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Schizophrenic Diblock Copolymer Functionalized Nanoparticles as Temperature Responsive Pickering Emulsifiers Mikhil Ranka, Hari Katepalli, Daniel Blankschtein, and T. Alan Hatton Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03008 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017
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Schizophrenic Diblock Copolymer Functionalized Nanoparticles as Temperature Responsive Pickering Emulsifiers Mikhil Rankaa,b, Hari Katepallia,b, Daniel Blankschteina, and T. Alan Hattona,c a Department of Chemical Engineering, Massachusetts Institute of Technology, 25 Ames Street, Cambridge, MA 02139 b These authors contributed equally to the manuscript c Corresponding author:
[email protected], 617-253-4588 Abstract Stimuli-responsive pickering emulsions have received considerable attention in recent years, and the utilization of temperature as a stimulus has been of particular interest. Previous efforts have led to responsive systems that enable the formation of stable emulsions at room temperature, which can subsequently be triggered to destabilize with an increase in temperature. The development of a thermoresponsive system that exhibits the opposite response however, i.e., one that can be triggered to form stable emulsions at elevated temperatures and subsequently be induced to phase separate at lower temperatures, has so far been lacking. Here, we describe a system that accomplishes this goal, by leveraging a schizophrenic diblock copolymer that exhibits both an upper and a lower critical solution temperature. The diblock copolymer was conjugated to 20nm silica nanoparticles, which were subsequently demonstrated to stabilize O/W emulsions at 65oC and trigger phase separation upon cooling to 25oC. The effects of particle concentration, electrolyte concentration, and polymer architecture were investigated, and facile control of emulsion stability was demonstrated for multiple oil types. Our approach is likely to be broadly adaptable to other schizophrenic diblock copolymers, and find significant utility in applications such as enhanced-oil recovery and liquid-phase heterogeneous catalysis, where stable emulsions are only desired at elevated temperatures. Introduction Emulsion systems in which colloidal particles adsorb to and stabilize an oil/water interface are known as Pickering emulsions.1 The presence of colloidal particles leads to unique properties in Pickering systems, because particles are known to exhibit very strong interfacial attachment.2,3 The free energy of desorption of a particle from an oil-water interface is given by: ∆ = 1 − |cos | where is the radius of the particle, is the three-phase contact angle made by the particle at the oil-water interface, and is the oil-water interfacial tension.3 Typical values of ∆ for detachment of particles from an oil-water interface range from 102 to 106 kBT, and therefore, in contrast to small molecules, particles attached to an oil-water interface cannot be displaced by weak thermal fluctuations of order kBT.3 When a sufficient number of particles adsorb at the interface, the resulting emulsion droplets become very resistant to coalescence. This, in turn, makes Pickering emulsions useful for a wide range of applications, where long-term stability is desired (food products, cosmetics, and drug delivery).4-9 In other applications, however, such as in liquid-phase heterogeneous catalysis, enhanced-oil recovery, and emulsion polymerization, stability is only desired transiently, and the ability to induce phase separation on demand is desirable.10-12 Towards this end, there have been many efforts directed at developing stimuli-responsive systems that destabilize in response to external stimuli such as pH, salinity, temperature, and magnetic fields. The destabilization process is typically driven by inducing a change in the oilparticle-water contact angle ( ), thereby forcing a loss of particle amphiphilicity and interfacial stability.13-16 The utilization of temperature as a stimulus is of particular interest as temperature can be cycled readily, and does not lead to a change in the material composition of the system. The most common approach to prepare thermoresponsive systems has been to functionalize nanoparticles with polymers that exhibit a lower critical solution temperature (LCST). By increasing temperature above the LCST, emulsions stable at room temperature, are destabilized due to polymer dehydration and subsequent particle flocculation.17-20 While there are many literature reports illustrating this concept, there are no reports of a system that behaves in the reverse direction, i.e., one where a stable emulsion can form at an elevated temperature and subsequently be induced to phase separate when cooled to room temperature. Such a system is likely to be of significant utility in a variety of applications. In enhanced-oil recovery, for example, one may envision its use in emulsifying underground oil pockets (at elevated temperatures), which once recovered above ground (at room temperature), may be readily destabilized and separated into extracted oil and water. Further, in liquid-phase heterogeneous catalysis, a biphasic reaction system may be stably emulsified
