Synergistic effect of organomodification and isocyanate grafting of layered double hydroxide in reinforcing properties of polyurethane nanocomposites

Article abstract of DOI:10.1039/c1jm13780h

The present work offers a novel approach for interlamellar surface modification of the hydroxyl groups of pristine and organomodified (dodecyl sulfate, DS and stearate, St) layered double hydroxides (LDHs) by grafting with isocyanate group of methylene di

Full text of DOI:10.1039/c1jm13780h

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PAPER
Synergistic effect of organomodification and isocyanate grafting of layered
double hydroxide in reinforcing properties of polyurethane nanocomposites†
*
Moumita Kotal and Suneel K. Srivastava
Received 5th August 2011, Accepted 13th September 2011
DOI: 10.1039/c1jm13780h
The present work offers a novel approach for interlamellar surface modification of the hydroxyl groups
of pristine and organomodified (dodecyl sulfate, DS and stearate, St) layered double hydroxides
(LDHs) by grafting with isocyanate group of methylene diphenylene diisocyanate (MDI) and
1
isophorone diisocyanate (IPDI). FTIR and H NMR studies confirm the grafting of MDI and IPDI
with LDHs. This has been further supported by XRD studies, which show a substantial increase in the
interlayer spacings of isocyanate grafted DS and St-LDHs. The exteꢀnt of grafting (wt%) has also been
estimated on the basis of the residue left in air atmosphere at 1000 C in thermogravimetric analysis
(TGA). The synergistic effect of organomodification and isocyanate grafting of layered double
hydroxide in reinforcing the properties of the polyurethane (PU) nanocomposites has also been
investigated and the findings have been compared with PU nanocomposites obtained by individually
grafted as well as organomodified LDHs. TEM and AFM studies establish better homogeneous
dispersion and exfoliation of MDI grafted St-LDH in PU matrix as compared to the other isocyanate
grafted LDHs. Among all the LDHs/PU nanocomposites, 3 wt% loaded MDI-grafted-St-LDH/PU
ꢀ
exhibits the maximum improvements in tensile strength (391%), thermal stability (26 C) and glass
transition temperature (11 ꢀC) and significant synergistic effect of organomodification and isocyanate
grafting of LDHs. The enhancement in the properties of all grafted organomodified LDHs/PU
nanocomposites is attributed to their nanolevel dispersion and interfacial interaction with PU.
including as flame retardants, additives in polymers, as precur-
Introduction
sors to magnetic materials, as storage and triggered release of
Over the past decade, interest in polymer nanocomposites con-
taining layered silicates has been focused more extensively due to
their favorable and often unique combination of remarkable
improved properties, e.g. mechanical,1–3 diffusion barrier,4–6
thermal stability7–9 and flame retardancy.10–12 Though, the
layered silicates with lower charge density facilitate the forma-
tion of exfoliated structures, the properties of the polymer/
layered silicate nanocomposites are difficult to control due to
their limitation in providing the variability in chemical compo-
sitions as well as the lack of control of their aspect ratio. Layered
double hydroxides (LDHs) are another type of clay with unique
properties that are not commonly observed in layered silicates,
and have also received more attention in recent years as novel
nanofiller in the preparation of polymer nanocomposites.13–18
Their novelty lies in their broad range of chemical composition,
ease of synthetic procedures in the laboratory, highly tunable
properties, high level of purity and their potential industrial uses,
functional anions, in biology and medicine, in catalysis, and in
environmental remediation.16,19–22 LDH can be represented by
III
the general formula [MII
M )
x(OH)2]x+(Anꢁ x/n$yH2O.13–18 The
1ꢁx
identities of the divalent and trivalent cations (MII and MIII,
respectively) and the interlayer anion (Anꢁ) together with the
value of the stoichiometric coefficient (x) may be varied over
a wide range, giving rise to a large class of isostructural materials.
In LDH, the hydroxide sheets are stacked with strong electro-
static interaction. As a result, the interlayer spacing between the
metal hydroxide layers (0.78 nm) is not enough to facilitate the
exfoliation of LDH layers and intercalation of the polymer
chains. In addition, the hydrophilic property of LDHs makes it
incompatible with most hydrophobic polymers. Therefore, the
appropriate interlamellar surface modification of LDH is
essential for the preparation of LDH based polymer nano-
composites.13–18 This has been achieved by making it hydro-
phobic by intercalating organic or polymeric anions, such
as borate23 and aromatic diphosphonate,24 porphyrin function-
alized with sulfonate or carbonate groups,25 alkyl or aryl
carboxylates: stearate,17 oleate,26 malonate,27 valerate,28 dithio-
benzoate,29 alkyl sulfates: dodecyl sulfate,13–16 dodecyl benzene
sulfate30 into the LDH interlayer by ion exchange, in situ poly-
merization; or by introducing them during LDH synthesis by
Inorganic Materials and Nanocomposite Laboratory, Department of
Chemistry, Indian Institute of Technology, Kharagpur, 721302, India.
E-mail: sunit@chem.iitkgp.ernet.in; Fax: (+91-3222)255303; Tel: (+91-
3222)283334
† Electronic supplementary information (ESI) available: [DETAILS].
See DOI: 10.1039/c1jm13780h
18540 | J. Mater. Chem., 2011, 21, 18540–18551
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a co-precipitation method. Recently, grafting of LDHs with
silane and toluene diisocyanate have been developed as an
effective means in the formation of oxane31,32 and carbamate33
bonds with the –OH groups of LDHs respectively. However, no
work is reported in the literature related to the synergistic effect
of organomodification and interlamellar grafting of LDHs in the
reinforcement of the polymer matrix through chemical bond
formation and effective exfoliation in the polymer matrix.
The present work deals with the preparation of long chain
organic anions, such as dodecyl sulfate (DS) and stearate (St)
intercalated LDH followed by the interlamellar grafting of
hydroxyl groups (–OH) of the modified LDH via covalent
bonding with the isocyanate (–N]C]O) group of methylene
diphenylene diisocyanate (MDI) and isophorone diisocyanate
(IPDI) through the urethane linkage as shown in Scheme 1 and 2.
As a result, the internal as well as the external surfaces of LDHs
are likely to be synergistically modified making them more
hydrophobic compared to the simple DS or St intercalated LDH.
The choice of MDI and IPDI in this work is mainly guided by the
fact that they are commercially available as major raw materials
and could promote their industrial applications in the formation
of novel advanced polymer/LDH nanocomposites. Therefore, we
thought that it would be very interesting to investigate the
synergetic effect of interlamellar grafting and organo-
modification of LDHs in PU matrix on the morphology, inter-
action, mechanical properties and thermal stability behavior of
the corresponding nanocomposites and compare these findings
with PU nanocomposites obtained by individually grafted as well
as organomodified LDH.
