Proper force calibration is a critical step in atomic and lateral force microscopies ͑AFM/LFM͒. The recently published torsional Sader method ͓C. P. Green et al., Rev. Sci. Instrum. 75, 1988 facilitates the calculation of torsional spring constants of rectangular AFM cantilevers by eliminating the need to obtain information or make assumptions regarding the cantilever's material properties and thickness, both of which are difficult to measure. Complete force calibration of the lateral signal in LFM requires measurement of the lateral signal deflection sensitivity as well. In this article, we introduce a complete lateral force calibration procedure that employs the torsional Sader method and does not require making contact between the tip and any sample. In this method, a colloidal sphere is attached to a "test" cantilever of the same width, but different length and material as the "target" cantilever of interest. The lateral signal sensitivity is calibrated by loading the colloidal sphere laterally against a vertical sidewall. The signal sensitivity for the target cantilever is then corrected for the tip length, total signal strength, and in-plane bending of the cantilevers. We discuss the advantages and disadvantages of this approach in comparison with the other established lateral force calibration techniques, and make a direct comparison with the "wedge" calibration method. The methods agree to within 5%. The propagation of errors is explicitly considered for both methods and the sources of disagreement discussed. Finally, we show that the lateral signal sensitivity is substantially reduced when the laser spot is not centered on the detector.

Photoluminescent polarizers that comprise uniaxially oriented photoluminescent species which absorb and emit light in highly linearly polarized fashion, can efficiently combine the polarization of light and the generation of bright colors. We here report the preparation and characterization of such polarizers by simple melt-processing and solid-state deformation of blends of a photoluminescent guest and a thermoplastic matrix polymer. The orientation behavior of a poly(2,5-dialkoxy-p-phenyleneethynylene) derivative (EHO-OPPE), 1,4-bis(phenylethynyl )benzene, and 1,4-bis(4-dodecyloxyphenylethynyl )benzene was systematically compared in different polyethylene grades. Experiments suggest that if phase-separation between the photoluminescent guest and the matrix polymer is reduced during the preparation of the pristine (i.e. unstretched) blend films, photoluminescent polarizers can be produced which exhibit unusually high dichroic properties at minimal draw ratios. In connection with this finding, an optimized, melt-processed blend based on 1,4-bis(4-dodecyloxyphenylethynyl )benzene and linear low-density polyethylene was developed that allows efficient manufacturing of photoluminescent polarizers which at draw ratios of only 10 exhibit dichroic ratios exceeding 50.

A series of new dichroic, photoluminescent alkoxy-substituted bis(phenylethynyl)benzene derivatives 1a-e, have been synthesized and characterized. All dyes are highly emissive and show bright-blue photoluminescence with quantum yields in solution ranging from 0.63 to 0.72. Oriented blend films comprising these dyes were prepared with ultrahigh-molecularweight polyethylene (UHMW PE) as the matrix material by solution-casting, and with linear low-density polyethylene (LLDPE) by melt-processing as well as guest diffusion. Despite the introduction of alkoxy substituents, not all dyes are compatible with UHMW PE and large-scale phase segregation was observed during the preparation of a number of UHMW PE blend films. In the case of the latter, films with relatively good dichroic properties are only obtained if the melting point of the dye is lower than or equal to the deformation temperature. On the other hand, melt-processing of 1a-e with LLDPE produces homogeneous blends that, after tensile deformation, give rise to highly dichroic photoluminescent films. In this case, the melting temperature of the dye was found to be less relevant for the orientation process. In view of a future application of these PL blends as photoluminescent polarizer, melt-processed blends based on octyloxy-and dodecyloxy-substituted bis(phenylethynyl)benzene derivatives (1c,d) and LLDPE embody the optimal combination of good accessibility, excellent luminescence characteristics and high polarization of the emitted light.

No abstract