Entropic effect implication for change in polymer coils swelling state in the demixing enthalpy recovery of aqueous poly(vinyl methyl ether) solutions

Time‐dependent demixing enthalpy recovery behavior of aqueous poly(vinyl methyl ether) (PVME) solutions exhibits distinct recovery characteristics in three concentration regions. The absence of recovery behavior below a water concentration of 38.3 wt % indicates that the PVME coil is in a globular state. The typically sigmoidal recovery behavior of demixing enthalpy above 38.3 wt % is ascribed to the reswelling of the collapsed polymer coils induced by the entropic effect. The increase in difference between the upper and lower limits indicates the continued swelling of the PVME coils. Above 65 wt %, a dominant diluting effect can be observed, and a much longer phase separation time is needed to reach the expected lower limit. In contrast, the recovery of demixing enthalpy in a wide range of water concentration (from 38.3 to 90 wt %) exhibits the same feature. The infrared spectroscopy results are in agreement with the above macroscopic differential scanning calorimetry results. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 142–151     

Molecular dynamics simulations of atomistic hydration structures of poly(vinyl methyl ether)

Molecular dynamics simulations have been performed on the aqueous solutions of poly(vinyl methyl ether) (PVME) at various concentrations. Both radial and spatial distribution functions are used to investigate the detailed hydration structures. The structures of water are found to get increasingly concentrated when polymers are introduced and the water motions are severely hindered by the polymer matrix. At low concentrations, larger populations of tt conformers in meso dyads than those at higher concentrationsare found and this phenomenon is believed to be due to the increasing in bonding of water molecule to two ether oxygens in meso dyad. At higher concentrations, the size and conformations of polymers are quite similar to those in bulk. A transition of hydrogen bond fractions between PVME and water at around the concentration of 0.3 is observed and this value is perfectly in agreement with the results of conformational analysis and Raman spectra. Second neighbor hydrogen bond statistics revealed the domination of complicated hydrogen bond networks at low concentrations, but single hydrogen bonds as well as isolated clusters composed of 2-4 water molecules are usual around each polymer repeat unit.     

Polymer‐solvent interactions during phase transition in poly(vinyl methyl eiher)/D2O solutions as studied by NMR methods

The structural‐dynamic changes and polymer‐solvent interactions during temperature‐induced phase transition in poly(vinyl methyl ether) (PVME)/D₂O solutions in a broad range of concentrations (0.1‐30 wt.‐%) were studied by ¹H NMR methods. In the whole concentration range the phase transition is manifested by line broadening (linewidth 350‐500 Hz) of a major part of PVME units, evidently due to the formation of globular‐like structures. Above the LCST transition, the fraction of phase‐separated PVME segments is equal to 0.8±0.1, independent of polymer concentration. While at low concentrations the transition is virtually discontinuous, at high concentrations the transition region is ∼ 3 K broad. Measurements of nonselective and selective ¹H spin‐lattice relaxation times T₁ of solvent (HDO) molecules evidenced that at elevated temperatures, where most PVME forms globular structures, a part of solvent molecules is bound to PVME forming a complex; the lifetime of the bound water (HDO) molecules is ≤2 s.     

Dynamics of Poly(vinylmethyl ether) Blends with a Strongly Interassociating Copolymer

We apply broadband dielectric relaxation spectroscopy to probe the dynamics of hydrogen bonded polymer blends. A copolymer consisting of 2,3-dimethylbutadiene (DMB) [86%] and p-(hexafluoro-2-hydroxyl-2-propyl)styrene (HFS) [14%] was synthesized and blended with poly(vinylmethyl ether) (PVME). The copolymer is capable of forming strong intermolecular hydrogen bonds, while minimizing the degree of intramolecular associations, and its blends with PVME are predicted to be miscible over the entire composition range. Two segmental processes, α and α₁, are present in blends containing 26, 50, and 76 weight percent copolymer. The slower process (α₁) is assigned to the segmental motion of the intermolecularly associated copolymer, and the faster process (α) to segmental motions of PVME modified by the HFS:DMB copolymer. A relaxation associated with residual water is present in the glassy state. A local process due to motions of the PVME ether groups (β) is also present in the glassy state, and does not change with blend composition.     

Binary polymer blends involving multiple specific interaction sites: Poly(vinyl methyl ether) blends with an ethyl methacrylate-co-4-vinyl phenol copolymer

We report the results of experimental infrared and thermal analysis studies of binary miscible blends of poly(vinyl methyl ether) (PVME) with an ethyl methacrylate-co-4-vinyl phenol (EMAVPh) copolymer. This is a complex blend system, involving multiple specific interaction sites. Using the concept of competing equilibria we show that the equilibrium constant describing hydrogen bonding between the OH group of EMAVPh and the ether oxygen of PVME can be determined from quantitative infrared spectroscopic analysis of the fraction of hydrogen bonded carbonyl groups present in miscible binary blends.     

