Morphological study of polymethyl methacrylate microcapsules filled with self-healing agents

Accepted Manuscript Title: Morphological study of polymethyl methacrylate microcapsules filled with self-healing agents Author: Fatemeh Ahangaran Mehran Hayaty Amir H. Navarchian PII: DOI: Reference:

S0169-4332(16)32836-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.12.116 APSUSC 34663

To appear in:

APSUSC

Received date: Revised date: Accepted date:

23-10-2016 11-12-2016 14-12-2016

Please cite this article as: Fatemeh Ahangaran, Mehran Hayaty, Amir H.Navarchian, Morphological study of polymethyl methacrylate microcapsules filled with self-healing agents, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.12.116 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.

Morphological study of polymethyl methacrylate microcapsules filled with selfhealing agents Fatemeh Ahangarana, Mehran Hayatya‫٭‬, Amir H. Navarchianb a b

Department of Applied Chemistry, Malek-ashtar University of Technology, Isfahan, 83145/115, Iran

Department of Chemical Engineering, Faculty of Engineering, University of Isfahan, Isfahan, 81746-73441, Iran Corresponding author’s e-mail: [email protected]; Tel: +98-3145912529

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Graphical abstract

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Highlights   

Study the effect of core materials, molecular weight of PMMA, type and concentration of emulsifier on theoretical and experimental morphologies of PMMA capsules. Investigation of surface morphology of PMMA capsules. Prediction the theoretical morphology of PMMA particles in different emulsion systems.

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Abstract Polymethyl methacrylate (PMMA) microcapsules filled with epoxy prepolymer, 3-aminomethyl-3,5,5trimethylcyclohexylamine, and pentaerythritol tetrakis (3-mercaptopropionate) as healing agents have been prepared separately through internal phase separation method for self-healing purposes. PMMA with two different molecular weights ( ̅ 1 = 36000 g/mol and ̅ 2 = 550000 g/mol) were used with two types of different emulsifiers (ionic and polymeric) to prepare microcapsules. The morphology of healing agent microcapsules was investigated using field emission scanning electron microscopy (FESEM). It was found that PMMA microcapsules separately filled with epoxy and amine had core-shell morphologies with smooth surfaces. The mercaptan/PMMA particles exhibited core-shell and acorn-shape morphologies. The surface morphology of mercaptan microcapsules changed from holed to plain in different emulsion systems. The spreading coefficient (S) of phases in the prepared emulsion systems were calculated from interfacial tension (σ) and contact angle ( ) measurements. The theoretical equilibrium morphology of PMMA microcapsules was predicted according to spreading coefficient values of phases in emulsion systems. It was also found that the surface morphology of PMMA microcapsules depended strongly on the nature of the core, molecular weight of PMMA, type and concentration of emulsifier.

Key words: Healing agent, internal phase separation, emulsion, equilibrium morphology, thermodynamic.

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1. Introduction Self-healing materials has attracted increasing interest of the research community, over the last decades, due to their efficiency in “automatically” healing damage without the need for detection or any type of manual intervention [1,2]. Encapsulation of healing agents is one of the most popular approaches for preparing the self-healing systems that can be carried out by various methods including internal phase separation [3-6]. Micro/nanocapsules containing healing agents and with polymeric shells, that dispersed in epoxy matrix are very useful for engineering applications [7-10]. One of the appropriate polymers used recently as polymer shell in these capsules is polymethyl methacrylate (PMMA) [11]. In the encapsulation process through internal phase separation method, organic phase includes a mixture of polymer (PMMA), a volatile solvent for the polymer and core material (healing agent), and the aqueous phase contains emulsifier. The organic phase is first added to aqueous phase to prepare an oil-in-water emulsion. When the volatile solvent gradually evaporates from the droplets under reduction of pressure and elevation of temperature, the organic phase composition changes from one-phase droplets to two-phase droplets. By complete evaporation of solvent, the polymer-rich phase separates in the tiny droplets, and eventually migrate to the oil/water interface and coalesce to form polymeric shell for particles. This is caused by the interfacial tension interaction between core, polymer, and aqueous phases [12,13]. The solvent choice and its properties for the technique of microencapsulation have important effects on the microparticle morphology [14,15]. The suitable solvent should be able to dissolve the chosen polymer, has high immiscibility with water, high volatility, low boiling point and low toxicity [16]. Methylene chloride is the most common solvent for the encapsulation using solvent evaporation technique. Its high saturated vapor pressure compared to other solvents promises a high solvent evaporation rate, which shortens the duration of fabrication of microspheres [16]. Very few studies have been reported on encapsulation of healing agents in PMMA shell. Li et al. encapsulated epoxy prepolymer and polyetheramine as healing agents in PMMA microcapsules and investigated the effects of some processing parameters on the encapsulation process [17,18]. Ahangaran et al. encapsulated a low viscosity epoxy and a mercaptan in separate PMMA shells and studied the effects of mixing modes and emulsifying agents on encapsulation yield, capsule mean diameter and core content [11]. 5

