H NMR study of thermotropic phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(vinyl methyl ether) mixtures

¹H NMR spectroscopy was used to investigate thermotropic phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(vinyl methyl ether) (PVME) mixtures. In all studied solutions (polymer concentrations c=0.1-10 wt%) two phase transitions were detected at temperatures roughly corresponding to different lower critical solution temperatures of PIPMAm and PVME. While the phase transition of PVME component (located at lower temperatures) is not affected by the presence of PIPMAm in the mixture, the phase transition temperatures of PIPMAm component (located at higher temperatures) are affected by the phase separation of the PVME component. Measurements of ¹H spin-spin relaxation of residual water (HDO) molecules revealed that above the phase transition, a certain portion of water molecules is bound to polymer globular structures. A major part of bound water is present in globular structures of predominating polymer component in the mixture.     

H NMR study of temperature-induced phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) mixtures and random copolymers

¹H NMR spectroscopy was used to investigate temperature-induced phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(N-isopropylacrylamide) (PIPAAm) mixtures and P(IPMAm/IPAAm) random copolymers of various composition on molecular level. While two phase transitions were detected for PIPMAm/PIPAAm mixtures, only single phase transition was found for P(IPMAm/IPAAm) copolymers. The phase transition temperatures of PIPAAm component (appears at lower temperatures) are not affected by the presence of PIPMAm in the mixtures; on the other hand, the temperatures of the phase transition of PIPMAm component (appears at higher temperatures) are affected by the phase separation of the PIPAAm component and depend on concentration of the solution. For P(IPMAm/IPAAm) random copolymers, a departure from the linear dependence of the transition temperatures on the copolymer composition was found for a sample with 75 mol% of IPMAm monomeric units.     

Effect of time on the hydration and temperature-induced phase separation in aqueous polymer solutions. H NMR study

Dehydration during temperature-induced phase separation in D₂O solutions of poly(vinyl methyl ether) (PVME), poly(N-isopropylmethacrylamide) (PIPMAm) and poly(N-isopropylacrylamide) (PIPAAm) was followed from time dependences of NMR spin-spin relaxation times T₂ of HDO. Both the time characterizing the exclusion of the water from mesoglobules (manifested by the increase in T₂ values) and the induction period which precedes the increase in T₂ values, increased in the order PVME < PIPMAm < PIPAAm. For D₂O solutions of PIPMAm/PVME (or PIPMAm/PIPAAm) mixtures a direct connection between the state of the mesoglobules (hydrated or dehydrated) formed by the component with lower LCST (PVME, PIPAAm) and the temperatures of the phase transition of the PIPMAm component was established by NMR spectroscopy.     

Thermal transitions of poly(N-isopropylmethacrylamide) in aqueous solutions

Thermal transitions of poly(N-isopropylmethacrylamide) (PIPMAm) in aqueous solutions were studied in a wide temperature range from −100 to 60 °C. The solutions of different polymer weight fractions (c=0.03-1) were investigated in cooling and heating scans of differential scanning calorimetry; four qualitatively different kinds of thermograms were identified in dependence of the fraction, c. The transitions are explained in a thermodynamic framework based on the phase diagram of the system. This diagram provides a complete understanding of thermal transitions in aqueous PIPMAm solutions as a function of c, in particular the existence of non-crystallizable water and the temperature induced phase transition that is due to the hydrophobic interaction in PIPMAm solutions.     

NMR study of temperature-induced phase separation and polymer-solvent interactions in poly(vinyl methyl ether)/D2O/ethanol solutions

The changes in the dynamic structure during temperature-induced phase transition in D₂O/ethanol solutions of poly(vinyl methyl ether) (PVME) were studied using NMR methods. The effect of polymer concentration and ethanol (EtOH) content in D₂O/EtOH mixtures on the appearance and extent of the phase separation was determined. Measurements of ¹H and ¹³C spin-spin and spin-lattice relaxations showed the presence of two kinds of EtOH molecules: besides the free EtOH expelled from the PVME mesoglobules there are also EtOH molecules bound in PVME mesoglobules. The existence of two different types of EtOH molecules at temperatures above the phase transition was in solutions with polymer concentration 20 wt% manifested by two well-resolved NMR signals (corresponding to free and bound EtOH) in ¹³C and ¹H NMR spectra. With time the originally bound EtOH is slowly released from globular-like structures. From the point of view of polymer-solvent interactions in the phase-separated PVME solutions both EtOH and water (HDO) molecules show a similar behaviour so indicating that the decisive factor in this behaviour is a polar character of these molecules and hydrogen bonding.     

