# Mechanical and Thermal Properties of Poly(urethane urea) Nanocomposites Prepared with Diamine-Modified Laponite

Authors: Joe-Lahai Sormana; Santanu Chattopadhyay; J. Carson Meredith
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Nanocomposites based on segmented poly(urethane urea) were prepared by reacting a poly(diisocyanate) with diamine-modified Laponite-RD nanoparticles that served as a chain extender. The nanocomposites were prepared at a constantNH2 to NCO mole ratio of 0.95, while varying the fraction of diamine-modified Laponite relative to the free diamine chain extender. Compared to neat poly(urethane urea), all nanocomposites showed increased tensile strength and elongation at break. As Laponite loading increased, tensile properties passed through a maximum at a particle concentration of 1 mass%, at which a 300% increase in tensile strength and 40% increase in elongation at break were observed. A maximum in urea and urethane hard-domain melting endotherms was also observed at this Laponite loading. Optimal mechanical and thermal properties coincided with a minimum in the size of the inorganic Laponite phase. Nanocomposites containing diamine-modified Laponite had higher tensile strengths than those with nonreactive monoamine-modified Laponite or diamine-modified Cloisite.


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## Body

## 1. Introduction

The synthesis and characterization of polymer composites with nanometer-size silicates have become an extremely active research area, and significant
improvements in mechanical, thermal, and barrier properties have been shown
relative to the pristine polymers. Many studies have illuminated the
difficulties of achieving nanometric dispersion of silicates in polymers,
especially for layered silicates [1, 2]. These difficulties arise apparently
from weak intermolecular interactions between the silicate and the polymer, relative
to the strong interactions between silicate layers.Synthetic Laponite is a silicate with well-controlled
dimensions (1×25nm disc) that forms less strongly-layered structures than
those found in natural silicates. Laponite is
readily modified with surfactants due to its complete exfoliation in
water. A number of studies have demonstrated physical blending as well as
chemical incorporation of Laponite in polymers [3–12]. The
addition of functionalized (reactive) surfactants to Laponite, followed by
incorporation into nanocomposites has not been reported extensively until
recently [8, 12–15].Despite the industrial importance of poly(urethane ureas)
(PUUs) as specialty engineering elastomers, few PUU-based nanocomposites have
been demonstrated [16–19].  PUU elastomers are block copolymers consisting
of alternating rubbery (soft) and crystalline urea (hard) segments that provide physical cross-links between soft
segments.  The mechanical and thermal
properties of PUUs depend on the size, crystallinity, and interconnectivity of
the hard domains, as well as mixing of hard segments in the soft segment phase
[20–23]. Depending on
the ratio of hard and soft segments, and reaction temperature, the hard domain
size can approach nanoscale dimensions on the order of 50 nm to
100 nm. While PUU elastomers have excellent elastic properties, they are limited
by the melting of hard domains at temperatures greater than ∼270°C.  One avenue for improving these properties is
by dispersing inorganic nanoparticles. Due to the complex
morphology of PUUs, the question arises how the presence of nanoparticles will
affect the PUU morphology, crystallinity, and hydrogen bonding [24]. PUU nanocomposites with improved
mechanical and barrier properties have been prepared by physically mixing
organically modified montmorillonite with presynthesized PUU in solution [19]. The
use of nanoparticles as participants in the chemical reaction to produce PUU
networks has also received attention [17, 20–23, 25, 26].  Studies have also reported the preparation of
polyurethane (nonurea) nanocomposites with the nanoparticle as a reacting component of the chain-extender [27–31] or the
prepolymer [32, 33]. These nanocomposites showed partial intercalation and
significant improvement in tensile strength and elongation at break.This paper reports on the surface modification of
Laponite-RD particles with amine functional groups and their use in preparing
Laponite-PUU nanocomposites. We describe an ion-exchange process for coating
the particles with organic diamines, resulting in a free amine group tethered
to the surface. These are characterized and compared to nanocomposites prepared
with monoamine-coated Laponite (no free amine groups) and nanocomposites made
with diamine-modified Cloisite-Na (montmorillonite). The relationship between
Laponite dispersion, PUU structure, as well as mechanical and thermal properties is
discussed.


## 2. Experimental Methods

### 2.1. Particle Modification

Laponite-RD and Cloisite-Na, having cationic exchange capacities of 75 mequiv/100 g and 92.6 mequiv/100 g, respectively,
were obtained from Southern Clay Products Inc. (Gonzales, Tex, USA), and used as received.  Laponite (Na+0.7[(Si8Mg5.5Li0.3)O20(OH)4]0.7−)
is a mica-type synthetic hectorite that hydrates in water to give a homogenous
dispersion of exfoliated nanometric disks (1nm×25–30nm) [34]. Cloisite-Na
is a natural montmorillonite clay with a lateral size of about 100–1000 nm.  Trimethylene glycol di-p-aminobenzoate (Air Products and Chemicals Inc., Allentown, Pa,
USA, Versalink 740-M, 98.2% purity, 314 g/mol)
and 4-benzylaniline (Lancaster Synthesis, Ward Hill, M, USA, 98% purity, 183 g/mol) were treated with excess concentrated
HCl to form ammonium salt. For the remainder of this paper, trimethylene glycol
di-p-aminobenzoate and 4-benzylaniline
will be referred to as diamine and monoamine, respectively.  The excess HCl was allowed to evaporate under
ambient conditions for several days, resulting in a dry salt.  The dried monoamine and diamine salts were
crushed in a mortar and washed 3 times with tetrahydrofuran (VWR, ACS grade) to
remove all untreated amines. The tetrahydrofuran (THF) was dried over molecular
sieve to remove water prior to use.  The purified
ammonium salts were dried and characterized with FTIR and TGA.The modification process involved a cation-exchange
reaction between ammonium salt and cations present naturally on the silicate
surface, as indicated in Figure1(a). Three different exchange reactions were
performed:  (I) monoamine salt and
Laponite-RD, (II) diamine salt and Laponite-RD, and (III) diamine salt and
Cloisite-Na.(a)
Schematic illustrating chemical structure of modified Laponites.  Structures are not drawn to scale and only
one side of modification is indicated.
DAML = diamine-modified Laponite and MAML = monoamine-modified Laponite.
(b) FT-IR spectra in the N–H stretch region of Laponite-RD particles
before and after modification with (a) diamine and (b) monoamine.
(a)(b)


