Cost effective poly(urethane-imide)-POSS membranes for environmental and energy- related processes

Abstract

The control of anthropogenic carbon dioxide (CO2) emissions is one of the most challenging environmental issues facing industrialized countries because of its implications to atmospheric CO2 levels and climatic change. Burning of fossil fuels is responsible for the majority of these CO2 emissions and, therefore, there is significant interest in developing technologies that will reduce CO2 emissions. The membrane-based separation processes are not only cost effective and environmentally friendly, but also with many novel polymeric materials available, offer much more versatility and simplicity in customized system designs. The ability to selectively pass one component in a mixture while rejecting others describes the perfect separation device. We have synthesized a set of poly(urethane-imide)-POSS (PUI) by the simple condensation reaction of isocyanate terminated polyurethane (PU) prepolymer and anhydride terminated polyimide (PI) prepolymer. The PUIs were characterized by TGA, SEM, and AFM analyses. Thermal stability of the PU was found to increase by the introduction of imide component. Gas permeation measurements were studied for O2, N2, and CO2 gases by employing different pressures using constant volume/variable pressure apparatus.

Full text

1 23
Clean Technologies and
Environmental Policy
Focusing on Technology Research,
Innovation, Demonstration, Insights
and Policy Issues for Sustainable
Technologies
ISSN 1618-954X
Clean Techn Environ Policy
DOI 10.1007/s10098-012-0500-7
Cost effective poly(urethane-imide)-POSS
membranes for environmental and energy-
related processes
D.Gnanasekaran & B.S.R.Reddy

1 23
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ORIGINAL PAPER
Cost effective poly(urethane-imide)-POSS membranes
for environmental and energy-related processes
D. Gnanasekaran •B. S. R. Reddy
Received: 19 February 2012 / Accepted: 31 May 2012
ÓSpringer-Verlag 2012
Abstract The control of anthropogenic carbon dioxide
(CO
2
) emissions is one of the most challenging environ-
mental issues facing industrialized countries because of its
implications to atmospheric CO
2
levels and climatic
change. Burning of fossil fuels is responsible for the
majority of these CO
2
emissions and, therefore, there is
significant interest in developing technologies that will
reduce CO
2
emissions. The membrane-based separation
processes are not only cost effective and environmentally
friendly, but also with many novel polymeric materials
available, offer much more versatility and simplicity in
customized system designs. The ability to selectively pass
one component in a mixture while rejecting others
describes the perfect separation device. We have synthe-
sized a set of poly(urethane-imide)-POSS (PUI) by the
simple condensation reaction of isocyanate terminated
polyurethane (PU) prepolymer and anhydride terminated
polyimide (PI) prepolymer. The PUIs were characterized
by TGA, SEM, and AFM analyses. Thermal stability of the
PU was found to increase by the introduction of imide
component. Gas permeation measurements were studied
for O
2
,N
2
, and CO
2
gases by employing different pressures
using constant volume/variable pressure apparatus.
Keywords Poly(urethane-imide)-POSS Membranes 
Environmental Carbon dioxide
Introduction
Global warming has been identified as one of the world’s
major environmental issues that need to be taken into
consideration on a global level. The global warming is
caused by the emission of greenhouse gases and most of
the component is carbon dioxide (CO
2
). The emissions of
CO
2
have been dramatically increased for the last 50 years
and yet continually increasing each year (Powell and Qiao
2006). This leads to an increasing CO
2
level in the atmo-
sphere which in turn, causes the global temperature to rise.
There are several points within stationary energy produc-
tion where CO
2
is produced and then emitted into the
atmosphere, such as in the production of natural gas from
an underground reservoir and in the production of synthesis
gas using fossil fuels and flue gas from electricity power
stations that are from direct combustion of fossil fuels
(Jung et al. 2004). Usually, CO
2
is the most abundant
contaminant in a typical natural gas feed, with some large
reservoirs containing over 50 % CO
2
. Carbon dioxide
emitted inevitably from the combustion of fossil fuels, has
attracted increasing attention because of its potential
impact on global climate change. With the rapidly
increasing interest in CO
2
separation to mitigate global
warming, separation of gases using polymeric membranes
will remain an active research area and the literature will
continue to grow as new discoveries are made in the art.