The morphology of polymeric micro/nanocapsules is an increasingly important subject in selfhealing application [19]. The surface morphology and interfacial adhesion between the microcapsule shell and epoxy matrix greatly affects the mechanical properties and self-healing efficiency of these composites [9,11,20]. In particular, formation of small holes or craters on the surface of PMMA microcapsules is a challenging issue of encapsulation of materials with PMMA shell. The surface morphology is affected by various factors of encapsulation process including type of polymer, emulsifier, solvent/non-solvent, temperature, and mixing mode [2124]. In our previous study, two different surface morphologies were observed for epoxy and mercaptan capsules with PMMA shell. The first was the rough surface in presence of ultrasonication, and the other was smooth surface in absence of ultrasonication. The formation of small holes or craters on the PMMA shell of the large mercaptan capsules was also observed [11]. The morphology of PMMA microcapsules formed by internal phase separation method depends on the interfacial energies. Briefly, if two immiscible liquid drops contact with a third immiscible liquid in an emulsion system, the resulting equilibrium morphology can be predicted by analyzing the contact angles, interfacial tensions, and spreading coefficients between the three phases [25]. Spreading coefficient is a measurement of the ability of one liquid to spontaneously spread across another [26]. Torza and Mason reported the earliest predictive approach of particle morphology using spreading coefficients based on the interfacial tensions measurements [25]. Loxley and Vincent predicted the morphology of the formed PMMA microcapsules according to the interfacial tension, contact angle, and spreading coefficients measurements in different emulsion systems. They suggested that the morphology of PMMA microcapsules depended strongly on the nature of non-solvent and emulsifier [13]. Saito et al. investigated the experimental and the theoretical morphologies of composite polystyrene (PS)/PMMA particles based on interfacial free energy considerations. They found that the experimental particle morphologies did not correspond to theoretical predictions because each polymer phase contained a small amount of the other polymer, and this in turn, affects the interfacial tensions [27]. Lavergne et al. studied the surface morphology of PMMA microcapsules with foamed shells based on the surface tension values of oil cores and optimal hydrophile-lipophile balance (HLBO) of different oil cores [28]. Wang et al. reported the encapsulation of hexadecane (HD) in PMMA shell and found that the morphology of microcapsules depended on the HD/polymer ratio and the governing interfacial 6

tensions, which in turn were influenced by the type and concentration of emulsifier [12]. Gonzalez et al. investigated theoretically the morphology of PMMA microcapsules with silicone liquid core by analyzing the contact angles and interfacial tensions of oil core, PMMA shell, and continuous phase [29]. In spite of above-mentioned studies, to the best knowledge of the authors, the surface morphology of PMMA microcapsules containing epoxy, cycloaliphatic amine, and mercaptan have not been investigated experimentally so far. There is no report also available on the equilibrium morphology of PMMA microcapsules and corresponding thermodynamic discussions. In addition, the effects of molecular weight (Mw) of PMMA, and type of emulsifier and its concentration on the morphology of these PMMA microcapsules have not been yet investigated. In this study, PMMA microcapsules containing low viscosity healing agents have been prepared separately by internal phase separation method within oil-in-water (O/W) emulsion droplets. The effect of the core material, Mw of PMMA, and type and concentration of emulsifier on the morphology of PMMA microcapsules were investigated by field emission scanning electron microscopy (FESEM). Furthermore, the spreading coefficient values of phases in emulsion systems were calculated from interfacial tension and contact angle measurements that were used to predict the morphology of healing agent/PMMA particles.

