Article pubs.acs.org/Macromolecules
Microencapsulation of Active Ingredients Using PDMS as Shell Material Roberto F. A. Teixeira,†,§ Otto van den Berg,†,§ Le-Thu T. Nguyen,†,§,∥ Krisztina Fehér,‡ and Filip E. Du Prez*,†,§ †
Department of Organic and Macromolecular Chemistry, Polymer Chemistry Research Group, and ‡Department of Organic and Macromolecular Chemistry, NMR & Structure Analysis Unit Group, Ghent University, Krijgslaan 281 S4bis, 9000 Gent, Belgium § SIM vzw, Technologiepark 935, B-9052 Zwijnaarde, Belgium ∥ Department of Polymer Materials, Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam National University, Ly Thuong Kiet 268, District 10, Ho Chi Minh City, Vietnam S Supporting Information *
ABSTRACT: We report an efficient and adaptable method for the microencapsulation of active ingredients by a polydimethylsiloxane (PDMS) shell material. The core−shell microcapsules were obtained by phase separation between the core component and the PDMS shell components after repartitioning of the common solvent THF between the PDMS/core material phase and the water phase. For the shell components, two commercially available functional PDMS polymers containing thiol and vinyl side groups were used. Photo-cross-linking in the presence of 2,2-dimethoxy-2-phenylacetophenone (DMPA) by thiol−ene radical addition was used to form a PDMS-thioether cross-linked shell. Variation of the PDMS component thiol to ene ratio resulted in different functionalities on the microcapsules surface and in the bulk, which was analyzed by attenuated total-reflection infrared spectroscopy (ATR-IR) and high-resolution magic-angle NMR-spectroscopy (HR-MAS NMR). Organically modified silica particles were mixed into the PDMS shell, resulting in better mechanical properties of the shell and control over the shell permeability, as measured on the one hand by tensile testing of representative PDMS bulk samples of identical composition as the actual shell material and on the other hand by leaching experiments of the core compounds, such as a tetrathiol and the UVabsorber octocrylene, followed by UV−vis.
■
increasing research field of self-healing materials in which damage-induced cracking is the healing trigger.13−15 Different examples of employing silicon-based organic polymers on microcapsules, being present either in the core or in the shell, have been described.16,17 Vincent and coworkers16 reported the preparation of modified silica-shell/ silicon-oil-core microcapsules. The core templates were prepared by soap free condensation polymerization of diethoxydimethylsilane (DEODMS) in water, forming a silicone oil/water emulsion, while the shells were formed through the condensation of tetraethoxysilane (TEOS) and DEODMS onto the silicone oil droplets. Wang and collaborators17 reported the fabrication of core−shell particles, also containing silicone in the core. Poly(N-isopropylacrylamide)/poly(dimethylsiloxane)-graf t-polyacrylates (PNIPA/ PDMS-g-PAA) core−shell composites in supercritical carbon dioxide were prepared in order to obtain smart microgels with pH-sensitive shells.
INTRODUCTION
Polydimethylsiloxane (PDMS) materials are the most common silicon-based organic polymers and have attracted much attention due to their unusual properties such as water repellency, high flexibility, low glass transition temperature, low surface energy, and biocompatibility.1−4 Because of their excellent properties, such materials are used in a wide range of applications including medical and pharmaceutical5,6 (e.g., contact lenses, parts of medical devices, drug delivery), cosmetics7 (e.g., present in shampoos), household, food, lubricating oils, electronics, etc. Organosiloxane materials have also been used in the synthesis of core−shell particles (microcapsules).5,8 Microcapsules represent reservoirs where an active ingredient (core) is surrounded or coated by a polymeric material or continuous film (shell) ranging in size from a few hundreds of nanometers to micrometers. Microencapsulation provides environmental protection of the active ingredient and can also provide control and triggered release of the core. Therefore, a vast number of applications9 using microcapsules can be found in the literature, including cosmetics,10 food,11 and drug delivery,12 or in the © XXXX American Chemical Society
Received: September 12, 2014 Revised: November 8, 2014
A
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
thiol side chain functional PDMS with vinyl side chain functional PDMS.
