pH-Sensitive Nanocapsules with Barrier Properties - American

Article pubs.acs.org/Macromolecules

pH-Sensitive Nanocapsules with Barrier Properties: Fragrance Encapsulation and Controlled Release Ines Hofmeister,†,‡ Katharina Landfester,† and Andreas Taden*,†,‡ †

Max Planck Institute for Polymer Research, 55128 Mainz, Germany Henkel AG & Co. KGaA, Adhesive Research, 40191 Düsseldorf, Germany

‡

S Supporting Information *

ABSTRACT: A facile synthesis method for polymer nanocapsules with high diffusion barrier and stimuli-responsive release properties is presented. The highly volatile fragrance α-pinene was used as hydrophobic model compound for the encapsulation process, which is based on a miniemulsion-analogous free radical polymerization process. The copolymer composition was systematically varied, and increasing contents of methacrylic acid as functional monomer in combination with high glass transition temperatures enabled unusually high encapsulation efficiencies of ≥90% for capsules with z-average diameters of 1000 nm and correspondingly high diffusion barrier properties.4,9−12 They can be easily opened by mechanical fracture as efficient release mechanism but unfortunately inherently display poor colloidal stability in liquid systems. Therefore, a limited number of techniques like layer-by-layer assembling, emulsion polymerization, the synthesis of star block copolymers, and the miniemulsion polymerization were applied for the synthesis of fragrance nanocapsules.13−17 However, these methods do not combine the creation of a core−shell structure with high encapsulation efficiencies, adjustable release kinetics, and stimuli responsive characteristics. © XXXX American Chemical Society

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EXPERIMENTAL METHODS

Materials. All monomers, methyl methacrylate (MMA, Merck, ≥99% stab.), butyl methacrylate (BMA, Merck, ≥99% stab.), methacrylic acid (MAA, Acros, 99.5% stab.), and 1,4-butanediol dimethacrylate (BDDMA, Aldrich, 95%) were used as received Received: July 6, 2014 Revised: August 4, 2014

A

dx.doi.org/10.1021/ma501388w | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Monomer Compositions, Estimated Tg of the Polymeric Shell, Nanocapsule Size (z-Average), Encapsulation Efficiencies, and Fragrance Release Values for the Samples S0−S11 MMA [wt %] BMA [wt %] MAA [wt %] BDDMA [wt %] Tga [°C] capsule diameterb [nm] PDIb EESCc [%] EEGCd [%] capsule loade [105] total releasef [%] relative releaseg [%]

