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Synthesis and Solution Behavior of Doubly Responsive Hydrophilic Block Copolymers

Synthesis and Solution Behavior of Doubly Responsive Hydrophilic Block Copolymers
Author: Xueguang Jiang
Publisher:
Total Pages: 282
Release: 2010
Genre:
ISBN:
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This dissertation presents the synthesis of stimuli-sensitive hydrophilic polymers, particularly doubly responsive hydrophilic block copolymers, by controlled radical polymerizations and the study of their solution behavior in water. By incorporating a small amount of stimuli-responsive groups into the thermosensitive block of a hydrophilic block copolymer, the lower critical solution temperature (LCST) of the thermosensitive block can be tuned by a stimulus and multiple micellization/dissociation transitions can be achieved by combining two external triggers. Chapter 1 describes the synthesis and thermosensitive properties of two new watersoluble polystyrenics with a short oligo(ethyl glycol) pendant from each repeat unit and the study of hydrophobic end group effects on cloud points of thermosensitive polystyrenics. Well-defined polymers were prepared from monomer-based initiators via nitroxide-mediated polymerization and the alkoxyamine end groups were removed by tri(n-butyl)tin hydride, yielding thermoresponsive polystyrenics with essentially no end groups. The results showed that hydrophobic end groups could significantly change the cloud points and the molecular weight dependences of cloud points of polystyrenics. Chapter 2 presents the synthesis of thermo- and light-sensitive hydrophilic block copolymers, poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate), and their responsive behavior in dilute aqueous solutions. Dynamic light scattering and fluorescence spectroscopy studies showed that these copolymers were molecularly dissolved in water at lower temperatures and self-assembled into micelles at temperatures above the LCST of the thermosensitive block. Upon UV irradiation, the o-nitrobenzyl group was cleaved and the LCST of the thermosensitive block was increased, causing the dissociation of micelles into unimers. The resultant copolymers underwent thermo-induced reversible micellization at higher temperatures. Chapter 3 describes multiple micellization/dissociation transitions of thermo- and pH-sensitive hydrophilic block copolymers, poly(ethylene oxide)-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid), in response to temperature and pH changes. The LCST of the thermosensitive block can be reversibly tuned and precisely controlled by solution pH. Chapter 4 presents the study on multiple sol-gel-sol transitions of a 20.0 wt % aqueous solution of poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) induced by temperature changes and UV irradiation. The solution underwent thermo-induced sol-gel-sol transitions. Upon UV irradiation to dissociate micelles, the gel was transformed into a free-flowing liquid, which upon heating underwent sol-gel-sol transitions again.

Synthesis of -dye-labelled Thermoresponsive Block Copolymers by Raft Polymerization

Synthesis of -dye-labelled Thermoresponsive Block Copolymers by Raft Polymerization
Author: Mariana Beija
Publisher:
Total Pages: 0
Release: 2009
Genre:
ISBN:
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Double hydrophilic diblock copolymers comprising a thermoresponsive block have gained increasing attention due to their capability of self-assembling in micelles by a temperature change. However, very few fluorescence studies were devoted to investigate their conformation and dynamics both at the air-water interface and in aqueous solutions. In this work, block copolymers composed of a thermoresponsive block of N,N- iethylacrylamide (DEA) and a hydrophilic block of N,N-dimethylacrylamide (DMA) or a reactive block [statistical copolymer of DMA and N-acryloxysuccinimide (NAS)] were prepared by RAFT polymerization. These block copolymers were functionalized at the hydrophilic chain-end by a Rhodamine B or Malachite Green dye using either a pre- or a post-polymerization strategy. In the first case, Rhodamine B and Malachite Green amino derivatives were synthesized for the preparation of dyelabelled chain transfer agent (CTA), which led directly the alpha-dye-labelled block copolymers. Alternatively, the block copolymers were prepared using a precursor CTA and further functionalized with the dye amino derivative. The thermoresponsive behaviour of these polymers and of amphiphilic block copolymers of DEA and N-decylacrylamide was studied at the air-water interface and in Langmuir-Blodgett films using AFM and confocal fluorescence microscopy. Fluorescence emission and anisotropy, light scattering and 1H NMR studies were performed to investigate their behaviour in aqueous solutions.

