Atomic force microscopies were used to quantitatively study and map the surface elastic properties of polymers and polypropylene/ethylene-propylene copolymer blends (PP/EP). Force curve and force modulation measurements were realized on polymers with moduli ranging from 10 to 3000 MPa. Both types of measurements enable us to classify polymers as a function of their rigidity. For rigid polymers, the results are in quantitative agreement with the predictions of a simple elastic model. The influence of a thin polymer layer on the surface elastic properties was also investigated. Force modulation responses are influenced by the subsurface elastic properties down to depths that may reach hundreds of nanometres. Force modulation microscopy was performed on the surface of physical blend compression-moulded plates and of 'reactor blend' injection-moulded plates. Images reveal soft regions embedded in a rigid matrix. For the physical blend, the force modulation measurements indicate that the rigidity on the EP nodules is close to that measured on pure EP, suggesting that the nodules are present at the outermost surface. Conversely, for the 'reactor blend', the EP nodules have an intermediary rigidity between those measured on bulk EP nodules and on pure PP, suggesting that EP nodules are under a skin of PP.
The contact angle of water was measured with the
Wilhelmy plate method on apolar polymers: soft
ethylene−propylene copolymer (EP), rigid polypropylene (PP), and
their blends PP/EP (80 wt % PP) and
EP/PP (20 wt % PP). Upon liquid displacement, the contact angles
of the soft materials, EP and EP/PP,
depended on viscoelastic energy dissipation due to solid deformation.
This was observed in different
situations: wetting and dewetting, liquid displacement due to the
relative motion of the plain liquid with
respect to the solid, and meniscus relaxation when that motion was
stopped. The wetting hysteresis of
polymer blends may be understood by considering viscoelastic energy
dissipation and the static contact
angles. The former depends on the elastic modulus while the latter
is determined by the more hydrophobic
phase and the less hydrophobic phase, for advancing and receding
contact angles, respectively.
Polypropylene/ethylene-propylene copolymer blends (PP/EP) were characterized by time-of-flight (ToF). SIMS, by dynamic wetting and by atomic force microscopy and force modulation spectroscopy (AFM-FMM).In a first step, model systems of compression-moulded PP/EP physical blends were prepared. With these materials, ToF-SIMS provides linear relationships between the selected peak and the EP content in the material. This indicates that both phases are present at the surface in the same concentrations as in the bulk. However, the EP signal is weak. In dynamic wetting measurements with water, for each tested blend composition the advancing contact angle corresponds to that of PP and the receding contact angle corresponds to that of EP. This method is thus suitable for detecting the presence of the two phases at the surface even for low concentrations of one consituent. The force modulation image gives elastic contrasts between EP and PP and is thus able to reveal the surface morphology. In a second step, these methods were used to characterize injection-moulded PP/EP reactor blends. Both AFM-FMM and ToF-SIMS suggest that a PP skin covers EP nodules at the surface. This is confirmed by electron microscopy examinations of cross-sections. This surface phase segregation leads to the presence of PP in the first 100 nm of the surface and could be explained by preferential wetting of the mould by the phase with the lower viscosity, PP in this case. However, dynamic wetting reveals that the PP skin is not complete. Thus, a low EP concentration may be found at the surface, in the form either of EP nodules or of EP molecules having migrated through the PP skin.
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