The elastic modulus of the poly(phosphonitrilic chloride) crystal

The Young's modulus for a crystal of poly(phosphonitrilic chloride) (poly-dichlorophosphazene) (NPCl₂)_n has been calculated using force constants derived from spectroscopy. Assuming that the molecule is a uniform helix the value of the modulus is 1.38 × 10⁹ dyne cm^?2 dyne cm^?2 = 0.1 N m^?2]; the result is 1.66 × 10¹⁰ dyne cm^?2 if a cis-planar structure is assumed for the molecule. Neither value is close to those obtained experimentally (1.8 × 10⁶ to 6.5 × 10⁶ dyne cm^?2). This is because experimental values relate to the amorphous polymer whereas the calculated values are those for the crystal. There is good agreement between the values calculated for the (NPCl₂)_n crystal and those for other polymer crystals.