Block Copolymer Blends Research Articles

Linear Viscoelasticity and Non‐Linear Elasticity of Block Copolymer Blends Used as Soft Adhesives

AbstractThe debonding of pressure‐sensitive‐adhesives from a rigid surface often occurs with the formation of a fibrillar structure bridging the surface and the adhesive. This fibrillar structure is responsible for the large (of the order of 1 kJ/m2) adhesion energy, which is measured, for this type of adhesive despite the very low level of applied stress. One way to account for this dissipated energy is viscous flow in the elongation process of the fibrils. However, for certain types of adhesives, the extension of the fibrils is essentially a quasi reversible elastic process and the energy is only dissipated rather rapidly when the fibrils are detached from the surface. It is therefore essential to characterize the elastic properties of theses adhesives not only in the linear viscoelastic regime but also in the large strain non‐linear elastic domain. We have therefore performed tensile tests of model adhesive films based on block copolymers. The respective roles played by the linear viscoelasticity and non‐linear elasticity in controlling the properties will be discussed.

Morphology and micromechanical deformation behavior of styrene–butadiene block copolymers. III. Binary blends of asymmetric star block copolymer with general‐purpose polystyrene

AbstractThe correlation between morphology, mechanical properties, and micromechanical deformation behavior of the blends consisting of an asymmetric styrene/butadiene star block copolymer (ST2‐S74, total styrene volume content ΦPS = 0.74) and general‐purpose polystyrene (GPPS) was investigated using transmission electron microscopy and uniaxial tensile testing. Addition of 20 wt % of GPPS to the block copolymer resulted in a drastic reduction in strain at break, indicating the existence of critical PS lamella thickness Dc. Above Dc lamellar block copolymers displayed a transition from ductile to brittle behavior, substantiating the mechanism of thin layer yielding proposed for lamellar star block copolymers. The blends showed a variety of deformation structures ranging from classical crazelike zones to those with internal shearlike components. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1208–1218, 2004

Morphology and micromechanical deformation behavior of styrene–butadiene block copolymers. IV. Structure–property correlation in binary block copolymer blends

AbstractThe structure–property correlation in blends consisting of styrene/butadiene block copolymers forming alternating polystyrene (PS) and polybutadiene (PB) lamellae, and PS domains in rubbery matrix was investigated by different microscopic techniques (transmission electron microscopy, scanning force microscopy, and scanning electron microscopy), uniaxial tensile testing, and dynamic mechanical analysis. Unlike the pure lamellar block copolymer, the blends showed predominantly disordered wormlike morphology formed by the intermolecular mixing. These structures allowed a precise control of stiffness/toughness ratio of the blends over a wide range. The blends showed a gradual transition from predominantly viscoplastic to elastomeric behavior with increasing triblock copolymer content. The results demonstrated that the binary block copolymer blends provide the unique possibility of tailoring mechanical properties on the basis of nanostructured polymeric materials. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1219–1230, 2004

Structure and mechanical properties of talc‐filled blends of polypropylene and styrenic block copolymers

AbstractThe structure–property relationships of isotactic polypropylene (iPP)/styrenic block copolymer blends filled with talc were examined by optical and scanning electron microscopy, wide‐angle X‐ray diffraction, and tensile‐ and impact strength measurements. The composites were analyzed as a function of the poly(styrene‐b‐ethylene‐co‐propylene) diblock copolymer (SEP) and the poly(styrene‐b‐butadiene‐b‐styrene) triblock copolymer (SBS) content in the range from 0 to 20 vol % as elastomeric components and with 12 vol % of aminosilane surface‐treated talc as a filler. Talc crystals incorporated in the iPP matrix accommodated mostly plane‐parallel to the surface of the samples and strongly affected the crystallization process of the iPP matrix. The SBS block copolymer disoriented plane‐parallel talc crystals more significantly than the SEP block copolymer. The mechanical properties depended on the final phase morphology of the investigated iPP blends and composites and supermolecular structure of the iPP matrix because of the interactivity between their components. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1255–1264, 2004

Crack toughness behavior of binary poly(styrene-butadiene) block copolymer blends

Fracture behavior of binary blends comprising styrene-butadiene block copolymers having star and triblock architectures was studied by instrumented Charpy impact test. The toughness of the ductile blends was characterized by the dynamic crack resistance concept (R curves). While the lamellar thermoplastic star block copolymer shows elastic behavior (small scale yielding and unstable crack growth), adding 20 wt% of a triblock copolymer (thermoplastic elastomer, TPE) leads to a strong increase in crack toughness. The stable crack propagation behavior of these blends was described by the crack resistance curve (R) concept of elastic-plastic fracture mechanics. This concept allows the determination of fracture mechanics parameters as resistance against stable crack initiation and propagation. Two brittle to tough transitions (BTT) are observed in the binary block copolymer blend: BTT1 at 20% TPE and BTT2 at about 60% TPE. The strong increase of toughness at 60 wt% TPE indicates a ‘tough/high-impact’ transition as a measure for the protection against stable crack initiation.

