Adsorption isotherms and dissipation of adsorbed poly(N-isopropylacrylamide) in its swelling and collapsed states

Poly(N-isopropylacrylamide) (PNIPAM) physisorbed on gold surfaces in aqueous solutions has been studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption isotherms of the polymer, that is, the adsorbed mass versus the concentration of PNIPAM in solution, show distinctly different behaviors at temperatures below and above a lower critical solution temperature (LCST). Below the LCST, PNIPAM forms a single compact layer in solutions with concentrations up to 100 ppm in weight; above the LCST, much thicker films of PNIPAM form in the same concentration range. Changes in the dissipation factor versus solvent concentration show a behavior similar to those in the isotherms. The difference in the adsorption behavior below and above the LCST can be qualitatively explained in terms of the conformation difference of the polymer in its swelling and collapsed states.     

Application of Ultrasonic Attenuation Measurements in the Studies on Macromolecular Conformational Behaviors

——Phase Behavior of the Aqueous Solution of Poly(vinyl methyl ether) Sensitive to Temperature and the Modification of the Behavior by Using Poly(acrylic acid) The phase behavior of the aqueous solution of poly(vinyl methyl ether) (PVME) sensitive to temperature and the modification of the behavior by using poly(acrylic acid) (PAA) have been studied by ultrasonic attenuation measurements and fluorescence probe techniques. It has been observed that PVME solution is transparent at room temperature and becomes turbid upon heating. The solution turns clear again as soon as the temperature is decreased to room temperature. The heating and cooling process can be repeated for many times. The phase behavior of the solution sensitive to temperature is attributed to the conformational changes of the polymer. PVME may adopt an open coil conformation at room temperature. With this conformation, the polymer is well miscible with the solvent, water, and thereby the system is a real solution. The polymer may adopt a compact coil conformation when the temperature is higher than a specific value, which is called the LCST (the lower critical solution temperature) of PVME. In this case, the polymer tangles to each other and forms various aggregates, which can scatter incident light and ultrasonic waves greatly, resulting in the phase separation. Introduction of PAA decreases the temperature sensitivity of the phase behavior of the polymer. The nature of the inhibition is attributed to the complexation of PAA with PVME and the strong hydrophilicity of PAA. Results from fluorescence probe studies are in accordance with those from ultrasonic attenuation measurements, indicating again that the ultrasonic attenuation method can be successfully used for the qualitative studies of polymer conformations and complexation between polymers.     

Application of Ultrasonic Attenuation Measurements in the Studies on Macromolecular Conformational Behaviors——Phase Behavior of the Aqueous Solution of Poly(vinyl methyl ether) Sensitive to Temperature and the Modification of the Behavior by Using Pol

Introduction Studies on the conformational and aggregation be-haviors of macromolecules in aqueous solution and at solid/liquid interfaces have been highlighted in colloid and interface science since the early 1990s.1-3 It was mentioned in the first part of this series of studies that development of new methods, which are characterized by being in time and non-destructive and can be used for monitoring the conformational and aggregation be-havior of macromolecules, is of critical importance fo…     

Synthesis and thermoresponsive property of end‐functionalized poly(N‐isopropylacrylamide) with pyrenyl group

