Synthesis and Characterization of Microencapsulated Phase Change

Research Article www.acsami.org

Synthesis and Characterization of Microencapsulated Phase Change Materials with Poly(urea−urethane) Shells Containing Cellulose Nanocrystals Youngman Yoo, Carlos Martinez, and Jeffrey P. Youngblood* School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: The main objective of this study is to develop microencapsulation technology for thermal energy storage incorporating a phase change material (PCM) in a composite wall shell, which can be used to create a stable environment and allow the PCM to undergo phase change without any outside influence. Surface modification of cellulose nanocrystals (CNCs) was conducted by grafting poly(lactic acid) oligomers and oleic acid to improve the dispersion of nanoparticles in a polymeric shell. A microencapsulated phase change material (methyl laurate) with poly(urea−urethane) (PU) composite shells containing the hydrophobized cellulose nanocrystals (hCNCs) was fabricated using an in situ emulsion interfacial polymerization process. The encapsulation process of the PCMs with subsequent interfacial hCNC-PU to form composite microcapsules as well as their morphology, composition, thermal properties, and release rates was examined in this study. Oil soluble Sudan II dye solution in methyl laurate was used as a model hydrophobic fill, representing other latent fills with low partition coefficients, and their encapsulation efficiency as well as dye release rates were measured spectroscopically in a water medium. The influence of polyol content in the PU polymer matrix of microcapsules was investigated. An increase in polyol contents leads to an increase in the mean size of microcapsules but a decrease in the gel content (degree of cross-linking density) and permeability of their shell structure. The encapsulated PCMs for thermal energy storage demonstrated here exhibited promising performance for possible use in building or paving materials in terms of released heat, desired phase transformation temperature, chemical and physical stability, and concrete durability during placement. KEYWORDS: microcapsules, cellulose nanocrystals, microcapsules, methyl laurate, phase change materials, heat storage, deicing

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INTRODUCTION

allows for reduction of the demand for deicing salt which causes premature damage in these applications.6−8 However, placement of PCMs into pavements or buildings must be achieved without considerable changes in construction practices. In addition to appropriate thermophysical properties and kinetics, the PCM to be utilized in the design of thermal storage applications should possess desirable chemical properties: (1) chemical stability; (2) complete reversible freeze/melt cycle; (3) no degradation after a large number of freeze/melt cycle;

Phase change materials (PCMs) produce a significant amount of thermal energy (latent heat or heat of fusion) through their phase transformation from liquid to solid. Organic, inorganic, or eutectic materials can be selected as a PCM which can be widely used to store thermal energy from ambient, solar, or applied heat sources. Recently, there is growing interest in PCMs due to their many applications; they are used in thermal protection, in energy storage systems, and in electronic active and passive cooling systems.1,2 Several studies have been conducted to incorporate PCMs in buildings and pavements. When cooled, the stored energy can be released because of PCMs’ high heat of fusion, thereby delaying or preventing ice formation on the surfaces of buildings and pavements.3−5 This © 2017 American Chemical Society

Received: May 17, 2017 Accepted: August 8, 2017 Published: August 8, 2017 31763

DOI: 10.1021/acsami.7b06970 ACS Appl. Mater. Interfaces 2017, 9, 31763−31776

Research Article

ACS Applied Materials & Interfaces

Reinforcing fillers with a significantly large aspect ratio has been widely used to avoid the loss of the inherent fracture toughness of elastomeric resins.35 Therefore, to improve the mechanical and barrier properties of microcapsules, we utilized the advantageous characteristics of cellulose nanocrystals (CNCs), such as high thermal conductivity, high stiffness, and low coefficient of thermal expansion with a high aspect ratio to reinforce polymeric materials, forming a dense network resulting in sustainable, ecofriendly, and versatile polymer composites.36−40 Some studies report that the addition of 5 or 10 wt % CNCs into waterborne polyurethane or poly(vinyl alcohol) leads to a significant increase in Young’s modulus/ tensile strength41,42 and enhancements of the barrier performance of poly(vinyl alcohol) membranes and sodium caseinate films.43,44 However, some key issues involved in the highly hydrophilic surfaces of CNCs are moisture uptake/swelling (coefficient of hygroscopic swelling: ∼0.30%strain/%moisture content), agglomeration of nanoparticles, and difficulty in redispersing agglomerated particles.45,46 Previously, we reported that modified cellulose nanocrystals (hCNCs), which were surface-grafted using a lactic acid as a reactive solvent for the esterification of CNC with fatty acids (FAs), were utilized in preparation of capsule shell structures and tung oil wood finishes because of their unique compatibility with hydrophobic polymeric matrixes or organic solvents (acetonitrile, acetone, tetrahydrofuran, 1-methoxy-2-propanol, and chloroform).46−48 The hCNC enhanced PU 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 during handling or postprocessing.48 In our previous work, the hCNC particles were directly embedded into the PU matrix by in situ polymerization; yet, it is possible that there could be significant mass loss of reinforcement as the nanoparticles can be trapped inside the core of the capsule during interfacial polymerization. Therefore, in this study, the hCNC particles and PU monomers were highly dispersed in an organic solution (ethyl acetate) and then converted to nanoparticles bonded to isocyanate-terminated PU prepolymers, which were used in preparation of the PU composite microcapsules. In this study, c-PCMs were fabricated through an interfacial polymerization of poly(urea−urethane) (PU) as a shell component. Methyl laurate was chosen as a core component because of its desirable melting point (about 4−5 °C) for deicing applications and hCNC as a reinforcing material, and PTMG (polytetramethylene glycol) as a reactive monomer was used to enhance the thermal, mechanical, and release properties of the PU shell structure of c-PCMs. The benefits of the hCNC plus PTMG enhanced PCM microcapsules are the suppression of their core material release to the outside of the capsule (via rupture, diffusion, etc.), increasing the thermal conductivity and the stiffness, and enduring the frequent volume change of the PCM core during the phase change. The morphologies of cPCMs having an hCNC enhanced PU shell were examined by scanning electron microscopy (SEM). The chemical characterization of c-PCMs was investigated by Fourier transform infrared spectroscopy (FT-IR). The thermal properties and thermal stability of the obtained c-PCMs were examined by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA).

