The role of responsive branched copolymer composition in controlling pH-triggered aggregation of “engineered” emulsion droplets: towards selective droplet assembly

Abstract

Stable polymer-functionalized oil-in-water emulsion droplets are prepared with precise surface compositions, which are defined by that of the branched copolymer surfactant. The influence of composition, specifically ratios of methacrylic acid (MA) to ethylene glycol (EG), on acid-triggered inter-polymer/inter-droplet hydrogen-bonding events is studied. It is demonstrated that simple variation of the EG : MA ratio can: (i) control the kinetics of, and thereby switch “on and off”, inter-droplet interactions, and (ii) control the stiffness and mechanical integrity of aggregated “engineered emulsions” under shear. We exemplify these points by demonstrating selective acid-triggered assembly of binary mixtures of contra-functionalized emulsion droplets. It is postulated that these results will allow fine-tuning of complex assembled engineered emulsion systems with the principles developed being applicable to other disperse systems (i.e., colloids and nanoparticles) and surfaces (i.e., for reversible adhesion).

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Introduction

Elucidating precise design rules for the controlled and reversible assembly of nanoscopic functional materials is becoming an increasingly active research endeavour. Much motivation for this stems from the exquisite exploitation of similar processes by natural biological systems,^1,2 but also in our attempts to develop increasingly advanced synthetic materials with adaptable properties.^3–7 The driving force for many assembly mechanisms is underpinned by non-covalent interactions such as hydrogen bonding, electrostatics, π–π stacking, hydrophobics, metal–ligand coordination and van der Waals forces. In nature self-assembly processes are driven by the subtle balance of multiple secondary interactions which generates unprecedented intricacy such as in the precise folding and aggregation of proteins. Likewise, in synthetic systems intermolecular interactions are being utilized in evermore complex situations in order to build-up hierarchical and often ordered structure.^8–15 The majority of assembled structures generate bulk strengths not as a result of the magnitude of the individual non-covalent interaction but from the combined, multiple and cooperative nature of these bonds which can amalgamate impressive and long-term stabilities with structural reversibility.

While the generation of increasing levels of complexity is important for the fundamental understanding of the natural systems we seek to emulate, the ability to replicate these selective assembly processes using viable and scalable synthetic strategies is an equally important, although arguably less glamorous, endeavour. Our group has an interest in addressing this consideration and in one approach we have been exploiting highly stable emulsion droplets as model surface-functionalized materials. Emulsion droplets have several key characteristics, which can be used for understanding inter-particle interactions. In particular they: (i) can be made very stable and with controlled surface functionality by judicious design of surfactant architecture and composition, respectively,¹⁶ (ii) are sufficiently large to allow facile imaging, (iii) can be prepared reproducibly on a significant scale, (iv) have inherent interest as delivery and encapsulation vehicles and as starting materials for more complex and functional materials syntheses,¹⁷ and (v) have distinct commercial applicability across extremely broad industrial sectors including drug delivery, encapsulation, cosmetics, food stuffs and coatings.

We recently demonstrated that architecturally and compositionally defined copolymers can be used to prepare highly stable and functional emulsion droplets. The presence of high concentrations of hydrophobic chain-ends within branched polymer architectures provides strong and multiple points of attachment for efficient droplet surface adhesion.¹⁶ Under these conditions the functionality present at this liquid–liquid interface can be used to define interactions between droplets. We have previously shown that oil droplets functionalized with poly(ethylene glycol) (PEG) and poly(methacrylic acid) (PMA) residues from a branched copolymer surfactant exist as highly stable and free-flowing droplet dispersions at neutral/basic pH due to the simultaneous steric and electrostatic stabilization afforded by the copolymers at this pH. On lowering the pH, the electrostatic stabilization is switched off as the PMA residues are protonated and the methacrylic acid (MA) residues form hydrogen-bonds with the ethylene glycol (EG) repeat units in the PEG chains. Providing the concentration of oil droplets in water is sufficiently high, the hydrogen bonds form not only around the droplets but also between droplets. This inter-droplet hydrogen bonding forms multiple cooperative bonding sites between droplets and results in emulsion gelation—so-called emulsion engineering¹⁸ (Fig. 1). A variety of complex, multi-component engineered emulsion structures can be fabricated using this strategy and the process can be scaled-up using a homogeneous pH-switch.¹⁹ The engineered emulsions can be disassembled back into individual discrete and stable droplets by simply increasing the solution pH. Importantly the branched copolymer surfactant which imparts the droplet stabilization and functionality is prepared via a single-step synthesis and using commercially available starting materials thus the entire process—from polymer synthesis to triggered emulsion droplet assembly—is amenable to scale up.^20,21

