BAB Triblock Copolymers Research Articles

Synthesis and characterization of the swelling and mechanical properties of amphiphilic ionizable model co-networks containing hydrophobic blocks.

Amphiphilic ionizable model co-networks based on near-monodisperse, linear ABA and BAB triblock and statistical copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable) and n-butyl methacrylate (BuMA, hydrophobic non-ionic) were synthesized using group-transfer polymerization in tetrahydrofuran (THF) with the use of the hydrophobic ethylene glycol dimethacrylate (EGDMA) as cross-linker. Seven model co-networks were prepared in which the architecture and copolymer composition were varied systematically. One randomly cross-linked copolymer co-network was also prepared. The co-networks were characterized in terms of their degree of swelling in water as a function of pH and in THF. An increase in the aqueous degree of swelling was observed below pH 6 because of the ionization of the DMAEMA residues. The aqueous degrees of swelling at low pH decreased with the co-network composition in hydrophobic BuMA units. The maximum aqueous degrees of swelling of the copolymer co-networks were architecture-dependent, with the co-networks comprising the statistical copolymer chains swelling about 4 times more than their triblock copolymer counterparts. This was attributed to microphase separation in the triblock copolymer co-networks, which reduced the effective chain length between cross-links due to the collapse of the hydrophobic blocks. The mechanical properties of the water-swollen co-networks at pH 3 and 9 were investigated by determining the co-network uniaxial compression modulus. This modulus was higher at pH 9 than 3, and increased linearly with the BuMA content.

Direct synthesis of mesoporous silica presenting large and tunable pores using BAB triblock copolymers: Influence of each copolymer block on the porous structure

Mesoporous silicas with large and tunable (from 4 to 40 nm) accessible pores have been successfully synthesized using laboratory-made BAB (A: hydrophilic block, B: hydrophobic block) type triblock copolymers as templates. A direct synthesis is carried out in the absence of swelling agent and without any further treatment. The polystyrene- b–poly(ethylene oxide)- b–polystyrene (PS- b–PEO- b–PS) copolymers were synthesized using living/controlled radical polymerization. The porous structure (mesopore size, size distribution and microporous volume) was characterized using electron transmission microscopy and nitrogen sorption measurements. This study is focused on the control of the porous structure as a function of the hydrophobic PS block length and the hydrophilic block length (whilst keeping the other block length constant). It was found that when the PS block length increases: (i) the mesopore size increases linearly with PS block length which thus proffers a fine tuning of the pore diameter from 4 to 40 nm, (ii) the micropore volume decreases. When the PEO block length increases, (i) the mesopore size decreases (ii) the micropore volume increases. All these results were explained considering (i) that both PS and PEO block lengths have an influence on the degree of stretching of the PS chains, thus on the micellar core radius (ii) that the mesopore size is directly connected to the micellar core radius, i.e. that PEO chains do not directly participate to mesoporosity (due to its particular loop configuration) but only to microporosity.

Synthesis and characterization of block copolymers containing poly(di(ethylene glycol) 2-ethylhexyl ether acrylate) by reversible addition–fragmentation chain transfer polymerization

Reversible addition–fragmentation chain transfer (RAFT) polymerization has emerged as one of the important living radical polymerization techniques. Herein, we report the polymerization of di(ethylene glycol) 2-ethylhexyl ether acrylate (DEHEA), a commercially-available monomer consisting of an amphiphilic side chain, via RAFT by using bis(2-propionic acid) trithiocarbonate as the chain transfer agent (CTA) and AIBN as the radical initiator, at 70 °C. The kinetics of DEHEA polymerization was also evaluated. Synthesis of well-defined ABA triblock copolymers consisting of poly(tert-butyl acrylate) (PtBA) or poly(octadecyl acrylate) (PODA) middle blocks were prepared from a PDEHEA macroCTA. By starting from a PtBA macroCTA, a BAB triblock copolymer with PDEHEA as the middle block was also readily prepared. These amphiphilic block copolymers with PDEHEA segments bearing unique amphiphilic side chains could potentially be used as the precursor components for construction of self-assembled nanostructures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5420–5430, 2007

