Variation with Chain Length

The charge mobility in sexithiophene was originally estimated from the SCLC of a symmetric sandwich configuration (see section 5.3.1.2). However, the most widely used technique now is that of field-effect. The first FET based on an oligothiophene (sexithiophene) was reported in 1989 by Horowitz and coworkers [64], with a mobility of a few 10 cm s V . Since then, the performance of oUgothiophene FETs 

These results would indicate an ultimate value of a few 10 cm s V . The question of whether this limit can be overcome is still open. Recent measurements on sexithiophene single crystals gave a mobiUty of 0.075 cm s [75]. 

Though this figure is significantly higher than that reported for polycrystaUine films, it does not indicate the possibility of a real improvement of charge transport through an improvement of the crystallinity. 

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The PMMA-fc-PMPS-fe-PMMA triblock copolymers prepared by the macroinitiator approach using ATRP [60] were only characterized using differential scanning calorimetry. The glass transition temperature (T of PMPS is usually difficult to observe but within the copolymers it was clearly evident at 125-130°C. The T s of the PMMA blocks increased with block length in a manner consistent with the variation with chain length for homopolymers of PMMA and were also clearly visible by DSC. The presence of two T s provides strong evidence for microphase separation of the blocks. 

For the van der Waals component no such analytical theory exists. Aqvist and co-workers assumed that a similar linear treatment would work for these interactions but with a different empirical factor, to be determined from calibration experiments. There was some indirect evidence that this approach would be reasonable. For example, the experimental free energies of solvation for various hydrocarbons (e.g. n-alkanes) depend in an approximately linear fashion on the length of the carbon chain. In addition, the mean van der Waals solute-solvent energies from molecular dynamics simulations did show a linear variation with chain length (the slope of the line varying according to the solvent). 

Rossi, P.F., Heats of adsorption of aliphatic alcohols on a-alumina at 25-200°C Variations with chain length, Adsorpt. Sci. Technol., 15(1), 69-78 (1997). 

FIGURE 3.7 Variation of KP with chain length of sodium alkyl sulfates. 

FIGURE 3 Variation of limiting heat of sorption (-AH0) with chain length nfor homologous series of linear paraffins. 

Fig. 17. Effects of mixing on polymer properties variation of MWD with chain length n, for different values of the dimensionless mixing time, ij.

Variation in Chemical Shifts with Chain Length and Ring Size... 

The increase of AE with chain length must originate from the difference between the monomer lattice constant and the length of oligomer/polymer repeat unit. Anticipating proportionality between 5AE/5n and lattice mismatch as suggested by studies under pressure (see 5.2), the variation of AE with n can be calculated for arbitrary conversion X from the low conversion value (8AE/8n)x=o ... 

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