Fig. 1 Schematic of the reversible transition from fluid emulsion dispersion to gelled engineered emulsion. Inter-polymer interactions between EG (blue) and MA (red) located on emulsion droplet surfaces are mediated by a pH-switch.

Herein we aim to elucidate the role of polymer composition—specifically the role of EG : MA ratio—in the acid-triggered engineered emulsion aggregation process. In particular we seek to identify how these variations alter the kinetics of droplet assembly from dilute solution and the mechanical properties of the resulting engineered emulsions. This study also seeks to formulate the design rules and principles behind selectively interacting hydrogen-bonding droplets while ensuring that simple, scalable and economically viable processes are exploited throughout. We use surface-immobilized polymers to monitor inter-polymer interactions and this approach also allows control over the interactions between substantially larger materials of commercial relevance. The process we describe and the principles we identify should be extendable to various other surface functionalised materials and polymers in solution.

Results and discussion

A series of branched copolymers were synthesized based on poly(ethylene glycol) methacrylate (PEGMA, M_n 1100 g mol⁻¹) and PMA with 1-dodecanethiol (DDT) chain-ends (Table 1). The molar ratio of PEGMA : MA was systematically varied such that the ratio of EG to MA varied and centred around 1 : 1 since this was previously used as a theoretical maximum hydrogen-bonding regime (Note: EG : MA hydrogen-bonds occur following precise stoichiometry).²² The dodecane chain-ends were chosen since these have previously been shown to interact favourably with dodecane oil phases—an important consideration in achieving substantial droplet stabilities. In all cases the branching monomer, ethylene glycol dimethacrylate (EGDMA), was present in feed molar equivalences of 10 relative to the total monofunctional monomer (MFM) which is nominally set to a molar value of 100. The branched polymers had theoretical compositions of the form MFM₁₀₀–EGDMA₁₀–DDT₁₀, where MFM can be MA, PEGMA or mixtures thereof. Table 1 shows the composition and characterization data for the branched copolymers. Compositions were determined by ¹H NMR spectroscopy of the polymers in which the MA residues were first esterified with TMS–diazomethane²³ and assume complete functionalization. Changing EG : MA ratios sometimes necessitates only a very subtle change of PEGMA : MA molar ratios and such small changes are within the error of the techniques we employed. Nevertheless, the polymers displayed consistent and systematic compositional variation throughout the series and stacked ¹H NMR spectra of esterified (co)polymers P1–P9 (ESI, Fig. S1†). Given the potential error in the characterization, target polymer compositions are used to define each copolymer (Table 1, P1–P9). Triple-detection GPC analyses of the copolymers modified with TMS–diazomethane using tetrahydrofuran eluent show that weight averaged molecular weight and polydispersity index are in close agreement for all MA-containing polymers which is consistent with previous studies on similar systems.¹⁸ Again, the GPC data recorded for such complex architecture polymers are extremely complicated and we do not believe that the recorded molecular weight parameters are absolute, however, similar molecular weight values are obtained for all copolymers which is good evidence that the polymerizations proceeded without complication (see overlayed intrinsic viscosity vs. molecular weight plots, ESI, Fig. S2†). Thus while the interpretation of this series of polymers is complex and opens to various degrees of error we have high confidence that within this series the relative ratio and molar masses follow a systematic and rational trend as predicted. Achieving similar molecular weights for the series of polymers is particularly important for the current study since it ensures that molar mass variations do not influence the emulsification behavior of these branched copolymer amphiphiles. Unfortunately reliable GPC data were not achievable for the 100% PEGMA branched copolymer using this eluent set-up and given the additional necessity to compare the series of polymers under identical conditions we did not seek alternative eluent systems. Mark–Houwink α-values are below 0.3 for all branched copolymers which indicates that all solution morphologies are compact as would be expected from branched architectures.

