No abstract

This report describes the synthesis of all-aromatic hyperbranched polyesters with phenol and acetate end groups. The synthesis was based on the melt condensation of tbe A¡B monomers 3,5-bis-(trimethylsiloxy)benzoyl chloride (2) and 3,5-diacetoxybenzoic acid (3). The trimethylsilyl groups of the polyesters from monomer 2 are hydrolyzed during workup, resulting in polymers with phenol terminal groups. Although the acetate groups of polymers prepared from 3 are quite stable and remain in the polymer, conditions were found where they could be hydrolyzed to give a phenolic polymer similar to that obtained from 2. The special structure of the monomers results in highly branched (hyperbranched) materials with a high number of terminal groups. Comparison of Mark-Houwink plots of linear polystyrene and a hyperbranched polyester sample and Mark-Houwink "a" values of less than 0.5 for numerous samples were consistent with highly branched structures. These polyesters are noncrystalline and are thermally stable to at least 350 °C. As predicted for such step-growth polymerizations, the molecular weight distributions broadened significantly at high conversions.

Melt condensation of 5-acetoxyisophthalic acid (1) and 5-(2-hydroxyethoxy)isophthalic acid (4) yielded hyperbranched aromatic polyesters with carboxylic acid terminal groups. The polymer of 1 has a Tt of 239 °C and thus required melt acidolysis polymerization temperatures of 240-260 °C to obtain M*s in the 20 000 to 80 000 range. The polyester based on 4, with a lower T" was obtained at around 190 °C using standard polycondensation techniques. Comparisons of Mark-Houwink plots of these polyesters with linear polystyrene were consistent with the expected highly branched structures. NMR studies revealed the degree of branching to be about 50%, similar to what has been observed in other hyperbranched polyester systems.The hyperbranched polyesters were easily converted to the ammonium or sodium salts and the viscosity behavior of these water-soluble hyperbranched polyelectrolytes was investigated.

Direct measurement of the slow α-relaxation modes of a metallic liquid near the glass transitionThe glass transition temperature T g of polystyrene spheres in aqueous suspension was investigated by differential scanning calorimetry. Spheres with diameters of 42-548 nm show an unambiguous glass transition very near the T g of bulk polystyrene. The magnitude of the observed heat capacity jump ⌬C p at the transition decreases as the sphere size decreases. These results are interpreted as indicating that the center portion of a sphere has bulklike dynamics while an outer shell has substantially faster dynamics than the bulk. The ⌬C p values are consistent with a mobile layer approximately 4 nm thick. In contrast, free-standing polystyrene films with thicknesses similar to these sphere diameters have been reported to show a single glass transition substantially below the bulk T g value.

We report viscosity, recoverable compliance, and molar mass distribution for a series of randomly branched polyester samples with long linear chain sections between branch points. Molecular structure characterization determines tau=2.47+/-0.05 for the exponent controlling the molar mass distribution, so this system belongs to the vulcanization (mean-field) universality class. Consequently, branched polymers of similar size strongly overlap and form interchain entanglements. The viscosity diverges at the gel point with an exponent s=6.1+/-0.3, that is significantly larger than the value of 1.33 predicted by the branched polymer Rouse model (bead-spring model without entanglements). The recoverable compliance diverges at the percolation threshold with an exponent t=3.2+/-0.2. This effect is consistent with the idea that each branched polymer of size equal to the correlation length stores k(B)T of elastic energy. Near the gel point, the complex shear modulus is a power law in frequency with an exponent u=0.33+/-0.05. The measured rheological exponents confirm that the dynamic scaling law u=t/(s+t) holds for the vulcanization class. Since s is larger and u is smaller than the Rouse values observed in systems that belong to the critical percolation universality class, we conclude that entanglements profoundly increase the longest relaxation time. Examination of the literature data reveals clear trends for the exponents s and u as functions of the chain length between branch points. These dependencies, qualitatively explained by hierarchical relaxation models, imply that the dynamic scaling observed in systems that belong to the vulcanization class is nonuniversal.