Light scattering and microcalorimetry studies on aqueous solutions of thermo-responsive PVCL-g-PEO copolymers

Dilute aqueous solutions of thermo-responsive poly(N-vinyl caprolactam)-graft-polyethylene oxide (PVCL-g-PEO) copolymers were studied by light scattering and high sensitivity differential scanning microcalorimetry. These copolymers are double hydrophilic at low temperatures, but become amphiphilic upon heating the solutions above the cloud point temperature of the PVCL segments (T_CP). The self-assembly properties of the copolymers are investigated by dynamic light scattering as a function of the temperature, degree of grafting and concentration. It was found that a certain critical polymer concentration is needed for the polymers to form stable aggregates. These structures are expected to consist of a hydrophobic PVCL core, stabilized by a hydrophilic PEO shell. The size of these aggregates increases with the degree of grafting. Microcalorimetry results revealed that the grafting of PVCL with hydrophilic PEO does not influence the phase transition enthalpy of PVCL.     

Mesoglobules of thermoresponsive polymers in dilute aqueous solutions above the LCST

The colloidal stability and characteristics of particles formed by homopolymers of poly(N-vinyl caprolactam), poly(N-isopropyl acrylamide) and poly(vinyl methyl ether) in dilute aqueous solutions above the lower critical solution temperature, LCST, was followed by means of dynamic and static light scattering. Depending on the solution concentration, the homopolymers precipitate or form stable dispersions of monodisperse spherical particles. To obtain colloidally stable aggregates, also called mesoglobules, no stabilising agent was added. The stability of the mesoglobules upon time and dilution at temperatures above the LCST suggests that the particle surfaces possess a hydrophilic character. The size of the formed particles depends on the concentration and the heating rate of the solutions. However, internal structure and shape of mesoglobules are affected neither by the way, how the mesoglobules were prepared, nor by molar mass of individual macromolecules. Mesoglobules of PNIPAM obey the M∝R^2.7 scaling law. Origin of stability of the dispersions vs. expected precipitation is discussed.     

Solvent isotope effect on sol–gel transition of methylcellulose studied by DSC

Methylcellulose (MC), a hydrophobically modified cellulose derivative, in an aqueous solution undergoes sol-to-gel and gel-to-sol transitions on heating and cooling, respectively. Using differential scanning calorimetry, MC in light (H₂O) and heavy (D₂O) water solutions has been investigated to elucidate the solvent isotope effect on the transitions. As a result, their transition temperatures are higher in H₂O by about 4 °C than D₂O. This phenomenon is rationalized in terms of the strength of the hydrophobic attractive interaction; the strength is enhanced by D₂O. We discuss the reason for the enhancement and the difference in the isotope effect between MC and a poly(N-isopropylacrylamide) polymer which shows an opposite trend to MC.     

Overlap concentration of an aqueous poly(N-vinylcaprolactam) solution in the dynamic and static states

The overlap concentration of poly(N-vinylcaprolactam)s (PVCLs) of low molecular weights was studied with dynamic and static methods (with viscosity and refractive-index measurements) at three temperatures (25, 30, and 35°C). This concentration, determined by viscometry, was lower than the overlap concentration determined by refractometry. Also, the influence of the temperature and molecular weight on the viscosity and refractive index was investigated, and the opposite effect of the temperature on the viscosity and refractive index of PVCL solutions was observed. The experimental overlap concentration values were compared with those calculated with the Rao, Huggins, and Fuoss equations. The results showed that the Huggins equation is the best equation to describe the dilute solution properties of this polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Nanosized colloidal particles from thermosensitive poly(methoxydiethyleneglycol methacrylate)s in aqueous media

