Cocrystalline copolyimides of poly(ethylene 2,6-naphthalate)

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S emicrystalline aromatic polyesters, notably poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN), have become major industrial materials for fiber, film, and packaging applications since the original discovery of PET in 1941. 1 High-temperature applications of PET are, however, limited to some extent by its low glass transition temperaure (T g = 80 °C).This limitation led to the commercial introduction of PEN as a more thermally resistant polyester, 2 especially in the form of biaxially oriented film. 3,4Although the T g of PEN (118 °C) is significantly greater than that of PET, its crystal melting point (268 °C) is only a few degrees higher, so that PEN is still readily processable in the melt. 2 Attempts have been made to increase the thermomechanical stability of polyesters still further by introducing rigid diimide comonomer residues. 5,6Although this approach can indeed result in an enhancement of T g it has also, until now, invariably led to complete loss of the crystallinity which is essential for achieving biaxial orientation.Even quite low levels of diimide comonomers (as little as 5 mol %) are generally sufficient to destroy the ability of the copolymer to crystallize. 5,6ere we report the synthesis of a new group of copolyesterimides in which biphenyl-3,4,3′,4′-tetracarboximide residues are incorporated into PET and PEN.Inclusion of this unit in PET at diimide levels above ca.5 mol % affords only amorphous materials, but copolymers with PEN retain thermal crystallizability across a wide composition range.This result strongly suggests that the biphenyldiimide and naphthalenedicarboxylate (but not terephthalate) units may be isomorphic (Figure 1), enabling their random copolymers to crystallize rapidly even at high levels of diimide.
Analysis of the resulting copolymers by 1 H NMR spectroscopy showed that, in general, the ratio of diimide to naphthalene-dicarboxylate residues was very close to the monomer feed ratio (see Supporting Information (SI)).Analysis by 13 C NMR spectroscopy also showed that the copolymer sequence distributions are entirely random, and size exclusion chromatography gave molecular weights (M w ) in the range 14−18 kDa.As with PET and PEN, 7 postpolymerization in the solid state at 200 °C under vacuum produces a significant enhancement of molecular weight in the new copolymers (e.g., M w = 16 kDa ⇒ 25 kDa for the 20% copolymer after 16 h of solid-state polycondensation).
Analysis of the new copolymers by DSC (see SI) showed that the ability of PET to crystallize is completely destroyed by the presence of the comonomer at levels of incorporation above ca.5 mol %.However, PEN responds very differently to copolymer formation, in that substantial crystallinity is retained up to at least 25 mol % of comonomer 1, at which level the melting point is actually higher than that of the homopolymer.Indeed, the melting point, glass transition temperature, and melt-crystallization temperature all increase progressively as the proportion of comonomer 1 rises from 5 to 25 mol %.Although both T g and T m increase significantly relative to the corresponding values for PEN, the copolymer melting points remain within the normal processing range for polyester films (280−320 °C).At 20 mol % of 1 for example, the T g (onset) is 145 °Can increase of some 27 °C in T g relative to PEN itselfand the melting point is 286 °C (see also Table 1).
X-ray powder patterns for the new series of PEN copolymers (2) are shown in Figure 2, and these confirm that significant levels of crystallinity are retained for all the compositions investigated (2−25 mol % diimide).Moreover it is clear that, although the α-PEN structure is still present with 2 mol % of diimide comonomer, at 5 mol % the structure transforms abruptly into a new crystal form which is then retained at higher levels of comonomer.A provisional structure for the new crystalline phase has been identified by X-ray powder and fiber diffraction interfaced to computational modeling 8 and is described below.
Because the new crystal form emerges at very low levels (≤5 mol %) of comonomer 1, the model was approximated as a polymorph of PEN, on the basis that the diffraction pattern simulated from the model would not be greatly affected by the presence or absence of 5% of diimide residues.It was established that the new structure was not the known βpolymorph of PEN as the characteristic β-phase reflections 9   groups adopt the anti-conformation (cf. Figure 1), and the chain itself is thus identical to that found in the α-phase of PEN.