(4) noncorrosiveness to the construction materials; and (5) nontoxic, nonflammable, and nonexplosive materials for safety.9 Organic PCMs should meet demands for complementary chemical, physical, and thermal properties with the concrete or asphalt, such as an optimum operating temperature, high latent heat fusion/heat capacity, low phase segregation, and safety to humans and the environment, yet they generally suffer from the drawbacks of supercooling, low heat conductivity, large volume changes, and decomposition upon melting.3,4,10 To overcome these undesirable organic PCM properties, some studies recommend the esters of long chain carboxylic acids or fatty acids as a promising PCM candidate in building applications.11,12 These materials, mixtures of methyl-esters (methyl laurate, methyl palmitate, and methyl stearate), can have a phase transition close to the icing temperature (2 to −10 °C) or human comfort temperature (∼20 °C), as well as relatively high latent heat capacity. They further posit that the charging rate of PCM storage can be improved by using a hermetic encapsulation method via interfacial polymerization since fatty acid esters are not stable in alkaline environments.10,13,14 A PCM must be encapsulated by a polymeric shell to retain its shape, improve its heat exchange ability, and prevent the PCM from leaking and being decomposed during the postprocessing or phase change process.10,15,16 This encapsulated PCM (c-PCMs) must last for about 10,000 thermal cycles (or about 20 years of operational service) without any damage. However, most encapsulated commercial products have insufficient rupture strength and antiosmosis performance to tolerate volume changes or diffusional oil leakage during this operating period. The diffusional core leakage through the intact microcapsule wall can deteriorate the mechanical properties of the concrete or asphalt. Furthermore, some studies report the effects of microcapsule size on the thermal, structural, and mechanical stability of c-PCMs.10,17 For example, the core loss of small capsules (below 100 μm diameter) at high temperatures was found to be more significant than that of large capsules during the expansion/ contraction of microcapsules over repeated phase-change cycles.18 Furthermore, small microcapsules (5−100 μm diameter) with small core volume fractions bring about supercooling because the number of nuclei in each capsule reduces as the capsule size decreases.19−21 Thus, the straightforward way to improve the stability of microcapsules is to increase their diameter and shell thickness.22 Conventional interfacial polymerization techniques have some practical limitations on increasing the wall thickness and diameter of microcapsules due to their emulsion instability, although such limitations are dependent on the conditions of the preparation.23 Melamine-formaldehyde (MF) resin,24,25 urea-formaldehyde (UF) resin, 26,27 poly(urea−urethane) (PU) resin,28−30 gelatin and Arabic gum,31 and acrylic polymers32,33 are typically used as microcapsule shell components for PCM encapsulation. However, in the case of MF, UF, or acrylic resin, any remaining monomer residues (i.e., formaldehyde or acrylic monomer) in the shell can be a threat to human health or the environment. On the other hand, poly(urea−urethane) (PU) has vast utility and potential in encapsulation technologies. It offers low toxicity, high elongation, high tensile/tear strength, wear resistance, flexibility, and high toughness, but low stiffness. Thus, the addition of nanoparticles into PU resins is a wellknown approach to overcome this deficiency; yet, the fracture toughness of such elastomeric resins is also inevitably reduced by the addition of a nanofiller with high stiffness and strength.34 31764


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Figure 1. Fabrication scheme of nanoreinforced, PCM microcapsules through hCNC embedded in a poly(urea−urethane) matrix.

Table 1. Monomer Compositions of Eleven Microcapsule Formulations Cases C0P00M60a C0P10M60 C0P20M60 C0P40M60 C5P00M60 C5P20M30 C5P20M40 C5P20M60 C5P20M80 C5P20M90 C5P40M60

hCNC

0.080 0.040 0.040 0.050 0.028 0.028 0.055

Gly

PTMG

0.090 0.075 0.047

0.200 0.250 0.550

0.070 0.070 0.080 0.050 0.050 0.052

0.180 0.180 0.190 0.123 0.123 0.470

EA

TDI

DETA

PCM

1.000 0.980 0.000 0.000 1.000 4.000 4.000 3.200 1.800 1.800 4.000

1.450 1.320 0.550 0.500 1.470 0.490 0.490 0.550 0.350 0.350 0.500

0.200 0.430 0.300 0.270 0.200 0.090 0.090 0.100 0.065 0.065 0.093

2.130 3.000 1.780 2.050 2.130 0.390 0.580 1.480 5.500 5.500 1.750

NCO/OH 4.32 2.02 2.00 2.01 2.01 2.00 2.03 2.03 2.01

NCO/NH2 1.40 2.70 1.05 1.05 1.43 1.04 1.04 1.04 1.03 1.03 1.03

a

Input based: C0 or 5:0 or 5 wt % hCNC; P00, 10, 20, or 40:0, 10, 20, or 40 wt % PTMG content; and M30, 40, 60, 80, or 90:30, 40, 60, 80, or 90 wt % methyl laurate.

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(CNC-g-PLA). The reaction pressure and temperature were decreased to 100 mmHg and increased to 190 °C, respectively for 30 min, and then an excess of lactic acid was removed by conducting the reaction under reduced pressure. Then, the reaction pressure was gradually decreased to 10 mmHg and was maintained until the distillation column top temperature fell below 35 °C, thereby preparing an oleic acid and PLA grafted CNC. Subsequently, the final purification of the hydrophobized CNCs (hCNCs) from the remaining free fatty acids and homo-PLA oligomers was conducted using dispersion-centrifugation (6000 rpm at 25 °C for 30 min.) 4 times with an excess of ethanol. Finally, the hCNC was kept in ethanol or freeze-dried using a lyophilizer (Labconco FreeZone Plus 4.5 L). According to 13C CP/ MAS solid-state NMR analysis, the degree of polymerization of the PLA is about 3 and the degree of substitution of grafted PLA and oleic acid are about 30 and 20%, respectively.46 Synthesis of hCNC Enhanced PU Microcapsules. For the preparation of hCNC-PU composite microcapsules, hCNCs particles were dispersed in PU monomers, methyl laurate, and ethyl acetate solution, and then chemically bonded to isocyanate-terminated PU prepolymers during polyurethane prepolymer synthesis in Figure 1 (contrary to a previous publication,48 which refers to our old method that hCNC particles were directly embedded into the PU matrix by in situ interfacial polymerization). An hCNC that was kept wet in its washing solvent (ethanol) was used for its dispersion in an oil phase. The percentage (wt %) of the hCNC in the wet dispersions was determined to be approximately 18 wt %. An appropriate amount of wet sample was used to obtain 5 g of hCNC. Poly(tetrahydrofuran) (PTMG) was introduced so that the final concentration of hCNC was kept at 20 wt %. The wet samples were redispersed in the PTMG using a stirrer and then its dispersing solvent (ethanol) was eliminated under reduced pressure in a rotary evaporator. Different reactive monomers of PTMG (polyol, Mn ∼ 1000), Gly (glycerol, chain extender), hCNC, and DETA (diethylenetriamine, cross-linker) as a shell component and a blue dye solution in methyl laurate (Sudan II, 2000 ppm) as a core component were used to fabricate c-PCMs. Different weight ratios of core/shell droplets including various contents of PTMG and hCNC were utilized as shown in Table 1. To prepare the stable oil droplet, an appropriate

