Nonlinear depression of the lower critical solution temperatures in aqueous solutions of thermo-sensitive random copolymers

We develop a theoretical model of cooperative hydration to clarify the molecular origin of the observed nonlinear depression of the lower critical solution temperature (LCST) in the aqueous solutions of thermosensitive random copolymers and find the monomer composition at which LCST shows a minimum. Phase diagrams of poly(N-isopropylacrylamide-co-N,N-diethylacrylamide) copolymer solutions are theoretically derived on the basis of the theory of cooperative hydration by introducing the microscopic structure parameter η which characterizes the distribution of the monomer sequences along the chains. We compared them with the experimental data of LCST of random copolymers with various monomer compositions and also of the diblock copolymers with equimolar monomer composition. The transition temperature shifts to lower than those of homopolymer counterparts when the monomer sequence of the chains has an alternative tendency. On the contrary, for the blocky polymers such as diblock copolymers, the transition temperature remains almost the same as those of the homopolymers. Thus, the nonlinear effect in phase separation appears when the average block length of the copolymers is shorter than the average sequence length of the cooperative hydration. The degree of hydration is calculated as a function of the temperature and polymer concentration for varied distribution of the copolymer compositions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1112–1123     

温度敏感性P(MEO2MA-co-OEGMA)共聚物的合成与性质

利用原子转移自由基聚合(ATRP)方法合成了组成递变的2-甲基-2-丙烯酸-2-(2-甲氧基乙氧基)乙酯(MEO₂MA)与寡聚乙二醇甲醚甲基丙烯酸酯(OEGMA)共聚物P(MEO₂MA-co-OEGMA). 核磁共振氢谱(¹HNMR)和凝胶渗透色谱(GPC)表征了聚合物的结构、分子量及其分布. 通过测定透光率、粘度、激光粒度分析了共聚物组成对共聚物低临界溶解温度(LCST)的影响, 考察了共聚物组成、浓度、盐浓度、盐种类、温度对其溶液相行为的影响. 结果表明: 所合成的共聚物具有温度敏感性, 其LCST 可以通过合成时共聚单体MEO₂MA与OEGMA投料比的改变来调控, 随着OEGMA量的增加共聚物的LCST升高, 共聚物溶液浓度升高其LCST减小, 随盐溶液浓度的增大共聚物的LCST降低, 共聚物的LCST降低主要受盐溶液中阴离子价数的影响; HCl的引入使共聚物水溶液的LCST降低; NaOH的引入使共聚物水溶液的LCST升高.     

The Interaction Between N–Isopropylacrylamide-Acrylic Acid-Ethyl Methacrylate Thermosensitive Polymers and Cationic Surfactants

The interaction between cationic surfactants and isopropylacrylamide-acrylic acid-ethyl methacrylate (IPA:AA:EMA) terpolymers has been investigated using steady-state fluorescence and spectrophotometric measurements to assess the effect of the polymer composition on the aggregation process and terpolymers’ thermosensitivities. Micropolarity studies using pyrene show that the interaction of cationic surfactants with IPA:AA:EMA terpolymers occurs at surfactant concentrations much smaller than that observed for the pure surfactant in aqueous solution. The critical aggregation concentration (CAC) values decrease with both the hydrocarbon length of the surfactant and the content of ethyl methacrylate. These results were interpreted as a manifestation of the increasing contribution of attractive hydrophobic and electrostatic forces between negatively charged polymer chains and positively charged surfactant molecules. The increase of ethyl methacrylate in the copolymers lowers the CAC due to the larger hydrophobic character of the polymer backbone. The cloud point determination reveals that the lower critical solution temperatures (LCST) depend strongly on the copolymer composition and surfactant nature. The binding of surfactants molecules to the polymer chain screens the electrostatic repulsion between the carboxylic groups inducing a conformational transition and the dehydration of the polymer chain.     

