Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PB_h-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45% PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization, is presented. Received 19 March 1999 and Received in final form 14 December 1999     

Kinetics of microphase segregation in one-dimensional symmetric diblock copolymer systems

An analytical one-dimensional model of the microphase separation in symmetric diblock copolymers is developed. Three stages of the process of the microphase segregation of a quenched diblock copolymer system into a lamella structure are predicted. The first stage involves a fast increase of the amplitude of the quasi-periodical lamella structure (with the average wave vector q₀) up to a nearly equilibrium value; the second stage is a slow phase diffusion process which is characterized by increasing coherency of the lamella structure; the third stage is the slow process of the lamella swelling, which is driven by the thermally activated process of the spontaneous deletion of excessive lamellas, the lamella size increasing from the initial value 2π/q ₀ to the equilibrium 2π/q _eq > 2π/q ₀ during the process. The last two stages are described with the specially introduced coarse-grained “amplitude-phase” approximation. It is shown that the relaxation of the gradient of the phase of the lamellar structure is the slowest relaxation process and, thus, can be used as an effective order parameter of the lamellar structure at the later stages of the microphase segregation. Received 10 March 2000 and Received in final form 5 June 2000     

Phase equilibria in random multiblock copolymers

A mean-field theory for domain structures in random multiblock copolymer melts is developed. We focus on the finite molecular weight effects resulting in a competition between macroscopic phase separation and microdomain formation in the system. We identify an essential parameter N ε controlling the phase behavior of the system, where N is the number of blocks per chain and ε is the composition asymmetry parameter (= the difference between the mean copolymer composition and its critical value). The phase diagram involving N ε and the reduced temperature as variables is obtained. The regions of coexistence of two or more phases are identified. We show that a superstructure formation on cooling is always pre-empted by a macroscopic phase separation of the macroscopically homogeneous (disordered) system yielding two homogeneous phases: H ₀↦H ₁ + H ₂. The third (lamellar) phase separates on further cooling. Then hexagonal and body-centred-cubic phases take over if N ε 1. As the Flory interaction parameter χ increases further, the standard transitions BCC↦HEX↦LAM take place. Received 13 July 2001     

Effects of polydispersity on the order-disorder transition of diblock copolymer melts

The effect of polydispersity on an AB diblock copolymer melt is investigated using lattice-based Monte Carlo simulations. We consider melts of symmetric composition, where the B blocks are monodisperse and the A blocks are polydisperse with a Schultz-Zimm distribution. In agreement with experiment and self-consistent field theory (SCFT), we find that polydispersity causes a significant increase in domain size. It also induces a transition from flat to curved interfaces, with the polydisperse blocks residing on the inside of the interfacial curvature. Most importantly, the simulations show a relatively small shift in the order-disorder transition (ODT) in agreement with experiment, whereas SCFT incorrectly predicts a sizable shift towards higher temperatures.     

Influence of polydispersity on the phase behavior of statistical multiblock copolymers with Schultz-Zimm block molecular weight distributions

In this paper we investigate in a systematic way the influence of polydispersity in the block lengths on the phase behavior of AB-multiblock copolymer melts. As model system we take a polydisperse multiblock copolymer for which both the A-blocks and the B-blocks satisfy a Schultz-Zimm distribution. In the limit of low polydispersity the expressions for the vertex functions are clarified by using simple physical arguments. For various values of the polydispersity the phase diagram is presented, which shows that the region of stability of the bcc phase increases considerably with increasing polydispersity. The strong dependence of the periodicity of the microstructure on the polydispersity and on the interaction strength is presented. Received 2 July 1998     

End-chain segment ordering in lamellar sublayers of a diblock copolymer

The segmental ordering within a lamellar polystyrene-polydimethylsiloxane diblock (PS-PDMS) was examined by NMR. A non-uniform stretching of the PDMS chains was evidenced. Although chain segments displaying isotropic reorientational motions are present along the chains, relaxation measurements on the free PDMS extremities demonstrate that these latter are oriented and submitted to a non-zero constraint. This specific result allows to precise some features of the order distribution within the lamellae. Received: 27 March 2003 / Accepted: 9 May 2003 / Published online: 4 June 2003 RID="a" ID="a"e-mail: deloche@lps.u-psud.fr     

