Wetting Dynamics of Liquid Polymers

"Everything flows and nothing abides". This quote from the Greek philosopher Heraclitus is the motto of a branch of science and technology called "dynamic wetting". To be specific, when a liquid is placed in contact with a solid substrate, the liquid front advances spontaneously towards equilibrium due to the out-of-balance Young’s force. As the liquid spreads, the dynamic contact angle relaxes to equilibrium state. In this framework, this doctoral dissertation discusses the physico-chemical aspects of the wetting dynamics of liquid polymers in contact with flat substrates and fibers.

Since the wetting behavior of molten drops of thermoplastic polymers is important in the processing of fiber-reinforced polymer composites and the ability of classical theories of dynamic wetting like the hydrodynamic approach (HD) and molecular-kinetic theory (MKT) to model molten polymers is unknown, we therefore investigated the spreading dynamics of polypropylene (PP), poly(vinylidene fluoride) (PVDF) and maleic anhydride-grafted polypropylene (MAPP) on flat glass substrates between 200°C and 260 °C. These polymers were chosen based on the fact that they have different physicochemical interactions with glass. The viscosities and surface tensions of these molten polymers were obtained by rheological measurements and the pendant drop method at different temperatures. Besides, the chemical interaction between MAPP and the glass substrate was validated by performing Fourier-Transform Infrared Spectroscopy-Attenuated Total Reflection experiments. In terms of spreading dynamics, it was evidenced that both HD and MKT showed excellent fitting agreements with the experimental data and the corresponding fitting parameters were meaningful, indicating that both models are relevant here. This triggered a question about the identification of the dominant energy dissipation channel for spreading dynamics of molten polymers on a flat substrate.

Next, a fiber substrate was used to uncover the dominant channel of dissipation for liquid polymers as the fiber geometry enhances the difference between the HD and MKT and therefore helps to distinguish between the dissipation regimes. Two types of polydimethylsiloxane (PDMS) with kinematic viscosities of 5 mm2/s (PDMS5) and 500 mm2/s (PDMS500) were selected as model liquids and a series of mixtures with different mass ratios were prepared. The dynamics of the capillary rise of PDMS mixtures on a fiber were studied, and the MKT and HD regimes were distinguished by their scaling laws. Besides, the MKT/HD transition of the PDMS mixtures was compared to the one of pure PDMS liquids. The MKT/HD transition of PDMS mixtures moved to a higher viscosity regime and we hypothesized that a surface segregation mechanism controlled this shift.

Finally, the transition interval between the MKT and HD regimes was further explored. Two types of PDMS liquids with kinematic viscosities of 20 mm2/s (PDMS20) and 50 mm2/s (PDMS50) at room temperature were selected. The capillary rise of these two liquids around a fiber was studied at different temperatures and the existence of a sharp transition between the MKT and HD regimes was demonstrated. Within this sharp transition region, the fiber roughness played an important role in the identification of the dominant channels of dissipation. Beyond this sharp transition region, the identification of the dominant channels of dissipation was not affected by the fiber roughness. A liquid rim ahead of the apparent contact line was observed and proposed to account for the viscosity dependence of the channels of dissipation.