Adsorption studies of polyethers Part II: adsorption onto hydrophilic surfaces

The adsorption of BAB-type triblock copolymers (B=poly(ethylene oxide); A=poly(propylene oxide)) from aqueous solution onto hydrophilic silica particles is described with particular reference to the role of the copolymer composition. The adsorbed amount and the layer thickness were determined by the standard depletion method and photon correlation spectroscopy, respectively. Snowtex-YL silica was used as the adsorbent. The results show an increase in the adsorbed amount with increasing molar masses of both PEO and PPO blocks. The adsorbed layer thickness is found to depend strongly on PEO block mass. Both these parameters (adsorbed amount and hydrodynamic layer thickness) show a maximum as a function of the mole fraction of the PPO block present in the copolymer. The conformation of the adsorbed layer is determined by the surface–copolymer interaction; principally by the interaction of the hydrophilic PEO block with the silica surface. A good qualitative agreement of the experimental results with theoretical predictions and self-consistent mean field calculations has been found.     

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.     

Adsorption of Diblock Copolymers of Poly(ethylene oxide) and Polylactide at Hydrophobized Silica from Aqueous Solution

The adsorption of a series of amphiphilic diblock copolymers of poly(ethylene oxide) (PEO) and poly(DL-lactide) (PL) at hydrophobized silica from aqueous solution was studied using time-resolved ellipsometry and reflectometry. The adsorbed amounts only display a weak dependence on the copolymer composition in both water and phosphate-buffered solution. For the short copolymers, the layer thickness decreases slightly with increasing length of the hydrophobic block. Furthermore, in comparison with the short copolymers, the layer thickness of the long copolymers is substantially higher. Upon degradation of the PL block, the adsorbed amount is found to decrease and approach that of the corresponding PEO homopolymer. Protein rejection studies indicate that the adsorption of fibrinogen is inhibited by copolymer preadsorption. The protein rejection is enhanced with increasing surface coverage of the preadsorbed copolymer, but largely independent of the length of the PL block and the PEO block. For all polymers investigated, essentially complete protein rejection is obtained above a critical surface coverage that is significantly lower than the saturation coverage of the copolymers. Removing the copolymer from bulk solution after preadsorption causes a partial desorption, resulting in reduced protein rejection. However, the protein rejection capacity with and without copolymer in the bulk solution is found to be similar at a given surface coverage. Contrary to the behavior of the intact copolymers, fibrinogen adsorption is found to be significant at surfaces pretreated with an extensively degraded copolymer and, in fact, quantitatively comparable to that at the hydrophobic surface in the absence of preadsorption. This finding, together with that of the effect of the copolymer composition on protein rejection, suggests that an efficient protein rejection is maintained until only a few L units remain in the copolymer, i.e., until nearly completed degradation. Copyright 2000 Academic Press.     

Thermal‐initiating potentialities of poly(methyl methacrylate) peroxide: Metamorphosis of block‐into‐block copolymer and comparative studies on surface texture and morphology

This is the first report concerning the use of vinyl polyperoxide, namely, poly(methyl methacrylate) peroxide (PMMAP), as a thermal initiator for the synthesis of active polymer PMMAP‐PS‐PMMAP by free‐radical polymerization with styrene. The polymerizations have been carried out at different concentrations of macroinitiator PMMAP. The active polymers have been characterized by ¹H NMR, DSC, thermogravimetric analysis, and gel permeation chromatography. PMMAP‐PS‐PMMAP is further used as the thermal macroinitiator for the preparation of another block copolymer, PMMA‐b‐PS‐b‐PMMA, by reacting the active polymers with methyl methacrylate. The block copolymers have been synthesized by varying the concentrations of the active polymers. The mechanism of block copolymers has been discussed, which is also supported by thermochemical calculations. Studies on the surface texture and morphology of the block copolymer of polystyrene (PS) and PMMA material have been carried out using scanning electron microscopy. Furthermore, in this article, a blend of the same constituent materials (PS and PMMA) in proportions (v/v) similar to that contained in block copolymers has been formulated, and the morphology and surface textures of these materials were also investigated. A comparative microscopical evaluation between two processing methods was done for a better understanding of the processing route dependence of the microstructures. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 546–554, 2001     

