B Bogdanov

Synthesis and thermal properties of poly(ethylene glycol)-poly(ϵ-caprolactone) copolymers

Abstract Three different types of polyester—polyether copolymers were synthesized by PEG initiated ring-opening polymerization of ϵ-caprolactone: an A-B and an A-B-A block copolymer and an (A)2-B star copolymer, where A is a poly(ϵ-caprolactone) (PCL) and B is a poly(ethylene glycol) (PEG) block. The monomer to initiator ratio was varied to obtain copolymers with different PCL block length and weight fraction ranging from 68–85 wt%. These PCL-PEG copolymers were characterized by means of g.p.c., n.m.r., d.s.c. and WAXD. In all types of copolymer the PCL constituent crystallizes first when cooling from the molten state. This leads to significant undercooling and imperfect crystallization of the PEG block. The mutual influence between PCL and PEG constituents is significantly stronger in the A-B-A block copolymers, having a central PEG block, and in the (A)2-B star copolymers. The effect of the PCL/PEG ratio on the melting and crystallization behaviour of the copolymers was investigated. The results obtained demonstrate that the thermal properties and morphology are affected significantly by the chain length of the PCL and PEG constituents in the copolymers and by the type of copolymers (sequence of the different blocks).

In vitro release characteristics of bioactive molecules from dextran dialdehyde cross-linked gelatin hydrogel films

Hydrogel films, prepared by cross-linking of gelatin with dextran dialdehydes (weight ratio 2:1), and containing either fluorescein isothiocyanate dextran (Mw 70000) or polypeptides were evaluated in terms of their release characteristics and mechanical properties upon increasing storage time at 4 degrees C. Important changes in release kinetics and mechanical properties of the cross-linked gelatin films were observed, especially during the first week after the hydrogel production. Rheological and NMR measurements showed that the mechanical properties of the gelatin hydrogel films were improved with increasing storage time. It appeared that the process of chemical cross-linking and physical structuring of the gelatin hydrogel matrix did not occur instantaneously and substantially influenced the polypeptide release patterns. Cross-linked gelatin hydrogels were found to be appropriate release systems for medium-term sustained delivery of biologically active epidermal growth factor (EGF), but release characteristics were strongly dependent on the nature of the protein which was incorporated.

Physical properties of poly(ester-urethanes) prepared from different molar mass polycaprolactone-diols

Abstract Segmented poly(ester-urethanes) (PEUs) based on poly(ϵ-caprolactone) (PCL) as a soft segment and a non-aromatic diisocyanate in the hard segment were synthesized. The soft segment crystallinity and other physical properties of the PEUs were studied. It was found that the crystallinity and rate of crystallization of the PCL continuous phase in the PEUs decreases and the glass transition temperature of PEUs increases in comparison with the PCL prepolymers. The restriction of the crystallization of the PCL soft segment depends on the hard segment concentration, length of the soft segment, and total molecular weight of the PEUs.

Hydrogels prepared by crosslinking of gelatin with dextran dialdehyde.

Hydrogels were prepared by reaction of gelatin with partial periodate oxidized dextran. It was found that the rate of gelation depends on the molecular weight and on the degree of oxidation of dextran, the type of gelatin and on the reaction conditions. Rheological measurements demonstrate that the gel strength is governed by two factors: chemical crosslinking by reaction with polyaldehyde and physical structuring of the gelatin. By proper selection of the reaction conditions, in particular storage temperature, the contribution of both processes to the final material strength can be varied. It was observed that short cryogenic treatment significantly enhances the chemical crosslinking.

Thermal Properties and Morphology of Poly(Ester-Urethanes) Prepared from Polycaprolactone-Diol

The soft segment crystallinity and morphology of poly(ester-urethanes) (PEUs) based on poly(ε-caprolactone) (PCL) as a soft segment and an aliphatic diisocyanate in the hard segment were studied. It was found that the restriction of the crystallization of the PCL soft segment depends on the hard segment concentration, the length of the soft segment, and the total molecular mass of the PEUs. The PEU based on a low molecular mass PCL (M=2000) is an amorphous elastic material during a long time after casting from solution or after melt crystallization. A soft-hard segment endothermal mixing transition (Tmix) of about 70-80°C is observed in the DSC curves of this PEU sample.

Thermal and rheological properties of gelatin-dextran hydrogels

Hydrogels prepared by crosslinkage of gelatin with dextran dialdehyde have been characterized by dynamic shear oscillation measurements at small strain. Isothermal as well as temperature scan measurements were performed. The results obtained demonstrated that the final polymer network is a result of a chemical gelatin-dextran dialdehyde interaction as well as a gelatin-gelatin (physical association) and a polymer-solvent interaction. This balance is strongly dependent on the composition of the system, the polymer concentration, the storage temperature and the storage time.We found that a short cryogenic treatment at −20°C of physically structured gels, significantly increases the chemical crosslinkage. DSC measurements at low cooling rate confirm these results and demonstrate a chemical reaction enthalpy contribution.