#### 2.1.1. Exchange Reaction I

Six grams of Laponite-RD were added to 600 g of
deionized water and stirred with a magnetic stirring bar under ambient
conditions for approximately 30 minutes. The clear Laponite-RD dispersion was
slowly added to a solution of excess monoamine salt (twice the exchange
capacity of Laponite-RD) in water. The mixture was stirred with a magnetic
stirring bar at room temperature for 24 hours. The monoamine-modified Laponite-RD
(MAML) particles were collected by
centrifuge and repeatedly washed and centrifuged with DI water until halide
anions were not detected. The presence of halide anions was monitored by adding
AgNO3 to the supernatant solution after centrifuge to form a white
AgCl precipitate. The final washed particles were dried under ambient
conditions for several days, subsequently crushed in a mortar, and screened
with a 325-mesh sieve.


#### 2.1.2. Exchange Reactions II and III

The
procedure for exchange reaction I was repeated with minor alterations. Both
exchange reactions II and III were carried out at60°C. In exchange
reaction III, Cloisite-Na was dispersed in hot water at 60°C, prior
to addition to the diamine salt solution. Exchange reactions II and III
produced diamine modified Laponite-RD (DAML)
and diamine modified Cloisite-Na (DAMC),
respectively. The organic surface content of the modified particles was
determined by the difference in weight between pure and modified particles from
thermogravimetric analysis in the temperature range 150–1000°C.


### 2.2. Synthesis of Poly(urethane urea)-Silicate Nanocomposites

PUU nanocomposites were prepared by first dispersing
modified particles in THF, followed by adding prepolymer to give solutions with
30 mass% prepolymer and 0.5 to 0.3 mass% particle. The prepolymer was
toluene-diisocyanate-(tetramethylene glycol) prepolymer (Air Products and Chemicals Inc.,
Allentown, Pa, USA,
AIRTHANE PET-90A,Mw•2350, NCO functionality = 2.0, mass% NCO =
3.58%).  All reactions, unless otherwise
stated, were carried out at 95 mole% of the stoichiometric amount of diamine
needed to react with all the isocyanate groups present in the prepolymer. The diamine
chain extender needed to achieve the desired stoichiometry was added from a 30
mass% stock solution in THF.  PUU/DAML
nanocomposites were prepared at different DAML weight fractions to optimize
DAML concentration in the nanocomposites. The reacting mixture was stirred with
a magnetic stirring bar for several minutes and the solution was coated on a
Piranha cleaned silicon wafer (Silicon Inc., Boise, Idaho, USA) using a knife-edge coater [35, 36]. Curing took
place directly following film casting, in a vacuum oven at 90°C for
6 hours.  For comparison purposes, both
neat PUU and PUU nanocomposites using MAML and DAMC were prepared using the
same procedures, with appropriate substitution of starting materials. All
nanocomposites were optically clear and indistinguishable to the naked eye from
neat PUU.


### 2.3. Characterization

All nanocomposite film thicknesses were measured by
visible-near infrared interferometry (Stellar Net EPP 2000) [37]. Thermogravimetric
analysis (TGA) was carried out at a heating rate of 10°C/min in a
nitrogen atmosphere from 30°C to 1000°C (TA Instruments
Model TGA-Q500). FT-IR spectra of monoamine, diamine, Laponite-RD, Cloisite-Na,
MAML, DAML, DAMC, and nanocomposite films were recorded at a resolution of 4cm−1 under ambient condition using a Bruker IRscope II connected to a Bruker IFS
66/V spectrometer. The sample area was purged with dry air to prevent the
appearance of atmospheric water bands in the spectra. All spectra were
corrected for thickness and peak analysis software package (Peakfit 4.11,
Jandel Scientific) was used to resolve the bands in the carbonyl (ν=1630–1760cm−1)
region to obtain the relative ratios of the peak areas, assuming Gaussian peak
shapes. Wide-angle X-ray diffraction (WAXD) spectra of the unmodified (Laponite-RD
and Cloisite-Na) and modified particle (MAML, DAML, and DAMC) powders were
recorded using a PANanalytical X’Pert diffractometer with nickel-filtered CuKα (λ=0.15406nm) radiation in a sealed
tube, operated at 40 kV and 200 mA at a scan rate of 6 deg/min. The WAXD
spectra of nanocomposite films, as prepared above, were collected with the same
instrument with the incident beam normal to the film plane. The gallery spacing
of the silicates, d, was determined
using Bragg’s equation: nλ=2dsin(θ).
Mechanical properties of pure PUU and PUU nanocomposite films were
determined using a commercial uniaxial tensile apparatus (INSTRON 5842) at a
strain rate of 30 mm/min. Differential scanning calorimetry (DSC) measurements
of the nanocomposites were performed on a Seiko DSC220C at a heating rate of 5°C/min
over the temperature range of −90 to 320°C in a nitrogen atmosphere.
Transmission electron microscopy (TEM) was performed using a JEOL JEM-100CX II TEM
at an acceleration voltage of 100 kV.
Samples for TEM were prepared by coating onto a PELCO Formvar support
film, stabilized with carbon on 200-mesh copper grid (Ted
Pella, Inc., Redding, Calif, USA). The grid-coated samples were
annealed at the same conditions as all others.