2. Experimental

2.1. Materials Low viscosity healing agents including epoxy prepolymer (EC157) with viscosity of 0.5-0.6 Pa.s at 25 °C (Elantas Co., Italy). The composition of epoxy prepolymer was 30-50% bisphenol A diglycidyl ether (DGEBA), 30 - 50% bisphenol F diglycidyl ether (DGEBF), 20-30% 1,6-bis (2,3-epoxypropoxy)

hexane

and

<5%

propylene

carbonate.

3-aminomethyl-3,5,5-

trimethylcyclohexylamine (W152MLR) with viscosity of 0.005-0.020 Pa.s at 25 °C (Elantas Co., Italy) and pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) with viscosity of 0.45 Pa.s at 25 °C (Sigma–Aldrich, USA), were separately used as microcapsule core. The last two materials were used as hardeners in different microcapsules. Fig. 1, indicates the molecular structure of these three chemicals. PMMA with two different molecular weights ( ̅ 1 = 36000 g/mol and ̅ 2 = 7

550000 g/mol) were procured from LG Co., Korea and Alfa aeser Co., USA, respectively and used as microcapsule shell. Two types of polyvinyl alcohols (PVAs) were also employed as polymeric surfactant: PVA1 (PVA; ̅ = 60000 g/mol, degree of hydrolysis ≥ 98.0 %) was obtained from Sigma–Aldrich Co., USA; and PVA2 (Alcotex B72; ̅ = 30000 g/mol, degree of hydrolysis> 96.5 %) obtained from Synthomer Co., UK. Sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB), as ionic emulsifiers, n-hexadecane (n-HD) as coemulsifier, and dichloromethane (DCM) as solvent were all obtained from Merck Co., Germany. All materials were used as received.

Fig. 1. Molecular structure of (a) epoxy, (b) amine, and (c) mercaptan.

2.2. Preparation of PMMA microcapsules The internal phase separation method was used for encapsulation of epoxy, amine, and mercaptan with PMMA as capsule shell [11]. In this method, PMMA (1 g) with specified Mw, together with specified core material (1 g) were dissolved separately in 30 ml DCM. Then, n-HD as co-emulsifier was added and the mixture was dispersed in 50 ml of specified emulsifier aqueous solution under high-speed (1500 rpm) mechanical mixing for 1 h at room temperature to obtain an oil-in-water (O/W) emulsion. The resultant emulsion was further poured into 80 ml of aqueous emulsifier solution, while the mechanical mixing was continued. After the evaporation of DCM, PMMA capsules containing oil cores were obtained. The PMMA microcapsules were filtered, washed several times with distilled water and dried at room temperature. All types of 8

emulsifiers in this process were employed at a concentration of 1 wt.% except PVA2 that was used at two concentrations of 1 and 2 wt.%.

2.3. Characterization The contact angle and surface tension measurements were determined by the pendant drop method at 20 (±2) °C using the pendant drop contact angle and interfacial tension measurement apparatus (CA-ES10) (Fars EOR technologies company, Iran). Interfacial tensions between PMMA and various liquid phases were determined by measuring the contact angle of each liquid (oil core or emulsifier aqueous phase) against the film of PMMA. The film had been formed onto clean glass microscope slides from a 5 wt.% solution of PMMA in DCM. At least three measurements were made for each liquid, and the mean contact angle was used to calculate the PMMA–liquid interfacial tension. The size and surface morphology of the PMMA capsules were analyzed by field emission scanning electron microscopy with different magnifications (FESEM; Sigma, Zeiss, Germany). The samples were coated by gold for this analysis and the capsule size measurement was performed by using image analysis softwares (Image J and particle size analyzer softwares).

3. Results and discussion

3.1. Morphology of prepared PMMA microcapsules The morphology of separately prepared PMMA microcapsules containing epoxy, mercaptan, and amine were investigated by FESEM images at different magnifications. Fig. 2 indicates the morphology of epoxy microcapsules that were prepared from separate aqueous emulsions containing either SDS or CTAB as emulsifier (1 wt.%). As observed, all capsules are formed in spherical shape, most probably due to the original micelle shapes [11], and with smooth surface morphology. In our previous study, the fractured surface of epoxy microcapsules embedded in epoxy matrix was studied by FESEM image [11]. Fig. 3, indicates that the epoxy microcapsules prepared via internal phase separation have a core-shell morphology. This is in correspondence with that obtained by Li et al., that spherical microcapsules were obtained with a smooth surface for epoxy encapsulated in PMMA shell when an anionic emulsifier (SDS) were used. These authors have reported however, a rough surface for microcapsules filled by polyetheramine when 9

