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Analysis of graphene-encapsulated polymer microcapsules with superior thermal and storage stability behavior
Accepted Manuscript Analysis of graphene-encapsulated polymer microcapsules with superior thermal and storage stability behavior Sourav Sarkar, Byungki Kim PII:
S0141-3910(17)30049-6
DOI:
10.1016/j.polymdegradstab.2017.02.012
Reference:
PDST 8172
To appear in:
Polymer Degradation and Stability
Received Date: 16 October 2016 Revised Date:
24 February 2017
Accepted Date: 26 February 2017
Please cite this article as: Sarkar S, Kim B, Analysis of graphene-encapsulated polymer microcapsules with superior thermal and storage stability behavior, Polymer Degradation and Stability (2017), doi: 10.1016/j.polymdegradstab.2017.02.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Analysis of Graphene-Encapsulated Polymer Microcapsules with Superior Thermal and Storage Stability Behavior
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Sourav Sarkar, Byungki Kim* School of Mechatronics Engineering, Korea University of Technology and Education
of Korea Sourav Sarkar: [email protected]
Phone no: +82-41-560-1143
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Fax no: +81-41-560-1253
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*Byungki Kim: [email protected]
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1600 Chungjeol-ro, Byeongcheon-myeon, Dongnam-gu, Cheonan, Chungnam, 31253 Republic
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Abstract A new series of urea-formaldehyde polymer microcapsules with two-component cores have
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been synthesized by an in-situ polymerization procedure. Graphene was used as one of the components in the core material along with a bisphenol-A epoxy resin. Preparation of graphene oxide was carried out by the Staudenmaier oxidation process, followed by further thermal
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treatment in a controlled environment to produce thermally treated graphene (TTG). The thermal and physical properties of the microcapsules were investigated by TGA (Thermogravimetric
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analysis) and, SEM (Scanning electron microscopy) among other methods. The chemical composition of the microcapsules was ascertained by FTIR (Fourier-transform infrared) spectroscopy. The resultant graphene-encapsulated microcapsules have shown an increasing degree of thermal stability in the range from 250°C to over 400°C, depending on the TTG wt%. They also exhibited good storage stability (> 90%) over different periods of time. It was
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determined that the thermal and physical properties of the microcapsules are closely linked to the core materials and the processing conditions. The processing conditions, in turn, can be varied to
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synthesize different types of microcapsules with different sizes, thermal stability and yield percentage values. The improved sway over the thermal degradation provided better control over
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the release rate of the core material under various conditions. Also, the unusually high thermal stability behavior of the polymer microcapsules’ shell wall demonstrates better survivability of these microcapsules under harsh external conditions which can be exploited in their use as highly stable self-healing agent carrier in high-performance polymer materials. Keywords: Polymer microcapsules, graphene, thermal stability, storage stability 1. Introduction 2
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Polymer microcapsules and microcapsule-based self-healing constitute the most prominent research branch in the autonomous self-healing approach in high-performance polymers.
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Microcapsule-based healing has been applied a number of times in different polymers, in which the microcapsules primarily work as containers to safely carry the self-healing agents inside the polymer matrix [1-5]. Generally, microcapsules are small particles with an active core
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surrounded by a shell material. This active core can be a variety of chemicals based on the end application, such as drugs, enzymes, dyes, water, or any type of self-healing agents. The
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mechanism of self-healing is dependent on the progression of micro cracks and their interaction with the healing agent containing microcapsules embedded in the polymer matrix. The microcapsules release the self-healing agents after getting ruptured by the sharp edges of the micro cracks during crack propagation in the polymer matrix. The subsequent polymerization of the healing agents triggered by interaction with either an embedded catalyst or curing agents
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from the polymer matrix helps to close the micro crack, thus repairing the polymer matrix internally and preventing further damage. This self-healing technique has the potential to
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significantly increase the life of polymer materials with minimal cost and human involvement. Survivability under harsh external environments along with long shelf life are some of the
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primary and essential characteristics needed before employing a certain type of microcapsules as self-healing delivery agents in a specific polymer matrix. Generally, an elevated curing temperature (100°C-175°C) is used during the curing process for high-performance polymers, which has proven detrimental to the survivability of most polymer microcapsules. Thus, increasing the thermal stability of polymer microcapsules that can survive under high curing temperatures is the most logical way forward. Apart from high thermal stability, microcapsules 3
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must also show adhesion with the polymer matrix to facilitate the release of healing agents after composite rupture.
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As high-performance polymer materials, various types of epoxy resins have been developed for a wide range of applications. These include, but are not limited to, uses as adhesives, coatings, and composite materials due to their impressive chemical-resistant properties as well as
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mechanical attributes such as high strength and modulus [6-9]. There have been many instances where epoxy resins have been used as a standard core material, i.e., self-healing agents in
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microcapsules, which can polymerize upon release [10]. The basic purpose of this work was to synthesize a special class of microcapsules that can withstand the harsh fabrication procedure of high-performance polymers. Kang et al demonstrated in 2015 that the overall thermal stability of polymer microcapsules can be increased to as high as 180°C by a protective PDA (polydopamine) coating [11]. We believe the thermal stability threshold can further be stretched
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well beyond this point. Toward this goal, we selected thermally treated graphene (TTG) as a nano-filler to be used along with epoxy resin to increase the thermal stability threshold of
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microcapsules. It has already been reported that core materials can have a very crucial role in shaping the thermal behavior of microcapsules [12]. Graphene is well known for its excellent
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thermal properties resulting from its unique sp2 bonded honeycomb surface and has been observed to increase the thermal stability of epoxy resin-based composite materials [13,14] We have applied an in-situ polymerization procedure to synthesize microcapsules with ureaformaldehyde (UF) polymer as a shell along with TTG attached to epoxy resin as active core materials. TTG was prepared by a thermal reduction technique with graphene oxide (GO) prepared by the Staudenmaier oxidation procedure from raw graphite. The ‘as prepared’ 4
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graphene-encapsulated microcapsules have shown incredible thermal stability to over 400°C in TGA. The chemical functional groups present in the microcapsules were evaluated using FTIR
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spectroscopy. It was deduced, after analyzing the experimental data, that the thermal, chemical and physical properties of microcapsules are closely dependent on various process parameters. All the relevant data concerning the close relationship between the process parameters and the
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prepared microcapsules have been established and presented in this article along with the most probable explanations. In this work, we were able to demonstrate for the first time that thermally
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treated graphene can be encapsulated inside the UF microcapsules under certain conditions, and the properties of those microcapsules can be controlled according to various requirements. In one of our previously reported study, GO had been encapsulated inside the UF microcapsules, rendering them more thermally stable than other types of microcapsules [15]. This work has utilized the superior thermal stability of graphene by adding it to the core material when
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synthesizing UF microcapsules with much higher thermal stability. TTG was one of the key components of the prepared microcapsules, contrary to previous works in which graphene had
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been added to the shell after microcapsules preparation by layer by layer assembly [16,17] The unusual high thermal stability of the microcapsules makes them ideal for application in high-
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2. Materials and Methods
Raw graphite (Code No. 230U, Size - 44 µm) was supplied by Asbury Carbons. Nitric acid (HNO3, Reagent grade, fuming >90%) and potassium chlorate (KClO3, 99+%, ACS Reagent) and sulfuric acid (H2SO4, ACS, 95%-99%) were purchased from Sigma Aldrich for graphene oxide (GO) preparation by the Staudenmaier oxidation procedure. EPON 828 was used as the 5
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standard epoxy resin and was purchased from Alfa Aesar. Acetone was bought from Sigma Aldrich and used as the solvent for attaching of the TTG with epoxy resin. For microcapsule
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preparation, urea (ACS, 99.0-100.5%) and ethylene maleic anhydride (EMA) were purchased from Sigma Aldrich. Resorcinol (ACS, 99.0-100.5% crystalline), ammonium chloride (NH4Cl, 98%+) and formaldehyde (HCHO, 37% w/w aq. Solution, stabilized with 7-8% methanol) were
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bought from Alfa Aesar. Sodium hydroxide (NaOH, 50% w/w aq. Solution), hydrochloric acid (HCl, ACS, 36.5-38% liquid) were bought from Alfa Aesar and were used to control the pH of
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the reaction. 2.1. Preparation of thermally treated graphene (TTG)
We used the Staudenmaier oxidation procedure to prepare GO from raw graphite in the presence of acid mixtures of HNO3 (nitric acid), H2SO4 (sulfuric acid) (mixing ratio 1:2) and KClO3 (potassium chlorate). The reaction continued under magnetic stirring for 96 hours at a
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stirring rate of 900 rpm before being stopped. Graphite oxide was formed when the reaction mixture changed color from black to green. The graphite oxide was then ultrasonicated to obtain
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few-layer graphene oxide (GO). GO was washed first with 5% HCl, followed by multiple distilled water washes until the pH value approached neutrality. The heavily oxygenated GO was
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then thermally treated using a simple hotplate procedure to obtain thermally treated graphene (TTG) [18].
