A Versatile Synthesis Platform To Prepare Uniform, Highly Functional

Mar 25, 2014 - Advances in the understanding of microgel properties and exploitation of their full potential for applications require control of the e...
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A Versatile Synthesis Platform To Prepare Uniform, Highly Functional Microgels via Click-Type Functionalization of Latex Particles Rahul Tiwari,† Daniel Hönders,† Susanne Schipmann,‡ Björn Schulte,† Paramita Das,† Christian W. Pester,† Uwe Klemradt,‡ and Andreas Walther*,† †

DWI−Leibniz Institute for Interactive Materials Research, Forckenbeckstr. 50, 52074 Aachen, Germany II. Institute of Physics B, RWTH Aachen University, 52056 Aachen, Germany

‡

S Supporting Information *

ABSTRACT: Advances in the understanding of microgel properties and exploitation of their full potential for applications require control of the extent and type of functionalization, while at the same time providing control of particle shape, that is, size and uniformity, and intrinsic particle properties such as hardness and degree of crosslinking. However, versatile and simple synthetic approaches to prepare highly uniform and densely functionalized microgels based on functional groups unsuitable for aqueous precipitation polymerization are still scarce. As an alternative platform approach, herein we report on the synthesis of uniform particles based on classical batch emulsion polymerization, which are later postfunctionalized via Cu-mediated Huisgen-type alkyne/azide click chemistry. We use propargyl acrylate (PGA) as a monomer and ethylene glycol dimethacrylate (EGDMA) as a crosslinker for the synthesis of narrowly dispersed latex particles in the size regime of 50−200 nm in radius. We demonstrate how particle hardness and swelling can be tuned as a function of the used ratio of monomer/crosslinker. Postmodifications in the interior of the particles are conducted in the swollen state in DMSO, and we add pH-responsive cationic moieties as a first attractive model functionality. Combined Raman spectroscopy and elemental analysis reveal the kinetics and degree of modification. Both depend on the degree of crosslinking, and we find densely functionalized particles exhibiting a conversion of the alkyne functionalities of up to 90%. After modification, the resulting microgels display a pH-dependent ionization and swelling behavior in water. The suggested route opens up new and versatile ways to prepare narrowly dispersed water-dispersible microgels with tailored hardness and high density of functional groups, based on readily available building blocks.

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INTRODUCTION Aqueous microgels are a unique class of colloidal particles with dimensions in the range of ca. 50 nm to several micrometers, combining responsiveness to environmental parameters as known from stimuli-responsive polymers with a particle character.1−9 In terms of size, they exceed other branched architectures such as hyperbranched polymers or crosslinked block copolymer micelles and add a strong particulate behavior to the property profile.10−14 However, they clearly contrast classical hard-sphere colloids by being soft and water-swollen, penetrable to solutes and guest molecules and exhibiting a diffuse boundary. Applications of microgels are pursued in a wide range of fields, from targeted delivery to photonic materials, as microlenses or as host for catalysts and chemical separation.15−21 Appealing is also their ability to stabilize interfaces, where they provide particulate stabilization similar to Pickering emulsions, but at the same time a soft and deformable interface with less restricted diffusion of molecules due to the network character of the microgel particles.4,22−24 It is absolutely mandatory to find pathways to control the functionalities (surface and interior) while maintaining a defined shape and uniform character to be able to deduce clear structure/property relationships or even to be able to target specific applications that can only be achieved by narrowly dispersed materials, e.g. sensors or filters, based on © 2014 American Chemical Society

colloidal crystals. The predominant route to microgels is aqueous precipitation polymerization.25−28 This technique is limited to polymers with lower critical solution behavior, and excess doping of additional functionalities, e.g., acid functions, into poly(Nisopropylacrylamide) can lead to the disappearance of the LCST behavior, heterogeneous compositions, and the failure of precipitation polymerization.29,30 Other alternatives to access functional microgels is the development of precipitation polymerization techniques31,32 in nonaqueous media or the use of miniemulsions33,34 as size-constraining templates to prepare crosslinked micro- and nanodroplets. However, the development of novel precipitation polymerizations in organic solvents requires knowledge of the phase diagrams, and often extensive experiments are required to optimize temperature, concentrations, and added stabilizer to yield narrowly dispersed particles. Similarly, although miniemulsions are extremely versatile, it is rather difficult to obtain uniform droplets.35 In terms of dispersity of the resulting particles, it has been shown that latex particles produced by classical emulsion polymerizations are of superior uniformity compared to particles based Received: December 10, 2013 Revised: February 28, 2014 Published: March 25, 2014 2257

