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Micromechanical properties of melamine–formaldehyde microcapsules by nanoindentation: Effect of size and shell thickness
Materials Letters 89 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Micromechanical properties of melamine–formaldehyde microcapsules by nanoindentation: Effect of size and shell thickness Jun-Feng Su a,b,n, Xin-Yu Wang a, Hua Dong b a b
Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China Department of Materials and Environment, CiTG, Delft University of Technology, 2628CN Delft, The Netherlands
a r t i c l e i n f o
abstract
Article history: Received 15 May 2012 Accepted 20 August 2012 Available online 29 August 2012
Melamine–formaldehyde (MF) microcapsules have been widely applied in many functional materials. Micromechanical properties are important characters determining the survival of the microcapsules during fabrication and application. In this study, nanoindentation was successfully applied as an effective method to measure the micromechanical properties of microcapsules containing paraffin. A series of MF-shell microcapsules had been fabricated with various size and thickness by controlling the core material stirring rates (1000–8000 r min 1) and core/shell weight ratios (1/1, 1/2 and 1/3). The load–displacement curves indicated that the MF shells had the ability of elastic–plastic deformation. Both of hardness and Young’s modulus of microcapsules depended on parameters of size and shell thickness. The results confirmed that larger microcapsule had higher modulus and thicker shell could increase the ability resisting deformation of microcapsules. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microcapsules Nanoindentation Melamine–formaldehyde Size Shell thickness
1. Introduction Melamine–formaldehyde (MF) resin may be one of the most applied shell materials to fabricate microcapsules [1]. Interestingly, in recent years MF resin has attracted more attention to encapsulate antimicrobial materials [2] self-healing materials [3], phase change materials [4] and electrophoretic materials [5]. In-situ polymerization is normally used to prepare the MF microcapsules with core–shell structure. It occurs in the continuous phase to form shells, rather than on both sides of the interface between the continuous phase and the core material. Beside the size, shell thickness and shell macrostructure, micromechanical character of shells is another important property need to be considered [6]. Mechanical properties of shells play an important role in many processes and an understanding, therefore, is indispensable for their application. In some cases, the microcapsules should have optimal mechanical strength. They must be strong enough to remain intact during manufacture and further processing, such as drying, pumping and mixing. In other cases, the core materials also should be controlled releasing or protecting. The shell mechanical properties are guides in the design of microcapsules and key issues of microcapsule manufacturing [7–9]. Based on the material science principle, it can be imaged that the mechanical property of microcapsule is determined by shell structure including molecular structure, molecular weight, shell
n Corresponding author at: Department of Materials and Environment, CiTG, Delft University of Technology, 2628CN Delft, The Netherlands. Tel./fax: þ31 15 27 89553. E-mail address: [email protected] (J.-F. Su).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.072
size and thickness. It has been reported that polymeric shell microcapsules are viscoelastic particles depending on size [10]. As the shell is thin film, its overall stiffness may be assessed through the product Eh0, where E is the bulk material elastic Young’s modulus and h0 the shell thickness. It is then clear that the production of a thicker shell will increase the overall stiffness of a single microcapsule. However, producing thicker shell may alter the bulk properties of the microcapsules in ways that are not easy to evaluate because, although essential, the membrane constitutive law is quite difficult to measure due to the smallness and fragility of artificial capsules. Because of tiny structure, there is still little knowledge focusing on relationship between micromechanical property and MF shells (size and thickness). At the same time, shell thickness is difficult to be controlled and measured. Several methods have been proposed to assess the mechanical properties of cell or microcapsule, which are all based on the measurement of deformation under a well-defined stress [11]. Among these methods, nanoindentation processes high resolution and depth sensing ability to measure micromechanical properties of microcapsules [12]. Hardness and Young’s modules are easy to be calculated by the load–displacement curves. Motivated by the importance of microcapsules application, the goal of this work was to investigate the micromechanical properties of MF-shell microcapsules affecting by size and thickness by nanoindentation.
