Abstract
Static and dynamic swelling properties of cylindrical poly(N-isopropylacrylamide) (PNIPA) gels in water as well as in liquid paraffin (non-solvent for PNIPA gels) have been investigated by using a laboratory-made magnetic force-driven rheometer. In static tests (under a constant stress), the length and diameter of the gel sample in water increase with time due to stress-induced swelling. Relaxation times of the stress-induced swelling in both directions are identical within experimental error. The Poisson ratio decreases from ∼0.5 to ∼0.2 with time just after the application of the magnetic force. The dynamic tests (under sinusoidal forces) reveal large differences in the swelling behavior of the gels in water and liquid paraffin. In liquid paraffin, amplitudes of strains parallel and perpendicular to elongation (ε‖ and ε⊥, respectively) are independent of angular frequency (ω), giving the absolute value of dynamic Poisson ratio (|μ∗|=ε⊥/ε‖) of ∼0.47. On the other hand, ε‖, ε⊥ and |μ∗| in water significantly depend on ω due to the stress-induced swelling with a characteristic time. The dynamic Poisson ratio at ω→∞ (short time limit) or ω→0 (long time limit) agrees well with the corresponding Poisson’s ratio obtained by the static tests. The phase lags of ε∗‖ and ε∗⊥ seeing from the stress wave (δ‖ and δ⊥, respectively) are independent of ω in liquid paraffin whereas ε∗‖ and ε∗⊥ move with keeping |δ⊥−δ‖|=π. In water, the δ‖ curve shows a minimum while the δ⊥ curve does a maximum at the same angular frequency. A characteristic time of swelling in the dynamic tests obtained as an inverse of the frequency accords well with the relaxation time in the static tests.
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Nosaka, S., Urayama, K. & Takigawa, T. Static and Dynamic Swelling Properties of Poly(N-isopropylacrylamide) Gels in the Swollen State. Polym J 37, 694–699 (2005). https://doi.org/10.1295/polymj.37.694
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DOI: https://doi.org/10.1295/polymj.37.694