Synthesis,thermo-responsive micellization and caffeine drug release of novel PBMA-<Emphasis Type="Italic">b</Emphasis>-PNIPAAm block polymer brushes

Brush-like block copolymers with poly(t-butyl methacrylate) (PBMA) and poly(N-isopropylacrylamide) (PNIPAAm) as side arms, PBMA-b-PNIPAAm, were designed and synthesized via a simple free radical polymerization route. The chemical structure and molecular weight of these polymer brushes were characterized and determined by nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectrometry (FTIR) and gel permeation chromatography (GPC). The micellar formation by these polymer brushes in aqueous solutions were detected by a surface tension technique, and the critical micelle concentration (CMC) ranged from 1.53 to 8.06 mg L⁻¹. The morphology and geometry of polymer micelles were investigated by transmission electron microscope (TEM) and dynamic light scattering (DLS). The polymer micelles assume the regularly-spherical core-shell structure with well-dispersed individual nanoparticles, and the particle size was in the range from 36 to 93 nm. The PNIPAAm segments exhibited a thermoreversible phase transition, so the resulting block polymer brushes were temperature-sensitive and the low critical solution temperature (LCST) was determined by UV-vis spectrometer at about 28.82–29.40°C. The characteristic parameters of the polymer micelles such as CMC, micellar size and LCST values were affected by their compositional ratios and the length of hydrophilic or hydrophobic chains. The evaluation for caffeine drug release behavior of the block polymer micelles demonstrated that the self-assembled micelles exhibited thermal-triggered properties in controlled drug release.     

N‐Isopropylacrylamide Copolymers with Acrylamide and Methacrylamide Derivatives of Cholic Acid: Synthesis and Characterization

By free radical polymerization, we have prepared a series of water‐soluble, thermosensitive copolymers based on N‐isopropylacrylamide and (meth)acrylamide derivatives of cholic acid, one of the bile acids. The copolymers contained 1–7 mol‐% of the (meth)acrylamide derivatives of cholic acid. The chemical composition in the copolymers was studied by NMR spectroscopy and was found to be close to the original composition of the comonomers in the feed prior to polymerization. The lower critical solution temperatures (LCST) of the polymer solutions were measured by means of differential scanning calorimetry and turbidimetry. The resulting copolymers exhibit systematic changes in their LCSTs as a function of their chemical composition, as the incorporation of hydrophobic comonomers leads to a lower LCST.     

Temperature‐induced aggregation of the copolymers of N‐isopropylacrylamide and sodium 2‐acrylamido‐2‐methyl‐1‐propanesulphonate in aqueous solutions

A series of random copolymers of N‐isopropylacrylamide (NIPAM) and sodium 2‐acrylamido‐2‐methyl‐1‐propanesulphonate (AMPS) was synthesized by free‐radical copolymerization. The content of AMPS in the copolymers ranged from 1.1 to 9.6 mol %. The lower critical‐solution temperature (LCST) of copolymers in water increased strongly with an increasing content of AMPS. The influence of polymer concentration on the LCST of the copolymers was studied. For the copolymers with a higher AMPS content, the LCST decreased faster with an increasing concentration than for copolymers with a low content of AMPS. For a copolymer containing 1.1 mol % of AMPS the LCST dropped by about 3 °C when the concentration increased from 1 to 10 g/L, whereas for a copolymer containing 9.6 mol % of AMPS the LCST dropped by about 10 °C in the concentration range from 2 to 10 g/L. It was observed that the ionic strength of the aqueous polymer solution very strongly influences the LCST. This effect was most visible for the copolymer with the highest content of AMPS (9.6 mol %) for which an increase in the ionic strength from 0.2 to 2.0 resulted in a decrease in the LCST by about 27 °C (from 55 to 28 °C), whereas for the copolymer containing 1.1 mol % of AMPS the LCST decreased only by about 6 °C (from 37 to 31 °C) when the ionic strength increased from 0.005 to 0.3. The reactivity ratios for the AMPS and NIPAM monomer pairs were determined using different methods. The values of r_AMPS and r_NIPAM obtained were 11.0–11.6 and 2.1–2.4, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2784–2792, 2001     

