Article pubs.acs.org/Langmuir
Microgel Colloidosomes Based on pH-Responsive Poly(tert-butylaminoethyl methacrylate) Latexes Andrew J. Morse,† Jeppe Madsen,† David J. Growney,† Steven P. Armes,*,† Peter Mills,‡ and Ron Swart§ †
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom Cytec Aerospace Materials, R414 The Wilton Centre, Wilton, Redcar and Cleveland TS10 4RF, United Kingdom § Allnex SA/NV, Anderlechstraat 33, 1620 Drogenbos, Belgium ‡
S Supporting Information *
ABSTRACT: Emulsion copolymerization of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) with divinylbenzene (DVB) cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded sterically stabilized poly[2-(tertbutylamino)ethyl methacrylate] (PTBAEMA) latexes at 10% solids at pH 9. Such particles proved to be an effective Pickering emulsifier at pH 10 for both n-dodecane and n-hexane. 1H NMR spectroscopy was used to follow the model reaction between the secondary amine of the TBAEMA monomer and the isocyanate groups of tolylene 2,4diisocyanate-terminated poly(propylene glycol) (PPG-TDI). Cross-linking the PTBAEMA latex particles adsorbed at the ndodecane/water interface using this oil-soluble PPG-TDI cross-linker at around 0 oC led to robust colloidosomes that survived an acid challenge. This resistance to demulsification was confirmed via laser diffraction studies following an in situ switch from pH 10 to 3, since no change was observed in either the oil droplet size or concentration (compared to non-cross-linked PTBAEMA-stabilized Pickering emulsions). Such PTBAEMA colloidosomes survived removal of the internal oil phase on washing with excess ethanol. However, because ethanol is a good solvent for the PTBAEMA chains, imaging the ethanol-treated colloidosomes via electron microscopy proved rather problematic due to partial film formation. Therefore, a series of TBAEMA/ styrene copolymer latexes (comprising 10, 30, 50, or 60 mol % styrene) were prepared via emulsion copolymerization at 70 °C in the presence of DVB and PEGMA. The higher glass transition temperatures exhibited by these copolymer particles (and their greater resistance to ethanol swelling) enabled better-quality electron microscopy images to be obtained. The presence of nitrogen atoms at the surface of these copolymer latex particles was confirmed via X-ray photoelectron spectroscopy studies; these secondary amine groups allow covalent cross-linking via PPG-TDI when adsorbed at the surface of n-dodecane droplets at TBAEMA comonomer contents as low as 40 mol %. After removal of the n-hexane oil phase by evaporation, fluorescence microscopy studies indicate that these colloidosomes undergo collapse in their latex form at pH 10 but regain their original spherical morphology in their cationic microgel form at pH 3.5.
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INTRODUCTION Pickering emulsions involve the self-assembly of colloidal particles at the interface between two immiscible liquids (typically oil and water).1−6 The driving force for particle adsorption is the reduction in interfacial area between the oil and aqueous phases. In principle, Pickering emulsifiers offer a number of advantages over conventional surfactants such as (i) more robust and reproducible formulations, (ii) reduced foaming problems, and (iii) lower toxicity (at least compared to certain surfactants). Recently, considerable attention has been devoted to the use of organic (e.g., polymer latex) particles as Pickering emulsifiers.7 In this case the surface wettability (or particle contact angle) can be readily tuned to obtain either oil-in-water (o/w) or water-in-oil (w/o) emulsions by controlling the surface chemistry.7 Pickering emulsion droplets may be stabilized to form so-called “colloidosomes” by polyelectrolyte adsorption,8 thermal annealing,9 gel trapping,9 covalent cross-linking,10,11 self-assembly,12 or solvent evaporation13 to lock in the initial latex super© XXXX American Chemical Society
structure. These microcapsules have attracted much interest owing to their potential use as an efficient delivery vehicle for the controlled delivery of oil-soluble or water-soluble actives.8,14 The first example of colloidosomes was reported in 1996 by Velev et al.,15 who described the adsorption of latex particles onto emulsion droplets to form hollow “supraparticles”. However, the term “colloidosome” was not coined until 2002, when Dinsmore et al.8 sintered micrometer-sized polystyrene (PS) latexes via thermal annealing above their glass transition temperature (Tg) to form microcapsules of tunable permeability.16 Covalent cross-linking of colloidosomes was recently reported by Thompson et al.,10 who utilized a near-monodisperse poly(glycerol monomethacrylate) (PGMA) macromonomer to prepare model sterically stabilized PGMA− PS latexes, which in turn stabilized o/w emulsions. The Received: August 22, 2014 Revised: September 27, 2014
A
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Mw/Mn of 1.10. Ammonium persulfate (APS; >98%), n-dodecane, nhexane, fluorescein isothiocyanate−dextran (FITC−dextran; molecular weight = 70 000 according to the manufacturer), and tolylene 2,4diisocyanate-terminated poly(propylene glycol) (PPG-TDI) were all purchased from Aldrich and used as received. Deionized water was obtained from an Elga Elgastat Option 3 system. Poly[2-(tert-butylamino)ethyl methacrylate] Latex Synthesis via Aqueous Emulsion Polymerization. The PEGMA-stabilized PTBAEMA latex syntheses have been described elsewhere.19 Full details of these can be found in Supporting Information. Chargestabilized PTBAEMA latexes were produced in both the absence and presence of a Nile blue-functionalized methacrylamide (NBM; 0.001 wt % based on monomer) comonomer recently synthesized within our group.20 Poly[2-(tert-butylamino)ethyl methacrylate-co-styrene] Synthesis via Aqueous Emulsion Copolymerization. PEGMAstabilized copolymer latexes were prepared via “one-pot” emulsion copolymerization of TBAEMA and styrene. PEGMA (1.00 g) was weighed into a 100 mL one-neck round-bottomed flask equipped with a magnetic follower. Deionized water (40.0 g) was added, followed by the appropriate