Biaxial Orientation Parameters

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

Compared with PET, PEN has five times more radiation resistance in air, four times more in O2 and ten times more resistance in vacuum under continuous-use temperature [10]. Cakmak and co-workers calculated the refractive index of PEN this parameter is highest (nc = 1.908) along the chain axis and lowest (nn = 1.36) normal to the naphthalene ring. Biaxially oriented PEN film has a... 

Table IV gives the essential parameters which were involved in successful biaxial orientation of this copolymer sample. The most im-...

Keywords extrusion die, die design, biaxially-oriented PP (BOPP), heat sealing, manifold, coextrusion, extrusion parameter, process control, flow. 

A concern for these simultaneous biaxial orientation processes relates to the clips separating from each other. As the clips separate in the oven, the edge of the web between the clips tends to bow in. This can result in lower yields. At the very edge of the web, the film is oriented purely in the machine direction. The orientation in the film changes in going from the edge toward the centerline. The breadth of this transition region can be controlled with process parameters and material properties. 

A particularly useful aspect of dichroic measurements is the chance to probe orientational order using more than one electronic transition in a molecule. Thus optical order parameters can be determined for different absorption bands, and if the transition moment directions are known, it is possible to determine both order parameters 5 and D. If it is assumed that there is a relationship between the uniaxial and biaxial order parameters, as given in Fig. 5 of Sec. 1 in this chapter, then it is possible to obtain both order parameters from the polarized spectra from a single absorption band [25]. This method has been applied [26] to the determination of order parameters of rigid aromatic probes, such as azulene, phenan-threne, and anthracene and related compounds. Dichroism measurements on impurity molecules in liquid crystal solvents have also been used [27, 28] to study inter-molecular interactions, and their influence on electronic absorption bands. Polarization effects of the type described above for sim-... 

A large fraction of the polymers that are commercially produced are fabricated into films. Films are produced by stretching the polymer along orthogonal axes, machine direction (MD), and in the transverse direction (TD), resulting in biaxial orientation. XRD is often used to characterize this biaxial orientation. There have been a few attempts to systematically characterize this biaxial orientation. The one proposed by White and Spruiell [69] can be readily used in practice. In the previous section, it was shown that uniaxial orientation can be described by one parameter, the degree of orientation of the chain axis with respect to the fiber axis. 

Similarly, it can be shown that a minimal description of biaxial orientation requires at least two parameters shown below ... 

The biaxial order parameter A represents the minor director and defines the degree of orientation along an axis that is perpendicular to the z-axis. In the isotropic phase = A = 0, in the uniaxial nematic phase 0, A = 0, and in the biaxial nematic phase 0, A y 0. 

The smaller the arc, the higher the orientation. If/is 1, then the fiber is completely oriented if/is 0, then it is randomly oriented and if/is -0.5, then orientation is perpendicular to the fiber direction. Orientation may also be present because a polymer has liquid crystalline properties and an orientation parameter may be similarly calculated. A combination of FTIR and XRD has been recently used in the analysis of the complex morphology of biaxially oriented syndiotactic polystyrene films [14]. 

The order parameters S for all three molecular axes or alternatively, the combination S plus D describe on the level of the first relevant polynomial term the orientational distribution of a rigid, non-cylindrical molecule in the uniaxial nematic phase. Additional order parameters come into play for biaxial phases (Straley, 1974). A concise overview on the concepts from statistical mechanics relevant to order parameters was given by Zannoni (1979). 

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