Phase Behavior of Diblock Copolymer–Homopolymer Ternary Blends with a Compositionally Asymmetric Diblock Copolymer

Abstract

The phase behavior of ternary polymer blends comprising poly(cyclohexylethylene) (C) and polyethylene (E) homopolymers and a compositionally asymmetric CE diblock copolymer with f C = 0.67 was investigated, where f C is the volume fraction of C. The morphology was established in the phase prism (volume fractions of C, E, and CE vs temperature) by optical transmission, small-angle X-ray scattering, and small-angle neutron scattering measurements. The locations of lamellar (LAM), hexagonally packed cylinders and gyroid ordered phases are shifted significantly toward lower fractions of the C homopolymer compared to previous results obtained from ternary polymer blends with a symmetric diblock copolymer (f C = 0.5). Conversely, the Scott line of critical points, which delineates the boundary between single-phase disorder and macroscopic phase separation, remains virtually unchanged, coincident with the fraction of the C homopolymer associated with the binary homopolymer blend critical composition. A central finding of this study is that the line of nearly congruent order−disorder transitions, where the LAM phase melts virtually directly into the disordered state, is decoupled in composition from that of the Scott line of critical points. A wide range of phase space between the compositions associated with the congruent transition and Scott line was identified as containing a microemulsion morphology. This study demonstrates that diblock copolymer compositional asymmetry significantly impacts the ordered phase regime but has a marginal effect on the region displaying macroscopic phase separation. It also provides useful guidance for tuning the interfacial curvature, a crucial factor in the formation of bicontinuous microemulsions.

Impact of Composition and Placement of Hydrogen-Bonding Groups along Polymer Chains on Blend Phase Behavior: Coarse-Grained Molecular Dynamics Simulation Study

“…The choice of voxel size is guided by g BC ( r ); we choose a cubic voxel length of 2 d , which is greater than B or C polymer bead size but smaller than the pairwise bead distance where g BC ( r ) reaches bulk value, allowing us to capture concentration fluctuations within microdomains in disordered microphase morphologies. Using eq and simulation box length L , S D ( k ) is calculated from the Fourier transform of f (φ j ) as follows To classify blend morphologies as bicontinuous microemulsions (BμE), we fit the S D ( k ) with the Teubner–Strey model, as done in previous work using the following equation where the fitting parameters a 2 , c 1 , and c 2 are used to calculate (using eqs –) the domain spacing d s or domain size d s /2 describing the periodicity of BμE morphology, the correlation length ζ describing how far the structural correlations persist, and the amphiphilicity factor f a that describes the ability of directional A–D interactions in stabilizing the microemulsion − We use the value of f a to quantify a morphology as lamellar when f a is close to −1 and as BμE when f a lies between 0 and −1. For this, we fit the Teubner–Strey model to S D ( k ) from each simulation trial separately and report the average value of f a from three trials.…”

Field-Theoretic Simulations for Block Copolymer Melts Using the Partial Saddle-Point Approximation

“…Naturally, this system has a far larger parameter space, and so studies have concentrated on symmetric blends where the diblock has a composition of , the two homopolymers have equal polymerizations , and the homopolymers have either equal concentrations or equal chemical potentials. For experimental convenience [ 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ], the diblock polymerization is generally set to with , such that the ODT occurs at over the full range of diblock volume fractions, . In this system, particular attention is paid to a bicontinuous microemulsion (B E), where the two homopolymers segregate into interweaving microdomains separated by a monolayer of diblock copolymer.…”

“…Experiments [ 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ] have proposed a somewhat different phase diagram, where the A+B and L regions are separated by a channel of B E rather than three-phase coexistence. It is difficult to fathom how this proposed phase diagram could possibly converge to the SCFT diagram, which it must, in principle, do as .…”

Phase Behavior of Salt-Doped A/B/AB Ternary Polymer Blends: The Role of Homopolymer Distribution

“…At finite molecular weights, the predicted LP is replaced by a fluctuation-stabilized disordered phase, characterized as a bicontinuous microemulsion (BμE) of interpenetrating A-rich and B-rich domains, with zero mean curvature and negative Gaussian curvature. , Recently, Matsen and co-workers used field-theoretic simulations to predict that the BμE always coexists with A and B homopolymer-rich phases. , Off the symmetric isopleth, other microstructures with nonzero curvature, such as double gyroid (GYR), hexagonally packed cylinders (HEX), and some cubic phases, have also been predicted and observed, which can be summarized in an isothermal phase triangle at a specific T . ,, For an ideally symmetric system where N A = N B = α N AB , the volume fraction of A in the copolymer f A = 0.5 (compositional symmetry), and A and B have identical statistical segment lengths, ε AB = ( b A / b B ) 2 = 1 (conformational symmetry), the isothermal phase diagram should have mirror symmetry around the symmetric isopleth. However, this symmetry can be broken when either the compositional or conformational symmetry restriction is relaxed. − The complex phase behavior also suggests a wide microstructural tunability through α, ε AB , f A , ϕ A /ϕ B , ϕ H , and T and offers the opportunity to construct microstructures with adjustable length scales without synthesizing copolymers of various compositions and molecular weights.…”