Zhibing Hu

Synthesis and Application of Modulated Polymer Gels

A class of environmentally responsive materials based on the spatial modulation of the chemical nature of gels is proposed and demonstrated here. The modulation was achieved by limiting the interpenetration of part of one gel network with another gel network. The gels so produced have an internally heterogeneous or modulated structure. Three simple applications based on the modulated gels are described here: a bigel strip, a shape memory gel, and a gel hand. The bigel strip bends almost to a circle in response to a temperature increase or an increase in solvent concentration. The shape memory gel changes its shape from a straight line to a pentagon to a quadrangle as the temperature increases. These transitions from one shape to another are reversible. The gel hand in water can grasp or release an object simply by an adjustment of the temperature.

Polymer gels with engineered environmentally responsive surface patterns

The polymer gels called hydrogels may be induced to swell or shrink (taking up or expelling water between the crosslinked polymer chains) in response to a variety of environmental stimuli, such as changes in pH or temperature, or the presence of a specific chemical substrate. These gels are being explored for several technological applications, particularly as biomedical materials. When hydrogels swell or shrink, complex patterns may be generated on their surfaces. Here we report the synthesis and controlled modulation of engineered surface patterns on environmentally responsive hydrogels. We modify the character of a gel surface by selectively depositing another material using a mask. For example, we use sputter deposition to imprint the surface of an N-isopropylacrylamide (NIPA) gel with a square array of gold thin films. The periodicity of the array can be continuously varied as a function of temperature or electric field (which alter the gels volume), and so such an array might serve as an optical grating for sensor applications. We also deposit small areas of an NIPA gel onthe surface of an acrylamide gel; the patterned area can be rendered invisible reversibly by switching the temperature above or below the lower critical solution temperature of the NIPA gel. We anticipate that these surface patterning techniques may find applications in display and sensor technology.

Swelling and mechanical behavior of poly(N-isopropylacrylamide)/Na-montmorillonite layered silicates composite gels

Abstract Clay–polymer hydrogel composites have been synthesized based on poly( N -isopropylacrylamide) (PNIPAM) gels containing 0.25–4xa0wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). The morphology of the composite gels has been studied using a polarized optical microscope. The size of Na-MLS aggregates increases with Na-MLS concentration. The swelling ratio of the Na-MLS/PNIPAM composite in water is increased at the low Na-MLS concentration but decreases as the concentration increases. Correspondingly, the shear modulus of the gel is found to exhibit a distinct minimum against clay concentration. For Na-MLS concentrations ranging from 2.0 to 3.2xa0wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. However, the gel volume change at the LCST is first increased and then decreased upon the increase of the Na-MLS. No pH induced phase transition is observed for the Na-MLS/PNIPAM composites. The experimental results can be explained by considering that Na-MLS is physically entrapped inside rather than chemically bonded into the gel.

Shape memory gels made by the modulated gel technology

Shape memory gels based on interpenetrating only part of one gel network with another gel network have been synthesized. These gels consist of two parts: a control element, which is responsive to a designated environmental stimulus, and a nonresponsive substrate element. By designing the pattern in the gelation process, a variety of shapes are obtained including “spiral,” “square,” “fish,” “numbers,” “alphabets,” and “tube.” The change between two different shapes can be controlled by external stimuli such as temperature and is reversible.

Bending of N‐isopropylacrylamide gel under the influence of infrared light

A CO2 infrared laser has been used to irradiate a straight cylindrical N‐isopropylacrylamide gel. It is found that the infrared laser not only induces the volume phase transition in the gel, but also causes the gel to bend toward the laser beam. When the laser is blocked, the gel becomes straight again. The transition between the straight and the bending gel is fully reversible. The maximum bending strain of the gel is comparable to that obtained for poly(vinyl alcohol)–poly(sodium acrylate) copolymer gel under the influence of an electric field. The bending effect has been systematically studied as a function of CO2 laser power, time, and the sample cell temperature. The relaxation behavior for the gel restoring its original shape after blocking the infrared irradiation follows an exponential form. It is suggested that the bending effect is caused by a temperature gradient which produces an osmotic pressure difference between the front surface area of the gel and the remainder of the gel.

