Preparation of photodegradable polyacrylamide hydrogels: Via micellar copolymerization and determination of their phototunable elasticity and swelling behaviors

Article abstract of DOI:10.1039/c6ra00556j

Polyacrylamide (PAAm) hydrogels were prepared by micellar copolymerization methodology in the presence of acrylamide and photo-decomposable hydrophobic crosslinker (diacrylated Irgacure-2959). After the synthesis of hydrophobically functionalized hydrogel

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Preparation of photodegradable polyacrylamide hydrogels via
micellar copolymerization and determination of their phototunable
elasticity and swelling behaviors
Fatma Selen1, Volkan Can2, 3, *, Gokhan Temel1, *
1 Yalova University, Polymer Engineering Department, Yalova, 77100, Turkey
2 Helmholtz Institute, LisaꢀMeitner Campus, HahnꢀMeitner Platz 1, Berlin, 14109,
Germany
3 Istanbul Technical University, Department of Chemistry, Maslak, Istanbul,
34469, Turkey
Abstract:
Polyacrylamide (PAAm) hydrogels were prepared by micellar copolymerization
methodology in the presence of acrylamide and photoꢀdecomposable hydrophobic
crosslinker (Diacrylated Irgacureꢀ2959). After the synthesis of hydrophobically
functionalized hydrogels via free radical copolymerization, gel samples are exposed
to UV irradiation for different time intervals between 0ꢀ17 hours in aqueous medium.
According to the results of swelling and compression tests, physical properties of
hydrogels were altered by UV exposure time. Swelling degrees of hydrogels
increased gradually with the irradiation time. Moreover, 17 h irradiated hydrogel
sample was entirely decomposed at the end of the swelling test. According to
Scanning Electron Microscopy images, UV irradiated hydrogel samples exhibit an
apparent degradation behavior.
Keywords:Photodecomposition; Hydrogel; Micellar copolymerization; Photoinitiator
Corresponding authors:
Tel: + 90 226 815 54 14 (E-mail: gtemel@yalova.edu.tr)
Tel: + 49 30 8062 43198 (E-mail: volkan.can@helmholtz-berlin.de)
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1. Introduction
Photoinduced polymerization is one of the most preferable and facile techniques to
reduce energy consumption and improve green chemistry applications.1, 2 Besides,
radical photopolymerization technique has been used in several commercial areas
such as curing of coatings, fabrication of smart materials, nanocomposites, etc.3, 4
Even though various polymeric materials have been produced by UV polymerization
techniques, new kind of macromolecular structures such as molecular weight
controlled polymers5, 6, hydrogels7ꢀ9, macromolecular photoinitiators10ꢀ12, star and
block copolymers13, 14 can be synthesized by various photoinduced polymerization
strategies. Norrish Type I (αꢀcleavage) photoinitiators such as irgacure, acetophenone
and acylphosphine oxide derivatives are known to be able to directly initiate the free
radical photopolymerization under UV light. Hence they have found wide
applications in UV curing and coating industry2, 15ꢀ17
.
Hydrophilic three dimensional crosslinked structures are able to absorb high
degree of water and these materials show resemblance with natural tissues due to their
soft and flexible structure. Since they have a high degree of water uptake
characteristic, biocompatibility and nonꢀtoxic properties, these water swellable
structures have found wide spread application areas in agriculture, drugꢀrelease
researches, wound treatment etc18ꢀ21. Especially photoꢀresponsive or lightꢀtriggered
hydrogels are promising materials and are used for cell migration, lightꢀstimuli drug
delivery, tissue engineering22ꢀ24. On the other hand, degradable hydrogels have great
potential in controlled release and tissueꢀengineering applications. Besides,
photodegradation is one of the most popular method to gain control over the
degradability of the polymeric materials and for this purpose, photodegradable groups
are inserted into the macromolecular structure 25, 26. There are several approaches on
this matter that utilize different polymerization methods, such as reversible addition
fragmentation chain transfer (RAFT), stepꢀgrowth/chainꢀgrowth polymerizations and
click chemistry techniques25ꢀ28. However, there is constant need of new strategies to
obtain photodegradable hydrogels for practical applications.