Synthesis and thermal,mechanical, and optical properties of A–B–A or A–B block copolymers containing poly(norbornene‐co‐1‐octene)

A–B–A block copolymers which consist of poly(norbornene‐co‐1‐octene) and atactic polypropylene (PP) segments were synthesized by using ansa‐fluorenylamidotitanium complex as a catalyst varying the ratio of norbornene, 1‐octene, and propylene. The copolymer was obtained quantitatively with high molecular weight (>100,000) and narrow molecular weight distribution (polydispersity index, <1.5). A–B block copolymers of poly(norbornene‐co‐1‐octene) and poly(methyl methacrylate) (PMMA) was also obtained by the same procedure. Mechanical and optical properties of these copolymer films, which were obtained by solution casting process, were also investigated. Introduction of PP soft segment greatly improved mechanical properties, keeping their high transparency. Introduction of PMMA block also increased the tensile strength. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 267–271     

Preparation and characterization of biodegradable poly(trimethylenecarbonate‐ε‐caprolactone)‐block‐poly(p‐dioxanone) copolymers

A series of poly(trimethylenecarbonate‐ε‐caprolactone)‐block‐poly(p‐dioxanone) copolymers were prepared with varying feed rations by using two step polymerization reactions. Poly(trimethylenecarbonate)(ε‐caprolactone) random copolymer was synthesized with stannous‐2‐ethylhexanoate and followed by adding p‐dioxanone monomer as the other block. The ring opening polymerization was carried out at high temperature and long reaction time to get high molecular weight polymers. The monofilament fibers were obtained using conventional melting spun methods. The copolymers were identified by ¹H and ¹³C NMR spectroscopy and gel permeation chromatography (GPC). The physicochemical properties, such as viscosity, molecular weight, melting point, glass transition temperature, and crystallinity, were studied. The hydrolytic degradation of copolymers was studied in a phosphate buffer solution, pH = 7.2, 37 °C, and a biological absorbable test was performed in rats. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2790–2799, 2005     

Physicochemical characterization of the thermo‐induced self‐assembly of thermo‐responsive PDMAEMA‐b‐PDEGMA copolymers

Diblock copolymers of poly[2‐(dimethylamino)ethyl methacrylate]‐block‐poly[di(ethylene glycol) methyl ether methacrylate], PDMAEMA‐b‐PDEGMA, were synthesized by reversible addition–fragmentation chain transfer polymerization. The block ratio was varied to study the influence on the lower critical solution temperature and the corresponding phase transition in water. Therefore, turbidimetry, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and laser Doppler velocimetry were applied. Additionally, asymmetric flow field‐flow fractionation (AF4) coupled to DLS and multiangle laser light scattering (MALLS) was established as an alternative route to characterize these systems in terms of molar mass of the polymer chain and size of the colloids after the phase transition. It was found that AF4–MALLS allowed accurate determination of molar masses in the studied range. Nevertheless, some limitations were observed, which were critically discussed. The cloud point and phase transition of all materials, as revealed by turbidimetry, could be confirmed by DSC. For block copolymers with block ratios in the range of 50:50, a thermo‐induced self‐assembly into micellar and vesicular structures with hydrodynamic radii (R_h) of around 25 nm was observed upon heating. At higher temperatures, a reordering of the self‐assembled structures could be detected. The thermo‐responsive behavior was further investigated in dependence of pH value and ionic strength. Variation of the pH value mainly influences the solubility of the PDMAEMA segment, where a decrease of the pH value increases the transition temperature. An increase of ionic strength leads to a reduction of the cloud point due to the screening of electrostatic interactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 924–935     

Aqueous solution properties of pH‐responsive AB diblock acrylamido–styrenic copolymers synthesized via aqueous reversible addition–fragmentation chain transfer

Here we report the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water‐soluble A blocks consisting of N,N‐dimethylacrylamide and pH‐responsive B blocks of N,N‐dimethylvinylbenzylamine. To our knowledge, this represents the first example of an acrylamido–styrenic block copolymer prepared directly in a homogeneous aqueous solution. The best blocking order [with poly(N,N‐dimethylacrylamide) as a macro‐chain‐transfer agent] yielded well‐defined block copolymers with minimal homopolymer impurities. The reversible aggregation of these block copolymers in aqueous media was studied with ¹H NMR spectroscopy and dynamic light scattering. Finally, an example of core‐crosslinked micelles was demonstrated by the addition of a difunctional crosslinking agent to a micellar solution of the parent block copolymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1724–1734, 2004     

