Relation between the Behavior of Water and Hydrogen Bonding in Poly(hexamethylene adipamide) Films with Different End Group Balance

Abstract

As an extension of the previous studies [M. Tomokiyo et al., submitted to Sen-i Gakkaishi], an attempt was made to elucidate the intrinsic role of terminal groups in the hydrogen bonding formation and the existing state of water in poly(hexamethylene adipamide) (nylon 66) with different molar fraction of amino end group fN. (fN=[NH2]/([NH2]+[COOH])) For this purpose, the heat-pressed nylon 66 films were subject to infrared (IR) and tan β-t analyses and at the same time the films which absorbed 10 wt% of water were subject to 1H NMR measurements to determine relaxation time T1 of water. Analyses on the change in the wave numbers of NH and CO stretching vibrations (κvNH, κvCO), the activation energy evaluated from the temperature dependence of κvNH and κvCO and the activation enthalpy and entropy for αa and αb relaxations indicated that the stronger hydrogen bond is formed for nylon 66 with larger fN, which corresponds to the higher molecular packing density and higher activation energy for flow of the melt for these polymers. 1H NMR revealed that water which penetrated into nylon 66 is composed of two electro-magnetically different species, one of which might be the ionized water by end groups, and those species are also composed of at least two components with shorter T1(T1,B) and with longer T1(T1,A). The existence of ionized water was supported by the pH measurement of water containing nylon 66. The both T1 were shorter for nylon 66 with lower fN, denoting that the nylon interacts more strongly with water, which is quite comparative to the facts that nylon 66 with low fN exhibits the lower resistance to the penetration of water for this polymer. These facts leaded to an idea that water in polyamide is ionized by the polar terminal end groups and based on this idea a tentative hydrogen bonding for nylon 66 with high and low fN (amino end rich and carboxyl end rich, respectively) is proposed.