N–Isopropylacrylamide (NIPAM) was polymerized using 1‐pyrenyl 2‐chloropropionate (PyCP) as the initiator and CuCl/tris[2‐(dimethylamino)ethyl]amine (Me₆TREN) as the catalyst system. The polymerizations were performed using the feed ratio of [NIPAM]₀/[PyCP]₀/[CuCl]₀/[Me₆TREN]₀ = 50/1/1/1 in DMF/water of 13/2 at 20 °C to afford an end‐functionalized poly(N‐isopropylacrylamide) with the pyrenyl group (Py–PNIPAM). The characterization of the Py–PNIPAM using matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry provided the number–average molecular weight (M_n,MS). The lower critical solution temperature (LCST) for the liquid–solid phase transition was 21.7, 24.8, 26.5, and 29.3 °C for the Py–PNIPAMs with the M_n,MS's of 3000, 3400, 4200, and 5000, respectively; hence, the LCST was dramatically lowered with the decreasing M_n,MS. The aqueous Py–PNIPAM solution below the LCST was characterized using a static laser light scattering (SLS) measurement to determine its molar mass, M_w,SLS. The aqueous solutions of the Py–PNIPAMs with the M_n,MS's of 3000, 3400, 4200, and 5000 showed the M_w,SLS of 586,000, 386,000, 223,000, and 170,000, respectively. Thus, lowering the LCST for Py–PNIPAM should be attributable to the formation of the PNIPAM aggregates. The LCST of 21.7 °C for Py–PNIPAM with the M_n,MS of 3000 was effectively raised by adding β‐cyclodextrin (β‐CD) and reached the constant value of ～26 °C above the molar ratio of [β‐CD]/[Py–PNIPAM] = 2/1, suggesting that β‐CD formed an inclusion complex with pyrene in the chain‐end to disturb the formation of PNIPAM aggregates, thus raising the LCST. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1117–1124, 2006     

Phase transition behavior of unimolecular micelles with thermoresponsive poly(N-isopropylacrylamide) coronas

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.     

Phase Separation of Poly(N‐isopropylacrylamide) in Mixtures of Water and Methanol: A Spectroscopic Study of the Phase‐Transition Process with a Polymer Tagged with a Fluorescent Dye and a Spin Label

The thermoreversible phase transition of poly(N‐isopropylacrylamide) randomly labeled with a spin label, 4‐amino‐2,2′,6,6′‐tetramethylpiperidine 1‐oxide (TEMPO), and a fluorescent dye, 4‐(pyren‐1‐yl)butyl (PNIPAM‐Py‐T), in different H₂O/MeOH mixtures was studied by turbidimetry, continuous‐wave electron paramagnetic resonance spectroscopy (CW‐EPR), and fluorescence spectroscopy. The macroscopic phase diagram of PNIPAM‐Py‐T in H₂O/MeOH measured by turbidimetry was identical to those of poly(N‐isopropylacrylamide) (PNIPAM) and of TEMPO‐labeled PNIPAM (PNIPAM‐T) in H₂O/MeOH mixtures. However, distinct differences among the three polymers were detected in their solvent‐dependent EPR and fluorescence‐spectroscopic properties. The EPR spectra were analyzed in terms of the isotropic hyperfine coupling constants, which monitor the variation in environmental polarity of the radical labels occurring for the conformational transitions of the polymer as a function of temperature, as well as the correlation time for reorientation motion, the increase of which is indicative of the increased viscosity of the radical environment and interactions occurring between the radical and other surface groups of the precipitated polymer, if compared to the soluble polymer. The fluorescence of Py in PNIPAM‐Py‐T displayed contributions from isolated excited pyrenes (monomer emission) and from preformed pyrene ground‐state aggregates (excimer emission). The quantum efficiencies of monomer and excimer emission were monitored as a function of solvent composition. By the two experimental approaches, we demonstrate the profound influence of the PNIPAM‐attached pyrene units in increasing the hydrophobicity of the nanodomains formed upon heat‐induced precipitation of PNIPAM‐Py‐T.     

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     

N-异丙基丙烯酰胺/丙烯酸胆甾醇酯共聚物研究

合成和表征了N 异丙基丙烯酰胺 (NIPAM)与丙烯酸胆甾醇酯 (CHA)的共聚物 .利用表面张力和荧光探针法研究了共聚物水溶液的表面活性性能 ,确定了其临界胶束浓度 (CMC) .利用浊度法和荧光探针法测定了共聚物的最低临界溶液温度 (LCST) .研究发现 ,在聚N 异丙基丙烯酰胺 (PNIPAM)分子链中引入疏水结构单元CHA会使其LCST下降 ;且随着共聚物中CHA含量的增加 ,LCST下降幅度增加 .在PNIPAM链段中引入少量的CHA就会使其表现出明显的两亲性 ,共聚物在水中能形成有壳核结构的稳定胶束 .通过将疏水化合物胆甾醇作为模拟药物包埋在胶束的疏水核中的研究 ,证实所得的胶束能包埋疏水药物 ,且随着包埋胆甾醇含量的增加 ,胶束平均粒径增大 .     