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on the miniemulsion technique.36 In fact, emulsion polymerization has been developed to an extent so that extremely uniform particles can be achieved. Further seeding techniques provide a facile way to prepare well-defined dumbbell and Janus particles, core−shell materials, or large micrometer-sized beads.37,38 Surprisingly, there is only a very rare amount of studies dealing with the application of emulsion polymerization and latex particles to the field of microgels, albeit it could be one of the simplest access routes.24,39−42 Herein, we will establish a versatile platform technology for the synthesis of highly functional, narrowly dispersed microgels based on an emulsion polymerization approach of a commercial monomer carrying an alkyne functionality. Such latex particles can be efficiently and selectively functionalized on both the surface and the interior of the particles with click-type ligation using Cu(I)-catalyzed alkyne/azide cycloaddition. Thereby we combine two simple and well-established methods of colloid chemistry and modular ligation. Emulsion polymerization enables a facile access to uniform and “easy-to-functionalize” microgels. Our approach contrasts earlier work dealing with (surface) modification of larger, reactive microspheres43−49 or the incorporation of a small amount of functionalities50 by focusing on modifications in the interior of completely functionalized latex nanoparticles as well as on material aspects of the resulting microgels. In the following, we will demonstrate in detail how the internal structure, i.e., the degree of crosslinking, can be tuned and how this influences particle hardness, the ability for swelling, and the degree, kinetics, and efficiency of click-type postmodification.

crosslinking density and thereby modification of particle hardness and the ability of the particle to swell as a function of the solvent quality. The network node density in the swollen state is also decisive for the second reaction step, which uses Huisgen Cu(I)-catalyzed alkyne/azide click chemistry51−53 to functionalize the pendant alkyne groups with functional azides. To allow easy access of the reactant, the particles are swollen in a good solvent to yield organogel particles. Following click modification, the functionalized particles can be transferred into water to yield the desired microgels. The general approach starts with the preparation of welldefined, narrowly dispersed latex particles. For this purpose we use a classical and straightforward batch emulsion polymerization employing sodium dodecyl sulfate (SDS) as surfactant to regulate the size and potassium peroxodisulfate as radical initiator (K2S2O8 (KPS)) at 60 °C. At first we will focus on how to tailor the particle size for two different degrees of crosslinking, described by the ratio of monomer/crosslinker. Here, we choose ratios of 10 and 250, yielding rather tightly and loosely crosslinked latex particles, respectively. EGDMA has been shown to be slightly more reactive than methyl acrylate in copolymerizations, which may lead to some depletion of EGDMA during the course of the emulsion polymerization and a slightly higher accumulation in the center of the particle.54 In our case, we traced the incremental incorporation by timedependent 1H NMR of the comonomers remaining in the mixture and also found a slightly faster polymerization of EGDMA, suggesting a slightly more crosslinked core (see Figure SI1, Supporting Information). We however selected EGDMA over the diacrylate analogue due to the more narrow size distributions of the resultant latex particles as found in initial screening experiments. Table 1 summarizes the size distribution analysis conducted via dynamic light scattering (DLS), smallangle X-ray scattering (SAXS), and statistical image analysis based on transmission electron microscopy (TEM). To allow for a conclusive analysis of the size distributions as a function of the surfactant concentration, all polymerizations were allowed to proceed for 48 h to ensure a high and comparable level of conversion, typically in the range of 90−95% (determined by gravimetry). Both series reveal the expected trend that an increase of surfactant concentration affords decreasing the overall particle size (Figure 1a).55−58 Higher SDS concentrations stabilize a larger amount of nuclei in the nucleation stage of the polymerization, which then yields overall smaller particles after monomer consumption when compared to lower SDS concentrations. The number-average radii obtained by statistical image analysis of the TEM images (Figure 1e,f) show that the particle dimensions can be tuned from roughly Rn,TEM = 40−160 nm for these combinations of monomer/crosslinker, hence covering an interesting size regime for functional nano- or microgels. All particles are characterized by an extremely low polydispersity index (PDI = Rw/Rn = 1.004−1.020). This can be macroscopically observed by the propensity of the latex particles to undergo crystallization into colloidal crystals with bandgaps in the visible light region and is further underscored by the SEM image displayed in Figure 1d. Additional SEM images with elemental mapping using energy-dispersive X-ray analysis are provided in Figure SI2. The width of the size distribution of the particles, as precisely evaluated by TEM, demonstrates a consistent narrowing for lower SDS concentrations. This can be explained by shorter and more defined nucleation stages.59,60 The DLS CONTIN plots, recorded after freeze-drying and

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RESULTS AND DISCUSSION The general concept is depicted in Scheme 1. Initially we use classical emulsion polymerization of propargyl acrylate (PGA) in combination with a crosslinker ethylene glycol dimethacrylate (EGDMA) to synthesize well-defined latex particles. The ratio of monomer/crosslinker (PGA/EGDMA) allows tuning of the Scheme 1. General Reaction Scheme for the Preparation of Uniform and Functional Microgels via Click-Type Ligation: (i) Synthesis of crosslinked Latex from PGA/EGDMA by Batch Emulsion Polymerization; (ii) Swelling of Freeze-Dried Latex Particles in Organic Solvent; (iii) Postmodification of Swollen Organogels via Click Chemistry To Yield Functional Microgels in Water

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Table 1. Tuning the Size as a Function of the Surfactant Concentration at Two Different crosslinking Densities: Monomer/ crosslinker = 10 and 250a SAXS characterization simple sphere sampleb

c(SDS) [mM]

Rn,TEM ± STD (PDI)c [nm]

PGA2501 PGA2502 PGA2503 PGA2504

0.05 0.10 0.15 0.20

160 ± 4 (1.004) 104 ± 4 (1.007) 71 ± 3 (1.009) 57 ± 4 (1.020)

