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Thermoresponsive Stability of Colloids in Butyl Acetate/Ethanol Binary Solvent Realized by Grafting Linear Acrylate Copolymers Lu Jin, Jonas Bemetz, Xia Meng, Hua Wu,* and Massimo Morbidelli* Institute for Chemistry and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: We have developed a new class of thermoresponsive colloids that can exhibit a sharp reversible transition between dispersion and aggregation in binary BuAc/EtOH solvents based on the UCST (upper critical solution temperature)-type phase separation. This is realized by grafting linear PMMA-BA (random) copolymer onto the colloidal particles. We have selected TiO2/PS hybrid spheres (HSs) as a model system to demonstrate our general design concept. By grafting the linear PMMA-BA copolymer onto the HS surface, with the molecular weight from 30 to 40 kDa, we found that the thermoresponsive transition between dispersion and aggregation is fast, sharp, and reversible. At high mass fractions of the HSs, we have even observed a sharp transition between dispersion and gelation (or phase separation). The transition temperature can be tuned by varying the binary solvent composition, BuAc/EtOH, and the molecular weight of the grafted linear copolymer in the range from 5 to 55 °C. One of the most important features of this work is that the thermoresponsive materials used in organic solvents are initially synthesized in water with widely applied conventional (instead of research-based) techniques, thus being well suited for industrial production. In addition, the proposed approach is rather general and applicable to realizing the thermoresponsive transition for various types of colloids and nanoparticles.

1. INTRODUCTION Stimuli-responsive materials have been gaining much research interest and have been exploited widely over the last few decades. Among them, synthetic polymers, because of the tailorable multifunctions on and along their backbones, can be made responsive to environmental stimuli (e.g., temperature, pH, solvent, etc.) and are most extensively utilized in practice.1,2 When this category of polymers or modified macromolecules is incorporated with inorganic nanoparticles (NPs), the obtained hybrid nanomaterials make use of both the stimuli-responsive characteristics of the polymers and the specific physicochemical properties of the NPs to prepare various smart materials for devices.1,3−9 The stimuli-responsive stability of colloidal (nano-) particles is one of the most widely applied phenomena in the preparation of nanomaterials that can switch between assembly (aggregation) and dispersion.10−20 In most of those applications, various synthetic polymers such as poly(N-isopropylacrylamide),21 polyacrylamide,22 poly[oligo(ethylene glycol)methacrylate],23 poly(methyl methacrylate),24,25 and poly[N(4-vinylbenzyl)-N,N-dialkylamine]26 have been utilized and grafted onto the particle surface to realize a great number of thermoresponsive systems (assemblies/dispersions) in the literature based on their lower/upper critical solution temperature (LCST/UCST) phase-separation behavior.19,27,28 Most of the prepared thermoresponsive colloids are based on the © XXXX American Chemical Society

LCST-type transitions in water or water/ethanol-based solvents, with many fewer examples that are based on UCSTtype transitions and involve other organic solvents. Although water-based dispersion systems are more environmentally friendly and particularly suitable in medical applications, organic-solvent-based dispersions are often necessary in many applications (e.g., in paints, coatings, bulk polymerization, and solution-based polymer composites). It should be also mentioned that to graft the polymers onto the particle surface, very delicate techniques such as ATRP29−31 and RAFT polymerization32−34 have been applied, whose applications on industrial scales are still unsatisfactory, especially at high particle concentrations. The objective of this work is to design a new class of colloidal systems with thermoresponsive stability in a binary organic solvent, butyl acetate/ethanol (BuAc/EtOH), with the UCSTtype transition. The proposed thermoresponsive polymer in BuAc/EtOH is the linear poly(methyl methacrylate-co-butyl acrylate) (PMMA-BA) (random) copolymer. The linear copolymer chains are grafted onto the surface of the desired particles whose colloidal stability needs to be thermoresponsive. As an example, we have selected a model colloidal system, Received: January 30, 2017 Revised: May 4, 2017