The deposition of organic polymeric materials onto the shell of core−shell particles can be employed by making use of a template core particle18 or by interfacial polymerization.19−22 The Chen group3 reported the preparation of a poly(methyl methacrylate) (PMMA) core and polydimethylsiloxane (PDMS) shell by successive seeding polymerization. Ding et al.23 have shown the successful microencapsulation of phase change materials using silica as shell material in a one-step procedure. Methyltrimethoxysilane (MTMS) and 3-aminopropyltrimethoxysilane were used as silica precursors, and no surfactants or dispersants were needed for stabilization due to the self-stabilization of the amine groups. Furthermore, different amounts of silica also allowed tuning of size and dispersity of the capsules. Gu et al.24 prepared core−shell particles by coprecipitation of Fe 2+ and Fe 3+ with NH4OH (core), with subsequent coating of an amino silane (shell) onto the surface of the magnetite nanoparticles. In order to improve the polymer shell physical properties, the use of inorganics such as silicates, zinc oxide particles, titanium, or magnesium oxide particles into the shell of core−shell structures have attracted much attention. Organosilicates have been incorporated in core−shell particles; e.g., Vincent et al.25 prepared core−shell particles by precipitation of a thin layer of silica from a supersaturated sodium silicate solution into crosslinked polydimethylsiloxane (PDMS) microgel core particles. The continuous growth of the shell and shell thickness was controlled by the addition of tetraethoxysilane with very specific ethanol and ammonia concentrations. Raston et al.26 showed an efficient method for the encapsulation and controlled release of nutraceuticals using mesoporous silica as the shell material. Other approaches to produce silica shelled microcapsules have also being reported, such as Pickering stabilization, reported by the Armes27,28 and the Bon group29−31 or emulsion templating using tetraethyl orthosilicate in acidic or basic conditions.32−34 To our knowledge, despite the numerous examples of core− shell particles using siloxane-based compounds, PDMS has not being used for the microencapsulation of reactive ingredients, such as the ones used in self-healing polymer applications.35−39 In this field, due to the high reactivity of the core materials, the type of shell is usually limited to melamine formaldehyde (MF), urea formaldehyde (UF), or polyurethane (PU) material. Sottos and co-workers36 reported the microencapsulation of isophorone diisocyanate (IPDI) via the interfacial polymerization of toluene diisocyanate-based urethane prepolymer with 1,4-butanediol in an oil-in-water emulsion obtaining a polyurethane type of shell. The same group has also reported a similar method for the microencapsulation of reactive amines by interfacial polymerization of an isocyanate and an amine stabilized by an inverse Pickering emulsion using nanoclay as stabilizer.37 Another example comes from Zhang and collaborators,35 who reported the microencapsulation of polythiol pentaerythritol tetrakis(3-mercaptopropionate) by in situ polymerization with melamine formaldehyde in the shell. Herein, we present an innovative method for the microencapsulation of hydrophobic active ingredients by a polydimethylsiloxane (PDMS) shell material, which allows straightforward functionalization of the microcapsule shell surfaces, control over the permeability, and physical strength of the shells. Moreover, it is a fast and broadly applicable method, which allows the encapsulation of reactive chemicals such as thiols, amines, and isocyanates. The PDMS shell is cured by application of a fast thiol−ene UV-curing reaction of
■
EXPERIMENTAL SECTION
Materials. 2,2-Dimethoxy-2-phenylacetophenone (DMPA), tetrahydrofuran (THF), poly(styrene-alt-maleic anhydride) partial methyl ester (Mw 350 000) (PSMA), sodium carbonate, methyl benzoate, mineral oil, hexamethylene diisocyanate, and octocrylene were purchased from Sigma-Aldrich at 99% or greater purity and used as received. Hexamethylene diisocyanate isocyanurate trimer (HDItrimer) (Tolonate HDT-LV) was provided by Perstorp. A bis-primary amine (Primamine 1074 liquid) was kindly supplied by Croda. Pentaerythritol tetra(3-mercaptopropionate) (95% purity) (tetrathiol) healing agent was obtained from Sigma-Aldrich. (4−6% Mercaptopropyl)methylsiloxane)−dimethylsiloxane copolymer (viscosity 75−150 cSt) (thiol PDMS) and