BMe Research Grant
BMe Research Grant - 2017
Nanotechnology developments have rapidly increased the interest in research on responsive polymer composites that contain carbon nanoparticles such as nanotubes or graphene and its derivatives. More and more data is available on the nanocomposites but the structural and dynamical properties of these materials are still not sufficiently understood. The potential applications are numerous (e.g. sensors, actuators, microfluidics, drug delivery vehicles, etc.), and this motivates further research [1-4].
During my PhD project I prepared composite materials of one of the most widely investigated temperature-responsive polymers, the poly(N-isopropylacrylamide) (PNIPA) (Fig. 1.) and studied their properties both on macroscopic and microscopic length scales.
Fig. 1: Poly(N-isopropylacrylamide - PNIPA
The Surface Chemistry Group focuses ‒ amongst many other issues ‒ on the preparation of solid and non-conventional adsorbents, the study of surface phenomena and application of different materials with tuned porosity and surface chemistry, in extensive collaborations in Hungary and abroad.
The Institut Laue-Langevin (ILL) is an international research centre at the leading edge of neutron science and technology. As the world’s flagship centre for neutron science, the ILL provides scientists with a very high flux of neutrons feeding some 40 state-of-the-art instruments.
Responsive hydrogels are three-dimensional, cross-linked polymers that can absorb large amount of water and are able to react to changes in their environment. Despite their numerous advantageous properties, they have certain drawbacks as well. Their poor mechanical strength for example, limits their use in load-bearing applications. Limited drug uptake and its non-uniform distribution within the system, particularly in the case of hydrophobic drugs, could also compromise certain applications. These challenges may conceivably be overcome by composite hydrogels. Carbon nanoparticles (CNP) (Fig. 2) are widely used for strengthening in polymer nanocomposites. Recently they also got into the focus of interest as components of responsive polymer hydrogel nanocomposites. In these systems CNPs may act not only as reinforcing agents, but due to their special structure, IR sensitivity and tuneable conductivity, they can also provide new or modified sensitivity to these complex systems.
Fig. 2: Graphene sheet assemblies (a) Multi-wall carbon nanotube (MWCNT); (b) graphene; (c) graphene nanoplatelets (d) graphite
Carbon nanotubes (CNT) got in the focus of attention after Iijima’s famous papers were published in Nature [5, 6], while graphene family raised interest in 2010, when the Nobel Prize in Physics was awarded jointly to Andre Geim and Konstantin Novoselov "for ground-breaking experiments regarding the two-dimensional material graphene" .
My PhD project aims at developing and characterising novel responsive, nanocomposite gels for various potential applications as a continuation of a work performed previously in the Surface Chemistry Group. The project is based on the proven synergy effect between CNPs and responsive polymer hydrogels. These nanocarbons themselves possess certain specific properties, therefore it is expected that the composite materials will exhibit not only improved mechanical properties, but enhanced or modified sensibility to different triggers. The morphological, chemical and mechanical characterisation of the synthesised nanocomposites involves both state-of-the-art and cutting edge methods.
PNIPA based composite systems were investigated with a variety of cross-link densities and/or with different co-monomers, as well as diverse conditions of synthesis, so it is not possible to evaluate the effects of the different nanoparticles. In addition, although the application of graphene oxide (GO) is very beneficial during the preparation in aqueous media (due to its high oxygen content), the incorporation of less hydrophilic CNPs is challenging. It is necessary to develop a new method which allows us to extend the limits of CNP incorporation.
The shape, size, surface chemistry, concentration and the preparation method, can all influence the final character of the resulting material. During my PhD I aimed at identifying the most important parameters with significant effect on the structure and responsiveness of composites and exploring the relations between them.
Structural and dynamical properties at a microscopic level are also of crucial importance for sensory, actuators related and controlled release applications. Such investigations could, however, prove difficult owing to the limited number of appropriate methods.
During my PhD, I prepared different CNP-PNIPA nanocomposite hydrogel systems. I used conventional and cutting edge methods to investigate the structure, response-kinetics and dynamics of the systems at micro- and macroscale level.
The mechanical properties of the gels were characterized by their compressive strength and by Young's moduli measured by uniaxial compression. Their temperature-sensitivity was determined by equilibrium swelling degree measurements at various temperatures and by differential scanning microcalorimetry (DSC).
