A series of polylactides (PLA) with different stereo sequences are prepared by the copolymerization of L‐lactide and DL‐lactide. It is confirmed that the glass transition temperature (Tg) of the PLA decreases with decreasing optical purity of the lactate units (%ee) according to the Fox's equation. Analysis of the FT‐IR spectra of these PLA samples reveals that the absorbance at 1 265 cm−1 (δCH + νCOC) decreases with increasing L‐content while the absorbance at 1 210 cm−1 (νasCOC + rasCH3) increases with increasing L‐content. These changes in absorbance are reasonably correlated with the randomness and helical nature of the L‐sequenced segments involved in PLA. Namely, the PLA chains with higher L‐content comprise a higher number of short helical blocks that are made of several L‐lactate units. This difference in helical nature causes the opposite dependences of Tg and density on the L‐content of PLA; i.e., the increased Tg and decreased density with increasing L‐content.FT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.magnified imageFT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.
Melt‐crystallized films of poly(L‐lactic acid) (PLLA) with Mv in the range of 3.8 ∼ 46 × 104 consisting of α‐form crystals were uniaxially drawn by solid‐state coextrusion. The effects of Mv, extrusion draw ratio (EDR), and extrusion temperature (Text) on the crystal/crystal transformation from α‐ to β‐form crystals and the resultant tensile properties of drawn products were studied. The crystal transformation proceeded with EDR and more rapidly for the higher Mv's. Furthermore, the crystal transformation proceeded most rapidly with EDR at a Text around 130 °C, independently of the Mv's. As a result of the optimum combination of processing variables influencing the the crystal transformation (Mv, Text, and drawability), highly oriented films consisting of β‐form crystals alone were obtained by coextrusion of higher Mv samples at Text's slightly below the melting temperature (150 ∼ 170 °C) and at higher EDR's > 11. Both the tensile modulus and strength increased rapidly with EDR. The modulus at a given EDR was slightly higher for the samples with higher Mv's. In contrast, the strength at a given EDR was remarkably higher for the higher M
v's. The highest tensile modulus of 8.0 GPa and strength of 500 MPa were obtained with the sample of the highest Mv of 46 × 104 coextruded at 170 °C to the highest EDR of 14.
Three types of copolymers of poly(L‐lactic acid) (PLLA) were synthesized by direct polycondensation of L‐lactic acid and phenyl‐substituted α‐hydroxy acids (L‐phenyllactic acid and D‐ and L‐mandelic acids). It was found that the glass transition temperature of the copolymers comprising L‐mandelic acid became significantly higher (from 58 to 69 °C) with increasing content of L‐mandelic acid (from 0 to 50 mol‐%) although the Mw decreased (from 87 000 to 4 000 Da). The cast films of the L‐mandelic acid containing copolymers showed improved tensile properties compared with those of the PLLA film. This may be due to a pinning effect of the L‐mandelic acid units on the helix formation of PLLA, although 30% of the units were racemized. The enzymatic degradability of the L‐mandelic acid containing copolymers was much higher than that of PLLA, as analyzed with Proteinase K® originating from Tritirachium album.Synthesis of copolymers of L‐lactic acid and phenyl‐substituted α‐hydroxy acids.magnified imageSynthesis of copolymers of L‐lactic acid and phenyl‐substituted α‐hydroxy acids.
Polyglactin was successfully synthesized by the melt/solid polycondensation of a mixture of glycolic acid (GA) and L-lactic acid (LA) mostly at a GA to LA monomer ratio of 90/10. In the polymerization procedure, a solid polycondensate was first prepared by melt-polycondensation at 150-190 • C, mechanically crushed into particles of various sizes (150-180, 180-210, 210-250, 250-300 and 300-355 µm), and subjected to solid-state post-polycondensation at 170 • C for 10-20 h. The polyglactin finally obtained was a colourless solid. Catalyst screening revealed that the single use of methanesulfonic acid gave the highest molecular weight of the product. Starting from the crushed melt-polycondensate with a diameter range of 180-250 µm, the highest number-average molecular weight of attained was 80 000 Da. This process can afford a facile route to large-scale synthesis of polyglactin with high molecular weight.
Poly‐L‐Lactide(PLLA) has been used as a bone fracture fixation material for several years. However, its mechanical properties are still not satisfied. To improve its mechanical properties, we examined the hydrostatic extrusion procedure on the PLLA rods made by Injection Molding process. The extrusion ratio was adjusted to 3, 6, 9, and 12. The molecular weight of the PLLA decreased from 260,000 to 200,000 after injection molding process, but it did not change during the hydrostatic extrusion procedure. The melting point of PLLA hydrostatic extrusion products were increased with the extrusion ratio, but the increment was not obviouse. Extrusion products having low extrusion ratio had α‐form crystal in them, extrusion products having high extrusion ratio had both of α and β‐form crystall in them. At extrusion temperature of 145°C, PLLA rods showed the best flowing trends in the pressure medium of PEG 400. Extrusion temperature is placed in the range of crystalline transition temperature and melting point of PLLA. At extrusion ratio 9∼12, the extrusion products showed the best mechanical properties. The highest bending strength of the extrusion product was over than 350MPa. It is far stronger than that of the human cortical bone (200MPa). SEM observations showed that the fiber structure began to appear at an extrusion ratio ER=3, and at the extrusion ratio ER=6, the chain axes of PLLA became aligned to the extrusion direction. The structure of extrusion products at the high extrusion ratio showed highly oriented fiber structure composed of micro‐fibril. At high extrusion ratio tranformation from α‐crystal to β‐crystal was also observed.
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