Morphology and microphase separation of star copolymers

Star copolymers have attracted significant interest due to their different characteristics compared with diblock copolymers, including higher critical micelle concentration, lower viscosity, unique spatial shape, or morphologies. Development of synthetic skills such as anionic polymerization and controlled radical polymerization have made it possible to make diverse architectures of polymers. Depending on the molecular architecture of the copolymer, numerous morphologies are possible, for instance, Archimedean tiling patterns and cylindrical microdomains at symmetric volume fraction for miktoarm star copolymers as well as asymmetric lamellar microdomains for star‐shaped copolymers, which have not been reported for linear block copolymers. In this review, we focus on morphologies and microphase separations of miktoarm (A_mB_n and ABC miktoarm) star copolymers and star‐shaped [(A‐b‐B)_n] copolymers with nonlinear architecture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1–21     

The relation between the microphase separation transition and the glass transition in diblock copolymers

The microphase separation transition (MST) has been studied for short chain diblock copolymers poly(styrene-b-isoprene) and poly(styrene-b-mma). A detailed analysis of small-angle x-ray scattering (SAXS) profiles in the homogeneous phase allows determination of the interaction parameter and the spinodal temperature T_s of the MST. T_s for the PS/PI diblocks is found to be lower than the glass transition temperature of their hard blocks. This results in a coupling of the MST and the glass transition. Using both structural (SAXS) and thermal differential scanning calorimetry (DSC) methods it is shown that an endothermal peak found in the DSC diagrams is related to the combined effect of the MST and the glass transition. © 1992 John Wiley & Sons, Inc.     

Simulations of microphase separation in crosslinked polymer blends

We investigate using bond‐fluctuation simulations the behavior of crosslinked symmetric binary A‐B polymer blends. The variation of the A‐B‐interaction parameter leads to microphase separation. We study it by focussing on the structure factor which we determine for different crosslink densities and for different values of the A‐B‐interaction parameter. The structure factor peaks at smaller values of the scattering vector q than predicted by de Gennes. This finding is in line with recent experiments, which, however, could not clarify its origin. We relate the finding to the topological disorder inherent in the network structure, which allows for large deformations during the separation process.     

Far-infrared studies of microphase separation in sulfonated ionomers

Far-infrared spectra of a series of un-neutralized and neutralized lightly sulfonated polystyrenes with varying sulfonation levels have been investigated to seek spectroscopic evidence for microphase separation known to control the physical properties of these polymers. Broad, strong absorbance bands, not found in the spectrum of unmodified polystyrene, are observed in the spectra of the sulfonated analogs. The effects on the far-infrared spectra both of sulfonation level and of the mass and charge of the neutralizing cation are discussed in terms of cation motion and the formation of ion-rich domains.     

The influence of disorder on a microphase separation of cross-linked polymer mixtures

We generalize the de Gennes' theory of the microphase separation of cross-linked polymer mixtures to take into account the spatial fluctuations of the elasticity constant c( r ) obeying a Poisson distribution. Within a meanfield analysis we found that for weak fluctuations of c( r ) the free energy possesses a minimum corresponding to a microphase separation. The size of the domains is enlarged in comparison to the homogeneous case.     

Photocontrolled microphase separation of block copolymers in two dimensions

A novel ABA-type triblock copolymer, where A and B correspond to azobenzene (Az) containing polymethacrylate and poly(ethylene oxide) (PEO), respectively, was synthesized by atom transfer radical polymerization. Langmuir-Blodgett monolayers showed characteristic microphase separation structures depending on the isomerization state of the Az unit. The trans-to-cis isomerization induced an anisotropic elongation of the domain of the Az polymer parallel to the rod maintaining the width. Thus, successful photocontrol of nanostructures formed by the block copolymer in the two dimensions was performed. A plausible model for the Az packing and PEO conformation is proposed.     

