Structure and thermal molecular motion at surface of semi-crystalline isotactic polypropylene films

Wettability of a Poly(vinylidene fluoride) Surface by a Pure Good Solvent and a Good Solvent/Nonsolvent Mixture: All-Atom Molecular Dynamics Study

This study investigated the wettability of poly(vinylidene fluoride) (PVDF) surfaces by a good pure solvent and a good solvent/nonsolvent mixture based on all-atom molecular dynamics (MD) simulations. In particular, droplets of pure N-methyl-2-pyrrolidone (NMP) and of mixed NMP/water molecules were brought into contact with both crystalline and amorphous PVDF surfaces. The contact angles of the macroscopic droplets on the crystalline surface were higher and those on the amorphous surface were lower than the experimental values. As the PVDF sheet surface is a mixture of crystalline and amorphous phases, the experimental contact angles being between those on crystalline and amorphous surfaces is reasonable. On the crystalline surface, the decrease in the contact angle with increasing NMP concentration in the droplets can be explained by the increase in the NMP density near the solid-liquid interface. On the amorphous surface, however, the contact angle is strongly affected by the swelling of PVDF by the mixed droplets at high NMP concentrations. The solvation free energy of PVDF in NMP is greater than that in water, suggesting that this may be a driving force of the swelling of the amorphous PVDF. Furthermore, when the Cassie equation for mixed crystalline and amorphous surfaces was assumed, the calculated contact angle corresponded well with the experimental value.

Molecular Dynamics Study on Wettability of Poly(vinylidene fluoride) Crystalline and Amorphous Surfaces

The present study investigates the effect of microscopic structure on the wettability of poly(vinylidene fluoride) (PVDF) surfaces using all-atom molecular dynamics simulations of water droplets brought into contact with both crystal and amorphous PVDF surfaces. For each case, computations were performed using five different droplet diameters, and the corresponding water droplet contact angles θ were obtained. Using the fact that the cosine of these contact angles for both surfaces are inversely proportional to the radius of the droplet contact surface ( r_dr( Z₀)), the contact angle θ_∞ of the macroscopic water droplet was obtained by extrapolating cos θ to 1/ r_dr( Z₀) = 0. The estimated values of θ_∞ on the crystal and amorphous surfaces were 96° and 86°, respectively, showing that the amorphous surface is less hydrophobic than the crystal surface. The contact angle of the crystalline/amorphous mixed surface was estimated using the Cassie equation to be 91°. This value agrees well with experimental measurement of the water contact angle on the PVDF film. Furthermore, the interaction energy, interface structure, and electrostatic potential were analyzed to clarify the reason for the lower hydrophobicity of the amorphous surface. This surface interacts more favorably with water than the crystal surface. Such an interaction reduces the excess free energy (interfacial tension) at the PVDF and water interface and makes the amorphous surface less hydrophobic. The amorphous interfacial region contains more water molecules than the crystal one, and water molecules are oriented toward the PVDF. This interface structure makes water strongly interact with the PVDF.

Sensing in-plane nanomechanical surface and sub-surface properties of polymers: local shear stress as function of the indentation depth

Dynamic force microscopy (DFM) is an excellent tool for the study of the compositional and nanomechanical properties of polymers that exploits the flexural eigenmodes of a cantilever comprising a sharp tip vibrating perpendicular to the sample surface. However, the in-plane nanomechanical properties of a specimen cannot be detected by this technique. Here, a bimodal approach was developed where flexural and torsional eigenmodes are driven simultaneously. The corresponding vibrational amplitude and phase shift of the vertical tip motion were utilized for topographical feedback and out-of-plane dissipative interaction acquisition, respectively, whereas the frequency shift and the drive amplitude of the lateral tip motion mapped the in-plane conservative and dissipative interactions of two heterogeneous polymers: an elastomeric polypropylene (ePP) and a polystyrene-block-polybutadiene diblock copolymer (SB). The shear stress at different sub-surface levels revealed an amorphous cover layer as well as a "slipping" plane on the ePP crystallites. The imaging of SB supported by dynamic force spectroscopy experiments showed that SB exhibits considerably different in- and out-of-plane nanomechanical properties at certain areas due to the complex polymer conformation of this diblock copolymer accompanied by inter- and intramolecular interactions that give rise to its viscoelastic behavior.

Bimodal frequency-modulated atomic force microscopy with small cantilevers

Small cantilevers with ultra-high resonant frequencies (1-3 MHz) have paved the way for high-speed atomic force microscopy. However, their potential for multi-frequency atomic force microscopy is unexplored. Because small cantilevers have small spring constants but large resonant frequencies, they are well-suited for the characterisation of delicate specimens with high imaging rates. We demonstrate their imaging capabilities in a bimodal frequency modulation mode in constant excitation on semi-crystalline polypropylene. The first two flexural modes of the cantilever were simultaneously excited. The detected frequency shift of the first eigenmode was held constant for topographical feedback, whereas the second eigenmode frequency shift was used to map the local properties of the specimen. High-resolution images were acquired depicting crystalline lamellae of approximately 12 nm in width. Additionally, dynamic force curves revealed that the contrast originated from different interaction forces between the tip and the distinct polymer regions. The technique uses gentle forces during scanning and quantified the elastic moduli Eam = 300 MPa and Ecr = 600 MPa on amorphous and crystalline regions, respectively. Thus, multimode measurements with small cantilevers allow one to map material properties on the nanoscale at high resolutions and increase the force sensitivity compared with standard cantilevers.

Subsurface imaging of soft polymeric materials with nanoscale resolution

Nondestructive depth-resolved imaging of ∼20-nm-thick surface layers of soft polymeric materials is demonstrated using amplitude modulation atomic force microscopy (AM-AFM). From a map of amplitude-phase-distance curves, the tip indentation into the specimen is determined. This serves as a depth coordinate for reconstructing cross sections and volume images of the specimen's mechanical properties. Our method reveals subsurface structures which are not discernible using conventional AM-AFM. Results for surfaces of a block copolymer and a semicrystalline polymer are presented.