purchased from Polysciences. Their concentration in the final solutions was about 5 × 10−4 v/v so that 40−50 particles were tracked simultaneously. The desired amount of microparticle dispersion was added to H2O before mixing with PHOS-g-P(PO-r-EO) for particle tracking experiments. Small-Angle Neutron Scattering (SANS). Small-angle neutron scattering (SANS) experiments were performed at HZB (reactor BER II) on the instrument V4.22 Three sample−detector distance/wavelength combinations 16 m/1.174 nm, 4 m/0.506 nm, and 1.35 m/0.506 nm were used in order to cover a q-range from 0.002 to 0.5 Å−1. Details on beam collimation and data correction can be found elsewhere.23,24 The BerSANS software developed by Keiderling23 was used for data reduction. Theoretically calculated I(q) was convoluted with a Gaussian distribution so that the resolution in scattering wave vector (Δq/q = Δλ/ λ = 10%) was taken into account.6 The data were fitted by minimizing the sum of the weighted square differences 2 conv (qi) − I exp(qi) ⎞ N ⎛I ⎟ χ 2 = ∑i = 1 ⎜ between the N theoretical and experexp δI (qi) ⎝ ⎠ imental data points by custom-made code in MATLAB with a Monte Carlo algorithm.25 The temperature of the samples was controlled with

3. RESULTS AND DISCUSSION 3.1. Morphology and Dynamics at Room Temperature. We used SANS in order to obtain the structure factor from semidilute solutions of PHOS-g-P(PO-r-EO). The hydrophobic PHOS main chain of the graft terpolymers tends to contract toward a globular conformation. At room temperature PEO is soluble while PPO is insoluble in water.28 Therefore, a single PHOS-g-P(PO-r-EO) macromolecule would form a unimeric micelle with a fairly compact hydrophobic core and a hydrated shell. The SANS data of Figure 1a reveal well-defined profiles that scale proportionally with concentration at q above ∼0.06 Å−1. This is proved as the scattering profiles fall on top of each other at q > 0.06 Å−1 when they are divided by the corresponding concentration (Figure 1b). At lower q a characteristic correlation peak is observed that shifts to higher values and becomes more pronounced at higher polymer solution content. This is a clear sign of interactions between the scattering particles which have a fairly unchanged shape and approach each other as the solution becomes more crowded. The model of interacting spherical core−shell micelles adequately fitted the data of Figure 1a. A simpler model for the shape of the scattering particles, i.e., polydisperse uniform spheres, was tested, and it was not adequate to fit the data (Figure S2 and relevant discussion in the Supporting Information). The data at room temperature (Figure 1a) were fitted by the function Imic(q) in eq 2. The form factor of core−shell micelles Icore−shell(q) (eq 3) is combined with the structure factor of hard spheres Shs(q) which captures the characteristic modulation (correlation peak) at low q. The analytical hard-sphere structure factor can be found elsewhere,9,29 and it is defined by the hard-sphere diameter B


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be equally fitted by a form factor of polydisperse spheres and a form factor of monodisperse ellipsoids.32 This would also be possible for our SANS data, but we chose the polydisperse spherical model because of the following reasons. The length of the main chain of PHOS-g-P(PO-r-EO) (DP = 146) is lower than the one of the side chains (DP = 160); the main chain is hydrophobic, and hence it tends to a globular conformation. The PO segments become gradually more hydrophobic with temperature (following section). Consequently, a spherically symmetric conformation is more possible to represent the morphology at all temperatures. The micellar neutron scattering length density (NSLD) profile ρ(r) of eq 3 consists of a uniform core ρcore = ρpol and a shell of radially varying density eq 4 while Nmic is the number of micelles per unit sample volume. The NSLD of the shell is coupled with the polymer volume fraction by ρsh(r) = φ(r)ρpol + (1 − φ(r))ρD2O. For the polymer a value of ρpol = 0.44 × 10−10 cm−2 was derived from a volume-weighted average of PPO and PEO on the grafted chains. The same value was assumed for the scattering length density of the core; i.e., ρcore = 0.44 × 10−10 cm−2. The PHOS main chain is a small fraction of the grafted polymer, and consequently the inner core of the micellar particles should consist of both PHOS and P(PO-r-EO) blocks. Hence, the SLD of the P(PO-r-EO) block dominates. For the solvent ρD2O = 6.4 × 10−10 cm−2 was used. The core−shell conformation would be similar to the one of star macromolecules with a large number of chains. In that case the volume fraction near the center of the star appears uniform (dense packing of chains), and a varying volume fraction profile appears only at higher distances from the center.33,34 The volume fraction in the shell is assumed to be a decaying function of r i.e. φ(r) = φ0(r/Rc)−α where Rc and Rm are the core and micellar radius, respectively.