a polymeric emulsifier (PVA) were used [17]. They suggested that the different surface morphology of epoxy and amine microcapsules is due to different chemical properties between the epoxy and amine [19]. In other similar studies, the morphology of the PMMA microcapsules containing other liquids were prepared from separate aqueous emulsions containing SDS and CTAB. The morphologies have been observed as “acorn-shaped” or “Janus-like” with smooth surface [12,30]. It is reported that when the engulfing of the core material with polymeric shell is partial, the acorn morphology is formed [25]. Comparing the FESEM images of a with b in Fig. 2, it is implied that the type of ionic emulsifiers has no significant effect on the surface morphology of epoxy microcapsules. Size distributions of microcapsules are drawn in Fig. 4 indicating that the average size and size distribution of epoxy microcapsules is affected by the type of emulsifier. Table 2 indicates that in the emulsion system with CTAB as emulsifier, the interfacial tension between the epoxy and aqueous phase (12.1 mNm-1) is less than that of the system with SDS (16.5 mNm-1), leading to reduction in droplet size and therefore smaller capsule diameter [30]. In the other word, when the interfacial tension between oil phase and aqueous phase decreases, the resistance of droplet to breakage decreases, leading to a decrease of the mean droplet size [31].

Fig. 2. FESEM images of epoxy/PMMA capsules with 1.00 KX magnification obtained with: (a) 1wt.% SDS, (b) 1wt.% CTAB .

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Fig. 3. FESEM image of fractured surface of epoxy microcapsules embedded in epoxy matrix [11].

Fig. 4. Size distributions of epoxy and amine microcapsules.

Fig. 5, shows PMMA capsules containing amine that were prepared from emulsion system containing SDS as emulsifier. The amine capsules have also a rather smooth surface morphology, and with very narrow and uniform size distribution (Fig. 4). Some authors have suggested that the most important process parameters influencing the size and size distribution of capsules are the composition of continuous organic phase, the volume ratio of the internal oil phase, the viscosity of the oil phase, the type and concentration of surfactant, and agitation rate [32-34]. When the viscosity of the oil phase decreases, the size distribution becomes narrower 11

and shifts to smaller diameters. In general, low-viscosity liquids exhibit a low resistance to breakage and deformation [32]. In the encapsulation process of amine with PMMA shell in the aqueous emulsion containing SDS, all the process parameters are the same as for encapsulation of epoxy and mercaptan, but amine has very lower viscosity than those, that results more uniform and narrow size distribution for amine capsules.

Fig. 5. FESEM images of amine/PMMA capsules with different magnifications obtained with 1wt.% SDS (a) 5.00 KX, (b) 100.00 KX.

In our previous study, the formation of small holes or craters on the PMMA shell of mercaptan microcapsules was a challenging issue [11]. Here, the surface morphology of mercaptan/PMMA microcapsules in different emulsion systems is further studied. The mercaptan/PMMA microcapsules were prepared in aqueous emulsion containing SDS. It can be seen from the FESEM image (Fig. 6) that the microcapsules have core-shell morphology and there are small holes or craters on the PMMA shell of the large mercaptan microcapsules.

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Fig. 6. FESEM image of mercaptan/PMMA capsules obtained with 1wt.% SDS.

The mercaptan/PMMA microcapsules were also prepared in emulsion system with CTAB as emulsifier (Fig. 7). The most of mercaptan capsules were produced in this condition with an acorn- shape morphology. A same morphology has already been reported by Loxley and Vincent for PMMA microcapsules with hexadecane core and in presence of CTAB [13]. The formation of acorn or bowl-like mercaptan/PMMA particles in the aqueous emulsion containing CTAB is probably attributed to reduction in the interfacial tension between the oil phase and aqueous phase, and the less tendency of the PMMA shell for engulfing of the mercaptan core [12]. In the extreme cases, the acorn particle may gradually rearrange to two separated droplets and oil core enters into the aqueous phase, leading to collapsing of PMMA capsule [13]. These microcapsules when compared with those obtained in presence of SDS, have smaller size and their size distribution is more uniform and narrower (Figs. 6 and 7).

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Fig. 7. FESEM images of mercaptan/PMMA capsules with different magnifications obtained with 1wt.% CTAB (a) 1.00 KX, (b) 20.00 KX.

