A Single Thermoresponsive Diblock Copolymer Can Form Spheres,Worms or Vesicles in Aqueous Solution

It is well‐known that the self‐assembly of AB diblock copolymers in solution can produce various morphologies depending on the relative volume fraction of each block. Recently, polymerization‐induced self‐assembly (PISA) has become widely recognized as a powerful platform technology for the rational design and efficient synthesis of a wide range of block copolymer nano‐objects. In this study, PISA is used to prepare a new thermoresponsive poly(N‐(2‐hydroxypropyl) methacrylamide)‐poly(2‐hydroxypropyl methacrylate) [PHPMAC‐PHPMA] diblock copolymer. Remarkably, TEM, rheology and SAXS studies indicate that a single copolymer composition can form well‐defined spheres (4 °C), worms (22 °C) or vesicles (50 °C) in aqueous solution. Given that the two monomer repeat units have almost identical chemical structures, this system is particularly well‐suited to theoretical analysis. Self‐consistent mean field theory suggests this rich self‐assembly behavior is the result of the greater degree of hydration of the PHPMA block at lower temperature, which is in agreement with variable temperature ¹H NMR studies.     

A Single Thermoresponsive Diblock Copolymer Can Form Spheres,Worms or Vesicles in Aqueous Solution

It is well‐known that the self‐assembly of AB diblock copolymers in solution can produce various morphologies depending on the relative volume fraction of each block. Recently, polymerization‐induced self‐assembly (PISA) has become widely recognized as a powerful platform technology for the rational design and efficient synthesis of a wide range of block copolymer nano‐objects. In this study, PISA is used to prepare a new thermoresponsive poly(N‐(2‐hydroxypropyl) methacrylamide)‐poly(2‐hydroxypropyl methacrylate) [PHPMAC‐PHPMA] diblock copolymer. Remarkably, TEM, rheology and SAXS studies indicate that a single copolymer composition can form well‐defined spheres (4 °C), worms (22 °C) or vesicles (50 °C) in aqueous solution. Given that the two monomer repeat units have almost identical chemical structures, this system is particularly well‐suited to theoretical analysis. Self‐consistent mean field theory suggests this rich self‐assembly behavior is the result of the greater degree of hydration of the PHPMA block at lower temperature, which is in agreement with variable temperature ¹H NMR studies.     

Vermicious thermo-responsive Pickering emulsifiers

Thermo-responsive vermicious (or worm-like) diblock copolymer nanoparticles prepared directly in n-dodecane via polymerisation-induced self-assembly (PISA) were used to stabilise water-in-oil Pickering emulsions. Mean droplet diameters could be tuned from 8 to 117 μm by varying the worm copolymer concentration and the water volume fraction and very high worm adsorption efficiencies (∼100%) could be obtained below a certain critical copolymer concentration (∼0.50%). Heating a worm dispersion up to 150 °C led to a worm-to-sphere transition, which proved to be irreversible if conducted at sufficiently low copolymer concentration. This affords a rare opportunity to directly compare the Pickering emulsifier performance of chemically identical worms and spheres. It is found that the former nanoparticles are markedly more efficient, since worm-stabilised water droplets are always smaller than the equivalent sphere-stabilised droplets prepared under identical conditions. Moreover, the latter emulsions are appreciably flocculated, whereas the former emulsions proved to be stable. SAXS studies indicate that the mean thickness of the adsorbed worm layer surrounding the water droplets is comparable to that of the worm cross-section diameter determined for non-adsorbed worms dispersed in the continuous phase. Thus the adsorbed worms form a monolayer shell around the water droplets, rather than ill-defined multilayers. Under certain conditions, demulsification occurs on heating as a result of a partial worm-to-sphere morphological transition.     

Capillary flow behavior of worm‐like micelles studied by small‐angle X‐ray scattering and small angle light scattering

A novel capillary flow device has been developed and applied to study the orientation of worm‐like micelles, among other systems. Small‐angle X‐ray scattering (SAXS) data from micelles formed by a Pluronic block copolymer in aqueous salt solution provides evidence for the formation of worm‐like micelles, which align under flow. A transition from a rod‐like form factor to a less persistent conformation is observed under flow. Flow alignment of worm‐like micelles formed by the low molar mass amphiphile system cetyl pyridinium chloride+sodium salicylate is studied for comparative purposes. Here, inhomogenous flow at the micron scale is revealed by streaks in the small‐angle light scattering pattern perpendicular to the flow direction. Copyright © 2006 John Wiley & Sons, Ltd.     