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them, we continuously fed 2.5 mL of the monomer mixture of MMA + BA (19:1 v/v), with additional 0.0, 0.5, 1.0, and 2.0% (by volume with respect to the monomer mixture) of the chain-transfer agent, DDM. Because the chain-transfer agent transfers the active center from a growing polymer chain to another molecule, the average molecular weight of the polymer decreases as the amount of DDM increases. The seeded emulsion polymerization was carried out in the presence of 50 mg of KPS under the same conditions as those for the HS latex preparation. Because the HSs are grafted with the linear polymer formed with different amounts of DDM, the obtained particles from the four batches are 0-LP-HSs, 0.5-LP-HSs, 1-LP-HSs, and 2-LP-HSs, respectively, where the number denotes the amount of DDM and LP is the abbreviation for linear polymer. The DLS measurements show that all four types of LP-HSs have practically the same diameter, ∼130 nm, indicating that the grafted linear copolymer in the collapsed form on the HS surface in water has a thickness of ∼13 nm. This is in good agreement with the theoretical estimation (∼16 nm), assuming that the linear copolymer is 100% grafted onto the HS surface. The typical TEM image in the case of 1-LP-HSs is shown in Figure 1b. It should be mentioned that with the monomer mixture of MMA + BA (19:1 v/v), the formed linear PMMA-BA copolymer contains a very small amount of BA. A small amount of BA is added to enhance the hydrophobicity of the polymer. It is known that the solubility of MMA in water is very significant. In the absence of BA, with watersoluble initiator KPS, the polymer chains can grow substantially in water before migrating to the surface of the HSs, which could reduce the grafting efficiency. On the other hand, if a large amount of BA is present in the PMMA-BA copolymer, then the glass-transition temperature of the polymer will be substantially reduced, which may lead to the formation of large agglomerates (due to the surface stickiness of the particles) during the following freeze-drying stage. The four latexes prepared with different amounts of DDM were thoroughly cleaned overnight under mild stirring in the presence of ion-exchange resin Dowex Marathon MR-3 to remove all of the ionic surfactants and oligomers both on the particle surface and in water. The cleaned latexes were finally freeze dried in the form of dried powders for further tests. 2.4. Characterization of the Grafted Linear Copolymer. On the basis of our design principle, as shown in Scheme 1, the prepared particles, LP-HSs, should have an inner TiO2/PS hybrid sphere on which the linear copolymer, PMMA-BA, is grafted. To verify how efficient the grafting is, we redispersed 4 mg of the dried 1-LP-HS powder in 4 mL of pure BuAc (which is verified to be an excellent solvent for the PMMA-BA copolymer) for at least 12 h. Then, the 1LP-HSs were separated by centrifugation (15 000 rpm, 10 min, Sigma 3K30, Switzerland). Although the obtained supernatant was rather transparent, when it was placed inside the light-scattering device the scattered light intensity was still very strong. This indicates that not only the dissolved linear copolymer but also a small amount of the 1LP-HSs remained in the supernatant, leading to a high scattered light intensity due to the large refractive index of the TiO2 component. To quantify the remaining 1-LP-HSs in the supernatant, we prepared the 1-LP-HS dispersions in pure BuAc at various concentrations and measured their scattered light intensity using a Brookhaven Instrument (BI-200SM, USA) at the detection angle of 90°. Thus, a calibration curve of the scattered light intensity as a function of the 1-LP-HSs concentration was obtained, as shown in Figure S1 in the SI, from which we are able to quantify the remaining 1-LP-HSs in the supernatant from the scattered light intensity. The obtained concentration of the 1-LP-HSs in the supernatant is 0.078 mg/mL. Then, the supernatant was dried to eliminate the solvent, BuAc, and the weight of the solid residual was measured. After subtracting the weight of the remaining 1-LP-HSs, we found that the dissolved linear copolymer is 7.7 wt % of the original dried powder. Assuming complete conversion of the monomers, we estimated that ∼18 wt % of the total linear PMMA-BA copolymer was dissolved (detached) from the HS surface. It follows that we have a good grafting efficiency of ∼82%. Three repetitions were performed to confirm the grafting efficiency. The same experiments were also performed for samples 1-LP-HS and 2-LP-HS, and very similar results