Figure 1. (a) SANS profiles from PHOS-g-P(PO-r-EO) in D2O at 10 (gray), 20 (blue), 40 (red), and 80 mg/mL (navy) (25 °C). Data have been shifted by the depicted multiplying factors for clarity. Lines are fits with eq 2. The error in scattered intensity is smaller than the data point size (see Supporting Information for more details). (b) Concentrationnormalized SANS profiles (same colors as in (a)).

(Dhs) and volume fraction (φhs). The term Iint(q) will be explained in the following. Imic(q) = Icore − shell(q)S hs(q) + Iint(q)

⎧ Icore − shell(q) = Nmic⎨4π ⎩

∫0

∞

(2)

(ρ(r ) − ρD O )r 2 2

2 sin qr ⎫ dr ⎬ qr ⎭

(3)

⎧ for 0 ≤ r < R c ρ ⎪ core ρ (r ) = ⎨ ⎪ ⎩ ρsh (r ) for R c ≤ r < R m

The core−shell scattering function is written as in eq 3. Spherical form factors are expected from bottle-brush copolymers with a main chain that is not much longer than the side chains30 or when the main chain is lyophobic.31 Graft copolymers in solution have been also modeled by ellipsoidal form factors. For example, data from amphiphilic graft copolymers under certain conditions can

poly (q ; R c , R m ) = Imic

∫0

+∞

∫0

+∞

∫0

Size polydispersity effects in core and micellar radius was introduced by a Gaussian distribution, i.e.

⎛ dR′c dR′m exp⎜ − ⎝ +∞

∫0

+∞

(4)

(

dR′c dR′m

We have used the same percentage polydispersity for both Rc and Rm.

2⎞

⎛

2⎞

) ⎟⎠ exp⎜⎝−( ) ⎟⎠I (q; R′ , R′ ) ⎛ ⎞ ⎛ ⎞ exp⎜ −( ⎟ ⎟ exp⎜ −( ) ) ⎝ ⎠ ⎝ ⎠ R c − R ′c 2 ΔR c

R c − R ′c 2 ΔR c

R m − R ′m 2 ΔR m

2

mic

R m − R ′m 2 ΔR m

c

m

2

A core−shell form factor shows Porod behavior at high q; i.e., the scattering profile scales as I(q) ∼ q−4. This is not the case in

Table 1. SANS Extracted Parameters for PHOS-g-P(PO-r-EO) at 25 °C concentration (mg/mL) parameter

10

20

40

80

Rc (nm) Rm (nm) a φhs Dhs (nm) dint

6.96 ± 0.19 16.2 ± 0.5 1.38 ± 0.07 0.0708 ± 0.0034 39.4 ± 1.2 2.05 ± 0.10

6.58 ± 0.19 15.4 ± 0.5 1.41 ± 0.07 0.132 ± 0.0066 31.1 ± 1.0 1.95 ± 0.09

6.55 ± 0.20 15.2 ± 0.4 1.49 ± 0.08 0.223 ± 0.011 31.0 ± 0.9 1.79 ± 0.08

6.58 ± 0.21 14.8 ± 0.5 1.02 ± 0.07 0.373 ± 0.012 19.6 ± 0.7 2.05 ± 0.08

C


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Macromolecules our SANS data as a power law of approximately I(q) ∼ q−2 is found (Figure 1a). Hence, the term Iint(q) has been superimposed to eq 2 and is given in eq 5. A low-q cutoff at q ≈ ξint−1 is used so that the term is significant only at high q. The power law is determined by the exponent dint and the prefactor Bint. This nontrivial contribution possibly comes from correlations of chain segments within individual micelles.35 3d int Bint ⎡ ⎛ qξint ⎞⎤ Iint(q) = d ⎢erf⎜ ⎟⎥ q int ⎣ ⎝ 6 ⎠⎦

