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.     

Collapse kinetics for individual poly(N-isopropylmethacrylamide) chains

The kinetics for the coil-to-globule transition of linear poly(N-isopropylmethacrylamide) (PiPMA) chains has been studied by use of the fluorescence and Rayleigh scattering with a fast laser pulse infrared heating. We have observed the two-stage kinetics in the collapse transition with the characteristic relaxation times, τ_fast and τ_slow, which are attributed to the nucleation and growth of pearls on the chain and the merging and coarsening of pearls to a globule, respectively. The collapse kinetics of PiPMA is similar to that of poly(N-isopropylacrylamide) which has one less methyl in each monomeric unit, indicating that the additional methyl groups in PiPMA chains slightly influence the kinetics. In other words, the pearls are not completely coarsened to form compact globules within τ_slow.     

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.     

Synthesis of thermoresponsive poly(N-isopropylmethacrylamide) and poly(acrylic acid) block copolymers via post-functionalization of poly(N-methacryloxysuccinimide)

Polymers exhibiting a thermoresponsive, lower critical solution temperature (LCST) phase transition have proven to be useful for many applications as “smart” or “intelligent” materials. A series of poly(N-isopropylmethacrylamide) (PNIPMAM) polymer, poly(N-isopropylmethacrylamide)-b-poly(acrylic acid) (PNIPMAM-b-PAA) diblock, and poly(acrylic acid)-b-poly(N-isopropylmethacrylamide)-b-poly(acrylic acid) (PAA-b-PNIPMAM-b-AA) triblock copolymer samples were synthesized via ATRP. A facile post-functionalization route was developed that uses an activated ester functionality to convert poly(N-methacryloxysuccinimide) (PMASI) blocks to LCST capable polyacrylamide, while poly(t-butyl acrylate) (PtBA) blocks were converted to water-soluble poly(acrylic acid) (PAA). The post-functionalization was monitored via ¹H NMR and ATR-FTIR. The aqueous solution properties were explored and the PNIPMAM polymers were shown to have a LCST phase transition varying from 35 to 60 °C. The ability to synthesize block copolymers that are thermoresponsive and water-soluble will be of great benefit for broader applications in drug delivery, bioengineering, and nanotechnology.     

Molecular motions and transitions in a poly(p-hydroxystyrene) derivative

The dielectric relaxation mechanisms in a poly(p-hydroxystyrene) derivative have been studied by Thermally Stimulated Depolarization Currents (TSDC) in the temperature range between −160 and 130°C. A broad relaxation was observed at low temperatures, from −160 up to 0°C, which obeyed to the zero entropy approximation. In contrast, the glass transition relaxation showed the usual behavior, a strong departure to the zero entropy prediction. Both relaxations have been studied in detail by the technique of thermal sampling. The thermal behavior of this poly(p-hydroxystyrene) derivative was also studied by Differential Scanning Calorimetry in the temperature range between 20 and 200°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1921–1926, 1999     

Order-disorder transitions in poly(3,4-ethylenedioxythiophene)

The commonly held perception that high conductivity in conducting polymers is linked to a high level of π-stacking order in the material is shown here to be of lesser importance in highly conducting poly(3,4-ethylenedioxythiophene) (PEDT), which has been prepared by chemical vapour phase polymerisation. Despite the fact that there is a highly energetic phase transition about 130 °C (110 J/g), and that this transition corresponds to a loss of the long-range π-stacking as observed in grazing angle XRD, the conductivity remains unchanged beyond the transition and only decreases by a factor of two when heating to above 200 °C. The XRD data suggest that order in two dimension remains above the phase transition measured by DSC and this order is sufficient to maintain a high level of electronic conductivity. Furthermore, as the ligand on the iron salt used in the synthesis is varied, the conductivity of the PEDT varies over two orders of magnitude. These phenomena cannot be explained by different degree of doping or crystallinity and it is proposed that the iron salt has an ordering effect during the vapour phase polymerisation.     

