x =V t

H = x/H

Step shear

Yielding of a strained chain entanglement network through disentanglement

when f retract> k B

T/l ent

disentanglement (1)

disentanglement (2)when f retract> k B

T/l ent

disentanglement (1)

disentanglement (2)

J. Chem. Phy. 127, xxxx (2007)

when f retract> k B

T/l ent

disentanglement (1)

disentanglement (2)when f retract> k B

T/l ent

disentanglement (1)

disentanglement (2)

J. Chem. Phy. 127, xxxx (2007)

01234567

-0.15-0.1-0.05 0 0.05 0.1 0.15

Sample internal recoil upon step sheart1=7.5 s

t1=14.5 s

t2= 28 s

t3= 34 s

t4= 37.5 s

H (

0.1

mm

)

V (0.1mm/s)

V=0.5 mm/s, H=0.7 mm, x=1.5 mm

Initial positions after shear at t=0

Are Polymeric liquids fragile solids?SGER: Re-examining the basic description of polymer entanglement in flow

Shi-Qing Wang, University of Akron, DMR 0603951

Among other activities, classical step shear (Fig. 1a) and extension experiments have been carried out to learn about the strength of polymer entanglement network. To one's great surprise, a strained polymer yields plastically as evidenced by internal macroscopic motions (Fig. 1b) detected by PTV (Fig. 4) after step shear and necking after uniaxial extension (Fig. 2). The phenomena indicate the origin of the cohesive breakdown of the chain entanglement network is due to elastic retraction force associated with the chain deformation, which also occurs during continuous flow deformation. Specifically, a chain disentanglement criterion can be proposed as shown in Fig. 3. See a list of publications below for details, where this grant support was acknowledged.

Fig. 1

a

b

Publication list1. Phys. Rev. Lett. 97, 187801.2. J. Rheol. 51, 217.3. Feature article, Macromol.

Mater. Engr. 292, 15.4. J. Chem. Phys., 127, xxxx.5. Phys. Rev. Lett. submitted.6-9. Four manuscripts submitted

to J. Rheol.10-11. Two manuscripts submitted to Macromolecules.

Fig. 3

Step uniaxial extension on a cylindrical filament

D0/D =L/L0

D0 ~ 1 mm

D

necking

Fig. 2

Laser

Objective lens

Glass cover slip

Fig. 4 Particle Tracking Velocimetry

0 %

110 %

t=20 s110 %

Over 20 movie clips can be viewed at http://www3.uakron.edu/rheology/

t=0 s

• Recent experimental discoveries reveal that molten plastics (a state encountered during processing, e.g., making of grocery bags) deform unevenly instead of flowing homogeneously, namely, the materials undergo "cohesive" breakdown before forced to flow.

• The PTV observations cause us to re-examine the experimental and theoretical foundations of polymer rheology: the elastic entanglement network is more fragile than commonly perceived.

• These results have broad implications on how to better describe polymer processing behavior with theoretical models as well as computer-based numerical simulations

• A more realistic understanding of the nonlinear viscoelastic behavior is expected to eventually result in more efficient polymer processing and better understanding of structure-property relationship.

Significant results can be summarized as follows

Broad impact on education and society

Are Polymeric liquids fragile solids?SGER: Re-examining the basic description of polymer entanglement in flow

Shi-Qing Wang, University of Akron, DMR 0603951

• Two graduate students (Pouyan Boukany and Sham Ravindranath) have been supported by this grant.

• Pouyan and Sham have been developing research skills that combine theoretical insight with experimental design.

• High school and college students take tour of PI's lab routinely where visually impressive results attract their attention to science.

• Great societal impact can be derived from the potential economical benefits that are anticipated in the long run when the new understanding help make better polymer products faster.

• The resulting knowledge will also alter the conventional content of the graduate curriculum concerning polymer physics.