This article focuses on the most recent polymeric
membrane designs, mainly nano-incorporated membranes
as shown in Fig. 1and facilitated transport membranes,
which provide improved CO
2
separation over the previous
polymeric designs. There are five possible mechanisms
for membrane separation (Paul and Yampolskii 1994;
Fritzsche and Kurz 1990) such as knudson diffusion,
molecular sieving, solution-diffusion separation, surface
D. Gnanasekaran B. S. R. Reddy (&)
Industrial Chemistry Laboratory, Central Leather Research
Institute (Council of Scientific & Industrial Research),
Chennai 600 020, India
e-mail: induchem2000@yahoo.com
123
Clean Techn Environ Policy
DOI 10.1007/s10098-012-0500-7
Author's personal copy

diffusion, and capillary condensation. Molecular sieving
and solution diffusion are the main mechanisms for nearly
all gas-separating membranes. Knudson separation is based
on gas molecules passing through membrane pores small
enough to prevent bulk diffusion.
Separation is based on the difference in the mean path of
the gas molecules due to collisions with the pore walls
which is related to the kinetic diameter (Table 1). Pores
within the membrane are of a carefully controlled size
relative to the kinetic diameter of the gas molecules. This
allows diffusion of smaller gases at a much faster rate than
larger gas molecules. In this case, the CO
2
/N
2
, selectivity is
greater than unity, as CO
2
has a smaller kinetic diameter
than N
2
. Surface diffusion is the migration of adsorbed
gases along the pore walls of porous membranes (Hill
1956; Hwang and Kammermeyer 1975). Polymeric mem-
branes are generally non-porous, and therefore gas per-
meation through them is described by the solution-
diffusion mechanism as shown in Fig. 2(Paul and Yam-
polskii 1994; Ganapathi-Desai and Sikdar 2000). This is
based on the solubility of specific gases within the mem-
brane and their diffusion through the dense membrane
matrix. Hence, separation is not just diffusion dependent
but also reliant on the physical–chemical interaction
between the various gas species and the polymer which
determines the amount of gas that can accumulate in the
membrane polymeric matrix. The ability to selectively pass
one component in a mixture while rejecting others
describes the perfect separation device. While no mem-
brane system truly behaves this way, membrane gas sepa-
ration do have a number of advantages over conventional
processes and a number of reviews examining their benefits
exist (Stern 1994; Powell and Qiao 2006; Maiser 1998;
Koros 2002; Baker 2002). In particular, this article has
focused on advances in polymeric membrane design for
improved CO
2
separation.
Experiment
Materials and methods
The Cy-POSS was synthesized in our laboratory as explained
in our previous work (Gnanasekaran et al. 2011). Hexam-
ethylene diisocyanate (Merk, 95 %) was used as received,
poly(dimethylsiloxane) bis(hydroxylalkyl) as terminated
(M
n
=5,600) (Aldrich, 99 %), and 4,40-(hexafluoroisopro-
pylidene) dipthalicdianhydride (Aldrich, 99 %) was purified
by sublimation under vacuum. Dibutyltin dilarurate
(Aldrich, 95 %), and tetrahydrofuran (Rankem) was distilled
using calcium hydride and sodium metal. All other chemi-
cals were analytical grade and used as received.
29
Si-NMR spectra of the samples were recorded on Jeol
ECA-500 NMR spectrometer at 99 MHz. The thermal
stabilities of the prepared membranes were determined
using Perkin-Elmer TGA-7 and TGA Q50-TA thermal
analyzers. The thermogravimetric analysis (TGA) curves
were recorded using 10–15 mg of samples at a heating rate
of 5 °C min
-1
under nitrogen atmosphere.
