Protein-polysaccharide based microencapsulated phase change material composites for thermal energy storage

Protein-polysaccharide based microencapsulated phase change material composites for thermal energy storage

Journal Pre-proof Protein-polysaccharide based microencapsulated phase change material composites for thermal energy storage Jitendra Singh, Jagadeesw...

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Journal Pre-proof Protein-polysaccharide based microencapsulated phase change material composites for thermal energy storage Jitendra Singh, Jagadeeswara Reddy Vennapusa, Sujay Chattopadhyay

PII:

S0144-8617(19)31199-3

DOI:

https://doi.org/10.1016/j.carbpol.2019.115531

Reference:

CARP 115531

To appear in:

Carbohydrate Polymers

Received Date:

25 August 2019

Revised Date:

16 October 2019

Accepted Date:

23 October 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Protein-polysaccharide based microencapsulated phase change material composites for thermal energy storage

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Jitendra Singh1, Jagadeeswara Reddy Vennapusa1, Sujay Chattopadhyay1*

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Department of Polymer and Process Engineering, IIT Roorkee Saharanpur Campus, Saharanpur-247001, India

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* Sujay Chattopadhyay: [email protected]; [email protected];

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Phone: +91 (132) 2714374 [O]; FAX: +91 (132) 2714310

Highlights

Synthesis of capric acid encapsulated composites utilizing protein-polysaccharide

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(GE/GA) interactions

Core: shell composition of 2:3 exhibited best mechanical/thermal properties



Thermal stability of composites remained unaffected with time/temperature of reaction

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with glutaraldehyde cross-linker, indicating occurrence of surface reactions only Amount of cross-linker strongly influenced thermal stability of the composite



Silica coating improved thermal stability and conductivity of the composite

Abstract:

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Microencapsulated phase change material (MPCM) composites of capric acid were synthesized utilizing protein (Gelatin, GE)-polysaccharide (Gum Arabic, GA) interactions as shell material. Mechanical and thermal stabilities of these MPCM composites were achieved using glutaraldehyde cross-linker and silica coating respectively. Thermal properties (enthalpy and, melting/crystallization and cyclic tests) were estimated using differential scanning calorimeter (DSC) while, thermal stability was obtained from thermogravimetric analyzer (TGA). 1

Morphology and particle size were analyzed using scanning electron microscope (SEM). FTIR and EDX (energy-dispersive X-ray) data interpreted nature of chemical bonds while crystalloid structures were obtained from XRD (X-Ray Diffraction). Based on morphology and thermal stability, the composite made with the core: shell ratio of 2:3 was chosen for analyzing the role of process parameters i.e. cross-linker amount and duration of cross-link reaction, and surfactant amount influencing encapsulation ratio. The composite tested and found stable for 50

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heating/cooling cycles.

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Keywords: phase change material, MPCM composite, thermal energy storage, coacervation

Nomenclature:

ΔHm

R η

Latent heat of solidifying, J.g-1 Encapsulation ratio, %

Encapsulation efficiency, % Theta

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θ

Latent heat of melting, J.g-1

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ΔHs

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List of symbols

°C

Degree Celsius

Abbreviations

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Phase change material

MPCM

Microencapsulated phage change material

CA

Capric acid

GE

Gelatin

GA

Gum Arabic

TEOS

Tetraethoxysilane

BS

Blank sample

HSM

Hot stage microscope

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PCM

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1. Introduction

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Increasing demand for human comfort and automation can be accomplished with the consumption of a higher amount of energy. Fossil fuels (e.g. crude oil, coal, and natural gas)

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have so far been applied to meet the energy demand but non-renewable nature, limited availability and environmental pollution due to burning are impediments to its utilization (H. Zhang, Sun, Wang, & Wu, 2011a). Therefore, energy from renewable sources (solar, wind and tidal) without any environmental impact is the need of the hours. Although energy from these

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sources is plenty, their intermittent nature needs to be addressed through appropriate storage and utilization option. Thus, effective storage of this energy is essential for smooth operation. Storing of latent heat in materials, called phase change materials (PCM), is a popular technique of energy conservation from the surrounding (Farid, Khudhair, Razack, & Al-Hallaj, 2004). Paraffin based hydrocarbons are found to have large latent heat capacity, chemically stability and available in high purity, hence, are used as popular PCMs. Fossil fuel origin and flammable property are 3

concerns for their large scale applications. Therefore, alternative materials of renewable nature e.g. fatty acids, fatty alcohols, fatty esters, etc. of large latent heat are being explored. All PCMs are associated with two undesirable properties i.e. poor thermal conductivity and leakage issue (after melting). These problems can be resolved by putting them in closed enclosures of various dimensions. Micron size capsules containing PCM are quite popular due to

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higher surface area and easy applications. Shell materials used to encapsulate PCMs are usually of higher thermal conductivity (T. X. Li, Lee, Wang, & Kang, 2013). Sol-gel technique (Şahan &

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Paksoy, 2017), emulsion polymerization (Alkan, Sari, Karaipekli, & Uzun, 2009), interfacial

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polymerization (Liang, Lingling, Hongbo, & Zhibin, 2009), suspension polymerization (W. Li et al., 2011), coacervation (Butstraen & Salaün, 2014; Katona, Sovilj, & Petrović, 2010; Krishnan,

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Kshirsagar, & Singhal, 2005) etc. are common techniques for microencapsulation. Widely varying shell materials e.g. SiO2 (Tahan et al., 2013), CaCO3 (Yu, Wang, & Wu, 2014),

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polystyrene (PS) (Döğüşcü, Altıntaş, Sarı, & Alkan, 2017), polymethylmethacrylate (PMMA) (Sari, Alkan, & Karaipekli, 2010), polyurea (PU) (Chen, Xu, Shang, & Zhang, 2009), melamine-

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formaldehyde (MF resin) (Cheong, Shin, Kim, & Lee, 2004), urea-formaldehyde (Han, Lu, Qiu, Tang, & Wang, 2015), gelatin- gum Arabic (Mayya, Bhattacharyya, & Argillier, 2003), gum Arabic-modified starch (Krishnan et al., 2005), etc. are mostly chosen based on targeted

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application.

