NMR Study on Polymer-Solvent Interactions during Temperature-Induced Phase Separation in Aqueous Polymer Solutions

Some possibilities of NMR spectroscopy (mainly spin-spin relaxation) in investigations of hydration and other polymer-solvent interactions during the temperature-induced phase separation in aqueous polymer solutions are described. A certain portion of water molecules bound in phase-separated mesoglobules was revealed. The residence time of the bound HDO for poly(vinyl methyl ether) (PVME)/D₂O solution (c = 6 wt%) is 1.2 ms. With time a slow release of originally bound water from the respective mesoglobules was observed. For highly concentrated PVME/D₂O solutions (c = 20–60 wt%), the residence time of bound HDO ≫ 2.7 ms and fractions of bound water unchanged even for 70 h were found. A similar behaviour as described above for water (HDO) was also found for EtOH molecules in PVME/D₂O/EtOH solutions.     

Phase Separation in Aqueous Polymer Solutions as Studied by NMR Methods

Some possibilities of ¹H NMR spectroscopy in investigations of structural-dynamic changes and polymer-solvent interactions during the temperature-induced phase transitions in aqueous polymer solutions are described. Results obtained recently on D₂O solutions of poly(vinyl methyl ether) (PVME), poly(N-isopropylmethacrylamide) (PIPMAm), negatively charged copolymers of N-isopropylmethacrylamide and sodium methacrylate, and PIPMAm/PVME mixtures are discussed. A markedly different rate of dehydration process in dilute solutions on the one hand, and in semidilute and concentrated solutions on the other hand, was revealed from ¹H spin-spin relaxation measurements.     

The Association of Tetraalkylammonium Ions with Poly(vinyl methyl ether) in Water and its Effect on Phase‐Separation Behavior as Studied by Micro‐Raman Spectroscopy

Summary: The thermosensitive phase‐separation of poly(vinyl methyl ether) in water has been investigated by micro‐Raman spectroscopy in the presence of tetraalkylammonium bromides. The equilibrium distribution of both polymer and salts to the polymer‐rich and solvent‐rich phases was determined as a function of temperature. Tetraalkylammonium ions with longer alkyl chains associate more strongly with PVME and raise the phase‐transition temperature due to an increase in hydrophilicity.

     

1H NMR study of phase transition of uncharged and negatively charged poly(N-isopropylmethacrylamide) in D2O solutions

¹H NMR spectroscopy has been applied to the analysis of dynamic-structural changes during temperature-induced phase transition of non-ionized poly(N-isopropylmethacrylamide) (PIPMAm) and ionized copolymers of N-isopropylmethacrylamide with sodium methacrylate, all in D₂O solutions with various polymer concentrations (c = 0.1-10 wt.-%) and ionic comonomer mole fractions (i = 0-10 mole %). It was found that the formation of compact globular-like structures during the phase transition is independent of polymer concentration for non-ionized samples; the presence of negative charges on the polymer chains leads to a dependence of the phase transition temperature on c and i. Virtually all PIPMAm segments are in globular-like structures for low polymer concentrations; for c ⩾ 1 wt.-%, this holds only for low content i of the ionic comonomer. An increase in c and i leads to a decrease in the fraction of polymer segments in globular-like structures; for samples with highest values of c and i, the phase transition was not observed.     

Stimuli-Responsive Polymers in Solution Investigated by NMR and Infrared Spectroscopy

Summary: Temperature-induced and solvent composition-induced phase separation in solutions of poly(N-isopropylmethacrylamide) (PIPMAm) and other thermoresponsive polymers as studied by NMR and infrared (IR) spectroscopy is discussed. The fraction p of phase-separated units (units with significantly reduced mobility) and subsequently, e.g., thermodynamic parameters characterizing the coil-globule phase transition induced by temperature, were determined from reduced integrated intensities in high-resolution ¹H NMR spectra. This approach can be especially useful in investigations of phase separation in solutions of binary polymer systems. Information on behaviour of water during temperature-induced phase transition was obtained from measurements of ¹H NMR relaxation times of HDO molecules. NMR and IR spectroscopy were used to investigate PIPMAm solutions in water/ethanol (D₂O/EtOH) mixtures where the phase separation can be induced by solvent composition (cononsolvency). Some differences in globular-like structures induced by temperature and solvent composition were revealed by these methods.     

