Low Cationic Proportion Ampholytic Polymer: Synthesis,Solution Properties and Interaction with Anionic Surfactant

Novel ampholytic terpolymer of N-vinylformamide (NVF), vinylamine (VAm) and sodium acrylate (NA) with low cationic proportion was obtained by hydrolyzing copolymer of NVF and NA (PNVFNA). Solution properties of the polymer were investigated by methods of turbidity and viscosity experiment. The effect of sodium dodecyl sulfate (SDS) on solution viscosity was also investigated. The results showed that the turbidity curves were bimodal, and pH 3.0 was determined as the isoelectric point (IEP). At high salt concentration, polyampholyte effect was noticeable at IEP in viscosity experiment. Although the proportion of VAm in polyampholyte was lower, the solution properties were influenced markedly in the presence of SDS. At IEP, the viscosity increased more rapidly as salt concentration increased when 10^-3 mol/L SDS contained in solution. A mechanism of SDS bound to positive sites on the collapsed globule surface was suggested.     

pH-Controlled association of PEG-containing terpolymers of N-isopropylacrylamide and 1-vinylimidazole

Temperature- and pH-controlled association of terpolymers of N-isopropylacrylamide (NIPA) with 1-vinylimidazole (VI) and polyethylene glycol (PEG) has been investigated by light scattering and atomic force microscopy (AFM) in situ. The polymers contained 0-15 mol% VI and 0-2 mol% PEG. The phase transition temperatures (T_p) have been in the range of 32-45 °C and exhibited significant dependence on the pH of solution in the pH range between 5 and 8. The T_p of the polymers increased with increasing VI content and with decreasing pH, confirming major effect of VI ionization status on T_p. The presence of PEG grafts in the polymer structure had augmenting effect on the magnitude of pH-responsiveness and on the pH-independent colloidal stability of the polymer particles formed above T_p. Incorporation of VI into the polymer structure had similar, but pH-dependent effect on colloidal stabilization of the polymer particles. The size of the particles formed after the phase transition is driven by the association of the collapsed NIPA segments in the globule conformation and it decreased with decreasing pH. The phase transition temperature of the polymers could be adjusted to increase from temperatures below, to temperatures above body temperature upon decreasing pH from 7 to 6, suggesting that such polymers could provide a material platform for a variety of biomedical applications. AFM analysis in situ showed a fully reversible formation of particles in the solutions of the polymers above their T_p.     

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with a hydrophobic comonomer

Hydrogels were prepared by free radical polymerisation in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with di-n-propylacrylamide (DPAM), di-n-octylacrylamide (DOAM) or di-dodecylacrylamide (DDAM) as hydrophobic comonomer. N,N-methylene bisacrylamide (BIS) and glyoxal bis(diallyacetal) (GLY) were used as crosslinkers. A series of copolymers with three different comonomer contents was synthesised and for some polymers three different crosslinker concentrations were employed. The swelling equilibrium of these hydrogels was studied as a function of temperature, hydrophobic comonomer species and content in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS). In pure water the gels showed a discontinuous volume phase transition at 33 and 30 °C for PNIPA and hydrophobically modified PNIPA copolymeric hydrogels, respectively. The swelling ratio r and the transition temperature (LCST) increased at low temperatures with the addition of SDS, this is ascribed to the conversion of non-ionic PNIPA gels into polyelectrolyte gels through the binding of SDS. At SDS concentration below 0.5 wt%, gels exhibited a single discontinuous volume transition at 36-38 °C. However, for SDS concentration above 0.5 wt%, two discontinuous volume transitions at 36-40 and 70 °C were observed. Additionally, the replacement of BIS by the novel octafunctional crosslinker glyoxal bis(diallylacetal) (GLY) yielded an increase in the swelling ratio.     