Appreciable chemical stability with paraffin based PCMs has resulted in synthesis of

different microencapsulated PCMs with various shell materials e.g. acrylic polymer/copolymers (Qiu, Li, Song, Chu, & Tang, 2012), carboxymethyl cellulose (CMC)-modified melamineformaldehyde (Hu, Huang, & Zhang, 2014), gelatine-gum Arabic (Onder, Sarier, & Cimen, 2008), gum arabic-chitosan (Butstraen & Salaün, 2014), etc. Silica incorporation in PMMA shell 4

of n-octadecane based nanoPCMs improved thermal conductivity (Fu et al., 2017; Zhu et al., 2018). Microencapsulation of PCM with reactive functional groups e.g. –COOH (fatty acid PCMs), –OH (fatty alcohols), –COOR (fatty esters) becomes more challenging due to their interactions with reaction medium as well as the shell material during synthesis. Thus, much lesser MPCM products have been reported in spite of their renewable (fatty acids etc.) nature

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(Bio-PCM). Microcapsules blends of triglycerides (core) using gum Arabic and chitosan as shells

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were synthesized (Butstraen & Salaün, 2014) using complex coacervation. The same technique

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was adopted to microencapsulate caprylic acid using MF/UMF/UF shell with 59.29 % core content (Konuklu, Unal, & Paksoy, 2014). Eutectic composition of capric-palmitic acids was

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encapsulated using polystyrene shell (Döğüşcü et al., 2017) where 47.3% of encapsulation ratio was reported. While 65.8% of encapsulation ratio was reported during encapsulation of capric-

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stearic eutectic using PMMA shell (Sari, Alkan, & Özcan, 2015). Mostly, synthetic polymers were used as shell materials to achieve thermal and

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mechanical stability and avoid leakage of molten PCM. Various polymerization techniques adopted were also of great concern considering environmental aspects as high BOD load of the waste streams generated in these processes. Improvement in thermal stability, thermal

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conductivity and encapsulation ratio were main focus for PCM encapsulation but, crucial parameters controlling microencapsulation process e.g. core: shell ratio, surfactant amount, cross-linker amount and duration of cross-link reaction influencing the morphology, size, encapsulation ratio, etc. have been lesser focused. Popularly used synthetic polymer shells made through polymerization are not environmentally benign due to involvement of various organic monomers etc. and part of it 5

enters into waste streams. These issues are overcome by encapsulating renewable capric acid (Bio-PCM) using a 1:1 mass ratio of protein (gelatin) - polysaccharide (gum Arabic) as shell (Espinosa-Andrews, Báez-González, Cruz-Sosa, & Vernon-Carter, 2007). Parameters influencing PCM encapsulation ratio were systemically explored. Thermal stability and thermal conductivity were improved by applying a secondary coating of silica. Gelatin-gum Arabic shell was used mostly in health care products but, its application to encapsulate PCM is being reported

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first time. Low cost, renewable and ecofriendly nature of gelatin/gum Arabic (GE/GA) as shell

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encouraged us to explore microencapsulate of capric acid.

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2. Experimental Section

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2.1. Materials: Capric Acid (C10H20O2, Mol. Mass 172.27, m.p. 30.66°C, b.p. 269oC, melting enthalpy of 160.84 J g-1) of 98.5% purity was purchased from M/s HiMedia Laboratories and

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was used as phase change material. Acetic acid (99.5%, Glacial, L.R.), shell materials gum arabic, gelatin and tetraethoxysilane (98.0%, TEOS) were also procured from M/s HiMedia

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Laboratories. Sodium Dodecyl Sulfate (Sr) as emulsifier was purchased from M/s Qualigens Fine Chemicals, India. 25% aqueous solution of Glutaraldehyde was purchased from M/s Loba Chemicals, India and used as such.

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2.2. Preparation of Organic-Inorganic hybrid (GE/GA-SiO2) microencapsulated PCM composite

2.2.1. Preparation of GE/GA (Organic) microencapsulated PCM composite A complex coacervation technique was adopted for preparing microencapsulated PCM (MPCM) composite (Onder et al., 2008). Materials used for making shell in complex coacervation were

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gelatin as a protein with positive charge and gum Arabic as a polysaccharide bearing negative charge. Two oppositely charged polymer materials were coacervate to form a composite shell over core material due to electrostatic interactions. Initially, an appropriate quantity of SDS was dissolved in 50 ml of water and temperature was maintained at 50°C with constant stirring speed 1800 rpm. Molten PCM was added dropwise into the SDS solution to obtain a stable oil-in-water emulsion. The stirring was then

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continued for 30 min before aqueous gelatin solution (~10 %) was added drop-wise into this

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stable emulsion. The whole mixture was then stirred for another 30 minutes keeping the

temperature of the mixture constant. Then, gum Arabic (~10 % aqueous, GA) was added

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dropwise to the emulsion (SDS + PCM + GE) and stirred continuously for 30 minutes. The pH

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of the above mixture was then adjusted to ~4.5 using dilute acetic acid which resulted in the formation of slurry by precipitating the solid mass out (Scheme 1). At this stage, the slurry

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temperature was brought down to 5°C and 1ml glutaraldehyde (25% solution) was added as cross-linker to cross-link the shell wall (unless otherwise mentioned, cross-linking reaction was

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performed for 9h). Schematic representation of different steps involved in microencapsulation of

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CA using GE/GA is presented in Scheme 1.

Scheme 1. Schematic representation of MPCM composite synthesis with GE/GA shell

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The solutions of GE and GA with 1:1 mass ratio were mixed to form the shell wall. The combined mass of GE and GA (shell mass) was varied as e.g. 1g, 2g, 3g and 4g. Further, the solutions with four varied masses of GE and GA were placed in separate vessels and to each vessel 2g of core (CA) was added to achieve core: shell ratios of e.g. 2: 1 (G1), 2: 2 (G2), 2: 3 (G3) and 2: 4 (G4) as reported in Table S1 ( see in supplementary data). Based on the mechanical stability and particle uniformity G3 composite sample was subsequently chosen for

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further investigations. Influence of (i) cross-linker, glutaraldehyde (25% aqueous solution)

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amount varied as e.g. 0.25ml (G3G0.25), 1.0ml (G3G1), 1.5ml (G3G1.5) and 2.0ml (G3G2); (ii) The duration of cross-linking reaction was altered e.g. 1h(G3T1), 3h(G3T3) 5h(G3T5), 7h

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(G3T7) and 9h (G3T9 or G3) , (iii) the amount of surfactant (SDS) varied e.g. 250mg (G3Sr1),

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400mg (G3Sr2), 600mg (G3Sr3) and 800mg (G3Sr4) and (iv) the amount of TEOS to alter SiO2 content deposited over shell wall (G3-SiO2). Table S1 shows details of all composite samples

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prepared with their compositions and process conditions applied. 2.2.2. Silica coating over GE/GA-Shell of MPCM composite

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SiO2 coating over GE/GA shell was developed using hydrolysis and polycondensation of TEOS (scheme 2) (H. Zhang, Sun, Wang, & Wu, 2011b). Acid hydrolysis of tetraethoxysilane (TEOS) was done according to the steps shown using scheme shown in Eq. 1 which, subsequently underwent polycondensation (Eq. 2) to form silica layer. Briefly, a suspension of 1g of MPCM

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composite (G3) was prepared in 60 ml DI water (de-ionized water) at 35°C with constant stirring of 500 rpm. 2 ml TEOS was subsequently added dropwise to this suspension and pH was maintained at ~2.5 by using dil. HCl (0.1 N). Further, the suspension was continuously stirred at 500 rpm for 40 h at constant temperature of 35°C. The obtained powder was filtered and dried at 40°C overnight.