light scattering on dilute solutions.19 Assuming that the main chain of the grafts is in a rather globular conformation, then the micellar radius at room temperature is a measure of the extension of the side chains. In the Daoud−Cotton36 model for starlike polymers the micellar radius is R ≈ N3/5f1/5b. Using N = 160, f = 146, and b = 0.35 nm for the number of monomers per star arm, number of arms per star, and monomer length, respectively, one obtains R ≈ 20 nm. This value is compatible with the observed Rm = 15.4 ± 0.6 as at room temperature PO units are not hydrophilic and drive the chains to more compact conformations. It is interesting to make some observations on the hard-core radius Rhs = Dhs/2 in comparison to the micellar radius Rm. At 10 mg/mL we find Rm < Rhs which can be an effect of size polydispersity (∼15% at 25 °C). At 20 and 40 mg/mL Rm ≈ Rhs, which means that the micelles interact as compact hard spheres. At 80 mg/mL Rm > Rhs (Table 1) which shows that at high concentration interdigitation between micelles is evident and reveals the soft nature of the micellar corona. In Figure 3 the VPTMR data at 25 °C are presented. The MSDs (Figure 3a) are clearly diffusive with Δr2(τ) ∼ τ, and

(5)

The resulting fitted parameters from SANS are presented in Table 1. The core radius does not seem to vary systematically (within experimental error) as a function of concentration. Its average value over the four concentrations is Rc = 6.67 ± 0.20 nm. The integrity of the micellar objects is verified by the expected scaling with concentration of Figure 2. The number density of

Figure 2. Number of micelles per unit volume (a) and scattering at high q from PHOS-g-P(PO-r-EO) in D2O (25 °C). Lines are linear fits. Fitted slopes are also shown.

micelles Nmic and the scattered intensity at high q, Iint(q = 0.3 Å−1) = Bint(0.3 Å−1)dint, which is determined by the internal correlations, are proportional to concentration. The correlation length that defines the onset of intramicellar scattering is concentration independent with a value ξint = 3.57 ± 0.34 nm. The characteristic exponent dint = 1.96 ± 0.12 can be attributed to Gaussian statistics of individual P(PO-r-EO) chains and contacts between chain segments.6,9 The exponent α of the shell volume fraction profile has a nonsystematic variation with concentration (Table 1). Except from the highest concentration its value is near α = 4/3, which is expected from neutral macromolecular spherical brushes.33 The low value found for 80 mg/mL points to a more uniform distribution of monomers. The micellar radius Rm shows a weak decrease, but this is near the limits of experimental uncertainty. This could be related to shrinkage of the micellar shells as the concentration increases and intermicellar interactions become stronger. The impact of concentration is more pronounced on the effective hard-sphere diameter Dhs which decreases significantly. Nevertheless, Rm is compatible with the micellar hydrodynamic radius at room temperature measured by dynamic

Figure 3. VPTMR results at 25 °C. (a) MSDs in PHOS-g-P(PO-r-EO) aqueous solutions (H2O) at 0 (□), 42.5 (■), 75 (○), and 100 mg/mL (●). Straight lines indicate slope 1. (b) Relative viscosity from PHOS-gP(PO-r-EO) aqueous solutions. The fits with the Batchelor (dashed blue line) and Krieger−Dougherty (red line) relation are also shown. Green points are bulk viscometry data. Uncertainty is smaller than point size (see Figure 6).

hence the solutions are viscous fluids at the probed time scales. The magnitude of the MSD systematically drops with concentration, illustrating the increase in viscosity (η) which is extracted by the Stokes−Einstein relation (eq 6). η=

kBT 6πDR

(6)

D is the probe particles diffusion coefficient which is taken from the experimentally observed 2D trajectories, i.e., Δr2(τ) = 4Dτ, kBT is the thermal energy, and R is the radius of the probe particles. Viscosity values are normalized with the viscosity of the solvent (ηs) and presented as relative viscosity ηr = η/ηs. This D