IR investigation of hydrogen bonds in weakly hydrated films of poly(vinyl alcohol)

A system of hydrogen bonds in weakly hydrated PVA films containing up to ≤8.5 wt % water is investigated via IR spectroscopy. It is shown that water molecules bind to only part of the hydroxyl groups of the polymer that are available for hydration and form the first hydrating layer. In a completely dehydrated film, practically every hydroxyl group of PVA forms hydrogen bonds with two other hydroxyl groups and serves as both a proton donor and a proton acceptor. In the hydrated film, one to three water molecules directly bind with one hydroxyl group of PVA.     

Hydrogen bonds in regioselectively substituted cellulose derivatives

Formation of hydrogen bonds in various cellulose derivatives, 2,3-di-O- and 6-O-substituted cellulose ethers, were characterized by FTIR and solid-state CP/MAS¹³C-NMR. The polymers were synthesized by regioselective substitution of hydroxyl groups and had a uniform structure. Since their three hydroxyl groups (OH) are selectively blocked, the cellulose derivatives appeared to form specific inter- and intramolecular hydrogen bonds. The characteristic OH stretching frequencies in IR spectra and the C-1 chemical shift in CP/MAS spectra of 6-O-substituted cellulose derivatives indicated existence of two equivalent intramolecular hydrogen bonds between ether oxygen and OH groups at 3-OH-O5′ and O6-HO-2′ [Figure 3(C)]. Influence of the substituents at the C-6 position on the formation was not significant except trityl group. Behavior of the hydrogen bonds in 6-O-tritylcellulose were also discussed. © 1994 John Wiley & Sons, Inc.     

聚N,N-二乙基丙烯酰胺低聚物水溶液的分子动力学研究

对50个单元构成的聚N,N-二乙基丙烯酰胺(PDEA)低聚物的水溶液体系进行了分子动力学的研究,分别模拟了300 K时的伸展链、310 K时的伸展链以及紧缩链与水构成的体系,对溶液中PDEA周围溶剂水分子的分布情况以及水分子形成氢键的情况进行了统计,结果表明在PDEA周围的水产生了比本体水更有序的结构,形成了更多的氢键,这种有序结构维持到第二水合层甚至更远.发生相分离后,PDEA与水分子形成的氢键大部分未被破坏,水合层中每个水分子形成的氢键数也没有明显变化,但水合层(形成有序结构的水分子)内水分子数目的减少使得总的氢键数目减少,从而造成体系能量增加及熵增加.同时还研究了聚合物及水分子的自扩散系数,表明PDEA影响周围水分子结构的同时,对水的动力学性质也产生了很大影响.     

1H NMR relaxation study of polymer-solvent interactions during thermotropic phase transition in aqueous solutions

The dynamic-structural changes and polymer - solvent interactions during the thermotropic phase transition in poly(vinyl methyl ether) (PVME)/D₂O solutions in a broad range of polymer concentrations (c = 0.1-60 wt.-%) were studied combining the measurements of ¹H NMR spectra, spin-spin (T₂) and spin-lattice (T₁) relaxation times. Phase separation in solutions results in a marked line broadening of a major part of polymer segments, evidently due to the formation of compact globular-like structures. The minority (∼15%) mobile component, which does not participate in the phase separation, consists of low-molecular-weight fractions of PVME, as shown by GPC. Measurements of spin-spin relaxation times T₂ of PVME methylene protons have shown that globular structures are more compact in dilute solutions in comparison with semidilute solutions where globules probably contain a certain amount of water. A certain portion of water molecules bound at elevated temperatures to (in) PVME globular structures in semidilute and concentrated solutions was revealed from measurements of spin-spin and spin-lattice relaxation times of residual HDO molecules.     

水和甲醇混合溶剂氢键相互作用及对高分子链溶解能力的理论研究

用密度泛函理论对水和甲醇混合溶剂体系的氢键结构进行了详细研究.通过构象和频率分析发现在水团簇中五聚体和六聚体环状结构最为稳定,同时发现一个全新的特征,即甲醇分子能与水五聚体和六聚体形成双氢键.根据各相互作用的稳定化能,分析了水和甲醇混合溶剂对PNIPAM溶解能力的影响,并对实验现象给予了合理解释.     

