Synthesis of Covalently Cross-Linked Colloidosomes from ... - MDPI

coatings Article

Synthesis of Covalently Cross-Linked Colloidosomes from Peroxidized Pickering Emulsions Nadiya Popadyuk 1 , Andriy Popadyuk 1 , Ihor Tarnavchyk 1 , Olha Budishevska 2 , Ananiy Kohut 2 , Andriy Voronov 1 and Stanislav Voronov 2, * 1

2

*

Department of Coatings and Polymeric Materials, North Dakota State University, NDSU Dept. 2760, P.O. Box 6050, Fargo, ND 58108-6050, USA; [email protected] (N.P.); [email protected] (A.P.); [email protected] (I.T.); [email protected] (A.V.) Department of Organic Chemistry, Lviv Polytechnic National University, vul. S. Bandery, 12, Lviv 79013, Ukraine; [email protected] (O.B.); [email protected] (A.K.) Correspondence: [email protected]; Tel.: +380-66-467-0898

Academic Editor: Jean-François Berret Received: 29 September 2016; Accepted: 20 October 2016; Published: 25 October 2016

Abstract: A new approach to the formation of cross-linked colloidosomes was developed on the basis of Pickering emulsions that were stabilized exclusively by peroxidized colloidal particles. Free radical polymerization and a soft template technique were used to convert droplets of a Pickering emulsion into colloidosomes. The peroxidized latex particles were synthesized in the emulsion polymerization process using amphiphilic polyperoxide copolymers poly(2-tert-butylperoxy-2-methyl-5-hexen-3-ine-co-maleic acid) (PM-1-MAc) or poly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic acid (PM-2-MAc), which were applied as both initiators and surfactants (inisurfs). The polymerization in the presence of the inisurfs results in latexes with a controllable amount of peroxide and carboxyl groups at the particle surface. Peroxidized polystyrene latex particles with a covalently grafted layer of inisurf PM-1-MAc or PM-2-MAc were used as Pickering stabilizers to form Pickering emulsions. A mixture of styrene and/or butyl acrylate with divinylbenzene and hexadecane was applied as a template for the synthesis of colloidosomes. Peroxidized latex particles located at the interface are involved in the radical reactions of colloidosomes formation. As a result, covalently cross-linked colloidosomes were obtained. It was demonstrated that the structure of the synthesized (using peroxidized latex particles) colloidosomes depends on the amount of functional groups and pH during the synthesis. Therefore, the size and morphology of colloidosomes can be controlled by latex particle surface properties. Keywords: pickering emulsion; covalently cross-linked colloidosomes; polyperoxide inisurf; emulsion polymerization; peroxidized latex particles

1. Introduction It is difficult to overestimate the importance of colloidal systems in current polymer technology and the development of polymer materials, including nanomaterials. As a result, constant efforts are being made in commonly used industry emulsion and suspension polymerization techniques to improve existing processes and to develop new advanced polymers and polymer materials [1,2]. These polymerization processes usually use templates from liquid droplets stabilized by molecular surfactants to yield tailored solid polymeric nanomaterials (nanoparticles). Over the past decade, considerable attention has focused on surfactant-free systems [3,4], as they provide an opportunity to reduce the environmental impact of polymer materials. More specifically, of considerable interest are Pickering emulsions, which are stable colloidal systems of two immiscible liquids, one of which is dispersed in the other with the assistance of solid particles [5–7].

Coatings 2016, 6, 52; doi:10.3390/coatings6040052

www.mdpi.com/journal/coatings

Coatings 2016, 6, 52

2 of 14

The phenomena of particle surface activity were first described by Pickering and Ramsden [8,9]. Several decades later, Velev [10,11] proposed a mechanism for the self-assembly of colloidal particles at the liquid–liquid interface. It was established that Pickering emulsions exhibit superior kinetic and thermodynamic stability compared to conventional emulsions. The stabilization mechanism of colloidal particles is based on the reduction of interfacial tension, which is similar to molecular surfactants; however, compared to surfactant molecules, colloidal particles are significantly larger, and that imparts specifics to the emulsification process and stability profile [12,13]. Using a Pickering emulsion opens up a broad range of opportunities not only for improving the properties and performance of already existing polymer colloids but also for developing new polymer composite materials and nanomaterials [14]. Colloidal structures obtained as a result of fully or partially solidifying droplets of Pickering emulsion are generally referred to as colloidosomes. There are reports in the literature of the synthesis of various types of colloidosomes, including “hairy” colloidosomes [15], “sensitive” colloidosomes [16,17], and “carbon nanotubosomes” [18]. The formation of covalently cross-linked colloidosomes via free radical and condensation polymerization mechanisms has been reported [19–21] to be an effective approach for the fabrication of various colloidosome morphologies, such as core-shell and capsule-like morphologies [20,22]. Due to the ability to finely tune colloidosome properties (density of particle packing at the surface, shell thickness and porosity, core type, composition, etc.), these structures can be engineered to efficiently encapsulate (absorb) and deliver (release) various cargos in a controlled fashion [23]. Reports have shown that colloidosomes can be employed as effective encapsulation tools for drug delivery or for the protection of components in the food and cosmetics industries [24–27]. In this study, peroxidized latex particles, i.e., latex particles with peroxide groups localized at their surface, were applied in the synthesis of cross-linked colloidosomes. The latexes were synthesized using amphiphilic polyperoxide copolymers that acted as initiators and surfactants (inisurf) simultaneously in the emulsion polymerization process. The amphiphilic polyperoxide copolymers employed in colloidosome synthesis, poly(2-tert-butylperoxy-2-methyl-5-hexen-3-ine-co-maleic anhydride) (PM-1-MA) and poly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic anhydride (PM-2-MA), were developed in the authors’ groups. PM-2-MA contains primary-tertiary peroxide groups in the macromolecules, whereas PM-1-MA has ditertiary peroxide groups. These peroxide groups have different reactivity and thermal stability levels. It was expected that the presence of carboxyl and peroxide functional groups (both derived from inisurf molecules) on the latex particles’ surfaces would provide: (i) the required surface properties of particles (hydrophilic-lipophilic balance, HLB), sustaining a Pickering emulsion mechanism for synthesis of colloidosomes [10]; (ii) the initiation of free radical reactions, polymer formation, grafting, and cross-linking within Pickering emulsion droplets; and (iii) an opportunity to further functionalize colloidosomes in post-polymerization reactions. The Pickering emulsion was composed of an oil phase from a monomer mixture, an initiator (AIBN), and a hydrophobic solvent hexadecane (HD) that was emulsified in an aqueous phase where the peroxidized latex particles, which exclusively acted as surface-active ingredients, were located. Once a Pickering emulsion was formed, it was heated to initiate free radical polymerization inside each oil droplet, yielding hollow (HD-filled) colloidosomes (Figure 1). It was demonstrated that the presence of functional groups on latex particle surfaces plays a crucial role in the stability of a Pickering emulsion (by controlling the HLB) and determines its success in colloidosome synthesis. By utilizing the peroxide functional groups of latex particles, colloidosomes can be further modified to obtain a covalently cross-linked shell and hollow (liquid) core.

Coatings 2016, 6, 52 Coatings 2016, 6, 52

3 of 14 3 of 14

Figure 1. Figure 1. Schematic Schematic of of the the colloidosomes colloidosomes synthesis: synthesis: (A) (A)Aqueous Aqueousand and oil oil phases phases before before homogenization; homogenization; (B) Pickering Pickering emulsion; emulsion; (C) (C) Colloidosomes Colloidosomes (polymeric (polymeric shell covered with with grafted grafted peroxidized peroxidized latex latex (B) shell covered particles and solvent-filled hollow interior). particles and solvent-filled hollow interior).

