Inkjet printing of polymer solutions and the role of chain entanglement Desheng Xu, a Veronica Sanchez-Romaguera, a Silvia Barbosa, a Will Travis, a Jos de Wit, b Paul Swan b and Stephen George Yeates* a Received 16th July 2007, Accepted 5th October 2007 First published as an Advance Article on the web 12th October 2007 DOI: 10.1039/b710879f The influence of polymer concentration, 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 polymer fluids in drop formation is indicative of the dominance of viscoelastic effects within the timescale of the process, in preventing ligament break- up at the pinch point compared with a water–glycerol–isopropanol blend Newtonian fluid of similar viscosity. This has previously been described in terms of the polymer chain undergoing a coil–stretch transition at the strain rates experienced in the inkjet process. When formulated at the coil 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 time continues to increase with increasing elasticity of the solution but the ligament rupture length decreases rapidly. In this regime chain entanglement becomes important, dramatically increasing the elastic nature of the ligament. Additionally 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 as intermolecular associations whilst the polymer chains are extended. These intermolecular associations act as physical crosslinks, thereby creating a transient network structure. This network structure is capable of having a finite large viscosity. Once the strain is removed the network will decay as the chains return to the more thermodynamically stable coil state. Introduction Inkjet printing has developed as an important technology for the defined spatial deposition of polymer solutions in applica- tions as diverse as graphics, textiles, digital electronics and displays. 1–6 For many emerging opportunities, it is necessary to increase the concentration of polymer in the deposited fluid in order to maximise either material throughput and/or functionality. However the addition of polymer to an ink has a strong impact on the nature of the drop generation and ejection process. 7–13 The influence of added polymer on drop formation and filament break-up has been studied as a function of both concentration, typically in the dilute regime up to the coil overlap concentration, c*, molecular weight and architecture. The coil overlap concentration is the point at which individual polymer chains in solution are just in contact. 14 Four different regimes have been observed in inkjet drop generation behaviour as a combined function of concentration and molecular weight. 1,9,10 The first regime occurs at very low concentrations and/or molecular weight, where a long ligament is formed that simultaneously breaks up along its axis to form several satellite droplets. This regime can often be highly chaotic and irreproducible in nature, leading to poor print quality. The second regime occurs upon increasing concentra- tion or molecular weight when only a few satellites appear at the end of the ligament. Raising concentration or molecular weight further yields a single droplet without a ligament, regime 3. This regime provides optimum print quality. Finally at high concentration or molecular weight the polymer solu- tion becomes highly visco-elastic and the droplet does not detach and returns into the nozzle, regime 4. The concentra- tion and molecular weight range over which the four regimes are identified is highly dependent upon the nature of the polymer, its molecular weight, architecture and the thermo- dynamic quality of the solvent. It has been proposed that drop break-up behaviour is in part related to the strain hardening resulting from the presence of polymer at high strain rate. 8–10,12 The microrheological explanation for strain hardening is a sudden transition of the polymer chain from a coiled to a stretched state, which is accompanied by a strong increase of the hydrodynamic drag. The coil–stretch transition occurs for linear polymers at a critical Weissenberg number W e,crit = e crit t 1 , where t 1 denotes the longest relaxation time and e crit the critical elongation rate. 15,16 The longest polymer chain relaxation time is typically described by the Zimm non-free-draining relaxation time, lz. 1,17 For the inkjet process the strain rate at the nozzle tip is such that the critical Weissenberg number is exceeded for all a Organic Materials Innovation Centre, School of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL. E-mail: stephen.yeates@manchester.ac.uk.; Fax: +44 161 275 4273 b Eastman Chemical Company, European Technical Centre, Knowsley Industrial Park, Kirkby, Liverpool, UK L33 7UF PAPER www.rsc.org/materials | Journal of Materials Chemistry 4902 | J. Mater. Chem., 2007, 17, 4902–4907 This journal is ß The Royal Society of Chemistry 2007