Hyperviscous diblock copolymer vesicles

Giant vesicles prepared from the diblock copolymer polybutadien-b-polyethyleneoxide (PB-PEO) exhibit a shear surface viscosity, which is about 500 times higher than those found in common phospholipid bilayers. Our result constitutes the first direct measurement of the shear surface viscosity of such polymersomes. At the same time, we measure bending and stretching elastic constants, which fall in the range of values typical for lipid membranes. Pulling out a tether from an immobilized polymersome and following its relaxation back to the vesicle body provides an estimate of the viscous coupling between the two monolayers composing the polymer membrane. The detected intermonolayer friction is about an order of magnitude higher than the characteristic one for phospholipid membranes. Polymersomes are tough vesicles with a high lysis tension. This, together with their robust rheological properties, makes them interesting candidates for a number of technological applications. Received 2 March 2001 and Received in final form 15 February 2002     

Phase behavior and structure of an ABC triblock copolymer dissolved in selective solvent

A mean-field lattice theory is applied to predict the self-assembly into ordered structures of an ABC triblock copolymer in selective solvent. More specifically, the composition-temperature phase diagram has been constructed for the system (C)₁₄(PO)₁₂(EO)₁₇/water, where C stands for methylene, PO for propylene oxide and EO for ethylene oxide. The model predicts thermotropic phase transitions between the ordered hexagonal, lamellar, reverse hexagonal, and reverse cubic phases, as well as the disordered phase. The thermotropic behavior is a result of the temperature dependence of water interaction with EO- and PO-segments. The lyotropic effect (caused by changing the solvent concentration) on the formation of different structures has been found weak. The structure in the ordered phases is described by analyzing the species volume fraction profiles and the end segment and junction distributions. A “triple-layer” structure has been found for each of the ordered phases, with each layer rich in C-, PO-, and EO-segments, respectively. The blocks forming the layers are not stretched. The dependence of the domain spacing on polymer volume fraction and temperature is also considered. Received 17 April 2002 Published online: 21 January 2003     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

One-dimensional optical reflectors based on self-organization of polymeric comb-shaped supramolecules

We demonstrate that complexation of dodecylbenzenesulphonic acid, DBSA, to a diblock copolymer of polystyrene- block-poly(4-vinylpyridine), PS- block-P4VP, leads to polymeric supramolecules PS- block-P4VP(DBSA)y (y = 1.0, 1.5, and 2.0), which self-organize with a particularly large lamellar periodicity in excess of 1000 A. The structures consist of alternating PS and P4VP(DBSA)y layers, where the latter contains smaller internal structure, probably lamellar. The DBSA side chains are bonded to the pyridines by protonation and hydrogen bonding and they effectively plasticize the material. In this way relatively well-developed structures are obtained even without annealing or macroscopic alignment. Transmission and reflectance measurements show that a relatively narrow and incomplete bandgap exists for supramolecules of high molecular weight block copolymer at ca. 460 nm.     

Microphase separation at two length scales

The possibility of microphase separation at two different length scales in monodisperse AB block copolymer melts consisting of a homopolymer A block and either a linear alternating AB copolymer block (poly(A)_m-block-poly(B-alt -A)_n) or an AB comb copolymer block poly(A)_m-block-poly(A-graft-B)_n, is investigated. An analysis of the structure factor reveals that in the parameter space of n and m three different cases can be distinguished: I) The structure factor has only one minimum corresponding to the short length scale (i.e. the characteristic length of the repeating unit of the alternating or comb block). II) The structure factor has only one minimum corresponding to the long length scale (the characteristic length of the blocks). III) Two minima are present leading to a competition between microphase separation at the short and the long length scale. Depending on the choice of n and m, one of these three possibilities will occur. Received 25 August 2000     

Polyelectrolyte brushes with specific charge distribution: Mean-field lattice theory