Thermosensitive copolymers having soluble and insoluble monomer units,poly(N‐vinylacetamide‐co‐methyl acrylate)s: Effect of additives on their lower critical solution temperatures

New thermosensitive polymers were synthesized by copolymerization between N‐vinylacetamide (NVA) and methyl acrylate whose homopolymers are soluble and insoluble in water, respectively. The lower critical solution temperature (LCST) of the obtained copolymers ranged between 59 and 83 °C, and the LCST increased with an increasing NVA content in the copolymers. The effectiveness of various salts addition on lowering the LCST of the copolymer solutions followed Hoffmeister series. NaCl and Na₂SO₄ addition linearly lowered the LCST with an increasing salts concentration, and slopes of the lines were almost constant regardless of the copolymer composition. The effectiveness of alcohols with various alkyl chain lengths on lowering the LCST did not follow the viscosity B coefficient values of the alcohols, which was probably the result of preferential adsorption of the alcohols to the copolymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2651–2658, 2004     

Modifying the thermosensitivity of copolymers of sodium styrene sulfonate and<Emphasis Type="Italic"> N</Emphasis>-isopropylacrylamide with dodecyltrimethylammonium chloride

The interactions between copolymers of sodium styrene sulfonate (SSS) and N-isopropylacrylamide (NIPAM), anionic polyelectrolytes, and dodecyltrimethylammonium chloride (DTAC), a cationic surfactant, were studied in aqueous solutions of various ionic strengths. The copolymers were found to be thermoresponsive, showing a lower critical solution temperature (LCST). The influence of the polymer composition, the surfactant concentration, and the ionic strength on the LCST was studied. The surfactant was found to interact strongly with the polymer, forming mixed polymer-surfactant micelles. The critical aggregation concentration (cac) of the polymer-surfactant system was found from fluorescence spectroscopy using pyrene as a fluorescent probe. A strong dependence of the anionic polyelectrolyte-cationic surfactant interactions on the structure of the ionic comonomer was observed.     

Synthesis of poly[N‐isopropylacrylamide‐g‐poly(ethylene glycol)] with a reactive group at the poly(ethylene glycol) end and its thermosensitive self‐assembling character

Poly[N‐isopropylacrylamide‐g‐poly(ethylene glycol)]s with a reactive group at the poly(ethylene glycol) (PEG) end were synthesized by the radical copolymerization of N‐isopropylacrylamide with a PEG macromonomer having an acetal group at one end and a methacryloyl group at the other chain end. The temperature dependence of the aqueous solutions of the obtained graft copolymers was estimated by light scattering measurements. The intensity of the light scattering from aqueous polymer solutions increased with increasing temperature. In particular, at temperatures above 40°C, the intensity abruptly increased, indicating a phase separation of the graft copolymer due to the lower critical solution temperature (LCST) of the poly(N‐isopropylacrylamide) segment. No turbidity was observed even above the LCST, and this suggested a nanoscale self‐assembling structure of the graft copolymer. The dynamic light scattering measurements confirmed that the size of the aggregate was in the range of several tens of nanometers. The acetal group at the end of the PEG graft chain was easily converted to the aldehyde group by an acid treatment, which was analyzed by ¹H NMR. Such a temperature‐induced nanosphere possessing reactive PEG tethered chains on the surface is promising for new nanobased biomedical materials. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1457–1469, 2006     

Effect of urea on the conformational behavior of poly(N-isopropylacrylamide)

The effect of urea on the conformational behavior of poly(N-isopropylacrylamide) (PNIPAM) in dilute aqueous solution has been investigated using fluorescence spectroscopy, fluorescence quenching and fluorescence anisotropy measurements via pyrene (Py) probe and acenaphthylene (ACE) label studies. It was demonstrated that urea promotes the partitioning of the hydrophobic probe, Py, towards the bulk aqueous phase at temperatures above the lower critical solution temperature (LCST) of the polymer due to swelling of the compact coil conformation. However, the compact coil structure of the polymer at temperatures greater than its LCST is not completely destroyed, even for urea concentrations up to 3 M, at which the phase transition is hardly observed. As expected, urea has little effect on the conformational behavior of PNIPAM at temperatures below its LCST. Received: 9 February 2000/Accepted: 13 June 2000     