Scheme 2 Schematic illustration of the preparation of interlamellar
grafting of hydroxyl groups of pristine, DS and St-LDH layers with
IPDI.
respectively. The hydroxyl value of Polyol-TG (mixture of
glycerol and trimethꢀylolpropane) was 58 mg KOH/g which was
dried for 24 h at 80 C before use. Methylenediphenylene diiso-
cyanate (MDI) and isophorone diisocyanate (IPDI) were
received from Aldrich Chemicals. Dibutyltin dilaurate (DBTL),
anhydrous toluene and dry terahydrofuran (THF) were obtained
from Aldrich Chemicals and E-Merck respectively. NaOH (S.D.
Fine chemicals, Boisar), Mg(NO3)2$6H2O, Al(NO3)3$9H2O (E.
Merck, India), sodium dodecyl sulfate (SDS) (SRL Pvt., Mum-
bai, India) and sodium stearate (SRL Pvt., Mumbai, India) were
used as received.
Experimental
Materials
The isocyanate terminated PU prepolymer and Polyol-TG were
kindly supplied by VSSC, ISRO, Trivandrum. The viscosity,
molecular weight and isocyanate content in the polyurethane
prepolymer were 2160 cps (at 25 ꢀC), 2300 g molꢁ1 and 4.4%
Preparation of pristine LDH, DS-LDH and St-LDH
Mg-Al-LDHs with exchangeable interlayer anions of nitrate
(NO3ꢁ) (pristine LDH) were synthesized by the co-precipitation
method as reported previously.13,14 DS-LDH and St-LDH were
prepared by an in situ method through drop-wise addition of
aqueous solution containing 3 : 1 molar ratio of Mg(NO3)2 and
Al(NO3)3 to sodium dodecyl sulfate or sodium stearate solution
under nitrogen atmosphere to minimize the contamination of
atmospheric CO2 i.e., to exclude carbonates from the LDHs.
Subsequently, 1 M NaOH solution was added to the above
solution by adjusting the pH at 8–9 and the mixture was aged at
ꢀ
80 C for 24 h and filtered. Finally, the precipitate was washed
five times with hot distilled water to remove surfactants, if any,
and vacuum dried at 60 ꢀC for 12 h and ground into powder.
Preparation of MDI and IPDI grafted pristine and
organomodified LDH
18 mmol of MDI was added to 18 mmol of 1-butanol in 80 mL
ꢀ
toluene at 60 C for 6 h with vigorous stirring. Additionally, 4
mmol of pristine LDH was added separately to 100 mL of
toluene and the mixture was sonicated for 45 min to disperse
Scheme 1 Schematic illustration of the preparation of interlamellar
grafting of hydroxyl groups of pristine, DS and St-LDH layers with
MDI.
ꢀ
pristine LDH completely and refluxed at 100 C for 30 min to
remove interlayer water. After that, MDI-butanol-toluene
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mixture containing two drops of DBTL was added into the
dispersed pristine LDH in toluene and reaction was refluxed at
100 ꢀC for 6 h. The product (MDI-g-pristine LDH) was filtered,
washed with toluene to remove unreacted MDI, vacuum dried
and ground into powders. Similar procedures were also adopted
for the preparation of the IPDI-g-pristine LDH, MDI-g-DS-
LDH, IPDI-g-DS-LDH, MDI-g-St-LDH and IPDI-g-St-LDH.
W2
W1
Gel content ð%Þ ¼
ꢃ 100
(1)
where, W1 ¼ initial weight of the polymer or polymer nano-
composites, W2 ¼ weight of the insoluble portion of the polymer
or polymer nanocomposites.
The volume fraction (Vr) of the nanocomposites in the swollen
mass was calculated by Florry–Huggins model of swelling,
Wrrꢁr 1
Wrrꢁr 1 þ Wsrꢁs 1
Preparation of MDI and IPDI grafted pristine and
organomodified LDH/PU nanocomposites
Vr ¼
(2)
where, rr and rs were the densities of polymer/nanocomposites
and solvent respectively, which were measured via the hydro-
static weighing method;
3 wt% MDI-g-pristine LDH was ultrasonically dispersed in dry
THF for 40 min under N2 atmosphere. Subsequently, the
dispersed MDI-g-pristine LDH was added into PU prepolymer
solution in THF followed by continuous stirring for 2 h at
ambient temperature under N2 atmosphere. Then, the equivalent
amount of polyol-TG (equivalent ratio of –NCO : –OH ¼ 1 : 1)
and 0.001 mol of DBTL were added with continuous stirring for
2 h. Finally, it was cast on Teflon Petri dish and kept at ambient
temperature for 24 h to obtain PU/MDI-g-pristine LDH nano-
composites. The same procedure was also extended using IPDI-
g-pristine LDH, MDI and IPDI grafted DS-LDH and St-LDH
for the formation of PU nanocomposites.
The cross link density (nc) and average molecular weights
between crosslinks (Mc) plays an important role in the
improvement of mechanical properties of the polymer and
polymer nanocomposites under equilibrium swollen conditions
and have been calculated according to the Flory–Rehner
equation:35
ꢀ
ꢁ
ꢁ lnð1 ꢁ VrÞ þ Vr þ cVr2
rr
nc ¼
and Mc ¼
0
1
2nc
1
(3)
VrC
B
@
V
V3 ꢁ
A
r
0
2
Characterizations
respectively, where c ¼ the Flory–Huggins polymer–solvent
interaction parameter; rr the density of the polymer/polymer
nanocomposites; Mc is the average molecular weight of network
chains between crosslinks; nc is the crosslink density and V0 the
molar volume of the solvent.
X-Ray diffraction (XRD) patterns were recorded at room
temperature on a PANalytical (PW 3040/60), ‘X’ Pert Pro (40
kV, 30 mA) with Cu-Ka radiation (l ¼ 0.1541 nm) with scan rate
of 2ꢀ minꢁ1. The distribution of grafted-LDH in PU matrix was
studied with a transmission electron microscope (JEM JEOL
2100-Japan) with an acceleration voltage of 200 kV. An Agilent
5100 atomic force microscope (AFM) instrument with a Si tip
from Budget Sensors was employed to obtain tapping-mode
images. Fourier transform infrared spectra were recorded on
a Perkin-Elmer (FTIR Spectrometer RXI) over the wave number
range of 400–4000 cmꢁ1. Nuclear magnetic resonance (1H NMR)
spectra were recorded on a Bruker 400 MHz spectrometer at
room temperature and d values relative to Me4Si (TMS). For this
purpose, all organomodified as well as grafted LDHs (MDI-g-
DS-LDH, MDI-g-St-LDH, IPDI-g-DS-LDH and IPDI-g-St-
LDH) were dissolved in DMSO-d6, whereas, MDI and IPDI
grafted pristine LDHs were made completly soluble on heating at
around 50 ꢀC. Tensile specimens were punched out from the cast
sheets using ASTM Die-C. The tests were carried out as per the
ASTM D 412-98 method in a universal testing machine
(Hounsfield-H10KS) at a cross head speed of 500 mm minꢁ1 at
Thermogravimetric analysis (TGA) were carried out on Red-
croꢀft 870 thermal analyzer, Perkin-Elmer with a heꢀating rate of
10 C minꢁ1 over a temperature range of 50–1000 C in air for
pristine, DS and St-LDH and their corresponding MDI and
IPDI grafted LDHs. TGA has also been carried out in an air
atmosphere using the same instrument over a temperature range
of 50–700 ꢀC for the corresponding nanocomposites of PU.
