Incorporation of low-mass compound to alter the orientation birefringence in cellulose acetate propionate

Abstract A blend of cellulose acetate propionate (CAP) with bisphenoxyethanolfluorene (BPEF), a low-mass compound (LMC), was prepared using melt-mixing method. The effect of BPEF addition on the orientation birefringence of CAP films was analyzed and compared to the blend added with tricresyl phosphate (TCP), an LMC known to increase the orientation birefringence of cellulose esters. Contrary to TCP, BPEF addition is found to decrease the positive birefringence of CAP. Moreover, it is observed that stress-optical law is not applicable for the CAP/LMC blend. The different effect on the orientation birefringence shown by BPEF and TCP is due to the different polarizability anisotropy. The decrease in orientation birefringence indicates that the polarizability ellipsoid of BPEF molecules is aligned in a direction perpendicular to the stretching direction, as opposed to that of TCP molecules which is parallel to the stretching direction. This alignment is thought to be resulted from the so called nematic interaction, in which LMC molecules are forced to orient to the stretching direction by the alignment of polymer chains. Furthermore, it is found that the magnitude of orientation birefringence of BPEF is larger than that of TCP at the same stress value, i.e., the same degree of orientation of CAP molecules, despite having a smaller intrinsic birefringence. This suggests that the nematic interaction with CAP chains of BPEF is stronger than TCP.