Molecular interactions arising in polyethylene-bentonite nanocomposites

Effect of pH Variations on the Yield Stress of Calcium Bentonite Slurry Treated with pH-Responsive Polymer

The pH-responsive polymers, such as polyacrylamide (PAM), show distinct conformational states according to the pH of their environmental groundwater. Therefore, the interactions between clay-polymer and polymer-water molecules, which determine the yield stress of bentonite-polymer composites, can be affected by the pH of groundwater. This study aims to evaluate the effect of pH variation on the yield stress of calcium bentonite treated with PAM. The yield stresses (τ_y) of untreated and PAM treated clays were measured with varying volume fractions of solid (VF = 10-23%) and under varying pH conditions (pH = 7.6-9.6). In addition, the zeta potential was measured for both untreated and treated clays to figure out the change in the surface charge of the mineral surface due to PAM treatment. The results of this study demonstrate that τ_y for treated clay is higher than that for untreated clay at a given VF, because van der Walls attraction dominates electrostatic repulsion in the case of treated clay. Due to the change in conformational states of PAM and the consequent change in surface charge that comes with varying pH, the pH-dependent change in τ_y of treated clay is significantly different from that of untreated clay.

Polyethylene Wax Modified by Organoclay Bentonite Used in the Lost-Wax Casting Process: Processing-Structure-Property Relationships

In this paper, the influence of bentonite (BNT) on rheological properties, linear shrinkage, hardness, and structure of polyethylene wax (PE-wax) used in the lost-wax casting process are studied. Experiments were conducted with PE-wax with different contents of bentonite (from 0.1 to 5 wt.%). The structural changes of modified polyethylene wax were evaluated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The addition of bentonite in a concentration range between 0 and 0.8 wt.% into PE-wax caused a decrease of the linear shrinkage and the hardness of PE-wax/BNT blends. Modified polyethylene wax containing greater than 1 wt.% of the filler showed an increase in viscosity. The DSC-based investigations confirmed good miscibility and a uniform structure of PE-wax with BNT. Moreover, an increase in melting temperature onset and its enthalpy observed for filler PE-wax material showed an influence of bentonite on the crystalline structure of the polymeric matrix. SEM observations of the fractured surfaces confirmed the homogeneous structure of blends with contents up to 0.8 wt.% BNT. In the case of higher filler concentrations, the presence of more numerous and large clusters of filler in the polymer matrix was observed due to the increase in the viscosity of the PE-wax/BNT melt composition during mixing. The PE-wax blend with 0.4 wt.% BNT gives better results of reduced linear shrinkage and lower hardness than unmodified material. Lastly, a new developed material (PE-wax/0.4 BNT) was subjected to technological tests, consisting of the preparation of the mold and manufacture of a high-quality aluminum cast, using the lost-wax method.

A versatile strategy for mechanically durable nanocomposite cryogels based on cationic (alkyl)methacrylates and hydrophilic bentonite via freezing (cryo)polymerization

A simple strategy for the preparation of organic-inorganic (alkyl)methacrylate-based nanocomposite gels was established. The preparation procedure was based on free-radical copolymerization of cationic monomer N,N-dimethylaminoethyl methacrylate (DMAEMA) in the presence of a low amount of ionic comonomer 2-acrylamido-2-methyl-propanosulfonic acid (AMPS), nanoclay bentonite (BENT) and diethyleneglycol dimethacrylate (DEGDMA) as a chemical crosslinker. The parameters of equilibrium volume swelling, initial swelling rate, diffusional exponent, and early- and late-time diffusion coefficients were evaluated from the swelling measurements in salt solutions. PDMAEMA retained its own characteristics regardless of the amounts of ionic comonomer AMPS and nanoclay bentonite. The prepared nanocomposite cryogels/hydrogels were found to be pH and temperature-responsive. The elastic moduli and compressibility of the nanocomposite NC/BENTm-Cgels were much higher than those of NC/BENTm-Hgels. The effective crosslink density distribution of the nanocomposite NC/BENTm-Hgels was evaluated from the elastic moduli data of the as-prepared state and the degree of crosslinking was described by a quadratic polynomial as a function of the clay concentration. At high clay content, the nanocomposite NC/BENTm gels exhibited a lower swelling degree, and thus a higher crosslinking density than the clay-free gels. Both NC/BENTm Hgels and Cgels exhibited obvious pH and temperature double sensitiveness; the equilibrium degree of swelling decreased as the solution's pH or the swelling temperature increased. The power law exponent values obtained from dynamic swelling in aqueous Na2SO4 solution indicated that the sorption mechanism of both the nanocomposite NC/BENTm-Hgel and NC/BENTm-Cgel samples was Fickian controlled. Overall, this study successfully establishes the thermodynamic relations between the physico-mechanical behavior and characteristic network parameters of (alkyl)methacrylate-based nanocomposite gels prepared at various BENT concentrations.