No abstract

We report the synthesis and morphological characterization of a miktoarm block copolymer architecture:  (PSα M−PIM) n −(PSM−PIα M) n , where M ∼ 20 000, n = 1, 2, and the arm asymmetry parameter α = 1, 2, or 4 (α is the ratio of the outer block molecular weight to that of the inner block). These block copolymers are symmetric in overall composition and exhibit n- and α-dependent microdomain morphologies. Alternating lamellae are observed for linear tetrablocks (n = 1), α = 1, 2, 4, and for inverse starblock (n = 2), α = 1, 2. An architecturally-induced morphological transition from lamellae to a tricontinuous cubic structure is observed with n = 2 and α = 4. The formation of the tricontinuous cubic microdomain structure in this compositionally symmetric system is thought to relieve the overcrowding of the four peripheral PS−PI junctions by providing a curved intermaterial dividing surface with a triply periodic microdomain structure, allowing some bridging by the interior blocks of the miktoarm star.

In this paper, we examine a new block copolymer architecture-cyclic block copolymers. The physical behavior of cyclic polymers, both in solution1 and in the bulk,2-3 has been the subject of continual research. Current issues concern their dynamics4-5 and topological characteristics.6-9 Due to the challenges present in the synthesis of cyclic polymers,10 the morphology of microphase-separated cyclic block copolymer systems has not been investigated previously. Recently, however, well-characterized cyclic polystyrene-poly(2-vinylpyridine) (PS-P2VP) and polystyrene-poly(dimethylsiloxane) (PS-PDMS) block copolymers have been synthesized from their linear triblock precursors.11-13 The morphology of these microphase-separated systems is the topic of the present study.The focus of our study is to probe the effect of loops versus bridges on the morphological characteristics of microphase-separated block copolymer systems. Cyclic A-B diblocks only assume doubly-looped chain conformations in the microphase-separated state, while their linear A-B-A triblock precursors are looped and bridged. We anticipate this difference to be manifest in the relative spacings of their respective microlattices. The microstructural characteristics of these systems are examined through transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) of systems exhibiting varying degrees of quench into the microphase-separated state. These observations are then compared to our theoretical predictions.Cyclic PS-P2VP and PS-PDMS were prepared by end-coupling their linear triblock precursors bis-anionic LiP2VP-PS-P2VPLi and LiPDMS-PS-PDMSLi with l,4-bis(bromomethyl)benzene and Cl2Si(CH3)2, respectively. The details of the synthesis of the precursor triblocks, coupling reactions, and characterization of the block copolymers are given elsewhere.11-13 The molecular characteristics of the cyclic diblock and linear triblock precursors used in this study are shown in

We present a general theoretical scheme which allows the characterization of microphase separation of A-B diblock copolymer systems at all degrees of segregation. Our method is based on the density functional theory of Melenkevitz and Muthukumar and uses the technique of density profile parameterization to greatly reduce the technical complexity of the solution. The microphase-separated systems are observed to pass through three stages of ordering as the system is quenched. These are the weak, intermediate, and strong segregation regimes. The phase diagram is calculated for three ordered morphologies: lamellae, hexagonally-packed cylinders, and body-centered-cubic spheres. We also characterize these microphases by the dependence of the lattice constant, D, and the interfacial width, a0, on the quench parameter xN. The theory correctly reproduces the behavior predicted by previous theories describing the weak and strong segregation regimes and establishes the experimental conditions for the validity of these regimes. In the intermediate regime, the effective exponent a describing the N dependence of D (D = Na) is larger than that in the strong segregation regime, a depends strongly on both block length and morphology in the intermediate regime. We attribute this behavior to chain stretching arising from the localization of junctions.

Intermaterial dividing surfaces (IMDS) in strongly segregated block copolymer systems are typically of constant mean curvature (CMC). This observation largely derives from the interplay of the thermodynamics of chain deformation and interfacial tension. Recently, a compositionally symmetric, linear poly(2-vinylpyridine)-b-polyisoprene-b-polystyrene (P2VP−PI−PS) triblock copolymer with each block having a M w ∼ 15 000, was observed to order in a hexagonal lattice of P2VP cylinders, surrounded by a PI annulus within a PS matrix. The PI/PS IMDS was of non-CMC, assuming a pseudohexagonal cross section. This striking morphological behavior originates from the nonuniform degree of chain deformation occurring in the microstructure and indicates that the PI/PS IMDS is strongly coupled to the shape of the Wigner−Seitz cell of the microdomain lattice. We further probe this phenomenon by attempting to alter the curvature of the PI/PS interface through incorporation of homopolystyrene (h-PS) into the PS matrix by preparing P2VP−PI−PS/h-PS (PIS/S) blends having a composition of 90/10 (v/v) and (M w)h - PS ∼ 4000−100 000. Using TEM and SAXS, we observe a transition in the PI/PS IMDS as a function of the molecular weight of h-PS. When (M w)h - PS< 50 000, the tethered PS blocks behave as “wet brushes”. In this regime, the non-CMC character of the PI/PS IMDS is preserved and a slight contraction in the domain spacing is observed due to a nearly uniform distribution of h-PS in the matrix. When (M w)h - PS = 50 000, behavior characteristic of the “dry brush” regime is seen. The PI/PS IMDS appears nearly circular in cross section with an accompanying increase in the domain spacing relative to that of the neat triblock. This behavior originates from a nonuniform distribution of h-PS in the matrix, with preferential segregation to the corners of the Wigner−Seitz cell. As the molecular weight of h-PS is further increased, the homopolymer is ejected from the microdomain structure and macrophase separates from the tethered PS blocks.