In this report, micellar copolymerization, which is a powerful method to
obtain multiꢀblock copolymers of both hydrophilic and hydrophobic monomers, was
used to achieve hydrophobically modified hydrogel structure (Scheme 1) 29ꢀ36. We
2

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introduce a new route for synthesis of a photodegradable hydrogel via micellar
copolymerization of a diacrylate functionalized photoinitiator and acrylamide in the
presence of redox initiator in the ambient temperature. A commercially available
photoinitiator, (Irgacureꢀ2959), was converted to a photocleavable crosslinker with a
simple esterification and this hydrophobic compound was successfully incorporated
into the hydrophilic polyacrylamide chains via micellar copolymerization
methodology. Resulting hydrophobically modified hydrogel was undergone photoꢀ
degradation in various UV irradiation doses and degradation behavior is followed in
the means of swelling and elasticity.
Scheme 1. General mechanism of micellar copolymerization
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2. Experimental
2.1. Materials
Irgacureꢀ2959 (98% Ciba), acryloyl chloride (97% contains <210 ppm MEHQ as
stabilizer, Aldrich), dichloromethane (DCM, ≥99.8%, Aldrich), triethylamine (99%,
Merck), acrylamide (AAm, Merck), sodium dodecylsulfate (SDS, Sigma), ammonium
persulfate (APS, Fluka), and
Fluka) were used as received.
N, N, N’, N’ꢀtetramethylethylenediamine (TEMED,
2.2. Synthesis of diacrylated Irgacure-2959 (DAI)
Acryloyl chloride (2.16 mL, 26.79 mmol) in DCM (3 mL) was added dropwise at 0
oC to a solution of Irgacureꢀ2959 (2.0 g, 8.93 mmol) and triethylamine (2.51 mL, 19
mmol) in DCM (15 mL). After the addition of acryloyl chloride solution, the resulting
mixture was stirred at room temperature overnight. Then the mixture was washed
several times with 5 % NaOH aqueous solution (200 mL), three times with distilled
water (100 mL) and a saturated aqueous solution of NaCl (100 mL). The organic
layers were dried over anhydrous Na2SO4, filtered, and then the solvent was
evaporated. Crude product was purified by column chromotography (EtAc/Hex: 1/4)
1
Yield: 40 %. H NMR (500 MHz, CDCl3), 7.96ꢀ 6.79 ppm (m, 4H, Ar H), 6.37–5.72
ppm (m, 6H, CH=CH2), 4.44ꢀ4.42 ppm (t, 2H, O=COꢀCH2), 4.17ꢀ4.15 ppm (t, 2H,
PhꢀOꢀCH2), 1.65 ppm (s, 6H, Cꢀ(CH3)2). FTꢀIR (ATR): ν= 3083, 3039, 2947, 1723,
1710, 1676, 1602, 1509, 1131 cm–1. GCꢀMS (EI, 70 eV): 219, 99, 76, 55.
O
O
O
Cl
O
O
O
OH
O
HO
Et3N/ CH2Cl2/ 0 O
C
O
O
Irgacure-2959
DAI
Scheme 2. Synthesis of diacrylated Irgacureꢀ2959 (DAI)
2.3. Gel preparation
The hydrogels were prepared by free radical micellar crosslinking copolymerization
of AAm and DAI in aqueous SDS solutions at 30 OC. The concentration of SDS in the
reaction solution was fixed at 7 w/v %, which is much above its critical micelle
concentration (0.24 w/v %) at the same temperature. The initial concentration of the
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total monomer (AAm+ DAI) was set to 5 w/v% throughout the experiments while the
DAI content in the monomer mixture was 2.5 mol % and APS (3.5 mM). Stock
solutions of APS and TEMED were prepared by dissolving 0.16 g of APS and 0.50
mL of TEMED each in 10 mL of distilled water.
General procedure of photodegradable hydrogel preparation: DAI (0.0562 g) was
added to the solution of SDS (0.7 g) in 7.0 mL of water and stirred to obtain a
transparent solution. After addition of AAm (0.444 g) and stock solutions of TEMED
(0.5 mL), solution was stirred until homogenous mixture obtained. Then, right after
the stock solution of APS (0.5 mL) was added to initiate the reaction, the volume is
rapidly completed to 10 ml in a flask. For the swelling and mechanical measurements,
the solution was transferred into several plastic syringes of 4 and 10 mm internal
diameters and the polymerization was conducted for 20 h at 30 OC.
