Formation and Growth of Mesoglobules in Aqueous Poly(N

Letter Cite This: ACS Macro Lett. 2018, 7, 1155−1160

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Formation and Growth of Mesoglobules in Aqueous Poly(N‑isopropylacrylamide) Solutions Revealed with Kinetic Small-Angle Neutron Scattering and Fast Pressure Jumps Bart-Jan Niebuur,† Leonardo Chiappisi,‡,§ Xiaohan Zhang,† Florian Jung,† Alfons Schulte,*,∥ and Christine M. Papadakis*,†

ACS Macro Lett. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/12/18. For personal use only.

†

Technische Universität München, Physik-Department, Fachgebiet Physik weicher Materie, James-Franck-Str. 1, 85748 Garching, Germany ‡ Institut Laue-Langevin, Large Scale Structures Group, 6, rue Jules Horowitz, 38042 Grenoble, France § Technische Universität Berlin, Stranski Laboratorium für Physikalische Chemie und Theoretische Chemie, Institut für Chemie, Straße des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany ∥ Department of Physics and College of Optics and Photonics, University of Central Florida, 4111 Libra Drive, Orlando, Florida 32816-2385, United States S Supporting Information *

ABSTRACT: The phase transition from swollen chains to polymer mesoglobules of an aqueous solution of poly(N-isopropylacrylamide) is investigated with kinetic small-angle neutron scattering with 50 ms time resolution in conjunction with millisecond pressure jumps across the coexistence line. The time-resolved study evidenced three distinct regimes: fractal clusters form during the first second and transform into compact mesoglobules. During the following ∼20 s, these grow by diffusion-limited coalescence. The final step consists of a slow growth characterized by an energy barrier of several kBT. The method opens opportunities for kinetic structural studies of multicomponent systems over wide length and time scales and gives a structural picture spanning from the chain collapse to mesoscopic phase separation.

P

relatively large volumes in the absence of thermal effects. While a number of studies using pressure jumps were carried out on polymeric systems as well as on protein and lipid solutions in combination with NMR,13 light scattering,14−18 and synchrotron small-angle X-ray scattering,19−22 the potential of kinetic SANS remains largely unexplored, except for high-pressure stroboscopic or pressure modulation devices.23,24 In the present work, we apply a sudden pressure jump to a solution of a thermoresponsive polymer and use kinetic SANS to study their aggregation process. In polymer solutions, the phase separation is strongly influenced by the dynamical asymmetry in the system, leading to a “moving droplet” state after a quench into the two-phase state, where coalescence is hindered due to the viscoelastic effect.25−30 However, the pathway from the molecularly dissolved state to the “moving droplet” phase has not been elucidated. To investigate the pathway from molecularly dissolved polymers to aggregates from collapsed chains, we chose poly(N-isopropylacrylamide) (PNIPAM). PNIPAM exhibits

hase transitions are ubiquitous in nature with their location depending on fundamental thermodynamic variables, temperature, and pressure. A possible consequence is aggregation. The controlled assembly of (functionalized) nanoparticles,1−3 colloids,4−6 or proteins7 enables the design of materials that have great potential in sensing, imaging, optoelectronics, synthetic biology, and medical therapy. Apart from the nature of the building blocks, the formation of aggregates depends on the strength of the attractive forces between them and may involve transient states. Determining the aggregation kinetics of molecules or particles is thus paramount for an understanding and control of the pathways.8−10 Structural information across many length scales (∼1−100 nm) can be obtained with small-angle neutron scattering (SANS), which provides a time resolution of milliseconds and the possibility of highlighting selected components by means of contrast variation. Rapid temperature changes across a phase transition have proven useful to induce phase transitions, and the resulting structural changes were often investigated using time-resolved scattering methods.8−10 However, due to the relatively large sample volume, temperature equilibration after a jump may take a few minutes to complete.11,12 In contrast, pressure jumps present a well-controlled way to trigger phase transitions because they allow a fast change of state even for © XXXX American Chemical Society

Received: August 10, 2018 Accepted: September 4, 2018

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DOI: 10.1021/acsmacrolett.8b00605 ACS Macro Lett. 2018, 7, 1155−1160

