Thermoresponsive complex amphiphilic block copolymer micelles investigated by laser light scattering

Poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) diblock copolymers form micelles in water. The introduction of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymer leads to the formation of mixed micelles through hydrophobic interaction. The dimension of the mixed micelles varies with the weight ratio (r) of PEO-b-PPO-b-PEO to PI-b-PEO. By use of laser light scattering, we have investigated the temperature dependence of the structural evolution of the micelles at different r. At r<10, the size of the mixed micelles decreases with temperature. At r>10, due to the excessive PEO-b-PPO-b-PEO chains in solution, as temperature increases, the mixed micelles aggregate into larger micelle clusters.     

Internal structure of a thin film of mixed polymeric micelles on a solid/liquid interface

The adsorption of mixed micelles of poly(4-(2-amino hydrochloride-ethylthio)-butylene)- block-poly(ethylene oxide), PAETB 49- b-PEO 212 and poly(4-(2-sodium carboxylate-ethylthio)-butylene)- block-poly(ethylene oxide), PCETB 47- b-PEO 212 on solid/liquid interfaces has been studied with light, X-ray, and neutron reflectometry. The structure of the adsorbed layer can be described with a two-layer model consisting of an inner layer formed by the coacervate of the polyelectrolyte blocks PAETB 49 and PCETB 47 ( approximately 1 nm) and an outer layer of PEO 212 blocks ( approximately 6 nm). The micelles unfold upon adsorption forming a rather homogeneous flat layer that exposes its polyethylene oxide chains into the solution, thus rendering the surface antifouling after modification with the micelles.     

Preparation of electrospun luminescent polyimide/europium nanofibers by simultaneous in situ sol-gel and imidization processes

Comicellization of a star block copolymer poly(ε-caprolactone)-block-poly(diethylamino)ethyl methacrylate (S(PCL-b-PDEAEMA)) and a linear block copolymer methoxy poly(ethylene glycol)-block-poly(ε-caprolactone) (mPEG-b-PCL) was developed to enhance the stability and lower the cytotoxicity of the micelles. The two copolymers self-assembled into the mixed micelles with a common PCL core surrounded by a mixed PDEAEMA/mPEG shell in aqueous solution. This core-shell structure was transformed to the core-shell-corona structure at high pH due to the collapse of the PDEAEMA segment. The properties of the polymeric micelles were greatly dependent on the weight ratio of the two copolymers and the external pH. As increasing the mPEG-b-PCL content, the size and the zeta potential of the mixed micelles were lowered while the pH-dependent stability and the biocompatibility were improved. Moreover, an increase in pH accelerated the release of indomethacin (IND) from the mixed micelles in vitro. These results augured that the mixed micelles could be applied as a stable pH-sensitive release system.     

Towards microphase separation in epoxy systems containing PEO/PPO/PEO block copolymers by controlling cure conditions and molar ratios between blocks. Part 2. Structural characterization

The influence of addition of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO–PPO–PEO) copolymers on final morphologies of modified epoxy matrices has been investigated as a function of PEO:PPO molar ratio and cure conditions by comparison with the cured epoxy blends only containing poly(ethylene oxide) (PEO) or poly(propylene oxide) (PPO) homopolymers. Atomic force microscopy (AFM) has been used to characterize structural features of blends. Whilst diglycidyl ether of bisphenol-A (DGEBA)/4,4’-diaminodiphenylmethane (DDM)/PPO system macrophase separates, the interactions between PEO and cured epoxy are responsible for miscibility of DGEBA/DDM/PEO system. Depending on PEO:PPO molar ratio, micro- or macrophase separated morphologies have been obtained for block copolymer modified epoxy matrices. Moreover, the influence of both copolymer content and cure temperature on final morphologies has also been investigated by both experimental and theoretical analysis.     

Interactions between bottle-brush polyelectrolyte layers: effects of ionic strength and oppositely charged surfactant

Interactions between cationic bottle-brush polyelectrolyte layers adsorbed on mica across salt and oppositely charged surfactant solutions were investigated with the interferometric surface force apparatus, and the results were compared with what is known for similarly charged linear polyelectrolytes. Ellipsometric measurements demonstrated that the bottle-brush polyelectrolytes, which contain 45 units long poly(ethylene oxide) side chains, are more readily desorbed than linear equivalents when the ionic strength of the solution is increased. It is argued that this is due to the steric repulsion between the poly(ethylene oxide) side chains that reduces the surface affinity. The preadsorbed bottle-brush polyelectrolyte layers were also exposed to sodium dodecyl sulfate (SDS) solutions. It was found that the presence of SDS affected the force profiles less than observed for similarly charged linear polyelectrolytes. This observation was attributed to excluded volume constraints imposed by the poly(ethylene oxide) side chains that reduces the accessibility of the charged polyelectrolyte segments and counteracts formation of large aggregates within the layer.     