vinylmethylsiloxane− dimethylsiloxane trimethylsiloxy-terminated copolymer (4−5 mol % vinylmethylsiloxane, viscosity 800−1200 cSt) (vinyl PDMS) were purchased from ABCR and used as received. Aerosil R 8200 fumed silica was purchased from Evonik. Synthesis of Microcapsules. All microcapsules, with different vinyl PDMS:thiol PDMS ratios and different quantities of hydrophobic silica particles, were synthesized at the same emulsification rate and UV light exposure time. A typical procedure is as follows: to a doubleneck round bottom flask (250 mL), PSMA solution (150 mL, 0.15 wt % solution using sodium carbonate to increase the pH to 7 and subsequently mixing overnight) was added and purged with nitrogen for 1 min. The solution was then stirred at 270 rpm with the help of an adapted overhead mechanical agitator (water phase). Separately, 8.05 g of thiol PDMS, 10.08 g of vinyl PDMS, and 36.00 g of core material (mineral oil, tetrathiol, tetrathiol with compatibilizer, and a mixture of methyl benzoate or octocrylene) were mixed together, and then 30 g of THF was added in order to obtain a homogeneous solution. 121 mg (0.472 mmol or 0.2 mol % of PDMS shell components) of DMPA photoinitiator was then added to the mixture, and this solution was purged with nitrogen over 1 min (oil phase). In the absence of oxygen, the obtained solution was dropwise added to the PSMA solution under agitation. Finally, the obtained emulsion was exposed to UV light at an approximate light intensity (365 nm) of 12 mW/cm2 in a Metalight Classic irradiation chamber for 5 min. Core Content Analysis. The core content of all microcapsules was analyzed by Soxhlet extraction using acetone as solvent. The core content of the PDMS microcapsules containing tetrathiol in the core was also analyzed by NMR and DSC (see Supporting Information). Scanning Electron Microscope (SEM). SEM images were performed on a TM-3000 Hitachi table top microscope, using Leit adhesive Carbon Tabs 12 mm from Agar Scientific. HR-MAS NMR. After removal of the core content of microcapsules using Soxhlet extraction, the obtained hollow microcapsules were ground and put in a 4 mm rotor (80 μL). Next, solvent (CDCl3) was added to allow the material to swell, which removes most of the dipolar line broadening typically associated with the solid state, while residual line broadening caused by susceptibility differences can be handled by spinning at the magic angle. All 1H NMR spectra were recorded on a Bruker Avance II 700 spectrometer (700.13 MHz) using a HR-MAS probe equipped with a 1H, 13C, 119Sn and a gradient channel. Samples were spun at a rate of 6 kHz. To characterize the gels, 1D 1H spectra were recorded. All spectra were measured with an acquisition time of 1.136 s, in which 32 768 fid points were obtained, leading to a spectral width of 20.6 ppm. For qualitative analysis, 8 transients were summed up with a recycle delay of 2 s. For quantification, 32 scans were used with 30 s recycling delay to guarantee full relaxation of the signal. Tensile Testing. Tensile testing was performed on a Tinius-Olsen H10KT tensile tester equipped with a 100 N load cell, using cylindrical specimen with an effective gage length of 25 mm and a diameter of 4.5 mm. The tensile tests were run at a speed of 10 mm/min. Test specimens were prepared by filling 1 mL polypropylene syringes with photocurable formulation and photocuring them at an approximate B
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Figure 1. Reaction scheme for the formation of a PDMS cross-linked shell by thiol−ene chemistry.40
Figure 2. (A) Optical microscope picture. (B) SEM picture of PDMS rubber shell microcapsules containing tetrathiol in the core. (C) SEM picture of a broken microcapsule containing tetrathiol in the core. light intensity (365 nm) of 12 mW/cm2 in a Metalight Classic irradiation chamber for 5 min, resulting in reproducible cylindrical specimens. UV−Vis. A weight amount of microcapsules was placed in a quartz cuvette containing 2.5 mL of methyl benzoate. Under continuous stirring conditions, the UV−vis spectra were recorded over time using a Carry 300 Bio UV−vis spectrophotometer from Variant. Each sample was analyzed 3 times, and during the absorbance measurements the microcapsules were floating on top of the cuvette and therefore not influencing the absorbance value. ATR-FTIR. The ATR-FTIR spectra of the microcapsules were obtained using a PerkinElmer Spectrum 1000 FTIR infrared spectrometer, using 16 scans per sample.