2.4. General procedure of UV irradiation of hydrogels
Due to the strong adhesion properties of hydrogels, hydrophobically modified
hydrogels were synthesized in plastic syringes. At the end of the reaction period,
samples were extracted on to a moisturized and flat surface and cut into appropriate
cylindrical pieces. Cut samples transferred into Pyrex tubes and 2 mL of distilled
water was added into tubes to prevent hydrogels dry during the UV exposure period.
2.5. Tensile mechanical tests
Uniaxial compression measurements were performed on cylindrical hydrogel samples
just after preparation and in equilibrium swollen states by using a Zwick Roell, 10 N
test machine at 25 OC under the following conditions: The hydrogel samples
subjected to the mechanical tests right after the synthesis were about 4.1 ± 0.3 mm of
diameter and 3 ± 0.2 mm of height. Dimensions of the network samples in
equilibrium swollen states was varied between 10 – 16 mm of diameter with the
similar diameter/height ratios as the samples at right after synthesis state. Before the
test, an initial compressive force of 0.001 N was applied to ensure a complete contact
between the gel and the plates. The tests were conducted at a constant speed of 0.3
mm/min. Load and displacement data were collected during the experiment. The
Young’s modulus, E, was calculated from the slope of stress−strain curves between 5
and 15% compressions. For reproducibility, at least five samples were measured for
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each gel and the results were averaged.
3. Results and Discussion
3.1. Synthesis and characterizations
Micellar copolymerization is highly efficient and facile technique to prepare
hydrophobically modified polymeric structures in aqueous media, and resulting
polymer exhibits multiꢀblock copolymeric structure possessing both hydrophobic and
hydrophilic moieties (Scheme 1) 29ꢀ36. In this contribution, micellar copolymerization
approach was utilized for the first time to prepare hydrogels which contain photoꢀ
decomposable crosslinker units. For this purpose, diacrylated Irgacureꢀ2959 (DAI)
was synthesized by simple esterification method under mild conditions (Scheme 2).
Mono and diacrylated Irgacureꢀ2959 mixture was formed after acrylation reaction and
desired difunctional crosslinker (DAI) was obtained after purification procedure.
According to GCꢀMS measurement, resulting photoꢀdecomposable crosslinker is in
1
high purity and MS peaks confirm the exact structure of obtained product. H NMR
and FTꢀIR results also confirm that diacrylation of Irgacureꢀ2959 was carried out
successfully (See supplementary information). Thereby obtained photoactive
hydrophobic compound (DAI) was used to achieve photoꢀcleavable polyacrylamide
based hydrogel.
3.2. Determination of gel fractions before and after UV irradiation
Polyacrylamide based photodegradable hydrogels were prepared as explained in
experimental section and the resulting hydrogel samples were exposed to varying
doses of UV light (2, 5, 7, 12, 17 h) in order to understand the effect of the
photodecomposable crosslinker on the hydrogel properties (Scheme 3).
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Scheme 3. Photodecomposition of DAI segments of hydrogel under UV irradiation.
Gel fractions were investigated by gravimetric method. Fig. 1 denotes the gel
fractions of hydrogel samples that are exposed to UV irradiation at different time
intervals. As it can be seen from the figure, nonꢀirradiated hydrogel has 91% gel
fraction where, the gel fraction of UV irradiated samples decreases by increase of
irradiation time. In addition, 12 h irradiated sample degraded approximately by 50%
that causes the network to dissolve after this point. Hence, 17 hours irradiated sample
was completely dissolved during swelling experiments and gel fraction of
corresponding hydrogel could not be determined. These results can be directly
attributed to the altered microꢀstructure of hydrogels due to the UV exposure.
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100
90
80
70
60
50
0
2
4
6
8
10
12
14
UV Radiation Time / h
Fig 1. Gel fractions versus different irradiation time intervals of photoꢀdecomposable
hydrogels.
