Shi-Qing Wang

Exploring molecular origins of sharkskin, partial slip, and slope change in flow curves of linear low density polyethylene

This paper explores the molecular mechanism for sharkskin formation on extrudate of linear low density polyethylenes (LLDPE) and investigates the rheological origin of a characteristic curvature (i.e., a slope change) in the flow curve of LLDPE. Rheological measurements, performed at various temperatures from 160 to 240 °C with a controlled‐pressure capillary rheometer and a variety of dies, suggest that the slope change in the flow curve, interpreted by many as demonstrating wall slip in the die land, arises from a combination of interfacial slip and cohesive failure due to chain disentanglement, first initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains within a certain stress range below the critical stress for the global stick–slip transition, a partial slip flow cannot sustain itself and occurs only periodically. This time‐dependent molecular entanglement–disentanglement fluctuation produces the sharkskin like extrudate in the regime where ...

Molecular Transitions and Dynamics at Polymer / Wall Interfaces: Origins of Flow Instabilities and Wall Slip

This article reviews recent results on capillary melt flow anomalies. Long standing controversies and debates in this field are illustrated by summarizing previous results and clarified with an extensive discussion of the most recent results. Explicit molecular mechanisms for flow instabilities are presented in contrast to a background of 40 years’ continuous and far ranging research. New experiments show that the widely observed extrusion anomalies (including oscillating flow, discontinuous flow transition and sharkskin) of linear polyethylenes (LPE) originate from interfacial molecular transitions, which may or may not be stable depending the specific flow conditions. A global flow instability (commonly known as oscillating capillary flow) evidently arises from a time-dependent oscillation of the global hydrodynamic boundary condition (HBC) between no-slip and slip limits at the capillary die wall. Other convincing observations show that sharkskin originates from a local instability of HBC at the die exit wall. The global and local interfacial instabilities both originate from a reversible coil-stretch transition involving interfacial unbound chains that are entangled with the adsorbed chains. In other words, local and global stress oscillations result in the observed macroscopic sharkskin-like and bamboo-like extrudate distortions respectively. A second molecular mechanism for wall slip is also clearly identified, involving stress-induced chain desorption off low surface energy walls. An organic coating of capillary die walls produces massive chain desorption and a large magnitude wall slip at rather low stresses, whereas bare metallic and inorganic surfaces (e.g., steel, aluminum, and glass) usually retain sufficient chain adsorption and prevent catastrophic slip up to the critical stress for the coil-stretch transition. The intricate interfacial flow instabilities exhibited by LPE are also shared by other highly entangled melts such as polybutadienes. In contrast, monodisperse melts with high critical entanglement molecular weight (M e ) such as polystyrene of M w =106 show massive wall slip on low energy surfaces but no measurable interfacial stick-slip transition before reaching the plateau around 0.2 MPa. Tasks for future work include (i) direct molecular probe of melt chain adsorption and desorption processes at a melt/wall interface as a function of the surface condition, (ii) new theoretical studies of chain dynamics in an entangling melt/wall interfacial region as well as in bulk at high stresses, (iii) test of universality of the established physical laws governing melt/wall interfacial behavior and flow for all polymers, and (iv) development of tractable experimental and theoretical methods to study boundary discontinuities and stress singularities.

Stick-slip transition in capillary flow of linear polyethylene: 3. Surface conditions

Effects of surface topology and energy on the stick-slip transition were studied in capillary flow of highly entangled polyethylene (PE) melts. Surface roughness was shown to increase the critical stress of the stick-slip transition because of the increased resistance to interfacial disentanglement. Lowering the surface energy of a smooth die wall by treatment with a fluorocarbon completely eliminates the stick-slip transition and produces massive interfacial slip at PE/wall boundary down to a stress level of 0.05 MPa. On the other hand, considerable roughness on the same low energy surface can produce a stick hydrodynamic boundary condition and restore the stick-slip transition despite the weak PE/wall interfacial interactions. Additionally, a slip-slip transition was found in the die with a nearly non-adsorbing wall that appears to involve a secondary chain-debonding process.

Interfacial molecular instability mechanism for sharkskin phenomenon in capillary extrusion of linear polyethylenes

A comprehensive study of sharkskin behavior in linear polyethylene extrusion is carried out to explore its molecular origin. Experimental characteristics are analyzed as a function of temperature, applied stress, and die surface condition. The experimental data favor an interfacial molecular instability (IMI) mechanism for sharkskin formation over a noninterfacial continuum mechanical mechanism. The effect of a local cooling of the die exit is demonstrated to be predictable by the proposed IMI mechanism. The IMI mechanism states that sharkskin occurs because of a local conformational transition at the die exit wall where the adsorbed chains entrap a layer of interfacial chains. This layer oscillates between entanglement and disentanglement states due to a reversible coil↔stretch transition. The corresponding oscillation of the exit wall boundary condition leads to cycles of local stress relaxation and growth and to periodic perturbation of the extrudate swell in the form of sharkskinlike surface roughenin...

