Search

Block polymers assemble into a variety of phase-separated morphologies based on volume fraction (φ) and interactions (χ) of the respective blocks. The arrangement of three different polymer blocks ...

Brush block copolymers (BBCPs) exhibit attractive features for use as templates for functional hybrid nanomaterials including rapid ordering dynamics and access to broad ranges of domain sizes; however, there are relatively few studies of the morphology of the BBCPs as a function of their structural variables and fewer still studies of BBCP composite systems. Here we report the structural diversity and phase behavior of one class of BBCP nanocomposites as a function of the volume fractions of their components and the side chain symmetry of the BBCPs. We conducted a systematic investigation of gold nanoparticle (NP) (∼2 nm) assembly in a series of poly(tert-butyl acrylate)-block-poly(ethylene oxide) (PtBA-b-PEO) BBCPs with a fixed side chain length of PtBA (Mn = 8.2 kg/mol) but with different PEO brush lengths (Mn = 5.0, 2.0, or 0.75 kg/mol) as well as volume fractions (fPEO = 0.200–0.484). The gold NPs are selectively incorporated within the PEO domain via hydrogen bond interactions between the 4-mercapto...

Well-ordered block copolymer nanocomposites with d-spacings as small as 15 nm and additive loadings greater than 70 mass % were formed upon blending functionalized polyhedral oligomeric silsesquioxanes (POSS) additives into disordered block copolymers containing poly(ethylene oxide) (PEO) as one of the blocks. The POSS additives were functionalized with maleamic acid or aminophenyl groups as ligands to enable selective hydrogen bonding with the PEO chains. The selective interaction of the additives with the PEO chains caused microphase separation of the block copolymer leading to formation of well-ordered morphologies as evidenced by small-angle X-ray scattering. Further addition of the additive induced transitions between cylindrical and spherical morphologies. While both ligands induced order, the maleamic acid ligand enabled higher levels of incorporation of POSS cages into the PEO phases of the composite. Differential scanning calorimetry and wide-angle X-ray diffraction were performed to confirm comp...

Micro- and nanoscale crystals of the thermochromic polydiacetylenes (PDAs) ETCD and PUDO exhibit electronic absorption maxima at wavelengths longer than those observed in bulk crystal specimens of the same materials. This observation is in contrast to the reported behavior of PDA−DCH where the absorption maximum shifts to shorter wavelengths as crystal size decreases. Infrared spectra and X-ray powder data indicate that the small crystals have the same composition and a structure similar to the bulk specimens. Differential scanning calorimetry (DSC) reveals a larger endothermic heat of the thermochromic phase transition in the small crystals vs bulk specimens. The crystal morphology and structure of microsized, nanosized, and bulk crystals of PDA−ETCD and PDA−PUDO were studied by transmission electron microscopy (TEM) and electron diffraction (ED). For both PDAs, no drastic changes in lattice parameters were observed as crystal size decreases from bulk to micro- and nanoscale.

We investigate the effect that process conditions have on the structural characteristics and mechanical response of a cylindrically phase-separated poly(styrene−ethylene-co-butylene−styrene) (SEBS) triblock copolymer system. Small-angle X-ray diffraction (SAXD) and transmission electron microscopy (TEM) have been used to study the microdomain and grain structure, respectively. It is observed that the structural changes that occur with different process conditions and thermal treatment alter the mechanical behavior in both monotonic tensile and cyclic tests. Results show that the d-spacing decreases with increasing temperature and increasing cooling rate from the melt state. These subtle changes in d-spacing are correlated with the elastic modulus. Such variations in domain spacing and size depend on the segregation power (χ) of the system. Grain size changes significantly with cooling rate and upon annealing fine-grain structures but has no significant effect on the material's modulus.

We have observed nematic textures in polarized optical microscopy and "plus" (+) patterns in depolarized light scattering (four lobes parallel to polarizer/analyzer axes) from a deeply quenched diblock copolymer melt with a cylindrical microstructure. Both of these features are similar to those found in nematic liquid crystalline polymers. In contrast, this same polymer sample exhibits a more conventional grain structure at shallower quench depths, characterized by an "X" pattern in depolarized light scattering (lobes at 45° to polarizer/ analyzer axes). A theoretical model is used to analyze the "plus" scattering patterns obtained from nematic textures in the single scattering limit. This model is used to quantify the nature of local correlations in our deeply quenched block copolymer melt. We show that the nematic texture is easy to align by shear flow while the granular structure is not.

