Darinka Christova

New thermo-responsive polymer materials based on poly(2-ethyl-2-oxazoline) segments

Abstract First, the solution behavior of poly(2-ethyl-2-oxazoline) (PEtOx) in water has been investigated. The dependence of the cloud points on the molecular weight and concentration indicates a typical Flory–Huggins (Type I) demixing behavior with a lower critical solution temperature (LCST). Secondly, the synthesis and properties of temperature-responsive hydrogels and segmented polymer networks, based on PEtOx bis-macromonomers, are reported. PEtOx hydrogels have been prepared by UV-induced radical polymerization of the corresponding α,ω-bis-acrylates. The networks exhibited a continuous shrinkage with increasing temperature. Series of segmented networks with LCST-behavior have been obtained by free radical copolymerization of PEtOx bis-macromonomer with the comonomers 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl acrylate (HPA) or methyl methacrylate (MMA). The LCST-behavior of the materials is controlled by varying the philicity of the comonomer and the fraction of PEtOx in the networks. PHEMA–PEtOx and PHPA–PEtOx hydrogels exhibited a large and reversible swelling–deswelling process, whereas the volume changes in PMMA–PEtOx swollen networks were small and occurred in a broad temperature interval.

Polyester-Based (Bio)degradable Polymers as Environmentally Friendly Materials for Sustainable Development

This review focuses on the polyesters such as polylactide and polyhydroxyalkonoates, as well as polyamides produced from renewable resources, which are currently among the most promising (bio)degradable polymers. Synthetic pathways, favourable properties and utilisation (most important applications) of these attractive polymer families are outlined. Environmental impact and in particular (bio)degradation of aliphatic polyesters, polyamides and related copolymer structures are described in view of the potential applications in various fields.

Amphiphilic segmented polymer networks based on poly(2-alkyl-2-oxazoline) and poly(methyl methacrylate)

Amphiphilic segmented polymer networks (SPNs) have been prepared by free radical copolymerization of methyl methacrylate with α,ω-bisacrylate terminated poly(2-methyl-2-oxazoline) and poly(2-ethyl-2-oxazoline). DSC and DMTA analysis demonstrated the influence of the copolymer composition, the molecular weight and nature of the poly(2-alkyl-2-oxazoline)s and the polymerization conditions on the final phase morphology of the networks. A comparative phase morphology study of the polymer blends and SPNs revealed the more compatible phase morphology of the networks. The compatibility could be further increased by UV-induced network formation. The investigation of the solvent uptake in different solvents clearly demonstrated the amphiphilic nature of the SPNs.

Study of the Potential of Amphiphilic Conetworks Based on Poly(2-ethyl-2-oxazoline) as New Platforms for Delivery of Drugs with Limited Solubility

Thermoresponsive amphiphilic conetworks comprising poly(2-ethyl-2-oxazoline) (PEtOx), 2-hydroxyethyl methacrylate, and 2-hydroxypropyl acrylate segments have been studied as new platforms for delivery of drug with limited solubility. Series of conetworks of varied composition were synthesized and swelling kinetics in aqueous media and ethanol were followed. The platforms were loaded with the hydrophobic drug ibuprofen by swelling in its ethanol solution. The structure and properties of the drug carriers were investigated by scanning electron microscopy and differential scanning calorimetry. The release kinetics profiles of ibuprofen from the studied platform were established. The investigation proved the feasibility of the PEtOx-based amphiphilic conetworks as highly effective platforms for sustained ibuprofen delivery.

Thermoresponsive hydrogels of hydrophobically modified polyglycidol

Abstract Synthesis, swelling properties and thermoresponsive behavior of a new class of hydrogels based on high molar mass polyglycidol are presented. High molar mass polyglycidol was synthesized by coordination polymerization. Thermosensitive poly(glycidol-co-glycidyl ethyl carbamate)s were obtained via a simple chemical modification of the polyglycidol with ethyl isocyanate. Polyglycidol and poly(glycidol-co-glycidyl ethyl carbamate)s were chemically crosslinked in DMF solution using oligomeric poly(ethylene glycol)-bis-(carboxymethyl ether chloride) as crosslinking agent. The hydrogels of degree of swelling of over 1000% were obtained. Thermoresponsive properties of the networks and the swellingdeswelling behavior of hydrogels in response to cyclic changes of the temperature were studied.

