This is a repository copy of Highly Stretchable and Highly Resilient Polymer-Clay
Nanocomposite Hydrogels with Low Hysteresis.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/118875/
Version: Accepted Version
Article:
Su, X., Mahalingam, S., Edirisinghe, M. et al. (1 more author) (2017) Highly Stretchable
and Highly Resilient Polymer-Clay Nanocomposite Hydrogels with Low Hysteresis. ACS
Applied Materials and Interfaces , 9 (27). pp. 22223-22224. ISSN 944-8244
https://doi.org/10.1021/acsami.7b05261
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1
Highly Stretchable and Highly Resilient
Polymer-Clay NanocompositeHydrogelswith
LowHysteresis
Xing Su,
1
Suntharavathanan Mahalingam,
2
Mohan Edirisinghe,
2
Biqiong Chen
1,
*
*Corresponding author. Email: biqiong.chen@sheffield.ac.uk.
1
Department of Materials Science and Engineering, University of Sheffield, Mappin Street,
Sheffield S1 3JD, United Kingdom.
2
Department of Mechanical Engineering, University College London, Torrington Place, London
WC1E 7JE, United Kingdom.
ABSTRACT: Highly stretchable and highly resilient polymer-clay nanocomposite hydrogels
were synthesized by in situ polymerization of acrylamide in the presence of pristine
montmorillonite (MMT) or chitosan-treated MMT nanoplatelets at an elevated temperature. Both
nanocomposite hydrogels can be stretched to a strain of no less than 1290%. The treatment of
clay with chitosan improves the tensile strength, elongation at break and energy at break of the
nanocomposite hydrogel by 237%, 102% and 389%, respectively, due to the strong chitosan-
MMT electrostatic interaction and the grafting of polyacrylamide onto chitosan chains. Both
hydrogels display excellent resilience with low hysteresis; with a maximum tensile strain of 50%
ultralow hysteresis is found, while with a maximum strain of 500% both hydrogels fully recover
their original state in just 1 minute. The superb resilience of the nanocomposite hydrogels is
attributed to the strong interactions within the hydrogels brought by chain branching, multiple
2
hydrogen bonding, covalent bonding and/or electrostatic force. The hydrogels can be fabricated
into different shapes and forms, including microfibers spun using pressurized gyration, which
may find a variety of potential applications in particular in healthcare.
KEYWORDS: nanocomposite hydrogel, montmorillonite, structure, mechanical properties,
elasticity, hysteresis
INTRODUCTION
Hydrogels are a class of polymeric, hydrophilic materials which are able to absorb and keep
water in their crosslinked network.
1
They may be fabricated into different forms including
microgels,
2-4
and used as agricultural products,
5
food ingredients
6
and biomaterials such as tissue
scaffolds,
7
drug delivery systems,
8
wound dressings
9
and biosensors.
10
However, conventional
chemically crosslinked hydrogels are often weak and/or brittle, which limits hydrogels from
wider applications.
11
To overcome this shortcoming, polymer-clay nanocomposite hydrogels have attracted much
attention in the last decade for synthesizing tough hydrogels because highly functionalized clay
can form physical, reversible and dense crosslinks which are effective on dissipating energy
2
and
stabilizing the network.
11
Haraguchi et al.
11,12
developed a series of poly(N-isopropyl
acrylamide)-synthetic hectorite nanocomposite hydrogels. Unlike conventional chemically
crosslinked hydrogels, these nanocomposite hydrogels could reach a tensile strength of 109 kPa
and a strain at break over 800%. The ionic interaction between initiator molecules and clay
surface was essential to hydrogel formation, which enables multiple polymer macromolecules to
grow on clay surface.
13
Polyacrylamide (PAM) was frequently used to synthesize nanocomposite
3
hydrogels because of its superb hydrophilicity, flexibility, simplicity in synthesis,
14
and
biocompatibility.
15
Montmorillonite (MMT), with the ideal chemical formula of
Al
2
Si
4
O
10
(OH)
2
.yH
2
O,
16
was often used as a complementary physical crosslinker of chemically
crosslinked PAM hydrogels.
17-20
MMT is also biocompatible and can be readily expelled from
the body after metabolism and excretion.
21
Until 2015, ultra-stretchable, self-healable and tough
PAM-MMT physically crosslinked hydrogels were synthesized, which presented an extremely
high strain at break of 12000%.
22
After stretched to a strain of 2000%, a residual strain of 500%
and a high hysteresis were observed after 1 minute, and a full shape recovery required 5 days’
storage at 25
o
C.
22
It is considered challenging to achieve low hysteresis with physically
crosslinked PAM-clay hydrogels because of temporary or even permanent breaking of some
organic-inorganic crosslinks.
23-24
High hysteresis often means reduced mechanical properties of hydrogels after being cyclically
loaded, so attempts were made to overcome hysteresis of hydrogels including well designed
hydrogels composed of copolymerized tetra-armed monomers
23-24
and physically crosslinked
PAM-synthetic hectorite nanocomposite hydrogels.
25
In the latter (with a starting weight ratio of
1:1 for the monomer:clay), a hysteresis of 19% in the first cyclic testing to a 1000% strain and a
reduced hysteresis of 9% in later cycles were found.
25
The high crosslinking density and
hydrophilic, flexible nature of PAM gave rise to a small internal friction force and thus low
hysteresis.
25
These promising results indicate that it may be possible to achieve low hysteresis
with PAM-MMT nanocomposite hydrogels.
Chitosan (CHI) is a biopolymer produced from chitin, which is rich in the crustacean and
mollusk shells.
26
Due to its excellent biocompatibility and biodegradability, chitosan is widely
used in healthcare applications.
27-29
It is positively charged in an acidic environment, which has
4
potential of strongly bonding to the negatively charged MMT surface
30-31
by electrostatic
interactions.
32-33
This work aimed to develop polymer-clay nanocomposite hydrogels with high extensibility,
excellent resilience and low hysteresis. Unlike previous work on PAM-clay
nanocomposites,
11,22,25
in situ polymerization of acrylamide (AM) in the presence of MMT or
chitosan-treated MMT (CHI-MMT) was conducted at a higher temperature of 60
o
C without a
catalyst, instead of room temperature with a catalyst, because it is favored for chain grafting and
branching at this temperature.
34
During graft polymerization, the radical initiator was expected to
attack the hydroxyl groups of the polysaccharide chains to generate alkoxy radicals, which would
also initiate the polymerization of AM.
35
A chitosan grafted PAM-MMT hydrogel was recently
synthesized by graft polymerization of AM onto chitosan at 60
o
C in the presence of MMT
platelets and a chemical crosslinker N,N'-methylenebisacrylamide.
19
However, the mechanical
properties of the hydrogels were not reported. In the present study, no chemical crosslinker was
used, and the mechanical properties of the nanocomposite hydrogels such as Young’s modulus,
tensile strength, strain at break, hysteresis ratio and rheological properties were investigated by
tensile, cyclic tensile and rheological tests. Their results were interpreted in depth with the
employment of the findings from X-ray diffraction (XRD), Fourier transform infared
spectroscopy (FTIR), dynamic scanning calorimetry (DSC), scanning electron microscopy
(SEM) and in vitro degradation tests. The mechanical properties of the fully swollen
nanocomposite hydrogels were also investigated, which are often overlooked in the literature. To
demonstrate the manufacturing versability of these hydrogels, various shapes of hydrogels were
prepared, together with hydrogel microfibers.