Conformation of polyacrylamide in aqueous solution with interactive additives and cosolvents

The viscosity of polyacrylamide (PAM) dilute aqueous solutions with NaCl, glucose, and SDS as additives was measured by Ubbelohde viscometry. There was linear relationship between reduced viscosity vs. PAM concentration in aqueous solutions. The Huggins constant k and intrinsic viscosity [η] were used to study the conformation of the polymer chains and the degree of polymer–solvent interaction. In addition, the viscosity of diluted PAM solutions in water with acetone, ethanol, DMF, and ethylene glycol as cosolvent was measured. It was found that the polymer chain conformation contracted as the acetone, ethanol, and DMF cosolvent composition ratio increased, but there was no distinguishing difference between water–ethylene glycol compositions. The solution properties of PAM were used to estimate the swelling properties of PAM gel in the same external conditions, as gel is formed by crosslinking of linear polymer. In good solvent the polymer chain should be expanded, and gel is expected to have large swelling ratio. In water cosolvent systems, when the linear polymer chain underwent coil–globule transition, PAM gel should have volume phase transition under corresponding external conditions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3122–3129, 2003     

Polymer intrusion into narrow pores at the interface between a poor solvent and adsorbing and non-adsorbing surfaces

We report thermodynamic and conformational properties of collapsed polymer globules in a poor solvent close to adsorbing and non-adsorbing surfaces containing a cylindrical pore that is either smaller or larger than the coil size in bulk solution. Configurational Bias Monte Carlo simulations at infinite dilution were used for this purpose. We find that above the critical adsorption transition changing from good solvent to poor solvent conditions causes an increase of the partition coefficient due to cooperative monomer adsorption on the pore wall. Below the adsorption transition, the reverse effect is observed. The radius of gyration components as well as conformational (free) energies and entropies as a function of the chain centre of mass (CM) position along the pore axis reveal strong anomalies in the vicinity of the pore entrance. Below the adsorption transition, the dense globule in a poor solvent swells and deforms when penetrating the pore and recollapses once it is fully confined. Above the adsorption transition in solvents above and below the Θ-point, minima occur in the free energy at chain CM positions just in front of the pore entry causing a barrier upon pore penetration. Upon approaching the small pore from the bulk poor solvent region, the chain conformational entropy and energy run through a maximum below and above the adsorption transition.     

A polyethylene chain investigated with replica exchange molecular dynamics simulation: Equilibrium lamellar thickness and melting point, ordering and free energy

In the present work, a polyethylene chain with N = 200 CH2 units was simulated using replica exchange molecular dynamics (REMD). Simulations were performed in a broad temperature range and for intra-chain interactions varying from the fully interacting to the ideal spring chain.Our work demonstrates that REMD is a very efficient method to obtain equilibrium data. It is found that the coil-to-globule transition is dominated by the vdW energy, whereas the globule-to-folded chain transition is accompanied by transitional behavior in the torsion and vdW energies. Our data clearly show that for the chain length considered here, the chain folded crystal to globule transition is a continuous transition. Nevertheless, we can establish with good accuracy the equilibrium transition temperature for the chain folded crystal to globule transition.A set of orientational order parameters was used to investigate the order in the polymer chain. At the globule-to-folded chain transition an abrupt change in the value of the order parameter is observed, whereas there is no or almost no change in the value of the order parameter at the coil-to-globule transition temperature. The (apparent) order in the disordered globular and coiled states indicated by some studied order parameters is related to the definition of the order parameter and depends on the chain length of the polymer.Below the equilibrium melting temperature the (largest principal component of the) radius of gyration and the equilibrium lamellar thickness of the folded chain crystal decrease with increasing temperature, which gives support to the theory of Muthukumar but is opposite to the prediction of classical crystallization theories. The agreement between simulations and theory may hint to universal behavior of the relative equilibrium thickness versus the relative super cooling.     

Synthesis and thermo-responsive behavior of fluorescent labeled microgel particles based on poly(N-isopropylacrylamide) and its related polymers

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) as an intramolecular fluorescent probe were prepared by emulsion polymerization. The thermo-responsive behavior of the VDP-labeled PNIPAM microgel particles dispersed in water was studied by turbidimetric and fluorescence analyses. The transition temperature of the VDP-labeled PNIPAM microgel particles in water determined by turbidimetric analysis was ca. 32.5 °C. The wavelength at the maximum fluorescence intensity of the VDP units linked directly to the microgel particles dramatically blue-shifted around the transition temperature. In addition it gradually blue-shifted even below the transition temperature where there was no change observed in the turbidity. These findings suggest that the gradual shrinking of microgel particles occurs with increasing temperature and the subsequent dramatic shrinking results in the increasing in the turbidity. The transition temperatures of VDP-labeled poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) microgel particles determined by turbidimetric analysis were ca. 23 and ca. 42.5 °C, respectively, and their thermo-responsive behavior was similar to that for the VDP-labeled PNIPAM system. In these three systems the microenvironments around the fluorescent probes above the transition temperatures became more hydrophobic than those below the transition temperature, and the estimated values of microenvionmental polarity around the VDP units on their collapsed states were almost the same.     