Reversible controlled assembly of thermosensitive polymer-coated gold nanoparticles

Aggregation of thermosensitive polymer-coated gold nanoparticles was performed in aqueous solution in the presence of a triblock copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic P123, PEO(20)-PPO(68)-PEO(20)). The gold nanoparticles, AuNPs, which are covered by thermosensitive statistical copolymers poly(EO(x)-st-PO(y)), aggregate when the temperature is higher than the phase transition temperature of the polymer, leading to a macroscopic precipitation. The presence of Pluronic chains in solution prevents the uncontrolled aggregation of the AuNPs at higher temperature than both the aggregation temperature of the AuNPs (T(agg)) and the critical micellization temperature (cmt) of the Pluronic. The size, the colloidal stability, and the optical properties of the AuNPs aggregates are modulated as a function of the P123-to-AuNP ratio, which constitutes the critical parameter of the system. Moreover, the AuNP aggregation is totally reversible upon decreasing the temperature below T(agg). Our approach constitutes an easy way to the formation of well-controlled nanoparticle aggregates with well-defined sizes. The resulting aggregates have been characterized by UV-vis spectroscopy, dynamic light scattering, and electron microscopy.     

On the stability of (highly aggregated) polyelectrolyte complexes containing a charged-block-neutral diblock copolymer

Using light scattering and cryogenic transmission electron microscopy, we show that highly aggregated polyelectrolyte complexes (HAPECs) composed of poly([4-(2-aminoethylthio)butylene] hydrochloride)49-block-poly(ethylene oxide)212 and poly(acrylic acid) (PAA) of varying lengths (140, 160, and 2000 monomeric units) are metastable or unstable if the method of preparation is direct mixing of two solutions containing the oppositely charged components. The stability of the resulting HAPECs decreases with decreasing neutral-block content and with increasing deviation from 1:1 mixing (expressed in number of chargeable groups) of the oppositely charged polyelectrolytes, most probably for electrostatic reasons. The difference between the metastable and stable states, obtained with pH titrations, increases with increasing PAA length and increasing pH mismatch between the two solutions with the oppositely charged components.     

THE EFFECT OF POLYMER ON THE CLOUD POINT OF A SERIES OF TRIBLOCK NONIONIC SURFACTANTS IN AQUEOUS SOLUTION

A series of triblock nonionic surfactants with different Propylene oxide and ethylene oxide chain lengths were synthesized. The triblock nonionic surfactants and poly(ethylene glycols) with different molecular weight were used, to find the effects of polymer chain length and size of the micelles on the cloud point of the surfactants. Two possible models are considered on the basis of cloud point changes of the solutions, to describe the polymer- surfactant interactions. One model suggests that flocculation depletion for the polymer chains exist between two regular micelles. This provides the driving force for the neighboring micelles to approach each other and destabilize the colloidal system. The flocculation effect is more important for polymers with a shorter chain block the approach of the micelles, since there is no typical polymer-surfactant association formed but just simple small molecule associations in which the steric and solvation effects of the polymer chains make the inter-micelles interactions repulsive. The other model considers that intra-chain micelles of polysoap are formed among the surfactant monomers and long polymer chains. The bridging attraction between two intra-chain micelles in such structures can enhance the collisions among the micelles, due to the exchange of amphiphilic monomers among the neighboring micelles.     

Mixed self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate in aqueous solution

Interaction of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers with anionic sodium dodecyl sulfate (SDS) has been investigated in aqueous solution. Formation of mixed micelles has been confirmed by surface tension measurements, whereas the influence of the surfactant on the copolymer self-assembling has been studied by measurement of the 1H NMR self-diffusion coefficients and by small-angle neutron scattering. As a rule, the surfactant decreases the heterogeneity of the micellar structures formed by the copolymer in water. Moreover, increasing the content of SDS results in the increasingly more important extension of the poly(ethylene oxide) (PEO) corona chains and the copolymer micelle deaggregation. The stability of the micelles against SDS increases with the length of the hydrophobic block. Preliminary two-dimensional NMR measurements with nuclear Overhauser enhancement have confirmed the spatial vicinity between SDS and the constitutive blocks of the copolymer.     