Simulation of a powder pattern from this structure gave a promising initial match with experimental X-ray powder data for the copolymers.Energy minimization of the structure using a reoptimized Dreiding II force field, 10 with unconstrained unit cell parameters gave a preliminary unit cell with dimensions a = 10.82,b = 6.75, c = 13.27Å, and β = 141.6°.Interactive, manual adjustment of the cell dimensions improved the fit between the simulated and experimental X-ray data very substantially, and the structure was then reminimized with cell dimensions fixed at the derived experimental values (a = 10.56,b = 6.74, c = 13.25 Å, β = 143.0°).
Examination of the resulting crystal structure (Figure 3) showed it to be consistent with the symmetry operations of space group C2/m.Energy minimization in this space group led to a structure giving the simulated powder pattern shown in Figure 3, superimposed on an experimental pattern for the 5 mol % copolymer 2 from which the amorphous background contribution has been subtracted.Some slight discrepancies remain (perhaps arising from the "homopolymer" approximation in the model), but the agreement in peak positions and relative intensities is clearly very good.
As shown in Figure 1, the coplanar diester (red) and diimide (green) residues have very similar overall dimensions and so could potentially be accommodated in the same crystal lattice, requiring only that the biphenyl unit also adopts a coplanar geometry.Although a torsion angle of zero at the biphenyl linkage would not represent the energy minimum in an isolated molecule, 11 this geometry is required by symmetry when an inversion center is present at the center of the biphenyl linkage.Indeed, coplanarity is relatively common in the crystal structures of biphenyl-containing molecules, 12,13 including specifically biphenyl-3,4,3′,4′-tetracarboxylic dianhydride. 14he proposed isomorphism is supported by the uniquely crystalline character of copolymer 2 at high levels of incorporation of monomer 1.We have found that analogous PEN copolymers based on other diimide residues, such as diphenylether tetracarboxylic diimide, entirely fail to crystallize once comonomer levels (of 1) exceed ca. 5 mol %.
Oriented film samples of the copolymers 2 gave good X-ray fiber diagrams, with the clearest pattern being obtained from the 20% diimide copolymer.The structure described above, obtained by diffraction modeling of powder data, gave a simulated fiber pattern in very good agreement with the experimental data (Figure 4).It is, however, noticeable that the [0 0 2] reflection on the second layer line in the experimental pattern is displaced slightly above the predicted line.Nonperiodic layer lines are characteristically associated with diffraction from copolymer crystallites containing random-  sequence chains, 15,16 and Figure 4 is thus also consistent with the proposed isomorphic character of the two comonomers involved here.We are currently developing more realistic structural models for copolymers 2 that incorporate cocrystalline biphenyldiimide residues, so that the proposed structure can be refined quantitatively, by Reitveld methods.
Finally, it may be mentioned that we have successfully synthesized PEN copolymers containing up to 20 mol % of comonomer 1 at the 7 kg scale (see SI) and that strong, biaxially oriented film samples have been obtained from these materials using standard polyester processing conditions.Detailed characterization of the new films is now in progress.
■ ASSOCIATED CONTENT * S Supporting Information Polymer synthesis and characterization information including DSC analyses, 1 H and 13 C NMR spectroscopic data, GPC chromatograms, and X-ray powder data.Details of computational modeling and diffraction simulation.Atomic coordinates (CIF) for the structural model shown in Figure 3.This material is available free of charge via the Internet at http://pubs.acs.org.

Notes
The authors declare no competing financial interest.

Figure 2 .
Figure 2. X-ray powder patterns (amorphous background subtracted) of PEN (α-form) and BPDI-PEN copolymers.Samples were cooled from 320 °C and then annealed at 180 °C for 2 h.

Figure 4 .
Figure 4. Simulated X-ray fiber pattern (colored contour lines) for the proposed copolymer structure in space group C2/m, superimposed on the experimental fiber pattern (greyscale) for a sample of copolymer 2 incorporating 20 mol % of monomer 1.

Table 1 .
Characterization Data for PEN and Its Copolymers with the Biphenyldiimide Monomer 1 a By GPC in hexafluoropropan-2-ol.b By DSC, second heating scan.c By DSC, first cooling scan.d Enthalpies of melting before and after annealing at 200 °C for 2 h.