Re-assembly behaviors of polystyrene-b-poly(acrylic acid) micelles

The re-assembly behaviors of spherical micelles of the polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer in different solvent mixtures were investigated using dynamic light scattering, transmission electron microscopy and atomic force microscopy. Depending on the nature of the solvent, PS-b-PAA micelles re-assembled from spheres to nanorings in toluene or to necklace-like aggregates in water induced by solvent evaporation. Systematic studies suggested that the re-assembly behaviors on a neutral surface are strongly correlated with the micellar surface components, the solvent polarity and the chain length of the micelle corona of the solvated blocks. We proposed that the formation of nanorings from PS-b-PAA micelles in toluene is mainly induced by the dewetting process of the solvent, while the necklace-like structure arises from the hydrogen bonding interactions among the partially dissociated PAA units.     

Adsorption kinetics and stability of poly(ethylene oxide)-block-polystyrene micelles on polystyrene surface

Herein, adsorption kinetics of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) micelles from aqueous solution on PS thin films was investigated by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). The stability of adsorbed micelles against water washing was enhanced by the strong physical interaction between the substrate-identical core blocks of micelles and the PS substrate. The adsorption kinetics was investigated and analyzed by a model considering both the effects of diffusion and micelles reorganization on the PS surface. The micelles adsorption process was well captured by this model which demonstrated that the micelles reorganized on the surface after adsorption. The fitting results exhibited that the micelles with longer core blocks had less tendency to reorganize on the surface while reorganization took more part in the adsorption process of the micelles with shorter core blocks. In the stability studies, micelles with shorter PS blocks could partially desorb from the PS substrate while micelles with longer PS blocks were totally hindered from desorption. Desorption process of micelles was evaluated by the hydrodynamics-induced rolling model. The results indicated that hydrodynamic force-induced partial slip of the micelles on the PS surface might be responsible for micelle desorption. Because of the hydrophilicity of adsorbed micelle layer, adsorption amount of BSA on the modified surface was greatly reduced comparing with that of the bare PS surface.     

Synthesis of poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene triblock copolymers by two-step atom transfer radical polymerization

The synthesis of ABC triblock copolymer poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene (PEO-b-PMMA-b-PS) via atom transfer radical polymerization (ATRP) is reported. First, a PEO-Br macroinitiator was synthesized by esterification of PEO with 2-bromoisobutyryl bromide, which was subsequently used in the preparation of halo-terminated poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) diblock copolymers under ATRP conditions. Then PEO-b-PMMA-b-PS triblock copolymer was synthesized by ATRP of styrene using PEO-b-PMMA as a macroinitiator. The structures and molecular characteristics of the PEO-b-PMMA-b-PS triblock copolymers were studied by FT-IR, GPC and ¹H NMR.     

Polymethylene-block-poly(dimethyl siloxane)-block-polymethylene nanoaggregates in toluene at room temperature

Aggregates of a polymethylene-block-poly(dimethyl siloxane)-block-polymethylene (PM-PDMS-PM) sample were prepared in toluene by cooling down a toluene solution of the sample slowly from 105 °C to room temperature. Atomic force microscopic (AFM) and electron microscopic (EM) studies revealed that the aggregates included triblock crystal plates with sizes of upto tens of micrometers and PM-PDMS-PM nanodiscs. Surprisingly and interestingly, we discovered that the topographic images of the nanodiscs obtained by tapping-mode AFM changed from nanodiscs at light tapping to strikingly beautiful nanodonuts at moderate tapping and then filled nanodonuts at heavy tapping. Justification for such observations is provided.     

Poly[(2-ethylhexyl acrylate)-ran-(tert-butyl acrylate)]-block-poly(2-cinnamoyloxyethyl acrylate) synthesis and properties

Poly[(2-ethylhexyl acrylate)-ran-(tert-butyl acrylate)]-block-poly(2-cinnamoyloxyethyl acrylate) or P(EXA-r-tBA)-PCEA was synthesized by atom transfer radical polymerization. Reactivity ratios of EXA and tBA for copolymerization were determined. The specific refractive index increments of six diblocks were measured as a function of their composition. The diblocks were thermally stable and formed micelles in an automobile engine oil. Such micelles may be useful as an anti-friction additive in lubricating oils.     

