Novel dendritic-poly(urethane-urea) hybrid thin films from hydrogen bond rich dendrons

Article abstract of DOI:10.1016/j.polymer.2014.08.017

Hydrogen bond rich segmented poly(urethane-urea) was synthesized from methylene diphenylisocyanate (MDI) and three generations of polyurea-malonamide dendrons as hard segment and polycaprolactone diol as soft segment for thin film applications. The prepar

Full text of DOI:10.1016/j.polymer.2014.08.017

Polymer 55 (2014) 5132e5139
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Novel dendritic-poly(urethane-urea) hybrid thin films from hydrogen
bond rich dendrons
*
Aravindaraj G. Kannan, Namita Roy Choudhury , Naba K. Dutta
Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes,
South Australia 5095, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2014
Received in revised form
Hydrogen bond rich segmented poly(urethane-urea) was synthesized from methylene diphenylisocya-
nate (MDI) and three generations of polyurea-malonamide dendrons as hard segment and poly-
caprolactone diol as soft segment for thin film applications. The prepared polymers were characterized
using spectroscopic, microscopic and thermal analyses. The formation of urethane linkage during the
prepolymer reaction and the urea linkage between prepolymer and the dendrons is confirmed by Fourier
transform infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (NMR) spectroscopy. FTIR
shows the presence of hydrogen bonding of eNH groups with both urethane carbonyl group from hard
segment and the ether group from the soft segment. However, the phase mixing of hard and soft seg-
ments decreases with the higher generation dendrons, as evidenced from FTIR. This observation was
confirmed by phase images of the atomic force microscopy (AFM). The coating when applied to clean
steel substrates via dip coating reveals uniform, dense and essentially defect free morphology. The work
demonstrates that the mechanical properties of the hybrid thin films are dependent on the generation of
the dendrons and provides a platform for surface engineering with tunable elastic modulus.
30 July 2014
Accepted 6 August 2014
Available online 15 August 2014
Keywords:
Dendritic hybrids
Segmented poly(urethane-urea)
Hydrogen bonding
©
2014 Elsevier Ltd. All rights reserved.
1
. Introduction
advanced and conventional application fields. Also their properties
are strongly influenced by their interfacial characteristics rather
than the bulk structures. Hence, it is essential to understand the
role of molecular architectures such as linear chain polymers,
dendrimers, etc. on interfacial characteristics to exploit their full
potential.
Dendrimers have recently emerged as promising candidates for
their potential applications in a broad range of areas, from drug
delivery agents, protective coatings and hybrids to catalysis. The
quantity and tailorability of the dendrimer's terminal functional
groups allow the molecular weight of the polymer to build up while
still possessing low viscosity, increased solubility and good rheo-
logical behaviour. Many developments have occurred since the first
dendritic structure was described by V o€ gtle et al. [1] in 1978, with
researchers focussing on developing synthetic methods that avoid
tedious deprotection/activation steps in order to produce high
yielding, multiple generation dendrimers in an efficient and cost-
effective manner [2e8]. While, polyurethane and polyamide den-
drimer syntheses using a highly selective addition process had been
realized [9,10] already, the requirement of catalysts and the gen-
eration of side products have proved this to be a less than ideal
synthesis route. Therefore, synthetic procedures that avoid tedious
Thin film coatings are widely used to tailor the surface proper-
ties of metallic and non-metallic components to impart character-
istics such as protection from environment, abrasion and corrosion
resistance, anti-friction and anti-flammability, lubrication,
biocompatible surfaces, etc. Traditionally, either ceramic or organic
thin films had been employed in fields as diverse as optics, mi-
croelectronics, tribology, corrosion and biomaterials. However, they
have inherent disadvantages such as brittleness and presence of
defects (ceramic thin films), mechanically weak films and poor
adhesion (organic thin films). This has resulted in a new class of
multifunctional hybrid thin films. The major advantages of such
multifunctional hybrid thin films are: homogeneity at near mo-
lecular level, materials with novel microstructure and high surface
to volume ratio of building blocks resulting in higher amount of
interaction between the components. The ability to tailor the
properties of such hybrid materials, at a molecular level, signifies
their enormous potential in a wide variety of technologically
*
Corresponding author. Tel.: þ61 8 8302 3719; fax: þ61 8 8302 3755.