The scanning electron microscopy (SEM) pictures were
taken on the flat surface and cross sections of the hybrid
membranes. Surface morphology of hybrid membranes
was studied using a Nanoscope III atomic force microscopy
(AFM) instrument and imaging was done in contact mode
Fig. 1 Structure of nanomaterial (POSS)
Table 1 Kinetic diameters of penetrants
Gas Lennard–Jones
diameter (A
˚)
Kinetic
diameter (A
˚)
He 2.57 2.60
H
2
2.91 2.89
N
2
3.61 3.64
O
2
3.43 3.46
CH
4
3.82 3.80
CO
2
3.99 3.30
D. Gnanasekaran, B. S. R. Reddy
123
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at room temperature in air. In these studies, we have used
the commercial tip of Si
3
N
4
provided by Digital Instru-
ments. Cantilever length is 200 lm with a spring constant
of 0.12 Nm
-1
.
Permeation measurements
The permeation properties of poly(urethane-imide)-POSS
(PUI) membranes were determined utilizing a constant
pressure/variable volume apparatus. The upstream pressure
was varied between 1 and 4 atm, whereas the downstream
pressure was the atmospheric pressure. Gas flow rates were
measured with a soap-film bubble flow meter. The tem-
perature was maintained at 30 °C(±1°C). Before each
experiment, both the upstream and downstream sides of
permeation cell were purged with penetrant gas.
Results and discussion
29
Si Solid-state CP/MAS NMR spectroscopy
The solid-state
29
Si NMR spectrum of the PUI hybrids
provides much more information about the type of Si units
present in the hybrids and the spectrum of PUI-20 is shown
in Fig. 3. The silicon atom corresponding to the Si–OH
shift disappeared in the CyPOSS-incorporated hybrids,
confirming that all the Si–OH groups were reacted with the
isocyanate groups. The urethane-connected Si atom showed
a signal at -65.5 ppm which was merged with T
3
-type
silicon atom present in the ring structure. This confirms the
formation of urethane linkages with silicon atom present in
the POSS molecules. The resonance signal at -20.2 ppm
was the characteristic signal representing Si atom present in
the PDMS backbone. The peak at 10.1 ppm corresponds to
the terminal Si atom (Si(CH
3
)–CH
2
) attached to the PDMS
chain and the CyPOSS molecule. From the spectral studies,
it was found that both the PDMS and CyPOSS retain their
cage structure even after hybridization.
TGA
The thermal stability of the PUI membranes was measured
by TGA at a heating rate of 10 °C min
-1
as shown in Fig. 4.
Polyurethane (PU) exhibited 50 % weight loss at 443 °C.
With the increase in PI content, the decomposition tem-
perature of the poly(urethane-imide) membrane increased
from 443 to 492 °C at 50 % weight loss. Compared to PU,
poly(urethane-imide) membranes exhibited better thermal
stabilities due to the presence of the heterocyclic imide
groups without phase separation in poly(urethane-imide)
(Park et al. 2006). In general, imide rings are considered to
be the most stable units among these linking groups. But,
urethane groups must be the most labile units and will
decompose first to start the initial thermal degradation.
Assuming different stabilities of the urethane and the imide
units, the first stage of weight loss might be attributed to the
early degradation of the urethane linkages. The reason for
Fig. 2 The gas separation
mechanism
PUI for environmental and energy-related processes
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these results could be that the more thermally sensitive PU
moiety began to decompose before PI moiety was degraded.
The pure PI with a tighter and more rigid coherence between
chains possess a good thermal stability, whereas PUI sam-
ples having a relatively increased amount of hard segment,
in the case of PUI-10 to PUI-20, had less chain coherence
because of fewer imides in the backbone chains.