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ACS Macro Letters lower critical solution temperature behavior; i.e., the chains are molecularly dissolved below the cloud point (one-phase state), whereas mesoglobules (moving droplets) are formed above.31−37 Mesoglobules can be induced not only by a temperature increase across the cloud point at constant (atmospheric) pressure31,34−36,38,39 but also by variation of pressure at constant temperature.40−42 At atmospheric pressure, spectroscopic methods have addressed the short-time regime (milliseconds) after a temperature jump, i.e., the chain collapse and formation of hydrophobic domains.43−47 Most of these studies addressed extremely diluted solutions. Investigations using scattering methods on more concentrated solutions were restricted to much longer time scales (seconds−hours), i.e., to the mesoglobules.11,12,31,38 A comprehensive study is still missing. We employ in situ, real-time SANS to investigate a semidilute solution of PNIPAM (36 kg mol−1) at 3 wt % in D2O (i.e., well above the overlap concentration, see the Supporting Information, SI) after a sudden pressure jump from the onephase state across the cloud point, which allows us to characterize the effects of the chain collapse and the mesoglobule formation and growth on a time scale from 0.05 to 1649 s. This work thus provides new insight into the mechanisms of polymer mesoglobule formation spanning time and size ranges not accessed before. Before the kinetic SANS experiment, the pressure-dependent coexistence curve of the 3 wt % PNIPAM solution in D2O was measured (Figure 1, for experimental details see the SI).

Figure 2. SANS data of the 3 wt % PNIPAM solution in D2O after the jump with Δp = −15 MPa (a) and Δp = −21 MPa (b). Black symbols: prerelease measurements. The growth regimes I (blue curves), II (red curves), and III (orange curves) are indicated on the left.

fluctuations, ξOZ, which amounts to ∼15 nm, as well as an amplitude, IOZ. After the pressure jump, the intensity decay in the high-q region corresponding to chain correlations is still observed. Upon crossing the coexistence line, the chain conformation of the polymers is expected to change from swollen to collapsed. In this regime, the scattering curves are still modeled by the Ornstein−Zernike structure factor to extract a correlation length. After 1.4 s for Δp = −15 MPa and 0.3 s for Δp = −21 MPa, an additional process is observed below q = 0.1 nm−1, whose intensity is rapidly increasing with time. It is attributed to the formation and growth of mesoglobules consisting of collapsed chains. To extract their size and surface structure in dependence on time, the Guinier−Porod model was used (eq S4 in the SI).49 It includes the radius of gyration of the mesoglobules, Rg, the Porod exponent m, describing the surface structure of the mesoglobules, as well as its amplitude Iagg0. We use this empirical model over the entire time range after the mesoglobule formation. (Fits of disperse homogeneous spheres are not stable in the entire time range.) After the formation of mesoglobules, chain scattering is still evident at q > 0.6 nm−1, albeit with much lower intensity. We attribute it to concentration fluctuations inside the mesoglobules, indicating that they contain water. For this contribution the Ornstein− Zernike structure factor was again used as a simple model. Over the entire time range, least-squares fits are in excellent agreement with the scattering curves (Figure S2 in the SI).

Figure 1. Temperature−pressure phase diagram of the 3 wt % PNIPAM solution in D2O measured by turbidimetry (for details, see the SI). The arrows indicate the start and target pressures of the jumps at 35.1 °C. Inset: Chemical structure of PNIPAM.

At atmospheric pressure, the cloud point is observed at 33.7 °C. It increases and reaches a maximum of 35.9 °C at ∼60 MPa. The phase transition was induced by reducing the pressure by Δp = −15 MPa or −21 MPa starting at 31 MPa and 35.1 °C. The kinetic SANS data are displayed in Figures 2a and 2b on logarithmic scales for momentum transfer and time. In spite of the low polymer concentration, a strong scattering signal is observed, which is due to the high scattering contrast in the SANS experiment. In the prerelease scattering curve, the decay in the high-q region (above 0.06 nm−1) is due to local correlations in the semidilute polymer solution and is therefore modeled by the Ornstein−Zernike structure factor (eq S3 in the SI).48 It comprises the correlation length of concentration 1156


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Figure 3. Time dependence of characteristic lengths and amplitudes obtained from fits to the SANS data for the two pressure jumps given at the top. Top panels: Amplitudes of the Ornstein−Zernike structure factor, IOZ, and of the Guinier−Porod form factor, Iagg0. Middle: Ornstein−Zernike correlation length ξOZ (left axis), radius of gyration of the mesoglobules, Rg (left axis), and Porod exponent m (right axis). Dotted black lines: Fits to Rg(t) with the logarithmic growth model to the final stages (eq 1). Bottom: Ornstein−Zernike correlation length ξOZ in regimes II and III.