Characterization of pore structure and pore wall crystal structure was performed on porous high-density polyethylene (HDPE) material using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction (ED). The porous HDPE material was obtained through crystallization from swollen cross-linked polyethylene gels (CSX process1) in supercritical propane. SEM showed an open-pore structure of micron-sized pores, large void fraction, and surface area as well as thin yet rigid pore walls, making this material a good candidate for a variety of applications. TEM revealed oriented lamellar structure in the pore walls which was much different from structures found in typical bulk HDPE as well as that of the cross-linked HDPE before CSX processing. Electron diffraction results confirmed the presence of oriented lamellar stacking. On the basis of this oriented lamellar structure, possible mechanisms for crystallization and pore formation are suggested.

Using a series of AnBn miktoarm star block copolymers with different numbers of arms (n = 1, 2, 4, and 16), the effect of molecular architecture on the grain growth kinetics was investigated by ann...

A novel bottom-up approach to obtain polymer nanocomposites using cubic silsesquioxanes (POSS) nanoparticles as building blocks is reported. The design is based on associative interaction between particles to form ordered nanostructure and limited crystal growth to render anisotropic shapes. Specifically, the affinity between POSS units causes these particles to aggregate and closely pack into a crystalline lattice. The organic polymer, covalently connected to each POSS unit, limits the crystallization into a two-dimensional lattice as demonstrated in random copolymers of polybutadiene and cubic silsesquioxanes. The copolymers were synthesized by ring-opening metathesis copolymerization of cyclooctadiene and POSS bearing a polymerizable norbornene group. The polymers were characterized using NMR, DSC, TEM, WAXD, and SAXS. The data from TEM and X-ray diffraction clearly show the formation of two-dimensional lamellar-like nanostructures of assembled cubic silsesquioxanes.

The living cationic sequential block copolymerization of isobutylene (IB) with tert-butyl vinyl ether (tBVE) was studied using a one-pot procedure in hexanes/CH2Cl2 and hexanes/CH3Cl solvent systems at −80 °C. It was carried out by the so-called capping-tuning technique, involving the capping of living PIB chain ends with 1,1-ditolylethylene (DTE) followed by addition of titanium(IV) isopropoxide (Ti(OIp)4) to lower the Lewis acidity before the introduction of tBVE. Homopolymerizations/model block copolymerizations of tBVE were carried out using 2-chloro-2,4,4-trimethylpentane (TMPCl) instead of living PIB. The living nature was exhibited by the linear plots of ln([M]0/[M]) vs time and number average molecular weight (Mn) vs conversion. 13C NMR spectroscopy indicated that the PtBVE is highly isotactic with close to 80% meso dyads. Well-defined PIB-b-PtBVE diblock copolymers were synthesized by the living cationic polymerization of IB followed by capping the living PIB ends with DTE and fine-tuning the Lew...

An electric field induced sphere-to-cylinder transition in thin films of asymmetric polystyrene-b-poly(methyl methacrylate) diblock copolymers was observed. In the absence of an applied electric field, thin films of the asymmetric diblock copolymer consisted of layers of spherical microdomains with poor in-plane long-range ordering. Under a ∼40V/μm applied electric field, hexagonally packed cylindrical microdomains normal to the surface were found. Cross-sectional transmission electron microscopy images of the intermediate stages of the alignment indicated that, under an electric field, the asymmetric diblock copolymer formed spherical microdomains that were deformed into ellipsoids and, with time, interconnected into cylindrical microdomains oriented in the direction of the applied electric field. Simulations suggest that improved long-range order of the cylindrical microdomains could be achieved by cycling the electrical field.

