Biomedical subjects

Ema Zagar

Publications and source records attributed to Ema Zagar.

3 recordsLinked to original sources

PubMed2006

Sequence distribution in microbial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) co-polyesters determined by NMR and MS.

The microstructure of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolyesters (PHBV) as well as a mixture of two PHBV copolyesters of different comonomer composition and sequence distribution was studied by 13C NMR based on dyad and triad analysis and multistage electrospray ionization mass spectrometry (ESI-MSn). Both techniques gave results that were in good agreement for all investigated samples. The effect of microstructure on PHBV thermal properties was investigated from the melting behavior of samples. A PHBV copolyester with randomly distributed hydroxyvalerate units (12.0 mol % HV) showed a single melting peak, whereas samples with nonrandom composition distribution showed multiple melting peaks in their thermograms. Such complex melting behavior suggested that the 12.9 and 27.1 mol % PHBV copolyesters were actually blends of several copolymers with widely different comonomer-unit composition.

Calorimetry, Differential Scanning↗

PubMed2004

Molar mass distribution of a commercial aliphatic hyperbranched polyester based on 2,2-bis(methylol)propionic acid.

The determination of absolute molar mass averages (MMA) and molar mass distribution (MMD) of the fourth generation hyperbranched polyester Boltorn H40 (Perstorp Specialty Chemicals AB), synthesized from 2,2-bis(methylol)propionic acid (bis-MPA) as the AB2 monomer and ethoxylated pentaerythritol as the B4 core molecule was studied in dependence on the type of solvent, preparation procedure and solution concentration. Due to a large number of polar hydroxyl groups, ester, and also some residual carboxyl groups, a very stable H-bond network is formed at room temperature, that can-not be completely disrupted by dissolving the sample in solvents such as tetrahydrofuran (THF), N,N-dimethylacetamide (DMAc), a mixture of THF and methanol (9:1, v/v), and a solution of 0.7% LiBr in DMAc. The H-bonds between the polar groups break down completely and the dissolution of Boltorn H40 on the molecular level is achieved only when the sample is thermally pretreated at a minimum 140 C for at least 20 min prior to dissolution in solvents THF/methanol or LiBr/DMAc. Thus, determined MMA and molar mass distribution (MMD) of Boltorn H40 are independent on the kind of the solvent and solution concentration.

Acetamides↗

PubMedDate not supplied

SEC-MALS characterization of microbial polyhydroxyalkanoates.

Characterization of poly-3-hydroxybutyric acid (PHB) and poly-3-hydroxybutyric-co-valeric acid (PHBV, 13% valerate) in chloroform was performed using size exclusion chromatography coupled to a multi-angle light scattering detector (SEC-MALS). Absolute molar mass averages, molar mass distribution, and the radius of gyration were determined. Three sample preparation methods were examined: dissolution in chloroform (1) at room temperature, (2) at 60 degrees C, and (3) after thermal pretreatment of samples (annealing at 180 degrees C with subsequent quenching in liquid nitrogen). Dissolution at 60 degrees C and dissolution of thermally pretreated samples gave molecularly dissolved PHB and PHBV. At 60 degrees C using acid free chloroform, there was no indication of degradation for up to 120 min dissolution time, whereas thermal degradation of polymers did take place during annealing at 180 degrees C. The degradation rate constants for number and weight average degree of polymerization at 180 degrees C were slightly higher for PHB (5.19 x 10(-5) min(-1), 4.95 x 10(-5) min(-1)) than for PHBV (4.99 x 10(-5) min(-1), 4.54 x 10(-5) min(-1)). The dependence of the radii of gyration on molar mass showed that both polymers form random coils in chloroform. The relationship between the absolute molar masses and relative SEC results was determined. DSC and NMR characterization also gave evidence of the progress of degradation.

Biodegradation, Environmental↗