Characterization and demulsification of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers

Four poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers with different molecular weights and PPO/PEO composition ratios were synthesized. The characterization of the PEO-PPO-PEO triblock copolymers was studied by surface tension measurement, UV-vis spectra, and surface pressure method. These results clearly showed that the CMC of PEO-PPO-PEO was not a certain value but a concentration range, in contrast to classical surfactant, and two breaks around CMC were reflected in both surface tension isotherm curves and UV-vis absorption spectra. The range of CMC became wider with increasing PPO/PEO composition ratio. Surface pressure Pi-A curves revealed that the amphiphilic triblock copolymer PEO-PPO-PEO molecule was flexible at the air/water interface. We found that the minimum area per molecule at the air/water interface increased with the proportion of PEO chains. The copolymers with the same mass fractions of PEO had similar slopes in the isotherm of the Pi-A curve. From the demulsification experiments a conclusion had been drawn that the dehydration speed increased with decreased content of PEO, but the final dehydration rate of four demulsifiers was approximate. We determined that the coalescence of water drops resulted in the breaking of crude oil emulsions from the micrograph.     

Surface properties of bottle-brush polyelectrolytes on mica: effects of side chain and charge densities

Surface properties of a series of cationic bottle-brush polyelectrolytes with 45-unit-long poly(ethylene oxide) side chains were investigated by phase modulated ellipsometry and surface force measurements. The evaluation of the adsorbed mass of polymer on mica by means of ellipsometry is complex due to the transparency of mica and its birefringence and low dielectric constant. We therefore employed a new method to overcome these difficulties. The charge and the poly(ethylene oxide) side chain density of the bottle-brush polymers were varied from zero charge density and one side chain per segment to one charge per segment and no side chains, thus spanning the realm from a neutral bottle-brush polymer, via a partly charged brush polyelectrolyte, to a linear fully charged polyelectrolyte. The adsorption properties depend crucially on the polymer architecture. A minimum charge density of the polymer is required to facilitate adsorption to the oppositely charged surface. The maximum adsorbed amount and the maximum side chain density at the surface are obtained for the polymer with 50% charged segments and the remaining 50% of the segments carrying poly(ethylene oxide) side chains. It is found that brushlike layers are formed when 25-50% of the segments carry poly(ethylene oxide) side chains. In this paper, we argue that the repulsion between the side chains results in an adsorbed layer that is non-homogeneous on the molecular level. As a result, not all side chains will contribute equally to the steric repulsion but some will be stretched along the surface rather than perpendicular to it. By comparison with linear polyelectrolytes, it will be shown that the presence of the side chains counteracts adsorption. This is due to the entropic penalty of confining the side chains to the surface region.     

The interaction between triton X series surfactants and poly (ethylene glycol) in aqueous solutions

 A series of Triton X surfactants with different ethylene oxide chain length and poly(ethylene glycols) with different molecular weight were used, to find the effects of polymer chain length and size of the micelles on the cloud point of the surfactants. Two possible models are considered on the basis of cloud point changes of the solutions, to describe the polymer–surfactant interactions. One model considers that intra-chain micelles of polysoap are formed among the surfactant monomers and long polymer chains. The bridging attraction between two intra-chain micelles in such structures can enhance the collisions among the micelles, due to the exchange of amphiphilic monomers among the neighboring micelles. The other model suggests that flocculation depletion for the polymer chains exists between two regular micelles. This provides the driving force for the neighboring micelles to approach each other and destabilize the colloidal system. The flocculation effect is more significant for polymer with a long chain. Polymers with a shorter chain block the approach of the micelles, since there is no typical polymer–surfactant association formed but just simple small molecule associations in which the steric and solvation effects of the polymer chains make the inter-micelle interactions repulsive. Received: 19 August 1997 Accepted: 11 December 1997     

Reversible restructuring of aqueous block copolymer assemblies through stimulus-induced changes in amphiphilicity

ABC triblock copolymers with a hydrophilic-thermomorphic-hydrophobic block sequence can assemble into distinct structures in water above and below the lower critical solution temperature for the thermomorphic block. Characterization of aqueous solutions of a representative poly(ethylene oxide)-block-poly(N-isopropylacrylamide)-block-polyisoprene copolymer by light scattering and transmission electron microscopy shows that small spherical micelles are formed at low temperatures and that large vesicles are favored at higher temperatures. The transformation from micelles to vesicles is reversible but slow, occurring over several weeks.     