Table 1 Branched copolymer composition

ID	Target polymer composition	Calculated monomer ratios^a	EG : MA ratio	M w ^b/g mol^−1	M n ^b/g mol^−1	PD ^b	α ^b	Surface tension^c/mN m^−1
a Determined by ^1H NMR on modified copolymers. Ratio does not include EGDMA and DDT incorporations due to overlapping peak resonances. b Determined by TD-GPC using THF eluent. c Measured using a Kibron Delta-8 parallel plate tensiometer and represents the surface tension of 2.0 w/v% aqueous solutions at pH 10.
P1	MA100–EGDMA10–DDT10	MA100	0 : 1	17 700	3700	4.7	0.29	42
P2	PEGMA0.83/MA99.17–EGDMA10–DDT10	PEGMA1.83/MA98.2	1 : 6	18 300	3300	5.6	0.27	39
P3	PEGMA1.25/MA98.75–EGDMA10–DDT10	PEGMA2.5/MA97.5	1 : 4	19 700	3400	5.9	0.26	41
P4	PEGMA2.5/MA97.5–EGDMA10–DDT10	PEGMA4.4/MA95.6	1 : 2	18 800	3700	5.1	0.26	42
P5	PEGMA5/MA95–EGDMA10–DDT10	PEGMA7.7/MA92.3	1 : 1	17 100	4100	4.1	0.23	39
P6	PEGMA10/MA90–EGDMA10–DDT10	PEGMA13.8/MA86.2	2 : 1	17 800	4100	4.4	0.22	43
P7	PEGMA20/MA80–EGDMA10–DDT10	PEGMA24.50/MA75.5	4 : 1	16 900	6900	2.4	0.21	37
P8	PEGMA30/MA70–EGDMA10–DDT10	PEGMA34.4/MA65.6	6 : 1	18 400	6500	2.8	0.21	48
P9	PEGMA100–EGDMA10–DDT10	PEGMA100	1 : 0	—	—	—	—	49

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Aqueous solutions of the branched copolymers (2.0 w/v%, pH 10) were homogenized with an equal volume of dodecane for 2 min at a rate of 24 000 rpm. The resulting emulsions were left to equilibrate for 24 h over which time the oil droplets creamed due to the density difference between the dodecane (0.75 g cm⁻³) and water (1.0 g cm⁻³). The creamed droplet phase comprised oil volume fraction, Φ_oil, between 0.69 and 0.74 which could be readily redispersed by simple agitation and no demulsification was observed over at least 4 months. Volume averaged droplet diameters (D_(4,3)) and spans were measured by laser diffraction (Table 2) and are reasonably consistent affording droplets with average diameters between 5.7 and 10.9 µm in all instances. The droplet sizes are lower than for similar branched particulate polymers²⁴ and the consistency of the diameters between polymer samples augurs well for more detailed analyses of their aggregation behaviour which minimizes any contribution of size effects from the aggregation and inter-droplet interactions.

Table 2 Composition of branched copolymer stabilized emulsions

ID	Target polymer composition	EG : MA ratio	Φ oil ^a	D (4,3) at pH 10^b/µm	Span at pH 10^b	Change in D(4,3) at pH 2^c/µm	Maximum G′^d/Pa	G′ = G″^d/strain %
a Determined by phase volume on creaming. b Determined by laser diffraction. c Determined by laser diffraction 10 min after reducing pH to 2 at fixed droplet concentration. d Determined by performing amplitude sweep on emulsions at pH 2.
E1	MA100–EGDMA10–DDT10	0 : 1	0.74	9.55	1.25	0.56	18 900	10.0
E2	PEGMA0.83/MA99.17–EGDMA10–DDT10	1 : 6	0.71	10.93	1.00	3.70	23 600	4.0
E3	PEGMA1.25/MA98.75–EGDMA10–DDT10	1 : 4	0.71	5.74	1.43	3.11	35 000	6.5
E4	PEGMA2.5/MA97.5–EGDMA10–DDT10	1 : 2	0.71	5.97	1.23	23.71	2300	14.4
E5	PEGMA5/MA95–EGDMA10–DDT10	1 : 1	0.71	5.66	1.18	39.06	900	28.6
E6	PEGMA10/MA90–EGDMA10–DDT10	2 : 1	0.69	7.05	1.18	0.03	10	28.2
E7	PEGMA20/MA80–EGDMA10–DDT10	4 : 1	0.71	7.74	1.17	0.11	30	5.0
E8	PEGMA30/MA70–EGDMA10–DDT10	6 : 1	0.71	7.91	1.10	0.15	70	4.5
E9	PEGMA100–EGDMA10–DDT10	1 : 0	0.71	6.88	1.24	0.11	20	15.9

