Compatibility versus reaction diffusion: Factors that determine the heterogeneity of polymerized adhesive networks

Heterogeneity and phase separation during network polymerization is a major issue contributing to the failure of dental adhesives. This study investigates how the ratio of hydrophobic crosslinkers to hydrophilic comonomer (C/H ratio), as well as cosolvent fraction (ethanol/water) influences the degree of heterogeneity and proclivity for phase separation in a series of model adhesive formulations.

Twelve formulations were investigated, with 4 different C/H ratios (7:1, 2.2:1, 1:1, 0.5:1) and 3 different overall cosolvent fractions (0, 10 and 20 wt%). The heterogeneity and phase behavior were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), dynamic mechanical analysis (DMA), small-angle x-ray scattering (SAXS) and atomic force microscopy (AFM).

In resins without cosolvent, all characterizations confirm reduced heterogeneity as C/H ratio decreases. However, when 10 or 20 wt% of cosolvent is included in the adhesive formulation, a higher degree of heterogeneity and even distinct phase separation with domains ranging from a few hundreds of nanometers to a few micrometers in size form. This is particularly noticeable at lower C/H ratios, which is surprising as HEMA is commonly considered a compatibilizer between hydrophobic crosslinkers and aqueous (co)solvents.

Our experiments demonstrate that formulations with lower C/H ratio and thus a lower viscosity experience later onsets of diffusion limitations during polymerization, which favors thermodynamically driven phase separation. Therefore, to determine or predict the resulting phase structure of adhesive materials, it is necessary to consider the kinetics and diffusion constraints during the formation of the polymer network and not just the compatibility of resin constituents.