TiO2/polystyrene (PS) hybrid spheres (HSs), to realize thermoresponsive aggregation/dispersion in BuAc/EtOH. In particular, we first synthesize the TiO2/PS HSs through miniemulsion polymerization in the aqueous phase. To graft linear PMMA-BA copolymer onto the HSs, we have performed seeded emulsion polymerization immediately after completing the HS synthesis. Then, the HSs with the grafted linear copolymer are precipitated from the aqueous phase and dried. The obtained dried powder exhibits thermoresponsive dispersion/aggregation in BuAc/EtOH. As can be seen, the applied polymerization techniques are only traditional miniemulsion and emulsion polymerizations, scalable to industrial applications, without involving any complex techniques (e.g., ATRP, RAFT, block copolymer, and dendritic polymer). It can be anticipated that by varying the linear copolymer chain length and the BuAc/EtOH composition one can obtain the thermoresponsive transition between aggregation and dispersion over a large range of temperature.

2. EXPERIMENTAL SECTION 2.1. Reagents. Potassium persulfate (KPS), butyl acetate (BuAc) (>99.0%, anhydrous), and Dowex Marathon MR-3 (ion-exchange resin, hydrogen and hydroxide forms) were purchased from SigmaAldrich Switzerland and were used as received. Chain-transfer agent 1dodecyl mercaptan (DDM) (>98.0%) and surface modifier 3(trimethoxysilyl)propyl methacrylate (MPS) were obtained from ABCR GmbH. Styrene (St) (>98.0%) from Sigma-Aldrich and methyl methacrylate (MMA) (>99.0%) and butyl acrylate (BA) from ABCR were purified by removing the inhibitor before polymerization. Analytical-grade ethanol (EtOH), chloroform, and 2-propanol were from ACROS Organics, and deionized water was used in all processes. 2.2. Synthesis of TiO2/PS Hybrid Spheres. TiO2 NPs ( 60%, the redispersed amount of the dried powder is negligible; i.e., in these cases, the BuAc/EtOH binary solvent becomes a poor solvent that cannot dissolve the grafted linear copolymer chains. As the used chain-transfer agent, DDM in LP-HSs, increases, the redispersion curve in Figure 3 shifts progressively toward a higher φE value. This is due to the fact that the solubility of the linear polymer increases as the polymer chain length (molecular weight) decreases. 3.3. Thermoresponsive Stability of LP-HSs in BuAc/ EtOH. Because the main objective of this work is developing colloids with thermoresponsive stability, the effect of temperature on the redispersion behavior of LP-HSs needs to be systematically investigated. Toward this aim, we have monitored in situ the size evolution of the dried powder dispersions in BuAc/EtOH along the temperature variation using DLS. The temperature increment (decrement) step is 1.0 °C, and at each temperature, the sample is stabilized for 5 min before carrying out the DLS measurements. Note that for the selected conditions, 8 min of stabilization time at each temperature has been tested as well, and no difference in the results with respect to those at 5 min of stabilization time can be found. A comparison is shown in Figure 4a in the case of φE = 70%. Let us first select the case 1-LP-HSs as an example for exploring the temperature effect. From Figure 3, the BuAc/ EtOH binary solvent is still a good solvent for 1-LP-HSs at T = 25 °C when the EtOH volume fraction is φE ≤ 40%. Thus, we have first examined the dispersion behavior of 1-LP-HSs in BuAc/EtOH at φE = 40% and at a temperature of around 25 °C. In particular, we started at T = 25 °C, at which 1-LP-HSs can be completely dispersed, and then the temperature was decreased progressively. Figure 4a shows the measured diameter (Dp) of the dispersed 1-LP-HSs as a function of temperature. It is seen that at φE = 40%, for T ≥ 7 °C, Dp is small and slightly decreases as the temperature decreases, but in all cases, the Dp value is somewhat larger than that of the original 1-LP-HSs (130 nm) in water. First, this result indicates that for T ≥ 7 °C, 1-LP-HSs have been completely redispersed in BuAc/EtOH at φE = 40%, forming a stable dispersion of individual primary particles. Second, the dissolved grafted linear copolymer chains are extended in the solvent, reducing the mobility of the particles, and it follows that the computed hydrodynamic diameter (Dp) from the Stokes−Einstein equation increases. Third, the slight decrease in Dp is consistent with the reduction in solvent quality for the grafted copolymer with decreasing temperature such that the copolymer chains would be less extended into the solvent. For T < 7 °C, however, the Dp value increases sharply as temperature decreases, accompanied by an observable apparent transition from a clear solution to an opaque suspension. This means that for T < 7 °C the dispersed 1-LP-HSs have lost their stability and started to aggregate, leading to the formation of clusters. Thus, T = 7 °C is basically the transition temperature of the UCST-based phase separation for the grafted linear copolymer in BuAc/ EtOH at φE = 40%; consequently, it is defined as the stabilization temperature (ST) for the formed colloidal dispersion.