Study of the phase transition of poly(N,N‐diethylacrylamide) in water by rheology and dynamic light scattering

Poly(N,N‐diethylacrylamide) (PDEA) possesses a lower critical solution temperature (LCST) in aqueous media. The solution properties of PDEA at various temperatures have been characterized with techniques such as rheology and dynamic light scattering. There is a decrease in the coil size before the phase transition due to a coil‐to‐globule transition. At the LCST, rheological and dynamic light scattering studies have also confirmed an aggregation phenomenon. This aggregation modifies the rheological properties of the polymer solutions. High frequencies hinder the phase‐transition process and reduce the LCST of the polymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1627–1637, 2003     

荧光探针法对N-异丙基丙烯酰胺-丙烯酰胺共聚物水溶液相变的研究

合成了多种不同配比的N 异丙基丙烯酰胺 (NIPAM )与丙烯酰胺 (AM )的共聚物 .用荧光探针法研究了共聚物溶液荧光光谱随温度升高而引起的变化 .研究表明 :共聚物的最低临界溶液温度 (LCST)取决于AM与NIPAM的组成比 .AM的比例越大 .LCST就越高 ,且有较好的正比关系 .通过以共价键连接于共聚物的荧光探针法 (标记法 )和以小分子探针混入共聚物水溶液的方法 (混入法 )测定上述体系LCST的结果比较 ,发现标记探针法具有较高的灵敏度 ,更适宜用来研究共聚物水相体系的相变问题 .     

Poly(N‐phenylmaleimide‐co‐acrylic acid)–copper(II) and poly(N‐phenylmaleimide‐co‐acrylic acid)–cobalt(II) complexes: Synthesis,characterization, and thermal behavior

The free‐radical copolymerization of N‐phenylmaleimide (N‐PhMI) with acrylic acid was studied in the range of 25–75 mol % in the feed. The interactions of these copolymers with Cu(II) and Co(II) ions were investigated as a function of the pH and copolymer composition by the use of the ultrafiltration technique. The maximum retention capacity of the copolymers for Co(II) and Cu(II) ions varied from 200 to 250 mg/g and from 210 to 300 mg/g, respectively. The copolymers and polymer–metal complexes of divalent transition‐metal ions were characterized by elemental analysis, Fourier transform infrared, ¹H NMR spectroscopy, and cyclic voltammetry. The thermal behavior was investigated with differential scanning calorimetry (DSC) and thermogravimetry (TG). The TG and DSC measurements showed an increase in the glass‐transition temperature (T_g) and the thermal stability with an increase in the N‐PhMI concentration in the copolymers. T_g of poly(N‐PhMI‐co‐AA) with copolymer composition 46.5:53.5 mol % was found at 251 °C, and it decreased when the complexes of Co(II) and Cu(II) at pHs 3–7 were formed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4933–4941, 2005     

Thermo and pH sensitive fluorescent polymer sensor for metal cations in aqueous solution

A new monomeric fluorophore N‐acryl‐N'‐(quinolin‐8‐yl)thiourea (AQT) was synthesized. Free radical copolymerization was carried out in order to prepare a water‐soluble fluorescent copolymer, based on N‐isopropylacrylamide. The fluorescent characteristic of the aqueous solutions of copolymers was investigated both in varied pH and in the presence of metal cations. The polymer‐ contained AQT was found to be a selective chemosensor for Ni, Pb, Co ions especially for Co ion. The obtained copolymers show thermo‐sensitive lower critical solution temperature (LCST) ranging from 32.5 to 37.5°C with varied ratios of AQT, N, N‐dimethylacrylamide and N‐isopropylamide. Copyright © 2007 John Wiley & Sons, Ltd.     

