Probes of the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)

Abstract

Cloud point, microcalorimetric, and fluorescent probe methods were in good agreement in determining the lower critical solution temperature (LCST) of an aqueous solution of a high molecular weight (Mn = 160,000), polydisperse (Mw/(Mn = 2.8) sample of poly(N-isopropylacrylamide) (PNIPAAM). The cloud point (32.2°C) concurred with the temperatures of abrupt changes observed in the emission spectra of micromolar quantities of solubilized fluorophores: the I1/I3 ratio of pyrene and the emission maxima of 1-pyrenecarboxaldehyde (pycho), sodium 2-(N-dodecylamino)naphthalene-6-sulfonate (C12NS), and ammonium 8-anilinonaphthalene-1-sulfonate (ANS) successfully monitored the formation of the precipitated polymer phase. The observed microcalorimetric endotherm had a peak maximum of 32.4 ± 0.1°C and an enthalpy on the order of the strength of hydrogen bonds. Poly(N-isopropylacrylamide) (PNIPAAM, 1) exhibits a lower critical solution temperature (LCST) in aqueous solution; our work has focused on characterizing perturbations of the accompanying change in polymer solubility by the addition of cosolutes and by copolymerization. C-O(1) NH / \ CH3 CH3The LCST is a consequence of the hydrogen bonding present in the system GL2). Although exothermic hydrogen bond formation between polymer and solvent molecules lowers the free energy of mixing, the specific orientations required by these bonds lead to a negative entropy change and thus to a positive contribution to the free energy.

Enhanced Retention of rhBMP-2In Vivo by Thermoreversible Polymers

“…The NiPAM LCST in our hands were $ 27 8C (in phosphate buffer), lower than the commonly reported LCST of 30 ± 33 8C (in water) [25]. Obviously it needs to be lower than the physiological temperature of 37 8C and the difference between the polymer LCST and the physiological temperature is expected to determine the polymer dissolution rate in vivo.…”

Temperature responsive flocculation and solid–liquid separations with charged random copolymers of poly(N-isopropyl acrylamide)

“…Even at 50°C, there is negligible settling observed for suspensions dosed with co-ionic PNIPAMs. The increase in settling rate that occurs when a suspension dosed with PNIPAM is heated above the LCST is a result of the phase separation attributed to the difference in free volume of the polymer and aqueous solvent [6,[31][32][33][34][35][36][37][38]. The polymer that has adsorbed on the surface at T below the LCST becomes insoluble as the temperature is raised and acts as a nucleation site for additional polymer deposition at T > LCST [4,39,40].…”

Poly(N-isopropylacrylamide) (PNIPAM) is never hydrophobic

“…The earliest fluorescent probe studies showed that pyrene accumulated in the lower polarity domains [16] and the local polarity was slightly greater than methanol [17]. Similarly, FTIR has confirmed some hydrogen bonding to amide groups above the LCST [18].…”