Xiaoping Yang Shi-Qing Wang Adel Halasa H. Ishida Fast flow behavior of highly entangled monodisperse polymers 1. Interfacial stick-slip transition of polybutadiene melts Received: 2 April 1998 Accepted: 1 June 1998 Xiaoping Yang · Shi-Qing Wang ( ✉ ) H. Ishida Department of Macromolecular Science Case Western Reserve University Cleveland, Ohio 44106-7202, USA e-mail: sxw13@po.cwru.edu Adel Halasa Goodyear Research Center Akron, OH 44305, USA Abstract A systematic experimen- tal investigation is carried out to clarify the nature of a well-known capillary flow phenomenon in linear monodisperse polybutadienes (PBd). By varying the surface condition and the die diameter, it is alluded that a spurt-like stick-slip transition actually results from a breakdown of chain entanglement between ad- sorbed and next-layer unbound chains. In other words, the transition is not a manifestation of any consti- tutive properties, as previously as- serted by Vinogradov and co- workers (1984). The melt viscosity dependence of the transition ampli- tude agrees with a Navier-de Gen- nes type analysis of wall slip. A comparison between the capillary flow and dynamic shear behavior of the same monodisperse PBd reveals that the interfacial stick-slip transi- tion occurs at a stress level that is only a third of the plateau stress given by the elastic plateau modulus G N 0 =1.0 MPa at 40 8C. The molecu- lar weight independence of the criti- cal stress for the transition provides a striking contrast with the transi- tion characteristics observed in linear polyethylenes and suggests a different state of PBd chain adsorp- tion on steel surfaces. Key words Interfacial stick-slip transition – wall ship – spurt flow Introduction Polymer rheology in fast flow has been an area of ac- tive investigation by polymer physicists and chemical engineers. A central topic is whether a constitutive in- stability could take place in shear flow, where the flow rate would become double valued at a single stress val- ue. Presumably, a constitutive instability would occur at a constant shear rate involving coexistence of entangle- ment and disentanglement states and shear band forma- tion (McLeish and Ball, 1986; Cates et al., 1993). Un- der controlled stress, a bulk flow transition would corre- spondingly take place at a critical level whose magni- tude would increase with the molecular weight as strongly as the melt viscosity does. If such flow behav- ior would ever happen, it would occur at very high stresses, comparable in value to the plateau modulus G N 0 . Recent experiments have clearly demonstrated that an interfacial stick-slip transition takes place in capil- lary flow of linear polyethylenes at stresses significantly below G N 0 (Wang et al., 1996 a, b, 1997 a, b), suggesting an interfacial instability may precede any constitutive instability. On the other hand, there has been insuffi- cient experimental characterization of any constitutive instability in melt flow. It is clear that without separating an interfacial insta- bility from inherent properties, the question of whether and how a constitutive instability occurs in fast flow cannot be addressed. To differentiate between interfacial and constitutive flow behavior, we must select a system capable of displaying an interfacial instability as well as Rheol Acta 37:415–423 (1998) © Steinkopff Verlag 1998 ORIGINAL CONTRIBUTION RA 909