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HydrolysispH 2.5 Si(OC2H5) + 4H2O

35°C

Si(OH)4 + 4C2H5OH…………………(1)

Polycondensation2SiO2 + 4H2O………………………..(2)

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Si(OH)4 + Si(OH)4

composite shell

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Scheme 2. Schematic of hydrolysis-polycondensation of TEOS to form SiO2 shell over MPCM

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2.3. Characterization of PCM composites

The surface morphology of microencapsulated PCM composite was obtained from FESEM (MIRA3, TESCAN). Dry powder sample(s) was placed on FESEM stub and sputtered with AuPd to impart electron conductivity to the sample surface. All images were taken under high

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vacuum (10-10 to 10-12 bar) and accelerated potential of 7-10 kV (kilovolts). Energy-dispersive Xray spectroscopy (EDX) is an analytical technique that is used for analysis of elemental composition of the sample. This analysis was performed using EDX (AMETEK EDAX) which was coupled with the field emission scanning electron microscopy (FESEM, MIRA3 TESCAN) at a voltage of 8 keV. Chemical interactions, if any, between core and shell material, were

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analyzed using FT-IR spectroscopy (PerkinElmer, USA). All samples were analyzed using ATR mode where sample was tested for wavenumbers between 500 cm-1 to 4000 cm-1 against air background. The X-Ray Diffraction (XRD) patterns of the samples were carried out using X-ray diffractometer (XRD, Rigaku Ultima IV, Japan) with CuKα radiation (0.154 nm) at a scanning speed of 4°/min in the range of 5°–80°. Thermal stability analysis of composite was performed using thermogravimetric analyzer (TGA 55, TA Instruments, USA). The sample was taken in a

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platinum crucible and it was placed inside furnace whose temperature was raised from 30°C to

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800°C at a constant heating rate (10°C/min) and inert atmosphere of N2 (60 ml·min-1) was purged to remove any volatile matter. With increase in temperature, sample gets

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evaporated/decomposed and the change in mass is recorded. DSC analysis of MPCM composites

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was done using DSC 25, TA Instruments, USA. Around 5.0 mg of sample was placed in DSC crucible and sealed using crimper after placing a lid over it. Each sample was exposed to

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heating/cooling cycles (minimum 3 cycles) between 10°C to 50ºC and the furnace temperature was uniformly varied at a constant rate of 5oC/min under a constant flow rate of inert nitrogen.

purposes.

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Always 2nd cycle data (melting and crystallization) were considered for analysis and reporting

Two important parameters were used to describe thermal properties of the MPCM composite i.e. encapsulation ratio (R) and encapsulation efficiency (η) (Tahan et al., 2013). These thermal

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properties of MPCM composite were calculated from the DSC thermogram using the equation (3) and (4)

% Encapsulation Ratio (%R)=

∆Hm.(MPCM composite) ∆Hm,(PCM)

×100

(3)

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% Encapsulation efficiency (η)=

∆Hm,(MPCM composite) + ∆H

c,(MPCM composite)

∆Hm,(PCM)

+ ∆H

×100

(4)

c,(PCM)

Where, ∆Hm,(MPCM composite ) and ∆Hs,(MPCM composite) are the enthalpies for MPCM composite during melting and solidification steps respectively where ∆Hm,(PCM ) and ∆Hs,(PCM) are enthalpies during melting and solidification of pure PCM respectively.

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3. Results and Discussion The systematic characterization and analysis of MPCM composites were carried out and

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presented in the following sections.

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3.1. FT-IR Analysis

IR analysis of each constituent of microcapsules i.e. capric acid (PCM), shell wall (GE/GA),

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blank sample (without core), MPCM composites and also silica-coated MPCM were carried out

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separately to understand nature of various functional groups present and probable interactions (bond vibration, stretching, hydrogen bonding etc.) occurring among themselves. The effect of

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shell mass variation on CA core was understood from IR spectra of composites prepared using different shell mass i.e. 1g - 4g but keeping the mass of core unchanged at 2g (G1-G4, TableS1). Equal mass of GE and GA (i.e. 50% mass basis) were mixed and four different samples were made by mixing 2g of CA in each of 1g, 2g, 3g and 4g of GE: GA mixture. Thus, core:

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shell compositions e.g. 2:1 (G1), 2:2 (G2), 2:3 (G3) and 2:4 (G4) were prepared. Any variation in molecular vibrations due to increased shell mass and core-shell interactions occurring were analyzed using FTIR spectra (Fig. 1). Peaks at 3293 cm-1 and 1624 cm-1 of GE corresponds to NH stretching of amine groups present in GE which is obvious because GE is protein while, peak at 1524 cm-1 corresponds to secondary amide linkage (N. Zhang et al., 2013). A broad peak at

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3293 cm-1 with GA is likely due to O - H stretching and possibility of hydrogen bonds while, the peak at 1600 cm-1 is probably due to C-N stretching of primary amide linkage. The strong and broad peak at 1027 cm-1 indicates presence of sugar linkages of GA.

GE

(a)

1624

1524

GA 1637

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G1

1524

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Transmittance (%)

CA

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BS

1600 1027

G2

2917

2849

3500

3000

1693

2500 2000 Wavenumber (cm-1)

1451 1300 931 722 1500

1000

500

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4000

546

G4

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3293

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G3

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SiO2 BS

(b)

Transmittance (%)

CA

G3-SiO2

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3293

3000

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3500

1300

1693 1639

2500 2000 Wavenumber (cm-1)

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4000

794

1538

2851 2917

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G3

1500

933 550

1069 1000

500

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Fig.1. FT-IR spectra of (a) GE (gelatin), GA (gum Arabic), GE/GA shell (without core) i.e. BS; CA (capric acid, core) and MPCM composite with 2g core and different shell mass i.e., with different core : shell ratios: e.g. G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 – 2:4 and (b) SiO2, BS, CA,

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G3 (C: S 2:3) and G3-SiO2 (1g G3: 2 ml TEOS) MPCM composite.

