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Sung A Kim published a paper titled: “Hierarchical Structure in Semicrystalline Polymers Tethered to Nanospheres” in Macromolecules.

Wednesday, February 19, 2014

Sung A Kim, a Ph.D. candidate from Lynden Archer’s group, recently published a paper titled: “Hierarchical Structure in Semicrystalline Polymers Tethered to Nanospheres” in the ACS journal Macromolecules.

Understanding structure-property relationships of polymer-nanoparticle composites is among the most fundamental questions in polymer science and technology today. When two or more components are combined to form such hybrid materials, at least one must undergo changes in structures, properties and dynamics to accommodate the other. Great attention has been given to understanding changes the organic polymer species undergo in polymer-nanoparticle composites, in particular to understand how molecular confinement between and tethering to inorganic particles changes structure, thermal transition and relaxations. These efforts have led to a significant body of knowledge about how large influences the nanoparticles can exert on crystallization, and thus structure and properties of semicrystalline polymers.

In their recent paper, Kim and Archer report a new approach for studying structural and dynamic transitions of confined polymers where bulk measurements performed on self-suspended nanoparticle-polymer hybrids are used to infer chain behaviors under confinement on very small, Angstrom and nanometer length scales. The authors created SiO2-poly(ethylene glycol) (PEG) hybrids that take advantage of the screened enthalpic interactions with the particle surface to study chain conformation, crystallization, and reorganization in 3 dimensions by using X-ray diffraction, vibrational spectroscopy, and thermal measurements.

Significantly, they report that crystallization behaviors of nanoparticle-tethered PEG chains could be very different depending on the length scale on which the behavior is viewed. Below the size of one hybrid unit (~30nm), nanoparticle-tethered PEG chains are reported to form more stable conformations than free, untethered PEG, while tethered PEG is more amorphous above length scales of order one hybrid unit.

On Angstrom and nanometer length scales, FT-IR and XRD analysis show that PEG exhibits more dominant TTG conformations and helix unit cell structures, which are more energetically stable, compared to the case for free PEG. On tens of nanometers length scales, tethered PEG is further reported to form only extended chain crystallites. As a consequence, nanoparticle-tethered PEG has only one melting temperature (Red, Inset diagram) while free PEG has 3 different melting transitions (Gray, Inset diagram) due to 3 crystallites of extended, once-folded, and twice-folded. On the other hand, tethered PEG is more amorphous on larger length scales, presumably as a result of the difficulty fitting the nanoparticle anchors into the PEG crystal lattice.

Because of PEG’s unique roles as a lithium-ion conducting polymer and in biomedical systems, this fundamental study on its structure and crystallization is expected to impact commercial practice in multiple areas, including electrolytes for Lithium batteries, gas and small molecule separation films for food and medicine packaging, and for sterically stabilized carriers for drugs and therapeutics delivery. 

Reference: Sung A Kim and Lynden A. Archer. “Hierarchical Structure in Semicrystalline Polymers Tethered to Nanospheres” Macromolecules 47, 687?694 (2014). DOI: 10.1021/ma4019922

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