Materials Sciences and Applicatio n, 2011, 2, 1007-1013
doi:10.4236/msa.2011.28136 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
Preparation and Characterization of
Poly(divinylbenzene) Microcapsules Containing
Preeyaporn Chaiyasat*, Am or n Chaiyasat**, Waraporn Boontung, Supaporn Promdsorn,
Sutanya Thipsit
Department of Chemistry, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Thanyaburi,
Email: *; **
Received March 17th, 2011; revised April 6th, 2011; accepted April 12th, 2011
Poly (divinylbenzene) (PDVB) microcapsules containing octadecane (OD) (PDVB/OD) used as heat storage material
were synthesized by suspension polymerization at 70˚C using benzoyl peroxide and polyvinyl alcohol as initiator and
stabilizer, respectively. Thermal properties and stability of PDVB/OD microcapsules were determined using differential
scanning calorimeter (DSC) and thermogravimetric analyzer. The morphology and structure of microcapsules were
characterized by optical microscope, scanning electron microscope and fourier transform infrared spectrophotometer.
From DSC analysis, the melting temperature of encapsulated OD (28˚C) was almost the same as that of bulk OD (30˚C)
while it was quite different in the case o f the solidification temperature (19˚C and 25˚C for encapsulated and bulk OD,
respectively). The latent heats of melting (184.0 J/g-OD) and solidification (183.2 J/g-OD) of encapsulated OD were
reduced from those of bulk OD (241.7 and 247.0 J/g, respectively). However, the prepared PDVB/OD microcapsules
are able to be used for heat storage applications.
Keywords: Microencapsulation, Microcapsule, Heat Storage Material, Octadecane, Suspension Polymerization, Poly
1. Introduction
The utilization of a latent heat storage system using heat
storage materials or phase change materials (PCMs) is an
effective method of advanced energy technologies, pre-
serving available energy and improving its consumption.
The heat storage materials are able to change the status
with a convinced temperature range. They can absorb
and emit the energy when the temperatures of the materi-
als overpass the temperature of phase change during
heating and cooling processes, respectively [1-10]. The
thermal energy storage systems typically select the ap-
plication temperatures approach phase transition tem-
peratures of PCMs. Paraffin waxes (the linear chain hy-
drocarbons or n-alkanes) such as tetradecane (TD), hexa-
decane (HD), octadecane (OD), nonadecane (ND), and
eicosane are useful as one group of numerous numbers of
heat storage materials that melt and solidify at a wide
range of temperatures, making them attractive for many
applications [11]. These PCMs are nontoxic, non -corrosi-
ve, chemically inert, easily obtained, and have no un-
pleasant odor. They have different phase change tem-
peratures, melting (Tm) and solid ification (Ts), depending
on the number of carbons in their structures [9,12]. They
have moderate thermal energy storage but low thermal
conductivity. Therefore, they require large surface areas
in applications. Encapsulation of these materials is asso-
ciated with many advantages such as provide large heat
transfer area, control the volume change of the storage
materials as phase change occurs and protect the PCM
against the influences of the outside environment [2,6,10,
13]. Therefore, it is more practical than the traditional
PCMs in many applications such as thermal insulation
[7], heat transfer [14], building materials [15] and the
wall energy storage capacity [16]. A numerous number
of studies have been carried out on micro and nanoen-
capsulation of PCMs using different polymers as the
capsule shell materials. The in situ polymerization to
fabricate the microcapsules and nanocapsules containing
Preparation and Characterization of Poly(divinylbenzene) Microcapsules Containing Octadecane
OD core with melamine-formaldehyde shell [17], resor-
cinol-modified melamine-formaldehyde shell [18] were
reported. The encapsulation of n-TD core with urea-for-
maldehyde polymer shell were also carried out by in situ
polymerization [19]. Three types of paraffin waxes ( n-HD,
n-OD and n-ND) were encapsulated through complex
coacervation of natural and biodegradable polymers, gum
arabic-gelatin mixture [20]. Paraffin waxes encapsulated
with styrene and methyl methacrylate copolymer were
prepared by suspension polymerization [21]. The prepa-
ration of polystyrene (PS) microcapsules containing