E-mail address: Namita.Choudhury@unisa.edu.au (N.R. Choudhury).
http://dx.doi.org/10.1016/j.polymer.2014.08.017
032-3861/© 2014 Elsevier Ltd. All rights reserved.
0

A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
5133
activation/deprotection steps and the use of catalysts and genera-
tion of side products were highly sought after [2e4].
group to allow the dendron to be covalently attached to other
functional polymer chains. These dendrons of generation 1, 2 and 3
are henceforth referred as G1-NH, G2-NH and G3-NH, respectively.
The availability of large number of urea and amide linkages in these
dendrons provides hydrogen-bonding sites, where inter-urea
hydrogen bonding linkages can contribute to three-dimensional
characteristics [3,11].
These dendrons, when incorporated into segmented poly-
urethanes, which are, as such, well known to exhibit micro-phase
separation due to their thermodynamic incompatibility between
Recently, Chen et al. [3] reported the use of a bifunctional
0
compound ([4-isocyanato-4 (3,3-dimethyl-2,4-dioxo-acetidino)-
diphenylmethane], MIA) which, on reaction with bis(2-aminoethyl)
amine in an alternating and successive fashion, produces polyurea/
malonamide dendrimers. This convergent synthetic route offers
high yields with easy purification steps, allowing dendrons up to
third generation to be produced. Scheme 1 shows the chemical
structure of generations 1, 2 and 3, each with a secondary amino
Scheme 1. Structure of G1-NH, G2-NH and G3-NH dendrons.

5134
A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
the soft and hard segments, can modify their phase-separation
behaviour via inter/intra urea-amide hydrogen bonding [12e15].
This interaction parameter provides the platform to combine
unique interfacial properties of dendritic macromolecules with the
processability and phase separation behaviour of linear polymers.
Apart from this, enhanced phase-separation behaviour can result
in improved physical and mechanical characteristics [4,16,17],
which makes them ideal choice for coatings, biomaterials and
other advanced applications. Also, the phase-separated
morphology of such hybrids is controlled by various factors such
as the type of chain extender, volume of each segment and the
type and chain lengths of soft segments [14,15,18,19]. These pos-
sibilities provide the platform to vary the mechanical properties
by simply varying the quantity/generation of the dendrons. Sur-
face engineering of metal substrate on the nanoscale is a critical
step for a range of novel biomedical applications. For example, in
biomedical engineering, improved fixation of bone implants has
been demonstrated upon imparting nanoscale physico-chemical
functionalities to the implant surface. Numerous advanced appli-
cations such as biomedical devices require substrate to be coated
to achieve biocompatibility, in which case the control of
morphology is crucial. The substrate induced phase-separation
behaviour could be also very different from that in the bulk
state. However, to date, to the best of our knowledge, no attempt
has been made to utilise these hybrids to form thin film coatings
with mechanical robustness.
hydrogen bonding. The PCL soft segment offers hydrophobicity
and biocompatibility, thereby leading to coatings that span a wide
field of applications from biomaterials to antifouling coatings. The
synthesized hybrid materials have been characterized using
Photo-acoustic Fourier Transform Infrared Spectroscopy (PA-
1
FTIR), H Nuclear Magnetic Resonance (NMR) Spectroscopy and
Thermogravimetric Analysis (TGA). In addition, the phase sepa-
ration as a result of multiple hydrogen bonding was investigated
with Atomic Force Microscopy (AFM) and Differential Scanning
Calorimetry (DSC). Also, the effect of phase separation on the
mechanical properties of the coatings was studied using nano-
indentation measurements.
2. Experimental section
2.1. Materials
Polycaprolactone diol (PCL diol, M.W. ¼ 530 g molꢀ1, hydroxyl
ꢀ
1
value ¼ 200e205 mg g , Aldrich, Australia) was dried under
ꢁ
vacuum at 30 C for 24 h. N,N dimethylacetamide (DMAc, Aldrich,
Australia) was freshly distilled while aniline (Aldrich, Australia),
triethylamine
(Aldrich,
Australia),
bis(2-aminoethyl)amine
(Aldrich, Australia) and methylene di-p-phenyl diisocyanate (MDI,
Aldrich, Australia) were all used as received.