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     

Micellar structure and mechanical properties of block copolymer‐modified epoxies

Amphiphilic block copolymers provide a unique means for toughening epoxy resins because they can self‐assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and vesicles, which form in blends containing epoxy and symmetric or asymmetric poly(ethylene oxide)–poly(ethylene‐alt‐propylene) (PEO–PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and spherical micelles were characterized by transmission electron microscopy and small‐angle X‐ray scattering (SAXS), respectively. SAXS data from the spherical micelles were fit to the Percus–Yevick model for a liquid‐like packing of spheres with hard‐core interactions. Mechanical properties of spherical‐micelle‐modified and vesicle‐modified epoxies in the dilute limit are compared. The glass‐transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact‐tension experiments were performed to determine the critical plane‐strain energy release rate for fracture. Vesicles were most effective in improving the epoxy fracture resistance. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2996–3010, 2001     

Solubility behavior of amphiphilic block and random copolymers based on 2‐ethyl‐2‐oxazoline and 2‐nonyl‐2‐oxazoline in binary water–ethanol mixtures

The solution properties of random and block copolymers based on 2‐ethyl‐2‐oxazoline (EtOx) and 2‐nonyl‐2‐oxazoline (NonOx) were investigated in binary solvent mixtures ranging from pure water to pure ethanol. The solubility phase diagrams for the random and block copolymers revealed solubility (after heating), insolubility, dispersions, micellization as well as lower critical solution temperature (LCST) and upper critical solution temperature behavior. The random and block copolymers containing over 60 mol % pNonOx were found to be solubilized in ethanol upon heating, whereas the dissolution temperature of the block copolymers was found to be much higher than for the random copolymers due to the higher extent of crystallinity. Furthermore, the block copolymer containing 10 mol % pNonOx exhibited a LCST in aqueous solution at 68.7 °C, whereas the LCST for the random copolymer was found to be only 20.8 °C based on the formation of hydrophobic microdomains in the block copolymer. The random copolymer displayed a small increase in LCST up to a solvent mixture of 9 wt % EtOH, whereas further increase of ethanol led to a decrease in LCST, which is probably due to the “water‐breaking” effect causing an increased attraction between ethanol and the hydrophobic part of the copolymer. In addition, the EtOx‐NonOx block copolymers revealed the formation of micelles and dynamic light scattering demonstrated that the micellar size is increasing with increasing the ethanol content due to the enhanced solubility of EtOx. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 515–522, 2009     

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

Small-angle X-ray scattering (SAXS) is used to characterize the in situ formation of diblock copolymer spheres, worms and vesicles during reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a poly(glycerol monomethacrylate) steric stabilizer. ¹H NMR spectroscopy indicates more than 99% HPMA conversion within 80 min, while transmission electron microscopy and dynamic light scattering studies are consistent with the final morphology being pure vesicles. Analysis of time-resolved SAXS patterns for this prototypical polymerization-induced self-assembly (PISA) formulation enables the evolution in copolymer morphology, particle diameter, mean aggregation number, solvent volume fraction, surface density of copolymer chains and their mean inter-chain separation distance at the nanoparticle surface to be monitored. Furthermore, the change in vesicle diameter and membrane thickness during the final stages of polymerization supports an ‘inward growth’ mechanism.

In situ small-angle X-ray scattering is used to monitor the formation of diblock copolymer spheres, worms and vesicles during reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate.     