On the microphase separation kinetics of symmetric diblock copolymers

Time-resolved small-angle x-ray scattering studies were performed on symmetric diblock copolymers of polystyrene and poly(methyl methacrylate), P(S-b-MMA). Freeze-dried powders of P(S-b-MMA) having a molecular weight of 8.4×10⁴ were rapidly heated to temperatures above the glass transition temperature to initiate the microphase separation. The microphase separation process was found to consist of a rapid, local microphase separation followed by a long-term coarsening process. The period characterizing the lamellar microphase separated structure was found to increase initiallv and then saturate at longer times. These results are discussed in light of recent theoretical developments.In celebration of his 65th birthday, this article is dedicated to Prof. E. W. Fischer whose methodic and thorough approach to research has been and continues to be a model for us to follow. Es freut mich, daß ich mit Herrn Fischer gearbeitet habe. Ich habe vieles von ihm gelernt. Ich hoffe, daß auch ich so fleißig sein werde, wenn ich so jung bin wie er.     

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.     

Phase transitions between smectic A phases as well as smectic C* phases and undulated structures in terminal non-polar compounds

Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.     

Macro-and microphase separation in nonaqueous solutions of binary multiblock copolymers

In terms of the random phase approximation, a global analysis of the thermodynamic stability of solutions of multiblock copolymers (A_nB_m)_k and (A_nB_m)_kA_n was performed. Phase diagrams for various parameters of the copolymer structure including the macrophase and microphase separation regions were obtained. Critical lines near which this approximation still holds and the microphase separation corresponds to the weak segregation mode were also constructed. The formation of ordered structures with a period of the order of a wavelength of visible light becomes possible under certain conditions. It was shown that the homogeneous state of solutions of multiblock copolymers in certain narrow ranges of values of the parameters breaks down through a simultaneous growth of fluctuations with substantially different wavelengths, a phenomenon which must lead to the appearance of structures ordered at two length scales. The width of such regions increases with an increase in the number of blocks k and a decrease in the degree of polymerization n + m of a block. The potential use of these multiblock-copolymer solutions as photonic crystals is discussed.     

Combinations of microphase separation and terminal multiple hydrogen bonding in novel macromolecules

The synthesis and characterization of terminal multiple hydrogen-bonded (MHB) polymers, such as poly(styrene) (PS), poly(isoprene) (PI), and microphase separated PS-b-PI block copolymers, possessing controlled molecular weights and narrow molecular distributions are described. Hydroxyl-terminated polymeric precursors were prepared using living anionic polymerization and subsequent quantitative termination with ethylene oxide. MHB polymers were synthesized in a controlled fashion via end-group modification of these well-defined macromolecular alcohols with excess isophorone diisocyanate and subsequent derivatization of the isocyanate-terminated polymeric intermediate with methyl isocytosine. The glass transition temperatures of the terminal MHB polymers were reproducibly higher than both nonfunctionalized and hydroxyl-terminated polymers at nearly equivalent number average molecular weights. Thin-layer chromatography analysis indicated that the interaction of terminal MHB polymers with silica was stronger as compared to both nonfunctionalized and hydroxyl-terminated polymers. Rheological characterization indicated that the melt viscosity at constant shear rate for various MHB polymers was more than 100 times higher than those for nonfunctionalized and hydroxyl-terminated polymers. Interestingly, the melt viscosity of MHB polymers was higher than those of nonfunctionalized polymers with twice the number average molecular weight. In addition, DSC and rheological characterization also suggested that terminal MHB polymers formed aggregates and not simple dimers in the melt state, and the aggregates were observed to completely dissociate at 80 degrees C.     

On the macro- and microphase separation of compatibilizers in immiscible polymer blends

Macro- and microphase separation of compatibilizing graft copolymers in melt-mixed polystyrene/polyamide-6 blends was studied by transmission electron microscopy and thermal analysis. Three different graft copolymers with main chains of polystyrene and side chains of poly(ethylene oxide) were used as additives at various concentrations. The polyamide-6 domain sizes decreased with increasing amounts of compatibilizing graft copolymers in the blends up to a saturation concentration, after which no further reduction was noted. Macrophase separation of the graft copolymers into discrete macrodomains 20–200 nm in size occurred at concentrations equal to or slightly lower than the saturation concentration. The macrodomains of the graft copolymers were microphase separated, and the sizes and shapes of the microdomains were found to largely depend on the graft copolymer structure and composition. As a consequence of microphase separation, poly(ethylene oxide) crystallinity was noted in blends with sufficiently high macrophase contents. Observations of a graft copolymer interphase between the polystyrene matrix and the polyamide-6 domains confirmed that the graft copolymer was present at the blend interfaces in some of the compatibilized blends. © 1996 John Wiley & Sons, Inc.     