the material that is to be dispersed in water allows simultaneous dispersion of both the core-component and the shell material. Addition of the surfactant PSMA to the water phase prevents coalescence and the formation of larger size capsules. Diffusion of THF from the still homogeneous dispersed THF/PDMS/ core droplets into the water phase leads to phase separation of the core component and the PDMS shell components, resulting in core−shell droplets with a liquid (oil) core and a liquid silicone shell. The presence of THF is a key issue in this process. Indeed, the absence of THF would not allow the formation of homogeneous droplets (once the PDMS and the hydrophobic core are not miscible), resulting in separate droplets of PDMS and oil core. By curing the PDMS oligomers under UV light for 5 min, PDMS-shell/oil-core microcapsules are obtained in a straightforward way (see Figure 2). Figure 2C shows a broken PDMS microcapsule with a defined shell consisting of PDMS cross-linked material and an inner core (black color inside), suggesting a core−shell structure. This encapsulation method can be successfully applied for hydrophobic compounds that have no affinity for water and are insoluble in PDMS. Hydrophobicity of the compound ensures that PDMS will be on the outside of the droplets. On the other hand, encapsulation of polar protic components is not possible. In such cases, solid silicone particles are isolated and do not contain any liquid core. In all cases, the obtained microcapsules
■
RESULTS AND DISCUSSION The synthesis of polydimethylsiloxane (PDMS) shell/oil core microcapsules is performed by making use of two available PDMS oligomers with thiol and vinyl side groups. In the presence of a radical photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA) and under UV exposure, a thiol−ene reaction is initiated and a cross-linked thiol−ene network is formed (see Figure 1). The core−shell particle formation is based on phase separation of the material to be encapsulated and the PDMS shell material. Addition of THF, a common solvent for both the core material and the PDMS (both vinyl and thiol PDMS), to C
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
had a diameter below ca. 80 μm as observed by optical microscope and SEM. The cross-linking degree of the PDMS shell network depends on the number of functional groups located on the PDMS oligomers and also on the molar ratios between the “thiol PDMS” and “vinyl PDMS” components. Muller et al.41 have reported an excellent study of the photochemical thiol− ene polyaddition kinetics of different silicone-based telechelic ene materials (vinyl, allyl, norbornyl, and many others). This kinetic study showed that the telechelic vinyl-substituted PDMS has a high reactivity and conversion in thiol−ene PDMS networks, while the norbornyl-substituted PDMS shows high reactivity but poor conversion. This remark agrees with observations of our group that demonstrates that radicalmediated thiol−ene reactions between thiol-functional polymers and ene-functional polymers are commonly not very efficient.42 Though, in a recent publication we have shown the feasibility of the reaction between thiol-functionalized polydimethylsiloxane (PDMS), telechelic vinyl-functionalized PDMS, and a dithiol PDMS chain-extender to efficiently form gel-like rubbery materials.43 In the present article, we varied the ratio of thiol to vinyl PDMS to obtain vinyl or thiol functionalities on the microcapsules shell surface, by making use of an excess of one of the components. Organic silica particles were added to the shell material in order to achieve better mechanical and physical properties and consequently to gain control over the permeability of the shell. The suggested microencapsulation strategy, using phase separation in combination with PDMS, is applicable to a vast number of hydrophobic organic materials, even to very reactive compounds such as the ones normally used in self-healing applications.35−39,44 For this study, we have encapsulated mineral oil, a mixture of methyl benzoate with 2-ethylhexyl 2cyano-3,3-diphenyl acrylate (octocrylene, UV-absorber), and a reactive self-healing agent (pentaerythritol tetra(3-mercaptopropionate) (tetrathiol)) with core contents around 40 wt %. Additionally, without optimization and only to demonstrate the versatility of the present methodology, we encapsulated hexamethylene diisocyanate isocyanurate trimer (HDI-trimer) and a bis-primary amine (Priamine 1074 liquid) with core contents around 10−15 wt %. All core contents were measured by Soxhlet extraction of dried powder samples using acetone as extraction solvent at 80 °C for 2 days. In the case of tetrathiol, NMR analysis of the extracted core was performed, showing the presence of free active thiol groups, similar to the pure NMR spectra (see Figure S1). Moreover, a Soxhlet extraction test after 1 month, performed on microcapsules stored in a closed vessel at room temperature, only showed a decrease of 5 wt % in the tetrathiol content. The encapsulation of a polythiol, such as the tetrathiol compound, is particularly difficult. As mentioned in the Introduction, the only reported encapsulation method for this compound makes use of a highly cross-linked melamine formaldehyde type of shell.35,44 Likewise, as mentioned above, we have been able to successfully encapsulate this compound, using the present approach at 0.15 wt % of PSMA content and 2:1 core:shell ratio (see Supporting Information). Stable microcapsules, dried as a powder and stored for numerous months, containing up to 30 wt % of pure tetrathiol were obtained. We further improved this microencapsulation efficiency up to 40 wt % of core content, making use of a custom-made compatibilizer between the tetrathiol active
ingredient and the PDMS shell. For that, we modified a small part of the tetrathiol core (in large excess) by linking one short PDMS chain (acryloxypropyl PDMS) to one free thiol group by radical thiol−ene addition chemistry (Figure 3 and synthesis
Figure 3. Synthesis of compatibilizer for tetrathiol/PDMS; synthesis of a modified tetrathiol core.