A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N‐isopropylacrylamide copolymers and related hydrogels

Linear and crosslinked polymers based on N‐isopropylacrylamide (NIPAAm) exhibit unusual thermal properties. Aqueous solutions of poly(N‐isopropylacrylamide) (PNIPAAm) phase‐separate upon heating above a lower critical solution temperature (LCST), whereas related hydrogels undergo a swelling–shrinking transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinimide (98/2 mol/mol) and two hydrogels with different hydrophilicities were prepared. Fourier transform infrared (FTIR) spectroscopy was employed to determine the transition temperature and provide insights into the molecular details of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the determination of the transition temperature for both the linear and crosslinked polymers. The transition temperatures for PNIPAAm and the gel resulting from the crosslinking with polylysine or N,N′‐methylenebisacrylamide (MBA) were in the same range, 30–35 °C. For the gels, the transition temperature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydrogels, implying a similar molecular reorganization during the transition. The C H stretching region suggests that the N‐isopropyl groups and the backbone both underwent conformational changes and became more ordered upon heating above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecular hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LCST of a gel crosslinked with MBA. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 907–915, 2000     

Microphase separation of cationic poly(N-isopropylacrylamide) copolymers in water: Effect of the migration of charges

The structures of aqueous copolymer solutions have been examined through small angle neutron scattering. The copolymers contained mostly N-isopropylacrylamide (NIPAM) monomers. Poly (NIPAM) solutions have a lower critical solution temperature (LCST), above which the macromolecules separate from water. A small fraction of ionizable N,N-[(dimethylamino) propyl] methacrylamide (MADAP) monomers was introduced into the macromolecules. This had dramatic consequences on the solution behavior at temperatures above the LCST of PNIPAM, where phase separation would have been expected for the homopolymer. When all MADAP monomers were ionized, it was found that the solutions resisted the phase separation. At short spatial scales, the chains were collapsed but at large scales they formed branched aggregates that did not separate out of water. When only half of the MADAP monomers are ionized, the electrical charges were able to redistribute themselves along the chains. In this case, the rise in temperature caused a microphase separation where the electrical charges were relocated on a fraction of the chains that remained in solution.The other chains (or section of chains) formed large nodules of a polymer rich phase.     

Ionic effects on the behavior of thermoresponsive PEO–PNIPAAm block copolymers

The temperature‐dependent aggregation and recovery of the copolymer poly(ethylene oxide)₂₂‐b‐poly(N‐isopropylacrylamide)₂₉ with a C₁₂ end‐cap in aqueous solutions of salts and acids are investigated. Salt solutions affected the critical aggregation temperature of the copolymer in a manner predictable according to the Hofmeister series, with the kosmotropic adipic ion lowering the critical aggregation temperature and the chaotropic iodide raising it. Also, both salts and acids increased the size of copolymer aggregates formed with heating, due to the electrostatic shielding of aggregated structures provided by the electrolytes. Additionally, the presence of ionic additives caused a thermohysteretic increase in the size of copolymer aggregates with temperature cycling. The transitions of polymer structure with increasing temperature were surprisingly sharp with the C₁₂ end‐cap present, and particularly broad in samples in which the end cap had been cleaved. This observation suggested that the hydrophobic end group was responsible for imparting some degree of order to the polymer at low temperatures, which allowed for rapid reconfiguration with increasing temperature. Finally, in addition to the transitions expected from the least critical solution temperature behavior of the polymer blocks, we have observed an unexpected additional transition which we attribute to the contraction of the poly(ethylene oxide) chains of the copolymer aggregates at higher temperatures. This work illustrates the importance of considering the environment and composition of thermoresponsive block copolymers in certain applications, particularly in solutions with even modest electrolyte concentrations (1–10 mM), as it can have a profound effect on transition temperatures and morphology. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 507–516     

Random copolymers of N-isopropylacrylamide and methacrylic acid monomers with hydrophobic spacers: pH-tunable temperature sensitive materials

