Preparation of polymer/LDH nanocomposite by UV-initiated photopolymerization of acrylate through photoinitiator-modified LDH precursor

Article abstract of DOI:10.1016/j.materresbull.2010.11.002

The exfoliated polymer/layered double hydroxide (LDH) nanocomposite by UV-initiated photopolymerization of acrylate systems through an Irgacure 2959-modified LDH precursor (LDH-2959) as a photoinitiator complex was prepared. The LDH-2959 was obtained by t

Full text of DOI:10.1016/j.materresbull.2010.11.002

Materials Research Bulletin 46 (2011) 244–251
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Preparation of polymer/LDH nanocomposite by UV-initiated
photopolymerization of acrylate through photoinitiator-modified LDH precursor
*
Lihua Hu, Yan Yuan, Wenfang Shi
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
The exfoliated polymer/layered double hydroxide (LDH) nanocomposite by UV-initiated photopoly-
merization of acrylate systems through an Irgacure 2959-modified LDH precursor (LDH-2959) as a
photoinitiator complex was prepared. The LDH-2959 was obtained by the esterification of 2-hydroxy-40-
(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) with thioglycolic acid, following by the
addition reaction with 3-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560), finally intercalation into
the sodium dodecyl sulfate-modified LDH. For comparison, the intercalated polymer/LDH nanocompo-
site was obtained with additive Irgacure 2959 addition. From the X-ray diffraction (XRD) measurements
and HR-TEM observations, the LDH lost the ordered stacking-structure and well dispersed in the polymer
matrix at 5 wt% LDH-2959 loading. The glass transition temperature of UV-cured exfoliated
nanocomposites increased to 64 8C from 55 8C of pure polymer without LDH addition. The tensile
strength was improved from 10.1 MPa to 25.2 MPa, as well the Persoz hardness enhanced greatly, while
the elongation at break remained an acceptable level.
Received 30 June 2010
Received in revised form 30 September 2010
Accepted 1 November 2010
Available online 5 November 2010
Keywords:
A. Layered compounds
A. Nanostructures
B. Intercalation reactions
C. X-ray diffraction
D. Mechanical properties
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
exfoliation LDH layers [5–7]. In order to increase the compatibility
of inorganic LDH filler with organic phase, some organic or
The polymer/inorganic nanocomposites have attracted much
attention for research and industrial uses in recent decades,
because of their enhanced thermal and mechanical properties,
improved barrier properties, and reduced flammability [1–3].
Moreover, the majority of previous studies have been focused on
polymer/clay nanocomposites, especially based on montmorillon-
ite-type layered silicates (MMTs) due to the lower charge density
of clay layer which is in favor of the formation of exfoliated
structure. However, the variability in the chemical composition of
naturally occurred layered clay make the control in properties of
the formed polymer/clay nanocomposite difficult.
polymeric anions, such as alkyl sulphonate [8], polyacylate [9],
poly(styrene sulfonate) [10] and poly(propylene carbonate) [11]
are used to modify LDH particles. Furthermore, several silane
coupling agents have been used to intercalate into LDH layers
successfully by the covalent linkage formation through the
reaction of hydroxyl group on LDH sheet with alkyloxysilane
group of silane coupling agent [12–14]. Qiu et al. [15] prepared the
exfoliated polystyrene/LDH nanocomposite through the in situ
atom transfer radical polymerization of styrene under the presence
of an initiator after the intercalation of dodecyl sulfate into the LDH
galleries.
The layered double hydroxide (LDH), known as a hydrotalcite-
In recent years, much work are concerning with UV cured
polymer/clay nanocomposites [16–19]. However, only little work
has been done for preparing polymer/LDH nanocomposites by
using UV irradiation [20,21]. In our laboratory, Lv et al. [22]
prepared the UV cured polymer/LDH nanocomposites, and
proposed that the intercalation of reactive groups into LDH
interlayers results in the formation of exfoliated structure of LDH.