Synthesis and Self-assembly of Multiple Thermoresponsive Amphiphilic Block Copolymers

Synthesis and Self-assembly of Multiple Thermoresponsive Amphiphilic Block Copolymers
Author: Jan Weiss
Publisher:
Total Pages: 157
Release: 2011
Genre:
ISBN:
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In the present thesis, the self-assembly of multi thermoresponsive block copolymers in dilute aqueous solution was investigated by a combination of turbidimetry, dynamic light scattering, TEM measurements, NMR as well as fluorescence spectroscopy. The successive conversion of such block copolymers from a hydrophilic into a hydrophobic state includes intermediate amphiphilic states with a variable hydrophilic-to-lipophilic balance. As a result, the self-organization is not following an all-or-none principle but a multistep aggregation in dilute solution was observed. The synthesis of double thermoresponsive diblock copolymers as well as triple thermoresponsive triblock copolymers was realized using twofold-TMS labeled RAFT agents which provide direct information about the average molar mass as well as residual end group functionality from a routine proton NMR spectrum. First a set of double thermosensitive diblock copolymers poly(N-n-propylacrylamide)-b-poly(N-ethylacrylamide) was synthesized which differed only in the relative size of the two blocks. Depending on the relative block lengths, different aggregation pathways were found. Furthermore, the complementary TMS-labeled end groups served as NMR-probes for the self-assembly of these diblock copolymers in dilute solution. Reversible, temperature sensitive peak splitting of the TMS-signals in NMR spectroscopy was indicative for the formation of mixed star-/flower-like micelles in some cases. Moreover, triple thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (A), poly(methoxydiethylene glycol acrylate) (B) and poly(N-ethylacrylamide) (C) were obtained from sequential RAFT polymerization in all possible block sequences (ABC, BAC, ACB). Their self-organization behavior in dilute aqueous solution was found to be rather complex and dependent on the positioning of the different blocks within the terpolymers. Especially the localization of the low-LCST block (A) had a large influence on the aggregation behavior. Above the first cloud point, aggregates were only observed when the A block was located at one terminus. Once placed in the middle, unimolecular micelles were observed which showed aggregation only above the second phase transition temperature of the B block. Carrier abilities of such triple thermosensitive triblock copolymers tested in fluorescence spectroscopy, using the solvatochromic dye Nile Red, suggested that the hydrophobic probe is less efficiently incorporated by the polymer with the BAC sequence as compared to ABC or ACB polymers above the first phase transition temperature. In addition, due to the problem of increasing loss of end group functionality during the subsequent polymerization steps, a novel concept for the one-step synthesis of multi thermoresponsive block copolymers was developed. This allowed to synthesize double thermoresponsive di- and triblock copolymers in a single polymerization step. The copolymerization of different N-substituted maleimides with a thermosensitive styrene derivative (4-vinylbenzyl methoxytetrakis(oxyethylene) ether) led to alternating copolymers with variable LCST. Consequently, an excess of this styrene-based monomer allowed the synthesis of double thermoresponsive tapered block copolymers in a single polymerization step.

Thermo- and PH-sensitive Hydrophilic Block Copolymers

Thermo- and PH-sensitive Hydrophilic Block Copolymers
Author: Thomas G. O'Lenick
Publisher:
Total Pages: 233
Release: 2011
Genre:
ISBN:
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This dissertation presents the synthesis of a series of thermo- and pH-sensitive hydrophilic block copolymers and the study of their solution behavior in water. By incorporating a small amount of weak acid or base groups into the thermosensitive block(s) of a hydrophilic block copolymer, the LCST of the thermosensitive block(s) can be modified by changing the solution pH. Accordingly, the critical micellization temperature (CMT) and the sol-gel transition temperature (T[subscript sol-gel]) of the block copolymer in water can be tuned. Chapter 1 describes the synthesis of thermo- and pH-sensitive poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid)-b-PEO-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) and the study of sol-gel transitions of its aqueous solutions at various pH values. The CMT of the 0.2 wt% solution and the T[subscript sol-gel] of the 12.0 wt% solution of this copolymer can be varied over a large temperature range. By judiciously controlling temperature and pH, multiple sol-gel-sol transitions were realized. Chapter 2 presents a systematic study of pH effect on rheological properties of micellar gels formed from 10.0 wt% aqueous solutions of thermo- and pH-sensitive poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid)-b-PEO-b-poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid). With the increase of pH, the sol-gel transition became broader. The plateau moduli (G[subscript N]) evaluated from frequency sweeps at T/T[subscript sol-gel] of 1.025, 1.032, and 1.039 decreased with the increase of pH from 3.00 to 5.40 with the largest drop observed at pH = ~ 4.7. The decrease in G[subscript N] reflects the reduction of the number of bridging chains. The ionization of carboxylic acid introduced charges onto the thermosensitive blocks and made the polymer more hydrophilic, facilitating the formation of loops and dangling chains. Chapter 3 presents the synthesis of PEO-b-poly(methoxydi(ethylene glycol) methacrylate-co-2-(N-methyl-N-(4-pyridyl)amino)ethyl methacrylate) with the thermosensitive block containing a catalytic 4-N, N-dialkylaminopyridine and the study of the effect of thermo-induced micellization on its activity in the hydrolysis of p-nitrophenyl acetate. The CMTs of this copolymer at pH of 7.06 and 7.56 were 40 and 37 °C, respectively. Below CMT, the logarithm of initial hydrolysis rate changed linearly with 1/T. Above CMT, the reaction rate leveled off, which is presumably because it was controlled by mass transport to the core of micelles above CMT.

