Regular Mixing Thermodynamics of Hydrogenated Styrene–Isoprene Block–Random Copolymers

Abstract

Random copolymerization of A and B monomers represents a versatile method to tune interaction strengths between polymers, as A<i>r</i>B random copolymers will exhibit a smaller effective Flory interaction parameter χ (or interaction energy density <i>X</i>) upon mixing with A or B homopolymers than upon mixing A and B homopolymers with each other, and the A<i>r</i>B composition can be tuned continuously. This approach can also be used to tune the segregation strength in A–A<i>r</i>B “block–random” copolymers. Simple models of polymer mixing thermodynamics suggest that the effective interaction energy density in such block–random copolymers should follow <i>X</i>_{A–A<i>r</i>B} = <i>f</i>_B²<i>X</i>_A–B, but this prediction has not been tested quantitatively. The present work systematically assesses the validity of this rule for thermally stable hydrogenated derivatives of styrene–isoprene block copolymers, through measurements of the order–disorder transition (ODT) temperature on near-symmetric diblock and diblock–random copolymers of varying composition and suitable molecular weight (M). Both hydrogenated derivatives wherein the styrene aromaticity is retained, and derivatives wherein the styrene units are saturated to vinylcyclohexane, are examined, and both are found to closely obey the <i>X</i>_{A–A<i>r</i>B} = <i>f</i>_B²<i>X</i>_A–B prediction, thereby confirming the utility of this simple relationship in designing block copolymers with targeted interaction strengths using only these two common monomers. The reduction in <i>X</i>_{A–A<i>r</i>B} over <i>X</i>_A–B permits the synthesis of polymers having much larger <i>M</i> and domain spacing <i>d</i> while maintaining a thermally accessible ODT; measured domain spacings are found to closely follow the expected scaling, <i>d</i> ∼ <i>X</i>^1/6<i>M</i>^2/3