Preparation and properties of a novel form-stable phase change material based on a gelator

Article abstract of DOI:10.1039/c4ta06508e

A series of gelators (Gm, m is the length of the alkyl tails, m = 2, 4, 6, 8, 10, 12, 14, 16 and 18) containing 4,4′-diaminodiphenylmethane moieties were synthesized. The chemical structures of Gm were confirmed by ¹H NMR and MS. The form-stable phase change materials (FSPCMs) were prepared by introducing Gm into paraffin. The minimum gelation concentration (MGC) and gel-to-sol transition temperature (T_GS) properties were tested by the "tube-testing method". It found that Gm (m = 2, 4, 6) was insoluble in paraffin, while the MGC and T_GS of Gm (m = 8, 10, 12, 14, 16, 18) increased with the increase of alkyl chain. The structure and morphology of the PCMs were systematically investigated by FT-IR, POM, 1D WXAD and SEM. Experimental results revealed that paraffin was restricted because the gelators could self-assemble into three-dimensional netted structures, leading to form the shape-stable PCMs without leakage even above their melting point. The thermal properties were studied by DSC. The research showed that the G18/paraffin FSPCMs exhibited excellent thermal stability and high heat storage density. The shape stability of G18/paraffin was investigated by rheological measurements, indicating that solid hard gel soft gel liquid was observed with the increase of temperature. This work is useful in the comprehensive academic research and industrial application of PCMs.

Full text of DOI:10.1039/c4ta06508e

Journal of
Materials Chemistry A
View Article Online
View Journal
PAPER
Preparation and properties of a novel form-stable
phase change material based on a gelator†
Cite this: DOI: 10.1039/c4ta06508e
*
*
Dang Wu, Wen Wen, Sheng Chen and Hailiang Zhang
A series of gelators (Gm, m is the length of the alkyl tails, m ¼ 2, 4, 6, 8, 10, 12, 14, 16 and 18) containing 4,40-
diaminodiphenylmethane moieties were synthesized. The chemical structures of Gm were confirmed by 1H
NMR and MS. The form-stable phase change materials (FSPCMs) were prepared by introducing Gm into
paraffin. The minimum gelation concentration (MGC) and gel-to-sol transition temperature (TGS
)
properties were tested by the “tube-testing method”. It found that Gm (m ¼ 2, 4, 6) was insoluble in
paraffin, while the MGC and TGS of Gm (m ¼ 8, 10, 12, 14, 16, 18) increased with the increase of alkyl
chain. The structure and morphology of the PCMs were systematically investigated by FT-IR, POM, 1D
WXAD and SEM. Experimental results revealed that paraffin was restricted because the gelators could
self-assemble into three-dimensional netted structures, leading to form the shape-stable PCMs without
leakage even above their melting point. The thermal properties were studied by DSC. The research
showed that the G18/paraffin FSPCMs exhibited excellent thermal stability and high heat storage density.
The shape stability of G18/paraffin was investigated by rheological measurements, indicating that solid
4 hard gel 4 soft gel 4 liquid was observed with the increase of temperature. This work is useful in
the comprehensive academic research and industrial application of PCMs.
Received 27th November 2014
Accepted 8th December 2014
DOI: 10.1039/c4ta06508e
www.rsc.org/MaterialsA
during the solid–solid phase transitions. However, the small
latent heats and super-cooling limit their application.9–11 To the
Introduction
best of our knowledge, numerous solid–liquid PCMs has been
studied for decades, such as, fatty acid, fatty alcohol, paraffin
wax, polyethylene glycol (PEG), due to their superior properties,
which have proper melting temperature ranges, high heat
storage capacity, good thermal stability, little or no super-
cooling, low vapor pressure, no or less volume change during
solid–liquid phase transition, non-toxicity, self-nucleating
behaviour, good thermal and chemical stability aer long-term
utility periods, low cost, commercially produced and so on.12–34
However, solid–liquid PCMs have to be placed in specially
designed devices/containers during application to prevent
leakage in their solid–liquid phase transition process, which
results in extra thermal resistance and cost.35–37
In order to solve the above problems, a wide class of form-
stable phase change materials (FSPCMs) which are composite
materials that contain a solid–liquid PCM (thermal energy
storage material) and supporting material (maintains the solid
shape of the FSPCMs) has aroused wide interest from
researchers in recent years.38 The main research of FSPCMs
focuses on nding efficient supporting materials which can
keep the composite material solid and stable without liquid
leakage even above the melting temperature of the PCM. There
are several methods to prepare FSPCMs: (1) encapsulating
solid–liquid PCMs into a polymeric structure by blending or situ
polymerization. There are many supporting polymers to be
used, such as polymethyl methacrylate (PMMA),39–41 high-
Thermal energy storage applications have gained a great
amount of attention in recent years because of the energy crisis,
the rising demand of energy consumption, high cost of fossil
fuels and the accompanied environmental problems. Thermal
energy storage as an effective use of thermal energy has been
applied in diverse areas, such as building heating/cooling
systems, solar energy collectors, and power and industrial waste
heat recovery.1–3
Phase change materials (PCMs) can store and release large
amounts of energy during phase change processes, the appli-
cation of which has been investigated as a prospective tech-
nique for storing thermal energy in recent decades.4–7 PCMs can
be classied into two major categories: inorganic compounds
and organic compounds. Inorganic PCMs include salt hydrates,
salts, metals and alloys, whereas organic PCMs are comprised of
paraffin, fatty acids/esters and polyalcohols.8 According to
phase change patterns, practically applied PCMs can be classi-
ed into solid–solid PCMs and solid–liquid PCMs. For solid–
solid PCMs, the advantages are that there is no liquid or gas
generated, so accordingly no recipient is needed to seal them
Key Laboratory of Polymeric Materials and Application Technology of Hunan Province,
Key Laboratory of Advanced Functional Polymer Materials of Colleges, Universities of
Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan
Province, China. E-mail: zhl1965@xtu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta06508e
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
density polyethylene (HDPE),42,43 low density polyethylene for supporting typical solid–liquid PCMs, this work widened the
(LDPE),44 polypropylene,45 etc. (2) Absorbing solid–liquid PCMs range of applications of gelators as well as offered a promising
into porous materials like expanded perlite,46,47 vermiculite,48 FSPCM with high thermal storage density and thermal
diatomite,49 expanded graphite.50 (3) Graing solid–liquid reversibility.
