Methods for assessment of biaxial chain orientation

Biaxiality is illustrated in Fig. 9.16. In this case, it is possible to find two orthogonal planes in both of which the chains have a preferential direction. The orientation function is a function of two angular variables. 

The biaxial orientation can be assessed by optical methods measuring the in-plane birefringence in the three orthogonal directions. The three Ah values may be derived by tensor summation of eq. (9.16)  

X-ray diffraction can also be used, the X-ray diffraction patterns being recorded at different angles. It is in principle possible to obtain the complete crystalline orientation function (= /( /), u)). Biaxiality may not only refer to the chain axis. In a single crystal of polyethylene, for example, the chain orientation is indeed very high (/ typically greater than 0.95), but 

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