FTIR spectrum of blank sample BS (i.e. only shell wall without PCM) shows a broad

peak at 3293 cm-1 (Fig. 1) mostly due to O - H stretching while peaks at 1637 cm-1 and 1451 cm-1 are likely due to primary and secondary amide linkages of the shell wall. The reaction of –NH2 groups of gelatin with –COOH of GA (García-Saldaña et al., 2016) possibly resulted in primary

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and secondary amide linkages. A low-intensity peak at 2917 cm-1 is likely due to C – H stretching found in BS. Two intense peaks at 2917 cm-1 and 2849 cm-1 with CA is likely due to stretching vibration of C – H bonds of – CH3 and – CH2. A high-intensity peak at 1693 cm-1 arises from carboxyl groups of CA (Song, Dong, Qu, Ren, & Xiong, 2014) while, peaks at 1430cm-1 and 1300 cm-1 correspond to bending or deformation of C – H bonds of –CH2 groups. The peak at 722 cm-1 might be because of in-plane rocking vibration of – CH2 group.

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IR spectra of all composites (G1 - G4) shows close resemblance with each other and no

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additional peak was observed due to variation in core: shell mass ratio. In fact, the intensity of characteristic peaks of CA got reduced with lower core: shell ratios. Neither appearance of any

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new peak nor shifting of an existing peak indicated inert nature of shell (GE/GA) wall selected

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for encapsulation of CA as core (PCM) i.e. no unwanted interaction was observed with higher shell mass which is certainly a desirable property of composite.

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Cross-linker can improve cross-link between gelatin-gum Arabic molecules and facilitate both thermal stability and rigidity in composite. Hence, cross-link reaction for various durations

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and different amounts were carried out cross-link. The remarkable change in surface morphology was observed with duration of crosslink reaction. Possibly, with increasing cross-linking duration no new peak appeared and the intensity of amide linkage (1637 cm-1) consistently remains constant (Fig. S1).

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Role of surfactant in MPCM composites was analyzed by applying different amount of

SDS surfactant without altering any other constituents of G3 and samples were denoted as G3Sr1, G3Sr2, G3Sr3, and G3Sr4 (Fig.S2). No change in characteristics peak of shell/core was observed in the composite. Therefore, higher surfactant concentrations didn’t show any unwanted chemical interaction between shell and core materials but the intensity of characteristic

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peaks of CA got reduced. Possibly, higher surfactant concentration resulted in dissolution of core CA and got washed out in various stages of synthesis which resulted in lowering of characteristic peak intensity (Fig. S2). Thermal stability and granular flow behavior of MPCM composites could be improved by silica coating over composites. Any chemical interaction of silica coating with GE/GA shell was analyzed using FTIR. The IR spectrum of pure silica (SiO2), blank sample BS (i.e. GE/GA

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shell), CA, G3 (without SiO2) and G3-SiO2 are separately presented in Fig.1b. Intense

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characteristic peaks below 1750 cm-1 are mostly contributed by CA molecules while the rest of the samples without ‘CA’ (SiO2, BS) showed relatively broad peaks in this range. IR spectrum of

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SiO2 shows two peaks at 1060 cm-1 and 794 cm-1 probably arising out of asymmetric and

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symmetric stretching vibration of Si – O – Si bonds (Song et al., 2014). The peak at 550 cm-1 corresponds to Si – O bending vibration, while the peak at 933 cm-1 arises from Si-OH. All these

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characteristic peaks of silica are common with G3-SiO2 composite as well. This confirms

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successful synthesis of SiO2 coating over microencapsulated G3 shell.

3.2. Surface Morphology of MPCM composites using SEM Understanding the surface morphology of MPCM composite is essential to interpret thermal stability, flowability, mechanical strength, and chemical reactivity during its subsequent

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applications. Appropriately dried MPCM composites of different core: shell ratio i.e. G1, G2, G3, G4, G3T1, G3T3, G3T5, G3T7, and G3-SiO2 were analyzed through SEM and images obtained are reported in Fig. 2. Morphology and microstructure of MPCM composite got altered with chosen core: shell ratio (Table S1). With G1, the mass of core being double that of the shell mass probably, shell mass was not enough to enclose whole amount of core which resulted in

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sticky oily appearance with even dried sample. After synthesis, the sticky mass was difficult to filter because the oily mass blocked pores of Whatman No. -1 filter paper and filtered mass became solid flakes after freeze-drying (Fig. 2a). With an equal proportion of core: shell composition as in G2 stickiness got reduced and samples could be filtered more easily than G1. This was reflected in much clearer images of micro-capsules (Fig. 2b). With further lowering of core: shell mass ratio (2:3) no sticky appearance was observed and samples could be easily

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filtered which after drying resulted in distinctly separate fine powdery micro-capsules from SEM

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images (Fig. 2c). This indicated a critical amount of shell mass was essential to encapsulate 2g of core used in composites and G3 (core: shell of 2:3) was found to be the right proportion for

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effective encapsulation. With further lowering of core: shell mass ratio (2:4), rigid flakes along

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with free-flow particles were obtained (Fig. 2d). Large excess of shell mass (GE/GA) possibly resulted in hydrogen-bonded GE-GA matrix which became rigid after drying. With higher shell

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mass and fixed core amount, the enthalpy content got lowered. Thus, G3 (2:3) composition was considered most appropriate for detailed analysis e.g. (i) silica coating for improved flow

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property and thermal conductivity, (ii) duration of cross-linking reaction, (iii) amount of crosslinker used, (iv) amount of surfactant applied, etc. Glutaraldehyde cross-linker addition, after coacervation step, was performed purposefully to improve cross-link density between protein (gelatin) and polysaccharides (gum Arabic)

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molecules. –CHO groups of cross-linker most probably reacted with –OH groups of polysaccharide, GA and amine groups of GE (Dinarvand, Mahmoodi, Farboud, Salehi, & Atyabi, 2005). Thus, both GE and GA got attached through glutaraldehyde chain. The mechanical stability of shell wall and particle integrity improved reducing clustering tendency. Contact time of cross-link reaction played important role in particle surface morphologies. Longer duration

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resulted in more cross-link formation, reduced shell porosity, lowered surface roughness and improved mechanical stability. Cross-link reactions were performed for various durations between 1h to 7h and samples were collected after each hour and labeled as G3T1(1h) to G3T7 (7h). Additionally, G3T9 or G3 composite was obtained from reaction of 9h duration. SEM images of G3T1 (2e), G3T7(2f), G3(or G3T9, 2g) clearly indicate that composites slowly change their morphology from very soft matrix with fewer capsules (Fig. 2e) to larger number of

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capsules with distinct shape, smoother surface and improved rigidity (2f and 2g). In the

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following section, improved mechanical stability resulted in gradual shifting of melting

role in developing morphology of composite.

