Structural studies on polymer whiskers by transmission electron microscopy. II. Unit cell of poly(p-hydroxybenzoic acid) whisker crystal at room temperature

A set of conspicuous reflections, which are not indexed using the cell constants reported so far for poly(p-hydroxybenzoic acid), were found in the electron diffraction (ED) patterns of its whisker crystal. A new unit cell [a = 1.487 nm, b = 0.572 nm, and c (chain axis) = 1.258 nm, α = β = γ = 90°] was proposed to explain the reflections in question, based on a set of the ED patterns viewed along the different zone axes, which were obtained by tilting the whisker around its long axis. The density of the whisker was experimentally measured to be 1.500 g/cm3, showing that four molecular stems are running through the new unit cell. From the extinction rule, the highest space group was assumed to be Pbca. In this case, the projection of the new unit cell onto the ab-plane gives a two-dimensional unit cell with pgm and is basically in agreement with those proposed so far.