2.2. Attachment of TTG with epoxy resin A measured amount of TTG was dispersed in acetone by an ultrasonicator for 30 minutes. The dispersed TTG in acetone was then added to epoxy resin (EPON 828) in a shear mixer for 15 minutes under 2000 rpm. Acetone was chosen because of its low boiling point and high 6
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volatility. Additionally, the presence of the >C=O group in its structure made it ideal to form a stable dispersion of TTG by stable bonding interactions with the active groups of TTG.
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2.3. Emulsification process Ethylene maleic anhydride (EMA) was chosen to be applied as an emulsifier in the encapsulation process. 2.5 wt % of EMA solution was prepared by adding it to deionized water.
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The solution was stirred with a mechanical stirrer for 20 hours at 30°C so that the emulsifier fully dissolved.
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2.4. In-situ polymerization process
The emulsifier solution was then added to distilled water and stored in a glass beaker. Urea, resorcinol and ammonium chloride were added to the aqueous medium under stirring. For detailed analysis, different batches of samples involving different amounts of core and shell materials accompanied by a varied degree of mechanical stirring rates were prepared. In the
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preparation process, the shell material of the resultant microcapsules was composed of urea and formaldehyde. Experimental data described in the discussion section also revealed the presence
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of water molecules in the capsule shell walls. Resorcinol was used as a crosslinking agent, while ammonium chloride reacted with the formaldehyde to facilitate the shell forming process of the
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microcapsules [19]. The pH of the solution was maintained at 3.5 by the addition of adequate amounts of NaOH (sodium hydroxide) and HCl (hydrochloric acid). The overall synthetic process of the in-situ polymerization procedure followed the pathway reported in our earlier work involving the encapsulation of GO [15]. After 4 hours of constant stirring, the prepared microcapsules in the aqueous medium were allowed to cool to room temperature for a few hours.
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After cooling, the microcapsules were collected by filtration, washed with distilled water and left to dry for 48 hours at room temperature.
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2.5. Characterization Graphene oxide (GO) and thermally treated graphene (TTG) were analyzed by FTIR spectroscopy on a Spectrum 100S instrument (PerkinElmer) to identify the functional groups
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composition on its surface. XRD of GO and TTG were performed on an EMPYREAN instrument (Panalytical Ltd.) with an angle ranging between 5° and 30° to observe the crystal
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structure. FTIR was again employed for chemical characterization of the microcapsules. Scanning electron microscopy (SEM, JSM-6010LA, JEOL) was used to evaluate the overall surface morphology and the degree of roughness of the shell of the prepared microcapsules. Thermal analysis of the microcapsules was carried out using thermogravimetric analysis on a TGA Q500 (TA Instruments) at a rate of 10°C/min. Electron diffraction studies of GO and TTG
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were carried out by a high resolution transmission electron microscope (HRTEM, Tecnai TF30 ST) which operated at 300kv.
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A high degree of storage stability is one of the most essential pre-requisites for practical applications of the microcapsules. The longer the microcapsules can hold the core content, the
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greater the stability. A stronger shell material is better for enhanced storage stability over a certain period of time. This was studied by the amount of weight loss suffered by the microcapsules in standard laboratory conditions over various durations of time. 2.6. Microcapsules’ core content calculation We employed an extraction procedure using acetone as the standard solvent to measure the core content of the prepared TTG-Epoxy resin encapsulated microcapsules. The calculated 8
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amount of microcapsules (Mi) were crushed in a mortar. The extraction solvent, acetone, was added to a mixture of both shell and core materials of crushed microcapsules. The mixture in
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acetone was filtered using filter paper; the filtrate portion contained the core material and the residue in the filter paper was the shell material. The shell material was then dried and weighed (S0). The final weight percentage of the shell material (S) was calculated, and the weight
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percentage of the core material (C) was found using that value. S = (S0 / Mi) 100% C = 100% – S 3. Results and discussions 3.1. Structural analysis of TTG
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Eqn (1)
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Structural analysis was carried out by XRD (X-ray diffraction) and is represented in Figure 1. In the XRD analysis, we see major peaks of GO and TTG at 11° and 24°, respectively, which reveal a decrease in the interlayer spacing in TTG compared to GO. According to a calculation
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using Bragg’s law, the interlayer distances between the carbon layers in GO and TTG are approximately 7Å and 3.6Å, respectively. Removal of most of the oxygen-containing active
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functional groups between the carbon layers devised a new 3D structure closely packed with the van der Waals force of attraction. Partial removal of oxygen-containing groups on the surface of GO is the reason behind this decreased interlayer spacing which also indicates the partial restoration of a sp2 hybridized hexagonal bonding network similar that of raw graphite on the carbon layer [20].
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Figure 1. XRD results of GO and TTG where XRD of GO was reproduced with permission from [15] (© 2016 Wiley) 3.2. Chemical analysis of TTG
Chemical analysis of GO and TTG involved FTIR (Fourier-transform infrared) studies, as
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shown in Figure 2. A detailed analysis shows the presence of oxygen-containing active functional groups on both the GO and TTG surfaces. While the presence of strong peaks at 914 cm-1 and 3319 cm-1 signifies the bending and stretching vibrations of hydroxyl (O-H) groups in
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GO, TTG shows no lower frequency vibrations for the -OH. Epoxide (C-O) and carbonyl (C=O) groups are detected by peaks at 1162 cm-1 and 1708 cm-1. The presence of C=C on the GO and
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TTG surface is identified by distinct peaks at 1640 cm-1 and 1614 cm-1. The presence of C-O on the TTG surface indicates the partial reduction of GO under external heat. The presence of these active groups on the TTG surface makes the thermally treated graphene a chemically active species that has a profound influence on the physical and chemical properties of the prepared microcapsules, as further analysis revealed.
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Figure 2. FTIR spectral analysis of GO and TTG where FTIR of GO was reproduced with permission from [15] (© 2016 Wiley) 3.3. Microencapsulation
The microencapsulation process by an in-situ polymerization process involved four distinct
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stages. These stages were significantly influenced by temperature and pH during the reaction. The TTG encapsulated UF polymer microcapsules were formed distinctly in a later part of the reaction. During the first 60 minutes, the epoxy droplets combined with dispersed TTG were
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visible in the aqueous medium. This stage exhibited a steady increase in temperature along with a decrease in the pH value. Over the total course of the microencapsulation process, the pH value
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deteriorated before finally becoming stable in the last 30 minutes of the reaction. The first stage was typically followed by a dark greyish-white milky suspension in the next hour of reaction which was the result of the newly formed urea-formaldehyde prepolymer. The dark greyish shade was due to the presence of TTG in the reaction medium. The microcapsules tentatively started to take shape around the 120-180 minute time period, and the pH appeared to be unchanging for a brief moment before embarking on another downward trend. The 11
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microcapsules finally formed during the last hour of the preparation procedure and the milky
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greyish-white suspension became transparent.
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Figure 3. Chemical reaction involving shell wall formation.
Figure 4. pH change during microencapsulation process.
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A coherent capsule shell structure started to form generally after 90 minutes of the start of the in-situ polymerization process. The prepolymer started to gain in molecular weight significantly as the reaction proceeded. The chemical reaction of the urea-formaldehyde polymer shell wall formation is shown in Figure 3. During the initial shell forming process, urea and formaldehyde react together in an aqueous medium to synthesize the mono-methylol urea derivative. The dimethylol urea derivative forms when the mono-methylol urea derivative reacts with free 12
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formaldehyde. These urea derivatives react together to ultimately produce the urea-formaldehyde (UF) polymer, thereby making the main building block of the capsule shell wall. The increase in
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molecular weight for the UF was followed by its deposition on the core material-water interface, thus starting the shell forming process. The deposition of the UF prepolymer on the TTG-epoxy resin-water interface was facilitated by the low stirring rate, which is discussed in detail later.