## 2.1. Particle Modification

Laponite-RD and Cloisite-Na, having cationic exchange capacities of 75 mequiv/100 g and 92.6 mequiv/100 g, respectively,
were obtained from Southern Clay Products Inc. (Gonzales, Tex, USA), and used as received.  Laponite (Na+0.7[(Si8Mg5.5Li0.3)O20(OH)4]0.7−)
is a mica-type synthetic hectorite that hydrates in water to give a homogenous
dispersion of exfoliated nanometric disks (1nm×25–30nm) [34]. Cloisite-Na
is a natural montmorillonite clay with a lateral size of about 100–1000 nm.  Trimethylene glycol di-p-aminobenzoate (Air Products and Chemicals Inc., Allentown, Pa,
USA, Versalink 740-M, 98.2% purity, 314 g/mol)
and 4-benzylaniline (Lancaster Synthesis, Ward Hill, M, USA, 98% purity, 183 g/mol) were treated with excess concentrated
HCl to form ammonium salt. For the remainder of this paper, trimethylene glycol
di-p-aminobenzoate and 4-benzylaniline
will be referred to as diamine and monoamine, respectively.  The excess HCl was allowed to evaporate under
ambient conditions for several days, resulting in a dry salt.  The dried monoamine and diamine salts were
crushed in a mortar and washed 3 times with tetrahydrofuran (VWR, ACS grade) to
remove all untreated amines. The tetrahydrofuran (THF) was dried over molecular
sieve to remove water prior to use.  The purified
ammonium salts were dried and characterized with FTIR and TGA.The modification process involved a cation-exchange
reaction between ammonium salt and cations present naturally on the silicate
surface, as indicated in Figure1(a). Three different exchange reactions were
performed:  (I) monoamine salt and
Laponite-RD, (II) diamine salt and Laponite-RD, and (III) diamine salt and
Cloisite-Na.(a)
Schematic illustrating chemical structure of modified Laponites.  Structures are not drawn to scale and only
one side of modification is indicated.
DAML = diamine-modified Laponite and MAML = monoamine-modified Laponite.
(b) FT-IR spectra in the N–H stretch region of Laponite-RD particles
before and after modification with (a) diamine and (b) monoamine.
(a)(b)


### 2.1.1. Exchange Reaction I

Six grams of Laponite-RD were added to 600 g of
deionized water and stirred with a magnetic stirring bar under ambient
conditions for approximately 30 minutes. The clear Laponite-RD dispersion was
slowly added to a solution of excess monoamine salt (twice the exchange
capacity of Laponite-RD) in water. The mixture was stirred with a magnetic
stirring bar at room temperature for 24 hours. The monoamine-modified Laponite-RD
(MAML) particles were collected by
centrifuge and repeatedly washed and centrifuged with DI water until halide
anions were not detected. The presence of halide anions was monitored by adding
AgNO3 to the supernatant solution after centrifuge to form a white
AgCl precipitate. The final washed particles were dried under ambient
conditions for several days, subsequently crushed in a mortar, and screened
with a 325-mesh sieve.


### 2.1.2. Exchange Reactions II and III

The
procedure for exchange reaction I was repeated with minor alterations. Both
exchange reactions II and III were carried out at60°C. In exchange
reaction III, Cloisite-Na was dispersed in hot water at 60°C, prior
to addition to the diamine salt solution. Exchange reactions II and III
produced diamine modified Laponite-RD (DAML)
and diamine modified Cloisite-Na (DAMC),
respectively. The organic surface content of the modified particles was
determined by the difference in weight between pure and modified particles from
thermogravimetric analysis in the temperature range 150–1000°C.


## 2.1.1. Exchange Reaction I

Six grams of Laponite-RD were added to 600 g of
deionized water and stirred with a magnetic stirring bar under ambient
conditions for approximately 30 minutes. The clear Laponite-RD dispersion was
slowly added to a solution of excess monoamine salt (twice the exchange
capacity of Laponite-RD) in water. The mixture was stirred with a magnetic
stirring bar at room temperature for 24 hours. The monoamine-modified Laponite-RD
(MAML) particles were collected by
centrifuge and repeatedly washed and centrifuged with DI water until halide
anions were not detected. The presence of halide anions was monitored by adding
AgNO3 to the supernatant solution after centrifuge to form a white
AgCl precipitate. The final washed particles were dried under ambient
conditions for several days, subsequently crushed in a mortar, and screened
with a 325-mesh sieve.


## 2.1.2. Exchange Reactions II and III

The
procedure for exchange reaction I was repeated with minor alterations. Both
exchange reactions II and III were carried out at60°C. In exchange
reaction III, Cloisite-Na was dispersed in hot water at 60°C, prior
to addition to the diamine salt solution. Exchange reactions II and III
produced diamine modified Laponite-RD (DAML)
and diamine modified Cloisite-Na (DAMC),
respectively. The organic surface content of the modified particles was
determined by the difference in weight between pure and modified particles from
thermogravimetric analysis in the temperature range 150–1000°C.


