Effect of the structure of the polymer matrix on the terthiophene chromophore dispersion in dichroic polyethelene films

Abstract: In the frame of an ongoing research project dealing with linear optical polarizers produced by dispersing terthiophene derivatives in ultra-high-molecular-weight polyethylene (UHMWPE) and successive orientation by uniaxial stretching of the derived films, the present paper describes the use of linear-low-density polyethylene (LLDPE) or high-density polyethylene (HDPE) as an alternative polymer matrix in relation also to the possibility of taking different preparation routes. The study was performed with the easily prepared 5,5"-bis-thiooctadecyl-2,2':5',2"-terthiophene as the chromophore which was blended in a Brabender mixer at 180degreesC and concentration 0.2-2% w/w with LLDPE or HDPE in the bulk. The morphology and optical properties of the films obtained by compression moulding were compared with those of a similar film in UHMWPE prepared by solution casting. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) analysis of the unoriented films of the former type were also performed. These data suggest the improved dispersion of the chromophore in case of the films obtained by melt processing. After uniaxial stretching up to 14-30 times the original length, molecular orientation was evaluated in different cases by measuring the dichroic ratio [R = A(parallel to)/A(perpendicular to)] and the order parameter [S = (R - 1)/ R + 2)]. Also the polarizer performance was detected by comparing the dependence of polarizing efficiency on single-piece transmittance of the different samples