PGA101 PGA102 PGA103 PGA104

0.05 0.10 0.15 0.20

114 ± 4 (1.005) 70 ± 3 (1.010) 65 ± 4 (1.020) 41 ± 5 (1.020)

⟨Rh⟩z,DLS (PDI)d [nm] monomer/crosslinker = 250 183 (1.05) 130 (1.04) 95 (1.06) 81 (1.07) monomer/crosslinker = 10 140 (1.01) 101 (1.03) 77 (1.05) 47 (1.07)

core linshell

Rsphere ± STD [nm]

Rcore ± STD [nm]

dRlinshell [nm]

185.0 ± 5.0 114.0 ± 4.5 83.0 ± 3.0 60.0 ± 3.5

185.0 ± 5.0 114.0 ± 4.5 83.0 ± 3.0 61.0 ± 3.5

2.0 2.0 2.0 2.0

122.5 ± 2.5 74.8 ± 2.5 66.0 ± 3.0 50.0 ± 2.5

122.5 ± 2.5 76.0 ± 2.5 66.0 ± 3.0 50.0 ± 2.5

1.5 1.0 1.5 1.0

a

Emulsion polymerization using the indicated ratios of monomer/crosslinker (250 and 10), different surfactant concentrations, and c(K2S2O8) = 0.2 mM at 60 °C. bThe subscript denotes the ratio of monomer/crosslinker. cPDI is defined as Rw/Rn. d⟨Rh⟩z from CONTIN analysis and PDI from second cumulant.

Figure 1. Analysis of particle size and distribution of PGA/EGDMA latex dispersions in dependence of the surfactant concentration. (a) Numberaverage radius of latex particles as a function of surfactant concentration obtained via TEM. The standard deviation is displayed as error bars to show the width of the distribution. (b, c) DLS CONTIN plots of PGA/EGDMA latex particles for PGA/EGMA = 10 (b) and 250 (c). (d) SEM image of PGA101 The inset shows a photograph demonstrating the photonic bandgap originating from the colloidal crystal packing. (e, f) Representative TEM images of PGA101 and PGA2501 (c(SDS) = 0.05 mM) and molar ratio PGA/EGDMA of 10 and 250, respectively. Additional SEM images with elemental mapping using energy-dispersive X-ray analysis are provided in Figure SI2.

using (i) the form factor for spherical particles as well as (ii) the form factor for spherical core−shell particles containing a thin shell of ca. 1−2 nm thickness with a higher scattering length density and a linearly decaying profile. Furthermore, for the PGA102, PGA103, and PGA104 samples, it was necessary to account for particle interactions with respect to the intensity distribution at low q-values, suggesting the formation of loose aggregates. Best results were obtained by fitting a structure factor of adhesive hard spheres (see Experimental Section). We attribute these isolated aggregation phenomena to insufficient redispersion during quick sample preparation and subsequent direct measurement  a problem which however only became apparent during off-site data treatment. Further details regarding sample preparation and data evaluation can be found in the

redispersion in water, display well-defined monomodal distributions and thereby demonstrate absence of coagulation and excellent colloidal stability and the ability to easily redisperse the particles after drying (Figure 1b,c). The intensity-weighted average radius determined by DLS (⟨Rh⟩z) is in agreement with the TEM data, considering that the intensity-weighted average is listed and a thin hydrated surface layer contributes when measured in water (Table 1).23,61 We also measured small-angle X-ray scattering data (SAXS) for selected samples to precisely verify size and size distribution of the prepared particles (Figure 2). Periodic undulations of the scattering curves indicate particles with a well-defined geometry, and we find oscillations well beyond the tenth minima, indicative of a high degree of uniformity. We conducted fitting procedures 2259

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Figure 2. Overview of SAXS data and corresponding fits for simple sphere (solid line) and core−shell models with a thin linearly decaying shell (dashed line) for PGA10 (a) and PGA250 (b) as a function of SDS. Scattering curves are separated by a factor of 10 to allow for a better visualization.

Table 2. Overview of the Influence of the Ratio of Monomer/crosslinker on the Particle Size and Degree of Swellinga sample

PGA/EGDMA

Rn,TEM ± STD (PDI)b [nm]

⟨Rh⟩z,DLS (PDI)c [nm] in water

⟨Rh⟩z,DLS (PDI) [nm]c in DMSO

PGA2 PGA10 PGA20 PGA30 PGA50 PGA100 PGA150 PGA200 PGA250 PGA500 PGA∞

2 10 20 30 50 100 150 200 250 500 no EGDMA

110 ± 4 (1.006) 114 ± 4 (1.005) 137 ± 4 (1.005) 143 ± 4 (1.003) 148 ± 5 (1.005) 153 ± 4 (1.004) 156 ± 3 (1.002) 158 ± 4 (1.002) 160 ± 4 (1.004) 157 ± 4 (1.003) 161 ± 4 (1.003)

137 (1.04) 140 (1.03) 147 (1.03) 159 (1.04) 167 (1.03) 170 (1.02) 176 (1.09) 178 (1.08) 183 (1.05) 197 (1.02) 195 (1.06)

160 (1.06) 167 (1.09) 215 (1.05) 243 (1.03) 258 (1.08) 265 (1.03) 283 (1.09) 298 (1.06) 309 (−d) 345 (−d) coag

a

Emulsion polymerization using the indicated ratios of monomer/crosslinker (2−500), at constant surfactant concentration (c(SDS) = 0.05 mM) and c(K2S2O8) = 0.2 mM at 60 °C. bPDI is defined as Rw/Rn. c⟨Rh⟩z from CONTIN analysis and PDI from second cumulant. dNot determined due to slight multimodality of the DLS CONTIN plot.

