Spectral interpretation of thermally irreversible recovery of poly(N-isopropylacrylamide-co-acrylic acid) hydrogel

The thermally induced volume phase transition process of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) hydrogel is studied using FT-IR spectroscopy in combination with the perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2Dcos) analysis. According to PCMW spectra analysis, an elevation of volume phase transition temperature (VPTT) due to an extra equilibrium of repulsive electrostatic interactions of acrylic acid moieties in hydrogel from 34 °C to ca. physiological temperature (37 °C) is determined. 2Dcos helps us to conclude that the dehydration of hydrogel responds earlier in the process of network collapse than hydrogen bond variations of AA and NIPAM moieties during heating, while the hydrogen bonds of NIPAM and AA moieties change before the network swelling in the cooling process. Furthermore, relatively stable inner hydrogen bonds of AA moieties restrict the complete expansion of PNIPAM-co-AA hydrogel, resulting in a unique irreversible recovery during cooling.     

Thermal inverse phase transition of azobenzene hydroxypropylcellulose in aqueous solutions

Water soluble 2-azobenzenoxy-ethoxy-hydroxpropylcelluloses (azo-EHPC) were synthesized by etherification reaction of bromoethoxy-azobenzene (BEA) with hydroxypropylcellulose (HPC) to study their phase transition behavior in aq. solution. The degree of substitution (DS) of the water soluble azo-EHPCs was less than 0.066. Their chemical structure and thermal property were characterized by proton nuclear magnetic resonance (¹H-NMR), fourier transform infrared spectroscopy (FT-IR), and differential scanning calorimetry. The azo-EHPC showed a reversible sol–gel transition behavior in its aq. solution, i.e. a clear azo-EHPC aq. solution became turbid when the solution temperature surpassed a lower critical solution temperature (LCST). The sol–gel transition phenomenon was investigated by optical microscopy and turbidimetric measurement. It was found that the LCST was related to the cis-/trans- conformation of the azobenzene side group, the type of cyclodextrin (CD), concentration of azo-EHPC, and NaCl concentration. The LCST of azo-EHPC was lower than that of HPC (36.6 °C) by at most 13.6 °C, and the LCST of trans-azo-EHPC was less than that of cis-azo-EHPC by ca. 3 °C. Additionally, the presence of CD in solutions displayed a positive effect on the LCST, i.e. increasing the LCST by 3–5 °C. And this impact was more profound on the azo-EHPC with higher DS values. The thermoreversible phase transition mechanism was discussed. We proposed that the effect of DS, conformation of azobenzene group, azo-EHPC concentration, salt concentration, and CD on the LCST of azo-EHPCs was a rearrangement of the hydrophilic/hydrophobic interaction between side azobenzene groups and water molecules.     

Hydration characterization of N-vinylcaprolactam polymers by absorption millimeter-wave measurements

Water molecules in the polymer hydration shell known as bound water lose their mobility in comparison with unperturbed water. This effect was quantified by absorption measurements in the millimeter-wave range of microwaves (1–10 mm, 30–300 GHz). Hydration measurements were performed for poly(N-vinylcaprolactam) (PNVCL) and copolymers of N-vinylcaprolactam (NVCL) and N-vinylimidazol (NVIA). The association of hydrophobic groups in PNVCL upon coil-to-globule transition was found to cause a decrease in the relative hydration number, which is the relative amount of bound water per solute molecule as measured by microwave method at 31 GHz. Millimeter-wave hydration measurements were confirmed by the determinations of specific heat capacity (c _p) with differential scanning calorimetry. Hydration determinations of NVCL/NVIA copolymers revealed that they associate via hydrophobic clustering with a decrease in hydration of hydrophobic groups.     

Investigation of water diffusion process in ethyl cellulose-based films by attenuated total reflectance Fourier transform infrared spectroscopy and two-dimensional correlation analysis

Two-dimensional correlation spectroscopy (2Dcos) analysis on the time-resolved attenuated total reflectance Fourier transform infrared spectroscopy has been employed to investigate the diffusion behavior of water in ethyl cellulose/triethyl citrate (EC/TEC) films with varied TEC content. The diffusion coefficients of water in the EC-based films are calculated from the diffusion curves according to the Fickian Diffusion Model. And they are observed to increase with TEC content, possibly caused by the increased free volume in the film matrix. In the 2Dcos analysis, the broad O–H stretching vibration region is split into at least four bands, which can be assigned to four different states of water molecules, that is, bulk water, cluster water, relatively free water and free water. Moreover, it is found that as water molecules disperse into the EC-based films, cluster water with moderate hydrogen bonds diffuses faster than bulk water with strong hydrogen bonds. The relatively free water and free water are formed during the diffusion process due to their interactions with the films matrix, which makes water molecules confined and restricted in limited space.     

