The effect of cross-linking density and dangling chains on surface molecular mobility of network polyurethanes

Chemically cross-linked polyurethanes (PUs) were synthesized with two different cross-linking densities and dangling chain concentrations to understand the difference between the molecular mobility of the bulk and free surface based on dynamic viscoelastic properties, lateral force microscopy (LFM) and scanning viscoelasticity microscopy (SVM) measurements. The PUs were synthesized from poly(oxypropylene) triol (PPT; M _n = 3000) and tolylene-2,4-diisocyanate (TDI) with a formulation ratio (= [NCO]/[OH]) of 0.85 and 1.20. The former PU has a lightly cross-linked structure with dangling chains, and the latter one has a densely cross-linked structure with allophanate groups and without dangling chains. The α-relaxation process, associated with glass transition of the PPT chains for the bulk and surface, was observed for both PUs in dynamic viscoelastic properties and lateral force microscopy measurements, respectively. The temperature region, at which the α-relaxation process was observed, for the surface was lower and broader in comparison with bulk and the trend was more obvious for the PU with dangling chains. These results can be explained well with the existence of dangling chains and their segregation at the top surface because of lower conformational entropy.