Phase transition of aqueous solutions of poly(N,N-diethylacrylamide-co-acrylic acid) by differential scanning calorimetric and spectrophotometric methods

 The phase transition of aqueous solutions of poly(N,N-diethylacrylamide-co-acrylic acid) (DEAAm–AA) is studied by differential scanning calorimetry (DSC) and UV–vis spectrophotometry. The copolymer aqueous solutions are shown to have well-defined lower critical solution temperatures (LCSTs). The LCST values obtained from the maximum of the first derivatives of the DSC and optical transition curves agree well. DSC can be used to measure the phase-transition temperature of more dilute polymer solutions. On increasing the AA composition in the copolymers, the LCST values of the copolymer increase, then decrease at higher AA composition. For the aqueous solution of the copolymers, the transition curve obtained by the spectrophotometric method is highly wavelength dependent. The LCST values are found to be concentration-dependent. The changes in the heat of the phase transition of the copolymer solutions measured from DSC are lower than that of the homopolymer PDEAAm solution. This is consistent with the suggestion that the polymer chains of the copolymers collapsed only partially at temperatures above the LCST. The added salt (sodium chloride) decreases the transition temperature of the polymer solution. Received: 14 November 2000 Accepted: 15 January 2001     

温度敏感性PNIPAM-b-PZLL-b-mPEG三嵌段共聚物的合成与自组装研究

通过大分子引发剂引发ε-苄氧羰基-L-赖氨酸-N-羧酸酐(Lys-NCA)开环聚合和大分子缩合的方法合成了聚(N-异丙基丙烯酰胺)-b-聚(ε-苄氧羰基-L-赖氨酸)-b-聚乙二醇单甲醚三嵌段共聚物(PNIPAM-b-PZLL-b-mPEG).用GPC和1H-NMR对其结构进行了表征.用芘荧光探针法证明了该三嵌段聚合物形成胶束的性质并测定了临界胶束浓度(CMC).动态光散射(DLS)研究表明,在固定PNIPAM-b-PZLL链段长度的情况下,mPEG分子量为2000时,胶束在温度高于临界溶解温度(LCST)时发生聚集,mPEG分子量为5000时,胶束在LCST以上没有发生聚集.     

Effect of ureas on the viscometric behavior of water-soluble isotactic poly(2-hydroxyethyl methacrylate)

Isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) is soluble and has the compact random coil structure in water solution. The effect of six ureas (thiourea, urea, methylurea, ethylurea, 1,3-dimethylurea, and tetramethylurea) on the viscometric behavior of aqueous solutions of isotactic PHEMA as a function of urea concentrations at 25°C has been investigated. The compact random coil structure is found to disappear as ureas are added. The result is discussed in terms of water structure breaking and making effect of solutes, and hydrophobic interactions. In addition, experiments were performed with aqueous isotactic PHEMA solutions at four different temperatures. Isotactic PHEMA in pure water has the most compact conformation at 20°C.     