TBAEMA/S comonomer mixture (total 5.0 g). DVB cross-linker was kept constant at 0.8 mol % for all copolymer latexes. Each flask was sealed with a rubber septum and the aqueous solution was degassed at ambient temperature via five vacuum/nitrogen cycles. The degassed solution was stirred at 250 rpm and heated to 70 °C with the aid of an oil bath. After 10 min, the appropriate amount of APS (1.0 wt % based on total monomer) dissolved in deionized water (5.0 g) was injected into the reaction vessel to initiate free radical copolymerization. The copolymerizing solution turned milky-white within 10 min, and stirring was continued for 24 h at 70 °C under a nitrogen atmosphere. Purification. All PEGMA-stabilized latexes were purified via repeated centrifugation/redispersion cycles to remove excess monomer(s), nongrafted PEGMA macromonomer, and initiator byproducts. Charge-stabilized latexes were exhaustively dialyzed to remove excess TBAEMA and APS initiator. In each case, the purity of the aqueous phase was monitored by surface tensiometry until it approached that of pure water (71 ± 1 mN·m−1). Colloidosome Preparation Using 2,4-Diisocyanate-Terminated Poly(propylene glycol) Cross-Linker. PPG-TDI oil-soluble cross-linker (1−5 mg/mL) was dissolved in n-dodecane (or n-hexane) in a 14 mL vial, and this solution was immersed in ice for 30 min. (Pickering emulsions were also prepared in the absence of PPG-TDI.) The appropriate latex was also cooled separately in ice for 30 min. The oil (4.0 mL) was then added to the latex (4.0 mL), which was surrounded in an ice jacket. Emulsification was achieved by use of an IKA Ultra-Turrax T-18 homogenizer. The resulting emulsion was allowed to stand unstirred for 30 min in an ice bath and then left for a further 30 min at 20 °C to ensure colloidosome formation. If n-hexane was used as the oil phase to produce colloidosomes, the protocol was the same as stated above. However, after standing for 1 h at 20 °C, the n-hexane was allowed to evaporate (aided by stirring) at either pH 10 or 3 (i.e., as either latex or microgel colloidosomes). Following this, the same volume of an aqueous solution of FITC−dextran (0.01 wt %; at the corresponding pH) was added to each of the colloidosome suspensions. These suspensions were stirred for a further 24 h at 20°C to allow dissolution of the FITC−dextran. Characterization. 1H NMR Spectroscopy. All 1H NMR spectra were recorded in CDCl3 on a 400 MHz Bruker Avance-400 spectrometer. Dynamic Light Scattering. Hydrodynamic diameters were measured at 25 °C on a Malvern Zetasizer NanoZS model ZEN 3600 instrument equipped with a 4 mW He−Ne solid-state laser operating at 633 nm. Back-scattered light was detected at 173°, and the mean particle diameter was calculated from the quadratic fitting of the correlation function over 30 runs of 10 s duration per run. All measurements were performed three times on 0.01% (w/v) aqueous latex dispersions. The pH of the deionized water used to dilute the latex was matched to that of the latex solution (around pH 10) and
hydroxy-functional PGMA chains were reacted with an oilsoluble tolylene-2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI) cross-linker present within the droplet phase to form urethane bonds between the PGMA stabilizer chains. Such colloidosomes proved to be sufficiently robust to survive an ethanol challenge, which removes the oil droplet phase. Walsh et al.11 replaced the PGMA macromonomer with a styrene-functionalized poly(ethylenimine) (PEI) macromonomer to produce an amine-functional PS latex that was used to stabilize o/w Pickering emulsions. The primary and secondary amines present on the surface of such latex particles could be covalently cross-linked to produce colloidosomes by use of PPG-TDI or poly(propylene glycol) diglycidyl ether (PPG-DGE). 10 However, this cross-linking had to be conducted at 0 °C to obtain well-defined microcapsules when n-dodecane was used as the oil phase. There have been two notable literature reports of stimulusresponsive colloidosomes.17,18 Dinsmore and co-workers17 described the preparation of thermoresponsive colloidosomes based on anionic poly(N-isopropylacrylamide-co-acrylic acid) microgels, with cross-linking achieved via electrostatic complexation with a cationic diblock copolymer. Similarly, Weitz and coworkers18 prepared thermosensitive colloidosomes using primary amine-functionalized poly(N-isopropylacrylamide) microgels, with covalent cross-linking occurring upon addition of glutaraldehyde. In both cases, a substantial reduction in colloidosome diameter was observed upon heating the microcapsules above the critical solution temperature for the microgels. Recently, we reported the synthesis of near-monodisperse lightly cross-linked poly[2-(tert-butylamino)ethyl methacrylate] (PTBAEMA) latexes via emulsion copolymerization.19 In the present study, we utilize the secondary amine functionality present on the TBAEMA residues to react with an oil-soluble cross-linker (PPG-TDI) to produce pH-responsive colloidosome microcapsules. Moreover, the weakly basic nature of the TBAEMA residues leads to the formation of novel pHresponsive microgel colloidosomes when the solution pH is lowered below their pKa. We also statistically copolymerize TBAEMA with styrene to produce a series of copolymer latexes of varying styrene content. In both cases, the secondary amine functionality could be reacted with PPG-TDI (predissolved in the oil phase) to produce colloidosome microcapsules. These cross-linked colloidosomes are sufficiently robust to survive both acid and ethanol challenges. In principle, the styrene comonomer is beneficial since it is significantly cheaper than TBAEMA. Its copolymerization also leads to a higher latex Tg, which facilitates electron microscopy studies. However, copolymerization of styrene with TBAEMA not only reduces the maximum degree of swelling of copolymer latex particles that can be achieved at low pH but also potentially prevents efficient cross-linking of styrene-rich copolymer particles adsorbed at the oil/water interface via the PPG-TDI crosslinker. These opportunities and limitations are explored herein.