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Kinetics of droplet aggregation

Having demonstrated that each of the branched copolymers produced stable droplets at pH 10 irrespective of the ratio of EG : MA we investigated their acid-triggered aqueous aggregation behavior. Dilute aqueous solutions of the emulsions (40 µL of creamed emulsion in 80 mL water) at pH 10 were analysed by laser diffraction for around 5 min. Following this nominal equilibration time (which persisted indefinitely in the absence of a pH switch) the aqueous dispersions were acidified by addition of HCl (1 M, 0.8 mL) and the D_(4,3) was monitored as a function of time for 10 min. An increase in D_(4,3) at constant concentration reflects an aggregation process on droplet collision with faster increases implying stronger propensity to form inter-droplet physical bonds between the polymer-functionalized surfaces. All EG-rich functional droplets (Samples E6–9, Table 2) showed negligible change in D_(4,3) which demonstrates that PEGMA residues of even 1100 g mol⁻¹ and in very small excess relative to MA residues provides an extremely efficient steric barrier to prevent droplet aggregation (Fig. 2a). Branched polymers P6–P8 (Table 1) all contain MA monomer thus at acidic pH EG–MA hydrogen-bond formation will occur, however, the presence of a small excess of EG completely isolates hydrogen-bonding to the same droplet. The presence of excess EG residues on droplet surfaces can therefore be used to effectively “switch off” inter-droplet interplay and isolate these interactions to occur in an exclusively intra-droplet fashion. This observation supports reports on the efficiency of PEG-moieties in restricting ionic²⁵ and covalent²⁶ cross-linking events within localized micellar domains.

Fig. 2 Change in volume average droplet diameters (D_(4,3)) as a function of time on acidification of dilute emulsion droplet dispersions. (a) EG-rich emulsion droplets (with EG : MA 1 : 1 as control). (b) MA-rich emulsion droplets (with EG : MA 1 : 1 as control), and (c) normalized volume average droplet diameter change as a function of EG % (relative to MA). Lines serve as guide to the eye.

Low molecular weight homopolymers of PMA are hydrophilic irrespective of solution pH—that is, they are water-soluble in both their anionic (high pH) and neutral (low pH) states. Consequently it is perhaps logical to reason that PEGMA/PMA stabilized emulsion droplets rich in MA moieties should remain dispersed at acidic pH. The MA-rich functional droplets (Samples E1–4, Table 2) were acidified having been initially dispersed in their as-made state at pH 10 using an identical procedure as the EG rich droplets. All droplets showed a detectable increase in D_(4,3) over a nominal 10 min period (Fig. 2b), however, only the 1 : 2 droplets showed signs of significant aggregation (average diameter increase of 24 µm over 10 min compared to 2.1 µm and 3.2 µm for EG : MA 1 : 6 and 1 : 4, respectively). The initial distinct increase followed by plateau formation for the 1 : 4 and 1 : 6 EG : MA droplets may occur due to the unavoidable shear required to cycle the emulsion dispersions through the detectors (1000 rpm). Thus under lower shear environments these droplets may continue to aggregate and this is consistent with rheology data and microscopy observations (see later). Nevertheless, in comparison to the EG-rich droplets the MA-rich droplets show a greater propensity to aggregate and therefore greater inter-droplet attraction at acidic pH when they are in their neutral form. This observation is consistent with the MA residues becoming more hydrophobic and providing less efficient stabilization or the formation of PMA dimers.²⁷Fig. 2c shows the average change in D_(4,3) after 10 minutes for all emulsion droplets displayed as a function of EG mole percent content. The maximum rate of aggregation occurs at stoichiometric EG : MA ratios which is consistent with our previous hypothesis that hydrogen-bonding is a key event in the inter-droplet aggregation process.¹⁸ Moreover, EG-rich surfaces display substantially greater inhibition of inter-droplet aggregation compared to MA-rich analogues due to the inherent differences in the hydrophilicities of these functional groups at acidic pH. Thus while PMA homopolymers remain hydrophilic at acidic pH the reduction in hydrophilicity and the potential for PMA dimmer formation are sufficient to prevent efficient stabilization and aggregation of microscopic oil droplets occurs. This result is important in terms of understanding general emulsion engineering (droplet aggregation) processes since it implies that hydrophobic interactions, in addition to EG–MA and PMA dimer hydrogen-bonding, contribute towards aggregation.