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temperature to higher values were confirmed. Thus, duration of cross-link reaction played crucial

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Incorporation of silica layer over MPCM composite using interfacial hydrolysispolycondensation of alkoxysilanes over GE/GA shell of G3 composite increased roughness (Fig.

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2h) compared to particles without silica coating. This may be attributed to irregular polycondensation and growth of silica particles resulted in non-uniformity and roughness of

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silica-coated composites.

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(b)

(d)

(e)

(f)

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(c)

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(h)

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(g)

Fig.2. FESEM Images of MPCM composite (a) G1 (core: shell or C : S 2:1), (b) G2 (C: S 2:2),

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(c) G3 (C: S 2:3), (d) G4 (C: S 2:4), (e) G3T1 (C: S 2:3, Crosslinking time T = 1h.), (f) G3T7 (C:

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S 2:3, Crosslinking time T = 7h.), (g) G3 (C : S 2:3) and (h) G3-SiO2 composite (1g G3: 2 ml

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TEOS).

3.3. Energy Dispersive X-ray Spectroscopy (EDX or EDAX)

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EDAX characterization is based on the fundamental principle that each element has a unique atomic structure and allows a unique set of peaks on its electromagnetic emission spectrum. Field emission scanning electron microscope (FESEM, MIRA3 TESCAN) with EDX was used

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to analyze G3 (Fig. 3a) and G3-SiO2 (Fig. 3b) samples. G3 didn’t show any silica peak while it is quite prominent with G3-SiO2 sample. Comparing EDX spectra of G3 composite (without silica coating) and G3-SiO2 (with SiO2 coating) the oxygen peak intensities (counts) increased possibly due to presence of SiO2 layer over GE/GA shell. SiO2 content of G3-SiO2 composite was estimated using EDX analysis was ~ 8.83% (mass) confirming appearance of thin silica layer over composite. 19

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(b)

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Counts (cnts)

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Counts (cnts)

(a)

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Fig. 3. EDX analysis of (a) G3 (C: S 2:3, without TEOS) and (b) G3-SiO2 (C: S 1g G3: 2 ml TEOS. 3.4. X-ray Diffraction (XRD)

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Fig.4 shows the XRD behavior of CA, MPCM composite (G3), Silica coated MPCM composite (G3-SiO2), only SiO2 and blank shell (BS). In case of SiO2 and BS spectra, only a broad band at 2θ = 15° ~ 35° appear indicating highly amorphous polymeric nature of both these shell wall materials (GE-GA and SiO2). XRD pattern of CA shows appearance of five sharp peaks at 2θ = 7.72°, 11.51°, 19.92°, 21.69°, and 23.82° which correspond to X-ray beam diffraction from 002, 003, 202, 110 and 200 crystal planes of solid capric acid CA molecules. This confirms during

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synthesis of MPCM composite crystal structure didn’t get destroyed which was consistent with

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earlier reported results (Geng, Wang, Wang, & Luo, 2016). XRD pattern of G3 composite shows presence of both crystallines (CA) and amorphous segments (shell wall, BS). With silica coating

-p

over G3 composite, G3-SiO2 shows existence of both crystalline CA and amorphous BS/SiO2

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shell (Fig. 4).

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4.0E5

21.69

3.5E5 11.51

23.82

7.72

19.92

2.5E5 CA

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2.0E5 G3

1.5E5

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Intensity (Counts)

3.0E5

1.0E5

-p

5.0E4

G3-SiO2 SiO2 BS

10

20

30

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0.0

40

50

60

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80

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Angle (2)

Fig.4. XRD pattern of core Capric acid, CA; MPCM with GE/GA, G3; silica-coated MPCM

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(G3-SiO2); only SiO2 and blank sample, BS (GE/GA shell).

Silica layer incorporation over MPCM shell increased amorphous content compared to G3 alone. XRD peaks of CA remained unaffected in both G3 and G3-SiO2 samples confirming core

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content and absence of any new material due to silica coating. 3.5. DSC Analysis of MPCM composite samples DSC measurement of each MPCM composite sample was performed to understand variation in thermal properties i.e. melting/crystallization temperatures and enthalpy change during phase

22

transformation. Any change in process parameter might affect thermal property and can be analyzed using DSC. Fig. 5a shows enthalpy variation with temperature occurring in pure PCM, MPCM composite samples made with reduced core: shell mass ratios (i.e. G1-G4) and blank sample, BS (without PCM). Only second cycle (heat/cool operation) data from DSC measurement are reported and analyzed. Melting of pure capric acid (PCM) initiated at 29.98°C, peak melting occurred at 31.74°C and associated enthalpy change was 160.8±3 (J g-1) while,

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crystallization phenomena started at 27.6°C, peak crystallization took place at 27.3°C and

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enthalpy of crystallization was 163.3±3 J g-1. Similar details of thermal properties of each sample may be referred from table S2. The blank sample BS, made of GE/GA shell cross-linked with

-p

glutaraldehyde and without PCM showed no melting/crystallization property as the shell was

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amorphous in nature and doesn’t undergo crystal formation and melting phenomena in the temperature range applied during DSC measurement.

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The amorphous behavior of GE/GA shell with negligible crystallinity was confirmed from XRD pattern (Fig. 4) as well and thus, didn’t have any enthalpy contribution in DSC. Both

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melting and crystallization temperatures of composite samples (G1 to G4) were in close agreement (~ ±0.5°C) with that of pure PCM indicating no unwanted complexation or core-shell interaction occurred during synthesis. The onset and peak temperatures of melting for all

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composites samples were lower than that of pure PCM. With the gradual increase in shell mass content, number of microcapsules started increasing per unit mass i.e. increase in surface area due to micron-sized capsules of PCM composites. Thus, surface area of heat transfer gradually increased with number of microcapsules/mass. This resulted in, melting of PCM occurs at relatively lower temperature compared to pure PCM used. In crystallization also, similar

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phenomena of rapid heat loss through surfaces of micron-sized composites resulted in lower crystallization temperature of composite compared to pure PCM.