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Changes in pH during the preparation process have a profound effect on the microcapsule development and surface morphology. Because it is an already well-known fact that either an
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acid or base can catalyze the addition phase of the polymerization process, the change in pH can accelerate or decelerate the reaction. The changes in pH during the reaction are shown in Figure 4. During the full course of the reaction, the pH decreased steadily before reaching a stable state at the end of the reaction. The considerable effect of pH on the surface morphology is also a widely reported phenomena [21]. In this experiment, the continuous downward trend in pH
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helped to increase the viscosity. High viscosity generally promotes greater deposition of shell material, making the microcapsule shell wall thicker and rougher. This swift increase in viscosity
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at the core material-water interface had a huge influence over the UF suspension in shear flow. Microcapsules prepared in higher pH value generally have smooth capsule shell walls, whereas
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thick and rough shell exteriors are noticed after preparation in lower pH values, as shown in Figure 5.
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3.4. Chemical analysis of microcapsules
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Figure 5. Shell morphology of microcapsules prepared under different pH value.
Figure 6 shows the FTIR spectra identifying all the active functional groups present in the microcapsules, both as part of the capsule shell wall and core materials. The presence of the epoxide group in the FTIR results for microcapsules approximately 910 cm-1 proves that the
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microcapsules are filled with epoxy resins. Apart from that, the existence of C-N, C=O were confirmed by their corresponding peaks at 1290 cm-1and 1716 cm-1. The broad peak at 3334 cm-1 may be due to the superposition of N-H and –OH groups [22]. Additionally, the presence of N-H
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was confirmed by peaks at 1645 cm-1 due to the presence of a primary amine (urea) and 831 cm-1 due to out of plane vibration. The presence of aromatic C=C was confirmed by a stretching
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vibration peak at around 1564 cm-1 which is most likely from phenyl group structure part of the TTG-epoxy resin core material of the microcapsules [23,24]. The alkene stretching vibration peak for C=C is detected at 1612 cm-1 (in-plane vibration) possibly as a result of mostly restored sp2 hybridized surface of TTG [25].
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3.5. SEM analysis of the microcapsules
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Figure 6. FTIR spectral analysis of the microcapsules.
Figure 7 shows the surface morphology of the TTG encapsulated microcapsules under SEM (Scanning Electron Microscopy) analysis. It confirms that the growth of the microcapsules with
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active core did not suffer any negative effect due to the presence of TTG. It reveals the rough exteriors of the larger ones due to the protruding nanoparticles of urea-formaldehyde, which enhanced and increased the surface adhesion. These larger microcapsules have higher surface
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area values as well. Typically, these microcapsules demonstrate smooth inner surfaces of the shell compared to the outer one. This is facilitated by a low stirring rate and a lower core/shell
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material ratio. We also see smaller microcapsules with smooth and uniform surfaces as a result of high stirring rates. The smaller microcapsules with uniform shapes exhibit a thin outer wall as a result of a lower rate of deposition by the urea-formaldehyde polymer at the water-core material interface.
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Figure 7. SEM images of the microcapsule surface morphology. 3.6. Mean diameter vs stirring rate
The mean diameter of the microcapsules is found to be inversely proportional to the stirring
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rate of the mechanical stirrer when all other process parameters are constant, which is shown in Figure 8. Every preparation yielded both larger and smaller diameter microcapsules. However, only the dominant sizes under specific stirring rates have been shown in the plot. During the
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microencapsulation process, smaller microcapsules are found near the magnetic stirrer, whereas larger diameter microcapsules exist in the region away from the stirrer. These conclusions are
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drawn from well-documented studies [1]. Generally, we considered a 20 µm diameter as the upper limit for smaller microcapsules and the lower limit for larger ones. The increase in the stirring rate causes an elevation of shear stress, which results in the decrease in diameter of the microcapsules; hence, smaller microcapsules predominate. A high stirring rate creates many sites of nucleation, which terminates larger microcapsules to smaller ones and provides uniform growth. On the other hand, the lower stirring rate helps synthesize larger microcapsules. 16
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Additionally, it is considered that a high stirring rate helps to contain the agglomeration of the UF polymer to an acceptable level while simultaneously facilitating the slow-moving but closely
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packed deposition of UF polymer at the core material interface.
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Figure 8. Size analysis vs stirring rate.
Figure 9. Mean diameter vs core/shell material ratio.
3.7. Mean diameter vs core/shell material Figure 9 shows the relationship between the core/shell material ratio and the average diameter of the microcapsules. The higher core/shell material ratio contributes to weaker shell growth and 17
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lower stability. A greater amount of TTG-epoxy resin core material in comparison to the UF shell material initially helps to expedite the process for smooth and thin capsule wall generation.
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However, the diameter size of the UF microcapsules drops drastically with a further increase in the core material. Theoretically, the increase in core material should have formed larger droplets of the core material during the microencapsulation procedure, paving the way for larger
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microcapsules. However, in reality, it was found to be counter-productive, as the lower amount of shell material could not withstand the outward pressure from the core. This makes the
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microcapsules with higher ratios of core materials unstable and prone to early fracture, even before the completion of the synthesis, proving that this was a less viable pathway for efficient synthesis. In the end, a higher core material ratio results in the predominance of smaller, smooth and uniform UF microcapsules. It is shown in Figure 10 that increasing the core materials in comparison to the shell resulted in smaller UF microcapsules with very smooth and thin outer
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shells as a result of the reduced deposition of shell materials. On the contrary, the lower core/shell ratio facilitated greater percentage of larger microcapsules with thick, rough capsule
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shells, which are considered to be better for their superior adhesive properties. Good adhesive properties are beneficial for the successful incorporation of the microcapsules in the polymer
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matrix, as they increase the mechanical properties in some cases as well as providing superior self-healing characteristics. Additionally, a greater amount of core material inhibits good dispersion in aqueous medium, causing a high amount of agglomeration and deals a severe impact to the yield percentage.
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Figure 10. Surface morphology depending on various core/shell material ratio. 3.8. Measurement of core materials post-preparation
After the completion of the microcapsule preparation and subsequent extensive drying, the amount of core materials encapsulated was calculated using the procedure described in the
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experimental section. We subjected various batches of microcapsules prepared under different conditions and process parameters to this evaluation. Table 1 shows the calculated results of these evaluations. From these experimental results involving different batches of microcapsule
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samples, it is concluded that increasing the UF shell material (keeping other process parameters constant) may adversely affect the growth of the microcapsules as well as the efficient
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encapsulation of core materials. Generally, the presence of a larger amount of urea and formaldehyde decreases the rate of the polymerization reaction between them, encumbering the whole process by not encapsulating the core material totally. As a result, microcapsules with larger amounts of shell materials have been found to carry a lesser amount of core materials with a diminished yield%. The lower amount of shell material is preferred for the preparation of these microcapsules, as it drives the rate of the condensation of the UF polymer along with promoting 19
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greater encapsulation of larger amounts of the TTG-epoxy core. Although this action has a negative impact on the UF network density, it does help to increase the deposition of shell
shell wall with better physical and adhesion properties.
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material on the core material-water interface. This facilitates the formation of thick and rough
Table 1. Core material measurement depending on different stirring rate, U-F ratio and their
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associated yield percentage rate. U-F ratio
Core material (%)
Yield percentage (%)
300
0.6
70
80
350
0.5
400
0.3
450
0.3
500
0.3
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Stirring rate (rpm)
82
90
92
80
87
65
70
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75
3.9. Thermal analysis of the microcapsules
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Figure 11 shows the thermal analysis involving TGA (Thermogravimetric Analysis) tests of the microcapsules, where the only variation was in TTG wt%, keeping all other process
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parameters constant. The microcapsules with the lowest amount of TTG in the core (1 wt% TTG) have a comparatively higher percentage of water in the shell. This specific batch of microcapsules lost those water molecules along with total weight loss of over 70% at approximately 100°C, followed by a major degradation of shell material at approximately 250°C when it lost over 60% of the remaining weight [26]. The total weight loss continued up to 390°C, when the microcapsules lost 95% of the initial weight. However, as the TTG wt% amount 20
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increases steadily in the subsequent batches of samples, the microcapsules tend to become more stable towards increased temperature. With 4 wt% and 7 wt% of TTG, the urea-formaldehyde
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microcapsules show thermal stability to approximately 300°C and 420°C, respectively. For the 4 wt% TTG encapsulated microcapsules, a mild weight loss happens at approximately 100°C due to evaporation of the water molecules, which is followed by another major weight loss of
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approximately 48% of the remaining weight at approximately 250°C. The final weight loss starts at approximately 390°C, and the final weight loss stands at 80% of the initial weight. For the
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next batch of microcapsules with 7 wt% TTG, the thermal stability further increases as the microcapsules barely lose any weight at approximately 100°C, implying that the amount of water molecules in the shell is negligible. The first major weight loss starts at approximately 300°C when the microcapsules lose approximately 25% of their initial weight. The next major weight loss begins at approximately 420°C, which constitutes a further weight loss of more than 70% of
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the remaining weight. At the end of the thermal analysis, the microcapsules with 7 wt% TTG lost
Figure 11. TGA analysis of the microcapsules.