Conformational relaxation dynamics of a poly(N-isopropylacrylamide) aqueous solution measured using the laser temperature jump transient grating method

We observed phase transition and phase relaxation processes of a poly(N-isopropylacrylamide) (PNIPAM) aqueous solution using the heterodyne transient grating (HD-TG) method combined with the laser temperature jump technique. The sample temperature was instantaneously raised by about 1.0 K after irradiation of a pump pulse to crystal violet (CV) molecules for heating, and the phase transition was induced for the sample with an initial temperature just below the lower critical solution temperature (LCST); the following phase relaxation dynamics was observed. Turbidity relaxation was observed in both the turbidity and HD-TG responses, while another relaxation process was observed only in the HD-TG response, namely via the refractive index change. It is suggested that this response is due to formation of globule molecules or their assemblies since they would have nothing to do with turbidity change but would affect the refractive index, which is dependent on the molar volume of a chemical species. Furthermore, the grating spacing dependence of the HD-TG responses suggests that the response was caused by the counter propagating diffusion of the coil molecules as a reactant species and the globule molecules as a product species and the lifetime of the globule molecules ranged from 1.5 to 5 seconds. Thus, we conclude that the turbidity reflects the dynamics of aggregate conditions, not molecular conditions. The coil and globule sizes were estimated from the obtained diffusion coefficient. The sizes of the coil molecules did not change at the initial temperatures below the LCST but increased sharply as it approaches LCST. We propose that the coil-state molecules associate due to hydrophobic interaction when the initial temperature was higher than LCST minus 0.5 K and that the globule-state molecules generated from the coil-state molecules showed a similar trend in temperature. The phase transition was also induced by heating under a microscope, and the relaxation process was followed using the fluorescence peak shift of a fluorescent molecule-labeled PNIPAM. The result also supports the existence of a globule molecule or its assembly remains for several seconds in the phase relaxation.     

"On-demand" control of thermoresponsive properties of poly(N-isopropylacrylamide) with cucurbit[8]uril host-guest complexes

The chain end complexation of a functional PNIPAM by a cucurbit[8]uril-viologen complex causes a shift in its lower critical solution temperature (LCST) by over 5 °C. An instantaneous phase change of the thermally responsive polymer beyond its LCST can be induced by addition of the aqueous cucurbituril host-guest complex. Subsequent decomplexation upon addition of a competitive guest releases the PNIPAM terminus and triggers complete reversibility.     

Molecular dynamics simulations of PNIPAM‐co‐PEGMA copolymer hydrophilic to hydrophobic transition in NaCl solution

Classical molecular dynamics simulations were carried out to investigate the hydrophilic to hydrophobic transition of PNIPAM‐co‐PEGMA close to its lower critical solution temperature (LCST) in 1 M NaCl solution. PNIPAM‐co‐PEGMA is a copolymer of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) methacrylate (PEGMA). The copolymer consists of 38 monomer units of NIPAM with two PEGMA chains attached to the PNIAPM backbone. The PNIPAM‐co‐PEGMA was observed to go through the hydrophilic?hydrophobic conformational change for simulations at temperature slightly above its LCST. Na⁺ ions were found to bind strongly and directly with amide O, even more strongly with the O atoms on PEGAMS chains, whereas Cl^? ions only exhibit weak interaction with the polymer. Significantly a novel caged stable metal‐organic complex involving a Na⁺ ion coordinated by six O atoms from the copolymer was observed after the PNIPAM‐co‐PEGMA copolymer went through conformational transition to form a hydrophobic folded structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Direct observation of the phase transition for a poly(N-isopropylacryamide) layer grafted onto a solid surface by AFM and QCM-D

The temperature-induced structural changes of a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) layer grafted onto a silica substrate were investigated in aqueous solution using an atomic force microscope (AFM) and a quartz crystal microbalance with dissipation (QCM-D). A PNIPAM layer was grafted onto the silicon wafer surface by free radical polymerization of NIPAM to obtain a high molecular weight polymer layer with low-grafting density overall. By AFM imaging, the transition of the grafted PNIPAM chains from a brush-like to a mushroom-like state was clearly visualized: The surface images of the plate were featureless at temperatures below the LCST commensurate with a brush-like layer, whereas above the LCST, a large number of domain structures with a characteristic size of approximately 100 nm were seen on the surface. Both frequency and dissipation data obtained using QCM-D showed a significant change at the LCST. Analysis of these data confirmed that the observed PNIPAM structural transition was caused by a collapse of the brush-like structure as a result of dehydration of the polymer chains.     

