The influence of block copolymer on droplet breakup and coalescence in immiscible polymer blends

The influence of a block copolymer on the droplet breakup and coalescence has been studied extensively. The role of the block copolymer as an emulsifier for immiscible polymer blends has been regarded as suppressing the interfacial tension between the two immiscible polymer blends by locating the block copolymer at the interface. These studies were typically accomplished by investigating the morphology after mixing the blend components in a conventional mixer, which imposes a highly complicated deformation field during the blending process. Such a complex deformation field poses difficulties in interpreting the breakup and coalescence processes.; We simplify the mixing process to obtain well-specified shear history by utilizing a controlled-strain rheometer with parallel plate geometry. Droplet breakup and coalescence are observed, respectively, after shearing at a high shear rate for a specified time, and then following a period of application of a low shear rate, respectively. Identification of the location of the block copolymer emulsifier is realized by selective staining of the PS segment of a poly(styrene-b-methylmethacrylate) (PS-b-PMMA) block copolymer in ternary blends with poly(styrene-co-acrylonitrile) (SAN) random copolymer and poly(cyclohexylmethacrylate) (PCHMA).; By choosing SAN as a blend component, the degree of exothermic interaction between SAN and PMMA block of the bcp can be varied by increasing the AN content of the SAN. As the AN content increases in the range of 15 to 33%, the degree of exothermic interaction with PMMA decreases. This change in the degree of exothermic interaction enables provides manipulation of swelling of the PMMA segment in contact with the SAN component of the blends.; A previous expression for the degree of swelling between a bcp segment and a blend component is improved and used to compute the swelling power ratio (S_r = S_out/S_in) which characterizes the spontaneous curvature at the droplet interface which originates from the difference in swelling between the outside and internal blocks of a symmetric bcp. For blend systems studied, SAN/PCHMA/bcp and SAN/PS/bcp, with varying the AN content of the SAN between 26, 29, and 33%, we observe that the droplet breakup and coalescence behavior obeys the following rule: Sr<0.4, internalemulsificationfai lure, 2.5>Sr>0.4, stableemulsification, Sr>2.5, externalemulsificationfai lure. ; For blend systems containing an asymmetric bcp we define an effective swelling ratio, S_r,eff = (N_outS_out)/(N _inS_in) which incorporates the effect of the chain length difference between the internal and external bcp segments where N_out and N_in are the degrees of polymerization of the external and internal bcp segments. We find that the effective swelling ratio utilized in the above rule successfully describes the breakup and coalescence behavior for immiscible polymer blends containing an asymmetric block copolymer.; The influence of swelling ratio on the phase inversion is observed by investigating the SAN/PS18k/bcp blend systems. It is found that the blend system having lower swelling ratio is preferential for phase inversion.