2.2. Synthesis of polyurea/malonamide dendrons
Hence, in this work, we have synthesized a series of novel
segmented polyurethane hybrids using polyurea/malonamide
dendritic structures comprising up to 3 generations and methy-
lene di-p-phenyl diisocyanate (MDI) as hard domains, with pol-
ycaprolactone diol (PCL diol) as soft domain (Scheme 2) for
coating applications. Our hypothesis is that the dendritic com-
ponents of the macromolecule will enhance the mechanical
properties through extensive hydrogen bonding from the large
array of urea-amide groups. The hard segments benefit from the
rigidity of MDI, due to the aromatic groups, while the incorpo-
ration of the dendrons leads to many urea groups to promote
Polyurea/malonamide dendrons were synthesized according to
the literature [3]. Briefly, a bi-functional building block, 4-
0
isocyanato-4 (3,3-dimethyl-2,4-dioxo-acetidino)diphenylmethane
(MIA) was synthesized from the reaction of MDI with isobutyryl
chloride in the presence of triethylamine. MIA was then reacted
with aniline, yielding a compound with an azetidine-2,4-dione
functional group. This functional group was subsequently reacted
with bis(2-aminoethyl)amine to provide the first-generation den-
drimer. A convergent route of dendrimer synthesis is seen through
a series of sequential and alternative additions of MIA and bis(2-
Scheme 2. Reaction scheme of synthesis of dendritic-poly(urethane-urea) hybrids.

A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
5135
aminoethyl)amine, producing dendrons of up to 3 generations ([G-
Multimode AFM with Nanoscope™ III controller (Veeco In-
1
], [G-2] and [G-3]).
struments Inc.) was used in tapping mode using silicon tips with
ꢀ
1
curvature radius of 10 nm and spring constant of 10 Nm . AFM
studies were conducted in air at room temperature. The results
were analysed using nanoscope version 5.30 r3 image analysis
software. Samples were prepared using solvent casting method on
silicon wafer substrates using DMSO as the solvent.
2
.3. Synthesis of isocyanate-terminated polyurethane prepolymer
In a flask fitted with a condenser, under an inert atmosphere,
PCL diol (10 g, 19 mmol) dissolved in DMAc (18 mL) was stirred at
ꢁ
6
0 C. MDI (9.4 g, 38 mmol) in the same solvent (20 mL) was added
dropwise and the reaction was carried out for 3 h. Further reaction
with polyurea/malonamide dendrons was continued with the same
set-up.
2.8. Mechanical properties of the hybrid thin film
The mechanical properties of the hybrid thin film on mild steel
substrate were evaluated using ultra-micro indentation system
(UMIS 2000, CSIRO Australia) with a Berkovich indenter. The cali-
bration of the indentation system was carried out with fused silica
(elastic modulus E ¼ 72 GPa) at 0.2 mN peak load. The indentations
were carried out after allowing enough time for the samples to
attain thermal equilibrium (drift rate < 1 nm/min) in the indenta-
tion chamber. Thermal drift during the experiment was deemed
negligible and no thermal corrections were made during analysis.
The nanoindentation loadedisplacement experiments were done
with step loading to the maximum load, hold period at the
maximum load for 30 s and the unloading. A series of 20 in-
2.4. Synthesis of dendritic-poly(urethane-urea) hybrid
eNH functionalized polyurea/malonamide dendrons, [G1eNH],
[G2eNH] or [G3eNH], in DMAc were added dropwise to the
isocyanate-terminated prepolymer solution in a stoichiometric
ꢁ
ratio of 2:1 (prepolymer:dendron). The solution was stirred at 60 C
under nitrogen for 6 h, after which, the gel was precipitated in
ꢁ
water, washed in DMAc, filtered and dried at 30 C under vacuum
for 4 h to give a series of dendritic-poly(urethane-urea) hybrid. The
synthesized dendritic-poly(urethane-urea) hybrids containing G1-
NH, G2-NH and G3-NH dendrons are henceforth referred as G1-PU,
G2-PU and G3-PU respectively.
dentations were made with the gap of 50 mm spacing. The average
elastic modulus and hardness values were determined from the
loadedisplacement data after making necessary corrections for
area factor and initial penetration depth.
