(Polystyrene-g-polyisoprene)-b-polystyrene comb-coil block copolymer in selective solvent

Comb-coil block copolymers consisting of two components polystyrene (PS) and polyisoprene (PI) were synthesized through combining TEMPO living free radical polymerization (LFRP) and anionic polymerization using “grafting-onto” strategy. Two typical samples with the same backbone but grafted with different numbers and lengths of branches, forming lamellar and cylinder phases, respectively, were investigated. As the selective solvent was added into these block polymers, the micro-structures transformed from disordered or weakly ordered structure into well-organized lamellar structure in the intermediate polymer concentration. The power law behavior of the lamellae space with respect to polymer concentration indicates that the two samples form the well-organized structure through different paths. The sample with longer branch length thus less branch points forms lamellae phase with smaller lamellae space. This difference at lamellae spacing is attributed to the two different ways that the chain assembles.     

Morphologies of diblock copolymer confined in a slit with patterned surfaces studied by dissipative particle dynamics

Diblock copolymers with ordered mesophase structures have been used as templates for nano-fabrication. Unfortunately, the ordered structure only exists at micromete rscale areas, which precludes its use in many advanced applications. To overcome this disadvantage, the diblock copolymer confined in a restricted system with a patterned surface is proved to be an effective means to prohibit the formation of defects and obtain perfect ordered domains. In this work, the morphologies of a thin film of diblock copolymer confined between patterned and neutral surfaces were studied by dissipative particle dynamics. It is shown that the morphology of the symmetric diblock copolymer is affected by the ratio of the pattern period on the surface to the lamellar period of the symmetric diblock copolymer and by the repulsion parameters between blocks and wall particles. To eliminate the defects in the lamellar phase, the pattern period on the surface must match the lamellar period. The difference in the interface energy of different compartments of the pattern should increase with increasing film thickness. The pattern period on the surface has a scaling relationship with the chain length, which is the same as that between the lamellar period and the chain length. The lamellar period is also affected by the polydispersity of the symmetric diblock copolymer. The total period is the average of the period of each component multiplied by the weight of its volume ratio. The morphologies of asymmetric diblock copolymers are also affected by the pattern on the surface, especially when the matching period of the asymmetric diblock copolymer is equal to the pattern period, which is approximately equal to the lamellar period of a symmetric diblock copolymer with the same chain length.     

A molecular dynamics simulation study of the influence of free surfaces on the morphology of self-associating polymers

Molecular dynamics simulations of thin films and bulk melts of model self-associating polymers have been performed in order to gain understanding of the influence of free surfaces on the morphology of these polymers. The self-associating polymers were represented by a simple bead-necklace model with attractive groups (stickers) at the chain ends (end-functionalized polymer) and in the chain interior (interior-functionalized polymer). The functionalized groups were found to form clusters in the melt whose size is representative of that found experimentally in many ionomer melts. While the size distribution and shape of the clusters in the thin films were found to be relatively unperturbed compared to their corresponding bulk melts, the morphology of the self-associating melts was found to be significantly perturbed by the free surfaces. Specifically, a strong depletion of stickers near the interface and the emergence of clearly defined layers of stickers parallel to the surface was observed. Increased bridging of clusters by the functionalized polymers was also observed near the free surface. We conclude that these effects can be associated with a high free energy for stickers in the low-density interfacial regime: stickers prefer to be in the higher-density interior of the film where relatively unperturbed sticker clusters can form.     

Solid-state morphologies of linear and bottlebrush-shaped polystyrene-poly(Z-l-lysine) block copolymers

The solid-state structures of polystyrene-poly(Z-l-lysine) block copolymers were examined with respect to the polymer architecture and the secondary structure of the polypeptide using circular dichroism, quantitative small- and wide-angle X-ray scattering, and electron microscopy. Linear block copolymers exhibit a hexagonal-in-lamellar structure where folded and packed polypeptide α-helices form lamellae which extend over an exceptional broad range of the composition diagram. Star- or bottlebrush-shaped copolymers are able to stabilize a larger interface area than linear ones which promotes the formation of undulated lamellar mesophases. Depending on the secondary structure of polypeptide segments, plane lamellar, superundulated lamellar, or corrugated lamellar phases are formed. These results indicate the importance of a secondary structure and packing of polymer chains for the formation of new phases and ordering far from the ‘classical’ phase behavior.     