PCMs onto the skeleton of high melting temperature poly-
mers.51–53 (4) Microencapsulating solid–liquid PCMs with
Experimental
Materials
different shells.54–56 (5) Encapsulating solid–liquid PCMs into
the SiO2 net structure.57–59 Although these methods for FSPCMs
have successfully prevented the leakage of solid–liquid PCMs,
the application range is still restricted due to some drawbacks:
(1) the latent heat is reduced by introducing of large amount of
supporting materials. (2) Some of FSPCMs cannot be processed
repeatedly once prepared. (3) The complex process of prepara-
tion will increase cost.
Paraffin was provided by Shanghai Huashen Recover Equip-
ment Co., Ltd. Gelators (Gm, m ¼ 2, 4, 6, 8, 10, 12, 14, 16 and 18)
were synthesized according to ref. 63. The composite PCMs
were prepared by mixing paraffin and Gm, heating to the
materials reached solution states, followed by cooling to room
temperature. All other compounds were analytical grade and
were used without any further treatment.
When analyzing the shortcomings of the FSPCMs mentioned
above, it's obvious that the introduction of supporting materials
will lead to the reduction of the energy storage density, so we are
interested in nding a new kind of supporting material, which
can the shape of the PCMs stable at low concentrations, high
latent heat and low cost.
Synthesis of gelators
According to the literature,63 the gelators (Gm, m ¼ 2, 4, 6, 8, 10,
12, 14, 16 and 18) containing 4,40-diaminodiphenylmethane
moieties were easily synthesized (see the ESI†). The synthetic
routes of Gm are shown in Scheme 1.
Low molecular mass organic gelators (LMOGs) are organic
molecules capable of immobilizing organic or aqueous solvents
when a solution containing a small amount of LMOG is cooled
below its gelation temperature.60 The resultant low-molecular-
weight gels (LMWGs) present the viscoelastic solid-like state
because the organic or aqueous solvents were restricted by
complex three-dimensional networks self-assembled by
LMOGs. Accordingly, the organic or aqueous solvents can be
replaced by solid–liquid PCMs, and the three-dimensional
networks self-assembled by LMOGs are assumed to support the
solid–liquid PCMs as they undergo solid–liquid transition.
Recently, Zhang and Tian have researched the properties of
gelatinous shape-stabilized PCMs by impregnating 1,3:2,4-di-(4-
methyl) benzylidene sorbitol (MDBS) or 1,3:2,4-di-(3,4-dimethyl)
benzylidene sorbitol (DMDBS) into paraffin and 1-tetradeca-
nol,61,62 which may open a new way to prepare form-stable
PCMs.
Preparation of Gm/PCMs
The mixture of gelator and paraffin were sealed in a test tube,
then heated at 160 ꢀC until a transparent solution was formed.
For example, the preparation of 2 wt% G18/paraffin is described
as follows: 0.02 g (0.113 mmol) G18 was added to 0.98 g paraffin
in a test tube to form a suspension. Aer being heated in an oil
bath at about 160 ꢀC, the mixture gradually turned into a
transparent solution. Then, the tube was cooled to room
temperature (25 ꢀC) until a white gel was formed (see Fig. 3). All
the other gels were prepared via a similar method.
Instruments and measurements
1H NMR measurements. All NMR measurements were per-
In this paper, we synthesized a series of gelators (Gm, m is formed on a Bruker ARX400 MHz spectrometer using CDCl3 as
the length of alkyl tails, m ¼ 2, 4, 6, 8, 10, 12, 14, 16 and 18) the solvent and tetramethylsilane (TMS) as the internal stan-
based on 4,40-diaminodiphenyl-methane with amide moieties, dard at ambient temperature. The chemical shis were reported
which could gel organic solvents with low concentration (typi- on the ppm scale.
cally <2 wt%) through hydrogen-bonding and p–p stacking
Minimum gelation concentration (MGC) and gel-to-sol
interactions.63 The chemical structures of Gm are shown in transition temperature (TGS) measurements. The MGC and TGS
Chart 1. Paraffin was used as the latent heat material due to its properties were tested by a “tube-testing method”:64 a weighted
high heat of fusion, proper range of phase change tempera- gelator was mixed with paraffin in a test tube (the samples were
tures, chemical resistance, commercial availability, and low weighed in at 1 g in total) with a screw cap (inside diameter was
cost. The gelator/paraffin composites (Gm/paraffin) were 14 mm) and heated until the gelator was dissolved. The
prepared and the thermal properties and thermal stability were resulting solution was cooled at 25 ꢀC for 2 h, and then put into
investigated. Since there were few reports about using gelators an oil-bath whose temperature was above the melting temper-
ature of solid–liquid PCMs. When upon inversion of the test
tube no uid ran down the walls of the tube, we judged it as a
“successful gelation”. The minimum concentration of gelator
necessary for gelation is the MGC, the unit of MGC is wt%. The
temperature that the gel turned into a sol is TGS
.
Fourier transform infrared (FT-IR) analysis. Infrared spec-
trogram of each specimen was obtained in transmittance mode
using a Nicolet Nexus 6700 FT-IR spectrophotometer with a 1
Chart 1 Chemical structures of Gm.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
Journal of Materials Chemistry A
Scheme 1 Synthesis route of Gm.
temperature,63 and then the specimens were examined under a
JEOL JSM-6610 scanning election microscope.