In the present work, the exfoliated UV cured polymer/LDH
nanocomposite was prepared by using a novel photoinitiator-
modified LDH precursor. A fragmental photoinitiator, 2-hydroxy-
40-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959)
was used to react with thioglycolic acid, 3-(2,3-epoxypropoxy)-
propyltrimethoxysilane (KH-560), and the sodium dodecyl sulfate-
modified LDH to form the LDH-2959 used as a photoinitiator
complex for acrylate systems. For comparison, the formulations
like material, is a so-called anion clay. The idea chemical structure
3+
can be represented by the formula [M2+
M
1ꢀx
x(OH)2]x+Anꢀ
x/
nꢁmH2O, where M2+ and M3+ are divalent and trivalent metal
cations, such as Mg2+, Al3+, respectively, and Anꢀ is an exchange-
able anion, such as CO32ꢀ, SO42ꢀ, and NO3ꢀ. Due to the highly
tunable properties, LDHs are considered as a new class of the most
favorable layered crystals for preparing polymer/layered crystal
nanocomposites [4]. However, the hydrophilic layer surface and
the strong interaction between LDH layers both impede organic
components to enter into the galleries, resulting in difficult
* Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630.
E-mail address: wfshi@ustc.edu (W. Shi).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.11.002

L. Hu et al. / Materials Research Bulletin 46 (2011) 244–251
245
with additive Irgacure 2959 were also prepared. The photopoly-
merization kinetics of acrylate systems initiated by LDH-2959, and
both LDH-2959 and Irgacure 2959 was investigated in detail. The
mechanical and thermal properties, and microstructures of the
formed nanocompsites were examined.
(–CH–CH2–S–), 1.62 (CH3–C–CH3), 1.46–1.33 (–Si–CH2–CH2–),
1.46–1.33 (–Si–CH2–CH2–).
FTIR (NaCl plate, cmꢀ1): 3600-3100 (–OH), 1735 (C55O of ester
group), 1667 (C55O abutted on the aromatic ring), 1600 (C55C of the
aromatic ring), 1046 (Si–O).
2. Experimental
2.2.3. Modification of LDH with SDS
The coprecipitation method was used to prepare the SDS-
modified LDH. 17.3 g SDS, 11.3 g (0.03 mol) Al(NO3)3ꢁ9H2O, and
23 g (0.09 mol) Mg(NO3)2ꢁ6H2O were dissolved into 300 mL of
deionic water, and stirred vigorously at 70 8C. The pH value of 10
was maintained by adding 1.0 M NaOH aqueous solution via a
peristaltic pump. The formed homogenous suspension was aged at
75 8C for 24 h, and then washed by deionic water to remove
unreacted SDS, obtaining a SDS-modified MgAl-LDH, named LDH-
DS, as a milk-white suspension.
2.1. Materials
Sodium dodecyl sulfate (SDS), Mg(NO3)2ꢁ6H2O, Al(NO3)3ꢁ9H2O
and sodium hydroxid were all purchased from Sinopharm
Chemical Reagent Co. (Shanghai, China). Thioglycolic acid was
purchased from Aldrich Chemical Co. 2-Hydroxy-40-(2-hydro-
xyethoxy)-2-methylpropiophenone (Irgacure 2959) and 1,6-
hexamethyldiol diacrylate (HDDA) were supplied by Ciba
Specialty Chemicals and Eternal Chemical Co., Taiwan, respec-
tively. 4-Dimethylamino pyridine (DMAP), p-toluene sulphonic
acid (PTSA) and dimethylacetamide (DMAC) were purchased
from Shanghai First Reagent Co. 4-Dimethylaminopyridine p-
toluenesulfonate (DPTS) was prepared from the reaction of
DMAP with PTSA according to the literature reported by Moore
and Stupp [23]. 3-(2,3-Epoxypropoxy)propyltrimethoxysilane
(KH-560) was purchased from Nanjing Yudeheng Chemical Co.
EB270, which is an aliphatic urethane acrylate with a molar
mass of 1500 g molꢀ1 and an unsaturation concentration of
1.33 mmol gꢀ1, was offered by Cytec Industries Inc., USA. All
chemicals were used as received without further purification
except for KH-560, which was purified by distillation under
reduced pressure before use.