Figure 3. Dependence of particle size of latex particles as a function of ratio of monomer/crosslinker. (a) Number-average radius of latex particles as a function of degree of crosslinking obtained via TEM. The standard deviation is displayed as error bars. The dashed line serves as a guide to the eye. (b) DLS CONTIN plots of differently crosslinked latex particles as indicated within the figure.

Experimental Section. A comparison of the two fits reveals that the data can be fitted more accurately with the core−shell type of fit. We relate the observation of a thin shell (ca. 1−2 nm) to the presence of polar sulfate groups, originating from the initiator (KPS) and the surfactant (SDS). The resulting values for the particle radius and PDI mostly fall in between DLS and TEM analysis, hence corroborating the values found by TEM and DLS, and reveal the presence of a stabilizing surfactant layer at the surface.

Interestingly, Figure 1a displays slight differences in the overall particles size at the same surfactant concentration. Latex particles with a monomer/crosslinker ratio of 10 are consistently smaller than the ones prepared with lower amounts of crosslinker in the reactions. Hence, it becomes obvious that the crosslinker takes part in the nucleation process and defines the amount of nuclei, which then grow to different overall size. We address this issue further by changing the monomer/crosslinker ratio to tune the particle crosslinking density and hardness at a constant SDS 2260

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Figure 4. Analysis of swelling behavior of PGA/EGDMA latex in DMSO in dependence of the degree of crosslinking. (a) DLS CONTIN plots of PGA2, PGA100, and PGA200 redispersed in H2O (H, solid line) and swollen in DMSO (D, dashed lines). (b) Swelling behavior of PGA/EGDMA as expressed by the volume ratio of VDMSO/VH2O. The dashed line is a guide to the eye.

species, thus ready for subsequent modification via click chemistry in a defined fashion. We note that slight agglomeration is present in the DLS CONTIN plots for particles with ratios of monomer/crosslinker above 250. Therefore, the CUMULANT analysis was not applied because it is restricted to systems with single species. The pure PGA∞ organogel suffers from somewhat stronger aggregation, which might be related to the absence of crosslinking. Yet, even those PGA∞ without any crosslinker maintain their particle-like character as found in TEM (not shown). This is due to a high level of entanglements, possible radical transfer onto the polyacrylate backbone, resulting in branching and slight crosslinking, or some side reaction of the alkyne group participating in the radical polymerization (also see below).63−65 A quantitative understanding of how the degree of crosslinking influences the ability of the organogel particles to swell can be obtained by comparing the actual volumes of the particles in the collapsed and the solvent-swollen organogel state. Interestingly, even particles with a high degree of crosslinking of 2 swell more than 1.6-fold of their initial volume when exposed to DMSO. This ratio VDMSO/VH2O increases further to a maximum ratio of approximately 5.5 at the lowest degree of crosslinking at monomer/crosslinker = 500 (Figure 4b). The ability of these organogel particles to swell increases rapidly from PGA2 to PGA30 and begins to level off at around PGA50 to lower crosslinking. This indicates that the extent of particle swelling and crosslinking is increasingly controlled by the properties of the PGA material rather than the addition of crosslinker EGDMA above monomer/crosslinker >100. The qualitative range of swelling is slightly lower compared to classical PNIPAM microgels.67 Functionalization of PGA/EGDMA Organogel Particles via “Click Chemistry”. We used Huisgen-type Cu(I)-catalyzed azide−alkyne cycloaddition to functionalize differently crosslinked PGAx organogels. In general, it is possible to selectively functionalize the surface or the full particle, also in sequence giving rise to core−shell structures, or potentially extend the strategy to side-selective functionalization after immobilization as Pickering particles to break the symmetry and prepare Janus particles and microgels.38 The functionalization of the particle interior requires to swell the particles in DMSO to ensure efficient diffusion of the reactants within the particle network. We selected 2-azido-N,N-dimethylethylamine (AzDMEA) as a first