The phase behavior of mixtures of polyisobutylene (PIB), polyethylene (PE), and a symmetric polyethylene-block-head-to-head polypropylene copolymer (PE−PP) was studied by transmission electron microscopy (TEM) and small-angle neutron and light scattering. The thermodynamic interactions between PE/PP and PE/PIB are repulsive (Flory−Huggins parameter χ > 0 and decreases with increasing temperature), while those between PP/PIB are attractive (χ < 0 and increases with increasing temperature). When the PE−PP copolymer is added to a 50/50 PE/PIB mixture, the resulting phase diagram in temperature−copolymer composition space exhibits many of the characteristics of “fish-shaped” phase diagrams found in oil/water mixtures stabilized by balanced surfactants. This is due to the interplay between the different χ parameters that characterize the system. Lamellar phases, single droplet microemulsions, and bicontinuous microemulsions were observed. The length scales of these structures and the locations of the phase tra...

To probe the effect of junction point functionality in miktoarm star block copolymer architecture on chain conformation and morphology, a series of AnBn miktoarm star copolymers where A arms are PS blocks and B arms are PI blocks were investigated. The overall series including a diblock and the star block copolymers can be represented by AnBn, where n = 1, 2, 4, and 16. These materials were produced by synthesizing a single batch of living PS arms and a single batch of living PI arms and then linking them together with chlorosilane coupling agents of different functionality. Thus, all PS arms are identical and all PI arms are identical across the entire series of materials. All stars in the series have equal numbers of PS and PI arms, and the volume fractions of all the samples in the series (nearly 0.50 PS by volume) are identical within experimental error. All the materials were found, via small-angle X-ray scattering and transmission electron microscopy, to form lamellar morphologies. A significant inc...

A series of five cyclic block copolymers of styrene and butadiene, having essentially the same molecular weight (52 ± 5 kg/mol) and PS volume fraction varying from 11 to 70%, were synthesized by cyclization of α,ω-dilithium polystyrene−polybutadiene−polystyrene triblock copolymers with bis(dimethylchlorosilyl)ethane. The cyclic block copolymers thus obtained have practically the same molecular weight and composition as their corresponding linear triblock copolymers. All materials were investigated via transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) techniques. In three cases where the cyclic and the corresponding linear block copolymer had the same morphology, the domain spacings of the cyclic block copolymers are found to be 84%−89% of those of their respective linear triblock copolymers. In the other two cases different morphologies are found in the cyclic and its corresponding triblock copolymer. Compared to their linear triblocks, the interfaces are curved away from the ...

Hybrid copolymers with dendritic−linear blocks are shown to exhibit many of the classic microphase-separated structures of linear−linear block copolymers. Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) were used to evaluate the morphology of a sixth generation (G6) poly(benzyl ether) dendron covalently bonded to linear polystyrene (PS) at the dendron focal point. Increasing the fraction of the linear block, φPS, through an increase in the molecular weight of the PS block revealed morphologies evolving from disordered to ordered lamellar to hexagonally close-packed dendron cylinders. Significantly, the observed morphologies are distinct from those expected for analogous linear−linear blocks at equivalent volume fraction, although the direction of progression follows expectation. Quantitative analysis suggests substantial molecular deformation or shape change in the dendritic phase. The possible role of conformational entropy in determi...

Aliphatic 22,12-polyurethane has been crystallized directly from the melt and isothermally at 105 °C from N,N-dimethylformamide to give chain-folded lamellae of thickness ≈ 14 nm. The crystals have been investigated using X-ray diffraction, electron microscopy (imaging and diffraction), and computer modeling. The diffraction data was used to establish that the chains crystallize in a single-chain, triclinic unit cell, and the structure bears a strong resemblance to chain-folded lamellar crystals of even−even nylons. The results show that, even for a molecule with relatively long alkane segments, the hydrogen bonds exercise a controlling influence on the crystallization process and crystalline structure. Two models were investigated: one based on an analogy with nylon 6 6 and the other derived using molecular dynamics; they both turned out to be closely related. The model generated using molecular dynamics predicted localized rotations at the termini of the ester−amide coupled units. The resolution of the...