Functional and site-specific macromolecular micelles as high potential drug carriers

Since several years, macromolecular micelles based on amphiphilic block copolymers have attracted much interest as drug carriers. These micelles show a long term blood circulation time resulting from their small diameter and the steric repulsion created by the poly(ethylene oxide) chains which constitute micelle corona, as well as from their high thermodynamic stability. Besides this long term blood circulation time generating a passive targeting, an active targeting, chemical or physical affinity targeting, might allow the preparation of more efficient drug carriers. In order to obtain such double targeting properties, we have prepared two kinds of macromolecular micelles. The first one is based on amphiphilic poly(ethylene oxide)/poly(β-benzyl -aspartate) ---PEO/PBLA--- block copolymers having hydroxy groups at the free end of PEO chains. As a result of their structure, such micelles have hydroxy groups on their outer-shell which can be further modified in order to introduce a targeting moiety (sugar, etc.). The characteristics (diameter, critical micellar concentration (cmc), drug loading capacity) have been determined. Moreover, doxorubicin loaded -hydroxy PEO/PBLA micelles have been shown to be slightly more cytotoxic than the corresponding -methoxy PEO/PBLA micelles. The second type of micelles is based on thermosensitive amphiphilic poly(N-isopropyl acrylamide)/polystyrene ---PIPAAm/PSt--- block copolymers. Such micelles have a small diameter and a low cmc in addition to thermosensitivity properties which are similar to those of PIPAAm.     

On the stability of the polymer brushes formed by adsorption of ionomer complexes on hydrophilic and hydrophobic surfaces

We have studied the effect of normal forces and shear forces on the stability and functionality of a polymer brush layer formed upon adsorption of polymeric micelles on hydrophilic and hydrophobic surfaces. The micelles consist of oppositely charged polyelectrolyte blocks (poly(acrylic acid) and poly(N-methyl 2-vinyl pyridinium iodide), and a neutral block (poly(vinyl alcohol)) or neutral grafts (poly(ethylene oxide)). The strength of the attachment of the micellar layers to various substrates was evaluated with Atomic Force Microscopy. Flow cell experiments allowed for the evaluation of long-term stability of coatings in lateral flow. Fixed angle optical reflectometry was used to quantify protein (BSA) adsorption on the micellar layers after their exposure to flow. The results show that adsorbed micellar layers are relatively weakly attached to hydrophobic surfaces and much stronger to hydrophilic surfaces, which has a significant impact on their stability. Adsorbed layers maintain their ability to suppress protein adsorption on hydrophilic surfaces but not on hydrophobic surfaces. Due to the relatively weak attachment to hydrophobic surfaces the structure of adsorbed layers may easily be disrupted by lateral forces, such that the complex coacervate-brush structure no longer exists.     

以两亲性共聚物为模板溶胶-凝胶法制备WO3多孔薄膜及其 氢致变色特性的研究

报道了以六氯化钨为原料, 以两亲性的无规共聚物聚苯乙烯-co-聚烯丙醇(PS-co-PAAL)和三嵌段共聚物聚氧乙烯-b-聚氧丙烯-b-聚氧乙烯(PEO-b-PPO-b-PEO)为模板, 采用溶胶-凝胶法制备了WO3多孔薄膜. 利用热重分析仪(TGA)、粉末X射线衍射仪(XRD)、扫描电子显微镜(SEM)、原子力显微镜(AFM)等表征手段, 研究了模板和热处理温度对薄膜孔结构的影响; 并通过H2敏测试以及利用紫外可见分光光度计(UV), 研究了薄膜的氢敏性能和着色前后的透射光谱. 结果表明, 以无规共聚物PS-co-PAAL为模板制得的WO3薄膜, 经400 ℃热处理后, 可得到呈交联网状的多孔结构, 并表现出最佳的氢致变色性能.     