2.5. Spectroscopic characterization
The spectroscopic characterization of the monomers and the
3. Results and discussion
progress of the reaction were monitored using Transmission mode
FTIR. Sodium chloride plates were used as window material. The
synthesized polymer was characterized using Photoacoustic Four-
ier Transform Infrared Spectroscopy (PA-FTIR). A Nicolet Magna™
IR spectrometer (model 750) equipped with a MTEC (model 300)
photoacoustic cell was used. The spectra were collected in the mid-
3.1. Spectroscopic characterization
FTIR was used to evaluate the structure of the dendritic mate-
rials, isocyanate-terminated prepolymer and the final structure of
the synthesized dendritic-poly(urethane-urea) hybrids. Fig. 1
shows the PA-FTIR spectra of the prepolymer, G1-NH dendron
and the final G1-PU sample. The prepolymer spectrum [Fig. 1 (a)]
ꢀ1
infrared region with 256 scans at a resolution of 4 cm and a
ꢀ
1
mirror velocity of 0.158 cms . Carbon black was used as a reference
ꢀ1
ꢀ1
and the system was purged with helium gas at a flow rate of
shows the presence of peaks at 2266 cm and 3319 cm , which
correspond to the isocyanate and stretching band of urethane
amide groups respectively. Correspondingly, the spectrum also
shows stretching of free urethane carbonyl; hydrogen bonded
urethane carbonyl and bending of amide groups, indicated by the
ꢀ1
1
1
5 cm3
s . H NMR was done on a Varian 200 MHz NMR spec-
trometer using deuterated dimethyl sulfoxide (DMSO) as the
solvent.
ꢀ
1
ꢀ1
ꢀ1
2.6. Thermal analyses
presence of peaks at 1734 cm , 1699 cm
and 1541 cm
ꢀ
1
ꢀ1
respectively. The peaks at 1066 cm and 1106 cm are assigned to
hydrogen bonded and free ether stretching band respectively
[20,21]. This shows that the eNH band of urethane is hydrogen
Thermal stability of the prepared hybrids was investigated using
Thermogravimetric Analyzer (TGA), TA Instruments (model 2950),
ꢁ
ꢀ1
ꢁ
at a heating rate of 10 C min from room temperature to 550 C
ꢀ
1
under a controlled nitrogen flow rate of 50 mL min . The mass of
the sample used was between 10 and 12 mg. The temperature of
the maximum weight loss was determined from Differential
Thermogravimetric (DTG) curves.
Differential Scanning Calorimetry (DSC) was performed using a
TA Instruments DSC (model 2920) with a heating and cooling rate
ꢁ
ꢀ1
ꢀ1
of 10 C min under nitrogen flow rate of 50 mL min . The mass of
the sample taken was between 8 and 10 mg and the sample was
ꢁ
ꢁ
cycled twice through a temperature range of ꢀ100 C to 200 C. The
transition temperatures were determined from the second heating
cycle. Liquid nitrogen was used to achieve sub-ambient tempera-
ture. The melting point was reported as the maximum of the
g
endothermic peak, while the glass transition (T ) temperature was
considered as the midpoint of the glass transition.
2.7. Morphological characterization
Atomic Force Microscopy (AFM) was used to characterize the
Fig. 1. FTIR spectra of (a) isocyanate terminated prepolymer, (b) G1-NH dendron and
topography and phase of the thin films coated on silicon wafer. A
(c) G1-PU.

5136
A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
bonded to both urethane carbonyl group and the ether linkage from
the soft segment. The hydrogen bonding between the soft segment
and the urethane amide group would increase the phase mixing
a shoulder peak to the carbonyl peak (1651 cmꢀ1) of the malona-
mide group. This observation can be attributed to the fact that as
the generation of the dendrons increases, the number of malona-
mide group increases. This reduces the ratio of urethane carbonyl
group to malonamide carbonyl group. Hence, a decrease in the
intensity of urethane carbonyl group is observed with the increase
in the generation of the dendritic structures. In case of ether band,
ꢀ1
between the hard and soft segments. The peaks at 2945 cm and
ꢀ
1
2
865 cm correspond to the asymmetric and symmetric stretch-
ing vibrations of CH
peaks at 1229 cm and 1190 cm correspond to OeC]O groups.
x
(x ¼ 2,3) from PCL-diol segment, whereas the
ꢀ
1
ꢀ1
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
The bands at 1457 cm , 1311 cm and 1413 cm are attributed to
CH bending vibration, CH wagging and the skeletal vibration of
the ratio of hydrogen bonded (1066 cm ) to non-hydrogen bonded
ꢀ1
2
2
(1106 cm ) ether decreases with the increase in the generation of
the dendritic structure in the polymer. This reduces the phase
mixing of hard and soft phases with the higher generations. This
can be attributed to the difficulty in molecular packing due to its
highly branched nature as the generation of dendrons increases.