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intensity and hydrodynamic radius. Therefore, intermolecular aggregation takes place above the LCST. This effect was accompanied by a continuous increase of hydrophobicity as measured by fluorescence emission of pyrene.19 In Figure 4, the SANS profiles from the four tested concentrations for different temperatures are presented. The

normalization is more useful in the next section because the effect of temperature on solvent viscosity needs to be taken into account. In Figure 3b, the relative viscosity is plotted as a function of concentration. The dependence is weak at low concentrations and becomes very strong at higher ones. This is typical for soft and hard colloidal particles in solution. The rheology of soft colloids has been extensively studied in the past two decades.36 Model soft-colloidal systems can be realized using star macromolecules or core−shell self-assembled diblock copolymer micelles.37 These systems offer the opportunity to bridge the gap between purely soft components, i.e., polymers, and hard systems, i.e., colloids.36 η = 1 + 2.5φeff + 5.9φeff 2 ηs (7) The zero-shear viscosity data of dispersed colloids are often described as a function of the effective volume fraction φeff. The effective volume fraction in the case of hard spheres is φeff = c/c*. In Vlassopoulos et al.37 the overlap concentration has been calculated using the hydrodynamic diameter of the soft colloids. This determination was proved suitable for a generic description of viscosity data. In other works, φeff has been estimated by applying the Batchelor relation (eq 7) at low concentrations. In Figure 3b, the fit with eq 7 (dashed blue line) resulted in c* ≈ 172 mg/mL, which is comparable to the overlap concentration we obtain from SANS using the micellar radius; i.e., c* = (M/NA)/ (4/3πRm3) is found at ∼120 mg/mL. The discrepancy between the two values for c* can be explained by the polydispersity of the micelles (∼15%). Hence an uncertainty in c* larger than 40% is introduced. This implies that smaller size micelles dominate the hydrodynamic interactions and a concentration higher than the average calculated c* is needed for the strong increase in viscosity to occur. We have to point out that VPTMR with thermally fluctuating probe particles is a linear viscoelasticity measurement as it is based on weak thermal perturbations.38 This way the extracted viscosity can be compared to the zero-shear viscosity obtained by bulk rheology. −2.5φm ⎛ φeff ⎞ η ⎜ ⎟ = ⎜1 − ⎟ ηs φm ⎠ ⎝

(8)

The Krieger−Dougherty formula (eq 8) has been proposed for the zero-shear viscosity data of hard colloids. In the case of soft colloids it predicts maximum packing volume fractions φm higher than 0.59 which is expected for hard spheres.36,37 This deviation is connected to interpenetration and “squeezing” of the soft particles.37 We used eq 8 (red line in Figure 3b) with c* ≈ 172 mg/mL and obtained φm ≈ 0.73. Similar values have been reported for star polymers39 and thermoresponsive microgels40 although in the latter case the intrinsic viscosity used was higher than 2.5. Consequently, the SANS picture, where in dilute concentrations the interaction between micelles is weak and interdigitation takes place at higher ones, is completed by the viscosity data. The concentration dependence of the relative viscosity is described as the one of soft spherical colloids (stars or core−shell micelles), and the interpretation between micelles is evident at high concentrations. 3.2. Temperature Response. The PHOS-g-P(PO-r-EO) grafted polymer under study has a LCST at about 40 °C in dilute solutions.19 This was observed by static and dynamic light scattering measurements by a strong increase in both scattered

Figure 4. SANS profiles from PHOS-g-P(PO-r-EO) at 10 (a), 20 (b), 40 (c) and 80 mg/mL (d) in D2O at 25 (gray), 37 (red), 38 (blue), 39 (navy), 41 (green), and 44 °C (black) (multiplied by 1, 2, 4, 8, 16, and 32, respectively, for clarity). Red lines are indicative fits for the different models. Straight lines indicate Porod’s law. The error in scattered intensity is smaller than the data point size (see Supporting Information for more details).

quality of the fits on all SANS data is presented in Figures S3−S6. The measurements were performed near the transition temperature. The SANS profiles are strongly affected by temperature. The clear feature at 0.06 Å−1 (25 °C) which originates from the well-defined spherical shape of the particles disappears at 37 °C. This is a sign of formation of particles of different (possibly simpler) morphology than the initial core−shell micelles or an effect of polydispersity. At the highest temperature (44 °C) there is evidence of scattering from large objects with well-defined interfaces (Porod scaling at low q) and correlations at q relatively E


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Macromolecules Table 2. Fitting Models for SANS Profiles at Different Temperatures temperature (°C) concentration (mg/mL)