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approximately 84% of their total initial weight. Although this batch (7 wt% of TTG) of microcapsules lost a greater amount of total weight than the previous one (4 wt% of TTG), they
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could withstand higher external heat (over 400°C) before starting the weight loss. The dearth of water molecules in the shell structure of the microcapsules with a higher TTG amount in the core
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materials is noticeable in the result.
Figure 12. Surface morphology of microcapsules under 20°C and 250°C temperature.
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Figure 12 shows the morphology of the three different batches of samples based on TTG wt%
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at 20°C and 250°C. Relatively high shell stability for microcapsules is observed, with 7 wt% TTG in the core, compared to the other two batches, as their shells remain largely intact at approximately 250°C. Whereas we see widespread decomposition of shell wall at high temperature in samples containing 1 wt% and 4 wt% TTG. The high thermal stability can be explained based on the hydrogen bond formation between the oxygen-containing functional groups on the TTG surface and the urea-formaldehyde shell. The presence of oxygen-containing 22
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functional groups on the TTG surface due to incomplete reduction has already been proven in the FTIR results in Figure 3, which is consistent with reported findings [27].
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On the other hand, GO with many more oxygen-containing functional groups on its surface than TTG could not increase the thermal stability of GO-microcapsules beyond 300°C as reported in an earlier study [15]. To analyze this apparent anomaly we carried out TGA analysis
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and electron diffraction studies of both GO and TTG to find out their difference in structure which might be the answer behind this unusual thermal stability. The TGA analysis of GO and
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TTG revealed a vast difference in thermal stability between the two sample as Figure 13 depicts. In case of GO, it lost over 15% of initial weight at around 100°C as a result of vaporization of water molecules trapped between the carbon layers. Note the loss of TGA temperature control due to the rapid evolution of water. Next phase of weight loss saw GO losing significant weight at 150°C-160°C due to evaporation of active oxygen-containing functional groups. The total
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weight loss stood at around over 75% of the initial weight. The GO sample essentially suffered partial burn and some of it turned into ash. But the TTG sample showed incredible thermal
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stability. It lost only around 5% of initial weight at 100°C and its total weight loss at the end of the experiment stood at around 15% of the initial weight. This significant difference in thermal
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stability showed a stark dissimilarity between the structures of the samples. This difference in structure was further proved in the study of selected area electron diffraction (SAED) as reveals Figure 13. The SAED image shows a diffuse ring pattern for the GO which is the evidence of GO having a disrupted structure as a result of the presence of huge amount of oxygen-containing functional groups. These functional groups rendered carbon surface in GO out of plane and delicate as a result of which GO is thermally less stable. This is also the reason behind the 23
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apparent lower thermal stability of GO encapsulated microcapsules [28]. The TTG has shown six-fold symmetry points in SAED analysis which happens as a result of hexagonal configuration
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on the carbon layer. This demonstrates crystallinity on the structure due to restoration of mostly sp2 hybridized carbon surface as a result of GO reduction [18,29]. The TTG surface is thermally more stable as a result of low amount of oxygen-containing functional group presence and high
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degree of structural integrity which is evident from the extraordinary level of thermal stability of TTG encapsulated microcapsules. In future, this unusual thermal stability can be conveniently
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employed to predict and manipulate the rate of the release of core materials from the microcapsules under various external conditions, including their application in high-performance
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polymers.
Figure 13. TGA and SAED analysis of GO and TTG. Note the loss of TGA temperature control due to the rapid evolution of water. 24
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3.10. Storage stability analysis Figure 14 shows that the microcapsules also exhibit increased storage stability with increasing
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TTG wt% at room temperature over an extended period of time. All three batches of samples with different encapsulated wt% of TTG values were able to hold over 90% of their initial weight after 60 days at room temperature, which proves the long-term reliability of their capsule
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shell material. The reinforcing effect of TTG through hydrogen bonding is believed to be the
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most probable reason of this extra stability in the shell structure.
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Figure 14. Storage stability analysis of the microcapsules. 3.11. Yield percentage of the microcapsules
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Figure 15 shows that the final yield of the microcapsules decreases considerably when the prepared microcapsules have a lower amount of shell material (UF) compared to core material (TTG-epoxy resin) with all other process parameters remaining constant. In this case, the weaker shell structure cannot provide a sufficient barrier to hold off the core material pressure, as mentioned previously. We also analyzed the relationships between the yield percentage and stirring rates during the preparation process. It is found that the yield percentage of the 25
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microcapsules increased when changing from a 300-rpm stirring rate to a 450-rpm stirring rate before decreasing with a higher stirring rate during preparation. The explanation lies in the fact
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that increasing the stirring rate to 450 rpm lowers the degree of agglomeration of the microcapsules and reduces the percentages of deformed ones. These factors drive up the yield percentage. However, too much agitation (>450 rpm) causes a high level of shear stress, as we
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discussed earlier, which is responsible for premature damage of microcapsules and thus lower
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yield percentage values.
Figure 15. Yield percentage result vs core/shell material ratio and stirring rate when all other
4. Conclusion
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process parameters are constant.
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In this work, UF microcapsules were prepared by an in-situ polymerization procedure. We have encapsulated graphene (TTG) for the first time inside UF microcapsules in conjugation with epoxy resin and studied their effect on the overall thermal stability of the microcapsules. The core material of the microcapsules did not suffer any negative impact during the aqueous phase synthesis, and graphene was successfully encapsulated. The various surface morphologies involving rough and smooth exteriors were studied, and the reasons for those different 26
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morphologies have been established based on different sets of process parameters during the encapsulation procedure. The increased thermal stability of the polymer shell wall makes the
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degradation behavior of the microcapsules more predictable. This unique character makes the microcapsules suitable to withstand the high curing temperature generally employed in the production of various polymers, as is revealed by the thermal analysis. All the process
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parameters were studied to optimize the process, with control over the yield, and are correlated with each other to better understand the effect graphene has on the thermal, chemical and
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physical properties of the microcapsules. It is revealed that nearly all aspects of the microcapsules’ character can be effectively controlled by adjusting the process parameters, as per requirements. These microcapsules have been proven to be appropriate to be used in highperformance polymers for the delivery of self-healing agents where exceptionally high capsule
Acknowledgements
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stability is required.
The work was generously supported by the Space Core Technology Program through the
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National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2013M1A3A3A02042257). The work was also funded by Basic Science
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Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry
of
Education,
Science
and
Technology
(Grant
Number:
NRF-
2016R1D1A1B03932101).
27
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Figure Captions
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Figure 1. XRD results of GO and TTG where XRD of GO was reproduced with permission from [15] (© 2016 Wiley).
permission from [15] (© 2016 Wiley).
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Figure 2. FTIR spectral analysis of GO and TTG where FTIR of GO was reproduced with
Figure 3. Chemical reaction involving shell wall formation. Figure 4. pH change during microencapsulation process.
Figure 5. Shell morphology of microcapsules prepared under different pH value.
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Figure 6. FTIR spectral analysis of the microcapsules. Figure 7. SEM images of the microcapsule surface morphology.
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Figure 8. Size analysis vs stirring rate.
Figure 9. Mean diameter vs core/shell material ratio.
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Figure 10. Surface morphology depending on various core/shell material ratio. Figure 11. TGA analysis of the microcapsules. Figure 12. Surface morphology of microcapsules under 20°C and 250°C temperature. Figure 13. TGA and SAED analysis of GO and TTG. Note the loss of TGA temperature control due to the rapid evolution of water. Figure 14. Storage stability analysis of the microcapsules. 32
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Figure 15. Yield percentage result vs core/shell material ratio and stirring rate when all other
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process parameters are constant.