2. Experimental procedure Microcapsulation by coacervation proceeds has been described in previous work [9]. It includes three main steps: (1) phase
2
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
separation of the coating polymer solution, (2) adsorption of the coacervation around the core particles and (3) solidification of the microcapsules. The resultant microcapsules were filtered and washed with pure water and dried in a vacuum oven. The average size is the mean value of 50 microcapsules measured from the scan electron microscopy (SEM) morphology image. About 2 g MF-shell microcapsules were mixed in 5 g epoxy resin. After the composite was dried at room temperature, it was carefully cut to sheets. The thickness of shells can be measured from the crosssection SEM images of microcapsules [4]. The Nano Indenters XP (MTS Nano Instruments) was applied to achieve the curves and then extract mechanical properties (hardness and Young’s modulus) of the microcapsules. A cone tip (3 mm radius) was used to measure hardness and Young’s modulus. The force and displacement resolutions were 50 nN and 0.01 nm, the holding time of indentation was 10 s. The surface approach velocity was 10 nm/s and the maximum displacement was 2000 nm. A record of these mechanical values can be plotted on a graph to create a load–displacement curve. The area of the residual indentation in the sample is measured and the hardness (H) is defined as the maximum load (Pmax) divided by the residual indentation area (Ar), or H¼
Pmax Ar
ð1Þ
The hardness is given by the equation above, relating the maximum load to the indentation area. Some important quantities are the peak load and displacement, Pmax and hmax, the residual depth after unloading, hf, and the slope of the initial portion of the unloading curve (S ¼dP/dh). S has the dimensions of force per unit distance, and so it is also known as the elastic stiffness of the contact. Young’s modulus (E) was calculated from the slope of the linear portion, dP/dh upon unloading [12], 1 1u 1u0 þ ¼ Er E E0 Er ¼
1=2
P
Ar
dP=dh
ð2Þ
ð3Þ
Where Er is the reduced modules, v is Poisson’s ratio.
3. Results and discussion In Fig. 1, the surface morphologies of microcapsules and shell thickness are presented. Fig. 1(a) shows the optical microscope morphology of dried microcapsules (core/shell ratio of 1/1, core material stirring rate of 4000 r min 1) with average size of about 22.5 mm. These microcapsules have compact and global shape in a
normal size distribution. The surface of microcapsules is smooth without defects as shown in Fig. 1(b). To determine the shell thickness, microcapsules were embedded in epoxy resin. The cross-section SEM morphologies of a typical single microcapsule are presented in Fig. 1(c,d). The shell thickness was measured directly. It must be noted that the shell may not be cut across its equator, so the thickness is an average data of at least 10 shells for each microcapsule sample. Fig. 2(a) shows the average sizes of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under various emulsion stirring rates in range of 1000–8000 r min 1. With the decreasing of stirring rates, the average diameters decreased sharply from 100.5 to 2.0 mm. Higher stirring rates will smash the core material into smaller droplets. Less core/shell ratio leads to a higher shell thickness value. This result accords with reported results and indicates that the average diameter is mainly determined by emulsion stirring rates [13]. In addition, it can be concluded that the MF prepolymer had cross-linked with a compact structure forming thin shells. Fig. 2(b) shows the data of shell thickness of microcapsules (core/ shell ratios of 1/1, 1/2 and 1/3) under core stirring rates of 2000, 4000, 6000 and 8000 r min 1, respectively. Microcapsule samples have the shell thickness data in a range from 4.5070.50 to 0.5070.10 mm. More shell material leads the shell to a thicker structure for each sample fabricated under the same core material stirring rate. Typical load–displacement curves of microcapsules are shown in Fig. 3. One sample microcapsules (core/shell ratio of 1/1, stirring rates of 2000, 3000 and 4000 r min 1) were tested by nanoindentation with different loads. Fig. 3(a–c) show the microcapsules are separately glued on glass plates. To measure Young’s modulus and hardness of microcapsules, the load in this study did not exceed the ‘yield point’ or ‘rupture point’. Once the yield point is passed, some fraction of the deformation may be permanent and nonreversible. In Fig. 3(d), the load–displacement curves upon loading (8, 11, and 14 mN) indicate that