Block copolymers with a polyvinyl and a polypeptide block: factors governing the folding of the polypeptide chains

AB block copolymers polystyrene-poly(γ-benzyl-l-glutamate) (SG) of various molecular weights and compositions were synthesized and studied by X-ray diffraction and infra-red spectroscopy. They exhibit lamellar mesophases in the solid state and in dioxane concentrated solution. Each sheet of the lamellar structure results from the superposition of two layers: one formed by the polyvinyl chains in a disordered conformation, the other formed by the polypeptide chains in an α-helix conformation arranged in a hexagonal array and generally folded. The comparison of the lamellar structure of copolymers polystyrene-poly(γ-benzyl-l-glutamate) (SG), polybutadiene-poly(γ-benzyl-l-glutamate) (BG) and polystyrene-poly(ε-carbobenzoxy-l-lysine) (SCK) showed that: (1) for the three types of copolymers (copolymers SG or BG or SCK) the number of folds of the polypeptide chains increases with the molecular weight of both the polyvinyl and the polypeptide blocks; (2) for copolymers of fixed molecular weight of the polyvinyl block and fixed degree of polymerization of the polypeptide block the number of folds of the polypeptide chains depends upon the nature of both the polyvinyl and the polypeptide blocks: a polypeptide chain is more folded when it is linked to a polystyrene chain than to a polybutadiene chain but a poly(ε-carbobenzoxy-l-lysine) chain is more rigid than a poly(γ-benzyl-l-glutamate) chain.     

Structure and dynamics of polymeric systems

Two problems that we encounter in the structure formation of polymeric systems are reviewed. One is the dynamics of phase separation of polymer blends and the other is the intramolecular structure formation of associating polymers. In the case of phase separation of polymer blends, we review the model and the simulation method that is suitable for large-scale computer simulations of phase separation of binary fluid mixtures. We also show that simulation results are in quantitative agreement with experimental results of polymer blends. In the case of associating polymers, we treat the intramolecular structure formation in single associating polymers. In order to study the structure formation in polymers with strong attractive interactions, we employ the multicanonical simulation method. We show that a two-step intramolecular conformational transition occurs in periodic associating polymers where associative groups are periodically placed along the chain. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and multiple flower-type micelles are formed via the transition. The number of the associative groups forming a micelle core is limited by the excluded volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition takes place. At this transition point, the two intramolecular micelles merge into one micelle.     

Three statistical mechanical arguments that favour chain folding in polymer systems of lamellar morphology

Three arguments are given that imply substantial chain folding in crystalline-amorphous polymer systems of lamellar morphology. These arguments assume random coil behaviour in the amorphous region. (1) The random walk character of the polymer chain portions comprizing the amorphous regions topologically constrains the number of crystalline stems that connect covalently with the amorphous regions to be small. Quantitative estimates are made and lower bounds on the amount of chain folding are given. Estimates of (2) the tightness of loops and (3) the average spatial separation between consecutive crystal stems (along a given chain) are made. The results favour substantial amounts of adjacent and near-adjacent re-entry. Existing experimental measurements on diblock copolymers consisting of crystallizable and non-crystallizable blocks provide a measure of the random walk character of the chains in the amorphous region. It is argued that such systems form lamellar structures with large amounts of chain folding in the crystalline regions as a condition of thermodynamic equilibrium.     

Structural and Dynamical Behaviour of Colloids with Competing Interactions Confined in Slit Pores

Systems with short-range attractive and long-range repulsive interactions can form periodic modulated phases at low temperatures, such as cluster-crystal, hexagonal, lamellar and bicontinuous gyroid phases. These periodic microphases should be stable regardless of the physical origin of the interactions. However, they have not yet been experimentally observed in colloidal systems, where, in principle, the interactions can be tuned by modifying the colloidal solution. Our goal is to investigate whether the formation of some of these periodic microphases can be promoted by confinement in narrow slit pores. By performing simulations of a simple model with competing interactions, we find that both the cluster-crystal and lamellar phases can be stable up to higher temperatures than in the bulk system, whereas the hexagonal phase is destabilised at temperatures somewhat lower than in bulk. Besides, we observed that the internal ordering of the lamellar phase can be modified by changing the pore width. Interestingly, for sufficiently wide pores to host three lamellae, there is a range of temperatures for which the two lamellae close to the walls are internally ordered, whereas the one at the centre of the pore remains internally disordered. We also find that particle diffusion under confinement exhibits a complex dependence with the pore width and with the density, obtaining larger and smaller values of the diffusion coefficient than in the corresponding bulk system.     