2. Materials 2. Materialsand andMethods Methods Styrene (St), divinylbenzene (DVB) (Sigma-Aldrich, St. Louis, MO, USA), Styrene (St), butyl butylacrylate acrylate(BA), (BA), divinylbenzene (DVB) (Sigma-Aldrich, St. Louis, MO, hexadecane (HD) (Alfa Aesar, Ward Hill, MA), 2,2’-azobis-isobutyronitrile (AIBN) (Aldrich, St. USA), hexadecane (HD) (Alfa Aesar, Ward Hill, MA), 2,2’-azobis-isobutyronitrile (AIBN) (Aldrich, Louis, MO, USA) were used. Amphiphilic copolymers poly(2-tert-butylperoxy-2-methyl-5-hexen-3St. Louis, MO, USA) were used. Amphiphilic copolymers poly(2-tert-butylperoxy-2-methyl-5-hexenine-co-maleic anhydride) (PM-1-MA) or poly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic 3-ine-co-maleic anhydride) (PM-1-MA) or poly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic anhydride (PM-2-MA) (PM-2-MA) were were synthesized synthesized as as described described elsewhere elsewhere [28–30]. [28–30]. All anhydride All monomers monomers were were purified purified by vacuum distillation. Other reagents and water (Milli-Q, 18 MΩ) were used as received. by vacuum distillation. Other reagents and water (Milli-Q, 18 MΩ) were used as received. Emulsion polymerization latex particles. Briefly, 10 Emulsion polymerization was wascarried carriedout outtotosynthesize synthesizeperoxidized peroxidized latex particles. Briefly, wt.% emulsions of of styrene inin anan aqueous 10 wt.% emulsions styrene aqueoussolution solutionofofPM-1-MAс PM-1-MAc(6–10 (6–10wt.% wt.%per perstyrene) styrene) at at pH pH 10 10 or or PM-2-MAс (1–7.5 wt.% per styrene) at pH 5.5–9.5 were prepared in a 50 mL single-neck round-bottom PM-2-MAc (1–7.5 wt.% per styrene) at pH 5.5–9.5 were prepared in a 50 mL single-neck round-bottom flask by by magnetic magnetic stirring stirring at at 1200 1200 rpm rpm for for 30 30 min. min. The argon under under flask The emulsions emulsions were were purged purged with with argon constant stirring stirring for for 20 min and and subsequently subsequently placed placed in in an an oil oil bath bath heated heated to to 85 85 ◦°C. constant 20 min C. The The polymerization polymerization was conducted under constant stirring to a conversion of 95%–98%. At the end the reaction, reaction, the the was conducted under constant stirring to a conversion of 95%–98%. At the end of of the latexes were a 5a mL dialysis bagbag (50 (50 kDakDa molecular cut-off) to remove the nonlatexes were cooled cooledand andplaced placedinto into 5 mL dialysis molecular cut-off) to remove the grafted inisurf. The bag was submerged into a 2000 mL beaker filled with distilled water adjusted to non-grafted inisurf. The bag was submerged into a 2000 mL beaker filled with distilled water adjusted pH 9.5 using NaOH. At intervals of 24 h, the water in the beaker was replaced to ensure a constant to pH 9.5 using NaOH. At intervals of 24 h, the water in the beaker was replaced to ensure a constant high concentration concentration gradient the dialysis dialysis process. process. After days, the the peroxidized peroxidized latexes latexes high gradient to to improve improve the After 14 14 days, were removed from the dialysis bag and stored at 4 °C. ◦ were removed from the dialysis bag and stored at 4 C. The resulting polymer was was then then either either (i) (i) stored stored at at 44 ◦°C The resulting polymer C or or (ii) (ii) precipitated precipitated by by aa freeze-thaw freeze-thaw cycle cycle and washed with 0.1 0.1 N materials. The and washed repeatedly repeatedly with N NaOH NaOH and and hexane hexane to to remove remove non-reacted non-reacted materials. The resulting resulting polymer was was collected, collected, dried dried at at room room temperature, temperature, and and stored stored at at 44 ◦°C to be be used used for for further further analysis. analysis. polymer C to Volume mean particle size and zeta-potential measurements were performed in dilute aqueous Volume mean particle size and zeta-potential measurements were performed in dilute aqueous dispersions ofoflatex latexparticles particles using Malvern Zetasizer Nano-ZS90 at The 25 °C. final represent numbers dispersions using Malvern Zetasizer Nano-ZS90 at 25 ◦ C. finalThe numbers represent an average of a minimum of five individual measurements. an average of a minimum of five individual measurements. A potentiometric titration (back (back titration titration using using 0.1 0.1 N N HCl A potentiometric titration HCl as as aa titrant) titrant) of of peroxidized peroxidized latexes latexes was was used to calculate the amount of carboxyl groups (originated from PM-1-MAс or PM-2-MAс) on the used to calculate the amount of carboxyl groups (originated from PM-1-MAc or PM-2-MAc) on the surface of latex particles. surface of latex particles. A thermal analysis of peroxidized latexes was used to calculate the amount of peroxide groups A thermal analysis of peroxidized latexes was used to calculate the amount of peroxide groups on the surface of latex particles. For this purpose, latex samples (15 mg) were heated at an underlying on the surface of latex particles. For this purpose, latex samples (15 mg) were heated at an underlying heating rate of 10 °C/min to 400 °C in air using a TA Instruments Q500 (TA Instruments, New Castle, heating rate of 10 ◦ C/min to 400 ◦ C in air using a TA Instruments Q500 (TA Instruments, New Castle, DE, USA). DE, USA). Gas-liquid chromatography was used to calculate the content of the peroxide monomer 2-tertGas-liquid chromatography was used to calculate the content of the peroxide monomer butylperoxy-2-methyl-5-hexen-3-ine (PM-1) units in both the copolymer PM-1-MA and the 2-tert-butylperoxy-2-methyl-5-hexen-3-ine (PM-1) units in both the copolymer PM-1-MA and the peroxidized polystyrene latex particles. A quantitative analysis of the products of a complete decomposition of the PM-1 units was carried out using a Selmihrom chromatograph with columns (3 × 3000 mm2) filled with an absorbent Inerton super (0.125–0.16 mm fraction) modified with 5 wt.% 

Coatings 2016, 6, 52

4 of 14

Coatings 2016, 6, 52

4 of 14

peroxidized polystyrene latex particles. A quantitative analysis of the products of a complete decomposition theThe PM-1 units was carried out using Selmihrom chromatograph with columns of Carbowax 40ofM. concentration of acetone and a2-methyl-2-propanol—the peroxide group 2 (3 × 3000 mm ) filled with an absorbent Inerton super (0.125–0.16 mm fraction) modified with 5 wt.% decomposition products—was determined using chloroform as an internal standard. of Carbowax 40 M. The concentration of acetone and 2-methyl-2-propanol—the peroxide group Pickering emulsions were formed by mixing 28 g of an aqueous phase (containing 0.3–1 wt.% of decomposition products—was determined using chloroform asof anHD, internal standard. peroxidized latex particles) with 2 g of an oil phase (consisting St and/or BA, DVB, and 0.06 g were formed by mixing g of anwere aqueous phase (containing AIBNPickering initiator).emulsions The mixtures of oil and aqueous28phases homogenized at 10,0000.3–1 rpmwt.% in a of peroxidized latex particles) with 2 g of an oil phase (consisting of HD, St and/or BA, DVB, pulsating regime (1 pulse consisting of 60 s of homogenizing +20 s of rest time) for 8 min using a and T25 0.06 g AIBN initiator). The mixtures of oil and aqueous phases were homogenized at 10,000 rpm in Ultra-Turrax homogenizer equipped with an S 25 N–25 G dispersing element (IKA, Wilmington, NC, aUSA). pulsating regime (1 pulse consisting of 60 s of homogenizing +20 s of rest time) for 8 min using a T25 Ultra-Turrax homogenizer equipped with were an S 25 N–25 G dispersing (IKA, Wilmington, NC, USA). The obtained Pickering emulsions transferred into a 50element mL single-neck round-bottom flask, The obtained Pickering emulsions were transferred into a 50 mL single-neck round-bottom purged with argon for 20 min, and subsequently polymerized at 80 °C with magnetic stirring atflask, 1200 ◦ C with magnetic stirring at purged with argonpolymerization, for 20 min, and subsequently polymerized at 80 rpm for 24 h. After colloidosomes were isolated from the reaction mixture, thoroughly 1200 rpm for an 24 acetone/ethanol h. After polymerization, colloidosomes were under isolated reaction mixture, washed with (1:1) mixture, and stored either anfrom H2O the or HD layer or in dry thoroughly washed with an acetone/ethanol (1:1) mixture, and stored either under an H O or HD 2 state. layerPickering or in dry state. emulsion stability was evaluated as a volume portion of an emulsified oil phase that Pickering emulsion stability was evaluated ashomogenization. a volume portion of an emulsified oil phase that does not float at room temperature in 15 min after does Digital not floatimages at roomoftemperature in 15 min after homogenization. latex particles were obtained using JEOL JSM-6490LV scanning electron Digital (JEOL imagesUSA, of latex particles MA, wereUSA). obtained using JEOL JSM-6490LV scanning electron microscopy Inc., Peabody, microscopy (JEOL USA, Inc., Peabody, MA, USA).