Imaging nanostructured fluids using cryo-TEM

Cryogenic transmission electron microscopy (cryo-TEM) has gained increasing popularity among researchers in the field of amphiphilic self-assembly as an experimental method of choice because of its unique ability to visualize nanostructures in complex fluids. Due to many recent technical improvements in the instrumentation, cryo-TEM experiments are now common. However, some limitations and possible artifacts are still a problem, and awareness of them remains a prerequisite for reliable operations and interpretation. The goal of this paper is to provide a review of the basic methods and procedures by which the cryo-TEM experiments are typically practiced. As examples, this paper also presents recent results derived from our cryo-TEM studies of micellization of block copolymer blends which clearly illustrate the unique usefulness of the technique for exploring the unexpected, complex, and/or heterogeneous nanostructures in amphiphilic solutions.

Relating microhardness to morphology in styrene/butadiene block copolymer/polystyrene blends

The microhardness behaviour of binary blends comprising a styrene/butadiene star block copolymer and polystyrene homopolymer (hPS) over a wide composition range is investigated. In particular, the interrelation between the morphology, tensile properties (such as yield stress σ Y and the Young's modulus, E) and the microhardness H is explored. As in the case of microphase separated block copolymers and binary block copolymer blends, as reported in preceding publications, a clear deviation in the microhardness behaviour from the additivity law is observed. The lamellar block copolymer system is compared with the nanostructure of semicrystalline polymers having a lamellar morphology. A dependence of H upon PS lamellar thickness is found. For the samples with lamellar morphology the hardness value was found to correlate with the mechanical parameters obtained by uni-axial tensile testing according to: H/ σ Y∼2.2 and E/ H∼22.

Fracture Behaviour of Styrene/Butadiene Block Copolymers having Different Molecular Architecture

AbstractThe crack toughness behaviour of styrene/butadiene block copolymers of triblock and star architectures was investigated using instrumented Charpy impact testing. In order to evaluate adequately the toughness behaviour of the investigated materials, different concepts of elastic‐plastic mechanics (J‐integral and crack‐tip opening displacement, CTOD concepts) were used. Although the lamellar block copolymers showed a remarkably enhanced ductility in the tensile test than the neat block copolymer having hexagonal PB cylinders in PS matrix, no pronounced difference in crack toughness was found. This behaviour implies that the tensile strain cannot be regarded as the only parameter defining the toughness value. A brittle/tough transition was observed in a lamellar star block copolymer on blending with a linear thermoplastic elastomeric SBS triblock copolymer.SEM micrograph showing the details of the stable crack propagation region in a binary block copolymer blend.magnified imageSEM micrograph showing the details of the stable crack propagation region in a binary block copolymer blend.

Microindentation hardness of SB-block copolymers relating to nano-mechanical mechanisms

Asymmetric styrene/butadiene block copolymers and their blends with polystyrene homopolymer are studied, using transmission electron microscopy and scanning force microscopy, to explore the influence of phase morphology on the microindentation hardness and the nano-mechanical deformation mechanisms. In contrast to polymer blends and random copolymers, in which microhardness generally follows the additivity law, the behaviour of the investigated block copolymer systems is found to significantly deviate from the hardness additivity behaviour. Owing to the modified architecture, the asymmetric star block copolymer having 74 vol% polystyrene possesses a lamellar morphology, which partly explains the observed low hardness values. An additional explanation is found in the large plastic homogeneous deformation of the polystyrene (PS) lamellae by means of a new micromechanical mechanism called thin layer yielding. In the blends of a star block copolymer with polystyrene homopolymer, a rapid change in the micromechanical deformation behaviour is found to cause a shift in the observed microhardness to larger values. A major conclusion of our investigations is that microhardness of the triblock and star block copolymers can only be explained in the light of the morphology and nano-mechanical deformation mechanisms involved.

Thin films of block copolymer blends for enhanced performance of acoustic wave-based chemical sensors.