Differential scanning calorimetry (DSC) measurement. DSC
traces of the polymer were obtained using a TA Q10 DSC
instrument. The temperature and heat ow were calibrated
using standard materials (indium and zinc) at cooling and
heating rates of 5 ꢀC minꢁ1. The sample with a typical mass of
about 5 mg was encapsulated in sealed aluminum pans.
Rheology measurement. A Rheometrics ARES rheometer (TA
ARES rheometer) was applied to measure the viscoelastic
properties of the samples based on oscillatory shear and
temperature. The experimental temperature was controlled
using forced N2 gas convection. Dynamic temperature ramp test
were performed in the region of 30 ꢀC to 160 ꢀC at 2 ꢀC minꢁ1
,
using a 25 mm diameter parallel-plate geometry with a
frequency of 1 rad sꢁ1 and a small strain amplitude. Isothermal
frequency sweeps were made using the same setup at frequen-
cies from 0.1 to 100 rad sꢁ1
.
Fig. 1 1H NMR spectrum of G18 in CDCl3.
Results and discussion
Synthesis and characterization of gelators and composite Gm/
PCMs
cm resolution in 32 scans collect. The samples were scanned in
the range of 500–4000 cmꢁ1 using KBr pellets.
Gm were synthesized according to the method discussed in ref.
63. The 1H NMR spectra of G18 is shown in Fig. 1. The MGC and
TGS test results of Gm/paraffin are shown in Fig. 2. It can be
found that, Gm (m ¼ 2, 4, 6) were insoluble in paraffin, while
paraffin can be gelated when introducing Gm (m ¼ 8, 10, 12, 14,
16, 18) at a low mass fraction (below 3 wt%), and the MGC of
Gm/paraffin decreased with the increase of the alkyl chain of
the gelators (see Fig. 2a). Moreover, G18 can even gelate paraffin
at a concentration of 0.5 wt%. This may result from the sol-
vophilic interaction of long alkyl groups to increase the inter-
action between gelator and paraffin. Meanwhile, the TGS of Gm/
paraffin at a certain mass fraction (3 wt%) decreased with the
increase of the alkyl chain of the gelators (see Fig. 2b). This can
partly be explained by the difference in melting points of the
Polarized optical microscope (POM) observation. Polarized
optical microscope observation was carried out using a Leica
DMLM-P microscope equipped with a Mettler FP82 hot stage
and a digital camera. Photographs were taken by a digital
camera at short intervals of time.
Wide angle X-ray diffraction (WAXD) analysis. One-dimen-
sional wide-angle X-ray diffraction (1D WAXD) experiments
were performed on a BRUKER AXS D8 Advance diffractometer
with 40 kV FL tubes as the X-ray source (Cu Ka) and a LYN-
XEYE_XE detector. Background scattering was recorded and
subtracted from the sample patterns.
Scanning election microscopy (SEM) observation. To inves-
tigate the nature of microstructures and morphologies, a SEM
sample was prepared by immersing the PCMs gels in petroleum
ether to extract the PCMs for 5 h followed by drying at room
pure Gm according to the Schroder–van Laar relation,65 which is
¨
that the TGS is related to the melting point of the pure gelator.66
It's worth mentioning that, the gelation process is reversible
(see Fig. 3). Compared to Gm/paraffin (m ¼ 8, 10, 12, 14, 16),
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 2 (a) MGC of Gm in paraffin; (b) TGS of 3 wt% Gm/paraffin by a tube-testing method.
Fig. 3 Photographs of gelation result of 3 wt% G18/paraffin FSPCM at
different temperature.
Fig. 4 IR spectra of (a) neat G18, (b) paraffin, (c) 3 wt% G18/paraffin.
about 3300 and 1659 cmꢁ1, respectively. In the spectrum of
paraffin (Fig. 4b) the peak at 2917 cmꢁ1 signies the symmet-
rical stretching vibration of its –CH3 group and the peak at 2849
cmꢁ1 represents the symmetrical stretching vibration of its
–CH2 group. The peaks at around 1463 cmꢁ1 belong to the
deformation vibration of –CH2 and –CH3, and the peak at 719
cmꢁ1 represents the rocking vibration of –CH2. In the spectrum
of composite G18/paraffin (Fig. 4c), the same peaks as G18 and
paraffin still existed, and no signicant new peak was observed,
indicating that the composite G18/paraffin was only a physical
interaction between paraffin and G18 and there was no chem-
ical reaction occurring between them.
G18/paraffin showed better compatibility, lower MGC and
excellent thermal stability, Thus, it is necessary to discuss G18/
paraffin in detail as the example of the whole paper.
Structure characterization of the form-stable Gm/PCMs
The FT-IR spectra of the neat G18, paraffin, 3 wt% G18/paraffin
are shown in Fig. 4. It was observed that IR peaks of H–N and
C]O moieties appeared at about 3300 and 1659 cmꢁ1
,
respectively. Similar spectral positions were observed both in
neat powders of G18 and G18/paraffin composite as shown in
Fig. 4. It could be inferred that hydrogen bonding was similar in
the two phases. However, the H–N stretching vibration band in
the gel state was broader than that of neat state for the disorder
in the gel brils. In the spectrum of G18 (Fig. 4a), it was
observed that IR peaks of H–N and C]O moieties appeared at
Fig. 5 shows the polarizing optical microscope (POM)
micrographs of the PCMs. As can been seen from Fig. 5a and b,
no birefringence phenomenon was observed when the paraffin
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 5 POM images (ꢂ200) of paraffin at 30 ꢀC (a) and 70 ꢀC (b); 3 wt% G18/paraffin at 30 ꢀC (c) and 70 ꢀC (d).
