Light scattering peaks from aqueous solutions of acrylamideacrylic acid copolymers

Aqueous solutions of acrylamide-acrylic acid copolymers of high molecular weight have been investigated by light scattering. It was shown that a scattering peak can be obtained if the samples used have been prepared by alcaline hydrolysis of polyacrylamide and if the solutions are filtered and sonicated. In the range of the lower concentrations, the position of this peak, q*, is found to vary with the polymer concentration C, as C^1/3, and to be independent of the charge density. The addition of very small amounts of salt provokes the disappearance of the peak. A phase diagram (charge density versus concentration of the polymer) has been established. It shows that these experiments were carried out in a concentration range intermediate between the dilute and the semidilute regimes (transition regime). The experimental scaling laws however correspond to the theoretical predictions for the dilute regime. Moreover, the q* values are much lower than those calculated by the random phase approximation theories.     

Turbidimetric studies of aqueous poly(vinyl alcohol) solutions

A freezing-thawing process of aqueous poly(vinyl alcohol), (PVA) solutions was undertaken to study the supermolecular PVA structures produced. Aqueous solutions of 2,5 to 15 wt.-% PVA were frozen at ?20°C for 45 to 120min and were subsequently thawed for long periods of time (up to 12h) at 23 ± 1°C. The transmittance of visible light was recorded as a function of thawing time. Determination of the parameters of the supermolecular particles in the PVA solutions was done by application of Klenin's theory. The average radii of the particles were in the order of 1,6–2,3 μm.     

Viscosity of aqueous and alkaline solutions of poly(ethylene oxide)

Viscosity measurements over a wide range of temperature were made on aqueous solutions of poly(ethylene oxide) samples of different molar mass and of low polydispersity. Above ca. 308 K the Mark-Houwink exponent decreases and tends to a value of 0,50 at a lower critical solution temperature of 376 ± 2 K. The same value for this θ-temperature is derived also as that at which the slope of Stockmayer-Fixman plots tends to zero. At 298 K the unperturbed dimensions [〈r²〉₀/M]^1/2 = 0,089 nm · g^?1/2 · mol^1/2 and their temperature coefficient d ln 〈r²〉₀/dT ≈ ? 1,5 (±0,5) · 10^?3 K^?1. In aq. KOH the intrinsic viscosity [η] is virtually uninfluenced by pH at low-medium pH, but at high pH, [η] falls sharply. Viscosity and phase separation measurements yield θ-conditions in 1,24 M aq. KOH at 298 K. The derived polymer-water interaction parameters χ increase from 0,449 at 276 K to 0,493 at 358 K, but the enthaplic component χ_H is incapable of yielding a meaningful value for the solubility parameter δ₂ of the polymer in such a hydrogen-bonding system. The calculated value of χ₂ was 18,9 (kJ · dm^?3)^1/2.     

Gelation kinetics in aqueous solutions of syndiotactic-rich poly(vinyl alcohol)

Aqueous solutions of Syndiotactic-rich poly(vinyl alcohol) (PVA) form thermally reversible gels. The volume of the solution decreases with gelling time on standing at isothermal condition. Assuming that the gelation is due to the crystallization of PVA, the kinetics of isothermal crystallization from 5 wt.-% aqueous solutions of PVAs having 64,3 mol-% syndiotactic diads was investigated as a function of temperature (45–90°C), using dilatometry. The maximum in the overall rate of crystallization appeared at 60°C, regardless of the degree of polymerization of PVA. Since the values of n in the Avrami equation vary from about 1 to 2, it was apparent that the crystallization from aqueous solution of the polymer proceeded in the fashion of one-dimensional growth and heterogeneous, diffusion controlled nucleation. The phase diagram revealed that the syneresis of the gel was caused by the crystalline phase separation which supports the gelation accompanying crystallization.     

Liquid crystals from side-chain mesogenic poly(vinyl ether-alt-maleic anhydride)

Alternating copolymers of maleic anhydride with vinyl ethers, bearing a 4′-methoxy-4-biphenylyl mesogenic group via an oligo(ethylene oxide) spacer, were synthesized and structurally characterized. From X-ray diffraction observations, a smectic E phase was observed. Smectic layers are single layers of ribbon-like polymer chains, with all pendant groups arranged in a single row and pending on the same side of the backbone. Mesogenic groups are oriented up and down at random, and polymer chains are aligned along the [110] direction of the rectangular lattice describing the packing of the pendant groups.     