details in the Supporting Information). This core modification increased the affinity between the PDMS shell and the core by reducing its surface tension, as determined by the comparison of the contact angles between a PDMS shell film and a droplet of pure tetrathiol and a PDMS shell film with a modified tetrathiol core droplet. The contact angles were 67° and 24°, respectively (mean average values of the first 10 s, see Table S1 of Supporting Information), confirming the much better affinity of the modified tetrathiol core. The stability, strength, functionality, and permeability of microcapsules are important factors that are governing the end use application of the microcapsules. In self-healing applications, for example, the microcapsules are required to be robust and stable for long periods of time without any leakage. On the other hand, in cosmetics and pharmaceuticals, there is a need for controlled release of the active ingredients over time. For such applications, a more permeable shell is required. In the present work, as a result of the PDMS nature being cross-linked to a low extent and having a low Tg, we expected our microcapsules to be less robust than the classical stiff, highly cross-linked melamine formaldehyde and polyurethane-based types of shells.38,39 Nevertheless, the physical strength of our PDMS microcapsule shells can be improved by the addition of hydrophobized silica nanoparticles. Tensile testing of representative PDMS bulk samples of identical composition as the actual shell material, containing 0, 5, 10, 15, 20, and 25 wt % of hydrophobized silica, showed a steady increase of the Emodulus with silica content from 1 MPa for the pure PDMS material to 2.5 MPa for the PDMS material containing 25 wt % of hydrophobized silica (Figure 4). Addition of more than 25% of hydrophobized silica resulted in a thixotropic, highly viscous formulation that was impossible to process. In relation to the microcapsule diameter, we did not observe a significant variation when silica nanoparticles are used. Besides the use of silica nanoparticles to vary the physical properties of the PDMS shells, it is expected that varying the thiol to vinyl PDMS ratio will also influence the E-modulus of the microcapsule shells as a result of changing cross-link density. To find out the maximum E-modulus, we prepared several cross-linked films, varying the thiol and ene molar ratios D
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Figure 6. Overall absorbance measurements as a function of time of PDMS microcapsules containing UV-active octocrylene in the core, immersed in a cuvette under stirring and with methyl benzoate as solvent. Three samples were measured, containing 0, 5, and 10 wt % of silica nanoparticles in the PDMS shells.
Figure 4. E-moduli of the PDMS shell material as a function of hydrophobized silica content. The error bars represent the 95% confidence interval (three replicates).
of the two PDMS shell components. The highest E-modulus was obtained for a thiol to ene molar ratio of approximately 1 as determined by NMR (Figure 5).
This indicates that the use of silica nanoparticles plays an important role not only in obtaining microcapsules with improved mechanical properties but also in gaining control over the shell permeability. We also found out that the release profile of the octocrylene sun protector follows the same path as the logistic dose response mathematical equation, which is usually referred to the drug delivery dose response of an individual:46,47 ABS = a +
b 1 + (t /c)d
in which ABS represents the absorbance; a, b, c, and d are constants; and t is time. R2 values of 0.994, 0.999, and 0.994 were obtained for respectively 0, 5, and 10 wt % of silica nanoparticles into the PDMS shells. Functionalities of Microcapsules Shell Surface. It was shown that by varying the ratio of the two PDMS shell components, the elastic modulus of the microcapsules can be modified. Additionally, by changing the thiol to ene ratio, it is expected that free thiol or vinyl groups will be present on the surface of the microcapsules when an off-stoichiometic ratio is applied. In other words, a straightforward functionalization of the shell surface is possible, and postmodification reactions can be easily applied, which can have a significant impact on the compatibility with applied surrounding media. To prove this concept, we have encapsulated mineral oil (a nonreactive core component), varying the two PDMS shell components with a molar ratio of thiol to ene of 0.5 and 3, respectively. After the microencapsulation, all mineral oil was removed by Soxhlet extraction with acetone and both microcapsules were analyzed by HR-MAS NMR. The obtained spectra, shown in Figure S4 (Supporting Information), clearly evidence the presence of vinyl bonds at 5.85 ppm for the microcapsules with high molar ratios of vinyl PDMS while this is not observed for the microcapsules with high content of thiol PDMS, in full agreement with the experiments. Moreover, a closer look at the HR-MAS spectrum of the sample with excess of thiol groups (between −0.4 and 2.8 ppm) shown in Figure 7 confirms the presence of free thiol groups, comparable to what we recently observed.43 Both α
Figure 5. Normalized NMR integral of the vinyl group signal of thiol− ene PDMS networks as a function of the molar thiol to ene ratio. The normalization was done with respect to the methyl silyl signal of PDMS. Triangles are the data points.
