Controlling the phase behavior of block copolymers via sequential block growth

Block copolymers remain one of the most extensively investigated classes of polymers due to their abilities to self-organize into various nanostructures and modify polymer/polymer interfaces. Despite fundamental and technological interest in these materials, only a handful of experimental phase diagrams exist due to the laborious task of preparing such diagrams. In this work, two copolymer series are each synthesized from a single macromolecule via sequential living anionic polymerization to yield molecularly asymmetric diblock and triblock copolymers systematically varying in composition. The phase behavior and morphology of these copolymers are experimentally interrogated and quantitatively compared with predictions from mean-field theories, which probe copolymer phase behavior beyond current experimental conditions.     

Influence of block lengths and symmetries of block copolymers on phase behavior of polymer A/polymer B/block copolymer ternary blends

Monte Carlo simulations were used to investigate the compatibilizing effects of diblock copolymers in A/B/A-B diblock copolymer ternary blends and triblock copolymers in A/B/triblock copolymer ternary blends, respectively. The volume fraction of homopolymer A was 19% and was the dispersed phase. The simulation results show that diblock copolymers with longer A-blocks are more efficient as compatibilizers, and symmetric triblock copolymers with a shorter middle block length are easily able to bridge each other through the association of the end blocks. This kind of triblock copolymers have relatively high ability to retard phase separation as compatibilizers.     

Miscibility studies on blends of Kraton block copolymer and asphalt

Miscibility phase behavior in blends of SBS copolymers (Kraton 1101) and asphalt has been investigated through establishment of thermodynamic phase diagrams. The observed phase diagram of the SBS/asphalt blend is an upper critical solution temperature (UCST) type with a maximum at about 20% copolymer and around 200 °C. The study on kinetics of phase decomposition has been carried out by means of time resolved light scattering at the 6/94 Kraton 1101/asphalt composition. Time-evolution of structure factor has been analyzed in the context of temporal scaling laws. The growth regime is seemingly dominated by the late stage of phase decomposition where the hydrodynamic effect is dominant. As typical for a deep off-critical quench, the cessation of domain growth occurs presumably through physical pinning.     

Effect of solvents on closed-loop phase behavior of block copolymers

We studied the effect of solvent selectivity on the closed-loop phase behavior of a polystyrene-block-poly(n-pentyl methacrylate) copolymer. It was found that the lower disorder-to-order transition temperature (LDOT) and upper order-to-disorder transition temperature (UODT) consisting of the closed-loop were very sensitive to the selectivity of the solvent. With the addition of very small amounts of non-selective solvents such as di-n-octyl phthalate and dimethyl phthalate, the LDOT increased rapidly, whereas the UODT decreased dramatically; thus, the immiscibility loop was shrunk greatly. On the other hand, both the LDOT and UODT decreased with increasing amount of dodecanol, a highly selective solvent to poly(n-pentyl methacrylate) block. However, the decrease in the LDOT was greater than that of the UODT, leading to an increased immiscibility loop.     

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Model diglycidyl ether of bisphenol-A based epoxy resins containing well-dispersed 15 nm block copolymer (BCP) nanoparticles were prepared to study the effect of matrix crosslink density on their fracture behavior. The crosslink density of the model epoxies was varied via the controlled epoxy thermoset technology and estimated experimentally. As expected, it was found that the fracture toughness of the BCP-toughened epoxy is strongly influenced by the crosslink density of the epoxy matrix, with higher toughenability for lower crosslink density epoxies. Key operative toughening mechanisms of the above model BCP-toughened epoxies were found to be nanoparticle cavitation-induced matrix shear banding for the low crosslink density epoxies. The toughening effect from BCP nanoparticles was also compared with core-shell rubber-toughened epoxies having different levels of crosslink density. The usefulness of the present findings for designing toughened thermosetting materials with desirable properties is discussed.     