The formation of well-defined colloidal particles (mesoglobules) from the thermosensitive polymer poly(methoxydiethyleneglycol methacrylate) was observed in dilute aqueous solutions (0.5–1.0 g/L) by turbidimetry and dynamic light scattering (DLS). DLS measurements were performed at 70 °C and showed a strong influence of polymer molecular weight: the hydrodynamic diameters of the mesoglobules increased from ca. 160 to 330 nm with a relatively small, i.e., from 6,400 to 14,000, increase in molecular weight. The addition of sodium dodecyl sulfate (SDS) at surfactant/polymer ratios (s/p, g/g) ranging from 0.3 to 0.5 practically inhibited the clouding of the solutions as the initial transmittance decreased by only 10–30%. Furthermore, a dramatic shift of the original cloud point values taken as a 10% decrease in transmittance, by approximately 20–60 °C was registered upon the surfactant addition. The presence of SDS resulted in size reduction by 52–90% as indicated by DLS.     

Formation of mesoglobules in aqueous media from thermo-sensitive poly(ethoxytriethyleneglycol acrylate)

A series of six poly(ethoxytriethyleneglycol acrylate) (PETEGA) homopolymers were synthesized by atom transfer radical polymerization, reversible addition-fragmentation transfer polymerization, and anionic polymerization in order to cover a molecular weight range from 7,000 to 40,000 Da. The polymers exhibited a lower critical solution temperature (LCST) behavior in water, which was observed by the occurrence of a cloud point (CP) at around 35 °C. The transmittance of visible light versus temperature dependence overlapped during the cooling and the heating cycles, showing almost a complete lack of hysteresis. Moreover, instead of the occurrence of an uncontrolled macroscopic phase separation, stable colloidal aggregates (mesoglobules) of narrow distribution in particle size were formed in water at temperatures above the LCST of PETEGA at 1 g L⁻¹ solutions. The dimensions of the mesoglobules ranged from 91 to 235 nm, and particle size was not influenced by the molecular weight of PETEGA. Temperature changes caused considerable variations of the mesoglobules dimensions, which were smaller at higher temperatures. The addition of an anionic surfactant simultaneously increased the CP values by 4–6 °C and lowered the dimensions of the mesoglobules.     

Thermal phase transition of poly(N‐propionylethyleneimine) hydrogel

This article presents the preparation of the hydrogel of poly(N‐propionylethyleneimine) and its interpenetrating polymer network (IPN) hydrogel containing polyacrylamide by means of γ‐ray radiation and a study of the phase transition temperature of these hydrogels. As a result, the hydrogel of the crosslinked poly(N‐propionylethyleneimine) exhibited swelling below and shrinking above the phase transition temperature (about 61°C), as well as the lower critical solution temperature (LCST) of the liner polymer–water system. The experiment also showed that the LCST of the IPN hydrogel could be adjusted by the incorporation of the second component polyacrylamide. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2457–2461, 1999     

Synthesis and study of new fluorinated poly(imide‐amide)s

New fluorinated poly(imide‐amide)s have been synthesized by solution polycondensation of various aromatic diamines containing a naphthalene unit with diacid chlorides having both imide and hexafluoroisopropylidene (6F) groups. These polymers are soluble in polar aprotic solvents, such as N‐methylpyrrolidone (NMP) or N,N‐dimethylformamide (DMF), and can be cast into flexible thin films from solutions. They show high thermooxidative stability with decomposition temperatures being above 425°C and glass transition temperatures being in the range of 235–305°C. The polymer films exhibit a low dielectric constant and tough mechanical properties.     

