Letter pubs.acs.org/macroletters
Poly(N‑isopropylacrylamide) Microgels under Alcoholic Intoxication: When a LCST Polymer Shows Swelling with Increasing Temperature Sebastian Backes,†,‡ Patrick Krause,† Weronika Tabaka,† Marcus U. Witt,†,‡ Debashish Mukherji,§ Kurt Kremer,§ and Regine von Klitzing*,†,‡ †
Stranski-Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany ‡ Institut für Physik, Technische Universität Darmstadt, Alarich-Weiss-Strasse 10, 64287 Darmstadt, Germany § Max-Planck Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.
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biomedicine, drug delivery,13−15 and sensors.16,17 The advantages of using microgels are their colloidal stability even above the VPTT and their fast stimuli responsiveness. However, in applications, polymers are mostly solvated in a mixture of solvents rather than in pure water. Therefore, it is crucial to understand their behavior in solutions of two or more components. A phenomenon of PNIPAM in mixed solvents is the cononsolvency effect. Cononsolvency occurs when organic solvents, such as short-chain alcohols, are added to an aqueous PNIPAM solution.18−28 While both water and alcohol are good solvents for PNIPAM, a small amount of alcohol added to the aqueous PNIPAM solution is enough to induce gel collapse. Upon further addition of alcohol in PNIPAM solution, the gel swells up again.29 Furthermore, for small volume fractions of alcohol in water (ϕa < 50 vol %), the VPTT of microgels (or LCST for a chain) decreases with increasing alcohol content.11,19,20,30 While there are several more explanations of cononsolvency based on various competing factors in complex ternary mixtures,11,20,27,31−40 some theoretical studies have shown the preferential adsorption of alcohol molecules at PNIPAM in comparison to the one of water.23−25,41,42 The majority of studies has concentrated on the LCST of PNIPAM. However, recent works also reported a UCST-like
hen a polymer chain collapses upon increase of solution temperature T, this transition point is referred to as a lower critical solution temperature (LCST). The LCST is usually associated with the breakage of hydrogen bonding between a monomer and the solvent molecules. In this process, the translational entropy gain of the expelled solvent molecules, because of the broken monomer−solvent hydrogen bonds, gets larger than the combined effect of the conformational entropy loss of the chain upon collapse and the interaction in bulk solution.1−6 Because a chain collapses upon increase of T, entropy gain becomes more relevant than enthalpic penalty and drives the coil-to-globule transition. Also intramolecular hydrogen bonding supports the collapse, but this is not essential since also polymers with only proton acceptors in the side chain show an LCST.7 On the other hand, when a polymer expands upon increase of T, this is referred to as an upper critical solution temperature (UCST) and is driven by the reduction of interaction energy between different solvent components.8,9 A standard polymer showing LCST behavior is poly(Nisopropylacrylamide) (PNIPAM) with LCST ≈ 32 °C in pure water.10,11 When PNIPAM chains are cross-linked, they form (spherical) microgel particles with radii ranging from tens of nanometers to micrometers. When these microgels are exposed to the external stimulus of changing T, they undergo a volume phase transition at about the same temperature of 32 °C (VPTT), as linear chains.3,12 Microgels are of particular interest because they possess tremendous potential in the field of © XXXX American Chemical Society
Received: August 1, 2017 Accepted: September 6, 2017
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Figure 1. Part (a) shows the hydrodynamic radius Rh of PNIPAM microgels measured by DLS as a function of ethanol volume fraction ϕe in water for two different temperatures 20 and 60 °C. Part (b) presents the change in hydrodynamic radius Rh with T measured by DLS for ϕe = 0, 10, and 50 vol %, respectively. Part (c) shows simulation data of the radius of gyration Rg with changing T for three different ϕe.