melted. Contrary to the neat paraffin, the G18/paraffin presents increase of concentration of G18. However, the peaks of G18
the birefringence phenomenon above the melting point of were covered by paraffin. The results suggested that the crystal
paraffin (Tm), as can be seen from Fig. 5c and d, showing that structure of the G18/paraffin composite was not destroyed by
the G18 formed an ordered structure in the solid–liquid PCMs. G18. As shown in Fig. 6b, the sharp peaks at 21.3ꢀ and 23.7ꢀ
Unlike the crystallization of neat G18 (Fig. S1†), a number of disappeared when heating to 60 ꢀC, showing that the melting of
crossed ber-like aggregations were observed in the G18/ paraffin in the G18/paraffin composite. The peaks at 20.8ꢀ and
paraffin composite. Although the paraffin component melted 21.3ꢀ didn't disappear until the temperature reached 150 ꢀC,
totally above the Tm, no obvious ow of PCMs was observed showing that G18 was dispersed in paraffin with an ordered
from POM micrograph, and the birefringence phenomenon structure, lead to the good supporting behavior of G18/paraffin.
ꢀ
remained until the temperature get to 150 C, suggesting that The results above were consistent with the POM results. Dif-
the support of the G18 gel network restricted the ow of melted fractograms of G18 in neat powder form and xerogel by
paraffin, resulting in form-stable composite PCMs with good extracting the paraffin in low angles are shown in Fig. 6c. The
thermal stability. Moreover, there was no difference in crystal XRD pattern of the xerogel revealed that a similar structure was
structure between neat paraffin and G18/paraffin (Fig.5a and c), adopted by the aggregates of the gelators in the gel state. The
indicating that G18 has little effect on crystal structure of PCMs. obtained long Bragg distances (d) of the xerogel were 4.88, 1.60,
The results were further supported by 1D WAXD. The 1D 1.19 and 0.08 nm (1, 1/3, 1/4, 1/6), respectively, which suggest-
WAXD patterns of paraffin, G18 and different mass fractions of ing that G18 self-assembles into a lamellar structure,67,68 the
the G18/paraffin composite PCMs are presented in Fig. 6a. In lamellar repeat was evaluated as 4.88 nm. The calculated van
the high 2q region of 15–25ꢀ, the two peaks appeared at 21.3ꢀ der Waals lengths (5.18 nm) of the fully extended G18 molecules
and 23.7ꢀ for paraffin and at 20.8ꢀ and 21.3ꢀ for G18, indicating calculated from the Materials Studio 6.0 (Accelrys, Inc.) model
the formation of crystal. The 1D WAXD patterns of the were slightly longer than the Bragg distances (d), which sug-
composite PCMs contain all the diffraction peaks of paraffin, gested that their long chains must be tilted with lamellar planes
whereas the peak intensities were relatively reduced with the or interdigitated with each other.69
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 6 1D WAXD patterns of (a) paraffin, G18 and G18/paraffin at room temperature; (b) 10 wt% G18/paraffin during the heating process; (c)
diffractograms of G18 in neat powder and xerogel by extracting the paraffin.
Fig. 7a and b show the SEM images of pure paraffin and the were also observed. The results indicating that Gm (m ¼ 8, 10,
G18/paraffin composite, respectively. Unlike the smooth and 12, 14, 16) self-assembled into a similar network to G18 in
uniform surface of paraffin in Fig. 7a, the dark and light regions paraffin. The dense entanglement of ribbons traps the solid–
were observed in G18/paraffin which represent the paraffin and liquid PCMs, so even if the environmental temperature is above
G18, respectively. The paraffin was uniformly dispersed in the their melting point, the composite FSPCMs remain in the solid
G18. The microstructures of the G18/paraffin xerogel prepared state.
by removing the paraffin were further observed in Fig. 7c and d.
The three-dimensional network was constructed by ribbon-like
microbers and the average diameter of the ribbons was about
Thermal properties of the Gm/PCM composites
400–1000 nm. Meanwhile, both the density and diameter of the
ribbons increased with the increase of the concentration of G18
(Fig. S2†). In addition, the microstructures of the Gm/paraffin
(m ¼ 8, 10, 12, 14, 16) xerogel were observed in Fig. S3.† Three-
dimensional networks constructed by ribbon-like microbers
The heating and cooling DSC curves of the G18/paraffin
composite PCM, pure paraffin and G18 are shown in Fig. 8.
From the gure, it can be seen that three peaks were detected in
the G18/paraffin composite DSC curve. The rst minor peak at
about 39 ꢀC should be attributed to the solid–solid phase
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 7 SEM images of (a) pure paraffin, (b) G18/paraffin composites, (c and d) xerogel of G18/paraffin composites at different magnifications.
Scale bar ¼ 5 mm for (a), (b) and (c), and 1 mm for (d), respectively.
transition of the paraffin and the second sharp peak at about 57 tube-testing method at the same gelator concentrations,
ꢀC corresponding to the solid–liquid phase transition of the showing a gel–sol transition.
paraffin.70 The third phase transition peak appeared at about
The thermal properties of the G18/paraffin composite with a
130 to 155 ꢀC was well correlated to TGS values obtained by the different mass ratio of G18 including melting temperature of
Fig. 8 DSC curves of G18/paraffin composites during heating process (a) and cooling process (b).