S0

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

82.5 7.5 10 0 106 166 0.06 88 88 6.13 88 100

80 5 10 5 109 186 0.09 91 90 8.92 90 100

75 10 10 5 103 168 0.07 90 78 6.50 72 92

72.5 12.5 10 5 100 176 0.11 73 70 6.07 67 96

67.5 17.5 10 5 95 192 0.09 73 28 7.87 28 100

60 25 10 5 87 182 0.09 30 18 2.76 18 100

50 35 10 5 77 154 0.09 30 8 1.67 3 38

35 50 10 5 63 178 0.08 8 6 0.69 0 0

87.5 5 2.5 5 102 170 0.07 77 70 5.77 2 3

85 5 5 5 104 164 0.05 83 80 5.58 12 15

82.5 5 7.5 5 107 172 0.05 88 85 6.82 41 48

81 5 9 5 108 175 0.06 89 86 7.27 65 76

a

Estimated by the Fox equation. bz-average (diameter) and PDI measured by dynamic light scattering (DLS). cEncapsulation efficiency determined by solid content of freeze-dried sample. dEncapsulation efficiency determined by headspace-GC (closed capsules; pH 3; 60 °C, 30 min). eCapsule load (molecules per capsule) calculated using EESC and z-average of the capsules, assuming the density of the miniemulsion to be 1 g/cm3. fReleased percentage of α-pinene after increasing the pH to 9 determined via headspace-GC (60 °C, 30 min) based on total weight of the utilized fragrance. g Released percentage of α-pinene after increasing the pH to 9 determined via headspace-GC (60 °C, 30 min) based on the EEGC result. without further purification. Sodium dodecyl sulfate (SDS, Lancaster, 99%), the ultrahydrophobe hexadecane (HD, Merck, ≥99%), and the initiator potassium peroxodisulfate (KPS, Merck, for analysis) were used as received. The fragrance α-pinene was received from Henkel fragrance center and used without further purification. Deionized water was used for all experiments. Synthesis of Fragrance Nanocapsules. A mixture of monomers MMA, BMA, MAA, and BDDMA (4 g) and 250 mg of the ultrahydrophobe hexadecane were dissolved in 2 g of the fragrance. A solution of 22 g of water with 23 mg of the surfactant SDS as emulsifier was then added to the clear hydrophobic mixture. For preemulsification, the mixture was homogenized 3 min with an UltraTurrax (16 000 rpm). The miniemulsion was prepared by ultrasonicating the emulsion for 120 s (pulsed: 10 s; 5 s pause) at 90% amplitude (Branson sonifier W450 Digital, 1/2 in. tip) under icecooling. The miniemulsion was charged in a round-bottom flask, and initiator solution (80 mg of the initiator KPS dissolved in 2 g of water) was added to initiate free radical polymerization. The polymerization was performed at 72 °C for 5 h under stirring. The pH after polymerization was approximately 2−2.5 for all samples. Instrumentation. Solid content (SC) was determined gravimetrically via freeze-drying the samples. Beforehand, two different theoretical solid contents were calculated: SC1 without the mass of the fragrance and SC2 including the fragrance mass. During the determination of the solid content via freeze-drying, the capsules stay intact and the encapsulated fragrance is included in the measured solid content SC2. Free α-pinene which has not been encapsulated is evaporated off in the process, and the amount of encapsulated fragrance EESC can be calculated using SC2 and SC1. The particle size was measured by dynamic light scattering (DLS) using a Malvern Instruments Zetasizer Nano at an angle of 173° (backscattering) and 25 °C. For the measurement the latex was diluted with deionized water until it was slightly opaque. The particle size is presented as the z-average [nm]. Transmission electron microscopy (TEM) was performed with a Jeol JEM-1400 transmission electron microscope, operating with an acceleration voltage of 40 to 120 kV. The samples were measured at −170 °C under low dose conditions. Scanning electron microscopy (SEM) was performed with a Zeiss Gemini 1530 scanning electron microscope; thereby an accelerating voltage of 0.2 kV was used. Atomic force microscopy (AFM) was performed with a Multimode Tuna TR microscope with an Olympus noncontact mode OMCLAC160TS-W2 cantilever. The diluted sample was measured on a silicon wafer substrate.

Differential scanning calorimetry (DSC) was measured to determine thermal transitions (e.g., the Tg) of the polymer particles. For the measurement, the freeze-dried latex sample was measured under nitrogen flow of 3 L h−1 in open pans with a heating/cooling rate of 2 K min−1. The efficiency of the encapsulation was quantitatively investigated by headspace gas chromatography. With this method, volatile substances can be analyzed over a solid or liquid sample. A quantitative analysis of the free fragrance above the sample can be made through the use of an external standard. The headspace-GC measurements were performed with an Agilent 7890 gas chromatograph and an Agilent G1888 headspace sampler with FID detector. An Agilent 19091 J-413 column was used with H2 as carrier gas. The oven temperature of the GC program was the following: 40 °C hold 2 min isotherm, ramp 1:10 °C min−1 to 150 °C hold 4 min isotherm, ramp 2:20 °C/min to 250 °C. The temperature in the headspace oven was 60 °C whereas the vial equilibration time was 30 min (unless otherwise stated) in high shaking mode. These measurement conditions were chosen to ensure equilibrium state between the liquid sample and the gas phase in headspace vials for most of the samples, unless otherwise stated. For headspace measurements 10% solutions from each sample were prepared by diluting them with deionized water. For the manual preparation, 10 μL of each diluted solution was given in a 20 mL headspace vial and promptly capped, to avoid evaporation of the analyte. The closed capsules with an intrinsic pH value of about 2−2.5 were measured in 2 mL of water which increased the pH to a value of around 3−3.5. For the measurement of the opened capsules, the diluted solutions of the samples were charged into headspace vials equipped with 2 mL of a buffer solution of pH 9. For data evaluation the maximum peak area was detected and compared to data from calibration.