SEM
The cross-sectional view of PU and PUI-10 hybrids is
shown in Fig. 5. All the PUI films show a microphase
separation of urethane hard segments and a micro/nano
level spheroidical aggregation of POSS-rich domains. The
aggregation of POSS molecules increases with increase in
the imide content. This may be due to the highly hydro-
phobic nature of POSS group. The cross-sectional view of
both the hybrids confirms the absence of formation of
microcracks or voids in the hybrid membranes. The cross-
sectional view of hybrid membranes demonstrated that the
microphase separation and POSS aggregation were not
only formed on the surface of the membranes, but also
formed throughout the hybrids. The type of aggregation
and the microphase separation of PUI hybrids were quite
different from the PU or PUI-10 hybrids.
AFM
The surface morphology and roughness of the formed PU
and PUI hybrid membranes were investigated by AFM. The
membranes PUI-10, PUI-20, and PUI-30 showed a rough
Fig. 3 Solid-state
29
Si NMR spectrum of PUI-20
Fig. 4 TGA curves of PUI membranes
Fig. 5 Cross-sectional view of SEM images of PU and PUI-10 membranes
D. Gnanasekaran, B. S. R. Reddy
123
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surface for the composite and the formation of a less
homogeneous surface compared to the PU membranes. The
surface topography, three-dimensional topographical image
and the phase images are given in Fig. 6. The extent of
projection of one phase and the surface roughness also
increased with increase in the PI content. Therefore, this
could be attributed to the existence of non-compatible
phases. It has been reported in the literature (Viville et al.
2001) that phase separation could cause surface roughness.
A similar correlation of surface roughness to the phase
separation has been reported for tetramethyl bisphenol A
polycarbonate and polystyrene blends (Cabral et al. 2001).
This further confirmed that the imide content increased the
roughness of the surface morphology.
Pressure and imide content dependency of permeability
of PUI hybrid membranes
The permeability and permselectivity values were deter-
mined from pure gas measurements of PUI membranes at
30 °C under various pressures (1–4 atm). Here, we have
studied the effect of various amounts of imide and feed
pressure on the gas transport properties. The permeability
of various types of PUI hybrids are given in Table 2.A
minimal change in N
2
and O
2
permeability with penetrant
pressure was observed for a pressure range of 1–4 atm. The
graphical analysis of this data of permeability versus
pressure showed a slight increase for N
2
and O
2
gases. In
the case of CO
2
, it was quite different from other two
gases, such as N
2
and O
2
. This may be due to two factors:
(i) More condensable nature of CO
2
gas. The increase in
pressure increases the adsorption of CO
2
gas on the surface
of the membrane and thereby diffusion of CO
2
gas was
more due to more condensable nature of CO
2
gas compared
to that of N
2
and O
2
gases. (ii) The plasticization effect of
CO
2
gas on the polymer membrane matrices. Reports have
shown that CO
2
gas molecule plasticizes the polymeric
membranes when there was increase in the pressure and
time (Duthie et al. 2007; Bos et al. 1998; Huang and Lai
1995).
For gases such as N
2
and O
2
, the permeability slightly
increases in the case of membranes having higher imide
content at higher pressures. This may be due to the intro-
duction of imide and POSS groups and this increases the
chain stiffness which in turn reduces the intrasegmental
mobility. This limits the degree of chain packing by
increasing the chain gap and serving as molecular spacers
and chain stiffeners in the polymer. The other factor that
may contribute to the higher permeability of the membrane
with 20 % of imide content was due to the lower degree of
Fig. 6 AFM image of PUI membranes
Table 2 Permeability properties of PUI membranes
Sample 1 Bar 2 Bar 3 Bar 4 Bar
O
2
N
2
CO
2
O
2
N
2
CO
2
O
2
N
2
CO
2
O
2
N
2
CO
2
PU 290 183 1,610 315 201 1,653 322 218 1,801 341 222 1,913
PUI-10 301 205 1,722 327 221 1,791 341 228 1,911 364 234 2,033
PUI-20 322 218 1,801 358 231 1,894 364 241 2,001 408 258 2,178
PUI for environmental and energy-related processes
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crosslinking. The decrease of the crosslinking degree usu-
ally results in an increase of membrane permeability since
the existence of a crosslinking network restricts the mobility
of the molecular chains. The aggregation of urethane/imide
groups would also have some effect on the gas permeabili-
ties of the membranes. Literature on PUs (Damian et al.