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
Table 1 The thermal characteristics of paraffin and G18/paraffin
Melting
Freezing
DHombsa (J gꢁ1
)
DHTmheb (J gꢁ1
)
TGS (ꢀC)
Tfa (ꢀC)
DHof bsa (J gꢁ1
)
DHTf heb (J gꢁ1
)
TSG (ꢀC)
a
Tm (ꢀC)
a
a
Paraffin
57.2
57.5
57.7
57.0
57.6
57.2
57.1
57.5
57.5
189.3
173.1
210.0
191.6
184.6
184.0
182.0
188.1
177.0
189.3
188.4
187.4
185.5
183.6
181.7
177.9
174.6
170.4
—
53.0
53.5
52.9
53.0
53.6
53.4
53.4
53.1
53.8
184.4
172.1
202.1
186.2
180.7
177.1
176.4
181.7
170.0
184.4
183.5
182.6
180.7
178.9
177.0
173.3
170.0
166.0
—
0.5 wt%G18/paraffin
1 wt%G18/paraffin
2 wt%G18/paraffin
3 wt%G18/paraffin
4 wt%G18/paraffin
6 wt%G18/paraffin
8 wt%G18/paraffin
10 wt%G18/paraffin
137.5
144.7
148.0
153.2
153.3
153.2
153.7
155.2
129.4
142.0
141.1
147.7
147.5
146.9
147.3
149.0
a
Evaluated by DSC during the second heating process at a rate of 5 ꢀC minꢁ1 under nitrogen atmosphere. b Calculated by multiplying the weight
percentage of paraffin in the composite PCM by the melting or freezing enthalpies of pure paraffin.
Fig. 9 Relationship between the TGS of G18/paraffin and the amount
Fig. 10 DSC curves of the 3 wt% G18/paraffin composite before and
of G18 additive from the tube-testing method (a) and DSC (b).
after thermal cycling.
paraffin (Tm), observed melting latent heat (DHombs), theoretical
melting latent heat (DHTmhe), gel–sol transition temperature
(TGS), freezing temperature (Tf), observed freezing latent heat
(DHof bs), theoretical freezing latent heat (DHTf he) and sol–gel
transition temperature (TSG) are summarized in Table 1. As seen
from Table 1, compared with that of the pure paraffin, there are
no differences for phase transition temperatures (Tm and Tf) of
the G18/paraffin composite, showing that G18 has no effect on
the phase transition temperature of paraffin. As can be seen in
Fig. 9, the TGS of G18/paraffin composite increased sharply with
the increase of mass fraction of G18, then increased only a little
when the mass fraction of G18 exceeds approximately 3 wt%.
Table 2 Comparison of thermal properties of the composite prepared with that of some FSPCMs in the literature
FSPCM
Mass ratio
Tfb (ꢀC)
DHmc (J gꢁ1
)
Reference
a
Tm (ꢀC)
SEBS/paraffin
EPDM/paraffin
HNT/paraffin
HDPE/paraffin
GO/paraffin
10/90
50/50
30/70
23/77
48.3/51.7
3/97
54.5
56.4
57.2
55.7
53.57
57.6
57.5
49.1
48.8
54.2
45.2
44.59
53.6
53.8
158.5
85.0
112.4
162.2
63.7
Q. Zhang et al.70
G. Song et al.71
J. Zhang et al.72
A. Sarı et al.73
M. Mehrali et al.74
Present study
G18/paraffin
G18/paraffin
183.6
170.4
10/90
Present study
a
Melting temperature of paraffin. b Freezing temperature of paraffin. c Melting latent heat of FSPCMs.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 11 (a) Frequency dependence of G0 (solid symbols) and G00 (open symbols) for 3 wt% G18/paraffin composite at indicated temperature.
Condition: 0.05% strain. (b) Temperature dependence of storage modulus (G0) and loss modulus (G00) for 3 wt% G18/paraffin composite at 30 ꢀC
to 160 ꢀC. Condition: 0.05% strain, 1 rad sꢁ1 frequency and 2 ꢀC minꢁ1 heating rate. (c) DSC curves of 3 wt% G18/paraffin composite during
heating process.
The results were coincident with the results from the tube- in Table 2. Based on the data in this table, it can remarkably be
testing method suggesting that the network was strong enough noted that G18/paraffin composite PCMs exhibit higher latent
to immobilize most of the paraffin at this concentration. heat than that of other composite PCMs. This result showed
Therefore, 3 wt% was selected as the proper adding amount of that the G18/paraffin composite PCM has potential for energy
G18. Meantime, the DHombs and DHfobs were consistent with the storage applications.
theoretical value based on the mass ratio of the paraffin in the
The multicycle DSC scans were performed to investigate the
composites (see Table 1). For example, the DHombs and DHmThe of 3 working reliability of G18/paraffin composites during a long-
wt% G18/paraffin were 184.6 J gꢁ1 and 183.6 J gꢁ1, respectively. term phase change process. Fig. 10 shows that the DSC curves of
These results suggest that both the paraffin and G18/paraffin 3 wt% G18/paraffin before and aer 50 thermal cycles. From the
composites exhibit similar thermal characteristics, because DSC curves, phase change temperatures for melting and
there was no chemical reaction between the PCMs and gelators freezing of the composite PCM before cycling were determined
in the preparation of G18/paraffin. Moreover, the comparison of at 58.7 and 52.0 ꢀC, respectively. Aer 50 thermal cycles, phase
latent heat of G18/paraffin prepared in the present study with change temperatures for melting and freezing of the composite
that of the different composite PCMs in literature was also given PCM changed to 58.3 and 52.6 ꢀC, respectively. The latent heat
Fig. 12 Temperature dependence of storage modulus for G18/paraffin composite at different mass fraction from 30 ꢀC to 160 ꢀC. Condition:
0.05% strain, 1 rad sꢁ1 frequency and 2 ꢀC minꢁ1 heating rate.
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
modulus (G00) measures the tendency of a material to ow under
stress.