Rheology of engineered emulsions

Having determined the roles of both EG and MA in the kinetics of acid-triggered emulsion droplet aggregation we investigated their influence on the mechanical properties of the engineered emulsions (emulsion gels) that are formed. EG-rich droplets did not aggregate (gel) at acidic pH and therefore analysis focuses on identifying the role of MA in the gel strength (Samples E1–5, Table 2). Rheometry was employed to measure the relative magnitude of the storage (G′) and loss (G″) moduli for the bulk gelled emulsions at acidic pH (pH < 2). A material is considered to be a gel in regimes where G′ > G″ whereas the material displays more liquid-like behavior when G″ > G′. In contrast to the laser diffraction kinetics data which were recorded at high dilution (Φ_oil ≈ 4 × 10⁻⁴) all MA-rich emulsion droplets aggregated on addition of acid at the higher droplet concentrations present within the creamed emulsion layer (Φ_oil 0.69–0.74). Following in situ droplet aggregation, amplitude sweeps (ω = 10 rad s⁻¹) were performed for each emulsion. The magnitude of G′ in the linear viscoelastic regime was taken as a measure of engineered emulsion stiffness and is shown in Fig. 3 as a function of MA mole percent incorporation. In contrast to the high dilution laser diffraction data, these concentrated rheology data confirm that excess MA induces greater engineered emulsion gel stiffness up to around 80 mol% MA after which the magnitude of G′ reduces but remains significantly higher than the 1 : 1 EG : MA-stabilized droplets. Maximum gel stiffness is achieved at around 80 mol% MA which suggests that the presence of some EG residues, and therefore EG–MA hydrogen bonding, are required for significant mechanical strengths to be achieved.

Fig. 3 Rheology data for EG/MA-functional emulsions at acidic pH. (a) Storage modulus (G′) as a function of MA content (expressed as a % relative to EG residues). (b) Crossover point (strain at which G′ = G″) as a function of MA content (expressed as a % relative to EG residues). Note: maximum G′ is dictated by MA concentration and maximum G′ = G″ is dictated by MA : EG stoichiometry (i.e., hydrogen-bonding potential).

In contrast to the magnitude of G′ in the linear viscoelastic regime which appears to be influenced strongly by MA incorporation (Fig. 3a), the strain at which the storage modulus deteriorates is more reliant on inter-droplet physical bonds. This is perhaps not surprising since the so-called crossover point (i.e., where G′ = G″) correlates to the point at which the mechanical integrity of the material breaks down and starts to flow. In the context of aggregated engineered emulsions this process requires severance of inter-droplet hydrogen bonds. As such we observe a maximum monolith strength between 50 and 60 mol% EG which correlates with the maximum theoretical hydrogen-bond formation regime (i.e., where EG : MA 1 : 1—see Fig. 3b).²⁸ In terms of designing next-generation engineered emulsions these combined results indicate that incorporation of hydrogen-bonding moieties and hydrophobic comonomer in the branched copolymer may provide a useful handle for optimization and that a play-off between engineered emulsion stiffness and strength under shear exists for these systems. Importantly, the branching synthesis route employed in this study appears to be ideally suited to these subtle compositional changes since the incorporation of highly diverse functionality is essentially unrestrictive.