Hence, we expect a higher phase separation between hard and soft
phases in G3-PU.
benzene ring respectively. These observations indicate that the
isocyanate group of MDI reacts with the hydroxyl group of the PCL-
diol resulting in polyurethane prepolymer endcapped with isocy-
anate groups.
Fig. 1 (b) shows the FTIR spectrum of G1-NH dendron, which
ꢀ
1
ꢀ1
shows peaks at 1651 cm and 1559 cm corresponding to the
carbonyl and amide groups of the malonamide linkages respec-
1H NMR was carried out to further elucidate the reaction of
polyurea-malonamide dendrons with isocyanate terminated pre-
ꢀ
1
tively [3]. The other peaks of importance are seen at 3305 cm and
ꢀ
1
1
3
045 cm respectively, which are attributed to NH band of the
malonamide linkage and aromatic ring structure. In case of G1-PU
Fig.1 (c)], the notable feature is the absence of the isocyanate peak,
polymer. Fig. 3 (a) and (b) show the representative H NMR spectra
of G2-NH dendron and its corresponding hybrid respectively. The
G2-NH dendron spectrum [Fig. 3 (a)] shows the resonance peak at
1.35 ppm, which is attributed to methyl protons (1) present in the
malonamide linkage; whereas the resonance peak at 2.69 ppm
corresponds to methylene protons (2) adjacent to secondary amino
functional (NH) group [3]. The dominant peak at 3.35 ppm arises
from the protons of water molecules, which is present as an im-
purity in the solvent DMSO, whose proton resonance peak is
observed at 2.49 ppm. The other resonance peak in the region of
3.12e3.23 ppm appears as a shoulder peak to the dominant peak at
3.5 ppm. This peak is attributed to methylene protons (3) adjacent
to tertiary amino function in the structure [22]. The chemical shift
at 3.80 ppm corresponds to the methylene protons (4) connecting
two aromatic ring structures [22,23]. The chemical shifts corre-
sponding to the aromatic (5) protons are evidenced in the region of
6.90e7.60 ppm. The resonance peak at 8.60 ppm arises from the
[
which is present in the prepolymer spectrum. Also, the G1-PU
spectrum shows the peaks corresponding to both the prepolymer
and G1-NH dendron, except the isocyanate peak. This shows that
the isocyanate terminated prepolymer reacts with secondary
amino-functionalized G1-NH dendron to form urea linkages. This
observation confirms the covalent interaction of the prepolymer
with the dendron structure.
The FTIR spectra of G1-PU, G2-PU and G3-PU are given in Fig. 2
(
a)e(c) respectively. The spectra of all three polymers show peaks
corresponding to carbonyl peaks of urethane and malonamide at
ꢀ ꢀ1 ꢀ1
1
1
734 cm , 1699 cm and 1651 cm respectively. In addition,
ꢀ
1
ꢀ1
amide peaks are observed at 1559 cm and 3305 cm and the
peaks corresponding to aromatic ring and aliphatic chains are also
observed in all three spectra. Furthermore, these spectra show the
ꢀ
1
1
absence of peak at 2266 cm corresponding to isocyanate peak
observed in the prepolymer. This observation shows that the iso-
cyanate terminated polymer completely reacted with the polyurea/
malonamide dendrons resulting in dendritic-poly(urethane-urea)
hybrid. The major difference among these three different hybrids
is the intensities of the peaks corresponding to hydrogen and non-
hydrogen bonded urethane carbonyl groups and soft segment ether
groups. G1-PU spectrum [Fig. 2 (a)] shows a strong peak corre-
proton (6) in the malonamide linkage. The H NMR spectrum of the
G2-NH dendron is well comparable to the literature values [3,23].