Polystyrene-poly(2-ethylhexylmethacrylate) block copolymers: Synthesis,bulk phase behavior,and thin film structure

Anionic polymerization was employed to synthesize well-defined diblock copolymers of polystyrene and poly(2-ethylhexylmethacrylate), PS-PEHMA. Diblock morphologies in bulk and in substrate-supported thin films were characterized by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively. PS-PEHMA diblocks exhibited thermotropic order-disorder transitions; one diblock showed a thermoreversible transition between lamellae and a higher-temperature morphology assigned as perforated lamellae. Unlike PS-poly(alkylmethacrylate) diblocks where the alkyl group is n-butyl or n-pentyl, PS-PEHMA diblocks showed a typical decreasing Flory interaction parameter with increasing temperature. Thin films of PS-cylinder-forming PS-PEHMA diblocks showed a strong preference for the cylinders to lie in the plane of the film; films of incommensurate thickness readily formed terraces. Films of commensurate thickness were easily aligned over macroscopic areas through the application of mechanical shear.     

Correlation between crystallization kinetics and melt phase behavior of crystalline-amorphous block copolymer/homopolymer blends

We show that the phase behavior of the strongly segregated blend consisting of a crystalline-amorphous diblock copolymer (C-b-A) and an amorphous homopolymer (h-A), which depends on the degree of wetting of A blocks by h-A, can be probed by the crystallization kinetics of the C block. A lamellae-forming poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) was blended with PB homopolymers (h-PB) of different molecular weights to yield the blends exhibiting ‘wet brush’, ‘partially dry brush’, and ‘dry brush’ phase behavior in the melt state. The crystallization rate of the PEO blocks upon subsequent cooling, as manifested by the freezing (crystallization) temperature (T_f), was highly sensitive to the morphology and spatial connectivity of the microdomains governed by the degree of wetting of PB blocks. As the weight fraction of h-PB reached 0.48, for instance, T_f experienced an abrupt rise as the system entered from the wet-brush to the dry-brush regime, because the crystallization in the PEO cylindrical domains in the former required very large undercooling due to a homogeneous nucleation-controlled mechanism while the process could occur at the normal undercooling in the latter since PEO domains retained lamellar identity with extended spatial connectivity. Our results demonstrate that as long as the C block is present as the minor constituent the melt phase behavior of C-b-A/h-A blends can also be probed using a simple cooling experiment operated under differential scanning calorimetry (DSC).     

Temperature-composition phase diagrams of binary blends of block copolymer and homopolymer

The temperature-composition phase diagrams for six pairs of diblock copolymer and homopolymer are presented, putting emphasis on the effects of block copolymer composition and the molecular weight of added homopolymers. For the study, two polystyrene-block-polyisoprene (SI diblock) copolymers having lamellar or spherical microdomains, a polystyrene-block-polybutadiene (SB diblock) copolymer having lamellar microdomains, and a series of polystyrene (PS), polyisoprene (PI), and polybutadiene (PB) were used to prepare SI/PS, SI/PI, SB/PS, and SB/PB binary blends, via solvent casting, over a wide range of compositions. The shape of temperature-composition phase diagram of block copolymer/homopolymer blend is greatly affected by a small change in the ratio of the molecular weight of added homopolymer to the molecular weight of corresponding block (M_H,A/M_C,A or M_H,B/M_C,B) when the block copolymer is highly asymmetric in composition but only moderately even for a large change in M_H,A/M_C,A ratio when the block copolymer is symmetric or nearly symmetric in composition. The boundary between the mesophase (M₁) of block copolymer and the homogeneous phase (H) of block copolymer/homopolymer blend was determined using oscillatory shear rheometry, and the boundary between the homogeneous phase (H) and two-phase liquid mixture (L₁+L₂) with L₁ being disordered block copolymer and L₂ being macrophase-separated homopolymer was determined using cloud point measurement. It is found that the addition of PI to a lamella-forming SI diblock copolymer or the addition of PB to a lamella-forming SB diblock copolymer gives rise to disordered micelles (DM) having no long-range order, while the addition of PS to a lamella-forming SB diblock copolymer retains lamellar microdomain structure until microdomains disappear completely. Thus, the phase diagram of SI/PI or SB/PB blends looks more complicated than that of SI/PS or SB/PS blends.     