microcapsules have a plastic deformation depending on size. The loading portion of the nanoindentation with nonlinear curve represents both elastic and plastic deformation; meanwhile shell represents elastic displacement being recovered during unloading. The displacement is about 0.2 to 0.3 mm. It was reported that MF microcapsules was compressed to rapture under load of 2.5 mN [14]; meanwhile Lee et al. [12] found that the MF microcapsules showed elastic deformation under load of 10.5 mN. The loads in this study have some deviation with literatures. The reason may be attributed to the difference of molecular weight and shell microstructure (thickness, density and porosity) of microcapsules. Fig. 4(a) shows the mean values of Young’s modulus for single microcapsule samples affected by size (core stirring rates of 2000, 4000, 6000 and 8000 r min 1) and shell thickness (core/shell ratios of 1/1, 1/2 and 1/3). It indicates that larger microcapsule
Fig. 1. Morphologies of MF-shell microcapsules, (a) optical microscope morphology of microcapsules fabricated with core/shell ratio of 1/1 under core material stirring rate of 4000 r min 1, (b) SEM morphology of with core/shell ratio of 1/1 under core material stirring rate of 4000 r min 1, (c) the cross-section SEM morphology of a single microcapsule embedded in epoxy, and (d) the measurement method of shell thickness by SEM.
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
Fig. 2. The (a) average diameter and (b) shell thickness data of MF-shell microcapsules fabricated under various core material stirring rates of 1000–8000 r min 1with core/ shell ratios of 1/1, 1/2 and 1/3.
Fig. 3. The morphology and load–displacement curves of microcapsules: (a,b,c) SEM morphologies of single-layer microcapsules (core/shell ratio¼ 1/1, core material stirring rates of 2000, 3000 and 4000 r min 1) glued on the glass plates for nanoindentation tests, (d) the load–displacement curves of different microcapsules.
3
Fig. 4. Micromechanical properties of microcapsules with various average diameter and shell thickness: (a) Young’s modulus and (b) hardness.
has higher modulus for each sample. It should be pointed out that this trend is similar the reported result [12]. The Young’s modules of microcapsule with core/shell ratio of 1/3 is in the range of 2.71– 1.90 GPa; meanwhile Young’s modules of microcapsule with core/ shell ratio of 1/1 is in the range of 2.25–1.64 GPa. More shell material can make the shell with higher thickness leading to higher stiffness of elastic microcapsules. It can be concluded that the micromechanical properties of microcapsules depend on the thickness and microstructure of shells. The characteristics of core material may be at the same time influence the load–displacement of microcapsules. During the deformation compression, the volume and Poisson’s ratio of polymer microcapsules may change continuously with the deformation because of the spherical geometry. In Fig. 4(b), the hardness of single microcapsule also has two impact parameters of core material disperse rates (size) and core/shell ratios (shell thickness). For each sample, the hardness has a slight increasing trend with the increasing of size. Higher shell proportion of microcapsules leads the shell to a higher hardness, which means that the thicker shell has higher ability to resist deformation.
4. Conclusion A series of MF-shell microcapsules have been successfully fabricated with various size and thickness. Nanoindentation has
4
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
been applied as an effective approach to measure the micromechanical properties of single microcapsule. Hardness and Young’s modulus results indicate that the MF shell acted as a polymer with elastic–plastic deformation. The size and shell thickness are both main influencing factors of micromechanical properties of the microcapsules.
Acknowledgments The authors acknowledge the financial support of the National Natural Science Foundation of China (no. 50803045). References [1] Su JF, Wang LX, Ren L. Preparation and characterization of double-MF shell microPCMs used in building materials. J Appl Polym Sci 2005;97(5):1755–62. [2] Ocepek B, Boh B, Sumig B, Tavcer PF. Printing of antimicrobial microcapsules on textiles. Color Technol 2012;128(2):95–102. [3] Hu J, Chen HQ, Zhang Z. Mechanical properties of melamine formaldehyde microcapsules for self-healing materials. Mater Chem Phys 2009;118(1):63–70. [4] Su JF, Huang Z, Ren L. High compact melamine–formaldehyde microPCMs containing n-octadecane fabricated by a two-step coacervation method. Colloid Polym Sci 2007;285(14):1581–91.