Structure and dynamics of methanol in water: a quantum mechanical charge field molecular dynamics study

An ab initio quantum mechanical charge field molecular dynamics simulation was carried out for one methanol molecule in water to analyze the structure and dynamics of hydrophobic and hydrophilic groups. It is found that water molecules around the methyl group form a cage-like structure whereas the hydroxyl group acts as both hydrogen bond donor and acceptor, thus forming several hydrogen bonds with water molecules. The dynamic analyses correlate well with the structural data, evaluated by means of radial distribution functions, angular distribution functions, and coordination number distributions. The overall ligand mean residence time, τ identifies the methanol molecule as structure maker. The relative dynamics data of hydrogen bonds between hydroxyl of methanol and water molecules prove the existence of both strong and weak hydrogen bonds. The results obtained from the simulation are in excellent agreement with the experimental results for dilute solution of CH(3)OH in water. The overall hydration shell of methanol consists in average of 18 water molecules out of which three are hydrogen bonded.     

Untersuchungen über das Verhalten organischer Mischphasen 9. Mitteilung Vergleich der binären Systeme Tetrahydrofuran mit Wasser,Methanol und Cyclohexan,Diäthyläther mit Wasser,Methanol und Cyclohexan und Tetrahydrofuran-Diäthyläther

Activities and mixing functions of the following binary systems at 25° C are discussed: 1. mixtures of tetrahydrofuran with water, methanol, and cyclohexane; 2. mixtures of diethyl ether with water, methanol, and cyclohexane, and 3. mixtures of tetrahydrofuran with diethyl ether. Comparison with similar systems shows that in systems containing methanol, the strongest interactions are formation and breaking of hydrogen bonds between alcohol molecules; interactions between methanol and ether molecules play a minor rǒle. Systems containing water exhibit two main kinds of interaction: formation and breaking of hydrogen bonds between water molecules, and formation of hydrogen bonds between water and ether molecules. Deviations from ideality are larger for diethyl ether than for tetrahydrofuran in water and methanol, and smaller in cyclohexane.     

Study of geometrical parameters in N-H...N type of hydrogen bonds

Geometrical parameters associated with N-H ... N types of hydrogen bonds have been analysed using crystal structure data on nucleic acids, amino acids and related compounds. Histograms depicting the frequency distribution of N-H ... N length (l) and H-N ... N angle (θ) have been drawn and conclusions on the favoured geometry of such bonds have been arrived at. The distribution ofl shows a pronounced maximum in the range between 2.9? and 3.0? with an overall average of 2.98 ?. The θ distribution shows a pronounced maximum for the hydrogen bond angle in the range 0°-10°, with a rapid fall-off in frequency for nonlinear hydrogen bonds. The frequency shows a cos⁶θ dependence as compared to cos²θ dependence term used earlier to predict the angular dependence of hydrogen bond potential energy in proteins and polypeptides.     

Infrared spectroscopic study on the crystallization of water in poly(vinyl methyl ether) aqueous solution during heating

The Fourier transform infrared (FTIR) results are consistent with the differential scanning calorimetric results and verify the anomalous crystallization of water in 50% poly(vinyl methyl ether) aqueous solution during heating. Below about ?34 °C, the water/polymer complex was not damaged, and the water still associated with the polymer. When heating to about ?34 °C, the associated water started to free from the unpolar (methyl group) and polar‐site (ether‐oxygen group) interaction fields of polymer gradually. Then crystallization of water was induced in this system at temperatures ranging from ?34 to ?24 °C. The FTIR data also indicate that the structure of water started to change first upon forming strong H bonds among water molecules, and then the dehydration of the polymer began to proceed subsequently when the anomalous crystallization of water occurred. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2772–2779, 2002     

An electrostatic model for hydrogen bonds in alcohols

We have measured the Raman spectra of liquid methanol at temperatures between 50° and –77°C. The weak O–H stretching bands appear, under amplification, more and more asymmetric as the temperature is lowered. They can be decomposed into three Gaussian components centered at about 3220, 3310, and 3400 cm^–1. The former, predominant at low temperature, corresponds to single, linear hydrogen bonds (LHB) between two molecules. The other two are assigned to branched hydrogen bonds, respectively bifurcated (BHB), between three molecules, and trifurcated (THB), between four molecules. We conclude that the molecular structure of liquid alcohols is not chain-like, as presumed so far, but a three-dimensional network featuring a mixture of single (LBH), and multiple hydrogen bonds (BHB, and THB). They are mainly electrostatic in nature, their relative proportions and geometry governed by the packing conditions for minimum energy. They form distinct trimers and tetramers in dilute solutions of alcohols in inert solvents and frozen matrices, and the latter even in the vapor.Deceased December 25, 1987.     