Thickness and composition dependence of the glass transition temperature in thin random copolymer films

Glass transition temperatures (T_g) of thin poly(styrene-co-methyl methacrylate) and poly(2-vinyl pyridine-co-styrene) films coated on a native oxide surface of Si wafer (100) were measured by ellipsometry. The thickness dependence of T_g can be properly fitted by previously suggested equation developed for homopolymers, based upon a continuous multi-layer model, although one component in thin random copolymer films demonstrates a slightly favorable interaction between a substrate and thin film, and another demonstrates a strongly favorable interaction. Surface and interface have a strong influence on T_g of thin film coated on substrate: the surface has the effect of reducing T_g, whereas the interface increases the T_g according to the degree of interaction between a substrate and thin film. This degree of interaction can be quantified as an interaction parameter (k), and is dependent on the composition of random copolymers. For the estimation of k values of thin random copolymer films, we proposed a parallel type additive function (1/k_ran=w₁/k₁+w₂/k₂) where w is a weight fraction of component.     

Crystallization and coalescence of block copolymer micelles in semicrystalline block copolymer/amorphous homopolymer blends

Crystallization of two oxyethylene/oxybutylene block copolymers (E₇₆B₃₈ and E₁₅₅B₇₆) from micelles in block copolymer/amorphous homopolymer blends was studied by differential scanning calorimetry (DSC) and time-resolved small angle X-ray scattering (SAXS). Unlike the simultaneous crystallization and formation of superstructure in crystallization from an ordered structure, crystallization of block copolymer from micelles can be divided into two steps. The core of the micelles firstly crystallizes individually, with first-order crystallization kinetics and homogeneous nucleation mechanism. The SAXS revealed that crystallization-induced deformation occurs for the micelles, which strongly depends on microstructure of the block copolymers. For the shorter block copolymer E₇₆B₃₈, larger deformation induced by crystallization was observed, leading to coalescence of the micelles after crystallization, while for the longer block copolymer E₁₅₅B₇₆ the micelles show little deformation and the morphology of micelle is retained after crystallization.     

Theoretical study on morphology of ABCD 4-miktoarm star block copolymer

A real-space implementation of the self-consistent field theory (SCFT) has been used to study the morphologies of ABCD 4-miktoarm star block copolymers. For the sake of numerical tractability, the morphologies and the phase diagrams of ABCD 4-miktoarm star block copolymers are investigated in two dimensions (2D) by varying the volume fractions of the blocks and the interaction parameters. Many interesting and complex morphologies occur and compared with ABCD linear block copolymers; ABCD 4-miktoarm star block copolymers have more regular disciplines. We found that systems with similar components have similar morphologies and at the weaker segregation, the minority components always cannot separate from the other blocks and they easily dissolve to form one phase with other block(s), but with the increase of the segregation degree (large ), the ordered phases can be well separated. With the help of our computational prediction, experimental researchers can work more purposefully and efficiently.     

Two-dimensional block copolymer photonic crystals

A high molecular weight polystyrene-block-polyisoprene copolymer was synthesized by anionic polymerization of styrene and isoprene monomers. Polystyrene cylindrical domains in a hexagonal lattice with relatively large periodicity and controlled orientation have been produced through roll casting of the polystyrene-block-polyisoprene copolymer with a total molecular weight of 1.0×10⁶ g/mol. The large periodicity and effective processing lead to reflectivity of about 70% in the visible regime, and generate a two-dimensional photonic crystal with a partial band gap.     

Metallo-supramolecular complexes mediated thermoplastic elastomeric block copolymer

Orthogonal multi-phase strategy has been applied to generate a novel type of thermoplastic elastomeric block copolymer which was obtained via anionic polymerization. Functional terpyridine groups which can potentially form metallo-supramolecular complexes with a variety of metal ions has been introduced to the polystyrene-b-polyisoprene (PS-PI) living chain ends through the termination reaction in an optimized condition. Iron(II), cobalt(II) and zinc(II) metal ions have been used to form metal-ligand complexes with the terpyridine end groups, which can phase separate from the polymer matrix to form hybrid clusters. The formation of metallo-supramolecular hybrid clusters have dramatic effects on the micro-phase separated structures of the PS-PI diblock copolymer, which have been characterized by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy. This type of hybrid material containing hard PS nanophase and metal-ligand clusters exhibit distinct mechanic properties such as increased modulus, higher yield strength and improved toughness, which is further discussed in light of nature of the metal-ligand bonds and the liability of the clusters. The utilization of metallo-supramolecular complex and its high tunability is potential to fabricate new types of supramolecular nanocomposite materials.     