Design and proof of reversible micelle‐to‐vesicle multistimuli‐responsive morphological regulations

Multistimuli‐responsive precise morphological control over self‐assembled polymers is of great importance for applications in nanoscience as drug delivery system. A novel pH, photoresponsive, and cyclodextrin‐responsive block copolymer were developed to investigate the reversible morphological transition from micelles to vesicles. The azobenzene‐containing block copolymer poly(ethylene oxide)‐b‐poly(2‐(diethylamino)ethyl methacrylate‐co‐6‐(4‐phenylazo phenoxy)hexyl methacrylate) [PEO‐b‐P(DEAEMA‐co‐PPHMA)] was synthesized by atom transfer radical polymerization. This system can self‐assemble into vesicles in aqueous solution at pH 8. On adjusting the solution pH to 3, there was a transition from vesicles to micelles. The same behavior, that is, transition from vesicles to micelles was also realizable on addition of β‐cyclodextrin (β‐CD) to the PEO‐b‐P(DEAEMA‐co‐PPHMA) solution at pH 8. Furthermore, after β‐CD was added, alternating irradiation of the solution with UV and visible light can also induce the reversible micelle‐to‐vesicle transition because of the photoinduced trans‐to‐cis isomerization of azobenzene units. The multistimuli‐responsive precise morphological changes were studied by laser light scattering, transmission electron microscopy, and UV–vis spectra. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Temperature and pH dual stimuli responsive PCL‐b‐PNIPAAm block copolymer assemblies and the cargo release studies

A new atom transfer radical polymerization (ATRP) initiator, namely, 2‐(1‐(2‐azidoethoxy)ethoxy)ethyl 2‐bromo‐2‐methylpropanoate containing both “cleavable” acetal linkage and “clickable” azido group was synthesized. Well‐defined azido‐terminated poly(N‐isopropylacrylamide)s (PNIPAAm‐N₃)s with molecular weights and dispersity in the range 11,000–19,000 g mol^?1 and 1.20–1.28, respectively, were synthesized employing the initiator by ATRP. Acetal containing PCL‐b‐PNIPAAm block copolymer was obtained by alkyne–azide click reaction of azido‐terminated PNIPAAm‐N₃ with propargyl‐terminated PCL. Critical aggregation concentration (CAC) of PCL‐b‐PNIPAAm copolymer in aqueous solution was found to be 8.99 × 10^?6 M. Lower critical solution temperature (LCST) of PCL‐b‐PNIPAAm copolymer was found to be 32 °C which was lower than that of the precursor PNIPAAm‐N₃ (36.4 °C). The effect of dual stimuli viz . temperature and pH on size and morphology of the assemblies of PCL‐b‐PNIPAAm block copolymer revealed that the copolymer below LCST assembled in spherical micelles which subsequently transformed to unstable vesicles above the LCST. Heating these assemblies above 40 °C led to the precipitation of PCL‐b‐PNIPAAm block copolymer. Whereas, at decreased pH, micelles of PCL‐b‐PNIPAAm copolymer disintegrate due to the cleavage of acetal linkage and precipitation of hydrophobic hydroxyl‐terminated PCL. The encapsulated pyrene release kinetics from the micelles of synthesized PCL‐b‐PNIPAAm copolymer was found to be faster at higher temperature and at lower pH. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1383–1396     

Surface functionalized nano‐objects from oleic acid‐derived stabilizer via non‐polar RAFT dispersion polymerization

Surface functionalization in a nanoscopic scaffold is highly desirable to afford nano‐particles with diversified features and functions. Herein are reported the surface decoration of dispersed block copolymer nano‐objects. First, side‐chain double bond containing oleic acid based macro chain transfer agent (macroCTA), poly(2‐(methacryloyloxy)ethyl oleate) (PMAEO), was synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization and used as a steric stabilizer during the RAFT dispersion block copolymerization of benzyl methacrylate (BzMA) in n‐heptane at 70 °C. We have found that block copolymer morphologies could evolve from spherical micelles, through worm to vesicles, and finally to large compound vesicles with the increase of solvophobic poly(BzMA) block length, keeping solvophilic chain length and total solid content constant. Finally, different thiol compounds having alkyl, carboxyl, hydroxyl, and protected amine functionalities have been ligated onto the PMAEO segment, which is prone to functionalization via its reactive double bond through thiol‐ene radical reactions. Thiol‐ene modification reactions of the as‐synthesized nano‐objects retain their morphologies as visualized by field emission‐scanning electron microscopy. Thus, the facile and modular synthetic approach presented in this study allowed in situ preparation of surface modified block copolymer nano‐objects at very high concentration, where renewable resource derived oleate surface in the nanoparticle was functionalized. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 263–273     