## 2.2. Synthesis of Poly(urethane urea)-Silicate Nanocomposites

PUU nanocomposites were prepared by first dispersing
modified particles in THF, followed by adding prepolymer to give solutions with
30 mass% prepolymer and 0.5 to 0.3 mass% particle. The prepolymer was
toluene-diisocyanate-(tetramethylene glycol) prepolymer (Air Products and Chemicals Inc.,
Allentown, Pa, USA,
AIRTHANE PET-90A,Mw•2350, NCO functionality = 2.0, mass% NCO =
3.58%).  All reactions, unless otherwise
stated, were carried out at 95 mole% of the stoichiometric amount of diamine
needed to react with all the isocyanate groups present in the prepolymer. The diamine
chain extender needed to achieve the desired stoichiometry was added from a 30
mass% stock solution in THF.  PUU/DAML
nanocomposites were prepared at different DAML weight fractions to optimize
DAML concentration in the nanocomposites. The reacting mixture was stirred with
a magnetic stirring bar for several minutes and the solution was coated on a
Piranha cleaned silicon wafer (Silicon Inc., Boise, Idaho, USA) using a knife-edge coater [35, 36]. Curing took
place directly following film casting, in a vacuum oven at 90°C for
6 hours.  For comparison purposes, both
neat PUU and PUU nanocomposites using MAML and DAMC were prepared using the
same procedures, with appropriate substitution of starting materials. All
nanocomposites were optically clear and indistinguishable to the naked eye from
neat PUU.


## 2.3. Characterization

All nanocomposite film thicknesses were measured by
visible-near infrared interferometry (Stellar Net EPP 2000) [37]. Thermogravimetric
analysis (TGA) was carried out at a heating rate of 10°C/min in a
nitrogen atmosphere from 30°C to 1000°C (TA Instruments
Model TGA-Q500). FT-IR spectra of monoamine, diamine, Laponite-RD, Cloisite-Na,
MAML, DAML, DAMC, and nanocomposite films were recorded at a resolution of 4cm−1 under ambient condition using a Bruker IRscope II connected to a Bruker IFS
66/V spectrometer. The sample area was purged with dry air to prevent the
appearance of atmospheric water bands in the spectra. All spectra were
corrected for thickness and peak analysis software package (Peakfit 4.11,
Jandel Scientific) was used to resolve the bands in the carbonyl (ν=1630–1760cm−1)
region to obtain the relative ratios of the peak areas, assuming Gaussian peak
shapes. Wide-angle X-ray diffraction (WAXD) spectra of the unmodified (Laponite-RD
and Cloisite-Na) and modified particle (MAML, DAML, and DAMC) powders were
recorded using a PANanalytical X’Pert diffractometer with nickel-filtered CuKα (λ=0.15406nm) radiation in a sealed
tube, operated at 40 kV and 200 mA at a scan rate of 6 deg/min. The WAXD
spectra of nanocomposite films, as prepared above, were collected with the same
instrument with the incident beam normal to the film plane. The gallery spacing
of the silicates, d, was determined
using Bragg’s equation: nλ=2dsin(θ).
Mechanical properties of pure PUU and PUU nanocomposite films were
determined using a commercial uniaxial tensile apparatus (INSTRON 5842) at a
strain rate of 30 mm/min. Differential scanning calorimetry (DSC) measurements
of the nanocomposites were performed on a Seiko DSC220C at a heating rate of 5°C/min
over the temperature range of −90 to 320°C in a nitrogen atmosphere.
Transmission electron microscopy (TEM) was performed using a JEOL JEM-100CX II TEM
at an acceleration voltage of 100 kV.
Samples for TEM were prepared by coating onto a PELCO Formvar support
film, stabilized with carbon on 200-mesh copper grid (Ted
Pella, Inc., Redding, Calif, USA). The grid-coated samples were
annealed at the same conditions as all others.


## 3. Results and Discusion

### 3.1. Particle Modification

Figure1(b) compares FT-IR spectra of diamine-modified
particles to the neat diamine and unmodified Laponite-RD in the N-H stretch
region.  These spectra confirm the successful
modification of Laponite and indicate the presence of free primary amine groups
that can serve as chain extenders for isocyanates in poly(urethane urea)
synthesis.  FT-IR results for diamine
modified Cloisite-Na (DAMC) and monoamine-modified Laponite (MAML) gave a similar
indication of successful modification, not shown. In particular, free amine
functionalities are not detected in the MAML FTIR spectra, as expected.TGA was used to estimate the amount of monoamine or diamine
tethered to the particles, shown in Figure2.
The organic amines contributed nearly 20 mass% of the DAML, 17% of the DAMC,
and 10% of the total weight of MAML. The pure monoamine and diamine (TGA graphs
not included) showed complete decomposition in this temperature range. The
reported CEC of Laponite, 75 mmol/100 g, can be used to estimate a capacity of
19 mass% diamine at full exchange (see Supplementary Material available online
at doi:10.1155/2008/869354), in close agreement with the TGA result of
20%. However, the diamine is predicted to comprise 22% of the modified Cloisite
mass. The lower TGA-determined value (17 mass%) suggests incomplete exchange
with Cloisite. The significantly lower mass fraction amine on the MAML is due
primarily to the lower molecular mass of the monoamine relative to the diamine.
Complete exchange is predicted to result in a monoamine mass fraction of 12%,
not far from the observed value of 10%. These quantities were used to determine
the amount of excess diamine needed for the desired poly(urethane urea) stoichiometry.Figure 2
TGA curves of unmodified and modified silicates.WAXD powder spectra for unmodified and modified Laponite
and Cloisite-Na are presented in Figure3. The WAXD of unmodified Laponite-RD shows
a single peak centered at 2θ=6.5Å (d001=13.6Å),
which is consistent with previously reported values [38, 39] and the absence of
surfactant contaminant in the as-supplied material (also absent in FTIR Figure 1(b)). In the two modified Laponite diffraction patterns, a broad set of
overlapping peaks from 1.5°<2θ<9°
is observed.  The breadth and presence of
overlapping subpeaks at angles above and below d001 suggest that a complex structure exists in the
modified Laponite. It is difficult to specify precisely the structures present
from the WAXD data. The low-angle peaks suggest that modification results in
gallery expansion (intercalation of amine modifier) relative to neat Laponite.
However, higher-angle peaks suggest that some unmodified clay remains. The
diffraction pattern for Cloisite-Na has a peak centered about 2θ=7.5° (d001 spacing of 11.7 Å).  The modified Cloisite-Na particle, DAMC,
shows a single diffraction peak at 2θ=4.5° (d001=19.6Å), which
indicates an expansion of the gallery spacing due to intercalated diamine.Figure 3
WAXD spectra of unmodified and modified Laponite-RD and Cloisite-NA silicate powders.


