Antti Helminen

Synthesis of polylactides in the presence of co-initiators with different numbers of hydroxyl groups

Abstract Linear and star-shaped polylactides were prepared by using alcohols with different numbers of hydroxyl groups as co-initiators. 1H-NMR analysis of low molecular weight polymers showed that the number of hydroxyl groups initiating polymerisation was near theoretical for 1,4-butanediol (2 OH groups) and pentaerythritol (4 OH groups). The numbers for polyglycerines were somewhat lower than theoretical (8 or 12 OH groups), but they clearly suggested the star-shaped structure, with more arms than in polymers initiated with pentaerythritol. The preparation of high molecular weight polymers showed that the polymerisation rate increases with the number of hydroxyl groups in the co-initiator. Along with the faster polymerisation, higher molecular weight polymers were obtained. High hydroxyl group content in the polymer did not cause a drop in the conversion level or enhanced backbiting during extended polymerisation. Furthermore, the co-initiator did not affect the thermal properties of the polymers except that slightly lower melting temperatures were measured for star-shaped than linear polylactides.

Chain extending of lactic acid oligomers. Effect of 2,2′-bis(2-oxazoline) on 1,6-hexamethylene diisocyanate linking reaction

Abstract New l -lactic acid polymers were synthesised with the use of highly effective carboxyl and hydroxyl reactive chain extenders. The chain extending behaviour of 2,2′-bis(2-oxazoline) (BOX) and 1,6-hexamethylene diisocyanate (HMDI) with two different oligomers was followed by means of acid value (AV) and molecular weight increments measured by size exclusion chromatography (SEC). Both chain coupling agents were found to react selectively with end-groups, forming oxamide and urethane groups as characterised by spectroscopy (FTIR and 1H NMR). Reaction between BOX and carboxyl groups of lactic acid oligomer led to hydroxyl terminated prepolymer with low AV, which provided significant increase of molecular weight in the HMDI linking reaction. In addition, an improvement in the thermal stability of the resulting polymers was observed with excess amount of BOX. The material properties of the polymers were characterised by dynamic mechanical thermal analysis (DMTA), dynamic rheometry and tensile testing. Introduction of oxamide groups into the polymer structure increased the chain stiffness, which was detected in mechanical properties and increment in glass transition temperature.

Self-reinforcement and hydrolytic degradation of amorphous lactic acid based poly(ester-amide), and of its composite with sol-gel derived fibers.

The self-reinforcing and hydrolytic degradation of an amorphous poly(ester-amide) (PEA) based on lactic acid have been studied and compared with those of poly-L-lactide (PLLA). The studied PEA-rods were self-reinforced (SR) by solid-state die drawing resulting double shear strength. The hydrolytic degradation of PEA was studied during exposure to phosphate buffered saline at pH 7.4 and at 37 °C for 18 weeks. The degradation and mechanical properties of PEA were also followed in a self-reinforced composite structure consisting of PEA and sol-gel derived SiO2-fibers (SGF, 8 wt %). The hydrolytic degradation of the SR-PEA-rods with and without SG-fibers was significantly faster than that of SR-PLLA-rods. The weight average molecular weight (Mw) of PEA decreased by 90% from the initial Mw during the first 6 weeks in hydrolysis, when the Mw of the PLLA decreased by 10%.