Slow knot formation by suppressed self-reptation in a collapsed polymer chain

Chain-expansion processes from knotted globules have been measured for poly(methyl methacrylate) (PMMA) in the mixed solvent tert-butyl alcohol (TBA) + water (2.5 vol %) by static light scattering. The solution was quenched from the Θ temperature of 41.5 C to 37.0 C, aged there for a time period t _p, and then returned rapidly to the Θ temperature. The chain-expansion process was determined as a time evolution of the expansion factor α2 after the temperature increase. The measurement was carried out by changing the aging time t _p from 240 to 7200 min, and the molecular weight from M _w = 4.0 × 106 to 1.5 × 107, by taking advantage of the extremely slow chain aggregation in the solution. The chain-expansion process obtained for M _w = 1.22 × 107 became slow with increasing t _p, which revealed the knot formation in single globules. The characteristic time of the chain expansion from globules aged for t _p = 7200 min was found to depend on the molecular weight as Mw2.7. This exponent, which is close to 3, demonstrated a disentanglement process due to self-reptation. The present data were compared with the previous data of the chain expansion from compact globules aged at 25.0 C. The comparison made at M _w = 1.22 × 107 and at the same values of t _p revealed that the chain expansion from the globules aged at 25.0 C was much faster than that from the globules at 37.0 C, indicating a lower knot density in the more compact globules. It was conjectured that the knot formation due to self-reptation would be suppressed in a compact globule because an entire conformational change required by knot formation would become difficult to occur in the confined space of high segment concentration, particularly for a long polymer chain. The chain collapse of PMMA in the mixed solvent has been observed to occur extremely slowly at the later stage. This slow process was explained by the suppressed self-reptation.