25

37

38

39

41

44

10 20 40 80

core−shell core−shell core−shell core−shell

spheres spheres spheres spheres

spheres spheres spheres spheres

spheres spheres core−shell core−shell

core−shell core−shell fractal clusters fractal clusters

spherical clusters spherical clusters fractal clusters fractal clusters

Table 3. SANS Extracted Sizes for PHOS-g-P(PO-r-EO) at Different Temperatures temperature (°C) 25

37

38

39

41

44

9.96 ± 0.23

11.0 ± 0.31 35.5 ± 0.4 69.2 ± 2.0

5.00 ± 0.22

Rc (nm) Rm (nm) Dhs (nm) Rclust (nm)

6.96 ± 0.19 16.2 ± 0.5 46.4 ± 1.2

8.93 ± 0.27

10 mg/mL 9.26 ± 0.25

41.1 ± 1.2

41.7 ± 1.2

42.4 ± 1.4

Rc (nm) Rm (nm) Dhs (nm) Rclust (nm)

6.58 ± 0.19 15.4 ± 0.5 31.1 ± 1.0

9.31 ± 0.22

20 mg/mL 10.8 ± 0.29

10.7 ± 0.21

36.1 ± 1.0

35.9 ± 1.1

36.5 ± 0.9

Rc (nm) Rm (nm) Dhs (nm) Rclust (nm) dclust

6.55 ± 0.20 15.2 ± 0.4 31.0 ± 0.9

13.7 ± 0.5

40 mg/mL 14.2 ± 0.6

43.1 ± 1.9

52.9 ± 2.2

Rc (nm) Rm (nm) Dhs (nm) Rclust (nm) dclust

6.58 ± 0.21 14.8 ± 0.5 19.6 ± 0.7

13.0 ± 0.5

80 mg/mL 13.7 ± 0.4

27.8 ± 0.9

28.8 ± 1.1

higher (0.04−0.05 Å−1) than the characteristic correlation peak (0.02−0.03 Å−1) at room temperature. Hence, the large clusters at high temperatures are evidently composed of particles that are smaller than the initial micelles. The increase of temperature resulted in qualitative different morphologies in the PHOS-g-P(PO-r-EO) solutions. As a consequence, the scattering profiles were not adequately fitted by eq 2. Hence, different models were tested for the analysis. The scattering of polydisperse homogeneous spheres Icore(q) fitted the data at intermediate temperatures (spheres, Table 2) and is calculated by eq 2 removing the shell term, i.e., setting ρsh(r) = 0 in eq 4. The profiles at the highest temperatures that show presence of clusters are modeled by eqs 9 and 10. The structure factor of hard spheres Shs(q) is combined with the structure factor that represents arrangements inside spherical (Ssph(q)) or fractal (Sfract(q)) clusters (Table 2). sph Iclust (q) = Icore(q)(Ssph(q) + S hs(q)) + Iint(q)

fract Iclust (q) = Icore(q)(Sfract(q) + S hs(q)) + Iint(q)

12.4 ± 0.26 34.7 ± 0.3 67.4 ± 2.1

9.35 ± 0.33 31.6 ± 0.9 5.43 ± 0.24 11.1 ± 0.34 39.9 ± 1.3

12.3 ± 0.25 37.0 ± 1.2 59.0 ± 2.2

3.03 ± 0.12

4.69 ± 0.15

13.9 ± 0.4 39.8 ± 1.3 3.27 ± 0.13

16.8 ± 0.5 101 ± 3 3.45 ± 0.14

12.7 ± 0.25 25.5 ± 0.8 43.5 ± 1.4

4.70 ± 0.12

3.71 ± 0.11

11.1 ± 0.4 35.0 ± 1.6 3.21 ± 0.14

15.3 ± 0.6 131 ± 4 4.20 ± 0.2

subparticles per cluster Nclust. The symbols have been chosen the same as for spherical clusters for the sake of presentation in Table 3. The characteristic exponent of the fractal clusters is dclust. Bclust is a non-free parameter set so that there is a smooth transition from the Guinier to the power-law regime.42 ⎤2 ⎡ sin(qR ) − qR clust clust cos(qR clust) ⎥ Ssph(q) = Nclust⎢3 (qR clust)3 ⎦ ⎣ (11)

