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Sustained Dye Release Using Polyurea-urethane/ Cellulose Nanocrystal Composite Microcapsules Youngman Yoo, Carlos Martinez, and Jeffrey Paul Youngblood Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04628 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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Sustained

Dye

Release

Using

Polyurea-urethane/Cellulose

Nanocrystal Composite Microcapsules

Youngman Yoo, Carlos Martinez, and Jeffrey P. Youngblood *. School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States * J. P. Youngblood. Email: [email protected]. Phone: +1 765-496-2294. Fax: +1 765 494-1204.

Keywords: Microcapsules; Cellulose nanocrystals; Surface modification; Release controlled; Diffusivity; Composite.

Abstract The aim of this study is to develop methods to reinforce polymeric microspheres with cellulose nanocrystals (CNCs) to make eco-friendly microcapsules for a variety of applications such as medicines, perfumes, nutrients, pesticides, and phase change materials. Surface hydrophobization treatments for CNCs were performed by grafting poly (lactic acid) oligomers and fatty acids (FAs) to enhance the dispersion of nanoparticles in the polymeric shell. Then, a straightforward process is demonstrated to design sustained release microcapsules by the incorporation of the modified CNCs (mCNCs) in the shell structure. The combination of the mCNC dispersion with subsequent interfacial polyurea (PU) to form composite capsules as well as their morphology, composition, mechanical properties, and release rates were examined in this study. The PU microcapsules embedded with the mCNC were characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The morphologies of the microcapsules were characterized by optical microscopy (OM) and scanning electron microscope (SEM). The rupture stress and failure behavior of the microcapsules were

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determined through single-capsule compression tests. Oil soluble Sudan II dye solution in mineral oil was utilized as a model hydrophobic fill, representing other latent fills with low partition coefficients, and their encapsulation efficiency was measured spectroscopically. The release rates of the encapsulated dye from the microcapsules were examined spectroscopically by both ethanol and 2-ethyl-1-hexanol medium at room temperature. The concentration of released dye was determined by using UV-vis absorption spectrometry (UV-vis). The mCNC embedded polyurea-urethane capsules have strong and dense walls, which function as excellent barriers against leakage due to their extended diffusion path length, and ensure enough mechanical strength from rupture for handling or post processing.

Introduction The sustained release of active ingredients from polymeric microcapsules is widely used in applications such as medicines, self-healing materials, perfumes, pesticides, and preserved nutrients. The use of microcapsules for their desired outcome (e.g., long-lasting scent) results from their ability to release active agents (e.g., fragrance) slowly and effectively into host systems over a certain period of time. The microcapsule shell should have a low permeability to inhibit the release of core components; yet in many cases the release is still too fast and the shelf life is not enough for most applications. The microcapsules need to be mechanically stable and preferably inexpensive. Additionally, the production procedure should be simple for achieving future scale up.1-4 There are many factors: core material size and solubility, polymeric networks and swelling, crack formation, water-filled pores, water penetration, and osmotic effects, which promote changes in a microcapsule shell matrix that leads to the release of microcapsule contents.3


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However, outside of rupture, according to Fick’s law, the resulting loss profiles depend mainly on diffusivity and shell thickness.5 For example, the ratio of molecular size of microcapsule fills to correlation length, which is a characteristic distance between joints of crosslinked chains in a polymeric matrix, has been considered as the primary structural parameter to affect the loss rate of microcapsules.6 For most polymer microcapsules, the thin and highly dense external wall can generally be utilized to control the loss profile, whereas a more porous structural inner layer can be applied to mechanically support the wall of a microcapsule.7-8 However, improving the diffusivity of microcapsule walls through adjustment of monomers or diluents can have negative effects on membrane stability and strength, compatibility with core components, and Environment/Health/Safety (EHS). Recently, several studies have been conducted to control diffusivity without deteriorating wall strength. These include microcapsules with double layer walls9, as well as heterogeneous systems such as copolymerization, blending, Layer-by-Layer (LbL)10-11, and hybridization (incorporation of nanoparticles and pores) of polymeric matrix. Likewise, the addition of inorganic particles such as montmorillonite clay12-14, cadmium or lead sulfide15, titanium dioxide16-17, molybdenum oxide18, gold19-20, and iron21, etc., onto preformed microcapsule membranes leads to diffusion barriers.16 Polymeric particles have also been introduced into polymers to enhance the mechanical stability of the resulting composite.22 However, many of these processes require demanding physical or chemical modifications of the polymeric matrix. Maintaining simplicity in design and process is required for achieving future scale-up. Furthermore, in light of global environmental issues, new enthusiasm exists for the use of sustainable and renewable materials in bio-medical and cosmetic applications. Recently, there is growing interest in cellulose nanocrystals (CNCs) due to their distinct benefits; they show high thermal conductivity and low coefficient of thermal expansion with a


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high aspect ratio, are very strong, stiff, and 100 % renewable.23-25 Therefore, the CNCs can be utilized to reinforce polymeric materials forming a dense network resulting in sustainable, ecofriendly, and versatile polymer composites.23,

26-29

CNCs can offer a new route to product

development and formulations of ecofriendly microcapsules used as a key material in these applications. However, there are crucial limitations in the CNC reinforced composites: aggregation of the nanoparticles, difficulty in breaking the aggregated particles, and their incompatibility with hydrophobic polymeric matrices.23, 30 Previously we reported that surface hydrophobization treatments for CNCs can be performed by using a lactic acid as a reactive solvent for the esterification of CNC with fatty acids (FAs), biodiesel, or plant oils.31 These scalable, green, one-pot aqueous surface hydrophobization methods improve the incorporation of sustainable nanoparticles into hydrophobic materials. There are many chemical methods for encapsulation processes: suspension, emulsion, dispersion, or precipitation in situ polymerizations and interfacial polycondensations.32-37 Furthermore, physical methods show ingenious ideas of magnification of microcapsules such as layer-by-layer (L-B-L) assembly, sol-gel encapsulation, supercritical CO2-assisted encapsulation, spraying, fluid-bed coating, centrifugal techniques, vacuum/electrostatic encapsulation2,

38-40

,

pressurized gyration41, and electrohydrodynamic atomization (EHDA)42. In this study, a new group of microcapsules containing modified CNCs is synthesized by using interfacial polymerization because of its cost-effectiveness, availability, simple fabrication procedures, and in situ loading of nanoparticles. For this purpose, a hydrophobic modified CNC has been synthesized from a lactic acid and oleic acid in an esterification and utilized for the synthesis of release-controlled PU microcapsules, which were loaded with an oil-soluble dye solution as a model system for other hydrophobic active agents. Fourier Transform infrared


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spectroscopy, scanning electron microscope, optical microscope, and dye release tests were conducted to analyze the chemical composition and physical properties of microcapsules. Additionally, it was found that controlling the quantity of modified CNCs is the most effective factor to control the release rate of the microcapsules’ contents. The barrier enhancement of the embedded CNCs and thus the time-controlled release of the loaded dye in a hydrophobic medium are examined.

