Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with Complex Macromolecular Architecture

Two star triblock terpolymers (PS-<i>b</i>-P2VP-<i>b</i>-PEO)₃ and one dendritic-like terpolymer [PS-<i>b</i>-P2VP-<i>b</i>-(PEO)₂]₃ of PS (polystyrene), P2VP (poly­(2-vinylpyridine)), and PEO (poly­(ethylene oxide)), never reported before, were synthesized by combining atom transfer radical and anionic polymerizations. The synthesis involves the transformation of the −Br groups of the previously reported Br-terminated 3-arm star diblock copolymers to one or two −OH groups, followed by anionic polymerization of ethylene oxide to afford the star or dendritic structure, respectively. The well-defined structure of the terpolymers was confirmed by static light scattering, size exclusion chromatography, and NMR spectroscopy. The self-assembly in solution and the morphology in bulk of the terpolymers, studied by dynamic light scattering and transmission electron microscopy, respectively, reveal new insights in the phase separation of these materials with complex macromolecular architecture

Phase Behavior of Binary Blends of High Molecular Weight Diblock Copolymers with a Low Molecular Weight Triblock

Binary blends of four different high molecular weight poly(styrene-b-isoprene) (SI) diblock copolymers with a lower molecular weight poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymer were prepared, and their morphology was characterized by transmission electron microscopy and ultra-small-angle X-ray scattering. All the neat block copolymers have nearly symmetric composition and exhibit the lamellar morphology. The SI diblock copolymers had number-average molecular weights, M̅n, in the range 4.4 × 105−1.3 × 106 g/mol and volume fractions of poly(styrene), ΦPS, in the range 0.43−0.49, and the SIS triblock had a molecular weight of M̅n ∼ 6.2 × 104 g/mol with ΦPS = 0.41. The high molecular weight diblock copolymers are very strongly segregating, with interaction parameter values, χN, in the range 470−1410. A morphological phase diagram in the parameter space of molecular weight ratio (R = Mndiblock/1/2Mntriblock) and blend composition was constructed, with R values in the range between 14 and 43, which are higher than previously reported. The phase diagram revealed a large miscibility gap for the blends, with macrophase separation into two distinct types of microphase-separated domains for weight fractions of SI, wSI R ∼ 30, morphological transitions from the lamellar to cylindrical and bicontinuous structures were also observed

Superlattice Structure from Self-Assembly of High‑χ Block Copolymers via Chain Interdigitation

Flexible and shape-tunable features of block copolymers (BCPs) with high Flory–Huggins interaction parameters (high χ value) have drawn intensive attention due to their rich phase behaviors. Herein, this work aims to examine a fascinating superlattice structure obtained from the self-assembly of high-χ BCP, polystyrene-block-polydimethyl­siloxane (PS-b-PDMS), as evidenced by reciprocal-space imaging from small-angle X-ray scattering (SAXS) and by real-space imaging from transmission electron microscopy (TEM). Surprisingly, an interesting reversible order–order transition from superlattice structure with chain interdigitation to typical lamellae with bilayer texture can be identified by in situ temperature-resolved SAXS. In contrast to the diblock (PS-b-PDMS)n (n = 1), the forming superlattice structure will be greatly impeded in star-block (PS-b-PDMS)n (n = 3 and 4) with equivalent arm length, suggesting a topological effect on self-assembly due to their star-shaped architecture. Accordingly, a lamellae-forming PS-b-PDMS with chain interdigitation (wet-brush-like chain packing) was proposed to be the origin of the forming superlattice structure. This finding provides an insight for the possible model with ladder-like structure and corresponding transformation mechanisms of high-χ BCPs. Also, the topological effect from star-block architecture may play an important role to justify the formation of such a unique self-assembled texture. These results implicitly explore the feasibility to acquire a superlattice structure from a simple coil–coil diblock copolymer

Retardation of Grain Growth and Grain Boundary Pinning in Athermal Block Copolymer Blend Systems

