Rod-coil Diblock Copolymers Research Articles

Liquid-crystalline ordering in rod-coil diblock copolymers studied by mesoscale simulations.

Using mesoscale dissipative particle dynamics (DPD) simulations, which ignore all atomistic details, we show the formation of lamella mesophases by cooling a fully disordered system composed of symmetric (A7B7) rod-coil diblock copolymers. Equilibration is achieved very rapidly using DPD, and isotropic, smectic A and crystalline phases of the rod-like blocks can be observed either by heating or cooling. An interesting pseudo-smectic phase can be characterized when the order-disorder transition temperature is above the clearing temperature. This phase gradually fades into a normal microphase-separated structure as the system is heated through the clearing temperature. Simulations of pure rods, however, show the formation of isotropic, nematic, smectic A and crystalline phases.

Synthesis of a novel hybrid liquid‐crystalline rod–coil diblock copolymer

AbstractA series of novel rod–coil diblock copolymers on the basis of mesogen‐jacketed liquid‐crystalline polymer were successfully prepared by atom transfer radical polymerization from the flexible polydimethylsiloxane (PDMS) macroinitiator. The hybrid diblock copolymers, poly{2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene}‐block‐polydimethylsiloxane, had number‐average molecular weights (Mn's) ranging from 9500 to 30,900 and relatively narrow polydispersities (≤1.34). The polymerization proceeded with first‐order kinetics. Data from differential scanning calorimetry validated the microphase separation of the diblock copolymers. All block copolymers exhibited thermotropic liquid‐crystalline behavior except for the one withMnbeing 9500. Four liquid‐crystalline diblock copolymers with PDMS weight fractions of more than 18% had two distinctive glass‐transition temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1799–1806, 2003

Synthesis and characterization of novel rod–coil diblock copolymers of poly(methyl methacrylate) and liquid crystalline segments of poly(2,5‐bis[(4‐methoxyphenyl)oxycarbonyl] styrene)

AbstractLiquid crystalline diblock copolymers with different molecular weights and low polydispersities were synthesized by atom transfer radical polymerization of methyl methacrylate (MMA) and 2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene (MPCS) monomers. The block architecture (coil‐conformation of MMA segment and rigid‐rod of MPCS segment) of the copolymer was experimentally confirmed by a combination of 1H nuclear magnetic resonance and gel permeation chromatograph techniques. The liquid crystalline behaviour of the copolymer was studied using differential scanning calorimetry and polarized optical microscope. It was found that the liquid crystalline behaviour was dependent on the number average molecular weight of the rigid segment. Only those copolymers with Mn(GPC) of the rigid block above 9200 g mol−1 could form liquid crystalline phases higher than the glass transition temperature of the rigid block. The random copolymers MPCS‐co‐MMA were also synthesized by conventional free radical polymerization. The molar content of MPCS in MPCS‐co‐MMA had to be higher than 71% to maintain liquid crystalline behaviour.© 2003 Society of Chemical Industry

Micelle Formation of a Rod−Coil Diblock Copolymer in a Solvent Selective for the Rod Block

Aggregation of an amphiphilic rod-coil diblock copolymer, poly(n-hexyl isocyanate)-b-poly(ethylene glycol) (PHIC-b-PEG), in toluene, a selective solvent for the rodlike PHIC block, was studied using light scattering and polarized optical microscopy. Static and dynamic light scattering data indicate the formation of cylindrical micelles with a hydrodynamic radius of 279 nm in very dilute solutions (weight fraction of polymer, w, from 0.0002 to 0.008) at 25 °C. The cylindrical micelles were determined to be disklike (planar), 0.9 μm in diameter and 20 nm in thickness, based on a comparison between the experimental data and the theory of scattering and self-diffusion of ellipsoids. Planar aggregates are expected if nematic interactions of the coronal rod block are dominant. The aggregate size decreases with increasing temperature. The micelles were observed visually in solution with the help of a polarized optical microscope due to the optical anisotropy of the PHIC block. Disklike objects with diameters in the micron range were observed to undergo translational and rotational motion in and out of the focal plane of the microscope.

