Polymer Dynamics, Interfaces and Rheology Group

Projects

[Polymer Processing behavior| Polymer dynamics in blends and solutions | Nonlinear Flow Behavior of Entangled Polymers | Rheological Studies of Filled Polymer Melts ]

Polymer Processing Behavior (Zhiyong Zhu)

 

A.     Extrudate Swell

Extrudate swell is a classic problem in polymer physics, directly relevant to polymer processing in industry.  The complexity associated with the phenomenon has often been overlooked and oversimplified.  In other words, extrudate swell is not well defined without specifying where and when a measurement is taken.  We have recently elucidated the role of exit in the extrudate formation process and explained why the measured extrudate swell is smaller at higher molecular weights under the extrusion condition of constant pressure.  Many questions still remain unanswered, for example:

What are the universal and specific features in die swell?  What are the important parameters to characterize the behavior of extrudate swell?

 How does the extrudate swell depend on the molecular weight distribution?   How to characterize the exit effect? 

B.     Sharkskin (Melt Fracture)

One of the most important tasks of both scientific and industrial significance in polymer processing is to elucidate the physical origin of a melt fracture phenomenon known as sharkskin.  Although the idea of associating the periodically distorted extrudate with an oscillating stick-slip boundary condition at the die exit is rather appealing since sharkskin just below the global stick-slip transition, there has hardly been any direct evidence for such a link.  Is sharkskin really due to rupture of the exiting melt surface?  What critical experiments can we do to unravel the molecular origin?  What do we mean by “melt rupture” for entangled polymers

C.     Rheological Characteristics of Biodegradable Polymers

 

Biodegradable polymers based on agricultural produces offer a promising and exciting future because they renewable.  We are carrying out rheological studies of biopolyesters that may replace petroleum-based synthetic polyolefins in a variety of applications.  Our objective is to predict rheological and processing behavior based on the molecular weight and its distribution and to match existing polyethylene resins with the new biodegradable polymers in terms of their flow properties.

Polymer Dynamics in Blends and Solutions (Shanfeng Wang)

 

The subject of polymer dynamics has remained a mainstream topic in polymer physics for over fifty years.  Efficient and successful polymer processing depends on our knowledge of polymer chain dynamics in solutions and blends.  Most applications of thermoplastics rely on entanglement to strengthen the polymeric materials.  The heuristic reptation theory developed by de Gennes, Doi and Edwards is extremely successful in describing entangled chain dynamics of monodisperse (narrow molecular weight distribution) systems. 

The next simplest case is a mixture of two different chain lengths, which is the first step toward a systematic treatment of polydispersity effect on entanglement dynamics.  Although the experimental findings regarding such mixtures have appeared since mid 1980s', a satisfactory theoretical formulation has not been available.  Experimentally, one finds that the short chains always speed up the overall relaxation process (associated with reptation of the long chains) at any finite concentrations, whereas one would theoretically expect such an effect to become negligible when short chains are sufficiently entangled and at high volume fractions of the long chains.  We have carried out rheological measurements using an advanced mechanical spectrometer to elucidate the essential physics that governs the perplexing experimental observations.  Some conceptual breakthrough has already been made to reconcile the well-known discrepancy between the theory and experiment.  The progress will significantly impact on our Solution Rheology Approach to rheological resolving component dynamics in polymer blends. 

In addition to rheological studies, we have been collaborating with Professor Ernst von Meerwall in our department to perform polymer diffusion studies using the pulse gradient spin echo NMR method.

 

Nonlinear Flow Behavior of Entangled Polymers (Prashant Tapadia and Amy Philips)

 

 Since constitutive models can only be worked out by prescribing a shear flow and examining the molecular response (deformation) to determine the resulting shear stress, most workers in the community are used to, experimentally speaking, thinking of rheological phenomena in terms of molecular deformation under a given shear rate.  Moreover, for entangled polymeric fluids, it is systematically assumed that flow phenomenon would be identical, independent of whether the flow is generated by applying a constant stress or a constant shear rate.  Because nobody has observed yield like behavior for entangled polymer solutions and melts, it has been well established and widely acclaimed that the steady state flow curve of stress vs. shear rate is monotonic. 