Chain Localization and Interfacial Thickness in Microphase-Separated Structures of Block Copolymers with Variable Composition Distributions

The effect of composition distribution on microphase-separated structures of AB diblock and BAB triblock copolymers was investigated at the molecular level by measurements of neutron reflectivity. Monodispersed three poly(styrene)-d8-block-poly(2-vinylpyridine) (DP) and three labeled triblock copolymers (PDP) and unlabeled counterparts were synthesized by a living anionic polymerization process designed to yield constant molecular weight with different volume fractions. The selective labeling method employs the blending of copolymers for a segmental distribution study while the labeled polymers are mixed for an interfacial study. Scattering length density profiles were observed by neutron reflectivity measurements for blended thin films produced by spin-coating and successive thermal annealing. It was confirmed that the blended thin films exhibit simple alternating lamellar structures, an observation consistent with the observed structures of bulk specimens. The localization phenomenon of longer and short...

Characterization of the Changes in Secondary Structure and Architecture of Elastin−Mimetic Triblock Polypeptides during Thermal Gelation

Vibrational spectroscopy and light scattering studies identify the conformational changes associated with gelation of elastin−mimetic triblock (EMT) copolymers. The EMT polypeptide is based on the VPGVG repeat of elastin and is constructed as a BAB triblock copolymer with relatively hydrophobic B-blocks. Laser Raman and ATR-FTIR spectroscopies probed changes in secondary structure associated with gelation in H2O and D2O. The amide I regions of these spectra suggest that the network-stabilizing interactions are intermolecular β-sheets with a significant fraction of the network containing β-turns and disordered structures. Peak frequency shifts in the ATR-FTIR spectra showed a conversion of extended hydrogen-bonded structures (polyproline II-like structures) to β-sheets during gelation. Molecular weights extrapolated through static light scattering at temperatures below the thermal transition were greater than for a single EMT chain. This suggests that some molecular organization exists prior to gelation, t...

Amphiphilic Polymethacrylate Model Co-Networks: Synthesis by RAFT Radical Polymerization and Characterization of the Swelling Behavior

Reversible addition−fragmentation chain transfer (RAFT) controlled radical polymerization was employed, for the first time, to prepare well-defined (model) amphiphilic polymer co-networks based on n-butyl methacrylate (BuMA, hydrophobic monomer) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable monomer) cross-linked with ethylene glycol dimethacrylate (EGDMA) and bearing elastic chains having the following comonomer distributions: BuMA-b-DMAEMA-b-BuMA and DMAEMA-b-BuMA-b-DMAEMA triblock and BuMA-co-DMAEMA statistical copolymers. Two randomly cross-linked (not model) amphiphilic co-networks were also synthesized, the one by RAFT and the other by conventional free radical polymerization. The amphiphilic triblock copolymer-based model co-networks were obtained by stepwise synthesis in three stages: (1) the synthesis of linear homopolymers bearing two active ends by using a bifunctional chain transfer agent (CTA), (2) the sequential addition of the second monomer on the macro-CTAs (homopolymers) to yield linear ABA or BAB triblock copolymers, and (3) the inter-linking of the linear copolymer chains at both ends using EGDMA cross-linker to form the final model co-networks. The homopolymer and copolymer precursors to the networks were characterized using gel permeation chromatography and 1H NMR spectroscopy. The swelling behavior of the networks was investigated in tetrahydrofuran and water. The co-networks swelled more in acidic than in neutral water due to the ionization of their DMAEMA units. The low pH aqueous swelling of the statistical co-network was higher than those of its triblock counterparts due to the lack of microphase separation with the statistical copolymer chains.