Differential scanning calorimetric (DSC) analysis have been
performed by a Perkin Elmer Pyris instrument in the temperature
range ꢁ70 to 0 C with a heating rate of 10 C minꢁ1 under N2
ꢀ
ꢀ
atmosphere.
Results and discussion
Fig. 1 displays the small angle XRD patterns of pristine LDH,
DS-LDH and St-LDH and their diisocyanate grafted counter-
parts in the 2q range of about 2–15ꢀ, respectively. It is observed
that the intensity of peak (003) in MDI-g-pristine LDH and
IPDI-g-pristine LDH decreases with respect to the pristine LDH,
though its position remains more or less unaltered (cf. Fig. 1a).
These observations clearly indicate that the reaction takes place,
which accounts for the observed easy compromise in LDH’s
crystallinity, defects and amorphous phase formation (as shown
in Fig. S2†).33 The diffractogram of MDI-g-pristine LDH and
IPDI-g-pristine LDH also shows the presence of a small hump in
the low angle region (2q < 10ꢀ). This in all probability is due to
the entry of a minute amount of MDI (or IPDI) in the interlayer
of pristine LDH, which interacts with –OH groups of LDH.33
ꢀ
(25 ꢂ 2) C.
Gel fraction, volume fraction of polymer or polymer nano-
composites in swollen gel and crosslink density were determined
by the following techniques. Previously weighed samples were
ꢀ
allowed to swell in THF at 30 C for 48 h and the equilibrium
swelling time was determined by a plot of mass uptake against
time. Then, the test species were taken out from THF, weighed
and dried to a constant weight in a vacuum oven at 70 ꢀC for 6 h.
The absence of the characteristic stretching frequency of –C–O–
C– in the FTIR spectra confirmed that the samples obtained under
these conditions were free from THF (as shown in Fig. S1†).34 The
gel fraction was calculated using the following expression,
18542 | J. Mater. Chem., 2011, 21, 18540–18551
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Fig. 1 X-Ray diffraction patterns of (a) pristine LDH, MDI-g-pristine LDH, IPDI-g-pristine LDH; (b) DS-LDH, MDI-g-DS-LDH, IPDI-g-DS-LDH
and (c) St-LDH, MDI-g-St-LDH, IPDI-g-St-LDH.
Table 1 records the d003 spacing of pristine and modified (DS
and St) LDHs along with their MDI and IPDI grafted LDHs.
The characteristic (003) peak of DS-LDH, in MDI and IPDI
grafted DS-LDH, is shifted from 3.85ꢀ (d ¼ 2.3 nm) to 2.78ꢀ (d ¼
3.17 nm) and 3.1ꢀ (d ¼ 2.84 nm), respectively (cf. Fig. 1b). This
suggests that MDI and IPDI enters successfully in the interlayer
space of modified LDH (DS-LDH), reacts with interlayer –OH
groups and further increases the interlayer space of DS-LDH.
Interestingly, from Fig. 1c, it is observed that the interlayer
spacing (3.75 nm) of St-LDH corresponding to the (003) peak is
relatively more in MDI (4.9 nm) and IPDI grafted St-LDH (4.41
nm) compared to the isocyanate grafted DS-LDH as mentioned
earlier. Based on these findings, it may be concluded that, the
hydrophobic surface and enlarged interlayer distance of DS-
LDH and St-LDH play an important role in interlamellar
covalent grafting of –OH groups of LDH.
absent in the case of 3 wt% filled MDI and IPDI grafted DS and
St-LDHs/PU nanocomposites suggesting uniform dispersion
and exfoliation of MDI or IPDI grafted DS-LDH or St-LDH
layers in PU matrix.37,38 However, XRD does not allow a more
detailed characterization of the structure of the nanocomposites,
as XRD alone can not detect layers exceeding 9 nm.13 Therefore
a complete characterization of the nanocomposite morphology
requires microscopic investigations to strengthen our views.
Fig. 3 shows TEM micrographs of PU nanocomposites with 3
wt% MDI and IPDI grafted pristine LDH, DS-LDH and St-
LDH. It is observed that the MDI and IPDI grafted pristine
LDH (cf. Fig. 3a and 3b) particles of the varying dimensions (50–
300 nm) are aggregated in PU. This may be attributed to the
strong attraction force between interlayer anions and MDI-g-
LDH (or IPDI-g-LDH) layers, which restricts its tendency to
intercalate in PU macromolecular chain.33,39 In fact, the hydro-
philic nature of isocyanate grafted pristine LDHs does not allow
it to be compatible with PU chains. It is also noticed from Fig. 3c
and 3d that formation of such aggregates is absent in case of the
MDI and IPDI grafted DS-LDH/PU nanocomposites, respec-
tively. However, some tactoids composed of several metal
hydroxide sheets (indicated by the arrows) with thicknesses 10–
30 nm are also present, along with exfoliated MDI-g-DS-LDH
and IPDI-g-DS-LDH layers in a PU matrix resulting in the
formation of partially exfoliated nanocomposites. Although the
significant exfoliation of individual layers of MDI or IPDI
grafted St-LDHs (thickness 2–3 nm and lateral dimension 30–60
nm) is also noticed in case of MDI or IPDI grafted St-LDH/PU
nanocomposites (cf. Fig. 3e and 3f), the MDI-g-St-LDH/PU
nanocomposite exhibited relatively better exfoliation and
homogeneous dispersion of LDH layers in PU matrix. However,
in either case, most of the exfoliated LDH layers are invariably
tilted with respect to the cutting section at some angle giving
thereby positive evidence of the nanoscale dispersion of MDI or
IPDI grafted St-LDH layers in the PU matrix. This observation
is further supported by the fact that the larger gallery spacing in
isocyanate grafted St-LDH leads to the reduced platelet–platelet
interaction, resulting its easier dispersion and higher degree of
exfoliation.