Spectral insights into microdynamics of thermoresponsive polymers from the perspective of two-dimensional correlation spectroscopy

Generalized two-dimensional correlation spectroscopy (2DCOS) and its derivate technique,perturbation correlation moving window (PCMW),have found great potential in studying a series of physico-chemical phenomena in stimuli-responsive polymeric systems.By spreading peaks along a second dimension,2DCOS can significantly enhance spectral resolution and discern the sequence of group dynamics applicable to various external perturbation-induced spectroscopic changes,especially in infrared (IR),near-infrared (NIR) and Raman spectroscopy.On the basis of 2DCOS synchronous power spectra changing,PCMW proves to be a powerful tool to monitor complicated spectral variations and to find transition points and ranges.This article reviews the recent work of our research group in the application of 2DCOS and PCMW in thermoresponsive polymers,mainly focused on liquid crystalline polymers and lower critical solution temperature (LCST)-type polymers.Details of group motions and chain conformational changes upon temperature perturbation can thus be elucidated at the molecular level,which contribute to the understanding of their phase transition nature.     

Hydration characterization of hydrophobically modified polymers by dielectric measurements in the millimeter range

Bound water molecules, the fraction of water molecules that are present in the hydration shells of polymers, lose their rotational mobility in the time scale of 10-50 ps, leading to additional ordering compared with unperturbed water. The amount of bound water per number of carbons in the nonpolar groups as measured by the method of waveguide dielectric resonance increased in the following order: hydrophobically modified chitosan < globular protein < hydrophobically modified polyacrylamide. The hydrophobic modification of acrylamide and chitosan polymers gave smaller variation in hydration than did both the coil-to-globule transition of poly(N-isopropylacrylamide) and the masking of nonpolar groups within the protein globule.     

Thermoreversible gelation of poly(ethylene oxide) biodegradable polyester block copolymers. II

Poly(ethylene oxide)-b-poly(L-lactic acid) (PEO-PLLA) diblock copolymers were synthesized via a ring opening polymerization from poly(ethylene oxide) and l -lactide. Stannous octoate was used as a catalyst in a solution polymerization with toluene as the solvent. Their physicochemical properties were investigated by using infrared spectroscopy, ¹H-NMR spectroscopy, gel permeation chromatography, and differential scanning calorimetry, as well as the observational data of gel-sol transitions in aqueous solutions. Aqueous solutions of PEO-PLLA diblock copolymers changed from a gel phase to a sol phase with increasing temperature when their polymer concentrations are above a critical gel concentration. As the PLLA block length increased, the gel-sol transition temperature increased. For comparison, diblock copolymers of poly(ethylene oxide)-b-poly(l -lactic acid-co-glycolic acid) [PEO-P(LLA/GA)] and poly(ethylene oxide)-b-poly(dl -lactic acid-co-glycolic acid) [PEO-P(DLLA/GA)] were synthesized by the same methods, and their gel-sol transition behaviors were also investigated. The gel-sol transition properties of these diblock copolymers are influenced by the hydrophilic/hydrophobic balance of the copolymer, block length, hydrophobicity, and stereoregularity of the hydrophobic block of the copolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2207–2218, 1999     

高分子研究中的一个知识问题——柔性链“从线团到小球”的构象变化

一条大分子链由许多小分子通过共价键连接而成.正是这一"连接"导致了大分子一些独特的物理性质及相关问题.本文希望阐明的就是这样一个小分子物理中没有的知识问题:小分子在溶剂中仅有溶或不溶2个状态;而自60年代起,理论学家们就预言一条柔性大分子链在溶解的状态下,其构象随着溶剂性质变差可以从无规线团蜷缩成一个单链小球.为了证明这一构象变化,实验学家们从70年代末起进行了大量的研究,直至90年代初期仍未观察到稳定的单链蜷缩小球.实验上这一长期悬而未解的问题困惑着众多研究者.甚至有理论学家在1993年报道,当今的样品制备和实验手段无法观察到一个热力学稳定的单链蜷缩小球.中国钱人元先生和一些其他研究者自80年代末期也开始关注与单链有关的问题.我们实验室从1993年开始另辟蹊径,通过制备和采用窄分布的热敏性水溶性高分子超长链,终于在1995年利用激光光散射首次观察到理论上预测的"线团到小球"的构象变化.随后,又揭示了变化过程中存在着一个全新的"融化球"构象以及在单链蜷缩小球中并无理论上预计的额外链互穿和打结.从得到的稳定单链蜷缩小球出发,我们又首次在实验上研究了"小球到线团"的过程,意外地观察到其在准理想状态附近滞后于"线团到小球"的构象变化,并证明该滞后可归于链蜷缩过程中形成的额外链内氢键.最后,借用红外纳秒脉冲激光加热的方法研究了"线团到小球"的蜷缩动力学,并发现其包含了在单个高分子链上"成核"和"粗化"先后2个过程.其中,"成核"过程与链长无关.经过近20年的努力,我们终于基本解决了这一近代高分子物理研究中与知识有关的重要问题,揭示了与其相关的一些大分子特有的物理性质.     