The performance of quartz crystal oscillator-based volatile organic compound (VOC) sensors has been enhanced by using coatings made from poly(styrene-block-ethylene-co-butylene-block-styrene) block copolymers blended with resins and homopolymers. Enhanced performance is characterized by a wider operational temperature range (-10 to +50 degrees C) over which the sensors displayed, concurrently, an analyte sensitivity of >0.2 Hz/ppm toluene, minimal energy loss (resistance <120 ohms), and response times of <20 min (time required to reach 90% of full response). Atomic force microscopy images are consistent with a process in which the additive associates with the polystyrene portions of the microphase-separated block copolymer. This association reinforces the rigidity of the polystyrene network while allowing the rapid uptake of VOCs by the softer polyethylene/butylene phase.

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.

Correlation between crystallization kinetics and microdomain morphology in block copolymer blends exhibiting confined crystallization

Correlation between crystallization kinetics and microdomain morphology in block copolymer blends exhibiting confined crystallization

Structural Design of Mesoporous Silica by Micelle-Packing Control Using Blends of Amphiphilic Block Copolymers

The formation of mesoporous silica materials has been studied using blends of diblock (CnH2n+1(OCH2CH2)xOH, CnEOx, n = 12 − 18 and x = 2−100) and Pluronic triblock (EOxPO70EOx, x = 5−100) amphiphilic block copolymers as the structure-directing agents and sodium silicate as the silica source. The mesostructure of the silica materials thus obtained, as determined by X-ray diffraction and transmission electron microscopy, changes as the volume of the hydrophilic EO group of the surfactant increases, from lamellar to two-dimensional hexagonal (p6mm), three-dimensional hexagonal, a cubic phase, and another cubic phase with Im3m symmetry. Particle morphologies of the mesoporous silica materials are correspondingly changed from sheetlike to irregular, facetted cubic shapes depending on the periodic mesoscale symmetry of the structure. It is reasonable that the structural transformations are due to the changes in hydrophobic surface curvatures, which can be controlled readily and precisely by using polymer blend...

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 (M 1) 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 (L 1+L 2) with L 1 being disordered block copolymer and L 2 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.

Correlation between crystallization kinetics and microdomain morphology in block copolymer blends exhibiting confined crystallization

AbstractThe crystallization kinetics of poly(ethylene oxide) (PEO) blocks in poly(ethylene oxide)‐block‐poly(1,4‐butadiene) (PEO‐b‐PB)/poly(1,4‐butadiene) (PB) blends were previously found to display a one‐to‐one correlation with the microdomain morphology. The distinct correlation was postulated to stem from the homogeneous nucleation‐controlled crystallization in the cylindrical and spherical PEO microdomains, where there existed a direct proportionality between the nucleation rate and the individual domain volume. This criterion was valid for confined crystallization in which the crystallization was spatially restricted within the individual domains. However, it was possibly not applicable to PEO‐b‐PB/PB, in that the melt mesophase was strongly perturbed upon crystallization. Therefore, it may be speculated that the crystal growth front developed in a given microdomain could intrude into the nearby noncrystalline domains, yielding the condition of cooperative crystallization. To establish an unambiguous model system for verifying the existence of microdomain‐tailored kinetics in confined crystallization, we crosslinked amorphous PB blocks in PEO‐b‐PB/PB with a photoinitiated crosslinking reaction to effectively suppress the cooperative crystallization. Small‐angle X‐ray scattering revealed that, in contrast to the noncrosslinked systems, the pre‐existing domain morphology in the melt was retained upon crystallization. The crystallization kinetics in the crosslinked system also exhibited a parallel transition with the morphological transformation, thereby verifying the existence of microdomain‐tailored kinetics in the confined crystallization of block copolymers. Homogeneous nucleation‐controlled crystallizations in cylindrical and spherical morphologies were demonstrated in an isothermal crystallization study in which the corresponding crystallinity developments followed a simple exponential rule not prescribed by conventional spherulitic crystallization. Despite the effective confinement imposed by the crosslinked PB phase, crystallization in the lamellar phase still proceeded through a mechanism analogous to the spherulitic crystallization of homopolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 519–529, 2002; DOI 10.1002/polb.10121

Micro- and Macrophase Separation in Phenolic Resol Resin/PEO-PPO-PEO Block Copolymer Blends: Effect of Hydrogen-Bonded PEO Length