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RESULTS AND DISCUSSION

Nanocapsule Design. The fragrance nanocapsules are synthesized via the polymerization-induced phase separation mechanism. In the miniemulsion droplets the monomers are dissolved in the core material, whereas after initiation, the growing polymer chains are incompatible with the hydrophobic fragrance which results in a phase separation.22,23 Based on this miniemulsion-analogous, free-radical polymerization approach, nanocapsules with a variable, but preferentially high, content of methacrylic acid (MAA) were synthesized. The hydrophobic fragrance α-pinene was used as model compound and B

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lower vapor pressures; the maximum temperature applied was 25 °C. Furthermore, headspace gas chromatography (headspace-GC) was performed using a headspace oven temperature of 60 °C. At pH 3 the nanocapsules are assumed to be in a “closed” and at pH 9 in an “open” state. In the following simply “pH 3” or the term “closed” capsule will be used to distinguish the systems without addition of a base or buffer. Accordingly, the encapsulation efficiency EEGC as determined via headspaceGC is given for the lower pH regime. The results reveal that EESC ≥ EEGC. The delta between these values can be explained by the different measurement conditions and becomes more pronounced when the Tg of the nanocapsules is lowered. As mentioned above, EESC and EEGC are measured at different temperatures, i.e., room temperature (RT) and 60 °C, respectively. We systematically varied the glass transition temperature, and the sample S4 can be seen as borderline case for high encapsulation efficiency at RT. Obviously at 60 °C the segment mobility of polymer chains of the capsule shell becomes significantly increased, which explains the strong differences. In fact, this result signifies that an adjustable release can be triggered not only by pH but also by temperature. Furthermore, headspace-GC measurements for closed and open capsules were used to determine the pH-induced fragrance release. Two different percentages were calculated for clarity reasons and to simplify the discussion: (a) total release, based on total weight of α-pinene utilized during sample preparation, and (b) relative release, based on the respective EEGC sample value. It can be postulated that the capsules in the open state display partial hydrogel-like characteristics; i.e., hydrogel colloids are known for stimuli-response and sensitivity toward guest molecules of different polarity.24−26 Alternatively, the deprotonation might also be considered as very strong hydroplasticization process.27,28 Mechanistically, the nanocapsule walls can absorb increased amounts of water upon deprotonation which correspondingly leads to conformational changes within the polymer shell, increased chain segment mobility, reduced diffusion barrier, and hence triggered fragrance release (see Figure 1). Based on this principle, the swelling degree can principally be considered as simple lever for controlled release. Unfortunately, we could not observe any particle size increase with our DLS

emulsified under high sheer conditions in conjunction with the monomers and hexadecane (HD) as ultrahydrophobe. Preliminary experiments showed high colloidal stability and full conversion when sodium dodecyl sulfate (SDS) as surfactant and potassium peroxodisulfate (KPS) as initiator were utilized. A weight ratio of 1:2 between fragrance and monomer mixture was applied throughout this investigation. The samples described in this contribution and their basic composition are summarized in Table 1, including important characteristic numbers, e.g., Tg (calculated based on Fox equation), average particle size, encapsulation efficiencies, and fragrance release values. Free radical copolymerization was chosen for the nanocapsule synthesis method to enable facile synthesis. As functional, pH-sensitive monomer, MAA was applied in contents of up to 10 wt % relative to the total monomer amount. By weight, methyl methacrylate (MMA) serves as the main monomer, accompanied by the more hydrophobic butyl methacrylate (BMA) in lower amounts, which was systematically varied in content to gradually lower the hydrophilicity and the Tg of the corresponding polymeric shells. The Tg values given in Table 1 are calculated based on the Fox equation, which were used to simplify the discussion on the general trends determining encapsulation efficiency and barrier performance of the shell. In addition, closer investigation and discussion in regard to thermal behavior can be found in the Supporting Information (see Figure SI 1 and Table SI 1), based on dynamic scanning calorimetry (DSC) measurements for the samples S1 and S5. To prohibit complete dissolution of MAA-enriched polymer chains upon deprotonation, the difunctional monomer 1,4butanediol dimethacrylate (BDDMA) was applied as crosslinker and kept at 5 wt % with respect to the total monomer amount throughout the investigation except for sample S0. The cross-linker is not mandatory to obtain a dense shell at low pH, as can be seen by comparison of the samples S0 and S1 (see Table 1), but potentially can contribute to the pH-induced release characteristics of the nanocapsules. BDDMA was chosen as rather hydrophobic component and identified to be adequate for the nanocapsule synthesis because it does not negatively interfere with the colloidal stability of the system. Dynamic light scattering measurements reveal average particle sizes which range between 150 and 200 nm and storage stabilities over several months. Interestingly, no significant trends related to monomer ratios, hydrophobicity, or content of MAA as surface-active component were identified in the set of experiments, which suggest the widely absence of homogeneous particle nucleation in combination with effective and fast colloidal stabilization of the nanodroplets prepared by high sheer forces. Similar to classical miniemulsion systems, droplet nucleation is therefore expected to be the dominating mechanism upon polymerization initiation. Controlled Release. In the protonated form, the polymeric shell in glassy state features a strong diffusion barrier for the fragrance. With increasing pH value (represented by “pH 9” in the following) the carboxylic acidic groups get deprotonated and a polyelectrolyte shell with substantially increased hydrophilicity is created. The encapsulation efficiency was determined by two different methods and under different conditions. By assuming quantitative monomer conversion, the weighted results obtained after freeze-drying were used to calculate the encapsulation efficiency EESC based on solid content. This value represents the encapsulation efficiency at