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S0141-3910(17)30049-6
DOI:
10.1016/j.polymdegradstab.2017.02.012
Reference:
PDST 8172
To appear in:
Polymer Degradation and Stability
Received Date: 16 October 2016 Revised Date:
24 February 2017
Accepted Date: 26 February 2017
Please cite this article as: Sarkar S, Kim B, Analysis of graphene-encapsulated polymer microcapsules with superior thermal and storage stability behavior, Polymer Degradation and Stability (2017), doi: 10.1016/j.polymdegradstab.2017.02.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Analysis of Graphene-Encapsulated Polymer Microcapsules with Superior Thermal and Storage Stability Behavior
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Sourav Sarkar, Byungki Kim* School of Mechatronics Engineering, Korea University of Technology and Education
of Korea Sourav Sarkar: [email protected]
Phone no: +82-41-560-1143
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Fax no: +81-41-560-1253
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*Byungki Kim: [email protected]
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1600 Chungjeol-ro, Byeongcheon-myeon, Dongnam-gu, Cheonan, Chungnam, 31253 Republic
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Abstract A new series of urea-formaldehyde polymer microcapsules with two-component cores have
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been synthesized by an in-situ polymerization procedure. Graphene was used as one of the components in the core material along with a bisphenol-A epoxy resin. Preparation of graphene oxide was carried out by the Staudenmaier oxidation process, followed by further thermal
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treatment in a controlled environment to produce thermally treated graphene (TTG). The thermal and physical properties of the microcapsules were investigated by TGA (Thermogravimetric
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analysis) and, SEM (Scanning electron microscopy) among other methods. The chemical composition of the microcapsules was ascertained by FTIR (Fourier-transform infrared) spectroscopy. The resultant graphene-encapsulated microcapsules have shown an increasing degree of thermal stability in the range from 250°C to over 400°C, depending on the TTG wt%. They also exhibited good storage stability (> 90%) over different periods of time. It was
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determined that the thermal and physical properties of the microcapsules are closely linked to the core materials and the processing conditions. The processing conditions, in turn, can be varied to
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synthesize different types of microcapsules with different sizes, thermal stability and yield percentage values. The improved sway over the thermal degradation provided better control over
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the release rate of the core material under various conditions. Also, the unusually high thermal stability behavior of the polymer microcapsules’ shell wall demonstrates better survivability of these microcapsules under harsh external conditions which can be exploited in their use as highly stable self-healing agent carrier in high-performance polymer materials. Keywords: Polymer microcapsules, graphene, thermal stability, storage stability 1. Introduction 2
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Polymer microcapsules and microcapsule-based self-healing constitute the most prominent research branch in the autonomous self-healing approach in high-performance polymers.
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Microcapsule-based healing has been applied a number of times in different polymers, in which the microcapsules primarily work as containers to safely carry the self-healing agents inside the polymer matrix [1-5]. Generally, microcapsules are small particles with an active core
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surrounded by a shell material. This active core can be a variety of chemicals based on the end application, such as drugs, enzymes, dyes, water, or any type of self-healing agents. The
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mechanism of self-healing is dependent on the progression of micro cracks and their interaction with the healing agent containing microcapsules embedded in the polymer matrix. The microcapsules release the self-healing agents after getting ruptured by the sharp edges of the micro cracks during crack propagation in the polymer matrix. The subsequent polymerization of the healing agents triggered by interaction with either an embedded catalyst or curing agents
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from the polymer matrix helps to close the micro crack, thus repairing the polymer matrix internally and preventing further damage. This self-healing technique has the potential to
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significantly increase the life of polymer materials with minimal cost and human involvement. Survivability under harsh external environments along with long shelf life are some of the
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primary and essential characteristics needed before employing a certain type of microcapsules as self-healing delivery agents in a specific polymer matrix. Generally, an elevated curing temperature (100°C-175°C) is used during the curing process for high-performance polymers, which has proven detrimental to the survivability of most polymer microcapsules. Thus, increasing the thermal stability of polymer microcapsules that can survive under high curing temperatures is the most logical way forward. Apart from high thermal stability, microcapsules 3
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must also show adhesion with the polymer matrix to facilitate the release of healing agents after composite rupture.
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As high-performance polymer materials, various types of epoxy resins have been developed for a wide range of applications. These include, but are not limited to, uses as adhesives, coatings, and composite materials due to their impressive chemical-resistant properties as well as
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mechanical attributes such as high strength and modulus [6-9]. There have been many instances where epoxy resins have been used as a standard core material, i.e., self-healing agents in
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microcapsules, which can polymerize upon release [10]. The basic purpose of this work was to synthesize a special class of microcapsules that can withstand the harsh fabrication procedure of high-performance polymers. Kang et al demonstrated in 2015 that the overall thermal stability of polymer microcapsules can be increased to as high as 180°C by a protective PDA (polydopamine) coating [11]. We believe the thermal stability threshold can further be stretched
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well beyond this point. Toward this goal, we selected thermally treated graphene (TTG) as a nano-filler to be used along with epoxy resin to increase the thermal stability threshold of
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microcapsules. It has already been reported that core materials can have a very crucial role in shaping the thermal behavior of microcapsules [12]. Graphene is well known for its excellent
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thermal properties resulting from its unique sp2 bonded honeycomb surface and has been observed to increase the thermal stability of epoxy resin-based composite materials [13,14] We have applied an in-situ polymerization procedure to synthesize microcapsules with ureaformaldehyde (UF) polymer as a shell along with TTG attached to epoxy resin as active core materials. TTG was prepared by a thermal reduction technique with graphene oxide (GO) prepared by the Staudenmaier oxidation procedure from raw graphite. The ‘as prepared’ 4
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graphene-encapsulated microcapsules have shown incredible thermal stability to over 400°C in TGA. The chemical functional groups present in the microcapsules were evaluated using FTIR
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spectroscopy. It was deduced, after analyzing the experimental data, that the thermal, chemical and physical properties of microcapsules are closely dependent on various process parameters. All the relevant data concerning the close relationship between the process parameters and the
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prepared microcapsules have been established and presented in this article along with the most probable explanations. In this work, we were able to demonstrate for the first time that thermally
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treated graphene can be encapsulated inside the UF microcapsules under certain conditions, and the properties of those microcapsules can be controlled according to various requirements. In one of our previously reported study, GO had been encapsulated inside the UF microcapsules, rendering them more thermally stable than other types of microcapsules [15]. This work has utilized the superior thermal stability of graphene by adding it to the core material when
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synthesizing UF microcapsules with much higher thermal stability. TTG was one of the key components of the prepared microcapsules, contrary to previous works in which graphene had
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been added to the shell after microcapsules preparation by layer by layer assembly [16,17] The unusual high thermal stability of the microcapsules makes them ideal for application in high-
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performance polymers.
2. Materials and Methods
Raw graphite (Code No. 230U, Size - 44 µm) was supplied by Asbury Carbons. Nitric acid (HNO3, Reagent grade, fuming >90%) and potassium chlorate (KClO3, 99+%, ACS Reagent) and sulfuric acid (H2SO4, ACS, 95%-99%) were purchased from Sigma Aldrich for graphene oxide (GO) preparation by the Staudenmaier oxidation procedure. EPON 828 was used as the 5
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standard epoxy resin and was purchased from Alfa Aesar. Acetone was bought from Sigma Aldrich and used as the solvent for attaching of the TTG with epoxy resin. For microcapsule
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preparation, urea (ACS, 99.0-100.5%) and ethylene maleic anhydride (EMA) were purchased from Sigma Aldrich. Resorcinol (ACS, 99.0-100.5% crystalline), ammonium chloride (NH4Cl, 98%+) and formaldehyde (HCHO, 37% w/w aq. Solution, stabilized with 7-8% methanol) were
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bought from Alfa Aesar. Sodium hydroxide (NaOH, 50% w/w aq. Solution), hydrochloric acid (HCl, ACS, 36.5-38% liquid) were bought from Alfa Aesar and were used to control the pH of
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the reaction. 2.1. Preparation of thermally treated graphene (TTG)
We used the Staudenmaier oxidation procedure to prepare GO from raw graphite in the presence of acid mixtures of HNO3 (nitric acid), H2SO4 (sulfuric acid) (mixing ratio 1:2) and KClO3 (potassium chlorate). The reaction continued under magnetic stirring for 96 hours at a
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stirring rate of 900 rpm before being stopped. Graphite oxide was formed when the reaction mixture changed color from black to green. The graphite oxide was then ultrasonicated to obtain
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few-layer graphene oxide (GO). GO was washed first with 5% HCl, followed by multiple distilled water washes until the pH value approached neutrality. The heavily oxygenated GO was
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then thermally treated using a simple hotplate procedure to obtain thermally treated graphene (TTG) [18].
