Designing Polymer Nanocomposites: Critical Role of the Interfacial Layer

Reference Speaker Authors
Alexei P Sokolov Sokolov, A.P.(Univeristy of Tennessee and oak Ridge National Laboratory); Polymer nanocomposites (PNC) play important role in many current technologies due to their light weight, high tunability of properties, and relatively low costs. It is now well recognized that interfacial layer controls macroscopic properties in PNCs. Polymer structure and dynamics in the interfacial layer are strongly affected by nanoparticles surface and polymer-nanoparticle interactions. In this plenary talk we overview recent studies on structure and dynamics of the interfacial layer in various polymeric nanocomposite materials. We overview broad array of experimental techniques and MD-simulations that provide detailed characterization of the interfacial layer at the nanometer scale. Dielectric relaxation studies and MD-simulations reveal a gradient in the interfacial layer dynamics, but no “glassy” or “dead“ layer [1-3]. Small angle X-ray scattering, dielectric spectroscopy and differential scanning calorimetry provide consistent estimates of the interfacial layer thickness in various PNCs [1,3-5]. The thickness of the layer increases upon cooling to Tg [3], and depends strongly on polymer rigidity [2,6], increasing from ~2nm in flexible polymers to ~5 nm in more rigid ones. At the same time, the increase in stregth of the polymer-nanoparticle interactions does not affect the thickness of the interfacial layer, but increases the slowing down of its dynamics [2]. Detailed studies revealed unexpected molecular weight dependence of PNCs properties ascribed to frustration in packing of long polymer chains in the interfacial region [4]. Analysis of mechanical properties using Brilloin light scattering and advanced scanning probe microscopy [5] revealed more than 2 times increase in polymer mechanical modulus in the interfacial layer. This huge increase was ascribed to stretching of the chain in the interfacial region. All these results clearly demonstrate the existence of the interfacial layer with much slower dynamics that propagates a few nm (~2-5 nm) into the polymer matrix, but no signs of any long range (larger than ~10nm) effects. Recent studies [7] also revealed unexpectedly large effects in nanocomposite properties caused by very small (~2nm) nanoparticles. In particular, these small nanoparticles lead to a very sharp temperature variations of viscosity. This effect has been ascribed to mobility of nanoparticles and relatively short chain-nanoparticle association time [7]. At the end, we present a general picture how microscopic parameters control the interfacial layer, and how by tuning the interfacial layer we can tune macroscopic properties of polymer nanocomposites. 1. A. Holt, et al., Macromolecules 47, 1837 (2014) 2. J.M.Y. Carrillo, et al., Macromolecules 48, 4207 (2015) 3. S. Cheng, et al., J. Chem. Phys. 143, 194704 (2015) 4. S. Cheng, et al., Phys. Rev. Lett 116, 038302 (2016) 5. S. Cheng, et al., Nano Letters 16, 3630 (2016) 6. S. Cheng, et al., Macromolecules (2017, in print). 7. S. Cheng, et al., ACS Nano 11, 752 (2017).
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