### 3.2. Nanocomposites

Uniaxial tensile stress-strain curves for pure PUU and PUU
nanocomposites with different amounts of DAML are presented in Figure4 and the
extracted mechanical properties are summarized in Figure 5 and Table 1. Both
the tensile strength and elongation at break increased with DAML concentration
up to a maximum at 1% DAML.
Interestingly, the modulus and shape of the stress-strain curve for the
DAML nanocomposites are indistinguishable from those of pure PUU up to strains of ≈300%. This suggests the mechanism by which DAML increases strength becomes
active only at high elongations, for example, enhanced strain crystallization.
The simultaneous increase in both tensile strength and elongation at break has
been reported for montmorillonite nanocomposites with polyurethanes [29, 30].  Ni et al. observed a two-fold increase in both
tensile strength and elongation for poly(tetramethylene glycol)-based PUUs
chain extended with a diamine-modified montmorillonite at 5% particle loading
[26].Table 1
Mechanical properties and endotherms of PUU nanocomposites.
Nanocomposite composition (mass% Silicate)NH2 from particlea (mole%)Tensile strength (MPa)Elongation atbreakb (%)ModulusE100b (MPa)Tgc(C°)Endod 1 Peak, T (C°)Endod 2 Peak, T (C°)Neat PUU016.7±1.7501±3411.1±1.2−721922740.5 mass% DAML0.833.0±3.9604±1711.7±0.9−701922701 mass% DAML1.748.8±1.4716±4011.6±1.0−70197265, 2771.5 mass% DAML2.626.4±2.7608±3710.4±1.4−691922733 mass% DAML5.416.0±2.1642±3510.9±0.8−681882771 mass% MAML022.1±3.0734±2113.2±1.4−681892711 mass% DAMC1.728.4±0.7751±3610.3±0.6−71189285aMole%  –NH2 groups contributed by particle out of total –NH2 moles in sample.
bError represents a 95% confidence level from at least 3 samples.
cSoft segment glass transition temperature.
dEndo means endothermic.Figure 4
Stress-strain curves for pure
PUU and PUU nanocomposites prepared with DAML at 1 and 3 wt.% DAML.Figure 5
Mechanical properties of
PUU-DAML nanocomposites prepared at different DAML concentrations.The amine moieties on diamine-modified Laponite compete
with free-(dissolved) amine chain extender to form urea linkages during network
formation. To investigate whether this competition leads to a significant
effect on the urea morphology, we used FTIR and DSC to investigate H-bonding
and thermal properties of the hard domains, respectively. PUU hydrogen bonding,
especially the bidentate urea-urea bonds, is critical to maintaining the
phase-separated morphology and mechanical strength of hard domains [23, 37, 40].
FT-IR spectra of neat and
nanocomposite PUU in the carbonyl region (shown in Supplementary Figure 1)
indicate that free and hydrogen-bonded peaks of neat PUU are essentially
identical to those found in the nanocomposites. The fractions of urea- and
urethane-specific hydrogen bonding (shown in Supplementary Figure 1) were
estimated from the carbonyl absorbances. The fraction of ordered and disordered
urea and urethane-involved hydrogen bonds for all nanocomposites were
statistically indistinguishable from one another and from the neat PUU.Table1 summarizes data from DSC thermographs (DSC traces
are shown in Supplementary Figures 2 and 3). The soft segment glass transition
temperatures of all nanocomposites are nearly identical to the glass transition
temperature of pure PUU.  This suggests that
the dispersed silicates do not have a significant effect on the soft phase-free
volume. A common feature of all the thermographs is the presence of two primary
melting endotherms: one at about 190°C and another at about 270°C.
The high-temperature endotherm at 270°C represents melting of the
crystalline urea hard domains, while the endotherm at 190°C is
associated with melting of urethane containing hard domains. The pure PUU shows
endotherms at 190 and 273°C,
with a slight shoulder peak at 288°C.
As DAML is added, the width, location, and number of peaks in the 270°C range change, whereas the 190°C endotherm shifts location slightly
but does not split into multiple peaks. The 1% DAML sample clearly shows two
distinct peaks in the urea hard-domain endotherm, at 265°C and 277°C,
above and below the pure PUU endotherm of 273°C. In addition, the 1% sample also shows a +5°C shift of the urethane endotherm,
the only sample that showed an increase in this endotherm. The appearance of
multiple endotherms in neat PU and PUU has been interpreted by others as
signifying the presence of urea crystal lamellae with different thicknesses or
crystallites with different polymorphism [22]. Similar endotherm increases
and splitting have been observed in other nanocomposite-PU studies [17, 41]. Tien
and Wei [17] reported that diamine-modified montmorillonite PU nanocomposites
showed a maximum melting endotherm at 1 mass% nanoparticle, which
returned close to the unmodified endotherm value at 5 mass% particle.Figure6 shows a comparison of elongation and break and
tensile strength versus values of the urea endotherm for all nanocomposite
samples. The strongest correlation is between elongation and urea endotherm,
which is linear for the DAML samples. On the contrary, elongation is not
correlated linearly with the urethane endotherm, but rather passes through a
minimum.  Still the highest elongation,
for the 1% DAML, did coincide with the highest urethane endotherm value. Both
the urea and urethane crystallization endotherms have very similar correlations
with tensile strength. All samples with the exception of the 3% DAML show a
linear correlation between tensile strength and both endotherms. The 3% DAML
sample shows a lower tensile strength than that predicted by the trend of the
other samples with the urea endotherm.Elongation-at-break and tensile strength
versus values of the urea endotherm for all nanocomposite samples.
(a)(b)WAXD spectra for the pure and
nanocomposite PUUs (Supplementary Figure 4) showed no diffraction peaks in any
of the nanocomposites. While the absence of peaks is consistent with
exfoliation of silicate layers in the PUU nanocomposites, this cannot be taken
as unambiguous evidence. There is, for example, the possibility of aggregates
consisting of randomly-oriented particles, especially considering the low-aspect
ratio of Laponite.  Representative TEM
micrographs of PUU and PUU-DAML nanocomposites prepared with 0%, 1%, and 3%
DAML are presented in Figure7. The
micrograph of the unmodified PUU (Figure 7(a)) shows no characteristic features
except for faint outlines of the hard-domain semicontinuous phase.
Nanocomposites prepared with 1 mass% DAML contained features resembling
circular discs, with a diameter of about 20–30 nm
(Figure 7(b)), distributed nonuniformly throughout the sample (Figure 7(c)).
The DAML phase in the TEM images appears as if it were oriented preferentially
in the direction of film spreading, although anisotropy cannot be confirmed
unambiguously from our sample preparation method. However, anisotropic particle
distributions have been observed for PEO-Laponite films prepared by spreading
from aqueous solution [9, 11].  Those
authors showed that the platelets become aligned in the film plane as the
network collapses during film drying [9, 11]. Although we cannot assign the
particle geometry (disc, sphere, cylinder) or confirm the presence of
exfoliation unambiguously, the shape and size of the inorganic content in the
1% DAML samples are consistent with the Laponite diameter. In
contrast, the TEM of the 3 mass% DAML sample indicates an aggregated inorganic
phase with size much larger than single Laponite.  In fact, at 3% DAML, diameters range from 60 nm to 120 nm with a median of 80 nm, as shown in Figure 7(c).Representative TEM micrographs of (a) unmodified PUU and
DAML-PUU nanocomposites containing (b) 1 wt.%
DAML and (c) 3 wt.% DAML.
(a)(b)(c)The data presented here indicate
a strong correlation between the size and dispersion of the inorganic phase, the
changes in hard segment endotherms and the PUU nanocomposite mechanical
properties.It is apparent that at 1% DAML, the inorganic
phase has a size on the order of 25 nm diameter, and that at 3% DAML, the size
increases to80±20nm.  Due to the sample
preparation and resolution of the TEM images, we cannot definitively conclude
whether or not the Laponite is exfoliated into single discs.  However, there is clearly a definitive
increase in the dispersed size of the inorganic phase (about 4-fold in
diameter) in going from 1% to 3% DAML. The larger combined surface area of the Laponite
phase at 1% DAML may expose the PUU to more confined surface-tethered urea
groups than at 3% DAML.The loss in
strength in the 3% DAML sample is correlated with the increase in the size of
the inorganic phase, compared to the 1% DAML samples.  Also correlated is the urea hard-domain
endotherm that was shifted to a maximum temperature at 1% DAML and returned to
values close to the neat PUU hard-domain crystallization at 3% DAML. None of these properties
(size, strength, endotherms) were related to changes in the fraction of
hydrogen bonds in urethane or urea groups.  These correlations suggest that ordering induced by
the tethered ureas colocated within a thin molecular plane on the Laponite
might lead to improved mechanical and thermal properties. Under this scenario,
urea crystallites tethered directly to Laponite might be expected to align
easier and form larger domains than the free bulk urea segments. However, it is
also possible that the nanoparticles may simply act as a hindrance to disordering
and melting as temperature is increased [17].  Both scenarios are consistent with our thermal
and mechanical observations but cannot be distinguished using the current data.


