Effect of oligomer length on the buckling of long and high aspect ratio microwalls UV embossed from oligomer/monomer mixtures

High aspect ratio microwalls separating long and deep microchannels are important for high sensitivity and high throughput microfluidic devices. Ultraviolet (UV) embossing is a feasible replication method for such microstructures. But buckling easily happens with thin and tall UV embossed microwalls with some formulations due to shrinkage stress. In this paper oligomer/monomer mixtures containing principally (68 wt.%) bisphenol A diacrylate with different degrees of ethoxylation (0, 3, 10 and 30) were used to study microwall buckling; these mixtures were denoted by E0, E3, E10 and E30, respectively. The effect of oligomer length on shrinkage, shrinkage stress, cross-linking kinetics and modulus and their inter-relations was studied. The fugitive multi-acrylated monomers present (trimethylolpropane triacrylate (20%) and dipropylene glycol diacrylate (10%)) caused the four mixtures to exhibit similar ultimate shrinkage stresses; this is contrary to the observation that long oligomers develop low shrinkage stress in neat oligomer systems. However, the E30 mixture network has the smallest modulus. The high shrinkage stress and low modulus of E30 mixture cross-linked network explains the buckling of long and high aspect ratio UV microwalls UV embossed from it. The in-plane shrinkage stress in the cured thin film was found to be the integrated effect of modulus and shrinkage rate built-up: only at high conversions did the stress developed drastically. Almost 100% conversion could be reached with the more flexible and mobile E10 and E30. E10 and E30 also have higher shrinkage values than E0 and E3.