Complex gels containing a hydrophobically modified polymer and phospholipid: component interactions, rheology and tribology

Polymers with phospholipids are widely used in personal care products due to their gelling and emulsifying properties. In this study we examine the underlying mechanisms through which these components interact using isothermal titration calorimetry (ITC) and their effects on rheological and tribological characteristics. Using a system containing a crosslinked hydrophobically modified polymer and phospholipids we find that the bulk rheology exhibits gel-like behavior with the elastic modulus increasing substantially upon phospholipid addition. We attribute this behavior to interactions between the hydrophobic moiety of the hydrophobically modified polymer and the phospholipids tails, resulting in a more interconnected network system, as evidenced from ITC measurements. In contrast, tribological behavior measured using a soft model contact consisting of polydimethylsiloxane (PDMS) show addition of phospholipids leads to lower friction coefficients at low entrainment speeds. Phospholipids are being adsorbed onto the PDMS surface, with the hydrated heads causing a decrease in the friction coefficients due to a hydration-lubrication mechanism. Results obtained for a system having an analogous polymer but without hydrophobes provide further credence to our hypotheses.