Formation of linear and crosslinked polyurethane nanoparticles that self-assemble differently in acetone and in water
Abstract
For the first time, stable water-borne polyurethane dispersions (PUDs) leading to mechanically strong polyurethane (PU) coatings were prepared without any chain extender. Elimination of this step and using of water-crosslinking gives promising applications of PUDs as biomaterials and production on an industrial scale.
The PU system consists of polycarbonate macrodiol (PCD), (2,2-bis(hydroxymethyl)) propionic acid (DMPA), 1,6-diisocyanatohexane (HDI) and N,N-diethylethanamine (TEA). Fourier transform infrared spectroscopy (FTIR), dynamic mechanical thermal analysis (DMTA), static light scattering (SLS), dynamic light scattering (DLS) and atomic force microscopy (AFM) enabled the study of the assembly of the PU chainsinanoparticles into supramolecular structures in acetone and in water.
In acetone, the self assembly of isocyanate-terminated pre-polymer chains occurs in the form of either PU solutions (linear PUs) or PU dispersions (crosslinked PUs). Due to a phase inversion, spherical linear or water-crosslinked PU dispersions are formed after water addition.
The isocyanate-to-total hydroxyl ratio is very important; a slight isocyanate (NCO) excess of 1.05 leads to linear rod-like nanoparticles in acetone followed by a core-shell structure after water addition. An isocyanate excess of 1.5 leads to compact sphere or micro gel particles initially crosslinked by moisture in acetone, and the final crosslinking and phase inversion occurs after water addition.
The chain lengths of the linear PUs and the mesh sizes in PU networks increase with the-PCD:DMPA ratio. Thus, the design of PU nanoparticles with desired sizes, shapes and crosslink densities is possible.
Unlike the substantially different shapes, sizes and compactness of PUs in acetone, all PUDs in water are spherical nanoparticles with an average diameter from 21 to 72 nm, a zeta potential from 40 to 65 mV and a size-dispersity index from 0.05 to 0.22.