Inter-droplet interactions in binary mixtures of contra-functional droplets

Having established that dilute dispersions of droplets stabilized by 100% MA (E1) and 100% EG (E9) branched polymers in isolation show negligible acid-triggered aggregation we sought to study whether binary mixtures of these droplets could induce an inter-droplet aggregation response. In principle, a binary emulsion droplet mixture of these two components should induce inter-droplet hydrogen-bond formation between these “contra-functional” droplets. The acid-triggered aggregation of dilute dispersions of binary mixtures of 100% EG and 100% MA was measured by laser diffraction as a function of time and compared to data of the pure EG and MA droplets in isolation (Fig. 4a). In contrast to the pure MA and EG droplets, which showed limited and no aggregation respectively, the binary mixture displayed a significant and immediate increase in D_(4,3). This synergistic aggregation provides additional confirmation that EG–MA hydrogen-bonding is an important component of the self-assembly process. In comparison to the aggregation of droplets stabilized by EG : MA 1 : 1 (where the EG and MA function are present on the same droplet) the rate of aggregation is slower. We hypothesize that this is due to droplets with both functionalities present on the same droplet being inherently more “sticky” since intra-droplet hydrogen bonding events will occur around every droplet surface. Also, the probability of EG residues being in close proximity with MA residues in the binary mixture is significantly lower compared to the EG : MA 1 : 1 (single-component) system and becomes a function of droplet concentration and relative ratios of the contra-functional droplets.

Fig. 4 (a) Change in volume average droplet diameters (D_(4,3)) as a function of time on acidification of dilute emulsion droplet dispersions comprising: 100% EG (open triangles), 100% MA (open squares) and EG:MA 1:1 (grey circles) as controls and a binary mixture of 100% MA and 100% EG (black circles). (b) Rheology of 100% MA (squares), 100% EG (diamonds), 1 : 1 EG : MA droplets (triangles), 50 : 50 binary mixture of 100% MA and 100% EG droplets (circles).

Acid-triggered gelation of binary mixtures of the droplets was also assessed using rheometry at high droplet concentrations (Fig. 4b). As control experiments Fig. 4b shows the amplitude sweep data for the 100% MA (E1), 100% EG (E9) and 1 : 1 EG : MA (E5) droplets. These data follow a trend as already discussed whereby the gel stiffness is dictated by the MA component—that is, higher MA content leads to a stiffer gel—and the strain at which the structures break down is maximised around stoichiometric EG : MA ratios. Binary mixtures of the contra-functional droplets in equal volumes were acidified on the rheometer plate and gel formation could be observed. Amplitude sweeps of the binary engineered emulsion showed an average gel stiffness in very close agreement with that obtained for the EG : MA 1 : 1 (E5) in the linear viscoelastic regime (G′ of 1100 Pa for the binary mixture and 900 Pa for single-component droplets). Thus under the conditions employed (i.e., droplet concentration, PEGMA chain length, etc.), gel stiffness appears to be defined by the overall bulk ratio of EG : MA as opposed to the locality of the MA and EG residues—that is, whether the functional groups are present on the same or on different droplets. In addition, the crossover points were also in good agreement for the binary and single-droplet engineered emulsion systems (29% and 24%, respectively) and both are significantly higher than the 100% EG and 100% MA control experiments (16% and 10%, respectively) which confirms that these binary systems induce a synergistic hydrogen-bonding effect on the engineered emulsion mechanical properties.