Experimental Section Materials. Zinc acetate dihydrate at 98 %, dibutyltin dilaurate at 95 %, DL-lactic acid (LA) at 85 % syrup, oleic acid at 90 %, tolylene-2,4-diisocyanate at 95 %, 2-ethyl-1-hexanol at 99.6 %, 1,6-hexanediol 99 %, poly(tetrahydrofuran) (avg. Mn ~1000), triethylenetetramine 97 %, methyl palmitate 97 %, anhydrous chloroform at 99 %, anhydrous ethyl acetate at 99.8 %, Sudan Blue II at dye content 98 %, Mowiol® 18-88 (poly vinyl alcohol), BioReagent light mineral oil (neat), and Triton™ X-100 (OP-10) at < 3 % polyethylene glycol were purchased from Sigma Aldrich, St. Louis, MO, USA. Ethanol (200 proof) and acetone (ACS grade) were purchased from VWR, West Chester, PA, USA. All reagents were used as received without further purification. 11.9 wt % never-dried CNC suspension in water, which is in sulfate half-ester form with 1 wt % sulfur and a sodium counterion, was manufactured by USDA Forest Service-Forest Products Laboratory (FPL), Madison, WI, USA and distributed by University of Maine, Orono, ME, USA.43 Chemical Hydrophobization of Cellulose Nanocrystals. Figure 1 shows a scheme of the one pot, aqueous surface modification method whose details can be found elsewhere.31 Briefly, an aqueous CNC suspension at 11.9 % (w/w) and additional deionized (DI) water were utilized to


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prepare a 5 % (w/w) CNC water suspension. An excess of 85 % (w/w) DL-lactic acid syrup (the ratio of the equivalent COOH of the lactic acid and OH of the dried CNC = 10) was introduced into the CNC water suspension, followed by ultrasonication for 1 min. The suspension mixture was added to a 500 mL 3-necked round flask equipped with a stirrer and a condenser. Then, a zinc acetate dihydrate catalyst (150 ppm based on the DL-lactic acid (LA) weight) was further added thereto, and polyesterification was conducted by water distillation at 180 ℃. After 70 to 99 % of the water was removed, an excess of oleic acid (the ratio of the equivalent COOH of the oleic acid and OH of the dried CNC = 2.8) and a dibutyltin dilaurate (DBTDL) catalyst (200 ppm based on the fatty acid weight) were further added to the intermediate products, composed of PLA oligomer and PLA oligomer grafted CNC (CNC-g-PLA). The reaction pressure and temperature were reduced to 100 mmHg and increased to 190 ℃, respectively, for 30 min, and then an excess of lactic acid was distilled by running the reaction under low vacuum. Then, reaction pressure was gradually decreased to 10 mmHg and was retained until the distillation column top temperature fell below 35 ℃, thereby preparing an oleic acid (C18) and PLA grafted CNC (CNC-g-PLA-C18). The resulting product was placed into a ceramic bowl for cooling. Subsequently, the final purification of the nanoscale particles from the remaining free fatty acid and homo-PLA oligomers was carried out using dispersion-centrifugation (6,000 rpm at 25 ℃ for 30 min.) several times with an excess of ethanol. In the end, the modified CNC was kept in ethanol or freeze-dried using a lyophilizer (Labconco FreeZone Plus 4.5 L). According to

13

C

CP/MAS solid-state NMR analysis, the degree of polymerization of the PLA is about three and the degree of substitution of grafted PLA and oleic acid are about 30 % and 20 %, respectively.


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Figure 1: One-pot esterification reaction scheme for hydrophobization of cellulose nanocrystals with lactic acid and oleic acid.

Preparation of mCNC Enhanced Polyurea-urethane (PU) Microcapsules. For the dispersion of a modified CNC, in an oil phase, a CNC hydrophobized with lactic acid and oleic acid (mCNC) that was kept “wet” in its washing solvent (ethanol) was utilized. The percentage (w/w) of the mCNC in the "wet” dispersions was first measured gravimetrically. An appropriate amount of “wet” sample was utilized to collect 5 g of mCNC. 2-ethyl-1-hexanol or poly(tetrahydrofuran) (PTMG) was introduced so that the final concentration of mCNC was kept at 20 % (w/w). The “wet” samples were dispersed in the 2-ethyl-1-hexanol or PTMG solvent using a vortexer; then, the flushing solvent (ethanol) was removed under reduced pressure by a rotary evaporator. For the synthesis of hollow PU microcapsules, an aqueous continuous phase was obtained by dispersing 2.0 % (w/w) copoly(vinyl alcohol-vinyl acetate) (PVA) and 0.2 % (w/w) OP-10 in distilled water. An appropriate amount 1.95 g of tolylene-2,4-diisocyanate monomer solution and 0.35 g of ethyl acetate with 3.2 mg of oil-soluble dye Sudan blue II were added to 1.95g of a different concentration (0, 5, 10, and 15 % (w/w)) of CNC-g-PLA-C18 suspension in 2-ethyl-1hexanol to obtain an oil dispersion phase. Then, the oil phase was added into 75 g of the aqueous solution and dispersed by mechanically stirring at 300 – 2500 rpm. The reaction was carried out


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for 4 h at 80 ℃. 100 mL of distilled water was added to the suspension to retain the monodispersed microcapsules with slow stirring. The microcapsule was decanted and flushed with distilled water and cyclohexane, respectively, to remove the remaining PVA, dye, surfactant, and unreacted monomers on the surface of microcapsules. Then, the microcapsules were dried in a vacuum oven at 25 ℃ for 24 h.44-45 For the synthesis of the liquid core PU microcapsules, different active hydrogens of PTMG (polyol, Mn ~1000), 1,6-HD (chain extender), and TETA (crosslinker) were utilized instead of a 2-ethyl-1-hexanol and a blue dye solution in mineral oil (Sudan II, 2000 ppm) as an oil-core component was initially introduced to the oil dispersion phase. Different weight ratios of core/shell emulsions including various contents of CNC-g-PLA-C18 were used as shown in Table 1. To make the stable oil emulsion, an appropriate amount of TDI, PTMG, HD, CNC-gPLA-C18, and 2g of ethyl acetate were dissolved in dye solution in mineral oil (Sudan II, 2000 ppm). The mixture was then added to 70 mL aqueous solution with 2.0 % (w/w) PVA and 0.2 % (w/w) OP-10. Emulsification was conducted at 50℃ for 30 min to remove the ethyl acetate solvent and produce the polyurethane membrane. Afterwards, 10 mL of 20 % (w/w) TETA solution was dropped into the above emulsion to form the crosslinked polyurea-polyurethane membrane and the reaction was carried out for 3 h. The following procedures were performed as previously described in the synthesis of hollow PU microcapsules. The encapsulation yield of microcapsules was determined by dividing the total mass of shell and core materials used to the whole mass of microcapsules obtained. The encapsulation yields were approximately calculated by eq 140: ܻ݈݅݁݀ ሺ%ሻ = ௐ