EXPERIMENTAL SECTION

Materials. Zinc acetate dihydrate at 98%, dibutyltin dilaurate at 95%, DL-lactic acid (LA) at 85% syrup, oleic acid at 90%, tolylene-2,4diisocyanate at 95%, glycerol at 99.5%, poly(tetrahydrofuran) (avg. Mn ∼ 1000), diethylenetriamine at 99%, methyl laurate at 99.5%, BioReagent light mineral oil (neat), anhydrous ethyl acetate at 99.8%, Sudan Blue II at dye content 98%, Mowiol 18−88 (poly vinyl alcohol), anhydrous calcium chloride at 96%, sodium hydroxide at 98%, and sodium dodecyl sulfate at 98.5% were purchased from Sigma-Aldrich, St. Louis, MO, United States. Ethanol (200 proof) and acetone (ACS grade) were purchased from VWR, West Chester, PA, United States. Polycarboxylate ether (PCE, Glenium 7700) was purchased from BASF Corporation, Ohio, United States. All reagents were used as received without further purification. Never-dried CNC (11.9 wt %, Lot no. 2016-FPL-CNC-065, dimension: 64 ± 5 nm (L) × 7 ± 1 nm (d), aspect ratio = 9.5, and crystallinity index 72%)46 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, United States. Chemical Hydrophobization of Cellulose Nanocrystals. CNCs were hydrophobized to graft poly(lactic acid) oligomers (PLA) and oleic acid side chain following a previous publication,46 which can be referred to for detailed explanations of the synthesis procedures. Briefly, an aqueous CNC suspension at 11.9 wt % was diluted with additional deionized (DI) water to prepare a 5 wt % CNC water suspension. An excess of 85 wt % DL-lactic acid syrup (the ratio of the equivalent COOH of the lactic acid and OH of the dried CNC = 10) was added to the CNC water suspension, followed by ultrasonication for 1 min. The suspension mixture was introduced into a 500 mL 3-necked round flask equipped with a condenser and a mixer. Then, a zinc acetate dihydrate catalyst (150 ppm based on the DL-lactic acid (LA) weight) was further added thereto, and polyesterification was implemented by water distillation at 180 °C. After 70−99% of the water was distilled, 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 introduced to the intermediate products, consisting of PLA oligomer and PLA oligomer grafted CNC 31765


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component or their pristine PU shells were fixed 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 deposit a 10 nm thick platinum layer. Scanning electron microscopy (SEM) of the microcapsules was conducted using a Phenom SEM (FEI Company, Hillsboro, OR, USA). Digital images of the microcapsules were analyzed using the freely available image analysis program, ImageJ (W. S. Rasband, US National Institutes of Health, Bethesda, MD, USA), to determine their diameters. Physical Breaking Test of Microcapsules. Each 1 g sample of microcapsules and solid-glass beads (5 g, borosilicate, diameter 3 mm) was added to 20 mL HDPE liquid scintillation vials, and then milled using a wheel mixer (30 rpm) for 6 h. Afterward, the pretreated microcapsules were immersed in a drop of mineral oil on a glass plate (25 × 76 × 1 mm). Another glass plate was carefully placed onto the surface of the treated microcapsules and a rubber sheet was applied to the top and bottom. A 9.8 N contact load was also carefully placed on the specimen for 30 s to fully fracture cracked and damaged capsules. After removing the compression force, the broken ratio was roughly estimated by checking the portion of broken microcapsules as shown in Figure 5 and Table 3. Likewise, physically broken microcapsules were achieved by both impact and compression forces via tumbling ball mill and subsequent weight load tests. Single compression tests were performed on individual microcapsules through a customized tester based on the setup developed by Keller and Sottos.52 The compression instrument is made of a DC motor actuator and controller (1046-PhidgetBridge 4-Input, USA) to compress the collected microcapsules at a strain rate of 50 μm/s and a 780 g load cell (3132-PhidgetBridge) 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 photographed from an overhead view through a Z16 APO macroscope (Leica, Germany). Release Studies. The diffusive permeability of the hCNC-PU membrane forming the wall of the microcapsules for a hydrophobic blue dye (Sudan II), were examined by checking the release rate of the dye into a water environment. For these tests, the dye and PCM-filled, pure PU (polymer only) and hCNC-PU microcapsules were utilized. The encapsulated dye was collected by bursting the microcapsules via mortar/pestle and rinsing with acetone. The collected dye solution was solvent-exchanged with mineral oil. Therefore, the amount of loaded dyes, M0 (mass of Sudan II [blue] dye loaded initially) was obtained from UV−vis absorbance spectroscopy and a standard curve. To determine Mt, the cumulative release mass of the dye at time t, the microcapsules were filtered and rinsed 3 times with distilled water. The collected microcapsules were dried to remove surface moisture at 60 °C for 4 h in the oven. The ratio of the solid phase (i.e., microcapsules) to the liquid medium (i.e., water) was maintained at 0.1 g/3 mL. Cement paste of Type I/II Lafarge cement (Mill certificate in SI) at a water/cement ratio of 0.45 was mixed in a FlacTek Speed Mixer for 120 s at 2000 rpm and then high alkali (pH = 13) supernatants were collected by centrifuging the cement paste for 100 min at 4000 rpm. The release studies of dye loaded microcapsules were performed in DI water, 5 wt % sodium dodecyl sulfate (SDS) aqueous, high alkali (the supernatants of cement paste, pH = 13), or water reducing agents (3 wt % polycarboxylate ether and 3 wt % calcium chloride) solutions at 25 °C under 100 rpm agitation. The released dye solutions were periodically collected and measured by a 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 water 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 h. 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

amount of tolylene-2,4-diisocyanate (TDI), PTMG, Gly, hCNC, Sudan II (2000 ppm based on the amount of methyl laurate) and methyl laurate were dispersed in ethyl acetate (EA). The mixture was stirred vigorously at 70 °C for 30 min and then introduced to 70 mL aqueous solution with 1.0 wt % copoly(vinyl alcohol-vinyl acetate) and 2 wt % SDS (sodium dodecyl sulfate). Suspension was implemented using a stirring impeller to make oil droplets at 40 °C. Afterward, 20 wt % DETA solution was dropped into the above emulsion to produce the cross-linked PU shell wall and the reaction was conducted at 60 °C for 3 h. The obtained microcapsule was decanted and washed with distilled water and cyclohexane, respectively, to remove the remaining PVA, dye, surfactant, and free monomers on the surface of the microcapsules; then, the microcapsules were dried in a vacuum oven at 25 °C for 24 h.49,50 The encapsulation yield of microcapsules was calculated by dividing the total weight of shell and core materials used by the total weight of microcapsules collected. The encapsulation yields were approximately determined by eq 1:

Yield (%) = Wcapsule WPTMG + WDETA + WTDI + WGly + Wmethyllaurate + WhCNC × 100

(1)

where Wcapsule is the weight of the prescribed, dried microcapsules, and WPTMG, WDETA, WTDI, WGly, and WhCNC are the weights of PTMG, Gly, DETA, TDI, and hCNC, respectively. Each 100 mg sample of microcapsules was crushed in a mortar/ pestle and dispersed in 10 mL of acetone, and the mixture was stirred vigorously for 12 h. After filtration via a PTFE syringe membrane filter (0.45 μm pore size, Fisher Scientific) and solvent-exchanging with 3 mL of mineral oil, the solution was measured spectrophotometrically and the content of dye fill in the microcapsules was determined by eq 2. The experiments were examined by conducting three measurements. The encapsulation efficiency of the microcapsules was determined by dividing the actual dye mass by the theoretical dye mass of the microcapsules.51 Efficiency (%) =

Actual dye content × 100 Theoretical dye content

(2)

