Sample thickness, significance

Effect of Sample Thickness. Sample thickness significantly affects the radiation-induced expansion of unstressed samples (Figure 14). The equilibrium asymptotic deflection values vary approximately as the square of the sample thickness for the 20- and 30-mil samples. The deflection values for all thicknesses above 0.010 follow the same curve at short times (—0.2 minute), where diffusion out of the sample has probably not yet become significant. 

Self-absorption and saturation effects also lead to suppression of the analytical signal. In gamma or X-ray analysis, self-adsorption is a function of sample thickness. Significant self-absorption in samples analyzed by X-ray emission could result in failure to detect elements present as traces or indicate their concentration to be below the actual level. 

Trade literature can provide a wealth of information. Users should, however, bear in mind that suppliers will naturally wish to emphasise data in the best possible light. For example, if the Izod impact strength increases sharply with decrease in sample thickness, then results may be quoted for thinner section test pieces. Whilst the facts may be stated, the underlying significance may not be fully appreciated by the casual reader. 

It is important to stress that ATR absorbance is strongly affected by the sample/crystal contact. Quantitative results are thus difficult to obtain even if the contact is maintained during the sample rotation that is required to record all four polarized spectra. A reference band that does not show significant dichroism is thus most often used to normalize the polarized absorbances in order to obtain quantitative data. For instance, the 1,410 cm-1 band of PET has often been chosen for that purpose, not only for ATR studies but also for specular reflectance (see below) and even transmission studies when the sample thickness is not uniform. It was shown that an appropriate normalization is possible even if no such reference band is available, by using a combination of two bands with orthogonal dichroism [34]. When performing ATR experiments, one should also make certain that the applied pressure does not create artifacts by affecting the structure of the sample. 

Figure 14. Effect of sample thickness on expansion of polystyrene during irradiation in the absence of significant stress...

Surprisingly, there appear to be no significant differences in creep rate values with increasing sample thickness for polystyrene, although the scatter of this set of data is large. No completely satisfactory explanation can be offered at this time. 

The water-treated sample exhibits inflammation and increased thickness, while the water-GTP-treated sample has significantly less inflammation and thickness. 

At 125°C. The stabilities of 2.5, 25, 60, and 120-mil thick plaques were also obtained at 125°C. (Table II). The resins are arranged in the relative order of increasing stabilities. The relative order of stability depends on sample thickness. The relative stability of A increases significantly with increasing thickness of sample, while that of I increases with decreasing sample thickness. At 2.5-mil thickness, Resin A, as one might expect, is less stable than the LTHA resins. In thicker sections, however, A is more stable than several of the LTHA resins. At 120-mil... 

A crystal plane normal to one of the optic zixes should be selected, otherwise elliptic polarization may result, and, the appzirent dichroism may depend on sample thickness. With a suitable sample, the significance of the dichroism still must be examined with caution. Even for a characteristic vibration (such as an N—H or C==0 stretching mode), the measurement indicates only the direction of the transition moment, whereas what is usually desired is the direction of the vibrating bond. The transition moment and the bond may not be parallel because of crystalline perturbations or because of fnframolecular interactions with other parts of the molecule. A portion of the crystalline perturbations can be eliminated by the dilute mixed crystal technique [discussed in Section 3.2.2 (see 975)], but this type of experiment has been performed only for a few cases. 

Despite the extensive photoconductivity data, the nature of photoexcitations in poly(phenylenevinylene), PPV, and its soluble derivatives has remained controversial. In part, the controversy arises from the conflict between the results obtained with fast time-resolved photoconductivity and those obtained by the more familiar steady-state photoconductivity the latter indicate a strong T-dependence for the np product. Recent experiments have resolved the apparent conflict [203]. The idea is rather simple If the sample is sufficiently thin that the photocarriers can be swept out before a significant fraction fall into traps, the steady-state photocurrent will provide information similar to that obtained at short times by transient photoconductivity (in the latter, sample thickness is not important since pre-trapping and trap dominated transport are separated in the time domain). 

Due to the significantly lower (10-100-fold) absorptivities of overtone and combination bands compared to fundamental absorptions, the sample thickness for NIR transmission measurements must be several hundred micrometers. 

---