Temperature‐Induced Transformation from Large Compound Vesicles to Worm‐like Aggregates by ABC Triblock Copolymer

Two triblock polymers, tetraaniline‐block‐poly(N‐isopropyl acrylamide)‐block‐poly(hydroxyethyl acrylate) (TA‐b‐PNIPAM‐b‐PHEA) and TA‐b‐PHEA‐b‐PNIPAM, were synthesized with unambiguous structure by a two step method. The difference of these two diblock polymers is the connection order of carboxyl group to block, e.g., carboxyl group to PNIPAM block for PNIPAM‐b‐PHEA and to PHEA block for PHEA‐b‐PNIPAM. Secondly, block tetraaniline was linked to the diblock polymer through amidation to yield the corresponding triblock copolymer. Both of them have almost the identical chemical compositions. The only difference is the connection order of each block in the triblock polymers. When they were self‐assembled at 45°C in a suitable solution, both of their aggregates have spherical shape with slight defects on their surface with the average diameter of about 400 nm. However, when their aggregate dispersion was cooled down to 20°C, only TA‐b‐PHEA‐b‐PNIPAM's morphology changed, forming worm‐like aggregates with the diameter of about 100–200 nm transformed from spherical aggregates. Both amphiphilic property and position of each block in this triblock copolymer are very essential for this morphology transformation. Since the worm‐like aggregates presented here by our group have hollow structure inside, its controlled release properties for doxorubicin were evaluated. Drug release experiment indicated that along with the temperature changes, the rearrangement of the intermediate layer structure caused morphology change in aggregate, thus accelerating the speed of drug release.     

Synthesis,micellization and gelation of temperature‐responsive star‐shaped block copolymers

A series of amphiphilic temperature‐responsive star‐shaped poly(D,L‐lactic‐co‐glycolic acid)‐b‐methoxy poly(ethylene glycol) (PLGA‐mPEG) block copolymers with different arm numbers were synthesized via the arm‐first method. Gel permeation chromatography data confirmed that star‐shaped PLGA‐mPEG copolymers had narrow polydispersity index, indicating the successful formation of star‐shaped block copolymers. Indirectly, the ¹H NMR spectra in two kinds of solvents and dye solubilization method had confirmed the formation of core‐shell micelles. Further, core‐shell micelles with sizes of about 30–50 nm were directly observed by transmission electron microscopy. Subsequently, the micellar sizes and distributions as a function of concentrations and temperature were measured. At various copolymer concentrations, individual micelles with size of 20–40 nm and grouped micelles with size of 600–700 nm were found. Micellar mechanism of star‐shaped block copolymers in aqueous solution was simultaneously discussed. In addition, sol–gel transition of star‐shaped block copolymers in water was also investigated via the inverting test method. The critical gel temperature (CGT) and critical gel concentration (CGC) values of two‐arm, three‐arm and four‐arm copolymer solutions were markedly higher than ones of one‐arm copolymer. Moreover, the same CGC values of copolymer solution with different molecular weight and the same arm composition were ~15 wt %, and CGT values increased from ~38 to ~47°C with increasing arm numbers. Finally, the temperature‐dependent micellar packing gelation mechanism of star‐shaped block copolymer was schematically illustrated. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature‐based fluorescence measurements of pyrene in block copolymer micelles: Probing micelle core glass transition breadths

Glass transition temperature (T_g) breadths are reported for polystyrene (PS) micelle cores in two series of micelle‐forming block copolymers [PS‐poly(ethylene oxide) and PS‐poly(methyl methacrylate)] with an ionic liquid solvent (1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)amide). An increased level of fluorescent molecules was induced within the cores upon rapid cooling followed by aging. Using fluorescence to monitor dye release with relaxation of this state upon heating, transition onset and end‐point temperatures were defined. The system with the lowest PS‐block molecular weight showed no evidence of a transition above 25 °C; however, in every other case, transitions were observed beginning at ～40‐45 °C and ending at ～60‐85 °C. These temperatures closely match PS‐block T_g results measured by differential scanning calorimetry in semidilute solutions of the same materials, suggesting that the transition temperature range correlates strongly to the transition of the cores from fully glassy to fully rubbery. Differences in transition end‐points were related to PS‐block molecular weights and relative copolymer fractions of PS. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Temperature‐dependent self‐assembly and rheological behavior of a thermoreversible pmma–PnBA–PMMA triblock copolymer gel