Polymerization-induced microphase separation of polymer-polyoxometalate nanocomposites for anhydrous solid state electrolytes

Solid-state electrolytes (SSEs) with high ionic conductivity, mechanical stability, and high thermal stability, as well as the stringent requirement of application in high-temperature fuel cells and lithium-ion batteries is receiving increasing attention. Polymer nanocomposites (PNCs), combining the advantages of inorganic materials with those of polymeric materials, offer numerous opportunities for SSEs design. In this work, we report a facile and general one-pot approach based on polymerization-induced microphase separation (PIMS) to generate PNCs with bi-continuous microphases. This synthetic strategy transforms a homogeneous liquid precursor consisting of polyoxometalates (POMs, H₃PW₁₂O₄₀, Li₇[V₁₅O₃₆(CO₃)]), poly(ethylene glycol) (PEG) macro-chain-transfer agent, styrene and divinylbenzene monomers, into a robust and transparent monolith. The resulting POMs are uniformly dispersed in the PEG block (PEG/POM) to form a conducting pathway that successfully realizes the effective transfer of protons and lithium ions, while the highly cross-linked polystyrene domains (P(S-co-DVB)) as mechanical support provide outstanding mechanical properties and thermal stability. As the POM loading ratio up to 35 wt%, the proton conductivity of nanocomposite reaches as high as 5.99 × 10^-4 S/cm at 100 °C in anhydrous environment, which effectively promotes proton transfer under extreme environments. This study broadens the application of fuel cells and lithium-ion batteries in extreme environments.     

Self-recovery of chiral microphase separation in an achiral diblock copolymer system

Macroscopic regulation of chiral supramolecular nanostructures in liquid-crystalline block copolymers is of great significance in photonics and nanotechnology. Although fabricating helical phase structures via chiral doping and microphase separation has been widely reported, the chiral memory and self-recovery capacity of asymmetric phase structures are the major challenge and still deeply rely on the presence of chiral additives. Herein, we demonstrate the first controllable chiral microphase separation in an achiral amphiphilic block copolymer consisting of poly(ethylene oxide) and azobenzene (Azo) groups. Chirality can be transferred to the fabricated helical nanostructures by doping with chiral additives (tartaric acid, TA). After the removal of the chiral additives and then performing cross-linking, the formed helical nanostructures will completely dispense with the chiral source. The supramolecular chirality and the micron-scale phase structure can be maintained under UV irradiation and heating-cooling treatment, enabling a reversible “on–off” chiroptical switch feature. This work is expected to avoid the tedious synthesis and expensive raw materials and shows a great application prospect in chiral separation and so on.

A chirality-storing copolymer MPS structure will overcome the external chiral source dependence and avoid tedious synthesis and expensive raw materials.     

Coupling of microphase separation and dewetting in weakly segregated diblock co-polymer ultrathin films

We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(L-lactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the co-polymer forms a lamellar structure in bulk with a long period of L ～ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100-140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ～ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.     

New polymer systems exhibiting microphase separation transition with the formation of nano-microstructures