Synthesis of four‐armed poly(ε‐caprolactone)‐block‐poly(ethylene oxide) by diethylzinc catalyst

Biodegradable, amphiphilic, four‐armed poly(?‐caprolactone)‐block‐poly(ethylene oxide) (PCL‐b‐PEO) copolymers were synthesized by ring‐opening polymerization of ethylene oxide in the presence of four‐armed poly(?‐caprolactone) (PCL) with terminal OH groups with diethylzinc (ZnEt₂) as a catalyst. The chemical structure of PCL‐b‐PEO copolymer was confirmed by ¹H NMR and ¹³C NMR. The hydroxyl end groups of the four‐armed PCL were successfully substituted by PEO blocks in the copolymer. The monomodal profile of molecular weight distribution by gel permeation chromatography provided further evidence for the four‐armed architecture of the copolymer. Physicochemical properties of the four‐armed block copolymers differed from their starting four‐armed PCL precursor. The melting points were between those of PCL precursor and linear poly(ethylene glycol). The length of the outer PEO blocks exhibited an obvious effect on the crystallizability of the block copolymer. The degree of swelling of the four‐armed block copolymer increased with PEO length and PEO content. The micelle formation of the four‐armed block copolymer was examined by a fluorescent probe technique, and the existence of the critical micelle concentration (cmc) confirmed the amphiphilic nature of the resulting copolymer. The cmc value increased with increasing PEO length. The absolute cmc values were higher than those for linear amphiphilic block copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 950–959, 2004     

Poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ϵ‐caprolactone) triblock copolymers: Synthesis and self‐assembly in aqueous solutions

Nontoxic and biodegradable poly(?‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(?‐caprolactone) triblock copolymers were synthesized by the solution polymerization of ?‐caprolactone in the presence of poly(ethylene glycol). The chemical structure of the resulting triblock copolymer was characterized with ¹H NMR and gel permeation chromatography. In aqueous solutions of the triblock copolymers, the micellization and sol–gel‐transition behaviors were investigated. The experimental results showed that the unimer‐to‐micelle transition did occur. In a sol–gel‐transition phase diagram obtained by the vial‐tilting method, the boundary curve shifted to the left, and the gel regions expanded with the increasing molecular weight of the poly(?‐caprolactone) block. In addition, the hydrodynamic diameters of the micelles were almost independent of the investigated temperature (25–55 °C). The atomic force microscopy results showed that spherical micelles formed at the copolymer concentration of 2.5 × 10^?4 g/mL, whereas necklace‐like and worm‐like shapes were adopted when the concentration was 0.25 g/mL, which was high enough to form a gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 605–613, 2007     

Equivalent temperature and specific ion effects in macromolecule-coated colloid interactions

Ensemble total internal reflection microscopy is used to measure reversible temperature- and specific-ion-mediated interaction potentials between macromolecule-coated colloids and surfaces. Potentials are measured between PEO-PPO-PEO (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) block copolymers adsorbed to hydrophobically modified silica colloids and glass or gold planar surfaces. Conditions investigated include temperatures from 20 to 47 degrees C and MgSO4 concentrations from 0.2 to 0.5 M. The solvent-quality-mediated copolymer layer collapse inferred by comparing measured potentials and the predicted van der Waals attraction, including effects of the adsorbed copolymer and surface roughness, displays good agreement with expected limits based on the PEO block contour length and the bulk PEO density. Superposition of all PEO layer collapse measurements onto a single universal curve, via a transformed temperature scale relative to a reference temperature in each case, indicates an equivalence of increasing temperature and increasing MgSO4 concentration when layer interactions and dimensions are mediated. Accurate knowledge of nanometer- and kT-scale interactions of copolymer-coated colloids as a function of temperature and MgSO4 concentration provides the ability to reversibly control the stability, phase behavior, and self-assembly of such particles.     