A series of linear, aliphatic m,n-polyurethanes [O−(CH2)m−O−C(O)−NH−(CH2)n−NH−C(O)]x derived from long-chain aliphatic diols HO−(CH2)m−OH, where m = 12, 22, or 32, and much shorter diisocyanates OCN−(CH2)n−NCO, where n = 4, 6, 8, or 12, were previously characterized and shown to have physical and thermal properties similar to polyethylene. The current study shows, however, that hydrogen bonding still exercises a controlling influence on the crystallization process of these long-chain, aliphatic polyurethanes. X-ray diffraction, electron diffraction, and infrared spectroscopy indicate that these long alkane segment polyurethanes have interchain and intersheet distances similar to that seen for polyamides and polyurethanes of higher hydrogen bonding densities. Hydrogen bonding controls the crystallization, packing, and morphology of these polyurethanes, resulting in a crystal structure analogous to that of aliphatic, even−even (syncephlic) polyamides and unlike that of polyethylene. Additionally, high-tempe...

Morphological characteristics and mechanical properties of a series of block-double-graft (BDG) copolymers and terpolymers polystyrene−[1,2-polybutadiene-g-X2] (X = 1,4-polybutadiene, polyisoprene, polystyrene, and polystyrene-b-polyisoprene diblocks) were investigated by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and tensile testing. All BDG materials have linear polystyrene−1,2-polybutadiene (PS-b-1,2-PBD) diblock copolymer backbones. Two identical branches are grafted at every randomly distributed tetrafunctional junction point on the 1,2-PBD part of the backbone. Standard microstructures, such as body-center-cubic spheres, hexagonally packed cylinders, and lamellae, are obtained at different total PS volume fractions. It is found that when the branches are polydienes, the BDG molecules form the same morphologies as their linear diblock counterparts. In such cases, phase separation occurs between the polystyrene domain and a combined diene microdomain formed by the bac...

The grain structure in lamellar block copolymer samples undergoing a disorder-to-order transition was studied by a combination of depolarized light scattering (DPLS) and transmission electron microscopy (TEM). The 4-fold symmetry of the DPLS profiles indicated the presence of anisotropic grains. A pattern recognition algorithm for analyzing the TEM micrographs of samples partially filled with anisotropic, ordered grains was developed. The volume fractions of sample occupied by ordered grains determined from light scattering and electron microscopy are in reasonable agreement. Both methods indicate that, on average, the characteristic length of the grains in the direction perpendicular to the lamellar planes was a factor of 2 larger than that in the plane of the lamellae. The absolute magnitudes of grain sizes determined by light scattering are about 50% larger than those determined by microscopy.

The crystallization behavior of functionalized polymethylene chains with carbamate esters placed periodically between alternating docosyl and octyl segments has been examined with infrared spectroscopy. Specific features of both the interactions and the chain conformation have been found. The crystallization kinetics, including the induction period, can be followed with time-resolved infrared spectroscopy (5 s time resolution) and occurs in three stages. The kinetics of local hydrogen-bonding rearrangement is quite different from the kinetics of methylene chain stem ordering. The interchain interactions are characterized by a broad distribution of states. The initial melt consists of highly interacting chains (75% hydrogen bonded). During crystallization, this broad asymmetric ensemble of interacting states changes continuously to one dominated by that characteristic of the ordered structure. Even with the time resolution achievable, the specific features of a crystallite nucleus could not be captured.

A morphological study of three I5S six-arm miktoarm star block copolymers is presented. These miktoarm stars are comprised of five arms of polyisoprene (PI) and one arm of polystyrene (PS) joined together at a single junction point. The strong segregation limit theory for the morphological behavior of miktoarm stars predicts that these materials should form spherical morphologies, but only lamellar and cylindrical morphologies were observed by TEM and SAXS. These results are similar to previously reported discrepancies between experimentally observed morphological behaviors of miktoarm stars and the predictions of the theory. Previous work has attributed the discrepancies to the neglect of the effect of the multifunctional junction points on calculated free energies. The current results suggest that, in addition to this, geometrical packing constraints prevent the formation of morphologies such as spheres and cylinders in highly asymmetric miktoarm stars in which the minor volume fraction component would ...