[5] Kim KS, Lee JY, Park BJ, Sung JH, Chin I, Choi HJ, Lee JH. Synthesis and characteristics of microcapsules containing electrophoretic particle suspensions. Colloid Polym Sci 2006;284(7):813–6. [6] Sun G, Zhang Z. Mechanical properties of melamine-–formaldehyde microcapsules. J Microencapsul 2001;18(5):593–602. [7] Brown EN, White SR, Sottos NR. Fracture testing of a self-healing polymer composite. Exp Mech 2002;42(4):372–9. [8] Su JF, Wang LX, Ren L, Huang Z. Mechanical properties and thermal stability of double-shell thermal-energy-storage microcapsules. J Appl Polym Sci 2007;103(2):1295–302. [9] Keller MW, Sottos NR. Mechanical properties of microcapsules used in a selfhealing polymer. Exp Mech 2006;46:725–33. [10] Kim K, Liu XY, Zhang Y, Cheng J, Wu XY, Sun Y. Elastic and viscoelastic characterization of microcapsules for drug delivery using a force-feedback MEMS microgripper. Biomed Microdevices 2009;11(2):421–7. [11] Walter A, Rehage H, Leonhard H. Shear-induced deformations of polyamide microcapsules. Colloid Polym Sci 2000;278:169–75. [12] Lee J, Zhang M, Bhattacharyya D, Yuan YC, Jayaraman K, Mai YW. Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsules by nanoindentation. Mater Lett 2012;76:62–5. [13] Su JF, Wang SB, Zhang YY, Huang. Physicochemical properties and mechanical characters of methanol-modified melamine–formaldehyde (MMF) shell microPCMs containing paraffin. Colloid Polym Sci 2011;289(2):111–9. [14] Sun G, Zhang Z. Mechanical strength of microcapsules made of different wall materials. Int J Pharm 2002;242(1-2):307–11.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Micromechanical properties of melamine–formaldehyde microcapsules by nanoindentation: Effect of size and shell thickness Jun-Feng Su a,b,n, Xin-Yu Wang a, Hua Dong b a b
Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China Department of Materials and Environment, CiTG, Delft University of Technology, 2628CN Delft, The Netherlands
a r t i c l e i n f o
abstract
Article history: Received 15 May 2012 Accepted 20 August 2012 Available online 29 August 2012
Melamine–formaldehyde (MF) microcapsules have been widely applied in many functional materials. Micromechanical properties are important characters determining the survival of the microcapsules during fabrication and application. In this study, nanoindentation was successfully applied as an effective method to measure the micromechanical properties of microcapsules containing paraffin. A series of MF-shell microcapsules had been fabricated with various size and thickness by controlling the core material stirring rates (1000–8000 r min 1) and core/shell weight ratios (1/1, 1/2 and 1/3). The load–displacement curves indicated that the MF shells had the ability of elastic–plastic deformation. Both of hardness and Young’s modulus of microcapsules depended on parameters of size and shell thickness. The results confirmed that larger microcapsule had higher modulus and thicker shell could increase the ability resisting deformation of microcapsules. & 2012 Elsevier B.V. All rights reserved.
