1975 Microtubular origin of mitotic spindle form birefringence
. Demonstration of the applicability of Wiener's equation.
For form birefringence
, the [[DELTA]n.sub.f] of polypropylene  was used as this value is not reported for PVA .
The total birefringence is due to the crystalline and amorphous phases in addition to a form birefringence
(which is generally negligible).
In the case of semicrystalline polymers, the overall residual birefringence ([DELTA]n) is the sum of amorphous ([DELTA][n.sub.a]) and crystalline ([DELTA][n.sub.c]) contributions and form birefringence ([DELTA][n.sub.f]) as 
In this research, for simplicity, it is assumed that the contribution of form birefringence ([DELTA][n.sub.f]) and thermal birefringence in the amorphous region ([DELTA][n.sub.a.sup.th]) to the total residual birefringence is neglected.
Otherwise, the SOR was violated due to the contribution of form birefringence in the case of immiscible blends.
where [DELTA][n.sub.i.sup.0] is the intrinsic birefringence of the i-polymer, [[phi].sub.i] is the volume fraction, [f.sub.i] is the orientation function, and [DELTA][n.sub.F] is the form birefringence. The orientation function f can be defined by:
where X is the function crystallinity, [DELTA][n.sub.am] and [DELTA][n.sub.c] are the amorphous and crystalline birefringences, and we neglect form birefringence
The measured birefringence of an oriented semicrystalline polymer can arise from three contributions- molecular orientation plus form birefringence
and distortion birefringence (14).
where the form birefringence
is generally neglected.
where [Delta]n is the birefringence measured, X crystalline fraction, [f.sub.c] crystalline orientation factor, [[[Delta].sub.c].sup.0] intrinsic birefringence for crystalline phase, [[[Delta].sub.am].sup.0] for amorphous phase and the form birefringence
Ohkoshi and Nagura (28) have suggested that the difference may come from the contribution of form birefringence
and trans conformations in the amorphous phase.