Multiple morphologies of the aggregates from self‐assembly of diblock copolymer with relatively long corona‐forming block in dilute aqueous solution

Reported here is self‐assembly behavior in selective solvent of diblock copolymers with relatively long corona‐forming block compared to core‐forming block. Three diblock copolymers, poly(ethylene glycol) monomethyl ether‐b‐poly(methacryloyl‐L ‐leucine methyl ester), also denoted as MPEG‐b‐PMALM copolymer, were prepared by fixing MPEG block with an average number of repeating units of 115, whereas varying PMALM block with an average number of repeating unit of 44, 23, 9, respectively. Multiple morphologies, such as sphere, cylinder, vesicle, and their coexisted structures from self‐assembly of these diblock copolymers in aqueous media by changing block nonselective solvent and initial polymer concentration used in preparation, were demonstrated directly via TEM observation. These results herein might, therefore, demonstrate as an example that a wide range of morphologies can be accessed not only from “crew‐cut micelles” but also from “star‐micelles” by controlling over preparation strategies. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 364–371, 2010     

Poly(3‐hexyl thiophene)‐b‐poly(N‐isopropyl acrylamide): Synthesis and its composition dependent structural,solubility, thermoresponsive,electrochemical, and electronic properties

Conjugated block copolymers consisting of poly(3‐hexyl thiophene) (P3HT) and a thermoresponsive polymer poly(N‐isopropyl acrylamide) (PNIPAM) with varying composition have been synthesized by facile click reaction between alkyne terminated P3HT and azide terminated PNIPAM. The composition‐dependent solubility, thermoresponsive property in water, phase behavior, electrochemical, optical, and electronic properties of the block copolymers were systematically investigated. The block copolymers with higher volume fraction of PNIPAM form thermoresponsive spherical micelles with P3HT‐rich crystalline cores and PNIPAM coronas. Both X‐ray and atomic force microscopic studies indicated that the blocks copolymers showed well‐defined microphase separated nanostructures and the structure depended on the composition of the blocks. The electrochemical study of the block copolymers clearly demonstrated that the extent of charge transport through the block copolymer thin film was similar to P3HT homopolymer without any significant change in the band gap. The block copolymers showed improved or similar charge carrier mobility compared with the pure P3HT depending on the composition of the block copolymer. These P3HT‐b‐PNIPAM copolymers were interesting for fabrication of optoelectronic devices capable of thermal and moisture sensing as well as for studying the thermoresponsive colloidal structures of semiconductor amphiphilic systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1785–1794     

Adsorption of polymers at the solution–solid interface. XIII. Adsorption and steric stabilization with styrene–2-vinylpyridine copolymers

Random and block copolymers of styrene and 2-vinylpyridine, covering the full range of composition, have been synthesized. The adsorption of these polymers from trichloroethylene solution on to precipitated silica has been studied and their ability to impart colloidal stability to the silica dispersions also investigated. Estimates of the layer thickness of adsorbed copolymers have been made. Polystyrene is not adsorbed from trichloroethylene and does not stabilize dispersions of precipitated silica. A random copolymer having 1% 2-vinylpyridine units is adsorbed but shows very little steric stabilization. Random copolymers of 2-vinylpyridine content greater than 10% and AB block copolymers of more than 6% 2-vinylpyridine behave very similarly in respect both of the quantity adsorbed and in their ability to stabilize silica suspensions. Layer thickness does not seem to depend on copolymer composition. Random copolymers with low to intermediate 2-vinylpyridine contents are better steric stabilizers in trichloroethylene than are the corresponding copolymers of methyl methacrylate with styrene: this is attributed in part to the longer sequences of adsorbable units in the vinylpyridine copolymers.     

End‐grafting copolymers of styrene and 4‐vinylpyridine on an interacting solid surface