Tuning Sol-gel Phase Diagrams of Doubly Thermosensitive Hydrophilic Diblock Copolymers in Water

Tuning Sol-gel Phase Diagrams of Doubly Thermosensitive Hydrophilic Diblock Copolymers in Water
Author: Naixiong Jin
Publisher:
Total Pages: 244
Release: 2013
Genre: Diblock copolymers
ISBN:
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This dissertation presents the synthesis of stimuli-responsive hydrophilic diblock copolymers and the study of their behavior in water under various conditions. The polymers were made by "living"/controlled radical polymerization. Chapter 1 presents a background of this dissertation. Chapters 2-4 describe a family of doubly thermosensitive diblock copolymers with a small amount of carboxylic acid groups incorporated into either one or both blocks. The lower critical solution temperature (LCST) of the weak acid-containing block increases with increasing pH due to the ionization of carboxylic acid. Chapter 5 presents the preparation of pH-sensitive diblock copolymer micelle-embedded agarose hydrogels. Chapter 2 describes the synthesis and solution behavior of poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate) (P(TEGMA-co-AA)-b-PDEGEA)). PTEGMA and PDEGEA are thermosensitive polymers with LCSTs of 58 and 9 °C [degree Celsius], respectively, in water. A 20 wt% aqueous solution of P(TEGMA-co-AA)-b-PDEGEA with pH of 3.11 underwent transitions from a free-flowing liquid, to a free-standing gel, to a hot liquid, and to a cloudy mixture upon heating. The Ts̳o̳l̳-̳g̳̳̳̳e̳̳l̳̳̳̳ [sol-to-gel transition temperature] and Tg̳̳̳̳e̳̳l̳̳̳̳-̳so̳l̳ [gel-to-sol transition temperature] are closely related to the LCSTs of the two blocks. Upon raising pH, the Tg̳̳̳̳e̳̳l̳̳̳̳-̳so̳l̳ increased, while the Ts̳o̳l̳-g̳̳̳̳e̳̳l̳̳̳̳ remained the same. Accordingly, only the upper boundary of the ̳so̳l̳-g̳̳̳̳e̳̳l̳̳̳̳ phase diagram shifted upward. Chapter 3 presents the tuning of Ts̳o̳l̳-g̳̳̳̳e̳̳l̳̳̳̳ of moderately concentrated aqueous solutions of doubly thermosensitive diblock copolymers by incorporating a small amount of AA groups into the lower LCST block and changing the solution pH. The AA content had a significant effect on the pH dependence of Ts̳o̳l̳-g̳̳̳̳e̳̳l̳̳̳̳-. Chapter 4 shows that by incorporating a small amount of carboxylic acid groups into both blocks of a doubly thermosensitive diblock copolymer, the C-shaped sol-gel phase diagram can be readily and reversibly shifted by changing the solution pH. Chapter 5 presents the fabrication of pH-sensitive diblock copolymer micelle-embedded agarose hydrogels. The gel properties were not significantly affected by the incorporation of the micelles even when the polymer concentration reached 5 mg/g. The pH-induced release of the payload from the core of micelles in a hybrid gel was studied. Chapter 6 presents conclusions and future work.