n-HD using casein instead of commercial stabilizer to
emulsify the mixture of core materials, monomers and
initiators during the suspen sion polymerization were also
reported [22]. For textile applications, melamine formal-
dehyde microcapsules containing eicosane were prepared
by in situ polymerization [23,24] and PS microcapsules
containing paraffin wax were synthesized by suspension
like polymerization [25]. Po lymethylmethacrylate micro-
capsules containing n-octaco-sane [26] and heptadecane
[27] were prepared by emulsion polymerization. The
capsules of PS as the shell and n-OD as the core were
prepared by miniemulsion polymerization [28]. Micro-
encapsulated n-OD with polyurea shells having different
soft segments was synthesized with interfacial polycon-
densation [29]. To improve the shell strength, crosslinked
polydivinylbenzene (PDVB) microcapsule particles with
encapsulated n-HD prepared by micro-suspension po-
lymerization utilizing the Shirasu Porous Glass membrane
emulsification technique for the preparation of compara-
tively monodisperse DVB/HD droplets were prepared
[30-32]. OD is an attractive PCM giving good capability
of heat absorption and emission in appropriate phase
change temperature range (23 - 28˚C) comfortable for the
human body. Moreover, its latent heat (241.2 J/g) is
higher than that of the other PCMS with similar phase
change temperature range [33]. As seen from the litera-
tures above, a numerous of different polymers were used
to prepare microencapsulated PCMs but there is no any
report study on microencapsulation of OD with PDVB
(the high mechanical properties polymer) shell
(PDVB/OD capsules). Therefore, in this work, OD and
PDVB were chosen to use as heat storage material and
particle shell, respectively, of the microcapsules prepared
by suspension polymerization. The morphology, chemi-
cal structure, thermal stability and thermal properties of
PDVB/OD capsules were characterized by scanning
electron microscope (SEM), optical microscope (OM),
fourier transform infrared spectrophotometer (FT-IR),
thermogravimetric analyzer (TGA) and differential scan-
ning calorimeter (DSC), respectively.
2. Experimental
2.1. Materials
DVB (Aldrich; purity, 80%) was washed with 1N sodium
hydroxide (NaOH) and distilled water to remove polym-
erization inhibitors before use. Poly (vinyl alcohol) (PVA)
(Aldrich; degree of saponification, 87 - 90%) was used as
received. Reagent-grade benzoyl peroxide (BPO) was
purified by recrystallization. OD (Merck; 99.5%) was
used as received.
2.2. M icroc ap sules Pr epar at i on
The microcapsules of PDVB/OD were prepared by sus-
pension polymerization as the procedure shown in Fig-
ure 1. The homogeneous organic phase of DVB and OD
at the ratio of 50:50% wt/wt (approximately 10% wt of
aqueous solution) were mixed with BPO (8% wt of mo-
nomer) and then added to the aqueous phase containing
PVA (1.5 g of PVA in 150 g of water). Emulsification
was carried out by high shear rate at the speed of 5,000
rpm for 5 min resulting in the organic phase drop
lets dispersed in the aqueous phase. The resulting emul-
sions were transferred to the rea ctor and poly meriz ed at
Figure 1. Schematic of the preparation process of PDVB/OD capsules with a suspension polymerization.
Copyright © 2011 SciRes. MSA
Preparation and Characterization of Poly(divinylbenzene) Microcapsules Containing Octadecane 1009
70˚C for 24 h under N2 atmosphere.
2.3. Characterization of Microcapsules
The particle diameters of microcapsules were obtained
from OM (SK-100EB & SK-100ET, Seek, Seek Inter
Corporation Ltd., Thailand) photos calculated from 200
particles by VIS Plus software. An OM and SEM
(JSM-6510, JEOL, JEOL Ltd., Japan) were used to in-
vestigate the morphology of the surface and the inner
structure of the microcapsules. For SEM observation, one
drop of the microcapsule dispersion was placed on a
nickel SEM stub and dried before Au-coated. Thermal
stability of OD, PDVB and microcapsules was studied
with TGA (TGA/SDTA 851e, Mettler-Toledo, Met-
tler-Toledo Ltd., Switzerland) using heating rate of
20˚C/min. The chemical structure of the polymer was
determined by FT-IR spectroscopy (PerkinElmer System
2000, PerkinElmer, USA). The dispersion sample was
filtered and then kept in vacuum oven overnight. The
dried sample was ground with dried potassium bromide
(KBr) powder and compressed into a disc. The pure OD
was directly m i xe d wi th KBr before measuremen t .