Figure 1. Schematic drawing of the release mechanism: Upon pH increase the glassy polymer shell of low permeability becomes deprotonated and transferred into a weak polyelectrolyte structure with partial hydrogel-character. The gradual absorption of water leads to conformational changes and hydroplasticization of the polymer network which triggers the fragrance release. C

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Figure 2. Amount of free α-pinene measured at 60 °C after 30 min via headspace-GC at pH 3 (closed capsules) and pH 9 (open capsules). On the left side the x-axis represents the Tg of the polymer material, whereas on the right side the x-axis shows the MAA content of the polymer shell.

performance seem to be strongly dependent on the polarity of the polymer capsule wall. An even more drastic influence can be found on the release behavior at pH 9. With increasing MAA content the total release changes from 2% to 90%, which corresponds to a relative release increase from 3% to 100%. An obvious explanation is that higher MAA contents facilitate the water uptake of the nanocapsule shells upon deprotonation and lead to an accelerated fragrance release. Additional headspace-GC measurements at different equilibration times were conducted for validation, and the results are shown in Figure 3. The higher the MAA content, the faster is the release upon deprotonation. The data support the proposed controlled release mechanism based on partial hydrogel-like behavior of the polymeric shell with an induced fragrance release due to swelling of the polymer network with water. In other words, a

measurement setup upon deprotonation as it can be expected for hydrogel swelling, presumably due to fragrance release and associated side effects. Therefore, we performed additional DLS experiments with dense particles, polymerized under the same conditions but without the presence of α-pinene as hydrophobic load. In fact, for copolymer particles with 5 wt % crosslinker and 10 wt % MAA an increase in particle size (z-average) from 125 to 138 nm was detected after swelling with water at pH 9, which strongly supports our suggested release mechanism. Figure 2 represents the headspace-GC results for two sets of experiments, where the free α-pinene content for selected nanocapsule systems at pH 3 and pH 9 were measured and compared. The difference between the close and the open state is shown as shaded light orange area and clearly indicates the general trends. The x-axis of the left diagram represents the estimated Tg values for the samples S7 to S1 as a result of varying contents of MMA and BMA, but with MAA and BDDMA kept constant at 10 and 5 wt %, respectively, relative to total weight of monomers. A clear Tg dependence can be observed and was expected to a certain degree, due to its obvious relationship to polymer segment mobility and impact on permeability. Please note that the samples with higher Tg also exhibit a more hydrophilic shell (more MMA and less BMA). The higher the Tg and the hydrophilicity is, the better is the encapsulation efficiency at pH 3. At elevated pH, however, the amount of free fragrance is close to the maximum value of 100% for all samples, which shows that under the experimental conditions systems based on 10 wt % MAA acid functionalization are able to provide effective release. In another set of experiments the degree of acid functionalization and its influence on encapsulation efficiency and release-behavior were investigated. MAA and MMA contents were varied whereas the BMA and BDDMA contents were kept constant. The estimated Tg values for the considered samples S1, S8, S9, S10, and S11 were above 100 °C in each case. The results demonstrate that with increasing the MAA content from 2.5 to 10% relative to the total monomer weight, the free amount of α-pinene is reduced from 30 to 10%; i.e., the EEGC values are improving significantly from 70 to 90%. Because all samples are expected to be in the glassy state during the evaluation, the encapsulation efficiency and the barrier

Figure 3. Amount of released α-pinene for nanocapsules with varying MAA content (from 2.5% to 10% MAA relative to total monomer weight) after 10, 30, and 60 min equilibration time in the headspace oven at 60 °C based on the EEGC result. D

