The structure of P(3HB-co-3HV) at a clay surface in acomposite is similar to the -form but more ordered, with chains in a planar zigzag conformation in parallel with the silica layers. The WAXS reflection (d = 0.480 nm) corresponds to the mean distance of P(3HB-co-3HV) chains in a pseudohexagonal structure. This crystal form disap-pears with annealing [188]. Antipov et al. [189] showedthat when gel spinning was used to obtain highly o rientedfibers, a third phase (in addition to the crystalline and amorphous) was present and was identified as a columnarmesophase with 2D-pseudohexagonal packing of the central axes of the polymers; it is reversibly strain induced and is a thermally stable phase formed only under mechanical stress