The Monte Carlo Simulation Of Separation Of Block Copolymer And Graft Copolymer By Liquid Chromatography At The Critical Condition

Block (graft) copolymers with two or more segments of polymer blocks (grafts) being joined together by covalent bond, which can combine good properties of different polymer block (grafts) have received much attention both in academic research and industrial application. On the other hand, beside the molecular weight distribution, block copolymers have other polydispersities like the block length distribution, chemical composition distribution and sequence distribution, while the graft copolymers have distributions in side chain number, length of side chain, grafting density. These distribution parameters are very important to investigate the properties of copolymers and to product the polymers with more excellent properties. SEC is a useful tool to measure molecular weight distribution of polymers, but it loss its effectiveness to characterize the multiple polydispersities of copolymers. Therefore, it is a challenging task to effectively separate and characterize block (graft) copolymers.Liquid chromatography at the critical condition (LCCC) is a new type of chromatographic technology that can make one kind of block in block copolymer or side chains in graft copolymer become "chromatographically invisible" at the critical condition of their corresponding homopolymers. So that the retention of block copolymer and graft copolymer are independent of this block or side chains, and the separation and characterization of block and graft copolymer are according to the length of other block or backbone. In realistic experiments, however, the block or side chains at the critical condition also have influence on the retention of block and graft copolymer, and this influence is increasing as the length of block or side chains is increased. The separation and characterization of block and graft copolymer by liquid chromatography at the critical condition have been simulated by Monte Carlo theory with random walk chain and self-avoid walk chain model. In the separation of A-b-B block copolymer, B block is at the critical condition, while the A block in LAC mode. The B block is less adsorbed with slit surface then the block A, so that the "pulling" effect make the partition coefficient of block copolymer K(A-b-B) smaller than partition coefficient of A homopolymers K(A) with the length of B block increasing. On the contrary, when the A block is in SEC mode, the B block is more adsorbed with wall than A block, the "anchoring" effect makes K(A-b-B) larger than K(A). Therefore, the B block at the critical condition also has influence on the retention of block copolymer because of the chain connectivity between blocks. Moreover, it is observed that the "anchor" or "pull" effect is decreased as the increasing of slit width. For the graft copolymer, the "pulling" or "anchoring" effect have also been observed when the side chains are at the critical condition. More deviation of K(A-g-B) from K(A) is observed with the increasing of side chain length. Different retention behavior is observed for the graft copolymer at different backbone lengths. When the backbone is in the LAC separation mode, more deviation of K(A-g-B) from K(B) is observed as the backbone length is increased. However, when the backbone is in SEC separation mode, the side chain length has little effect on the elution of the graft copolymer. In summary, we investigated LCCC separation of the block copolymer and graft copolymer. It is found that the block or the side chain at the critical condition can also influence the retention of block copolymer or graft copolymer, which is attributed to the chain connectivity of different blocks. In the silt pore, the different blocks have different polymer-surface interactions with the pore surface, which makes them influence each other during the elution.