Materials Letters 76 (2012) 62–65

Contents lists available at SciVerse ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r .com/ locate /mat le t

Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsulesby nanoindentation

Jim Lee a,c,⁎, Mingqiu Zhang b, Debes Bhattacharyya a,⁎, Yan Chao Yuan b,Krishnan Jayaraman a, Yiu Wing Mai c

a Centre for Advanced Composite Materials, Department of Mechanical Engineering, The University of Auckland, Auckland 1142, New Zealandb Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Materials Science Institute, Zhongshan University, Guangzhou 510275, PR Chinac Centre for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia

⁎ Corresponding author at: Centre for Advanced CompMechanical Engineering, the University of Auckland, Au

E-mail addresses: jim.lee@auckland.ac.nz, jl11018@g

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.02.052

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 October 2011Accepted 11 February 2012Available online 17 February 2012

Keywords:FunctionalBiomimeticPolymersIndentationElastic properties

Nanoindentation is a widely used method for measuring the micromechanical properties of thin films andmicro scale materials. Self-healing polymeric materials have the built-in capability to substantially recovertheir load transferring ability after damage. One of the main self-healing strategies incorporates microencap-sulated healing agents within a polymer matrix to produce a polymer composite capable of self-healing. Inthis study, microcapsules containing, respectively, epoxy (resin) and mercaptan (hardener) were investigat-ed with poly (melamine-formaldehyde) (PMF) as the shell material. The micromechanical behavior of micro-capsules was tested using nanoindentation. The results show that the PMF shell material behaves as aviscoelastic plastic material. The modulus and hardness of the microcapsules were determined quantitatively.The size and loaded-component of microcapsules (i.e., hardener or resin) have a significant effect on themicromechanical properties of the microcapsules.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Nanoindentation possesses high resolution and depth sensingability. It has become an important tool in measurements of nano-mechanical properties for small scale materials [1]. Since the 1800swhen Friedrich Mohs introduced a simple scratch tester, the use ofrelative hardness as a way to characterize mechanical properties hasbecome commonplace; however, the advancement of miniaturizeddevices has led to the development of nano-indenters. Nanoindenta-tion is widely used to measure micromechanical properties of mate-rials. The load–displacement curve (Fig. 1) is used to calculate themechanical properties of the sample [2–5]. Owing to the small samplevolume under nanoindentation, the test can be highly localized to aspecific microstructural feature.

Polymer composites often experience micro-cracks during theirservice [6]. The concept of self-healing is applicable where the dam-age can be repaired by the materials already contained within thestructure. Self-healing polymers have built-in capability to recoversubstantially their lost load transfer ability after damage [7–9]. Thecapacity to self-heal results in prolonged material service life, lessmaintenance, and hence potential cost reductions [7].

The field of self-healing materials is a relatively new one, beginningin early 1990s, with most of the research occurring in the past decade

osite Materials, Department ofckland 1142, New Zealand.mail.com (J. Lee).

l rights reserved.

[6–8]. Considering the feasibility of mass production and breadth of ap-plication, the routes based onmicrocapsulation are very promising [10].When a micro-crack is formed from internal stress or physical damage,the microcapsules are supposed to rupture, release the healing agents,react to form a polymer network and glue the crack faces [7,11].Hence, it is important to understand the micromechanical property ofthemicrocapsules to achieve self-healing. To the best of our knowledge,the micromechanical properties of self-healing microcapsules are notavailable in the literature. The goal of this work was to study themicro-mechanical behavior of epoxy and mercaptan-loaded microcapsuleswith PMF shells by nanoindentation.

2. Experimental work

2.1. Microcapsules and sample preparation

This procedure involved dispersing a core substance in a film-forming polymer solution [11,12]. The microcapsules containingepoxy and mercaptan (hardener) with poly (melamine-formaldehyde)(PMF) as the shell material (Fig. 2) were prepared by the methodreported by Yuan et al. [10,13,14]. The microcapsule sizes wereexamined using a Malvern Master sizer 2000 Version 5.54 and by scan-ning electron microscopy (XL30 ESEM-FEG, Philips).

The microcapsules were dispersed in water, carefully separated ona smooth surface, and strongly glued onto a rigid substrate by superglue. The microcapsules must be separated from one another fornanoindentation purpose.

Load(

P)

Displacement (h)

P max

dP/dh

hr he

hp

ht

ha

Loading

Unloading

Recovery

hold

Fig. 1. Representative loading–unloading curve obtained from a nanoindentation testof polymer.

