Thermal transitions of poly(vinyl alcohol) hydrogel sensed by a fluorescent probe

Abstract

ABSTRACT:Fluorescence from the fluorescein molecule has been used to study of phase transition processes in five poly(vinyl alcohol) (PVA) hydrogels in the temperature range 75-350 K. These hydrogels were prepared with PVA with molecular weight ϭ 124,000 -186,000, 99ϩ% hydrolyzed, by repeated freezing-thawing cycles in five compositions: 9.9, 11.1, 13.3, 16.5, and 20.9 PVA w/w %. Plots of total fluorescence intensity versus temperature, normalized with the fluorescence signal at 75 K, and of the emission wavelength maximum versus temperature indicated that the dynamic of the guest deactivation was dependent on the hydrogel thermal transitions. These thermal transition processes were compared to three model systems: uranine (fluorescein di-sodium salt) in water, fluorescein in water/ethanol (85%/15% v/v), and fluorescein/PVA homopolymer. Small spectral shifts were obtained for these model systems over the entire temperature range attributed to the hot band effect. Nevertheless, larger spectral shifts were obtained for hydrogels, indicating that shifts of the chemical dissociation equilibrium was occurring in this last case. Slope changes of both intensity and wavelength maximum plots versus temperature occurred at the same temperature values, and we attributed the onset of the hydrogel thermal transition as the sol-gel transition occurring at T Х 370 K; at 320 K we observed the glass transition temperature of PVA plasticized by water molecules; at T Х 240 Ϯ 5 K we attributed the melting point of water molecules bonded to the PVA chains (freezable bond molecules); at T Х 270 Ϯ 5 K we observed the melting point of free water molecules (bulk water); and, at 135-145 K the water glass transition.

Toxicity, Biocompatibility, pH-Responsiveness and Methotrexate Release from PVA/Hyaluronic Acid Cryogels for Psoriasis Therapy

“…The low-temperature peaks were evidenced indicating the presence of freezable bound water within the polymeric network which show transition temperatures at very low values i.e., at −19 °C for HA/PVA3c and −4 °C for HA/PVA2c in accordance with results reported by other authors [ 38 , 40 , 41 ]. Transition temperatures at higher temperature were assigned to the relaxation mechanism of the glycosidic bonds in HA and structural rearrangements due to the loss of the residual water [ 42 ].…”

Poly(vinyl alcohol)/chitosan/montmorillonite nanocomposites for food packaging applications

“…There can be observed the glass transition temperature of PVA at about 30–40°C and very well evidenced water-related glass transitions. Comparable results were found by Dibbern–Brunelli and Atvars 36 who studied phase transition processes in PVA hydrogels. They attributed the sol–gel transition occurring at T > 96°C; and the glass transition temperature of PVA plasticized by water molecules is observed at 46°C; while the transition at T > −30°C to the melting point of water molecules bounded to the PVA chains (freezable bound water) also they attributed that at T > −3°C to the melting point of free water molecules.…”

“…The effect of the water content on the PVA glass transition involves an opposite effect, since it should promote the PVA polymer plasticization by the increase in the free volume cavity size and consequently with a decrease in T g . 36…”

Relaxations in Poly(vinyl alcohol) and in Poly(vinyl acetate) Detected by Fluorescence Emission of 4-Aminophthalimide and Prodan

“…Interaction of excited states of dyes with polymer matrixes is important in applications such as displays, memories and switches, sensors, and photochromic materials . On the other hand, the use of fluorescent probes has proven to be useful for diverse studies in polymer science, such as diffusion and swelling, , characterization of the heterogeneity of environments, − phase segregation, − thermal transitions, , detection of free volume, − and the study of curing processes. − Excited states can be used to monitor fast dynamics in these media, when time-resolved fluorescence spectroscopy can be used, , or long-term relaxations such as those involved in polymer curing can be monitored by steady-state emission spectroscopy. The production of the excited state involves a very fast and great energy input into the system.…”

“…These moieties associate in pools controlling the folding of the polymer carbon backbone. On the other hand, the polymer chains are maintained together by H-bonding, and this fact is responsible for the relatively high glass transition temperature of PVA, which lays in the vicinity of 350 K. , The structure and the extension of the H-bond interactions can be conveniently studied by IR absorption spectroscopy. , Nevertheless, this technique cannot render information on the heterogeneity and on the dynamics of these interactions. On the other hand, TRES of suitable fluorescent probes can be an adequate technique to obtain this later information.…”