The bulk morphology of a crystalline/amorphous diblock copolymer under different thermal conditions was studied. The diblock copolymer, poly((ethylene oxide)-b-1,4-polybutadiene), forms a microphas...

This work is part of an extensive study of model nonlinear block copolymer/homopolymer blends. Effects of graft molecular architecture on the morphology of block copolymer/homopolymer blends have been examined. The single graft Y-shaped block copolymers used in the study are I2S block copolymers, which have two low polydispersity (PDI) polyisoprene arms and one low PDI polystyrene arm joint at a single junction point. Previously reported linear diblock copolymer/homopolymer blend systems showed that the order−order transitions (OOTs) occur at about the same volume fractions as in pure linear diblock copolymers. The OOT occurs at the same volume fraction regardless of the direction from which it is approached, i.e., blending homopolymer A with a diblock which forms A cylinders in a B matrix to push it toward lamella or blending B homopolymer with a lamellar diblock to push it back toward cylinders. This study shows that when a homopolymer is blended with an I2S block copolymer, the OOTs split so that they ...

The evolution of the morphology of a crystalline/amorphous diblock copolymer poly(ethylene oxide-b-1,4 butadiene) (P(EO-b-BD)) upon crystallization in thin films was studied via interference optical microscopy. Two-dimensional crystallization confined within the PEO lamellar layers was observed with retention of the microphase-separated lamellar morphology formed in the melt state. The morphology was further characterized by TEM and electron diffraction which showed it to consist of alternating layers of PEO and PBD with PEO crystalline chains oriented perpendicular to the lamellar layers of the microphase-separated structure. Multiple parallel layers of crystalline PEO were found by electron diffraction to be in orientational registry even though they were separated by approximately 10 nm thick layers of amorphous PBD.

T-junction grain boundaries were studied in a blend of polyisoprene homopolymer and a single graft block copolymer I2S with two equal length blocks of polyisoprene and one arm of polystyrene linked at a common junction point. The overall polyisoprene volume fraction in the blend was 0.52, and its equilibrium morphology was lamellar. While T-junctions were previously observed to be quite rare compared to other tilt grain boundary morphologies such as chevrons and omegas, they were found in abundance in the blend used in the current study. The T-junctions in the blend show a number of distinctive characteristics including enlarged semicylindrical end caps terminating polystyrene lamella and an increase in the spacing of the lamella as they near their termination at the T-junction. Simple free energy calculations show that the homopolymer present in the blend stabilizes the cylindrical curvature of the end caps, rendering the T-junction morphology more stable in blends than in neat block copolymers. This agr...

An investigation of the morphological behavior of a series of graft copolymers having multiple regularly spaced, tetrafunctional branch points has been carried out. The behavior of these materials, comprised of polyisoprene backbones with two polystyrene arms grafted to the backbone at each branch point, is shown to be effectively modeled by considering the behavior of smaller, architectural subunits based on the local environment of each junction point (constituting block copolymer). Morphological behavior was characterized using TEM and SAXS. Well-ordered cylindrical and lamellar morphologies were observed. Several samples appear to form cylindrical domains in disordered arrangements. Samples predicted to form spheres form instead a microphase-separated “mesh” morphology. Lamellar grain size and shape were also investigated, and lamellar grain orientation correlation lengths were determined. These measurements show a decrease in grain size with increasing number of branch points per molecule. They also ...

A morphological study of three A5B, six-arm miktoarm star block copolymers is presented. The miktoarm stars are comprised of five arms of polyisoprene and one arm of polystyrene joined together at a single junction point. The strong segregation limit theory for the morphological behavior of miktoarm stars predicts that these materials should form cylindrical (two samples) and lamellar (one sample) morphologies, but only lamellar morphologies were observed by TEM and SAXS. These results are similar to previously reported discrepancies between miktoarm star morphological behavior and the predictions of the theory. Combining the data of this study with that of previous morphological studies of miktoarm star materials, we can track the increasing discrepancy between the experimentally observed morphology and theoretical predictions as the molecular asymmetry parameter, e, increases. The A5B materials in this study were also observed to form exceptionally well-ordered morphologies.