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EXPERIMENTAL SECTION
Materials. 2-(tert-Butylamino)ethyl methacrylate (TBAEMA; 97%; Aldrich), styrene (S; >99%, Aldrich) and divinylbenzene (DVB; 80 mol % 1,4-divinyl content; Fluka) were treated with basic alumina to remove inhibitor and stored at −20 °C prior to use. The monomethoxy-capped poly(ethylene glycol) methacrylate macromonomer (PEGMA, kindly donated by GEO Specialty Chemicals, Hythe, U.K.) had a mean degree of polymerization (DP) of 45 and an B
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Figure 1. Reaction scheme for formation of sterically stabilized PTBAEMA or P(TBAEMA/S) copolymer latexes by aqueous emulsion copolymerization at 70 °C and subsequent acid-induced swelling behavior in aqueous solution at 20 °C to form cationic microgels.
Figure 2. 1H NMR spectra recorded for (a) TBAEMA monomer, (b) 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI) cross-linker, and (c−g) TBAEMA/PPG-TDI at molar ratios of (c) 10:1, (d) 4:1, (e) 2:1, (f) 1:1, and (g) excess PPG-TDI. All spectra were recorded in CDCl3. was ultrafiltered through a 0.20 μm membrane so as to remove any dust. Transmission Electron Microscopy. Images were recorded on a Phillips CM100 instrument operating at 100 kV and equipped with a Gatan 1k charge-coupled deice (CCD) camera. Dilute latex dispersions (0.01 wt %) were prepared at pH 10 and dried onto a carbon-coated copper grid at room temperature. X-ray Photoelectron Spectroscopy. Studies were conducted on latexes dried onto silicon wafers (at 20 °C for 24 h) by use of a Kratos Axis Ultra DLD instrument equipped with a monochromatic Al Kα Xray source (hν = 1486.6 eV) operating at a base pressure in the range 10−8−10−10 mbar. Field Emission Scanning Electron Microscopy. Scanning electron microscopy (SEM) studies were performed on a FEI Sirion field mission gun scanning electron microscope with a beam current of 244 μA and a typical operating voltage of 20 kV. Latexes were dried directly onto carbon tape and allowed to dry overnight before being sputter-coated with a thin layer of gold prior to examination in order to prevent sample charging. Pickering Emulsion/Colloidosome Characterization. Laser Diffraction. A Malvern Mastersizer 2000 instrument equipped with a small-volume Hydro 2000SM sample dispersion unit (ca. 50 mL), a HeNe laser operating at 633 nm, and a solid-state blue laser operating at 466 nm was used to size the emulsion droplets at pH 10. The
stirring rate was adjusted to 1000 rpm in order to avoid creaming of the emulsion during analysis. The mean droplet diameter was taken to be the volume mean diameter (D4/3), which is mathematically expressed as D4/3 = ∑Di4Ni/∑Di3Ni. The standard deviation for each diameter provides an indication of the width of the size distribution. After each measurement, the cell was rinsed once with ethanol, followed by three rinses with water. The glass walls of the cell were carefully wiped with lens cleaning tissue to avoid cross-contamination, and the laser was aligned centrally on the detector. This set-up allowed continuous measurements to be made after the sample chamber pH had been adjusted from 10 to 3. This allowed the droplet stability to be assessed with regard to any changes in pH. Acid Challenge. Sufficient HCl (0.1 M) was added to 1.0 mL of either the PTBAEMA-stabilized Pickering emulsions or colloidosome suspensions to lower the solution pH to 3. This protocol allowed any demulsification to be readily assessed by visual inspection. Optical Microscopy. A drop of the diluted emulsion was placed on a microscope slide and viewed on an optical microscope (James Swift MP3502, Prior Scientific Instruments Ltd.) connected to a PC laptop to record images. This technique was used to estimate the mean droplet diameter. The response of the o/w emulsion droplets following in situ acidification of the aqueous phase was also assessed using this equipment. C
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Figure 3. Reaction scheme for formation of pH-responsive Pickering emulsions and cross-linked colloidosomes at 50 vol % using PEGMA− PTBAEMA latex at pH 10. Route A represents an o/w pH-responsive Pickering emulsion formed via homogenization at 20 °C with oil. Route B represents colloidosomes formed from an o/w Pickering emulsion using PPG-TDI following homogenization at 0 °C. In both cases, homogenization was conducted at 12 000 rpm for 2 min. Fluorescence Microscopy. A drop of colloidosome suspension was placed on a microscope slide and viewed on an Olympus upright epifluorescence microscope equipped with a Hamamatsu ORCA-ER monochrome camera and Volocity software. This technique was used to view fluorescently labeled PTBAEMA colloidosomes following evaporation of n-hexane at either pH 10 or 3, as well as addition of FITC−dextran following n-hexane evaporation.
