Episode 11: Polymer necking revisited

In this podcast episode, MRS Bulletin’s Laura Leay interviews assistant professor Christos Athanasiou and postdoctoral researcher Danqi Sun from Georgia Institute of Technology about their research on introducing a new material state through polymer necking. After applying different strain rates to poly(ethylene terephthalate) (PET) then testing the material at different orientations, they discovered changes in mechanical behavior. They attributed these behaviors to changes in the microstructure. This work was published in a recent issue of the Journal of the Mechanics and Physics of Solids.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay.  Polymers are widely used, recyclable materials but there are fundamental unknowns surrounding their properties. New curiosity-driven research from the Georgia Institute of Technology has focused on what happens when yielding occurs under uniaxial tension, and has led to a surprising discovery: After the material undergoes necking, its response under certain conditions is independent of strain rate. This behavior is markedly different from the virgin material, which is known to have a response that depends on strain rate.

CHRISTOS ATHANASIOU: We pull the material under uniaxial tension, and then normally when we see that the material yields, this means that basically it kind of fails. In our case we focused on this material – the yielded material – and then we characterized this material as a new phase. So it’s kind of like a cold-drawing process where we apply the force – we yielded the material – we created a new material state out of the same polymer.

LAURA LEAY: That was Assistant Professor Christos Athanasiou. Using poly(ethylene terephthalate) in a large load cell, different strain rates were applied. The necked part of the sample was then cut out and re-tested at differing orientations. Re-testing with the sample-oriented transverse to the original draw direction, the polymer exhibited reduced yield stress and secondary necking. In contrast, material re-tested along the original draw direction was found to have a yield stress more than double that of the virgin material and an increased elastic modulus. Lead author of the study, postdoctoral researcher Danqi Sun, explains further testing to link this behavior to changes in the microstructure.

DANQI SUN: Along the parallel direction, or the draw direction, this material is enhanced by more than two times. So we were very curious; how about the other direction and we found that this material is highly anisotropic. We wanted to see if this highly-oriented material state leads to its extraordinary mechanical behavior and the rate independence so we also did some materials characterization like DSC to see if the crystallization or the highly oriented microstructure leads to this mechanical behavior.

LAURA LEAY: The differential scanning calorimetry revealed that although the virgin material exhibited both a cold crystallization peak and a melting peak, the necked PET produced only a melting peak which indicated that crystallization had occurred as a result of being put under load. 

DANQI SUN: During the stable necking process, the polymer chains in the yielded material have aligned along the draw direction and this leads to an enhanced intermolecular interaction between the polymer chains. This interaction suppressed chain mobility so its rate-dependence and other viscoelastic behavior is suppressed. And also this material state leads to enhanced mechanical behavior along the draw direction.

LAURA LEAY: Putting the material under strain, which causes necking, leads to a new material state which could be used as new way of manufacturing polymeric materials with highly desirable properties.

CHRISTOS ATHANASIOU: The initial PET is rate-dependent – how fast you apply the load gives you a different response. But now the new material state is rate-independent which is pretty unique. Also it’s anisotropic and in one direction it has very high strength – high specific strength. And this strength can be used, for example, you can think of making composites and using this as a barrier film or you can think of using composite layers where you make this new material state then you lay materials one on top of the other in different orientations and then you can create a composite with very high strength.

LAURA LEAY: So far work has focused on conditions below the glass transition temperature where polymer chains have low mobility, requiring a large load to be applied. Future work could consider the effect of temperature to optimize processing of the polymer. This work was published in a recent issue of the Journal of the Mechanics and Physics of Solids. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.