Unjamming a Polymer Glass

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David Weitz Nature 323, 214-215 (2009).

Soft Matter Keywords

Polymer, Jamming

Summary

This paper is a perspective article on a report by Lee et al [1] where they probe the shear induced flow in glasses on very small scales. By using optical photobleaching of small dye molecules embedded in a lightly crosslinked PMMA slab, they can determine the fluidity of the polymer. With this technique, they find that the fluid like properties observed when the glass is sheared occur due to a different mechanism than what is seen when the polymer glass is melted.


Figure 1 - Shear flow. Schematics comparing shear-induced flow in a granular material (left) and a glassy polymer (right).


Results

The polymer is subjected to a uniaxial tension, i.e. is pulled apart by its two ends. Initially, when the strain is small, the structural relaxation rate increases slightly and can be described by the Eyring Model. However, as the strain and strain rate are increased, the relaxation rate of the probe molecules increases by several orders of magnitude, suggesting that the mobility has increased by up to a factor of 2000, and the polymer is flowing.


The data are consistent with the idea that deformation induces mobility that allows flow to occur.


Figure 2 - A strong correlation is seen between the strain applied (vertical axis) and mobility of the dye molecules (horizontal axis) [1].

Conclusions and Soft Matter Discussion

When a polymer such as polycarbonate is is stretched slightly (less than 1%) at room temperature, it responds like a very stiff, ideal spring. When the force is released, the material returns to its original state. At larger the polymer appears to flow and can be pulled without further increase of the applied force. If the force is released in this regime, the glass does not return to its original length but has acquired a “permanent set”. If polycarbonate is deformed beyond 130% of its original length, it typically fractures and breaks. In order to understand

Mobility plays a central role in current efforts to describe polymer glass deformation, yet before this work, there were no quantitative microscopic measurements of molecular mobility during deformation. The experiments described in the paper by Lee do establish that shear does induce melting of the glass, and that the resulting material has many features similar to that of a liquid.

References

[1] Hau-Nan Lee, et al. Direct Measurement of Molecular Mobility in Actively Deformed Polymer Glasses Science 323, 231 (2009);

--Cassidy 16:54, 16 September 2009 (UTC)


Unjamming a Polymer Glass (2nd Entry - Ben Yang)

Soft Matter Keywords

Melting, Glass, Shear Stress, Jamming

Summary

Under the jamming picture, there are 3 ways that glasses can melt. Dr. Weitz proffers the illustration of a bucket of sand. A bucket of dry sand acts like a solid; if you step inside the bucket, the sand will support your weight. The three ways that sand will start to flow are shear stress, temperature, and volume fraction. Shear stress flow can be illustrated by tipping the bucket; in this case, gravity provides a shear stress and the sand flows out of the bucket. In contrast, when you step onto the sand in the bucket, you are exerting a normal stress, which the sand can sustain. Temperature increase can be modeled by shaking the bucket of sand; in this case, each grain of sand is gaining some random movement, analogous to an increase in temperature. Volume fraction refers to the percentage of total volume that the solid particles occupy. For example, if we immerse the sand in enough water, then its volume fraction will decrease because it is occupying a smaller percentage of the total volume of the solution. In this case, once you add enough water, the sand grains will be far enough apart from each other and just flow like the water.

This perspective article discusses how Lee et al. in their article "Direct measurement of Molecular Mobility in Actively Deformed Glasses" have come up with a technique to measure the fluidity of glassy polymers. They inserted dye molecules into a polymer (PMMA) and used confocal fluorescent microscopy to track the properties of the material under stress. This research is important because it has helped confirm the conjecture that the jamming picture can be extended from grains to glasses, while simultaneously illustrating how flow resulting from stress has a different nature than flow from melting.

Relevance to Soft Matter

The mechanics of grains, and their transitions from solid to liquid have been widely studied, both in theory and in computer simulation. If the jamming picture applies sufficiently well to polymer glasses, then much of this knowledge can be carried over from grains to glasses, a system which is not as well understood. Understanding transitions is important in soft matter because often applications will involve manipulating or avoiding transitions. This research shows that it is possible to make a glass flow by applying stress, which could be useful; in addition, it shows that this flow will have different characteristics than flow caused by temperature increase. Therefore, applications could make use not only of the state change in the matter, but also the different types of flow possible. Furthermore, this research opens up the question of whether this optical technique could be extended to studying other types of soft matter materials.

References

Weitz, D.A. "Unjamming a Polymer Glass." Science, vol. 323, p. 214-215. (2009)

Lee, H., Paeng, K., Swallen, S.F., Ediger, M.D. "Direct Measurement of Molecular Mobility in Actively Deformed Polymer Glasses." Science, vol. 323, p. 214-215. (2009)