Copolymers of styrene and 4‐vinylpyridine with a styrene fraction f varying from 1 to 0 were grafted onto a silicon substrate in the melt. The grafting reaction and the stability of the grafted chains were investigated by Fourier transform infrared and X‐ray photoelectron spectroscopy. The thickness and surface morphology of the grafted copolymer layers were characterized with ellipsometry and atomic force microscopy (AFM). The copolymer chains were successfully grafted to the surface of the silicon substrate by a reaction between the hydroxyl groups of the nitroxide moiety at the end of the copolymers and the silanol groups on the surface of the silicon wafer. A measurement of the thickness of the grafted copolymer layers showed that the ratio of grafted‐layer thickness to the unperturbed chain radius of gyration decreased with the increasing fraction of 4‐vinylpyridine in the copolymer; this indicated that the grafted layer was strongly attracted to the substrate. In addition, an accelerated grafting process was observed at grafting times ranging from 48 to 72 h for pure poly(4‐vinylpyridine) and copolymers with f values of 0.3 and 0.5. AFM observation revealed that the grafted layers densely and homogeneously covered the silicon substrate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1332‐1343, 2005     

pH and ionic strength responsive polyelectrolyte block copolymer micelles prepared by ring opening metathesis polymerization

Well‐defined amphiphilic block copolymers were prepared by ring opening metathesis polymerization and their stimuli responsive behavior of formed micelles in aqueous solution was investigated. The hydrophobic core of the micelles consists of either a poly[5,6‐bis(ethoxymethyl)bicyclo[2.2.1]hept‐2‐ene]‐block with a glass transition T_g at room temperature or a poly[endo,exo[2.2.1]bicyclohept‐5‐ene‐2,3‐diylbis (phenylmethanone)] with a T_g of 143 °C. For the polyelectrolyte shell, the precursor block poly[endo,exo[2.2.1]bicyclohept‐5‐ene‐2,3‐dicarboxyclic tert‐butylester] was transformed into the free acidic block by cleavage of the tert‐butyl groups using trifluoroacetic acid. Micellar solutions were prepared by dialysis, dissolving the copolymers in dimethyl sulfoxide which was subsequently replaced by water. All polymers form micelles with radii between 10 and 20 nm at a pH‐value below 5, where the carboxylic acid groups are in the protonated state. The block copolymer micelles show a strong increase of the hydrodynamic radius with increasing pH‐value, due to the repulsion among the formed carboxylate anions resulting in a stretching of the polymer chains. In this state, the micelles exhibit responsive behavior to ionic strength where a contraction of the micelles is observed as the carboxylate charges are balanced by sodium ions, whereas no changes of the hydrodynamic radius on addition of salt are observed at low pH. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1178–1191, 2009     

Absorption of polymers at the solution–solid interface. XII. Stabilization of dispersed silica by absorbed styrene–methyl methacrylate copolymers

The ability of styrene–methyl methacrylate copolymers to stabilize silica dispersions has been investigated. Random, block, and graft copolymers covering the entire composition range have been employed in carbon tetrachloride, trichloroethylene, and benzene solutions. Equilibrium sediment volumes and dispersion turbidities provide adequate and concordant estimates of stabilization efficiency. Polystyrene is not adsorbed by precipitated silica from trichloroethylene or benzene and does not stabilize dispersions in these liquids; although adsorbed from carbon tetrachloride, there is no stabilization. Poly(methyl methacrylate) is an efficient dispersion stabilizer, and its performance is independent of molecular weight over a wide range. Random copolymers having styrene contents in excess of ca. 60% do not stabilize in trichloroethylene but do so in carbon tetrachloride, although well adsorbed in both cases. With this major exception, and that of a low-styrene graft copolymer in carbon tetrachloride, copolymers of all structures and compositions stabilize well, better than poly(methyl methacrylate) in the solvents examined. A substantial degree of surface coverage is necessary for optimum stabilization. Subsidiary solution adsorption and layer thickness measurements are also reported.     

Combining atom‐transfer radical polymerization and ring‐opening polymerization through bifunctional initiators derived from hydroxy benzyl alcohol—Preparation and characterization of initiators,macroinitiators, and block copolymers