Self‐assembly and mechanical properties of triblock copolymers in a mid‐block selective solvent are of interest in many applications. Herein, we report physical assembly of an ABA triblock copolymer, [PMMA–Pn BA–PMMA] in two different mid‐block selective solvents, n‐butanol and 2‐ethyl‐1‐hexanol. Gel formation resulting from end‐block associations and the corresponding changes in mechanical properties have been investigated over a temperature range of ?80 °C to 60 °C, from near the solvent melting points to above the gelation temperature. Shear‐rheometry, thermal analysis, and small‐angle neutron scattering data reveal formation and transition of structure in these systems from a liquid state to a gel state to a percolated cluster network with decrease in temperature. The aggregated PMMA end‐blocks display a glass transition temperature. Our results provide new understanding into the structural changes of a self‐assembled triblock copolymer gel over a large length scale and wide temperature range. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 877–887     

Preparation and phase behavior of poly(4‐trimethylsilylstyrene)‐block‐polyisoprene

Three poly(4‐trimethylsilylstyrene)‐block‐polyisoprenes (TIs), the molecular weights of which were 82,000, 152,000 and 291,000 (TI‐82K, TI‐152K, and TI‐291K), were synthesized by sequential anionic polymerizations. The component polymers were a miscible pair that presented a lower critical solution temperature phase diagram if blended. The TI phase behavior was investigated with transmission electron microscopy. The order–disorder transition could be observed at a temperature between 200 °C (the ordered state) and 150 °C (the disordered state) for the block copolymer TI‐152K. The block copolymer TI‐82K presented the disordered state at 200 °C, whereas TI‐291K was in the ordered state at 150 °C. With the Flory–Huggins interaction parameter between poly(4‐trimethylsilylstyrene) and polyisoprene, which was evaluated by small‐angle neutron scattering for the block copolymers, the TI phase behavior could be reasonably explained by mean‐field theory. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1214–1219, 2005     

Preparation of methoxy poly(ethylene glycol)/polyester diblock copolymers and examination of the gel‐to‐sol transition

Diblock copolymers consisting of methoxy poly(ethylene glycol) (MPEG) and poly(?‐caprolactone) (PCL), poly(δ‐valerolactone) (PVL), poly(L ‐lactic acid) (PLLA), or poly(lactic‐co‐glycolic acid) (PLGA) as biodegradable polyesters were prepared to examine the phase transition of diblock copolymer solutions. MPEG–PCL and MPEG–PVL diblock copolymers and MPEG–PLLA and MPEG–PLGA diblock copolymers were synthesized by the ring‐opening polymerization of ?‐caprolactone or δ‐valerolactone in the presence of HCl · Et₂O as a monomer activator at room temperature and by the ring‐opening polymerization of L ‐lactide or a mixture of L ‐lactide and glycolide in the presence of stannous octoate at 130 °C, respectively. The synthesized diblock copolymers were characterized with ¹H NMR, IR, and gel permeation chromatography. The phase transitions for diblock copolymer aqueous solutions of various concentrations were explored according to the temperature variation. The diblock copolymer solutions exhibited the phase transition from gel to sol with increasing temperature. As the polyester block length of the diblock copolymers increased, the gel‐to‐sol transition moved to a lower concentration region. The gel‐to‐sol transition showed a dependence on the length of the polyester block segment. According to X‐ray diffraction and differential scanning calorimetry thermal studies, the gel‐to‐sol transition of the diblock copolymer solutions depended on their degrees of crystallinity because water could easily diffuse into amorphous polymers in comparison with polymers with a crystalline structure. The crystallinity markedly depended on both the distinct character and composition of the block segment. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5784–5793, 2004     

An insight into the schizophrenic self‐assembly of thermo and proton sensitive graphene oxide grafted block copolymer

