Flow‐induced chain fracture of isolated linear macromolecules in solution

The recently developed elongational flow technique is applied to the examination of flow‐induced chain rupture of macromolecules in solution. For both closely monodisperse atactic polystyrene (a‐PS) and poly(ethylene oxide) (PEO), the critical strain rate for extension ( {article}{empty}{document}ε c{document} ) is found to depend upon molecular weight M as {article}{empty}{document}ε c M- 1.5{document} , consistent with ideal Zimm dynamics. When the chains are subjected to strain rates beyond {article}{empty}{document}ε f{document} the scission products correspond closely to one‐half of the initial molecular weight. The critical fracture stress depends upon molecular weight as {article}{empty}{document}ε f M- 2{document} , enabling the prediction of the ultimate chain length which can be extended without fracture ( {article}{empty}{document}ε c = ε f{document} ). For a‐PS this corresponds to M = 3 × 10 7 . These findings are well accounted for by Stokes' Law applied to an extended bead–rod model. The calculated flow‐induced force in the chain corresponds closely to the rupture force of a covalent backbone bond calculated from a modified Arrhenius rate equation. During the prefracture stage ( {article}{empty}{document}ε f > ε > ε c{document} ) a‐PS shows anomalies in the flow‐induced birefringence, which suggest that the Phenyl side groups are becoming reoriented due to the progressive increase in free volume as the chemical backbone bonds stretch and the bond angles open.