Abstract
The purpose of this study is to highlight the surface and size effects of the nanopores on the thermodynamics and kinetics of gelation. The effects have been probed by applying differential scanning calorimetry to poly(vinylidene fluoride) solutions in tetraethylene glycol dimethyl ether (tetraglyme) and γ-butyrolactone. Nanoconfinement has been accomplished by introducing gels into native and organically modified silica nanopores (4–30 nm). Nanoconfinement has produced two major effects. First, the heat of gelation has decreased three to four times compared to that for the bulk systems. Second, the temperature of gelation has increased by ~40 °C (tetraglyme based systems) and ~70 °C (γ-butyrolactone based systems), the increase being stronger in native nanopores. The effects are discussed in terms of acceleration of gelation due to heterogeneous nucleation at the confining surface, and retardation of gelation due to constricted polymer chain mobility in the middle of the pore volume. Calorimetric data have been subjected to isoconversional kinetics analysis. The obtained temperature dependencies of the activation energies of gelation have been interpreted in the frameworks of the nucleation model of Turnbull and Fisher. The results suggest that nanoconfinement leads to a lowering of both the free energy of nucleation and activation energy of diffusion.
Highlights
FTIR and XRD analyses have been conducted in order to try to establish the crystalline form of the crystallites and see if it remains unchanged between the bulk and nanoconfined gels. This is a very challenging task, especially in the case of nanoconfined gels when the instrumental signal is complicated by the presence of silica and is very weak due to a small extent of “crystallinity”. The latter can be estimated by comparing the heats of gelation reported further and the heat of melting of 100% crystalline polyvinylidene fluoride (PVDF), which is
If we look at the increases in Tp for the PVDF/TG/organically modified (OM)
The study has been driven by the hypothesis that gelation of polymer solutions confined to nanopores can be affected by the size and surface effects
Summary
A gel is a special state when the solution loses its ability to flow, i.e., turns into the so-called soft solid. Gels are formed at the expense of crosslinking of polymer chains that form a three-dimensional network that entraps large amounts of a solvent. Nanogels are soft materials, whose size is less than 0.5 μm [1]. Because of their small size, nanogels find promising applications in nanomedicine as drug-delivery, imaging, and therapeutic materials [2]. Nanogels can circulate inside the body, entering the cells for burst release to deliver drugs and target diseases more effectively [3,4,5]. Photolithographic, micromolding, microfluidic techniques as well as free radical and emulsion polymerization are several methods for the fabrication of nanogels [4]
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