dissolution rate of polymers in a solvent decreases as the polymer molecular weight increases.37 From the above results of the three cases, 0.5-LP-HSs, 1-LP-HSs, and 2-LP-HSs in the presence of DDM, we may conclude that when the molecular weight of the grafted linear PMMA-BA copolymer chains on the HS surface is smaller than 50 kDa the copolymer chains can quickly dissolve, leading to excellent solubility in BuAc. Instead, in the case of 0-LP-HSs in the absence of DDM, the weightaverage molecular weight (109 kDa) of the linear copolymer chains is too large to allow them to be completely disentangled and dissolved in BuAc in a reasonable time. As further confirmation that the redispersion of the LP-HS powders is due to the presence of the grafted linear copolymer, we have also examined the redispersion behavior of the HSs without grafting the linear copolymer (data not shown). As expected, the redispersion of the dried HS powder is practically impossible. Even after substantial sonication, most of the HS powder is still segregated, and the small amount of the dispersed HS powder is composed of large clusters whose PSD distribution does not move toward smaller sizes with time. Therefore, the powder of the HSs without the grafted linear copolymer is not redispersible in BuAc. 3.2. Redispersion Behavior of Dried Powders of LPHSs in BuAc/EtOH. The cosolvent selected for the thermoresponsive dispersion system is EtOH. The choice is related to the fact that EtOH is a poor solvent for PMMA. Because the powder of the 0-LP-HSs is extremely difficult to redisperse in BuAc and the powder of the HSs is practically not redispersible, in the following text we examine only the remaining three cases: 0.5-LP-HSs, 1-LP-HSs, and 2-LP-HSs. First, we have monitored the redispersion behavior of the three dried powders as a function of the composition of the binary solvent, BuAc/EtOH, at a fixed temperature, T = 25 °C. In a typical experiment, the powder was added in a set of the BuAc/EtOH solvent with different volume fractions of EtOH, φE. The mass fraction of the powder in the BuAc/EtOH solvent was fixed at ϕ = 0.1 wt %. After sonication with a probe sonicator (Branson, digital sonifier, Switzerland) for 10 min, the dispersions were stored for about 12 h to allow possible large clusters to have sufficient time to settle down. Then, from each dispersion, we took the supernatant to quantify the solid content through a UMT 5 comparator balance (Mettler Toledo). Figure 3 shows the redispersed amount of LP-HSs in BuAc/ EtOH as a function of φE. Let us first consider the case of 0.5-

Figure 3. Fraction of dispersed LP-HSs in BuAc/EtOH as a function of the EtOH fraction, φE. D

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the form of clusters. From the results in Figure 4b, we can observe that at a given φE value, the ST value increases as the length (molecular weight) of the grafted linear copolymer chains increases. Alternatively, at a fixed ST value, the stability range can extend to a higher φE value as the length of the grafted linear copolymer chains decreases. Therefore, through control of the length of the grafted linear copolymer chains, one can easily obtain a colloid that is stable in the desired temperature range and solvent composition. 3.4. Stability Reversibility of the Thermoresponsive Colloids. To demonstrate the stability reversibility of the thermoresponsive LP-HSs in BuAc/EtOH, we have performed two sets of experiments at low and high mass fractions of LPHSs, ϕ = 0.1 and 10 wt %, respectively. In the first set, we have selected the BuAc/EtOH solvent at φE = 60% as the test solvent, and we first redisperse the LP-HS powder in the solvent at T > ST. Then, we progressively cool the system to have T < ST so that LP-HSs are destabilized, and the average diameter of the dispersion is monitored in situ by DLS, as done in Figure 4a. When all of the LP-HSs are fully aggregated, we start to heat the system progressively until reaching a steadystate colloidal dispersion. Figure 5 shows both the cooling and