In order to study the self‐assembly of block copolymer grafted from graphene oxide (GO) by the fluorescence of GO, poly(ε‐caprolactone) (PCL)‐block‐poly(dimethyl aminoethyl methacrylate) (PDMAEMA) is grafted from its surface using consecutive ring opening (ROP) and atom transfer radical polymerization (ATRP). GO‐g‐(PCL₁₃‐b‐PDMAEMA₁₁₇) (GPCLD) at pH 9.2 exhibits cloud point (T_c) at 32 °C. At pH 9.2 HRTEM images indicate schizophrenic morphology from vesicle at 26 °C to annular ring at 30 °C followed by giant size aggregation at 38 °C. But the reference block copolymer (PCL₁₄‐b‐PDMAEMA₁₂₆, PCLD), synthesized using benzyl alcohol as ROP initiator, exhibits only core–shell morphology whose size increases with rising temperature at pH 9.2. GPCLD solution exhibits good photoluminescence (PL) property arising from GO at pH 9.2 and PL‐intensity increases abruptly during phase transition. Both T_c and size of GPCLD assembly can be reversibly tuned by CO₂ and N₂ gas purging. ¹H NMR spectra exhibit a gradual shift of resonance peaks of the protons on CO₂ bubbling. Thus at pH 9.2 and at 38 °C the GPCLD acts as a good CO₂ sensor. Additionally, the GPCLD vesicle can load hydrophobic guest molecules which can be released by triggering with CO₂. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3878–3887     

Synthesis,self‐assembly,and thermosensitivity of amphiphilic POEGMA‐PDMS‐POEGMA triblock copolymers

In this article, the synthesis and self‐assembly of a novel well‐defined biocompatible amphiphilic POEGMA‐PDMS‐POEGMA triblock copolymer were studied. The copolymer was synthesized by atom transfer radical polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA) using α,ω‐dibromo polydimethylsiloxane macroinitiator (Br‐PDMS‐Br). Br‐PDMS‐Br was synthesized through the esterification of α,ω‐hydroxypropyl polydimethylsiloxane and 2‐bromoisobutyryl bromide. The structures of the copolymers were confirmed by proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography. The copolymers showed reversible aggregation in response to temperature cycles with a lower critical solution temperature (LCST) between 61 and 66 °C, as determined by ultraviolet‐visible spectrophotometry and dynamic light scattering. The LCST values increased in proportion to the length of the hydrophilic block and were lower than that of the POEGMA homopolymer. The self‐assembly behavior of the copolymers in aqueous solution was investigated by fluorescence spectroscopy and transmission electron microscopy. The critical micelle concentration value (1.08–0.26 10^?6 mol L^?1) decreased as the length of the POEGMA chain increased. The POEGMA‐PDMS‐POEGMA copolymers can easily self‐assemble into spherical micelles in aqueous solution. Such biocompatible block copolymers may be attractive candidates as ‘‘smart'' thermo‐responsive drug delivery systems. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2684‐2691     

Synthesis and self‐assembly of highly incompatible polybutadiene–poly(hexafluoropropylene oxide) diblock copolymers

A versatile coupling reaction for the preparation of polybutadiene–poly‐(hexafluoropropylene oxide) (BF) diblock copolymers is described. Six diblock copolymers with different block lengths were characterized by nuclear magnetic resonance spectroscopy and size exclusion chromatography; all six had total molecular weights below 15,000. Microphase separation of the block copolymers in the bulk state was established by small‐angle X‐ray scattering (SAXS) and differential scanning calorimetry. SAXS data suggest that the diblocks are characterized by an unusually large Flory‐Huggins interaction parameter, χ, on the order of 10. However, extraction of χ from the order–disorder transition gave large (order 1) but significantly different values, thereby suggesting that these copolymers are too small and too strongly interacting to be described by block copolymer mean‐field theory. Dynamic light scattering was used to analyze dilute solutions of the title block copolymers in four selective organic solvents; the sizes of the micelles formed were solvent dependent. The micellar aggregates were large and nonspherical, and this is also attributed to the high degree of incompatibility between the two immiscible blocks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3685–3694, 2005