Multi-cycle reversible control of gas permeability in thin film composite membranesviaefficient UV-induced reactions

Article abstract of DOI:10.1039/d0cc08238d

This communication presents a new, UV-induced mechanism to reversibly control the permeability of ultra-thin polymer coatings. Photoreversible [2+2] cycloaddition reactions were utilised to adjust the crosslinking degree and glass transition temperature of a coating. Consequently, a 300%, reversible change in the coating's oxygen permeability was achieved without loss of performance. Ultimately, the findings demonstrate the capability of using low UV doses to reversibly and efficiently regulate mass transport through ultra-thin coatings fabricated in a facile manner.

Full text of DOI:10.1039/d0cc08238d

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COMMUNICATION
Multi-cycle reversible control of gas permeability
in thin film composite membranes via efficient
UV-induced reactions†
Cite this: Chem. Commun., 2021,
57, 3391
Received 19th December 2020,
Accepted 25th February 2021
b
b
Abdalrahman U. Alrayyes,a Ze-Xian Low,
Huanting Wang
*
ac
and Kei Saito
*
DOI: 10.1039/d0cc08238d
rsc.li/chemcomm
This communication presents a new, UV-induced mechanism to Reversibility allows for optimisation of the oxygen environment
reversibly control the permeability of ultra-thin polymer coatings. of the packaged good, which in turn optimises its quality and
Photoreversible [2+2] cycloaddition reactions were utilised to shelf-life as it goes through the packaging and storage processes.
adjust the crosslinking degree and glass transition temperature of Another application requiring a switchable oxygen permeability is
a coating. Consequently, a 300%, reversible change in the coating’s polymer nanocarriers as gas transport vehicles.9 That is, a nano-
oxygen permeability was achieved without loss of performance. carrier could reversibly hold and release oxygen when triggered by a
Ultimately, the findings demonstrate the capability of using low UV remote signal, acting as an oxygen carrier without sacrificing its
doses to reversibly and efficiently regulate mass transport through structural integrity. Within the context of smart membrane triggers,
ultra-thin coatings fabricated in a facile manner.
UV light offers remarkable advantages over other stimuli such as
temperature, magnetism, pH, and others. The high levels of spatial
Much of the recent membrane research is heavily invested into and temporal resolution,10 the reactions’ limited by-products, the
smart, multifunctional films for applications in gas and mole- inessential need for additional chemicals, and the unnecessary
cule separation, catalysis, drug delivery, sensing and others.1–6 physical/chemical contact between the trigger and the substance11
Undoubtedly, materials that can be tailored to respond to all make UV an ideal trigger for a variety of applications sensitive to
specific triggers to control heat and mass flow, energy, wetting all other stimuli. One approach, other than the prominent use of
properties and overall shape7 are essential towards the creation cis–trans photoisomerisation, to photo-control a membrane’s gas
of advanced functional materials. More specifically, there are permeance is by using the crosslinking/de-crosslinking mechanism
applications that require reversible tuning of the oxygen perme- seen in [2+2] photocycloaddition reactions in the solid state,12 as
ability of a polymer film from high to low barrier using remote exhibited by coumarin. Coumarin has been extensively studied for
signals. An often overlooked application in stimuli responsive its ability to undergo efficient [2+2] cycloaddition reversible
polymer films is that of packaging, where careful control of the reactions,13–17 such as its use in self-healing polymers,15 shape
gas environment around a packaged good is necessary to pro- memory polymers,18 biomedical applications,19 and others.20
long its shelf-life, and enhance or maintain its sensory Inducing UV-triggered permeance changes in an explicitly photo-
characteristics.8 Some packaged goods, such as red meat, often active polymer film can be challenging, since photomasking effects,
need dynamically changing oxygen environments (low or high low reaction efficiencies and oligomer formations can all hinder
amounts of oxygen) based on their location in the supply chain. the penetration of gases through the matrix. To mitigate such
issues, a high density of photoactive moieties achieved by a ‘star-
shaped 4-arm’ monomer structure, coupled with a flexible polymer
backbone able to show acute changes in the glass transition
temperature (Tg), and a small monomer size are needed.
a School of Chemistry, Monash University, Clayton, VIC 3800, Australia.