In Fig. 11a, the storage modulus and loss modulus of 3 wt%
G18/paraffin composite are shown at four different tempera-
tures, namely, 100, 140, 145 and 150 ꢀC for the applied
frequency range 0.1–100 rad sꢁ1. It is apparent from the gure
that both the G0 and G00 were parallel to the frequency axis and
G0 > G00 at 100 ꢀC and 140 ꢀC, characterizing the system as in the
gel state. At 145 ꢀC, the composite behaved as a sol (G00 > G0) for
the low frequency region (below 2.4 rad sꢁ1) and above that
frequency the system behaved as a gel (G0 > G00). Finally at 150
ꢀC, the composite behaved completely as a sol (G00 > G0). These
results were consistent with the results from “tube-testing
method” and DSC (Fig. 11b). Fig.11c shows the effect of
temperature on storage modulus and loss modulus of 3 wt%
G18/paraffin composites at a heating rate of 2 ꢀC minꢁ1. To
deeply investigate the phase change behavior of G18/paraffin,
we can divide the different changing tendencies of G0 and G00
into several segments.67 It can be seen that two peaks of G0 and
G00at around 39 ꢀC and 57 ꢀC agreed well with the transition
temperature of composites from DSC (Fig. 11b), due to a solid–
solid phase transition and solid–liquid transition of the paraffin
component. However, with temperature increasing gradually
above 57 ꢀC, both G0 and G00 decreased because paraffin
underwent a solid–liquid phase transition, but G0 remained
larger than G00 and the value kept nearly invariant up to 118 ꢀC,
this implies that paraffin can be contained in the network of
G18 even above its melting point and the composite PCMs
remains in a gel state. We denominate this solid-like gel with a
higher modulus as a hard gel. With the increase of temperature,
both G0 and G00 began to decrease steadily, but still G0 > G00, in
the temperature range of 118 to 147 ꢀC, suggesting the trend
that the G18/paraffin composite PCMs were losing solid-like
characters. This gel with a lower modulus was denominated as a
so gel. A crossover point of G0 and G00 traces was found at
around 147 ꢀC, which is the symbol of the gel–sol transition
from rheological measurements, aer that G00 would be larger
than G0, the whole system presents a viscous-dominant
behavior.75–77
Fig. 13 Frequency dependence of G0 (solid symbols) and G00 (open
symbols) for G18/paraffin composite PCMs of different concentration
of G18. Condition: 0.05% strain, 70 ꢀC.
Fig. 14 “Phase diagram” of G18/paraffin composite during heating.
of fusion varied from 186.9 to 181.7 J gꢁ1, while the latent heat
of freezing changed from 187.6 to 186.9 J gꢁ1. The phase change
temperature and latent heat of the composite PCM only varied
slightly, suggesting that the composite PCM could always
maintain stable phase change temperatures and enthalpies over
the multicycle phase transitions. Therefore, the prepared form-
stable composite PCM has a high working reliability to perform
energy storage-release repeatedly at an almost stable
temperature.
The temperature at which G0 and G00 began to decrease was
considered as transition temperature from a hard gel to so gel
and was denoted as T1, and the critical temperature when G0 was
equal to G00 was denoted as T2. With reference to Fig. 12, with
the increase of mass percentage of G18 in the composites, both
T1 and T2 shied to higher temperatures. Moreover, G0
increased along with the increase of mass fraction of gelator
G18, indicating that higher concentration of gelator in PCMs
tend to form more stable gels.
Frequency sweeps were conducted at low strain (0.05%), well
within the linear region. Testing temperature was 70 ꢀC, at
which a solid–liquid PCMs phase transition occurred. Consis-
Rheology measurement
To examine the phase transition behavior and shape stability of tent with Fig. 13, it can be seen that G18/paraffin examples
materials, the G18/paraffin composite was characterized by exhibit a weak frequency dependence from 0.1 to 100 rad sꢁ1
,
rheological measurements. The storage modulus (G0) is an and the elastic modulus G0 remained larger than the viscous
important parameter which characterizes the strength of gels modulus G00, and there was almost no crossover. The value of G0
and can estimate the degree of resistance of the PCMs gels also increased with the increase of mass percentage of G18 in
against mechanical disturbance. On the other hand, the loss the composites. These dynamic mechanical results showed that
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
Journal of Materials Chemistry A
the composite G18/paraffin are in a gel state, i.e., the paraffin
was trapped by the gel brillate network as the solid–liquid
phase transition occurred, which resulted in FSPCMs.
Acknowledgements
This research was nancially supported by the National Nature
Science Foundation of China (51373148), the Opening Project of
Key Laboratory of Polymeric Materials and Application Tech-
nology of Hunan Province (PMAT201401).
According to the analyses above, we charted a “phase
diagram” of G18/paraffin FSPCMs presented in Fig. 14. The
vertical axis represents temperature while the horizontal axis
represents the content of G18 in the composites. Tm, T1, T2 were
determined by the former rheological analysis of the G18/
paraffin composites. As shown in Fig. 14, when the temperature
was below about 57 ꢀC, the melting point of paraffin, both pure
paraffin and G18/paraffin composites were in solid state. As the
temperature increased to above the melting point, the pure
paraffin melted into liquid, while the composites formed a hard
gel. Under these conditions, the paraffin was trapped by the gel
brillate network of G18. When the temperature further
increased above T1, the composites turned into a so gel and
their rigidity decreased. In this case, the gel network started to
collapse but the shape of the samples was partially maintained,
and the overall state was still similar to solid rather than liquid.
When the temperature increased to above T2, the composites
became uid, losing their shape stability completely. Mean-
while, with the increase of G18 content, both T1 and T2 of the
G18/paraffin FSPCMs shied to higher temperatures. However,
when the adding amount of G18 was more than 3 wt% or so, the
T1 and T2 basically reached a plateau, so 3 wt% was considered
as the optimal adding amount of G18 to keep the G18/paraffin
composite stable.
Notes and references
´
1 B. Zalba, J. M. Marın, L. F. Cabeza and H. Mehling, Appl.
Therm. Eng., 2003, 23, 251–283.
2 A. Sari and A. Biçer, Sol. Energy Mater. Sol. Cells, 2012, 101,
114–122.
3 A. Najjar and A. Hasan, Energy Convers. Manage., 2008, 49,
3338–3342.