Formation and structure of ionomer complexes from grafted polyelectrolytes

We discuss the structure and formation of Ionomer Complexes formed upon mixing a grafted block copolymer (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₁-b-PAPEO₁₄) with a linear polyelectrolyte (poly(N-methyl 2-vinyl pyridinium iodide), P2MVPI), called grafted block ionomer complexes (GBICs), and a chemically identical grafted copolymer (poly(acrylic acid)-co-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₈-co-PAPEO₂₂) with a linear polyelectrolyte, called grafted ionomer complexes (GICs). Light scattering measurements show that GBICs are much bigger (~70–100 nm) and GICs are much smaller or comparable in size (6–22 nm) to regular complex coacervate core micelles (C3Ms). The mechanism of GICs formation is different from the formation of regular C3Ms and GBICs, and their size depends on the length of the homopolyelectrolyte. The sizes of GBICs and GICs slightly decrease with temperature increasing from 20 to 65 °C. This effect is stronger for GBICs than for GICs, is reversible for GICs and GBIC-PAPEO₁₄/P2MVPI₂₂₈, and shows some hysteresis for GBIC-PAPEO₁₄/P2MVPI₄₃. Self-consistent field (SCF) calculations for assembly of a grafted block copolymer (having clearly separated charged and grafted blocks) with an oppositely charged linear polyelectrolyte of length comparable to the charged copolymer block predict formation of relatively small spherical micelles (~6 nm), with a composition close to complete charge neutralization. The formation of micellar assemblies is suppressed if charged and grafted monomers are evenly distributed along the backbone, i.e., in case of a grafted copolymer. The very large difference between the sizes found experimentally for GBICs and the sizes predicted from SCF calculations supports the view that there is some secondary association mechanism. A possible mechanism is discussed.     

Near-infrared light-triggered dissociation of block copolymer micelles using upconverting nanoparticles

We demonstrate a novel strategy enabling the use of a continuous-wave diode near-infrared (NIR) laser to disrupt block copolymer (BCP) micelles and trigger the release of their "payloads". By encapsulating NaYF(4):TmYb upconverting nanoparticles (UCNPs) inside micelles of poly(ethylene oxide)-block-poly(4,5-dimethoxy-2-nitrobenzyl methacrylate) and exposing the micellar solution to 980 nm light, photons in the UV region are emitted by the UCNPs, which in turn are absorbed by o-nitrobenzyl groups on the micelle core-forming block, activating the photocleavage reaction and leading to the dissociation of BCP micelles and release of co-loaded hydrophobic species. Our strategy of using UCNPs as an internal UV or visible light source upon NIR light excitation represents a general and efficient method to circumvent the need for UV or visible light excitation that is a common drawback for light-responsive polymeric systems developed for potential biomedical applications.     

Reduction of protein adsorption to a solid surface by a coating composed of polymeric micelles with a glass-like core

Adsorption studies by optical reflectometry show that complex coacervate core micelles (C3Ms) composed of poly([4-(2-amino-ethylthio)-butylene] hydrochloride)(49)-block-poly(ethylene oxide)(212) and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)(47)-block-poly(ethylene oxide)(212) adsorb in equal amounts to both silica and cross-linked 1,2-polybutadiene (PB). The C3Ms have an almost glass-like core and atomic force microscopy of a dried layer of adsorbed C3Ms shows densely packed flattened spheres on silica, which very probably are adsorbed C3Ms. Experiments were performed with different types of surfaces, solvents, and proteins; bare silica and cross-linked 1,2-PB, NaNO(3) and phosphate buffer, and lysozyme, bovine serum albumin, beta-lactoglobulin, and fibrinogen. On the hydrophilic surface the coating reduces protein adsorption >90% in 0.1 M phosphate buffer, whereas the reduction on the coated hydrophobic surface is much lower. Reduction is better in phosphate buffer than in NaNO(3), except for the positively charged lysozyme, where the effect is reversed.     

Application of solid phase peptide synthesis to engineering PEO–peptide block copolymers for drug delivery

This work describes a simple, versatile solid-phase peptide-synthesis (SPPS) method for preparing micelle-forming poly(ethylene oxide)-block-peptide block copolymers for drug delivery. To demonstrate its utility, this SPPS method was used to construct two series of micelle-forming block copolymers (one of constant core-composition and variable length; the other of constant core length and variable composition). The block copolymers were then used to study in detail the effect of size and composition on micellization. The various block copolymers were prepared by a combination of SPPS for the peptide block, followed by solution–phase conjugation of the peptide block with a proprionic acid derivative of poly(ethylene oxide) (PEO) to form the PEO-b-peptide block copolymer. The composition of each block component was characterized by mass spectrometry (MALDI and ES-MS). Block copolymer compositions were characterized by ¹H NMR. All the block copolymers were found to form micelles as judged by transmission electron microscopy (TEM) and light scattering analysis. To demonstrate their potential as drug delivery systems, micelles prepared from one member of the PEO-b-peptide block copolymer series were physically loaded with the anticancer drug doxorubicin (DOX). Micelle static and dynamic stability were found to correlate strongly with micelle core length. In contrast, these same micellization properties appear to be a complex function of core composition, and no clear trends could be identified from among the set of compositionally varying, fixed length block copolymer micelles. We conclude that SPPS can be used to construct biocompatible block copolymers with well-defined core lengths and compositions, which in turn can be used to study and to tailor the behavior of block copolymer micelles.