Berkeley Lab investigates stronger polymer nanocomposites
The study with the US Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) developed an opto-mechanical sensing technique based on tetrapod quantum dots.
Fluorescent tetrapod quantum dots or tQDs serve as stress probes that allow measurements of polymer fiber tensile strength with limited impact on their mechanical properties.
Earlier this year, we covered a study saying bio-nanocomposites needed more improvement if they were to match conventional packaging.
Stronger polymer nanocomposites
Nanocrystals could lead the way to the future design of strong polymer nanocomposites, according to the researchers.
Polymer nanocomposites contain fillers of nanoparticles dispersed throughout the polymer matrix.
These materials have potential for biomedical and material applications but rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.
The research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium sulfide (CdS) arms.
The tQDs were incorporated into the polymer fibers via electrospinning in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers
Electrospinning method
Paul Alivisatos, Berkeley Lab director and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley, led the study.
“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20-percent by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos said.
“The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”
Current drawbacks
He added that understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites.
“All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness.”
Berkeley Lab researchers met the challenge of minimally impact the polymer fibers by combining semiconductor tQDs of CdSe/CdS.
When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were observed as a shift in the fluorescence of the tQDs even at low particle concentrations.
As particle concentrations were increased, a greater fluorescence shift per unit strain was observed so the tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way, said the researchers.
Andrew Olson, a member of Alivisatos’ research group, said: “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”
Another potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.
