Surface Modification of Nanoplatelets in Polymer Nanocomposites.

摘要

In polymer nanocomposites (PNCs), it is very important that the nanofiller (typically an inorganic) is well-dispersed in an organic polymer matrix. Optimal compatibility results in end-use products such as packaging materials with enhanced properties. In practice, however, this is challenging to achieve because "ideal" dispersions require the complete uniform separation, termed exfoliation, of the nanofiller within the matrix. As a result, different nanofillers including calcium niobate (CN) and montmorillonite (MMT) are first studied for their ability to exfoliate in aqueous suspensions by measuring the organic cation uptake. The cation exchange capacity, or CEC, determined that CN needs further work before introduction into a polymer matrix, while MMT is more suitable for directly making PNCs. As hypothetical solutions, both nanofillers are surface modified (after ion-exchange) and evaluated by characterization testing.;To improve compatibility, the covalent grafting of phenylphosphonate (PPA) onto exfoliated, protonated calcium niobate (HCN) was explored. PPA can be readily grafted onto the face surfaces of exfoliated HCN, which has reactive apical oxygen atoms. HCN can be fully exfoliated in aqueous solutions of tetrabutylammonium hydroxide (TBAOH), denoted as TBACN. The effect of reflux conditions on the dispersion state of TBACN suspensions was investigated, along with PPA grafting onto both non-exfoliated HCN and exfoliated TBACN dispersed in deionized (DI) water, TBA solution, and various alcohols. The characterization results confirm the grafting of PPA on HCN and TBACN, quantify the extent of PPA grafting, and identify various grafting modes (mono-, bi-, and tridentate). All of these aspects are found to be dependent on the layered materials' exfoliation state, suspension processing conditions, and solvent composition.;The factors that control the dispersion of exfoliated MMT in poly(vinyl alcohol) (PVOH) during solution blending and solvent evaporation were also studied in making polymer nanocomposite films. Dynamic light scattering (DLS) indicates that addition of dilute suspensions of fully exfoliated MMT platelets to aqueous PVOH solutions results in undesired particle aggregation and thus poor MMT dispersion in cast films. It is believed that PVOH bridging induces MMT platelet aggregation. To counteract bridging aggregation, the novel idea of pretreating the MMT surface with a small amount of compatible polymer prior to solution blending with PVOH was explored. It is hypothesized that "pretreating" the MMT platelet surfaces with adsorbed polymer in dilute suspensions will protect the platelets from bridging aggregation during solution blending and solvent evaporation. DLS shows that pretreated MMT platelets are less susceptible to aggregation during blending with PVOH solutions. Characterization results compare the crystalline structure, thermal properties, dynamic mechanical properties, gas permeability, and dissolution behavior of MMT/PVOH films incorporating untreated versus pretreated MMT.