Volume phase transition of “smart” microgels in bulk solution and adsorbed at an interface: A combined AFM, dynamic light, and small angle neutron scattering study
Introduction
Since the first report on the preparation of poly(N-isopropylacrylamide) microgels [1], numerous studies discussing different aspects of these interesting functional polymer materials have been published. The most interesting feature in the behavior of N-isopropylacrylamide (NIPAM) based systems certainly is the volume phase transition [2], [3]. This phenomenon was already investigated in some detail in macroscopic gels [4], [5], [6] and also in different microgels [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [44]. A general overview on microgels can be found in Refs. [17], [18], or in Ref. [19] with a focus on scattering methods. In macroscopic gels, the volume phase transition can be very slow and in some cases it takes several days until the equilibrium state is reached [20]. Microgels react faster upon changes in temperature, ionic strength, solvent quality, or pH. Therefore, they are well suited to investigate the volume transition. Moreover, due to their colloidal character [21] they are interesting model systems to study processes like formation of mesoscopic crystals [22], [23], [24], [25], [26]. Maybe their most visionary potential lies in the possible construction of actuators on the nanometer scale, which are driven by physico-chemical processes like swelling.
In the present study, we describe the properties of poly(N-isopropylacrylamide)-co-vinylacetic acid (PNIPAM-co-VA) microgels with different contents of vinylacetic acid. The local structure of the polymer network is studied by means of small angle neutron scattering (SANS). The swelling behavior is investigated by dynamic light scattering (DLS) and moreover by tapping mode atomic force microscopy (AFM). DLS allows to obtain the average degree of swelling in bulk solution, whereas AFM is used to investigate the transition of individual microgel particles attached to an interface. AFM is an excellent tool for this approach.
For applications of such attached particles as sensors or for actuators it is of course necessary to conserve reversibility of the transition. Linear PNIPAM-copolymer chains adsorbed at an interface and in the thin polyelectrolyte multilayers were found to collapse irreversibly [27]. The present study was motivated by the idea that microgels due to their shape stability might conserve reversibility of the transition also in the adsorbed state.
The volume transition of PNIPAM microgels as observed in bulk solution is usually found to be continuous. However, in solution usually the ensemble averaged size of the microgels is followed and therefore it is not clear whether the continuous character of the volume phase transition arises from the polydispersity of the particles (only apparently continuous) or is a property of a single PNIPAM microgel. The present work also aims to answer this question.
Section snippets
Microgel synthesis and characterization
N-isopropylacrylamide (NIPAM; synthesis grade, purity 97%), N,N′-methylene bis-acrylamide (BIS; synthesis grade, purity 99%), vinylacetic acid (VA; synthesis grade), and potassium persulfate (KPS; purity 99%) were obtained from Sigma–Aldrich. In contrast to macroscopic gels it was shown previously that for microgels re-crystallization of the chemicals does not lead to significantly different particle properties [28] and hence, all chemicals were used without further purification. The microgel
DLS
The swelling behavior of the four different microgels was investigated by means of DLS. The microgels are rather large. Therefore, the angular dependence of the decay of the correlation functions was studied first. According to Γ = DTq2 a plot of Γ vs. q2 should be linear. In Supplementary data the data for the sample VA1 are given.
The linear fit does not go through zero. This can be attributed to growing intra-particle interference at larger scattering angles. However, the effect is small and of
Conclusions
The combination of DLS, SANS, and AFM gives a detailed view at various aspects of the swelling behavior of PNIPAM-co-vinylacetic acid microgel particles.
All experiments indicate that the volume phase transition temperature is not influenced by the VA content of the particles and is located between 32 °C and 34 °C.
The AFM experiments clearly show that the volume phase transition of the PNIPAM-co-VA microgels is still reversible for adsorbed microgels, but the swelling capacity (given as α)
Acknowledgments
We are grateful to Wim Pyckhout-Hintzen for help with the SANS experiments and to the FZ Jülich for providing the beamtime. L.Z. and F.M. acknowledge funding from the Sonderforschungsbereich 569 ‘Hierarchic Structure Formation and Function of Organic–Inorganic Nano Systems’.
T.H. acknowledges funding by the EUROCORES program within the project Higher levels of self-assembly of ionic amphiphilic block-copolymers (SONS-AMPHI).
References (47)
- et al.
Colloids Surf
(1986) - et al.
Colloids Surf A Physicochem Eng Aspects
(1999) - et al.
Colloids Surf A
(2002) Adv Colloid Interface Sci
(2000)- et al.
J Colloid Interface Sci
(2002) - et al.
Colloids Surf A
(2000) Computer Physics Com
(1982)- et al.
Colloids Surf A
(2002) Advances in polymer science
(1993)Advances in polymer science
(1993)
J Chem Phys
J Chem Phys
J Chem Phys
Colloid Polym Sci
J Macromol Sci Phys
Macromolecules
Ber Bunsen-Ges Phys Chem
Colloid Polym Sci
Phys Chem Chem Phys
J Am Chem Soc
Phys Rev E
Angew Chem Int Ed
Cited by (114)
Probing of microgel–enzyme films on graphite substrates by means of atomic force microscopy and amperometry
2024, Mendeleev CommunicationsIn situ single particle characterization of the themoresponsive and co-nonsolvent behavior of PNIPAM microgels and silica@PNIPAM core-shell colloids
2023, Journal of Colloid and Interface ScienceCharged microgels adsorbed on porous membranes - A study of their mobility and molecular retention
2019, Journal of Membrane ScienceCitation Excerpt :Below the pKa value the acid groups are protonated and the size of the microgel decreases. Adsorbed to solid surfaces microgels flatten [31] thereby exhibiting different phase transition dynamics and properties [30,32] as compared to the dispersed state. Yet, adsorbed microgel systems are less investigated and mostly adsorbed microgels or films on impermeable media such as gold, silicon wafer or mica are analyzed [30,31,33,34].
Monolayer microgel composite membranes with tunable permeability
2018, Journal of Membrane ScienceCitation Excerpt :Above the VPTT, the polymer network collapses causing the microgel to shrink towards the PS core and also perpendicular to the membrane surface. The latter is a result of the steric hindrance due to strong adsorption of the microgels [57,58]. Only a thin hydrophobic PNIPAM layer covers the PAN pores and the hydraulic resistance decreases with the thickness of the gel layer.