Fig. 2a and Fig. 2b show the XRD patterns of neat PU/MDI
and IPDI grafted LDHs (pristine and organomodified) nano-
composites. A broad diffraction peak at around 19.7ꢀ corre-
sponds to the neat PU,36 which is also present in PU/MDI(or
IPDI) grafted pristine LDH nanocomposites. In addition,
a sharp peak (2q ¼ 11.3ꢀ) also appears in the diffractogram
similar to MDI (or IPDI) grafted pristine LDH. These findings
clearly demonstrate that the PU chains are not intercalated in
the interlayers of the MDI or IPDI grafted pristine LDHs. On
the contrary, the (003) basal diffraction peak (Fig. 1) as well
as the weak (110) peak (as shown in Fig. S3†) are completely
Table 1 The layer spacing (d003) of pristine and modified (DS and St)
LDHs along with their MDI and IPDI grafted LDHs measured by X-ray
diffraction
Samples
Interlayer Spacing/nm
Pristine LDH
0.78
0.79
0.78
2.30
3.17
2.84
3.75
4.90
4.41
MDI-g-pristine LDH
IPDI-g-pristine LDH
DS-LDH
MDI-g-DS-LDH
IPDI-g-DS-LDH
St-LDH
Fig. 4 shows the tapping-mode AFM images of MDI (or
IPDI)-g-St-LDH/PU nanocomposites. It further confirms the
exfoliation of morphologically irregular 2-D ultrathin sheets of
MDI-g-St-LDH
IPDI-g-St-LDH
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Fig. 2 X-Ray diffraction patterns of (a) neat PU and its nanocomposites of MDI grafted pristine LDH, DS-LDH and St-LDH; (b) neat PU and its
nanocomposites of IPDI grafted pristine LDH, DS-LDH and St-LDH.
Fig. 3 TEM images of (a) PU/MDI-g-pristine LDH, (b) PU/IPDI-g-pristine LDH, (c) PU/MDI-g-DS-LDH, (d) PU/IPDI-g-DS-LDH, (e) PU/MDI-g-
St-LDH and (f) PU/IPDI-g-St-LDH nanocomposites showing the nature of dispersion of MDI and IPDI grafted pristine, DS and St-LDH layers in PU
matrix.
organomodified grafted LDHs with average lateral dimensions
in the range of 30–60 nm in PU matrix. The height profile of the
images revealed that the sheets consist of fairly flat terraces with
little wrinkling and an average thickness of around 3 nm, which
also agreed well with that detected by TEM. All these findings
unambiguously account for the unilamellar nature of the exfo-
liated nanosheets of MDI (or IPDI)-g-St-LDH in PU.40,41
Fig. 4 Tapping mode AFM images of 3D plot of (a) PU/MDI-g-pSt-LDH, (b) PU/IPDI-g-St-LDH nanocomposites deposited on a mica substrate.
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FTIR studies of MDI, IPDI (as shown in Fig. S4†) and their
physical mixtures with DS- and St-LDH at zero time (as shown
in Fig. S5†) suggests no grafting. In order to establish the
formation of –NHCOO linkages between isocyanate and
hydroxyl groups, FTIR spectra of the pristine, modified (DS and
St) LDHs and MDI and IPDI grafted pristine and modified
LDHs after 6 h of reaction have been recorded and are displayed
in Fig. 5. A broad absorption band is observed in all LDHs in the
range of 3400–3600 cmꢁ1 corresponding to the –OH stretching
(1550 cmꢁ1), amide carbonyl stretching mode (1646 cmꢁ1) and
–C]O stretching vibrations of urethanes (1703 cmꢁ1).33,42,43 In
either case, it is found that the band corresponding to the
–N]C]O group (2272 cmꢁ1) is absent due to the chemical
reaction of diisocyanate with the hydroxyl group of LDHs sug-
gesting no absorption/intercalation of free isocyanate on its
surface.44 Similar observations have also been inferred in the
IPDI grafted DS-LDH. However, the absorption band at 3300
cmꢁ1 due to –NH stretching vibrations is absent, whereas a new
band corresponding to the –NH bending vibration appears at
1527 cmꢁ1, suggesting the partial grafting of pristine LDH with
IPDI. Finally, the hydrolytic stability of the –NHCOO linkages
in MDI grafted St-LDH has also been proven by FTIR of MDI
grafted St-LDH and its suspension in water after 72 h (as shown
in Fig. S6†). It shows that the positions of the peaks earlier
observed for the –NH stretching and –NH bending vibration
mode in Fig. 5h remains more or less unaltered. The presence of
the peak at 1700 cmꢁ1 corresponding to –CO of urethane group
in Fig. S6b† confirms that the amine formation has not taken
place due to the nonavailability of the free isocyanate in MDI
grafted St-LDH. Similar findings have also been observed in case
of other isocyanate grafted LDHs in water.
vibration of the surface bound hydroxyl group of the LDHs.13
A
new band appears in all the isocyanate grafted LDHs at about
3326 cmꢁ1 due to the –NH stretching mode42,43 overlapping
partly with the broad absorption band of –OH group. The peak
at 1700 cmꢁ1 corresponds to the –C]O stretching vibration of
the urethane linkage of the surface of grafted St-LDH. An
additional absorption band is observed at around 1539 cmꢁ1 due
to the newly appeared –NH bending vibration mode of the amide
II bond for MDI grafted St-LDH. The new stretch at 1633 cmꢁ1
can be assigned to the amide carbonyl stretching mode (i.e. the
amide I vibrational stretch).42,43 The bands detected in the lower
frequency region of 800–600 cmꢁ1 are due to the M–O vibra-
tional modes.13 Such observations have also been noticed in case
of the MDI grafted pristine and DS-LDH. The presence of
absorption bands at 2851, 2920 and 2952 cmꢁ1 in grafted DS and
St-LDH arises due to the deformation vibration of –CH3 and
–CH2 of the long alkyl chain of LDH.15,17 All these findings
clearly suggest that LDH is organomodified followed by grafting
with MDI. The grafting of IPDI in St-LDH has been confirmed
by the presence of the absorption bands corresponding to the
–NH stretching vibration (3296 cmꢁ1), –NH bending vibrations
1H NMR spectroscopic analysis has also been used as another
confirmatory evidence for the grafting of –OH in LDHs with
1
–NCO of MDI or IPDI. Fig. 6(a,b) represents the H NMR
spectra of aromatic diisocyanate grafted LDH (MDI-g-St-LDH)
and aliphatic diisocyanate grafted LDHs (IPDI-g-St-LDH). The
aromatic diisocyanate grafted LDHs show the presence of the
characteristic signals at about 9.4 and 8.5 ppm corresponding to
the amide protons placed in a different environment45 as shown
earlier in Scheme 1. Also the signals due to the protons of phenyl
rings appear at 7.0 and 7.3 ppm. However, in case of aliphatic
diisocyanate grafted LDHs, amide protons appear at around 7.2
ppm due to the shielding effect of the carbonyl group (Scheme 2).
