Study of polymer nanocomposites with chemically heterogeneous interfaces

Abstract

In bilayer thin films, PEO was always used as the second polymer layer, mimicking the matrix polymer in the 3-dimensional bulk nanocomposites. Three different polymers were used as the first polymer layer (the layer “sandwiched” between PEO and silica substrate): poly(2–vinyl pyridine), P2VP; poly(methyl methacrylate), PMMA; and polystyrene, PS. These three polymers were chosen because they present a variation in: (i) interactions with silica and PEO, (ii) miscibility with PEO, and (iii) dynamic asymmetry against PEO. Thermal analysis results showed the presence of multiple glass transition temperatures indicating a complex interfacial structure and dynamics. The degradation behavior and the characteristic relaxation times were also found to be altered providing a clear evidence of changing properties at the interfaces due to confinement.

In a groundbreaking study, Akcora group has shown that PEO nanocomposites with dynamically asymmetric, heterogeneous interfaces present a unique and reversible thermal-stiffening behavior above the glass transition temperature of the adsorbed polymer (Senses, E., Isherwood, A. and Akcora, P., Reversible Thermal Stiffening in Polymer Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 14682–14689.). However, chemically heterogeneous interfaces can be fragile under severe shear fields that are common in continuous polymer processes. In the current study, polyethylene oxide based nanocomposites that are reinforced with poly(2–vinyl pyridine)- or polycarbonate-adsorbed silica nanoparticles were processed using a single-screw extruder to address the effect of processing on the structure and properties of these nanocomposites. Although the as-prepared nanocomposite samples showed decent nanoparticle dispersion, nanoparticle clusters formed upon extrusion.

Current models developed for traditional binary polymer nanocomposites (with homogeneous interfaces) are not sufficient to explain the complex nature of polymer nanocomposites with heterogeneous interfaces. The current work aims to take the first step in investigating the role of extrusion processing on the structure and properties of select polymer nanocomposites with heterogeneous interfaces.In addition, an attempt has been made to study polymer nanocomposites with dynamically asymmetric, heterogeneous interfaces in a 2-dimensional geometry in order to eliminate certain variables that are present in 3-dimensional, bulk nanocomposites (such as nanoparticle size distribution, nanoparticle agglomeration, interface thickness distribution, and confinement or particle-to-particle distance distribution) and enable better control of remaining variables (such as interface layer thickness and confinement distance). Thermal properties and dynamics of single and bilayer polymer thin films sandwiched between silica substrates are studied via Modulated Differential Scanning Calorimetry, Thermogravimetric Analysis, and Broadband Dielectric Spectroscopy.

These studies present a pathway for the characterization of dynamically asymmetric, chemically heterogeneous interfaces via MDSC, TGA, and Broadband Dielectric Spectroscopy; and thereby, the opportunity to understand the physics responsible for the unique thermal-stiffening behavior these polymer nanocomposites present. Based on the results obtained in the current study, a set of future studies focusing on the effect of confined layer thickness and polymer molecular weight is proposed with the goal of being able to design polymer nanocomposites with specific properties.