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Figure 4. Electron micrographs at different magnifications from sample S1 prepared from closed capsules at pH 3: (A, B) transmission electron micrographs (TEM); (C) scanning electron microscope (SEM) image and from opened capsules at pH 9; (D, E) TEM; (F) SEM. For triggered fragrance release, NH3 was utilized as a volatile base.

interlocking of the nanocapsule structures after complete evaporation of all volatiles. Figure 4C,F shows the corresponding SEM images of sample S1. Interestingly fused particles are less obvious and collapsed nanocapsules structures are more prominent instead, which has to be explained by the different sample preparation and measuring conditions. For nanocapsules after fragrance release considerably fewer collapsed core−shell structures are observed compared to images prepared from systems at pH 3 in the closed state. Presumably increased polymer shell permeability can limit the mechanical rupture of the nanocapsule walls under high vacuum conditions, and additionally the nanocapsule integrity should benefit from the previous release of the highly volatile α-pinene. Furthermore, AFM measurements were conducted for sample S1, and the corresponding images are shown in the Supporting Information (Figure SI-3). The results are in accordance with the electron microscope pictures but do not provide additional insights.

strong hydroplasticization takes place and increased polymer segment mobility enables faster diffusion. Additionally, Figure SI-2 and Table SI-2 in the Supporting Information summarize and compare the release kinetics including the behavior for the closed capsules at lower pH value. As seen before at pH 3 the barrier performance is better for samples with higher MAA contents. Nanocapsule Morphology. The sample morphology was investigated by TEM, SEM, and AFM measurements. All samples were examined at their primarily pH 3 to visualize the closed capsules. Furthermore, pH 9 was adjusted by adding volatile ammonia solution to investigate the capsules after fragrance release. TEM measurements were performed under low-dose electron beam and cryogenic conditions of −170 °C to prevent PMMA degradation. Figures 4A,B,D,E show the resulting TEM images for sample S1. Independent from the capsule size, core−shell morphologies can be detected for the closed capsules as well as for the capsules after fragrance release. This result signifies that droplet nucleation and polymerization-induced phase separation are dominant for particle formation and suggest that homogeneous nucleation can be neglected. The nanocapsules prepared at elevated pH value appear to be partially fused together, which can be seen as indication that swelling of the polymer shells indeed takes place and that the nanocapsule walls become softened in that process. Accordingly, strong capillary forces between particles in contact can lead to partial interpenetration of polymer segments during drying, even when a cross-linking agent was used for nanocapsule synthesis, and finally to

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CONCLUSION Nanoencapsulation was performed with the hydrophobic fragrance α-pinene as model substance, based on a miniemulsion-analogous, polymerization-induced phase separation process with significant contents of methacrylic acid as watersoluble, pH-sensitive functional monomer. As expected, high glass transition temperature and restricted motion of polymer segments were identified as prerequisites to limit the diffusion E

