Dichroic film measurement using

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

The Herman function is equal to 1 when the chain axis is parallel to the film orientation, 0 when the system is randomly oriented, and 1/2 when the chain axis is perpendicular to the film orientation direction. This function can be calculated from measurements of the dichroic ratio by using [20]... 

Normal incidence transmission IRLD measurements are used to study thin films (typically 100 pm thickness and less, depending on the molar extinction coefficient of the bands) with in-plane uniaxial orientation. Two spectra are recorded sequentially with the radiation polarized parallel (p) and perpendicular (s) to the principal (machine) direction of the sample. The order parameter of the transition moment of the studied vibration is calculated from either the dichroic ratio (R — Ap/As) or the dichroic difference (AA = Ap—As) as ... 

Fig. 22(b) shows polarized visible absorption spectra of mixed LB films of MS-C2o binary and MS-C20-AL18 ternary systems prepared by using an aqueous subphase containing Cd2+ ions, where the ratio in the mixed system is [MS] [C20] [AL18] = l 2 x (x — 0 and 1) [77-84]. The thick and thin lines in Fig. 22(b) refer to the spectra, Atl and A , measured by linearly polarized light with the electric vector parallel and perpendicular to the dipping direction of the substrate, respectively. For x = 0, a sharp absorption peak is observed at 590 nm, which is red-shifted from the MS monomer peak at around 540 nm. The dichroic ratio R of the 590 nm band is R> 1, where R is defined as A /A . For x= 1.0, on the other hand, a sharp blue-shifted band with R< 1 appears at 505 nm. Fig. 22(c) refers to the cases of the corresponding binary and ternary systems fabricated under the subphase without the Cd2+ ions [84], Unlike the results in Fig. 22(b), remarkably red- and blue-shifted bands are not observed... 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

The necessary instrumentation for making dichroic measurements depends on the technique used. The most common method is to use a single polarizer and an ordinary infrared or ultraviolet spectrophotometer. The sample (thin film) is then placed in the sample beam preferably before the polarizer and generally with the stretch axis at 45° to the entrance slit of the monochromator. This inclination of the stretch axis is to minimize the effect of machine polarization. Such effects have been discussed by Makus and Shareliff (42) and Jones (28). The electric vector of the polarizer is then aligned either parallel or... 

Measurements f dichroic ratios at fundamental absorption frequencies usually require very thin specimens or films (typical thickness range 2 too. to 0-1 mm) in order to avoid complete absorption of the radiation. Such specimens require careful preparation using either melt... 

A better stretch ratio, and thus also a higher degree of orientation, can be obtained by casting a solution of P30T on a polyetylene (PE) foil, and stretching the structure as a whole. This method is also preferred, since films with a thickness suitable for UVA IS/NIR spectroscopy can be used. Fig. 3 shows the band at 1510 cm-i of a P30T/PE structure, which was stretched to a final stretch ratio 1 / 1q = 3. The measured dichroic ratio is D = 14, which gives... 

DICHROIC MEASUREMENTS OF POLYMER FILMS USING INFRARED SPECTROMETRY... 

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