Syntheses of well-defined poly(siloxane)-b-poly(styrene) and poly(norbornene)-b-poly(styrene) block copolymers using functional alkoxyamines

Functional alkoxyamines, 1-[4-(4-lithiobutoxy)phenyl]-1-(2,2,6,6-tetramethylpiperidinyl-N-oxyl)ethane (2) and 1-[4-(2-vinyloxyethoxy)phenyl]-1-(2,2,6,6-tetramethylpiperidinyl-N-oxyl)ethane (3) were prepared, and well-defined poly(hexamethylcyclotrisiloxane)-b-poly(styrene)[poly(D₃)-b-poly(St)] and poly(norbornene)-b-poly(St) [poly(NBE)-b-poly(St)] were prepared using the alkoxyamines. The first step was preparation of poly(D₃) and poly(NBE) macroinitiators, which were obtained by the ring-opening anionic polymerization of D₃ using 2 as an initiator and the ring-opening metathesis polymerization of NBE using 3 as a chain transfer. The radical polymerization of St by the poly(D₃) and poly(NBE) macroinitiators proceeded in the ‘living’ fashion to give well-defined poly(D₃)-b-poly(St) and poly(NBE)-b-poly(St) block copolymers.     

Water-dispersible fluorescent nanospheres from poly(solketal acrylate)-block-poly(2-hydroxyethyl acrylate)

Reported in this paper is the preparation of fluorescent nanospheres from poly(solketal acrylate)-block-poly(2-hydroxyethyl acrylate) or PSA-PHEA. The strategy involved the chemical derivation of the PHEA block to graft the fluorophore fluorosceinamine (Fl) first. A selective solvent for PSA was then used to induce micelle formation from the diblock with PSA as the corona and the fluorescein-tagged PHEA block as the core. Nanospheres with fluorescent cores were obtained after PHEA core crosslinking with succinyl chloride. Water-dispersible nanospheres were prepared after removing the acetonide groups from the PSA block by hydrolysis to yield poly(glyceryl acrylate). Such nanospheres may find applications in fluorescent in situ hybridization assays.     

Synthesis and properties of novel biodegradable triblock copolymers of poly(5-methyl-5-methoxycarbonyl-1,3-dioxan-2-one) and poly(ethylene glycol)

Novel biodegradable triblock copolymers of poly(5-methyl-5-methoxycarbonyl-1,3-dioxan-2-one) (PMMTC) with poly(ethylene glycol) (PEG), PMMTC-b-PEG-b-PMMTC, were synthesized by the ring-opening polymerization of MMTC in bulk, using the dihydroxyl PEG as initiator and Sn(Oct)₂ as catalyst. The triblock copolymers with different compositions were characterized by IR and ¹H NMR, their molecular weight was measured by gel permeation chromatography (GPC). The results showed that the molecular weight of triblock copolymers increased either with the increase of the molar ratio of MMTC in feed while the PEG chain length kept constant, or by lengthening the backbone chain of PEG block with the same ratio of MMTC in feed. The hydrophilicity of copolymers was greatly improved by incorporation of PEG block into polycarbonate. The in vitro hydrolytic/enzymatic degradation and controlled drug release properties of the triblock copolymers were also investigated.     

Functionalization of amphiphilic coil-rod-coil triblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) with florescence moiety and C60

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride in the presence of pyridine. This led to the introduction of an active methylene group that is a site for further functionalization in the rod block. The Michael addition reaction of the triblock copolymer with 7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rod block. The fluorescent labeled copolymers were isolated in ∼94% yields. Similarly C₆₀ pendent was introduced to the rod block by the Bingel reaction. The yields of C₆₀ functionalized copolymers were ∼54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphologies, such as micelles and vesicles by simply changing the solvent. For the C₆₀ functionalized block copolymer, structural constraints in micelles and vesicles prevented C₆₀ pendents to aggregate.     