The concept of microphase separation was up to now widely applied mainly to the conformational transitions in block-copolymer solutions and melts. However, recently it became obvious that this concept has a much more general meaning. It was shown that microphase separation transition can be observed in random copolymers, interpenetrating polymer networks, polyelectrolyte mixtures, poor solvent polyelectrolyte solutions, ionomer solutions and melts, polymer blends and solutions with nonlocal entropy of mixing. In all these examples the emerging microdomain structures correspond to the nanometer scale, therefore the study of these effects can lead to the new ways of obtaining polymer materials with controlled nano-microstructure. In this presentation the review of our recent findings on microphase separation in some of the above-mentioned systems will be presented. 1. The problem of microphase separation in the systems containing weakly charged polyelectrolytes (polyelectrolyte mixtures and poor solvent polyelectrolyte solutions) will be considered. From the methodic point of view, it will be shown that this problem can be solved by direct minimization of the free energy, without the use of “weak segregation” or “strong segregation” assumptions which are common in the theory of block-copolymers. The final phase diagrams exhibit wide macroscopic phase separation regions, which is their main difference from the corresponding phase diagrams for block-copolymer systems. The formation of microdomains is thus coupled with macroscopic phase separation: in most of the cases microdomain structure is formed in one of the coexisting phases after macroscopic phase separation takes place [1] - [2]. 2. The formation of the multiplet structure in ionomer melts and solutions can be also considered as the microphase separation in the random copolymer system with the formation of the “micelles” (or clusters) of ionic links. The parallels with micelle formation in block-copolymer systems can be established if one considers a new “superstrong segregation regime” for block-copolymer microstructures. This regime can be indeed observed for diblock copolymers with one ionomeric and one neutral block [3]. 3. The microphase separation transition in ordinary polymer blends and solutions is also possible. The conditions for this effect are: (i) significant entropic contribution to polymer/polymer or polymer/solvent miscibility, (ii) the nonlocal character of this contribution with a high value of the nonlocality radius. It is argued that one can expect that the entropy nonlocality radius increases in the vicinity of the glass transition for the blend or polymer solutions (in the latter case solvent molecules act like “poor solvent plasticisers”). Computer simulation data supporting the theoretical prediction of microphase separation transition in these systems will be presented [4] - [5].     

Influence of structure on smectic A-smectic A phase separation

The critical temperature at the consolute point where two smectic A phases of different compositions phase separate has been measured for binary systems which combine one compound from a newly synthesized series of fluorinated mesogens with one homologue from the commercially available cyanobiphenyl family. Taking advantage of the regular evolution of the layer spacings within a series of homologues, this systematic analysis identifies a clear connection between miscibility and difference in layer spacings of the smectic A phases of the pure compounds. However, additional data obtained on significantly different systems show the non-universal character of this analysis and suggest that factors in addition to this structural difference are relevant to this phenomenon.     

Dissipative particle dynamics simulation of microphase separation behaviors in graft-diblock copolymer films

Dependence of the microphase separation behaviors of graft-diblock copolymers (A _x )g(B _y ) in thin films on composition fraction, thickness of film and A–B repulsing strength is investigated preliminarily by dissipative particle dynamics. Several kinds of ordered mesostructures have been observed and the simulated phase diagrams show evident asymmetries, besides, the center of lamellas region shifts away from f _A = 0.5. Some of the mesostructures in the film can correspond to those in bulk. Decreasing the thickness of film as well as strengthening the A–B repulsion help the mesostructures enhance the degree of order.     

Synthesis of all-conjugated diblock copolymers by quasi-living polymerization and observation of their microphase separation

We designed and synthesized the all-conjugated diblock copolymers poly(3-hexylthiophene-block-3-(2-ethylhexyl)thiophene)s (P(3HT-b-3EHT)s) via a modified Grignard metathesis (GRIM), a type of quasi-living polymerization, and studied their microphase-separated structures. The P(3HT-b-3EHT)s synthesized had well-controlled molecular weights and very narrow polydispersity indices (PDIs), which demonstrates the usefulness of GRIM polymerization for the synthesis of semiconducting block copolymers. P(3HT-b-3EHT)s self-organized to form clear microphase-separated patterns upon thermal treatment, as observed by AFM. Interestingly, the enhancement of the interchain interaction of the P3HT segments compared with the P3HT homopolymer was clearly observed from the UV-vis spectra, despite the fact that the amount of crystalline P3HT fraction was reduced to 83% of the total polymer amount in P(3HT-b-3EHT). It is suggested that the relatively unconstrained, amorphous segments of P3EHT can enhance the crystallization of P3HT segments to form an ordered self-organized nanostructure.