Interdiffusion and phase behavior at homopolymer/random copolymer interfaces

Interdiffusion in polymer bilayers of polystyrene (PS) and the statistically random copolymer poly(styrene-r-4-bromostyrene) (PBS), (C₈H_(8−x)Br_x)_N, where x is the mole fraction of brominated repeat units in the copolymer and N the degree of polymerization, was studied using Rutherford backscattering spectroscopy (RBS). PS/PBS bilayers with 0.04<x<0.63 and the ratio N_PS/N_PBS varied from 0.06<N_PS/N_PBS<18.1 were examined. PBS volume fraction versus depth profiles were obtained from the evolution of the bromine peak in the RBS spectra. It is shown that as the phase boundary is approached, interdiffusion occurs until layer compositions indicative of binodal conditions are reached. These observations are in very good agreement with phase diagrams obtained using Flory-Huggins theory and a PS/PBS interaction parameter measured using small angle X-ray scattering. For N_PS/N_PBS≠1, the mobility is dictated by the faster diffusing (lower N) component, resulting in an interface which moves toward the faster diffusing component. This result is consistent with fast mode theory; equilibrium conditions correspond to the asymmetry of the phase diagrams. Mutual diffusion coefficients were determined by comparison of the RBS data to a mean-field interdiffusion model using the fast mode expression for mobility. The mutual diffusion coefficient is shown to decrease with increasing N and x and increase with temperature. The implications of this miscibility dependence of the interdiffusion behavior, based on both composition of the copolymer and degree of polymerization, are discussed in the context of strengthening homopolymer/random copolymer interfaces.     

Visualization of block copolymer distribution on a sheared drop

We have visualized a fluorescently-labeled poly(styrene-b-methylmethacrylate) (NBD-PS-b-PMMA) block copolymer on the surface of a polymethylmethacrylate (PMMA) drop in a polystyrene (PS) matrix. Confocal microscopy revealed that the block copolymer distributed uniformly on the drop surface before deformation. However, in shear flow the copolymer concentration was higher at the tips and edges of the drop. Visualization of drop deformation using a counter-rotating apparatus showed enhanced drop deformation for a drop with block copolymer resulting in larger area generation. Drops with block copolymer showed widening even for shear strains exceeding 10, in contrast to bare drops, which first widened and then shrank. These results agree qualitatively with the observed distribution of fluorescent block copolymer. Copolymer concentration is highest in the regions of high curvature, where lowering interfacial tension should be most effective in retarding drop retraction. Block copolymer on these highly curved surfaces is found to be very effective since the exact theory for zero interfacial tension by Cristini fits our drop widening results well.     

Effect of nanoparticle surface functionality on microdomain orientation in block copolymer thin films under electric field

The electric field induced microdomain orientations has been an interesting research topic. In this article, the effect of nanoparticle surface functionality on microdomain alignments in block copolymer/nanoparticle hybrid thin films was investigated with transmission electron microscopy experiments. The presence of gold nanoparticles influenced the microdomain orientation behaviors of block copolymer/nanoparticle thin films. The possibility for complete alignment normal to the substrate was illustrated by controlling electric field strength, concentration, and surface ligands of nanoparticles. This work provides basic and essential data to understand the properties and behaviors of emerging block copolymer/nanoparticle hybrid thin films.     