Miscibility of poly(vinyl methyl ether) and poly(styrene-co-2-vinylnaphthalene) blends by FT-IR spectroscopy and Tg measurements

Miscibility of blends consisting of poly(vinyl methyl ether) (PVME) and poly(styreneco-2-vinylnaphthalene) [P(S-co-2VN)] was investigated by means of Fourier transform infrared (FT-IR) spectroscopy and thermal analysis. Copolymers containing 21, 51, and 84 wt % of styrene were synthesized by radical polymerization. Based on optical clarity and glass transition temperatures, it was shown that the miscibility in P(S-co-2VN)/PVME blends is largely affected by compositions of the copolymers as well as concentrations of the blend. From the FT-IR results, the relative intensity at 1100 cm^?1 peak of COCH₃ band of PVME and the position of naphthyl ring of 2VN were sensitive to the miscibility of the blends. It was observed that blends of PVME with P(S-co-2VN) of 84 wt % styrene or P(S-co-2VN) of 51 wt % styrene are miscible over the entire concentration ranges of the blends. Blends of PVME with P(S-co-2VN) containing 21 wt % of styrene are immiscible below 65 wt % PVME. In the miscible P(S-co-2VN)/PVME blends, there was observed a large shift in the naphthyl frequency at a characteristic wavelength of 748 cm^?1. © 1993 John Wiley & Sons, Inc.     

Solid Polymer Electrolytes Studied by NMR Spectroscopy and DFT Calculations

Summary: Results obtained recently on polymer electrolytes poly(ethylene oxide) (PEO)/LiCF₃SO₃ and poly(2-ethyl-2-oxazoline) (POZ)/AgCF₃SO₃ by a combination of solid-state ¹³C and ¹H NMR spectroscopy and DFT quantum-chemical calculations are discussed. Essentially the same local structure was found for the amorphous and crystalline phases of semicrystalline PEO/LiCF₃SO₃ polymer electrolyte. The amorphous POZ/AgCF₃SO₃ complex has a defined stoichiometry with two POZ monomeric units per one AgCF₃SO₃. A close contact between the metal salt and polymer was determined for both investigated systems from the Lee-Goldburg cross-polarization ¹H → ¹³C dynamics.     

Sequence analysis of poly (ether sulfone) copolymers by 13C NMR

Random and block copolymers of poly (ether sulfone) (PES) and poly (ether ether sulfone) (PEES) were synthesized by the nucleophilic polycondensation of 4,4′‐dichlorodiphenyl sulfone (DCDPS) with 4,4′‐dihydroxydiphenyl sulfone (DHDPS) and hydroquinone (HQ). Chemical structures of these copolymers were characterized by ¹³C NMR. The monomer molar fraction, sequential distribution, and degree of randomness of the copolymers were determined through analyses of the resonances of quaternary carbons in the DCDPS unit. Experimental results show that the molar fractions of the comonomer determined by ¹³C NMR analyses are close to the charged values in the synthetic step. Moreover, these copolymers, which were prepared by different polymerization methods, revealed different number‐average sequential length and degree of randomness. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1624–1630, 2005     

Thermal diffusivity study of polystyrene/poly(vinyl methyl ether) blends by flash radiometry