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.     

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.     

Dilute solution behaviour of polymers near the phase separation temperature

Light scattering and viscosity measurements performed in the temperature range between the θ-point and the temperature of phase separation with dilute solutions of polystyrene in cyclohexane and poly(acrylic acid) in dioxane have indicated a different mechanism of precipitation in the two systems. While polystyrene molecules give rise to progressively growing clusters the poly(acrylic acid) coils assume compact forms before precipitating. Phase separation is achieved in the former case by extensive clustering, in the latter by particle coalescence. The dimensions that the compact form of the poly(acrylic acid) molecule can reach before precipitation sets in correspond to a solid-like density inside the molecular domain. From the experimental data the temperatures characteristic for cluster formation (T_c) and for coil to globule transition (T_g) have been obtained.     

Temperature sensitive poly(N-t-butylacrylamide-co-acrylamide) hydrogels: synthesis and swelling behavior

A series of temperature sensitive hydrogels was prepared by free-radical crosslinking copolymerization of N-t-butylacrylamide (TBA) and acrylamide in methanol. N,N′-methylenebis(acrylamide) was used as the crosslinker. It was shown that the swelling behavior of the hydrogels can be controlled by changing the amount of TBA units in the network chains. Hydrogels immersed in dimethylsulfoxide (DMSO)-water mixtures exhibited reentrant swelling behavior, in which the gels first deswell then reswell if the DMSO content of the solvent mixture is continuously increased. In water over the temperature range of 2-64 °C, hydrogels with less than 40[percnt] TBA by mole were in a swollen state while those with TBA contents higher than 60[percnt] were in a collapsed state. Hydrogels with 40-60[percnt] TBA exhibited swelling-deswelling transition in water depending on the temperature. The temperature interval for the deswelling transition of 60[percnt] TBA gel was found to be in the range from 10 to 28 °C, while for the 40[percnt] TBA gel, the deswelling started at about 20 °C and continued until the onset of the hydrolysis of the network chains at around 64 °C. It was shown that the Flory-Rehner theory of swelling equilibrium provides a satisfactory agreement to the experimental swelling data of the hydrogels, provided that the sensitive dependence of the χ parameter on both temperature and polymer concentration is taken into account.     

The Effect of Aniline Hydrochloride Hydrotrope on the Phase Behavior of SDS/Water System

The aim of this work was to determine the detailed phase behavior of the sodium dodecyl sulfate/aniline hydrochloride/water system as a function of concentration of sodium dodecyl sulfate (SDS), aniline hydrochloride (AHC) to sodium dodecyl sulfate molar ratio (R = [AHC]/[SDS]) and temperature. Phase behavior information was obtained via polarizing microscopy, differential scanning calorimetry (DSC), cryo-scanning electron microscopy (Cryo-SEM) and oscillatory linear rheological measurements with good agreement among these techniques. It is well known that SDS in water forms spherical micelles at concentrations lower than 40 wt% and temperatures above its Krafft temperature (T _k = 16–21 °C). In this region, the SDS/water system exhibits Newtonian rheological behavior, which is characteristic of spherical micellar solutions. The addition of the hydrotrope, aniline hydrochloride, to SDS aqueous solutions produces a viscosity increase in this system as R augments, and a maximum of about five orders of magnitude was found at R = 0.47 for 5 wt% SDS at 20 °C. Moreover, the system shows a transition from viscous to strong viscoelastic behavior. These changes in the rheological behavior are produced by the transitions from sphere to rodlike micelles, which are induced by the hydrophobicity of AHC causing it to be absorbed into the core and the hydrophilic interface of the micelles, which screens the repulsions between the charged head groups.     