concentration. We varied this ratio in great detail to fully understand how different crosslinking densities influence the swelling of the latex particles in good solvents for the network, thus from a collapsed latex particle to an organogel particle. This ratio is important to consider for postfunctionalization in the interior of the particle and defines the particle hardness. The statistical analysis of the TEM data clearly reveals an increasing size of the latex particles at lower crosslinker concentrations (Table 2 and Figure 3a). The particles grow from Rn,TEM = 110 to 160 nm when increasing the monomer/ crosslinker ratio from 2 to 500 and approach the value of pure PGA∞ without added EGDMA crosslinker. Figure 3a illustrates how the dependence levels off at ratios above 50 and reaches a plateau value around 200. Smaller particles at the end of an emulsion polymerization are caused by a higher amount of particle nuclei in the beginning and thus relate back to the stability of the oligomer radicals formed at the very start of the polymerization in the aqueous phase and also to the concentration of dissolved monomer in water.62 The addition of crosslinker leads to lower oligomer stability and thus more particle nuclei for higher EGDMA concentrations, which then grow to an overall lower size. In all cases, we find very welldefined latex particles with narrow distributions as expressed in the standard deviations and PDI values obtained by TEM and the PDI values obtained by CUMULANT analysis of the DLS. This demonstrates that the crosslinking densities can be varied in a wide ratio without compromising control over the homogeneity of the latex particles. Such well-defined latex particles allow us to quantitatively study how the degree of crosslinking influences the swelling of the particles in organic solvents. We selected DMSO as swelling solvent for its ability to solubilize a wide range of compounds of different polarity in subsequent postmodification reactions. At this point we have to rely on DLS to understand the differences in particle sizes, as drying in TEM would lead to undesirable changes to the dimensions. Figure 4a shows a direct comparison of the CONTIN plots for different degrees of crosslinking (PGA/EGDMA = 2, 100, 200). All average values ⟨Rh⟩z,DLS,DMSO are also summarized in Table 2. The point-by-point analysis of the average values and the CONTIN plots display a consistent increase of the particle size when exposed to DMSO (Figure 4a). The persistent narrow distributions in DMSO also confirm that the latex particles can be dispersed as organogel particles with little to no aggregation in DMSO and are present as single 2261

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Scheme 2. Modification of Latex Particles via “Click Chemistry” Using AzDMEA To Give Rise to Weak Polyelectrolyte Behavior

Figure 5. Kinetics of alkyne/azide cycloaddition and internal modification as a function of degree of crosslinking. (a−c) Time-dependent evolution of Raman spectra during Cu(I)-catalyzed click type ligation of AzDMEA to PGA∞, PGA250, PGA2. (d) Degree of functionalization from elemental analysis and Raman spectroscopy during modification of PGA2, PGA10, PGA250, and PGA∞ (PGA20 omitted for clarity, see Figure SI4). Open and closed symbols represent degree of functionalization from Raman spectroscopy and elemental analysis, respectively. The dashed lines serve as a guide to the eye.

PMDETA, CuBr/PMDETA, and pristine CuSO4 (PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine; TBTA = tris[(1benzyl-1H-1,2,3-triazol-4-yl)methyl]amine)68,69 as well as a Ru catalyst (pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride). Figure SI3 reveals a similar performance of the Cu-based systems, yet with the Cu(I)/TBTA system slightly outperforming the other systems. Although the system without ligand only show a slightly lower conversion (possible

attractive model compound for the bulk functionalization, as it is able to function as a weak electrolyte group with pH-dependent ionization behavior (Figure 6a). We performed a screening of typical easily available catalyst systems at fixed conditions employing a 3-fold molar excess of azide to alkyne for PGA250 organogel particles redispersed in DMSO with ascorbic acid as reducing agent at 60 °C (Scheme 2) and the following catalyst systems: CuSO4/TBTA, CuSO4/ 2262

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replacement of ligand with DMSO70−72), we found that using TBTA has an advantage in work-up of the reaction by producing a white and clean material with less washing and redispersion cycles. SEM-EDX of the resulting materials shows absence of Cu(I) for the Cu(I)/TBTA catalyst and the appearance of nitrogen signals within the functionalized microgels (Figure SI5). More precise elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed the remaining Cu content to be below 0.1 wt % (compared to the mass of the resulting microgels) when using TBTA, while the system without ligand showed ca. 0.2 wt %. Smaller ligands such as PMDETA do not increase the reaction rate but in fact show the lowest conversions. The Ru catalyst is unsuitable as it only displays 1/10 of the conversion compared to the Cu(I)-based systems at the same reaction time. This may be attributed to a lower rate of the catalyst or increased steric hindrance of the complex in entering the gel network. Because of these reasons, we selected the Cu(I)/TBTA system for the further reactions. Further tuning of the conditions, e.g., rising the temperature to 80 °C and increasing the alkyne−azide molar ratio of 1:8 and reaction time up to 15 days, did not significantly increase the conversion We only find an increase in conversion from 65% to 66%. This already points to some limitations in terms of accessibility of the functionalities within the gel network. To quantify the influence of the crosslinking density on the kinetics and extent of functionalization, we analyzed organogel particle dispersions across the full spectrum ranging from loosely to tightly crosslinked structures (PGAx, x = 250, 20, 10, and 2) and also pure PGA∞. We used Raman spectroscopy and elemental analysis to follow the conversion of the alkyne groups. Representative Raman spectra are depicted in Figure 5a−c for three different PGAx organogel particles. All spectra are normalized to the −CO band located at ∼1720 cm−1, which remains constant throughout the reaction. Distinct differences can be observed for loosely and tightly crosslinked particles. The most notable difference can be observed in the unlike decrease in the intensity of the ν(CC)str band originating from the alkyne group at ∼2100 cm−1.73 Other changes are visible for the vibrational bands at 1031, 1230, 1381, and 1540 cm−1, which emerge due to the triazole ring73,74 and the (C−H)str of methyl groups at ∼2800 cm−1, and are getting stronger with time. While the loosely crosslinked organogel particles PGA250 and PGA∞ display a high and rapid conversion, the tightly crosslinked PGA2 only undergoes minor changes. These differences clearly reflect the unlike swelling behavior of the underlying gel networks, and limited diffusion of the reactants inside the tightly crosslinked PGA2 network can be concluded. Furthermore, we measured the elemental content of carbon, nitrogen and hydrogen, (and partly sulfur) and used the ratio of nitrogen to carbon to quantify the degree of functionalization (cf. Figure 5d). A plateau is reached after a convenient reaction time of only 1 day, and degrees of functionalization of 65% are found for the loosely crosslinked organogels (PGA250 and PGA∞). Results obtained for heavily crosslinked PGA2 are in stark contrast to the high degree of modification of loosely linked organogels. These only show a degree of functionalization of less than 10%, even after the full span of the reaction time of 5 days. Intermediately crosslinked organogels, PGA10 and PGA20, quickly achieve appreciable functionalization densities of 30% and 51%. Figure SI4 displays a comparison of the values obtained in the plateau region after 5 days of reaction as a function of the degree of crosslinking. The trend of increasing conversion for