4 A. Sharma, V. V. Tyagi, C. R. Chen and D. Buddhi, Renewable
Sustainable Energy Rev., 2009, 13, 318–345.
5 A. Jamekhorshid, S. M. Sadrameli and M. Farid, Renewable
Sustainable Energy Rev., 2014, 31, 531–542.
6 Y. Tian and C. Y. Zhao, Appl. Energy, 2013, 104, 538–553.
7 C. Chen, L. Wang and Y. Huang, Chem. Eng. J., 2009, 150,
269–274.
8 Z. Zhang and X. Fang, Energy Convers. Manage., 2006, 47,
303–310.
9 X. Wang, E. Lu and W. Lin, Energy Convers. Manage., 2000,
41, 135–144.
10 K. Chen, X. Yu, C. Tian and J. Wang, Energy Convers.
Manage., 2014, 77, 13–21.
11 Y. Jiang, E. Ding and G. Li, Polymer, 2002, 43, 117–122.
´
12 M. Delgado, A. Lazaro, J. Mazo and B. Zalba, Renewable
Sustainable Energy Rev., 2012, 16, 253–273.
13 P. B. Salunkhe and P. S. Shembekar, Renewable Sustainable
Energy Rev., 2012, 16, 5603–5616.
Conclusions
In this work, novel FSPCMs (Gm/paraffin (m ¼ 8, 10, 12, 14, 16, 14 H. Fauzi, H. S. Metselaar, T. M. I. Mahlia and M. Silakhori,
18)) possessing high heat storage density and excellent thermal Sol. Energy, 2014, 102, 333–337.
stability were successfully prepared by introducing 4,40-dia- 15 N. Zhu, Z. Ma and S. Wang, Energy Convers. Manage., 2009,
minodiphenylmethane-based gelators into paraffin. FT-IR, 50, 3169–3181.
POM, WAXD and SEM measurements suggested that the 16 Y. Wang, T. D. Xia, H. Zheng and H. X. Feng, Energy and
paraffin acted as a latent heat storage material while gelators Buildings, 2011, 43, 2365–2370.
served as the supporting material, and the paraffin was 17 C. Wang, L. Feng, W. Li, J. Zheng, W. Tian and X. Li, Sol.
restricted by a three-dimensional netted structure of gelators to Energy Mater. Sol. Cells, 2012, 105, 21–26.
avoid leakage even above their melting point. The G18/paraffin 18 M. Xiao, B. Feng and K. Gong, Energy Convers. Manage., 2002,
composites exhibited high heat storage density and excellent 43, 103–108.
thermal stability according to the DSC and rheology measure- 19 M. M. Farid, A. M. Khudhair, S. A. K. Razack and S. Al-Hallaj,
ments. The mass percentage of paraffin can reach 97 wt% Energy Convers. Manage., 2004, 45, 1597–1615.
without obvious leakage of paraffin above the melting point and 20 A. M. Khudhair and M. M. Farid, Energy Convers. Manage.,
the composites can keep their shapes in a wide range of 2004, 45, 263–275.
temperature due to the excellent supporting ability of the gel 21 D. Rozanna, T. G. Chuah, A. Salmiah, T. S. Y. Choong and
network from G18. As a result, the TGS of 3 wt% G18/paraffin M. Sa'ari, Int. J. Green Energy, 2005, 1, 495–513.
FSPCMs was 150 ꢀC, which was well above the melting point of 22 S. D. Sharma and K. Sagara, Int. J. Green Energy, 2005, 2, 1–
paraffin, and values of the melting and freezing enthalpies were 56.
184.6 and 180.7 J gꢁ1, making the G18/paraffin composite an 23 V. V. Tyagi and D. Buddhi, Renewable Sustainable Energy Rev.,
effective and promising material to store or release thermal 2007, 11, 1146–1166.
energy in practical applications. This research on Gm/paraffin 24 V. V. Tyagi, S. C. Kaushik, S. K. Tyagi and T. Akiyama,
composites provided a new method for the preparation of Renewable Sustainable Energy Rev., 2011, 15, 1373–1391.
FSPCMs, and meanwhile broadened the application eld of 25 M. Kenisarin and K. Mahkamov, Renewable Sustainable
gelators.
Energy Rev., 2007, 11, 1913–1965.
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
´
´
`
26 M. M. Kenisarin, Renewable Sustainable Energy Rev., 2010, 14, 54 L. Bayes-Garcıa, L. Ventola, R. Cordobilla, R. Benages,
955–970.
T. Calvet and M. A. Cuevas-Diarte, Sol. Energy Mater. Sol.
Cells, 2010, 94, 1235–1240.
55 A. Sari, C. Alkan, A. Karaipekli and O. Uzun, Sol. Energy,
2009, 83, 1757–1763.
56 J. Li, P. Xue, H. He, W. Ding and J. Han, Energy and
Buildings., 2009, 41, 871–880.
57 B. Tang, J. Cui, Y. Wang, C. Jia and S. Zhang, Sol. Energy,
2013, 97, 484–492.
27 A. Pasupathy, R. Velraj and R. V. Seeniraj, Renewable
Sustainable Energy Rev., 2008, 12, 39–64.
28 F. Agyenim, N. Hewitt, P. Eames and M. Smyth, Renewable
Sustainable Energy Rev., 2010, 14, 615–628.
29 R. Baetens, B. P. Jelle and A. Gustavsen, Energy and Buildings,
2010, 42, 1361–1368.
30 C. Y. Zhao and G. H. Zhang, Renewable Sustainable Energy
Rev., 2011, 15, 3813–3832.
31 L. F. Cabeza, A. Castell, C. Barreneche, A. de Gracia and
58 G. Fang, H. Li and X. Liu, Mater. Chem. Phys., 2010, 122, 533–
536.