How different mesophases affect the interactive crystallisation of a block co-oligomer

An effective method for fabrication of long range ordered micro- and nanostructures on surfaces is to control the interactive crystallisation of block copolymers. In this study, the influence of different initial mesophases of a double crystalline polyethylene-block-poly (ethylene oxide) (PE-b-PEO) diblock co-oligomer on the interactive crystallisation process was studied using synchrotron radiation X-ray diffraction (SAXS/WAXD), in situ optical microscopy and differential scanning calorimetric analysis (DSC). According to the applied annealing procedure, different PE-b-PEO initial mesophases, i.e., disordered, cylindrical and spherical, have been induced. In all cases, the subsequent PEO crystallisation disrupted these initial microdomains and transformed them into crystalline lamellar morphologies with the same long periods. However, the different initial mesophases significantly affected the PEO crystallisation kinetics due to different topological confinements. An initial disordered mesophase induced the highest PEO crystallisation rate because PEO nucleation and crystal growth were limited only by chain diffusion. For an initial spherical or cylindrical mesophase, decreased PEO crystallisation rates were observed. Here, the chain diffusion was decreased by the microdomain structure. For an initial cylindrical mesophase, the earlier formed PE crystals act as a template for the subsequent PEO crystallisation and, thus, increased the PEO crystallisation as compared to the spherical mesophase where the PE was amorphous. This study demonstrates that the topological confinement of the block copolymer's initial mesophase strongly influences the crystallisation kinetics and, thus, the structures formed at the surface of drop-casted films.     

Micelles and networks formed by symmetric triblock copolymers in dilute solutions that are poor solvents for the terminal blocks

Summary A simulation for symmetric triblock copolymers in dilute solution shows a rich variety of structures when the solvent produces a net attractive interaction between the terminal blocks. Depending on the strength of this interaction, the system can form transitory aggregates, large independent micelles, or branched structures in which bridging soluble internal blocks connect dense particles formed by the aggregation of insoluble terminal blocks. The formation of the branched structure with the ABA triblock copolymer can be seen under conditions where the corresponding AB diblock copolymer would provide steric stabilization for a polymer colloid.     

Self‐assembled structures in blends of disordered and lamellar block copolymers: SAXS,SANS and TEM study

BACKGROUND: The phase behaviour of copolymers and their blends is of great interest due to the phase transitions, self‐assembly and formation of ordered structures. Phenomena associated with the microdomain morphology of parent copolymers and phase behaviour in blends of deuterated block copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA), i.e. (dPS‐block‐dPMMA)₁/(dPS‐block‐PMMA)₂, were investigated using small‐angle X‐ray scattering, small‐angle neutron scattering and transmission electron microscopy as a function of molecular weight, concentration of added copolymers and temperature. RESULTS: Binary blends of the diblock copolymers having different molecular weights and different original micromorphology (one copolymer was in a disordered state and the others were of lamellar phase) were prepared by a solution‐cast process. The blends were found to be completely miscible on the molecular level at all compositions, if their molecular weight ratio was smaller than about 5. The domain spacing D of the blends can be scaled with M_n by D ～ M_n^2/3 as predicted by a previously published postulate (originally suggested and proved for blends of lamellar polystyrene‐block‐polyisoprene copolymers). CONCLUSIONS: The criterion for forming a single‐domain morphology (molecularly mixed blend) taking into account the different solubilization of copolymer blocks has been applied to explain the changes in microdomain morphology during the self‐assembling process in two copolymer blends. Evidently the criterion, suggested originally for blends of lamellar polystyrene‐block‐polyisoprene copolymers, can be employed to a much broader range of block copolymer blends. Copyright © 2008 Society of Chemical Industry     