1
The H NMR spectrum of the G2-PU [Fig. 3 (b)] shows new
resonance peaks at 1.21 ppm, 1.67 ppm, 2.29 ppm, 3.60 ppm,
4.23 ppm and 9.20 ppm, in addition to the resonance peaks present
in the G2-NH dendron spectrum. The resonance peak observed at
2 2
3.60 ppm arises from the methylene protons in the CH eOeCH (a)
linkage from the reacted PCL-diol. The protons of the methylene
group (b) connected to the urethane linkage resonate at 4.23 ppm
[22,24]. The other chemical shifts with peak positions at 1.21 (d),
1.67 (c, e) and 2.29 (f) are assigned to methylene protons of PCL
present in different environments [21,25]. Finally, the resonance
peak observed at 9.20 ppm is attributed to the proton in poly-
urethane NH (g) [22,26]. Hence, the new signals present in the
spectrum of G2-dendritic-poly(urethane-urea) hybrids are attrib-
uted to the presence of PCL-diol, which are connected to the den-
drons through urethane linkage. This observation confirms the
reaction of prepolymer with the dendron forming dendritic-
poly(urethane-urea) linkage and further substantiates the FTIR
results.
ꢀ
1
sponding to non-hydrogen bonded (1734 cm ) and hydrogen
ꢀ1
bonded urethane carbonyl groups (1699 cm ); whereas the G2-
NH incorporated hybrid shows a decreased intensity of these two
peaks. In the case of G3-PU, the urethane carbonyl peaks appear as
3.2. Thermal stability studies
Thermogravimetric analysis was carried out to examine the
thermal stability of the monomers and the prepared dendritic-
poly(urethane-urea) hybrids. Their corresponding weight loss
plots as a function of temperature are given in Fig. 4. The onset of
degradation of the monomers and the hybrids are given in Table 1.
The components corresponding to the degradation peaks are
assigned based on the theoretical and experimental calculations of
the weight percentage of the individual components. The
Fig. 2. PA-FTIR spectra of (a) G1-PU, (b) G2-PU and (c) G3-PU.

A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
5137
Fig. 3. Representative 1H NMR spectra of (a) G2-NH dendron and (b) G2-PU samples.
ꢁ
theoretical weight percent of individual components are calculated
by considering the repeating unit of the polymer chain and the
second step is observed at 350 C. The initial step is attributed to
the degradation of malonamide linkage and the second stage is
assigned to the degradation of urea linkage and partial degradation
of the aromatic ring structure. The residue yield observed for G2-
NH dendron is 12.13%. Similarly, the thermal degradation of G3-
NH occurs in two stages.
stoichiometric feed ratio. The onset of degradation of MDI occurs at
ꢁ
2
14 C and the complete degradation of the same monomer occurs
ꢁ
in a single step before 287 C. In case of PCl diol monomer, the
thermal degradation occurs in two stages with the initial maximum
ꢁ
weight loss observed at 338 C and the subsequent maximum
The decomposition temperatures of the final polymers are
shifted to higher temperature in comparison to respective mono-
mers. The thermal degradation of G2-PU occurs in two steps. The
maximum degradation temperature of the initial degradation step
is observed at 276 C, which corresponds to the degradation of the
malonamide linkage as observed from G2-NH dendron. The second
degradation step occurs over a broad range of temperature
(306 Ce460 C) indicating the presence of more than one linkage
such as urea, urethane and the aromatic ring structure. In case of
G3-PU, the degradation occurs in a single step with the maximum
ꢁ
weight loss occurs at 432 C. During the first stage, ether linkage
and the associated alkyl groups of the PCL diol monomer undergo
degradation followed by the degradation of the ester linkage in the
ꢁ
second stage. The onset of degradation of G2-NH dendron occurs at
ꢁ
2
46 C with the degradation occurring in two steps. The maximum
ꢁ
weight loss of the initial degradation step occurs at 277 C with the
weight loss of 29.7%; whereas the maximum weight loss during the
ꢁ
ꢁ
ꢁ
weight loss occurring at 373 C. This maximum weight loss tem-
perature is higher than the monomers and G2-PU sample. This
shows that the thermal degradation property of the prepared
polymer is dependent on the degree of branching.
Table 1
Thermal properties of the monomers and the resulting hybrids.
ꢁ ꢁ ꢁ ꢁ ꢁ ꢁ
d g m g m g
T / C T / C T hard/ C T soft/ C T soft/ C T hard/ C
Sample/thermal
behaviour
PCL diol
G2-NH
G2-PU
G3-NH
G3-PU
278
246
263
272
287
ꢀ58 50
e
e
e
e
e
130
e
e
e
e
e
e
ꢀ30
e
58
e
123
e
128
e
ꢀ38
56
125
Fig. 4. Thermogravimetric curves of monomers and the synthesized hybrids.

