Surface Segregation of Low Surface Energy Polymeric Dangling Chains in a Cross-Linked Polymer Network Investigated by a Combined Experimental–Simulation Approach

Abstract

The surface properties of polymeric materials are generally determined by the chemical groups present at their surface. For low surface energy polymeric films the preferential location of the low surface energy chemical groups at the surface is crucial for the low-adhesion properties which are of high interest for various engineering fields. Controlling the surface segregation of such chemical groups would allow maintaining the materials properties at high performance level all through its service lifetime. In this work we used a combined experimental–simulation approach to study the surface segregation and the bulk distribution of low surface energy polymeric dangling chains, chemically bonded to a cross-linked poly(urethane) network. The surface properties of the cross-linked polymeric films, prepared with different experimental parameters, were investigated by contact angle (CA) measurements and X-ray photoelectron spectroscopy (XPS). A dissipative particle dynamics (DPD) method was used to model the distribution of the low surface energy dangling chains, at the surface and in the bulk of the cross-linked systems, equivalent to the ones prepared experimentally. The combined results show with excellent agreement the segregation of the low surface energy polymeric dangling chains toward the air–polymer interface. The influence of different experimental parameters, such as fluorine concentration and dangling chains molecular mobility, on the surface segregation is discussed. DPD simulations revealed further details of the polymeric films structure: the formation of a depletion zone beneath the top surface and the presence of highly dynamic clusters of the polymeric dangling chains in the bulk of the network, but not at its surface.

Self-replenishing ability of cross-linked low surface energy polymer films investigated by a complementary experimental-simulation approach

“…The presence of a depletion zone was already reported in our first study of the surface self-segregation which occurs during film formation. 27 From the F-beads profiles in Figure 1(b), it is also possible to observe that after damage the F-beads concentration remains at the average bulk value of the un-damaged film. This allows multiple healing events throughout the film by inflicting repeated damaging procedures (i.e., cutting at different depths of the simulation box) and allowing the system to recover.…”

“…The synthesis of the TMP-PCL x polyester precursors was made according to the method previously described. 27,28 The precursors synthesized with different degree of polymerization (DP) and respective characterization are summarized in Poly(ester urethane) films were prepared from a mixture of the PCL precursors prepared (TMP-PCL x ) and a triisocyanate cross-linker, using NMP as solvent (45% wt solid content). The molar ratio of NCO/OH was kept at 1.1 to ensure the full conversion of OH groups.…”

“…The self-replenishing ability decreases with increasing length of the polymeric segments in-between cross-links of the bulk network from ∼92 down to 75%. The caprolactone based systems, as used in this study, have been reported to have large CA hysteresis due to surface-rearrangements 27 or permanent damages on the network by extensive contact with water, e.g., by hydrolyses of the ester bonds. These aspects could be playing an important role here, since the final CA for the films made with the longest polymeric segment (DP = 48), reaches values below 90 • (i.e., hydrophilic) ( Figure 5(b)).…”

“…In a previous work 27 we reported a combined experimental-simulation study to investigate the surface-segregation occurring during the film formation (cross-linking) of these model low surface energy systems. A number of parameters were varied systematically in analogous experimental and simulation set-ups.…”

“…We showed that the combination of a significant fluorine concentration in the formulation and a polymeric spacer with a specific length allows reaching an optimum hydrophobicity upon film formation. 27 Using the dual approach we also gained additional insight into the dynamics and network structure of the films, such as the existence of a depletion zone beneath the top surface-segregated layer and the presence of "clusters" of the dangling chains in the bulk.…”