Hollow polymer microcapsule embedded transparent and heat-insulating film

We herein report a facile and scalable approach to manufacturing optically transparent and heat-insulating films by incorporating hollow poly(methyl methacrylate) microcapsules into a transparent polymeric matrix. The microcapsule was prepared via emulsion polymerization. The size of the microcapsules could be easily controlled from ∼1 to 3 μm by varying the polymerization time in a narrow size distribution. The microcapsules were then mixed with a UV-curable transparent liquid resin and cured by a subsequent light irradiation. The current approach could enhance the thermal barrier property of the films without a significant reduction in the optical transparency. The solid film possessing 30 wt% microcapsules, for example, exhibited a high visible light transmittance (∼80% as measured by UV-vis spectroscopy) and the thermal conductivity was reduced to 0.06 W mK−1, corresponding to 46% of the capsule free film. To quantify and verify this result, theoretical models describing a heat transfer in a hollow microsphere composite were used, and the model showed a good agreement with our experimental observations.


Introduction
To safely accommodate global increases in population and per capita energy consumption, a signicant need for immediate energy conservation has been emphasized all around the world.An effective way to conserve energy is to improve the thermal insulation of buildings.Based on a US Department of Energy (DOE) report, for example, 23% of the electricity used in the US is for the energy supply for space heating, ventilation, and cooling only. 1 In particular, the energy used to offset unwanted heat losses and gains through windows in buildings was estimated to cost the US $$20 billion, which is one-fourth of all the energy used for space heating and cooling. 2 This is because windows typically occupy $20% of the building surface area and it loses more heat in winter and gains more heat in summer than any other surface in the building, ascribing to their high thermal conductivity.Along with the need for thermal insulation, a visible transparency is a prerequisite to the materials used as windows.Therefore, much attention has been focused on improving both the thermal barrier rate and the optical transparency of building windows. 3 conventional approach to minimizing the heat loss through windows is placing insulating materials with low thermal conductivities onto or instead of the window glass.Less conductive gas (such as air or argon) lled glazed windows have been widely adapted for the thermal insulation. 4However, the process ow including adjustment of the gas mixture securely is somewhat complicated and uneconomical. 5Also, the glazed windows may form cracks due to a thermal stress during the usage.3][14] Many other insulating materials and manufacturing methods including nano-size hollow silicate particles, 15 nitrogen-doped graphene aerogels, 16 ceramic bres/ whiskers frameworks, 17 and urea formaldehyde foam 18 have been proposed for thermal insulation but usage of them as an insulating window is still limited by optical transparency, complexity of the process, and/or the manufacturing cost.
To this end, we herein describe a facile and scalable approach to manufacturing optically transparent, heatinsulating lms by incorporating hollow poly(methyl methacrylate) (PMMA) microcapsules into a transparent polymeric matrix.By incorporating hollow PMMA microcapsules into the transparent lms, less thermally conductive air could be efficiently included into the polymeric matrix, thus enhancing the thermal barrier property of the resulting lms without a significant reduction in the optical transparency.Using a simple and scalable synthetic route, the hollow PMMA microcapsules were prepared.The capsule sizes were easily tuned by varying the polymerization time in a narrow size distribution.The microcapsules were then mixed with a UV-curable transparent liquid resin and cured by a subsequent light irradiation.Therefore, the current approach could adjust both the optical transparency and the thermal conductivity by simply varying the microcapsule loading.

Hollow PMMA microcapsule preparation and characterization
Hollow PMMA microcapsule was prepared following a previously published procedure. 19Briey, AOT (0.9 wt%, 0.45 g), PVP (4 wt%, 4 g), and MeOH (85.45 wt%, 108 mL) were added to the argon purged reactor vessel and allowed to stir for 10 min at room temperature to facilitate the dissolution of AOT and PVP.Then, MMA (10 wt%, 10 mL) and AIBN (0.1 wt%, 0.1 g) were added to the reactor.The reactor was heated to 58 C and stirred.The weight percentages above are given with respect to the total solution weight.As the reaction proceeded for at least 2 h, the clear solution color turned into translucent solution, indicative of a hollow PMMA microcapsule formation.The obtained PMMA microcapsule was washed with DI-water several times and then freeze dried from water to get ne powder.
Solution state proton nuclear magnetic resonance ( 1 H-NMR) measurement was performed using an Agilent NMR 600 MHz spectrometer.Deuterated chloroform (CDCl 3 ) was used as the solvent.Microcapsule morphologies were characterized by both polarized optical microscopy (POM, ZEISS, AXIO Imager A2m microscope stand) and scanning electron microscopy (SEM, Nova NanoSEM, FEI, USA) analysis.