We demonstrate microphase-separated thermosets based on blends of phenolic resol resin and poly(ethylene oxide)-block-poly(propylene oxide)-block- poly(ethylene oxide) (PEO-PPO-PEO), i.e., so-called Pluronics. Three triblock copolymers are used (PE 9200, PE 10300 and PE 9400) where the molecular weights of the PPO blocks are nearly equal and the weight fractions of the PEO blocks f PEO are 0.20, 0.30 and, 0.40, respectively. The blends are prepared in a particularly straight-forward way using aqueous solutions and thermal crosslinking. Structure formation is characterized using transmission electron microscopy and small-angle X-ray scattering. PPO turns out to be sufficiently repulsive to allow microphase separation in the bulk crosslinked phase and the tendency for macrophase separation upon curing is reduced due to the hydrogen bonding between the PEO and resol. The weight fraction of PEO-PPO-PEO in the present blends has been limited to a relatively small value, i.e., 20 wt.-%, and spherical microphase-separated structure is observed for f PEO = 0.40 with a long period of the order 120 A. Macrophase separation manifests upon curing if the weight fraction of the PEO blocks is smaller, i.e., f PEO = 0.30 or f PEO = 0.20. In addition, in order to prevent macrophase separation, the molecular weights of PEO blocks and resol resin before curing are of the same order. In that respect, the system behaves qualitatively similar to the corresponding thermoplastic homopolymer/ block copolymer blends.

Two-dimensional phase separation of block copolymer and homopolymer blend studied by scanning near-field optical microscopy

The phase separation of dye-labeled poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) (PODMA) in two-dimensional monolayers was investigated by scanning near-field optical microscopy (SNOM). The energy transfer (ET) efficiency between dyes introduced to the polymers was mapped with SNOM for a PiBMA/PODMA homopolymer blend monolayer. The ET fluorescence images revealed that the phase boundary had a width of a few hundred nanometers, which was considerably larger than that expected in a three-dimensional bulk state. The micro-phase separation of a diblock copolymer PiBMA- block-PODMA was also discussed. Ribbon-like lamellar structures with a width of ca. 300 nm were observed, and the block polymer chain took a highly elongated conformation in two dimensions.

Superlattices in block copolymer blends with attractive interactions

AbstractBlends of microphase separated polystyrene-block-polybutadiene-blockpoly( methacrylic acid) triblock copolymers (SBA) with polystyrene-block-poly(2- vinylpyridine) (SV) or poly(2-vinylpyridine)-block-poly(cyclohexyl methacrylate) (VC) diblock copolymers were prepared and characterized. Attractive segmental interactions through hydrogen bonds between A and V could be monitored by infrared spectroscopy and dynamic mechanical analysis. Common superlattices were obtained by casting from a mixed solution of these block copolymers in tetrahydrofuran and were investigated by transmission electron microscopy. Varying the amount of hydrogen bonding donors via controlled saponification of an SBT triblock copolymer (T: poly(tert-butyl methacrylate)) lead to different superlattices in blends with a VC diblock copolymer.

Morphology and crack resistance behavior of binary block copolymer blends

Fracture behavior of binary blends comprising of styrene–butadiene block copolymers having star and triblock architectures was studied via instrumented Charpy impact test. The toughness of the ductile blends was characterized by dynamic crack resistance curves ( R-curves). This study represents a systematic investigation of crack resistance behavior of nanometer structured binary block copolymer blends and the development of a new material with a combination of high toughness and transparency, usually not observed in incompatible polymer blends. While the lamellar star block copolymer shows an elastic behavior (small-scale yielding and unstable crack growth), adding of 20 wt% of the triblock copolymer leads to a stable crack growth and at 60 wt% of the triblock copolymer the strong increase of toughness values indicate a tough/high-impact transition, demonstrating the existence of novel toughening concepts for polymers based on nanometer structured materials.

Templating of cylindrical and spherical block copolymer microdomains by layered silicates

The influence of a highly anisotropic layered silicate (organically modified montmorillonite) in directing the mesoscopic self-assembly of a block copolymer blend is studied as a model for the development and tailoring of templated inorganic–organic hybrid materials. The potential for nanometer thick layers to induce large-scale mesoscopic ordering of cylindrical and spherical microdomains in asymmetric block copolymers is studied using a combination of rheology, electron microscopy, and small angle neutron scattering. Spherical microdomains arranged on a bcc lattice are templated by the anisotropic layered silicate and the kinetics of their growth are dramatically accelerated by the presence of even 0.1 wt.% (0.04 vol.%) of the filler. However, for cylindrical microdomain ordering, the kinetics are essentially unaffected by the addition of layered silicates and the development of three-dimensional mesoscopic order is possibly even disrupted. These results suggest that for the development of three-dimensional well-ordered nanostructures, the surface defining the pattern has to be significantly larger than the leading dimension of the structure being templated.