Investigation of the thermal aggregation in aqueous poly(N-isopropylacrylamide-co-itaconic acid) solutions

Copolymers of N-isopropylacrylamide (NIPAAM) and itaconic acid (IA) were prepared via free radical polymerisation using potassium persulfate (K₂S₂O₈) in distilled-deionised water at 50°C in nitrogen atmosphere. The dependence of the aggregation process on pH and IA content and the copolymer composition were examined by means of UV-VIS, FTIR, and ¹H NMR spectroscopy, gel permeation chromatography (GPC), and viscosity measurements. Cloud points of 0.5 wt.-% aqueous solutions of the copolymers and of PNIPAAM were measured in the range of pH 1–9 and found to decrease with increasing NIPAAM content and decreasing pH. The differences in the phase transition temperatures were supposed to result from interactions between the isopropyl group and unionised carboxylic acid groups or electrostatic interactions between the carboxylate groups, depending on the pH and hydrophilic constituent content.     

Grafting of maleic anhydride onto poly(tetrahydrofuran)

A well-defined modification of poly(tetrahydrofuran)s was realized by grafting with maleic anhydride. The grafted products were investigated by Fourier-transform infrared (FTIR) spectroscopy and ¹³C nuclear magnetic resonance (NMR) spectroscopy. FTIR spectroscopy evidenced a characteristic shift of the carbonyl valence vibrations of cyclic anhydrides to higher wave numbers due to grafting. The grafting of maleic anhydride, which occurred mainly onto poly(tetrahydrofuran) carbons in α-position, was proved by means of ¹³C NMR spectroscopy. Furthermore, it was found that approximately 10% of all graftings took place onto poly(tetrahydrofuran) carbons in β-position. All the grafted units were monosubstituted succinic anhydride units. Indications for the formation of poly(maleic anhydride) graft units were not found. The quantitative determination of grafted anhydride units was carried out by titration after hydrolysis to the acid form. Remarkably high values of percentage grafting up to 20 wt.-% of maleic anhydride were obtained using an initiator concentration of only 2.5 mol-% related to maleic anhydride. The high values of percentage grafting at an initail maleic anhydride concentration of 20 wt.-% were due to a nearly complete conversion of the maleic anhydride. If the initial amount of maleic anhydride was higher than 20 wt.-% phase separation took place during the reaction. This phase separation prevented complete conversion of the maleic anhydride.     

Dilute solution properties of poly(allylammonium chloride) in aqueous sodium chloride solutions

Ten fractions of poly(allylammonium chloride) were investigated at 25°C in sodium chloride solutions of various concentrations, c_s, by viscosity and light scattering techniques. The molecular weight (M_w)-intrinsic viscosity [η] relationships were established at four NaCl concentrations. The unperturbed dimensions estimated from the Stockmayer-Fixman plot are independent of c_s. The second virial coefficient A₂ and the excluded volume parameter B as well as [η] were found to be proportional to c and to have negative values in the limit of infinite c_s. In addition, it was found that the electrostatic part of the expansion factor can be expressed in terms of a reduced parameter (M_w/c_s)^1/2 in a fairly wide range of M_w and c_s.     

Light scattering studies of poly(N-isopropylacrylamide) microgel particles in mixed water-acetic acid solvents

Properties of poly(N-isopropylacrylamide) (PNIPAM) microgel particles were examined in mixtures of water and acetic acid at 20°C using dynamic light scattering in order to investigate the cononsolvency phenomenon. It was found that the cononsolvency exhibited by the PNIPAM-water-acetic acid ternary system results in five regions in response to changes in solvent composition. Upon the addition of acetic acid to the aqueous dispersion medium, the regions appeared successively: volume change from a swollen to a collapsed state, unstable microgel aggregates (in water rich range), precipitation, unstable microgel aggregates (in acetic acid rich range), and swelling. The volume change from swollen to collapsed states starts before the phase-separation composition is achieved. Both volume phase transition and swelling are continuous but undergo in a sharp way. In a very narrow range of solvent composition, the microgel aggregates formed are unstable, while the immiscible phase was found to span a wide range. The degree of swelling is dependent on the crosslinker concentration in the solutions of very higher acetic acid content. The cononsolvency of the mixed water-acetic acid solutions for PNIPAM microgel particles is not only involved in changes of the interactions between their components but also in the partial neutralisation of surface charges of the PNIPAM microgel particles.     

