A chitosan dressing reduced hemorrhage and improved survival after severe liver injury in swine. Further studies are warranted.

A series of eight thermoplastic polyurethane elastomers were synthesized from 4,4-methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) chain extender, with poly(hexamethylene oxide) (PHMO) macrodiol soft segments. The PHMO molecular weights employed ranged from 433 g/mol to 1180 g/mol. All materials contained 60% (w/w) of the macrodiol. The materials were characterized by differential scanning calorimetry (DSC) following up to nine different thermal treatments. In addition, three of the materials were selected for characterization by small-angle x-ray scattering (SAXS) following similar thermal treatments. The DSC experiments showed the existence of five hard segment melting regions (labelled T1-T5), which were postulated to result from the disordering or melting of sequences containing one to five MDIderived units, respectively. Evidence for urethane linkage dissociation and reassociation during annealing at temperatures above 150ЊC is presented. This process aids in the formation of higher melting structures. Annealing temperatures of 80-100ЊC provided the maximum SAXS scattering intensity values. Materials containing longer soft segments (and, therefore, longer hard segments) were observed to develop and sustain higher melting hard domain structures and also develop maximum average interdomain spacing values at higher annealing temperatures. Another additional series of three PHMO-based polyurethanes having narrower hard segment length distributions, was synthesized and characterized by DSC in the as-synthesized and annealed states. The resulting DSC endotherms provided further evidence to suggest that the T1-T5 endotherms were possibly due to melting of various hard segment length populations.

ABSTRACT:The compatibilizing effect of poly(hexamethylene oxide) (PHMO) on the synthesis of polyurethanes based on ␣,-bis(6-hydroxyethoxypropyl) poly(dimethylsiloxane) (PDMS) was investigated. The hard segments of the polyurethanes were based on 4,4Ј-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol. The effects of the PDMS/ PHMO composition, method of polyurethane synthesis, hard segment weight percentage, catalyst, and molecular weight of the PDMS on polyurethane synthesis, properties, and morphology were investigated using size exclusion chromatography, tensile testing, and differential scanning calorimetry (DSC). The large difference in the solubility parameters between PDMS and conventional reagents used in polyurethane synthesis was found to be the main problem associated with preparing PDMS-based polyurethanes with good mechanical properties. Incorporation of a polyether macrodiol such as PHMO improved the compatibility and yielded polyurethanes with significantly improved mechanical properties and processability. The optimum PDMS/PHMO composition was 80 : 20 (w/w), which yielded a polyurethane with properties comparable to those of the commercial material Pellethane™ 2363-80A. The one-step polymerization was sensitive to the hard segment weight percentage of the polyurethane and was limited to materials with about a 40 wt % hard segment; higher concentrations yielded materials with poor mechanical properties. A catalyst was essential for the one-step process and tetracoordinated tin catalysts (e.g., dibutyltin dilaurate) were the most effective. Two-step bulk polymerization overcame most of the problems associated with reactant immiscibility by the end capping of the macrodiol and required no catalysts. The DSC results demonstrated that in cases where poor properties were observed, the corresponding polyurethanes were highly phase separated and the hard segments formed were generally longer than the average expected length based on the reactant stoichiometry. Based on these results, we postulated that at low levels (ϳ 20 wt %) the soft segment component derived from PHMO macrodiol was concentrated mainly in the interfacial regions, strengthening the adhesion between hard and soft domains of PDMS-based polyurethanes.

No abstract