3. Results and Discussion 3. Results and Discussion The main goal of this work was to develop a fabrication route for colloidosome synthesis on the 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. basis of Pickering emulsions stabilized using peroxidized latex particles. To achieve this goal, two objectives were set: (i) to synthesize colloidosomes using peroxidized 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 particles and (ii) to investigate the ways that the presence of hydrophilic functional groups on latex particle surfaces impacts Pickering emulsion stability. latex particle surfaces impacts Pickering emulsion stability. Thus, the synthesis of colloidosomes from Pickering emulsions involves the following steps: (i) Thus, the synthesis of colloidosomes from Pickering emulsions involves the following steps: synthesis of peroxidized latex particles as reactive building blocks in the colloidosome formation; and (i) synthesis of peroxidized latex particles as reactive building blocks in the colloidosome formation; (ii) localization of the peroxidized latex particles at the interface, i.e., formation of an oil-in-water 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. Pickering emulsion and the synthesis of colloidosomes using the peroxide groups at the interface. 3.1. Synthesis and Characterization of Peroxidized Latexes 3.1. Synthesis and Characterization of Peroxidized Latexes An amphiphilic PM-1-MA copolymer used as an inisurf was synthesized through a free-radical 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 copolymerization of a peroxide monomer 2-tert-butylperoxy-2-methyl-5-hexen-3-ine (PM-1) with maleic anhydride (Scheme 1). maleic anhydride (Scheme 1).

Scheme 1. Synthesis of of PM-1-MA PM-1-MA containing containing peroxide Scheme 1. Synthesis peroxide and and anhydride anhydride groups. groups.

The subsequent hydrolysis hydrolysis of of the the synthesized synthesizedPM-1-MA PM-1-MAcopolymer copolymerleads leadstotothe theformation formationofofa groups (at (atpH pH< 7)>(Scheme 2). 2).



Coatings 2016, 6, 52

5 of 14

Coatings 2016, 6, 52 Coatings2016, 2016,6,6,5252 Coatings

5 of 14 5 of 1414 5 of

Scheme 2. Reactions groupsofof PM-1-MA. Scheme Reactionsof ofthe the anhydride anhydride groups Scheme 2. 2.Reactions of the anhydride groups ofPM-1-MA. PM-1-MA. Scheme 2. Reactions of the anhydride groups of PM-1-MA.

In a similar way, an amphiphilic PM-2-MA copolymer was synthesized through a free-radical In In a similar way, an PM-2-MA copolymer was synthesized synthesized througha afree-radical free-radical way, an amphiphilic amphiphilic PM-2-MA copolymer synthesizedthrough a similar way, amphiphilic PM-2-MA copolymer was copolymerization of aan peroxide monomer N-(tert-butylperoxymethyl)acrylamide (PM-2) with maleic copolymerization of a peroxide monomer N-(tert-butylperoxymethyl)acrylamide (PM-2) with maleic copolymerization of a3).peroxide monomer N-(tert-butylperoxymethyl)acrylamide (PM-2) with maleic anhydride (Scheme anhydride (Scheme 3).3). anhydride (Scheme

Scheme 3. Synthesis of PM-2-MA containing peroxide and anhydride groups.

Scheme SynthesisofofPM-2-MA PM-2-MA containing containing peroxide andanhydride anhydridegroups. groups. Scheme 3.3.Synthesis peroxideand

The capability of the peroxide groups in the PM-2-MA copolymer to undergo thermal decomposition was demonstrated by a groups thermogravimetric analysis. copolymer Figure 2 indicates that the thermal PM-2The capability of the peroxide in the PM-2-MA to undergo The capability capability of of the the peroxide peroxide groups groups in in the PM-2-MA PM-2-MA copolymer copolymer to to undergo thermal thermal The MA samples undergo weight loss at C. Thethe recorded weightFigure loss is due to the undergo decomposition decomposition was demonstrated by130–210 a thermogravimetric analysis. 2 indicates that the PM-2decomposition was byby a thermogravimetric analysis. Figure 2 indicates that the PM-2decomposition wasdemonstrated demonstrated a in thermogravimetric analysis. Figure indicates that the of the primary–tertiary peroxide the units of the copolymer. MA samples undergo weight loss groups at 130–210 C.PM-2 The recorded weight loss is due to2the decomposition

◦ C. The MA samples undergoundergo weight loss at 130–210 C. The recorded weight loss isweight due to loss the decomposition PM-2-MA samples weight lossinatthe 130–210 is due to the of the primary–tertiary peroxide groups PM-2 units of therecorded copolymer. of the primary–tertiary peroxide groups in the PM-2 units of the copolymer. decomposition of the primary–tertiary peroxide groups in the PM-2 units of the copolymer.

Figure 2. Thermogravimetric analysis of PM-2-MA. 





Figure 2. Thermogravimetric analysis of PM-2-MA.

Figure 2. Thermogravimetric analysis of PM-2-MA.

Coatings 2016, 6, 52 Coatings 2016, 6, 52

6 of 14 6 of 14

An inisurf inisurf PM-2-MAc PM-2-MAc with with carboxyl carboxyl groups groups or or carboxylate carboxylate (salt) (salt) groups groups was was formed formed via via the the An hydrolysis of the PM-2-MA copolymer at different pH levels (Scheme 4). hydrolysis of the PM-2-MA copolymer at different pH levels (Scheme 4).

Scheme Scheme 4. 4. Inisurf Inisurf formation formation by by hydrolysis hydrolysis of of an an amphiphilic amphiphilic PM-2-MA PM-2-MA copolymer copolymer at at different different pH pH levels levels (blue (blue and and orange orange represent represent the the hydrophilic hydrophilic and and hydrophobic hydrophobicmoieties moieties of of PM-2-MA, PM-2-MA,respectively). respectively).

