Structure and Mechanical Properties

High- and low-density polyethylene (HDPE and LDPE), iso-tactic and atactic polypropylene (iPP and aPP), and polymer blends were investigated regarding the friction coefficient and the elastic modulus via SFM. Structural changes and the mechanical property changes around the glass transition temperature have been monitored during temperature runs. The enhanced ordering of the backbone correlates to the increased surface modulus. Additionally, the elastic modulus, the time-dependent relaxation process, and the friction properties were measured as a function of pressure. Loads between 1 and 1000 nN were applied. At low pressure the deformation of the polymer is elastic. With increasing pressure there is a phase transition to a plastic behavior attributed to a polymer alignment effect. Friction properties were investigated concerning the contact pressure and contact area, revealing an increased elastic modulus with increased density and crystallinity and a linear increase with contact pressure. Stretching of LDPE was shown to lead to surface roughening and alteration of surface mechanical properties. These experiments show that a material subject to complex mechanical stresses will change continuously its surface mechanical properties as the nature of the stress changes [102].

Polyethylene and the influence of low molecular weight additives that are added to prevent oxidation were investigated via friction versus load curves. For loads of 1000 nm, the lower friction of the polymer with additives gives way to friction properties of the different samples that are identical [102].

Polyurethane copolymers with hydrophobic and hydrophilic side chains were investigated regarding the mechanical properties changing with the hydrogen bonding between soft (SS) and hard segments (HS). The adhesion force showed a maximum at 57% HS correlated with the highest number of nonassociated urethane groups. The friction mechanisms were investigated as well, showing different mechanisms for the different compositions [102]. The surface structure of six different polycarbonate polyurethane copolymers was investigated with SFM in topography and phase modes. The stoichiometry of the reagents and the chemistry of the hard segment were changed, varying the contribution of diisocyanate, poly(hexyl, ethyl) carbonate, and butane diol. The 1,6-hexane diisocyanate showed a stronger phase separation and the highest values of mean square roughness (RMS) with 3.3 to 10.0 nm with regard to methylene bis(p-phenyl isocyanate) and methylene bis(p-cyclohexyl isocyanate) that have RMS values of 0.8 respectively 0.25 nm. These values have to be viewed with respect to the image processing of a third-order flatten. This is only obvious in this article as the image processing is usually not mentioned, and therefore the values of the other investigations must be examined closely as well. The polymers presented relatively stiff rodlike structures with dimensions of 12 nm in width and 170 nm in length [202].

Investigations on polyurethane and rubber were done with force indentation curves [28].

Differences between polystyrene (PS) and poly(methyl-methacrylate) (PMMA) could be determined by measuring the adhesion differences of the different polymers by PFM [95]. Polymer blends of poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) with isotactic and atactic polypropylenes were characterized observing the morphology according to different thermal treatments. It was possible to monitor microphase separations with tapping mode in the phase image [203]. Interfaces between PMMA and polystyrene (PS) could be identified with HM-LFM [84] as well as interfaces between poly(acrylonitrile-co-styrene) and polybutadiene respectively between polypropylene and poly(propylene-co-ethylene) [85].

Fractographic investigations looking at the surfaces after a fracture of dental composites consisting of silicon dioxide fillers in a matrix of dimethacrylate resins are presented in [107]. The pristine surfaces and the results of different preparation methods for poly(ethylene glycol) (PEGMA) grafted poly(tetrafluoro ethylene) (PTFE) surfaces were investigated with SFM. The PEGMA treated surfaces exhibited a larger RMS of 144 nm with regard to 129 nm RMS of the pristine PTFE [204]. Hexafluoroethane and tetraflu-oroethylene films produced by glow discharge plasma deposition show with SFM a pinhole free and smooth surface with a RMS roughness of 0.4 to 2 nm. Scanning in contact mode with moderate applied loads yields rippled films. At small scan sizes, and high velocities and load, the film disrupts. The surface modulus was estimated between 1.2 and 5.5 GPa [205]. Poly(vinylidene difluoride) (PVDF) and poly(vinylidene fluoride/hexafluoropropylene) [P(VDF-HFP)] polymers were radiation grafted with polystyrene (PS) yielding an increase in surface roughness upon irradiation and a smoothing of the valleys by further functionaliza-tion by chlorosulfonation and sulfonamidation [206].

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