Synthesis of the (hydroxypropyl)cellulose–titania hybrid nanocomposite as seen by Raman spectroscopy

The nanocomposite of the (hydroxypropyl)cellulose–titania hybrid was prepared using the (hydroxypropyl)cellulose (HPC) and (tetra‐isopropyl)orthotitanate (TIPT) modified by the methacrylic acid (MAA) as a sol–gel precursor. The Raman investigations of the TIPT with MAA mixtures revealed that these mixtures formed an intermolecular complex determined by the non‐hydrolytic condensation of the constituents. In spite of this, the MAA/TIPT precursor can be used as a solvent for the HPC. The high viscosity of the homogenous liquid of HPC in the TIPT/MAA system was obtained and the sol–gel process under an influence of the moisture from the air could be conducted. The Raman investigations of HPC–titania hybrid showed octahedrally coordinated titania atoms [TiO₆] embodied in the HPC environment. Although the chelating bond between the octahedrally coordinated titania [TiO₆] and the MAA still occurred. The nanosize properties of TiO₂ prepared by calcinations of HPC–titania hybrids were studied by the Raman spectroscopy, X‐ray investigations and the scanning electron microscopy (SEM). Copyright © 2007 John Wiley & Sons, Ltd.     

Tube assembly built by cholesterol-based organogel

In this paper, tubular structure has been obtained by self-assembly of cholesterol derivative via sol–gel process. The tube structure and the formation mechanism have been studied and certified by scanning electron microscopy, infrared, ultraviolet-visible spectroscopy (UV–vis) and x-ray diffraction (XRD) data. It is predicated that the tube has been formed by the curl of sheet structure, and the aggregates are composed of two molecules of 1 with H aggregate in laminar structure at micro level.     

Attenuated total reflectance/fourier transform infrared studies on the phase‐separation process of aqueous solutions of poly(n‐isopropylacrylamide)

Temperature‐induced phase separation of poly(N‐isopropylacrylamide) in aqueous solutions was studied by attenuated total reflectance (ATR)/Fourier transform infrared spectroscopy. The main objectives of the study were to understand, on a molecular level, the role of hydrogen bonding and hydrophobic effects below and above the phase‐separation temperature and to derive the scenario leading to this process. Understanding the behavior of this particular system could be quite relevant to many biological phenomena, such as protein denaturation. The temperature‐induced phase transition was easily detected by the ATR method. A sharp increase in the peaks of both hydrophobic and hydrophilic groups of the polymer and a decrease in the water‐related signals could be explained in terms of the formation of a polymer‐enriched film near the ATR crystal. Deconvolution of the amide I and amide II peaks and the O? H stretch envelope of water revealed that the phase‐separation scenario could be divided, below the phase‐separation temperature, into two steps. The first step consisted of the breaking of intermolecular hydrogen bonds between the amide groups of the polymer and the solvent and the formation of free amide groups, and the second step consisted of an increase in intramolecular hydrogen bonding, which induced a coil–globule transition. No changes in the hydrophobic signals below the separation temperature could be observed, suggesting that hydrophobic interactions played a dominant role during the aggregation of the collapsed chains but not before. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1665–1677, 2001     

An insight into the sequential order in 2D correlation spectroscopy using polymer transitions: Boltzmann Sigmoid,Gaussian Cumulative,Lorentz Cumulative,and Asymmetric Sigmoid. Findings in experiments and simulations