Miscibility and complexation behavior of poly(cyanomethyl methacrylate) and poly(2-cyanoethyl methacrylate) with tertiary amide polymers

The miscibility and complexation behavior of poly(cyanomethyl methacrylate) (PCYMMA) and poly(2-cyanoethyl methacrylate) (PCYEMA) with various tertiary amide polymers was studied. PCYMMA and PCYEMA form interpolymer complexes with poly(N-methyl-N-vinylacetamide) (PMVAc) or poly(N-vinyl-2-pyrrolidone) (PVP) in tetrahydrofuran (THF) solutions. PCYMMA also forms complexes with poly(N,N-dimethylacrylamide) (PDMA) in THF solutions. However, PCYEMA does not form complexes with PDMA in THF solutions, but the THF-cast blends are miscible over the entire composition range. Both PCYMMA and PCYEMA do not form complexes with poly(2-ethyl-2-oxazoline) (PEOx) in THF solutions and are only miscible with PEOx when the blend contains greater than 60 wt % PCYMMA or 80 wt % PCYEMA. On the other hand, both PCYMMA and PCYEMA do not form complexes with PMVAc, PVP, or PDMA in N,N-dimethylformamide (DMF) solutions. The compositions of the complexes consist of simple mole ratios of the component polymers, and the glass-transition temperatures of the complexes are higher than those of the DMF-cast blends of similar compositions. Fourier-transform infrared spectroscopy provides further evidence on the miscibility behavior through changes in the amide carbonyl absorption bands of each tertiary amide polymer in the blends as well as in the cyano absorption band of PCYEMA. © 1995 John Wiley & Sons, Inc.     

Hydrogels from 2-(dimethylamino)ethylacrylate with 2-acrylamido-2-methyl-1-propanesulfonic acid: synthesis,characterization, and water-sorption properties

A novel series of copolymer hydrogels of 2-(dimethylamino)ethylacrylate (DMAEA)/2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) were prepared by solution free radical polymerization at different feed monomer mol ratios. The monomer reactivity ratios were determined by Kelen–Tüdös method. According to that, the monomer reactivity ratios for poly(DMAEA-co-AMPS) were r₁ = 0.125 and r₂ = 2.85, (r₁ × r₂ = 0.356). The effect of reaction parameters, including the concentration of cross-linking reagent N,N′-methylene-bis-acrylamide (MBA) and initiator ammonium persulfate (APS), the monomer concentration, pH, temperature, salt solutions, and solvent polarity on the water absorption have been also studied. The hydrogels achieved water-absorption values of 430 g of water/g of xerogel for the copolymer 1:2 richest in AMPS moiety. This copolymer is also very stable to the temperature effect. The optimum pH for the copolymers is 7. Aqueous solutions of the copolymers showed lower critical solution temperature behaviour (LCST). The phase transition temperatures of aqueous solutions of these copolymer increased with increasing of hydrophilic AMPS unit content in the copolymers. The glass transition temperature (Tg) of hydrogels showed a decrease by increasing of comonomer DMAEA content.     

Synthesis and characterization of novel poly(amide imide)s

Novel poly(amide imide)s having inherent viscosities in the range 0.89–1.56 dL/g were prepared by the method of low temperature polycondensation in solution of N,N-dimethylacetamide (DMA) from aromatic diamines and a new monomer: N-[4″-(chloroformylphenyl)-4′-iminobenzoyl]trimellitimide chloride. The polymers were soluble in polar aprotic solvents and essentially amorphous. They showed glass transition temperatures in the range 310–350°C and decomposition temperatures of 450–460°C. Polymer films, fabricated by casting polymer solutions in DMA, showed tensile strengths in the range 70–90 MPa and moduli values between 3000–4000 MPa. © 1996 John Wiley & Sons, Inc.     