Figure 4. (a) Average diameter of the particles in the 1-LP-HS dispersions in different BuAc/EtOH binary solvents as a function of temperature from the cooling experiments and (b) the ST values from (a) as a function of φE in BuAc/EtOH. ϕ = 0.1%.

Figure 5. Reversibility of the transition between aggregation and a stable dispersion monitored by DLS along cooling and heating experiments. ϕ = 0.1% and φE = 60%.

It should be noted that, as detailed in Part C of the SI, in the applications of the Stokes−Einstein equation to compute Dp, the viscosity data of Hasan et al.38 for the given binary solvent systems have been used and extrapolated to the temperature range of this work. Thus, the uncertainty generated by the extrapolation in the used viscosity data may significantly alter the estimated Dp values. On the other hand, these errors, though somewhat altering the absolute Dp values, do not affect our estimation of the transition temperature for the UCST-type phase separation. Figure 4a also shows the experiments for 1-LP-HSs carried out in BuAc/EtOH at φE = 50, 60, and 70%. As expected, as the volume fraction of the poor solvent (EtOH) in BuAc/EtOH increases, the ST value increases. For T ≥ 7 °C, in all cases, similar to the case at φE = 40%, the Dp value decreases slightly as the temperature decreases. The minimum Dp value is somewhat different, depending on φE, but in an irregular mode. This may be related to the uncertainty generated by the extrapolation in the used viscosity data, as discussed above. The ST curve of 1-LP-HSs versus the EtOH volume fraction, φE in BuAc/EtOH, has been generated from the data in Figure 4a and reported in Figure 4b, where those of 0.5-LP-HSs and 2LP-HSs are also shown. (The Dp vs temperature curves in the cases of 0.5-LP-HSs and 2-LP-HSs are given in Figure S3 of the SI.) Each ST curve divides the plane into two regions: the above-left one where the colloidal dispersions are stable and the bottom-right one where the colloidal dispersions are unstable in

heating results for all three samples: 0.5-LP-HSs, 1-LP-HSs, and 2-LP-HSs. As can be seen, although there is slight hysteresis between cooling and heating in some cases, the average diameters of the stable dispersions at T > ST are practically identical. The cooling and heating cycle can be repeated many times, thus confirming the stability reversibility of the thermoresponsive colloids. The hysteresis between cooling and heating leads to a significant difference in the ST value in the case of 2-LP-HSs. This may be related to the corresponding ST value, which is relatively low. At low temperature, the transition from entangled polymer chains to disentangled (dissolved) chains may need more energy, and the heating ST value shifts toward a higher temperature. For the second set of experiments at ϕ = 10 wt %, we took the case of 1-LP-HSs as an example and added the 1-LP-HS powder to BuAc/EtOH at φE = 60%, and because the ST value in this case is 38 °C, the obtained suspension was heated to 50 °C to form a uniform dispersion, as shown in Figure 6a. Then, the vial containing the uniform dispersion of 1-LP-HSs was immediately inserted and held in an ice−water bath for 5 s, and we observed a sudden transition from the liquidlike dispersion to a solidlike gel. A picture of the ceased-flow, solidlike gel is shown in Figure 6b. Gelation occurs obviously because the temperature was suddenly reduced to below ST such that the E

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temperature increases as the mass fraction of EtOH in BuAc/ EtOH increases. For a fixed BuAc/EtOH composition, the transition temperature increases as the molecular weight of the grafted linear copolymer increases. In the tested range of the EtOH mass fraction in BuAc/EtOH, from 40 to 70%, and of the molecular weight of the grafted linear copolymer, from 30 to 40 kDa, the transition temperature varies from 5 to 55 °C. Finally, it should be emphasized that the thermoresponsive materials used in organic solvents are initially synthesized in water with conventional techniques without involving any complex techniques such as ATRP, RAFT, block copolymer, and dendritic polymer, thus they are well suited for industrial production. In addition, the proposed approach is rather general and applicable to various types of colloids and nanoparticles to realize their thermoresponsive transition between dispersion and aggregation in BuAc/EtOH (or BuAc/EtOH-like) through the surface grafting of linear PMMA-BA (or PMMA-BA-like) copolymer chains.

