The main goal of this work was to develop a fabrication route for colloidosome synthesis on the basis of Pickering emulsions stabilized using peroxidized latex particles.
To achieve this goal, two objectives were set: (i) to synthesize colloidosomes using peroxidized latex particles and (ii) to investigate the ways that the presence of hydrophilic functional groups on latex particle surfaces impacts Pickering emulsion stability.
Thus, the synthesis of colloidosomes from Pickering emulsions involves the following steps: (i) synthesis of peroxidized latex particles as reactive building blocks in the colloidosome formation; and (ii) localization of the peroxidized latex particles at the interface, i.e., formation of an oil-in-water Pickering emulsion and the synthesis of colloidosomes using the peroxide groups at the interface.
3.1. Synthesis and Characterization of Peroxidized Latexes
An amphiphilic PM-1-MA copolymer used as an inisurf was synthesized through a free-radical copolymerization of a peroxide monomer 2-tert
-butylperoxy-2-methyl-5-hexen-3-ine (PM-1) with maleic anhydride (Scheme 1
The subsequent hydrolysis of the synthesized PM-1-MA copolymer leads to the formation of a surface active initiator (inisurf) with carboxyl groups (at pH < 7) or carboxylate (salt) groups (at pH > 7) (Scheme 2
In a similar way, an amphiphilic PM-2-MA copolymer was synthesized through a free-radical copolymerization of a peroxide monomer N-(tert
-butylperoxymethyl)acrylamide (PM-2) with maleic anhydride (Scheme 3
The capability of the peroxide groups in the PM-2-MA copolymer to undergo thermal decomposition was demonstrated by a thermogravimetric analysis. Figure 2
indicates that the PM-2-MA samples undergo weight loss at 130–210 °C. The recorded weight loss is due to the decomposition of the primary–tertiary peroxide groups in the PM-2 units of the copolymer.
An inisurf PM-2-MAc with carboxyl groups or carboxylate (salt) groups was formed via the hydrolysis of the PM-2-MA copolymer at different pH levels (Scheme 4
Amphiphilic PM-1-MAc and PM-2-MAc copolymers (Scheme 2
and Scheme 4
) were used as inisurfs in emulsion polymerization to synthesize peroxidized latex particles. To provide particles with surface activity and the ability to stabilize a Pickering emulsion in colloidosome synthesis, these particles must exhibit amphiphilic behaviors, i.e., combine hydrophilic and hydrophobic molecular moieties at the surface. In the case of peroxidized latexes, hydrophilic properties are provided by the hydrolysis of PM-1-MA or PM-2-MA maleic anhydride groups (Scheme 2
and blue area in Scheme 4
) and form the carboxyl groups of PM-MAc at pH < 7 or PM-MAc (salt) at pH > 7 inisurf macromolecules, whereas non-polar fragments of the peroxide monomers in the copolymer (Scheme 2
and orange area in Scheme 4
) and polystyrene (core material of latex) provide hydrophobic properties to the particle’s surface.
To investigate the ability to vary the nature of the peroxide groups and the amount of carboxyl groups on the surface of latex particles, and thus their reactivity and surface activity, a series of peroxidized latexes were synthesized at pH 10 using PM-1-MAc at various concentrations (6, 8, and 10 wt.% based on monomer weight) and at pH 5.5, 7.5, and 9.5 using PM-2-MAc at various concentrations (0.3, 1, 2.5, 5, and 7.5 wt.% based on monomer weight).
