Understanding the nanostructure and
nanomechanical properties of
surface layers of erucamide, in particular the molecular orientation
of the outermost layer, is important to its widespread use as a slip
additive in polymer materials. Extending our recent observations of
nanomorphologies of erucamide layers on a hydrophilic silica
substrate, here we evaluate its nanostructure on a more hydrophobic polypropylene surface. Atomic force microscopy
(AFM) imaging revealed the molecular packing, thickness, and surface
coverage of the erucamide layers, while peak force quantitative nanomechanical
mapping (QNM) showed that erucamide reduced the adhesive response
on polypropylene. Synchrotron X-ray reflectivity (XRR) was used to
probe the out-of-plane structure of the surface layers. Static contact
angle measurements further corroborated on the resulting wettability,
also demonstrating the efficacy of erucamide physisorption in facilitating
control over polypropylene surface wetting. The results show the formation
of erucamide monolayers, bilayers and multilayers, depending on the
concentration in the spin-cast solution. Correlation of AFM, XRR and
wettability results consistently points to the molecular orientation
in the outermost layer, i.e. with the erucamide tails pointing outward
for the surface nanostructures with different morphologies (i.e.,
bilayers and multilayers). Rare occurrence of monolayers with exposed
hydrophilic head groups were observed only at the lowest erucamide
concentration. Compared with our previous observations on the hydrophilic
surface, the erucamide surface coverage was much higher on the more
hydrophobic propylene surface at similar erucamide concentrations
in the spin-cast solution. Furthermore, the structure, molecular orientation
and nanomechanical properties of the spin-cast erucamide multilayers
atop polypropylene were also similar to those on industrially relevant
polypropylene fibers coated with erucamide via blooming. These findings shed light on the nanostructural features of the
erucamide surface layer underpinning its nanomechanical properties,
relevant to many applications in which erucamide is commonly used
as a slip additive.
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