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butyl and CH2 groups, respectively, attached to the secondary amine. These signals exhibit the largest shifts [1.46 (tert-butyl) and 3.4 (CH2) ppm] after reaction with the isocyanate groups on the PPG-TDI cross-linker. The CH2 signal also becomes obscured by the poly(propylene glycol) backbone signals present in PPG-TDI. The -OCH2 signal at 4.23 ppm in spectrum a is also shifted to 4.33 ppm. Spectra c−g shown in Figure 2 indicate the progress of the reaction once TBAEMA and PPG-TDI are mixed together at various molar ratios. As more PPG-TDI is added, the monomer signal at 4.23 ppm shifts gradually to 4.33 ppm. The shift in the two protons (peak e) adjacent to the secondary amine group (originally at 2.85 ppm) is not discernible, since this signal is obscured by the polymer backbone at 3.40 ppm. Increasing the PPG-TDI concentration in the reaction solution leads to the disappearance of the peak at 2.85 ppm, since all of the secondary amine eventually reacts with isocyanate groups on PPG-TDI. The PPG-TDI cross-linker was chosen because it is significantly less volatile (and hence less toxic) than smallmolecule diisocyanates. Its oil solubility also ensures that the cross-linking is confined within the individual oil droplets, rather than between adjacent droplets (see route B, Figure 3).10,11 The water-insoluble PPG-TDI also prevents partitioning between the oil droplets and the aqueous continuous phase. The reaction between secondary amine groups on TBAEMA residues and isocyanate groups on PPG-TDI produces robust urea cross-links between neighboring PTBAEMA particles surrounding the emulsion droplets, so the initial latex superstructure is locked in place to form colloidosome microcapsules. The PPG-TDI cross-linker is estimated to be around 5 nm in size. Thus cross-linking probably occurs only at the point of nearest approach of the close-packed latex particles at the oil/water interface. Because the oil phase is a poor solvent for PEG, the PEGMA stabilizer chains are collapsed on the oil side of the latex particles (which is where the PPG-TDI is located). This promotes cross-linking, since the latex particles can pack more closely together. It is perhaps also worth emphasizing that the spatial location of this cross-linking
RESULTS AND DISCUSSION
Latex Preparation and Characterization. It is worth briefly commenting on the series of sterically stabilized lightlycross-linked PEGMA−PTBAEMA latexes that have been reported previously.19 These syntheses were conducted via aqueous emulsion copolymerization of TBAEMA with DVB cross-linker in the presence of a hydrophilic PEGMA macromonomer at approximately 10% solids (see Figure 1). Mean latex diameters ranged from 150 to 220 nm depending on the initial DVB cross-linker concentration. Such PEGMA− PTBAEMA latexes produce stable o/w Pickering emulsions when homogenized at pH 10 with n-dodecane. In contrast, no stable Pickering emulsions could be obtained when these PTBAEMA particles were homogenized in their microgel state at pH 3. Thus these Pickering emulsifiers exhibit pH-dependent behavior. Moreover, complete demulsification of the Pickering emulsion formed at pH 10 occurs upon lowering the aqueous solution pH from 10 to 3. This is because the PTBAEMA latex becomes highly swollen cationic microgel particles under these conditions, which leads to their desorption from the oil/water interface. Spectroscopic Studies of Model Reaction of TBAEMA Monomer with PPG-TDI Cross-Linker. 1H NMR spectroscopy was used to monitor the model reaction between the secondary amine on the TBAEMA monomer and the isocyanate groups present on PPG-TDI (see Figure 2). Spectra a and b show the TBAEMA monomer and PPG-TDI, respectively [N.B. signals between 3.0 and 4.0 ppm in spectrum b correspond to the poly(propylene glycol) backbone.] The signals at 1.16 and 2.85 ppm in spectrum a correspond to tertD
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Table 1. Effect of Varying Synthesis Parameters on Mean Diameters of Poly[2-(tert-butylamino)ethyl methacrylate] and Poly[(2-tert-butylamino)ethyl methacrylate-co-styrene] Latexesa entry
PEGMA (wt %)
DVB (mol %)
APS (wt %)
TBAEMA:S molar ratio
comonomer conversionb (%)
theor Tg (°C)
1 2 3 4d 5 6 7 8 9e
10.0 5.0 5.0 0.0 10.0 10.0 10.0 10.0 10.0
0.8 2.4 4.0 0.8 0.8 0.8 0.8 0.8 0
2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
100:0 100:0 100:0 100:0 90:10 70:30 50:50 40:60 0:100
95 96 95 98 89 90 93 90 95
39 39 39 39 44 55 68 75 104
intensity-avg diameterc (nm) 200 155 160 220 340 215 200 250 450
± ± ± ± ± ± ± ± ±
30 15 15 50 95 60 45 80 180
Prepared using PEGMA stabilizer, DVB cross-linker, and APS initiator at 70 °C. The weight percent values given in columns 2 and 4 are based on total comonomer in the reaction mixtures. All latexes were prepared at pH 10 via emulsion (co)polymerization at 10% solids. Theoretical Tg values were calculated using the Flory−Fox equation. bDetermined by gravimetry. cDetermined by dynamic light scattering at 20 °C. dThis latex was also synthesized in the presence of Nile blue methacrylamide (NBM). eThis PEGMA−PS latex was prepared as a reference material for XPS studies. a
reaction is at the underlying latex surface, that is, below the chemically inert, solvated PEGMA steric stabilizer chains. In contrast, the cross-linking strategies described by Thompson et al.10 and Walsh et al.11 involved reaction of hydroxy- or aminefunctional steric stabilizer chains with the PPG-TDI reagent. In this sense, the spatial location of the cross-linking reaction described herein is similar to that described by Croll et al.21 The mechanical integrity of these colloidosomes can be readily tested by an acid challenge. Unlike PTBAEMAstabilized Pickering emulsions that readily demulsify upon addition of acid19 (see route A, Figure 3), such cross-linked PTBAEMA-based colloidosomes were expected to survive the addition of excess acid or ethanol. In the former case, a latex-tomicrogel transition was expected, but the swollen cross-linked microgel layer should remain intact to produce a microgel colloidosome (see route B, Figure 3). Acid-induced swelling of PTBAEMA latexes (entries 1−4, Table 1) was monitored by dynamic light scattering (DLS), as discussed previously.19 A latex-to-microgel transition was observed at around pH 8 for PTBAEMA particles prepared with 0.80 mol % DVB (see Figure 4a). The highly swollen cationic PTBAEMA microgels formed below pH 7.5 had hydrodynamic diameters of 600−700 nm, which correspond to a volumetric expansion factor of more than 27. However, increasing the DVB cross-linker content to 2.4 or 4.0 mol % reduced the swollen microgel diameter significantly (to less than 300 nm), and the critical pH for the latex-to-microgel transition shifted from 7.9 to approximately 7.5. Copolymerization of styrene (S) with TBAEMA produced copolymer latexes with hydrodynamic diameters of 225−340 nm (see Table 1). Styrene (S) appeared to have little effect on the final particle diameter when used up to 60 mol %, but above this value the particle size distribution became significantly broader. Acidinduced swelling of selected PEGMA−P(TBAEMA/S) copolymer latexes (entries 5, 7, and 8, Table 1) was monitored by DLS (see Figure 4b). Increasing the amount of styrene in the copolymer latex syntheses resulted in smaller swollen microgels at low pH, as expected. For interparticle cross-linking to be successful, the secondary amine groups must be present on the latex particle surface at a sufficiently high concentration. Copolymer latex surface compositions were assessed by X-ray photoelectron spectroscopy (XPS). This surface analytical technique has a typical sampling depth of 2−5 nm,22,23 which allows elemental surface compositions to be obtained for the near-surface of charge-
Figure 4. Variation of hydrodynamic diameter with solution pH for (a) (solid black squares) 0.8 mol % cross-linked PEGMA−PTBAEMA (entry 1, Table 1); (solid green inverted triangles) 2.4 mol % crosslinked PEGMA−PTBAEMA (entry 2, Table 1); and (solid blue triangles) 4.0 mol % cross-linked PEGMA−PTBAEMA (entry 3, Table 1). (b) PEGMA-P(TBAEMA/S) copolymer latexes with TBAEMA:S molar ratios of (open black squares) 90:10 (entry 5, Table 1); (open red circles) 50:50 (entry 7, Table 1); and (open blue diamonds) 40:60 (entry 8, Table 1).