Using all-atom simulation trajectories, first the normalized radial ethanol mole fraction xe*(r) = xe(r)/xe around the polymer backbone is calculated for two different T. The main panel of Figure 2 shows x*e (r) with radial distance r from the
reswelling for high ethanol concentrations, i.e., ϕa > 45 vol %.30,35 This is unexpected, given that systems dominated by hydrogen-bonded interactions usually exhibit a LCST. Therefore, it is worthwhile to investigate the UCST-like behavior of PNIPAM and if the reswelling is indeed a UCST transition. In this study the cononsolvency effect of PNIPAM is analyzed by the combination of experimental investigations of microgels and simulations of linear chains. The study is focused in particular on the temperature dependency of cononsolvency. It is shown that PNIPAM is not always in a collapsed state above its LCST or VPTT. Instead, for T > 50 °C, PNIPAM shows reswelling in aqueous solutions containing methanol, ethanol, and propanol. This is in contrast to ref 30, where a UCST was only observed for linear PNIPAM but not for microgels. The present analysis indicates that the delicate microscopic interaction details, together with the bulk solution properties, play a key role in dominating the reswelling of a standard LCST polymer, which is not a UCST-like swelling. The cononsolvency behavior of PNIPAM microgels in aqueous ethanol mixtures is studied at T = 20 °C using dynamic light scattering (DLS) (see Supporting Information for synthesis and experimental details). Figure 1a shows the variation of hydrodynamic radii Rh with ethanol volume fraction ϕe. For T = 20 °C the microgels show a swelling−collapse− swelling scenario with ϕe.18−21 Up to ϕe = 30% they shrink, and beyond ϕe = 50% they reswell. Figure 1b shows the effect of T on cononsolvency by presenting the change in Rh with varying T for three different ϕe. For ϕe = 0 vol %, PNIPAM collapses around 32 °C and around 28 °C for ϕe = 10 vol %, and for ϕe = 50 vol % it is difficult to identify a VPTT within the range of T (for more details at different ϕe see Figure S1 in the Supporting Information). Between 32 °C < T < 50 °C a plateau in Rh is observed. Moreover, for T > 50 °C the microgels reswell in mixtures of water and ethanol. While there is a weak indication of reswelling for ϕe = 10 vol %, it becomes more pronounced for ϕe > 50 vol %. To understand the experimentally observed conformational behavior in Figure 1(a) and Figure 1(b), all atom molecular dynamics simulations of a PNIPAM chain of length Nl = 32 for three different ϕe were performed (see Supporting Information for simulation details). Figure 1c shows the simulation data of the change in radius of gyration Rg with T. Simulation data qualitatively capture the observed experimental trend in Figure 1(b), while there is a shift in T of about ∼10 °C compared to the experimental data.
Figure 2. Main panel shows simulation data of the normalized ethanol mole fraction xe*(r) = xe(r)/xe as a function of radial distance from the polymer surface. Here xe is the reference mole fraction of methanol in bulk solution. The results are shown for ϕe = 40 vol % (or xe = 0.17) and for two different temperatures T. Note that for convenience xe is presented, and not ϕe as used throughout this manuscript. The inset shows the total number of hydrogen bonds between monomer of a pw pe polymer with solvent molecules Ntotal hb = Nhb + Nhb (black), PNIPAM− pp PNIPAM hydrogen bond Nhb (green), and the excess of NIPAm− pe total ethanol hydrogen bond N pe hb = Nhb/(xeNhb ) (red) as a function of T. pw pe Here Nhb and Nhb are PNIPAM−water and PNIPAM−ethanol hydrogen bonds, respectively.
PNIPAM backbone (for xe = 0.17). In the first solvation shell between 0.2 < r < 0.6 nm, xe*(r) is almost twice as high as in bulk. To rationalize this issue, the number of hydrogen bonds between different solution components is calculated, such as pp pe the NIPAM−NIPAM N hb , NIPAM−ethanol N hb , and pw total NIPAM−water Nhb (see inset of Figure 2). While Nhb changes by ∼30% between the lowest and highest temperatures, Npp hb remains constant. Furthermore, the excess of pe NIPAM−ethanol hydrogen bonds Nhb ∼ 1.8, which is consistent with factor ∼1.9−2.0 in coordination number in the first hydration shell (see the main panel of Figure 2). This 1043
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Figure 3. Hydrodynamic radius Rh as a function of temperature as observed from DLS experiments for (a) aqueous methanol, (b) aqueous ethanol, and (c) aqueous propanol mixtures for alcohol volume fraction 40 vol %.