ௐ೎ೌ೛ೞೠ೗೐

೛೚೗೤೚೗/೒೗೤೎೚೗ ାௐ೅ಶ೅ಲ ାௐ೅ವ಺ ାௐ೏೤೐ ାௐ೘೔೙೐ೝೌ೗ ೚೔೗ ାௐ೘಴ಿ಴


× 100

ሺ1ሻ

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where Wcapsule is the weight of the prescribed, dried microcapsules, and Wpolyol/glycol, WTETA, WTDI, Wdye, and WmCNC are the weights of PTMG, 1,6-HD, TETA, TDI, and mCNC, respectively. 100 milligrams of individual microcapsules was broken via mortar/pestle and dispersed in 10 ml of acetone, and the mixture was stirred vigorously for 12 hours. After filtration through a PTFE syringe membrane filter (0.45 µm pore size, Fisher Scientific) and solvent-exchanging with 3ml of mineral oil, the solution was analyzed spectrophotometrically and the content of dye fill in the microcapsules was calculated by eq 2. The experiments were performed in triplicate. The encapsulation efficiency of microcapsules was calculated by dividing the actual dye amount to the theoretical dye amount of microcapsules40. ‫ ݕ݂݂ܿ݊݁݅ܿ݅ܧ‬ሺ%ሻ =

‫ݐ݊݁ݐ݊݋ܿ ݁ݕ݀ ݈ܽݑݐܿܣ‬ × 100 ܰ‫ݐ݊݁ݐ݊݋ܿ ݁ݕ݀ ݈ܽ݊݅݉݋‬

ሺ2ሻ

Characterization of Microcapsules. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR transmission spectra was utilized to qualitatively analyze the chemical composition of pristine CNC, mCNC, pure PU microcapsule, and mCNC-PU microcapsules. (FTIR Spectrum 100, PerkinElmer, Waltham, MA, USA). All spectra collected from the samples and KBr pellets varied from 450 to 4000 cm−1 with a resolution of 4 cm−1 over 10 scans. ThermoGravimetric Analysis (TGA). TGA analyses were conducted to analyze qualitatively the chemical composition of pristine CNC, mCNC, pure PU microcapsule, and mCNC-PU microcapsules at 10 ℃ min-1 from room temperature to 700 ℃ under nitrogen flow using a TA Instrument Q50 thermogravimetric analyzer (TA Instruments, USA). Optical Characterizations. Bright-field and cross-polarized images of the microcapsules were obtained on an inverted microscope (Axio Observer, Zeiss America, Peabody, MA, USA)


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and captured using a color camera (Retiga 4000R Fast cooled color 12 bit, Q-Imaging, Surrey, BC, Canada). Digital images of the microcapsules were analyzed by using the freely available image analysis program, ImageJ (W. S. Rasband, US National Institutes of Health, Bethesda, MD, USA), to obtain their diameters as a function of both time and coefficient of variation (CV). Scanning Electron Microscopy (SEM). Each microcapsule was placed onto a scanning electron microscope (SEM) mount and coated for 30 s by an Emitech, K550X Sputter coater (Quorum Technologies Ltd., East Sussex, UK) to create 10-nm thick platinum layer. Scanning electron microscopy (SEM) of the microcapsules was performed using a Phenom SEM (FEI Company, Hillsboro, OR, USA). Compression Tests. Single compression tests were performed on individual microcapsules through a customized tester according to the setup suggested by Keller and Sottos.46 The compression instrument is made of a DC motor actuator and controller (M-230.25DC and C863 Mercury, Physik Instrumente, Germany) to compress the collected microcapsules at a strain rate of 50 µm/s and a 100g load cell (GSO,TransducerTechniques,USA) to measure the applied force. A NI-9237 DAQ module (National Instruments, USA) and the motor controller are utilized to acquire live force and displacement data respectively. The in situ deformed microcapsules were captured from an overhead view through a Z16 APO macroscope (Leica, Germany). Release Studies. The permeability of the PU membrane forming the wall of the microcapsules for a hydrophobic blue dye (Sudan II), were determined by observing the release rate of the dye into an ethanol and 2-ethyl-1-hexanol environment. For these tests, the liquidfilled, pure PU (polymer only) and liquid-filled mCNC-PU microcapsules were utilized. The encapsulated dye was extracted by washing with acetone. The extracted dye solution was


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solvent-exchanged with 2-ethyl-1-hexanol. Therefore, the amount of loaded dyes, M0 (mass of Sudan II [blue] dye loaded initially) was determined from UV-vis absorbance spectroscopy and a standard curve. To obtain Mt, the cumulative release mass of the dye at time t, the microcapsules were filtered and washed 3 times with distilled water. The filtered microcapsules were dried to remove surface moisture at 60 ℃ for 4 h in the oven and taken in a release container containing 3 mL 2-ethyl-1-hexanol. The ratio of the solid phase (i.e. microcapsules) to the liquid medium (i.e. mineral oil) was maintained at 0.1 g/3 mL. The release studies of dye loaded microcapsules was conducted in ethanol or 2-ethyl-1-hexanol at room temperature under 100 rpm agitation. The released dye solutions were periodically collected and measured by UV-vis spectrometer. Their absorbance at 600 nm wavelength can be transferred into concentration using a standard dye solution curve. After the measurement, each sample medium was replaced by 3 mL of fresh ethanol or 2-ethyl-1-hexanol for the following test. A sample taken in the size of 0.1 g from individual PU microcapsules used in the release studies was dispersed in 10 ml of dehydrated chloroform and stirred therein for 16 hours. The resultant dispersion was filtered through a JIS standard 100 metallic mesh net (100 mm×100 mm). After washing the resultant gel with chloroform three times, it was poured into a dish and dried at 80° C to obtain the dried gel part of the PU microcapsule [W1 (g)]. The experiments were performed twice. The solvent-insoluble component content was calculated in accordance with the following numerical eq 3. ‫ ݐ݊݁ݐ݊݋ܥ ݈݁ܩ‬ሺ%ሻ = ଴.ଵభ × 100 ௐ

