HPLC elution behavior of isotopically different block copolymers was investigated.
A series of three diblock copolymers of hydrogenous polystyrene and deuterated polystyrene (hPS-b-dPS), in which the length of hPS-block is fixed at 18 kg/mol and the length of dPS-block is varied from 17 kg/mol to 80 kg/mol, was synthesized and their retention behavior in liquid chromatography at critical condition (LCCC) was investigated using a C18 coated silica stationary phase and a mixed solvent of CH2Cl2 and CH3CN. At this LC separation condition, hPS is retained slightly longer than dPS. The LCCC separations were performed at both LCCC conditions of dPS and hPS established with the same stationary and mobile phases, but at different column temperatures. Since the chromatographic retention difference between dPS and hPS is very small, it was possible to elute the block copolymers at both exclusion and interaction modes at the critical condition of each individual block. We found that the block at its critical condition is not fully “invisible” in both cases. In the LCCC separation at the critical condition of dPS, hPS-b-dPS elutes after the injection solvent peak (interaction mode) due to the stronger interaction of the hPS block. Although they have the same hPS block, they do not coelute but elute in the order of decreasing total molecular weight like an elution in exclusion mode. It clearly demonstrates that the dPS block is not “invisible” at the critical condition but influences the retention of the block copolymers. Monte Carlo simulations of the partition coefficient of A-b-B into a slit pore were performed to give insight on the elution behavior of A-b-B at the critical condition of the B block. The simulation shows that a block under the critical condition influences the retention of the other “visible” block whether the “visible” block is eluted in exclusion or interaction mode. When A is eluted in the interaction mode and B is at its critical condition, the partition coefficient is found to decrease with the increase of the “invisible” B block length, conforming to the observed elution behavior in experiments.