On the contrary, in the case of aromatic diisocyanate grafted
LDHs, the amide proton located in the deshielding zone of the
aromatic ring appears at a relatively higher signal. The methylene
group attached to –NHCOO shows a signal at about 3.8 ppm.
All these observations confirm that the grafting of –NCO with
–OH results in the formation of urethane linkages. In addition,
Fig. 5 FTIR spectra of (a) pristine LDH, (b) MDI-g-pristine LDH, (c)
IPDI-g-pristine LDH, (d) DS-LDH, (e) MDI-g-DS-LDH, (f) IPDI-g-
DS-LDH, (g) St-LDH, (h) MDI-g-St-LDH and (i) IPDI-g-St-LDH.
Fig. 6 1H NMR spectra of (a) MDI-g-St-LDH and (b) IPDI-g-St-LDH.
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the signal at around 4.1 ppm in both cases confirms the presence
of –OH groups in grafted LDHs,46 which subsequently reacted
with –NCO terminated prepolymer. Similar findings have also
been found for other MDI and IPDI grafted LDHs (as shown in
Fig. S7†).
appear in PU/MDI-g-St-LDH nanocomposites suggesting the
existence of MDI-g-St-LDH in the PU matrix.13 The charac-
teristic stretching frequency of neat PU at 3312 and 1720 cmꢁ1
corresponding to –NH and –CO groups shifted to 3291 and
1706 cmꢁ1, respectively, signifying clearly the formation of H-
bonding between PU and MDI-g-St-LDH.48,49 Similar obser-
vations have also been observed for PU nanocomposites of
MDI (and IPDI) grafted pristine, DS-LDH and IPDI grafted St
LDH.
The possibility of urethane linkage formation between –NCO
group of PU prepolymer and ungrafted –OH group of MDI-g-
DS (or St)-LDH and IPDI-g-DS (or St)-LDH has also been
established by monitoring the intensity of –NCO and –CO peaks
(as shown in Fig. S8†). The –NCO terminated PU prepolymer is
characterized by the presence of an absorption peak at 2270
cmꢁ1. Further, it is observed that the intensity of the –NCO peak
decreases after 2 h reaction of MDI grafted DS (or St)-LDH or
IPDI grafted DS (or St)-LDH with PU prepolymer. Addition-
ally, the intensity of the peak at 1725 cmꢁ1, corresponding to the
–CO group, also increases gradually due to the formation of
urethane linkages.
Mechanical properties are tested to study the effect of inter-
lamellar grafting of pristine and organomodified LDHs in the
formation of PU nanocomposites. The mechanical data related
to the tensile strength (TS), elongation at break (EB) and
modulus at different percentage of strain for PU and its nano-
composites are presented in Table 2 and shown in Fig. 8. It is
clearly demonstrated that, the isocyanate-grafted organo-LDHs
impart improvement in tensile strength as well as elongation at
break for PU nanocomposites. The maximum improvement in
the tensile strength is observed for PU nanocomposites of MDI
grafted St-LDH (391%) and IPDI grafted St-LDH (318%). Such
significant improvement in mechanical properties is likely to be
due to chemical interaction of the prepolymer with grafted LDH,
as observed in case of isocyanate-grafted MMT/PU nano-
composites.43 In addition, the crosslinking effect of polyol TG
also contributes to the enhancement in mechanical properties.
The observed improvements in the tensile strength may also be
attributed to the homogeneous dispersion of IPDI grafted St-
LDH and MDI grafted St-LDH layers in PU matrix as is evident
from the TEM and AFM in Fig. 3 and 4. In case of MDI-g-DS-
LDH/PU and IPDI-g-DS-LDH/PU nanocomposites, the tensile
strength is improved by 270% and 241%, respectively. In order to
prove that the isocyanate effect is real, PU nanocomposites of
DS-LDH and St-LDH at similar loading (3 wt%) have also been
prepared following the similar procedures and the respective
mechanical data is provided in Table 2 (as shown in Fig. S9†). It
is clearly evident that the improvement in tensile strength is
higher for the PU nanocomposites of isocyanate grafted orga-
nomodified LDHs than the individual grafted or organomodified
LDH/PU nanocomposites. Hence, it is anticipated that the
synergistic effect of isocyanation and long chain modification of
LDHs results in the formation of the crosslinked PU nano-
composites providing significant improvements in the tensile
strength.
Fig. 7 shows the FTIR spectra of MDI-g-St-LDH, neat PU
and PU/MDI-g-St-LDH nanocomposites. The absence of the
peak at 2270 cmꢁ1 of –NCO group suggests that the curing of
PU prepolymer with polyol-TG is completed.47 In case of MDI-
g-St-LDH, the bands detected in the lower frequency region of
800–600 cmꢁ1 corresponding to the M–O vibrational modes
Fig. 7 FTIR spectra of (a) MDI-g-St-LDH (b) neat PU and (c) PU/
MDI-g-St-LDH nanocomposite.
Table 2 Mechanical data of polyurethane and its nanocomposites with 3 wt% MDI and IPDI grafted LDH, DS-LDH and St-LDHa
Modulus/MPa
Samples
100%
200%
300%
TS/MPa
EB (%)
PU
0.28 ꢂ 0.01
0.32 ꢂ 0.01
0.35 ꢂ 0.01
0.40 ꢂ 0.02
0.43 ꢂ 0.02
0.48 ꢂ 0.03
0.62 ꢂ 0.02
0.38 ꢂ 0.01
0.35 ꢂ 0.02
0.41 ꢂ 0.01
0.43 ꢂ 0.03
0.45 ꢂ 0.03
0.53 ꢂ 0.03
0.60 ꢂ 0.03
0.65 ꢂ 0.02
0.88 ꢂ 0.03
0.49 ꢂ 0.02
0.52 ꢂ 0.03
0.54 ꢂ 0.01
0.56 ꢂ 0.02
0.58 ꢂ 0.02
0.70 ꢂ 0.02
0.80 ꢂ 0.02
0.84 ꢂ 0.03
1.14 ꢂ 0.02
0.64 ꢂ 0.03
0.67 ꢂ 0.03
1.10 ꢂ 0.6
1.76 ꢂ 0.3
2.48 ꢂ 0.7
3.75 ꢂ 0.8
4.07 ꢂ 0.4
4.60 ꢂ 0.6
5.40 ꢂ 0.5
3.20 ꢂ 0.7
2.60 ꢂ 0.4
626 ꢂ 27
850 ꢂ 32
800 ꢂ 15
900 ꢂ 40
872 ꢂ 20
906 ꢂ 22
822 ꢂ 29
866 ꢂ 24
895 ꢂ 35
PU/IPDI-g-LDH
PU/MDI-g-LDH
PU/IPDI-g-DS-LDH
PU/MDI-g-DS-LDH
PU/IPDI-g-St-LDH
PU/MDI-g-St-LDH
PU/DS-LDH+
PU/St-LDH+
a
Stress–strain plots+ of PU/DS-LDH and PU/St-LDH are provided under Fig. S9.†
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Fig. 8 Stress–strain plots of (a) neat PU and its nanocomposites of MDI grafted pristine, DS and St-LDH; (b) neat PU and its nanocomposites of IPDI
grafted pristine, DS and St-LDH.