This article presents thermal diffusivity (D) measurements by flash radiometry for the polymer blend of polystyrene (PS) and poly(vinyl methyl ether) (PVME) with lower critical solution temperature (LCST) phase diagram. Dependence of D on PS content measured at 100°C coincides a phase diagram determined by a cloud point measurement. D value for the blend decreases with increasing PS content and has minimum value at the PS content around 20 wt % from which D increases again with increasing PS content. If the concentration fluctuation between two components in the miscible states at the temperature close to LCST causes the remarkable phonon scattering, the composition dependence of D would resemble the phase diagram. D for the sample in the phase-separated state is larger than that for the miscible state. The larger D in the phase-separated sample would be due to the decrease of the total surface area microscopically contacted to the counter component in the phase-separated state. Dependence of D on temperature for the phase-separated sample is quite different from that of the miscible one. On an isothermal measurement of D for PS/PVME (10 : 90) at 110°C just below the cloud point, D started to increase at time above 100 min and leveled out above 250 min. Isothermal observation of sample film by a differential interference contrast microscopy showed the creation of some structure due to the nucleation and growth of interface at 225 min and it became obvious above 250 min. Thus, the increase in D at 110°C implies that D can sensitively reflect the change in microscopic structures which follows the nucleation and growth of interface. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1869–1876, 1997     

Immiscibility,upper critical solution temperature,and miscibility in blends of poly(vinyl ether)s with polyacrylics: Effects of pendant groups

Various phase behavior of blends of poly(vinyl ether)s with homologous acrylic polymers (polymethacrylates or polyacrylates) were examined using differential scanning calorimetry, optical microscopy (OM), and Fourier‐transformed infrared spectroscopy. Effects of varying the pendant groups of either of constituent polymers on the phase behavior of the blends were analyzed. A series of interestingly different phase behavior in the blends has been revealed in that as the pendant group in the acrylic polymer series gets longer, polymethacrylate/poly(vinyl methyl ether) (PVME) blends exhibit immiscibility, upper critical solution temperature (UCST), and miscibility, respectively. This study found that the true phase behavior of poly(propyl methacrylate)/PVME [and poly(isopropyl methacrylate)/PVME)] blend systems, though immiscible at ambient, actually displayed a rare UCST upon heating to higher temperatures. Similarly, as the methyl pendant group in PVE is lengthened to ethyl (i.e., PVME replaced by PVEE), phase behavior of its blends with series of polymethacrylates or polyacrylates changes correspondingly. Analyses and quantitative comparisons on four series of blends of PVE/acrylic polymer were performed to thoroughly understand the effects of pendant groups in either polyethers (PVE's) or acrylic polymers on the phase behavior of the blends of these two constituents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1521–1534, 2007     

Impact of Magnesium(II) on Beryllium Fluorides in Solutions Studied by 19F NMR Spectroscopy

Beryllium fluorides are widely used in protein phosphorylation studies to get stable transition state analogs or near attack conformers, which has attracted much attention. BeF₃^? is one of the optimal phosphoryl (PO₃^?) analogs for its identical geometry and charge, and Mg²⁺ naturally participates in the phosphoryl binding in biological systems. In solutions, BeF₃^? coexists with other beryllium fluorides (BeF₄^2?, BeF₂ and BeF⁺) and magnesium fluorides, and there are equilibriums between these species. In this article, ¹⁹F NMR spectroscopy was applied to the investigation of the impact of magnesium(II) on beryllium fluorides. It has been found that when Mg²⁺ was introduced into the solutions, the chemical shifts, the intensities and the line widths of ¹⁹F signals of various beryllium fluoride complexes were changed. After ionic strength correction, these effects were remarkable only for BeF₄^2? and BeF₃^?, especially BeF₄^2?, when the concentration of the fluoride ion is relatively low. Mechanism of the effects is proposed which involves ion pair formation between Mg²⁺ and beryllium fluorides.     