Complex formation between poly(vinyl ether) of ethylene glycol and poly(acrylic acid) in aqueous and organic solutions

Interpolymer reactions between poly(acrylic acid) and poly(vinyl ether) of ethylene glycol were studied by viscometric and spectroturbidimetric methods in aqueous and organic solutions of different nature. It is shown that the formation of interpolymer complexes strongly depends on the strength of the polymer-solvent interaction. A decrease of the thermodynamical quality of the solvents should be favorable for the complexation process.     

Photoresponsive liquid crystals prepared from poly(biphenylyl vinyl ether-alt-maleic anhydride)s

Photoresponsive groups (azobenzene and cinnamic acid derivatives) were incorporated into alternating copolymers of maleic anhydride and vinyl ethers with pendant 4′-methoxy-4-biphenylyl moieties bound via ethylene glycol spacers. The resulting polymers were examined with respect to their liquid crystalline behavior and photochemical reactivity in relation to the isothermal phase transition behavior.     

Molecular association in aqueous solutions of high molecular weight poly(N-vinyl-2-pyrrolidone)

A viscosimetric method was used to investigate the molecular association in aqueous solutions of poly(N-vinyl-2-pyrrolidone), (PVP), of molecular weight 700 000. Limiting viscosity numbers [η] and Huggins constants k_H of the polymer solutions were observed to decrease upon addition of denaturing agents such as thiourea and guanidinium sulfate. Guanidinium sulfate was found to be even more effective in its denaturing action as compared to urea and thiourea. The decrease in [η] and k_H values increased with increasing concentrations of the denaturing agents. This behaviour was explained on the basis of the rupture of bridges formed by water molecules among different PVP chains, hydrogen bonding being responsible for these intermolecular associations. While no change in the [η] values was observed for PVP in 2 M thiourea solutions upon increasing the temperature from 25°C to 35 and 45°C, for the same increase in temperature the [η] values of aqueous PVP solutions showed substantial decreases. This is also attributed to the breaking of hydrogen bonds existing between PVP molecules.     

Biodegradable and thermoreversible hydrogels of poly(ethylene glycol)-poly(epsilon-caprolactone-co-glycolide)-poly(ethylene glycol) aqueous solutions

The aqueous solutions of triblock copolymers of poly(ethylene glycol)-poly(epsilon-caprolactone-co-glycolide)-poly(ethylene glycol) [PEG-P(CL-GA)-PEG] undergoing sol-gel transition as the temperature increases from 20 to 60 degrees C were successfully prepared. The thermogelling block copolymers were synthesized by subtle control of the hydrophilic/hydrophobic balance and the chain microstructures. The amphiphilic block copolymer formed micelles in aqueous solution, and the micelle aggregated as the temperature increased. The sol-gel transition of the copolymer aqueous solutions was studied focusing on the structure-property relationship. GA was incorporated into the polymer chain to prevent crystallization of PCL component and increase the polymer degradation. It is expected to be a promising long-term delivery system for pH-sensitive drugs, proteins, and genes.     

Stereosequencing in poly(leucine) obtained from stereoselective polymerization of racemic leucine N-carboxy anhydride

Polymerization of mixtures of D - and L -isomers of α-amino acid N-carboxy anhydrides follow normal copolymerization behavior, and sometimes even ideal copolymerization, which is conventionally interpreted as leading to a random distribution of monomer units along the polymer chains. Equations are dereived which relate the fraction of isotactic L -diads to experimental data from the previously reported methods involving sequential and stereospecific cleavage with enzymes. The application of these equations to poly(D ,L -leucines), which were prepared by polymerization of racemic leucine N-carboxy anhydride with chiral and achiral primary amines as initiators, indicated that the polymers contain between 94 and 98% isotactic diads. The poly(leucines) thus consist of long stereoblocks despite the observed ideal copolymerization behavior.     