Table 2 shows that the modulus at 100, 200 and 300% is
significantly increased for PU nanocomposites consisting of
MDI and IPDI grafted pristine and DS and St-LDH and is
found to be maximum for MDI-g-St-LDH/PU nanocomposites.
This is probably due to the development of shear zones in
nanocomposites under stress and strain conditions13,37 or high
aspect ratio of LDH leading to its enhanced interaction with
PU.16,17 In addition, the nanoreinforcement effect from the MDI-
g-St-LDH on the PU matrix also enhances the modulus. Table 2
also shows the relatively lower elongation at break for all MDI
grafted LDHs/PU nanocomposites in comparison to their cor-
responding IPDI grafted LDHs/PU nanocomposites. However,
the maximum decreases in the percent elongation (31.3%) is
observed in case of MDI-g-St-LDH/PU nanocomposites. This,
in all probability, is due to the fact that the PU chains in the
nanocomposites are more restricted by the MDI grafted St-LDH
layers, owing to the increased stiffness as well as crosslink
density, which results in the decreased degree of freedom as
evident from the DSC analysis discussed later.50
corresponding data is presented in Table 4. It is observed that the
gel content and crosslink density of the polymer and its nano-
composites increases gradually with increasing grafting efficiency
of LDHs as discussed later. This leads to an increase in the
availability of –NHCOO linkages and consequently the reduc-
tion in the accessibility of the –OH groups, causing thereby
greater available crosslinking of –NCO group of prepolymer
with –OH groups of polyol-TG.52 Thus, the available vacant sites
decrease for solvent uptake during swelling. The highest cross-
link density and gel content in the PU nanocomposite of MDI
grafted St-LDH also supported the observed maximum rein-
forcement in the mechanical properties. Also, the respective
exfoliation and partial exfoliation of MDI (or IPDI)-g-St-LDH
and MDI (or IPDI)-g-DS-LDH layers in the PU matrix, as
evident from TEM in Fig. 3(c–f), also inhibits the solvent
molecules to penetrate into the PU matrix. Interestingly, the
increased crosslink density and decreased average molecular
weight between crosslinks of the PU/MDI grafted LDHs nano-
composites compared to the IPDI grafted LDHs may possibly be
due to the relatively lower reactivity of aliphatic diisocyanate.
In order to analyze the organic modification and isocyanate
grafting of pristine LDH, DS-LDH, St-LDH and their
Table 3 represents the maximum percentage improvement in
tensile strength and elongation at break in PU nanocomposites
formed by 3 wt% each of DS-LDH, MDI (or IPDI) grafted DS-
LDH, St-LDH and MDI (or IPDI) grafted St-LDH. It is noted
that the grafting of isocyanate with the hydroxyl groups of
organomodified LDH in general improves the tensile strength of
corresponding PU nanocomposites, though isocyanate grafted
St-LDH/PU nanocomposites exhibits the maximum improve-
ment (391% and 318%), compared to PU/DS-LDH or PU/St-
LDH nanocomposites (as shown in Fig. S9†). This is because of
the stronger reinforcement of PU by isocyanate grafted St-LDH
than that by St-LDH because of the presence of chemical
bonding between isocyanate grafted St-LDH and PU.43,51
Additionally, the incorporation of MDI (or IPDI) grafted St-
LDH in the PU matrix dramatically improves the tensile strength
compared to PU nanocomposites of MDI grafted organo-MMT
(48%).43
Table 3 Data related to the maximum improvement (%) in mechanical
properties of 3 wt% filler loaded PU nanocomposites compared with that
of neat PUa
Maximum improvement in
Fillers
TS (%)
EB (%)
PU/IPDI-g-LDH
PU/MDI-g-LDH
PU/DS-LDH+
PU/IPDI-g-DS-LDH
PU/MDI-g-DS-LDH
PU/St-LDH+
60
125
191
241
270
136
318
391
36
28
38
44
39
43
45
31
PU/IPDI-g-St-LDH
PU/MDI-g-St-LDH
The network properties of PU and its nanocomposites could
be explained on the basis of gel content (%), crosslink density
(nc), average molecular weight between crosslinks (Mc) and the
a
Stress–strain plots+ of PU/DS-LDH and PU/St-LDH are provided
under Fig. S9.†
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Table 4 Gel content (%), volume fraction (Vr), density (rr), crosslink density (nc), average molecular weight between crosslinks (Mc) of PU and its
nanocomposites
Sample
rr/gcmꢁ3
Gel content (%)
Vr
nc ꢃ 105/g ccꢁ1
Mc ꢃ 10ꢁ5/g molꢁ1
Neat PU
DS-LDH/PU
St-LDH/PU
MDI-g-LDH/PU
IPDI-g-LDH/PU
MDI-g-DS-LDH/PU
IPDI-g-DS-LDH/PU
MDI-g-St-LDH/PU
IPDI-g-St-LDH/PU
1.066
1.095
1.085
1.082
1.075
1.132
1.122
1.205
1.391
68
78
77
75
70
84
80
89
86
0.0622
0.0733
0.0723
0.0707
0.0681
0.0931
0.0862
0.1252
0.1033
1.966
2.676
2.583
2.499
2.331
4.255
3.658
7.712
5.214
0.2711
0.2045
0.2100
0.2164
0.2305
0.1330
0.1533
0.0781
0.1334
ꢀ
corresponding MDI and IPDI grafted LDHs were subjected to
thermal degradation analysis in air from room temperature to
1000 ꢀC to ensure the complete oxidation and removal of any
carbon char remaining. The decomposition of IPDI and MDI
grafted pristine LDH, DS-LDH and St-LDH plays an important
role in determining the thermal stability of the PU nano-
composites. Fig. 9(a–c) displays the thermograms (TGA and
DTG curves) of pristine LDH, DS-LDH, St-LDH and their
corresponding MDI and IPDI grafted LDHs. Pristine LDH and
their MDI and IPDI grafted pristine LDH shows two step
decompositions, whereas thermal decomposition for DS-LDH,
IPDI-g-DS-LDH, MDI-g-DS-LDH, St-LDH, IPDI-g-St-LDH,
and MDI-g-St-LDH takes place in several steps. Pristine
LDH, IPDI-g-pristine LDH and MDI-g-pristine LDH exhibit
their respective first step decomposition peak at around 200, 206
and 252 ꢀC as inferred from DTG and the corresponding weight
loss in TG referred to the expulsion of the physisorbed and
intercalated water molecules.13–16,32 In addition, the second peak
in DTG appears at around 364, 368 and 388 C signifying the
decomposition of metal hydroxide layers and volatilization of
isocyanate. On the contrary, DS-LDH, St-LDH and their IPDI
and MDI grafted LDHs show their first decomposition
temperature at around 87, 80, 88, 82, 167 and 175 ꢀC respectively
due to the loss of physisorbed water molecules on the nongallery
surfaces of modified LDHs. In all probability, such drastic
reduction in their respective first decomposition temperature
compared to the pristine LDH and their MDI (or IPDI) grafted
LDHs may be attributed to the weaker interaction of water
molecules with dodecyl sulfate (and stearate) anion and
hydroxide sheets located in the interlayers of LDH through
hydrogen bonding.53 In addition, the second decomposition
peaks for DS-LDH, St-LDH, MDI-g-DS-LDH, IPDI-g-DS-
LDH, MDI-g-St-LDH and IPDI-g-St-LDH are observed at
around 240, 189, 288, 233, 273 and 258 ꢀC respectively due to the
loss of surfactant anions (DS and St).13–16,54 The weight loss in
TG above 340 ꢀC is attributed to the volatilization of isocyanate,
Fig. 9 TGA and DTG plots of (a) pristine LDH, IPDI-g-pristine LDH and MDI-g-pristine LDH, (b) DS-LDH, IPDI-g-DS-LDH and MDI-g-DS-
LDH (c) St-LDH, IPDI-g-St-LDH and MDI-g-St-LDH.
