Ali Güner

Degradation of PLA, PLGA homo‐ and copolymers in the presence of serum albumin: a spectroscopic investigation

Poly(D,L-lactide-co-glycolide) samples with different glycolide contents, ie 85:15, 75:25 and 50:50 mole ratios of lactide to glycolide, and poly(L-lactide) were obtained from a commercial source. Polymer films of 15 µm thickness were prepared by a solvent-casting method. Degradation studies were conducted at 37 °C in pH 7.4 phosphate buffered saline and in bovine serum albumin solution. The degradation behaviour of the films was followed by UV and FTIR spectrophotometry and viscometric and gravimetric measurements. Spectroscopic investigations showed that during the first month of degradation, both the chain length and the chain chemical structure changed, especially in the presence of bovine serum albumin. The short chains are attributed to the formation of new ester groups; however, the end-groups may be aldehyde- or ketone-like structures. While the intrinsic viscosities of all polymers decreased continuously after being exposed to the degradation media, insignificant mass loss occurred during the experimental investigation. © 2000 Society of Chemical Industry

Unperturbed dimensions and the theta temperature of dextran in aqueous solutions

Intrinsic viscosity measurements were carried out on dextran samples (of different molecular weights) in aqueous solutions at 25, 28, 31, 34, 37, 40, and 43°C. The extrapolation methods were used for the data; they gave unperturbed dimensions, K0, of the chain. The unperturbed root-mean-square end-to-end distance 〈r2〉[STACK]1/20[ENDSTACK] calculated for the polymer samples in water indicate that the polymer coils are slightly contracted in this solvent as the temperature is increased. The long-range interaction parameter, B, was also determined. In aqueous dextran solutions, this showed a significant decrease in the long-range interactions between 25 and 43°C. The values of Θ = 317.82 and 316.57 K were obtained from the temperature dependence of the interaction parameter B in the Kurata–Stockmayer–Fixman and Berry equations. Calculated values were interpreted mainly on the basis of hydrogen-bond formation between polymer segments and dextran–water molecules in solution.

Crosslinking of dextran with some selective Cl-, P- and N-containing functional substances in aqueous solutions

Abstract Crosslinking reactions of Dextran (Dx) ( M n of 5.0×10 5 and 2.0×10 6 g mol −1 ) with some selective Cl-, P- and N -containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1 3 ) and N , N ′-methylenebisacrylamide (MBAM) as well as with 60 Co-γ-irradiation were carried out in the basic aqueous solutions (2.8 N NaOH) at 25–50°C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found. Some swelling parameters such as initial rate of swelling ( R S , %min −1 ), crosslink density ( ρ , g ml −1 ), equilibrium swelling ( S eq , %) and average molecular weight of crosslinked fragments ( M c , g mol −1 ) for the Dx/crosslinker (CL) systems studied were determined by using of kinetic methods and well known Flory-Rehner equation. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature and concentration of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that R S and ρ values increase while the S eq and M c values decrease depending on the nature of CLs in the following raw: γ-ray-ECH-MBAM-POCl 3 . Concentration of MBAM does not influence the initial rate of swelling while effect of CL concentration is strongly advented in the Dx/ECH and Dx/POC1 3 systems. Effectiveness of CLs is also confirmed by TGA analysis of crosslinking polymers. The formation of unsaturated crosslinked fragments, which are easily polymerized or photocyclodimerized as affected of u.v.-irradiation, was confirmed by Fourier transform infrared (FT-IR) spectroscopy. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems was also described.

Cloud points and Θ temperatures of aqueous poly(N-vinyl-2-pyrrolidone) solutions in the presence of denaturing agents

Abstract Cloud point curves have been determined for various concentrations of aqueous solutions of poly(N-vinyl-2-pyrrolidone) in the presence of a variety of denaturing agents (e.g. thiourea, guanidinium chloride and guanidinium carbonate). From these θ temperatures have been found. The resulting dependences of the critical temperature theta (between 320 and 350 K) on the molar concentration can be expressed as a sequence showing the increasing effect of denaturants in salting out the polymer. The increasing order of effectiveness of denaturing agents increasing the θ temperature is guanidinium carbonate > guanidinium chloride > thiourea. The change of θ temperature by denaturing agents was found to be the result of the changes taking place in the hydrophobic/ hydrophilic interactions among the polymer, solvent and additive denaturing agents, and of the influence of denaturing agent—polymer interactions.

The important role of thermodynamic interaction parameter in the determination of theta temperature, dextran/water system

Abstract The interaction parameter χ was evaluated through the sum of the individual values of enthalpy and entropy dilution parameters, χ H and χ S , for different molecular weights of dextran samples. Subsequently, ((1/2)− χ ) versus temperature relation yielded the theta temperature interpolated to 316.5±0.3 K. Similarly, the dependence of second virial coefficient, A 2 , on temperature was also studied from which another theta temperature was determined equal to 317.0±0.5 K. These two individual theta temperatures evaluated via interpolation methods employing interaction parameter and second virial coefficient were quite in accordance with that calculated as 317.2±0.6 K from intrinsic viscosity measurements achieved between the temperature intervals of 25°C and 43°C for the same system, previously.