Characterization of Microcapsules. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR transmission spectra was used to qualitatively check the chemical composition of the shell component of neat CNC, hCNC, polyurea microcapsules, poly(urea−urethane) microcapsules, and hCNC-PU microcapsules. (FTIR Spectrum 100, PerkinElmer, Waltham, MA, USA). All spectra obtained from the samples and KBr pellets range from 450 to 4000 cm−1 with a resolution of 4 cm−1 over 12 scans. Differential Scanning Calorimeter (DSC). Thermal properties of the new composite microcapsules: melting and crystallization points and latent heats and their encapsulation efficiency were measured by DSC. As a result, the effect of hCNC enhancement on the performance of the c-PCMs was investigated. The analyses were carried out at a 5 °C/min heating rate under a constant flow of nitrogen gas at a flow rate of 60 mL/min. Reproducibility was checked by performing three measurements. ThermoGravimetric Analysis (TGA). TGA analyses were performed to determine qualitatively the chemical composition of bulk PTMG, hCNC, polyurea microcapsules, poly(urea−urethane) microcapsules, and hCNC-PU microcapsules at 10 °C min−1 from room temperature to 700 °C under nitrogen flow using a TA Instrument Q50 thermogravimetric analyzer (TA Instruments, USA). Scanning Electron Microscopy (SEM). The c-PCMs maintained their morphologies when dried and resuspended in a liquid medium. The c-PCMs were immersed in melted paraffin and was solidified at room temperature. Then, it was carefully razor cut to achieve the cross-section. Afterward, the half-shelled microcapsules were rinsed with fresh ethanol to remove the core component and dried at 40 °C for 4 h. Individual microcapsules containing methyl laurate as a core 31766


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Figure 2. Scanning electron micrographs of (a) C0P00M60 (0% CNC-0% PTMG-60% methyl laurate), (b) C0P10M60 (0% CNC-10% PTMG60% methyl laurate, but excess TDI used), (c) C0P40M60 (0% CNC-40% PTMG-60% methyl laurate), (d) C5P20M30 (5% CNC-20% PTMG30% methyl laurate), (e) C5P20M90 (5% CNC-20% PTMG-90% methyl laurate), (f−h) cross sections of C0P10M60, C5P20M30, and C5P20M90 microcapsules. performed twice. The solvent-insoluble component content was calculated in accordance with the following numerical eq 3.

Gel Content (%) =

W1 × 100 0.1

active hydrogen monomers are introduced to carry out the interfacial polymerization. In this system, TDI was extended either with Gly to form urethane-hard segments or with diethylenetriamine DETA to form urea hard segments and then polymerized with PTMG to form soft segments, starting with readily available poly(urea−urethane) raw materials, and thereby the oil droplet formation stability and PU reactivity/ cross-linking density were appropriately controlled. According to our previous publication describing how the encapsulation occurs in detail,48 TDI as an isocyanate and hCNC/dye suspended in methyl laurate and PTMG were used as the oil dispersion phase. A toluene diamine hydrolyzed from TDI, multifunctional amine, or polyol/glycol were utilized as active hydrogen sources. However, urea reaction rates typically surpass urethane reaction rates.57,58 Likewise, competitive polyaddition reaction of a mixture of amine and alcohol to isocyanate is mainly attributed to unequal incorporation of these two active hydrogen sources. Thus, in order to avoid the biased reaction of the two reagents, an isocyanate-terminated prepolymer was utilized in this system, which is the reaction product of TDI and PTMG/Gly, where the equivalent ratio of NCO groups on the isocyanate to HO groups on the PTMG polyol plus the chain extender Gly is between 1.5:1 and 2.5:1, and the molar ratio of the polyol to chain extender is between 2:1 and 30:1. hCNC can be incorporated into the isocyanateterminated prepolymer chain according to the process shown in Figure 1. The fabrication of hCNC-embedded PU microcapsules filled with a methyl laurate component was implemented according to the following procedures. The surface hydrophobization of the CNCs was carried out to make them oil-dispersible for incorporation into the prepolymer. Therefore, the hCNC, PTMG, Gly, and TDI were mixed in methyl laurate solution in EA and their urethane reaction was carried out with 100 ppm of

(3)

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RESULTS Encapsulation. The poly(urea−urethanes) are synthesized by reacting hydroxy-terminated polyethers or polyesters and low molecular weight chain extenders (diamines or diols) with aliphatic or aromatic polyisocyanates. Since a chain extender and soft segment react competitively with a diisocyanate, it is important that the relative rates of the different reactions be adjusted to create the desired properties of PU resins. The polyol used in this study is polytetramethylene glycol (PTMG), owing to its low cost, ease of handling, and improved hydrolytic stability/flexibility over other polyols. TDI was chosen as a reactive aromatic isocyanate to prepare rigid PU. Multifunctional low molecular weight glycol (Gly) and diethyltriamine (DETA) were used as chain extenders and a cross-linker to obtain the desired degree of cross-linking density in PU synthesis.53 In particular, DETA is used as the amine monomer to achieve a dense and thick shell structure, owing to the moderate diffusivity caused by its appropriate chain length during the fabrication of the shell.54 Synthesis of the c-PCMs was conducted by interfacial polymerization with the microcapsule shell fabricated on the surface of methyl laurate oil droplets through poly addition of the monomer’s active hydrogens and multifunctional isocyanates. In a common encapsulation with poly(urea−urethane) (PU) by interfacial polymerization,55,56 the core material is mixed with the isocyanate to create walls and optional hydrophobic solvents in the oil phase. To disperse the oil phase into an aqueous continuous phase, the mixture is mechanically stirred and later 31767


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Figure 3. Scanning electron micrographs of (a) C0P20M60 (0% CNC-20% PTMG-60% methyl laurate), (b) C5P20M40 (5% CNC-20% PTMG40% methyl laurate), (c) C5P20M60 (5% CNC-20% PTMG-60% methyl laurate), (d) C5P20M80 (5% CNC-20% PTMG-80% methyl laurate), and (e−h) cross sections of C0P20P60, C5P20M40, C5P20M60, and C5P20M80 microcapsules.