lower degrees of crosslinking is fully reflected across a wide range of crosslinking densities. The consistency of this relationship convincingly demonstrates that the network node density, which has been elucidated by the swelling experiments (Figure 4b), is the decisive parameter in controlling the extent of functionalization by determining the accessibility for the reactants and catalyst. This is further underscored by the fact that increasing temperature, excess of azide, and prolonging the reaction time up to 15 days does not increase the conversion for a given sample. It is interesting to realize that the calculated degree of functionalization of 65% for the loosely crosslinked PGA organogel particles by elemental analysis is lower compared to the almost quantitative degree of functionalization, as deduced by the disappearance of the alkyne bonds found in Raman spectroscopy. The degree of functionalization from Raman spectroscopy is calculated to 90% for PGA250 and PGA∞ (Figure 5d). These differences may be attributed to three factors. First, the values calculated based on elemental analysis assume that all carbon content originates from the monomer, while in reality a part of the carbon content is also associated with the surfactant SDS. The presence of sulfur is confirmed by energy-dispersive Xray analysis in the SEM (see Figure SI2), and further elemental analysis confirms its presence, yet with low concentration 8), the zeta potential becomes negative, as the amount of protonated amine (pKa ≈ 7.5)76 groups is low, and the remaining fraction of sulfate groups (surfactant, initiator) dominates the behavior. PGA2-AzDMEA containing only ca. 7 mol% of modifications consistently has lower zeta potential values and also a lower isoelectric point than densely modified PGA250-AzDMEA and PGA∞-AzDMEA, which reflects the lower degree of functionalization. To understand the influence of the degree of crosslinking on pH-induced swelling and to assess the quality of the formed dispersions, we measured the particle size and its distribution of PGA250-AzDMEA and PGA2-AzDMEA by DLS at different pH values and complement these data by cryogenic TEM in acidic conditions for PGA250-AzDMEA. Figure 6b displays the exemplary DLS CONTIN plots of the loosely crosslinked PGA250 latex before modification as well as the PGA250-AzDMEA microgels at pH 10, 7, and 3. In all cases we observe monomodal distribution functions, demonstrating well-dispersed, homogeneous, and stable colloidal dispersions. The distribution functions undergo a continuous shift from the pristine, nonfunctionalized latex particle, PGA250 (⟨Rh⟩z,DLS = 183 nm, measured at pH 7), to the collapsed state of the PGA250AzDMEA at pH 10 (⟨Rh⟩z,DLS = 202 nm), which expands to the 2264

