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Change of Free Volume in Polymer Gels As Studied by Positron Annihilation Lifetimes

100%
Ito K., Ujihira Y., Yamashita T., Horie K.
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vol. 95
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issue 4
551-556
EN
Variations of a free volume size of a poly(N-isopropylacrylamide) gel and a polyacrylamide gel during a volume phase transition induced by a change of external environment - temperature, solvent composition, pH, and ion strength, are described. The free volume size is probed by positron annihilation lifetime technique. Variations of the free volume size suggest that a nanoscopic environment of the gels is affected by the free volume of the solvent phase and an interaction between the polymer chain and the solvent molecule in the swollen state, and by the aggregation of the polymer chain and a balance of the interaction among the polymer chain and the solvent molecules in the collapsed state. It is revealed that the free volume size reflects the essential change of the nanoscopic environment in the gels which could induce the macroscopic volume phase transition.
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Free Volume Study on Three Types of Shape Memory Polymers by Positron Annihilation

100%
Ujihira Y., Li H., Ito K.
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vol. 95
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issue 4
677-682
EN
Temperature dependence of nanoscopic environment of shape memory polymers such as polynorbornene (T_{g}≈313 K), polyurethane (T_{g}≈321 K=dynamic viscoelasticity, destruction of hydrogen bond ≈353 K) and styrene-butadiene copolymer (T_{m}=333 K) is observed in terms of free volume parameters - average size, numerical concentration and size distribution - estimated from positronium lifetime measurement, and mechanisms of their shape recoveries are discussed at a molecular level.
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