1730 and 1705 cm−1 and another two urea carbonyl bands at 1695 and 1640 cm−1 are assigned to free carbonyl and carbonyl groups which form hydrogen bonds with urethane or urea NH groups.61 There is a strong band at higher frequency regions due to the hydrogen bonded N−H stretching vibration at 3300 cm−1 and free N−H stretch at 3400 cm−1. A distinct shoulder beside the N−H stretch at 3400 cm−1 appeared at 3500 cm−1, which is related to the hydroxyl group of hCNC, exhibiting the incorporation of the hCNC into the PU matrix. Morphology. The SEM images of the microcapsules produced by reaction of TDI with various active hydrogen sources are shown in Figure 2. From Figures 2a−e, it is seen that these five types of microcapsules (C0P00M60, C0P10M60, C0P40M60, C5P20M30, and C5P20M90 microcapsules) show different morphologies. The C0P00M60 microcapsules (produced with only DETA as an active hydrogen) and C0P10M60 microcapsules (fabricated with excess TDI and a small amount of PTMG including DETA) have a very thin outer membrane and a coarse, porous inner shell structure (Figures 2a and b), owing to the rapid reaction of DETA with TDI at the interface of oil droplets. In addition, the outer shell of the C0P40M60 microcapsules (synthesized with DETA as well as initially added 40% PTMG and a small quantity of Gly) is irregular and contains a small hole due to a high viscosity of the prepolymer used, while it is seen that the outer shell of the C5P20M30 microcapsules (having a 20% core content and synthesized with postadded DETA as well as TDI-terminated prepolymer) became more regular, due to their optimal reactivity with TDI, but showed some agglomeration (see Figure 2d). Furthermore, it is found that the outer shell of the C5P20M90 microcapsules (having a 90% core content) became rougher and their cross section could not be imaged since their thin membrane was fused thermally during the paraffin wax molding process. Therefore, optimal content of both PTMG and the core

dibutyltin dilaurate catalyst. With this preparation of the prepolymer, the reactive oil components (the mixture of TDI and PTMG/Gly) were promptly created, incorporating the hCNC particles inside the prepolymer structure. When the desired degree of TDI and PTMG/Gly/hCNC monomer conversion was achieved, the mixture solution (hCNCincorporated, NCO terminated prepolymer solution in EA and methyl laurate) was introduced into an aqueous solution and cross-linked poly(urea−urethane) [PU] microcapsules with robust and dense walls were successfully manufactured by introducing DETA to the emulsions to initiate a polyaddition polymerization between the prepolymer in the dispersion phase and DETA in the continuous phase. These methods have been proposed in many studies to describe the encapsulation processes for poly(urea−urethane) microcapsules.56,59,60 FT-IR analysis was conducted to determine the chemical composition of the shell polymer containing chemical bonds. FT-IR spectra of the bulk hCNC and PTMG, and the individual chloroform-insoluble gel portion of C0P00M60 (0% PTMG), C0P20M60 (20% PTMG), and C0P40M60 (40% PTMG) pure PU microcapsules, and C5P20M60 (20% PTMG) 5% hCNC embedded PU microcapsules are shown in Figure S2. A new peak of the hCNC and C5P20M60 is seen at 1760 cm−1, which was not shown in the pristine CNC, C0P20M60, and C0P40M60. This new peak may be generated from the emerging carbonyl groups of the ester group either in polylactic acid or in the fatty acid ester, demonstrating the esterification success of the CNC and the incorporation success of hCNC into the polymer shell matrix. The urea and urethane bond linkages were generated by reacting TDI with active hydrogen sources in the aqueous solution and are verified by examining the absorbance peaks at around 3300 cm−1 (NH) and 1730−1640 cm−1 (COurethane and COurea) for the corresponding microcapsules. Two urethane carbonyl bands at 31768


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ACS Applied Materials & Interfaces Table 2. Gel Content and Encapsulation Yield/Efficiency of Eleven Microcapsule Formulations Cases

PTMG%

hCNC%

Methyl laurate%

calculated (g)

collected (g)

yield%

Efficiency%

gel%

C0P00M60 C0P10M60 C0P20M60 C0P40M60 C5P00M60 C5P20M30 C5P20M40 C5P20M60 C5P20M80 C5P20M90 C5P40M60

0 10 20 40 0 20 20 20 20 20 40

0 0 0 0 5 5 5 5 5 5 5

60 60 60 60 60 30 40 60 80 90 60

3.78 5.04 2.96 3.42 3.83 1.26 1.45 2.45 3.02 6.12 2.92

3.15 4.49 2.63 2.97 3.08 1.20 1.33 2.17 2.55 4.40 2.57

83 89 89 87 80 95 92 89 85 72 88

79 82 89 91 83 97 97 95 92 88 90

48 42 27 20 58 61 55 36 1 0 28

Figure 4. Influence of PTMG and core content on the microcapsule characteristics of (a) size (plus oil droplet size), (b) shell thickness, (c) gel%, and (d) encapsulation efficiency%.

formation of the urea linkage. However, the C0P20M60, C5P20M40, C5P20M60, and C5P20M80 microcapsules have a regular core/shell microstructure. It is also found that the thickness ranges from 20 to 150 μm, depending on their microcapsule size. When PTMG and Gly are utilized as reactive hydrogens with the postaddition of DETA, the core/shell microstructure of the microcapsules is certainly distinct, and the PCM core component has enough space to prevent supercooling, including a thinner, denser, and more uniform poly(urea−urethane) shell component compared to a porous polyurea shell structure by the direct reaction of TDI with DETA.62 The timing of DETA-addition played an important

component is crucial to achieve the appropriate shape and surface of the c-PCMs. The SEM images of microcapsules (C0P20M60, C5P20M40, C5P20M60, and C5P20M80) fabricated at various weight fractions of the methyl laurate (40, 60, and 80 wt %) and PTMG (20 wt %) reveal that they are spherically shaped and rigid and have a smooth outer surface (Figure 3). Furthermore, SEM images of cross sections of c-PCMs were taken to observe the inner microcapsule morphology and determine shell thickness (Figure 3). The walls of the C0P00M60 and C0P10M60 microcapsules are too porous and sparse to distinguish a regular core/shell microstructure due to the fast 31769


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ACS Applied Materials & Interfaces Table 3. PU Composition Factors and Breaking Ratio of c-PCMs remarks before tumbling after tumbling a

C0P20M60

C5P20M60

C5P20M80

C5P20M60

C0P00M60

C5P00M60

No CNC 8.0%a 48.4%

Standard 4.7% 5.0%

too thin shell 6.0% 45.9%

big size 0.0% 10.2%

brittle polyurea 36.4% 82.8%

brittle polyurea 20.8% 85.0%

Number (%) of microcapsules below radius of 20 μm.

Figure 5. Images of (a) initial and (b) broken C0P40M60, (c) initial and (d) broken C5P40M60, and (e) initial and (f) broken C5P40M80 c-PCMs after tumbling and a 9.8 N load compression. Images of (g) initial and (h) broken C5P40M60, (i) initial and (j) broken C0P00M60, and (k) initial and (l) broken C5P00M60 c-PCMs after tumbling and a 9.8 N load compression.

