Enhancement of mechanical properties of triblock copolymers by random copolymer middle blocks

Symmetric styrene-b-styrene-co-butadiene-b-styrene (S-SB-S) tri-block copolymers with varying middle and outer block composition have been studied. We report our findings based on a systematic variation of the effective interaction parameter (χ) by adjusting the composition of the random copolymer in the middle block and of the outer blocks (in terms of PS-chain length) which allows us to explore the χ-parameter space with regard to molecular architecture more thoroughly than in SBS triblock copolymers. A variation in the S/B middle block composition or in the PS outer block content leads to a change in phase behaviour and morphology simultaneously accompanied by significant changes in mechanical properties, varying from elastomeric to thermoplastic property profile. Despite high PS contents of 55-75 wt.% these S-SB-S triblock copolymers reveal high strain at break values between 650% and 350% which is in striking contrast to the conventional SBS triblock copolymers where only about 10% strain at break have been reported to be achieved with similar PS-content (∼75 wt.%).     

Solution properties of styrene-p-chlorostyrene diblock copolymers—I. Conformational behaviour in dilute solutions

The conformational behaviour of styrene-p-chlorostyrene diblock copolymers in dilute solutions was studied and compared with that of the corresponding triblock copolymers. Eight styrene-p-chlorostyrene diblock copolymers, of almost equimolar composition but with different molecular weights, were prepared using an anionic polymerization technique. The intrinsic viscosities of the copolymers were measured in non-selective solvents, such as toluene and 2-butanone, and in a selective solvent, cumene. The osmotic second virial coefficients of the diblock copolymers were measured in toluene. The data were analysed on the basis of two parameter theories. The unperturbed dimensions for the diblock copolymers can be expressed as a composition average of those for the parent homopolymers and the long-range interaction parameters of the diblock copolymers in toluene, 2-butanone and cumene are smaller than those of the triblock copolymers of the same composition. It means that the diblock copolymer chains in these 3 solvents had a more compact conformation than the triblock copolymers of the same composition and molecular weight.     

Influence of chain architecture on phase behaviour of styrene-(styrene/butadiene)-styrene triblock copolymers and their binary blends

The phase behaviour of symmetric (LN4) and asymmetric (LN3) triblock copolymers based on styrene-b-(styrene-co-butadiene)-b-styrene (S-SB-S) and their blends have been studied using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) and were correlated with rheological properties. A direct control over the final morphology and segregation strength for the block copolymer blends was achieved by blending of LN3 and LN4. The interaction parameter (χ) for LN4 is extracted by fitting the SAXS patterns at temperatures well above the ODT in consistency with Leibler mean-field structure-function for ABA triblock copolymers. A weak temperature dependency of χ has been observed which revealed that the phase behaviour in LN4 is mainly controlled by the entropic term. In the low frequency regime a non-terminal flow behaviour was observed in LN3 revealing the persistence of ordered structure within the experimental temperature range whereas a terminal flow behaviour with composition fluctuation was observed in LN4. G′ vs. G″ plots indicated a solid-like elastic melt behaviour for LN3 whereas presence of ODT over a broad temperature range was observed for LN4. ODT is observed to increase non-linearly with increase in LN3 content in the blends. ODT behaviour of the blends further reveals that the blends shift from weak-segregation to intermediate-segregation strength with the increase in LN3 content. The improvement in the state of ordering along with the change in morphology with the increase of LN3 content is attributed to co-surfactant effect between the PS end-blocks of LN3 and LN4 inside PS-rich phase.     

Synthesis and crystallization behaviors of poly(styrene-b-isoprene-b-ε-caprolactone) triblock copolymers

The triblock copolymers, poly(styrene-b-isoprene-b-ε-caprolactone)s (PS-b-PI-b-PCL) have been synthesized successfully by combination of anionic polymerization and ring-opening polymerization. Diblock copolymer capped with hydroxyl group, PS-b-PI-OH was synthesized by sequential anionic polymerization of styrene and isoprene and following end-capping reaction of EO, and then it was used as macro initiator in the ring-opening polymerization of CL. The results of DSC and WAXD show big effect of amorphous PS-b-PI on the thermal behaviors of PCL block in the triblock copolymers and the lower degree of crystalline in the triblock copolymer with higher molecular weight of PS-b-PI was observed. The real-time observation on the polarized optical microscopy shows the spherulite growth rates of PCL₂₇, PCL₃₂₈ and PS-b-PI-b-PCL₃₄₄ are 0.71, 0.46 and 0.07 μm s⁻¹, respectively. The atomic force microscopy (AFM) images of the PS₉₀-b-PI₆₆-b-PCL₂₈ show the columns morphology formed by it’s self-assembling.     