´
A. I. Fernandez, Renewable Sustainable Energy Rev., 2011, 59 W. Wang, X. Yang, Y. Fang and J. Ding, Appl. Energy, 2009,
15, 1675–1695. 86, 170–174.
32 F. Kuznik, D. David, K. Johannes and J. J. Roux, Renewable 60 P. Dastidar, S. Okabe, K. Nakano, K. Iida, M. Miyata,
Sustainable Energy Rev., 2011, 15, 379–391.
33 L. Fan and J. M. Khodadadi, Renewable Sustainable Energy
Rev., 2011, 15, 24–46.
34 V. A. A. Raj and R. Velraj, Renewable Sustainable Energy Rev.,
2010, 14, 2819–2829.
35 J. L. Zeng, J. Zhang, Y. Y. Liu, Z. X. Cao, Z. H. Zhang, F. Xu
and L. X. Sun, J. Therm. Anal. Calorim., 2008, 91, 455–461.
36 J. A. Mole, A. S. Luyt and I. Krupa, Thermochim. Acta, 2010,
500, 88–92.
N. Tohnai and M. Shibayama, Chem. Mater., 2005, 17, 741–
748.
61 X. Zhang, P. Deng, R. Feng and J. Song, Sol. Energy Mater. Sol.
Cells, 2011, 95, 1213–1218.
62 T. Tian, J. Song, L. Niu and R. Feng, Thermochim. Acta, 2013,
554, 54–58.
63 J. He, B. Yan, B. Yu, R. Bao, X. Wang and Y. Wang, J. Colloid
Interface Sci., 2007, 316, 825–830.
´
64 H. Svobodova, Z. Wimmer and E. Kolehmainen, J. Colloid
37 G. Fang, H. Li, Z. Chen and X. Liu, J. Hazard. Mater., 2010,
181, 1004–1009.
38 M. Silakhori, H. S. C. Metselaar, T. M. I. Mahlia, H. Fauzi,
Interface Sci., 2011, 361, 587–593.
65 S. Grassi, E. Carretti, L. Dei, C. W. Branham, B. Kahr and
R. G. Weiss, New J. Chem., 2011, 35, 445–452.
S. Baradaran and M. S. Naghavi, Energy Convers. Manage., 66 R. Schmidt, F. B. Adam, M. Michel, M. Schmutz, G. Decher
´
and P. J. Mesini, Tetrahedron Lett., 2003, 44, 3171–3174.
2014, 80, 491–497.
39 Y. Wang, T. D. Xia, H. X. Feng and H. Zhang, Renewable 67 H. Yui, H. Minamikawa, R. Danev, K. Nagayama, S. Kamiya
Energy, 2011, 36, 1814–1820.
40 C. Alkan and A. Sari, Sol. Energy, 2008, 82, 118–124.
41 F. Chen and M. P. Wolcott, Eur. Polym. J., 2014, 52, 44–52.
and T. Shimizu, Langmuir, 2008, 24, 709–713.
68 J. L. Li, X. Y. Liu, C. S. Strom and J. Y. Xiong, Adv. Mater.,
2006, 18, 2574–2578.
42 X. Wang, Q. Guo, J. Wang, Y. Zhong, L. Wang, X. Wei and 69 Y. Xiong, Q. Liu, H. Wang and Y. Yang, J. Colloid Interface
L. Liu, Renewable Energy, 2013, 60, 506–509. Sci., 2008, 318, 496–500.
43 Y. Cai, Q. Wei, F. Huang and W. Gao, Appl. Energy, 2008, 85, 70 Q. Zhang, Y. Zhao and J. Feng, Sol. Energy Mater. Sol. Cells,
765–775. 2013, 118, 54–60.
44 M. J. Hato and A. S. Luyt, J. Appl. Polym. Sci., 2007, 104, 2225– 71 G. Song, S. Ma, G. Tang, Z. Yin and X. Wang, Energy, 2010, 35,
2236. 2179–2183.
45 C. Alkan, K. Kaya and A. Sari, J. Polym. Environ., 2009, 17, 72 J. Zhang, X. Zhang, Y. Wan, D. Mei and B. Zhang, Sol. Energy,
254–258. 2012, 86, 1142–1148.
46 A. Sari, A. Karaipekli and C. Alkan, Chem. Eng. J., 2009, 155, 73 A. Sari, Energy Convers. Manage., 2004, 45, 2033–2042.
899–904.
74 M. Mehrali, S. T. Latibari, M. Mehrali, H. S. C. Metselaar and
M. Silakhori, Energy Convers. Manage., 2013, 67, 275–282.
75 A. I. Van Den Bulcke, B. Bogdanov, N. De Rooze,
E. H. Schacht, M. Cornelissen and H. Berghmans,
Biomacromolecules, 2000, 1, 31–38.
47 D. Sun, L. Wang and C. Li, Mater. Lett., 2013, 108, 247–249.
48 A. Karaipekli and A. Sari, Sol. Energy, 2009, 83, 323–332.
49 S. Karaman, A. Karaipekli, A. Sari and A. Biçer, Sol. Energy
Mater. Sol. Cells, 2011, 95, 1647–1653.
50 A. Sari and A. Karaipekli, Sol. Energy Mater. Sol. Cells, 2009, 76 D. Dasgupta, S. Manna, A. Garai, A. Dawn, C. Rochas,
93, 571–576.
J. M. Guenet and A. K. Nandi, Macromolecules, 2008, 41,
779–787.
51 Q. Cao and P. Liu, Eur. Polym. J., 2006, 42, 2931–2939.
52 J. C. Su and P. S. Liu, Energy Convers. Manage., 2006, 47, 77 A. Garai and A. K. Nandi, J. Polym. Sci., Part B: Polym. Phys.,
3185–3191.
2008, 46, 28–40.
53 Q. Meng and J. Hu, Sol. Energy Mater. Sol. Cells, 2008, 92,
1260–1268.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2015