Compatibilizing effects of block copolymer mixed with immiscible polymer blends by solid-state shear pulverization: stabilizing the dispersed phase to static coarsening

A continuous, industrially scalable process called solid-state shear pulverization (SSSP) leads to compatibilization of polystyrene (PS)/high-density polyethylene (HDPE) blends by addition of a commercially available styrene/ethylene-butylene/styrene (SEBS) triblock copolymer. Partial or full compatibilization is characterized by a reduction or elimination of coarsening of the dispersed-phase domains during high-temperature (190 °C), static annealing. In the case of a 90/10 wt% PS/HDPE blend, processing with 3.5 wt% SEBS block copolymer by SSSP yields a coarsening rate that is reduced by a factor of 10 (six) relative to a melt-mixed blend without copolymer (with 3.5 wt% SEBS block copolymer). Addition of 5.0 wt% SEBS block copolymer to the 90/10 wt% PS/HDPE blend during SSSP yields a reduction in coarsening rate by a factor of thirty relative to a melt-mixed blend without copolymer. With an 80/20 wt% PS/HDPE blend, pulverization with 10 wt% SEBS block copolymer yields cessation of coarsening when the average dispersed-phase domain diameter reaches 1.6-1.7 μm. The implications of these results for developing a new, technologically attractive method for achieving compatibilization of immiscible polymer blends are discussed.     

Tuned phase behavior of weakly interacting polystyrene-block-poly(n-pentyl methacrylate) by selective solvent

We investigated, via small angle X-ray scattering, depolarized light scattering, rheometry, and transmission electron microscopy, the phase behavior of the mixture of a symmetric polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing the closed-loop phase behavior and excellent baroplasticity, and dodecanol, a PnPMA-selective solvent. We found that the addition of a selective solvent is simple, but very effective to obtain various microdomains including hexagonally packed cylinders and gyroids. Also, with increasing temperature, the mixtures showed multiple ordered-to-ordered transitions (OOTs) in addition to upper ordered-to-disordered transition (UODT). The first observation of gyroid microdomains in PS-b-PnPMA is very important, although they have been widely reported in many block copolymers, for instance, PS-block-polyisoprene copolymer (PS-b-PI) and PS-block-poly(d,l-lactide) copolymer (PS-b-PLA). Since the gyroid microdomains of PS-b-PnPMA show excellent baroplasticity, external pressure instead of temperature could easily change the microdomains.     

羟化含氟嵌段共聚物的合成及其自组装行为研究

用α-溴代丙酸乙酯（EPN—B）／氯化亚铜（CuCl）／联二吡啶（bpy）作为催化引发体系,环己酮为溶剂,进行甲基丙烯酸2,2,2-三氟乙酯（TFEMA）的原子转移自由基聚合（ATRP）,得到单分散PTFEMA—X预聚体。并以此预聚体为大分子引发剂引发甲基丙烯酸-β-羟乙酯聚合,得到分子质量可控、分子质量分布窄的聚甲基丙烯酸2,2,2-三氟乙酯-b-聚甲基丙烯酸-β-羟乙酯嵌段共聚物,用FTIR、^1H—NMR、GPC等对产物的结构与性能进行了表征。同时利用动态激光光散射（DLS）对嵌段共聚物的自组装行为进行了研究。     

Dynamic formation of SEBS copolymer submicrometric structures

Highly ordered A-B-A block copolymer arrangements in the submicrometric scale, resulting from dewetting and solvent evaporation of thin films, have inspired a variety of new applications in the nanometric world. Despite the progress observed in the control of such structures, the intricate scientific phenomena related to regular patterns formation are still not completely elucidated. SEBS is a standard example of a triblock copolymer that forms spontaneously impressive pattern arrangements. From macroscopic thin liquid films of SEBS solution, several physical effects and phenomena act synergistically to achieve well-arranged patterns of stripes and/or droplets. That is, concomitant with dewetting, solvent evaporation, and Marangoni effect, Rayleigh instability and phase separation also play important role in the pattern formation. These two last effects are difficult to be followed experimentally in the nanoscale, which render difficulties to the comprehension of the whole phenomenon. In this paper, we use computational methods for image analysis, which provide quantitative morphometric data of the patterns, specifically comprising stripes fragmentation into droplets. With the help of these computational techniques, we developed an explanation for the final part of the pattern formation, i.e. structural dynamics related to the stripes fragmentation.