Sfract(q) = Nclust

3dclust ⎛ (qR )2 ⎞ Bclust ⎡ ⎛ qR clust ⎞⎤ clust ⎟ + d ⎢erf⎜ exp⎜− ⎟⎥ 3 ⎝ ⎠ q clust ⎣ ⎝ 6 ⎠⎦

(12)

Table 2 reveals qualitative differences between low (10 and 20 mg/mL) and high concentrations (40 and 80 mg/mL) regarding the temperature response of PHOS-g-P(PO-r-EO) solutions and the representative models. At all concentrations the interacting core−shell micelles (25 °C) are transformed into homogeneous spheres (37 and 38 °C). At low concentrations the homogeneous-sphere morphology remains up to 39 °C. The core−shell morphology is found again at 41 °C, and eventually at 44 °C the system forms large spherical clusters of interacting homogeneous spheres. At high concentrations the spherical shape at 38 °C changes to core−shell at 39 °C, i.e., at lower temperature than the low concentrations. Cluster formation also

(9) (10)

The structure factor of the spherical clusters is given in eq 11 and is defined by the cluster radius Rclust and the number of subspheres per cluster Nclust. It has been proposed by Hansen et al.41 for describing spherical clusters of casein micelles. The fractal structure factor of eq 12 is the Beaucage model41 and contains the cluster gyration radius Rclust and the number of F


Article

Macromolecules

Scheme 1. Formation of Clusters in Aqueous Solutions of PHOS-g-P(PO-r-EO) for Low (a−c) and High (d−f) Concentrationsa

a

The morphologies at room temperature (a, d), at temperatures near the region of viscosity increase (b, e), and at high temperatures (c, f) are presented. PHOS and P(PO-r-EO) are illustrated by black and gray respectively.

at high temperatures for 10 and 20 mg/mL. At 40 and 80 mg/mL polydispersity is lower (∼20%). This is related to the non homogeneous collapsing of macromolecular grafts and their random interassociation caused by intense hydrophobic interactions. A schematic representation of the morphology in PHOS-g-P(PO-r-EO) aqueous solutions as a function of concentration and temperature is drawn in Scheme 1. The bulk viscosity from PHOS-g-P(PO-r-EO) aqueous solutions (Figure 5) shows mostly Newtonian behavior at the accessible shear rates. Data from 10 mg/mL are not shown because they do not have any significant temperature or shear rate dependence. It is notable that at temperatures within the viscosity transition (Figure 5 and following discussion) strong clear shear thinning is observed. This highlights the formation of a complex viscoelastic fluid which will be discussed in the following. In 80 mg/mL shear thinning is not observed (Figure 5c), but in this case measurements within the transition regime are restricted to one or two shear rates or impossible (indeed measurements between 41 and 42 °C are outside the viscometer range). In order to plot the values from the different concentrations as a function of temperature (Figure 6), the extrapolated values to the lowest shear rate from every experiment was used. The viscosity of PHOS-g-P(PO-r-EO) solutions as a function of temperature is shown in Figure 6. The viscosity is independent of temperature up to 39 °C (except from 80 mg/mL where there is an initial drop). In Figure 3 the data at room temperature are compared to VPTMR and have reasonable agreement. The viscosity experiences a strong maximum between 40 and 43 °C for concentrations higher than 20 mg/mL. At 80 mg/mL the viscosity values overcome the limits of the instrument’s capability at their maximum. For that reason no measurements were possible between 41 and 42 °C.

occurs at lower temperature, and remarkably the formed clusters are fitted by a fractal model and not a spherical one. Observing the numerical results of Table 3, a detailed picture can be drawn. It is more convenient to consider temperatures that are lower than the ones where the clusters are formed, first. The radius of the uniform core Rc increases with temperature for low concentrations (10 and 20 mg/mL) and somewhat more strongly for high concentrations (40 and 80 mg/mL). At the same time the number of particles per unit volume Nmic decreases (not shown). This indicates aggregation between particles into larger ones. At the extremes of this temperature regime, where the core−shell model applies, Rm is comparable to Dhs/2. For 40 mg/mL at 39 °C and 80 mg/mL at 25 and 39 °C there are obvious signs of interdigitation (Rm > Dhs/2). At the intermediate temperatures the radius of the solid spheres Rc is clearly lower than Dhs/2, except from 80 mg/mL. A plausible explanation is the presence of a diffuse shell that is not dense enough to produce measurable contrast. At 80 mg/mL Rc is comparable to Dhs/2, which is related to a high interpenetration. The clusters that appear at 44 °C for low concentration and at 41 °C for high concentrations consist of spherical subparticles with small Rc (Table 3). For low concentrations the uniform spheres are rather free of a brush shell because Rc ≈ Dhs/2. On the contrary, at high concentrations Rc < Dhs/2. Within the clusters the spherical subparticles are apparently connected with P(PO-rEO) blocks creating a 3D network. This explains the surface fractal morphology of the clusters at higher concentrations. These structures are significantly larger than the compact spherical clusters that are formed at lower concentrations (Table 3). The number of subparticles within the clusters Nclust is much higher for 40 and 80 mg/mL in comparison to 10 and 20 mg/mL. It has to be noted that the polydispersity in core (and micellar) sizes (ΔR/R) changes from ∼15% at room temperature to ∼25% G