Micellar Growth in Cetylpyridinium Chloride/Alcohol System: Role of Long Chain Alcohol,Electrolyte and Surfactant Head Group

The microstructural transition of aqueous 0.1 M cetylpyridinium chloride (CPC) in the combined presence of salt KBr and long chain alcohol (C₉OH-C₁₂OH) has been studied as a function of alcohol concentration, electrolyte concentration and temperature. The viscosity of the CPC/KBr micellar system showed a peaked behavior with alcohol concentration (C ₀), due to alcohol induced structural transition, which was confirmed by dynamic light scattering (DLS) and rheological analysis. Besides C ₀, the chain length of alcohol (n) was found to show a remarkable effect on the micellization behavior of CPC/KBr system. It was observed that the ability of alcohol to induce micelle growth diminishes with n, which was well supported by viscosity, rheology and DLS measurements. To examine the effect of the electrolyte on the micellar growth, the salt concentration was varied from 0.05 to 0.15 M and it was observed that with increase in [KBr], the peak position shifts towards lower C ₀. The effect of temperature on the micellar system showed interesting phase behavior for CPC/KBr/Decanol. The system exhibited a closed solubility loop with an upper critical solution temperature (UCST) > the lower critical solution temperature (LCST), reminiscence of nicotine-water system. The role of surfactant head group on the structural evolution was revealed by comparing the present results with our previous report for similar micellar system, CTAB/KBr/long chain alcohol.     

Contributions to the thermodynamics of polymer hydrogel systems

In this work, an extended version of a quasichemical thermodynamic model is presented. The swelling behavior of crosslinked acrylamide polymer gels and N-substituted derivatives, such as N-isopropylacrylamide and N-tert-butylacrylamide has been compared to predictions from such model which takes into account the specific hydrogen bonding interactions encountered in these systems. The calculated volume transition temperature of the poly(N-isopropylacrylamide) gel is 0.8 °C lower than the experimental value and the predicted solvent volume fraction in the collapsed and swollen gel states are about 2% larger than the corresponding experimental data measured at the transition point. Applying the same energy parameters obtained from regressing poly(N-isopropylacrylamide) gel swelling pressure data, the model has also been capable to correctly represent the major features found in the swelling behavior of linear poly(N-tert-butylacrylamide) and poly(N-tert-butylacrylamide) gels, after the model parameters that characterize the molecular structure were changed in accord to each polymer repetitive unit.     

Study on conformational transition phenomena of poly(methyl methacrylate) in acetonitrile near theta conditions

The coil–globule transition for poly(methyl methacrylate) (PMMA) has been studied in a theta solvent, acetonitrile (Θ = 45 °C). The viscosity of PMMA was measured as a function of temperature in the range 26–55 °C. The contraction and expansion of the molecular chains are determined using the measured viscosity values. The temperature dependence of the intrinsic viscosity can be represented by a master curve in a versus |τ|M _w^1/2 (g^1/2 mol^−1/2) plot, where τ = |T − Θ|/T is the reduced temperature and M_w‐is the weight‐average molecular weight. A universal plot of reduced viscosity versus reduced blob parameter (N/N_c) shows the attainment of the collapsed state below the theta temperature. The dimensions of PMMA in acetonitrile (M_w = 3.15 × 10⁶ g mol⁻¹) decrease to 63 % at 26 °C of those in the unperturbed state. The results in this work are compared with those previously published. © 2000 Society of Chemical Industry     

Application of time–temperature superposition principle to polymer transition kinetics

The time–temperature equivalence equation is deduced simply in view of the transition kinetics of a polymer. The independent variables time and temperature are separated in the two sides of the resulting equation. Thus, a physical property of the polymer, which is temperature dependent, can be matched with the theoretical calculated curve from a supposed model of transition kinetics in which only time is involved as the independent variable. By comparing different models, one may judge which model is probably more correct. The procedure of data fitting is described. As an application example, the measured viscosity data at different temperatures for the coil–globule transition of poly(N‐isopropylacrylamide) in aqueous solution is tested to judge its transition mechanism. A transition mechanism involving a two‐stage reversible reaction fits the experimental data in a satisfactory way. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1767–1772, 2006     

Flow analysis of aqueous solution of silk fibroin in the spinneret of Bombyx mori silkworm by combination of viscosity measurement and finite element method calculation

