Kalorimetrische Untersuchungen zum Lösungsverhalten von Polyacrylsäure und Poly(natriumacrylat) in Wasser,Formamid und Ethylenglykol

Heats of solution and heats of dilution of poly(acrylic acid) (PAAcH), poly(sodium acrylate) (PAAcNa), and poly(acrylamide-co-sodium acrylate) (1:1) (PAAm/PAAcNa) in water, formamide (FA), and ethylene glycol (EG) as measured at 25°C, are reported. While for PAAcH/H₂O and PAAcH/EG only minor caloric effects are observed, the heats of solution for PAAcNa/H₂O are strongly exothermic as a result of dissociation of the polysalt. Heats of dilution in this system are endothermic because of the hydrolysis reaction. PAAcH/FA shows large negative enthalpies on dissolution as well as on dilution. This is due to the dissoziation of the COOH-groups which is much more pronounced in FA than in the aqueous system because formamide has a greater ionizing capability. Comparison between the behaviour of copolymer PAAm/PAAcNa and physical admixing of the homopolymers confirms the specific neighbouring group interaction of carboxylate and carbonamide groups along the same chain. Comparing the calorimetric results for PAAcH with those for the low-molecular-weight model compound propionic acid, it becomes evident that the differences are mostly due to the different state of order of the pure compounds, namely, due to the chain structure, the COOH-groups in the polymers are not as flexible as in the monomers. Therefore, in the pure polymer less hydrogen bonds are formed than in the pure propionic acid. The calorimetric results are discussed in the light of previous work on viscosimetric dilute solution behaviour of these systems, showing the consistency of the different experimental findings.