Twisting orientation and the role of transient states in polymer crystallization

A possible connection is suggested and explored between non-planer crystal habits in banded polymer spherulites and disordered chain folding in polymers crystallized relatively rapidly from the melt. It is proposed that, when lateral growth faces and fold surfaces are not orthogonal (because chain stems are tilted with respect to the lamellar normal), different degrees of disorder develop at opposite fold surfaces. Resulting differences in surface stress give rise to bending moments, but these are likely to be of transient existence. It is shown that, on this basis, an appealingly simple rationale can be developed to account for the complex and hitherto puzzling observations of Bassett and Hodge on polyethylene spherulites, including S-bending and non-uniform axial twisting in lamellae, and also an empirical correlation between these deformations. Much depends, however, upon interactions between interleaved crystals and upon relaxation of bending moments. Existing evidence in support of the rationale is outlined. Implications with respect to polymers other than polyethylene, and to kinetics of crystallization in general, are discussed briefly. Calculations concerning axial twisting under the influence of surface stresses suggest that the twisted crystals incorporate twist boundaries, possibly formed by aggregation of dislocations generated during the growth of what must initially be relatively disordered crystals. The ‘chiral’ factor determining handedness of twisting in a given crystal is the direction in which chain stems tilt with respect to the lamellar normal.     

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.     

Synthesis and self-assembling properties of α,ω-hydroxy-poly(ethylene oxide) end-capped with 1-isocyanato-3-pentadecylcyclohexane

A novel hydrophobic compound, 1-isocyanato-3-pentadecylcyclohexane (PDC) issued from a renewable resource material (Cashew Nut Shell Liquid), is used to end-cap poly(ethylene oxide) (PEO) of different molecular weights. The synthesis, characterization and self-assembling properties of these new associating polymers are reported. In aqueous solution, PDC modified PEOs self-associate into micellar structures which are characterized by neutron scattering. Using a star-polymer model with sticky hard sphere interactions, it is shown that the characteristics of the micelles mainly depend on the size of the PEO chain. In the case of PDC stickers, the aggregation number decreases from 25 to 17 with increasing molecular weight of PEO, from 10 to 20 kg/mol, respectively. The temperature directly impacts the interaction potential between the hydrophilic coronas of PEO which become less repulsive with increasing temperature. Rheological measurements, performed in dilute solution, clearly demonstrate that PEO micelles self-associate into open supramolecular structures. The correlation length of these loose clusters increases with polymer concentration and the hierarchical self-assembly follows the rules of the percolation theory. The sol/gel transition takes place at the overlap concentration of clusters. In semi-dilute solution, the viscoelastic properties of PDC end-capped PEOs are well described by the Maxwell model with a single relaxation mode. The characteristic time of the network follows an Arrhenius temperature dependence with an activation energy of 70 kJ/mol, in very good agreement with the size of PDC stickers.     

Ordering effects on the photopolymerization of a lyotropic liquid crystal

The synthesis of polymers bearing the highly ordered nanostructure of lyotropic liquid crystal (LLC) phases has recently been of great interest. This work describes the polymerization behavior and structural evolution of a cationic amphiphile in various LLC phases. The type and degree of LLC phases formed from this monomer depend strongly on the composition and temperature. By adding a nonpolymerizable surfactant a variety of LLC phases are formed including hexagonal, bicontinuous cubic, and lamellar morphologies while maintaining a constant monomer concentration. The highly ordered lamellar LLC phase exhibits the fastest polymerization rate with the slowest occurring in the hexagonal phase. The polymerization rates of the bicontinuous morphology were intermediate to the lamellar and hexagonal phases. The faster polymerization kinetics is due to diffusional limitations imposed on the propagating polymer by the highly ordered lamellar LLC phase. Also, the order of this LLC system has a strong dependence on temperature. At higher temperatures, the degree of LLC order and correspondingly the polymerization rate decrease. The original LLC phase morphology appears to be retained to the greatest extent in the faster polymerizing lamellar phase. The original nanostructure is also retained in the hexagonal and cubic LLC phases but with some slight changes in structure. This LLC structure is preserved at temperatures well exceeding the thermal phase transitions of the unpolymerized LLC samples.     

