Segmented poly(tetramethylene oxide) zwitterionomers and their homologous ionenes. 3. Structural study through SAXS and SANS measurements
Résumé
The structure in bulk of a series of segmented poh(tetramethylene oxide) (PTMO) zwitterionomers of the ammonioalkanesulfonate (N+(CH2)pSO3-, p= 3, 4) or alkoxydicyanoethenolate (N+(CH2)pOCOC-(CN)2, p = 2, 3) type and of their homologous iooenes was analyzed through small angle X-ray and neutron scattering measurements in their amorphous state. Segregation of the polar units from the PTMO matrix resulting in sharp interfaces systematically occurs in all the systems. These two-phase structures are thermally stable and still persist in the liquid state up to at least 140 and 100 °C for sulfonates and dicyanoethenolates, respectively. However, long range order is observed only in the case of zwitterionomers bearing a bifunctional junction, with a typical transition from a lamellar structure for the shorter PTMO segments (Mno ∼ (1.8-2.2) × 103, high zwitterion density, lamellar spacings between 7 and 8 nm with a thickness of the zwitterionic layer of about 1 nm) to a hexagonal structure for the longer ones (Mno > 3.6 × 103, low zwitterion density, radius of the zwitterionic cylinder of about 1.2 nm). In the lamellar structure, supported by transmission electron microscopy which shows coherence domains spreading over more than 100 nm. the PTMO subchain is extended by about 50% with respect to its unperturbed dimensions. Such an extension does not occur in the less sterically constraining hexagonal structure. The swelling of the ammonioethoxydicyanoethenolate zwitterionomers by tetrahydrofuran, a selective solvent of the PTMO segments, results in physical gels (swelling ratios Q ≤ 5) with a progressive loss of long range order. It involves either an inhomogeneous swelling process or a gradual fragmentation of the zwitterionic microdomains of constant and strongly anisotropic shape.