聚甲基丙烯酸甲酯与聚醋酸乙烯酯共混的红外光谱研究

用红外光谱(FTIR)研究了聚甲基丙烯酸甲酯(PMMA)与聚醋酸乙烯酯(PVAc)共混体系相容性,在160℃以上共混体系发生相分离;分相体系与非分相体系的FTIR谱明显不同;共混体系的FTIR谱不能从两统组分红外光谱简单加和得到;结果表明大分子构象发生了变化,PMMA/PVAc体系相容可能是大分子构象熵变所致。     

Hydration of Temperature-Responsive Polymers Observed by IR Spectroscopy

Summary: Phase separation of aqueous solutions of temperature-responsive polymers such as poly(N-isopropylacrylamide) (PNiPAm) and poly(N,N-diethylacrylamide) (PdEA) was investigated. IR spectroscopy was proved to be a quite useful method observing changes in the hydration states of individual chemical groups of the polymers. For example, the analyses of the amide I and II bands provides important information concerning the hydrogen bonding of the CO group and the N-H group, respectively. Changes in the C H stretching bands indicate hydration changes of the alkyl groups. In addition, molecular simulations based on quantum chemistry give a strong support to the interpretation on molecular level. Our recent progress on this subject was presented and the relationship between the molecular interactions and the macroscopic phase behaviors was discussed.     

Solid‐state 13C and 129Xe NMR study of poly(vinyl alcohol) and poly(vinyl alcohol)/lactosilated chitosan gels

Dry and hydrated poly(vinyl alcohol) (PVA) gels with 55% (a‐PVA) and 61% (s‐PVA) syndiotacticity and related PVA/lactyl chitosan (LC) blends have been investigated with ¹²⁹Xe and cross‐polarization/magic‐angle‐spinning ¹³C NMR techniques. Although the dry gels exhibit two broad ¹²⁹Xe resonances in the slow‐to‐intermediate exchange limit, both hydrated gels show three resonances. The corresponding dry blends exhibit two signals, the chemical shifts and line widths of which change with respect to those of pure PVA, whereas one (a‐PVA/LC) or two (s‐PVA/LC) signals appear in the spectra of the hydrated blends. A comparative analysis of the data demonstrates that LC rearranges the domains of the polymeric matrix in both the dry and hydrated blends according to the syndiotacticity of the PVA chains. Information on the molecular motions of the amorphous and swollen polymeric domains in the kilohertz range has been obtained from an analysis of the spin‐lattice relaxation times. These data indicate that the dynamics and arrangement of the PVA chains in the gels are strongly affected by their tacticity and the addition of the copolymer LC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3123–3131, 2003     

Methyltrimethoxysilane Sol-Gel Polymerization in Acidic Ethanol Solutions Studied by 29Si NMR Spectroscopy

²⁹Si solution NMR has been used to study the acid-catalyzed polymerization of methyltrimethoxysilane (MTMS) in ethanol. The complex reaction pathways have been analyzed, and the general trends described. New interpretations are given to the rate constants of condensation reactions. The steric effects and inductive effects are found to work in coordination to reduce the reactivity of the silicon sites with the increasing connectivity. The extensive cyclization is attributed to the low steric effects of a tetramer ring closure compared to other end/end species condensations.     

Preparation and properties of poly(alkyl vinyl ether-2-ethyl-2-oxazoline) diblock copolymers

A method for the synthesis of well-defined poly(alkyl vinyl ether–2-ethyl-2-oxazoline) diblock copolymers with hydrolytically stable block linkages has been developed. Monofunctional poly(alkyl vinyl ether) oligomers with nearly Poisson molecular weight distributions were prepared via a living cationic polymerization method using chloroethyl vinyl ether together with HI/ZnI₂ as the initiating system and lithium borohydride as the termination reagent. Using the resultant chloroethyl ether functional oligomers in combination with sodium iodide as macroinitiators, 2-ethyl-2-oxazoline was polymerized in chlorobenzene/NMP to afford diblock copolymers. A series of poly(methyl vinyl ether–2-ethyl-2-oxazoline) diblock materials were found to have polydispersities of ≈ 1.3–1.4 and are microphase separated as indicated by two T_g's in their DSC thermograms. These copolymers are presently being used as model materials to study fundamental parameters important for steric stabilization of dispersions in polar media. © 1993 John Wiley & Sons, Inc.     

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.     

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.