Keywords: Microcapsules Nanoindentation Melamine–formaldehyde Size Shell thickness
1. Introduction Melamine–formaldehyde (MF) resin may be one of the most applied shell materials to fabricate microcapsules [1]. Interestingly, in recent years MF resin has attracted more attention to encapsulate antimicrobial materials [2] self-healing materials [3], phase change materials [4] and electrophoretic materials [5]. In-situ polymerization is normally used to prepare the MF microcapsules with core–shell structure. It occurs in the continuous phase to form shells, rather than on both sides of the interface between the continuous phase and the core material. Beside the size, shell thickness and shell macrostructure, micromechanical character of shells is another important property need to be considered [6]. Mechanical properties of shells play an important role in many processes and an understanding, therefore, is indispensable for their application. In some cases, the microcapsules should have optimal mechanical strength. They must be strong enough to remain intact during manufacture and further processing, such as drying, pumping and mixing. In other cases, the core materials also should be controlled releasing or protecting. The shell mechanical properties are guides in the design of microcapsules and key issues of microcapsule manufacturing [7–9]. Based on the material science principle, it can be imaged that the mechanical property of microcapsule is determined by shell structure including molecular structure, molecular weight, shell
n Corresponding author at: Department of Materials and Environment, CiTG, Delft University of Technology, 2628CN Delft, The Netherlands. Tel./fax: þ31 15 27 89553. E-mail address: [email protected] (J.-F. Su).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.072
size and thickness. It has been reported that polymeric shell microcapsules are viscoelastic particles depending on size [10]. As the shell is thin film, its overall stiffness may be assessed through the product Eh0, where E is the bulk material elastic Young’s modulus and h0 the shell thickness. It is then clear that the production of a thicker shell will increase the overall stiffness of a single microcapsule. However, producing thicker shell may alter the bulk properties of the microcapsules in ways that are not easy to evaluate because, although essential, the membrane constitutive law is quite difficult to measure due to the smallness and fragility of artificial capsules. Because of tiny structure, there is still little knowledge focusing on relationship between micromechanical property and MF shells (size and thickness). At the same time, shell thickness is difficult to be controlled and measured. Several methods have been proposed to assess the mechanical properties of cell or microcapsule, which are all based on the measurement of deformation under a well-defined stress [11]. Among these methods, nanoindentation processes high resolution and depth sensing ability to measure micromechanical properties of microcapsules [12]. Hardness and Young’s modules are easy to be calculated by the load–displacement curves. Motivated by the importance of microcapsules application, the goal of this work was to investigate the micromechanical properties of MF-shell microcapsules affecting by size and thickness by nanoindentation.
2. Experimental procedure Microcapsulation by coacervation proceeds has been described in previous work [9]. It includes three main steps: (1) phase
2
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
separation of the coating polymer solution, (2) adsorption of the coacervation around the core particles and (3) solidification of the microcapsules. The resultant microcapsules were filtered and washed with pure water and dried in a vacuum oven. The average size is the mean value of 50 microcapsules measured from the scan electron microscopy (SEM) morphology image. About 2 g MF-shell microcapsules were mixed in 5 g epoxy resin. After the composite was dried at room temperature, it was carefully cut to sheets. The thickness of shells can be measured from the crosssection SEM images of microcapsules [4]. The Nano Indenters XP (MTS Nano Instruments) was applied to achieve the curves and then extract mechanical properties (hardness and Young’s modulus) of the microcapsules. A cone tip (3 mm radius) was used to measure hardness and Young’s modulus. The force and displacement resolutions were 50 nN and 0.01 nm, the holding time of indentation was 10 s. The surface approach velocity was 10 nm/s and the maximum displacement was 2000 nm. A record of these mechanical values can be plotted on a graph to create a load–displacement curve. The area of the residual indentation in the sample is measured and the hardness (H) is defined as the maximum load (Pmax) divided by the residual indentation area (Ar), or H¼
Pmax Ar
ð1Þ
The hardness is given by the equation above, relating the maximum load to the indentation area. Some important quantities are the peak load and displacement, Pmax and hmax, the residual depth after unloading, hf, and the slope of the initial portion of the unloading curve (S ¼dP/dh). S has the dimensions of force per unit distance, and so it is also known as the elastic stiffness of the contact. Young’s modulus (E) was calculated from the slope of the linear portion, dP/dh upon unloading [12], 1 1u 1u0 þ ¼ Er E E0 Er ¼
1=2
P
Ar
dP=dh
ð2Þ
ð3Þ
Where Er is the reduced modules, v is Poisson’s ratio.