In order to investigate the effect of emulsifier type, mercaptan/PMMA microcapsules were prepared also in presence of PVA1 as a polymeric emulsifier. The observed morphology of mercaptan/PMMA microcapsules in this system have holes and dimples on their surfaces (Fig. 8). This is probably ascribed to difference in the interfacial tensions compared to other emulsion systems. The mercaptan/PMMA microcapsules with PVA1 have large size with a broad size distribution in comparison with that prepared in presence of ionic emulsifiers (SDS and CTAB) (Fig. 9).

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Fig. 8. FESEM images of mercaptan/PMMA capsules with different magnifications obtained with 1wt.% PVA1 (a) 1.00 KX, (b) 5.00 KX.

Fig. 9. Size distributions of mercaptan/PMMA microcapsules in various emulsion systems.

The effect of PVA2 as another polymeric emulsifier on the morphology of mercaptan/PMMA microcapsules was also investigated at 1% and 2% concentrations (Figs. 10 and 11). The mercaptan/PMMA microcapsules in this system are highly aggregated (Fig. 10) that indicates the poor colloidal stability of emulsion containing PVA2 [13]. The surface of microcapsules there are covered by several dimples (Fig. 10). This surface morphology may be due to the presence of interfacial tension gradients in emulsion system [35].

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Fig. 10. FESEM images of mercaptan/PMMA capsules with golf ball-like morphology at different magnifications obtained with 1wt.% PVA2. (a) 1.00 KX, (b) 5.00 KX, (c) 20.00 KX.

Comparing Figures 10 and 11, one can conclude that when the concentration of PVA2 in aqueous emulsion increases from 1 wt. % to 2 wt. %, the aggregation of mercaptan microcapsules reduces. This can be justified by the fact that the oil–water interfacial tension is lowered by increasing the concentration of surfactant. The surface morphology of mercaptan/PMMA microcapsules in Fig. 11 (2 wt. % PVA2) has slightly deeper depressions or dimples compared to those obtained with 1 wt.% PVA2 (Fig. 10). Some authors have suggested that the presence of dimples on the surface of PMMA microcapsule can probably be due to impacts between the capsules [28]. It seems that the 16

mercaptan/PMMA microcapsules prepared in in presence of PVA2 as emulsifier have “golf balllike” morphology. A same morphology has been reported for the composite polymer particles with PMMA seed particles prepared by seeded dispersion polymerization (SDP) [36]. The authors have suggested that this morphology is observed when the shell polymer in seed particles has a stronger affinity in nature to the medium and the mobility of polymer is low. The hydrophilicity of the surface layer of the seed particle also influenced the formation of dimples at the surface of the golf ball-like particle [36-38].

Fig. 11. FESEM images of mercaptan/PMMA capsules with golf ball-like morphology at different magnifications obtained with 2 wt.% PVA2. (a) 5.00 KX, (b) 10.00 KX.

The effect of molecular weight of PMMA on the surface morphology of mercaptan microcapsules was also studied by comparing the microcapsules prepared by the current PMMA (Mw = 36000 g/mol) with those prepared by a higher molecular weight (Mw = 550000 g/mol) in emulsion system containing 1 wt.% SDS, as illustration. As shown in Fig. 12, the latter have very smooth surface morphology without any holes or craters on their surface, in contrast to microcapsules prepared using PMMA with low Mw in the same emulsion system (Fig. 6). The prepared mercaptan/PMMA microcapsules using PMMA with high Mw in this emulsion system are also very adherent. It seems that when the Mw of PMMA increases, the viscosity of the oil phase is increased as well, resulting in more viscous stresses that resist droplets against breaking up, that in turn, leads to aggregated microcapsules with greater sizes [39,40]. It is suggested that 17

in emulsion encapsulation systems, micellar structures can be influenced by molecular weight. The morphological transition of micelles takes place upon changing the molecular weight. The high molecular weight materials may lead to formation of spherical, almost monodisperse micelles with high aggregation. In the previous studies at low molecular weights, cylindrical micelles were observed. This is probably due to the low molecular weight micelles have dynamic and deformable nature, therefore the structural stability of micelles increases with increasing molecular weight [41,42]. In our recent work, we encapsulated epoxy and mercaptan using PMMA with low Mw, and found smooth and porous surface morphologies for epoxy and mercaptan capsules, respectively [11]. It is implied consequently that the Mw of PMMA has very significant effect on the surface morphology of mercaptan/PMMA microcapsules.

