Forced wetting of polymer liquids: The role of chain entanglements on the contact-line friction.

Hypothesis: Despite the importance of wetting on the processing of polymers and key role of chain entanglements on the mechanical properties of polymer parts, the understanding of the role of chain entanglements during wetting is still rudimentary. It is hypothesized that by progressively changing the molecular weights (M_n) of polymer liquids, different relations between contact-line friction (ζ) and M_n would be obtained without (below the critical entanglement molecular weight, M_c) and with chain entanglements (above M_c).

Experiments: Forced wetting of polydimethylsiloxane (PDMS) liquids with different M_n around a polyethylene terephthalate (PET) fiber is studied. The dynamics is modelled by the molecular-kinetic theory (MKT) and the influence of chain entanglements on the relation between ζ and M_n is discussed.

Findings: Two regimes are indeed observed for the contact line friction versus molecular weight and are modelled by ζ ∼ M_n^δ with δ = 2.18 ± 0.27 for M_n < M_c and δ = 3.78 ± 0.51 for M_n > M_c. The solid-liquid interaction activation energy Δg_s^∗ is small ∼0.013 J/m² regardless of M_n, as it is mainly controlled by the interfacial interaction between the PDMS chains and the fiber. In contrast, the viscous interaction activation energy Δg_vis^∗ presents a significant increase when M_n reaches M_c, with Δg_vis^∗ surpassing Δg_s^∗ up to 21.6 %. It is proposed that the dominating role of Δg_vis^∗ regulates the relationship between ζ and M_n when M_n > M_c.