A simple, one‐step procedure has been developed for the preparation of bifunctional initiators capable of polymerizing monomers suitable for atom‐transfer radical polymerization (ATRP) and ring‐opening polymerization (ROP). These bifunctional initiators were employed for making narrow disperse poly(styrene) macroinitiators, which were subsequently used for the ROP of various lactides to yield poly(styrene‐block‐lactide) copolymers. Thermogravimetric analysis (TGA) of these block copolymers are interesting in that it shows a two‐step degradation curve with the first step corresponding to the degradation of poly(lactide) segment and the second step associated with the poly(styrene) segment of the block copolymer. This nature of the block copolymer makes it possible to estimate the block copolymer content by TGA in addition to the ¹H NMR spectroscopic analysis. Thus, this study for the first time highlights the possibility of making porous materials by thermal means which are otherwise obtained by base hydrolysis. The bifunctional initiators were prepared by the esterification of 3‐hydroxy, 4‐hydroxy, and 3,5‐dihydroxy benzyl alcohols with α‐bromoisobutyryl bromide and 2‐bromobutyryl bromide. A mixture of products was obtained, which were purified by column chromatography. The esterified benzyl alcohols were employed in the polymerization of styrene under copper (Cu)‐catalyzed ATRP conditions to yield macroinitiators with low polydispersity. These macroinitiators were subsequently used in the ROP of L ‐, DL ‐, and mixture of lactides. The formation of block copolymers was confirmed by gel permeation chromatography (GPC), spectroscopic and thermal characterizations. The molecular weight of the block copolymers was always higher than the macroinitiator, and the GPC chromatogram was symmetrical indicating the uniform initiation of ROP by the macroinitiators. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 102–116, 2008     

Self‐Assembly of Poly(vinylpyridine‐b‐oligo(ethylene glycol) methyl ether methacrylate) Diblock Copolymers

Poly(oligoethylene glycol)‐poly(2‐vinylpyridine) is a model diblock for studying the effect of block‐localized charge on block copolymer self‐assembly because in the absence of charge the polymers are perfectly miscible, and upon protonation of the vinylpyridine block the polymer undergoes an order–disorder transition. Seven model block copolymers with molecular weights of approximately 60 kDa containing poly(2‐vinylpyridine) volume fractions spanning 0.069–0.700 were synthesized using reversible addition fragmentation transfer polymerization and then studied to understand the effect of protonation level, diblock composition, and temperature on the location of the ordering transition and the type of nanostructures formed in a charge asymmetric system. All of the polymers displayed lower critical solution‐type behavior, with the order–disorder transition temperature decreasing with increasing acid content. Polymers with symmetric compositions showed the highest degree of incompatibility for a given degree of protonation, and the observed morphologies for all polymers were consistent with those observed at similar compositions for classical hydrophobic block copolymers. The observed protonation‐induced phase transition can be explained by the shift of the Flory–Huggins parameter due to the alternation of the identity of monomers, consistent with the prediction of Nakamura and Wang's theory. The use of polyvalent ions promotes self‐assembly at lower concentrations, consistent with ionic crosslinking effects between polymer chains that are promoted at high concentration due to exchange entropy in crosslinked polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1181–1190     

Phase‐transition characteristics of amphiphilic poly(2‐ethyl‐2‐oxazoline)/poly(ε‐caprolactone) block copolymers in aqueous solutions

Amphiphilic diblock and triblock copolymers of various block compositions based on hydrophilic poly(2‐ethyl‐2‐oxazoline) (PEtOz) and hydrophobic poly(ε‐caprolactone) were synthesized. The micelle formation of these block copolymers in aqueous media was confirmed by a fluorescence technique and dynamic light scattering. The critical micelle concentrations ranged from 35.5 to 4.6 mg/L for diblock copolymers and 4.7 to 9.0 mg/L for triblock copolymers, depending on the block composition. The phase‐transition behaviors of the block copolymers in concentrated aqueous solutions were investigated. When the temperature was increased, aqueous solutions of diblock and triblock copolymers exhibited gel–sol transition and precipitation, both of which were thermally reversible. The gel–sol transition‐ and precipitation temperatures were manipulated by adjustment of the block composition. As the hydrophobic portion of block copolymers became higher, a larger gel region was generated. In the presence of sodium chloride, the phase transitions were shifted to a lower temperature level. Sodium thiocyanate displaced the gel region and precipitation temperatures to a higher temperature level. The low molecular weight saccharides, such as glucose and maltose, contributed to the shift of phase‐transition temperatures to a lower temperature level, where glucose was more effective than maltose in lowering the gel–sol transition temperatures. The malonic acid that formed hydrogen bonds with the PEtOz shell of micelles was effective in lowering phase‐transition temperatures to 1.0M, above which concentration the block copolymer solutions formed complex precipitates. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2400–2408, 2000