Amphiphilic PM-1-MAc PM-1-MAc and and PM-2-MAc PM-2-MAc copolymers copolymers (Schemes (Schemes 22 and and 4) 4) were were used used as as inisurfs inisurfs in in Amphiphilic emulsion polymerization to synthesize peroxidized latex particles. To provide particles with surface emulsion polymerization to synthesize peroxidized latex particles. To provide particles with surface activity and and the the ability ability to to stabilize stabilize aa Pickering Pickering emulsion emulsion in in colloidosome colloidosome synthesis, synthesis, these these particles particles activity must exhibit exhibitamphiphilic amphiphilicbehaviors, behaviors,i.e., i.e.,combine combinehydrophilic hydrophilic and hydrophobic molecular moieties must and hydrophobic molecular moieties at at the surface. In the case of peroxidized latexes, hydrophilic properties are provided by the the surface. In the case of peroxidized latexes, hydrophilic properties are provided by the hydrolysis hydrolysis ofor PM-1-MA ormaleic PM-2-MA maleic anhydride groups (Scheme andin blue area in 4) of PM-1-MA PM-2-MA anhydride groups (Scheme 2 and blue 2area Scheme 4) Scheme and form andcarboxyl form the carboxyl groups of PM-MAc at 7pH 7 or PM-MAc (salt) at pH inisurfmacromolecules, macromolecules, the groups of PM-MAc at pH < or 7> 7inisurf whereas non-polar fragments of the peroxide monomers in the copolymer (Scheme 2 and orange area whereas non-polar fragments of the peroxide monomers in the copolymer (Scheme 2 and orange in Scheme 4) and4)polystyrene (core material of latex)ofprovide hydrophobic properties to the particle’s area in Scheme and polystyrene (core material latex) provide hydrophobic properties to the surface. particle’s surface. Toinvestigate investigate the the ability ability to to vary vary the the nature nature of of the the peroxide peroxide groups groups and and the the amount amount of of carboxyl carboxyl To groups on on the the surface surface of of latex latex particles, particles, and and thus thus their their reactivity reactivity and and surface surface activity, activity, aa series series of of groups peroxidized latexes were synthesized at pH 10 using PM-1-MAс at various concentrations (6, 8, and peroxidized latexes were synthesized at pH 10 using PM-1-MAc at various concentrations (6, 8, 10 wt.% based on on monomer weight) and at atpH at various various and 10 wt.% based monomer weight) and pH5.5, 5.5,7.5, 7.5,and and9.5 9.5 using using PM-2-MAс PM-2-MAc at concentrations (0.3, (0.3, 1, 1, 2.5, 2.5, 5, 5, and and 7.5 7.5 wt.% wt.%based basedon onmonomer monomerweight). weight). concentrations The water soluble copolymers PM-1-MAc and PM-2-MAc exhibit surface surface activity activity that that decreases decreases The water soluble copolymers PM-1-MAc and PM-2-MAc exhibit thesurface surfacetension tension water-air interface to 30–40 mN/m. In an aqueous medium, they form the at at thethe water-air interface to 30–40 mN/m. In an aqueous medium, they form micellar micellar structures, which are able to solubilize hydrophobic substances [28]. This allows for structures, which are able to solubilize hydrophobic substances [28]. This allows for the applicationthe of applicationand of PM-1-MAc PM-2-MAc as polymeric surfactants in emulsion The polymerization. The PM-1-MAc PM-2-MAcand as polymeric surfactants in emulsion polymerization. peroxide groups peroxide groupssurfactant in the polymeric surfactant macromolecules enable free their ability to initiate free in the polymeric macromolecules enable their ability to initiate radical processes [31–33]. radical processes [31–33]. The amphiphilic nature of the PM-1-MAc and PM-2-MAc macromolecules The amphiphilic nature of the PM-1-MAc and PM-2-MAc macromolecules with the hydrophilic units with the hydrophilic of maleicunits acidofand hydrophobic units of the peroxide monomer of maleic acid and theunits hydrophobic the the peroxide monomer facilitates the stabilization of facilitates the stabilization of latex particles in emulsion polymerization. The solubilization of the latex particles in emulsion polymerization. The solubilization of the styrene monomer in emulsion styrene monomer in emulsion followed layers by theat formation of adsorption layersoccurs at the polymerization followed by thepolymerization formation of adsorption the surface of latex particles surface of latex particles occursand duethetohydration hydrophobic interactions and the hydration oftowards ionized due to hydrophobic interactions of ionized carboxylate groups oriented carboxylate groups oriented towards an aqueous phase (Figure 3). The stability of the latex particles an aqueous phase (Figure 3). The stability of the latex particles is achieved due to the combination of is achieved due to the stabilization combination [28]. of steric and electrostatic stabilization [28]. steric and electrostatic An advantage advantage of of the the application application of of PM-1-MAc PM-1-MAc and and PM-2-MAc PM-2-MAc as as polymeric polymeric inisurfs inisurfs is is the the An presence of peroxide groups in the hydrophobic PM units, which are regularly distributed along the presence of peroxide groups in the hydrophobic PM units, which are regularly distributed along macromolecules. During thermolysis, the peroxide groups decompose to form tert-butoxy and the macromolecules. During thermolysis, the peroxide groups decompose to form tert-butoxy macroradicals, which are able to initiate radical reactions of styrene polymerization [28,34]. The and macroradicals, which are able to initiate radical reactions of styrene polymerization [28,34]. 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 

Coatings 2016, 6, 52

7 of 14

The combination Coatings 2016, 6, 52 of the PM-1-MAc and PM-2-MAc macroradicals with the growing polystyrene chains 7 of 14 leads to the attachment of the PM-1-MAc and PM-2-MAc macromolecules to the surface of latex particles and andtotothe the formation of polyperoxide the polyperoxide PM-1-MAc and PM-2-MAc layers covalently particles formation of the PM-1-MAc and PM-2-MAc layers covalently grafted grafted to the polystyrene particle surface (Figure 3). to the polystyrene particle surface (Figure 3).

Figure Emulsion polymerization Figure 3. 3. Emulsion polymerization of of styrene styrene using using PM-1-MAc PM-1-MAc and and PM-2-MAc PM-2-MAc as as polymeric polymeric inisurfs. inisurfs.

presence of PM-1-MAc PM-1-MAc resulted resulted in in polystyrene polystyrene The emulsion polymerization of styrene in the presence latex particles with peroxidized surfaces and average diameters in the range of 165 nm to 220 nm (Table 1). 1). (Table Table using varying concentrations of Table1.1. Physico-chemical Physico-chemicalcharacteristics characteristicsofoflatex latexparticles particlessynthesized synthesized using varying concentrations PM-1-MAc in the emulsion polymerization of styrene at pH 10. 10. of PM-1-MAc in the emulsion polymerization of styrene at pH

Sample of theof the Sample Peroxidized Peroxidized LatexLatex Particles Particles

Synthetic Conditions Peroxidized Latexes Synthetic Conditions forfor Peroxidized Latexes of the Content Content of the (Latex Particles) (Latex Particles) Dh Dh PM-1 Units in PM-1 Units in (nm) 2 (nm) 2 Initial[PM-1-MAc] [PM-1-MAc] T T Conversion Initial Conversion a Sample (%) 3 a Sample (%) 3 (%) (◦ C)(°C) (%) 1 (%) 1 (%)

polySt-PM-1-MAc-1 70 70 97 181 polySt-PM-1-MAc-1 66 97 181 3.2 3.2 polySt-PM-1-MAc-2 8 70 98 180 3.2 polySt-PM-1-MAc-2 8 70 98 180 3.2 polySt-PM-1-MAc-3 10 70 97 166 2.8 polySt-PM-1-MAc-3 106 97 166 2.6 2.8 polySt-PM-1-MAc-4 80 70 99 212 polySt-PM-1-MAc-5 80 80 97 207 polySt-PM-1-MAc-4 68 99 212 2.8 2.6 polySt-PM-1-MAc-6 10 80 98 218 1.7 polySt-PM-1-MAc-5 8 80 97 207 2.8 Notes: 1 Using solid content determination; 2 Using dynamic light scattering; 3 Using gas-liquid chromatography. polySt-PM-1-MAc-6 10 80 98 218 1.7 Notes: 1 Using solid content determination; 2 Using dynamic light scattering; 3 Using gas-liquid chromatography.

The presence of the peroxide groups in the grafted PM-1-MAc macromolecules was proven by the thermogravimetric analysis of groups the polystyrene latex. PM-1-MAc Weight lossmacromolecules at 150–220 ◦ C accompanied The presence of the peroxide in the grafted was proven by by athe well-pronounced exothermic effect is characteristic of the decomposition of the ditertiary peroxide thermogravimetric analysis of the polystyrene latex. Weight loss at 150–220 C accompanied by groups of PM-1 [28,31,35]. a well-pronounced exothermic effect is characteristic of the decomposition of the ditertiary peroxide

groups of PM-1 [28,31,35]. 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 

Coatings 2016, 6, 52

8 of 14

Coatings 6, 52 The2016, ratio of

of 14 the low-molecular-weight products (i.e., acetone and 2-methyl-2-propanol) of8 the peroxide group decomposition (Scheme 5) was determined using gas-liquid chromatography, which enabled quantifying quantifying the the PM-1 PM-1 units of enabled units in in the the peroxidized peroxidized polystyrene polystyrenelatex latex(Table (Table1). 1).The Thepresence presence the PM-1-MAc chains in the polystyrene latex confirms a covalent grafting of the PM-1-MAc of the PM-1-MAc chains in the polystyrene latex confirms a covalent grafting of the PM-1-MAc macromoleculestotothe thesurface surfaceofofthe thepolystyrene polystyrenelatex latexparticles. particles. macromolecules

Scheme5.5.Decomposition Decomposition of PM-1 units. Scheme ofthe theperoxide peroxide PM-1 units.