Application of FTIR Microscopy in Combinatorial Experimentation on Polymer Blends

Summary: A novel combinatorial, high-throughput experimentation (HTE) setup has been developed, which allows for rapid mapping of the phase behavior of blends of homopolymers and block copolymers. The principle is based on the preparation of composition (ϕ)-temperature (T) gradient films. Linear ϕ gradients were obtained over a large composition range, as shown by FTIR microscopy. The applicability of this combinatorial approach was demonstrated by studying the phase behavior of a poly(styrene-co-acrylonitrile) (SAN)/poly(methyl methacrylate-co-ethyl acrylate) (PMMA-EA) blend with varying EA content and a poly(styrene-b-butadiene-b-methyl methacrylate) (SBM) triblock copolymer.     

Synthesis and micelle formation of triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) in aqueous solution

Amphiphilic triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) (PMMA-b-PEO-b-PMMA) with well-defined structure were synthesized via atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) initiated by the PEO macroinitiator. The macroinitiator and triblock copolymer with different PMMA and/or PEO block lengths were characterized with ¹H and ¹³C NMR and gel permeation chromatography (GPC). The micelle formed by these triblock copolymers in aqueous solutions was detected by fluorescence excitation and emission spectra of pyrene probe. The critical micelle concentration (CMC) ranged from 0.0019 to 0.016 mg/mL and increased with increasing PMMA block length, while the PEO block length had less effect on the CMC. The partition constant K_v for pyrene in the micelle and in aqueous solution was about 10⁵. The triblock copolymer appeared to form the micelles with hydrophobic PMMA core and hydrophilic PEO loop chain corona. The hydrodynamic radius R_h,app of the micelle measured with dynamic light scattering (DLS) ranged from 17.3 to 24.0 nm and increased with increasing PEO block length to form thicker corona. The spherical shape of the micelle of the triblock copolymers was observed with an atomic force microscope (AFM). Increasing hydrophobic PMMA block length effectively promoted the micelle formation in aqueous solutions, but the micelles were stable even only with short PMMA blocks.     

Synthesis and characterization of aba-type block copolymer of poly(ϵ-caproplactone) with poly(ethylene glycol), by mean of activation end groups

Poly(?-caprolactone)-b-poly(ethylene glycol)-b-poly(?-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymer were synthesized by mean anionic activation of the hydroxyl end groups of poly(ethylene glycol) in presence of diphenylmethylsodium. Copolymers were characterized by SEC, FT-IR and ¹H-NMR spectroscopy, TGA and DSC. Size exclusion chromatographic analysis of obtained copolymers indicated incorporation of CL monomer into PEG without formation of PCL homopolymer. Characterization by FT-IR and ¹H NMR spectroscopy of the resulting polymeric products, with respect to their structure, end-groups and composition, showed that they are best described as ester-ether-ester triblock copolymers, whose compositions can be adjusted changing the feeding molar ratio of PEG to CL. The thermal stability of triblock copolymers was less that PEG precursor, but higher that PCL homopolymer. Analysis by mean DSC showed that all copolymers were semi-crystalline and their thermal behavior depending on their composition.     

Nucleation and crystallization of PS-b-PEO-b-PCL triblock copolymers

We have recently prepared a series of Polystyrene-b-Poly(ethylene oxide)-b-Polycaprolactone (PS-b-PEO-b-PCL or SEOCL) triblock copolymers of varying compositions and molecular weights. These ABC triblock copolymers present the peculiarity that two of the three blocks are able to crystallize upon cooling from an already phase segregated melt. When either of the crystallizable blocks or both are a minor phase, a fractionated crystallization process develops. The confinement of crystallizable blocks in the nanoscopic scale enables the clear observation in some cases of exclusive crystallization from homogeneous nuclei of two components within the triblock copolymer. The homogeneous nature of the nucleation was deduced since the supercooling attained is the maximum possible before vitrification of the material takes place. The self-nucleation domains were also found to depend on the composition and molecular weight of the copolymers. The block copolymers exhibited a marked decrease in crystalline memory and when the crystallizable blocks constitute minor phases, the self-nucleation domain disappears. The reason behind this behavior is that only at lower self-nucleation temperatures the density of self-nuclei becomes high enough to include at least one crystal fragment per confined microdomain in view of their vast numbers (e.g., 10¹⁶/cm³).     