The time-dependent behaviour during the volume phase transition was investigated by the temperature-jump induced shrinkage kinetics. During this measurement, gel disks were equilibrated at 20 °C, then plunged into warm water (40 or 50 °C). The mass was measured as a function of time.
The interactions between the CNPs and the polymer matrix was studied by solid state NMR spectroscopy and thermal analysis in order to determine the presence of first or second order bonds.
To describe their structure on the nanoscale level, small-angle neutron scattering (SANS) (Fig. 3) measurements were used. SANS methods are widely used, complementary techniques. Via SANS the measured signal comes from the polymer matrix only, the nanoparticles are practically invisible, due to their contrast.
Fig. 3: Small-angle neutron scattering instrument setup
The dynamics below the volume phase transition temperature (VPTT, ~ 34 °C) were examined by neutron spin-echo spectroscopy (NSE). NSE is a remarkable technique, pioneered at the ILL by a Hungarian physicist, Ferenc Mezei, which offers a highly sensitive method for following very slow dynamic processes in soft matter. It can also be used for non-transparent systems, with the highest energy resolution among all types of neutron spectrometers. It has proved to be an excellent tool for investigating dynamics in polymer gels.
PNIPA gel composites of various CNT, GO and reduced graphene oxide (RGO) contents were prepared under identical conditions by incorporating the CNPs into the polymer matrix and for RGO containing systems via applying a post-synthesis reduction of the GO already incorporated into the polymer.
The quality and quantity of the incorporated CNP influenced the swelling and the mechanical properties of the composites.
Direct incorporation of RGO is strongly limited by its reduced hydrophilicity. We found that the aggregation tendency of CNT and RGO reduced prior to the polymerisation resulted in only small structural modification in the nanocomposites. The swelling and mechanical properties remained almost unchanged compared to the pure PNIPA gel. The elastic modulus of PNIPA gel is enhanced by GO, while the swelling degree of the GO@PNIPA systems decreases significantly. These properties showed strong dependence on the concentration of GO and were conserved in the post-synthesis reduction.
The VPTT was not influenced significantly by the CNP incorporation, but significant differences were observed in the time-dependence of the thermal response of the systems (Fig. 4). GO loading slows down the macroscopic shrinkage of the gel and the time scale of the response is extended. CNT-content and direct incorporation of RGO into the polymer matrix hardly affects the timescale of the macroscopic response at the beginning of the process, but results in a slower overall shrinkage.
Fig. 4: Temperature shock (20 -> 50 °C) induced shrinkage kinetics of pure PNIPAM gel (g), and (a) CNT@PNIPAM (blue datasets) and (b) GO@PNIPAM (red datasets) nanocomposite samples
Thermogravimetric and NMR observations revealed strong interactions between GO and the polymer matrix, which was not influenced by the post-reduction treatment. Our results, however, show no clear evidence of covalent bonds being present. At the same time, RGO and CNT interact only weakly when incorporated directly into the gel.
The polymer-polymer correlation length (ξ), which characterises the distance between the cross-links determined by SANS experiments, decreases upon CNP incorporation due to the hypernodal structure developing around the CNPs (Fig. 5).
Fig. 5: Suggested internal structure of GO-containing composites
NSE measurements enable characterisation of the dynamical properties of these systems. In the pure PNIPA gel the experimental intermediate scattering function curves measured by NSE decay to zero (Fig. 6), indicative of practically ergodic behaviour, i.e. there is no frozen-in component, what we can observe is a simple diffusion movement.
Fig. 6: Experimental intermediate scattering function curves of pure PNIPA gel at 25 °C measured by NSE
In spite of the differences visible at the macroscopic scale, NSE results at the microscopic level, by contrast, indicate that the motion of the polymer chains is only partially affected by the nanoparticle incorporation below the VPTT. However, more pronounced differences can be seen above it.
Thermoresponsive hydrogels have enormous potential e.g. as sensors, actuators, and pollution control remedies or in drug delivery systems. Nevertheless, their application is often restricted by physical limitations (poor mechanical strength and uncontrolled thermal response).
Our results show that the incorporated carbon nanoparticles can improve the properties of the PNIPA gel. Both their chemistry and their concentration are promising means for tuning the thermal responsivity broadening the application fields of such nanocomposite systems for specific applications.
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