Exploring stress overshoot phenomenon upon startup deformation of entangled linear polymeric liquids

This work explores the picture associated with stress overshoot during sudden continual (i.e., startup) external deformation of entangled polymeric liquids and proposes a specific scaling form to depict the intermolecular interactions responsible for chain deformation. Following a previously proposed idea that the stress overshoot in startup deformation is a signature of yielding, we search for ingredients that should go into the description of the force imbalance at the yield point and show that the expression for the intermolecular locking force fiml, derived from the characteristics associated with the yield point, can be tested against experiment. New rate-switching experiments support the proposed formula for fiml.

From elastic deformation to terminal flow of a monodisperse entangled melt in uniaxial extension

Using a well-entangled monodisperse styrene-butadiene random-copolymer (SBR) melt as a model system, we illustrate generic features of uniaxial extension behavior that may be shared by all well-entangled thermoplastic and elastomeric materials. Depending on the imposed extensional rate, the same sample may behave like a viscous liquid or an elastic “solid.” Analogous to the recently revealed shear inhomogeneity, the SBR melt inevitably undergoes cohesive failure in the form of sample breakage whenever the Weissenberg number is much greater than unity, making it challenging to reach steady state. In the elastic deformation regime where the external deformation rate is faster than Rouse relaxation rate, the sample undergoes a finite amount of uniform stretching before yielding occurs in a period much shorter than the terminal relaxation time. Steady flow can be achieved only in the terminal regime where entangled chains utilize directed molecular diffusion to achieve rearrangement and enable uniform flow.

FT-IR IMAGING OF THE INTERFACE IN MULTICOMPONENT SYSTEMS USING OPTICAL EFFECTS INDUCED BY DIFFERENCES IN REFRACTIVE INDEX

For phase-separated multicomponent polymeric systems, characterization of the interface between the components is particularly challenging. We have observed an optical effect in the infrared that can be used to image the interface specifically. This method yields images of the interfaces based on the interfaces showing apparent absorption arising from changes in refractive index at frequencies far from the specific frequencies associated with the components of the mixture. This method has been applied to multicomponent samples of polymer-dispersed liquid crystals where the nature of the interface can be specifically altered by the application of an electric potential across the sample. Effects of this optical phenomenon on spectra from such multicomponent systems are discussed, and factors that complicate quantitative analysis of data from interfacial regions have been pointed out.

Steady state measurements in stress plateau region of entangled polymer solutions: Controlled-rate and controlled-stress modes

Despite decades of efforts, reliable measurements of nonlinear flow behavior of well-entangled polymers in continuous shear have been challenging to obtain. The present work attempts to accomplish three important tasks: (A) overcome this challenge by adopting a strategy of decoupling rheological measurements from the outer meniscus region in a cone-partitioned plate (C/PP) setup; (B) determine whether well-entangled solutions indeed undergo a flow transformation under creep that can be taken to phenomenologically define an entanglement-disentanglement transition (EDT); (C) provide the velocity profiles of such solutions undergoing either controlled-stress or controlled-rate shear by carrying out in situ particle-tracking velocimetric (PTV) measurements. Upon removing any influence of edge fracture and sample loss, we are able to reach steady state during continual shear and elucidate more reliably the nonlinear flow behavior of well entangled polymer solutions with little ambiguity. Three well-entangled s...

Shear banding or not in entangled DNA solutions depending on the level of entanglement

(Received 28 November 2007; final revision received 6 October 2008 Synopsis Entangled DNA solutions are ideal as a model system to examine nonlinear shear flow behavior. Even when the number of entanglements per chain, Z, is higher than 100, the solution is still soft enough with an elastic plateau modulus under 100 Pa and is thus amenable to experimental study by commercial rotational rheometry without ambiguity and uncertainty. We have investigated nonlinear flow behavior of three entangled DNA solutions with Z=24, 60, and 156, respectively, using a combination of particle-tracking velocimetric PTV and conventional rheometric measurements. We explore questions such as a whether shear banding also occurs in moderately entangled solutions, b whether creep results in development of nonlinear velocity profile, c whether shear banding produced in startup shear and creep persists at long times in steady state, and d whether these entangled solutions exhibit homogeneous shear at the upper end of the stress plateau region. We found that the first DNA solution Z=24 only shows transient weakly inhomogeneous shear and steady linear velocity profile. In the more entangled solutions Z=60 and 156, shear banding is observed in startup rate- and stress-controlled shear in the shear thinning regime. Shear homogeneity eventually returns at the upper end of the stress plateau shear

Wall slip and absence of interfacial flow instabilities in capillary flow of various polymer melts

We conduct a comprehensive experimental study of melt/wall interfacial slip behavior of various polymer melts in capillary flow. Polymers under study include two polystyrenes of different molecular weights (MW=280 000 and MW=2×106, respectively), two low-density polyethylenes (LDPE), poly(ethylene vinyl acetate) (EVA), and polypropylene (PP). The experimental results reveal an important finding that has greatly extended our previous knowledge of the roles of melt/wall interfacial interactions and molecular entanglements in dictating capillary melt flow behavior. None of the six polymers exhibits an interfacial stick–slip transition or shows any sign of wall slip in bare aluminum dies. Yet, all are found to display a sizable wall slip when the strength of polymer/surface adsorption is reduced by lowering the die wall surface energy with a fluoropolymer coating. The degree of wall slip as characterized by the Navier–de Gennes extrapolation length is demonstrated to be explicitly proportional to the melt vis...