2.2. Attachment of TTG with epoxy resin A measured amount of TTG was dispersed in acetone by an ultrasonicator for 30 minutes. The dispersed TTG in acetone was then added to epoxy resin (EPON 828) in a shear mixer for 15 minutes under 2000 rpm. Acetone was chosen because of its low boiling point and high 6
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volatility. Additionally, the presence of the >C=O group in its structure made it ideal to form a stable dispersion of TTG by stable bonding interactions with the active groups of TTG.
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2.3. Emulsification process Ethylene maleic anhydride (EMA) was chosen to be applied as an emulsifier in the encapsulation process. 2.5 wt % of EMA solution was prepared by adding it to deionized water.
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The solution was stirred with a mechanical stirrer for 20 hours at 30°C so that the emulsifier fully dissolved.
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2.4. In-situ polymerization process
The emulsifier solution was then added to distilled water and stored in a glass beaker. Urea, resorcinol and ammonium chloride were added to the aqueous medium under stirring. For detailed analysis, different batches of samples involving different amounts of core and shell materials accompanied by a varied degree of mechanical stirring rates were prepared. In the
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preparation process, the shell material of the resultant microcapsules was composed of urea and formaldehyde. Experimental data described in the discussion section also revealed the presence
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of water molecules in the capsule shell walls. Resorcinol was used as a crosslinking agent, while ammonium chloride reacted with the formaldehyde to facilitate the shell forming process of the
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microcapsules [19]. The pH of the solution was maintained at 3.5 by the addition of adequate amounts of NaOH (sodium hydroxide) and HCl (hydrochloric acid). The overall synthetic process of the in-situ polymerization procedure followed the pathway reported in our earlier work involving the encapsulation of GO [15]. After 4 hours of constant stirring, the prepared microcapsules in the aqueous medium were allowed to cool to room temperature for a few hours.
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After cooling, the microcapsules were collected by filtration, washed with distilled water and left to dry for 48 hours at room temperature.
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2.5. Characterization Graphene oxide (GO) and thermally treated graphene (TTG) were analyzed by FTIR spectroscopy on a Spectrum 100S instrument (PerkinElmer) to identify the functional groups
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composition on its surface. XRD of GO and TTG were performed on an EMPYREAN instrument (Panalytical Ltd.) with an angle ranging between 5° and 30° to observe the crystal
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structure. FTIR was again employed for chemical characterization of the microcapsules. Scanning electron microscopy (SEM, JSM-6010LA, JEOL) was used to evaluate the overall surface morphology and the degree of roughness of the shell of the prepared microcapsules. Thermal analysis of the microcapsules was carried out using thermogravimetric analysis on a TGA Q500 (TA Instruments) at a rate of 10°C/min. Electron diffraction studies of GO and TTG
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were carried out by a high resolution transmission electron microscope (HRTEM, Tecnai TF30 ST) which operated at 300kv.
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A high degree of storage stability is one of the most essential pre-requisites for practical applications of the microcapsules. The longer the microcapsules can hold the core content, the
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greater the stability. A stronger shell material is better for enhanced storage stability over a certain period of time. This was studied by the amount of weight loss suffered by the microcapsules in standard laboratory conditions over various durations of time. 2.6. Microcapsules’ core content calculation We employed an extraction procedure using acetone as the standard solvent to measure the core content of the prepared TTG-Epoxy resin encapsulated microcapsules. The calculated 8
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amount of microcapsules (Mi) were crushed in a mortar. The extraction solvent, acetone, was added to a mixture of both shell and core materials of crushed microcapsules. The mixture in
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acetone was filtered using filter paper; the filtrate portion contained the core material and the residue in the filter paper was the shell material. The shell material was then dried and weighed (S0). The final weight percentage of the shell material (S) was calculated, and the weight
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percentage of the core material (C) was found using that value. S = (S0 / Mi) 100% C = 100% – S 3. Results and discussions 3.1. Structural analysis of TTG
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Eqn (1)
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Structural analysis was carried out by XRD (X-ray diffraction) and is represented in Figure 1. In the XRD analysis, we see major peaks of GO and TTG at 11° and 24°, respectively, which reveal a decrease in the interlayer spacing in TTG compared to GO. According to a calculation
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using Bragg’s law, the interlayer distances between the carbon layers in GO and TTG are approximately 7Å and 3.6Å, respectively. Removal of most of the oxygen-containing active
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functional groups between the carbon layers devised a new 3D structure closely packed with the van der Waals force of attraction. Partial removal of oxygen-containing groups on the surface of GO is the reason behind this decreased interlayer spacing which also indicates the partial restoration of a sp2 hybridized hexagonal bonding network similar that of raw graphite on the carbon layer [20].
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Figure 1. XRD results of GO and TTG where XRD of GO was reproduced with permission from [15] (© 2016 Wiley) 3.2. Chemical analysis of TTG
Chemical analysis of GO and TTG involved FTIR (Fourier-transform infrared) studies, as
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shown in Figure 2. A detailed analysis shows the presence of oxygen-containing active functional groups on both the GO and TTG surfaces. While the presence of strong peaks at 914 cm-1 and 3319 cm-1 signifies the bending and stretching vibrations of hydroxyl (O-H) groups in
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GO, TTG shows no lower frequency vibrations for the -OH. Epoxide (C-O) and carbonyl (C=O) groups are detected by peaks at 1162 cm-1 and 1708 cm-1. The presence of C=C on the GO and
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TTG surface is identified by distinct peaks at 1640 cm-1 and 1614 cm-1. The presence of C-O on the TTG surface indicates the partial reduction of GO under external heat. The presence of these active groups on the TTG surface makes the thermally treated graphene a chemically active species that has a profound influence on the physical and chemical properties of the prepared microcapsules, as further analysis revealed.
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Figure 2. FTIR spectral analysis of GO and TTG where FTIR of GO was reproduced with permission from [15] (© 2016 Wiley) 3.3. Microencapsulation
The microencapsulation process by an in-situ polymerization process involved four distinct
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stages. These stages were significantly influenced by temperature and pH during the reaction. The TTG encapsulated UF polymer microcapsules were formed distinctly in a later part of the reaction. During the first 60 minutes, the epoxy droplets combined with dispersed TTG were
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visible in the aqueous medium. This stage exhibited a steady increase in temperature along with a decrease in the pH value. Over the total course of the microencapsulation process, the pH value
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deteriorated before finally becoming stable in the last 30 minutes of the reaction. The first stage was typically followed by a dark greyish-white milky suspension in the next hour of reaction which was the result of the newly formed urea-formaldehyde prepolymer. The dark greyish shade was due to the presence of TTG in the reaction medium. The microcapsules tentatively started to take shape around the 120-180 minute time period, and the pH appeared to be unchanging for a brief moment before embarking on another downward trend. The 11
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microcapsules finally formed during the last hour of the preparation procedure and the milky
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greyish-white suspension became transparent.
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Figure 3. Chemical reaction involving shell wall formation.
Figure 4. pH change during microencapsulation process.
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A coherent capsule shell structure started to form generally after 90 minutes of the start of the in-situ polymerization process. The prepolymer started to gain in molecular weight significantly as the reaction proceeded. The chemical reaction of the urea-formaldehyde polymer shell wall formation is shown in Figure 3. During the initial shell forming process, urea and formaldehyde react together in an aqueous medium to synthesize the mono-methylol urea derivative. The dimethylol urea derivative forms when the mono-methylol urea derivative reacts with free 12
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formaldehyde. These urea derivatives react together to ultimately produce the urea-formaldehyde (UF) polymer, thereby making the main building block of the capsule shell wall. The increase in
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molecular weight for the UF was followed by its deposition on the core material-water interface, thus starting the shell forming process. The deposition of the UF prepolymer on the TTG-epoxy resin-water interface was facilitated by the low stirring rate, which is discussed in detail later.