1997; Yoshino et al. 2000; Yang et al. 2004; Lee et al. 2004)
reports that the increase of phase separation between hard
and soft segments leads to the increase in gas permeability
(Fig. 7). The combination of these functional groups (imide,
POSS, and PDMS) and aggregation of urethane and imide
groups contribute to an increase in permeability. But, per-
meability increases in the case of CO
2
with higher pressures.
For PU, the increase in the permeability value from 290 to
341 Barrer for O
2
, from 183 to 222 Barrer for N
2
, and from
1,610 to 1,913 Barrer for CO
2
were observed. In the case of
PUI-10, the permeability values for O
2
,N
2
, and CO
2
increased from 301 to 364 Barrer, from 205 to 234 Barrer,
and from 1,722 to 2,033 Barrer, respectively. The increase in
the permeability values for O
2
,N
2
, and CO
2
gases from 322
to 408 Barrer, from 218 to 258 Barrer, and from 1,801 to
2,178 Barrer, respectively, for PUI-20.
Selectivity of the PUI membranes
The O
2
/N
2
and CO
2
/N
2
gas pair selectivities of membranes
under different pressures (1–4 atm) were measured. The
O
2
/N
2
gas pair selectivities of PU, PUI-10, and PUI-20
membranes were found to be in the range of 1.47–1.53,
1.49–1.51, and 1.50–1.55 for CO
2
/N
2
, and 8.12–8.30,
8.36–8.79, and 8.33–8.81, respectively. The O
2
/N
2
gas pair
selectivity of PU membrane was lower than the other PU
hybrid membranes. For the POSS and the imide-incorpo-
rated membranes, gas pair selectivities increased with
increase in pressure. The incorporation of nonporous inor-
ganic nanoparticles either remains unaltered or decreases
with improved permeabilities as reported in literature (Nunes
et al. 1999; Rios-Dominguez et al. 2006). We have observed
surprisingly that the gas pair selectivities increased with
increase in pressure. The POSS cage molecules in the
membrane matrix leads to less control of sieving small gases
such as N
2
and O
2
mainly due to nanogaps created by POSS.
The selectivities of CO
2
/N
2
gas pair for PUI-20 of urethane/
imide POSS membranes increases with increase in pressure.
The increase in imide content also leads to an increase in the
CO
2
/N
2
selectivities.
Conclusions
The surface morphology and thermal properties of PUI
membranes were characterized using SEM, AFM, and TGA,
and those properties were corroborated well with the per-
meation measurements. The effect of POSS nanoparticle,
rigid imide, and mixed soft segment on gas transport prop-
erties were studied in detail. Separation of CO
2
is an
emerging technology used to reduce the impact of fossil fuel
combustion. Chemists can play an important role in the
development of this technology and one such major role is
the development of novel polymeric materials for the CO
2
/
N
2
selectivity. The CO
2
and N
2
gas transport properties of a
number of polymeric membranes have been discussed. The
PUI polymers offer very high permeabilities and modest
selectivities, which have lead to excellent gas transport
properties. The gas transport studies of the membranes
confirm that the POSS molecules aid the permeation of
penetrant molecules. The permeation data in this work is to
study the complete transport studies. We do hope that the
complete gas transport study will give a clear idea about the
transport behavior in the urethane/imide POSS membranes.
Acknowledgments D. Gnanasekaran thanks Department of Science
and Technology, New Delhi (No. SR/S1/PC- 33/2006) for the Junior
Research Fellowship and CSIR for the Senior Research Fellowship.
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