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(7) Levrand, B.; Ruff, Y.; Lehn, J.-M.; Herrmann, A. Chem. Commun. 2006, No. 28, 2965−2967. (8) Herrmann, A. Chem.Eur. J. 2012, 18 (28), 8568−8577. (9) Hwang, J.-S.; Kim, J.-N.; Wee, Y.-J.; Yun, J.-S.; Jang, H.-G.; Kim, S.-H.; Ryu, H.-W. Biotechnol. Bioprocess Eng. 2006, 11 (4), 332−336. (10) Jacquemond, M.; Jeckelmann, N.; Ouali, L.; Haefliger, O. P. J. Appl. Polym. Sci. 2009, 114 (5), 3074−3080. (11) Rodrigues, S. N.; Martins, I. M.; Fernandes, I. P.; Gomes, P. B.; Mata, V. G.; Barreiro, M. F.; Rodrigues, A. E. Chem. Eng. J. 2009, 149 (1−3), 463−472. (12) Monllor, P.; Bonet, M. A.; Cases, F. Eur. Polym. J. 2007, 43 (6), 2481−2490. (13) Theisinger, S.; Schoeller, K.; Osborn, B.; Sarkar, M.; Landfester, K. Macromol. Chem. Phys. 2009, 210 (6), 411−420. (14) Sansukcharearnpon, A.; Wanichwecharungruang, S.; Leepipatpaiboon, N.; Kerdcharoen, T.; Arayachukeat, S. Int. J. Pharm. 2010, 391 (1−2), 267−273. (15) Sadovoy, A. V.; Lomova, M. V.; Antipina, M. N.; Braun, N. A.; Sukhorukov, G. B.; Kiryukhin, M. V. ACS Appl. Mater. Interfaces 2013, 5 (18), 8948−8954. (16) Lertsutthiwong, P.; Noomun, K.; Jongaroonngamsang, N.; Rojsitthisak, P.; Nimmannit, U. Carbohydr. Polym. 2008, 74 (2), 209− 214. (17) Kreutzer, G.; Ternat, C.; Nguyen, T. Q.; Plummer, C. J. G.; Månson, J.-A. E.; Castelletto, V.; Hamley, I. W.; Sun, F.; Sheiko, S. S.; Herrmann, A.; Ouali, L.; Sommer, H.; Fieber, W.; Velazco, M. I.; Klok, H.-A. Macromolecules 2006, 39 (13), 4507−4516. (18) Dundua, A.; Landfester, K.; Taden, A. Polymer 2014, 55, 3543− 3550. (19) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17 (3), 908−918. (20) Landfester, K.; Weiss, C. Encapsulation by Miniemulsion Polymerization. In Modern Techniques for Nano- and Microreactors/reactions; Caruso, F., Ed.; Springer: Berlin, 2010; Vol. 229, pp 1−49. (21) Rajot, I.; Bône, S.; Graillat, C.; Hamaide, T. Macromolecules 2003, 36 (20), 7484−7490. (22) Cao, Z.; Ziener, U. Curr. Org. Chem. 2013, 17 (1), 30−38. (23) Luo, Y.; Zhou, X. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (9), 2145−2154. (24) Seiffert, S. J. Polym. Sci., Part A: Polym. Chem. 2014, 52 (4), 435−449. (25) Jagadeesan, D.; Nasimova, I.; Gourevich, I.; Starodubtsev, S.; Kumacheva, E. Macromol. Biosci. 2011, 11 (7), 889−896. (26) Dai, H.; Chen, Q.; Qin, H.; Guan, Y.; Shen, D.; Hua, Y.; Tang, Y.; Xu, J. Macromolecules 2006, 39 (19), 6584−6589. (27) Feng, J.; Winnik, M. A. Macromolecules 1997, 30 (15), 4324− 4331. (28) Tsavalas, J. G.; Sundberg, D. C. Langmuir 2010, 26 (10), 6960− 6966.

of fragrance molecules through the polymeric shell, which already can be considered as first simple measure to adjust the nanocapsule permeability. Furthermore, the capsule shell polarity, represented by the monomer ratio between MMA and BMA and more significantly by the degree of acid functionalization, was identified as main factor to obtain surprisingly high fragrance encapsulation efficiencies of more than 90% with nanocapsule sizes below 200 nm. The polymer shell composition can be easily adopted to tailor the fragrance release kinetics, which can be triggered by pH change and/or temperature. A particularly strong dependence on the MAA content was observed and can be explained by the transition toward partly hydrogel-like behavior upon deprotonation. Such pH-responsive capsules can for example be used in fabric conditioner formulations which exhibits pH values around 3, where the capsules are existent in their closed state. During the washing process the pH increases to neutral or slightly alkaline values which consequently can trigger the fragrance release. In summary, our contribution enables the facile synthesis of colloidally stable nanocapsules with high loading efficiency and adjustable release kinetics. Finally, it should be noted that our nanoencapsulation approach is principally not restricted to volatile fragrances and expected to work also in conjunction with other hydrophobic substances that do not interfere with free radical polymerization, e.g., catalysts, curing agents, initiators, healing agents, corrosion inhibitors, indicators, dyes, scavengers, blowing agents, etc.

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ASSOCIATED CONTENT

S Supporting Information *

Dynamic scanning calorimetry measurements showing the thermal behavior of the nanocapsules, additional headspace-GC data for release kinetics, and AFM pictures of the nanocapsules. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] or taden@mpip-mainz. mpg.de (A.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ingo Lieberwirth for performing cryo-TEM measurements. We furthermore acknowledge Gunnar Glasser for his help with SEM studies and Rüdiger Berger for his AFM expertise. The authors also thank Elke Veit for supporting laboratory work. Many thanks go to Ursula Huchel (Henkel Fragrance Center) for giving valuable suggestions and for providing the fragrances.

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REFERENCES

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