The effect of filler addition on the grain coarsening characteristics of block copolymer materials is analyzed for the particular case of a lamellar poly­(styrene-<i>b</i>-isoprene)-type block copolymer and polystyrene as well as polystyrene-grafted nanoparticle fillers. Filler addition is shown to reduce the rate of grain growth and to induce grain size distributions that deviate from the log-normal type that is characteristic for pristine block copolymer systems. The retardation of grain growth is shown to be associated with the segregation of filler additives into high energy grain boundary defectsa process that bears similarities to the segregation of impurity atoms within grain boundary structures in ceramics or metals. The analysis of grain boundary energy, grain size distribution, and grain coarsening kinetics suggests two major mechanisms for the interference of filler additives with grain coarsening: First, the segregation of fillers into boundary regions lowers the relative grain boundary energy and hence the driving pressure for grain growth. Second, the formation of particle aggregates along grain boundaries gives rise to a “pinning pressure” that counteracts grain growth and that limits the ultimate grain size during thermal annealing. This is in contrast to pristine block copolymer systems in which continuous grain growth is observed during thermal annealing. The results highlight the fundamental differences between structure evolution in pristine and mixed block copolymer systems and suggest that thermal annealing (in the absence of structure-guiding fields) is an inefficient path to facilitate the controlled growth of large grains in athermal block copolymer blend materials

Extreme Plasticity, Adhesion, and Nanostructural Changes of Diblock Copolymer Microparticles in Cold Spray Additive Manufacturing

Using the laser-induced projectile impact testing (LIPIT), the extreme plastic and adhesive responses of polystyrene-polydimethylsiloxane block copolymer (BCP) microparticles are investigated to provide the ultra-high-strain-rate behavior of individual BCP feedstock powders during their collisions with a stationary substrate in the cold spray additive manufacturing process. The onset of BCP microparticle adhesion to the substrate is precisely predicted by the maximum coefficient of dynamic friction, quantified from the angled collisions, and by the spectra of the coefficients of restitution. This finding confirms the direct correlation between friction and adhesion mechanisms in the ultra-high-strain rate regime and its significance in the consolidation process of BCP feedstock powders. Furthermore, the impact-induced adiabatic shear flows create structural ordering of initially disordered nanostructures of the block copolymers consisting of glassy and rubbery domains while generating a temperature rise beyond their glass transition temperatures. In addition to the conventional strain-hardening effect in homopolymers, nanoscale morphological ordering can provide another strain-hardening mechanism of BCP feedstock microparticles in the cold spray of additive manufacturing

Direct Visualization of Order–Order Transitions in Silicon-Containing Block Copolymers by Electron Tomography

Here, we aim to comprehend the mechanism of the order–order transition (OOT) from nonequilibrium, metastable phase to equilibrium phase. Polystyrene-block-polydimethylsiloxane (PS-PDMS) block copolymer (BCP) bulks with metastable cylinder (C) and double gyroid (G) phases can be obtained from lamellae (L) forming PS-PDMS by simply tuning the selectivity of casting solvent. The recovery of the intrinsic L phase can be achieved by thermal annealing through OOT. Time-resolved small-angle X-ray scattering (SAXS) experiments are carried out to reveal the variation of the structural evolution in reciprocal space during annealing. The structural evolution in real space is directly visualized by using electron tomography (i.e., 3D transmission electron microscopy (TEM)). As a result, combining the time-resolved scattering experiments and the morphological observations from electron tomography offers new insights into the phase behaviors of the OOT of BCPs

Controlled Orientation of Plasma-Treated Diblock Copolymer Films from the Responsive Functionalized Substrate through Solvent Annealing

This study demonstrates a new technique for controlled orientation of nanostructured block copolymer (BCP) thin films through solvent annealing using polystyrene-block-polydimethyl­siloxane (PS-b-PDMS) as a representative BCP system. A two-step substrate functionalization of an intrinsic oxide layer (SiO2) wafer is performed by using hydroxyl-terminated PS (PS-OH) followed by hydroxyl-terminated PDMS (PDMS-OH). By varying the grafting percentage of the PS and PDMS brushes on the substrate, it is possible to give different degrees of stretching and recoiling of grafted PS and PDMS, respectively, using PS-selective solvent for solvent annealing, resulting in roughness variation; that is termed a responsive functionalized substrate. With the appropriate roughness of the functionalized substrate under solvent annealing, the development of perpendicularly oriented cylinders of PDMS in the nanostructured PS-b-PDMS thin films can be driven from the bottom of the film. Moreover, by taking advantage of air plasma treatment, it is possible to generate a top-capped neutral layer on the film surface, giving induced perpendicular cylinders from the top surface of the thin film after solvent annealing. Consequently, it is possible to attain the formation of film-spanning perpendicular cylinders of PDMS in the PS-b-PDMS thin film under solvent annealing through the self-alignment process of the perpendicularly oriented cylinders from the top and the bottom surface of the thin film