S3) during the interfacial polymerization despite the change in agitation rates from 400 to 1500 rpm. Typically, the diameter of microcapsule decreases with the increase of stirring rate. However, Figure 4a shows the reverse tendency, which may be attributed to the greater effect of PTMG content on the viscosity of oil droplets (Figure S3). This influence led to bigger droplets of hCNC-PU microcapsules (average radius 183 vs 496 um) even at a higher stirring rate (800 vs 1500 rpm). As previously reported in other investigations of the thickness/diameter and rupture force of polymeric shell microcapsules, the design and synthesis of sturdy polymeric shell microcapsules having a thicker shell are still a significantly challenging work (see Table S1 in the Supporting Information). However, in this study, we suggested that a hCNC enhanced PU microcapsule functions as nanoreinforced (high rupture strength) and controlled-permeabilitty shell microstructures. Likewise, the shell thickness of hCNC-PU microcapsules ranges from 10 to 150 μm, which is considerably thicker than the average shell thickness of pure PU microcapsules (from 10 to 30 μm). Overall, the shell thickness of hCNC-PU microcapsules was well-tailored by a weight ratio of core/shell materials. It is seen that the formed microcapsule has a compact surface without any disfigurement with up to 80 wt % core; however, the thin shell layer of the microcapsules having 90 wt % core is rough and porous. This may be due to the fact that

role in the microstructure of the microcapsules which are fully cross-linked or less cross-linked, respectively, by addition of DETA either too early or too late. Likewise, the gel % content (chloroform-insoluble portion) of the PU microcapsule shell was determined and the results are shown in Table 2. The gel % contents of the microcapsules, which is a stand-in for the degree of cross-linking density of a polymeric shell, are certainly distinct according to their microstructures (C0P00M60, C0P10M60, C0P20M60, and C0P40M60). However, the urea linkage formation reaction between TDI and DETA at the interface of the emulsions could lead to an increase in gel % content (cross-linking density) of C0P00M60 and C0P10M60 microcapsules.60 Likewise, PCM microcapsules can be further cross-linked; yet, the loading efficiency and phase change performance of the core components deteriorates. The effects of the content of PTMG on gel content are anticipated by changing the urethane/urea bonds ratio. The gel content of PU microcapsules (Figure 4) increased from 20 to 48% with increasing PTMG (wt %) contents (0%, 10%, 20%, and 40%), owing to a urea cross-linking reaction. The size distribution of the microcapsules was calculated by statistical treatment of SEMs (Figure S1), ranging from 20 to 400 μm radius. Figure 4a and b clearly suggest that the diameter and thickness increased by introducing the hCNC and PTMG to the PU matrix due to their viscous oil dispersions (Figure 31770


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ACS Applied Materials & Interfaces the oil droplets become larger with increasing core/shell weight ratio, leading to a thin shell of the microcapsules. Subsequently, the thick and compact shell can serve as an excellent barrier to leakage and allow improved mechanical stability from rupture in postprocessing. Mechanical Properties of c-PCMs. The mechanical properties and wear resistance were measured through single compression tests and glass bead milling tests, respectively. hCNC-PU composites, prepared by using hCNC-incorporated prepolymer, have higher mechanical strength than that of those obtained by in situ polymerization at the same hCNC loading weight, because the former fabrication method allows more hCNC particles to be chemically bonded to PU chains and highly dispersed in the PU matrix than by the latter method. This is probably because hCNC particles in heterogeneous emulsion systems lead to nonneglectable mass loss of hCNCreinforcement and more aggregation during the in situ polymerization method than by the prepolymer solution method, thereby decreasing mechanical performance. Table 3 and Figure 5 show the breaking behaviors of individual samples with different hCNC/PTMG contents, microcapsule sizes, and wall thicknesses (core content). The broken ratio decreases with hCNC-reinforced, poly(urea− urethane) C5P40M60/C5P40M80 microcapsules compared to no CNC-poly(urea−urethane) C0P40M60 and polyurea C0P00M60/C5P00M60 microcapsules. Figures 5a, b, c, and d show the results of the breaking behaviors of two similar-sized microcapsules with and without hCNC particles. Figure 5b, of the pure PU microcapsules, shows more debris and fragments than Figure 5d, of the hCNC(5 wt %)-PU microcapsules, having a higher breaking ratio. Figure 5f, of hCNC(5 wt %)-PU microcapsules with thinner wall thicknesses (input-based 80% core), shows a roughly higher breaking ratio than Figure 5d, of the hCNC(5 wt %)-PU microcapsules with thicker walls (input-based 60% core). Additionally, there is no significant difference between Figures 5d and h (bigger microcapsules) in the given diameter range (100−1000 μm), which is also attributed to their thicker walls (a ratio range of the shell thickness to radius of hCNC(5 wt %)-PU microcapsules = 2−5 compared to that of pure PU microcapsules = 10−60). There is a noticeable hCNC effect between Figures 5j and l; yet, both polyurea samples with and without hCNC show a considerably higher breaking ratio than that of poly(urea−urethane) microcapsules having PTMG soft linkages (Figures 5a−h). Thus, microcapsule wall thickness and elasticity affect the breaking behavior of microcapsules. Single microcapsule compression tests were conducted to determine force to failure and elastic modulus of c-PCMs containing 5 wt % hCNC. The mechanical properties of cPCMs measured in compression, compared to pure PU (polymer-only) microcapsules, are shown in Figure 6. To compare the rupture behavior of c-PCMs with slightly different radius, the following normalization is applied: ε (fractional deformation) = [D (diameter of microcapsule) − d (displacement)]/D of c-PCMs. Region I in Figure S4c corresponds to the transducer tip compressing microcapsule; thus, the force being imposed on the microcapsule increased until the fracture point where the microcapsule ruptured.63 At failure, brittle polyurea C0P00M60 and C5P00M60 microcapsules were shattered or smashed to pieces at a compressional deformation of ε ≈ 0.4 (Figures S4b-5 and 6), whereas elastic poly(urea− urethane) C0P20M60, C5P20M60, C0P40M60, and C5P40M60 microcapsules only burst or collapsed at

Figure 6. Mechanical properties and rupture behavior of (a) C0P20M60, (c) C0P40M60, and (f) C0P00M60 with pure PU (open symbols) walls and (b) C5P20M60, (d) C5P40M60, (e) C5P40M60-2, and (g) C5P00M60 with 5 wt % hCNC-PU (closed symbols) walls in typical force-deformation curves (ε is the fractional deformation: ε = [D (diameter of microcapsule) − d (displacement)]/ D of c-PCMs. Green arrows indicate fracture point as observed by video.

deformations as high as ε ≈ 0.6−0.9 (Figures S4b-1 to 4). The indent tip moved down to compress the fragments of the microcapsule and thus the force being imposed on the microcapsule increased to region II (Figure S4c).64 These curves display how the rupture force and elastic modulus of cPCMs (around 0.2−0.4 N and 0.4−0.8 N/mm, respectively) containing 5 wt % hCNC are different from the pure PU microcapsules (about 0.04−0.1 N and 0.05−0.20 N/mm, respectively). The results describe that the fracture strength of c-PCMs increases by up to 3−4 times via hCNC reinforcement. Furthermore, Figure 6d, of C5P40M60 (produced by the isocyanate-terminated PU prepolymer method in this study), shows a higher rupture force/modulus than Figure 6e, of C5P40M60-2 (fabricated by the in situ polymerization method in our previous work48), since the former synthesis method allows more hCNC particles to be chemically bonded to PU chains and highly dispersed in the PU matrix than by the latter method. In particular, a remarkable enhancement in elastic poly(urea−urethane) C5P20M60 and C5P40M60 microcapsules containing 5 wt % hCNC is attributed to the fairly higher resistance to fracture of the hCNC, which leads to enhanced energy dissipation by a decreased Young’s modulus at the hCNC particle−PU matrix interface, the so-called “interphase”.34−65 It is very appealing that the fracture strength, as well as rigidity, increases with the hCNC content in the PU matrix, since in traditional nanocomposites higher rigidity is generally obtained at the cost of increased brittleness. The higher fracture strength may be due to the high bond strength (well-dispersed hCNC) and Young’s modulus (larger plastic zones) of the exfoliated structure. Overall, the high modulus and strength of the rod shape of hCNC lead to PU microcapsules with improved structural rigidity and strength and maintain high strain tolerance. Therefore, the mechanical properties of PU microcapsules can be improved and tailored by controlling the amount of hCNC and PTMG in the shell structure. 31771