In this paper, we found the curves of infrared spectral intensity at specific wavenumbers of several polymer transitions can be accurately fitted by one of Boltzmann Sigmoid, Gaussian Cumulative, Lorentz Cumulative, or Asymmetric Sigmoid functions. These transitions include the melting of iPP, the Brill transition of PA66, the epoxy curing, the oxidation of SBS, and the melting of HDPE. These functions were obviously different from other important functions, which were earlier introduced into generalized 2D correlation spectroscopy, such as sinusoidal, exponential, and Lorentzian. The properties of the Boltzmann Sigmoid, Gaussian Cumulative, Lorentz Cumulative functions were studied using the simulated infrared spectra. The sequential order is only controlled by the parameter reflecting the center point location, while other parameter values have no relationship. The influences of the parameters in Asymmetric Sigmoid on the sequential order were also studied using the simulated IR spectra. Within the transition range, it was found the values of several waveform parameters co-determine the sequential order. We concluded that the MW2D or PCMW2D method should first be employed to determine a rational transition range before using 2D correlation infrared spectroscopy to study the mechanism of the polymer transitions. The clear physical meaning of the sequential order is the “earlier” or “later” of the transition points. As long as the experimental range (external perturbation) is wide enough and the data precision is good, the sequential order is absolutely reliable within the transition range. The results discussed throughout this paper have proven that the sequential order rules are absolutely correct. The content of the present study will solve the controversy on the sequential order rules to a large extent.     

Organic–inorganic nanocomposites of (2‐hydroxypropyl)cellulose as a precursor of nanocrystalline zinc oxide layers

The sol–gel method of synthesis of the hybrid nanocomposite films of ZnO/(2‐hydroxypropyl) cellulose (HPC) on silica glass is presented. The sol phases were prepared for different weight ratios of zinc acetate dihydrate to HPC in the presence of triethylamine (TEA). Raman spectrum of the mixture of ZnAc and HPC indicates coordinating interaction between zinc ion and HPC. The generation of ZnO nanoparticles in the HPC matrix proceeds in situ through the annealing of the gel phase at a temperature of 160°C. Identification of ZnO nanoparticles in the HPC matrix is done by using photoluminescence (PL), UV–Vis, and Raman spectroscopy. The films of ZnO/HPC nanocomposite are transparent in the visible light and show a higher energy value of absorption edge compared with ZnO in the bulk. Nanocrystalline films of ZnO were obtained by the calcination of ZnO/HPC nanocomposite at 500°C. ZnO films possess a good transparency for the visible light and high absorbance for UV light. Nanocrystallite sizes of ZnO particles were estimated from the X‐ ray lines broadening. The properties of ZnO layers were studied by the evaluation of PL, X‐ray investigation and atom force microscope (AFM) scanning, and the optical absorption edge. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrophobic molecules slow down the hydrogen-bond dynamics of water

We study the spectral and orientational dynamics of HDO molecules in solutions of tertiary-butyl-alcohol (TBA), trimethyl-amine-oxide (TMAO), and tetramethylurea (TMU) in isotopically diluted water (HDO:D(2)O and HDO:H(2)O). The spectral dynamics are studied with femtosecond two-dimensional infrared spectroscopy and the orientational dynamics with femtosecond polarization-resolved vibrational pump-probe spectroscopy. We observe a strong slowing down of the spectral diffusion around the central part of the absorption line that increases with increasing solute concentration. At low concentrations, the fraction of water showing slow spectral dynamics is observed to scale with the number of methyl groups, indicating that this effect is due to slow hydrogen-bond dynamics in the hydration shell of the methyl groups of the solute molecules. The slowing down of the vibrational frequency dynamics is strongly correlated with the slowing down of the orientational mobility of the water molecules. This correlation indicates that these effects have a common origin in the effect of hydrophobic molecular groups on the hydrogen-bond dynamics of water.     

Semiflexible amphiphilic polymers: cylindrical-shaped, collagenlike, and toroidal structures

A coarse-grained model is used to study the conformational properties of semiflexible polymers with amphiphilic monomer units containing both hydrophilic and hydrophobic interaction sites. The hydrophobically driven conformational transitions are studied using molecular dynamics simulations for the chains of varying stiffness, as characterized by intrinsic Kuhn segment lengths that vary over a decade. It is shown that the energy of hydrophobic attraction required for the realization of the coil-to-globule transition increases with increasing chain stiffness. For rather stiff backbone, the coil-to-globule transition corresponds to a first order phase transition. We find that depending on the chain stiffness, a variety of thermodynamically stable anisometric chain morphologies are possible in a solvent selectively poor for hydrophobic sites of amphiphilic monomer units. For flexible chains, the amphiphilic polymer forms a cylindrical globule having blob structure with nearly spherical blobs. With increasing stiffness, the number of blobs composing the globule decreases and the shape of blobs transforms into elongated cylinder. Further increase in stiffness leads to compaction of macromolecules into a collagenlike structure when the chain folds itself several times and different strands wind round each other. In this state, the collagenlike structures coexist with toroidal globules, both conformations having approximately equal energies.     