基于星型杂臂环糊精聚合物的纳米胶束: 构筑及包合特性

通过胺化反应和原子转移自由基聚合(ATRP),合成了以β-环糊精为“核”,以1条聚乙二醇和2~4条聚N-异丙基丙烯酰胺为“臂”的双亲水性星型杂臂聚合物(MPEG-CD-PNIPAMx)。通过1H NMR,13C NMR和凝胶渗透色谱/多角度激光光散射联用(SEC/MALLS)对其结构进行了表征。对1H NMR峰面积积分计算得聚N-异丙基丙烯酰胺“臂”数为2~4。通过紫外-可见分光光度计测得该星型大分子的较低溶液临界温度(LCST)为37℃。MPEG-CD-PNIPAMx在其水溶液温度达到LCST以上时呈现两亲性,并通过疏水相互作用自组装成以聚N-异丙基丙烯酰胺为“核”,以β-环糊精及聚乙二醇为“壳”的纳米级胶束粒子。通过MPEG-CD-PNIPAMx及其胶束粒子在芘溶液中的荧光光谱,发现胶束粒子对疏水性客体小分子的包合可发生在处于壳层的β-环糊精的疏水性空腔和胶束粒子的疏水性内核。     

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N‐isopropylacrylamide)

The lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N‐isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18‐kDa sample had a 0.6 °C higher LCST. A 56‐kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl‐containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with M_w values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006     

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.     

Saccharide‐structure effects on poly N‐isopropylacrylamide phase transition in aqueous media; Reflections on protein stability

Protein stability in aqueous solutions is important in numerous fields, particularly biotechnology and food‐science. To shed new light on the protective effect of carbohydrates on proteins, we studied saccharide‐structure effects in aqueous solutions on the coil‐to‐globule transition occurring at the lower critical solution temperature (LCST) of poly‐N‐isopropylacrylamide (PNIPA), an isomer of polyleucine, as a simple model representing certain key behaviors of proteins (e.g., denaturation/renaturation). We systematically selected sugars and polyols to relate structural and physical characteristics of these carbohydrates to their effect on PNIPA solutions. Using isothermal titration‐microcalorimetry, we showed that no significant binding of saccharides to the polymer occurs. Using micro‐DSC, we studied the decreasing polymer LCST temperature with rising carbohydrate concentration. Beyond the expected observation that steric exclusion is important, we observed previously‐unreported significant differences among the effects of isomeric aldohexoses and also among the effects of isomeric diglucoses on PNIPA LCST. We found good correlation between the sugar hydration number and its effect on LCST. We conclude that the larger and denser the hydrated cluster a carbohydrate forms, the worse a cosolvent is for the polymer, and the stronger it's lowering effect of the coil‐to‐globule transition. Such favoring of the compact globule state provides a protective effect against denaturation of globular proteins. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2307–2318, 2008     

Two-stage collapse of unimolecular micelles with double thermoresponsive coronas

Phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive coronas is studied. Successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMA) were conducted using fractionated fourth-generation hyperbranched polyester (Bolton H40) based macroRAFT agent. At lower temperatures (<20 degrees C), dendritic macromolecules H40-poly(N-isopropylacrylamide)-poly(2-(dimethylamino)ethyl methacrylate) (H40-PNIPAM-PDMA) exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core, swollen PNIPAM as the inner shell, and swollen PDMA as the corona. PNIPAM and PDMA homopolymers undergo phase transitions at their lower critical solution temperatures (LCST), which are found to be 32 degrees C for PNIPAM and 40-50 degrees C for PDMA, respectively. Upon continuously heating through the LCSTs of PNIPAM and PDMA, such dendritic unimolecular micelles exhibit two-stage thermally induced collapse. This process is reversible with a two-stage reswelling upon cooling. Laser light scattering, micro-differential scanning calorimetry, and excimer fluorescence measurements are used to investigate the double phase transitions.