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(for enhanced reaction surface area) at an elevated reaction temperature, and subsequently restored to a phase separated state when cooled. The objective of this Communication is to describe a system that does, in fact, accomplish thermoresponsive control in this ‘reverse’ direction. Our approach leverages the thermoresponsive nature of polyzwitterions (exhibiting a UCST) in combination with that of N-alkyl methacrylamides (exhibiting a LCST) to design schizophrenic diblock copolymer-functionalized nanoparticles that gain surface activity at elevated temperatures, and lose surface activity when cooled to ambient temperatures. Thermoresponsive schizophrenic diblocks are a distinct class of block copolymers with known ability to invert their micelle structure upon changes in temperature.21-26 They consist of a UCST block whose aqueous solubility increases with increasing temperature, coupled to an LCST block whose aqueous solubility decreases with increasing temperature.27 The opposing solubilities of the two blocks with increasing temperature is what fundamentally drives the inter-conversion between the two distinct micellar structures, and in our case, gain or loss of nanoparticle surface activity. RAFT polymerization was utilized to synthesize a low polydispersity schizophrenic diblock consisting of poly(sulfobetaine methacrylamide) (polySBMA) as the UCST block (16oC), and poly(N-isopropyl methacrylamide) (polyNIPMAM) as the LCST block (42oC). The diblock was subsequently conjugated to 20 nm diameter silica nanoparticles via siloxane conjugation. At an ambient temperature of 25oC, both blocks are hydrophilic and water soluble, ensuring that the functionalized nanoparticles lack surface activity, and partition completely into the water phase. On the other hand, at an elevated temperature of 65oC, the dehydration of the N-alkyl methacrylamide block competes with the hydration of the polyzwitterionic block, resulting in interfacially-active nanoparticles that can stabilize an oil-in-water emulsion at elevated temperatures, as shown in Figure 1. Synthesis Scheme: The synthetic scheme for developing functionalized nanoparticles began with the synthesis of a low dispersity schizophrenic diblock copolymer, where the reversible chain transfer agent 4-cyano-4(phenylcarbonothioylthio)pentatonic acid was used to conduct a sequential one-pot RAFT polymerization. The polymerization was conducted in trifluoroethanol (TFE), and block lengths of 200 for the poly(SBMA) block and 50 for poly(NIPMAM) were targeted. 10 mol % of -(trimethoxysilyl) propyl methacrylate was also copolymerized in the poly(NIPMAM) block to enable permanent immobilization of the polymeric layer to the nanoparticles via polySBMA
polyNIPMAM
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Figure 1: Schizophrenic diblock copolymer-functionalized nanoparticles with tunable surface activity.
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siloxane conjugation. This was necessary to prevent desorption of the polymeric layer, and to ensure that particles do not lose function after a one-time exposure to elevated temperatures. The use of TFE as a solvent is also noteworthy, because the ability to synthesize block or triblock architectures is quite often limited by the lack of availability of a common solvent for all relevant monomers. TFE was found to be unique in this respect, since it provides simultaneously polar (due to fluorine groups) and hydrophobic (due to ethanolic backbone) properties as a solvent, and therefore, enables successful solvation of all three monomers SBMA, NIPMAM, and -(trimethoxysilyl) propyl methacrylate.28 Lastly, it is important to note that by selective copolymerization of the silanol monomer within the second block only, the grafted polymer was always oriented to position the LCST block toward the particle core (as shown in Figure 1). Such segregation was found to be critical in order to maintain cyclability. The importance of polymer architecture is discussed further below. Results and Discussion The ability of the nanoparticles to stabilize or destabilize emulsions can be characterized in terms of two main parameters: (1) separation time to attain oil-water phase separation, and (2) system cyclability (the ability of the particles to stabilize/destabilize emulsions repeatedly). Particle concentration, electrolyte concentration, and polymer architecture were found to be the primary factors controlling the two parameters. Oil-in-water emulsions (20/80 (v/v)) were prepared with three different oils of varying viscosity to demonstrate the ability of the particles to stabilize the emulsions under different conditions. Figure 2 shows optical microscopy images of stabilized Pickering emulsion droplets at 65oC for three different oil phases (hexadecane, cyclohexane, and silicone oil), demonstrating the broad ability of the particles to stabilize oil phases with different properties. The hexadecane-inwater mixture was chosen as a model system to further study the emulsion stabilization and destabilization process. Videos demonstrating the lack of emulsion formation at room temperature, and successful emulsion formation at 65oC (for the hexadecane-in-water system), are included as Supplementary information.