Monte Carlo simulation of the aggregation of rod–flexible triblock copolymers in a thin film

The aggregation of rod–flexible ABA and BAB triblock (A was rod block and repulsive with block B) copolymers in a thin film was studied as a function of varying the rigidity ( η) and the length of the rod block by Monte Carlo simulation. The rigidity of block A was defined as η = R c/ R max in this study. R c was the end-to-end distance below which the conformation of the block was not allowed, whereas R max was the longest end-to-end distance that the block could be. If η = 0 the block was flexible, whereas if η = 1 the block was a straight rod. The simulation results showed that the ABA triblock copolymer film were likely to form lamella structure with increasing the rigidity ( η) of block A. The lamellas were parallel each other and perpendicular to the film surface. However, the aggregation of BAB triblock copolymers tended to change from lamella to cylinder structure with increasing the rigidity ( η) of block A. Typical lamella and cylinder co-exist structure was obtained at η = 0.504 for the BAB copolymer film. On the other hand, the simulation results indicated that the film changed from disorder to order, then to disorder structure with increasing the relative length of B block for both ABA and BAB copolymer films. Well ordered lamella for ABA copolymer and well ordered cylinder for BAB copolymer structures could be obtained when the composition of block B is around 50%.

Effect of Molecular Weight Distribution on Microphase-Separated Structures from Block Copolymers

The effect of molecular weight distribution on microphase-separated structures for both AB diblock and BAB triblock copolymers was investigated in comparison with that of composition distribution. Monodisperse poly(styrene-b-2-vinylpyridine) (SP) and poly(2-vinylpyridine-b-styrene-b-2-vinylpyridine) (PSP) parent block copolymers were synthesized by living anionic polymerizations whose volume ratios were all designed to be 0.5/0.5. Three parent copolymers were blended variously with both number-average molecular weight and composition kept constant but having different molecular weight distribution. Microphase-separated structures of sample films obtained by solvent-casting followed by annealing were observed by transmission electron microscopy and small-angle X-ray scattering. It has been found that both SP and PSP block copolymers show simple lamellar structures even when the molecular weight distribution is relatively wide and that lamellar domain spacing increases with increase in polydispersity index of the blend system. Furthermore, the increment for PSP triblock is larger than that for SP diblock as was the result for the study on composition distribution. The microdomain expansion can be caused by the localization of polydisperse block chains in both phases, which was commonly observed for both the composition distribution system and the molecular weight distribution system.

Effect of Composition Distribution on Microphase-Separated Structure from BAB Triblock Copolymers

The effects of composition distribution on microphase-separated structures formed by monodisperse BAB triblock copolymers were investigated. Monodisperse nine parent BAB triblock copolymers consisting of polystyrene for A and poly(2-vinylpyridine) for B were prepared by anionic living polymerization. These nine copolymers were designed such that polystyrene volume fraction, φs, ranged from 0.1 to 0.9, and they were blended to produce samples with various composition distributions but with constant average composition at φs of 0.5. Periodic simple alternating lamellar structures were observed for solvent cast and well-annealed blend films as long as composition distribution is relatively low. It has been found that microdomain spacing increases with increasing composition distribution of copolymer samples up to 1.25 in terms of Mw(S)/Mn(S), where Mw(S) and Mn(S) are weight-average and number-average molecular weights of polystyrene blocks, respectively, as was the case of the AB diblock copolymer system, t...

Synthesis of Sterically Stabilized Polystyrene Latex Particles Using Cationic Block Copolymers and Macromonomers and Their Application as Stimulus-Responsive Particulate Emulsifiers for Oil-in-Water Emulsions

2-(Dimethylamino)ethyl methacrylate (DMA) was block copolymerized with methyl methacrylate (MMA) using group transfer polymerization to give four AB diblock, ABA triblock, and BAB triblock copolymers of low polydispersity (Mw/Mn < 1.20). In addition, a near-monodisperse styrene-functionalized DMA-based macromonomer was synthesized via oxyanionic polymerization using a potassium 4-vinylbenzyl alcoholate initiator. These five well-defined, tertiary amine methacrylate-based copolymers were evaluated as steric stabilizers for the synthesis of polystyrene latexes via emulsion and dispersion polymerization. The most efficient steric stabilizers proved to be the DMA-MMA diblock copolymer and the DMA-based macromonomer. The polystyrene latexes were characterized in terms of their particle size and morphology, stabilizer content, surface charge, and surface activity using dynamic light scattering, scanning electron microscopy, 1H NMR spectroscopy, aqueous electrophoresis measurements, and surface tensiometry, respectively. The pH-dependent surface activity exhibited by selected latexes suggests potential applications as stimulus-responsive particulate emulsifiers for oil-in-water emulsions.