## 3.1. Particle Modification

Figure1(b) compares FT-IR spectra of diamine-modified
particles to the neat diamine and unmodified Laponite-RD in the N-H stretch
region.  These spectra confirm the successful
modification of Laponite and indicate the presence of free primary amine groups
that can serve as chain extenders for isocyanates in poly(urethane urea)
synthesis.  FT-IR results for diamine
modified Cloisite-Na (DAMC) and monoamine-modified Laponite (MAML) gave a similar
indication of successful modification, not shown. In particular, free amine
functionalities are not detected in the MAML FTIR spectra, as expected.TGA was used to estimate the amount of monoamine or diamine
tethered to the particles, shown in Figure2.
The organic amines contributed nearly 20 mass% of the DAML, 17% of the DAMC,
and 10% of the total weight of MAML. The pure monoamine and diamine (TGA graphs
not included) showed complete decomposition in this temperature range. The
reported CEC of Laponite, 75 mmol/100 g, can be used to estimate a capacity of
19 mass% diamine at full exchange (see Supplementary Material available online
at doi:10.1155/2008/869354), in close agreement with the TGA result of
20%. However, the diamine is predicted to comprise 22% of the modified Cloisite
mass. The lower TGA-determined value (17 mass%) suggests incomplete exchange
with Cloisite. The significantly lower mass fraction amine on the MAML is due
primarily to the lower molecular mass of the monoamine relative to the diamine.
Complete exchange is predicted to result in a monoamine mass fraction of 12%,
not far from the observed value of 10%. These quantities were used to determine
the amount of excess diamine needed for the desired poly(urethane urea) stoichiometry.Figure 2
TGA curves of unmodified and modified silicates.WAXD powder spectra for unmodified and modified Laponite
and Cloisite-Na are presented in Figure3. The WAXD of unmodified Laponite-RD shows
a single peak centered at 2θ=6.5Å (d001=13.6Å),
which is consistent with previously reported values [38, 39] and the absence of
surfactant contaminant in the as-supplied material (also absent in FTIR Figure 1(b)). In the two modified Laponite diffraction patterns, a broad set of
overlapping peaks from 1.5°<2θ<9°
is observed.  The breadth and presence of
overlapping subpeaks at angles above and below d001 suggest that a complex structure exists in the
modified Laponite. It is difficult to specify precisely the structures present
from the WAXD data. The low-angle peaks suggest that modification results in
gallery expansion (intercalation of amine modifier) relative to neat Laponite.
However, higher-angle peaks suggest that some unmodified clay remains. The
diffraction pattern for Cloisite-Na has a peak centered about 2θ=7.5° (d001 spacing of 11.7 Å).  The modified Cloisite-Na particle, DAMC,
shows a single diffraction peak at 2θ=4.5° (d001=19.6Å), which
indicates an expansion of the gallery spacing due to intercalated diamine.Figure 3
WAXD spectra of unmodified and modified Laponite-RD and Cloisite-NA silicate powders.


