Kinetics of Volume Phase Transition in Poly(N-isopropylacrylamide-co-acrylic acid) Gels

Abstract

The shrinking/swelling behavior across the volume phase transition temperature, Tc, has been investigated for cylindrical gels made of poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) and poly(N-isopropylacrylamide) (NIPA). NIPA/AAc gel shrank at Tc ) 43°C by quasistatic heating. On the other hand, the gel underwent a shrinking transition at a lower temperature, i.e., Tc ≈ 40°C, when a temperature jump was applied to the gel from 20°C. More interestingly, the shrinking process consisted of three stages: (i) a uniform shrinking stage where the gel diameter decreases exponentially, (ii) a plateau stage where the gel shrinks further from both ends of the cylinder while the middle (swollen) part remains in the swollen state, and (iii) a collapsing stage where the middle part of the gel shrinks linearly with time. This sequential process was found to be characteristic of weakly charged gels with a large aspect ratio. To the contrary, the swelling process was fitted with a single exponential function given by the Tanaka-Fillmore theory on gel swelling. This interesting shrinking behavior was discussed by comparing the results of swelling kinetics. In the case of NIPA homopolymer gel, the shrinking process was strongly decelerated due to phase separation, while the swelling process was well reproduced by the TanakaFillmore theory.

Hydrogels for Soft Machines

“…On the other hand, a rapidly shrinking gel tends to form a dense skin that inhibits water loss from the interior, slowing the volume change and possibly leading to fracture of the gel under the resulting shrinkage stresses. The effect of such skin formation on deswelling kinetics has been studied by Hirose and Shibayama, [112] who showed that pure NIPAM gels form a dense layer and shrink much more slowly than weakly charged copolymers of NIPAM and acrylic acid, which retain more mobility for water in the collapsed state.…”

Polymer Gel Actuators: Fundamentals

“…This type of approach addresses the problem that a rapidly shrinking gel tends to form a dense skin that inhibits water loss from the interior, both slowing the volume change and possibly leading to fracture of the gel under the resulting shrinkage stresses. The effect of such skin formation on deswelling kinetics has been studied by Hirose and Shibayama, who showed that pure NIPAM gels form a dense layer and shrink much more slowly than weakly charged copolymers of NIPAM and acrylic acid that retain more mobility for water in the collapsed state [98].…”

How much weighs the swelling pressure

“…After the maximum in the m app curve at a temperature of 25.4 °C the deswelling of the PNIPAM governs the scene. This happens about 8 degrees below the LCST which is quite an astonishing result because so far, only moderate changes in the PNIPAM solution state have been described in this temperature range [24][25][26][27][28][29][30]. Moreover, 25 °C is frequently used as starting or reference temperature for deswelling studies [26,[29][30][31][32][33].…”