2.2.4. Intercalation of TMS-2959 into LDH-DS
For intercalating TMS-2959 into LDH, a proper amount of water
suspension containing 3.0 g LDH-DS was dispersed into 200 mL of
toluene, and then distilled by azeotropy to remove water,
obtaining a LDH-DS suspension in toluene. Then 200 mL of DMAC
was mixed with 200 mL of LDH-DS toluene suspension, and
vigorously stirred at 100 8C until LDH-DS uniformly dispersed in
DMAC. After toluene was removed under vacuum, 15 g TMS-2959
was added, stirred at 120 8C for 30 h under nitrogen atmosphere.
The product, named LDH-2959, was collected by filtrating washed
repeatedly with toluene to remove the unreacted TMS-2959, and
finally dispersed in toluene for further use.
2.2.5. Preparation of formulations for UV curing
2.2. Synthesis
The primary formulation utilized in this study was consisted of
a 7:3 (w/w) mixture of EB270 as an oligomer to HDDA as a
monomer, and LDH-2959 loadings of 1, 3, 5 wt% as toluene
suspensions, respectively. The formulation was stirred until a
complete dispersion was achieved, and then distilled to remove
toluene under vacuum. For preventing any unexpected polymeri-
zation, all the operations were performed in the dark. Every
formulation was divided into two samples, including one with only
LDH-2959 addition, referred as a-1, a-3 and a-5 (series A), and the
other added with additional 3 wt% Irgacure 2959 based on series A,
and referred as b-1, b-3 and b-5 (series B). And the formulation for
preparing the pure polymer film as a reference was consisted of a
7:3 (w/w) mixture of EB270 to HDDA and 3 wt% Irgacure 2959.
2.2.1. Synthesis of thioglycolic acetate-modified 2959 (TA-2959)
Thioglycolic acid (11.04 g, 0.12 mol), Irgacure 2959 (22.4 g,
0.1 mol) and DPTS (0.672 g) were mixed with 50 mL of toluene in a
150 mL three-neck flask with a Dean-Stark trap, and stirred
overnight at 125 8C under refluxing in nitrogen atmosphere. Then
toluene was removed by rotary evaporation under reduced
pressure. The crude product was diluted with 60 mL of CH2Cl2,
washed with saturated NaHCO3 aq. (5ꢂ 100 mL) and distilled
water (3ꢂ 100 mL). The organic phase was dried with anhydrous
MgSO4 overnight, and then filtrated and distilled, obtaining the
product, named TA-2959, as a yellowish liquid (yield: 91%).
1H NMR (300 MHz, CDCl3):
d (ppm) 8.04–7.98 (–CH55CH–C–
CO–), 6.93–6.87 (–O–C–CH55CH–), 4.49–4.43 (–COO–CH2–CH2–),
4.24–4.19 (–COO–CH2–), 3.27–3.23 (HS–CH2–), 2.0–1.94 (HS–
CH2–), 1.57 (CH3–C–CH3).
FTIR (NaCl plate, cmꢀ1): 3600–3100 (–OH), 2571 (–SH), 1735
(C55O of ester group), 1667 (C55O abutted on the aromatic ring),
1600 (C55C of the aromatic ring).
2.2.6. Preparation of UV-cured polymer/LDH nanocomposite
The above formulations were exposed to a medium pressure
mercury lamp (2 KW, Fusion UV Systems, USA) situated at 10 cm
above the moving belt in air to form tack free films (Scheme 1).
2.3. Measurements
2.2.2. Synthesis of trimethoxysilane-modified 2959 (TMS-2959)
The 1H NMR spectra were recorded with a DMX-300 MHz
instrument (Bruker, Germany) using CDCl3 as a solvent. The FTIR
spectra were obtained with a Nicolet MAGNA-IR 750 spectrometer.
The UV–Vis spectra were measured with a SHIMADZU UV-2401pc
instrument in dichloromethane with the concentration of
1.25 ꢂ 10ꢀ2 g Lꢀ1. The X-ray diffraction (XRD) patterns were
recorded using a Rigaku D/Max-rA rotating anode X-ray diffrac-
In
a
typical synthesis, into
a
50 mL round-bottom flask
mixture of KH-560
equipped with
a
mechanical stirrer,
a
(4.727 g, 0.02 mol), TA-2959 (5.96 g, 0.02 mol), triethylamine
(0.179 g) as a catalyst and CH2Cl2 (30 mL) was added, and stirred
at 40 8C under nitrogen atmosphere. Completion of the reaction
was confirmed by the disappearance of characteristic –SH peak at
2570 cmꢀ1 in the FT-IR spectrum. Finally, a modified Irgacure 2959
by KH-560, named TMS-2959, was obtained as a yellowish liquid
(yield: 97%).
tometer equipped with a Cu Ka tube and Ni filter (l = 0.1542 nm).