ሺ3ሻ


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Results Encapsulation. In a typical encapsulation with polyurea-urethane (PU) by interfacial polymerizatoin47-48, the active agent is mixed with the isocyanate to form walls and optional hydrophobic solvents in the oil phase. To disperse the oil phase into an aqueous continuous phase, the mixture is mechanically agitated to which active hydrogen sources are added later to execute the interfacial polymerization. Microcapsule size and distribution can be selected by the emulsion technique.48 Of course, more stable and smaller sized emulsions can also be produced by application of ultrasonication; but these microcapsules are too small to be analyzed for their morphology and mechanical properties. Therefore, mechanical agitation was utilized in this synthesis. This study’s purpose is to describe how the encapsulation occurs in detail (Figure 2). Establishing the mechanisms of microcapsule membrane formation and growth is essential to adjusting microcapsule performance with regards to diffusivity and strength. The two major wall growth mechanisms are the moving boundary mechanism and the stationary boundary mechanism.15 In this system, TDI as an isocyanate and mCNCs/dye suspended in 2-ethyl-1hexanol or PTMG/mineral oil were utilized as an oil dispersion phase. A toluene diamine hydrolyzed from TDI, multifunctional amine, fatty alcohol, or polyol/glycol were active hydrogen sources. Competitive interfacial polyaddition of a mixture of amine and alcohol to TDI should thus lead to sequential incorporation of these two active hydrogen sources. Urea reactions typically proceed much more rapidly than urethane reaction.49-50 In the case of the hollow microcapsules, the polyurea produced initially should be rich in aromatic urea moieties, whereas polyurea-urethane created later should be rich in aliphatic urethane moieties (Figure 2). Hence, the encapsulation mechanism in this system can be described as follows. The water-soluble diamine monomer, which is hydrolyzed from TDI, reacts with the oil-soluble TDI monomer,


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which is made possible by the diffusion of the diamine through the initially produced polymeric wall. Then, the wall thickness grows toward the oil phase. The fatty alcohol in the oil phase also is consumed by the urethane reaction as an active hydrogen and then the PU shell grows thicker. The modified CNC nanoparticles, which are amphiphilic and exist mainly at the interface of water-hydrophobic core, are spontaneously embedded into the polymeric shell during the filmforming reaction. At the same time, the CO2 gas (hollow microcapsules) and hydrophobic ingredients (liquid core microcapsules) are entrapped in the core. The reaction area in the interfacial wall formation moves inward during the reaction, stacking up continuing layers on the interior microcapsule film. The preparation of mCNC-embedded PU microcapsules filled with a dye solution in mineral oil was carried out according to the process shown in Figure 2b. Although the surface modification of the CNCs was conducted to make them oil-soluble in emulsion systems, the mCNCs have an amphiphilic surface. Therefore, the mCNCs, PTMG, and TDI reside at the interface of emulsion droplets. With this emulsion system, the reactive oil components (the mixture of TDI and PTMG/1,6-HD) can be immediately solidified into a polymeric shell, followed by embedding the mCNC particles inside the shell film and the encapsulation of the dye solution in mineral oil. When a certain degree of TDI and PTMG/1,6-HD monomer conversion was achieved, crosslinked polyurea-urethane (PU) microcapsules with strong and dense walls were successfully fabricated by adding TETA to the emulsions to initiate an interfacial polymerization between PTMG-TDI in the oil phase and TETA in the water phase. These mechanisms have been utilized in many studies to demonstrate the encapsulation processes for polyurea-urethane microcapsules.48, 51-52


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Encapsulation yield and efficiency of the microcapsules were calculated and the results are shown in Table 1. The encapsulation yield was found to be between 83 % and 91 %; the encapsulation efficiency was found to be 86-97 % and 88-95 % for hollow microcapsules and liquid-filled microcapsules respectively. All the dye cannot diffuse into emulsion polymerization droplets across the continuous phase; thus a small amount of dye remained out of the microcapsules. The dye that had not been encapsulated into microcapsules became suspended in the aqueous phase or existed on the surface of the microcapsules. Liquid-filled microcapsules have comparatively less encapsulation efficiency due to the loss of core components during their emulsion processes; yet it does not mean that they should not be taken into consideration for in vitro release studies. Accordingly, there was no distinct difference of yield and efficiency between the hollow microcapsules and the liquid-filled microcapsules or between pure PU microcapsules and mCNC-PU microcapsules. Above all, the dye encapsulation efficiency of in situ polymerized microcapsules can be affected mainly by the core to shell ratio.


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Figure 2 Encapsulation mechanism of nanoreinforced, controlled release microcapsules through CNCs embedded in polyurea-urethane matrix.

Microcapsule Chemical Compositions. The polymeric shell of these microcapsules was composed of a polyurea-urethane, which is slightly chemically-crosslinked or simply crosslinked by hydrogen bonds of urea groups.53 Microcapsule walls formed from linearly polymerizable monomers or a small amount of crosslinker (the ratio of the equivalent NCO of the TDI and NH or NH2 of the TETA below 2.0), resulting in a moving boundary mechanism, would tend to result in highly permeable and weak walls without a dense interpenetrating network. In particular, the slightly crosslinked PU microcapsules can be produced as follows. After forming linear polyurethane microcapsules, TETA was post-added to these polyurethane microcapsules. Likewise, since addition of TETA either too early or too late may make the microcapsules fully crosslinked or less crosslinked, respectively, the timing of TETA-addition is important.