Synthesis and Associating Properties of Poly(ethoxyethyl glycidyl ether)/Poly(propylene oxide) Triblock Copolymers

A number of ABA and BAB triblock copolymers of ethoxyethyl glycidyl ether (EEGE) and propylene oxide (PO) were prepared by sequential anionic polymerization. The copolymers were characterized by nuclear magnetic resonance and gel permeation chromatography. The total number-average molecular weights were in the range 1000−9000, whereas the degrees of polymerization of the PPO and PEEGE blocks varied from 2 to 34 and from 3 to 17, respectively. The copolymers were comprised of blocks with different lower critical solution temperatures (LCSTs) in aqueous media. Their self-association in aqueous environment was studied by cloud point (CP) measurements, dye solubilization, light scattering, 1H nuclear magnetic resonance, and scanning electron microscopy. Although the behavior of the copolymers was found to depend on their architecture, the PO/EEGE ratio, and the degrees of polymerization of the different blocks, the role of the PEEGE blocks was decisive: it is PEEGE that determines the LCST properties of the ...

Block Copolymers of Methyl Vinyl Ether and Isobutyl Vinyl Ether With Thermo‐Adjustable Amphiphilic Properties

AbstractThe thermo‐adjustable hydrophilic/hydrophobic properties of AB, ABA and BAB block copolymers in which A is poly(methyl vinyl ether) (PMVE) and B is poly(isobutyl vinyl ether) (PIBVE) have been investigated. The block copolymers were prepared by “living” cationic polymerization using sequential addition of monomers. The polymerizations were carried out with the system acetal/trimethylsilyl iodide as initiator and ZnI2 as activator. The initiating system based on diethoxyethane leads to AB block copolymers whereas the initiating system based on tetramethoxypropane leads to ABA or BAB triblock copolymers. Well‐defined block copolymers of different composition with controlled molecular weights up to approx. 10 000 have been prepared. When IBVE is added to living PMVE, PIBVE‐blocks form only in the presence of an additional amount of ZnI2, which is attributed to the fact that part of the ZnI2 is inactive because of complex formation with PMVE. At room temperature, the combination of hydrophilic (PMVE) and hydrophobic (PIBVE) segments provides the copolymers with surfactant properties. Above the lower critical solution temperature (LCST) of PMVE, situated around 36 °C, the PMVE‐blocks become hydrophobic and the amphiphilic nature of the block copolymers is lost. The corresponding changes in hydrophilic/hydrophobic balance have been evaluated by investigation of the emulsifying properties of the block copolymers for water/decane mixtures as a function of the temperature. Below the LCST, the block copolymers have emulsifying properties similar to or better than those of the commercial PEO‐PPO block copolymers (Pluronic®). Either oil‐in‐water or water‐in‐oil emulsions can be obtained, depending on the polymer architecture and the water/decane volume ratio. The emulsifying properties are strongly reduced or completely lost above 40 °C. Emulsions obtained with a PMVE36‐b‐PIBVE54 block copolymer for a water/decane (v/v) ratio of 85/15 remained stable for more than six months.50/50 and a 85/15 water/decane w/o emulsion (15 g/l) with the PMVE36‐b‐PIBVE54 block copolymer at 20 °C.magnified image50/50 and a 85/15 water/decane w/o emulsion (15 g/l) with the PMVE36‐b‐PIBVE54 block copolymer at 20 °C.