Varying of the synthesized PM-2-MAc copolymer were used in used a series Varyingconcentrations concentrations of the synthesized PM-2-MAс copolymer were inofaemulsion series of polymerizations of styrene of at styrene pH 9.5 at (Table 2). (Table The expectation was thatwas the that PM-2-MAc would emulsion polymerizations pH 9.5 2). The expectation the PM-2-MAс act as an the polymerization process, initiating polymerization by generating free would actinisurf as an during inisurf during the polymerization process, initiating polymerization by generating radicals at elevated temperatures and ensuring micelle formation and the stabilization of growing free radicals at elevated temperatures and ensuring micelle formation and the stabilization of polymer particles. growing polymer particles. ofof latex particles synthesized using varying concentrations of Table Table2.2.Physico-chemical Physico-chemicalcharacteristics characteristics latex particles synthesized using varying concentrations PM-2-MAc in the emulsion polymerization of styrene at 85at◦85 c and pH 9.5. of PM-2-MAc in the emulsion polymerization of styrene °C and pH 9.5.

Initial Initial[PM-2-MAc] [PM-2-MAc] 1 /–OO– (mmol/g) (%)1/–OO– (%) (mmol/g)

[PM-2-MAc] at the the [PM-2-MAc]Localized Localized at LatexParticle Particle Surface Surface (%) Latex (%) 22

1.0/0.05 1.0/0.05 2.5/0.13 2.5/0.13 5.0/0.27 5.0/0.27 7.5/0.40 7.5/0.40 1

0.73 0.73 1.76 1.76 3.99 3.99 6.21 6.21

ζ-Potential DDhh PDI ζ-Potential PDI (nm) (mV) (nm) (mV) 196 196 162 162 146 146 155 155

0.050 0.050 0.015 0.015 0.041 0.041 0.033

± 2.1 −58−58 ± 2.1 −57−57 ± 1.9 ± 1.9 −54 ± 2.0 −54 ± 2.0 −54 ± 1.8 0.033 −54 ± 1.8

–OO– Localized –OO– Localized at at the Surface the Surface 2,3 (mmol/g) (mmol/g) 2,3 0.04/0.01 0.04/0.01 0.09/0.07 0.09/0.07 0.21/0.19 0.21/0.19 0.32/0.29 0.32/0.29

Notes: In reaction feed, per styrene monomer; 2 Using potentiometric titration (as no decomposition occurs);

2 Using potentiometric titration (as no decomposition 3 Using1 thermogravimetric Notes: In reaction feed, per styrene monomer; analysis (considering decomposition). occurs); 3 Using thermogravimetric analysis (considering decomposition).

Figure 4A illustrates the size variations of the peroxidized particles obtained at 85 ◦ C as a function Figure 4A illustrates the size variations of the peroxidized particles obtained at 85 °C as a of pH and initial PM-2-MAc concentration. In general, the size of latex particles decreases with function of pH and initial PM-2-MAс concentration. In general, the size of latex particles decreases an increasing PM-2-MAc concentration and with a decreasing pH. The latter can be explained by the with an increasing PM-2-MAс concentration and with a decreasing pH. The latter can be explained fact that a higher concentration of inisurf naturally results in a larger number of micelles (nucleating by the fact that a higher concentration of inisurf naturally results in a larger number of micelles sites for particle growth in emulsion polymerization); thus, a greater number of smaller particles is (nucleating sites for particle growth in emulsion polymerization); thus, a greater number of smaller formed. The HLB of inisurf macromolecules depends on the pH of the solution. At a higher pH, inisurf particles is formed. The HLB of inisurf macromolecules depends on the pH of the solution. At a macromolecules are more hydrophilic, and thus more are required to form a micelle (the opposite is higher pH, inisurf macromolecules are more hydrophilic, and thus more are required to form a true for a lower pH). Consequently, the pH changes the total number of micelles in polymerization micelle (the opposite is true for a lower pH). Consequently, the pH changes the total number of and results in variations in latex particle numbers and sizes (Figure 4A). micelles in polymerization and results in variations in latex particle numbers and sizes (Figure 4A). As shown in Tables 1 and 2, the inisurf macromolecules at elevated temperatures undergo covalent As shown in Tables 1 and 2, the inisurf macromolecules at elevated temperatures undergo attachments (grafting) to latex particle surfaces during polymerization. A potentiometric titration covalent attachments (grafting) to latex particle surfaces during polymerization. A potentiometric of peroxidized latexes was performed to determine the amount of carboxyl groups on the particles’ titration of peroxidized latexes was performed to determine the amount of carboxyl groups on the surfaces. The obtained data reveal that only 60%–80% of carboxyl groups in grafted macromolecules particles’ surfaces. The obtained data reveal that only 60%–80% of carboxyl groups in grafted are indeed located on the particle surface. Thus, a significant part of the anhydride groups was macromolecules are indeed located on the particle surface. Thus, a significant part of the anhydride localized in the particle core during the synthesis and may not have undergone hydrolysis. Figure 4B groups was localized in the particle core during the synthesis and may not have undergone shows the zeta potential measurements of peroxidized latexes and provides information regarding the hydrolysis. Figure 4B shows the zeta potential measurements of peroxidized latexes and provides overall effect of inisurf concentration and pH on a particle’s charge. The data show that the amount information regarding the overall effect of inisurf concentration and pH on a particle’s charge. The of carboxyl groups on the surface of latex particles depends almost linearly on the concentration of data show that the amount of carboxyl groups on the surface of latex particles depends almost linearly on the concentration of the inisurf (Figure 5B); however, the density of the functional groups on the surface differs based on inisurf concentrations, possibly due to changes in latex particle sizes (Figure 5A). 

Coatings 2016, 6, 52

9 of 14

the inisurf (Figure 5B); however, the density of the functional groups on the surface differs based on inisurf concentrations, possibly due to changes in latex particle sizes (Figure 5A). Coatings 2016, 6, 52 9 of 14 Coatings 2016, 6, 52

9 of 14

Figure 4. 4. (A) Diamenter Diamenter and (B) (B) ζ-potential variations variations of peroxidized peroxidized latex particles particles synthesized at at Figure Figure 4.(A) (A) Diamenterand and (B)ζ-potential ζ-potential variationsof of peroxidizedlatex latex particlessynthesized synthesized at 85°C at different pH levels and PM-2-MAс concentrations. ◦ 85 C at at different different pH 85°C pH levels levels and and PM-2-MAc PM-2-MAс concentrations. concentrations.

Figure 5. Amount of: (A) carboxyl groups; and (B) PM-2-MAс copolymers on peroxidized latex Figure 5.5. Amount Amount of: of: (A) (A) carboxyl carboxyl groups; groups; and (B) PM-2-MAс copolymers on peroxidized latex Figure particles synthesized at different pH levels. and (B) PM-2-MAc copolymers on peroxidized latex particles synthesized at different pH levels. particles synthesized at different pH levels.