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Therefore, the TETA-addition was conducted 0.5 h after the initial reaction time because reactive isocyanate groups of TDI remains in the system to produce the slightly crosslinked PU (polyurea-urethane) shell. Accordingly, the chloroform-insoluble portion of the PU microcapsule shell (gel % content) was calculated and the results are shown in Table 1. The gel % content of the prescribed PU microcapsules, which represents the degree of crosslinking density of a polymeric membrane, are not that distinct (hollow microcapsules: ~1 % and liquid core microcapsules: 24-36 %). However, an increase in gel % content with increased mCNC content could be caused by the reaction between –NCO terminal group of PTMG-TDI and –OH on the surface of mCNCs at the interface of the emulsions, not caused by an increase in the crosslinking density.52 Crosslinking density of the microcapsules can be further increased technically; yet, the encapsulation yield and loading efficiency should be considered and eventually they may decrease. However, the corresponding mCNC enhanced PU microcapsule wall formed at the oilwater interface afterward serves as a nanoreinforced and controlled-release membrane for the encapsulated fills, yielding high encapsulation. Therefore, this nanocomposite has a high rupture strength, even though it is somewhat brittle, breakable, and less tough, consisting of only a linear or slightly crosslinked polymer. Furthermore, the release of active agents in the liquid-filled microcapsules can be mainly controlled by percolating network structures of CNCs in conjunction with a slightly crosslinked polymer network as will be shown below. Likewise, their mechanical strength and permeability can be tailored by incorporating the CNCs into the polymer matrix. To the best of our knowledge, this is the first CNC embedded microcapsule synthesis through interfacial polymerization in water-in-oil emulsions. FT-IR spectra of the pristine CNC, mCNC (CNC-g-PLA-C18), hollow PU microcapsule shell (no mCNC), and hollow, 7.5 % (w/w) mCNC embedded PU microcapsule shell are shown


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in Figure S1 of the Supporting Information. The urea linkage was produced by reacting TDI with active hydrogen in the aqueous solution and is verified by observing the absorbance peaks at around 3300 cm−1 (NH) and 1700 cm−1 (C=O) for the resultant microcapsules. There is no band at 2250 cm−1 related to unreacted –NCO, demonstrating the urea reaction completion. A strong band at higher frequency regions are observed because of hydrogen bonded N-H stretching vibration at 3300 cm−1 and free N-H stretch at 3400 cm−1. A significant shoulder beside the N-H stretch at 3400 cm−1 appeared at 3500 cm−1, which corresponds to the hydroxyl group of modified CNCs, demonstrating the incorporation of the CNC into the PU shell. Thermogravimetric analysis (TGA) was utilized to check the existence of mCNC nanoparticles in hollow PU composite microcapsules. The thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves for pure CNC and CNC-g-PLA-C18 are shown in Figure S2 of the Supporting Information. Incorporation of dye and mCNC into hollow PU microcapsules produces four weight loss regions indicating that decomposition of the three materials happens as mainly individual events.


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Table 1 Monomers, DSC analysis, gel content, and encapsulation yield/efficiency of nine types of differently loaded microcapsules

a) Only 2000 ppm of dried Sudan II dye included based on total weight of polymer used b) 2000 ppm of Sudan II dye solution in mineral oil c)

Purified and dried microcapsules after encapsulation

Morphology. The liquid core or hollow microcapsules maintained their morphologies when dried and re-suspended in a liquid medium. SEM images of broken hollow or liquid core microcapsules were taken to observe the inner microcapsule morphology and determine shell thickness (Figure 3). Their thickness ranges from 10 to 70 µm, depending on their microcapsule size. The SEM images of both microcapsules reveal that they are spherically shaped, rigid, and have a smooth outer (Figure 4). The size distribution of the microcapsules was determined by statistical treatment of SEMs (Figure S3 in the Supporting Information). Agitation rates can be adjusted to achieve a range of microcapsule sizes using a mechanical agitator, resulting in relatively large microcapsules with a radius of 40 - 500 µm. However, the broad size distribution of the microcapsules can be improved by other stable emulsification processes: microfluidic channels or membrane emulsification.54-55 With 7.5 % CNC-g-PLA-C18, microcapsule shape becomes less spherical and more oval-shaped because of this high viscosity (Figure 4). A closer


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inspection of the microcapsule shell reveals that the CNC particles are deeply embedded into the shell, but the CNC particles of 7.5 % CNC microcapsules come to the surface of the polymer membrane leading to a rugged surface (Figures 4d). The effect of the mCNC content on the average microcapsule diameter and shell wall thickness is also shown in Figure S4 of the Supporting Information, which definitely indicates that the diameter and thickness increased linearly with increasing the mCNC content despite the change in agitation rate from 800 to 2500 rpm. Likewise, at higher mCNC contents, the emulsified oil droplets were bigger, indicating that viscosity of the oil droplets became larger. Therefore, the amount of shell components encapsulating each oil droplet was also larger, resulting in a thicker shell membrane of the corresponding microcapsules, when the amount of core components was fixed. There is a widespread and critical issue of poor resistance performance of PU microcapsules, which should be improved for handling or post processing. The surest way to improve the resistance performance of microcapsules is by increasing the shell thickness56; yet, the design and preparation of robust PU microcapsules having a thicker shell is still considerably challenging as previously reported in other investigations of the thickness of PU microcapsules.53,

57-61

(see

Table S1 in the Supporting Information). However, in this study, the shell is quite thick, and the shell thickness of mCNC-PU microcapsules ranges from 20 to 70 µm compared to the average shell thickness of pure PU microcapsules from 10 to 30 µm. The thick shell functions as an excellent barrier to leakage and leads to enhanced mechanical stability to rupture from post processing.


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Figure 3 Scanning electron micrographs of hollow microcapsules containing a) 0, b) 2.5, c) 5.0, and d) 7.5 wt % CNC-g-PLA-C18, and liquid core microcapsules containing e) 0, f) 2.5, g) 5.0, and h) 7.5 wt % CNC-g-PLA-C18.


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Figure 4 Scanning electron micrographs of hollow microcapsules containing a) 0, b) 2.5, c) 5.0, and d) 7.5 wt % CNC-g-PLA-C18, and liquid core microcapsules containing e) 0, f) 2.5, g) 5.0, and h) 7.5 wt % CNC-g-PLA-C18.

Mechanical Properties. Images of an mCNC enhanced microcapsule and a pure microcapsule, dried overnight at 50℃ and then placed between microscope slides are shown in Figure 5. The 200 g weight was softly loaded on the sample for 1 min. The pure microcapsules in Figure 5a do not show brightness. On the other hand, the CNC embedded hollow microcapsules in Figure 5c exhibit bright regions when placed between crossed polarizers. As CNCs wish to maximize interfacial contact they are distributed tangentially (i.e. they lay at the interface62). This arrangement allows the native birefringence of the CNCs to show classic “Maltese cross” patterns in polarized microscopy.54,

63

In addition, CNC reinforcement allows these

microcapsules to resist compression with elastic deformation followed by rupture of the microcapsule wall which brings about ceramic-like fragments. Whereas, pure microcapsules (polymer only) are likely to be pliable compared to mCNC microcapsules and collapse or burst when compressed.


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Figure 5 Polarized OM images of microcapsules with pure PU a) before and b) after bursting and 5 wt% mCNC/PU walls c) before and d) after compression.