Synthesis and characterization of poly(ethylene glycol)-b-poly (l-lactide)-b-poly(l-glutamic acid) triblock copolymer

A biodegradable amphiphilic triblock copolymer of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(l-glutamic acid) (PEG-b-PLLA-b-PLGA) was obtained by catalytic hydrogenation of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(γ-benzyl-l-glutamic acid) (PEG-b-PLLA-b-PBLGA) synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG-NCA) with amino-terminated MPEG-b-PLLA-NH₂ as a macroinitiator. MPEG-b-PLLA-NH₂ converted from MPEG-b-PLLA-OH first reacted with tert-Butoxycarbonyl-l-phenylalanine (Phe-^NBOC) and dicyclohexylcarbodiimide (DCC) and then deprotected the tert-butoxycarbonyl group. MPEG-b-PLLA-OH was prepared by ROP of l-lactide with monomethoxy poly(ethylene glycol) in the presence of stannous octoate. The triblock copolymer and its diblock precursors were characterized by ¹H NMR, FTIR, GPC and DSA (drop shape analysis) measurements. The lengths of each block polymers could be tailored by molecular design and the ratios of feeding monomers. The triblock polymer PEG-b-PLLA-b-PLGA containing carboxyl groups showed obviously improved hydrophilic properties and could be a good potential candidate as a drug delivery carrier.     

Analysis of poly(ethylene oxide)-b-poly(propylene oxide) block copolymers by MALDI-TOF mass spectrometry using collision induced dissociation

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used to analyse the block length of commercially available block copolymers of poly(ethylene oxide) and poly(propylene oxide) (PEO-b-PPO) based on the fragmentation behaviour in collision induced dissociation (CID) experiments.MALDI-CID-TOF² analysis is a complex procedure depending on a number of different experimental parameters. Therefore, a step-by-step procedure was used starting with PEG and PPG standards, PEG-PPG blends and endgroup-functionalized PPGs, to understand the fragmentation behaviour of the different species. These results showed that characteristic fragment patterns of the homopolymers and PEG-PPG mixtures can be obtained that facilitate the interpretation of the fragment spectra of PEO-b-PPO di- and triblock copolymers. It was found that di- and triblock copolymers can be differentiated by their fragment spectra. In addition, the sequence of monomer units in the diblock copolymers could be determined.     

Self-association of double-hydrophilic copolymers of acrylic acid and poly(ethylene oxide) macromonomer

Poly(ethylene oxide) (PEO) end-capped by a methacrylate unsaturation was copolymerized with acrylic acid by RAFT with dibenzyltrithiocarbonate as a chain transfer agent. Tapered triblock copolymers consisting of a poly(acrylic acid) (PAA) inner block and comb-like outer blocks of PEO macromomers were formed as result of the comonomers reactivity ratios. Composition of these copolymers and length of the PEO branches were varied. Dynamic light scattering (DLS) was used to characterize the aggregates formed in water and to investigate their response to stimuli, such as pH, temperature and ionic strength. In parallel, the morphology of the aggregates was directly observed by transmission electron microscopy (TEM). Well-defined aggregates were formed in the 5<pH<8 range, with a morphology strongly depending on the copolymer composition. At pH<5, the copolymers were poorly soluble and no well-defined structure was observed, whereas free chains were formed at pH>8 as consequence of the complete ionization of the PAA block.     

Stabilization of nano-TiO2 aqueous dispersions with poly(ethylene glycol)-b-poly(4-vinyl pyridine) block copolymer and their incorporation in photocatalytic acrylic varnishes

In this work, TiO₂ nanoparticles were dispersed and stabilized in water using a novel type of dispersant based on tailor-made amphiphilic block copolymers of poly(ethylene glycol)-block-poly(4-vinyl pyridine) (mPEG-b-P4VP) prepared by atom transfer radical polymerization (ATRP). The performance of this new block copolymer as dispersant was compared to a polyelectrolyte dispersant commonly used for TiO₂, sodium salt of polyacrylic acid (Na-PAA). The effect of dispersion technique and type and amount of dispersant on deagglomeration and stability of the TiO₂ aqueous suspensions were studied. After incorporation in a standard waterborne acrylic varnish formulation, dry film transparency, photocatalytic activity, and nanoparticle cluster size were also evaluated. The results show that mPEG-b-P4VP copolymer with appropriate block lengths can have a better performance than Na-PAA in terms of aqueous dispersion stabilization and cluster size reduction in the acrylic matrix. This translates into higher film transparency and photocatalytic performance.     