6 CA G1 G2 G3 G4 BS

(a)

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2

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0

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Heat flow (W/g)

4

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-2

15

20

25 30 35 Temperature (C)

40

45

50

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10

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-4

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6 (b)

G3 G3-SiO2

4

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0

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Heat flow (W/g)

2

-p

-2

15

20

25 30 35 Temperature (C)

40

45

50

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10

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-4

Fig.5. DSC thermograms of (a) BS (blank sample, without capric acid), CA (capric acid) and

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MPCM composite prepared with varying core/shell ratio (G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 2:4) and (b) MPCM composite G3 (0ml TEOS) and G3-SiO2 (2ml TEOS). With increased shell mass i.e. with reduced core: shell mass ratios, specific enthalpy

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content of MPCM composites got reduced. This observation was partially consistent with earlier reports (Fang, Li, Liu, & Wu, 2010). Keeping aside the exception of G3 sample a consistent drop in specific enthalpy content in the sequence: G1(C: S 2:1) >G2 (2:2)> G4 (2:4) may be attributed to higher shell (having zero enthalpy contribution) mass and fixed core PCM mass. G1 and G2 composite samples showed inadequate encapsulation of PCM by the shell mass used which resulted in significant PCM staying un-encapsulated (as indicated through SEM images in 25

section 3.2) and incurred losses during washing/drying stages. This situation didn’t arise with G3 sample where PCM could get properly encapsulated and composite particles were of non-sticky appearance. Thus, encapsulation ratios (R, Eq. 3) reported in Table S2 varied between ~59% (G1) to 29% (G4) with core: shell mass ratios. Amount of cross-linker altering thermal properties of MPCM composite is best

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understood from DSC analysis and results obtained with 0.25ml, 1.0ml, 1.5ml, 2.0 ml glutaraldehyde (G3G0.25, G3G1, G3G1.5, and G3G2) are presented in Fig. S3. The onset and

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peak temperatures for both melting/crystallization steps remained unaffected with cross-linker

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amount. This confirmed that cross-linker reacted externally with GE/GA molecules of shell surface which caused negligible impact on thermal performance of PCM. A small rise in

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enthalpy content of MPCM composites was noted during both melting and crystallization stages. Possibly, higher cross-linker amount increased crosslink density, reduced shell porosity and

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reduced PCM leakage during synthesis.

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Crosslink reactions were carried out for nine hours after addition of 1.0 ml of glutaraldehyde into known mass of MPCM composite derived after complex coacervation step and aliquots were collected after every hour. Longer exposure of cross-linker with GE/GA shell is expected to increase crosslink density in the shell wall improving mechanical stability and

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minimizing leakage of PCM from composite. For representative purposes, DSC results of composite samples (G3T1, G3T3, G3T5, G3T7, and G3) collected after 1h, 3h, 5h, 7h, and 9h of crosslink reactions are presented in Fig. S5. The onset of peak melting/crystallization temperatures remained unaffected with duration of crosslink reaction. Enthalpy of melting/crystallization was within ±3.0 J g-1, indicating crosslink reaction occurred externally. In

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a separate investigation, crosslink reaction was performed at 50oC for 9h to explore any variation in particle morphology or thermal properties but, no appreciable change in those properties were observed. Composite samples prepared without crosslink reactions were of sticky nature compared to samples with cross-linker. Neither duration nor temperature of crosslink reaction showed influence in thermal or morphological properties of MPCM composite samples. Possibly, crosslink reaction occurs immediately after addition of glutaraldehyde and thereafter no

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effect with time/ temperature was noted. This was confirmed from a separate experiment where

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glutaraldehyde solution was added separately into two homogeneous solutions of (i) GE and (ii) GE-GA (1:1) mixture and an instantaneous gelling occurred in both solutions maintained

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separately at room temperature and at 50oC. But, no gelling was observed with GA alone. DSC

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results of these samples (Fig.S4) also confirmed insignificant impact on the duration of crosslink reactions. The percentage encapsulation of PCM also remained unaffected with duration of

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crosslink reaction.

Surfactant plays a crucial role in controlling the size, number/volume and stability of

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micelles in solution besides influencing solution viscosity, miscibility or random phase, etc. Thus, surfactants can influence various properties of composite. Four different composite samples e.g. G3Sr1, G3Sr2, G3Sr3, G3Sr4 were prepared by adding 250mg, 400mg, 600mg and

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800mg of SDS surfactant while all other parameters were kept same as G3. DSC results of these samples showed melting and crystallization peaks remained unchanged with surfactant quantity but both melting/crystallization enthalpies got reduced with surfactant amount (Fig. S5). This shows amount of PCM retained by micelles (subsequently transformed into composites) got reduced with surfactant amount. Usually, higher amount of surfactant increases foaming tendency and increased solubility of PCM in water i.e. increasing quantity of PCM prefers to be 27

in aqueous phase which resulted in loss of PCM and lower enthalpy. This observation is in agreement with an earlier report (Mayya et al., 2003). Possibly higher surfactant amount resulted aggregated micelles which resulted in reduced encapsulation of PCM which resulted in reduced enthalpy (melting/crystallization) and % encapsulation. G3 composite performed superior to all other amounts chosen hence, this was chosen for subsequent silica coating formation over it.

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Silica coating over shell of G3 composite was achieved through hydrolysis of TEOS and

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polycondensation reactions according to Eq.1 and Eq.2 (Zhu et al., 2018). Thermograms of

GE/GA-SiO2 MPCM composite is shown in Fig.5b. With SiO2 coating over G3 composite,

-p

melting/crystallization temperatures remained unchanged but, enthalpy contents of both

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melting/crystallization got drastically reduced due to addition of SiO2 layer (with zero enthalpy content). Cyclic (heating/cooling) stability analysis of G3 composite was carried out (G3C1,

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G3C10, G3C20, G3C30, G3C40, G3C50) using DSC instrument and thermograms obtained are reported in Fig. S6. Negligible change in DSC thermograms between 1st to 50th cycles indicate

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appreciable thermal stability and reported in Table S3.