E-mail: kei.saito@monash.edu
b Department of Chemical Engineering, Monash University, Clayton, VIC 3800,
The following communication reports a new and practical
method to photo-trigger reversible oxygen permeability tuning
for at least 4 cycles for thin film composite membranes without
loss of photo-control efficiency. An ultra-thin layer composed of
a photoresponsive monomer was spin-coated onto a PDMS/
LDPE membrane. The oxygen permeance of the composite
membrane was reversibly controlled by successive UV light
Australia
c Graduate School of Advanced Integrated Studies in Human Survivability,
Kyoto University, Higashi-Ichijo-Kan, Yoshida-nakaadachicho 1, Sakyo-ku,
Kyoto, 606-8306, Japan
† Electronic supplementary information (ESI) available: Experimental methods,
reaction pathways, characterisation of monomers and UV reaction (NMR, FTIR,
MS, SEC, TGA), supporting SEM images, additional permeability data, theoretical
background on permeability, Tg and crosslinking. See DOI: 10.1039/d0cc08238d
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irradiation of different wavelengths. Under 365 nm, the mem- regarding the effects of crosslinking and Tg on polymeric mem-
brane’s oxygen permeance was impeded, and reversibly, under branes’ gas permeability is presented in the ESI.†
254 nm, the membrane’s oxygen permeance was eased. The
Factors affecting the permeance of nonporous membranes,
transitions were realised by reversibly controlling the cross- as is the case in this communication, are the glass transition
linking density and Tg of the film using [2+2] photocycloaddi- temperature and crystallinity.22 For the effect of crosslinking
tion polymerisation reactions. While there have been studies density on gas permeance, studies have shown that the degree
pertaining to the Tg and crosslinking relationship with gas of crosslinking, which accompanies a change in polymer den-
permeability, there have been no reports, to our knowledge, of sity, affects the permeability and permselectivity of a polymer
using both factors to dynamically change the permeance membrane.23,24 It was explained that crosslinking causes a
to create a smart, ultra-thin polymer film using UV light only. reduction in chain mobility due to inter-chain covalent
A ‘4-arm’, siloxane based monomer with coumarin moieties bonds.23,25 Within the context of the effect of Tg on perme-
terminating each arm was synthesized and chosen in this study ability, two general theories can be applied. First is that
to create the ultra-thin, non-porous UV-active film. The mono- transitioning above the Tg has no effect on permeability.
mer characterisation data such as its NMR, FT-IR and mass According to Barrer’s equation,26
ꢀ
ꢁ
spectra are presented in Fig. S3 and S4 of the ESI.† It is well
known that siloxane-based materials are much more permeable
than their carbon counterparts, mainly due to the high
flexibility of the Si–O bond, which permits easier intermolecu-
lar movement of polymer chains and thus, greater free
volumes.21 Hence, such bonds were integrated into the mono-
mer structure to have a very low Tg (at least 10 1C below room
temperature) when the film was de-crosslinked, which was a
critical factor in attaining a large permeance change (300%
increase or decrease) while also using a small amount of UV
power. Through the facile fabrication of the monomer layer
(illustrated in Fig. 1b) and the highly efficient, reversible
cycloaddition reaction in the solid state, the chosen monomer
was effective in creating a practical, photo-switching film
able to demonstrate oxygen permeance control for multiple cycles
while maintaining a nonporous structure. A theoretical discussion
Ed
D ¼ Do ꢀ e ꢁ
(1)
RT
diffusivity is dominated by the activation energy, which
‘‘involves the hindered rotation of a few contiguous monomer
segments to allow the passage of the penetrant’’.27 An increase
in temperature allows for increased segmental mobility, which
in turn allows for a greater zone of activation, leading to an
increase in both energy and entropy of activation.27 Therefore,
an increase in temperature at the Tg has the same effect as it
would without the Tg transition. However, previous studies
show that this effect can vary based on penetrant size and
nature of the polymer,27 where smaller voids require a large
amount of monomer rearrangement, making it a free volume
dependent process. The rate of change of free volume changes
at the Tg, making the temperature transition above the Tg
a
Fig. 1 (a) 4ASil chemical structure before and after crosslinking with UV light. (b) The methodology in the creation of a UV-switchable membrane
composed of an ultra-thin photoactive film onto a porous substrate. A gutter layer of PDMS was spin-coated onto porous LDPE and thermally cured.
Next, a layer of 4ASil was spin-coated onto the PDMS/porous LDPE membrane. Finally, the composite membrane was subjected to consecutive
irradiation with 365 and 254 nm UV light.
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substantial factor in permeability results.28 However, for larger
void sizes, the activation energy argument is true.
By irradiation with a higher energy wavelength (254 nm), the
network de-crosslinked, forming back the flexible, soft and
In investigating the gas transport properties before and after small monomer, which showed a higher O2 permeance of
crosslinking, the material was spin-coated as an ultra-thin layer 75.6. Permeance values are in (10ꢁ10 mol mꢁ2 sꢁ1 Paꢁ1) units.