Fig. 12. FESEM images of mercaptan/PMMA capsules with different magnifications obtained using PMMA with high Mw in aqueous emulsion containing 1wt.% SDS. (a) 10.00 KX, (b) 50.00 KX.

3.2. Theoretical equilibrium morphology of PMMA capsules As observed in previous section, the FESEM images indicated different morphologies for the PMMA microcapsules. In this section, we theoretically discuss on the equilibrium morphology of these microcapsules filled with healing agents, in order to find a basis for the prediction of experimental observations. The morphology with minimum surface free energy (Gs) is thermodynamically most stable morphology, and this is called equilibrium morphology [19]. The difference in the equilibrium morphologies of PMMA microcapsules in different emulsion 18

systems can be described according to total interfacial free energy change. The Gibbs free energy change (ΔG) is expressed as a combination of the terms describing enthalpy, entropy, and interfacial free energy changes. The surface free energy for any morphology of a microcapsule in emulsion system can be explained as: ∑

(1)

where σi/j and Aij are the interfacial tension between the core i and shell j, and the surface area of i/j interface in the configuration, respectively [25]. The interfacial tension between the core and aqueous phases (

) for example, can be

calculated according to following equation: | in which

and

|

(2)

are surface tensions of core and aqueous phases, respectively.

The interfacial tensions between the PMMA and the various liquids such as healing agents and aqueous phases containing emulsifier (

) were determined by measuring

the mean contact angle of each liquid against a film of PMMA (

) using

Young’s equation (Eqs. 3 and 4). (3) (4) The surface tension of PMMA (

) in the literature was reported as 41.1 mNm-1 at 20 °C

[43]. In the encapsulation processes, four emulsifiers (SDS, CTAB, PVA1, and PVA2) were separately used to disperse the oil phase in the aqueous phase. The contact angles and surface tensions of healing agents and emulsifier aqueous phases were measured using the pendant drop method and the results are shown in Table 1.

Table 1. The measured contact angles ( ) and surface tensions ( ) for healing agents and emulsifier aqueous phases. Core

Aq. emulsifier

Core/PMMA

19

W/PMMA

Core/air

W/air

(wt.%)

(◦)

(◦)

(mNm-1)

(mNm-1)

Epoxy

1% SDS

23.3

26.8

43.3

59.9

Epoxy

1% CTAB

23.3

30.2

43.3

55.5

Amine

1% SDS

15.5

26.8

34.4

59.9

Mercaptan

1% SDS

29.8

26.8

43.7

59.9

Mercaptan

1% CTAB

29.8

30.2

43.7

55.5

Mercaptan

1% PVA1

29.8

57.0

43.7

73.6

Mercaptan

1% PVA2

29.8

41.1

43.7

66.9

Mercaptan

2% PVA2

29.8

43.5

43.7

55.4

In the Table 1, it can be seen that the surface tension of aqueous solutions of emulsifier is reduced generally as compared to the surface tension of water in 20 °C (72.75 mNm-1) [44]. The reduction of surface tension of aqueous solution with ionic emulsifier (SDS and CTAB) is more than that of the polymeric emulsifiers (PVA1 and PVA2) [45]. It can be also seen from the data in Table 1 that the smallest and largest contact angles are attributed to amine and aqueous phase containing 1 wt.% PVA1, respectively. The low contact angle of amine means that its drop spreads widely on the PMMA film and wet almost completely the polymer surface. According to previous studies, the contact angle of aqueous solution of emulsifier on the surface of PMMA film depends on the concentration and composition of aqueous solution [46]. The aqueous solution on the solid surface spreads when the difference between the surface tension of solid surface and the interfacial tension between solid surface and aqueous solution is equal to or higher than that of the surface tension of aqueous solution [29]. The possible equilibrium morphology of PMMA microcapsules containing healing agents were predicted using the interfacial tensions and spreading coefficients measurements according to the relations that suggested by Torza and Mason [25]. Spreading or wetting describe the process that occur when a liquid partially or completely wets solid or immiscible liquid surfaces. The spreading coefficient (S) is a factor that determines spontaneous spreading for a drop of oil placed on a liquid substrate with ambient gas [47]. The spreading coefficients

for

each phase in emulsion system (core, W or PMMA) can be determined using following equation [25]: (5) There are only three possible combinations of Si values: 20