After ∼30 s for Δp = −15 MPa and ∼10 s for Δp = −21 MPa (regime III), the growth of the mesoglobules slows down, suggesting the appearance of an energy barrier that hinders the coalescence of mesoglobules. After the larger jump, the system is further away from the coexistence line, which is expected to lead to a faster phase separation. However, the chains also lose their mobility sooner, and this is the time at which the slow process starts. The Porod exponent m decreases gradually to 4, indicating a smoothening of the surface51 caused by the slow permeation of water through the dense PNIPAM shell. At the longest time measured (∼1650 s), Rg is ∼90−100 nm. The slowing down of the mesoglobules’ growth is described by Rg(t) ∼ log(t). This relation is characteristic for hindered growth, where aggregation is hampered by an energy barrier ε.53 More precisely, the hindered-growth model predicts:

The time dependence of characteristic lengths and amplitudes obtained from the fits is depicted in Figure 3. At the earliest times (regime I), mesoglobules are absent. The amplitude of the scattering due to concentration fluctuations (or chain scattering), IOZ, increases (Figure 3a,b); i.e., the local concentration fluctuations become more pronounced. Since IOZ does not increase exponentially, phase separation by spinodal decomposition can be ruled out.50 The correlation length ξOZ increases from ∼15 nm to ∼30 nm, indicating the formation of small clusters. We conclude that phase separation proceeds by nucleation and growth. When the metastable phase is reached (regime II, after ∼1.4 s for Δp = −15 MPa and ∼0.3 s for Δp = −21 MPa), the amplitude of the scattering from mesoglobules, Iagg0, starts to rise. The radius of gyration, Rg, also increases with time, which may be attributed to a high mobility of the still hydrated chains.37 Both findings are in agreement with those on significantly more dilute solutions.31,38 The Porod exponent m, describing the surface structure of the mesoglobules (Figure 3c,d) has an initial value of ∼2; i.e., the initially formed clusters are loose mass fractals. It subsequently rises to values in the range of 5−6, indicating compact mesoglobule surfaces with a concentration gradient.51 This gradient is due to the rapid water diffusion out of the nearsurface layer, leading to a dense shell. Water is trapped inside the mesoglobules,52−54 which is consistent with the persistence of chain scattering with ξOZ ≈ 6 nm (Figure 3e,f). (The transient depression of IOZ and ξOZ may be an artifact in model fitting because ξOZ and Rg are still quite close to each other.) In the beginning of regime II, Rg(t) ∝ t1/3 (Figure 3c,d); i.e., the coalescence is diffusion limited.55

R (t ) =

R 0 ijj u t yzz zz lnjj u jj 3 τlog zz k {

(1)

Here R0 is the initial mesoglobule radius, u = ε/kBT the reduced energy barrier (kB is Boltzmann’s constant and T the absolute temperature), and τlog the average time between collisions of mesoglobules of radius R0.53 We assume values of R0 of 27 and 40 nm for Δp = −15 MPa and −21 MPa, respectively. The energy barriers obtained from the fits (Figure 3c,d) are ε = 6.2 kBT and 6.3 kBT for Δp = −15 MPa and −21 MPa, respectively. These values are much larger than the thermal energy, implying that aggregation is strongly hindered in regime III. The values are comparable to the ones reported in ref 53. An energy barrier much higher than kBT was previously 1157


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wavelength. The sample with thickness 2 mm was mounted in a temperature-controlled copper beryllium pressure cell, where it is separated from the pressurizing medium by a movable piston.57 The pressure cell and the pressure generator were connected by a pneumatically driven valve (PDV), enabling fast pressure changes.13,58,59 Before the jump, the PDV was open, and the pressure in the whole system was set to the desired initial value. The PDV was closed, and the prerelease curve was measured. The pressure in the external system was set to a lower value, and the pressure in the sample cell was rapidly decreased by opening the pneumatic valve with an electronic trigger. The response time of the PDV is less than 100 ms. We estimate that 0.1 mL of solution crosses the pneumatic valve of a diameter of 1 mm in ∼10−5 s without significant oscillations in pressure.16 For details on the experiment and the raw data treatment, see the SI.