## 3.2. Nanocomposites

Uniaxial tensile stress-strain curves for pure PUU and PUU
nanocomposites with different amounts of DAML are presented in Figure4 and the
extracted mechanical properties are summarized in Figure 5 and Table 1. Both
the tensile strength and elongation at break increased with DAML concentration
up to a maximum at 1% DAML.
Interestingly, the modulus and shape of the stress-strain curve for the
DAML nanocomposites are indistinguishable from those of pure PUU up to strains of ≈300%. This suggests the mechanism by which DAML increases strength becomes
active only at high elongations, for example, enhanced strain crystallization.
The simultaneous increase in both tensile strength and elongation at break has
been reported for montmorillonite nanocomposites with polyurethanes [29, 30].  Ni et al. observed a two-fold increase in both
tensile strength and elongation for poly(tetramethylene glycol)-based PUUs
chain extended with a diamine-modified montmorillonite at 5% particle loading
[26].Table 1
Mechanical properties and endotherms of PUU nanocomposites.
Nanocomposite composition (mass% Silicate)NH2 from particlea (mole%)Tensile strength (MPa)Elongation atbreakb (%)ModulusE100b (MPa)Tgc(C°)Endod 1 Peak, T (C°)Endod 2 Peak, T (C°)Neat PUU016.7±1.7501±3411.1±1.2−721922740.5 mass% DAML0.833.0±3.9604±1711.7±0.9−701922701 mass% DAML1.748.8±1.4716±4011.6±1.0−70197265, 2771.5 mass% DAML2.626.4±2.7608±3710.4±1.4−691922733 mass% DAML5.416.0±2.1642±3510.9±0.8−681882771 mass% MAML022.1±3.0734±2113.2±1.4−681892711 mass% DAMC1.728.4±0.7751±3610.3±0.6−71189285aMole%  –NH2 groups contributed by particle out of total –NH2 moles in sample.
bError represents a 95% confidence level from at least 3 samples.
cSoft segment glass transition temperature.
dEndo means endothermic.Figure 4
Stress-strain curves for pure
PUU and PUU nanocomposites prepared with DAML at 1 and 3 wt.% DAML.Figure 5
Mechanical properties of
PUU-DAML nanocomposites prepared at different DAML concentrations.The amine moieties on diamine-modified Laponite compete
with free-(dissolved) amine chain extender to form urea linkages during network
formation. To investigate whether this competition leads to a significant
effect on the urea morphology, we used FTIR and DSC to investigate H-bonding
and thermal properties of the hard domains, respectively. PUU hydrogen bonding,
especially the bidentate urea-urea bonds, is critical to maintaining the
phase-separated morphology and mechanical strength of hard domains [23, 37, 40].
FT-IR spectra of neat and
nanocomposite PUU in the carbonyl region (shown in Supplementary Figure 1)
indicate that free and hydrogen-bonded peaks of neat PUU are essentially
identical to those found in the nanocomposites. The fractions of urea- and
urethane-specific hydrogen bonding (shown in Supplementary Figure 1) were
estimated from the carbonyl absorbances. The fraction of ordered and disordered
urea and urethane-involved hydrogen bonds for all nanocomposites were
statistically indistinguishable from one another and from the neat PUU.Table1 summarizes data from DSC thermographs (DSC traces
are shown in Supplementary Figures 2 and 3). The soft segment glass transition
temperatures of all nanocomposites are nearly identical to the glass transition
temperature of pure PUU.  This suggests that
the dispersed silicates do not have a significant effect on the soft phase-free
volume. A common feature of all the thermographs is the presence of two primary
melting endotherms: one at about 190°C and another at about 270°C.
The high-temperature endotherm at 270°C represents melting of the
crystalline urea hard domains, while the endotherm at 190°C is
associated with melting of urethane containing hard domains. The pure PUU shows
endotherms at 190 and 273°C,
with a slight shoulder peak at 288°C.
As DAML is added, the width, location, and number of peaks in the 270°C range change, whereas the 190°C endotherm shifts location slightly
but does not split into multiple peaks. The 1% DAML sample clearly shows two
distinct peaks in the urea hard-domain endotherm, at 265°C and 277°C,
above and below the pure PUU endotherm of 273°C. In addition, the 1% sample also shows a +5°C shift of the urethane endotherm,
the only sample that showed an increase in this endotherm. The appearance of
multiple endotherms in neat PU and PUU has been interpreted by others as
signifying the presence of urea crystal lamellae with different thicknesses or
crystallites with different polymorphism [22]. Similar endotherm increases
and splitting have been observed in other nanocomposite-PU studies [17, 41]. Tien
and Wei [17] reported that diamine-modified montmorillonite PU nanocomposites
showed a maximum melting endotherm at 1 mass% nanoparticle, which
returned close to the unmodified endotherm value at 5 mass% particle.Figure6 shows a comparison of elongation and break and
tensile strength versus values of the urea endotherm for all nanocomposite
samples. The strongest correlation is between elongation and urea endotherm,
which is linear for the DAML samples. On the contrary, elongation is not
correlated linearly with the urethane endotherm, but rather passes through a
minimum.  Still the highest elongation,
for the 1% DAML, did coincide with the highest urethane endotherm value. Both