Figure 6. Pictures of (a) a stable dispersion of 1-LP-HSs in BuAc/ EtOH at φE = 60%, ϕ = 10 wt %, and T = 50 °C, (b) the gel formed from (a) after sudden cooling to T < ST in 5 s, and (c) phase separation from (a) after slow cooling to T < ST in 3 min.

fractal clusters were formed and grew quickly to fill the space, interconnecting and forming the gel. Again, such a transition between a uniform dispersion and gelation is fully reversible. It should be mentioned that during the cooling process, if the temperature was decreased slowly, the clusters formed during the aggregation may restructure to form compact, nonfractal objects, leading to phase separation and precipitation, as shown in Figure 6c. Apart from the kinetic effect, the softness of the grafted surface copolymer chains around ST can also ease the restructuring. Therefore, to obtain a fractal gel, fast quenching is necessary to allow the formed fractal clusters to quickly interconnect before having the possibility of restructuring.

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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.langmuir.7b00301. Preparation of TiO2/PS hybrid spheres. Calibration curve for quantifying the trace amount of LP-HSs dispersed in BuAc. Estimation of the dynamic viscosity of the BuAc/EtOH binary solvents as a function of temperature at various molar fractions of EtOH. Dependence of the average diameter of the particles in BuAc/EtOH on temperature. GPC analysis of the dissolved linear PMMA-BA copolymer. Hansen solubility parameters of the PMMA-BA copolymer in the BuAc/ EtOH binary solvents. (PDF)

4. CONCLUDING REMARKS In this work, we have designed a new class of thermoresponsive colloids in binary BuAc/EtOH solvent mixtures that can exhibit reversible dispersion/aggregation over a large range of the transition temperature for the UCST-based phase separation. The applied polymer is linear PMMA-BA (random) copolymer, which is found to have a sharp UCST-type transition in BuAc/ EtOH. Such thermoresponsive colloids have been synthesized initially in water using only the conventional seeded emulsion polymerization technique. We have selected a model colloidal system, TiO2/PS hybrid spheres (HSs), to demonstrate our general design concept. In particular, we first synthesize the TiO2/PS HSs through miniemulsion polymerization in water, followed immediately by grafting linear PMMA-BA copolymer on the HSs through seeded emulsion polymerization. Then, the HSs with the grafted linear copolymer are precipitated from the aqueous phase and dried. The obtained dried powder is used to perform the study on the thermoresponsive dispersion/ aggregation in BuAc/EtOH. It is found that when the molecular weight of the grafted linear PMMA-BA copolymer is smaller than 50 kDa, because of the good dissolution rate of the grafted copolymer, the thermoresponsive transition between dispersion and aggregation is fast, sharp, and reversible. At the high mass fractions of the HSs, we have even observed a sharp transition between dispersion and gelation. When the molecular weight of the grafted linear copolymer is larger than 100 kDa, the dissolution rate of the copolymer even in pure BuAc is too slow to be practical, and thus we are unable to realize the thermoresponsive systems. The temperature for the UCST-type transition can be tuned by varying the binary solvent composition, BuAc/EtOH, and the molecular weight of the grafted linear copolymer. For a given molecular weight of the copolymer, the transition

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AUTHOR INFORMATION

Corresponding Authors

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

Jonas Bemetz: 0000-0002-8998-9702 Hua Wu: 0000-0002-2805-4612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Swiss National Science Foundation (grant no. 200020_165917). L.J. acknowledges the China Scholarship Council for partial financial support (grant no. 201206090022).

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DOI: 10.1021/acs.langmuir.7b00301 Langmuir XXXX, XXX, XXX−XXX