Self-assembly Behavior in Hydrophilic Block Copolymers

Self-assembly Behavior in Hydrophilic Block Copolymers
Author: Clara Valverde Serrano
Publisher:
Total Pages: 0
Release: 2011
Genre:
ISBN:
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Block copolymers are receiving increasing attention in the literature. Reports on amphiphilic block copolymers have now established the basis of their self-assembly behavior: aggregate sizes, morphologies and stability can be explained from the absolute and relative block lengths, the nature of the blocks, the architecture and also solvent selectiveness. In water, self-assembly of amphiphilic block copolymers is assumed to be driven by the hydrophobic. The motivation of this thesis is to study the influence on the self-assembly in water of A b B type block copolymers (with A hydrophilic) of the variation of the hydrophilicity of B from non-soluble (hydrophobic) to totally soluble (hydrophilic). Glucose-modified polybutadiene-block-poly(N-isopropylacrylamide) copolymers were prepared and their self-assembly behavior in water studied. The copolymers formed vesicles with an asymmetric membrane with a glycosylated exterior and poly(N-isopropylacrylamide) on the inside. Above the low critical solution temperature (LCST) of poly(N-isopropylacrylamide), the structure collapsed into micelles with a hydrophobic PNIPAM core and glycosylated exterior. This collapse was found to be reversible. As a result, the structures showed a temperature-dependent interaction with L-lectin proteins and were shown to be able to encapsulate organic molecules. Several families of double hydrophilic block copolymers (DHBC) were prepared. The blocks of these copolymers were biopolymers or polymer chimeras used in aqueous two-phase partition systems. Copolymers based on dextran and poly(ethylene glycol) blocks were able to form aggregates in water. Dex6500-b-PEG5500 copolymer spontaneously formed vesicles with PEG as the "less hydrophilic" barrier and dextran as the solubilizing block. The aggregates were found to be insensitive to the polymer's architecture and concentration (in the dilute range) and only mildly sensitive to temperature. Variation of the block length, yielded different morphologies. A longer PEG chain seemed to promote more curved aggregates following the inverse trend usually observed in amphiphilic block copolymers. A shorter dextran promoted vesicular structures as usually observed for the amphiphilic counterparts. The linking function was shown to have an influence of the morphology but not on the self-assembly capability in itself. The vesicles formed by dex6500-b-PEG5500 showed slow kinetics of clustering in the presence of Con A lectin. In addition both dex6500-b-PEG5500 and its crosslinked derivative were able to encapsulate fluorescent dyes. Two additional dextran-based copolymers were synthesized, dextran-b-poly(vinyl alcohol) and dextran-b-poly(vinyl pyrrolidone). The study of their self-assembly allowed to conclude that aqueous two-phase systems (ATPS) is a valid source of inspiration to conceive DHBCs capable of self-assembling. In the second part the principle was extended to polypeptide systems with the synthesis of a poly(N-hydroxyethylglutamine)-block-poly(ethylene glycol) copolymer. The copolymer that had been previously reported to have emulsifying properties was able to form vesicles by direct dissolution of the solid in water. Last, a series of thermoresponsive copolymers were prepared, dextran-b-PNIPAMm. These polymers formed aggregates below the LCST. Their structure could not be unambiguously elucidated but seemed to correspond to vesicles. Above the LCST, the collapse of the PNIPAM chains induced the formation of stable objects of several hundreds of nanometers in radius that evolved with increasing temperature. The cooling of these solution below LCST restored the initial aggregates. This self-assembly of DHBC outside any stimuli of pH, ionic strength, or temperature has only rarely been described in the literature. This work constituted the first formal attempt to frame the phenomenon. Two reasons were accounted for the self-assembly of such systems: incompatibility of the polymer pairs forming the two blocks (enthalpic) and a considerable solubility difference (enthalpic and entropic). The entropic contribution to the positive Gibbs free energy of mixing is believed to arise from the same loss of conformational entropy that is responsible for "the hydrophobic effect" but driven by a competition for water of the two blocks. In that sense this phenomenon should be described as the "hydrophilic effect".

New Hydrogel Forming Thermo-responsive Block Copolymers of Increasing Structural Complexity