Bombyx mori silk fibers possess outstanding mechanical properties in spite of being spun at room temperature and from the aqueous solution. Therefore, the mechanism of the structural transition has been studied with great attention, but still not be well understood. In this study the flow simulation of the silk fibroin aqueous solution using finite element method was performed on the basis of both the relationship between the viscosity and shear rate of the silk fibroin solution prepared from the silk gland, and the detailed structure of the spinneret including silk press part of the silkworm obtained from the optical micrographs. The viscosity of the silk fibroin solution decreased with power-law till the shear rate, about 1.5 s⁻¹ with increasing shear rate. Then the viscosity increased reversely which is speculated due to the fiber formation as a result of aggregation of the molecules. In the flow simulation analysis, the initiation site of the fiber formation was calculated by regulating the extrusion pressure. The fiber formation occurs in 550 μm from the spigot at 1 MPa and in 600 μm from the spigot at 50 MPa. The extrusion pressure in the range from 1 MPa to 50 MPa induces the fiber formation in the stiff plates (550-600 μm from the spigot), that is, the silk press part in the spinneret.     

Two-dimensional Fourier transform rheological study on thermosensitivity of poly(N,N-diethylacrylamide) in aqueous solutions

The phase transition of a thermo-responsive polymer, poly(N,N-diethylacrylamide) (PDEA) above its critical overlap concentration (c*) has been studied by two-dimensional Fourier transform (FT) rheology using Large Amplitude Step Shear Oscillation (LASSO). This technique allows the separation of the linear and nonlinear contributions to different relaxation processes and the determination of their time scale and amplitude through the time response of the shear stress after step strain experiments. The interchain interactions increase at the onset of the phase transition at 29 °C, indicated by an increased non-linear contribution at short relaxation times as compared to the single phase condition. During the phase separation of a concentrated solution above the phase transition temperature, the polymer-rich phase can form a transient network created by the hydrophobic interactions between the collapsed polymer chains. The non-linear behavior of a phase-separated system well above the transition temperature (at 33 °C) reflects the stretching of the bridging chain segments between larger aggregated domains and the coalescence of aggregates broken during the step in strain. Relaxation time distributions have been fitted in the LASSO spectra by the nonlinear regularization (NLREG) technique and the relaxation times have been attributed with various linear and non-linear processes below and above the phase transition temperature.     

Solvent dependence of the viscous and viscoelastic properties of a high molecular weight polystyrene in moderately concentrated solution

The steady-flow viscosity and viscoelastic behaviour of two solutions of a sensibly monodisperse polystyrene of high molecular weight (M_w = 498 000) have been measured over a temperature range of 100°C for identical concentrations of 20.55 wt.%. Toluene and methyl ethyl ketone were chosen as the two low viscosity solvents having, respectively, good and marginal thermodynamic affinities. Dynamic viscoelastic measurements were made at a frequency of 41 kHz using travelling torsional waves. At this frequency, both solutions exhibit behaviour characteristic of the rubbery region, and the ratio of the dynamic viscosity normalised by dividing by the corresponding solvent viscosity is independent of the solvent until the onset of the glass transition region with decreasing temperature. The storage shear modulus of the toluene solution in the rubbery region is higher than for the MEK solution, indicating a higher entanglement density in the better solvent and a larger polymer radius. Some features of the results in the poor solvent (MEK) appear to indicate that, as the temperature decreases, partial exclusion of the solvent leads to the formation both of stronger entanglements and of macromolecular aggregates or bundles, as suggested by Dreval and others^6–8,11,22.     

Elongational flow studies of the conformation of DNA molecules in the globular state

Elongational flow‐induced birefringence of a T4‐phage DNA aqueous solution was measured with changing NaCl and polyethylene glycol (PEG) concentrations. DNA molecules are known to manifest a coil–globule transition with increasing PEG concentration. At certain PEG concentrations near the critical concentration of the transition, the globular DNA solution, which was expected to be nonbirefringent, showed flow‐induced birefringence. Strain‐rate dependence of the birefringence intensity, having a critical strain rate, was similar to that of the flexible polymer chain that manifests the coil–stretch transition. The flow‐induced birefringence pattern, however, suggested that the globular DNA molecules were rigid and optically anisotropic. At the critical strain rate, the globular DNA molecules in the solution of the particular PEG concentration were considered to collapse nonadiabatically to an optically anisotropic and mechanically rigid conformation. The overall shape of the collapsed conformation of the globular DNA was estimated to be an ellipsoid with an aspect ratio of about 0.7. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1351–1358, 2000