Thermally induced hydrogel formation in aqueous solutions of poly(N-isopropylacrylamide) and fluorocarbon-modified poly(oxyethylene)s

A number of perfluorooctyl-modified poly(oxyethylene)s (F_QPEOs) of molar masses of approximately 2000 and 4000 were synthesised using a two-step procedure: (a) preparation of PEO precursors bearing tertiary amino groups (NPEOs) and (b) their quaternisation with perfluorooctyl iodide to obtain one-ended (F_QE_n), telechelic (F_QE_nF_Q) or PEO modified with a C₈F₁₇-group in the mid of the polyether chain (E_nF_QE_n). F_Q is abbreviation of the C₈F₁₇-group attached to a PEO chain (E_n) through a quaternary ammonium group. The aqueous solution properties of F_QPEOs were studied by viscosity measurements at 25 °C. The cationic charge affects the association ability of the F_Q group and the one-ended F_QE_n behaves like PEO modified with a C₆F₁₃-group through urethane linkage. Temperature induced interactions in aqueous mixtures of F_QPEOs with poly(N-isopropylacrylamide) (PNIPAM) were studied by turbidity and rheological measurements. In the semi-dilute regime, the aqueous polymer mixtures exhibit a large viscosity enhancement upon heating as soon as the temperature reaches 35 °C. In the range from 35 to 40 °C the mixtures of PNIPAM with F_QE_n and E₅₀F_QE₅₀ form gels with viscoelastic response. The solution of PNIPAM mixed with the telechelic F_QE₁₀₀F_Q reveals shear-thickening at 35 and 40 °C.     

Thermal stabilities of poly(N-alkyl maleimides)

The thermal stabilities of polymaleimide (PMI), poly(N-methyl maleimide) (PMMI), poly(N-ethyl maleimide) (PEMI), poly(N-n-propyl maleimide) (PNPMI) and poly(N-isopropyl maleimide) (PIPMI) were studied over the temperature range 300° to 400°C under a constant flow of nitrogen. At the initial stage of degradation the following results were obtained: (1) the main chain of PMMI is the most stable among the poly(N-alkyl maleimides); (2) the weight loss of PEMI, PNPMI and PIPMI is larger than that of PMI and PMMI; (3) the five-membered imide of PMI is the most unstable among the poly(N-alkyl maleimides); (4) the crosslinkages are formed more easily in PMI and PMMI than in PEMI, PNPMI and PIPMI.     

Reentrant conformation transition in poly(N,N-dimethylacrylamide) hydrogels in water-organic solvent mixtures

Conformational changes in poly(N,N-dimethylacrylamide) (PDMA) networks swollen in aqueous solutions of organic solvents are studied both experimentally and theoretically. PDMA hydrogels of various charge densities were prepared by free-radical crosslinking copolymerization. Swelling behavior of the hydrogels was investigated in aqueous organic solvent mixtures as functions of solvent species and the concentration. With increasing volume fraction ? of acetone, tetrahydrofuran, or 1,4-dioxane in the aqueous solution, PDMA hydrogels exhibit reentrant conformation transition. During this transition, the gel first deswells in the range of ? between 0.4 and 0.9, and then rapidly reswells if ? is monotonically increased. The reswelling of the collapsed PDMA gel occurs in a narrow of ? above ?=0.97. It was shown that the reentrant transition in PDMA gels requires moderate hydrogen bonding organic solvents, so that the hydrophobic interactions between PDMA and the organic solvent dominate the swelling process. The results were interpreted using the theory of equilibrium swelling. The interaction parameters in the gel system as well as the partition parameter of the organic solvent between the gel and the solution phases were calculated.     

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.     

A study of thermoassociative gelation of aqueous cationic poly(N-isopropyl acrylamide) graft copolymer solutions

In this work thermoassociative gel formation of a new family of aqueous temperature-responsive copolymer solutions has been investigated. This was achieved using a cationic poly(N-isopropyl acrylamide) (PNIPAm) graft copolymer recently prepared [Liu R, De Leonardis P, Cellesi F, Tirelli N, Saunders BR. Langmuir 2008;24:7099]. The PDMA⁺_x-g-(PNIPAm_n)_y copolymers have x and y values that originate from the macroinitiator; the value for n corresponds to the PNIPAm arm length. DMA⁺ is quarternarized N,N-dimethylaminoethyl methacrylate. The copolymer solutions exhibited cloud point temperatures (T_clpt) of about 33 °C, which were not significantly affected by x/y ratio or the value for n. Thermoassociative gel formation occurred above T_clpt at copolymer concentrations (C_copol) greater than or equal to 4 wt.%. This is a reasonably low C_copol value and is a consequence of the graft copolymer architecture employed. We investigated the effect of temperature, C_copol and copolymer structure on gelation and gel elasticity using variable - temperature dynamic rheology. For PDMA⁺₃₀-g-(PNIPAm₂₁₀)₁₄ solutions at 39 °C it was found that G′ (elastic modulus) scales with C_copol according to G′ ∼ C_copol^3.85. The data suggested that a significant proportion of PNIPAm units is not directly involved in network formation. Thermoassociative gel formation and the gel properties for these systems appear to be governed by a balance between electrostatic repulsion involving the DMA⁺ units (favouring spatial extension of the copolymer backbones) and attractive hydrophobic interactions between PNIPAm side chains (favouring associative crosslink formation).     