The high resolution transmission electron microscope (HR-TEM)
images were obtained with JEOL-2011 instrument, operated at an
acceleration voltage of 200 kV. The samples were ultramicrotomed
with a diamond knife on a LKB Pyramitome to give 60-nm thick
slices. The photopolymerization kinetics analysis was carried out
on a modified CDR-1 DSC apparatus (Shanghai Balance Instrument
1H NMR (300 MHz, CDCl3):
d (ppm) 8.09–8.06 (–CH55CH–C–
CO–), 6.98–6.94 (–O–C–CH55CH–), 4.54–4.45 (–COO–CH2–CH2–),
4.29–4.26 (–COO–CH2–), 4.01–3.88 (–CH–CH2–S–), 3.76–3.31
(CH3–O–Si–, –CH2–CH2–O–CH2–, –S–CH2–COO–), 3.13–3.09

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L. Hu et al. / Materials Research Bulletin 46 (2011) 244–251
samples were analyzed to determine an average value in order to
obtain the reproducible result. The pendulum hardness was
determined using a QBY pendulum apparatus (Tianjin Instrument
Co., China), while for the pencil hardness a QHQ-A pencil hardness
apparatus (Tianjin Instrument Co., China) was used.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route of LDH-2959 and the structural illustration
of polymer/LDH nanocomposite are shown in Scheme 1. As
depicted, TA-2959 was firstly prepared by the esterification
reaction of thioglycolic acid with Irgacure 2959. Fig. 1a shows
the 1H NMR spectrum of TA-2959. The peaks at 2.0–1.94 ppm are
assigned to the proton of –SH, while other proton-corresponding
peaks can be also distinguished clearly. KH-560 reacted with TA-
2[(T)F$DIG] 959 for introducing trimethoxysilane group into TA-2959 to
Scheme 1. Synthetic route of LDH-2959 and illustration of polymer/LDH
nanocomposites.
Co., China) with a UV spot cure system BHG-250 (Mejiro Precision
Co., Japan). The incident light intensity at the sample pan was
measured to be 2.4 mW cmꢀ2 by a UV power meter. The final
unsaturation conversion (Pf) was calculated by the formula, Pf = Ht/
H
, where Ht is the heat effect within t seconds, H is the heat
1
1
effect for 100% unsaturation conversion. The DSC curve was unified
by the weight of sample (g). The polymerization rate was defined
by J gꢀ1 sꢀ1, namely, the heat of polymerization per second for 1 g
samples. For calculating the unsaturation conversion, the value of
polymerization heat 4H0 = 86 J mmolꢀ1 per acrylic double bond
was considered. The thermogravimetric analysis (TGA) was
performed on a Shimadzu TGA-50H thermoanalyzer with 5-mg
sample under a N2 flow rate of 8 ꢂ 10ꢀ5 m3 minꢀ1 at a heating rate
of 10 8C minꢀ1 from room temperature to 600 8C. The tensile
storage modulus (E0) and tensile loss factors (tan
d) of UV cured
films were measured by a dynamic mechanical thermal analyzer
(Diamond DMA, PE Co., USA) at a frequency of 5 Hz and a heating
rate of 5 8C minꢀ1 in the range of ꢀ100 to 250 8C with
25 mm ꢂ 5 mm ꢂ 1 mm specimens. The crosslink density (
ne), as
the molar number of elastically effective network chain per cube
centimeter of the film, was calculated from the lowest storage
e = (E0rubb
modulus in the rubbery plateau region according to: n /
3RT), where E0rubb is the lowest elastic storage modulus, R is the
ideal gas constant, and T is the temperature in K corresponding to
the lowest storage modulus. The mechanical properties were
measured with an Instron Universal tester (model 1185, Japan) at
25 8C with a crosshead speed of 25 mm minꢀ1. The dumb-bell
shaped specimens were prepared according to ASTM D412-87. Five
Fig. 1. 1H NMR spectra of (a) thioglycolic acetate-modified 2959 (TA-2959) and (b)
trimethoxysilane-modified 2959 (TMS-2959) in CDCl3.