A Monte Carlo simulation for the micellization of ABA- and BAB-type triblock copolymers in a selective solvent. II. Effects of the block composition

The micellization behaviors of two types of ABA and BAB triblock copolymers with different block compositions in a solvent selective for block A are investigated by using the grand canonical Monte Carlo simulation combined with the multiple histogram method. The block composition and sequence are found to significantly control the micelle formation and the association behavior for both types of triblock copolymers. The thermodynamic analysis shows that ΔH0 and ΔS0 decreases as the length of insoluble block B decreases, indicating that the driving force toward micellization decreases with decreasing the B block length for both types of copolymers. On the other hand, BAB copolymers show less negative ΔH0 and more negative ΔS0 than ABA at all the compositions except at fB=0.20, where the reduced capability of BAB copolymer to form micelle leads to increase the amount of dangling chains in the micelle, yielding less negative ΔS0. The basic micellar parameters such as the micellar size, the micellar shape, and the distribution of segments in a micelle are examined as a function of block composition and sequence. It is evident that the micelle formation of triblock copolymer follows the closed association mechanism, irrespective of the block composition and sequence.

A Monte Carlo Simulation for the Micellization of ABA- and BAB-Type Triblock Copolymers in a Selective Solvent

The micellization behaviors of ABA and BAB triblock copolymers in a solvent selective for block A are investigated by means of grand canonical Monte Carlo (GCMC) simulation. The basic micellar parameters such as the micellar size, its distribution, the micellar shape, and the internal structure of micelles are obtained and compared for both systems. The critical micelle concentrations (cmc) are quantitatively estimated from the osmotic pressure obtained by the GCMC simulation combined with the multiple histogram method. The results of simulation show that the chain architecture of the triblock copolymers shows large differences in their association behavior in a selective solvent, although both copolymers have the same composition and total chain length. The thermodynamic analysis confirms that an additional entropy loss due to the looping conformation of the middle block of BAB copolymer induces a higher cmc value and therefore reduces the capability to self-assemble into micelles. As a result, the BAB c...

Self-assembled structures of block copolymers in selective solvents reproduced by lattice Monte Carlo simulation

A lattice Monte Carlo (MC) simulation was applied to the study of block copolymers in selective solvent or amphiphilic surfactant solution on the segment level, hydrodynamic interactions being neglected. The code was found to be very efficient, employing a partial reptation mode as the elementary movement of the self-avoiding lattice chains. Typical self-assembled structures of block copolymers such as micelle, lamellae, hexagonal cylinder and bicontinuous networks have been successfully reproduced without any priori specification of structure. Order–disorder and order–order transitions of diblock copolymers are systematically studied by adjusting the temperature, the concentration or the block length ratio in a series computer simulations. The structural differences between micelles composed of ABA and BAB triblock copolymers are also explicitly revealed by direct visualisation of the underlying chain configurations. The simulation results are consistent with the experimental observations in the literature. This simulation approach is thus a very useful tool in the extensive investigation of self-assembled structures. It has the advantage that both micro-domains and chain configurations can be studied with only a comparatively modest call on computational resources.

Mesoscopic patterns of block and graft copolymers in condensed systems

Equilibrium structures of two kinds of two-component copolymers with equivalent chemical contents but with different chain architectures in bulk were compared. They are BAB triblock copolymers and AB2 star-branched graft copolymers. These copolymers have been confirmed to show quite different morphological change with composition. Deformation manner of B block chains of lamellar microphase-separated BAB triblock copolymers depend on B contents, however, the volumes of the deformed coils are always kept constant to have those of the unperturbed chains irrespective of their architectures. The observed polystyrene/poly(2-vinylpyridine) interfacial thickness is fairly thin though it is much thicker than the theoretically-predicted one.