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Changes in pH during the preparation process have a profound effect on the microcapsule development and surface morphology. Because it is an already well-known fact that either an
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acid or base can catalyze the addition phase of the polymerization process, the change in pH can accelerate or decelerate the reaction. The changes in pH during the reaction are shown in Figure 4. During the full course of the reaction, the pH decreased steadily before reaching a stable state at the end of the reaction. The considerable effect of pH on the surface morphology is also a widely reported phenomena [21]. In this experiment, the continuous downward trend in pH
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helped to increase the viscosity. High viscosity generally promotes greater deposition of shell material, making the microcapsule shell wall thicker and rougher. This swift increase in viscosity
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at the core material-water interface had a huge influence over the UF suspension in shear flow. Microcapsules prepared in higher pH value generally have smooth capsule shell walls, whereas
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thick and rough shell exteriors are noticed after preparation in lower pH values, as shown in Figure 5.
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3.4. Chemical analysis of microcapsules
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Figure 5. Shell morphology of microcapsules prepared under different pH value.
Figure 6 shows the FTIR spectra identifying all the active functional groups present in the microcapsules, both as part of the capsule shell wall and core materials. The presence of the epoxide group in the FTIR results for microcapsules approximately 910 cm-1 proves that the
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microcapsules are filled with epoxy resins. Apart from that, the existence of C-N, C=O were confirmed by their corresponding peaks at 1290 cm-1and 1716 cm-1. The broad peak at 3334 cm-1 may be due to the superposition of N-H and –OH groups [22]. Additionally, the presence of N-H
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was confirmed by peaks at 1645 cm-1 due to the presence of a primary amine (urea) and 831 cm-1 due to out of plane vibration. The presence of aromatic C=C was confirmed by a stretching
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vibration peak at around 1564 cm-1 which is most likely from phenyl group structure part of the TTG-epoxy resin core material of the microcapsules [23,24]. The alkene stretching vibration peak for C=C is detected at 1612 cm-1 (in-plane vibration) possibly as a result of mostly restored sp2 hybridized surface of TTG [25].
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3.5. SEM analysis of the microcapsules
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Figure 6. FTIR spectral analysis of the microcapsules.
Figure 7 shows the surface morphology of the TTG encapsulated microcapsules under SEM (Scanning Electron Microscopy) analysis. It confirms that the growth of the microcapsules with
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active core did not suffer any negative effect due to the presence of TTG. It reveals the rough exteriors of the larger ones due to the protruding nanoparticles of urea-formaldehyde, which enhanced and increased the surface adhesion. These larger microcapsules have higher surface
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area values as well. Typically, these microcapsules demonstrate smooth inner surfaces of the shell compared to the outer one. This is facilitated by a low stirring rate and a lower core/shell
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material ratio. We also see smaller microcapsules with smooth and uniform surfaces as a result of high stirring rates. The smaller microcapsules with uniform shapes exhibit a thin outer wall as a result of a lower rate of deposition by the urea-formaldehyde polymer at the water-core material interface.
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Figure 7. SEM images of the microcapsule surface morphology. 3.6. Mean diameter vs stirring rate
The mean diameter of the microcapsules is found to be inversely proportional to the stirring
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rate of the mechanical stirrer when all other process parameters are constant, which is shown in Figure 8. Every preparation yielded both larger and smaller diameter microcapsules. However, only the dominant sizes under specific stirring rates have been shown in the plot. During the
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microencapsulation process, smaller microcapsules are found near the magnetic stirrer, whereas larger diameter microcapsules exist in the region away from the stirrer. These conclusions are
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drawn from well-documented studies [1]. Generally, we considered a 20 µm diameter as the upper limit for smaller microcapsules and the lower limit for larger ones. The increase in the stirring rate causes an elevation of shear stress, which results in the decrease in diameter of the microcapsules; hence, smaller microcapsules predominate. A high stirring rate creates many sites of nucleation, which terminates larger microcapsules to smaller ones and provides uniform growth. On the other hand, the lower stirring rate helps synthesize larger microcapsules. 16
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Additionally, it is considered that a high stirring rate helps to contain the agglomeration of the UF polymer to an acceptable level while simultaneously facilitating the slow-moving but closely
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packed deposition of UF polymer at the core material interface.
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Figure 8. Size analysis vs stirring rate.
Figure 9. Mean diameter vs core/shell material ratio.
3.7. Mean diameter vs core/shell material Figure 9 shows the relationship between the core/shell material ratio and the average diameter of the microcapsules. The higher core/shell material ratio contributes to weaker shell growth and 17
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lower stability. A greater amount of TTG-epoxy resin core material in comparison to the UF shell material initially helps to expedite the process for smooth and thin capsule wall generation.
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However, the diameter size of the UF microcapsules drops drastically with a further increase in the core material. Theoretically, the increase in core material should have formed larger droplets of the core material during the microencapsulation procedure, paving the way for larger
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microcapsules. However, in reality, it was found to be counter-productive, as the lower amount of shell material could not withstand the outward pressure from the core. This makes the
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microcapsules with higher ratios of core materials unstable and prone to early fracture, even before the completion of the synthesis, proving that this was a less viable pathway for efficient synthesis. In the end, a higher core material ratio results in the predominance of smaller, smooth and uniform UF microcapsules. It is shown in Figure 10 that increasing the core materials in comparison to the shell resulted in smaller UF microcapsules with very smooth and thin outer
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shells as a result of the reduced deposition of shell materials. On the contrary, the lower core/shell ratio facilitated greater percentage of larger microcapsules with thick, rough capsule
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shells, which are considered to be better for their superior adhesive properties. Good adhesive properties are beneficial for the successful incorporation of the microcapsules in the polymer
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matrix, as they increase the mechanical properties in some cases as well as providing superior self-healing characteristics. Additionally, a greater amount of core material inhibits good dispersion in aqueous medium, causing a high amount of agglomeration and deals a severe impact to the yield percentage.
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Figure 10. Surface morphology depending on various core/shell material ratio. 3.8. Measurement of core materials post-preparation
After the completion of the microcapsule preparation and subsequent extensive drying, the amount of core materials encapsulated was calculated using the procedure described in the
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experimental section. We subjected various batches of microcapsules prepared under different conditions and process parameters to this evaluation. Table 1 shows the calculated results of these evaluations. From these experimental results involving different batches of microcapsule
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samples, it is concluded that increasing the UF shell material (keeping other process parameters constant) may adversely affect the growth of the microcapsules as well as the efficient
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encapsulation of core materials. Generally, the presence of a larger amount of urea and formaldehyde decreases the rate of the polymerization reaction between them, encumbering the whole process by not encapsulating the core material totally. As a result, microcapsules with larger amounts of shell materials have been found to carry a lesser amount of core materials with a diminished yield%. The lower amount of shell material is preferred for the preparation of these microcapsules, as it drives the rate of the condensation of the UF polymer along with promoting 19
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greater encapsulation of larger amounts of the TTG-epoxy core. Although this action has a negative impact on the UF network density, it does help to increase the deposition of shell
shell wall with better physical and adhesion properties.
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material on the core material-water interface. This facilitates the formation of thick and rough
Table 1. Core material measurement depending on different stirring rate, U-F ratio and their
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associated yield percentage rate. U-F ratio
Core material (%)
Yield percentage (%)
300
0.6
70
80
350
0.5
400
0.3
450
0.3
500
0.3
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Stirring rate (rpm)
82
90
92
80
87
65
70
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75
3.9. Thermal analysis of the microcapsules
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Figure 11 shows the thermal analysis involving TGA (Thermogravimetric Analysis) tests of the microcapsules, where the only variation was in TTG wt%, keeping all other process
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parameters constant. The microcapsules with the lowest amount of TTG in the core (1 wt% TTG) have a comparatively higher percentage of water in the shell. This specific batch of microcapsules lost those water molecules along with total weight loss of over 70% at approximately 100°C, followed by a major degradation of shell material at approximately 250°C when it lost over 60% of the remaining weight [26]. The total weight loss continued up to 390°C, when the microcapsules lost 95% of the initial weight. However, as the TTG wt% amount 20
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increases steadily in the subsequent batches of samples, the microcapsules tend to become more stable towards increased temperature. With 4 wt% and 7 wt% of TTG, the urea-formaldehyde
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microcapsules show thermal stability to approximately 300°C and 420°C, respectively. For the 4 wt% TTG encapsulated microcapsules, a mild weight loss happens at approximately 100°C due to evaporation of the water molecules, which is followed by another major weight loss of
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approximately 48% of the remaining weight at approximately 250°C. The final weight loss starts at approximately 390°C, and the final weight loss stands at 80% of the initial weight. For the
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next batch of microcapsules with 7 wt% TTG, the thermal stability further increases as the microcapsules barely lose any weight at approximately 100°C, implying that the amount of water molecules in the shell is negligible. The first major weight loss starts at approximately 300°C when the microcapsules lose approximately 25% of their initial weight. The next major weight loss begins at approximately 420°C, which constitutes a further weight loss of more than 70% of
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the remaining weight. At the end of the thermal analysis, the microcapsules with 7 wt% TTG lost
Figure 11. TGA analysis of the microcapsules.
