No 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.

We study the slowing down of polymer dynamics in canonical nanocomposites made of strongly interacting mixtures of poly-2-vinyl pyridine and silica nanoparticles (radius R = 7 ± 2 nm) by means of low-field NMR relaxometry. We demonstrate that this technique enables the accurate quantification of the fraction of immobilized (irreversibly adsorbed) polymer on to the filler surface, in a manner that complements data from broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). We rationalize the slowing down using two previously developed approaches to model the immobilized layer. The first one is based on a glass-transition temperature (T g) gradient, while the second one assumes a single interfacial component with a distribution of relaxation times. While both models convincingly fit the NMR data providing us with robust and complementary conclusions, the former approach is found to fit more accurately previously published DSC experiments. Quantitatively, NMR results indicate that the segmental relaxation of the immobilized layer is ca. 1 decade slower than in the bulk polymer, while earlier BDS analyses were equivocal on this point, indicating either 1 or 2 decades. These discrepancies highlight the difficulties and potential model dependencies in quantifying the dynamics of the minority polymer fraction in the immobilized layer, necessitating a systematic multi-technique approach to properly characterize dynamical heterogeneities in polymer-based materials.

A detailed calorimetric study on an epoxy-based nanocomposite system was performed employing bisphenol A diglycidyl ether cured with diethylenetriamine as the polymer matrix and a taurine-modified MgAL layered double hydroxide as the nanofiller.