2.4. Thermal Properties Measurement
The latent heats of solidification (Hs), melting (Hm), Ts,
and Tm of OD encapsulated in microcapsule particles in
aqueous solution (solid content: ca 10%) were measured
in an aluminum pan with a DSC (DSC 822, Metler-Tole-
do, Mettler-Toledo Ltd., Switzerland) under a N2 flow
with the scanning temperature range and rate of 0 - 40˚C
and 5˚C/min, respectively. To compare Hs and Hm of the
encapsulated OD having different wt% in the capsule
particles and also pure OD, the Hs and Hm values were
used in the unit of joule per 1 g of encapsulated OD
(J/g-OD). They were calculated from the cooling/h eating
peak area of DSC thermogram and OD content obtained
from TGA analysis using the following equ ation (1).
J/g-OD = (A/B) × 100 (1)
A = Hs or Hm of encapsulated OD in microcapsule dis-
persion obtained from DSC thermogram (J/g-sample) B
= % OD in microcapsule dispersion obtained from TGA
3. Results and Discussion
3.1. Microcapsule Formation
The preparation process of the PDVB capsules contain-
ing OD as heat storage material by suspension polymeri-
zation was shown in Figure 1. Aqueous phase containing
PVA as stabilizer was added with oil phase consisting of
OD, DVB and BPO. PVA molecules strongly adsorb
onto the oil-water interface and stabilize the oil droplets
generated by high shear rate. The polymerization took
place within the oil droplets after the temperature
reached to 70˚C. OD is well miscible with DVB mono-
mer in the early stage of polymerization resulting in ho-
mogeneous phase in monomer droplets. When the PDVB
was formed, the oil phase became gradually less com-
patible until reach the critical chain length of PDVB,
phase separation occurred. Because the interfacial ten-
sion of PDVB-water is lower than that of OD-water, the
PDVB move to the interface of oil droplets to form the
polymer shell. At the end of polymerization, the polymer
capsules of OD core encapsulated by PDVB shell were
obtained. When the dispersions were kept for a while, all
of the PDVB/OD capsules floated on the top of the dis-
persion solution because the total density of the capsules
is lower than that of water. This indicates that the
PDVB/OD capsules were successfully prepared.
3.2. Morphology of Microcapsule and
Particle Size
The PDVB/OD microcapsules were observed by OM and
SEM as shown in Figure 2. The SEM photograph indi-
cates that most of the particle shapes were spherical with
smooth outer surface even the microcapsule sizes were
polydisperse consistent with the result from the optical
micrograph. This is general characteristic of the polymer
particles obtained from conventional suspension polym-
erization. Moreover, the OD core was completely en-
capsulated with PDVB shell as shown in the optical mi-
crograph. The average diameter of the microcapsules is
approximately 28 µm, measured from the optical micro-
graphs calculated from 200 particles by VIS plus 3.0
software. Because quite small microcapsules were ob-
tained, PDVB/OD capsules therefore afford a large sur-
face area per unit volume. This is the reason why the
microcapsules containing the heat storage material are
more attractive than the pure one.
3.3. Chemical Structure Characterization of
FT-IR spectra of OD, PDVB and PDVB/OD microcap-
sules were shown in Figure 3. In the case of pure OD
(Figure 3(a)), the characteristic absorption peak at 720
cm–1 corresponds to the in-plane rocking vibration of the
methylene group. The absorption peak at 1460 and 1360
cm–1 are associated with C-H stretching vibrations of me-
thylene bridges. The alkyl C-H stretching vibrations of
methyl and methylene groups are observed at 2920 and
2850 cm–1. In the case of PDVB spectrum (Figure
3(b)), the characteristic absorption peaks at 700 - 750 cm –1
are benzene ring deformation vibration. The absorption
Copyright © 2011 SciRes. MSA
Preparation and Characterization of Poly(divinylbenzene) Microcapsules Containing Octadecane
Figure 2. SEM photo (a) and optical micrograph (b) of PDVB/OD prepared by suspension polymerization.
Figure 3. FT-IR spectra of OD (a), PDVB (b) and PDVB/OD capsule (c).
peaks at 1500, 1600 cm–1 are associated with benzene
ring C = C stretching vibration. The aliphatic C-H stret-
ching vibration is observed at 2920 cm–1 whereas 3030
cm–1 showed the characteristic peaks of aromatic C-H
stretching vibration. All the above characteristic peaks of
bulk OD and PDVB are able to observe in the spectrum
of the PDVB/OD microcapsules (Figure 3(c)), and no
new peak is observed. This result indicates that the OD
was incorporated in the PDVB.
