The principal refractive indices of monoclinic crystal of isotactic polypropylene were evaluated by simulaneous measurements of x-ray diffraction, infrared dichroism and birefringence.
The _??_o_??_al birefringence of crystalline polymer can be represented as follows; on the basis of simple additivity of the crystalline and non-crystalline contributions. In uniaxially oriented system, the cystalline birefringence,
Δc can be further represented in terms of the orientation factors and refractive indices of three orthogonal principal axes of oriented crystallites as follows: where
na′,
nb and
nc are the refractive indices along the orthogonal axes,
a′,
b- and
c-axis, respectively, and
fα and
fβ are the orientation factors for
a′- and
b-axis with respect to the stretching axis of bulk polymer.
When the orientation factor of non-crystalline chain segments can be directly determined from a measurable quantity, such as infrared dichroism, the non-crystalline birefringence,
Δa can be calcuated by using proper value of intrinsic birefringence of the segment; the crystalline birefringence can be separated from the total birefringence. From the crystalline birefringence thus separated and the oriention factors of crystal
a′- and
b-axis, the values of (
na′-
nc) and (
nb-
nc) can be evaluated.
Along the above experimental procedure using well-annealed and relatively low-stretched isotactic polypropylene films, for which the crystal
a′-and
b-axis showed negative orientation of different degree from each other i.e.,
fα′_??_
fβ the fowllowing conclusings were reached:
1) The 1155cm
-1 band in infrard spectrum was found to be parallel dichroic non-crystalline band and the the angle between the non-crystalline chain segment axis and the transition moment of the band to be 42.8°.
2) Among the literatured values of intrinsic birefringence of monoclinic crystallite and noncrystalline chain segment, the values given by Samuels as
Δ0c:33.1×10
-3 and
Δ0σ:46.8×10
-3 seemed to be the most proper.
3) The principal refractive indices of monoclinic isotactic polypropylene crystal along the
a′-,
b- and
c-axis were found as follows.
na′=1.5067,
nb=1.5070
nc=1.5419.
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