Upper critical solution temperature—type cononsolvency of poly(N,N-dimethylacrylamide) in water—organic solvent mixtures

The behaviour of linear poly(N,N-dimethylacrylamide) (PDMAM) chains was studied by turbidimetry and viscometry in mixtures of water with the polar organic solvents methanol, dioxane and acetone. The swelling-deswelling behaviour of PDMAM gels in the same solvent mixtures was also investigated. Contrary to the behaviour in water-methanol mixtures, in water-dioxane and water-acetone mixtures a significant shrinkage of polymer chains and deswelling of polymer gels, followed by phase separation, was observed for high organic solvent fractions. Cononsolvency phenomena were found to be temperature-dependent, as demixing occurred upon decreasing temperature. This upper critical solution temperature (UCST) phase separation behaviour in mixed solvents was studied by turbidimetry and compared to the well-known lower critical solution temperature (LCST) behaviour of poly(N-isopropylacrylamide) (PNIPAM) in similar solvents mixtures.     

N-methyl-2-pyrrolidone as a solvent for poly(vinyl alcohol)

NMP (N-methyl-2-pyrrolidone) and its mixtures with water have been investigated as solvents for PVA [poly(vinyl alcohol)]. Judging from the higher exponent in the Mark-Houwink equation, NMP appears to be a better solvent than water. The equation found is for solutions in NMP at 30°C. Although the viscosity of NMP–water mixtures goes through a maximum when the molar ratio of NMP to water is 1:2, the intrinsic viscosity for PVA exhibits neither a maximum nor a minimum in mixtures ranging from pure water to pure NMP. As in aqueous solutions, the intrinsic viscosity in NMP solutions decreases slightly with increasing temperatures from 20 to 50°C.     

Control of the lower critical solution temperature—type cononsolvency properties of poly(N-isopropylacrylamide) in water—dioxane mixtures through copolymerisation with acrylamide

The behaviour of the homopolymers poly(N-isopropylacrylamide) (PNIPAM), polyacrylamide (PAM) and random copolymers of N-isopropylacrylamide (NIPAM) with acrylamide (AM) was studied by turbidimetry and viscometry in mixtures of water with dioxane. It was found that the well-known lower critical solution temperature-type cononsolvency properties of PNIPAM in water-dioxane mixtures, observed in the water-rich region, can be effectively controlled by copolymerisation of NIPAM with AM. Thus, the cononsolvency properties of the copolymers in water-dioxane mixtures are shifted to higher temperatures and restricted within a narrower solvent composition region as the acrylamide content of the copolymers increases. A significant decrease of the reduced viscosity of the systems exhibiting phase separation properties was observed upon heating, indicative of the collapse of the (co)polymer chains as temperature approaches the corresponding cloud point temperature. Furthermore, when temperature is fixed close to the cloud point temperature, the reduced viscosity decreases with increasing the volume fraction of dioxane, φ, as far as the solvent mixtures are rich in water. On the contrary, the reduced viscosity of PNIPAM in dioxane-rich mixtures is found significantly higher, indicative of an expansion of the polymer chain, as compared to the reduced viscosity of this polymer in the two pure solvents.     

Thermo- and pH-sensitivity of aqueous poly(N-vinylpyrrolidone) solutions in the presence of organic acids

The phase transition in poly(N-vinylpyrrolidone) (PVP) aqueous solutions is shown to occur at heating upon addition of organic acids such as isobutyric, isovaleric, and, especially, trichloroacetic (TCA) ones. The cloud point temperature (T_c) of PVP solutions drops from 70 to 6 °C when the TCA concentration rises from 0.2 to 0.3 mol/l. A decrease in T_c is even more drastic when HCl is also added though HCl addition to the system without TCA does not result in phase separation. These phenomena are explained by the reversible coordination between the non-ionized form of TCA and PVP units via hydrogen bonding. An increase in the medium acidity depresses TCA dissociation, resulting in an increase in PVP-TCA associate concentration. Calculations based on the pK_a values of TCA confirm this suggestion. The similar behavior is observed with poly(N-vinylcaprolactam) systems. The amount of TCA bound to PVP has been determined by means of separation of the precipitate by centrifugation at temperatures above T_c and subsequent titration of TCA in the polymer with NaOH. It is shown that the precipitate contains one TCA molecule per 3-6 VP units, this value decreasing down to 1.25-2 upon HCl addition to the system.