In addition to the improvement of the mechanical properties of the PDMS shells, the use of silica nanoparticles could also decrease the transport of species across the shell, depending on the nature of the core species and the medium. To further investigate how these nanofillers influence the permeability, a series of microcapsules containing a mixture of methyl benzoate and octocrylene (Mw: 361.5) in the core were prepared. Octocrylene is a UV-absorber used in sunscreen lotions45 while methyl benzoate was used as a diluting solvent. For measuring the permeability of the PDMS microcapsules, we immersed them in a UV−vis cuvette, containing 2.5 mL of methyl benzoate. Then, by osmosis, the UV-active octocrylene should diffuse from the microcapsule core toward the cuvette with methyl benzoate. The increase of absorbance in the cuvette was measured over time. Figure 6 shows the UV−vis measurement data, from which it can be confirmed that an increase of the hydrophobized silica nanoparticles content of the PDMS microcapsule shells slows down the diffusion of octocrylene. E
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
groups, hexamethylene diisocyanate should not show any reactivity; hence, a thio−urethane bond and urea formation are not expected after the washing procedure. Figure 8 shows the ATR-FTIR spectra obtained for both types of microcapsules before (black line) and after (red line) the reaction with hexamethylene diisocyanate. The peak at 1730 cm−1 only appears on the microcapsules containing “free thiols”, suggesting a CO stretch bond attributed to the formation of urea and a thio−urethane. These results are in agreement with recent investigations of our group,50,51 and the overlap of the two peaks in the same region can explain the relative broad peak obtained.
■
CONCLUSIONS In conclusion, an efficient and versatile method to produce PDMS-shell/oil-core particles using two PDMS shell components, one having free thiol side groups and the other having free vinyl side groups, was reported. The core−shell particle synthesis is based on the phase separation of the active oil-core and PDMS-shell components mixed together in THF. In the presence of a water containing surfactant, THF diffuses to the water, and phase separation between the PDMS and oil core occurs within a stabilized droplet. In the presence of DMPA initiator and UV exposure, a thiol−ene cross-linking reaction occurs between the two PDMS oligomers forming a PDMS shell network. Within this method, different hydrophobic ingredients were successfully encapsulated, such as mineral oil, a mixture of methyl benzoate with 2-ethylhexyl 2-cyano-3,3diphenyl acrylate, and a reactive self-healing agent (tetrathiol). It was demonstrated that variations of the thiol to ene molar ratios of the PDMS components have an impact on the elastic modulus of the thiol−ene networks, where the highest Emodulus was obtained for a thiol−ene molar ratio of approximately 1. In addition, the organic silica particles can be mixed within the PDMS shell network, resulting in better physical properties (higher E-modulus) of the shell and control over the shell permeability.
Figure 7. Selected region of a HR-MAS 1H NMR spectrum of thiol− ene PDMS microcapsules, prepared with excess of “thiol PDMS”, swollen in deuterated chloroform.