The observed changes of zeta potential with respect to pH during synthesis can be explained by The observed changes of zeta potential with respect to pH during synthesis can be explained by the fact part ofchanges the anhydride groups iswith not accessible as wascan determined by the Thethat observed of zeta potential respect to for pHhydrolysis, during synthesis be explained by the fact that part of the anhydride groups is not accessible for hydrolysis, as was determined by the potentiometric titration. 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 potentiometric titration. Thus, it was observed that increasing the inisurf concentration in latex synthesis results in a significantly smaller size of that the resulting particles and aconcentration larger amount carboxyl groups on the Thus, it was observed increasing the inisurf in of latex synthesis results in significantly smaller size of the resulting particles and a larger amount of carboxyl groups on the The size of thesize particles determined by theand pHaduring emulsionofpolymerization, is asurface. significantly smaller of theisresulting particles larger amount carboxyl groupsand on itthe 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. 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 of carboxyl groups on theofsurface of peroxidized latex particles also affected by the amount amount of carboxyl groups on the surface particles. As a result, a variable amount of carboxyl groups on the surface of peroxidized latex particles allows controla over particle surface properties (including Asfor a result, variable amount of carboxyl groups on theHLB). surface of peroxidized latex particles allows for control over particle surface properties (including HLB). During the next step, a colloidosome synthesis was attempted allows for control over particle surface properties (including HLB). using peroxidized latex particles. During the next step, a colloidosome synthesis was attempted using peroxidized latex particles. During the next step, a colloidosome synthesis was attempted using peroxidized latex particles. 3.2. Colloidosome Preparation 3.2. Colloidosome Preparation 3.2. Colloidosome Preparation In this study, the “soft template” approach [10] was used in which liquid droplets of Pickering In this study, the “soft template” approach [10] was used in which liquid droplets of Pickering In this study, “soft template” approach [10] was used in which liquid1). droplets of Pickering emulsion serve as the templates for cross-linked colloidosomes synthesis (Figure emulsion serve as templates for cross-linked colloidosomes synthesis (Figure 1). 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 To synthesize colloidosomes, a combined initiation mechanism was applied, i.e. initiation in a To synthesize colloidosomes, a combined initiation mechanism was applied, i.e., initiation in bulk template (with AIBN) and initiation from the surface of the peroxidized latex particles (with the bulk template (with AIBN) and initiation from the surface of the peroxidized latex particles (with the ainisurf). bulk template (withon AIBN) and initiation the surface of thepolystyrene, peroxidized poly(butyl latex particles (with the Depending the composition offrom a monomer mixture, acrylate), or 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 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, 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 their macromolecules were localized at the interface and were involved in the formation of the colloidosomes’ polymer walls (Figure 1C). The peroxide groups localized at the interface (on the colloidosomes’ polymer walls (Figure 1C). The peroxide groups localized at the interface (on the surface of peroxidized latex particles) in turn generate free radicals, which participate in radical surface of peroxidized latex particles) in turn generate free radicals, which participate in radical

Coatings 2016, 6, 52

10 of 14

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 1C). The peroxide groups localized at the interface (on the surface of peroxidized Coatings 2016, 6, 52 10 of 14 latex particles) in turn generate free radicals, which participate in radical reactions of initiation, chain transfer, and combination. Due totransfer, the above-mentioned radical polymer chainsradical from the reactions of initiation, chain and combination. Due processes, to the above-mentioned monomers andpolymer the divinylbenzene cross-linker both in the bulk template andgrew fromboth the in interface. processes, chains from the monomersgrew and the divinylbenzene cross-linker the template andformation from the interface. This resulted in the formation of intermolecular cross-links and Thisbulk resulted in the of intermolecular cross-links and the covalent immobilization of the covalent immobilization of the latexstabilizer, particle, which as a Pickeringwalls. stabilizer, within the latexthe particle, which acted as a Pickering withinacted the colloidosome colloidosome walls. The formation and stability of a Pickering emulsion depend on the latex particles’ surface activity The formation and of control a Pickering emulsion the latex particles’ surface that (surface HLB). Therefore, thestability ability to the HLB of the depend particle on surface is crucial. To confirm activity (surface HLB). Therefore, the ability to control the HLB of the particle surface is crucial. To peroxidized latex particles can be used in Pickering emulsion formation for covalently cross-linked confirm that peroxidized latex particles can be used in Pickering emulsion formation for covalently colloidosome synthesis, a series of Pickering emulsions was prepared. 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 First, the formation and stability of a series of formulated Pickering emulsions were studied as as a afunction It was was found foundthat thatthere thereisisananoptimum optimum range in which superior functionof of pH pH (Figure (Figure 6). 6). It pHpH range in which superior Pickering emulsion stability can be achieved. Notably, at pH < 3 and pH > 9, no emulsion formation Pickering emulsion stability can be achieved. Notably, at pH < 3 and pH > 9, no emulsion formation was was observed, which cancan bebe explained (pH 9) of ionized carboxyl groups surfaces,causing causing particles to coagulate due of ionized carboxyl groupson onthe thelatex latex particles’ particles’ surfaces, thethe particles to coagulate due to a to detrimentalHLB HLBchange changeof of inisurf inisurf macromolecules actact as as particle stabilizers. a detrimental macromoleculesthat that particle stabilizers.

Figure 6. Fraction of remaining emulsified oil phase of Pickering emulsions prepared at various pH

Figure 6. Fraction of remaining emulsified oil phase of Pickering emulsions prepared at various pH levels (recorded after a 24 h rest period). levels (recorded after a 24 h rest period).

Immediately after formation, a Pickering emulsion “creams” by floating an emulsified oil phase Immediately after formation, a Pickering emulsion an emulsified oilthe phase on top of an excessive aqueous phase. This effect can be“creams” explainedby byfloating the different densities of on top of an and excessive aqueous phase. This effect can be explained At by this the point, different of the aqueous oil phases (a large size prevents droplet suspension). the densities formulation quality canoil be phases assessed(abylarge inspecting the aqueous phasesuspension). for the presence excessive aqueous and size prevents droplet Atofthis point,latex the particles. formulation Opalescence and turbidity would signify an inefficient particles. By adjusting theparticles. pH, quality can be assessed by inspecting the aqueous phaseuse for of thelatex presence of excessive latex shear rate,and andturbidity concentration of latex particles, it is possible to form clear (particle-free) aqueous Opalescence would signify an inefficient use of latex aparticles. By adjusting the pH, phase, that all particles areparticles, involved in stabilization. shear rate,indicating and concentration of latex it droplet is possible to form a clear (particle-free) aqueous For the synthesis of colloidosomes, latexes developed at pH 7.5 at 85 °C in the presence of 1 wt.% phase, indicating that all particles are involved in droplet stabilization. (based on monomer weight) inisurf PM-2-MAc were chosen. It was the assumption that while having For the synthesis of colloidosomes, latexes developed at pH 7.5 at 85 ◦ C in the presence of a smaller amount of carboxyl groups, these latexes exhibit sufficient surface activity to stabilize a 1 wt.% (based on monomer weight) inisurf PM-2-MAc were chosen. It was the assumption that while Pickering emulsion. having a After smaller amount of carboxyl emulsion, groups, these latexes exhibit stabilize formulating a Pickering polymerization was sufficient conductedsurface at 80 °Cactivity to yield to hollow a Pickering emulsion. colloidosomes decorated with peroxidized latex particles. Table 3 presents data on the properties of After formulating a Pickering emulsion, polymerization was conducted at 80 ◦ Ccharacteristics to yield hollow the synthesized colloidosomes. The obtained results indicate that several morphological colloidosomes decorated with peroxidized latex particles. Table 3 presents data on the properties of colloidosomes can be controlled by variations in oil phase compositions (e.g., monomer/HD ratio) of and the pH levels of the aqueous the synthesized colloidosomes. The phase. obtained results indicate that several morphological characteristics 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. 

Coatings 2016, 6, 52

11 of 14

of colloidosomes can be controlled by variations in oil phase compositions (e.g., monomer/HD ratio) and the pH levels of the aqueous phase. Table 3. Synthetic conditions and properties of colloidosomes based on PM-2-MAc.

Sample

pH

Oil Phase Composition (wt.%) St

DVB

HD

AIBN

Colloidosome Dimensions D (µm)

Shell (µm)

7.5 28 12 57 3 10 ± 8 1.7 ± 0.4 11 of 14 CS4 3.5 28 12 57 3 10 ± 4 1.3 ± 0.3 3. Synthetic of colloidosomes based CS3 Table4.5 28conditions 12and properties 57 3 15 ± 3 on PM-2-MAc. 1.5 ± 0.4 CS2 4.5 14 6 77 3 10 ± 2 0.8 ± 0.2 Oil Phase Composition (wt.%) Colloidosome Dimensions Sample pH St DVB HD AIBN D (m) Shell (m) At the same time, using CS1 the size 7.5 of28colloidosomes 12 57can be adjusted 3 10  8 the pH 1.7of the 0.4 aqueous phase and the homogenization parameters CS4 3.5 28(pulse12length,57sonication 3 intensity), 10  4 whereas 1.3 shell  0.3 thickness depends on the total initial concentration of monomers. SEM of different colloidosomes CS3 4.5 28 12 57Figure 73 shows 15  3 images1.5 0.4 CS2 4.5 14 6 77 3 10  2 0.8  0.2 (Table 3) synthesized using peroxidized latex particles. Coatings 2016, CS1 6, 52

Figure SEM photographsofofcolloidosomes colloidosomes synthesized synthesized from PM-2-MAc) latex Figure 7. 7. SEM photographs fromperoxidized peroxidized(inisurf (inisurf PM-2-MAc) latex particles: (A) CS2; (B) CS3; (C) CS1. particles: (A) CS2; (B) CS3; (C) CS1.