Extended chain lamellae self-assembled in liquid crystal phase of soluble polydiacetylenes through chain segregation

This review article highlights our efforts in the past years toward a fundamental understanding of the supramolecular structures of a soluble but rigid polydiacetylene. In our earlier works we presented evidence to indicate that this polymer formed a lyotropic phase in solutions. TEM, AFM and SEM observations on solidified thin films showed an extended chain lamellar structure which is surmised to be self-assembled in the lyotropic phase through chain segregation. Our results revealed that the supramolecular structures formed are closely associated with the structures formed in the lyotropic state. Due to three attributes of rigid macromolecules at the molecular level-high molecular weight, broad molecular weight distribution, and worm-like chain conformation, the supramolecular structures observed are also greatly different from those of conventional nematics or smectics. Dedicated to Professor Dr. Gerhard Wegner, Max-Planck-Institute for Polymer Research, Germany, together with my wishes on the occasion of his 60^th birthday.     

Polycondensation in liquid crystalline phases of nonionic surfactants. Kinetics and morphology

We have investigated acid-catalyzed polycondensation of alkoxysilane monomers in liquid crystalline phases of nonionic CnEm surfactants. The liquid crystalline phase is retained when the monomers polymerize. The high molecular weight molecules formed phase separate from the mesophase and are subsequently organized by it to form micron-sized particles. A variety of particle morphologies are formed by organization of the polymer particles in the mesophase. For condensation of dimethyldimethoxysilane (DMS, with trimethoxysilane, TMS as crosslinker) in hexagonal and lamellar phases, specific reaction conditions, viz. slow condensation kinetics and low crosslink density, give rod-like particles in hexagonal phases and sheet-like structures in lamellar phases. However, when higher acid concentrations are used, the reaction kinetics accelerates and irregular particles form. Irregular particles also form when the fraction of trifunctional crosslinker is increased, and finally complex flower-like structures form for condensation of trimethoxysilane in the hexagonal phase. The particle morphology formed is crucially dependent on the details of the polycondensation rate, crosslinker density and surfactant-monomer/oligomer interactions.     

Influence of S/B middle block composition on the morphology and the mechanical response of polystyrene-poly(styrene-co-butadiene)-polystyrene triblock copolymers

Polystyrene-poly(styrene-co-butadiene)-polystyrene triblock copolymers (PS-P(S-co-B)-PS) having different styrene contents (from 30 wt.% to 80 wt.%) in the statistical copolymer middle block and different block architectures (20-60-20 and 30-40-30) were characterised to study the influence of S/B middle block composition and segregation strength on the morphology and mechanical behaviour. The morphological investigations, i.e. TEM and SAXS, exhibited ordered lamellar and lamellar-like morphologies for both block architectures at low styrene contents between 30 wt.% and 50 wt.% in the S/B middle block. The increase in the styrene content in the middle block to 70 wt.% resulted in phase separated structures without long range order due to the enhanced miscibility between the PS and P(S-co-B) phase as observed from dynamic mechanical analysis. Further it was observed that the glass transition of the butadiene-rich phase is mainly determined by the S/B composition of the statistical copolymer block as confirmed by the Fox-equation. The alteration of the glass transition of the PS-rich phase and the observed PS-softening with raise in styrene content might be correlated to the increasing interphase width due to the enhanced miscibility as shown by calculations based on a simple model for diblock copolymers. Tensile testing revealed a transition from ductile to semi-ductile to brittle behaviour that strongly depends on the styrene content in the S/B middle block, chain architecture and the resulting morphology. Block copolymers (BCPs) with lamellar structure exhibited ductile behaviour with extensive strain hardening, whereas BCPs forming segregated structures without long range order were semi-ductile or brittle depending on the type of block architecture and on the hard-phase content. The transition in the mechanical behaviour was confirmed by fracture mechanical investigations based on the essential work of fracture approach and SEM-characterizations.     

Effect of polymer architecture on metal nanoclusters

Synthesis of metal nanoclusters in polymeric media has been shown to yield small clusters with a narrow size distribution. Embedding such clusters in the three-dimensional structures formed by diblock copolymers will allow the development of ordered structures with high optical and magnetic contrast between the different regions. In this paper we investigate the effect of homopolymer and diblock copolymer properties on the cluster size. We find that in homopolymer solutions, the cluster size reaches a minimum at a specific chain molecular weight (MW). In the case of diblock copolymers, the cluster size is set by the MW of the block with the stronger affinity to the metal surface.     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.