J. Non-Newtonian Fluid Mech., 80 (1999) 183–197 Measuring uniaxial extensional viscosity using a modified rotational rheometer J.M. Maia *, J.A. Covas, J.M. No ´ brega , T.F. Dias, F.E. Alves Department of Polymer Engineering , Uni 6ersity of Minho, 4800 Guima ra ˜ es, Portugal Received 21 November 1997; received in revised form 16 April 1998 Abstract Knowledge of the extensional behaviour of polymer melts is extremely important due to the industrial relevance of extens ion al flows in common pro ces sin g tec hniqu es and seq uen ces suc h as blo w mould ing , fil m blo win g, fibr e spinning, melt flow through extrusion dies and injection mould filling. One of the main problems both researchers and industrialists come across is the fact that, unlike shear flows, steady-state extensional flows are not easy to generate and maintain experimentally. This fact limits the extent to which one can characterise the materials and, therefore, the deg ree of optimisation of the pro ductiv e pro ces s. In this pap er, a modifi cation to a commercially availa ble controlled rate rotational rheometer is proposed in order to produce a cheap, easy to set-up, flexible extensional rhe omete r. This is bas ed on the wel l-k nown Mei ssner- typ e ext ens ion al rhe omete r and mak es use of the acc ura te velocity control and torque measurement possibilities of the rotational apparatus. In this case, the adaptation was per forme d on a TA Ins tru men ts Wei sse nberg Rhe ogo nio met er, but the idea is app lic abl e to most oth er similar devices. The feasibility of the modification will be discussed and confirmed, results being presented for two materials at diff erent tempera tures. Thes e incl ude the calcu latio n of tran sient uniaxia l exten sion al visco sity and a stud y of rupture conditions. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Modified rotational rheometer; Meissner; Transient uniaxial extensional viscosity; Deformation at rupture 1. Introduction Extensional flow is one of the major modes of deformation imposed in most industrial polymer processing sequences. This characteristic makes the understanding of the extensional behaviour of these systems mandatory for modelling and for optimisation purposes. According to Rides [1], this knowledge is mainly needed up to strains of 2, strain rates of 5 s −1 , and temperatures of 260°C. * Corresponding author. Fax: +351 53 5102 49. 0377-0257/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0377-0257(98)00086-X