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ACS Applied Materials & Interfaces Thermal Properties of c-PCMs. The thermal properties for the c-PCMs were measured by DSC, and the transient temperature response for individual microcapsules was examined through the solidification process in order to measure their cooling rate. The phase change enthalpies and phase change temperatures of bulk methyl laurate, bulk methyl laurate mixed with 10 wt % hCNC, and c-PCMs encapsulated by using poly(urea−urethane) (PU) polymers were measured through DSC, and the cooling and heating phase changes are shown in Figure 7. The methyl laurate core/capsule shell material

encapsulation yield. The mixtures were examined in terms of the phase change of the melted/solidified methyl laurate. Thermal enthalpy changes of the microcapsules during heating and following cooling (between −30 and 20 °C at the rate of 5 °C min−1) are given as data in Table 4. In the DSC chart, the peaks represent the solid−liquid transition. The latent heat of melting of pure methyl laurate and its mixture with hCNC was found to be 224.1 and 183.3 J/g, and the effective latent heat of melting of the PU capsules (C5P20M80, C5P20M60, C0P20M60, and C5P20M40) was 148.4, 113.2, 106.5, and 49.8 J/g, calculated using the integration area of the peaks. In each sample, the phase transition peak of the pure methyl laurate almost overlaps with that of the capsules. This data indicates the content of the methyl laurate in the capsules is from 22 to 66 wt % and the chemical properties of core materials are not subjected to any opposite effect in encapsulation. Therefore, the encapsulation efficiencies were defined as eq 4 and shown by the results presented in Table 4 and Figure 4d: Efficiency (%) =

ΔHm , capsule ΔHm , bulk

× 100 (4)

where Hm,capsule denotes the enthalpy of microcapsules and that of the bulk methyl laurate measured by DSC analysis. With the content of ingredient core dye solutions in emulsions, these results show a great relationship between the enthalpies of the methyl laurate used and that measured in the capsules, which is considered evidence of a high encapsulation yield (Figure 4d). In addition, encapsulation yield and efficiency of the microcapsules (via spectroscopically measured encapsulated dye content) were also determined and the results are shown in Table 2. The encapsulation yield was calculated to be between 72% and 95%; the encapsulation efficiency through the spectroscopy method was found to be 75−96%. The dye cannot diffuse into oil dispersions across the water phase; thus, a small amount of dye resided outside of the microcapsules. The dye that had not been encapsulated in the microcapsules existed on their outer surfaces. The spectroscopic results may show slightly higher encapsulation efficiency than those of the DSC analysis due to the existence of free (not encapsulated) dye on the outsides of the microcapsules or dye embedded inside the shell structure during their emulsion processes; yet, the difference is not significant. In particular, in the case of C5P20M40 (40% core content), the microcapsule displays a conspicuous gap of its efficiency data between the two methods since the significantly thick shell component as well as the core portion of C5P20M40 can contain the dye and the core component is too small for high thermal energy storage. Accordingly, no apparent difference of yield and efficiency between pure PU microcapsules and hCNC-PU microcapsules was observed. Above all, the PCM encapsulation efficiency of in

Figure 7. DSC curves of (a) bulk methyl laurate, bulk methyl laurate mixed with 10 wt % hCNC, and c-PCMs encapsulated by using poly(urea−urethane) (PU) polymers microcapsules; (b) C5P20M60 microcapsules during 10 thermal cycling.

mixtures (C0P20M60, C5P20M40, C5P20M60, and C5P20M80) were manufactured at various weight fractions of the methyl laurate (40, 60, and 80 wt %) to obtain the highest

Table 4. Thermal Properties of Microcapsules Containing Methyl Laurate PCMs bulk methyl laurate methyl laurate + 10% hCNC C5P20M80 C5P20M60 C0P20M60 C5P20M40

Latent heat of melting (J/g) 224.10 183.23 148.43 113.18 106.53 49.81

± ± ± ± ± ±

0.05 0.06 0.10 0.33 0.03 0.29

Latent heat of freezing (J/g) 211.07 ± 0.23 173.9 ± 0.20 138.37 ± 0.13 101.89 ± 0.26 96.69 ± 0.46 58.69 ± 0.61 31772

Melting point (°C) 5.48 4.91 6.58 6.12 5.95 2.26

± ± ± ± ± ±

0.05 0.13 0.02 0.01 0.08 0.16

Freezing point (°C) −0.78 −0.92 −1.64 −3.19 −4.40 −11.69

± ± ± ± ± ±

0.13 0.13 0.12 0.08 0.20 0.31

Core content (%) 100 82 66 51 48 22

Efficiency (%)

73 85 83


Research Article


Figure 8. Percent cumulative concentration of Sudan II blue dye released from microcapsules with different hCNC and PTMG contents to the surroundings (aqueous SDS, NaOH, and TCE/CaCl2 solutions) over time at room temperature and an agitation of 100 rpm.

situ polymerized microcapsules can be adjusted mainly by the core to shell ratio. The melting and solidification parameters of individual samples collected from DSC measurements are listed in Table 4. Among the heating thermogram of all the samples, the peak shoulders of the methyl laurate encapsulated within the PU microcapsules were slightly broader than that of bulk methyl laurate. Therefore, the melting peak temperature of the c-PCMs is slightly higher than that of bulk methyl laurate because of the low thermal conductivity of the PU shell material. Likewise, the lower thermal conductivity results in lower heat transfer rate from the shell outside to the core inside, influencing the phase change temperatures of the c-PCMs. Furthermore, it is observed that the freezing point of the hCNC enhanced PU microcapsules (C5P20M60) is slightly higher than that of the pure PU microcapsule (C0P20M60) because of an increase in the thermal conductivity using hCNC, even though the pure PU microcapsule has a thinner shell (Figure 4b) and thus is expected to have a better thermal conductivity.66 Accordingly, the mixture of methyl laurate and 10 wt % hCNC shows a slightly higher freezing point than that of the bulk methyl laurate. However, it is found that multiple peaks emerge in the cooling thermogram of the 5 wt % hCNC enhanced PU microcapsule (C5P20M60, 40% core) due to the lower conductivity of its significantly thick shell. Above 60 wt % core content, the crystallization of c-PCMs (a high core/shell weight ratio) is not considerably inhibited by encapsulation because of the core space (which contains a large number of nuclei) created by the large and elastic shell components. Likewise, the thermal property of the capsules is similar to that of pure methyl laurate. Although we can technically increase the core contents in the capsules, emulsion formation and the capsule production may likely become more difficult (Figure 4d). Figure 7b shows the DSC thermogram of C5P20M60 microcapsule (5% hCNC-60% core) during 10 thermal cycling. The melting point and crystallizing point of the microcapsules during 10 cycling were only slightly altered within the range 6.07−6.15 °C and −2.96 to −3.46 °C, respectively. Furthermore, the latent heats of melting and crystallization of the microcapsule during 10 cycling were found in the range 112.2−113.9 and 115.8−123 J/g. The c-PCMs had excellent thermal stability and reproducibility in terms of the change in