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Macromolecules

Article

(TBTA, Aldrich, 97%), and ascorbic acid (ACROS Organics, 99%) were used as received. Synthesis of crosslinked Poly(PGA) Latex. The latex particles based on propargyl acrylate (PGA) and ethylene glycol dimethacrylate (EGDMA) as a crosslinker were synthesized by simple batch emulsion polymerization at 60 °C. A typical recipe is as follows: a 25 mL glass bottle was charged with 19 g of water, SDS (0.05 mM) as surfactant, and PGA/EGDMA molar ratio of 10 were added. Then the mixture was deoxygenated for 10 min by bubbling with nitrogen gas and placed in an oil bath with constant stirring at 500 rpm. After equilibration for 30 min at 60 °C, 1 mL of a stock solution of potassium peroxodisulfate initiator (0.2 mM) was injected into the reaction mixture. The contents of the bottle turned white within few minutes, and the reaction was continued for 48 h. Samples for gravimetry were constantly withdrawn from the reaction mixture to monitor the conversion. The white dispersion was purified by repeated centrifugation and redispersed in Milli-Q water for several times to remove the unreacted monomer and excess of surfactant. The suspension was stored with 5−10 wt % solid content. Synthesis of 2-Azido-N,N-dimethylethylamine (AzDMEA). The synthesis procedure was adapted from the literature.79 Sodium azide (2.71 g, 41.65 mM) and 2-chloro-N,N-dimethylethylamine hydrochloride (2 g, 13.88 mM) were dissolved in 50 mL of water and stirred at 80 °C for 16 h. The reaction mixture was concentrated to ∼1/3 of the initial volume (the temperature of the water bath should not exceed 50 °C). Then it was cooled down (ice bath), and 50 mL of diethyl ether was added followed by the addition of 2.6 g of Na2CO3 on portions keeping the temperature below 10 °C. The mixture was stirred vigorously for 30 min. The organic layer was separated, and the same procedure of diethyl ether and sodium carbonate was repeated with aqueous phase to extract the remaining product from it. The combined organic layers were dried over Na2SO4, and the solvent was evaporated by a rotatory evaporator. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 3.33 [N−N+N−CH2], 2.44 [CH2−N], 2.18 [N−(CH3)2] Raman spectroscopy (cm−1): 2101 (N−N+N), 2772 and 2821 (−N− (CH3)2). Click Modification into Cationically Charged PGAx-AzDMEA Microgels. PGAx (30 mg, 0.27 mM) organogels and AzDMEA (93 mg, 0.81 mM) and ascorbic acid (11.7 mg, 0.068 mM) were mixed in 12 mL of DMSO. The solution was degassed with nitrogen for 10 min. After purging nitrogen, 0.014 mM or 1.4 mL of copper(II)−TBTA complex (10 mM in 55% aqueous DMSO) was added and stirred for 5 days at 60 °C. Then the clicked organogel was purified by centrifugation several times, washed with acetone, and dried in a high vacuum. Characterization. Dynamic Light Scattering. The particle sizes and size distribution were measured by dynamic light scattering (DLS) using an ALV-DLS at a fixed scattering angle of 90° and temperature of 20 °C. Very dilute concentration were used for sample preparation and filtered through 1.2 μm syringe filter. Transmission Electron Microscopy and Cryo-TEM. For morphological observation, zero-loss energy-filtered transmission electron microscopy (TEM) images were recorded with a LIBRA 120 operating at 80 kV and 120 kV using a bottom mounted slow-scan CCD camera. For sample preparation, one drop of colloidal solution (0.1 wt % solid content) was placed on a plasma-treated 300-mesh carbon-coated copper grid (EMS), and excess solution was soaked by dust-free tissue paper. No additional staining was applied. Particle diameters and distribution were determined from micrographs using “ImageJ” software with at least 200 particles being counted. Cryogenic TEM samples were prepared by rapid vitrification from aqueous dispersion (1 g/L) using plasma-treated lacey grids and a vitrobot system. Field Emission Scanning Electron Microscopy (FE-SEM). The morphology of colloidal crystals was observed using a Hitachi S4800. The sample was sputtered with a thin layer of gold/palladium. FE-SEM-EDX was performed on a Hitachi SU-9000 using 1 kV for imaging and 12 kV for EDX measurements. 1 H NMR spectra were recorded on a Bruker AC400 FT NMR spectrometer operating at 400 MHz. Raman spectroscopy was performed on a Bruker RFS 100/S with Nd:YAG laser with a wavelength of 1064 nm. We used 1000 scans with 200 mW and a spectral resolution of 4 cm−1.

definition of the distinct interface in emulsion polymerization (hydrophobic particles in water) versus the rather fuzzy interface in precipitation polymerization (collapsed gel network in water) during the synthesis. Present research in our lab focuses on understanding these differences between the different synthesis methods further and on applications of such readily functionalizable microgels.

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CONCLUSION We demonstrated a simple and versatile platform strategy to prepare highly uniform nano- or microgels with tunable size and crosslinking density by starting from commercially readily available compounds. The approach combines two efficient chemical strategies: (i) emulsion polymerization to prepare extremely uniform particles with polydispersity indices well below 1.05 (TEM) and (ii) Cu-catalyzed click-type coupling of alkynes and azides as a straightforward modular ligation method for functionalization. We showed that narrowly dispersed particles can be prepared in a facile fashion on a large scale within an attractive nanometer-scale size regime. The particle hardness/ability for swelling can be widely tuned by the amount of surfactant and the ratio of monomer/crosslinker. The degree of swelling, as expressed by the volume ratio of collapsed state vs solvent swollen state, extends from 1.5 to over 5.5 from tightly to loosely crosslinked microgels. The different network densities decisively influence the degree of modification in the postmodification reaction. Loosely and uncrosslinked latex particles show bulk degrees of functionalization of 70−90%, with a small fraction of alkyne groups remaining unreacted and another part likely consumed in a slight side reaction (formation of double bonds due to attack of radical during emulsion polymerization). Herein, we demonstrated the capabilities of our approach to prepare pH-dependent microgels with a high density of tertiary amine groups, largely exceeding the typical degrees of modification in classical precipitation copolymerization of PNIPAM microgels. The resultant microgels display homogeneous populations in water and display pH-dependent swelling and changes in zeta-potential. The extent of both parameters can be controlled by the degree of crosslinking and modification with amine groups. Overall, the combination of near-perfect uniformity, easy preparation strategy, and dense functionalization with simple click chemistry represents a new and viable method to prepare highly and densely functionalized nanoparticles and microgels. The strategy is particularly suitable to prepare highly uniform model particles for detailed structure− property relationships because several “children” particles can be prepared from exactly the same well-defined “parent” latex particle.