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247
Fig. 2. FTIR spectra of TA-2959 and TMS-2959.
Fig. 4. FTIR spectra of LDH-DS and LDH-2959.
obtain the TMS-2959. The peaks at 2.0-1.94 ppm disappeared
completely in the 1H NMR spectrum of TMS-2959 (Fig. 1b), which
indicates that -SH groups of TA-2959 have been consumed and
TMS-2959 was synthesized successfully. Fig. 2 represents the FTIR
spectra of TA-2959 and TMS-2959. The peak at 2571 cmꢀ1 for –SH
group in the spectrum of TMS-2959 disappeared compared with
the spectrum of TA-2959. Contrariwise, the peaks for the C55O
group of ester at 1735 cmꢀ1, C55C group of the aromatic ring at
1600 cmꢀ1, and Si–O group at 1046 cmꢀ1 appeared. The results of
FTIR analysis further confirmed the molecular structures of TA-
2959 and TMS-2959.
vibration of sulfate is observed in LDH-DS spectrum, demonstrat-
ing that SDS was intercalated into the LDH interlayer. The
absorption peaks at 1735 cmꢀ1 for C55O group of ester group,
and at 1044 cmꢀ1 for Si–O group are observed from the FTIR
spectrum of LDH-2959, indicating that TMS-2959 has been grafted
into the LDH-DS interlayer. In addition, the stretching vibration of
sulfate for LDH-DS at 1220 cmꢀ1 shifted to 1112 cmꢀ1 for LDH-
2959 after grafting TMS-2959 into the LDH-DS interlayer due to
the change of chemistry environment around SDS.
The XRD patterns for MgAl-LDH and LDH-DS are shown in Fig. 5.
The UV–Vis spectra of Irgacure 2959 and TMS-2959 were
The starting scattering angle 2
MgAl-LDH shows a basal spacing of 0.78 nm (2
LDH-DS shows 2.67 nm (2 = 3.38). The enlarged spacing of LDH-
DS indicates the intercalation of SDS into MgAl-LDH. The starting
scattering angle 2 for Fig. 6 was set as 0.88.As shown in Fig. 6, after
TMS-2959 reacted with LDH-SDS, the basal spacing greatly
increases from 2.67 nm to 5.2 nm, indicating that a new complex
photoinitiator LDH-2959 was successfully prepared. Furthermore,
for the samples a-5 and b-5, containing 5 wt% LDH-2959 and
additional 3 wt% Irgacure 2959, respectively, no XRD diffraction
was observed, compared to LDH-2959 with the basal spacing of
u
was set as 28. The unmodified
measured in ethanol with the concentration of 1.25 ꢂ 10ꢀ2 g Lꢀ1
,
u
= 11.48), while the
as shown in Fig. 3. It can be seen that TMS-2959 has the similar
absorbance spectra, but smaller molar extinction coefficient
compared with Irgacure 2959. This indicates that the obtained
TMS-2959 can initiate the photopolymerization of acrylate
systems but with low photoinitiating efficiency due to its lower
molar percentage of chromophore moiety.
Fig. 4 shows the FTIR spectra of LDH-DS and LDH-2959. Both
samples display a broad absorption peak between 3600 and
3100 cmꢀ1, which is attributed to the O–H stretching vibration.
And the peaks at 2920 and 2850 cmꢀ1 assigned to the aliphatic C–H
stretch vibration are also found in both spectra. Moreover, the
absorption peak at 1220 cmꢀ1 attributed to the stretching
u
u
5.2 nm (2u = 1.78). Though the absence of XRD diffraction peak
doesn’t imply that the exfoliation has happened, it indicates the
occurrence of random orientation for a-5 and b-5.
[()TD$FIG]
The contents of organo-modifiers and other volatile materials
(e.g. water) in the samples were determined by TGA in N2
[)(TD$FIG]
Fig. 3. UV–Vis spectra of Irgacure 2959 and TMS-2959 (ethanol, 1.25 ꢂ 10ꢀ2 g Lꢀ1).
Fig. 5. XRD patterns of MgAl-LDH and LDH-DS.