Solvation thermodynamics of water in nonpolar organic solvents indicate the occurrence of nontraditional hydrogen bonds

Experimental data for the solvation of water in nonpolar organic solvents indicate that the process is spontaneous under the Ben-Naim standard conditions, due to a large and negative enthalpy change. The process is analyzed by considering that the solvation Gibbs energy change is given by the sum of two opposing terms: the work to create a suitable cavity and the work to turn on the attractive solute-solvent interactions. Basic calculations point out unequivocally that, beyond the van der Waals contributions, additional favorable interactions occur between water and the surrounding solvent molecules. These additional favorable interactions should be nontraditional hydrogen bonds such as those between the delocalized pi-electron cloud of the aromatic ring and the hydrogen atoms of water, and those between the CH groups of both aliphatics and aromatics and the oxygen atom of water.     

最小冰聚体结构单元模型及其对水性质影响的量化分析

水的一些奇特性质主要源于水分子之间存在的氢键, 但在分子尺度上的氢键结构和数据仍是目前研究和争论的焦点. 统计分析了目前文献中普遍采用的水分子和氢键结构数据, 并在此基础上应用AutoCAD图形软件模拟出(H2O)10结构的最小冰聚体结构单元(Minimum Ice Structural Unit, MISU)模型, 以及由MISU聚合而成的冰晶体三维模型. 根据MISU模型, 可以计算得到冰在0 ℃融化为水、水由0 ℃加热至100 ℃、水在100 ℃汽化为水蒸气的三相转化过程中分别需要吸收5.86, 4.40和24.94 kJ•mol－1的能量以断裂16.7%, 12.5%和70.8%的氢键. 如若不考虑氢键的影响, 那么计算得到水的融化热和汽化热分别为0.15和15.73 kJ•mol－1, 与VIA族氢化物H2S, H2Se, H2Te的融化热和汽化热基本呈线性关系. 另外, 由MISU模型计算得到冰在0 ℃融化为水时, 密度由923.17 kg•m－3增至999.89 kg•m－3, 亦与实际测量数据基本一致.     

Reconsideration on hydrogen bond strengthening or cleavage of photoexcited coumarin 102 in aqueous solvent: a DFT/TDDFT study

In this work, the excited-state hydrogen bonding dynamics of photoexcited coumarin 102 in aqueous solvent is reconsidered. The electronically excited states of the hydrogen bonded complexes formed by coumarin 102 (C102) chromophore and the hydrogen donating water solvent have been investigated using the time-dependent density functional theory method. Two intermolecular hydrogen bonds between C102 and water molecules are considered. The previous works (Wells et al., J Phys Chem A 2008, 112, 2511) have proposed that one intermolecular hydrogen bond would be strengthened and the other one would be cleaved upon photoexcitation to the electronically excited states. However, our theoretical calculations have demonstrated that both the two intermolecular hydrogen bonds between C102 solute and H(2)O solvent molecules are significantly strengthened in electronically excited states by comparison with those in ground state. Hence, we have confirmed again that intermolecular hydrogen bonds between C102 chromophore and aqueous solvents are strengthened not cleaved upon electronic excitation, which is in accordance with Zhao's works.     

Influence of water on the glass transition of oligoether in segmented polyurethane

The dynamic mechanical method was employed to study the influence of dissolved water in segmented polyurethane (4,4-diphenylmethane-diisocyanate-oligoethylene glycol) on the viscoelastic behavior and molecular mobility of oligoether in the glass transition range. The molecules of water were shown to decrease the number of pseudo-crosslinks and to increase the amount of relaxing elements by destroying hydrogen bonds between NH-groups in the rigid segments and oxygen in the ether group. As a result, the dynamic shearing modulus increases in the glassy state at an invariable glass temperature.     

Improvement in semipermeable membrane performance of wholly aromatic polyamide through an additive processing strategy

A new concept for the method to provide semipermeability in ultrathin and single‐component wholly aromatic polyamide membranes has been developed for the first time. It was found that water molecules could permeate through the membrane prepared not from polyamides containing flexible ether, bulky binaphthyl, or fluorene rigid units, but one with carboxylic acid groups under a reverse osmosis mode. However, the enhancement of water transport properties by introducing the hydrophilic group of polyamide was not substantial. Therefore, polyamide membranes were prepared from the solution containing aqueous additives in order to weaken hydrogen bonds between polymer chains and thereby to suppress the aggregation of the polymer chains. As a result, water flux was dramatically improved with slightly improved NaCl rejection. Our analyses based on attenuated total reflectance Fourier transform infrared spectroscopy and solid‐state carbon polarization and magic angle spinning nuclear magnetic resonance (¹³C CPMAS NMR) spectroscopy confirmed that the aggregation of polymer chains due to the hydrogen bonds among the amide linkages was suppressed by the co‐ordination of the aqueous additives to the amide linkage. The state of water in the membranes analyzed by differential scanning calorimetry also supported the formation of pores. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1275–1281