3. Results and discussion In Fig. 1, the surface morphologies of microcapsules and shell thickness are presented. Fig. 1(a) shows the optical microscope morphology of dried microcapsules (core/shell ratio of 1/1, core material stirring rate of 4000 r min 1) with average size of about 22.5 mm. These microcapsules have compact and global shape in a
normal size distribution. The surface of microcapsules is smooth without defects as shown in Fig. 1(b). To determine the shell thickness, microcapsules were embedded in epoxy resin. The cross-section SEM morphologies of a typical single microcapsule are presented in Fig. 1(c,d). The shell thickness was measured directly. It must be noted that the shell may not be cut across its equator, so the thickness is an average data of at least 10 shells for each microcapsule sample. Fig. 2(a) shows the average sizes of microcapsules (core/shell ratios of 1/1, 1/2 and 1/3) under various emulsion stirring rates in range of 1000–8000 r min 1. With the decreasing of stirring rates, the average diameters decreased sharply from 100.5 to 2.0 mm. Higher stirring rates will smash the core material into smaller droplets. Less core/shell ratio leads to a higher shell thickness value. This result accords with reported results and indicates that the average diameter is mainly determined by emulsion stirring rates [13]. In addition, it can be concluded that the MF prepolymer had cross-linked with a compact structure forming thin shells. Fig. 2(b) shows the data of shell thickness of microcapsules (core/ shell ratios of 1/1, 1/2 and 1/3) under core stirring rates of 2000, 4000, 6000 and 8000 r min 1, respectively. Microcapsule samples have the shell thickness data in a range from 4.5070.50 to 0.5070.10 mm. More shell material leads the shell to a thicker structure for each sample fabricated under the same core material stirring rate. Typical load–displacement curves of microcapsules are shown in Fig. 3. One sample microcapsules (core/shell ratio of 1/1, stirring rates of 2000, 3000 and 4000 r min 1) were tested by nanoindentation with different loads. Fig. 3(a–c) show the microcapsules are separately glued on glass plates. To measure Young’s modulus and hardness of microcapsules, the load in this study did not exceed the ‘yield point’ or ‘rupture point’. Once the yield point is passed, some fraction of the deformation may be permanent and nonreversible. In Fig. 3(d), the load–displacement curves upon loading (8, 11, and 14 mN) indicate that microcapsules have a plastic deformation depending on size. The loading portion of the nanoindentation with nonlinear curve represents both elastic and plastic deformation; meanwhile shell represents elastic displacement being recovered during unloading. The displacement is about 0.2 to 0.3 mm. It was reported that MF microcapsules was compressed to rapture under load of 2.5 mN [14]; meanwhile Lee et al. [12] found that the MF microcapsules showed elastic deformation under load of 10.5 mN. The loads in this study have some deviation with literatures. The reason may be attributed to the difference of molecular weight and shell microstructure (thickness, density and porosity) of microcapsules. Fig. 4(a) shows the mean values of Young’s modulus for single microcapsule samples affected by size (core stirring rates of 2000, 4000, 6000 and 8000 r min 1) and shell thickness (core/shell ratios of 1/1, 1/2 and 1/3). It indicates that larger microcapsule
Fig. 1. Morphologies of MF-shell microcapsules, (a) optical microscope morphology of microcapsules fabricated with core/shell ratio of 1/1 under core material stirring rate of 4000 r min 1, (b) SEM morphology of with core/shell ratio of 1/1 under core material stirring rate of 4000 r min 1, (c) the cross-section SEM morphology of a single microcapsule embedded in epoxy, and (d) the measurement method of shell thickness by SEM.
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
Fig. 2. The (a) average diameter and (b) shell thickness data of MF-shell microcapsules fabricated under various core material stirring rates of 1000–8000 r min 1with core/ shell ratios of 1/1, 1/2 and 1/3.
Fig. 3. The morphology and load–displacement curves of microcapsules: (a,b,c) SEM morphologies of single-layer microcapsules (core/shell ratio¼ 1/1, core material stirring rates of 2000, 3000 and 4000 r min 1) glued on the glass plates for nanoindentation tests, (d) the load–displacement curves of different microcapsules.