Pattern formation of constrained acrylamide/sodium acrylate copolymer gels in acetone/water mixture

Pattern formation and evolution in constrained acrylamide/sodium acrylate (PAAM/SA) gels have been investigated in acetone/water mixture. The constraint is achieved by cross linking the gel slabs onto a rigid substrate. All samples are initially kept in water with the hexagonal pattern develop on the surface before being immersed in the acetone/water mixture. Depending on the solvent composition and ionic strength of the sample, different patterns, i.e., hexagonal, grains, and bubbles, have been observed. These patterns are formed at acetone concentration below, near, and above the concentration at which the gel volume phase transition occurs. The wavelengths of hexagonal and bubble patterns are found to be the same, while that of the grain pattern is four times smaller. It is suggested that the shrinking patterns are formed due to the dense gel surface produced during the shrinking process.

The phase transition and shear modulus of ionic N-isopropylacrylamide gels in concentrated salt solutions

The swelling and mechanical properties of thermally sensitive N-isopropylacrylamide (NIPA) hydrogels containing 0–35 mM sodium acrylate comonomer have been investigated at room temperature. The swelling medium is an aqueous solution with an NaCl concentration varying from 0 to 3.0M. As the NaCl concentration increases, the ionic NIPA gels shrink. In a dilute sodium chloride solution with NaCl less than 0.1M, the conformational change of the gels is a simple process of osmotic deswelling. As the sodium chloride concentration increases above 0.8M, the gels undergo a shrinkage phase transition. Corresponding to the shrinking of the gels, the shear modulus of the gels gradually increases at the transition point. It is found that the phase transition significantly affects the turbidity of the NIPA gels.

Temperature and time dependencies of surface patterns in constrained ionic N‐isopropylacrylamide gels

Surface patterns in ionic N‐isopropylacrylamide (N‐IPA) gels have been investigated under external constraint. Hexagonal, grain, and bubble patterns have been observed at temperatures below, near, and above the phase transition temperature Tc. It is found that the behavior of these patterns depends not only on temperature, time, and external constraint, but also on the thermal path. The time dependence of the gel thickness has revealed that there is a plateau period and that the end of the plateau period corresponds to the onset of the bubble pattern. This observation is in agreement with previous measurements of free spherical N‐IPA beads. The experiments show that each hexagonal cell evolves into a bubble for N‐IPA gels, while a bubble can be present in the middle of a hexagonal cell for acrylamide gels. The mechanism for the shrinking patterns will be also discussed.

Rubber elasticity of polyacrylamide gels in high network concentration

Abstract Shear modulus of polyacrylamide (PAAM) gels has been measured with the network concentration ranging from 0.02 to 1.0 g/cm3. In the low concentration region, the shear modulus is a scaling function of network concentration with the exponent equal to 1/3, which is expected by classical rubber elasticity theory. As the concentration increases, the behaviour of shear modulus deviates the scaling law significantly. This deviation has been observed from all six series of gels with cross-linking concentrations varying from 0.61 to 4.91 mol%. The modified Mooney theory which incorporated a second higher order term is used to explain the results. This second term is believed to be a result of gel inhommogeneity. The measured shear modulus data can be fitted well with the new theory, with two exponents equal to 0.33 and around 2.0–2.5, respectively. The crossover concentration from the classical behaviour to the non-classical behaviour was found to be cross-linking concentration dependent.

Bending of bi‐gels

The bending of bi‐gels has been studied as a function of temperature, acetone concentration, and NaCl concentration. The bi‐gels consist of acrylamide on one side, and the interpenetrating polymer networks of acrylamide and N‐isopropylacrylamide on the other side. The maximum bending strain achieved by the bi‐gel is about 0.85. The bending mechanism of the bi‐gels reported here is due to the engineered structure heterogeneity, in contrast with previous homogeneous gel bending which is induced by external gradient fields. The kinetic behavior of the bi‐gel is discussed. A theoretical model is used to estimate the collective diffusion coefficient Dc of the IPN side of the bi‐gels in pure water and in salt solution. The results agree with those obtained by the light‐scattering method.