5138
A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
Fig. 5. DSC thermograms of (a) PCL-diol monomer, (b) G2-NH, (c) G3-NH, (d) G2-PU and (e) G3-PU.
3
.3. Thermal transitions
g g
FTIR. While, there is an increase in the T of soft segments, the T of
ꢁ
ꢁ
the hard segments of G2-PU and G3-PU decreased by 7 C and 3 C
respectively. This shows that the phase mixing of soft phase with
the hard phase introduces much free volume, thereby reducing the
Thermal transitions of the monomers and the final dendritic
poly(urethane-urea) are determined using DSC. Thus determined
thermal transitions are used to understand the phase mixing/sep-
aration behaviour of the prepared polymeric materials. Fig. 5 shows
the DSC thermograms of PCL diol, G2-NH, G3-NH, G2-PU and G3-
PU samples and the transition temperatures are listed in Table 1.
g
T of the hard phase.
3.4. Morphological characterization of microphase separation
ꢁ
PCL diol shows both glass transition (at ꢀ58 C) and melting point
ꢁ
ꢁ
ꢁ
(
at 50 C), whereas G2-NH and G3-NH show T
g
at 130 C and 128 C
In order to confirm the phase-separated morphology as pre-
respectively. There are three transitions in both G2-PU and G3-PU
samples showing the T of both hard and soft segments and the
melting point of the soft segment. G2-PU shows T of the soft and
dicted by FTIR and DSC results, surface morphologies of the syn-
g
thesized
segmented
dendritic-poly(urethane-urea)
were
g
ꢁ
characterized using AFM. Fig. 6 (a)e(c) show the phase images of
G1-PU, G2-PU and G3-PU samples respectively. The bright regions
in the phase image denote the hard domains, whereas the dark
regions are soft domains. G1-PU sample shows more uniform
distribution of soft segment domains on hard domains. Cross-
sectional analysis of the phase plot reveals the hard segment
domain widths of 27 ± 8 nm. Similar type of segmented poly-
urethanes show phase separated morphology on comparable
length scales [13,27,28]. In case of G2-PU sample, the size of hard
domains is much higher than the G1-PU sample, of the order of
ꢁ
hard segments respectively at ꢀ30 C and 123 C and melting
ꢁ
transition of the soft segment at 58 C. In the case of G3-PU, T
g
of the
ꢁ
ꢁ
soft segment is observed at ꢀ38 C and the melting point at 56 C,
ꢁ
whereas the T
g
of the hard segment is observed at 125 C. The
presence of thermal transitions of both soft and hard segments in
the final polymer indicates the phase separation between hard and
soft segments. On the other hand, the glass transition temperatures
ꢁ
of the soft segment of G2-PU and G3-PU show an increase of 28 C
ꢁ
and 20 C respectively in comparison to the PCL-diol monomer. The
T
g
of the soft segment is increased due to the imposed limitations of
freedom of rotation by a certain degree of phase mixing among the
hard and soft segments. The increase in T is more predominant for
48
±
9 nm. G3-PU samples show randomly distributed
morphology with higher hard segment domains. Earlier studies
[28,29] indicated that the increase in the hard phase volume
fraction leads to the lower degree of phase mixing between hard
and soft segments. Similar observation has been made in the
present case indicating greater degree of phase separation with
higher generations.
g
G2-PU than G3-PU, which shows that the phase mixing is more
pronounced in lower generation dendritic polymer than the higher
generation ones. This can be due to the fact that the molecular
packing is difficult for higher generation dendrons as evidenced by
Fig. 6. AFM phase images of (a) G1-PU, (b) G2-PU and (c) G3-PU samples.

A.G. Kannan et al. / Polymer 55 (2014) 5132e5139
5139
hybrid increases, resulting in segmented hybrid structures. This
segmented morphology leads to higher physical cross-link density
and intermolecular interactions among the hard segments, thereby
resulting in enhanced mechanical properties. This work provides a
platform for novel surface engineering with variable elastic
modulus/hardness, which is in great demand in the field of bio-
engineered surfaces.
Acknowledgements
The authors gratefully acknowledge the financial support of the
Australian Research Council's Special Research Centre (grant no:
S00001491) for carrying out this work.
Fig. 7. Nanoindentation loading-hold-unloading curves of G1-PU, G2-PU and G3-PU
hybrids.
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