stabilized PTBAEMA and PEGMA−PTBAEMA latexes. Figure 5 shows the XPS survey spectra recorded for (a) chargestabilized PTBAEMA latex and (b) PEGMA−PTBAEMA latex E
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sonication in n-dodecane (4.0 mL) and cooled to ∼0 °C in an ice bath. An equal volume of aqueous PEGMA−PTBAEMA latex particles (1.2 wt %, pH 10; entry 1, Table 1) was also added to a vial and cooled to ∼0 °C for 30 min. The oil was added via pipet to the vial containing the latex and homogenized for 2 min at 12 000 rpm within an ice jacket. This emulsion was allowed to stand in ice for 30 min, followed by a further 30 min at 20 °C. It is emphasized that liquids were precooled to 0 °C prior to homogenization, because the same reaction conducted at 20 °C allowed the PPG-TDI to react with the secondary amine too quickly, resulting in a viscous cross-linked gel and no Pickering emulsion. Presumably, the secondary amine groups became cross-linked with the PPGTDI during homogenization but prior to the formation of a stable emulsion. In contrast, a stable Pickering emulsion was formed when homogenization was conducted at 0 °C in the presence of PPG-TDI, confirming that this cross-linker had little or no effect on the wettability of the oil and that PEGMA−PTBAEMA latexes were still adsorbed at the o/w interface. Full emulsification of the oil phase was achieved, despite the presence of excess latex particles in the aqueous phase following creaming of the less dense oil-containing colloidosomes. However, laser diffraction studies indicated a significant difference in the oil droplet diameter following homogenization in the presence of PPG-TDI cross-linker compared to the Pickering emulsions prepared in its absence. Laser diffraction reported a mean n-dodecane droplet diameter of 230 ± 90 μm, compared with 50 ± 38 μm for PTBAEMA Pickering emulsions prepared at the same latex concentration. This increase in oil droplet diameter suggests that the PPG-TDI does have some secondary effect on droplet formation during homogenization. Presumably, the rapid reaction of the secondary amine (as observed when homogenization was performed at 20 °C) with the PPG-TDI occurs before the formation of stable oil droplets. This problem was also reported when n-dodecane was utilized as the oil phase for colloidosome formation with PEI-stabilized PS latex and PPG-TDI crosslinker.11 The resistance to demulsification for these PTBAEMA colloidosomes was compared to that of the corresponding Pickering emulsions prepared in the absence of PPG-TDI via an acid challenge. Approximately 1.0 mL of each emulsion was placed in a 2 mL vial and treated with approximately 5 drops of 0.10 M HCl to induce the latex-to-microgel transition. The Pickering emulsion demulsified within a few tens of seconds, whereas the cross-linked colloidosomes showed no signs of demulsification (see digital photographs, Figure 6). In situ demulsification was also observed when acid was added directly to the optical microscope slide (see Figure 6a, right-hand OM image). In contrast, PPG-TDI cross-linked colloidosomes exhibited no demulsification in the presence of acid (see Figure 6b). This acid challenge was also monitored on diluted emulsions by use of a Malvern Mastersizer instrument fitted with a smallvolume Hydro 2000SM sample dispersion unit. The relationship between oil droplet concentration and droplet diameter over 60 min for PTBAEMA-stabilized Pickering emulsions prepared in the absence of PPG-TDI has been previously reported.19 The solution pH was adjusted to pH 10 prior to addition of the colloidosome emulsion. Five measurements were recorded over 15 min at pH 10 to confirm the excellent stability of the oil droplets prior to the solution pH being
Figure 5. X-ray photoelectron survey spectra and (inset) core-line C 1s spectra recorded for (a) charge-stabilized PTBAEMA (entry 4, Table 1); (b) PEGMA−PTBAEMA (entry 1, Table 1); (c−f) PEGMA−P(TBAEMA/S) copolymer latexes with TBAEMA:S molar ratios of (c) 90:10 (entry 5, Table 1) (d) 70:30 (entry 6, Table 1) (e) 50:50 (entry 7, Table 1), and (f) 40:60 (entry 8, Table 1); and (g) PEGMA−PS particles (entry 9, Table 1). Particles were dried onto silicon wafers at 20 °C. Spectra are vertically offset for clarity.