A microphase-separated core−shell cylinder morphology has been observed, via transmission electron microscopy and small-angle X-ray scattering, in diblock copolymers of polystyrene (PS) and poly(1,3-cyclohexadiene) (PCHD). The structures consist of PS cylindrical cores surrounded by PCHD cylindrical annuli which are then hexagonally packed in a matrix of PS. The materials were produced by anionic polymerization of styrene followed by 1,3-cyclohexadiene. Characterization by size exclusion chromatography revealed a main peak due to diblock with a very narrow molecular weight distribution. However, a significant amount of PS homopolymer (about 30%) was present in the as-synthesized materials. The as-synthesized materials with homopolymer present produced core−shell morphologies, and these structures became much more regular when the homopolymer was removed by fractionation. After fractionation, the pure core−shell forming diblocks had PCHD volume fractions of around 0.37 and polydispersities well under 1.1.

The morphologies of two series of model graft copolymers were studied using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Both series of materials have monodisperse polybutadiene (PB) backbones and monodisperse polystyrene (PS) graft blocks. In one series there are on average five trifunctional junction points randomly distributed along the PB backbone. Each junction point grafts one PS block to the backbone. In the other series there are on average four tetrafunctional junction points randomly distributed along the PB backbone. Each junction point grafts two PS blocks to the backbone. A range of overall PB and PS volume fractions was investigated for both series. These materials simulate a controlled and known degree of architectural disorder. Current theory cannot rigorously predict the morphological behavior for these complex molecular architectures. However, it is found that an approximate extension of existing theory utilizing the constituting block copolymer (fundam...

Microstructures of block copolymers in the strong segregation limit are characterized by well-defined interfaces separating the different block materials into domains on a nanometer scale. In this paper, we address the effects of architectural and conformational asymmetry of the blocks on the interfacial curvature characteristics and on the degree of long-range order in the cylindrical morphologies. Experimental (TEM and SAXS) curvature data from polyisoprene−polystyrene (I2S) simple graft block copolymers and from polyisoprene−poly(tert-butyl methacrylate) (PtBMA) linear, conformationally asymmetric diblock copolymers are presented and compared to data from polyisoprene−polystyrene linear diblock copolymers. The experimental data are elucidated by a simple curvature free energy model which accounts for core-space-filling without explicitly specifying the shape of the microdomain. This model allows the prediction of preferred interfacial curvature characteristics as a function of molecular architecture. Good agreement is obtained between the theoretically calculated mean and Gaussian curvatures and the experimentally measured values. A key finding is that the degree of frustration, as measured by the difference between the free energy of the preferred curvature of a given block copolymer and that of the nearest accessible space-filling structure (such as the cylindrical structure), is correlated with the degree of long-range order.

The rheo-optical response of solutions of silk in aqueous 9 M LiBr of concentrations ranging from 7.5 to 15 wt % was investigated. These samples were found to be birefringent under flow, with the steady state birefringence having a concentration scaling of c 3.9 . This scaling behavior suggests a wormlike conformation of the silk molecules in solution. The relaxation was fast, and Maxwellian behavior was observed at low shear rates. No dichroism was detected suggesting that electrostatic screening suppresses structure formation under shear.

The relation between rheology and the time dependent morphology of a phase-separating binary blend of polystyrene and poly(vinyl methyl ether) was investigated by heating a sample from the single-phase (at 90 °C) into the two-phase regime (at 124 °C, 16 K above the LCST) and maintaining its temperature there while measuring the evolution of the dynamic moduli G‘ and G‘‘. Morphological changes occurred slowly so that there was sufficient time to cycle the dynamic mechanical measurements repeatedly over five decades in frequency. The morphology was observed on length scales from 1 mm down to 1 nm by conventional optical microscopy combined with digital image analysis, Hoffman modulation microscopy, TEM, and WISE NMR with spin diffusion. NMR shows that major compositional changes occur mostly in the first 20 min and then the composition remains constant at about 60:40 PS/PVME for the PS-rich matrix and 5:95 PS/PVME for the PVME-rich microdomains. The PVME-rich microdomains are separated by thin layers of the...

