Size control of poly(2-oxy-6-naphthoy) whiskers by the addition of oligomers during polymerization

The method of controlling the size of a poly(2-oxy-6-naphthoyl) (PON) whisker was examined with a focus on lengthening the whisker. PON whiskers were prepared by the polymerization of 2-acetoxy-6-naphthoic acid in liquid paraffin at 330°C. The whiskers were very symmetrical, and both tips were very sharp. Growth of the whiskers along the length took place by which oligomer lamellae piled up along the long axis of the needlelike crystals with spiral growth. During steady growth until 8 min, the tip angle of the whiskers was constant at 80°. However, it became significantly sharper (22°) at 30 min, and later the trunk part of the whiskers stopped increasing. To depress the sharpening of the tips and to extend the steady-growth period for longer whiskers, oligomers were added to the polymerization system after 8 min, just before the tip angle became sharper, to keep the concentration of the oligomers in the polymerization system constant. The addition of the oligomers extended the steady-growth period, and the length increased from 12.5 to 17.3 μm. Although the width of the whiskers increased very slightly with the addition of the oligomers, the axial ratio of the whiskers increased from 19.4 to 24.7.