Novel multifunctional water- and oil-repellent,antibacterial, and flame-retardant cellulose fibres created by the sol–gel process

This research aimed to prepare cotton fibres with novel multifunctional water- and oil-repellent, antibacterial, and flame-retardant properties. A three-component equimolar sol mixture, which included 1H,1H,2H,2H-perfluorooctyltriethoxysilane, 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride, and P,P-diphenyl-N-(3-(trimethoxysilyl)propyl) phosphinic amide, was applied to the cotton fabric using the sol–gel process. The presence of the coating on the cotton fibres was confirmed by Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy. The functional properties of the coated cotton fabric were determined from liquid contact angle measurements and antibacterial activity, burning behaviour, and thermo-oxidative stability studies. The results demonstrate that a unique, compatible, and uniform organic-inorganic hybrid polymer network was formed on the fabric surface, which preserved its simultaneous hydrophobic (water contact angle of 135 ± 2°), oleophobic (n-hexadecane contact angle of 117 ± 1°), and bactericidal (bacterial reduction of 100 %) properties and incorporated the enhanced thermo-oxidative stability of the modified cellulose fibres.     

Moving-window two-dimensional correlation infrared spectroscopy study on structural variations of partially hydrolyzed poly(vinyl alcohol)

In the present study, the molecular chain changes and structural transitions of partially hydrolyzed poly(vinyl alcohol) (PVA) having a 12 mol% acetate unit were analyzed by moving-window two-dimensional (MW2D) correlation infrared spectroscopy combined with differential scanning calorimetry and thermogravimetric analysis. The results show the glass-transition temperature (T _g ) of PVA is clearly distinguished by MW2D correlation infrared spectroscopy, and the acetate groups start to be eliminated around the melting temperature, whereas the free water molecules in PVA are eliminated above T _g. The correlation movements of the O–H stretching modes, including the free hydroxyl groups and the hydrogen bonds, are clearly determined using MW2D correlation infrared spectroscopy. The spectral variations in the C=O stretching region caused by the elimination of the acetate unit from polymer chains are also discussed on the basis of the results of the MW2D correlation analysis. Such results cannot be obtained by traditional infrared spectroscopy owing to the complex overlapping peaks.

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

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

Light-scattering study of the coil-to-globule transition of linear poly(N-isopropylacrylamide) ionomers in water

The coil-to-globule transition of two poly(N-isopropylacrylamide) (PNIPAM) ionomers with different ionic contents (0.8 and 4.5 mol %), but similar weight average molar masses, in deionized water was investigated by a combination of static and dynamic light scattering. In spite of the large difference in their ionic contents, both the ionomers have a nearly same lower critical solution temperature (LCST, ∼ 32.5°C). At temperatures higher than the LCST, the ionomer chains undergo a simultaneous intrachain coil-to-globule transition and interchain aggregation to form nanoparticles thermodynamically stable in water. The average size of the nanoparticles decreases respectively as the ionic content increases and the ionomer concentration decreases. The interchain aggregation can be completely suppressed in an extremely dilute ionomer solution (< ∼ 5 × 10⁻⁶ g/mL), so that the intrachain coil-to-globule transition leads to the collapse of the ionomer chains into individual single-chain nanoparticles. Our results clearly indicate that there is a hysteresis in the colling process (the globule-to-coil transition). © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1501–1506, 1998     

Sol–gel synthesis of mesoporous silicas containing albumin and guanidine polymers and its application to the bilirubin adsorption

The organic–inorganic composite materials based on mesoporous silica were synthesized using sol–gel method. The surface area of silicas was modified by bovine serum albumin (BSA) and guanidine polymers: polyacrylate guanidine (PAG) and polymethacrylate guanidine. The mesoporous silicas were characterized by nitrogen adsorption–desorption analysis, Fourier transform infrared spectroscopy, transmission electron microscopy. The obtained materials were used as adsorbents for selective bilirubin removal. It was shown that the structural properties and surface area of modified materials depend on the nature of polymers. Incorporation of polymers in silica gel matrix during sol–gel process leads to the formation of mesoporous structure with high pore diameter and a BET surface area equals to 346 m²/g for SiO₂/BSA and 160 m²/g for SiO₂/PAG. Analysis of adsorption isotherms showed that modification of silica by BSA and guanidine polymers increases its adsorption ability to bilirubin molecules. According to Langmuir model, the maximum bilirubin adsorption capacity was 1.18 mg/g.