Article

Macromolecules

the samples are milky-white (especially at 40 and 80 mg/mL). The clusters are at this point independent of each other with well-defined interfaces and no interconnections. In Mortensen and Pedersen the phase behavior of PEO-bPPO-b-PEO triblock copolymers was influenced by the micellization of the individual chains as a function of temperature and concentration. Temperature induces a strong aggregation into micelles that leads to a crystalline structure of hard spheres. At very high temperatures the spherical structures change into prolate ellipsoids and the decrease of intermicellar interactions cause melting of the cubic lattice.28 A sol−gel transition in poly(ε-caprolactone)−poly(ethylene glycol)−poly(ε-caprolactone) at body temperatures has been related to bridging connections between micelles. This broad transition (from 25 to 40 °C) was in pair with a gel−sol transformation at high temperatures (from 40 to 55 °C). The second transition was explained by a possible destruction of the micelles coming from strong thermal motion of ε-caprolactone block.5 To conclude, our system passes from moderately viscous solutions of interacting soft colloids to interacting clusters of small spherical subparticles that maximize viscosity and finally to an opaque dispersion of independent well-defined clusters. MSDs obtained by VPTMR at 80 mg/mL indicate that some viscoelastic character can be detected at elevated temperatures (Figure 7). The fractal morphology and hierarchical organization

Figure 5. Viscosity from PHOS-g-P(PO-r-EO) aqueous solutions (H2O) at 20 (a), 40 (b), and 80 mg/mL (c). Figure 7. VPTMR MSDs in PHOS-g-P(PO-r-EO) 80 mg/mL aqueous solution (H2O) at 26.7 (□), 40.9 (■), 41.5 (○), and 42.1 °C (●). Red lines are fits with eq 13. Straight lines indicate slope 1.

are the cause of self-similar viscoelasticity in critical gels.43,44 A power law, different than Δr2(τ) ∼ τ1, is observed at short times for the two highest temperatures at 80 mg/mL. A power law in the MSD Δr2(τ) ∼ τa obtained by VPTMR is the signature of self-similar viscoelasticity in the time domain. This is equivalent to a power-law in the viscoelastic moduli G′ ∼ G″ ∼ ωa or in other words self-similarity in the frequency domain.45 a rel

Figure 6. Extrapolated viscosity from PHOS-g-P(PO-r-EO) aqueous solutions (H2O) at 10 (black), 20 (red), 40 (blue), and 80 mg/mL (green).

Δr 2(τ ) = A(1 − e−(τ / τrel) ) + 4Dτ

(13)

The MSD from 80 mg/mL has viscous behavior up to 40.9 °C (Figure 6). At this regime the viscosity is extracted from eq 6. For the two highest temperatures accessible by VPTMR the MSD is

Based on the picture obtained by SANS, the formation of clusters is related to the increase in viscosity. In the region of the strong transition the system is in the onset of forming (10 and 20 mg/mL) or have already formed clusters (40 and 80 mg/mL). The interconnections and steric interactions of these clusters are responsible for the viscosity increase. At 10 mg/mL there is no observable change in the viscosity within the critical temperature range because in this case the concentration and size of the formed clusters are not big enough to critically affect viscosity. At 44 °C the viscosity drops to values near the viscosity of water and

Table 4. Viscoelastic Parameters from MSDs in 80 mg/mL at Temperatures within the Viscosity Transition temperature (°C)

