Ink-jet printing of polymer solutions in the semi-dilute regime

The influence of polymer concentration on going from the dilute through the overlap into the concentrated regime during drop on demand inkjet printing is investigated for a range of cellulose ester (CE) polymers from visual examination of ligament stretching as a function of applied wave form. The physical behaviour of the fluids in drop formation is indicative of the dominance of viscoelastic effects within the timescale of the process, in preventing ligament breakup at the pinch point compared with a glycerol / water / iso-propanol blend Newtonian fluid of similar viscosity. When formulated at the overlap concentration all polymers showed qualitatively similar behaviour with respect to time and length of ligament at rupture irrespective of polymer molecular weight. Beyond the overlap concentration the ligament rupture times continue to increase with increasing elasticity of the solutions but the ligament rupture lengths decrease rapidly. In this regime coil overlap and entanglement effects become increasingly important, dramatically increasing the elastic nature of the ligament. It is proposed that in the case of weakly associating polymers such as cellulose esters, the effective relaxation time is further increased due to the possibility that on chain extension intramolecular H-bonds are broken and may reform in the extended form as intermolecular associations. These intermolecular associations act as physical crosslinks, thereby creating a transient gel structure. Because these associations are transient in nature, this gel structure is capable of having a finite large viscosity. Once the strain is removed the gel will decay as the chains return to the more thermodynamically stable coil state.