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Figure 2: Bright-field optical microscopy images of particle-stabilized emulsions maintained at 65oC, with 20% (v/v) oil volume fraction, and particle concentration of 0.4 wt%.
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Separation Time: The ease of oil-water phase separation is arguably the most important property of the system, and both the particle concentration and the salinity of the aqueous phase were found to have a pronounced effect on separation rates. In order to quantify separation time, transmission measurements of the aqueous phase were made every ten minutes until a transmission of at least 90 percent was attained. Figure 3 clearly shows that, at a given salinity, an increase in particle concentration leads to an increase in the separation time. For example, under a 0.1 API Brine solution condition (API Brine is defined as 8 wt% NaCl and 2 wt% CaCl2), with increasing particle concentration from 0.4 wt% to 1.2 wt%, the separation time increased from 70 to 130 minutes. The observed trend is likely due to changes in the particle assembly at the oil/water interface of the emulsion droplet. Indeed, colloidal monolayers, which are known to form at lower particle concentrations, can transition to stacked multilayer configurations at higher particle concentrations.29 The latter configuration makes phase separation a significantly slower kinetic process because, in addition to the need for particle desorption from the oil/water interface, attractive interparticle interactions within the multilayer need to be overcome in order to promote droplet coalescence. This in turn, leads to longer separation times. Videos demonstrating the difference in separation times as a function of particle concentration are provided as Supplementary Information. The second parameter that directly affects separation times is the electrolyte concentration in the aqueous phase. As clearly shown in Figure 3, an increase in the electrolyte concentration decreases separation times; i.e., the presence of electrolyte significantly accelerates the separation process. For example, Figure 3 shows that, at a particle concentration of 0.4 wt%, a separation time of 70 minutes decreases to 10 minutes when the electrolyte concentration increases from 0.1 API to API. The same qualitative behavior is observed at all three particle concentrations investigated. The behavior observed in response to an increase in the electrolyte concentration may be attributed to the ‘antipolyelectrolyte’ behavior exhibited by zwitterionic polymers.30 This behavior can be attributed to the ability of a polymer chain to increase, rather than decrease, its aqueous solubility when exposed to an electrolyte environment. Note that this response to the addition of an electrolyte is in direct contrast to the response observed in traditional polyelectrolytes, whose aqueous solubility decreases when an electrolyte is added. This phenomenon originates from the charge screening of intra-chain interactions that occur upon addition of electrolyte, causing the polymer chain to swell osmotically and hydrate.21 In applications where high electrolyte environments are commonplace (such as in enhanced-oil recovery), this is uniquely advantageous, because the enhanced hydration afforded by an electrolyte-rich environment results in easier and faster separation. Videos demonstrating the difference in separation times as a function of electrolyte concentration are provided as Supplementary Information. Furthermore, the broad applicability of the novel functionalized nanoparticles in emulsifying/demulsifying different types of oils is indicated by the separation times observed for cyclohexane and silicone oil emulsions, which are tabulated in the Supplementary Information.
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1.2 wt % 100
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Figure 3: Separation times for schizophrenic diblock copolymer functionalized nanoparticles as a function of particle concentration and electrolyte concentration.