attraction, known from colloidal science.43 Here, because of the broken hydrogen bonds at high temperatures, the expelled solvent molecules induce a depletion effect onto the system, leading to an effective attraction between NIPAm monomers of a microgel. Furthermore, such depletion effects are directly related to the bulk solution number density ρn. The higher the ρn, the stronger the depletion effects. Therefore, this hypothesis has been tested by studying the swelling behavior in different aqueous solutions, namely, methanol, ethanol, and 2-propanol, as shown in Figure 3. Here, ρmethanol > ρethanol > ρpropanol for a n n n given volume fraction. As expected, it is found that Rmethanol = h 125 nm, Rethanol = 162 nm, and Rpropanol = 230 nm at 60 °C. h h Here it is also important to stress that the swelling behavior observed here is not a standard UCST like swelling, in contrast to the findings.30 Instead, it can be explained by the addition of good solvent (ethanol) to a poor solvent (water) for PNIPAM. The observed preferentiability also explains why a plateau between 32 °C < T < 50 °C is seen. From the inset of Figure 2 it can be seen that the total number of hydrogen bonds Ntotal hb shows an almost continuous decrease until about 45 °C. Ntotal hb changes by about 30% between the lowest and highest temperature. Therefore, within the range of 32 °C < T < 45 °C a microgel still contains a large amount of solvent molecules. So far, the excess ethanol coordination close to the PNIPAM chain has been investigated using molecular simulations. Experimentally, up to ϕe = 30 vol % a plateau minimum of Rh ∼ 90 nm between 30 °C < T < 50 °C has been found. This is lower than Rh ∼ 130 nm known from the minimum induced by cononsolvency at 20 °C (see Figure 1). An explanation for this is the breaking of hydrogen bonding between PNIPAM and solvent molecules at T > VPTT, which leads to a small number of solvent molecules encapsulated within the collapsed structure. Cononsolvency is in contrast caused by preferentiability of PNIPAM monomers toward ethanol molecules, whereby a higher number of solvent molecules remains within the gel. This explanation of the cononsolvency effect is supported by the increasing plateau minimum value between 30 °C < T < 50 °C for increasing ϕe for ϕe > 20% (see Figure S1 and Figure S2 in Supporting Information). Additional experimental evidence of this theory is provided by dynamic force measurements using atomic force microscopy on PNIPAM microgels with the comonomer acrylic acid poly(NIPAM-co-AAc). Poly(NIPAM-co-AAc) is chosen for the surface measurements because of their larger sizes compared to
analysis suggests that a preferential binding of ethanol molecules with the PNIPAM microgel units is a driving force behind the decrease in VPTT with increasing ϕe. For example, VPTT shifts from 32 °C in pure water to 28 °C at ϕe = 10 vol %. Note that the interaction energy between ethanol and a monomer of PNIPAM is about 6−8kBT in comparison to NIPAm−water, that is, 2−4kBT, leading to an interaction contrast of about 4kBT per monomer.24 Preferentiability of PNIPAM toward ethanol in comparison to water remains unaltered irrespective of T (see red curve in Figure 2). Therefore, if there is always an excess of ethanol even for T > 60 °C, why should PNIPAM swell and not remain collapsed? A closer investigation of the phase diagram of PNIPAM in pure water reveals that, for T > 32 °C, PNIPAM is insoluble when the solvent quality changes from good to poor.10,11 However, no such LCST or VPTT behavior is known in pure ethanol. This supports the assumption that ethanol is always a good solvent for PNIPAM within the known regime of T ≤ 60 °C. This leads to a situation at 60 °C where good solvent molecules (ethanol) for a polymer are added into a poor solvent solution (water) for the same polymer for T > VPTT. In this situation Rh of the PNIPAM microgel continuously increases with increasing ϕe as shown in the red curve of Figure 1a. Furthermore, for higher ϕe, most Rh values are smaller at 60 °C in comparison to the data for T = 20 °C. In this context, it should be mentioned that water and alcohol are almost perfectly miscible at 60 °C (see Figure S2 in the Supporting Information). Had the bulk solution phase separated, one would have expected the ethanol molecules to prefer complete enrichment of the PNIPAM backbone and thus expel water molecules from the first solvation shell. This would have led to a complete expansion of the collapsed PNIPAM microgel for the full ϕe range. The above observation raises a question about the reason for smaller Rh values at T = 60 °C and higher ϕe compared to T = 20 °C. The smaller Rh values suggest an increased attraction between the monomer beads. This can be understood by looking into Ntotal hb data in the inset of Figure 2, which shows that Ntotal hb decreases with increasing T. Thus, they expel ∼30% solvent molecules close from the polymer backbone. From the theory of depletion forces, it is known that the expelled solvent molecules often induce a repulsive force between monomer and the solvent molecules. This in turn leads to an effective attractive interaction between monomers. This attraction is known as depletion (or reduced coordination) induced 1044