Glass transition temperatures of ABA poly(styrene-b-isoprene) block copolymers by differential scanning calorimetry

The glass transition behavior of two sets of ABA poly(styrene-b-isoprene) block copolymers was examined by differential scanning calorimetry. In one series, the triblock copolymers had different total molecular weights and the same (30 wt %) polyisoprene content, in the other, the molecular weight was constant (30,000 g/mol) and the elastomer content was the variable. For all triblock copolymers studied, the data show an inward shift for the glass transition temperatures T_g of the corresponding homopolymers. This shift increases for the rigid-phase T_g as the polystyrene block length decreases. Depending on their molecular characteristics, two, three, or only one T_g were found. The third T_g was interpreted in terms of the existence of an interphase. Some of these conclusions could be confirmed by transmission electron microscopy.     

Microphase separation in oxyethylene/oxybutylene copolymers with diblock and triblock architectures

EBE and BEB triblock copolymers were prepared and characterized. Microphase separation in the melt state was studied, and the results combined with those for EB and BEB copolymers reported previously. The microphase separation temperature (MST) was determined from the temperature dependence of SAXS. There was a large difference in MST between the diblock and triblock copolymers as expected from theory. The Flory‐Huggins parameter (χ) was independent of block architecture for all three series provided that the E block lengths in the EBE copolymers exceeded 65.     

Syntheses,characterization, and properties of multiblock copolymers consisting of polyisobutylene and poly(L‐lactide) segments

The syntheses of {‐poly(L ‐lactide) (PLLA)‐b‐polyisobutylene (PIB)‐}_n multiblock copolymers were accomplished for the first time by chain extension of PLLA‐b‐PIB‐b‐PLLA triblock copolymers. Well‐defined PLLA‐b‐PIB‐b‐PLLA triblock copolymers with predictable M_ns, low PDIs (1.10–1.18) and excellent blocking efficiencies were prepared by anionic ring‐opening polymerizations of L ‐lactide initiated with hydroxyallyl telechelic PIB (HO‐Allyl‐PIB‐Allyl‐OH) in toluene at 110 °C. The triblock copolymers were successfully chain extended with 4,4′‐methylenebis(phenylisocyanate) (MDI) to obtain the multiblock copolymers with good gravimetric yields of ～86 to 96%. The chain‐extended polymers were soluble in a range of common organic solvents. The block copolymers showed two glass transition temperatures in differential scanning calorimetric analysis for the PIB and PLLA blocks indicating microphase separation, which was supported by atomic force microscopy images. The as‐synthesized compression molded multiblock copolymers exhibited tensile strengths in the range of 8–24 MPa with elongations at break in the range of 2.5–400%. The static and dynamic mechanical properties showed a strong dependence on the relative PLLA content in the copolymer. The dynamic mechanical analysis also indicated microphase separation at higher PLLA compositions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3490–3505, 2009     

Crystallization and melting behaviors of polystyrene-b-poly(ethylene-co-butene) block copolymers

Non-isothermal and isothermal crystallization behaviors of polystyrene-b-poly(ethylene-co-butene) (PSt-b-PEB) block copolymers with different compositions and chain lengths were investigated by differential scanning calorimetry (DSC). The results show that crystallization of PEB block is strongly dependent on the composition. Crystallization temperature (T_c), melting temperature (T_m) and fusion enthalpy (ΔH_f) increase rapidly with PEB volume fraction (V_E) for block copolymers with V_E below 50%, but there is little change when PEB block becomes the major component. Glass transition temperature (T_g) of the PSt block and order-disorder transition temperature (T_ODT) of block copolymers also have a weak effect. The isothermal crystallization kinetics results show that Avrami exponent (n) was strongly dependent on the composition and crystallization temperature. For the block copolymers with V_E below 38.7 vol%, the values of n vary between 0.9 and 1.3, indicating that crystallization is confined. For the PSt-b-PEB block copolymers with V_E higher than 50%, fractionated crystallization behavior is usually observed. A two-step isothermal crystallization procedure is applied to these block copolymers. It is found that breakout crystallization occurs at higher T_c, but confined at lower T_c. Two overlapped melting peaks are observed for the block copolymers with fractionated crystallization behavior after two-step crystallization, and only the higher melting peak corresponding to breakout crystallization can be used to derive equilibrium melting temperature.     