Light scattering and in vitro biocompatibility studies of poly (vinyl pyrrolidone) derivatives with amino-acid-dependent groups

Two poly (vinyl pyrrolidone) (PVP) families with amino-acid residues (glycine, beta-alanine, gamma-aminobutiric acid and epsilon-aminocaproic acid) on the base of the co-polymer N-vinyl pyrrolidone and allyl-glycidyl ether (VP-AGE) and on the base of epoxidized PVP (EPVP) were synthesized. Static and dynamic light scattering measurements of these PVP derivatives in water showed that their structure/ behavior were similar to that of PVP. The bioreactivity was also similar to that of PVP. Further investigation of the immunoreactive properties of the derivatives in in vitro proliferation assays with fresh normal human peripheral blood lymphocytes and monocytes led to the determination of a costimulatory profile for each derivative in terms of polyclonal stimulation, specific antigen presentation, and immunoglobulin secretion. This profile allows the selection of an appropriate derivative as a carrier that would suit the immunoreactivity needs of the immobilized ligand.     

Notes on light scattering from dilute poly(vinyl chloride) solutions in a binary theta-solvent,tetrahydrofuran/water

Tetrahydrofuran (THF) with 8–9 vol.-% of water (depending on the polymer tacticity) is at 25°C a Θ-solvent for poly(vinyl chloride). The refractive index increment (dn/dc)_μ (i.e. measured after dialysis) for this system is 0.108₅ and 0.112 ml/g for λ₀ = 546 and 436 nm, respectively. The light scattering measurements yield correct molecular weights of the polymer. The measurements, however, require special care. Evaporation of THF and polymer aggregation in solvents with even slightly negative second virial coefficients are the major source of error.     

In vitro release of clomipramine HCl and buprenorphine HCl from poly adipic anhydride (PAA) and poly trimethylene carbonate (PTMC) blends

Controlled drug-delivery technology is concerned with the systematic release of a pharmaceutical agent to maintain a therapeutic level of the drug in the body for modulated and/or prolonged periods of time. This may be achieved by incorporating the therapeutic agent into a degradable polymer vehicle, which releases the agent continuously as the matrix erodes. In this study, poly trimethylene carbonate (PTMC), an aliphatic polycarbonate, and poly adipic anhydride (PAA), an aliphatic polyanhydride, were synthesized via melt condensation and ring-opening polymerization of trimethylene carbonate and adipic acid, respectively. The release of clomipramine HCl and buprenorphine HCl from discs prepared with the use of PTMC-PAA blends in phosphate buffer (pH 7.4) are also described. Clomipramine HCl and buprenorphine HCl were both used as hydrophilic drug models. Theoretical treatment of the data with the Peppas model revealed that release of clomipramine HCl (5%) in devices containing 70% PTMC or more followed a Fickian diffusion model. However, the releases of buprenorphine HCl (5%) in the same devices were anomalous. For devices containing 50% and more PAA, surface erosion may play a significant role in the release of both molecules.     

Vapour-liquid equilibria of concentrated poly(styrene-co-butadiene) and poly(styrene-co-acrylonitrile) solutions

Isopiestic vapor sorption measurements were carried out for concentrated solutions of poly(styrene-co-butadiene) (SBR) and poly(styrene-co-acrylonitrile) (SAN) in a number of aromatic solvents in the temperature range of 343 to 398 K. Henry constants were determined by means of inverse gas-liquid chromatography in the temperature range of 373 K (SBR) or 423 K (SAN) to 473 K, to obtain additional data at infinite dilution for the solvents, and for two of the monomers, styrene and 1,3-butadiene, in SBR. The chain-of-rotators (COR) equation of state model was used to calculate the thermodynamic quantities applying a group-contribution approach. In general, good agreement is observed between experimental and calculated results.     

Phase behavior and miscibility in blends of poly(sebacic anhydride)/poly(ethylene glycol)

In this research, poly(sebacic anhydride) was synthesized via melt-condensation. The polymer was then blended with poly(ethylene glycol) in various ratios by solvent casting, to obtain the desired polymer blends. A differential scanning calorimeter was employed to investigate the crystalline behavior of the blends. Blends with under 10% poly(ethylene glycol) were found to consist of two partially miscible polymers in the amorphous phase. This compatibility of the two polymers may induce the crystallization of the poly(sebacic anhydride) component from decreasing the glass transition temperature of the poly(sebacic anhydride) component in the blending system due to the presence of poly(ethylene glycol) chain segments. Furthermore, phase separation occurred and the crystallinity of poly(sebacic anhydride) diminished when at least 20% poly(ethylene glycol) was present in the blends. These results were verified by the variation in the absorptions of carbonyl groups in infrared spectra; these spectra exhibit the changes in crystallinity of poly(sebacic anhydride) in the blends.