Atom-Transfer Radical Polymerization to Synthesize Novel Liquid Crystalline Diblock Copolymers with Polystyrene and Mesogen-jacketed Liquid Crystal Polymer Segments

The synthesis of a series of new rod-coil diblock copolymers with different molecular weights and low polydispersity was achieved by atom transfer radical polymerization. The block architecture (coil-conformation of styrene segment and rigid-rod conformation of 2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene segment) of the diblock copolymers was experimentally confirmed by proton nuclear magnetic resonance ( 1 H NMR) and differential scanning calorimetry (DSC). The liquid crystalline behavior of the copolymers was studied using DSC and a polarized optical microscope (POM). It was found that the liquid crystalline behavior was dependent on the molecular-weight of the rigid segment. Only those copolymers with Mn of the rigid block beyond 9,300 g/mol could form liquid crystalline phases above the glass transition temperature of the rigid block.

A surface-reactive rod-coil diblock copolymer: nano- and micropatterned polymer brushes.

A diblock copolymer consisting of poly(3-(triethoxysilyl)propylisocyanate) (PIC) and polystyrene(PS) was synthesized by anionic polymerization. A polymeric monolayer of the block copolymer was formed on silica substrates by various grafting techniques such as immersion, casting, or contact-printing. The PIC block adheres covalently to Si substrates in an in-plane fashion due to its extended rodlike conformation and reactivity to the silica. The polystyrene blocks aggregate to form mounds on the surface resulting in a new type of nanopatterned polymer brush. The self-limiting adsorption of the rod coils results in a thickness of about 5 nm regardless of the solution concentration and coating method. This particular property allowed microcontact printing directly onto silicon or glass substrates. The resulting surface morphology consisted of nanoscale domains given by the block copolymer and uniform thickness micropatterns transferred from the stamp within the printed area. This study offers a simple new method to prepare a covalent polymeric monolayer with nano- and micropatterns, which can be performed directly onto various silicon-based substrates.

Supramolecular Materials via Block Copolymer Self-Assembly

This review discusses the potential of block copolymer type macromolecular building blocks for the preparation of self-assembled materials. Three different classes of block copolymer type architectures will be distinguished: (i) coil–coil diblock copolymers, (ii) rod–coil diblock copolymers, and (iii) rod–coil diblock oligomers. The basic principles that underlie the self-assembly of each of these different building blocks will be discussed. These theoretical considerations are complemented with examples from recent literature that illustrate the potential of the different types of block copolymers to prepare (functional) supramolecular materials. Finally, several strategies will be presented that could allow the preparation of stimuli-sensitive self-assembled materials, i.e., materials whose properties can be reversibly manipulated under the action of appropriate external stimuli.

Steric interaction of an incoming particle with grafted rods: exact solutions and unusual force profiles.

We examine the problem of a grafted rod or rods compressed by a sphere, concentrating on the steric force exerted by the rods on the sphere. We show that this problem can be solved exactly to yield simple and nontrivial expressions for the repulsive force. In particular, there are several different regimes and in some of these the force exhibits surprising maxima and minima as a function of compression. This has applications to systems of stiff grafted polymers, particularly biopolymers. One experimental realization of our system might be a rod-coil diblock copolymer with the coil grafted to a solid surface.

Smectic Rod−Coil Melts Confined between Flat Plates: Monolayer−Bilayer and Parallel−Perpendicular Transitions

We study theoretically the equilibrium properties of a rod-coil diblock copolymer melt confined between two flat plates. We assume that the system forms lamellae in the bulk. The plates may have a preference for either the rods or coils. We investigate whether the preferred alignment of the lamellae is parallel or perpendicular to the plates and how the lamellar spacing changes compared to the bulk lamellar spacing. We find that this system is much more constrained than the analogous coil-coil system and for certain values of the rod-coil interfacial tensions that perpendicular lamellae exist for quite thick films. For dissimilar bounding plates we find the antisymmetric parallel morphology may exist.

Nitroxide-mediated ?living? free radical synthesis of novel rod-coil diblock copolymers with polystyrene and mesogen-jacketed liquid-crystal polymer segments

Nitroxide-mediated ?living? free radical synthesis of novel rod-coil diblock copolymers with polystyrene and mesogen-jacketed liquid-crystal polymer segments

Nitroxide‐mediated ‘living’ free radical synthesis of novel rod–coil diblock copolymers with polystyrene and mesogen‐jacketed liquid‐crystal polymer segments