Film preparation and characterization
An UV-curable optically transparent resin (NOA 65) was purchased from Norland Products, Inc.The resin contains a photo-initiator, Irgacure 651.The microcapsule was added into the NOA 65 and allowed to stir for 10 min at room temperature.The percentage of microcapsule added varied from 10-30 wt% with respect to the NOA 65 resin.Films were bar coated onto the poly(ethylene terephthalate) (PET) lm with thickness of 100 mm and then UV-cured using a mercury lamp with broadband output from 200-600 nm (Lichtzen UV Cure-60PH) for 2 minutes.The lm was dried in vacuum for overnight at room temperature.
UV/vis spectra of lms were acquired with a JASCO V-670 UV-Visible Spectrophotometer.Thermal conductivity was measured by the transient plane source method (Hot-disk AB, TPS-2500s).

Results and discussion
A schematic describing how the hollow PMMA microcapsules were prepared is shown in Fig. 1.The narrow size distributed, hollow PMMA microcapsules were prepared via emulsion polymerization 19 using 2,2 0 -azobisisobutylronitrile (AIBN) and methanol as the initiator and solvent, respectively.AIBN, MMA monomer, dioctylsulfosuccinate sodium salt (AOT), poly(vinyl pyrrolidone) (PVP), and methanol were weighed into the reactor.AOT, a surfactant, formed spherical micelles in methanol, templating the amphiphilic interface.Aer the reactor was purged with nitrogen gas, the polymerization was carried out at 58 C by stirring at the speed of 400 rpm.Upon heating, the AIBN initiators generated radicals, triggering the polymerization at the amphiphilic interface.In order to inhibit coagulation of the emulsion, PVP was used as a stabilizer. 20The obtained transparent PMMA microcapsules were washed with The size of the microcapsule could be controlled in a narrow size distribution by simply varying the polymerization time.As scanning electron micrographs (SEM) shown in Fig. 2ad display, the microcapsule size increased monotonically with increasing the polymerization time, indicating the PMMA shell gets thicker.Aer 20 h of the polymerization, the maximum microcapsule size was achieved ($2.6 mm) and longer polymerization times did not alter the capsule size further.The average capsule diameters shown in Fig. 2e were taken from 30 diameter measurements from the corresponding SEM images.Since the capsule morphologies are dictated by the micelle structures, the capsule morphologies can also be affected in response to changes in other factors such as the surfactant and dispersant types, the amount of surfactants, and etc.
In order to further conrm the hollow spherical morphology of the microcapsules, microencapsulation experiment using liquid crystals (LCs) was conducted.The synthesized PMMA capsules (0.3 g) were re-dispersed in a mixture of ethanol and 0.25 wt% sodium dodecyl sulfate aqueous solution (in a 1 : 4 volume ratio) by 30 min of sonication.The microcapsules were swollen with an LCs/methylene chloride (0.3/2.7, g/g) emulsion by ultrasonic homogenizing in a 0.25% sodium dodecyl sulfate aqueous solution/ethanol mixture.LCs used were 1 : 1 molar mixture of 4-(trans-4-n-propylcyclohexyl)ethoxybenzene and 4-(trans-4-n-propylcyclohexyl) butoxybenzene and they were synthesized following the procedure described elsewhere. 21The swelling was carried out by stirring with a magnetic bar at ambient conditions for 3 h until all of the emulsion droplets disappeared completely.The methylene chloride in the swollen PMMA particles was completely evaporated with a rotary evaporator at 30 C for 1 h.The dispersion was passed repeatedly through a sintered glassing lter (average 2 mm pore diameter) and dried under a vacuum overnight at ambient temperature.Finally, ne LCs-loaded PMMA microcapsules were obtained.Representative optical micrograph images with and without a cross-polarizer on the identical spot of the microcapsule array are shown in Fig. 3.The cross-polarized optical micrograph shown in Fig. 3b clearly displays a birefringence (green color) located only at the center of each capsule, indicating the LCs was successfully encapsulated in the microcapsules.This result reveals that the prepared microcapsules possessed hollow spherical morphology, consistent with the identically prepared microcapsule reported elsewhere. 19,22,23This fact also highlights their potential usages in numerous industrial elds including drug delivery, self-healing materials, fragrance release, nutrient preservation, and etc. [24][25][26][27][28][29][30] As a practical application using the hollow PMMA microcapsules, optically transparent, heat-insulating lms were manufactured so that the lms can be used as a window insulation lm.A schematic describing the process is shown in Fig. 4. Various amounts of the microcapsules were mixed with a UV-curable transparent liquid resin (Norland Optical Adhesive or NOA 65).The percentage of microcapsule was varied from 0-30 wt% with respect to the NOA 65 resin.The microcapsules embedded optical resins were bar coated onto a 100 mm thick poly(ethylene terephthalate) (PET) substrate and then cured with a subsequent light irradiation.The lms manufactured by this method, therefore, could include the hollow spherical capsules with their maintained morphologies.A high optical transparency is a crucial aspect for the materials used as a window insulation lm.As the representative photographs in Fig. 5a-d display, the lms processed as described in Fig. 4 with various amounts of microcapsules up to 30 wt% exhibited a high optical transparency without optical distortions.Accordingly, the lms can be directly attached to or used instead of a glass window.All lms were 80 mm thick coated on the 100 mm PET substrate.Incorporated microcapsules were well dispersed in the matrix as evidenced by the optical micrographs shown in Fig. 6 and S2 in ESI.