3.6. TGA analysis of MPCM composite

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Thermal stability analysis of MPCM composites using TGA (thermo-gravimetric analysis) instrument explains the temperature range the composite is applicable. Fig. 6 shows TGA data of pure Bio-PCM (CA, m.p. 31.74°C and b.p. 269°C), blank shell BS (without CA core) and different MPCM composites (i.e. G1, G2, G3 and G4). Detailed temperature and mass loss data are tabulated in Table S4. Nearly 100% mass loss was observed with pure CA between 110.9°C181.4°C indicating rapid evaporation of CA molecules due to its low b.p. at 269°C. With blank 28

sample, BS (without PCM) ~ 4.5% mass loss till 100oC and negligible loss thereafter between 100°C - 220°C indicates evaporation of initial moisture associated with the shell wall [composed of GE/GA (1:1)] and appreciable thermally stability (till 220°C) of the GE-GA matrix. In spite of appreciable thermal stability of BS shell wall composites (G1-2:1, G2-2:2, G3-2:3, and G42:4) showed evaporation of Bio-PCM at much lower temperature compared to the temperature at which evaporation of pure Bio-PCM occurred. The increased surface area with MPCM

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composite samples possibly facilitated heat transfer followed by melting, porous diffusion of

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molten Bio-PCM through shell wall and evaporation of CA molecules from particle surface.

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Thus, melting and evaporation of encapsulated Bio-PCM occurred earlier than that of pure CA. The blank sample, BS showed higher thermal stability compared to MPCM composite

4.5% mass loss between 30°C -100°C mostly due to evaporation of moisture present

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(i)

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samples and it showed three stages of decompositions as:

in hygroscopic GE/GA shell, (ii)

55.4% mass loss between 200oC - 385oC resulted out of functional group

(iii)

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decomposition of GE/GA molecules and Residual 40.1% mass loss between 390oC - 600oC occurred because of thermal

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degradation of -C-C- chains.

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100 (a)

BS CA G1 G2 G3 G4

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Mass (%)

60

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0

400 Temperature (C)

600

800

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200

30

100 CA G3 G3-SiO2

(b)

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Mass (%)

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400 Temperature (C)

600

800

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200

Fig.6a. TGA thermograms of (b) BS (blank sample, without capric acid), CA (capric acid) and MPCM composite prepared with varying core/shell ratio (G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 2:4) and (b) CA (core), GE/GA microencapsulated PCM composite G3 (C : S: TEOS 2 : 3 : 0ml

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), G3-SiO2 (G3 : TEOS 1g : 2ml).

With higher shell mass but fixed amount of core PCM, mass percent of core Bio-PCM

got reduced in composites: G1(200%)-G2(100%)-G3(67%)-G4(50%) which got reflected in percentage mass loss of PCM due to evaporation. At higher temperature, every composite

31

sample showed three stages of thermal decomposition and the temperature ranges precisely coincided with that of BS. The sequence in mass loss percentage with composites, during stage 1 (~30oC-172oC), were: G1 - 70.2%; G2 – 52.9%; G3 – 60.3%; G4 – 39.7% (Table S4) were same as that of Bio-PCM used for synthesis. Large difference in PCM content initially taken for composite synthesis and amount evaporated during stage-1 heating (~30oC-172oC) accounts for losses occurred during synthesis stages. Percent loss of PCM gets lowered with increasing

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proportion of shell mass used in composite, possibly appreciable amount of shell mass could

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easily encapsulate the core Bio-PCM and reduced its losses during synthesis. Interestingly, allcomposite samples (G1, G2, G3, and G4) showed rapid PCM evaporation starting at ~100oC and

-p

are independent of the shell mass content i.e. increased shell mass couldn’t impart appropriate

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thermal resistance to composite samples. This is most likely due to appearance of porosity in the shell wall of composite dried through lyophilization technique. This technique improved the non-

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sticky property of the material but made it porous in nature. Figure 2 indicates appearance of porosity with lyophilized samples having no characteristic shape and is the typical due to

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sublimation of frozen water. Composite surface morphology is purely dependent on the drying method adopted. Figure 2 revealed that particles were relatively spherical with irregular surface but, were agglomerated/adhered which resulted in porous structure of MPCM composites. Usually, absence of cryo-protectant during lyophilization results in aggregation, deformation and

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appearance of collapsed structures (Yüksel & Şahin-Yeşilçubuk, 2018). This resulted in inconstancy in evaporation temperature of PCM presented in G1 to G4. TGA thermograms were quite consistent with mass percent of Bio-PCM present in core. The second and third stages of thermal decomposition occurring between 200°C-600°C (G129.8%; G2-45.7%; G3-39.5% and G4-61.1%) is attributed to decomposition of functional groups

32

of protein (GE) and polysaccharides (GA) molecules. PCM evaporation started (with each composite) at ~ 100°C but, the final temperature to arrive at the residual mass gradually shifted towards lower temperatures i.e. ~172.5oC, 164.3oC, 171.5oC, 152.5oC, etc. probably increased shell mass facilitated more microcapsule formation i.e. increased surface area and better heat transfer. The residual mass for all sample reduced to zero beyond 600°C, indicating complete

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degradation of carbon chains of GE/GA molecules of shell wall. Cross-linker creates link between reactive functional groups of GE/GA molecules and

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improves mechanical stability. TGA behaviors of composite samples made with different

-p

amounts of cross-linker (25% aqueous solution of glutaraldehyde) i.e. 0.0ml, 0.25ml, 1.0ml, 1.5ml and 2.0ml and the percent mass loss noted (between 30°C-164°C) were: 55.5%, 51.8%,

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52.8%, 53.0% and 54.8% respectively (Fig. S7). Highest amount of PCM evaporation with composite made with zero crosslinker amount is due to presence of highest porosity in shell wall

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which allowed easy evaporation of PCM through shell wall. A higher quantity of cross-linker improved crosslink density and improved retention of encapsulated PCM which gets reflected in

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PCM evaporation loss. Between 195°C–600°C nearly ~47% mass loss occurred due to degradation of shell wall.

The duration of crosslink reaction (i.e. time of exposure a cross-linker be in contact with

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microencapsulated PCM) may influence the number of cross-links formed with GE/GA shell. Hence, crosslink reactions were performed for different durations e.g. 1h, 3h, 5h, 7h and 9h (G3T1, G3T3, G3T5, G3T7, G3T9 or G3) and TGA analysis were done after drying. Duration of crosslink reaction had negligible impact on thermal stability of MPCM composites (Fig. S8). All composite samples showed ~55.8% PCM evaporation between 30oC - 170oC. As established before, rapid crosslink reactions resulted in specific number of crosslinks which didn’t change 33

with duration afterward. Thus, duration of cross-link reaction had negligible impact on TGA results. Amount of surfactant decides effectiveness of micellization (control population of micelles per unit volume and size distribution) and entrapment efficiency of PCM as a core. Different amount of surfactant were selected e.g. 250mg (G3Sr1), 400mg (G3Sr2), 600mg (G3Sr3) and 800mg

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(G3Sr4). TGA data (Fig. S11) showed 52.9, 53.1, 49.3 and 42.3 % of PCM loss between 30°C170°C. Higher surfactant amount resulted in improved solubility of Bio-PCM in water which got

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washed out during synthesis, thus, observed trend (baring G3Sr2) in PCM evaporation is

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justified (Fig. S9).