Injectable hydrogels of poly(?-caprolactone-co-glycolide)-poly(ethylene glycol)-poly(?-caprolactone-co-glycolide) triblock copolymer aqueous solutions

Thermogelling triblock copolymers of poly(?-caprolactone-co-glycolide)-poly(ethylene glycol)-poly(?-caprolactone-co-glycolide) [P(CL-GA)-PEG-P(CL-GA)] were successfully prepared by control of the hydrophilicity/hydrophobicity balance and chemical compositions of the copolymers. The aqueous solutions of the copolymers underwent sol-gel transition as the temperature was increased from 20 to 60 °C. The amphiphilic copolymer formed micelles in water and a gel was formed by aggregation of micelles. The structure parameters played a critical role in determining sol-gel transition behavior. Either increasing [GA]/[CL] ratio or decreasing P(CL-GA) block length could induce the increase of the lower sol-gel transition temperature. Glycolide (GA) was incorporated into the polymer chain to increase the polymer degradation rate. Sustained release of rifampicin for approximately 32 days was obtained from the gel. It is believed to have potential applications in drug delivery and tissue engineering.     

Interaction of poly(N-acryloyl-amino acids) with saccharides in aqueous media

The interaction of a series of poly(N-acryloyl-amino acids) (pAXaa) with saccharides has been investigated by ¹H NMR. ¹H NMR for methyl-β-d-galactopyranoside (MβGal) in the presence of pAXaa indicated that hydrophobic interaction or hydrogen bonding was not considerable in the interaction of the polymers with MβGal in aqueous media whereas CH-π interaction was relatively important. ¹H NMR for several saccharides in the presence of poly(N-acryloyltryptophan) (pATrp) indicated that pATrp interacted more strongly with the β-anomers than with the α-anomers presumably because of the triple CH-π interactions of the three axial protons in the β-anomers. In the interaction of pATrp with MβGal, MβGal interacted with two or more Trp residues because Trp residues were localized on the polymer chain.     

1H NMR study of temperature-induced phase transitions in aqueous solutions of poly(N-isopropylmethacrylamide)/poly(N-vinylcaprolactam) mixtures

¹H NMR spectroscopy was used to investigate the temperature-induced phase transitions in aqueous solutions of poly(N-isopropylmethacrylamide)/poly(N-vinylcaprolactam) (PIPMAm/PVCL) mixtures to find out if the phase transition of the given component (PIPMAm or PVCL) is affected by the presence of the second component. Our results that PVCL and PIPMAm transitions are in polymer mixtures shifted by ~2 K towards higher temperatures in comparison with neat polymers and depend on polymer concentration show that such effect exists. Spin–spin relaxation times of water (HDO) indicate that in solutions with c ≥ 1 wt% a portion of water is predominantly bound in PVCL mesoglobules even at temperatures above the LCST transition of PIPMAm component. Water is with time released from these mesoglobules without any induction period so indicating that it is mostly indirectly bound water. We assume that there is a direct connection between character of the bound water and the transition temperatures.     

Nuclear magnetic relaxation in poly(N-amyl maleimide) and poly(N-dodecyl) maleimide

Longitudinal relaxation times in the laboratory (T₁) and rotating (T_1ρ) frames have been measured for poly(N-amyl maleimide) and poly(N-dodecyl maleimide). The results are compared with information previously published using dielectric relaxation techniques. A total of three relaxation processes have been detected corresponding to methyl rotation, motion within the substituent alkyl chain and out-of-plane deformation of the maleimide ring. An attempt to fit the T₁ data in the dodecyl derivative to theoretical equations is described.     

Effect of salt on the elastic modulus of poly(N-isopropylacrylamide) gels

The measurement of tensile modulus of poly(N-isopropylacrylamide) (PNIPA) gel in the solution of NaCl, NaI, LiNO₃ and NaNO₃ was carried out. It was confirmed that the tensile modulus of PNIPA gel in the solution of salt depends on the volume of gel regardless of the kind and concentration of salts. This result leads us to the conclusion that the addition of salt effect only on the mixing contribution to the Flory's type free energy of gel especially in the swollen state. Therefore, our result is in agreement with a recent remarkable discovery that the volume of PNIPA gel depends only on the chemical potential of water in spite of the kind of additives. On the other hand, it was found that on the volume phase transition point and in the deswollen state, the elasticity of PNIPA gel depended on the concentration and kind of the salt because the viscoelasticity emerged due to the shrinkage of polymer network.     