Although the surface charge of latex particles is essential for the electrostatic stabilization of 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 droplets in the a Pickering emulsion, it can also disruptive it is toosurfaces large. covered Notably,with unlike other samples, colloidosomes prepared at pH 7.5be did not featureifsmooth grafted samples, the colloidosomes pH 7.5 signs did not smooth surfaces with grafted latex particles. Their outerprepared surfacesatshowed of feature phase separation duringcovered synthesis and an latex particles. Their outer surfaces showed signs of phase separation during synthesis and an obvious obvious lack of latex particles, which is possibly due to an excessive particle charge at pH 7.5. lack of The latexsame particles, which was is possibly an excessive charge at pH 7.5.latex particles approach applieddue fortothe synthesis particle of colloidosomes from The same approach wasPM-1-MAc. applied for The the synthesis colloidosomes from latex particles synthesized synthesized with inisurf syntheticofconditions and characteristics of the formed with inisurf PM-1-MAc. The synthetic conditions and characteristics of the formed colloidosomes are colloidosomes are summarized in Table 4. summarized in Table 4. Table 4. Synthetic conditions 1 and properties of colloidosomes based on PM-1-MAc. Table 4. Synthetic conditions 1 and properties of colloidosomes based on PM-1-MAc. Monomer conc. in Total Colloidosome Monomer/DVB Hexadecane Oil Phase (mol/L) Monomer Monomer conc. in Polymer Ratio (mol/mol) conc. (mol/L) Total Colloidosome Monomer/DVB Hexadecane conc. (mol/L) St Phase BA(mol/L) DVB Oil Monomer Polymer Ratio (mol/mol) conc. (mol/L) polySt-BA-1 1.3 0.7 2.0 1/1 1.66 conc.4.0 (mol/L) St BA DVB polyBA-2 – 2.0 2.0 4.0 1/1 1.47 polySt-BA-1 1.3 0.7 2.0 4.0 1/1 1.66 polyBA-3 – 1.5 1.5 3.0 1/1 1.96 polyBA-2 – 2.0 2.0 4.0 1/1 1.47 polySt-4 1.0 4.0 3/1 1.76 polyBA-3 –3.0 1.5– 1.5 3.0 1/1 1.96 polySt-4 3.0 –– 1.0 4.0 3/1 1.76 polySt-5 2.0 2.0 4.0 1/1 1.67 polySt-5 2.0 –– 2.0 4.0 1/1 1.67 polySt-6 2.25 0.75 3.0 3/1 2.18 polySt-6 2.25 – 0.75 3.0 3/1 2.18 1

D (m) D (µm) 4–20 5–30 4–20 10–35 5–30 1.7–6 10–35 1.7–6 6–18 6–18 1–3 1–3

Notes: Ratio of aqueous:oil phase as 9:1; Pickering stabilizer concentration 0.36 wt.%; reaction time 150 min.; 1 Ratio of aqueous:oil phase as 9:1; Pickering stabilizer concentration 0.36 wt.%; reaction time 150 min.; Notes: temperature 80 °C. temperature 80 ◦ C.

As determined by SEM measurements, the diameter of the synthesized colloidosomes varied in the range of 1.3 m to 35 m depending on the synthetic conditions and compositions of the reactive mixture. Figure 8A 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].

Coatings 2016, 6, 52

12 of 14

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 8A 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 8B) clearly shows the presence of the polymer particles within the colloidosome wall. As evidenced by Figure 8C, the colloidosome walls Coatings an 2016, 6, 52 12 of 14 define 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 8C). The cross-linked polymer namely, the formation of the spongy walls of the colloidosomes (Figure 8C). The cross-linked polymer chains then lose their mobility, which leads to a decrease in their precipitability and an increase in 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 the wall thickness. Under certain conditions, the filling of the entire colloidosome cavity with the spongy polymer was observed (Figure 8D). On the other hand, the development of broken (or not spongy polymer was observed (Figure 8D). On the other hand, the development of broken (or not fully fully formed) colloidosomes was observed when the content of the monomers in the template formed) colloidosomes was observed when the content of the monomers in the template decreased. decreased. This indicates that the walls formed under these conditions are either not strong enough This indicates that the walls formed under these conditions are either not strong enough or that the or that the cross-linking degree is low. cross-linking degree is low.

Figure SEM photographs photographs of of colloidosomes colloidosomes synthesized synthesized from from peroxidized peroxidized (inisurf (inisurf PM-1-MAc) PM-1-MAc) latex latex Figure 8. 8. SEM particles: (A,B,C) polySt-4; (D) polySt-BA-1. particles: (A,B,C) polySt-4; (D) polySt-BA-1.

4. Conclusions 4. Conclusions Pickering Pickering emulsions emulsions were were first first synthesized synthesized using using new new Pickering Pickering stabilizers: stabilizers: peroxidized peroxidized latex latex particles with a grafted layer of polymeric inisurfs poly(2-tert-butylperoxy-2-methyl-5-hexen-3particles with a grafted layer of polymeric inisurfs poly(2-tert-butylperoxy-2-methyl-5-hexen-3-ineine-co-maleic acid)and andpoly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic poly[N-(tert-butylperoxymethyl)acrylamide]-co-maleic acid.acid. Covalently cross-linked co-maleic acid) Covalently crosscolloidosomes were synthesized on peroxidized particle basis. The formation of colloidosomes was linked colloidosomes were synthesized on peroxidized particle basis. The formation of colloidosomes achieved through the radical copolymerization of vinylofmonomers in a disperse phase of the Pickering was achieved through the radical copolymerization vinyl monomers in a disperse phase of the emulsion. The copolymerization was initiated both in a template bulk and from the surface of the Pickering emulsion. The copolymerization was initiated both in a template bulk and from the surface Pickering stabilizer. of the Pickering stabilizer. Peroxidized latex particles with variable amounts of of carboxyl and peroxide Peroxidized latex particles with variable amounts carboxyl and peroxide groups groups on on particle particle surfaces allow for the formation of Pickering emulsions and covalently cross-linked colloidosomes surfaces allow for the formation of Pickering emulsions and covalently cross-linked colloidosomes

with several different morphologies. It was found that the surface charge of the particles is essential for maintaining the stability of the droplets in a Pickering emulsion and thus for colloidosome formation. The developed synthetic technique allows the fabrication of carrier colloidosome capsules free of surfactant contamination that can potentially leak out during colloidosome application. Acknowledgments: Ihor Tarnavchyk acknowledges the financial support of the Fulbright Scholar Program.

Coatings 2016, 6, 52

13 of 14

with several different morphologies. It was found that the surface charge of the particles is essential for maintaining the stability of the droplets in a Pickering emulsion and thus for colloidosome formation. The developed synthetic technique allows the fabrication of carrier colloidosome capsules free of surfactant contamination that can potentially leak out during colloidosome application. Acknowledgments: Ihor Tarnavchyk acknowledges the financial support of the Fulbright Scholar Program. Author Contributions: Stanislav Voronov and Andriy Voronov conceived and designed the experiments; Nadiya Popadyuk, Andriy Popadyuk, and Ihor Tarnavchyk performed the experiments; Nadiya Popadyuk, Andriy Popadyuk, Ihor Tarnavchyk, Olha Budishevska, Ananiy Kohut, Stanislav Voronov and Andriy Voronov analyzed the data; Ananiy Kohut, Stanislav Voronov and Andriy Voronov wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

14. 15. 16. 17. 18. 19.