Synthesis of well-defined macrocyclic block copolymers using living coupling agent method

Macrocyclic poly(styrene-b-butadiene) (SB) block copolymers were prepared by coupling a living poly(styrene-b-butadiene-b-styrene) (SBS) block copolymer using a living coupling agent, 1,3-bis(1-phenylethylenyl)benzene (DDPE), or a difunctional electrophile, dimethyldichlorosilane. The living poly(styrene-b-butadiene-b-styrene) block copolymer was generated from an addition product of sec-butyllithium and DDPE. A living heteroarmed star block copolymer has been prepared by coupling two moles of monolithium polystyrene with one mole of DDPE followed by reinitiation and polymerization of the butadiene monomer. The dilithium 4-armed star block copolymer was then coupled using dimethyldichlorosilane to form a cyclic polybutadiene with two attached polystyrene branches.     

Morphologies and domain sizes of microphase-separated structures of block and graft copolymers of different types

Well-defined block and graft copolymers of different types with different compositions and molecular weights, such as styrene(S)-2-vinylpyridine(P) diblock copolymers, SP star-shaped block copolymers, PSP triblock copolymers, styrene(S)-isoprene(I) multiblock copolymers of the (SI)_n type, ISP triblock copolymers, SPP graft copolymers and their deuterated samples were prepared. Variations of the morphologies with compositions, molecular weight dependences of the lamellar domain sizes and conformations and distributions of block chains in the lamellar domains were studied in the strong segregation limit. Besides typical morphologies such as spherical, cylindrical and lamellar structures, ordered bi- and tri-continuous structures were found between cylindrical and lamellar structures for SP diblock copolymers, PSP and ISP triblock copolymers, respectively. The composition ranges of morphologies are different for the block and graft copolymers of different types. The molecular weight dependences of lamellar domain sizes are about the same, but their magnitudes are not always the same for the block and graft copolymers of different types. These results are well explained by the theories of Helfand-Wasserman and Semenov. Block chains in lamellae are extended along the direction perpendicular to lamellae, but they are contracted along the parallel direction. The former result is well explained by the theories, but the latter is not. Chains adjacent to the junction points between different block chains are localized near the domain interface, but chains at the free-ends of block chains are widely distributed in the domain with the maximum at the center of domain.     

Oligomers Derived from 2-Oxazolines

This paper describes the synthesis and properties of oligomer chains derived from 2-oxazolines. First, poly(styrene-g-N-acetyl-ethylenimine) was prepared, and its hydrolysis gave poly(styrene-g-ethylenimine) which showed good chelating properties. Secondly, ABA type triblock copolymers were prepared in which an N-acylethylenimine chain is used as A block and ethylene oxide chain is employed as B block. These triblock copolymers showed good compatibility with Nylon 6, which were shown to posecess effective anti-electrostatic properties for Nylon 6. Thirdly, AB type block copolymers from 2-oxazolines have been prepared by using living polymerization technique. These block copolymers are soluble in water and showed good surfactant nature as reflected by surface tension (γ), when A block is consisted from N-acetyl- or N-propionylethylenimine chain (hydro-philic) and B block is made of N-tridecanoyl or N-aroylethylenimine chain (lipophilic). Finally, graft copolymers of cellulose diacetate having N-acetylethylenimine chain were prepared. It has been found by using a rheovibron that these graft copolymers are compatible with poly(vinyl chloride).     

Macromolecular dynamics of sulfonated poly(styrene-b-ethylene-ran-butylene-b-styrene) block copolymers by broadband dielectric spectroscopy

Macromolecular dynamics of sulfonated poly(styrene-b-ethylene-ran-butylene-b-styrene) (sSEBS) triblock copolymers were investigated using broadband dielectric spectroscopy (BDS). Two main relaxations corresponding to the glass transitions in the EB and S block phases were identified and their temperature dependences were VFT-like. T_g for the S block phase shifted to higher temperature due to restrictions on chain mobility caused by hydrogen bonded SO₃H groups. While the EB block phase T_g appeared to remain constant with degree of sulfonation in DMA experiments, it shifted somewhat upward in BDS spectra. A low temperature relaxation beneath the glass transition of the EB block phase was attributed to short range chain motions. The Kramers–Krönig integral transformation was used to calculate conductivity-free loss permittivity spectra from real permittivity spectra to enhance true relaxation peaks. A loss permittivity peak tentatively assigned to relaxation of internal S-EB interfacial polarization was seen at temperatures above the S block phase glass transition, and the temperature dependence of this relaxation was VFT-like. The fragilities of the EB and S block domains in sulfonated SEBS decreased after sulfonation. The temperature dependence of the dc conduction contribution to sSEBS loss spectra also followed VFT-like behavior and S block segmental relaxation time correlated well with conductivity according to the fractional Debye–Stokes–Einstein equation.     