Thermal and mechanical stabilities of microencapsulated PCM can be improved applying

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silica coating over MPCM composite. Acid hydrolysis cum polycondensation of TEOS resulted

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in SiO2 coating. Comparing TGA thermograms of G3 and G3-SiO2 improvement in thermal stability is evident (Fig. 6b), this is in consistent with earlier reports (Zhu et al., 2018) of improvement in thermal properties of nanoPCMs with n-octadecane core and PMMA-SiO2

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hybrid shell. Even silica-coated PCM underwent three stages of thermal decomposition i.e. 53% (30°C - 185°C), 22% (185°C – 385°C) and 11% (390°C - 600°C) as was observed with blank

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shell, BS.

3.7. Leak test of composite samples using Hot Stage Microscope Leak test is essential to understand the leakage issue with MPCM composite. This test was performed using hot stage microscopy (HSM). MPCM composite samples were placed over hot stage which can be heated in a controlled manner. The temperature of hot stage was raised (10o

34

min-1) to 50°C and was held for 10 min to monitor any physical transformation occurring with the sample. HSM analysis of CA, G1, G2, G3, and G4 was separately carried out at two temperatures i.e. 25 °C and 50 °C. Images were captured at 5x magnification and presented in Fig. 7. It showed capric acid got melted at 50°C but, no composite showed melting/leakage behavior while exposed at 50°C indicating, encapsulated PCM could comfortably stay inside composite shell. This test supported TGA data and negligible PCM loss was recorded at 80°C.

G1

G2

G3

G4

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CA

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Thus, GE/GA shell improved thermal stability and prevented PCM leakage till 80oC.

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25 ºC

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Fig.7. Hot stage microscopy images obtained with G1, G2, G3 and G4 composites at temperatures 25°C and 50°C to understand leakage behavior of MPCM composite.

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Conclusion: A novel capric acid based micro encapsulated PCM (MPCM) composite was synthesized using protein-polysaccharide interactions as shell materials adopting complex coacervation. Inadequate shell mass for given mass of core resulted in inefficient encapsulation (e.g. G1 and G2 composite samples, where major PCM mass remained outside microcapsules) and significant PCM loss during washing/drying stages. A consistent drop in specific enthalpy

35

content according to the sequence: G1(C: S 2:1) >G2 (2:2)> G4 (2:4) might be due to higher shell mass for a given core PCM mass. This situation didn’t arise with G3 sample where PCM could get properly encapsulated and composite particles were of non-sticky appearance. The composite G3 (core: shell, 2:3) showed high encapsulation ratio and good energy storage ability. So, G3 (core: shell, 2:3) was selected for detailed investigations. Existence of hydrogen bonding between GE and GA functional groups plays important role in imparting thermal stability to the

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shell wall. The thermal stability of composites (G3G0.25 to G3G4) could be significantly

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improved by increasing cross-linker quantity but, the duration of cross-link reaction didn’t much influence the thermal stability. Higher surfactant amount facilitated PCM dissolution in water

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which, lowered the PCM content inside composite without affecting the thermal stability.

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Incorporation of silica layer over MPCM composite improved free-flow behavior and thermal stability but obviously lowered the enthalpy content (per unit mass). The thermal stability of

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composite (G3-SiO2) improved by ~20°C with SiO2 coating over GE/GA shell (G3). Melting and crystallization temperatures were close to pure PCM but % encapsulations were in the range:

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G1(58.89%), G2(50.25%), G3(53.71%), G4(28.80%) and G3-SiO2 (35.55 %). The thermal properties of G3 composite (after 50 rapid heat/cool cycles) remained unaffected besides overcoming leakage/contamination/losses issues of PCM after placing composite at 50ºC for 10min over hot stage microscope. High latent heat storage ability, good thermal stability and

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zero leakage of MPCM composite showed its potential as thermal energy storage material. Hope, this green encapsulate technique using Bio-PCM will encourage many more researches in this field.

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Acknowledgment: Financial support to execute the experimental work is gratefully acknowledged to MHRD (Ministry of Human Resources Development) Plan grant (2017-18) and

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IIT Roorkee (No. IITR/SRIC/244/FIG-Sch-A), India.

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List of Figures

Fig.1. FT-IR spectra of (a) GE (gelatin), GA (gum Arabic), GE/GA shell (without core) i.e. BS; CA (capric acid, core) and MPCM composite with 2g core and different shell mass i.e., with different core : shell ratios: e.g. G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 – 2:4 and (b) SiO2,

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BS, CA, G3 (C: S 2:3) and G3-SiO2 (1g G3: 2 ml TEOS) MPCM composite.

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Fig.2. FESEM Images of MPCM composite (a) G1 (core: shell or C : S 2:1), (b) G2 (C: S

2:2), (c) G3 (C: S 2:3), (d) G4 (C: S 2:4), (e) G3T1 (C: S 2:3, Crosslinking time T = 1h.), (f)

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G3T7 (C: S 2:3, Crosslinking time T = 7h.), (g) G3 (C : S 2:3) and (h) G3-SiO2 composite (1g

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G3: 2 ml TEOS).

Fig.3. EDX analysis of (a) G3 (C: S 2:3, without TEOS) and (b) G3-SiO2 (C: S 1g G3: 2 ml

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TEOS.

Fig.4. XRD pattern of core Capric acid, CA; MPCM with GE/GA, G3; silica coated MPCM

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(G3-SiO2); only SiO2 and blank sample, BS (GE/GA shell). Fig.5. DSC thermogram of (a) BS (blank sample, without capric acid), CA (capric acid) and MPCM composite prepared with varying core/shell ratio (G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 -

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2:4) and (b) MPCM composite G3 (0ml TEOS) and G3-SiO2 (2ml TEOS). Fig.6. TGA thermograms of (b) BS (blank sample, without capric acid), CA (capric acid) and MPCM composite prepared with varying core/shell ratio (G1 - 2:1, G2 - 2:2, G3 - 2:3, and G4 2:4) and (b) CA (core), GE/GA microencapsulated PCM composite G3 (C : S: TEOS 2 : 3 : 0ml ), G3-SiO2 (G3 : TEOS 1g : 2ml).

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Fig.7. Hot stage microscope images obtained with G1, G2, G3 and G4 composites at

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temperatures 25°C and 50°C to understand leakage behavior of MPCM composite.

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