attributed to the viscoelastic effect (entanglement force) hindering the coalescence of two mesoglobules: the interaction time τi during collisions is very small compared to the entanglement time τe needed for merging,28 which is equivalent to an energy barrier much higher than kBT.56 The collision times are τlog = 2.2 × 10−6 s and 5.2 × 10−4 s obtained for Δp = −15 MPa and −21 MPa, respectively. They compare to the encounter and coalescence time τD estimated from the relation τD = 3η/(8C0kBT) with η being the viscosity of the solvent and C0 the initial number density of mesoglobules (see the SI).55 The hindered-growth regime is similar to the moving droplet phase25−28 where the growth is described to follow a power law with small exponent (α ≤ 0.1).26 This model is also consistent with the behavior of Rg in the final stage of regime III. Thus, the hindered-growth model points to the underlying mechanism of the moving droplet phase, namely, an energy barrier. In conclusion, we have demonstrated that in situ, real-time SANS following a pressure jump initiating a phase transition allows us to obtain a comprehensive view on processes at time scales ranging from tens of milliseconds to thousands of seconds and length scales of ∼1−100 nm, which not only give the overall size but also detailed information about the inner structure of the mesoglobules (Figure 4). This is an advantage

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00605. Estimation of C*. Determination of the coexistence line. Details of the kinetic SANS experiment. Modeling of SANS data. Encounter and coalescence time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alfons Schulte: 0000-0003-0824-8572 Christine M. Papadakis: 0000-0002-7098-3458 Author Contributions

Figure 4. Schematic representation of the formation and structural evolution of the mesoglobules during a pressure jump. The black squares indicate similar length scales in each regime.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

of SANS over other methods, such as time-resolved spectroscopy, light/small-angle X-ray scattering, or time-resolved microscopy. Employing a simple model system, a semidilute aqueous solution of the thermoresponsive polymer PNIPAM, we show that during the first ∼0.3−1.4 s after crossing the phase boundary the collapse of the chains leads immediately to the formation of fractal clusters. This finding is consistent with spectroscopic measurements that report the formation of hydrophobic domains.43−47 The loose clusters not only grow rapidly by diffusion-limited coalescence but also compactify and form a dense shell with water trapped inside the clusters. After ∼10−30 s, the mesoglobules become more homogeneous, and their growth is dominated by an energy barrier of several kBT. These results elucidate how mesoglobules (or moving droplets) are formed from molecularly dissolved chains, and they demonstrate that their growth kinetics is intimately linked to their inner structure. Thus, kinetic SANS in conjunction with sudden pressure changes is an enabling technique for structural studies of soft, complex, multicomponent systems over wide length and time scales.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A. S. acknowledges support by an August-Wilhelm Scheer visiting professorship at TU Munich. L. C. thanks Technische Universität Berlin and the ILL for postdoctoral funding though a three-year cooperation agreement. The high-pressure SANS cell was developed within the Integrated Infrastructure Initiative for Neutron Scattering and Muon spectroscopy (NMI3-II) supported by the EU 7th Framework Programme (FP7). We thank K.-L. Claude and S. Pinzek for help with the initial experiments and R. Schweins for valuable advice. We acknowledge ILL for allocation of beamtime at instrument D11 (DOI:10.5291/ILL-DATA.9-11-1827) and the support of the sample environment (SANE) and the instrument control (SCI) teams of the ILL, in particular C. Payre and J. Maurice.

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REFERENCES

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Poly(N-isopropylacrylamide) (PNIPAM) with molar mass Mn = 36 kg mol−1 (Đ = 1.26) was dissolved at a concentration of 3 wt % in D2O. SANS experiments were performed at instrument D11 at the Institut Laue-Langevin (ILL), Grenoble, France, covering a q range of 0.02−3 nm−1. The momentum transfer q is given by q = 4π sin(θ/2)/λ where θ is the scattering angle and λ = 0.6 nm the neutron 1158


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