The synthesis of rod-coil diblock copolymers with narrow polydispersity was achieved for the first time by TEMPO-mediated 'living' free radical polymerization of styrene and 2,5-bis((4- methoxyphenyl)oxycarbonyl)styrene. The block architecture of the two diblock copolymers thus prepared, MPCS-block-St (5400/2400) and MPCS-block-St (10 800/8700), was confirmed by GPC, 1 H and 13 C NMR and DSC studies. The liquid-crystalline behaviour of the copolymers was studied by DSC and polarized optical microscope. It was observed that both copolymers showed two distinct glass transitions, corresponding to polystyrene and poly(-2,5-bis((4-methoxyphenyl)oxycarbonyl)styrene). Above the glass transition temperature of rigid block, liquid-crystalline phase was formed. The clearing point of the phase is higher than the polymer decomposition temperature. # 2000 Society of Chemical Industry

Self-assembly of ordered microporous materials from rod-coil block copolymers

Rod-coil diblock copolymers in a selective solvent for the coil-like polymer self-organize into hollow spherical micelles having diameters of a few micrometers. Long-range, close-packed self-ordering of the micelles produced highly iridescent periodic microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes whose diameter, periodicity, and wall thickness depended on copolymer molecular weight and composition. Addition of fullerenes into the copolymer solutions also regulated the microstructure and optical properties of the microporous films. These results demonstrate the potential of hierarchical self-assembly of macromolecular components for engineering complex two- and three-dimensional periodic and functional mesostructures.

Liquid-crystalline behavior of rod-coil diblock copolymers

The exact mean-field phase behavior of the Semenov–Vasilenko model for rod-coil diblock copolymers is studied by applying self-consistent field techniques. The behavior depends on three quantities: the rod/coil immiscibility χN, the coil volume fraction f, and the ratio ν of the characteristic coil to rod dimensions. When χN≲5, the rods and coils mix producing a nematic phase, and at larger χN, they microphase separate forming a lamellar phase. The nonlamellar phases expected at f≳0.7 are not treated here. The lamellar phase is typically a smectic-C structure with monolayers of tilted rods. A thorough understanding of the model is achieved by closely examining segment distributions and various contributions to the free energy. The tilt angle θ is generally controlled by a competition between rod/coil interfacial tension and stretching of the coils. Lowering f reduces the latter contribution, causing θ→0 and producing a continuous transition to a smectic-A structure. Beyond that, there is a tendency to form structures with the rods arranged in bilayers, but this is strongly suppressed by a large steric penalty. We suggest that small amounts of solvent can greatly alleviate this penalty, and therefore could significantly affect some aspects of the phase behavior.

Self-assembled aggregates of rod-coil block copolymers and their solubilization and encapsulation of fullerenes

Amphiphilic poly(phenylquinoline)-block-polystyrene rod-coil diblock copolymers were observed to self-organize into robust, micrometer-scale, spherical, vesicular, cylindrical, and lamellar aggregates from solution. These diverse aggregate morphologies were seen at each composition, but their size scale decreased with a decreasing fraction of the rigid-rod block. Compared to coil-coil block copolymer micelles, the present aggregates are larger by about two orders of magnitude and have aggregation numbers of over 10(8). The spherical and cylindrical aggregates have large hollow cavities. Only spherical aggregates with aggregation numbers in excess of 10(9) were formed in the presence of fullerenes (C60, C70) in solution, resulting in the solubilization and encapsulation of over 10(10) fullerene molecules per aggregate.

Grafted rod-coil polymers in poor solvents: turnip and jellyfish surface micelles

We consider a system of rod-coil diblock copolymers which are permanently tethered to a flat surface by the end of the coil block. This system is placed in a solvent which is poor for both the rods and the coils. At low grafting densities the polymers form separate globules on the surface. As the grafting density is increased these globules can fuse to form surface micelles. At intermediate grafting densities a ‘turnip’ should form, while at the higher grafting densities the system resembles a series of ‘jellyfish’.

Self-Assembled Smectic Phases in Rod-Coil Block Copolymers

Rod-coil block copolymers are self-assembling polymers that combine the physics of orientational ordering of rodlike polymers and the microphase separation of coil-coil block copolymers. Several new solid-state morphologies were observed in a series of anionically synthesized model poly(hexyl isocyanate-b-styrene) rod-coil diblock copolymers examined by transmission electron microscopy and selected-area electron diffraction. The rod-coils formed smectic C-like and O-like morphologies with domain sizes ranging from tens of nanometers to almost 1 micrometer. Both structural and orientational changes were found for increasing rod volume fractions. In addition, some morphologies exhibited spontaneous long-range orientational order over many tens of micrometers.