3.3. Swelling and mechanical behaviors of hydrogels
Fig. 2 depicts the variation of swelling ratio with swelling time of photoꢀ
decomposable hydrogels. Samples show a swellingꢀdeswelling pattern due to the
ionic contribution of SDS moiety. Initially, gels contain an approximate concentration
of 7% SDS, which causes the gel act as an ionic network. However, as the swelling
process continues, SDS molecules are washed out to decrease the ionic content of the
gel. Therefore, gels swell up to a maximum value then start to deswell and reach
lower equilibria. While nonꢀirradiated sample has the lowest swelling ratio, water
absorption capacities of UV irradiated samples are found to increase with the UV
exposure time due to the decomposition of photodegradable crosslink units. On the
other hand, maximum irradiated sample, 17 h, start falling apart into pieces during the
swelling process and no deswelling could be observed.
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120
100
80
60
40
20
0
0-UV
2-UV
5-UV
7-UV
12-UV
17-UV
0
1
2
3
4
5
6
7
Time / Days
Fig 2. Swelling ratio m/mo of photoꢀdecomposable hydrogels with different time of
UV irradiation shown as a function of the swelling time.
As the variation of the elastic moduli investigated, the nonꢀirradiated sample is found
to exhibit the highest elastic modulus value, since it does not undergo any
photodecomposition process (Fig. 3). While the same sample reached equilibrium
swollen state, elastic modulus value dramatically decreased as expected. Furthermore,
elastic moduli continue to decrease as UV exposure time of hydrogel samples were
increased. In addition, modulus of 17 hours irradiated hydrogel could not be
investigated since sample was decomposed before end of the swelling experiment.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
12
14
UV - Exposure Time / h
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Fig 3. Elastic moduli of each sample versus different irradiation time intervals at
equilibrium swollen state. Open and filled symbols represent the moduli of the gels at
after preparation and equilibrium swollen states respectively.
3.4. Macroscopic and morphologic studies of hydrogels
The morphologies of nonꢀirradiated and irradiated hydrogels were investigated using
scanning electron microscopy (SEM). In Fig. 4, nonꢀirradiated hydrogel sample has
porous structure due to the lyophilization process and it shows thick walled room
structure (Fig. 4a). However, UV exposed hydrogel samples demonstrate thinner
interface and irregular porous structures. Another point seen from SEM images is the
decrease of wall thickness with increasing UVꢀexposure time (Fig. 4b & 4c). 12 hours
UV exposed hydrogel exhibits much more decomposed material and separation of
polymeric layers can clearly be observed in Fig. 4c.
a)
b)
c)
Fig 4. Scanning electron microscopy images of a) nonꢀirradiated, b) 7h irradiated and
c) 12 h irradiated hydrogel samples.
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Fig. 5 shows the hydrogel samples at swollen equilibrium states that were at the same
dimensions initially and exposed to UV irradiation at different time intervals. It is
obvious that swelling degree of the hydrogels increase significantly with the UV
exposure time. 12 h exposed hydrogels are quite weak and 17 h irradiated hydrogels
decompose almost completely at the end of swelling process. Figure exhibits a good
representation of the macroscopic behavior of the photodegradable crosslinked
hydrogels.
Fig. 5. Equilibrium swollen state photograph of photodegradable hydrogel samples
before and after irradiation.
4. Conclusion
Photoꢀdecomposable crosslinker was readily synthesized by endꢀfunctionalization of
commercially available Irgcureꢀ2959 photoinitiator with acrylate units. Afterwards,
the resultant photodegradable crosslinker is succesfully polymerized with acrylamide
monomer via free radical micellar copolymerization in order to achieve hydrogels
with phototunable properties. The study showed that physical and swelling properties
of obtained hydrogels were strongly depended on the dose of UV light exposure.
Furthermore, the samples that are exposed to UV more than 12 hours are found to be
almost completely decomposed. We believe that these kinds of hydrogels may find
many potential applications in various areas such as photoꢀtriggered materials
preparation, drug delivery systems and degradable thermosets.
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Acknowledgements
This study was supported by Yalova University Scientific Research Foundation
(2013/YL/032).
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Graphical Abstract
Fatma Selen, Volkan Can, Gokhan Temel
Photo-decomposable hydrophobic crosslinker was synthesized and utilized to obtain photo-
tunable hydrogels via free radical micellar copolymerization.