methylene-silyl signals (labeled as γ and β′) are present and are resolved from other signals related to the α thiomethylene signals (thiol and thioether, labeled as α and α′), the β thiomethylene (labeled as β, overlap with the water signal), and SH signal (overlap with a signal from a minor impurity, most presumably due to some traces of mineral oil). Furthermore, the presence of free thiol groups on the surface of the microcapsules was also confirmed by ATR-FTIR analysis. For that, we have reacted both types of microcapsules with hexamethylene diisocyanate in the presence of triethylamine, acting as catalyst48,49 and using chloroform as solvent, followed by water and acetone washing to remove all the unreacted compounds. Taking into account that free thiols can react with isocyanates in a 1:1 ratio, a diisocyanate such as hexamethylene diisocyanate can react with the free thiols of the PDMS shell bearing free isocyanate groups, which can be converted into urea groups upon their reaction with water in the medium (during the washing). Additionally, a thio−urethane bond can be formed from the reaction between the isocyanate and thiol groups. In the case of microcapsules enriched with free vinyl
■
ASSOCIATED CONTENT
S Supporting Information *
Information of tuning the core:shell ratio and surfactant concentrations; procedure for the synthesis of the “tetrathiol core with compatibilizer”; contact angle measurements; graph
Figure 8. Left: microcapsules synthesized with excess of “thiol PDMS” before (black) and after (red) reaction with hexamethylene diisocyanate. Right: microcapsules synthesized with excess of “vinyl PDMS” before (black) and after (red) reaction with hexamethylene diisocyanate. F
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
(29) Teixeira, R. F. A.; McKenzie, H. S.; Boyd, A. A.; Bon, S. A. F. Macromolecules 2011, 44, 7415−7422. (30) Colard, C. A. L.; Teixeira, R. F. A.; Bon, S. A. F. Langmuir 2010, 26, 7915−7921. (31) Colver, P. J.; Colard, C. A. L.; Bon, S. A. F. J. Am. Chem. Soc. 2008, 130, 16850−16851. (32) Song, L.; Ge, X.; Wang, M.; Zhang, Z. J. Non-Cryst. Solids 2006, 352, 2230−2235. (33) Zoldesi, C. I.; van Walree, C. A.; Imhof, A. Langmuir 2006, 22, 4343−4352. (34) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768−771. (35) Yuan, Y. C.; Rong, M. Z.; Zhang, M. Q. Polymer 2008, 49, 2531−2541. (36) Yang, J.; Keller, M. W.; Moore, J. S.; White, S. R.; Sottos, N. R. Macromolecules 2008, 41, 9650−9655. (37) McIlroy, D. A.; Blaiszik, B. J.; Caruso, M. M.; White, S. R.; Moore, J. S.; Sottos, N. R. Macromolecules 2010, 43, 1855−1859. (38) Billiet, S.; Hillewaere, X. K. D.; Teixeira, R. F. A.; Du Prez, F. E. Macromol. Rapid Commun. 2013, 34, 290−309. (39) Teixeira, R. F. A.; Hillewaere, X. K. D.; Billiet, S.; Du Prez, F. E. Chemistry of crosslinking processes for self-healing polymers. In SelfHealing Polymers: from Principles to Applications; Wiley-VCH: Weinheim, 2013; ISBN: 978-3-527-33439-1. (40) Cramer, N. B.; Davies, T.; O’Brien, A. K.; Bowman, C. N. Macromolecules 2003, 36, 4631−4636. (41) Müller, U.; Kunze, A.; Herzig, C.; Weis, J. J. Macromol. Sci., Part A: Pure Appl. Chem. 1996, 33, 439−457. (42) Koo, S. P. S.; Stamenović, M. M.; Prasath, R. A.; Inglis, A. J.; Du Prez, F. E.; Barner-Kowollik, C.; Van Camp, W.; Junkers, T. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1699−1713. (43) van den Berg, O.; Nguyen, L.-T. T.; Teixeira, R. F. A.; Goethals, F.; Ö zdilek, C.; Berghmans, S.; Du Prez, F. E. Macromolecules 2014, 47, 1292−1300. (44) Hillewaere, X. K. D.; Teixeira, R. F. A.; Nguyen, L.-T. T.; Ramos, J. A.; Rahier, H.; Du Prez, F. E. Adv. Funct. Mater. 2014, 24, 5575−5583. (45) do Nascimento, D. F.; Silva, A. C.; Mansur, C. R.; Presgrave, R. F.; Alves, E. N.; Silva, R. S.; Ricci-Junior, E.; de Freitas, Z. M.; dos Santos, E. P. J. Nanosci. Nanotechnol. 2012, 12, 7155−7166. (46) Cheng, J. C.; Schultheiss, T. E.; Wong, J. Y. C. Int. J. Radiat. Oncol., Biol., Phys. 2008, 71, 1436−1443. (47) Carde, P.; MacKintosh, F. R.; Rosenberg, S. A. J. Clin. Oncol. 1983, 1, 146−153. (48) Shin, J.; Matsushima, H.; Chan, J. W.; Hoyle, C. E. Macromolecules 2009, 42, 3294−3301. (49) Iwakura, Y.; Okada, H. Can. J. Chem. 1960, 38, 2418−2424. (50) Espeel, P.; Goethals, F.; Driessen, F.; Nguyen, L.-T. T.; Du Prez, F. E. Polym. Chem. 2013, 4, 2449−2456. (51) Nguyen, L.-T. T.; Gokmen, M. T.; Du Prez, F. E. Polym. Chem. 2013, 4, 5527−5536.