>0

(6)

<0

(7)

<0

(8)

When the oil core ends up entirely inside the PMMA shell to form a spherical drop, the condition (6) is satisfied and particles have core–shell morphology. When the partial engulfing of oil core and PMMA shell occurs condition (7) is satisfied and acorn-shaped particles are formed. Finally, two separate droplets will form when the inequality (8) is governed [25]. The internal phase separation mechanism and three possible equilibrium morphologies for particles in encapsulation system

of

oil

phase

with

polymeric

shell

are

depicted

in

Fig.

13.

Fig. 13. Internal phase separation and three possible morphologies for particles in encapsulation system of oil cores with polymeric shell. The term n.v.n.s is an abbreviation for non-volatile non-solvent [12,13].

The mentioned relations between the spreading coefficients of core, aqueous and PMMA phases were used to predict the theoretical equilibrium morphology of prepared particles in the encapsulation systems of epoxy, amine, and mercaptan in PMMA shells. In the Table 2, the calculated interfacial tensions, spreading coefficients, predicted morphologies, and observed experimental morphologies of PMMA microcapsule are presented.

Table 2. Measured interfacial tensions ( ), calculated spreading coefficients (S), and predicted and observed equilibrium morphologies for prepared particles in encapsulation systems of healing agents with PMMA shell.

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Core

Aq. emulsifier

Core/W

Core/PMMA

Morphology

W/PMMA

(mNm-1)

(mNm-1)

(mNm-1)

SCore

SW

SPMMA

(wt.%)

Predicted

Observed

Epoxy

1% SDS

16.5

1.3

-12.3

<0

<0

>0

Core-shell

Core-shell

Epoxy

1% CTAB

12.1

1.3

-0.6

<0

<0

>0

Core-shell

Core-shell

Amine

1% SDS

25.4

7.9

-12.3

<0

<0

>0

Core-shell

Core-shell

Mercaptan

1% SDS

16.1

3.2

-12.3

<0

<0

>0

Core-shell

Core-shell

Mercaptan

1% CTAB

11.8

3.2

-0.6

<0

<0

>0

Core-shell

Acorn

Mercaptan

1% PVA1

29.9

3.2

1.0

<0

<0

>0

Core-shell

Core-shell

Mercaptan

1% PVA2

23.2

3.2

-9.3

<0

<0

>0

Core-shell

Core-shell

Mercaptan

2% PVA2

11.7

3.2

0.9

<0

<0

>0

Core-shell

Core-shell

As mentioned, the surface tension of aqueous emulsion decreases more by ionic emulsifiers when compared with polymeric emulsifiers [45]. In addition, the interfacial tension between oil phase and aqueous phase (

) with ionic emulsifiers is lower than that of the polymeric

emulsifiers [13]. As observed in Table 2, the interfacial tension between aqueous phase and PMMA film in the most of cases is negative. The negative interfacial tension indicates that the free energy of emulsion has been lowered by increase in the interfacial area [48,49]. It can be seen that theoretically-predicted equilibrium morphology is core-shell for all cases that is in agreement with experimental observations obtained from FESEM images of microcapsules considering the spreading coefficient values of corresponding phases. The only exception is mercaptan/CTAB system which a core-shell morphology is predicted again according to thermodynamic relations, while the experimental result indicated the acorn morphology. The difference between the thermodynamic predictions and experimental results can be due to the reduction of interfacial tension between the oil phase and aqueous phase, and reduction in the tendency of the PMMA shell for engulfing the mercaptan core. Table 3 lists a number of morphological studies of PMMA microcapsules filled with oil materials. The authors found that the morphology of PMMA microcapsules is influenced by the nature of oil core, oil/polymer ratio, the emulsifier type and its concentration, surface tension of oil cores, and interfacial tensions between phases in emulsion systems [12,13,28]. In some studies has been suggested that the presence of holes on the surface of PMMA microcapsules can be due to the solvent loss from PMMA shell, and encapsulated tiny water droplets in polymer phase [13,28]. 22