Comparison of morphologies and mechanical properties of crosslinked epoxies modified by polystyrene and poly(methyl methacrylate)) or by the corresponding block copolymer polystyrene-b-poly(methyl methacrylate)

Polystyrene (PS, M_n=28,400, PI=1.07), poly(methyl methacrylate) (PMMA, M_n=88,600, PI=1.03), and PS (50,000)-b-PMMA (54,000) (PI=1.04), were used as modifiers of an epoxy formulation based on diglycidyl ether of bisphenol A (DGEBA) and m-xylylene diamine (MXDA). Both PS and PMMA were initially miscible in the stoichiometric mixture of DGEBA and MXDA at 80 °C, but were phase separated in the course of polymerization. Solutions containing 5 wt% of each one of both linear polymers exhibited a double phase separation. A PS-rich phase was segregated at a conversion close to 0.02 and a PMMA rich phase was phase separated at a conversion close to 0.2. Final morphologies, observed by scanning electron microscopy (SEM), consisted on a separate dispersion of PS and PMMA domains. A completely different morphology was observed when employing 10 wt% of PS-b-PMMA as modifier. PS blocks with M_n=50,000 were not soluble in the initial formulation. However, they were dispersed as micelles stabilized by the miscible PMMA blocks, leading to a transparent solution up to the conversion where PMMA blocks began to phase separate. A coalescence of the micellar structure into a continuous thermoplastic phase percolating the epoxy matrix was observed. The elastic modulus and yield stress of the cured blend modified by both PS and PMMA were 2.64 GPa and 97.2 MPa, respectively. For the blend modified by an equivalent amount of block copolymer these values were reduced to 2.14 GPa and 90.0 MPa. Therefore, using a block copolymer instead of the mixture of individual homopolymers and selecting an appropriate epoxy-amine formulation to provoke phase separation of the miscible block well before gelation, enables to transform a micellar structure into a bicontinuous thermoplastic/thermoset structure that exhibits the desired decrease in yield stress necessary for toughening purposes.     

Synthesis, characterization, and bulk crosslinking of polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) block terpolymers

We report on the synthesis and characterization of triblock terpolymers, polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (PB-b-P2VP-b-PtBMA; BVT), via sequential living anionic polymerization in THF at low temperatures using sec-butyl lithium as initiator. In this work, 18 different BVT terpolymers were produced with volume fractions Φ_B : Φ_V : Φ_T in the range of 1 : 0.4…1.2 : 0.2…4.6. All polymers exhibit a very narrow molecular weight distribution (PDI < 1.1). They were characterized in terms of bulk morphology using small-angle X-ray scattering and transmission electron microscopy, unveiling mostly lamellar patterns or hexagonally arranged cylindrical structures. Some polymers displayed a partial gyroid structure coexisting with lamellar parts or cylinders with a non-continuous shell around the PB core and could serve as an interesting template for the facile generation of multi-compartmental self-assembled structures. In one case the middle block, P2VP, is forming a helix around the PB core. Crosslinking of the polybutadiene compartment of the bulk morphologies with an UV-photoinitiator was performed, followed by sonication-assisted dissolution of the aggregates to elucidate further use of the terpolymers for the generation of soft polymeric nanoparticles with controlled functionality. In that way, core-crosslinked cylindrical micelles could be generated and characterized.     

Characterization and emulsifying properties of block copolymers prepared from lactic acid and poly(ethylene glycol)

Block copolymers were prepared by the direct polycondensation of an aqueous lactic acid solution on monomethoxy or dihydroxyl poly(ethylene glycol) (PEG) in the absence of a catalyst. The resulting poly(lactic acid) (PLA)–PEG diblock and PLA–PEG–PLA triblock copolymers were characterized by various analytical techniques, including matrix‐assisted laser desorption/ionization time of flight mass spectrometry (MALDI‐TOF MS), gel permeation chromatography, and ¹H‐NMR. The molecular structure between PLA–PEG and PLA–PEG–PLA could be distinguished after the calculation of the repeat unit masses and end‐group masses through the MALDI‐TOF MS spectra. Interestingly, both copolymers could serve as a hydrophilic emulsifier to stabilize the squalene/water interfaces and yield narrowly distributed oil‐in‐water nanoparticles. In contrast, the prepolymer PEG failed to stabilize the squalene/water interface under the same homogenization conditions. These features are of great interest for applications as bioactive agent delivery, especially for candidate vaccine antigens and lipophilic anticancer drugs. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Stereocomplex‐induced gelation properties of polylactide/poly(ethylene glycol) diblock and triblock copolymers