Two series of random copolymers of N-isopropylacrylamide (NIPAAm) and comonomers derived from methacrylic acid with a different number of methylene units as hydrophobic spacers (n=4, 7 and 10) were synthesized via free radicals. The first series was prepared having the acid groups methoxy-protected while the second series have the acid groups free. The NIPAAm-copolymers of both series were prepared varying the comonomer content from 5 to 20 mol%. All the copolymers were characterized by FTIR, DSC, ¹H NMR and SLS. The aqueous solution behaviour of the copolymers having methoxy-protected acid groups shows that the comonomer increases the hydrophobicity of the copolymer chain and decreases its lower critical solution temperature (LCST). For the copolymers having free acid groups, hydrogen-bonding is responsible for a further decrease in the LCST of these copolymers in pure water. In buffer solutions, every acid comonomer have a critical ionization degree (α_crit) above which the LCST increases with increasing comonomer content while at an ionization degree lower than α_crit the LCST decreases with increasing comonomer content. In dependence of comonomer content, number of methylene units in the spacer and the pH of the buffer solution, the LCST of the copolymers can be varied widely, showing that these random copolymers have pH-tunable temperature sensitivity.     

Synthesis and characterization of poly(p-arylene sulfide ketone/Schiff base) copolymers

Poly(p-arylene sulfide ketone/Schiff base) copolymers(PASK/SB) were prepared by solution polycondensation of 4,4’-diflurobenzophenone(DFBP) and N-phenyl(4,4’-diflurodiphenyl) ketimine(DFBI) with sodium sulfide in the presence of sodium hydroxide under normal pressure.Elemental analyses,FT-IR,NMR,DSC,TGA and XRD were used to characterize the resultant copolymers.It was found that the copolymers had good thermal properties with glass transition temperature(T_g) of 155.0-172.0°C,melting temperature(T_m) of 298-344°C,5%weight loss temperatures(T_d) of 471.0-501.5°C.These copolymers were almost amorphous with the content of DFBI beyond 30%.The polymer with 100% DFBI had excellent solubility,and it could dissolve in some solvents such as tetrahydrofuran(THF) and N-methyl-2- pyrrolidone(NMP).The processability of polymers was improved.Meantime the viscosity of PASK made from hydrolysis of PASK/SB(H-PASK/SB) was greatly improved from 0.135 dL/g to 0.605 dL/g.     

Synthesis and characterization of temperature‐sensitive block‐graft PNiPAAm‐b‐(PαN3CL‐g‐alkyne) copolymers by ring‐opening polymerization and click reaction

This study synthesized thermo‐sensitive amphiphilic block‐graft PNiPAAm‐b‐(PαN₃CL‐g‐alkyne) copolymers through ring‐opening polymerization of α‐chloro‐ε‐caprolactone (αClCL) with hydroxyl‐terminated macroinitiator poly(N‐isopropylacrylamide) (PNiPAAm), substituting pendent chlorides with sodium azide. This was then used to graft various kinds of terminal alkynes moieties by means of the copper‐catalyzed Huisgen's 1,3‐dipolar cycloaddition (“click” reaction). ¹H NMR, FTIR, and gel permeation chromatography (GPC) was used to characterize these copolymers. The solubility of the block‐graft copolymers in aqueous media was investigated using turbidity measurement, revealing a lower critical solution temperature (LCST) in the polymers. These solutions showed reversible changes in optical properties: transparent below the LCST, and opaque above the LCST. The LCST values were dependant on the composition of the polymer. With critical micelle concentrations (CMCs) in the range of 2.04–9.77 mg L^?1, the block copolymers formed micelles in the aqueous phase, owing to their amphiphilic characteristics. An increase in the length of hydrophobic segments or a decrease in the length of hydrophilic segments amphiphilic block‐graft copolymers produced lower CMC values. The research verified the core‐shell structure of micelles by ¹H NMR analyses in D₂O. Transmission electron microscopy was used to analyze the morphology of the micelles, revealing a spherical structure. The average size of the micelles was in the range of 75–145 nm (blank), and 105–190 nm (with drug). High drug entrapment efficiency and drug loading content were observed in the drug micelles. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011