the urea and urethane crystallization endotherms have very similar correlations
with tensile strength. All samples with the exception of the 3% DAML show a
linear correlation between tensile strength and both endotherms. The 3% DAML
sample shows a lower tensile strength than that predicted by the trend of the
other samples with the urea endotherm.Elongation-at-break and tensile strength
versus values of the urea endotherm for all nanocomposite samples.
(a)(b)WAXD spectra for the pure and
nanocomposite PUUs (Supplementary Figure 4) showed no diffraction peaks in any
of the nanocomposites. While the absence of peaks is consistent with
exfoliation of silicate layers in the PUU nanocomposites, this cannot be taken
as unambiguous evidence. There is, for example, the possibility of aggregates
consisting of randomly-oriented particles, especially considering the low-aspect
ratio of Laponite.  Representative TEM
micrographs of PUU and PUU-DAML nanocomposites prepared with 0%, 1%, and 3%
DAML are presented in Figure7. The
micrograph of the unmodified PUU (Figure 7(a)) shows no characteristic features
except for faint outlines of the hard-domain semicontinuous phase.
Nanocomposites prepared with 1 mass% DAML contained features resembling
circular discs, with a diameter of about 20–30 nm
(Figure 7(b)), distributed nonuniformly throughout the sample (Figure 7(c)).
The DAML phase in the TEM images appears as if it were oriented preferentially
in the direction of film spreading, although anisotropy cannot be confirmed
unambiguously from our sample preparation method. However, anisotropic particle
distributions have been observed for PEO-Laponite films prepared by spreading
from aqueous solution [9, 11].  Those
authors showed that the platelets become aligned in the film plane as the
network collapses during film drying [9, 11]. Although we cannot assign the
particle geometry (disc, sphere, cylinder) or confirm the presence of
exfoliation unambiguously, the shape and size of the inorganic content in the
1% DAML samples are consistent with the Laponite diameter. In
contrast, the TEM of the 3 mass% DAML sample indicates an aggregated inorganic
phase with size much larger than single Laponite.  In fact, at 3% DAML, diameters range from 60 nm to 120 nm with a median of 80 nm, as shown in Figure 7(c).Representative TEM micrographs of (a) unmodified PUU and
DAML-PUU nanocomposites containing (b) 1 wt.%
DAML and (c) 3 wt.% DAML.
(a)(b)(c)The data presented here indicate
a strong correlation between the size and dispersion of the inorganic phase, the
changes in hard segment endotherms and the PUU nanocomposite mechanical
properties.It is apparent that at 1% DAML, the inorganic
phase has a size on the order of 25 nm diameter, and that at 3% DAML, the size
increases to80±20nm.  Due to the sample
preparation and resolution of the TEM images, we cannot definitively conclude
whether or not the Laponite is exfoliated into single discs.  However, there is clearly a definitive
increase in the dispersed size of the inorganic phase (about 4-fold in
diameter) in going from 1% to 3% DAML. The larger combined surface area of the Laponite
phase at 1% DAML may expose the PUU to more confined surface-tethered urea
groups than at 3% DAML.The loss in
strength in the 3% DAML sample is correlated with the increase in the size of
the inorganic phase, compared to the 1% DAML samples.  Also correlated is the urea hard-domain
endotherm that was shifted to a maximum temperature at 1% DAML and returned to
values close to the neat PUU hard-domain crystallization at 3% DAML. None of these properties
(size, strength, endotherms) were related to changes in the fraction of
hydrogen bonds in urethane or urea groups.  These correlations suggest that ordering induced by
the tethered ureas colocated within a thin molecular plane on the Laponite
might lead to improved mechanical and thermal properties. Under this scenario,
urea crystallites tethered directly to Laponite might be expected to align
easier and form larger domains than the free bulk urea segments. However, it is
also possible that the nanoparticles may simply act as a hindrance to disordering
and melting as temperature is increased [17].  Both scenarios are consistent with our thermal
and mechanical observations but cannot be distinguished using the current data.


## 4. Conclusions

Segmented poly(urethane urea) nanocomposites were prepared
using commercial Laponite and Cloisite silicates modified with reactive diamine and nonreactive
monoamine groups.  A 3-fold
increase in tensile strength is observed with 1% by mass of diamine-modified
Laponite, relative to unmodified Laponite. This is associated with a marked
increase in PUU urea melting endotherm.  As
the loading of diamine-modified Laponite increases from 1% to 3% by mass, the
size of Laponite inorganic phase increases dramatically, along with a return of
urea hard segment endotherms and nanocomposite mechanical properties to values
near unmodified PUU.  Work is ongoing
toward using high-resolution and orientation-dependent TEM and WAXD define precisely
the size, shape, and dispersion of the Laponite phase.



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*Source: 869354-2008-10-29.xml*