dried at 20 °C on silicon wafers. Both spectra contain N 1s signals at 396 eV, which indicates the presence of nitrogen atoms at the surface of the particles. The nitrogen surface contents are 5.1 at. % and 4.8 at. % for charge-stabilized and PEGMA−PTBAEMA latexes, respectively. This reduction in surface nitrogen concentration suggests a PEGMA surface concentration of 5.8%. Both spectra also contain C 1s (283 eV) and O 1s (530 eV) signals as well as Cl 2s (267 eV), Cl 2p (198 eV), and Na KLL (469 eV). The presence of the latter signals is thought to be due to background electrolyte (NaCl). It is also noteworthy that the low Tg film-forming nature of the PTBAEMA latexes shields underlying Si 1s and Si 2s signals from the underlying silicon wafer. This is in contrast to higher Tg latex particles that crack upon drying, revealing the underlying silicon signals. The core-line C 1s signal is partially resolved into four sub-peaks for both latexes, revealing the presence of C−C, C−O, C−N, and CO species at the surface of the latex particles (see Figure 5 inset). This is due to the presence of ester carbonyls in the PTBAEMA residues, as well as the secondary amine groups. PEGMA−PTBAEMA-Based Colloidosomes. Oil-soluble cross-linker (PPG-TDI, 1−5 mg/mL) was dissolved via F
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Figure 6. Digital photographs and optical microscopy images obtained for PEGMA−PTBAEMA latex-stabilized n-dodecane-in-water emulsions after homogenization (12 000 rpm, 2 min) before and after an acid challenge (a) in the absence of PPG-TDI cross-linker, and (b) in the presence of PPGTDI cross-linker (1 mg/mL).
colloidosome particles were fully cross-linked. However, no macroscopic oil was present in the sample chamber after the pH jump (unlike that found in Pickering emulsions), confirming that the majority of these colloidosomes survive an acid challenge. The mechanical integrity of these colloidosomes was also tested via an “alcohol challenge”, whereby the oil phase was completely removed by use of excess alcohol (either ethanol or 2-propanol).10,11,24 PTBAEMA-based Pickering emulsions do not survive such a challenge. However, in the case of the colloidosomes, the cross-linked microcapsule shell is expected to remain intact upon removal of the internal oil phase. Unfortunately, the relatively low Tg of PTBAEMA caused partial coalescence and/or film formation upon drying, making electron microscopy imaging difficult. Another problem is that the ethanol used to remove the oil phase is a solvent for PTBAEMA, leading to particle swelling. This makes it difficult to image individual colloidosomes prepared from lightly crosslinked particles (entry 1, Table 1) via optical microscopy (OM), but the colloidosomes could be observed by SEM (see Figure S1a in Supporting Information) following drying at 20 °C from ethanol. Although these latex superstructures could be observed, higher magnification images were not possible due to the drying of swollen colloidosome shells. Therefore, a new approach was investigated in order to limit the degree of swelling for the PTBAEMA microgel colloidosomes following an ethanol challenge. Various other solvents were investigated in an attempt to remove the internal oil phase without causing swelling of the PTBAEMA particles, but unfortunately no suitable solvent was identified. Instead, the DVB cross-linker content was increased to limit the swelling of the PTBAEMA microgels (see entries 2 and 3, Table 1). Increasing the DVB cross-linker content to 2.4 mol % reduced the swollen microgel diameter significantly (to less than 300 nm; see Figure 4a). The most notable difference is that the structural outline of these colloidosomes can now be more clearly visualized (see Figure
lowered (see Figure 7a). Acid (0.10 M HCl) was then added directly to the sample chamber, and changes in oil droplet
Figure 7. Volume-average droplet diameter distribution curves obtained by laser diffraction for (a) PTBAEMA-stabilized ndodecane-in-water colloidosomes at pH 10 and (b) the resulting size distribution after the pH was lowered to 3 using HCl. Measurements were recorded every 3 min for 60 min, with the acid being added after the first five measurements.
diameter and concentration were observed in situ at pH 3. The covalently cross-linked colloidosomes showed no reduction in oil droplet concentration (as observed for PTBAEMAstabilized Pickering emulsions); instead, the concentration remained constant for the entire duration (see Figure 7b). A slight shoulder at lower diameter began to appear, indicating a modest degree of demulsification and suggesting that not all G
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Figure 8. Scanning electron microscopy images obtained for PPG-TDI cross-linked colloidosomes prepared with (a) 4.0 mol % DVB cross-linked PEGMA−PTBAEMA (entry 3, Table 1), (b) PEGMA-P(TBAEMA/S) latex at pH 10 (50:50 TBAEMA:S molar ratio; entry 7, Table 1), and (c) PEGMA-P(TBAEMA/S) latex at pH 10 (40:60 TBAEMA:S molar ratio; entry 8, Table 1) after washing with excess ethanol. All mixtures were homogenized for 2 min at 12 000 rpm and allowed to stand for 1 h. Image d is a higher magnification SEM image of image c, which confirms the particulate nature of the colloidosome shell.