 

We have recognized that at stress levels comparable to the plateau modulus of entangled polymers the flow behavior may be different, depending on whether controlled stress or controlled shear rate is applied to the samples.  To challenge the conventional wisdom, we designed a model system to interrogate whether it is possible to induce an entanglement-disentanglement transition by applying a sufficiently high shear stress.  The system needs to be sufficiently entangled and yet soft with a very low plateau modulus, so that a commercial rheometer can produce an adequate range of stresses.  We have studied highly entangled polymer solutions at a concentration as low as 5 wt. % so that its plateau modulus is only 1500 Pa.  Our experiments reveal that entangled polymers behave differently, depending on whether the flow is produced by applying a constant stress or a constant shear rate.  Specifically, beyond a stress level comparable to the plateau modulus, entangled polymers were found to undergo a yield like bulk flow transition [Phys. Rev. Lett. 91, 198301 (2003)].  In the controlled rate mode, a stress plateau appears in steady state within a certain range of the applied shear rate.  The difference in the flow behavior indicates that there is no longer a one-to-one correspondence between shear stress and shear rate in steady state.  More importantly, it would suggest that the shear rate across the sample may not be uniform.  These findings challenge the widely used assumption that the shear rate is uniform in simple shear even in the stress plateau region, and therefore have far-reaching implications in terms of the search for a suitable constitutive description of entangled polymer flow that can be applied in numerical simulations.

 

Rheological Studies of Filled Polymer Melts

 

A.     Reinforcement Mechanism (Zhiyong)

 

The subject of rheological characteristics of filled (amorphous) polymer melts is a relatively old research topic, but extremely important to industrial applications.  Phenomenologically, one finds that the storage modulus G' of the composite materials can be much greater than that of the pure polymer matrix and the zero-shear viscosity h significantly higher than h0 of the neat polymer.  Quantitatively speaking, the filler enhancement of the viscoelasticity depends on the state of filler dispersion, filler size, filler-filler interactions and filler-polymer interactions.  Sharp increases occur presumably at the point of filler-filler percolation.  The onset of the percolation depends on many factors including the structure of the dispersing medium. The reinforcement mechanism is thought to have been well understood: filler-filler association or networking contributes to the enhanced dynamic elasticity of the neat polymer.  Recently, the work of Sternstein and Zhu [Macromolecules 35, 7262 (2002); Composites Sci. and Tech. 63, 1113 (2003)] seems to indicate that at filler loadings lower than the percolation threshold a different reinforcement mechanism operates: the presence of fillers and nano-scale spacing between them produces "trapped chain entanglements that alter the mobility and dynamic stiffness of the matrix chains" to boost the elastic response of the filled melt. 

We are designing and carrying out careful rheological investigations into a variety of filled polymer systems to interrogate the Sternstein mechanism and to more explicitly identify the molecular origin of the filler reinforcement.

 

B.     Payne Effect (Zhiyong)

 

In filled melts and crosslinked rubbers, a rheological phenomenon known as the Payne effect is ubiquitous, corresponding to an observed decrease of G' with increasing strain amplitude.  The conventional explanation is that the large applied strain causes the filler-filler networking to break down, resulting in lowered G'.  The recent work of Sternstein and Zhu claim that the Payne effect is due in part to the removal of the polymer-filler interactions, resulting in diminishing "trapped entanglement".  More recently, work from French group claimed that the Payne effect originates from the existence of a glass transition temperature gradient at the filler surfaces.

We believe the origin of the Payne effect is elsewhere at all loadings (the Payne can be observed well below the commonly assumed percolation threshold).  Our understanding will be closely related to our findings about the reinforcement mechanism.

C.     Rheology of Nano-clay Polymer Composites (Yu Zhong)

 

Rheological characterization is a most useful method to interrogate the state of dispersion or level of exfoliation in nano-clay based polymer composites.  Specifically, at a few weight percents the nano-clay particles would have negligible reinforcement effect on the matrix properties such as the storage modulus G' if they are not exfoliated to a significant extent.  On the other hand, upon exfoliation the nano-clay particles can form network like associations to produce a significant level of G'.  We have carried out rheological studies to confirm the validity of this rheological approach and to provide a host new set of experimental systems for which the reinforcement mechanism and Payne effect will be studied.