thermal properties after a period of no fewer than 10 thermal cycling. TGA peaks of the bulk hCNC and PTMG, and the individual chloroform-insoluble gel portion of C0P20M60 (20% PTMG) and C0P40M60 (40% PTMG) pure PU microcapsules, and C5P20M60 (20% PTMG plus 5% hCNC embedded) PU microcapsules are shown in Figure S5.67,68 These TGA results have shown that hCNC-PU microcapsules have significantly better thermal stability in the temperature range of 250−450 °C than neat polyurea or polyurethane resin.68 These results are consistent with the results from the mechanical data of PU microcapsules toughened with hCNC. The possible explanation is that the compatibility of hCNC particles enables them to be closely combined or chemically connected with polymer matrix to create a composite structured wall that can show higher thermal stability and strength to resist crack/failure during the postprocessing or phase change process. Release Properties. Controlled permeation behavior of microcapsule membranes is crucial for the c-PCMs, which are highly dependent on the outer wall microstructure. To check the effect of the embedded hCNC on the penetration property of the microcapsules, dye loaded microcapsules (C0P00M60, C0P20M60, and C5P20M60) were produced with and without hCNC and PTMG as a shell component. To load the dye into c-PCMs, a Sudan II dye solution in methyl laurate (a concentration of 2000 ppm) was used as a core material. Antiosmosis tests were performed using these dye-PCM loaded microcapsules. In this manner, the permeability of the PU membrane forming the outer wall of the c-PCMs with a hydrophobic blue dye (Sudan II), was determined by observing the release rate of the dye into 5 wt % SDS (sodium dodecyl sulfate), NaOH (sodium hydroxide, pH 13), and 3 wt % PCE (polycarboxylate ether)/3 wt % CaCl2 (calcium chloride) aqueous solution environments, representing the supernatant liquid of Type I/II cement pastes and water reducing admixtures. The release study was utilized to evaluate the water durability of the c-PCMs for the deicing pavement applications. The release data was collected via time-resolved measurements of the UV−vis absorbance of the outer bulk solution over a specific period of time using a UV−vis spectrometer. The cumulative release rate of the core components is calculated by eq 5: 31773


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ACS Applied Materials & Interfaces Cumulative Dye Release Percentage (%) =

Wt × 100 W0

thickness of the microcapsules increased as the core/shell weight ratio decreased. In addition, the capsule size and gel content of the c-PCMs increased as the PTMG content increased. The composite hCNC enhanced c-PCMs demonstrated here are suitable for encapsulation and for improving the energy efficiency of buildings or for the deicing of pavements. This is because methyl laurate was successfully encapsulated within the PU, poly(urea−urethane), shells and the c-PCMs showed much better phase change and antiosmotic behaviors, as well as much clearer core−shell structures and higher encapsulation efficiencies than those of pure polyurea or poly(urea−urethane) microcapsules. Due to the microstructural compactness of the hCNC−PU microcapsule shell wall, the methyl laurate as a PCM core component can be protected via the encapsulation from diffusion loss even in high alkaline environments during production, storage, and use of the microcapsules.

(5)

where W0 and Wt are the initial weight and cumulative release weight of Sudan II (blue) dye at time t, respectively. (W0 is measured by bursting the microcapsules via mortar and pestle, collecting the dye with acetone, exchanging the solvent with mineral oil, and analyzing the UV−vis absorbance of the extracted solution.) Figure 8 exhibits the cumulative release curves of individual microcapsules, which represent the durability of the PU shells synthesized by using hCNC as a release barrier. It was found that the hCNC-PTMG-poly(urea−urethane) microcapsules (C5P20M60 and C5P40M60) show a better extended release rate than that of pure polyurea microcapsules. In particular, the C5P20M60 microcapsule fabricated with 5% hCNC and 20% PTMG in 5 wt % SDS aqueous solution lost 50 wt % dye at a longer period of 4 days and the C5P40M60 kept more than 90 wt % dye, which may be attributed to a small amount of dye remaining out of the microcapsules. The dye that had not been encapsulated into microcapsules existed on the surface of the microcapsules. Furthermore, all microcapsules preserved their core content in the alkaline aqueous solution (pH = 13) environment of cement pastes or water reducing admixtures. These results reveal that the incorporation of the PTMG and hCNC could significantly affect the microstructural compactness of their microcapsule shell wall. Usually, adjusting the polymeric membrane morphology is of importance, because the microstructure strongly controls the diffusivity of the microcapsule outer wall. For example, leaking of core components through the shell layer can be caused by the porous shell structures of polyurea microcapsules (see SEM images in Figures 2a and f), which are attributed to a poor sealing performance of the microcapsules. Accordingly, the loosely knitted outer polyurea membranes contain capillaries or lacunas, bringing about high release rates. On the other hand, a nanoparticle-barrier enhanced smooth, tight poly(urea− urethane) shell microstructure exhibits a release rate with low permeability. This agrees with the SEM photos of the c-PCMs having different outer shell structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06970. Particle size distribution, FT-IR spectra, and images of encapsulated phase change materials used in this study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +1 765-496-2294; Fax: +1 765 494-1204. ORCID

Youngman Yoo: 0000-0001-7250-0993 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Science Foundation Scalable Nanomanufacturing program under award CMMI-1449358, and the Forest Products Laboratory under awards 11-JV-11111129-118 and 11-CR11111129-109.

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CONCLUSION Encapsulated phase change materials (c-PCMs) based on methyl laurate core and poly(urea−urethane) shells containing modified cellulose nanocrystals (hCNCs) were successfully fabricated by using interfacial polymerization of NCOterminated polyol, which is composed of polytetrahydrofuran (PTMG), glycerol (Gly), and oleic acid-poly(lactic acid) grafted CNC (hCNC) with tolylene-2,4-diisocyanate (TDI), as an oil-soluble monomer, and active hydrogen diethylenetriamine (DETA), as a water-soluble monomer. These microcapsules have high rigidity (rupture force of 0.1−0.31 N and elastic modulus of 0.3−0.63 N/mm) and provide barrier enhancement against the osmotic release of core components. The c-PCMs were fabricated from a core/shell weight ratio of 30/70 to 90/10. In addition, the radius and thickness of these microcapsules range over 20−400 μm and 10−150 μm, respectively, and their morphologies show a denser and more smooth outer surface than those of only DETA-TDI polyurea microcapsules. The c-PCMs exhibited a phase change of methyl laurate at 0−6 °C. The core/shell weight ratio analyzed using the heat of fusion of methyl laurate was only slightly less than calculated from the feed. The encapsulation efficiency and shell

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REFERENCES

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Synthesis and Characterization of Microencapsulated Phase Change

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