work. Entry 5 is a PEGMA-P(TBAEMA/S) latex prepared with a 90:10 TBAEMA:S comonomer molar ratio, which produced particles with a hydrodynamic diameter of 340 ± 95 nm (as judged by DLS). Despite the addition of 10 mol % S, these copolymer particles still proved difficult to image by transmission electron microscopy (TEM) (see Figures S2a, S2b in Supporting Information) and SEM (images not shown) because of their film-forming nature. Therefore the incorporation of 30, 50, and 60 mol % styrene into the copolymer latex formulation was investigated. These syntheses were also performed by “one-pot” emulsion copolymerization in the presence of 0.8 mol % DVB and 10 wt % PEGMA macromonomer. Entries 6, 7, and 8 in Table 1 are PEGMAP(TBAEMA/S) latexes prepared with 70:30, 50:50, and 40:60 TBAEMA:S comonomer molar ratios, with respective hydrodynamic diameters of 215 ± 60, 225 ± 45, and 240 ± 75 nm. Figure S2 in Supporting Information shows TEM images obtained for these copolymer latexes. Better-quality images were observed for particles containing a higher proportion of styrene, as expected. Despite this, some film formation was still evident. Stable o/w Pickering emulsions were formed when all of these copolymer latex particles were homogenized in turn with n-dodecane in the absence of PPG-TDI at 20 °C. The initial latex concentration was adjusted to 1.1 wt % prior to homogenization. Following emulsification, the oil droplet
S1b in Supporting Information). However, higher magnification images proved difficult, as the ethanol-swollen particles still possessed some film-forming character upon drying. OM proved difficult for imaging the colloidosomes, which is believed to be related to a reduction in the refractive index of the ethanol-swollen colloidosomes following washing. Figure 8a shows an SEM image obtained when the DVB cross-linker content was increased to 4.0 mol %. Again, the particulate shell morphology proved difficult to image, most likely the result of plasticization as well as the low Tg of PTBAEMA. Nevertheless, intact colloidosome structures could be observed on the SEM stub. A final attempt to visualize the latex superstructure within the colloidosome shells involved partial replacement of TBAEMA monomer with a higher Tg comonomer, namely, styrene. PS homopolymer has a Tg of 104 °C16 and is known to withstand an ethanol challenge upon removal of the internal oil phase. 10,11 Styrene comonomer also offers a potential commercial benefit for the production of such microcapsules since it is significantly cheaper than TBAEMA. Copolymerization of 2-(tert-Butylamino)ethyl Methacrylate with Styrene. Colloidal P(TBAEMA/S) particles were prepared by “one-pot” emulsion copolymerization at pH 9 in the presence of PEGMA macromonomer and 0.8 mol % DVB cross-linker (see Figure 1). Entries 5−8 in Table 1 summarize the statistical copolymer latexes prepared in this H
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Figure 9. Fluorescence microscopic images of cross-linked colloidosomes prepared with NBM-labeled PTBAEMA latex particles. These colloidosomes were prepared with n-hexane as the oil phase, which was allowed to evaporate at either pH 10 (panels A and B) or pH 3 (panels C and D). Following this, FITC−dextran aqueous solution (0.1 wt %, an equal volume to the evaporated n-hexane) was added to the suspension. Images A and C were recorded with a Texas red filter to visualize the NBM-labeled PTBAEMA particles, whereas images B and D were recorded with a diamidino-2-phenylindole (DAPI)−FITC filter to enable the location of FITC−dextran to be observed.
diameters were comparable to o/w Pickering emulsions prepared using PEGMA−PTBAEMA particles under similar conditions. However, an important question is whether the surface concentration of copolymerized TBAEMA residues is sufficient to enable colloidosome formation. Figure 5 shows XPS survey spectra recorded for a series of PEGMA-P(TBAEMA/S) copolymer latexes prepared at various TBAEMA:S molar ratios, as well as a PEGMA−PS latex (Figure 5g) for comparison. A N 1s signal was observed at 396 eV for spectra c−f, indicating the presence of nitrogen atoms at the surface of the latex particles. The respective nitrogen surface contents are (c) 4.3, (d) 3.9, (e) 2.7, and (f) 1.5 at. %. This progressive reduction in surface nitrogen concentration is consistent with the lower proportion of TBAEMA comonomer employed in these copolymer latex syntheses. As expected, spectrum g has no N 1s signal since these polystyrene latex particles were prepared under the same conditions but in the absence of TBAEMA. All spectra also contain C 1s (283 eV) and O 1s (530 eV) signals. The inset in Figure 5 shows the
respective C 1s core-line spectra for each of the above samples: close inspection indicates presence of C−C, C−O, C−N, and CO subpeaks. Once XPS had confirmed that the TBAEMA residues were present at the surface of all copolymer latexes, the particles were emulsified with n-dodecane to form stable Pickering emulsions. A 4.0 mL aliquot of the 50:50 TBAEMA/S molar ratio (entry 7, Table 1, 1.0 wt %, pH 10) was added to a vial and cooled to 0 °C. PPG-TDI (3.0 mg/mL) was sonicated in 4.0 mL n-dodecane and cooled to 0 °C in an ice bath. Following homogenization at 12 000 rpm for 2 min, the emulsion was allowed to stand in ice for 30 min, followed by a further 30 min at 20 °C. No stable colloidosomes were formed, presumably because of the reduced nitrogen content at the particle surface. Thus, the same protocol was repeated via homogenization at 20 °C and the resulting stable emulsion was allowed to stand at 20 °C for 1 h to allow cross-linking to occur. Laser diffraction studies indicated an oil droplet diameter of 138 ± 100 μm, which was supported by OM studies (data I
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recorded when n-dodecane was employed as the oil phase. Following slow evaporation of the n-hexane at 20 °C, an aqueous solution of FITC−dextran (4.0 mL, 0.05 wt %) was added to both the suspensions at either pH 10 or 3. (N.B. the pH of the FITC−dextran solution was adjusted to either 10 or 3 prior to addition to the colloidosome suspensions.) Fluorescence microscopic images indicate the collapsed nature of the latex colloidosomes at pH 10 upon removal of the nhexane (see Figure 9A). Furthermore, the FITC−dextran polymeric dye does not penetrate these microcapsules (see Figure 9B). In contrast, the microgel colloidosomes, formed upon lowering the solution pH to 3, regain their original spherical morphology following oil evaporation rather than remaining collapsed (see Figure 9C). In addition, FITC− dextran fully penetrates the latter colloidosomes, confirming the highly porous nature of the cationic microgel shell (see Figure 9D). These observations suggest a convenient loading mechanism for these novel microgel colloidosomes.
