Tuning 3D refractive indices via constrained uniaxial stretch in cellulose triacetate films plasticized with triethyl citrate.

Tuning 3D refractive indices of polymers is urgently needed for optical films, but it is quite challenging. Here, we proposed a simple constrained uniaxial stretch method that successfully tuned the 3D refractive indices in cellulose triacetate (TAC) films plasticized with triethyl citrate (TEC). Our results suggest that, under constrained uniaxial stretch, the main chains and side groups prefer to orientate in the stretch direction and the constrained direction, respectively. Such a unique chain arrangement differentiates the refractive indices in three directions of the film. The branched small molecule TEC is also crucial for tuning refractive indices, which promotes chain activity and enhances the chain orientation under stretching, leading to a considerable change in refractive indices before samples fracture. The polymer film we fabricated possesses a direction-dependent optical performance, where the refractive index in the film thickness direction is between that of the stretch direction and constrained direction. This work provides a fundamental understanding on the chain structure and optical performance of polymer films. The constrained uniaxial stretch method, in general, should also be applicable to tuning other 3D physical properties through tuning the direction-dependent orientation of polymers.

Regulation of orientation birefringence for cellulose acetate film: The role of crystallization and orientation.

Cellulose acetate (CA) based films are widely used in liquid crystal displays due to their outstanding transparency, and a certain orientation birefringence of CA films is required when they are used as retardation films. The regulation of orientation birefringence is usually from the perspective of stretch-induced orientation, while the effects of crystallization behaviors of CA films remain obscure. In this study, the roles of crystallization and orientation on the orientation birefringence of CA films were elucidated. For cellulose diacetate films, the orientation birefringence is dominated by the orientation degree. In comparison, apart from the orientation degree, crystallinity is another key variable to regulate the orientation birefringence of cellulose triacetate and plasticized cellulose triacetate films, originating from the birefringence heterogeneity of the crystalline and amorphous phases. These results provide valuable guidelines for the production of CA-based optical films with excellent optical performance.

Effects of shear on epitaxial crystallization of poly(ε-caprolactone) on reduced graphene oxide.

Epitaxial crystallization of poly(ε-caprolactone) (PCL) on reduced graphene oxide (RGO) was investigated by shearing at different shear rates of 3 s-1 and 75 s-1 and different shear temperatures of 65 °C, 70 °C and 75 °C, respectively. Two dimensional wide angle X-ray diffraction (2D WAXD) results show that the crystallinity and the orientation degrees of the (110) plane of PCL/RGO nanocomposites with shear are higher than those without shear, but the imposed shear field has no obvious effect on the crystal structure of the PCL matrix. Two dimensional small angle X-ray scattering (2D SAXS) results suggest that the imposed shear field makes PCL chains epitaxially crystallize on RGO surfaces to form thicker lamellae. Thereby the melt points of PCL/RGO nanocomposites with shear are higher than that without shear from the differential scanning calorimetry (DSC) results. These results indicate that the imposed shear field can enhance the orientation of the PCL matrix, and promote epitaxial crystallization of PCL chains on RGO surfaces. Higher shear temperature is the requirement for PCL chains to epitaxially crystallize well on RGO at low shear rate, although it is not required for the samples at high shear rate.