† To further demonstrate the optical transparency of the lms, UV-vis absorbance spectra were obtained for the lms containing 0, 10, and 30 wt% microcapsules and are shown in Fig. 5e.Visible light (a wavelength range from 390 nm to 800 nm) transmittance reduced with increasing microcapsule loadings, attributing to a slight refractive index mismatch.Refractive indices are 1.52, 1.49, and 1.00 for the NOA 65, 31 PMMA, 32 and air, respectively.This indicates that higher air content in the microcapsule would result in a lower visible light transmittance.The haze values estimated by UV-vis spectroscopy were 9.9% and 23% for the lms containing 10 and 30 wt% microcapsules, respectively.Since the capsule free lm on the PET substrate exhibited $90% transmittance in a visible light range, the free standing lm containing 30 wt% microcapsules (the maximum amount of the capsules used in this study) is anticipated to possess $80% visible light transmittance, which the value is higher than those of many commercial window insulation lms. 33 addition to a high optical transparency, low thermal conductivities are also required for the materials to be used as a window insulation lm.The transparent lms with a thermal conductivity ranging from 0.03 to 0.10 W mK À1 are considered as heat insulating lms. 34Fig. 7 shows thermal conductivities of the lms as a function of loaded microcapsule volume faction.The thermal conductivities were measured by the transient plane source method (Hot-disk AB, TPS-2500s).An increase in the loaded capsule volume results in a reduction in the thermal conductivities.
Theoretical models would enable efficient optimization of experimental variables to precisely achieve a better material design.6][37][38] To understand the dependence of the loaded microcapsule volume fraction, 4 f , on thermal conductivities, the experimental results were compared to a theoretical model of heat transfer in hollow microsphere composites, k. 39 The model used in this study is expressed as follow (eqn (1)): where k p , k m , and k a are the thermal conductivites of the matrix (optical resin), PMMA shell (0.18 W mK À1 ), 32 and air (0.025 W mK À1 ) while the r m , r a , and r s are the densities of the PMMA shell (1180 kg m À3 ), 32 air (1.226 kg m À3 ), and the mircocapsule.The density of the mircocapsule could be estimated using the eqn (2): where V m and V s are the volumes of a PMMA shell and the mircocapsule, respectively.The resulting k values were then corrected by substituting k and a literature value for a thermal conductivity of PET (k PET ; 0.24 W mK À1 ) 32 into a basic equation of thermal conduction in parallel layers (eqn (3)) with known layer thicknesses in order to account for the sample geometry.
Here, L refers to the thickness of layer i.By tting eqn (3) with the experimental value for capsule free lm on the PET substrate, k p was estimated to be 0.14 W mK À1 .
Finally, the predicted k total along with the experimentally observed values were plotted in Fig. 7a.Three model ts (denoted as Model 1 in Fig. 7a) are provided corresponding to the lm lled with PMMA microcapsules (i.e.no air), half air-lled microcapsules in volume, and pure air spheres (i.e.no PMMA).The model ts predicted the lower thermal conductivities as the loaded microcapsule includes less PMMA (i.e.  higher air fraction).All model ts exhibited decaying thermal conductivities as a function of the microcapsule loadings, qualitatively agreeing with experimental observations.However, this model effort did not show any quantitative agreement with the experimental data.The experimental values were higher than the predicted values at low capsule loadings, while the model values predicted higher than the experimental values at high capsule loadings.Since the model assumes a system with homogeneously dispersed spheres, sphere aggregation may result in the discrepancy between the model and the experiment.
Since we've conrmed that the incorporated microcapsules were well dispersed in the matrix in in-plane direction by the optical micrograph investigation (see Fig. 6), we postulated that the out-of-plane sphere aggregation could result in the discrepancy between the model and the experiment.The out-ofplane sphere aggregation could occur due to a density difference between the capsule and the liquid matrix during the lm fabrication.Accordingly, new model ts for predicting thermal conductivities of the lms as a function of 4 f were given (denoted as Model 2 in Fig. 7b).To simplify the system with the out-of-plane sphere aggregation, this new model assumed a parallel multilayered geometry with four layers including PMMA, air, matrix, and the PET substrate.Therefore, the values were predicted by using only eqn (3).Similar to the results shown in Fig. 7a, the lower conductivities were predicted as increasing the capsule loading and also as increasing the air volume fraction in each capsule.It is noteworthy that a reasonable agreement between data and the model was achieved with 85 volume percent air-lled microcapsules.The quality of agreement obtained with the 85 volume percent air-lled capsules suggests the lms in this study possessed highly aggregated capsules in out-of-plane direction rather than well dispersed ones.With the sphere aggregation, the lower thermal conductivities could be achieved than the samples with homogeneously dispersed capsules, implying the lms would be more practically benecial for being used as a window insulation lm.Note all reported values in Fig. 7 were taken from the lms on the PET substrate with a relatively high thermal conductivity.The thermal conductivity of the free standing lm was reduced to 0.06 W mK À1 upon incorporating 30 wt% microcapsules (54% reduction compared to the capsule free lm), reaching the values suitable for the heat insulating lm usage. 34