Single microcapsule compression tests were performed to determine force to failure and elastic modulus of hollow microcapsules containing 5 wt% CNC-g-PLA-C18 as one representative example of CNC embedded microcapsules. The mechanical properties of the mCNC-PU shell microcapsules measured in compression are shown in Figure 6 and compared to pure PU (polymer-only) microcapsules.46,

64

Point I corresponds to the transducer probe

compressing the microcapsule; thus the force being imposed on the microcapsule increased until point II where the microcapsule ruptured. At point II, the brittle composite microcapsules started


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to split into several fragments already at a compressional deformation of ε ≈ 0.2, whereas the pure PU microcapsules only burst at deformations as high as ε ≈ 0.3 – 0.4. The probe continued to move down to compress the fragments of the microcapsule and thus the force being imposed on the microcapsule dropped to point III. These curves display how the rupture force and elastic modulus of the mCNC-PU shell microcapsules (around 1.0 – 1.4 N and 5.8 – 7.5 N/mm, respectively) are different from the pure PU microcapsules (about 0.1 N and 0.2 – 0.3 N/mm, respectively). The results show that force to failure of microcapsules increases by up to 10 times through CNC reinforcement even considering bending effects in the walls of pliable pure microcapsules (polymer only).65-66 Force to failure increases with microcapsule size, or more precisely, shell thickness. However, the mCNC-microcapsules can easily be crushed, releasing the core active agents in a burst. Rupture strength and elastic modulus of the mCNC-PU hollow microcapsules are much higher than those of the pure hollow microcapsules, indicating that the mCNC-reinforced microcapsules are brittle and breakable because of their high stiffness. In other words, the composite microcapsules can be easily picked up by hand or by using tweezers without inducing any obvious physical damage (rigidity insures avoidance of premature breaking), but they can be easily cracked and broken above a specific stress. The high crystallinity and Young’s modulus (rigid) rod shape of CNCs lead to microcapsules with improved structural rigidity and strength, but low strain to failure. The most typical polymeric walls often remain nonbreakable and provide high permeability for the active core components even after further chemical cross-linking.64 This means that the mechanical properties of microcapsules can be tailored by incorporating CNCs into their shell structure. Breakable microcapsules can be used in self-healing materials, cigarette filters, or encapsulated fragrances.46, 67-68


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Figure 6 Mechanical properties and rupture behavior of hollow microcapsules with pure PU and 5 wt% mCNC/PU walls. a, b) Typical force-deformation curves (ɛ is the fractional deformation: ɛ=[D (diameter of microcapsule) – d (displacement)]/D of mCNC/PU hollow microcapsules a) compared to pure PU hollow microcapsules with polymer-only shells b). mCNC/PU hollow microcapsules fracture as rigid, brittle materials, whereas typical PU hollow microcapsules yield collapsed, pliable skins upon compression. c) Photo images of mCNC/PU hollow microcapsules with thick (shattering resistance) and thin (fragmented shattering) walls upon deformation, failure, and post fracture at corresponding times (t). d) Schematic diagrams


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showing deformation of microcapsules with a hollow core under single compression set-up (50 µm/s): micromanipulation with two parallel plates.

Release Properties. To check the effect of the embedded mCNC on the release of the microcapsules, dye loaded microcapsules were produced with varying contents of CNCs. To load the dye into a microcapsule, a Sudan II dye solution in mineral oil (a concentration of 2000 ppm) was utilized as a core component. Dye release studies have been conducted using these dye loaded microcapsules. In this manner, the microcapsules were dispersed in ethanol and 2-ethyl-1hexanol, respectively, at room temperature and stirred for one day to enable the release of the dye. The release was studied via time-resolved measurement of the UV-vis absorbance of the outer bulk solution over a specific period of time using a UV-vis spectrometer (Figure S5 in the Supporting Information). The relative release rate of core components is calculated by eq 4: ‫ ݁݃ܽݐ݊݁ܿݎ݁ܲ ݁ݏ݈ܴܽ݁݁ ݁ݕܦ ݁ݒ݅ݐ݈ܽݑ݉ݑܥ‬ሺ%ሻ =

‫ܯ‬௧ × 100% ‫ܯ‬଴

ሺ4ሻ

where ‫ܯ‬଴ ܽ݊݀ ‫ܯ‬௧ are initial mass and cumulative release mass of Sudan II (blue) dye at time t respectively. (‫ܯ‬଴ is measured by breaking the microcapsules via mortar and pestle, extracting the

dye with acetone, solvent-exchanging with mineral oil, and measuring the UV-vis absorbance of the extracted solution.)

Figure 7 shows the experimentally determined release of Sudan II dye from liquid core, pristine PU microcapsule and mCNC-PU microcapsules in well agitated (100 rpm) media; plotting the cumulative dye release percent as a function of immersion time. The release rates are significantly linear in the initial stages with some curvature at subsequent times due to a decrease in the osmotic pressure inside/outside the capsules. With the ethanol solvent, the diffusion rate of


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encapsulated dye in the outer PU shell is considerately high at the beginning of diffusion regardless of the effect of mCNC. The diffusion process is dramatically faster in the hydrophilic ethanol, which can wet and swell the immersed microcapsules.52 Therefore, the cumulative release fraction achieved 90 % after releasing dye for only 1 hour as shown in Figure 7a. On the other hand, the monitored release rates are significantly different for the pure PU microcapsules and other mCNC microcapsules in 2-ethyl-1-hexanol. This controlled dye release may be caused by the hydrophilic nature of the PU polymer, which retards the penetration of the dissolution carrier into the microcapsules leading lower dissolution and diffusion of the dye molecule from the microcapsules. The pure microcapsules were permeable to the dye and released it during a short period of time. However, the curves indicate that the mCNC enhanced microcapsules were less permeable to the dye during the release period. In comparison with the release curve without the mCNC, the core release exhibits gradual decreases corresponding to its mCNC content in the polymeric shell. Whereas the pure PU microcapsules displayed 50 % core release in less than 30 min, it took 120 min for the 7.5 % mCNC enhanced microcapsules to complete 50 % core release, illustrating a remarkable decline in the average loss profile from nanocomposite microcapsules. This progressive attenuation in the release profile was caused by the embedded mCNC barrier layer. In addition, the remaining core-dye contents after full release of the dye from PU microcapsules are found to be 3–6 % in accord with the full release content of dye in Figure 7. Hence, the release rate is controlled sharply with increasing mCNC content in the shell, showing the possibilities of fine-tuning active agent release using nanoparticles. In this study, the data can therefore be explained in terms of a permeation model. The dissolution of the dye and its diffusion are likely the primary rate determining steps for dye release. The slow release of dye in the presence of mCNCs is apparently because of the barrier enhancement effect


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of needle-like nanoparticles for which diffusion of dye molecules is hindered causing controlled release in nanocomposites. Likewise, the dispersed mCNCs in PU matrix are considered to enhance the barrier abilities of the shell components by producing a tortuous path that suppresses the rate of diffusion of dye from the matrix resulting in lower release rates in nanocomposites as compared to pristine polyurea-urethane. Furthermore, the high aspect ratio and better interacting system in nanocomposites control the diffusion rate of dye from the PU shell causing suppressed release in nanocomposites. To prove this theory, the specific release mechanism and mathematical modeling are needed for dye release kinetics.