Lavergne et al. suggested oil cores with surface tension close to 25 mN m-1, such as long alkyl chains gave capsules with foamed shell, whereas oil cores with larger surface tension, around 30 mN m-1 and aromatic or polar oils gave capsules with plain surface [28]. In this study, the surface tension measurements indicated that the healing agents had surface tension more than 30 mN m-1 (Table 1), and all of them were polar. It was observed that the capsules containing epoxy and amine cores had smooth surface but the mercaptan-filled capsules had holes, craters, and in some cases depressions or dimples on their surfaces. It seems that there is no general relations between the value of surface tension of core materials and formation of craters and holes on the surface of PMMA capsules. The experimental results also indicated that the appearance of holes, craters or depressions on the surface of mercaptan/PMMA microcapsules were very influenced by the type and concentration of emulsifier, and M w of PMMA shell.

Table 3. Literature review of morphological studies of PMMA microcapsules filled with oil materials Morphology Author(s)

Core

Aq. emulsifier (wt.%)

Wang et al. [12]

Loxley and Vincent [13]

Lavergne et al. [28]

Predicted

Observed

Hexadecane

0.5 PVP a

Hollow sphere

Hexadecane

0.05 SDS

Bowl-like

Hexadecane

0.1 SDS

Hemisphere

Hexadecane

0.25 SDS

Hexadecane

1 PMAA

Hexadecane

Surface Morphology

Truncated sphere b

Core-shell

Core-shell

Plain

1 PVA

Acorn

Core-shell

Holed

Hexadecane

1 SDS

Acorn

Acorn

Hexadecane

1 CTAB

Acorn

Acorn

Decane

1 PMAA

Core-shell

Core-shell

Octanol

1 PMAA

Acorn

Acorn

Benzyl acetate

0.8 PVA

Core-shell

Plain

Geraniol

0.8 PVA

Core-shell

Plain

Aurantiol

0.8 PVA

Core-shell

Plain

Xylene

0.8 PVA

Core-shell

Plain

Acetophenone

0.8 PVA

Core-shell

Plain

Cyclohexanone

0.8 PVA

Core-shell

Plain

23

Plain

a

Miglyol

0.8 Lutrol F68

Core-shell

Foamed

Miglyol

0.8 SDS

Core-shell

Foamed

Miglyol

0.8 Brij 98

Core-shell

Foamed

Miglyol

0.8 PVA

Core-shell

Foamed

Pentane

0.8 PVA

Core-shell

Foamed

Hexane

0.8 PVA

Core-shell

Foamed

Heptane

0.8 PVA

Core-shell

Foamed

Octane

0.8 PVA

Core-shell

Foamed

Decane

0.8 PVA

Core-shell

Foamed

Cyclohexane

0.8 PVA

Core-shell

Foamed

b

Polyvinyl pyrrolidone, polymethacrylic acid.

It can be concluded that the surface morphology or appearance of holes and craters on the capsule surface cannot be predicted by the thermodynamic relations because the thermodynamic can predict the equilibrium morphology or configuration of capsules (core-shell, acorn, and two separate droplets). In addition, the experimental results showed the surface morphology of PMMA capsules are very sensitive to the chemical natures of the core material, molecular weight of PMMA, and emulsifier type and its concentration.

4. Conclusions PMMA microcapsules containing epoxy, cycloaliphatic amine, and mercaptan as core materials were prepared by internal phase separation method. The morphology of healing agent/PMMA capsules were investigated experimentally and theoretically. FESEM images indicated that the epoxy/PMMA and amine /PMMA particles had core–shell morphology with smooth surfaces. The mercaptan/PMMA particles in the most of cases exhibited core–shell morphology but the microcapsules had holes, craters or depressions on their surfaces. The experimental results also indicated the surface morphology of mercaptan/PMMA microcapsules was very influenced by the type and concentration of emulsifier, and Mw of PMMA shell. Theoretical equilibrium morphology of healing agent/PMMA particles was predicted according to spreading coefficient values of phases in different emulsion systems. The predicted theoretical morphology of all healing agent/PMMA microcapsules was core-shell that this indicates the results of experimental and theoretical morphologies were in good agreement with together. In this study was found that the surface morphology or presence of holes and craters on the surface of healing agent/PMMA 24

capsules cannot be predicted by the thermodynamic relations. As an important result, it seems that the thermodynamic relation can predict the equilibrium morphology or configuration of particles in emulsion systems.

Acknowledgements The authors would like to thank Malek-ashtar University of Technology (MUT) for financial support for this study.

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