LATTICE MONTE-CARLO INVESTIGATIONS ON COPOLYMER SYSTEMS, 1 - DIBLOCK COPOLYMERS

Symmetric diblock copolymers in dilute solution were examined by means of Monte Carlo simulations on a cubic lattice with respect to chain-a nd block dimensions, shape, local structure and number of contacts. Th e solvent was either a common good one, a common theta-solvent or a se lective one for the two blocks. In all cases, repulsive interactions a re operative between the blocks. In addition, the underlying homopolym ers (athermal and theta) were divided into two parts (and treated as a block copolymer) for comparison. Chain-length was varied from 40 to 1 280 segments leading to the expected values for the critical exponent 2 upsilon approximate to 1.2 for good solvent quality and 2 upsilon ap proximate to: 1.0 for theta-solvent. Copolymers in a selective solvent scale with an intermediate exponent, 2 upsilon approximate to 1.13. T he deviation of the mean squared dimensions of the copolymers from the sum of those of two homopolymers of the same length and for the same solvent quality as the blocks is largest for block copolymers in a com mon theta-solvent (where it exceeds 20%), while the blocks themselves have mostly the same dimensions as their underlying homopolymers of eq ual length. The shape of the copolymers, expressed by the parameter de lta (asphericity) becomes more rod-like with increasing chain-length i f there are (compact) theta-blocks in the molecule which are subject t o mutual repulsive interaction. In these cases, delta exceeds the valu e of the homopolymers in the limit of infinite chain-length. The numbe r of contacts per segment approaches a limiting value with increasing chain-length which is approximate to 0.20 for athermal chains and athe rmal blocks. For theta-chains and theta-blocks, a limiting value is no t yet reached within the range of chain-lengths investigated. The numb er of contacts per segment between two different blocks quickly tends to zero with increasing chain-length.