The water soluble copolymers PM-1-MAc and PM-2-MAc exhibit surface activity that decreases the surface tension at the water-air interface to 30–40 mN/m. In an aqueous medium, they form micellar structures, which are able to solubilize hydrophobic substances [28
]. This allows for the application of PM-1-MAc and PM-2-MAc as polymeric surfactants in emulsion polymerization. The peroxide groups in the polymeric surfactant macromolecules enable their ability to initiate free radical processes [31
]. The amphiphilic nature of the PM-1-MAc and PM-2-MAc macromolecules with the hydrophilic units of maleic acid and the hydrophobic units of the peroxide monomer facilitates the stabilization of latex particles in emulsion polymerization. The solubilization of the styrene monomer in emulsion polymerization followed by the formation of adsorption layers at the surface of latex particles occurs due to hydrophobic interactions and the hydration of ionized carboxylate groups oriented towards an aqueous phase (Figure 3
). The stability of the latex particles is achieved due to the combination of steric and electrostatic stabilization [28
An advantage of the application of PM-1-MAc and PM-2-MAc as polymeric inisurfs is the presence of peroxide groups in the hydrophobic PM units, which are regularly distributed along the macromolecules. During thermolysis, the peroxide groups decompose to form tert
-butoxy and macroradicals, which are able to initiate radical reactions of styrene polymerization [28
]. The combination of the PM-1-MAc and PM-2-MAc macroradicals with the growing polystyrene chains leads to the attachment of the PM-1-MAc and PM-2-MAc macromolecules to the surface of latex particles and to the formation of the polyperoxide PM-1-MAc and PM-2-MAc layers covalently grafted to the polystyrene particle surface (Figure 3
The emulsion polymerization of styrene in the presence of PM-1-MAc resulted in polystyrene latex particles with peroxidized surfaces and average diameters in the range of 165 nm to 220 nm (Table 1
The presence of the peroxide groups in the grafted PM-1-MAc macromolecules was proven by the thermogravimetric analysis of the polystyrene latex. Weight loss at 150–220 °C accompanied by a well-pronounced exothermic effect is characteristic of the decomposition of the ditertiary peroxide groups of PM-1 [28
The ratio of the low-molecular-weight products (i.e., acetone and 2-methyl-2-propanol) of the peroxide group decomposition (Scheme 5
) was determined using gas-liquid chromatography, which enabled quantifying the PM-1 units in the peroxidized polystyrene latex (Table 1
). The presence of the PM-1-MAc chains in the polystyrene latex confirms a covalent grafting of the PM-1-MAc macromolecules to the surface of the polystyrene latex particles.
Varying concentrations of the synthesized PM-2-MAc copolymer were used in a series of emulsion polymerizations of styrene at pH 9.5 (Table 2
). The expectation was that the PM-2-MAc would act as an inisurf during the polymerization process, initiating polymerization by generating free radicals at elevated temperatures and ensuring micelle formation and the stabilization of growing polymer particles.
A illustrates the size variations of the peroxidized particles obtained at 85 °C as a function of pH and initial PM-2-MAc concentration. In general, the size of latex particles decreases with an increasing PM-2-MAc concentration and with a decreasing pH. The latter can be explained by the fact that a higher concentration of inisurf naturally results in a larger number of micelles (nucleating sites for particle growth in emulsion polymerization); thus, a greater number of smaller particles is formed. The HLB of inisurf macromolecules depends on the pH of the solution. At a higher pH, inisurf macromolecules are more hydrophilic, and thus more are required to form a micelle (the opposite is true for a lower pH). Consequently, the pH changes the total number of micelles in polymerization and results in variations in latex particle numbers and sizes (Figure 4
As shown in Table 1
and Table 2
, the inisurf macromolecules at elevated temperatures undergo covalent attachments (grafting) to latex particle surfaces during polymerization. A potentiometric titration of peroxidized latexes was performed to determine the amount of carboxyl groups on the particles’ surfaces. The obtained data reveal that only 60%–80% of carboxyl groups in grafted macromolecules are indeed located on the particle surface. Thus, a significant part of the anhydride groups was localized in the particle core during the synthesis and may not have undergone hydrolysis. Figure 4
B shows the zeta potential measurements of peroxidized latexes and provides information regarding the overall effect of inisurf concentration and pH on a particle’s charge. The data show that the amount of carboxyl groups on the surface of latex particles depends almost linearly on the concentration of the inisurf (Figure 5
B); however, the density of the functional groups on the surface differs based on inisurf concentrations, possibly due to changes in latex particle sizes (Figure 5
The observed changes of zeta potential with respect to pH during synthesis can be explained by the fact that part of the anhydride groups is not accessible for hydrolysis, as was determined by the potentiometric titration.
Thus, it was observed that increasing the inisurf concentration in latex synthesis results in a significantly smaller size of the resulting particles and a larger amount of carboxyl groups on the surface. The size of the particles is determined by the pH during emulsion polymerization, and it is also affected by the amount of carboxyl groups on the surface of particles.
As a result, a variable amount of carboxyl groups on the surface of peroxidized latex particles allows for control over particle surface properties (including HLB).
During the next step, a colloidosome synthesis was attempted using peroxidized latex particles.
3.2. Colloidosome Preparation
In this study, the “soft template” approach [10
] was used in which liquid droplets of Pickering emulsion serve as templates for cross-linked colloidosomes synthesis (Figure 1
To synthesize colloidosomes, a combined initiation mechanism was applied, i.e., initiation in a bulk template (with AIBN) and initiation from the surface of the peroxidized latex particles (with the inisurf). Depending on the composition of a monomer mixture, polystyrene, poly(butyl acrylate), or poly(styrene-co
-butyl acrylate) (all polymers cross-linked with divinylbenzene) were formed in the bulk template during polymerization. The formed polymers are insoluble in hexadecane. As a result, their macromolecules were localized at the interface and were involved in the formation of the colloidosomes’ polymer walls (Figure 1
C). The peroxide groups localized at the interface (on the surface of peroxidized latex particles) in turn generate free radicals, which participate in radical reactions of initiation, chain transfer, and combination. Due to the above-mentioned radical processes, polymer chains from the monomers and the divinylbenzene cross-linker grew both in the bulk template and from the interface. This resulted in the formation of intermolecular cross-links and the covalent immobilization of the latex particle, which acted as a Pickering stabilizer, within the colloidosome walls.