3
Fig. 4. Micromechanical properties of microcapsules with various average diameter and shell thickness: (a) Young’s modulus and (b) hardness.
has higher modulus for each sample. It should be pointed out that this trend is similar the reported result [12]. The Young’s modules of microcapsule with core/shell ratio of 1/3 is in the range of 2.71– 1.90 GPa; meanwhile Young’s modules of microcapsule with core/ shell ratio of 1/1 is in the range of 2.25–1.64 GPa. More shell material can make the shell with higher thickness leading to higher stiffness of elastic microcapsules. It can be concluded that the micromechanical properties of microcapsules depend on the thickness and microstructure of shells. The characteristics of core material may be at the same time influence the load–displacement of microcapsules. During the deformation compression, the volume and Poisson’s ratio of polymer microcapsules may change continuously with the deformation because of the spherical geometry. In Fig. 4(b), the hardness of single microcapsule also has two impact parameters of core material disperse rates (size) and core/shell ratios (shell thickness). For each sample, the hardness has a slight increasing trend with the increasing of size. Higher shell proportion of microcapsules leads the shell to a higher hardness, which means that the thicker shell has higher ability to resist deformation.
4. Conclusion A series of MF-shell microcapsules have been successfully fabricated with various size and thickness. Nanoindentation has
4
J.-F. Su et al. / Materials Letters 89 (2012) 1–4
been applied as an effective approach to measure the micromechanical properties of single microcapsule. Hardness and Young’s modulus results indicate that the MF shell acted as a polymer with elastic–plastic deformation. The size and shell thickness are both main influencing factors of micromechanical properties of the microcapsules.
Acknowledgments The authors acknowledge the financial support of the National Natural Science Foundation of China (no. 50803045). References [1] Su JF, Wang LX, Ren L. Preparation and characterization of double-MF shell microPCMs used in building materials. J Appl Polym Sci 2005;97(5):1755–62. [2] Ocepek B, Boh B, Sumig B, Tavcer PF. Printing of antimicrobial microcapsules on textiles. Color Technol 2012;128(2):95–102. [3] Hu J, Chen HQ, Zhang Z. Mechanical properties of melamine formaldehyde microcapsules for self-healing materials. Mater Chem Phys 2009;118(1):63–70. [4] Su JF, Huang Z, Ren L. High compact melamine–formaldehyde microPCMs containing n-octadecane fabricated by a two-step coacervation method. Colloid Polym Sci 2007;285(14):1581–91.
[5] Kim KS, Lee JY, Park BJ, Sung JH, Chin I, Choi HJ, Lee JH. Synthesis and characteristics of microcapsules containing electrophoretic particle suspensions. Colloid Polym Sci 2006;284(7):813–6. [6] Sun G, Zhang Z. Mechanical properties of melamine-–formaldehyde microcapsules. J Microencapsul 2001;18(5):593–602. [7] Brown EN, White SR, Sottos NR. Fracture testing of a self-healing polymer composite. Exp Mech 2002;42(4):372–9. [8] Su JF, Wang LX, Ren L, Huang Z. Mechanical properties and thermal stability of double-shell thermal-energy-storage microcapsules. J Appl Polym Sci 2007;103(2):1295–302. [9] Keller MW, Sottos NR. Mechanical properties of microcapsules used in a selfhealing polymer. Exp Mech 2006;46:725–33. [10] Kim K, Liu XY, Zhang Y, Cheng J, Wu XY, Sun Y. Elastic and viscoelastic characterization of microcapsules for drug delivery using a force-feedback MEMS microgripper. Biomed Microdevices 2009;11(2):421–7. [11] Walter A, Rehage H, Leonhard H. Shear-induced deformations of polyamide microcapsules. Colloid Polym Sci 2000;278:169–75. [12] Lee J, Zhang M, Bhattacharyya D, Yuan YC, Jayaraman K, Mai YW. Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsules by nanoindentation. Mater Lett 2012;76:62–5. [13] Su JF, Wang SB, Zhang YY, Huang. Physicochemical properties and mechanical characters of methanol-modified melamine–formaldehyde (MMF) shell microPCMs containing paraffin. Colloid Polym Sci 2011;289(2):111–9. [14] Sun G, Zhang Z. Mechanical strength of microcapsules made of different wall materials. Int J Pharm 2002;242(1-2):307–11.