A fundamentally important concept in self-assembly—especially in natural biological systems—concerns selectivity of inter-molecular interactions. It is clearly desirable in broader synthetic applications to control the proximity and inter-play of specific molecules and colloids using readily achievable function and simple triggers. Having ascertained that there is a synergistic effect of the aggregation of mixtures of pure EG and pure MA droplets we investigated whether we could observe selective aggregation events between these contra-functional droplets. 100% MA polymer-stabilized emulsion droplets were prepared using a crystallizable oil (1-dodecanol) as the internal phase. Below the melting temperature (∼24 °C) 1-dodecanol droplets become highly non-spherical are therefore readily identifiable in the presence of “standard” spherical dodecane droplets functionalized with 100% EG. Fig. 5a and b show light micrographs of droplet dispersions of the 100% EG/dodecane and 100% MA/1-dodecanol droplets at pH 10, respectively—both droplets were entirely dispersed and the 1-dodecanol droplets were non-spherical. On lowering the solution pH the 100% EG/dodecane droplets remained completely dispersed since the surface function is unresponsive in isolation (Fig. 5c) whereas the 100% MA/1-dodecanol droplets aggregated due to their reduced hydrophilicity (Fig. 5d). A binary mixture of the contra-functional droplets was prepared at basic pH and on lowering the pH light microscopy revealed a substantial proportion of droplets in this dilute regime had aggregated, moreover a number of assembled structures comprised non-spherical 100% MA/1-dodecanol droplets interacting selectively with spherical 100% EG/dodecane droplets (Fig. 5e and f). Although there was evidence of MA–MA droplet interactions, a proportion (unquantified) of the assembled structures comprised MA-droplets interacting selectively with EG-droplets and this augurs extremely well as a proof-of-concept demonstration that controlled and selective aggregation between simple and readily scaleable materials is achievable.

Fig. 5 Light microscopy images of 100% MA/1-dodecanol droplets and 100% EG/dodecane droplets. (a) 100% EG/dodecane (spherical) droplet dispersion at pH 10. (b) 100% MA/1-dodecanol (non-spherical) droplet dispersion at pH 10. (c) 100% EG/dodecane (spherical) droplet dispersion at pH 2. (d) 100% MA/1-dodecanol (non-spherical) aggregated droplets at pH 2. (e) Binary mixture of contra-functional droplets of spherical 100% EG/dodecane droplets interacting selectively with non-spherical 100% MA/1-dodecanol droplets. (f) Schematic of the selective contra-functional droplet assembly observed in (e): blue spherical droplets represent 100% EG/dodecane and non-spherical red droplets represent 100% MA/1-dodecanol.

Conclusions

A one-pot branched vinyl polymerization process has permitted the facile synthesis of a series of compositionally controlled copolymer surfactants based on methacrylic acid and poly(ethylene glycol). These copolymers can be used as highly efficient emulsifiers for the preparation of stable oil-in-water emulsions at basic pH. The effect of systematically varying the MA to EG molar ratios present on the droplet surfaces on acid-triggered droplet aggregation was assessed. The aggregation kinetics of dilute droplet dispersions shows that equimolar ratios of EG : MA promote fastest aggregation and that excess MA component retains droplet aggregation albeit at slower rates. Excess PEG residues entirely preclude inter-droplet, inter-polymer hydrogen bonding events at acidic pH, which eliminates droplet aggregation. These design rules provide the basis for a conceptual demonstration that binary mixtures of contra-functionalized droplets (100% MA droplets and 100% EG droplets) can aggregate in a selective manner. Rheometry studies show that higher MA contents on droplet surfaces produce stiffer aggregated emulsion gels and the strain at which the structures breaks down is maximised around stoichiometric EG : MA ratios. We therefore demonstrate that emulsion droplets stabilized with structurally similar branched copolymers and with functionality that varies only very subtly can be used to control—at a high level—triggered inter-droplet interactions. We are currently investigating the use of surface functionalization strategies to controllably and selectively associate various multi-responsive soft materials using scaleable and accessible synthetic processes.

Acknowledgements

Prof. S. P. Rannard is thanked for access to the laser diffraction instrument.

Notes and references

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28. Data point for 100% PEGMA emulsion is omitted as the G′ and G″ values are too low to reliably infer crossover points.

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Footnotes

† Electronic supplementary information (ESI) available: Polymer and emulsion synthesis and characterization data. See DOI: 10.1039/c0py00277a

‡ This paper is part of a Polymer Chemistry issue highlighting the work of emerging investigators in the polymer chemistry field. Guest Editors: Rachel O'Reilly and Andrew Dove.

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