Poly(hexamethylene terephthalate)—I. Interpretation of its diffraction patterns

X-ray diffraction, electron microscopy and electron diffraction studies of poly(hexamethylene terephthalate) reveal a variety of structures and morphologies. Polymer micro-single crystals, grown from nitrobenzene-hexane or nitrobenzene-decane solutions exist in three crystal structures. Two forms are triclinic, the third is monoclinic. The single chain triclinic unit-cell (form I) has dimensions $\text{a = 5.217, b = 5.284, c = 15.738}\text{A}\text{?}\text{(}\text{fibre axis}\text{) α = 129.4, β = 97.6, γ = 95.6°}$. The double chain triclinic unit-cell, (form II), has all dimensions and angles equal to those of form I but for b which is twice as large. There are six chains in the monoclinic unit-cell (form III) with dimensions $\text{a = 9.100, b = 17.560, c = 15.74}\text{A}\text{?}\text{(}\text{fibre axis}\text{), α = 127.8°}$. All three forms have a common value of the fibre repeat, $\text{c = 15.74}\text{A}\text{?}$, indicating that in all three forms the polyester presents itself in a fully extended conformation. The differences between the various polymorphs arise from different packing modes of the chains. In the fibre, both the monoclinic and the triclinic forms coexist.