Synthesis and characterization of dual stimuli-responsive block copolymers based on poly(N-isopropylacrylamide)-b-poly(pseudoamino acid)

The current study synthesized amphiphilic thermal/pH-sensitive block copolymers PNiPAAm-b-PHpr by condensation polymerization of trans-4-hydroxy-l-proline (Hpr) initiated from hydroxy-terminated poly(N-isopropylacrylamide) (PNiPAAm) as the macroinitiator in the presence of the catalyst, SnOct₂. ¹H NMR, FTIR, and gel permeation chromatography (GPC) characterized these copolymers. Their solutions showed reversible changes in optical properties: transparent below a lower critical solution temperature (LCST) and opaque above the LCST. The LCST values depended on the polymer composition and the media. With critical micelle concentrations (CMCs) in the range of 1.23-3.73 mg L⁻¹, the block copolymers formed micelles in the aqueous phase owing to their amphiphilic characteristics. Increased hydrophobic segment length or decreased hydrophilic segment length in an amphiphilic diblock copolymer produced lower CMC values. The current work proved the core-shell structure of micelles by ¹H NMR analyses of the micelles in D₂O. Transmission electron microscopy analyzed micelle morphology, showing a spherical core-shell structure. The micelles had an average size in the range of 170˜210 nm (blank), and 195˜280 nm (with drug). Observations showed high drug entrapment efficiency and drug-loading content for the drug micelles.     

Thermal transitions in poly (N-alkyl-3,6-carbazolylene)s

The development and nature of the transcystalline-like superstructure produced by pulling the glass fiber (GF) from the isothermal crystallizing (T_c≈135°C) isotactic polypropylene (PP) melt was studied thermooptically. In the vicinity of the pulled GF α row-nuclei form which induce selective β-nucleation resulting in β-cylindrites. The presence of α row-nuclei was evidenced by selective melting of the β-phase having lower melting temperature than the α. Because of the above change in the crystal modification this phenomenon can not be termed by transcrystallinity. The thermal stability and β-nucleation ability of the primary α row-nuclei depend both on the applied shear stress (i.e. molecular orientation) and pulling temperature (T_pull) under the testing conditions: T_αβ<T_c<T_βα and T_αβ<T_pull<>T _βα, where T_αβ and T_βα are the lower and upper threshold temperature of the β-formation, respectively. It was suggested that the observed αβ-bifurcation may be related with epitaxial growth. Paper presented on the conference “Microphenomena in Advanced Composites” (28 June–1 July 1992, Herzlia, Israel)     

Thermo-sensitive sol-gel transition of poly(depsipeptide-co-lactide)-g-PEG copolymers in aqueous solution

A series of biodegradable graft copolymers composed of poly(ethylene glycol) side-chains and a poly(depsipeptide-co-dl-lactide) backbone (PDG-dl-LA-g-PEG) were prepared as a novel thermo-gelling system. An aqueous solution of PDG-dl-LA-g-PEG (20 wt%) with a certain PEG length and composition showed instantaneous temperature-sensitive gelation at 33 °C. The sol-gel transition temperature (T_gel) could be controlled from 33 to 51 °C by varying the PEG length and compositions without a decrease in mechanical strength of the hydrogels. The 20 wt% hydrogel was eroded gradually in PBS at 37 °C for 60 days. This research provides a molecular design approach to create biodegradable thermo-gelling polymers with controllable T_gel and mechanical toughness.     

Thermal properties of hexadecane encapsulated in poly(divinylbenzene) particles

The thermal properties of n‐hexadecane (HD) encapsulated in crosslinked capsule particles containing a water and/or air domain were studied from the viewpoint of heat‐storage applications. The capsule particles were prepared by the microsuspension polymerization of divinylbenzene at 70°C with the self‐assembling of phase‐separated polymer method that we developed. In the differential scanning calorimetric thermograms, pure HD had a single solidification temperature (T_s) peak at 15°C, whereas the encapsulated HD containing a water domain had two peaks at 6 and 1°C. That is, the encapsulated HD containing the water domain required a longer time and lower temperature to complete the solidification than pure HD, which was negative for heat‐storage applications. However, once the particles were dried and the water domain was replaced with air, the problem with the partially lowered T_s improved. The air domain was also found in the encapsulated HD core after solidification because of the shrinkage of HD. The presence of the air domain did not affect the thermal stability of the encapsulated HD. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009