of permeability of PDMS microcapsules in D2O followed by NMR; TGA and DSC graphs of microcapsules. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: fi
[email protected] (F.E.D.P.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The presented research is funded by SIM and IWT through a SIM ICON project within the SIBO program. The authors thank Prof. José C. Martins for the help with the HR-MAS NMR measurements. The 700 MHz part of the Interuniversity NMR Facility was funded by a FFEU-ZWAP grant of the Flemish government.
■
REFERENCES
(1) Turner, J. S.; Cheng, Y. L. Macromolecules 2000, 33, 3714−3718. (2) Lim, K. T.; Webber, S. E.; Johnston, K. P. Macromolecules 1999, 32, 2811−2815. (3) Deng, X.; Liu, B.; Cao, S.; Luo, R.; Chen, H. Appl. Surf. Sci. 2007, 253, 4823−4829. (4) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489−497. (5) Klose, P. Germany WO2013189477A1. (6) Simmons, A.; Padsalgikar, A. D.; Ferris, L. M.; Poole-Warren, L. A. Biomaterials 2008, 29, 2987−2995. (7) Nakade, M.; Kameyama, K.; Ogawa, M. J. Mater. Sci. 2004, 39, 4131−4137. (8) Prokopowicz, M.; Szewczyk, A.; Sawicki, W. J. Mol. Struct. 2014, 262, 1056−1057. (9) Esser-Kahn, A. P.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Macromolecules 2011, 44, 5539−5553. (10) Lotan, T.; Eckhouse, S. US20056923976. (11) Friedman, S. P.; Mualem, Y. Fert. Res. 1994, 39, 19−30. (12) Manna, U.; Patil, S. Langmuir 2009, 25, 10515−10522. (13) Murphy, E. B.; Wudl, F. Prog. Polym. Sci. 2010, 35, 223−251. (14) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Nature 2001, 409, 794−797. (15) Shchukin, D. G.; Mohwald, H. Chem. Commun. 2011, 47, 8730− 8739. (16) O’Sullivan, M.; Zhang, Z.; Vincent, B. Langmuir 2009, 25, 7962−7966. (17) Cao, L. Q.; Chen, L. P.; Cui, P. Y.; Wang, J. D. J. Appl. Polym. Sci. 2008, 108, 3843−3850. (18) Park, J. H.; Bae, S. Y.; Oh, S. G. Chem. Lett. 2003, 32, 598−599. (19) Alkan, C.; Sari, A. Sol. Energy 2008, 82, 118−124. (20) Chu, L. Y.; Xie, R.; Zhu, J. H.; Chen, W. M.; Yamaguchi, T.; Nakao, S. J. Colloid Interface Sci. 2003, 265, 187−196. (21) Sánchez, L.; Sánchez, P.; Lucas, A.; Carmona, M.; Rodríguez, J. Colloid Polym. Sci. 2007, 285, 1377−1385. (22) Zhang, X.; Tao, X.; Yick, K.; Wang, X. Colloid Polym. Sci. 2004, 282, 330−336. (23) Jin, Y.; Lee, W.; Musina, Z.; Ding, Y. Particulogy 2010, 8, 588− 590. (24) Ma, M.; Zhang, Y.; Yu, W.; Shen, H. Y.; Zhang, H. Q.; Gu, N. Colloids Surf., A 2003, 212, 219−226. (25) Goller, M. I.; Vincent, B. Colloids Surf., A 1998, 142, 281−285. (26) Clifford, N. W.; Iyer, K. S.; Raston, C. L. J. Mater. Chem. 2008, 18, 162−165. (27) Fielding, L. A.; Armes, S. P. J. Mater. Chem. 2012, 22, 11235− 11244. (28) Lovett, J. R.; Fielding, L. A.; Armes, S. P.; Buxton, R. Adv. Funct. Mater. 2014, 24, 1290−1299. G
dx.doi.org/10.1021/ma501897j | Macromolecules XXXX, XXX, XXX−XXX