Figure 7 Percent cumulative concentration of Sudan II blue dye released from microcapsules with different mCNC contents to the surroundings: a) ethanol and b) 2-ethyl-1-hexanol over time at room temperature and an agitation of 100 rpm.

There are four types of controlled release systems in the drug delivery system according to a core-shell system and a continuous matrix.5 The core-shell structure can be further divided into a “nonconstant activity source” and a “constant activity source.” In the first case, drug


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concentration in the core is lower than its solubility and its release rate decreases with time. Because all dye molecules in this study are quickly dissolved, the PU microcapsule functions as a core-shell structure with a nonconstant activity source with first order kinetics.5, 45, 69 Three mass transport processes happen in a series: (1) mineral oil diffusion, (2) dye dissolution, and (3) dye diffusion. Among these three processes, the dye diffusion rate is much slower compared to the other steps’ rate and it is therefore the rate determining step.5 The release rate controlling PU membrane in hydrophobic medium does not significantly swell, shrink, or dissolve, and does not considerably change in dye permeability to hydrophobic medium during the release period. The diffusion pathway length and diffusion coefficient of dye is therefore constant. The released dye also does not significantly reduce the release of dye still remaining in the microcapsule core. With some assumptions of perfect sink in the surrounding bulk fluid, controlled release through a polymeric network, constant diffusion path length, and constant diffusion coefficient, the dye release amount can be theoretically quantified by the mathematical expression of mass transfer proposed by Crank69-71: ‫ܯ‬௧ − ‫ܯ‬଴ ‫ݐܭܦܣ‬ 3‫ܴܭܦ‬଴ ‫ݐ‬ = 1 − exp ൬− ൰ = 1 − exp ቆ− ቇ ‫ܯ‬ஶ − ‫ܯ‬଴ ܸ‫ܮ‬ ሺܴ଴ − ܴ௜ ሻ × ܴ௜ ଶ

ሺ5ሻ

where ‫ܯ‬௧ and ‫ܯ‬ஶ are cumulative mass of dye at t time and infinity (at plateau) respectively;

‫ ܸ ݀݊ܽ ܣ‬are total surface area and volume of microcapsule; ܴ଴ and ܴ௜ are inner and outer radius of microcapsule; L is length of the diffusion pathway (time independent); and ‫ ܦ‬and ‫ ܭ‬are

apparent diffusion coefficient (constant) and partitioning coefficient from the microcapsule core into the polymeric film shell. This mass transfer follows power law and looks to obey Fickian diffusion; thus n is 0.5. It is expected that the release rate (Mt/M∞) increases linearly at the initial stage as a function of square


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root of time, t1/2. Diffusion coefficients were calculated from the slope of a) graph in Figure S8 of the Supporting Information using eq 6 and eq 7.72 ‫ܯ‬௧ 4 ‫ݐ∙ܦ‬ = k ∙ ‫ݐ‬௡ = ∙ ඨ ‫ܮ‬ ߨ ‫ܯ‬ஶ

ሺ6ሻ

0.04919 ∙ ‫ܮ‬ଶ ‫=ܦ‬ ‫ݐ‬଴.ହ

ሺ7ሻ

In this manner, the diffusion coefficient of the dye through the pristine PU film in mineral oil was determined to be 3.6×10-12 m2/s. The diffusion coefficients of the dye corresponding to different types of PU microcapsules estimated from such plots are tabulated in Table S2 of the Supporting Information. Among the samples considered, a variation in D from 9.9×10-13 to 3.6×10-12 m2/s is seen, which fits well with previously published investigations of mineral oil diffusion into polymer films.72-73 A partition coefficient (K) is a measure of the relative solubility of the dye in PU membrane vs mineral oil and is described elsewhere.74-75 In this study, a low K value indicates a dye molecule is not soluble in PU matrix. This partition coefficient can be calculated using the following eq 8: ‫=ܭ‬

‫ܥ‬௘௦௛௘௟௟ ‫ܥ‬௘௖௢௥௘

ሺ8ሻ

where the numerator is the concentration of the dye in the PU membrane and the denominator is the concentration of the dye in a microcapsule core at equilibrium. The partition coefficient of dye in PU membrane was estimated by plotting curves with the predetermined diffusion coefficient and different K values (Figure S6 in the Supporting Information). The K value of the pristine PU membrane converges to 0.15. Furthermore, the dye release profile data was fitted


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through Higuchi, zero-order, Hixson-Crowell, first-order, and Korsmeyer-Peppas models, respectively40, 69. We used the Regression analysis in Excel and evaluated the accuracy of the fit by calculating the adjusted R2 as a fitting parameter (see Figure S7 in the Supporting Information). The curve of the Sudan II release from the PU microcapsules shows a great adjusted r-square value (adj. R2=0.989) for exponential decay which fits a first order kinetic. Using this D and K value, when fitting this equation to the experimental data, there is a perceived agreement between theory and experimental data (dotted lines and solid line in Figure S5 in the Supporting Information). These release studies provide support for the existence of mCNC barriers, which control dye release mainly by diffusion. To prove this hypothesis, the theoretically estimated dye release kinetics were determined using eq 5. In this calculation, the influence of equation parameters such as film wall thickness and microcapsule size can be examined. Release rate relies only on the partition and diffusion coefficient, and thus is a function only of the membrane structure in these experiments. In Figure 8, release rates according to different diffusion lengths were calculated using the predetermined D and K value and plotted as a function of measured shell thickness (L).5 It is assumed that whereas the partition coefficient is liable to be only a weak function of the mCNC content, the diffusivity is likely to be a strong function. The curves display the theoretically calculated release rate of dye from microcapsules embedded with 0, 2.5, 5.0, and 7.5 % (w/w) of mCNC. To check the cogency of the mathematical calculations, experimentally determined dye releases from the microcapsules in mineral oil and applied theoretical predictions were compared. The curves of the dye-release from the PU microcapsules show an exponential curve progression (adj. R(0%)2=0.989, R(2.5%)2=0.950, R(5%)2=0.959, and R(7.5%)2=0.990) indicating a first order kinetic. Clearly, the theoretical calculation based on a