The formation and stability of a Pickering emulsion depend on the latex particles’ surface activity (surface HLB). Therefore, the ability to control the HLB of the particle surface is crucial. To confirm that peroxidized latex particles can be used in Pickering emulsion formation for covalently cross-linked colloidosome synthesis, a series of Pickering emulsions was prepared.
First, the formation and stability of a series of formulated Pickering emulsions were studied as a function of pH (Figure 6
). It was found that there is an optimum pH range in which superior Pickering emulsion stability can be achieved. Notably, at pH < 3 and pH > 9, no emulsion formation was observed, which can be explained by either an absence (pH < 3) or an excessive amount (pH > 9) of ionized carboxyl groups on the latex particles’ surfaces, causing the particles to coagulate due to a detrimental HLB change of inisurf macromolecules that act as particle stabilizers.
Immediately after formation, a Pickering emulsion “creams” by floating an emulsified oil phase on top of an excessive aqueous phase. This effect can be explained by the different densities of the aqueous and oil phases (a large size prevents droplet suspension). At this point, the formulation quality can be assessed by inspecting the aqueous phase for the presence of excessive latex particles. Opalescence and turbidity would signify an inefficient use of latex particles. By adjusting the pH, shear rate, and concentration of latex particles, it is possible to form a clear (particle-free) aqueous phase, indicating that all particles are involved in droplet stabilization.
For the synthesis of colloidosomes, latexes developed at pH 7.5 at 85 °C in the presence of 1 wt.% (based on monomer weight) inisurf PM-2-MAc were chosen. It was the assumption that while having a smaller amount of carboxyl groups, these latexes exhibit sufficient surface activity to stabilize a Pickering emulsion.
After formulating a Pickering emulsion, polymerization was conducted at 80 °C to yield hollow colloidosomes decorated with peroxidized latex particles. Table 3
presents data on the properties of the synthesized colloidosomes. The obtained results indicate that several morphological characteristics of colloidosomes can be controlled by variations in oil phase compositions (e.g., monomer/HD ratio) and the pH levels of the aqueous phase.
At the same time, the size of colloidosomes can be adjusted using the pH of the aqueous phase and the homogenization parameters (pulse length, sonication intensity), whereas shell thickness depends on the total initial concentration of monomers. Figure 7
shows SEM images of different colloidosomes (Table 3
) synthesized using peroxidized latex particles.
Although the surface charge of latex particles is essential for the electrostatic stabilization of droplets in a Pickering emulsion, it can also be disruptive if it is too large. Notably, unlike other samples, the colloidosomes prepared at pH 7.5 did not feature smooth surfaces covered with grafted latex particles. Their outer surfaces showed signs of phase separation during synthesis and an obvious lack of latex particles, which is possibly due to an excessive particle charge at pH 7.5.
The same approach was applied for the synthesis of colloidosomes from latex particles synthesized with inisurf PM-1-MAc. The synthetic conditions and characteristics of the formed colloidosomes are summarized in Table 4
As determined by SEM measurements, the diameter of the synthesized colloidosomes varied in the range of 1.3 μm to 35 μm depending on the synthetic conditions and compositions of the reactive mixture. Figure 8
A shows SEM images of colloidosomes polySt-4 (Table 4
). The polydispersity of the colloidosome diameter is evidently dependent on a dispersing process during the Pickering emulsion formation; the same behavior was noted in Ref. [20
The morphology of the synthesized colloidosomes (Figure 8
B) clearly shows the presence of the polymer particles within the colloidosome wall. As evidenced by Figure 8
C, the colloidosome walls define an internal enclosure, or cavity. The application of divinylbenzene as a low-molecular-weight cross-linker allows for the formation of cross-linked polymers with a three-dimensional structure, namely, the formation of the spongy walls of the colloidosomes (Figure 8
C). The cross-linked polymer chains then lose their mobility, which leads to a decrease in their precipitability and an increase in the wall thickness. Under certain conditions, the filling of the entire colloidosome cavity with the spongy polymer was observed (Figure 8
D). On the other hand, the development of broken (or not fully formed) colloidosomes was observed when the content of the monomers in the template decreased. This indicates that the walls formed under these conditions are either not strong enough or that the cross-linking degree is low.