Fast temperature-responsive nanocomposite PNIPAM hydrogels with controlled pore wall thickness: Force and rate of T-response

Abstract

Fast thermoresponsive porous hydrogels based on the PNIPAM–silica nanocomposite were prepared and optimized in order to obtain increased force (pressure) response to temperature stimuli. This was achieved by increasing pore wall thickness via changing the monomers concentration during the hydrogels’ cryogenic synthesis. The mechanically strong and compact pore structure of the best gel made it possible to increase the force (pressure) response 3 times and the shear modulus 4 times, while the volume ratio of swollen to deswollen state remained unchanged, and the kinetics of the T-response was only moderately slower than in the case of the analogous ultra-fast responsive soft gels. The porous structure of the hydrogels (SEM), the dispersion and particle size of the nanofiller in the pore walls (TEM), as well as the swelling behavior and time-resolved force response to T-change are discussed in this work.

Introduction

Poly-(N-isopropylacrylamide) based hydrogels belong to the most studied stimuli-responsive systems. Materials of this type were discovered to display a marked volume transition under certain conditions [1], [2]. The volume transition can be caused by factors like: change in solvent mixture composition [3], [4], change of pH [5], change of temperature [6], electric [7] or magnetic field [8]. The first thermally inducted volume transition of a linear PNIPAM hydrogel was described more than 20 years ago [9]. The T-responsivity of PNIPAM makes it a very attractive material for many applications like: mechanical actuators [10], nano-valves [11], drug delivery systems [12], [13], enzyme immobilization systems [14], etc. One of the main factors which decide about the practical use of PNIPAM is the rate of response (swelling and deswelling) to environmental stimuli. In case of homogeneous hydrogel samples, the response rate is controlled by solvent diffusion. This is a very slow process which strongly depends on the specimen’s dimensions and on its density. Only very small and thin homogeneous samples can display reasonably short response times. In case of larger samples, the so called “skin effect” slows down the stimuli-responsivity: a surface layer of the hydrogel deswells and becomes denser and less permeable for solvent. The introduction of an interconnecting pore system into the hydrogel is one of the best methods to avoid slow responsivity of larger 3-D samples (solvent transport through convection instead of diffusion). Highly porous hydrogels can exhibit fast swelling, and even superabsorbent properties, due to interconnected pores and due to eventual capillary effects [15], [16], [17]. Several methods have been used for the preparation of highly porous hydrogels, e.g. addition of pore forming agents [18], [19], [20], gas-blowing [21], [22] freeze drying (followed by re-hydration) [23], polymerization of phase separated systems above the lower critical solution temperature (LCST) [24], polymerization-induced phase separation [25], incorporation of dendrimers [26] introduction of side chains [27], polymerization at room temperature under vacuum (solvent bubbles as porogen) [28], impregnation of the hydrogel with microgels [29], and by using solvent crystals as pore templates during a polymerization at a “solvent freezing” temperature [30], [31], [32], [33], [34]. Usually, materials of this type possess poor mechanical properties, and hence their application potential is highly limited. The presence of additional non-reactive substances in the reaction mixture during hydrogel preparation via the “solvent-freezing” method can strongly alter the porous structure of the final product, by influencing solvent ice crystal size and amount [35].

Materials with advanced properties are obtained, if PNIPAM is synthesized in the form of nanocomposites: PNIPAM-poly(l-lactide) block copolymer with inorganic polyhedral oligomeric silsesquioxane (POSS) as star core can self-assemble to temperature-sensitive vesicles in solution [36]; Comb-like polymers with amphiphilic PNIPAM side chains and polar hydrophobic backbone were successfully used to transfer CdSe and ZnS quantum dots (QD, stabilized prior to coating by a surface layer of trioctylphosphine oxide) into the aqueous phase. The water-soluble PNIPAM–QD assemblies display a lower critical solution temperature, which makes possible their repeated dissolution/precipitation. They display also temperature-sensitive photoluminescence (reversible change of color and intensity) which correlates with the assemblies’ swelling degree [37].

In previous work [38], [39] done by some of the authors, a two-step synthesis of silica-filled PNIPAM hydrogels nanocomposites [39] with a very short time of response to temperature was developed, for the potential application as mechanical actuators. The synthesis is an improved “solvent freezing” method (see above), which leads to especially efficient pore systems. During the first step, the crosslinked PNIPAM network grows in a homogeneous aqueous solution. Subsequently, in the second step, which occurs before gelation, the mixture is cooled down below the freezing point. At this stage, a part of water (solvent) crystallizes and the polymerization reaction continues in the concentrated solution around the solvent crystals. The ice crystals play the role of templates of the future porous structure while the concentrated solution around them gelates, and a well-crosslinked polymer network filled with ice crystals is finally formed.

Very fast T-responsivity – also for reswelling – was enabled by incorportaing in situ formed silica nanoparticles into the PNIPAM matrix [39]. This was a considerable improvement in comparison to classical porous hydrogels, which typically display a considerably slower re-swelling than deswelling. The nanoparticles obviously mechanically stabilize the porous structure of hydrogels against collapse (“gluing together”) during deswelling as suggested in Scheme 1: The incompressible nanoparticles play a prominent structural role in the deswollen state, after the strong decrease of matrix volume. Furthermore, the addition of nano-SiO₂ also markedly increased the gels’ moduli.

In this work, the authors’ aim was to obtain materials with a strong force response to temperature stimuli, via synthesizing nanocomposite PNIPAM–SiO₂ gels with thicker pore walls. The samples were to be prepared by modifying the previously developed two-step procedure [39]: the increased pore thickness had to be achieved by an increased monomers concentration in the reaction mixture, which would lead to the growth of a smaller crystals during the freezing stage, and hence to smaller pores with stronger walls. Due to the higher monomer concentration, the pore walls were expected to be more efficiently crosslinked. The silica nanofiller content was to be optimized for the best strong-walled porous gel.