Phase transitions in polyphosphazene films: Poly[bis(trifluoroethoxy)phosphazene]

Abstract

The morphology of cast and annealed films from poly[bis(trifluoroethoxy)phosphazene] (PBFP, +(CF3CH20bP=N+,] has been studied by electron, scanning electron and optical microscopy. Several polymorphic forms and one mesophase (hexatic) have been established. From THF solution, PBFP has a chain folded morphology (a-orthorhombic) which transforms into a chain extended 2D hexagonal mesophase at T(1). It remains in this state until Bnal melting occurs at T, = 245 "C approximately. Upon cooling the melt, it quickly reverts to the hexagonal mesophase again, but then passes into a chain extended (3D) y-orthorhombic modification. This material is much more stable thermodynamically than the a-form, which is friable or brittle. PBFP cannot be quenched to a glassy form from the molten state, but it can be disordered as assessed by its reduced T(1) enthalpy and less well defined morpholqgy assessed by electron microscopy. Another crystal modification (B-monoclinic) has been found in solution cast low molecular weight PBFP films.

Morphology of polyphosphazenes: Oriented and unoriented

“…The phase aboven T(1) is believed to be a 2D thermotropic phase which changes to a 3D orthorhombic phase where the unit cell is twice as large as the found in the lamellar THF solution cast sample [14] formed initially. The crystallographic modifications that have been established [15,18] recently are confirmed here from electron diffraction of thin films cast at temperature above T(1).…”

“…Several of these papers [6][7][8][9][10][11] have been devoted to improving our understanding of these "inorganic" polymers which have been fabricated from (i) dilute solution (ii) relatively concentrated solutions, and from (iii) the molten and/or thermotropic states. Several techniques [12][13][14][15] including electron, scanning electron, and optical microscopy, X-ray diffraction (wide and small angle), dilatometry, creep and dynamic mechanical testing and differential scanning *) Paper presented at: Friihjahrstagung der Deutschen Physikalischen Gesellschaft und der Polymer-Gruppe Schweiz, Lausanne 18.-20. M~irz, 1985.…”

“…At any rate, distinctive features are being elucidated providing a dearer but still incomplete picture of structure-property relationship in these macromolecules. Work with polymer fractions is required to established definitive correlations regarding chain extension believed to occur upon passing through T(1), now documented as a first order transition [14,15]. Polyphosphazenes have the structure:…”

High temperature behavior of poly[bis(2,2,2-trifluoroethoxy)phosphazene]

“…Thermotropic behavior and the inter-chain separation between molecules are influenced by side group size and chemical nature 15,16,25,26) . Whenever PBFP is heated through T (l) it transforms into a chain extended 2D mesophase which upon cooling transforms into the 3D orthorhombic chain extended form that becomes highly crystalline 5) . The rate of crystallization of PBFP is controlled by athermal nucleation and growth at sub-T m and at sub-T (l) temperatures, respectively 9) .…”

A Study of the T(1) Transition of Poly(bis(trifluoroethoxy)phosphazene) (PBFP) Using Solid-State Nuclear Magnetic Resonance Spectroscopy

“…5 The interchain spacing has been assessed by X-ray and electron 'also Materials Science and Engineering Department diffraction 6 which confrms the hexagonal arrangement in the 2D 8-state. 6 , 7 Whenever PBFP is cooled from the molten state it returns very rapidly to the 2D hexagonally packed phase and finally crystallizes into a 3D orthorhombic structure below T(1)** (i.e., different from the original monoclinic form).…”