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simple first order equation serves as a reasonable prediction of the corresponding dye release trends being a good fit for each experimental set. Likewise, this remarkable agreement between theoretical prediction and experimental results supports further the hypothesis that the dye releases from the microcapsules studied are primarily determined by dye diffusion through the pure polymeric or the mCNC enhanced shells. As long as diffusion occurs mainly through PU resin domains, with the mCNC particles acting as barriers, the release rates can be sustained within a given period time. A distinctive correlation between the mCNC content and its diffusion length was found by comparing the experimental vs. theoretical release value. Figure 8 shows the relatively strong dependence of the release rate on the mCNC content. For example, the diffusion rate of 2.5 % CNC microcapsules having a length of 40 μm is lower than that of the theoretical value having a length of 190 µm. Nonetheless, pure PU microcapsules do not show any extended diffusion rate. The 7.5 % microcapsules show 17 times longer diffusion length than that of pure microcapsules; thus they need more than 10 hours for their release saturation. It turns out that the presence of fillers affects diffusivity by increasing the tortuosity in the nanocomposite membrane; thus, the permeability of polymeric shell microcapsule systems can be tuned by varying the content of mCNCs.


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Figure 8 Theoretically predicted (curve) and experimentally confirmed (symbols) dye release kinetics of mCNC enhanced microcapsules in 2-ethyl-1-hexanol.

Conclusion This study demonstrates that new release-controlled microcapsules can be fabricated through polyurea-urethane (PU) interfacial polymerization, utilizing modified CNCs as shell-forming materials. These microcapsules, composed of an active agent core and a PU shell embedded with modified CNCs, have high compressive strength and provide barrier enhancement against the release of core components. The radius of microcapsules ranges from 40 to 500 µm. The composite mCNC enhanced microcapsules described here can be suitable for the encapsulation and controlled delivery of hydrophobic fills, because release from these microcapsules can be


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governed by the hydrophilicity of carrier medium and concentration of the enhanced mCNCs. The results demonstrate that reinforcing a microcapsule shell with modified CNCs is found to have a significant effect on rupture behavior under compression; an obvious show of a higher rupture strength and elastic modulus, and low strain to failure as breakable composite microcapsules. Because of the close packing of the cellulose nanoparticles in the membrane, the encapsulation can protect the core component from diffusion loss during production, storage, and use of the microcapsules. The relative dye release rate decreases with time because of a decrease in osmotic pressure. The data show the trend anticipated of a diffusional release mechanism. Therefore, release rates strongly depend on the mCNC content, which is because of increased diffusion path length. Consequently, the mCNC enhanced system is a potential candidate for secure encapsulation of oil-soluble compounds, such as dyes, perfumes, phase change materials (PCMs), or drugs.

Supplementary Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FT-IR analysis, TGA analysis, particle size distribution, diameter/shell wall thickness at different agitation rates and mCNC content, UV-vis absorption data, plot of Mt/M∞ versus square root of time, different kinetic models for the dye release profile data, and table of diameter/shell wall thickness and diffusion coefficients of pristine CNC and modified CNCs used in this study (PDF). Acknowledgements The authors acknowledge for financial support the National Science Foundation Scalable Nanomanufacturing program under award CMMI-1449358, the Forest Products Laboratory


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under awards 11-JV-11111129-118 and 11-CR-11111129-109. The authors declare no competing financial interests.

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For Table of Contents Use Only

Sustained

Dye

Release

Using

Polyurea-urethane/Cellulose

Nanocrystal Composite Microcapsules

Youngman Yoo, Carlos Martinez, and Jeffrey P. Youngblood *.

Synopsis New release controlled and nanoreinforced microcapsule systems through the incorporation hydrophobized CNCs into polymeric wall membrane are discussed.


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Figure 1: One-pot esterification reaction scheme for hydrophobization of cellulose nanocrystals with lactic acid and oleic acid. 510x115mm (96 x 96 DPI)


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Figure 2

Encapsulation mechanism of nanoreinforced, controlled release microcapsules through CNCs embedded in polyurea-urethane matrix. 619x353mm (96 x 96 DPI)


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Figure 3 Scanning electron micrographs of hollow microcapsules containing a) 0, b) 2.5, c) 5.0, and d) 7.5 wt % CNC-g-PLA-C18, and liquid core microcapsules containing e) 0, f) 2.5, g) 5.0, and h) 7.5 wt % CNC-gPLA-C18. 530x280mm (96 x 96 DPI)


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Figure 4 Scanning electron micrographs of hollow microcapsules containing a) 0, b) 2.5, c) 5.0, and d) 7.5 wt % CNC-g-PLA-C18, and liquid core microcapsules containing e) 0, f) 2.5, g) 5.0, and h) 7.5 wt % CNC-gPLA-C18. 530x279mm (96 x 96 DPI)


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Figure 5 Polarized OM images of microcapsules with pure PU a) before and b) after bursting and 5 wt% mCNC/PU walls c) before and d) after compression. 263x264mm (96 x 96 DPI)


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Figure 6 Mechanical properties and rupture behavior of hollow microcapsules with pure PU and 5 wt% mCNC/PU walls. a, b) Typical force-deformation curves (ɛ is the fractional deformation: ɛ=[D (diameter of microcapsule) – d (displacement)]/D of mCNC/PU hollow microcapsules a) compared to pure PU hollow microcapsules with polymer-only shells b). mCNC/PU hollow microcapsules fracture as rigid, brittle materials, whereas typical PU hollow microcapsules yield collapsed, pliable skins upon compression. c) Photo images of mCNC/PU hollow microcapsules with thick (shattering resistance) and thin (fragmented shattering) walls upon deformation, failure, and post fracture at corresponding times (t). d) Schematic diagrams showing deformation of microcapsules with a hollow core under single compression set-up (50 µm/s): micromanipulation with two parallel plates. 541x543mm (96 x 96 DPI)


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Figure 7 Percent cumulative concentration of Sudan II blue dye released from microcapsules with different mCNC contents to the surroundings: a) ethanol and b) 2-ethyl-1-hexanol over time at room temperature and an agitation of 100 rpm. 564x271mm (96 x 96 DPI)


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Figure 8 Theoretically predicted (curve) and experimentally confirmed (symbols) dye release kinetics of mCNC enhanced microcapsules in 2-ethyl-1-hexanol. 393x301mm (96 x 96 DPI)


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Synopsis New release controlled and nanoreinforced microcapsule systems through the incorporation hydrophobized CNCs into polymeric wall membrane are discussed. 347x95mm (150 x 150 DPI)


Cellulose

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