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EXPERIMENTAL SECTION

Materials. Ultrapure Milli-Q water with a resistivity of 18.2 MΩ cm was used for all experiments. Propargyl acrylate (98%) and ethylene glycol dimethacrylate (98%) were obtained from Sigma-Aldrich and used after passing through a column of basic alumina to remove the inhibitor. According to 1H NMR, the propargyl acrylate contains only 95 mol% of alkyne functionalities versus acrylate groups. The initiator potassium peroxodisulfate (Fluka >99%), surfactant sodium dodecyl sulfate from Bio-Rad (Electrophoresis purity reagent), sodium azide (Aldrich, 99%), 2-chloro-N,N-dimethylethylamine hydrochloride (Aldrich, 98%), Na2CO3 (BDH Prolabo, VWR 99.5%), CuSO4·5H2O (Aldrich, 99%), CuBr (Aldrich, 98%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride (Aldrich), tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine 2265

dx.doi.org/10.1021/ma402530y | Macromolecules 2014, 47, 2257−2267

Macromolecules

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Zeta-Potential Measurements. Zeta potentials of modified microgels were measured using a Zetasizer Nano from Malvern, connected to a MPT-2 titrator and degasser in a pH range of 3−10. Small-angle X-ray scattering of latex particles was performed at two different places using ca. 1 wt % suspensions in water. (i) The PGA10 series was measured at the small-angle X-ray MiNaXS beamline P03 at HASYLAB, Hamburg, Germany. Scattering data were recorded at 13 keV (0.954 Å) with a beam size of 22.0 × 17.1 μm2 and a Pilatus 300k detector at a distance of 8.63 m. (ii) The PGA250 series was measured at the ID2 SAXS beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) at 12.5 keV (0.99 Å) and beam size of 200 × 400 μm2. A Kodak FReLoN detector was used at a distance of 4.1 m. Obtained small-angle radiation scattering data were fitted using SASFit. The SASFit simulation software and user guide is available from https:// kur.web.psi.ch/sans1/SANSSoft/sasfit.html. We used two different form factors to describe the scattering intensity profiles: (i) simple sphere model and (ii) core−shell model with a linearly decaying scattering length density of a thin shell.80 A Gaussian distribution was used to model the size distribution. The form factor, P(q), describes the simple sphere model

⎛ sin(qR ) − qR cos(qR ) ⎞2 ⎟ I ∝ P(q) = ⎜3 (qR )3 ⎝ ⎠

∫0

∞

4πr 2

ASSOCIATED CONTENT

S Supporting Information *

Data comprise time-dependent feed composition of PGA/ EGDMA at crosslinking degree of 2 and 50 during the reaction via 1H NMR, SEM-EDX elemental mapping of PGA250 latex, PGA250 microgel, degree of functionalization from elemental analysis and Raman spectroscopy after 5 days of modification of PGA2, PGA10, PGA20, PGA250, and PGA∞. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the SFB 985 - Functional Microgels and Microgel Systems, TP-C4. The authors thank DESY (Hamburg, Germany) and ESRF (Grenoble, France) for granting SAXS beam time. We thank Jose Guillermo Torres Rendon for SEM measurement and also Thomas Heuser for help with preparing Scheme 1.

(1)

where R is the radius of particle. The second model describes core−shell morphology with a slightly higher scattering length density in the thin shell. We hypothesized that a thin shell could be visible in SAXS due to the presence of sulfur (SO4−) and partly condensed counterions Na+/ K+, originating from the surfactant and initiator. Both species are confined to the surface of the latex particles due to the polarity of the groups imparting colloidal stabilization. Scattering intensity is calculated by

I(q) ∝

Article

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REFERENCES

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sin qr η(r ) dr qr

assuming a linearly decaying density profile of the shell

⎧ ηc r R + ΔR ⎩ sol where R is the core radius, ΔR the shell thickness, and ηc, ηsh, and ηsol the scattering length densities of the core, the shell, and the solvent, respectively. For the pair interaction potential we used Baxter’s model for adhesive hard spheres with an infinitely narrow attractive well. The potential is given by

⎧∞ 0 < r < R′ ⎪ ⎪ ⎛ 12τ(R − R′) ⎞ U (r ) = ⎨ ln⎜ ⎟ R′ < r < R ⎠ kBT R ⎪ ⎝ ⎪ ⎩0 r>R and then taking the limit R − R′ → 0.81 τ is a measure of the adhesive strength, also known as stickiness parameter; a fixed value of τ = 0.1 was used in the simulations. We had to take these interactions into account as more concentrated (1 wt %) samples were prepared quickly by centrifugation and redispersion starting from 0.1 wt % dispersion. More efficient redispersion with sonication or longer stirring would likely disengage the remaining aggregates from the centrifuged particles, as no aggregation is found in the PGA250 series, which was prepared directly with sufficient homogenization and without intermediate centrifugation cycle. 2266

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