A novel polymer system containing charged diblock polymers grafted onto a planar surface, simple salt and solvent has been considered in the framework of a mean-field lattice theory. On the basis of predicted volume fraction profiles of polymer segments, free ends, block junctions, and small ions, a detailed picture of the system has emerged. It is found that the structure of the polymer brush is decisively dependent of the relative charge of the blocks. For certain conditions, bimodal profiles appear which demonstrate the simultaneous presence of two types of chain conformations, one coiled and one stretched. Received 30 August 2000     

Confinement of columnar diblock copolymers: Simulations, theory and applications

The inherent nanoscale morphologies of self-organizing diblock copolymer melts are now being investigated for a variety of technological applications. To obtain global, well-oriented, regular patterns requires suitably confining and aligning the melt between two flat plates. Here we consider such confinement for an asymmetrical diblock melt, which forms columns of the minority phase in a matrix of the majority phase. We investigate this system with a combination of numerical simulations and strong segregation theory and make suggestions as to when perpendicular orientation should prevail over parallel orientation of the columns. Received 22 May 2001 and Received in final form 14 February 2002     

Block copolymer adsorption from a homopolymer melt to an amine-terminated surface

Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS) matrix is studied as a function of substrate type and non-adsorbing block degree of polymerization. The block copolymer is poly(deutero styrene)-block-poly(methyl methacrylate) and the substrates are silicon oxide, SiO_x, and SiO_x functionalized with (3-aminopropyl)triethoxysilane (APTES). We have determined the equilibrium volume fraction-depth profiles for such films, and compared them with volume fraction profiles generated by self-consistent mean-field (SCMF) theory and find good agreement between the experimental and theoretical data. SCMF calculations show that the segmental interaction energy between PS matrix chains and APTES is two orders of magnitude stronger than that between PS and SiO_x.     

Theory of microphase separation on side-chain liquid-crystalline polymers with flexible spacers

We model a melt of monodisperse side-chain liquid-crystalline polymers as a melt of comb copolymers in which the side groups are rod-coil diblock copolymers. We consider both excluded-volume and Maier-Saupe interactions. The first acts among any pair of segments while the latter acts only between rods. Using a free-energy functional calculated from this microscopic model, we study the spinodal stability of the isotropic phase against density and orientational fluctuations. The phase diagram obtained in this way predicts nematic and smectic instabilities as well as the existence of microphases or phases with modulated wave vector but without nematic ordering. Such microphases are the result of the competition between the incompatibility among the blocks and the connectivity constraints imposed by the spacer and the backbone. Also the effects of the polymerization degree and structural conformation of the monomeric units on the phase behavior of the side-chain liquid-crystalline polymers are studied.     

Dynamics and order-disorder transitions in bidisperse diblock copolymer blends

We employ the dynamic extension of self-consistent field theory (DSCFT) to study dynamics and order-disorder transitions (ODT) in AB diblock copolymer binary mixtures of two different monodisperse chain lengths by imitating the dynamic storage modulus G′ corresponding to any given morphology in the oscillatory shear measurements. The different polydispersity index (PDI) is introduced by binary blending AB diblock copolymers with variations in chain lengths and chain number fractions. The simulation results show that the increase of polydispersity in the minority or symmetric block introduces a decrease in the segregation strength at the ODT, (χN)_ODT, whereas the increase of polydispersity in the majority block results in a decrease, then increase and final decrease again in (χN)_ODT. To the best of our knowledge, our DSCFT simulations, for the first time, predict an increase in (χN)_ODT with the PDI in the majority block, which produces the experimental results. The simulations by previous SCFT, which generally speaking, is capable of describing equilibrium morphologies, however, contradict the experimental data. The polydispersity acquired by properly tuning the chain lengths and number fractions of binary diblock copolymer blends should be a convenient and efficient way to control the microphase separation strength at the ODT.     

Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO4-5 zeolite) host materials

This paper discusses the self-assembly of block copolymers into vesicular morphology. After a brief state of art of the field, a system based on an amphiphilic poly(butadiene)-b-poly(-L-glutamic acid) (PB-b-PGA) diblock copolymer in aqueous solution is discussed in detail. The aggregation behavior of this block copolymer has been investigated by means of fluorescence spectroscopy, dynamic (DLS) and static (SLS) light scattering as well as transmission electron microscopy (TEM). The diblock copolymer was found to form well-defined vesicles in water. The size of these so-called polymersomes or peptosomes could be reversibly manipulated as a function of both pH and ion strength. Depending on the pH of the aqueous solution, the hydrodynamic radii of these vesicles were found to vary from 100 nm to 150 nm. By cross-linking the 1,2-vinyl double bonds present in the polybutadiene block, the ability to transform a transient supramolecular self-organized aggregate into a permanent “shape-persistent stimuli-responsive nanoparticle” has been demonstrated. Received 25 June 2002 and Received in final form 22 October 2002 Published online: 11 March 2003     

Hydrodynamic-flow-driven phase evolution in a polymer blend film modified by diblock copolymers

We have studied surface-directed phase separation in thin films of deuterated polystyrene and poly(bromostyrene) (with 22.7% of monomers brominated) using ³He nuclear reaction analysis, dynamic secondary ion mass spectroscopy and atomic force microscopy combined with preferential dissolution. The crossover from competing to neutral surfaces of the critical blend film (cast onto Au) was commenced: polyisoprene-polystyrene diblock copolymers were added and segregated to both surfaces reducing in a tuneable manner the effective interactions. Two main stages of phase evolution are characterised by i) the growth of two surface layers and by ii) the transition from the four-layer to the final bilayer morphology. For increasing copolymer content the kinetics of the first stage is hardly affected but the amplitude of composition oscillations is reduced indicating more fragmented inner layers. As a result, a faster mass flow to the surfaces and an earlier completion of the second stage were observed. The hydrodynamic flow mechanism, driving both stages, is evidenced by nearly linear growth of the surface layer and by mass flow channels extending from the surface layer into the bulk. The final bilayer structure, formed even for the surfaces covered by strongly overlapped copolymers, is indicative of long-range (antisymmetric) surface forces. Received 15 March 2000 and Received in final form 9 February 2001     

Control of amphiphilic block copolymer morphologies using solution conditions

It is well known that the morphology of block copolymer aggregates depends on polymer properties such as the molecular weight, the relative block length, and the chemical nature of the repeat unit. Recently, we have shown that if aggregates are allowed to self-assemble in solution, then in addition to the above factors, a high degree of control over the aggregate architecture can be achieved by adjusting the solution conditions. Factors such as the water content in the solvent mixture, the solvent nature and composition, the presence of additives (ions, surfactants, and homopolymer) and the polymer concentration were successfully employed to control the aggregate shape and size. In this paper, we review a series of studies performed in our group to show how solution properties can control the architecture of aggregates prepared from a given copolymer. The control mechanism is explained in terms of the effect of each property on the forces that govern the formation of any given morphology, namely the core-chain stretching, corona-chain repulsion and interfacial tension. Received 30 April 2002 and Received in final form 3 September 2002 Published online: 21 January 2003     

Statistics of copolymer sequences obtained by polymeranalogous reactions of homopolymer globules

For the first time the macrokinetics of polymeranalogous reaction in a homopolymer globule is considered theoretically. Using mathematical apparatus of the theory of markovian stochastic processes we examined the statistical characteristics of the chemical structure of the emerging copolymers. It is shown that when the reaction is kinetically-controlled the sequence distribution of monomeric units in copolymer chains follows Bernoullian statistics. In the opposite regime, when the reaction is diffusion-controlled it is proved that the resulting primary structure is described by the Levy flight distribution. Such copolymers with nonmarkovian character of the alteration of units in macromolecules can not be synthesized in the bulk or in concentrated solutions by means of standard synthetic polymerization procedures. Received 14 June 2002 Published online: 16 April 2003 RID="a" ID="a"e-mail: khokhlov@polly.phys.msu.ru