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Table 5 Weight residue (Wres), degree of modification (Dmod), degree of grafting (Dg) and mass ratio (Mr) data of MDI-g-pristine LDH, IPDI-g-pristine
LDH, DS-LDH, St-LDH and their corresponding MDI and IPDI grafted LDHsa
Samples
Wres (wt%)
Dmod (wt%)
Dg (wt%)
Mr
MDI-g-LDH
IPDI-g-LDH
DS-LDH
MDI-g-DS-LDH
IPDI-g-DS-LDH
St-LDH
48.23
50.35
41.46
32.37
35.63
32.13
20.17
24.65
—
—
10.16
6.21
—
21.93
14.07
—
0.11 : 1
0.06 : 1
0.29 : 1
0.36 : 0.29 : 1
0.21 : 0.29 : 1
0.67 : 1
22.77
22.77
22.77
40.15
40.15
40.15
MDI-g-St-LDH
IPDI-g-St-LDH
37.23
23.28
0.99 : 0.67 : 1
0.50 : 0.67 : 1
a
Wres of LDH ¼ 53.68.
Fig. 10 Thermograms of (a) neat PU and its MDI grafted pristine, DS- and St-LDH nanocomposites; (b) neat PU and its IPDI grafted pristine, DS and
St-LDH nanocomposites.
loss of remaining dodecyl sulfate and stearate.15,16,54 The extent
of grafting of LDHs has been calculated on the basis of the
residue left in TGA in air atmosphere at 1000 ꢀC45,55 and the
corresponding data related to the weight residue, degree of
modification, degree of grafting and mass ratio are recorded in
Table 5 (detailed calculations are given in supplementary
information†). It shows that the degree of isocyanate grafting is
maximum (37.23 wt%) for MDI grafted St-LDH, which is also
supported by its highest interlayer spacings.
PU and individually organomodified LDH/PU nanocomposites
(as shown in Fig. S10†). Similar improvements in the onset
decomposition temperature have also been observed in case of
EPDM/DS-LDH13 and EVA/Borate-LDH23 nanocomposites
respectively.
Fig. 10 also shows that the thermal decomposition of neat PU
and its nanocomposites takes place in two steps. The first stage
(50–260 ꢀC) corresponds to the depolymerisation originating
from the failure of urethane links releasing thereby the polyol
and isocyanate followed by volatilization of monomers.16 The
second stage in the TG refers to the decomposition of urea
groups (260–420 ꢀC) followed by the high temperature
Fig. 10 represents the influence of 3 wt% loaded pristine
LDH, DS and St-LDHs grafted with MDI and IPDI on the
thermal stability of PU nanocomposites in an air atmosphere.
When MDI and IPDI grafted pristine LDHs are used as fillers
ꢀ
in PU, they are thermally stable up to 256 C compared to the
252 and 206 ꢀC for MDI and IPDI grafted pristine LDHs
respectively. It is to be noted that the total weight loss of 19.0
(ꢄ6 wt% for neat PU and 13.0 wt% for MDI-g-LDH) and
24 wt% (ꢄ6 wt% for neat PU and 18 wt% for IPDI-g-LDH)
observed separately up to its onset decomposition temperature
at 256 ꢀC shown in Fig. 9 and 10 do not match with the total wt.
loss (ꢄ5 wt%) recorded for grafted pristine LDH/PU nano-
composites in Fig. 10. It is anticipated that the presence of the
metal like Mg in MDI or IPDI grafted LDH can cause catalytic
degradation of the PU matrix56 and we can account for such
a mismatch in the wt. loss. Similar observations have also been
noted for the MDI and IPDI grafted DS-LDH/PU and
St-LDH/PU nanocomposites with their relatively higher onset
decomposition temperature (265–274 ꢀC) compared to its neat
Table 6 Dynamic TGA data for PU and its nanocomposites containing
3 wt% MDI and IPDI grafted pristine LDH, DS-LDH and St-LDHa
Samples
Td(5%)/ꢀC
Td(50%)/ꢀC
Wres at 700 ꢀ
C
PU
248
256
259
265
270
268
274
261
257
367
375
379
380
384
383
388
377
380
1.2
2.4
2.6
4.4
4.6
4.8
5.1
3.7
4.0
PU/IPDI-g-LDH
PU/MDI-g-LDH
PU/IPDI-g-DS-LDH
PU/MDI-g-DS-LDH
PU/IPDI-g-St-LDH
PU-MDI-g-St-LDH
PU/DS-LDH*
PU/St-LDH*
a
Thermograms* of PU/DS-LDH and PU/St-LDH are provided under
Fig. S10.†
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Fig. 11 DSC scans of (a) neat PU and its nanocomposites of MDI grafted pristine, DS and St-LDH; (b) neat PU and its nanocomposites of IPDI
grafted pristine, DS and St-LDH.