Thin films from hydrophilic poly(N,N-dimethyl acrylamide) copolymers as optical indicators for humidity

In the present paper we study thin films from poly(N,N-dimethyl acrylamide)-poly(ethylene oxide) (PDMAA/PEO) copolymers of different composition and structure in order to implement them as sensitive media for optical indicators for humidity. PDMAA/PEO di- and triblock copolymers were synthesized via redox polymerization in aqueous media. Thin films were deposited on silicon substrates by spin coating method using polymers solutions with appropriate concentrations. Refractive index, extinction coefficient and thickness of the films are calculated from reflectance spectra of the films deposited on silicon substrates using non-linear curve fitting method. Sensing properties of the films were tested by films exposure to different humidity levels followed by in-situ monitoring of the changes in the optical properties. The influence of the polymer structure and postdeposition annealing on the optical and sensing properties of the films was investigated. The potential application of selected polymers for optical sensing of humidity were demonstrated and discussed.

Glutaraldehyde-crosslinked poly(glycidol-block-ethylene oxide-block-glycidol) networks with temperatureresponsive swelling behaviour

Abstract In order to obtain temperature-responsive hydrogel networks, poly- (glycidol-block-ethylene oxide-block-glycidol) copolymers were chemically crosslinked by using glutaraldehyde and tetraethoxysilane in a sol-gel reaction. The crystallinity of copolymer networks prepared has been estimated by differential scanning calorimetry. Thermo-responsive properties were evaluated by measuring the equilibrium swelling degree for a series of copolymer hydrogels as a function of temperature.

Color sensing of humidity using thin films of hydrophilic cationic opolymers

The possibility for color sensing of humidity at room temperature using thin films from hydrophilic cationic block copolymers of different compositions and thickness is demonstrated. Diblock copolymers of composition poly [(ethylene oxide)-b-(N, N-dimethyl acrylamide)] (PEO-b-PDMAA), the PDMAA block being modified with different amounts of quaternary ammonium groups, are synthesized via redox copolymerization in aqueous media. Silicon wafers and Bragg stacks are used as substrates for spin-coating of thin polymers films with thickness in the range 50–200 nm. The color change is quantified through calculation of CIE tristimulus color coordinates using reflectance spectra of the films measured at relative humidity (RH) of 5% and 97%. The influence offilm thickness, substrate type and chemical modification on the humidity sensitivity and color change of the films is demonstrated and discussed.

Influence of Pelvis Width and Leg Length on the Wear Behavior of UHMWPE Hip Cup

Total hip joint replacement considered one of the success sections of the modern medicine, but still the wear of total hip prostheses is a significant clinical problem and this problem might be increasingly complex with adding the influence of pelvis width and leg length during the preclinical testing. This study aims to analyze the effect of the leg length (femur and tibia) and the pelvis width on the wear behavior of ultra-high molecular weight polyethylene (UHMWPE) hip cup. The experiments have been carried out on a total leg joint’s simulator (TLJS), which was particularly designed for this purpose according to ISO 14242-1 standard. The TLJS consists of three units: leg components (femur, tibia, and foot), quick-return mechanism, and cam mechanism were used to reproduce the flexion–extension and external–internal movement, respectively, and upper body was used to stimulate the pelvis construction and applied load. The wear behavior of the tested cups (in weight loss form) was evaluated under dry lubricating conditions, and then the wear mechanism of worm cups was examined by using scanning electron microscopy. The results showed that the length of the femur, tibia, and pelvis width have a positive effect on the wear rate of the tested cups due to flexion and pelvic moment which is resulted by the ground reaction force and pelvic rotation force.

Structure analysis and thermosensitive properties of copolymers prepared from 2-ethyl-2-oxazoline and 2-(4-aminophenyl)-2-oxazoline

AbstractThermosensitive polymers play an important role in controlled drug delivery and other biomedical and biotechnology applications. Thermosensitive polymers and copolymers that contain free reactive groups provide further possibilities in the field of targeted therapy. A high concentration of free amino groups in copolymers allows for the immobilization of various biosubstances, e.g., drugs, proteins or polysaccharides. Copolymers containing free amino groups were prepared via the cationic copolymerization of 2-ethyl-2-oxazoline with 2-(4-aminophenyl)-2-oxazoline. The mechanism of copolymerization was studied via a model polymerization of 2-ethyl-2-oxazoline in the presence of ethyl 4-aminobenzoate (EAB) as the source of the aromatic amino group. These model polymerizations showed that EAB incorporated into the polymer via a termination step. The studies also showed that the molar mass of the formed polymers depended on the ratio of initiator to EAB. The composition of the prepared copolymers was determined by NMR and absorbance spectroscopy. The prepared copolymers exhibited decreasing phase transition temperatures with increasing concentrations of aromatic units, which were confirmed by optical transmittance measurements. The influence of the concentration of copolymers, the presence of surfactants and the presence of an inorganic salt on the thermosensitive properties of the prepared copolymers was also demonstrated.