63J. Lee et al. / Materials Letters 76 (2012) 62–65

2.2. Nanoindentation testing

The XP indenter (MTS Nano Instruments) was used at room tem-perature (~20 °C). A cone tip (~3 μm radius) was used to measureYoung's modulus and hardness. After the sample stool was loaded,the microscope was turned on to target the single microcapsule. Atthe beginning of the test, the tip was slowly driven toward the singlemicrocapsule surface. The force and displacement resolutions were50 nN and 0.01 nm, respectively. The holding time of indentationwas 10 s. The surface approach velocity was 10 nm/s and the maxi-mum displacement was ~2000 nm.

A schematic of the loading and unloading process and the pa-rameters are shown in Fig. 1. The nanoindentation data were cor-rected for frame compliance and thermal drift. The indentationdepths ht, he, hr total depth at load Pt, elastic depth rebound atunloading and residual impression depth, respectively. ha is thedisplacement of the surface at the perimeter and hp the contact in-dentation depth. Hardness, H, was calculated from the data alongthe loading curve and was defined as the maximum load, Pmax, di-vided by the residual indentation area, Ar or

H ¼ PmaxAr

:::::::::::::::::::: ð1Þ

(a) (

Fig. 2. SEM micrograph of (a) epoxy and

Young's Modulus E was calculated from the slope of the linear por-tion, dP/dh upon unloading.

1Er

¼ 1−vE

þ 1−voEo

;Er ¼ ΠAr

� �1=2dP=dh :::::::::::::::::::: ð2Þ

where Er is the reduced modulus, E (E ) and v (v ) are Young'smodulus and Poisson's ratio respectively for the material (indenter)[2,5,15,16]. The loading slope change was used to calculate the micro-capsule fracture load.

3. Results and discussion

In Fig. 2, micrographs of epoxy and hardener-loaded microcap-sules are presented. The microcapsules were prepared by in situ poly-merization in an oil-in-water emulsion with PMF as shell materials,epoxy and hardener (mercaptan) as the core substance [10,13,14].PMF is a thermosetting polymer [14]. There are some other materialsin addition to the highly dispersed microcapsules. These are the pow-ders formed by melamine-formaldehyde oligomers by successive po-lymerization, liquid–liquid phase separation and deposition in theaqueous volume phase [13]. The microcapsule sizes are in the range10–150 μm for epoxy and 20–300 μm for hardener and their respec-tive mean diameters are ~50 and 100 μm [9].

The Young's modulus and hardness of themicrocapsules were mea-sured 3 to 5 times. Typical load–displacement curves of epoxy-loadedand hardener-loadedmicrocapsules are shown in Figs. 3 and 4. This be-havior is similar to that of bio-cells and liquid cells respectively [18]. Theloading portion of the nanoindentation with linear and nonlinear curverepresents both elastic (recovery) and plastic (inelastic) deformations[5,17]. During unloading, the portion mainly represents the elastic be-havior with elastic displacement being recovered [5,15].The displace-ment is around 0.2 μm which is about the thickness of shell [14]. Thedisplacement is small compared to the average microcapsule size of50 to 100 μm.

In Fig. 3, the load–displacement curve upon loading increases bi-linearly until hold, then decreases sharply upon unloading (~14.6%deformation recovery). These characteristics may correspond to theunevenness of the inner or outer surfaces of the shell of the microcap-sule. The creep in polymer materials under constant load is controlledby the available free volume for polymer chains and molecular unitsto move [5]. The average Young's modulus and hardness of theepoxy-loaded PMF shell materials are 4.66 GPa and 0.138 GPa, re-spectively. For comparison the average values of the modulus andhardness of cured epoxy (Aeropoxy) are 4.46 and 0.188 GPa at 25 °C

b)

(b) hardener-loaded microcapsules.

0

5

10

0 1000 2000 3000

L

o

a

d

(

m

N)

Displacement (nm)

Elastic-plasticloading Elastic-

unloading

hold

Fig. 3. The load–displacement curve of epoxy-loaded microcapsule. The loading por-tion of the nanoindentation represents both elastic and plastic deformations.

(a)

(b)

1

2

3

M

o

d

u

l

u

s

(

G

P

a)

Particle size (µm)

0

0.1

0.2

50 100 150

50 100 150

H

a

r

d

n

e

s

s

(

G

P

a

)

Particle size (µm)

Fig. 5. Modulus (a) and hardness (b) as a function of hardener-loaded microcapsule

64 J. Lee et al. / Materials Letters 76 (2012) 62–65

under an indentation load of 3 mN [5], which is less stiff but harderthan the PMF shells. In Fig. 4, the load–displacement curve is non-linear upon loading and becomes elastic on unloading with 22.2%deformation recovery. The modulus and hardness of the hardener-loaded microcapsules are 2.83 GPa and 0.093 GPa. Hence, the epoxy-loaded PMF shell materials are stiffer than the hardener-loaded PMFshell materials. The PMF shells with different loading components in-side the microcapsules may have a significant effect on the microme-chanical properties of the microcapsules made of PMF shells. This maybe associated with the different physical properties of the shell withdifferent loaded-components inside the microcapsules under thesame puncture load or stress for the application of the capsules.Fig. 5 shows that the modulus and hardness values of hardener-loaded microcapsules increase with their size. The size has a signifi-cant effect on the modulus, however, only a slight effect on hardness.

size.