New Hydrogel Forming Thermo-responsive Block Copolymers of Increasing Structural Complexity
Author: Anna Miasnikova
Publisher:
Total Pages: 0
Release: 2012
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ISBN:
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This work describes the synthesis and characterization of stimuli-responsive polymers made by reversible addition-fragmentation chain transfer (RAFT) polymerization and the investigation of their self-assembly into "smart" hydrogels. In particular the hydrogels were designed to swell at low temperature and could be reversibly switched to a collapsed hydrophobic state by rising the temperature. Starting from two constituents, a short permanently hydrophobic polystyrene (PS) block and a thermo-responsive poly(methoxy diethylene glycol acrylate) (PMDEGA) block, various gelation behaviors and switching temperatures were achieved. New RAFT agents bearing tert-butyl benzoate or benzoic acid groups, were developed for the synthesis of diblock, symmetrical triblock and 3-arm star block copolymers. Thus, specific end groups were attached to the polymers that facilitate efficient macromolecular characterization, e.g by routine 1H-NMR spectroscopy. Further, the carboxyl end-groups allowed functionalizing the various polymers by a fluorophore. Because reports on PMDEGA have been extremely rare, at first, the thermo-responsive behavior of the polymer was investigated and the influence of factors such as molar mass, nature of the end-groups, and architecture, was studied. The use of special RAFT agents enabled the design of polymer with specific hydrophobic and hydrophilic end-groups. Cloud points (CP) of the polymers proved to be sensitive to all molecular variables studied, namely molar mass, nature and number of the end-groups, up to relatively high molar masses. Thus, by changing molecular parameters, CPs of the PMDEGA could be easily adjusted within the physiological interesting range of 20 to 40°C. A second responsivity, namely to light, was added to the PMDEGA system via random copolymerization of MDEGA with a specifically designed photo-switchable azobenzene acrylate. The composition of the copolymers was varied in order to determine the optimal conditions for an isothermal cloud point variation triggered by light. Though reversible light-induced solubility changes were achieved, the differences between the cloud points before and after the irradiation were small. Remarkably, the response to light differed from common observations for azobenzene-based systems, as CPs decreased after UV-irradiation, i.e with increasing content of cis-azobenzene units. The viscosifying and gelling abilities of the various block copolymers made from PS and PMDEGA blocks were studied by rheology. Important differences were observed between diblock copolymers, containing one hydrophobic PS block only, the telechelic symmetrical triblock copolymers made of two associating PS termini, and the star block copolymers having three associating end blocks. Regardless of their hydrophilic block length, diblock copolymers PS11 PMDEGAn were freely flowing even at concentrations as high as 40 wt. %. In contrast, all studied symmetrical triblock copolymers PS8-PMDEGAn-PS8 formed gels at low temperatures and at concentrations as low as 3.5 wt. % at best. When heated, these gels underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurs. The gel-sol transition shifted to markedly higher transition temperatures with increasing length of the hydrophilic inner block. This effect increased also with the number of arms, and with the length of the hydrophobic end blocks. The mechanical properties of the gels were significantly altered at the cloud point and liquid-like dispersions were formed. These could be reversibly transformed into hydrogels by cooling. This thesis demonstrates that high molar mass PMDEGA is an easily accessible, presumably also biocompatible and at ambient temperature well water-soluble, non-ionic thermo-responsive polymer. PMDEGA can be easily molecularly engineered via the RAFT method, implementing defined end-groups, and producing different, also complex, architectures, such as amphiphilic triblock and star block copolymers, having an analogous structure to associative telechelics. With appropriate design, such amphiphilic copolymers give way to efficient, "smart" viscosifiers and gelators displaying tunable gelling and mechanical properties.

Developments in Block Copolymer Science and Technology

Developments in Block Copolymer Science and Technology
Author: Ian W. Hamley
Publisher: John Wiley & Sons
Total Pages: 388
Release: 2004-03-05
Genre: Technology & Engineering
ISBN: 0470843357
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Focuses on recent advances in research on block copolymers, covering chemistry (synthesis), physics (phase behaviors, rheology, modeling), and applications (melts and solutions). Written by a team of internationally respected scientists from industry and academia, this text compiles and reviews the expanse of research that has taken place over the last five years into one accessible resource. Ian Hamley is the world-leading scientist in the field of block copolymer research Presents the recent advances in the area, covering chemistry, physics and applications. Provides a broad coverage from synthesis to fundamental physics through to applications Examines the potential of block copolymers in nanotechnology as self-assembling soft materials

Synthesis and Characterization of Stimuli Responsive Block Copolymers, Self-assembly Behavior and Applications

Synthesis and Characterization of Stimuli Responsive Block Copolymers, Self-assembly Behavior and Applications
Author: Michael Duane Determan
Publisher:
Total Pages: 296
Release: 2006
Genre:
ISBN:
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Water-soluble stimuli responsive block copolymers are a rapidly emerging class of materials with great potential in biomedical and technological applications. In this work a novel class of pentablock copolymers are synthesized via atom transfer radical polymerization techniques and their stimuli-responsive self-assembly properties are characterized. Aqueous solutions of these materials are observed to form micelles and hydrogels in response to changes in both temperature and pH. Cryogenic transmission electron microscopy (cryo-TEM) and small angle neutron and X-ray scattering (SANS and SAXS) techniques are used to investigate the nanoscale structures formed by these pentablock copolymers in solution. The gel structure and mechanical properties are investigated with SANS and rheological techniques. The multi-responsive properties of these materials are utilized to formulate a stimuli responsive drug delivery formulation that exhibits thermoreversible gelation and pH dependent release rate of model drugs.