not shown). Regardless of the copolymer latex concentration used for homogenization, creaming of the emulsion droplets upon standing always produced a turbid underlying aqueous phase. This indicates that latex adsorption is not particularly efficient, since excess non-adsorbed particles always remain in the continuous phase. The latex adsorption efficiency was not determined in the present work, but in principle this parameter could be determined via either gravimetry or turbidimetry measurements on the aqueous supernatants after creaming. These colloidosomes were subjected to an ethanol challenge in order to remove the internal oil phase, leaving behind the intact microcapsule. A SEM image of a dried colloidosome prepared from a PEGMA-P(TBAEMA/S) latex (TBAEMA:S molar ratio = 50:50; entry 7, Table 1) is shown in Figure 8b. These colloidosomes are easier to visualize than the PTBAEMA-based colloidosomes, since ethanol does not solvate the copolymer chains. The surviving microcapsules provide good evidence for effective interparticle cross-linking. The structures appear to be hollow when viewed on the microscope slide (see Figure S3 in Supporting Information). However, the colloidosomes collapse following evaporation of ethanol, allowing higher contrast images. An ethanol challenge conducted on Pickering emulsions prepared from the same copolymer latex in the absence of any PPG-TDI produced no such structures (data not shown). In all cases, the colloidosomes formed collapsed 2D “pancakes” rather than hollow, spherical structures as a result of the ultrahigh vacuum conditions required for SEM studies. Although better quality images were obtained with these copolymer latexes, it still proved difficult to observe the individual copolymer latex particles within the colloidosome shell. Since XPS confirmed the presence of nitrogen on the surface of the copolymer latex prepared with 60 mol % styrene, these particles were also investigated as Pickering emulsifiers with PPG-TDI present in the oil phase. A 5.0 mL aliquot of the latex solution (entry 8, Table 1; 1.6 wt %, pH 10) was added to a vial. PPG-TDI (25 mg) was sonicated in 5.0 mL of n-dodecane and homogenized at 12 000 rpm for 2 min with the latex in order to allow cross-linking to occur at 20 °C. After standing at 20 °C for 1 h, laser diffraction studies indicated a mean oil droplet diameter of 160 ± 90 μm for the emulsion, which was confirmed by OM studies (data not shown). This emulsion was subjected to an ethanol challenge in order to remove the internal oil phase, leaving behind the colloidosome microcapsule. Figure 8c shows a typical SEM image obtained for colloidosomes prepared according to entry 8 (see Table 1) after an ethanol challenge. These colloidosomes are also much easier to visualize than the PEGMA−PTBAEMA colloidosomes, since ethanol does not solvate the styrene-rich copolymer chains. Moreover, higher magnification images (see Figure 8d) confirmed that these colloidosome shells were composed of copolymer latex particles. Finally, n-hexane was utilized as an oil phase in order to investigate the latex superstructure of the colloidosomes following oil removal at either pH 10 or 3. Under these conditions, the colloidosome shell should comprise either nonswollen latex particles (pH 10), or swollen microgels (pH 3). Nile blue-labeled charge-stabilized PTBAEMA particles (4.0 mL, 1.0 wt %, pH 10) were homogenized with n-hexane (4.0 mL, containing 1.0 mg/mL PPG-TDI) to produce colloidosomes. Following homogenization at 0 °C, the colloidosomes were allowed to stand for 1 h at 20 °C to allow cross-linking to occur. The mean oil droplet diameter was similar to those
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CONCLUSIONS A series of sterically stabilized, lightly cross-linked PTBAEMAbased latexes were prepared via emulsion (co)polymerization. Such particles have relatively narrow particle size distributions and act as effective Pickering emulsifiers for model oils such as n-dodecane. The secondary amine group on the TBAEMA residues can be utilized to prepare colloidosomes via reaction with an oil-soluble polymeric diisocyanate cross-linker added to the oil phase prior to emulsification. Unlike the precursor Pickering emulsions, such colloidosomes proved to be sufficiently robust to survive a challenge with excess ethanol, which removes the oil droplet phase. Upon treatment with acid, the cross-linked latex particles within the colloidosome shell become highly swollen, producing a new type of highly permeable cationic microgel colloidosome. Copolymerization of styrene with TBAEMA produced copolymer latexes that exhibited lower degrees of swelling at low pH but possessed higher Tg values, which facilitated colloidosome imaging via electron microscopy. X-ray photoelectron spectroscopy studies confirmed the presence of surface nitrogen atoms attributed to the TBAEMA residues, and successful colloidosome crosslinking could still be achieved for TBAEMA contents as low as 40 mol %. However, robust colloidosomes could not be obtained at higher styrene contents since the surface concentration of the TBAEMA comonomer residues was too low for efficient cross-linking. Fluorescence microscopic studies confirmed that these colloidosomes can regain their original spherical morphology after evaporation of the internal oil phase in their microgel form but not in their latex form. As expected, the colloidosome shell proved to be much more permeable toward a water-soluble polymeric dye in its microgel form at low pH than in its latex form at high pH.
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ASSOCIATED CONTENT
S Supporting Information *
Additional text describing PTBAEMA latex synthesis via aqueous emulsion polymerization, and three figures showing SEM images obtained for PPG-TDI cross-linked PTBAEMA colloidosomes, TEM images of PEGMA-P(TBAEMA/S) copolymer latexes, and optical microscopy images recorded after an ethanol challenge of PEGMA-P(TBAEMA/S) colloidosomes. This material is available free of charge via the Internet at http://pubs.acs.org. J
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(20) Madsen, J.; Canton, I.; Warren, N. J.; Themistou, E.; Blanazs, A.; Ustbas, B.; Tian, X.; Pearson, R.; Battaglia, G.; Lewis, A. L.; Armes, S. P. Nile blue-based nanosized pH sensors for simultaneous far-red and near-infrared live bioimaging. J. Am. Chem. Soc. 2013, 135, 14863− 14870. (21) Croll, L. M.; Stöver, H. D. H.; Hitchcock, A. P. Composite tectocapsules containing porous polymer microspheres as release gates. Macromolecules 2005, 38, 2903−2910. (22) Hofmann, S. Practical surface analysis: State of the art and recent developments in AES, XPS, ISS and SIMS. Surf. Interface Anal. 1986, 9, 3−20. (23) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley: Chichester, U.K., 1992; p 295. (24) Williams, M.; Armes, S. P.; York, D. W. Clay-based colloidosomes. Langmuir 2011, 28, 1142−1148.
AUTHOR INFORMATION
Corresponding Author
*E-mail S.P.Armes@sheffield.ac.uk Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank EPSRC for a Ph.D. studentship for A.J.M. and both METRC and Cytec (Drogenbos, Belgium) for additional financial support. Dr Claire Hurley is thanked for XPS analysis.
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REFERENCES
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