Biodegradable amphiphilic block copolymers containing functionalized PEO blocks: Controlled synthesis and biomedical potentials

A series of controllable amphiphilic block copolymers composed of poly(ethylene oxide) (PEO) as the hydrophilic block and poly(?-caprolactone) (PCL) as the hydrophobic block with the amino terminal group at the end of the PEO chain (PCL-b-PEO-NH₂) were synthesized. Based on the further reaction of reactive amino groups, diblock copolymers with functional carboxyl groups (PCL-b-PEO-COOH) and functional compounds RGD (PCL-b-PEO-RGD) as well as the triblock copolymers with thermosensitive PNIPAAm blocks (PCL-b-PEO-b-PNIPAAM) were synthesized. The well-controlled structures of these copolymers with functional groups and blocks were characterized by gel permeation chromatography (GPC) and ¹H NMR spectroscopy. These copolymers with functionalized hydrophilic blocks were fabricated into microspheres for the examination of biofunctions via cell culture experiments and in vitro drug release. The results indicated the significance of introducing functional groups (e.g., NH₂, COOH and RGD) into the end of the hydrophilic block of amphiphilic block copolymers for biomedical potentials in tissue engineering and controlled drug release.     

Synthesis and characterization of semicrystalline triblockcopolymers of isotactic polystyrene and polydimethylsiloxane

iPS‐b‐PDMS‐b‐iPS triblock copolymers were prepared by hydrosilylation of vinyl‐terminated isotactic polystyrenes (iPS) with α,ω‐bis(dimethylsilane)‐terminated poly(dimethylsiloxane)s (PDMS). As a function of the molecular weights of the two components, the triblock copolymer composition was varied between 9.0 and 98 wt % iPS. The resulting triblock copolymers remained soluble during block copolymer synthesis due to slow iPS crystallization in solution. At iPS content exceeding 31 wt %, the iPS crystallization was achieved by postpolymerization annealing and melt processing. The triblock copolymers melted above 200 °C with melting temperatures very similar to those of the corresponding iPS homopolymers. Nanostructure and microstructure formation of both amorphous and semicrystalline triblock copolymers were examined by means of light microscopy, atomic force microscopy, and TEM measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Electrostatic force microscopy measurements of CdSe-PS nanoparticles and CdSe-PS/poly(styrene-b-butadiene-b-styrene) nanocomposites

Electrostatic force microscopy (EFM) measurements were performed to analyze the conductive properties of CdSe nanoparticles functionalized with polystyrene (PS) brushes and embedded in a poly(styrene-b-butadiene-b-styrene) triblock copolymer. CdSe nanoparticles were synthesized aqueously and functionalized with polystyrene chains by the grafting through technique. CdSe-PS nanoparticles obtained after 5 and 8 h of polymerization were analyzed, in order to study the effect of the molecular weight of PS chains on conductive properties. EFM results showed the maintenance of the conductive properties of CdSe nanoparticles through functionalization reactions and even when they were confined in the block copolymer. Due to the low differences between the values obtained in the response of the samples to the charged tip, no effect of the molecular weight of brushes was confirmed.     

Hydrogen Bonding Complexes of Poly(styrene-co-2-hydroxyethyl acrylate) and Poly (styrene-co-4-vinylpyridine)

Summary: Random copolymers of poly(styrene-co-4-vinylpyridine) (S4VP) and poly (styrene-co-2-hydroxyethyl acrylate) (SHEA) of different compositions were prepared and characterized. An investigation of the effects of solvent and densities of the interacting species incorporated within these copolymers showed that novel and various hydrogen bonding interpolymer complexes of different structures were elaborated when these copolymers are mixed together. The specific interactions that occurred within the SHEA copolymers and the elaborated complexes were evidenced by FTIR qualitatively from the appearance of a new band at 1604 cm⁻¹ and quantitatively using appropriate spectral curve fitting in the carbonyl and pyridine regions. The intermolecular hydrogen bonding interactions that occurred between the hydroxyl groups of the SHEA and the nitrogen atom of the pyridine groups in the S4VP are stronger than the self-associations within the SHEA. In the solid state, a DSC analysis showed that the variation of the glass transition temperatures of these materials with the composition behaved differently with the densities of interacting species and were analyzed quantitatively. A thermal stability study of the synthesized copolymers and of their different mixtures carried by thermogravimetry confirmed a similar behaviour.