H

parameter

41.5

42.1

A (μm2) τrel (s) arel

(1.8 ± 0.1) × 10−3 0.29 ± 0.02 0.76 ± 0.01

(3.4 ± 0.1) × 10−3 1.3 ± 0.02 0.69 ± 0.01 DOI: 10.1021/acs.macromol.7b02711 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules fitted by eq 13. The first term captures the initial subdiffusive region as A(1 − e

−(τ /τrel)arel

)≈A

4. CONCLUSIONS We investigated the thermally triggered self-assembly in PHOSg-P(PO-r-EO) aqueous solutions by SANS and rheological methods. At room temperature the PHOS-g-P(PO-r-EO) macromolecules are defined as spherical core−shell micelles, and the viscosity is governed by soft-colloidal behavior. The increase in temperature results to a system of larger spherical homogeneous spheres and ends up in clusters of spherical subparticles. At a critical region between 40 and 43 °C a strong maximum in viscosity appears, and it is related to the interactions between clusters. At even higher temperatures the solutions viscosity drops to values lower than the ones at room temperature, and the solutions are no longer transparent. Viscoelastic characteristics are detected at the highest tested concentration within the critical temperature regime. The elucidation of the relation between morphological and viscosity transitions in this thermoresponsive graft-type macromolecular system provides the foundation for its possible use in biomedical and industrial applications and presents guidelines for further polymeric materials development with enhanced characteristics.

a rel

( ) τ τrel

for short times. The

exponential term is necessary for best fit, and it follows the quasiplateau at intermediate times (more evident at ∼1 s at 42.1 °C). A characteristic exponent arel and a relaxation time τrel are obtained by this term (Table 4). The second term represents the terminal diffusive trend which follows at high lag times, and the low frequency viscosity is measured. The obtained viscosities are presented in Figure 8 together with the values measured at 40 mg/mL. At temperatures higher than ∼43 °C the solutions are opaque, and the diffuse background makes the probe particles invisible.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02711. Representative uncertainties in meaured SANS intensities; test of fits with uniform sphere form factor; fits of the whole set of SANS data (PDF)

Figure 8. Viscosity obtained by VPTMR from PHOS-g-P(PO-r-EO) aqueous solutions (H2O) at 40 (blue) and 80 mg/mL (green). Uncertainty is smaller than point size (see Figure 3b).

■

AUTHOR INFORMATION

Corresponding Author

The increase in temperature enhances intermolecular associations and clustering which has been shown by SANS. At 80 mg/mL the interpenetration of micellar shells at room temperature leads to a stronger dependence on temperature because intermicellar associations are preferred from shrinkage of individual graft macromolecules. At the two highest temperatures viscoelasticity appears. The characteristic relaxation time increases significantly from 41.5 to 42.1 °C. At the same time the elastic component that is significant at τ < τrel is enhanced; i.e., arel drops. This is a signature of the formation of a network of interconnected micelles that ends up in fractal clusters at even higher temperatures (>43 °C). This network is the cause of the power law Δr2(τ) ∼ τarel at short lag times. The MSDs at concentrations lower than 80 mg/mL are diffusive (Δr2(τ) ∼ τ) with no sign of viscoelasticity (within the probed time scales). However, bulk viscosity data revealed viscoelasticity (in the form of shear thinning) at both 20 and 40 mg/mL. As can be seen in Figure 8, the increase in viscosity at ∼40 °C is also followed by the VPTMR viscosity at 40 mg/mL PHOS-g-P(PO-r-EO). At 20 mg/mL (not shown) the temperature thickening transition is not detected by VPTMR, contrary to bulk viscometry. This could have the same origin with the apparent insensitivity of VPTMR on viscoelasticity at 20 and 40 mg/mL within the critical temperature regime. One explanation could be the formation of inhomogeneities at length scales comparable to or larger than the probe-particle size.46 In any case the combination of the two rheological methods undoubtedly demonstrates the transition under study and describes its main features, which satisfies completely the purposes of the current study.

*E-mail: [email protected] (A.P.). ORCID

Aristeidis Papagiannopoulos: 0000-0002-5662-9866 Junpeng Zhao: 0000-0002-2590-0027 Charl J. Jafta: 0000-0002-9773-6799 Notes

The authors declare no competing financial interest.

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REFERENCES

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