The helicoid section morphology allows a diblock copolymer lamellar phase to maintain microphase separation across a twist grain boundary. The interface between the two microphases in the grain boundary region approximates a stack of sections of the helicoid minimal surface. Grain boundary energies were calculated for the helicoid section morphology both as a function of diblock chain characteristics and as a function of grain boundary twist angle. The basic approach to grain boundary energy calculation is to formulate a general expression for local free energy density as a function both of chain characteristics and of the local curvature of the interface. The local energy density is then integrated over the mathematical model for the Scherk grain boundary. Two general methods of calculation were used, and the results where then compared. First, a self-consistent field model was formulated in which average energies per chain were calculated for all the possible interfacial curvature environments encounter...

This paper reports on the synthesis and morphological characterization of graft block copolymers in which a single polystyrene (PS) arm was grafted at an asymmetric position along a polyisoprene (PI) backbone. These materials represent a model series of asymmetric simple graft (ASG) block copolymer structures. The synthesis of these materials was carried out with methods developed for three-arm “miktoarm” star copolymers using anionic polymerization high-vacuum techniques with cholorosilane linking agents. The three arms were two polyisoprene blocks with different degrees of polymerization and one deuterated polystyrene block, which formed an asymmetric simple graft structure (ASG). Molecular characterization was performed using size exclusion chromatography (SEC) with refractive index and UV detection, membrane osmometry, and low-angle laser light scattering. These techniques confirmed that the materials exhibited narrow molecular weight distributions and low compositional heterogeneity. The morphologies...

The morphological behavior of three well-defined miktoarm star block copolymers having 16 arms/molecule was characterized using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) techniques. The molecules, called Vergina stars, have 8 arms of polyisoprene and 8 arms of polystyrene radiating from a single junction point. The samples, with polystyrene volume fractions of 0.37, 0.44, and 0.47 and total molecular weights ranging from 330 000 to 894 000, were all found to microphase separate into lamellar morphologies. In this respect all three samples, in agreement with theory, behave in the same way as linear diblock copolymers of the same relative volume fractions. Incorporating results from previous studies in the literature of miktoarm block copolymers containing trifunctional and tetrafunctional branch points, as well as the new Vergina star data, the scaling behavior of the area per junction versus junction functionality was investigated.

Well-defined graft copolymers with polyisoprene backbones and polystyrene branches, having trifunctional branch points, of the type S2IS2 (H-shaped) and (SI)I(SI) (π-shaped) were synthesized by anionic polymerization high-vacuum techniques. The synthetic strategy involves the preparation of the outer parts of the molecules, carrying one reactive Si−Cl bond, followed by coupling with difunctional living poly(isoprenyllithium) chains. In this way, the number and placement of the branches can be precisely controlled. Molecular characterization of the fractionated samples by size exclusion chromatography with UV and RI detection, membrane osmometry, low-angle laser light scattering, and 1H-NMR spectroscopy confirmed that the materials exhibit narrow molecular weight distributions and low compositional heterogeneity. The strongly microphase-separated morphologies of these two samples were characterized using TEM and SAXS. The π architecture with a PS volume fraction of 0.21 was found to form body-centered cubi...

A new equilibrium morphology consisting of randomly oriented wormlike micelles dispersed in a continuous matrix is observed in a neat, strongly segregated I2S simple graft block copolymer. The equilibrium nature of the worm phase is determined via a set of selective solvent casting and prolonged annealing experiments. Transmission electron microscopy (TEM) experiments on quenched samples allow a unique opportunity to directly observe the transition of a kinetically trapped, nonequilibrium folded-layer morphology, formed by casting the sample with a solvent selective for polyisoprene (PI), into the equilibrium, randomly oriented worm phase through an intermediate folded-lace morphology. The folded-lace intermediate is similar to the “mesh” structure previously observed by Hashimoto et al. in starblock/homopolymer blends.1 The simple graft block copolymer, formed by grafting a single polystyrene (PS) chain onto the center of a polyisoprene backbone, introduces a 2:1 PI/PS arm number asymmetry in the microph...