Nanocomposites of Polystyrene‑<i>b</i>‑Poly(isoprene)‑<i>b</i>‑Polystyrene Triblock Copolymer with Clay–Carbon Nanotube Hybrid Nanoadditives

Polystyrene-<i>b</i>-polyisoprene-<i>b</i>-polystyrene (PS-<i>b</i>-PI-<i>b</i>-PS), a widely used linear triblock copolymer of the glassy-rubbery-glassy type, was prepared in this study by anionic polymerization and was further used for the development of novel polymer nanocomposite materials. Hybrid nanoadditives were prepared by the catalytic chemical vapor deposition (CCVD) method through which carbon nanotubes were grown on the surface of smectite clay nanolayers. Side-wall chemical organo-functionalization of the nanotubes was performed in order to enhance the chemical compatibilization of the clay–CNT hybrid nanoadditives with the hydrophobic triblock copolymer. The hybrid clay–CNT nanoadditives were incorporated in the copolymer matrix by a simple solution-precipitation method at two nanoadditive to polymer loadings (one low, i.e., 1 wt %, and one high, i.e., 5 wt %). The resulting nanocomposites were characterized by a combination of techniques and compared with more classical nanocomposites prepared using organo-modified clays as nanoadditives. FT-IR and Raman spectroscopies verified the presence of the hybrid nanoadditives in the final nanocomposites, while X-ray diffraction and transmission electron microscopy proved the formation of fully exfoliated structures. Viscometry measurements were further used to show the successful incorporation and homogeneous dispersion of the hybrid nanoadditives in the polymer mass. The so prepared nanocomposites exhibited enhanced mechanical properties compared to the pristine polymer and the nanocomposites prepared by conventional organo-clays. Both tensile stress and strain at break were improved probably due to better interfacial adhesion of the clay–CNT hybrid of the flexible rubbery PI middle blocks of the triblock copolymer matrix

Factors Controlling the Enhanced Mechanical and Thermal Properties of Nanodiamond-Reinforced Cross-Linked High Density Polyethylene

A systematic investigation of the factors influencing the notable enhancement of the mechanical and thermal properties of nanodiamonds (NDs)-reinforced cross-linked high density polyethylene (PEX) is presented in this work. The effects of crystal structure and molecular conformation as well as filler dispersion and adhesion with the matrix were found to govern the mechanical properties of the final composites. A considerable increase in the strength, toughness, and elastic modulus of the materials was found for the composites with filler content below 1 wt %. For higher NDs concentrations, the properties degraded. When filler concentration does not exceed 1 wt %, enhanced adhesion with the matrix is achieved, allowing a more successful load transfer between the filler and the matrix, thus enabling an effective reinforcement of the composites. The higher degree of crystallinity along with larger crystal size are also positively influencing the mechanical properties of PEX. Higher filler concentrations, on the other hand, lead to the formation of larger aggregates, which lead to lower adhesion with the matrix, while they also constitute stress concentrators and therefore reduce the positive reinforcement of the matrix. The thermal conductivity of the composites was also found to be significantly increased for low-filler concentrations. This enhancement was less significant for higher NDs concentrations. It is concluded that this reinforcement is due to the heat capacity increase that NDs incorporation causes in PEX. Additionally, a thermal stability enhancement was found for the composite with minimum filler content

Continuous Equilibrated Growth of Ordered Block Copolymer Thin Films by Electrospray Deposition

Deposition of block copolymer thin films is most often accomplished in a serial process where material is spin coated onto a substrate and subsequently annealed, either thermally or by solvent vapor, to produce a well-ordered morphology. Here we show that under appropriate conditions, well-ordered block copolymer films may be continuously grown under substrate equilibrated conditions by slow deposition of discrete subattoliter quantities of material using electrospray. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature, block-substrate selectivity, and flow rate of the feed solution. For a PEO cylinder-forming poly(styrene-b-ethylene oxide) block copolymer, we uncover a wide temperature window from 90 to 150 °C and an ideal flow rate of 2 μL/min for ordered film deposition from dilute acetone solutions. PEO cylinders aligned with their long axes perpendicular to the film–air interface at optimal spray conditions. Using poly(styrene-b-methyl methacrylate) deposited onto neutrally selective surfaces, we show that the substrate-equilibrated process results in vertically oriented microdomains throughout the film, indicating a preservation of the initial substrate-dictated morphology during the film deposition. Electrospray offers a new and potentially exciting route for controlled, continuous growth of block copolymer thin films and manipulation of their microstructure