4. Summary

Nanoindentation has been used as an effective method to deter-mine the micromechanical properties of the microcapsules. The mi-crocapsules with a PMF shell behave like a viscoelastic plasticmaterial. The epoxy-loaded PMF shell material is stiffer but softerthan cured epoxy. It is also much stiffer than the hardener-loadedPMF shell material. The modulus and hardness of hardener-loadedmicrocapsules both increase with size.

0

5

10

0 1000 2000 3000

L

o

a

d

(

m

N)

Displacement (nm)

Elastic-plasticloading

Elastic-unloading

Hold

Fig. 4. The load–displacement curve of hardener-loaded microcapsule.

Acknowledgment

This work was supported by the Foundation for Research, Scienceand Technology for New Zealand Science & Technology Post-DoctoralFellowship. Thanks also go to the laboratory staff and Prof. StoykoFakirov at CACM, Dr. Michelle Dickinson, Department of Chemicaland Materials Engineering, The University of Auckland, New Zealand.JL was a Visiting Scholar to the CAMT, The University of Sydney, whenparts of this work were conducted.

References

[1] Huang G, Lu H. Measurements of two independent viscoelastic functions bynanoindentation. Experimental mechanics. Exp Mech 2007;47(1):87–98.

[2] Doerner MF, Nix WD. A method for interpreting the data from depth-sensing in-dentation instruments. J Mater Res 1986;1(4):601–9.

[3] Koch T, Evaristo M, Pauschitz A, Roy M, Cavaleiro A. Nanoindentation and nano-scratch behaviour of reactive sputtered deposited W–S–C film. Thin Solid Films2009;518(1):185–93.

[4] Oliver WC, Pharr GM. An improved technique for determining hardness and elas-tic modulus using load and displacement sensing indentation experiments.J Mater Res 1992;7:1564–83.

[5] Tehrani M, Safdari M, Al-Haik MS. Nanocharacterization of creep behavior of mul-tiwall carbon nanotubes/epoxy nanocomposite. Int J Plast 2010;27(6):887–901.

[6] Yuan YC, Yin T, Rong MZ, Zhang MQ. Self healing in polymers and polymer compos-ites. Concepts, realization and outlook: a review. eXPRESS Polym Lett 2008;2(4):238–50.

[7] Murphy EB, Wudl F. The world of smart healable materials. Prog Polym Sci2010;35(1–2):223–51.

[8] Wu DY, Meure S, Solomon D. Self-healing polymeric materials: a review of recentdevelopments. Prog Polym Sci 2008;33(5):479–522.

[9] Lee J, Bhattacharyya D, Zhang MQ, Yuan YC. Fracture behaviour of a self-healingmicrocapsule-loaded epoxy system. eXPRESS Polym Lett 2011;5(3):246–53.

65J. Lee et al. / Materials Letters 76 (2012) 62–65

[10] Yuan YC, Rong MZ, Zhang MQ, Chen J, Yang GC, Li XM. Self-healing polymeric ma-terials using epoxy/mercaptan as the healant. Macromolecules 2008;41(14):5197–202.

[11] Liu X, Sheng X, Lee JK, Kessler MR. Synthesis and characterization of melamine-urea-formaldehyde microcapsules containing ENB-based self-healing agents.Macromol Mater Eng 2009;294(6–7):389–95.

[12] Nesterova T, Dam-Johansen K, Kiil S. Synthesis of durable microcapsules for self-healing anticorrosive coatings: a comparison of selected methods. Prog Org Coat2010;70(4):342–52.

[13] Yuan YC, Rong MZ, Zhang MQ. Preparation and characterization of poly(melamine-formaldehyde) walled microcapsules containing epoxy. Acta PolymSin 2008;5:472–80.

[14] Yuan YC, Rong MZ, Zhang MQ. Preparation and characterization of microencapsu-lated polythiol. Polymer 2008;49:2531–41.

[15] Flores A, Baltá Calleja FJ. Mechanical properties of poly(ethylene terephthalate)at the near surface from depth-sensing experiments. Philos Mag A 1998;78(6):1283–97.

[16] Gao F, Qian Y. Micromechanical properties of heterogeneous aluminium–siliconbrazed joint. Mater Lett 2004;58(22–23):2861–6.

[17] Seltzer R, Mai Y-W. Depth sensing indentation of linear viscoelastic-plastic solids:a simple method to determine creep compliance. Eng Fract Mech 2008;75(17):4852–62.

[18] http://nanoindentation.co.uk/liquidcell.asp.