Comparison of molecular association of dextran and periodate-oxidized dextran in aqueous solutions

A viscosimetric method was used to investigate the molecular association of dextran, 20% and 40% periodate-oxidized dextrans in aqueous solutions. Intrinsic viscosity numbers, [η], found by Huggins and Kraemer equations, decreased in the presence of denaturing agents like urea, thiourea and guanidinium sulfate. Intrinsic viscosity values also decreased with increasing temperature. This decrease has been even less in the presence of denaturing agents.

Unperturbed dimensions and the theta temperature of dextran in ethylene glycol solutions

Abstract The unperturbed molecular dimensions of dextran samples (of different molecular weights) have been evaluated in ethylene glycol solutions from viscosity measurements at 25°C, 30°C, 35°C, 40°C and 45°C. The unperturbed dimension, K 0 , has been determined from extrapolation methods, i.e, Kurata–Stockmayer–Fixman (KSF), Berry and Inagaki–Suzuki–Kurata (ISK) equations. The hydrodynamic expansion factor, α η , as well as the unperturbed root-mean-square end-to-end distance, 〈 r 2 〉 0 1/2 , found for dextran samples in ethylene glycol solutions, indicated that the polymer coils are contracted as the temperature is raised from 25°C to 45°C. The long-range interaction parameter, B , was also evaluated and a significant decrease is found for the dextran/ethylene glycol system between 25°C and 45°C. The theta temperatures, Θ , were determined as 328.53, 328.02 and 325.93 K from the temperature dependence of the interaction parameter, respectively.

Spectrophotometric behavior of polyvinylpyrrolidone in aqueous solutions. II. The effects of denaturing agents

Electronic spectral behavior of aqueous polyvinylpyrrolidone solutions have been investigated by UV-vis spectrophotometry. n → π * electronic excitations of the polymer were observed to shift longer wavelengths with a variety of denaturing agents. The shifting effect of denaturing agents on the λ max increased in the order guanidinium carbonate guanidinium sulfate > guanidinium chloride > urea. Intrinsic viscosities of different concentrations of aqueous solutions of polyvinylpyrrolidone with the same denaturing agents have also been determined. Intrinsic viscosity number, [η], of polymer solutions decreased with the addition of denaturing agents. The slope, k H [η] 2 , of the polymer was also observed to decrease in the presence of denaturing agents. Shift to the longer wavelengths and decrease in the viscosity characteristics can be explained by the break of the molecular association between the polymer and the water molecules.

Effect of phenolic cosolutes on the main parameters, phase separation and theta temperature of dilute aqueous poly(N-vinyl-2-pyrrolidone) solutions

Abstract The effect of phenolic cosolutes (phenol, catechol, resorcinol, hydroquinone and phloroglucinol) on the main physicochemical parameters, cloud points (Tcp), phase diagram and theta temperature (Θ) of dilute aqueous poly(N-vinyl-2-pyrrolidone) (PVP) solutions was studied experimentally using various concentrations of cosolutes and polymers at different temperatures. The resulting dependence of Tcp and Θ temperatures (between 300 and 343 K) on the molar concentrations can be determined as sequences showing the decreasing effect of cosolute species in salting out the polymer. The decreasing order of effectiveness of phenolic cosolutes in reducing both Tcp and Θ temperatures is phloroglucinol>resorcinol>hydroquinone>catechol>phenol. It is shown that thermodynamic incompatibility of aqueous PVP/phenolic cosolute solutions strongly depends on the effect of the character of phenolic cosolute, changes taking place in the interactions among polymer, solvent and additive phenolic cosolutes and on the change of water structure in the mixture.

Swelling behavior of poly(N‐vinyl‐2‐pyrrolidone) and poly(N‐vinyl‐2‐pyrrolidone)/K2S2O8 hydrogels in urea solutions

Hydrogels with varying cross-linking densities and average molecular weights between two consecutive cross-links were prepared from the binary poly(N-vinyl-2-pyrrolidone)/water and ternary poly(N-vinyl-2-pyrrolidone)/water/K2S2O8 systems by irradiation with γ rays at ambient temperature. Both hydrogel systems were employed for diffusion and swelling experiments in various urea solutions at room temperature. Diffusion of urea solutions into hydrogels has been found to be of the non-Fickian type, except for the samples of PVP/K2S2O8 hydrogels irradiated at higher doses in dilute urea solutions. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate constant, equilibrium water/urea content, and diffusion constant values were evaluated for PVP and PVP/K2S2O8 hydrogel systems at two different urea concentrations. With regard to swelling data, it was determined that the dilute urea solution can penetrate much more effectively into the PVP/K2S2O8 hydrogel system.