The morphological behavior of a series of well-defined A2B simple graft or “Y” architecture block copolymers is characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). This model architecture is formed by grafting a polystyrene block onto the center of a polyisoprene backbone. The volume fraction windows in which specific strongly segregated microphase-separated morphologies are observed are shifted to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers. These findings are in good agreement with recently calculated theoretical phase behavior for simple graft, A2B, block copolymers. However, a new morphology, not found in neat linear diblock copolymers, is also observed. This A2B material is microphase separated into wormlike micelles but not ordered on a lattice. This morphology is found at high PS graft volume fraction (φs = 0.81), where the two PI chains per molecule are initially forced to the concave side of the...

A look inside the silk spinning process along the length of a silk gland has been achieved by the cryogenic quenching and subsequent microtoming of live silk-spinning animals, Nephila clavipes (spider) and Bombyx mori (silkworm). Observations made using transmission electron microscopy, electron diffraction and atomic force microscopy indicate a cholesteric liquid crystalline phase of aqueous silk fibroin in the early duct portion of the major silk-producing gland in both species. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide evidence for the cholesteric intermediate phase. The fracture surface produced by the diamond microtoming knife follows the twist of the director field, yielding thin sections with an undulating surface topography which produces a characteristic banding, on the order of 200−600 nm, in TEM and AFM images. Electron diffraction results also support the picture of the aqueous silk existing as a cholesteric at an intermediate stage in the spinning process.

Tilt grain boundaries in poly(styrene-b-butadiene) lamellar diblock copolymers were characterized using transmission electron microscopy (TEM). Three distinct tilt grain boundary morphologies were observed: the chevron, the omega boundary, and the T-junction. All three boundary types are found to have intermaterial dividing surfaces, separating the polystyrene and polybutadiene microphases, that approximate surfaces of constant mean curvature. The chevron and omega morphologies are observed for symmetric tilt boundaries, and a transition occurs from the chevron to the omega as tilt is increased at constant boundary width. A transition from either the chevron or the omega boundary to the T-junction occurs as the boundary becomes more asymmetric

The Scherk surface morphology allows a diblock copolymer lamellar phase to maintain microphage separation across a twist grain boundary. The interface between the two microphases in the Scherk grain boundary approximates a minimal surface consisting of a doubly periodic array of saddle surfaces. Grain boundary energies were calculated for the Scherk surface morphology as a function of diblock chain characteristics and as a function of grain boundary twist angle. The basic approach to grain boundary energy calculation is to formulate a general expression for the local free energy density as a function of chain characteristics and of the local curvature of the interface

Grain boundary morphologies in poly(stgrene-b-buldiene) lamellar diblock copolgmers were characterized using TEM. Two types of tilt grain boundaries were observed in which microphase separation of the two block was maintained in the grain boundary region by intermaterial dividing surfaces that approximate classically known minimal surfaces. The geometry of these interfaces was demonstrated by comparing experimental TEM images with rag tracing computer simulations of the model surfaces as the projection direction was systematically varied in both the experimental and simulated images

Smectic ordering has been observed in perfectly monodisperse poly(γ-benzyl α,L-glutamate) (PBLG) which was synthesized using recombinant DNA technology. These PBLG molecules form rigid α-helical rods 11.45 nm in length. In the present study, transmission electron microscopy (TEM) and electron diffraction reveals a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in a cholesteric or twisted smectic. Detailed examination of the relative orientation of the banding in the morphology images and the reflections in the electron diffraction patterns, indicating interchain and intrachain correlations, leads to the conclusion that the structure observed is a twisted smectic phase. The relationship between the twist and the layering is found to be that of the twist grain boundary (TGB) phase. However, our data does not allow us to determine whether the monodisperse PBLG structure is blocky with discrete twist boundaries as in a true TGB or is a more continuously twisting structure. Thus, we will refer to the phase as TGB-like. Conventional, polydisperse PBLG is well-known to form cholesteric phases as a result of the chirality of the helical rod. The formation of a TGB-like phase in monodisperse PBLG is consistent with the superposition of a smectic-A layering resulting from the uniform rod length on the twisted texture present in the cholesteric.