NMR and Thermodynamic Study of Phase Transition in Aqueous Solutions of Thermoresponsive Amphiphilic Polymer

Abstract

1H NMR techniques combined with differential scanning calorimetry (DSC) experiments are employed to investigate the temperature-induced phase transition in aqueous poly(vinyl methyl ether) solutions. NMR data were used to construct van't Hoff plots, and changes in the enthalpy and entropy characterizing the phase transition were determined. It follows from a comparison of NMR and DSC thermodynamic parameters (¦H values) for solutions with c = 130 wt % that the size of the cooperative units (domains) undergoing the transition corresponds to the whole polymer chain.An aqueous poly(vinyl methyl ether) (PVME) solution exhibits a lower critical solution temperature (LCST) of around 308 K, above which the solution separates into two phases.1,2 On the molecular level, this phase separation is assumed to be a macroscopic manifestation of a coilglobule transition followed by aggregation, as shown for acrylamide-based polymers in water by light scattering and small neutron scattering. 37 The phase transition in PVME/water solutions has been investigated using cloud point, calorimetric, viscometric, viscoelastic, infrared, and Raman measurements.1,2,815 Maeda 9 found three distinct PVME concentration regions differing in the number of water molecules forming a complex with PVME. The phase diagram was revealed for aqueous PVME solutions from modulated-temperature differential scanning calorimetry (DSC) experiments. 14,16,17 In past decades, we applied 1 H NMR spectroscopy to investigate the structural and dynamical changes during temperature-induced phase separation in PVME/D 2 O solutions. 1823The phase separation is manifested in the NMR spectra by marked line broadening for a major part of the PVME units due to the formation of rather compact globule-like structures. 18,19 Above the phase separation, the fraction of PVME units involved in collapsed globule-like structures was approximately 0.85 for solutions in the concentration range 0.130 wt %. 19The data obtained from NMR spectroscopy were used to construct van't Hoff plots, and thermodynamic parameters of the transition in polymer solutions and hydrogels were determined. 2426 On the basis of the phenomenological model in which a macromolecule is viewed as consisting of independent cooperative units, the cooperativity of the transition can be related to a comparison of the effective (van't Hoff) enthalpy and the calorimetric enthalpy of a polymer molecule. 27,28 In the present work, our attention is focused on the thermodynamic investigation of the phase transition in aqueous solutions of PVME. The thermodynamic parameters obtained by NMR analysis using van't Hoff plots and by DSC are compared, and are used for characterization of the mesoglobules as a function of the polymer concentration. From a methodical point of view, we combined the measurements of 1 H NMR spectra (mainly integrated intensities) and calorimetric measurements.Aqueous PVME (M w = 60500 g mol ¹1, M w /M n ¥ 3) solutions were prepared in a broad polymer concentration range of c = 190 wt %. DSC mea...

Smart Poly(lactide)-b-poly(triethylene glycol methyl ether methacrylate) (PLA-b-PTEGMA) Block Copolymers: One-Pot Synthesis, Temperature Behavior, and Controlled Release of Paclitaxel

“…This suggests that mobility restriction is more significant close to and along the hydrophobic polymer backbone. Similar behavior was previously observed for PTEGMA homopolymer [31], PHPMA-b-PDEGMA, and POEGMA-based copolymers [44,51] and is in contrast to other thermo-responsive polymer-based systems, e.g., poly(N-isopropyl acrylamide) (PNIPAm) [52,53], poly(vinyl methyl ether) (PVME) [54], poly(N,N′-diethylacrylamide) [55], and PIPOx [56], in which all polymer signals were suppressed in the same way. The transition behavior is confirmed by the sudden increase in the intensity of the scattered light observed by DLS at the begging of the transition temperature (Figure S2).…”

Additive Effects on Phase Transition and Interactions in Poly(vinyl methyl ether) Solutions

“…Enthalpy values ∆H DSC can be compared with those determined from NMR. Assuming that the phase transition is a cooperative process and the polymer chain consists of cooperative units (domains) which undergo transition as a whole [29][30][31], the values ∆H NMR are thus related to mol of cooperative units. The ratio ∆H NMR /∆H DSC represents the average number of monomer units in one cooperative domain and can provide a quantitative analysis of the degree of cooperativity (cf.…”