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Fig. 6. XRD patterns of LDH-2959 and UV cured polymer/LDH nanocomposites.
Fig. 8. Photopolymerization rates of the formulations without and with LDH-2959
addition (series A).
atmosphere. As shown in Fig. 7, the weight loss of 13.9% for MgAl-
LDH, 12.7% for LDH-DS, and 11.6% for LDH-2959 below 200 8C were
attributed to the loss of physically adsorbed and interlayer water.
At above 200 8C, the weight loss of 42% for LDH-DS and 38.4% for
LDH-2959 were resulted from the decomposition and combustion
of intercalated organo-modifiers. After heating to 600 8C, the
weight residuals for LDH-DS and LDH-2959 were 45.3% and 54.2%,
respectively. The more residuals for the latter than the former was
interpreted to due to the incorporation of silane group, forming
more char.
and Pf increase distinctly with increasing the LDH-2959 loading,
due to the increased content of photoinitiating chromophore
group. It can be concluded that the LDH-2959 can be used as a
photoinitiator complex to initiate the photopolymerization of
acrylate resin.
As shown in Figs. 10 and 11, the Rmp ax and Pf for series B samples
are comparable with those for the pure resin without LDH addition.
This can be explained that the additional Irgacure 2959 plays a
dominative role in initiating the photopolymerization. Otherwise,
the Rmp ax and Pf increase slightly as the LDH-2959 loading
increased.
3.2. Photopolymerization behavior initiated by LDH-2959
3.3. Morphology of the UV cured nanacomposite
The photoinitiating performance of LDH-2959 was investi-
gated by Photo-DSC measurement in air at room temperature
(25 8C). Figs. 8 and 9 show the photopolymerization rates at the
peak maximum (Rmp ax) and the final unsaturation conversion (Pf)
for series A samples, respectively. It can be seen that the
formulation containing photoinitiator Irgacure 2959 has the
highest Rmp ax and Pf, as well the shortest irradiation time to reach
the Rmp ax, compared with the formulations using LDH-2959. This
can be interpreted as follows: the photoinitiating chromophore
group of LDH-2959 was linkaged to the LDH interlayer, which
weakens the remotion of formed photoinitiating radicals, and
thus decreases the reactivity of double bond. However, the Rmp ax
Fig. 12 shows the HR-TEM micrographs of cured films with
5 wt% LDH-2959 and adding additional Irgacure 2959. The dark
lines are the intersections of LDH platelets. It can be seen that for
both samples a-5 and b-5 LDH dispersed in the polymer matrix.
However, for sample a-5, the LDH layers lost the ordered stacking-
structure and show the completely exfoliation after UV curing. This
can be explained by the fact that the sample a-5 only containing
LDH-2959 exhibited a relative lower photopolymerization rate
(Fig. 8), which was propitious to further expand the LDH
intergallery to form the exfoliated structure. The cured sample
b-5 film shows a different morphology. The LDH layes were not
[)(TD$FIG]
[()TD$FIG]
Fig. 9. Unsaturation conversion in the UV cured polymeric film and nanocomposites
Fig. 7. TGA curves of MgAl-LDH, LDH-DS and LDH-2959.
(series A).

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249
Fig. 10. Photopolymerization rates of the formulations without and with LDH-2959
Fig. 11. Unsaturation conversion in the UV cured polymeric film and
addition (series B).
nanocomposites (series B).
completely exfoliated, showing the partly intercalation into the
thin tactoids. The sample b-5 containing not only LDH-2959 but
also additional 3 wt% Irgacure 2959 was provided with higher
photopolymerization rate (Fig. 10), resulting in the formation of
cross-linking network at early stage, and thus retardance of the
interlayer spacing enlarging. As a results, the intercalated structure
for the sample b-5 was formed.
posites. The storage modulus (E0) curves and loss factor (tan
curves are shown in Figs. 15 and 16. The data are listed in Table 1.
The temperature associated with the peak position of tan curve is
d)
d
defined as the glass transition temperature (Tg). E0, which is a
measurement of material stiffness, can be used to provide the
information regarding the degree of cure and cross-linking density.
