Structure, Mechanical Properties, and Dynamics of Polyethylenoxide/Nanoclay Nacre-Mimetic Nanocomposites

Abstract

Nacre-mimetic nanocomposites based on high fractions of synthetic high-aspect-ratio nanoclays in combination with polymers are continuously pushing boundaries for advanced material properties, such as high barrier against oxygen, extraordinary mechanical behavior, fire shielding, and glass-like transparency. Additionally, they provide interesting model systems to study polymers under nanoconfinement due to the well-defined layered nanocomposite arrangement. Although the general behavior in terms of forming such layered nanocomposite materials using evaporative self-assembly and controlling the nanoclay gallery spacing by the nanoclay/polymer ratio is understood, some combinations of polymer matrices and nanoclay reinforcement do not comply with the established models. Here, we demonstrate a thorough characterization and analysis of such an unusual polymer/nanoclay pair that falls outside of the general behavior. Poly(ethylene oxide) (PEO) and sodium fluorohectorite form nacre-mimetic, lamellar nanocomposites that are completely transparent and show high mechanical stiffness and high gas barrier, but there is only limited expansion of the nanoclay gallery spacing when adding increasing amounts of polymer. This behavior is maintained for molecular weights of PEO varied over four orders of magnitude and can be traced back to depletion forces. By careful investigation via X-ray diffraction and proton low-resolution solid-state NMR, we are able to quantify the amount of mobile and immobilized polymer species in between the nanoclay galleries and around proposed tactoid stacks embedded in a PEO matrix. We further elucidate the unusual confined polymer dynamics, indicating a relevant role of specific surface interactions.

Competition between nucleation and confinement in the crystallization of poly(ethylene glycol)/ large aspect ratio hectorite nanocomposites

“…Apparently, the PEG:Hec ratio applied does not perfectly meet the ratio requested for the hybrid material and consequently some Hec-only domains (Figure 3a, white arrow) are forced to segregate despite thermodynamics favouring the hybrid structure as recently suggested by Walther et al. 32 These Hec domains are, however, far thinner than the coherence length of the X-ray beam and therefore do not show up in the diffraction pattern. (ϕ= filler content).…”

Glass Transition Temperature Regulates Mechanical Performance in Nacre‐Mimetic Nanocomposites

“…Traditionally, commercial hydrophilic polymers like polyvinylalcohol (PVA), polyvinylamine (PVAm) or polyethylene oxide (PEO) were used, which however presently limits the understanding as their properties are not tailor‐made for understanding the overall mechanical behavior. [ 17,26,27 ] Detailed engineering of the polymer structure allowed to introduce new functionalities and dynamics within the internal structure of the nanocomposites through supramolecular bonds in dissociative networks. [ 21,28,29 ] Despite these advances, it is interesting to note that there are several fundamentally important issues that have not been addressed at all so far.…”

Recyclable and Light-Adaptive Vitrimer-Based Nacre-Mimetic Nanocomposites

“…While there have been experimental developments toward nacre-mimetics with sequential layer-by-layer techniques and freeze-casting, one of the most facile, scalable and versatile techniques to realize artificial brick-and-mortar architectures is the colloidal self-assembly route using core/shell nanoplatelets that form lamellar phases upon drying. , Our previous work introduced this concept, and we investigated in detail how platelet size, glass transition temperature, and cross-linking influence the material properties. − Other groups reported on graphene-based or MXene nacre-mimetic structures following similar evaporative self-assembly. ,− We also revealed that tailored dynamics of hydrogen-bonded polymers ( i.e. , dissociative dynamic covalent bonds ) can, in fact, be used to engineer sacrificial bonds and dynamic cross-links, thereby promoting combinations of stiffness and toughness beyond what covalent (nondynamic) cross-linking could offer. − Moreover, light was proven to be a useful tool to break supramolecular bonds and weaken the mechanical properties of the nanocomposites through an induced photothermal effect. , …”