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above a transition temperature of 50 °C, which is independent of the ϕe and/or the sizes of the alcohol molecules. While linear chains have been shown to exhibit a UCST-like behavior before in mixtures of water and ethanol,30 a reswelling of microgels in mixtures of methanol, ethanol, and 2-propanol has been observed for the first time. Molecular dynamics simulations of linear PNIPAM chains reveal a delicate interplay between the preferential binding of the better solvent with the monomer, the solvent quality contrast, and the bulk solution number density. Measurements of the complex modulus of adsorbed microgels support our explanations of the collapse mechanisms behind temperature- and cononsolvency-induced collapse. A detailed analysis of the mechanical behavior as well as the swelling properties of microgels adsorbed to surfaces is of great importance for a deeper understanding of their responsive properties.46,47 A comparison between the herein presented microgels in bulk solutions and adsorbed microgels will therefore be subject to further studies.
the PNIPAM microgels, while still showing similar conosolvency behavior in aqueous ethanol (see Figure S3 in Supporting Information). By these measurements the complex modulus G = G′ + iG″ with the shear modulus G′ and the loss modulus G″ could be determined.44,45 P(NIPAM-co-AAc) microgels are chosen because of their larger size (Rh ∼ 600 nm), which facilitates force measurements. Their cononsolvency behavior is similar to that of pure PNIPAM gels, with the minimum Rh also being found at ϕe = 30 vol %. The lowest G′ values of 98 kPa were obtained at 20 °C in pure water. G′ in 30 vol % ethanol was found to be 156 kPa, whereas G′ in pure water at 50 °C is 604 kPa. The lower G′ value for a collapsed microgel under the influence of cononsolvency compared to the microgel collapse with increasing T in pure water illustrates two different collapse mechanisms. In the first case, because of the preferential binding of the ethanol molecules (see Figure 2), there exists a large amount of interstitial ethanol molecules forming ethanol bridges, and the gel remains rather soft. The possible microscopic mechanism of ethanol bridges can be rationalized by the −OH group of ethanol making preferential hydrogen bonds with the amide group of a NIPAM monomer (see the red curve in the inset of Figure 2), while the hydrophobic group of ethanol binds to the isopropyl group of another NIPAM monomer far along the polymer contour. Furthermore, not just one ethanol molecule is forming bridges, but it is a rather collective effect of several ethanol molecules forming these sticky contacts.42 In the second case, however, solvent molecules are expelled, and the gel becomes dry or much stiffer. This can be attributed to the fact that, at high temperatures, more than 30% of the hydrogen bonds break down, reducing the number of interstitial solvent molecules significantly. In summary, this study combines experiments and simulation for a detailed microscopic investigation of the effect of temperature T and ethanol volume fraction ϕe on the complex phase diagram of PNIPAM-based microgels. A representative phase diagram with their respective mechanisms is shown in Figure 4. The microgels show the expected swelling−collapse− swelling transition of PNIPAM with changing alcohol content at a given T < VPTT, which can be explained by preferential adsorption of ethanol. The VPTT consequently decreases for increasing ϕe. The system shows an unexpected reswelling
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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/acsmacrolett.7b00557. Details about the material synthesis, dynamic light scattering, dynamic force measurements, and the allatom molecular dynamics simulations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +49 (0)6151 1625647. ORCID
Sebastian Backes: 0000-0001-5400-4937 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the DFG (KL1165/15) for financial support. Carlos Marques is acknowledged for bringing this collaboration together and also for many stimulating discussions.
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