ꢀ
degradation (420–700 C) of these stabilized structures to yield
small quantity of carbonaceous char. Table 6 records the thermal
data of neat PU and its nanocomposites with MDI and IPDI
grafted pristine LDH, DS-LDH and St-LDH, when temperature
at 5 and 50% wt. loss (Td(5%), Td(50%)) are selected as a point of
comparison. The maximum increase in the onset decomposition
Thus the significant increase in Tg,soft is observed in PU
nanocomposites of MDI-g-St-LDH (11 ꢀC) and IPDI-g-St-LDH
(8 ꢀC), which may be due to the stronger confinement of the
segmental motions of intermolecular chains of PU within the
grafted St-LDH galleries compared to grafted pristine LDHs and
DS-LDHs.50 As a result, the mobility of the PU chains attached
to grafted LDH in the interfacial layers is restricted, which
increases the requisite thermal energy for the occurrence of glass
transition in the nanocomposite. Therefore, the relaxation
behavior is restricted depending on the extent of exfoliation of
the grafted LDH as well as the interfacial interaction between PU
and grafted LDH. However, the endotherms appear at ꢁ39 and
ꢀ
temperatures of 26 and 21 C are found in case of PU/MDI-St-
LDH and PU/IPDI-g-St-LDH nanocomposites respectively. The
inset of the Fig. 10a and Fig. 10b show that the thermal stability
ꢀ
ꢀ
of neat PU (367 C) is improved to 388 and 383 C at 50% wt.
loss for the PU/MDI-g-St-LDH and PU/IPDI-g-St-LDH nano-
composites. The improvements in thermal decomposition
temperature of PU nanocomposites at the same wt. loss possibly
arises due to the nanoscale dispersion of MDI or IPDI grafted
modified LDH, which hinders the release of volatile degradation
products and reduces the initiation of PU chain scission.13–16
Interestingly, Table 6 shows that the improvement in thermal
stability of 3 wt% filled MDI grafted St-LDH/PU nano-
composites is higher compared to that of the PU/St-LDH
nanocomposite (as shown in Fig. S10†). Additionally, the wt.
reside in general increases for all the composites and is maximum
for MDI or IPDI grafted St-LDH/PU nanocomposites. In all
likelihood, the enhanced char residue produced by the decom-
position of diisocyante in MDI or IPDI grafted St-LDH and the
decomposition products (MgO and a spinel like structure
MgAl2O4) of LDH makes a definite contribution in the degra-
dation process and may find application as fire safety
materials.11,13
ꢀ
ꢁ40 C for 3 wt% loaded PU/DS-LDH and PU/St-LDH nano-
composites respectively, which is lower than their corresponding
isocyanate grafted PU nanocomposites (as shown in Fig. S11†).
Therefore, it is anticipated that the synergistic effect of organo-
modification and isocyanate grafting of LDHs is responsible for
such enhancement of Tg of PU nanocomposites.
Conclusions
The hydroxyl groups of pristine and organically modified (DS
and St) LDHs have been grafted with isocyanate group of MDI
and IPDI and used in the preparation of polyurethane nano-
composites by in situ polymerization and characterized to
investigate the synergistic effect of organomodification and
grafting of LDHs in reinforcing the properties of PU nano-
composites. These findings have also been compared with PU
nanocomposites of the individually grafted pristine LDH as well
Fig. 11 represents the DSC curves of neat PU and its nano-
composites of MDI and IPDI grafted pristine, DS and St-
modified LDHs (3 wt%). It shows that the glass transition
temperature of the soft segment phase (Tg,soft) of PU, MDI-g-
pristine LDH, MDI-g-DS-LDH and MDI-g-St-LDH nano-
composites are ꢁ42, ꢁ41.6, ꢁ36 and ꢁ31 ꢀC respectively (cf.
Fig. 11a). On the other hand, Tg,soft of the PU nanocomposites of
IPDI grafted pristine LDH, DS-LDH and St-LDH nano-
composites are found to be ꢁ42, ꢁ38 and ꢁ34 ꢀC respectively (cf.
Fig. 11b). It is also noted from Fig. 11 that Tg,soft of MDI or
IPDI grafted pristine LDH/PU nanocomposites remains more or
less similar to the neat PU due to their poor intercalation in the
PU matrix, which is also evident from the TEM images in Fig. 3
(a–b) shown earlier.
1
as organomodified LDH. FTIR and H NMR studies provide
the successful evidence for the isocyanate grafting of inter-
lamellar hydroxyl groups of LDHs by MDI or IPDI, which also
supports the observed substantial increase in the interlayer
spacings based on XRD analysis. Further, the extent of isocya-
nate grafting of pristine, DS and St-LDH has been estimated on
the basis of TGA studies. TEM studies reveal the better homo-
geneous dispersion and effective exfoliation of MDI (or IPDI)
grafted St-LDH in PU matrix as compared to the MDI (or IPDI)
grafted DS-LDH, whereas inhomogeneous distribution of MDI
(or IPDI) grafted pristine LDH layers takes place in the corre-
sponding PU nanocomposites. The average thickness of exfoli-
ated grafted St-LDH nano sheets in PU matrix as determined by
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the height-profile tapping-mode AFM image is found to be
about 3 nm which is also supported by TEM. Among the 3 wt%
of the DS or St modified LDH/PU, isocyanate grafted DS and
St-LDH as well as grafted pristine LDHs/PU nanocomposites,
MDI-g-St-LDH/PU shows the maximum improvements in the
tensile strength (391%) due to its better interaction with PU,
which is also supported by its crosslink density. TGA studies
show the maximum increase in thermal stability by 26 and 21 ꢀC
at 5 and 50% wt. loss for MDI-g-St-LDH/PU nanocomposites.
DSC studies reveal that MDI-g-St-LDH/PU and IPDI-g-St-
LDH/PU exhibit a maximum shift in glass transition tempera-
ꢀ
ture (Tg,soft) by 11 and 8 C with respect to the neat PU. Such
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significant enhancements in the properties are due to the
synergistic effect of organomodification and isocyanate grafting
of LDH, nanolevel dispersion of MDI-g-St-LDH layers in the
PU matrix and the improvement in the interfacial interaction
among themselves. All these findings clearly demonstrate that
the interlamellar grafting of the hydroxyl groups of organo-
modified LDHs with MDI or IPDI provides an effective way to
prepare thermally stable and mechanically improved crosslinked
PU nanocomposites for multifaceted future applications in
various fields like adhesion, lamination, aerospace and flame
retardancy.
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Acknowledgements
The authors are grateful to CSIR and DRDO, New Delhi, India,
for the financial support and also to Vikram Sarabhai Space
Centre, Thiruvananthapuram, India for providing PU prepol-
ymer (ISRO-PU 2000). The authors would like to thank
Professor R. Mukherjee, Department of Chemical Engineerig,
IIT Kharagpur for AFM analysis.
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This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 18540–18551 | 18551