Influence of a Crown Ether Comonomer on the Temperature-Induced Phase Transition of Poly(N-isopropylacrylamide) Hydrogels

Abstract

Copolymerization of thermosensitive hydrogels based on poly(N-isopropylacrylamide) (PNIPA) is a possible route to enhanced storage capacity of guest molecules. This article describes the synthesis of the amphiphilic crown ether N9-propenoyl-3,6,12,15-tetraoxa-9,21-diazabicyclo[15.3.1]heneicosa-1(21),17,19-triene (CE) and its incorporation into a PNIPA hydrogel (PNIPA/CE). Mechanical measurements on the gel show that the CE units contribute to the elasticity of the network, but the swelling ratio in water is reduced compared to the unmodified system. The comonomer reduces the temperature of the volume phase transition (VPT), TVPT, and broadens the transition. Both the enthalpy and the entropy associated with the VPT decrease. Scattering measurements indicate that the local structural features on the scale of 10 A are unchanged, but the CE units form large clusters, the size of which increases with rising temperature. In the phase-separated state above TVPT these clusters are distributed on the polymer-water interface.

Kinetics of Jammed Systems: PNIPA Gels

“…All the gels were prepared with a molar ratio of N‐isopropyl acrylamide to cross‐linker (N,N′‐methylene bisacrylamide) 150:1. The samples were synthesized in the form of disks of swollen diameter d 0 ≈ 7 mm and thickness 1.5 mm, with different degrees of inhomogeneity, as described elsewhere 14, 15…”

“…In the latter case, the formation of hydrogen bonds in the collapsed state suffices to prevent extended ordering of the chains above T VPT . In Figure 4, sample 1 was washed after synthesis and kept in the swollen state; sample 2 was dried in a desiccator, then reswollen; sample 3 was copolymerized with a crown ether in order to enhance the structural inhomogeneity 15. For the three PNIPA hydrogels of increasing network inhomogeneity, Figure 4 shows that the disk diameter decreases linearly with time until the equilibrium volume is reached (in the logarithmic scale of this figure, the linear decays appear curved).…”

Complexation of poly(acrylic acid) and poly(ethylene oxide) investigated by enhanced Rayleigh scattering method

“…Because the ERS behavior is similar to the light scattering, the ξ static (the static correlation length of the density fluctuation of the gel network, i.e., the size of the blobs at polymer chains) in a polymer solution or a gel network is correlative to the scattering light intensity ( I ) and the scattering vector ( q ) by the Ornstein‐Zernike equation 34–36 …”

Aggregate formation of cellulose derivative-surfactant in aqueous solutions investigated by enhanced Rayleigh scattering spectroscopy

“…the size of the blobs at polymer chains) in a polymer solution or a gel network is correlated to the scattering light intensity and the scattering vector (q) by the Ornstein-Zernike equation (Collings et al 1999;Kosik et al 2008;Li et al 2010a, b;Nasimova et al 2004).…”