The determination of single-chain properties for heterogeneous molecules such as a rigid-flexible block copolymer requires the use of solvents capable of reducing selective interactions involving the blocks and the diluent. Molecular dispersion is displayed by a two-block copolymer of benzoyl-terminated poly(p-benzamide) (poly(imin0-1,4-phenylenecarbonyl) and anilino-terminated poly(m-phenyleneisophthalamide) in 96 wt.-% sulfuric acid. For a series of such copolymers, intrinsic viscosities were measured avoiding degradation effects due to H2S04. The intrinsic viscosity ([a]) of the copolymers was found to decrease with increasing length of the flexible block, but [a] remained larger than the value corresponding to an equimolar blend of rigid and flexible homopolymers. These results are explained by theoretical considerations within the framework of the hydrodynamic behavior of single rodlike and single coiled macromolecules. a) part 1 : cf. ref. l). b, Structure-based IUPAC name for poly(p-benzamide) is: poly(imino-1 ,Cphenylenecarbonyl).
Diblock copolymers were prepared by condensation of
benzoyl-terminated poly(p-benzamide)
[PhCO(PBA)] and anilino-terminated
poly(m-phenylene isophthalamide) [PhNH(MPD-I)].
The copolymers
were freed from unreacted prepolymers and also partially fractionated
by selective extraction techniques.
The formation of the nematic phase was investigated for three
copolymers having a similar length of the
rigid PhCO(PBA) block and a fraction β of flexible segment
covering a range from 0.5 to 0.7. In the
N,N-dimethylacetamide (DMAc) containing 3% LiCl solvent the
critical volume fraction v
2
i of the
PhCO(PBA) prepolymer was largely unaffected by copolymerization with a
similar length of the flexible PhNH(MPD-I) block. At the largest β value only a 25% increase in the
critical composition of PhCO(PBA) was
observed. The mesophase appears capable of admitting much longer
flexible sequences than anticipated.
These results are qualitatively in line with a reduced flexibility
of the flexible segment due to the self-consistent orientational field of the mesophase. However,
comparison of data in aggregating and
nonaggregating solvents suggests that mesophase formation should be
described in terms of supramolecular assemblies rather than molecularly dispersed units.
SUMMARY Copolymers were prepared by the condensation of rigid benzoyl-terminated poly@-benzamide) and flexible anilino-terminated poly(m-benzamide) prepolymers. The use of two monomers of the AB type and of end-capped prepolymers resulted in the formation of diblock molecules uncontaminated by triblock or multiblock sequences. Moreover, selective extraction techniques in N,N-dimethylacetamide (DMAc) with varying amounts of LiCl resulted in the complete elimination of unreacted prepolymers, as evidenced from material balance and 'H NMR data. The above extraction technique also allowed a fractionation of the copolymers in terms of their rigidflexible compositional distribution ratio. The molecular weight-intrinsic viscosity dependence of poly(y-benzamide) determined in a nonaggregating solvent using light scattering yielded a persistence length of 8 A, suggesting a relatively large chain flexibility. A tailored copolymer with a fraction B of flexible residues = 0.70 was fractionated, determining the solubility, the viscosity, and the critical composition for mesophase formation in DMAcLiCl and in 96% H2S04 solutions. The results, in line with those previously reported for a related system, reveal that the addition of the flexible segment has a little effect on the critical concentration of poly@-benzamide), whereas the biphasic gap is strongly reduced. Moreover, the solubility of the copolymer is substantially increased over that of the rigid homopolymer. The net result is an unexpected widening of the range of stability of the pure mesophase, with significant implications from both a fundamental and an applied standpoint.
Copolymers were prepared by the condensation of rigid benzoyl‐terminated poly(p‐benzamide) and flexible anilino‐terminated poly(m‐benzamide) prepolymers. The use of two monomers of the AB type and of end‐capped prepolymers resulted in the formation of diblock molecules uncontaminated by triblock or multiblock sequences. Moreover, selective extraction techniques in N,N‐dimethylacetamide (DMAc) with varying amounts of LiCl resulted in the complete elimination of unreacted prepolymers, as evidenced from material balance and 1H NMR data. The above extraction technique also allowed a fractionation of the copolymers in terms of their rigid/flexible compositional distribution ratio. The molecular weight‐intrinsic viscosity dependence of poly(m‐benzamide) determined in a nonaggregating solvent using light scattering yielded a persistence length of 8 Å, suggesting a relatively large chain flexibility. A tailored copolymer with a fraction β of flexible residues = 0.70 was fractionated, determining the solubility, the viscosity, and the critical composition for mesophase formation in DMAc/LiCl and in 96% H2SO4 solutions. The results, in line with those previously reported for a related system, reveal that the addition of the flexible segment has a little effect on the critical concentration of poly(p‐benzamide), whereas the biphasic gap is strongly reduced. Moreover, the solubility of the copolymer is substantially increased over that of the rigid homopolymer. The net result is an unexpected widening of the range of stability of the pure mesophase, with significant implications from both a fundamental and an applied standpoint.
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