Conclusions
In conclusion, optically transparent, heat insulating lms were fabricated by incorporating hollow PMMA microcapsules into a UV-curable, transparent polymeric matrix.Using a simple and scalable synthetic route, the hollow PMMA microcapsules were prepared.The capsule sizes were easily tuned by varying the polymerization time.As the lms possessed higher microcapsule contents, the thermal conductivity was monotonically decreased without a signicant reduction in the optical transparency.At 30 wt% microcapsule incorporation, the thermal conductivity and the visible light transparency were reduced by 54% and 20%, respectively, compared to the capsule free lm.Additionally, models describing heat transfer in hollow microsphere composites were used and compared with our experimental observations.The model effort further revealed the lower thermal conductivity could be achieved with the lm possessing out-of-plane hollow sphere aggregation rather than well dispersed ones.In addition to providing fundamental insights, we envision this study could guide future material design in this area.

Fig. 2
Fig. 2 Representative SEM images of the hollow microcapsules after (a) 2 h, (b) 6 h, (c) 10 h, and (d) 20 h of polymerizations.The images were taken at room temperature.The scale bars in this figure correspond to 5 mm.(e) Average microcapsule diameter as a function of polymerization time.The error bars represent the standard deviations of the diameter distributions.

Fig. 3
Fig. 3 Representative optical micrograph image of liquid crystal (LCs) loaded microcapsule: (a) without a cross-polarizer and (b) with a cross-polarizer.The scale bars in this figure indicate 10 mm.

Fig. 4
Fig. 4 Schematic describing a preparation of the microcapsules embedded transparent film.A UV-curable, transparent liquid resin containing various amounts of microcapsule with a thickness of 80 mm were bar coated onto a 100 mm thick poly(ethylene terephthalate) (PET) substrate and then cured by UV irradiation.The capsule with a diameter of 2.6 mm was used.

Fig. 5
Fig. 5 Photograph of the transparent films (80 mm thick) with (a) 0 wt%, (b) 10 wt%, (c) 20 wt%, and (d) 30 wt% hollow microcapsule on a 100 mm thick PET substrate.The weight percentages are given with respect to the matrix weight.(e) UV-vis absorption spectra of the transparent film (80 mm thick) containing 0, 10, and 30 wt% microcapsule on the 100 mm thick PET substrate. k

Fig. 7
Fig. 7 Thermal conductivity of the film as a function of the incorporated microcapsule volume fraction, 4 f .Emptied circles are experimental data.The experimental points in both figures are the identical data.80 mm thick films were bar coated on the 100 mm thick PET substrates for all samples.Error bars represent standard deviations calculated from three measurements on the identically prepared samples.(a) Three model fits corresponding to the film filled with PMMA microcapsule (i.e.no air), half air-filled microcapsule in volume, and pure air sphere (i.e.no PMMA) based on eqn (1) are provided.The reported thermal conductivity values in this figure were corrected using eqn (3) in order to account for the sample geometry.(b) Four model fits corresponding to the film filled with PMMA microcapsule (i.e.no air), 15 volume% air-filled microcapsule, 85 volume% air-filled microcapsule, and pure air sphere (i.e.no PMMA) are provided based on only eqn (3) assuming a parallel multilayered geometry.