Studies on equilibrium structures of complex polymers in condensed systems

The thermodynamic-equilibrium structures of four copolymers with different chain architectures are compared. They are AB diblock, BAB triblock, and ABC triblock copolymers and AB2 star-branched graft copolymers. Their morphological transitions meaningfully differ from one another, reflecting the difference in chain architecture. Two-component polymers barely form bicontinuous structures; however, symmetric ABC triblock copolymers easily form tricontinuous structures composed of two surfaces parallel to a Gyroid minimal surface. All four ABC equilibrium structures have a superlattice structure. Block polymer chains in microphase-separated bulk are elongated in the longitudinal direction in lamellar microphases. Although the deformation manner of the chains restricted in microphases depends on the chain architectures, the volumes of the deformed coils are always the same as those of the unperturbed chains regardless of their architectures. The measured polystyrene/poly(2-vinylpyridine) interfacial thickness is fairly thin, but the observed value is much thicker than the theoretically predicted one. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1645–1655, 2000

Interfacial structures of block and graft copolymers with lamellar microphase-separated structures

Chain conformations of A-polymers of BAB two-component triblock and ABB graft copolymers, composed of polystyrene (=A) and poly(2-vinylpyridine) (=B), with lamellar microphase-separated structures were investigated by small-angle neutron scattering (SANS), and interfacial thicknesses of AB diblock and BAB triblock copolymers were compared by using neutron reflectivity (NR). The middle A-block of BAB triblock copolymer has preferentially “loop” conformation when the volume fraction, φ A, of A-chain is high above 0.5, while it does preferentially “bridge” conformation when φ A is low around 0.3. A-graft chain of ABB copolymer has slightly larger dimension along the direction parallel to lamellar interface than A-chain of AB diblock copolymer. The interfacial thickness of BAB triblock copolymer is almost constant irrespective of φ A, and comparable with that of AB diblock copolymer.

Metal-Centered Star Block Copolymers: Amphiphilic Iron Tris(bipyridine)-Centered Polyoxazolines and Their Chemical Fragmentation to Bipyridine-Centered BAB Triblock Copolymers

Amphiphilic metal-centered six-arm star block copolymers were prepared by the sequential addition of two different oxazoline monomers to the hexafunctional metalloinitiator, [Fe(4,4‘-bis(chloromethyl)-2,2‘-bipyridine)](PF6)2. Polymers of the form [Fe{bpy(PEOX−PUOX)2}3]X2, [Fe{bpy(PUOX−PEOX)2}3]X2, and [Fe{bpy(PEOX−PPOX)2}3]X2 (PEOX = poly(2-ethyl-2-oxazoline); PUOX = poly(2-undecyl-2-oxazoline); PPOX = poly(2-phenyl-2-oxazoline); X = Cl, I, or PF6) were generated by reaction of the appropriate 2-R-2-oxazolines (R = Et, Ph, undecyl). In some cases sodium iodide was added to activate the initiator, and dipropylamine was used for termination. Since the labile core polymers fragment during GPC analysis, the iron was removed by reaction with aqueous K2CO3 for MW characterization by multiangle laser light scattering in CHCl3 solution. The resulting bpy-centered telechelic BAB triblock copolymers, bpy(PEOX−PUOX)2, bpy(PUOX−PEOX)2, and bpy(PEOX−PPOX)2, exhibit narrow molecular weight distributions (PDIs < 1.1) and molecular weights close to targeted values. For polymers prepared from ethyl and undecyl monomers, differential scanning calorimetry (DSC) measurements were performed for Fe-centered star homopolymers and block copolymers as well as for the corresponding metal-free bpy-centered linear polymers. DSC runs of block copolymers show Tg and Tm values that correlate well with those observed for the respective ethyl- and undecyloxazoline homopolymers.

Segregation Study of a BAB Triblock Copolymer at the A/B Homopolymer Interface

The segregation of a poly[(2-vinylpyridine)-b-styrene-d 8 -b-(2-vinylpyridine)] triblock copolymer (PVP-dPS-PVP) to a planar interface between the homopolymers PS and PVP is determined by forward recoil spectrometry (FRES). Compared to a diblock copolymer of dPS-PVP with the same mole fraction of PVP (∼0.15) and total degree of polymerization (∼500), we find that the triblock copolymer interfacial excess z i * at saturation is roughly half that of the diblock copolymer. Self-consistent mean field theory produces a segregation isotherm in fair agreement with the experimental results for the triblock copolymer using the same value of the Flory parameter χ deduced for the diblock copolymer