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Figure 4: Cyclability as a function of particle and electrolyte concentration. (a) Electrolyte concentration fixed at API Brine conditions with varying particle concentration (b) Particle concentration fixed at 0.4 wt% and electrolyte concentration varied System Cyclability: Since schizophrenic diblocks are known to respond to temperature changes reversibly, the surface activity of functionalized nanoparticles was also expected to increase or decrease reversibly. This was tested by subjecting the particles to five emulsification/demulsification cycles at different electrolyte and particle concentrations, and tracking the quality of the separation via transmission measurements of the aqueous phase. In order to quantify the quality of the separation, transmission values were recorded at the time required to attain 90% sample transmission during the first cycle. Figure 4(a) and 4(b) show that, under most conditions, the transmission values did not deteriorate more than 10%, indicating that the particles foster strong cyclability in emulsion stabilization/destabilization processes by adsorbing to the interface and stabilizing the emulsion at high temperature, and then desorbing from the interface to facilitate reversion back to a phase-separated state on cooling of the system. It is noteworthy, however, that, under conditions of high particle concentrations and low electrolyte concentrations, significant deterioration in the quality of the separation was observed. For example, in Figure 4(a) at a particle concentration of 1.2 wt%, the normalized transmission values were found to decrease from 96% to 82%, while at the lowest electrolyte concentration of 0.1API the normalized transmission values were found to decrease from 93% to 81%. The observed deterioration may be due to the stable clustering of a growing fraction of particles with each cycle, since both a stacked multilayer particle configuration (due to a high particle concentration) and low polymer hydration (due to a low electrolyte concentration) are more likely to prevent the particles from reverting back to their original dispersed state. Effect of Polymer Architecture: The effect of polymer architecture on the overall behavior of the system was evaluated by comparing the effects of the block copolymer discussed above with those of a random copolymer also synthesized in this work (synthesis and polymer grafting details are provided in the Supplementary Information). Important differences between the two polymer architectures were observed, with the random copolymerfunctionalized nanoparticles exhibiting significantly slower separation times. Data and videos demonstrating the difference in separation times as a function of polymer architecture are provided as Supplementary Information. The observed differences between the two polymer architectures can be attributed directly to the arrangement of the NIPMAM units along their respective polymer backbones. In the diblock case, the polymer was purposely engineered to ensure that the poly(NIPMAM) block was always segregated towards the core of the nanoparticle. This segregation actively hindered particle clustering by sterically limiting associative interactions between the poly(NIPMAM) blocks on different nanoparticles. The reduced tendency towards particle clustering, in turn, leads to a better tolerance to cyclability and shorter separation times. On the other hand, in the random copolymer case, NIPMAM is distributed throughout the polymer backbone instead of being segregated towards the core. Accordingly, this lack of segregation facilitates associative interactions between NIPMAM units on different nanoparticles, actively leading to particle clustering above the LCST (note that similar observations on free polymer clustering in solution were reported by Shen et al.31). This increased tendency towards particle clustering, in turn, leads to poorer cyclability and longer separation times for the random copolymer functionalized nanoparticles.
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Conclusion We have created unique polymer-functionalized nanoparticles that responsively exhibit increased surface activity at elevated temperatures, and decreased surface activity at room temperature. We demonstrated that the functionalized nanoparticles can function successfully as thermoresponsive Pickering emulsifiers by enabling stable emulsion formation at high temperature and facile phase separation at room temperature. The direction of the temperature response is opposite to that previously reported in the literature, and was achieved by leveraging the unique behavior of schizophrenic diblock copolymers. Additionally, the system was shown to be cyclable under a broad range of conditions, and successful emulsification/demulsification behavior was demonstrated for multiple oil types. The approach pursued here can be generalized broadly to a large class of known schizophrenic copolymers, and can be extended to functionalization of other nanoparticle cores such as iron oxide, graphene, and magnetite. Overall, we demonstrated that schizophrenic diblock copolymer-functionalized nanoparticles enable unique temperature control of Pickering emulsion systems, and therefore, are likely to find important applications in diverse fields such as enhanced-oil recovery, catalysis, and emulsion polymerization. Supporting Information Block and Random Copolymer Synthesis, Separation Time Protocol, Illustrative Videos of Separation, Polymer Conjugation Protocol, System Cyclability Protocol Acknowledgements This work was supported by ENI under the MIT Energy Initiative. We thank Johannes Elbert for his assistance with polymer synthesis. References 1.
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Table of Content (TOC) Graphic
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