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approximately 84% of their total initial weight. Although this batch (7 wt% of TTG) of microcapsules lost a greater amount of total weight than the previous one (4 wt% of TTG), they
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could withstand higher external heat (over 400°C) before starting the weight loss. The dearth of water molecules in the shell structure of the microcapsules with a higher TTG amount in the core
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materials is noticeable in the result.
Figure 12. Surface morphology of microcapsules under 20°C and 250°C temperature.
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Figure 12 shows the morphology of the three different batches of samples based on TTG wt%
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at 20°C and 250°C. Relatively high shell stability for microcapsules is observed, with 7 wt% TTG in the core, compared to the other two batches, as their shells remain largely intact at approximately 250°C. Whereas we see widespread decomposition of shell wall at high temperature in samples containing 1 wt% and 4 wt% TTG. The high thermal stability can be explained based on the hydrogen bond formation between the oxygen-containing functional groups on the TTG surface and the urea-formaldehyde shell. The presence of oxygen-containing 22
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functional groups on the TTG surface due to incomplete reduction has already been proven in the FTIR results in Figure 3, which is consistent with reported findings [27].
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On the other hand, GO with many more oxygen-containing functional groups on its surface than TTG could not increase the thermal stability of GO-microcapsules beyond 300°C as reported in an earlier study [15]. To analyze this apparent anomaly we carried out TGA analysis
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and electron diffraction studies of both GO and TTG to find out their difference in structure which might be the answer behind this unusual thermal stability. The TGA analysis of GO and
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TTG revealed a vast difference in thermal stability between the two sample as Figure 13 depicts. In case of GO, it lost over 15% of initial weight at around 100°C as a result of vaporization of water molecules trapped between the carbon layers. Note the loss of TGA temperature control due to the rapid evolution of water. Next phase of weight loss saw GO losing significant weight at 150°C-160°C due to evaporation of active oxygen-containing functional groups. The total
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weight loss stood at around over 75% of the initial weight. The GO sample essentially suffered partial burn and some of it turned into ash. But the TTG sample showed incredible thermal
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stability. It lost only around 5% of initial weight at 100°C and its total weight loss at the end of the experiment stood at around 15% of the initial weight. This significant difference in thermal
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stability showed a stark dissimilarity between the structures of the samples. This difference in structure was further proved in the study of selected area electron diffraction (SAED) as reveals Figure 13. The SAED image shows a diffuse ring pattern for the GO which is the evidence of GO having a disrupted structure as a result of the presence of huge amount of oxygen-containing functional groups. These functional groups rendered carbon surface in GO out of plane and delicate as a result of which GO is thermally less stable. This is also the reason behind the 23
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apparent lower thermal stability of GO encapsulated microcapsules [28]. The TTG has shown six-fold symmetry points in SAED analysis which happens as a result of hexagonal configuration
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on the carbon layer. This demonstrates crystallinity on the structure due to restoration of mostly sp2 hybridized carbon surface as a result of GO reduction [18,29]. The TTG surface is thermally more stable as a result of low amount of oxygen-containing functional group presence and high
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degree of structural integrity which is evident from the extraordinary level of thermal stability of TTG encapsulated microcapsules. In future, this unusual thermal stability can be conveniently
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employed to predict and manipulate the rate of the release of core materials from the microcapsules under various external conditions, including their application in high-performance
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polymers.
Figure 13. TGA and SAED analysis of GO and TTG. Note the loss of TGA temperature control due to the rapid evolution of water. 24
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3.10. Storage stability analysis Figure 14 shows that the microcapsules also exhibit increased storage stability with increasing
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TTG wt% at room temperature over an extended period of time. All three batches of samples with different encapsulated wt% of TTG values were able to hold over 90% of their initial weight after 60 days at room temperature, which proves the long-term reliability of their capsule
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shell material. The reinforcing effect of TTG through hydrogen bonding is believed to be the
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most probable reason of this extra stability in the shell structure.
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Figure 14. Storage stability analysis of the microcapsules. 3.11. Yield percentage of the microcapsules
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Figure 15 shows that the final yield of the microcapsules decreases considerably when the prepared microcapsules have a lower amount of shell material (UF) compared to core material (TTG-epoxy resin) with all other process parameters remaining constant. In this case, the weaker shell structure cannot provide a sufficient barrier to hold off the core material pressure, as mentioned previously. We also analyzed the relationships between the yield percentage and stirring rates during the preparation process. It is found that the yield percentage of the 25
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microcapsules increased when changing from a 300-rpm stirring rate to a 450-rpm stirring rate before decreasing with a higher stirring rate during preparation. The explanation lies in the fact
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that increasing the stirring rate to 450 rpm lowers the degree of agglomeration of the microcapsules and reduces the percentages of deformed ones. These factors drive up the yield percentage. However, too much agitation (>450 rpm) causes a high level of shear stress, as we
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discussed earlier, which is responsible for premature damage of microcapsules and thus lower
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yield percentage values.
Figure 15. Yield percentage result vs core/shell material ratio and stirring rate when all other
4. Conclusion
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process parameters are constant.
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In this work, UF microcapsules were prepared by an in-situ polymerization procedure. We have encapsulated graphene (TTG) for the first time inside UF microcapsules in conjugation with epoxy resin and studied their effect on the overall thermal stability of the microcapsules. The core material of the microcapsules did not suffer any negative impact during the aqueous phase synthesis, and graphene was successfully encapsulated. The various surface morphologies involving rough and smooth exteriors were studied, and the reasons for those different 26
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morphologies have been established based on different sets of process parameters during the encapsulation procedure. The increased thermal stability of the polymer shell wall makes the
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degradation behavior of the microcapsules more predictable. This unique character makes the microcapsules suitable to withstand the high curing temperature generally employed in the production of various polymers, as is revealed by the thermal analysis. All the process
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parameters were studied to optimize the process, with control over the yield, and are correlated with each other to better understand the effect graphene has on the thermal, chemical and
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physical properties of the microcapsules. It is revealed that nearly all aspects of the microcapsules’ character can be effectively controlled by adjusting the process parameters, as per requirements. These microcapsules have been proven to be appropriate to be used in highperformance polymers for the delivery of self-healing agents where exceptionally high capsule
Acknowledgements
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stability is required.
The work was generously supported by the Space Core Technology Program through the
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National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2013M1A3A3A02042257). The work was also funded by Basic Science
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Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry
of
Education,
Science
and
Technology
(Grant
Number:
NRF-
2016R1D1A1B03932101).
27
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Figure Captions
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Figure 1. XRD results of GO and TTG where XRD of GO was reproduced with permission from [15] (© 2016 Wiley).
permission from [15] (© 2016 Wiley).
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Figure 2. FTIR spectral analysis of GO and TTG where FTIR of GO was reproduced with
Figure 3. Chemical reaction involving shell wall formation. Figure 4. pH change during microencapsulation process.
Figure 5. Shell morphology of microcapsules prepared under different pH value.
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Figure 6. FTIR spectral analysis of the microcapsules. Figure 7. SEM images of the microcapsule surface morphology.
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Figure 8. Size analysis vs stirring rate.
Figure 9. Mean diameter vs core/shell material ratio.
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Figure 10. Surface morphology depending on various core/shell material ratio. Figure 11. TGA analysis of the microcapsules. Figure 12. Surface morphology of microcapsules under 20°C and 250°C temperature. Figure 13. TGA and SAED analysis of GO and TTG. Note the loss of TGA temperature control due to the rapid evolution of water. Figure 14. Storage stability analysis of the microcapsules. 32
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Figure 15. Yield percentage result vs core/shell material ratio and stirring rate when all other
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process parameters are constant.
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