3.4. Thermal Stability of PDVB/OD Capsule
The TGA thermograms of bulk OD, PDVB and
PDVB/OD capsule were show n in Figure 4 and Table 1.
The decomposition of OD started at approximately
165˚C and completely lost its weight at 305˚C while the
decomposition temperature range of PDVB is 330˚C to
495˚C. The TGA curves of OD and PDVB were sharp
and showed only one step. In the case of PDVB/OD
capsules TGA curve, it was mainly consisted of three
steps: 50 - 124, 180 - 295 and 350 - 480 ˚C corresp onding
to the decomposition of water, OD and PDVB respec-
tively. In comparison with OD, the decomposition tem-
Figure 4. TGA thermograms of (a) PDVB; (b) OD and (c)
PDVB/OD microcapsule.
perature of encapsulated OD was slightly higher than that
of bulk OD due to the encapsulation.
3.5. Thermal Properties of Microcapsule
The thermal properties of the OD encapsulated in PDVB
microcapsules were measured using DSC. The Hm (184.0
J/g-OD) and Hs (183.2 J/g-OD) of encapsulated OD were
lower than those of the bulk OD (241.0 and 247.0 J/g of
Copyright © 2011 SciRes. MSA
Preparation and Characterization of Poly(divinylbenzene) Microcapsules Containing Octadecane1011
Table 1. TGA data of OD, PDVB and PDVB/OD micro-
Degradation interval (˚C) Weight loss (%)
OD 162-305 100
PDVB 330-495 92
PDVB/OD 50 - 124 (1st step) 76
180 - 295 (2nd step) 12
350 - 480 (3rd step) 10
Hm and Hs, respectively) as shown in Figure 5. This
seems to be general case for PCMs microcapsules con-
sistent with the other report s [17,18,22,26, 27,29-32, 34,35] .
The possible reason of the reduction of Hm and Hs of en-
capsulated PCMs is that the phase separation b etween the
polymer shell and PCM core was incomplete as in the
case of PDVB/HD microcapsules prepared by suspension
polymerization [32]. The capsules may have some unex-
hausted monomers or oligomer incorporated at the inter-
face between polymer shell and PCM core as in the case
of PS/HD microcapsules prepared by suspension polym-
erization [22]. They may act as the compatibilizer and
help the miscibility between polymer and PCM material
leading to the decreasing of the phase separation in oil
phase. However, to overcome this problem, the copoly-
merization with more hydrophilic monomers is the good
idea as copolymerization of PDVB with methyl acrylate,
ethyl acrylate and butyl acrylate [32]. In the case of
phase transition temperature, Tm of encapsulated OD
(28.3˚C) was almost the same as that of bulk OD (30˚C).
In contrast, Ts was shifted to the lower temperature com-
paring to bulk OD. This occurrence is namely super-
cooling. Supercooling leads to the reduction of the Ts
resulting in the releasing of latent heat at a lower tem-
perature or a wider temperature range. This effect may
limits to the applications. To prevent supercooling, the
nucleating agents were incorporated in the PCMs core.
However, increasing nucleating agent content decreased
the latent heat [36,37]. At the present, there is still no
good solution to overcome this problem. As the total
surface area of the microcapsules increased comparing to
bulk OD, these PDVB/OD capsules are quite acceptable
for energy storage applications even having a slightly
lower latent heats.
4. Conclusions
PDVB/OD microcapsules were successfully prepared by
suspension polymerization. The optical micrographs sho-
wed that OD had been completely encapsulated inside
the PDVB microcapsules with average size of 28 µm.
The regular spherical particles with smooth outer surface
were observed by SEM. All the characteristic peaks of
OD and PDVB were observed in the FT-IR spectrum of
Figure 5. DSC thermograms of encapsulated OD in
PDVB/OD capsule (solid line) and bulk OD (dotted line)
(scanning rate 5˚C/min).
the PDVB/OD capsules without any new peaks. The la-
tent heats, Hm and Hs, of OD in the cap sule were slightly
lower than those in bulk OD. As a result, the prepared
PDVB/OD microcapsules are able to be used for heat
storage applications because of their large surface area
and still remain good thermal properties of encapsulated
5. Acknowledgements
This work was supported by The National Research
Council, Thailand (No. 23666).
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