onto a porous substrate. Due to the softness and flowability of Gas permeance towards other gases (H2, N2, CO2 and CH4) and
the photoresponsive monomer ‘4ASil’, a dense layer of poly- ideal selecitivities are presented in Table S2 of the ESI.†
dimethylsiloxane (PDMS) was spin-coated onto the porous low-
density polyethylene (LDPE) first. The membrane fabrication of reaction efficiency with every cycle due to the formation of
methodology is shown in Fig. 1b. oligomers during crosslinking/de-crosslinking (Fig. S5b of the
Although [2+2] cycloaddition reactions are subject to decay
Since the [2+2] reactions are reversible, it was expected that ESI†), that phenomenon did not translate to a lower permeance
the same logic could be extended to the case of permeance switching efficiency in this case. That is because partial cross-
switching. The oxygen permeance photoreversibility is exhibited linking (0.04 J cmꢁ2 of 365 nm irradiation) was enough to
in Fig. 2a, where the oxygen permeance was reversibly tuned for at create a high barrier film, and hence, there was at least 70% of
least 4 cycles. Critical to that success was two factors. Firstly was unreacted monomer able to undergo the cycloaddition reaction
the drastic photo-modification of the polymer matrix between in consecutive cycles. In the case of cycle 3, the previously
highly crosslinked and loosely oligomerised, which was a means asserted 254 nm UV dose of 1.05 J cmꢁ2 was not enough to
to reversibly control the free volume and the polydispersity completely revert the permeance back to the higher value
(Fig. S12 of the ESI†); both of which are crucial factors to control expected from previous cycles. However, further irradiation
a membrane’s permeability. Secondly was the choice of a material allowed the film to reach the expected permeance value. This
that exhibited an acute change in its Tg when crosslinked and de- may have been due to an accumulation of partially crosslinked
crosslinked, which was accomplished by the monomer’s siloxane material with every cycle, which eventually created some resis-
core and star shape. The changes in Tg were necessary to control tance to oxygen flow through the film. Once the accumulated
the adsorption of penetrant molecules within the polymer film’s oligomers were further de-polymerised in cycle 3 with a larger
matrix. By doing so, the reversible cycloaddition polymerisation irradiation dose, the film was able to undergo permeance
reaction created a rigid, highly crosslinked polymer with a low O2 reversibility with the expected irradiation dose of 1.05 J cmꢁ2
,
permeance of 26.5 (365 nm).
as seen in cycle 4.
The thickness of the 4ASil film used in this study was found
to be B375 nm as indicated by cross-sectional SEM imaging in
Fig. 2b (right), with a typical topological photo presented in
Fig. 2a (left). In addition, Fig. S10 of the ESI,† depicts SEM
images of 4ASil after photocrosslinking (polymerization) and
de-crosslinking (de-polymerisation). Observed differences were
miniscule, which may be because crosslinking was only partial
(0.04 J cmꢁ2 irradiation versus 11.60 J cmꢁ2 needed for full
crosslinking, as seen in Fig. 3a). Hence, there was not much of a
bulk material migration since the underlying layer was mainly
still a monomer.
Fig. 3a and b show the UV-Vis spectra of 4ASil after cross-
linking and de-crosslinking respectively, which along with the
FTIR spectra presented in Fig. S4b of the ESI,† prove that the
photoreversible [2+2] reaction of coumarin took place. The peak
at 325 nm corresponded to the double bond at carbons 2 and 3
of the coumarin, where a maximum absorbance was exhibited
as a monomer (red line). Under 365 nm irradiation (Fig. 3a), the
325 nm peak was reduced as crosslinking proceeded via the
formation of the cyclobutane ring.29 Conversely, the peak
increased when the reverse reaction proceeded via 254 nm irra-
diation cleavage of the cyclobutane ring (Fig. 3b). The poly-
merisation conversion was 89%, while the de-polymerisation
conversion was 75%.
Fig. 3c and d display the DSC analysis of 4ASil after cross-
linking and de-crosslinking respectively. At a high crosslinked
polymer Tg of 133.4 1C, it was expected that the polymer would
show high barrier characteristics at room temperature, which
coupled with the high crosslinking density of the star-shaped
molecules, explained the permeance results. Moreover, the
Fig. 2 (a) Photoreversible changes in oxygen permeance for 4ASil by
irradiation with 254 nm (black) and 365 nm (red) for multiple cycles. p:
polymerisation under 365 nm irradiation, dp: de-polymerisation under
254 nm irradiation. All irradiation values are in units of [J cmꢁ2]. In terms
of time, the irradiation doses translate to a duration of 3 minutes of 365 nm
irradiation for crosslinking, and a duration of 10 minutes of 254 nm
irradiation for de-crosslinking. (b) A topological SEM image of 4ASil, with
a 2 mm scale bar (left), and a cross-sectional image of the composite
membrane, with a 5 mm scale bar.
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to reversibly switch a film’s oxygen permeance, such as in
packaging and in gas transport vehicles.
The authors acknowledge funding by the Meat and Livestock
Australia (MLA) under Monash Food and Dairy GRIP. The
authors would also like to acknowledge the Monash Centre
for Electron Microscopy (MCEM), and the Monash Analytical
Platform (MAP) for access to the SEM and MS instruments.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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3394 | Chem. Commun., 2021, 57, 3391–3394