Truong, N.P.; Whittaker, M.R.; Anastasaki, A.; Haddleton, D.M.; Quinn, J.F.; Davis, T.P. Facile production of nanoaggregates with tuneable morphologies from thermoresponsive P(DEGMA-co-HPMA). Polym. Chem. 2016, 7, 430–440. [CrossRef] Truong, N.P.; Quinn, J.F.; Anastasaki, A.; Haddleton, D.M.; Whittaker, M.R.; Davis, T.P. Facile access to thermoresponsive filomicelles with tuneable cores. Chem. Commun. 2016, 52, 4497–4500. [CrossRef] [PubMed] Truong, N.P.; Dussert, M.V.; Whittaker, M.R.; Quinn, J.F.; Davis, T.P. Rapid synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerization. Polym. Chem. 2015, 6, 3865–3874. [CrossRef] Sakai, T. Surfactant-free emulsions. Curr. Opin. Colloid Interface Sci. 2008, 13, 228–235. [CrossRef] Binks, B.P. Particles as surfactants. Curr. Opin. Colloid Interface Sci. 2002, 7, 21–41. [CrossRef] Rossier-Miranda, F.J.; Schroen, C.G.P.H.; Boom, R.M. Colloidosomes: Versatile microcapsules in perspective. Colloids Surf. A 2009, 343, 43–49. [CrossRef] Saraf, S.; Rathi, R.; Kaur, C.C.D.; Saraf, S. Colloidosomes and advanced vesicular system in drug delivery. Asian J. Sci. Res. 2001, 4, 1–15. [CrossRef] Pickering, S.U. Emulsions. J. Chem. Soc. 1907, 91, 2001–2021. [CrossRef] Ramsden, W. Separation of solids in the surface-layers of solutions and ‘suspensions’. Proc. R. Soc. Lond. 1903, 72, 477–486. Velev, O.D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates. 1. Microstructured hollow spheres. Langmuir 1996, 12, 2374–2384. [CrossRef] Velev, O.D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates. 2. Ball-like and composite aggregates. Langmuir 1996, 12, 2385–2391. [CrossRef] Dismore, A.D.; Hsu, M.F.; Nicolaides, M.G.; Marquez, M.; Bausch, A.R.; Weitz, D.A. Colloidosomes: selectively permeable capsules, composed of colloidal particles. Science 2002, 298, 1006–1009. [CrossRef] [PubMed] Taubman, A.B.; Koretskiy, A.F. Stabilization of emulsions with solid surfactants and coagulative structure formation. In Achievements in Colloidal Chemistry; Rebinder, P.A., Fuks, G.I., Eds.; Nauka: Moscow, Russia, 1976; pp. 255–262. Kim, S.H.; Yi, G.R.; Kim, K.H.; Yang, S.M. Photocurable Pickering emulsions for colloidal particles with structural complexity. Langmuir 2008, 24, 2365–2371. [CrossRef] [PubMed] Noble, P.F.; Cayre, O.J.; Alargova, R.G.; Velev, O.D.; Paunov, V.N. Fabrication of “Hairy” Colloidosomes with Shells of Polymeric Microrods. J. Am. Chem. Soc. 2004, 126, 8092–8093. [CrossRef] [PubMed] Lawrence, D.B.; Cali, T.; Hu, Z.; Marquez, M.; Dinsmore, A.D. Temperature-responsive semipermeable capsules composed of colloidal microgel spheres. Langmuir 2007, 23, 395–398. [CrossRef] [PubMed] Kim, J.W.; Frenandes-Nieves, A.; Dan, N.; Utada, A.S.; Marquez, M.; Weitz, D.A. Colloidal assembly route for responsive colloidosomes with tunable permeability. Nano Lett. 2007, 7, 2876–2880. [CrossRef] [PubMed] Panhuis, M.; Paunov, V.N. Assembling carbon nanotubosomes using an emulsion-inversion technique. Chem. Commun. 2005, 13, 1726–1728. [CrossRef] [PubMed] Yin, D.; Zhang, Q.; Zhang, H.; Yin, C. Fabrication of covalently-bonded polystyrene/SiO2 composites by Pickering emulsion polymerization. J. Polym. Res. 2010, 17, 689–696. [CrossRef]

Coatings 2016, 6, 52

20. 21. 22. 23. 24.

25. 26. 27. 28. 29.

30. 31.

32.

33. 34. 35.

14 of 14

Chen, T.; Colver, P.J.; Bon, S.A.F. Organic-inorganic hybrid hollow spheres prepared from TiO2 -stabilized Pickering emulsion polymerization. Adv. Mater. 2007, 19, 2286–2289. [CrossRef] Thompson, K.L.; Arnes, S.P.; Howse, J.R.; Ebbens, S.; Ahmad, I.; Zaidi, J.H.; York, D.W.; Burdis, J.A. Covalently cross-linked colloidosomes. Macromolecules 2010, 43, 10466–10474. [CrossRef] Wang, H.; Zhu, X.; Tsarkova, L.; Pich, A.; Möller, M. All-silica colloidosomes with a particle-bilayer shell. ACS Nano 2011, 5, 3937–3942. [CrossRef] [PubMed] San Miguel, A.; Scrimgeour, J.; Curtis, J.E.; Sven, H. Smart colloidosomes with dissolution trigger. Soft Matter 2010, 6, 3163–3166. [CrossRef] Zhang, K.; Wu, W.; Guo, K.; Chen, J.F.; Zhang, P.Y. Magnetic polymer enhanced hybrid capsules prepared from a novel Pickering emulsion polymerization and their application in controlled drug release. Colloids Surf. A 2009, 349, 110–116. [CrossRef] Shilpi, S.; Jain, A.; Gupta, Y.; Jain, S.K. Colloidosomes: An emerging vesicular system in drug delivery. Crit. Rev. Ther. Drug Carrier Syst. 2007, 24, 361–391. [CrossRef] [PubMed] Porta, F.; Kros, A. Colloidosomes as single implantable beads for the in vivo delivery of hydrophobic drugs. Part. Part. Syst. Char. 2013, 30, 606–613. [CrossRef] Zhao, Y.; Pan, Y.; Nitin, N.; Tikekar, R.V. Enhanced stability of curcumin in colloidosomes stabilized by silica aggregates. LWT-Food Sci. Technol. 2014, 58, 667–671. [CrossRef] Voronov, S.; Kohut, A.; Tarnavchyk, I.; Voronov, A. Advances in reactive polymeric surfactants for interface modification. Curr. Opin. Colloid Interface Sci. 2014, 19, 95–121. [CrossRef] Popadyuk, A.; Tarnavchyk, I.; Popadyuk, N.; Kohut, A.; Samaryk, V.; Voronov, S.; Voronov, A. A novel copolymer of N-[(tert-Butylperoxy)methyl]acrylamide and maleic anhydride for use as a reactive surfactant in emulsion polymerization. React. Funct. Polym. 2013, 73, 1290–1298. [CrossRef] Popadyuk, A.; Tarnavchyk, I.; Popadyuk, N.; Kohut, A.; Samaryk, V.; Voronov, S.; Voronov, A. Reinforcing latex coatings with reactive latex particles. Prog. Org. Coat. 2014, 77, 2123–2132. [CrossRef] Voronov, S.; Tokarev, V.; Seredyuk, V.; Bednarska, O.; Oduola, K.; Adler, H.; Puschke, C.; Pich, A. Polyperoxidic surfactants for interface modification and compatibilization of polymer colloidal systems. II. Design of compatibilizing layers. J. Appl. Polym. Sci. 2000, 76, 1228–1239. [CrossRef] Adler, H.-J.; Pich, A.; Henke, A.; Puschke, C.; Voronov, S. New Core-Shell Dispersions with Reactive Groups. In Polymer Colloids. Science and Technology of Latex Systems; Daniels, S., Sudol, E.D., El-Aasser, M.S., Eds.; ACS Symp. Ser.: Washington, DC, USA, 2001; pp. 276–292. Voronov, S.; Tokarev, V.; Petrovska, G. Heterofunctional Polyperoxides. Theoretical Basis of Their Synthesis and Application in Compounds; State University “Lvivska Polytechnika”: Lviv, Ukraine, 1994. Voronov, S.; Tokarev, V.; Datsyuk, V.; Kozar, M. Peroxidation of the interface of colloidal systems as new possibilities for design of compounds. Prog. Colloid Polym. Sci. 1996, 101, 189–193. Tokarev, V.S.; Voronov, S.A.; Adler, H.; Datsyuk, V.V.; Pich, A.Z.; Shevchuk, O.M. Application of peroxide macroinitiators in core-shell technology for the coating improvement. Macromol. Symp. 2002, 187, 155–164. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).