For series A, as the content of LDH-2959 increased from 1 wt% to
5 wt%, the Tg value increased from 56.7 to 63.9 8C, the storage
modulus increased from 39.1 to 58.3 MPa. These results can be
attributed to the exfoliation morphology with well dispersion of
LDH in the polymer matrix. The bulky LDH inorganic core might
restrict the segmental motion of polymer chain, and thus higher
temperature was required to provide the requisite thermal energy
for the occurrence of glass transition in the nanocomposite. In
addition, the nanoreinforcement effect from the LDH core on the
polymer matrix also enhanced the storage modulus. The same
results were obtained for the series B nanocomposites with lower
values than series A samples (Fig. 16), as listed in Table 1. This can
be explained that the series A samples propossess the exfoliation
structure, and thus more uniformly dispersion in the polymer
matrix compared to series B samples with the intercalated
structure.
3.4. Thermal properties of the UV cured nanocomposites
Figs. 13 and 14 show the TGA curves of UV cured pure polymer
and polymer/LDH nanocomposites. It can be found that the
nanocomposites have the similar thermal behavior as the pure
polymer. However, for the nancomposite as the LDH loading
increased, the onset temperature of thermal decomposition
slightly raised. Moreover, the char residue also increased along
with LDH-2959 added, indicating that the LDH promoted the
charring process. On the other hand, as LDH-2959 loading
increased, the crosslinking density increased, resulting in the
enchancement in the thermal property.
The dynamic mechanical thermal analysis (DMTA) was used to
investigate the mechanical properties of polymer/LDH nanocom-
[()TD$FIG]
Fig. 12. HR-TEM micrographs of UV cured nanocomposites at 5 wt% LDH loading for a-5 (a) and b-5 (b).

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Fig. 15. DMTA curves of UV cured polymer and nanocomposites (series A).
Fig. 13. TGA curves of UV cured polymer and nanocomposites (series A) under N2
flow.
[()TD$FIG]
[)(TD$FIG]
Fig. 14. TGA curves of UV cured polymer and nanocomposites (series B) under N2
Fig. 16. DMTA curves of UV cured polymer and nanocomposites (series B).
flow.
The influence of LDH content on the mechanical properties and
hardness of UV cured polymer/LDH nanocomposites are listed in
Table 2. The tensile strength increases along with the increase of
LDH-2959 loading. Furthermore, the tensile strength of series A
samples are higher than that of series B. This is consistent with the
results obtained by DMTA. However, the elongation at break of all
the nanocomposites decreases slightly compared with that of the
pure polymer. This can be attributed to that the polymer chains in
the nanocomposites are restricted by the LDH, resulting in the
decreased degree of freedom. The pendulum and pencil hardness
of series A and series B samples both increase with increasing LDH
loading due to the nanoreinforcement effect of LDH.
Table 1
E0rubb, Tg and ne of UV cured polymer and nanocomposites from DMTA.
E0rubb (MPa)
ne (10-3 mol cmꢀ3
)
˚
Tg (C)
Sample
Pure polymer
36.1
39.1
51.2
58.3
38.6
42.5
49.1
55.1
56.7
59.1
63.9
56.1
57.5
59.2
3.78
4.09
5.36
6.11
4.04
4.45
5.14
a-1
a-3
a-5
b-1
b-3
b-5
Table 2
Mechanical properties and hardness of UV cured polymer and nanocomposites.
Sample
Tensile strength (MPa)
Elongation at break (%)
Persoz hardness (s)
Pencil hardness (H)
Pure polymer
10.1(1.1)
13.3(1.3)
17.8(1.2)
25.2(1.1)
10.9(1.4)
11.6(1.2)
13.1(1.3)
16(0.2)
14(0.3)
13(0.2)
13(0.4)
16(0.6)
15(0.3)
13(0.5)
91(1.5)
99(1.8)
2(0)
a-1
a-3
a-5
b-1
b-3
b-5
2(0.5)
4(0.4)
4(0.4)
2(0.5)
3(0)
110(1.3)
124(1.4)
96(2.0)
104(2.5)
115(1.9)
3(0.5)
Standard deviation listed in parenthesis.

L. Hu et al. / Materials Research Bulletin 46 (2011) 244–251
251
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The financial support of National Natural Science Foundation of
China (No. 50973100) is gratefully acknowledged.