The ring‐opening polymerization of L ‐ or D ‐lactide was realized in the presence of dihydroxyl or monomethoxy poly(ethylene glycol) (PEG) with a number‐average molecular weight of 2000. The resulting low‐molar‐mass poly(L ‐lactide) (PLLA)/PEG and poly(D ‐lactide) (PDLA)/PEG triblock and diblock copolymers were characterized with nuclear magnetic resonance (NMR), differential scanning calorimetry, size‐exclusion chromatography, and X‐ray diffractometric analysis. Bioresorbable hydrogels were successfully prepared from aqueous solutions containing both copolymers because of interactions and stereocomplexation between the PLLA and PDLA blocks. Gelation was evaluated with the tube inverting method and rheological measurements. A phase diagram was realized with gel–sol transitions as a function of concentration. The rheological properties of the hydrogels were investigated under various conditions through changes in the copolymer concentration, temperature, time, and frequency. It was concluded that the hydrogels constituted a dynamic and evolutive system because of the continuous formation/destruction of crosslinks and degradation. Further studies are underway to elucidate the degradation behavior and the potential of these substances as drug carriers or cell culture scaffolds. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structure and swelling behaviour of epoxy networks based on α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)

Structure and swelling behaviour of hydrophilic epoxy networks prepared from α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene) and diglycidyl ether of brominated Bisphenol A in dependence on the initial molar ratio of reactive amino and epoxy groups has been investigated by small- and wide-angle X-ray scattering (SAXS and WAXS), differential scanning calorimetry (DSC) and dynamic mechanic analysis (DMA). Anomalous swelling behaviour of the networks in water has been found. The anomaly is attributed to the changing microphase separation in the networks controlled by their composition and crosslinking density, and inhomogeneous swelling on nanometer space scale.     

脂肪族聚丁二酸丁二醇-聚乙二醇嵌段共聚物的制备及性能研究

采用熔融缩聚法以钛酸四丁酯为催化剂,使单体发生酯交换反应,成功制备了一系列以聚乙二醇(PEG)为亲水软段,以聚丁二酸丁二醇酯(PBS)为硬段的嵌段共聚物,采用1H-NMR确定了共聚物的结构组成;采用DSC、吸水性测试及水解降解试验对嵌段共聚物性能表征,结果表明共聚物中两种链段的含量与原料投料比一致,具有可调控性。由于PEG的引入,使共聚物结晶性下降,亲水性和降解性得到显著改善。     

Amphiphilic poly(acrylic acid-b-styrene-b-isobutylene-b-styrene-b-acrylic acid) pentablock copolymers from a combination of quasiliving carbocationic and atom transfer radical polymerization

Amphiphilic poly(acrylic acid-b-styrene-b-isobutylene-b-styrene-b-acrylic acid) (PAA-PS-PIB-PS-PAA) block copolymers were prepared using a combination of quasiliving carbocationic and atom transfer radical polymerization (ATRP) techniques. Poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymer macroinitiators with targeted molecular weights and high degrees of chain end functionality (F_n>1.7) were prepared by quasiliving carbocationic polymerization of isobutylene followed by sequential addition of styrene. Poly(tert-butyl acrylate-b-styrene-b-isobutylene-b-styrene-b-tert-butyl acrylate) (PtBA-PS-PIB-PS-PtBA) pentablock terpolymers with targeted molecular weights and low polydispersities (PDIs) were synthesized from the PS-PIB-PS macroinitiators via ATRP of tBA using either a Cu(I)Cl/1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA) or Cu(I)Cl/tris[2-(dimethylamino)ethyl]amine (Me₆TREN) catalyst system. Deprotection of the tert-butyl groups using trifluoroacetic acid at 25 °C resulted in the formation of PAA-PS-PIB-PS-PAA pentablock terpolymers. Comonomer composition of the final terpolymers, determined by ¹H-NMR spectroscopy, was very close to theoretical.