Microstructure and Thermomechanical Properties of Glassy Polylactides with Different Optical Purity of the Lactate Units

Abstract

Microstructure and thermomechanical properties of polylactides (PLA) with different optical purity (OP) of the lactate units were studied in detail. Thermal analysis by DSC revealed that the melting (Tm) and glass transition (Tg) temperatures of PLA can be simulated by the theoretical equations of Flory and Fox, respectively, both decreasing with decreasing OP. The sequential analysis of the PLA samples by 13C NMR spectroscopy suggested that the meso‐sequence is more favorably formed than the racemic sequence in the L‐rich samples. Their glassy specimens made by injection‐moltding showed characteristic behaviors in dynamic mechanical analysis (DMA); the storage modulus (G ′) of the L‐rich PLA specimens (L ratio = 99.0, 96.1, and 95.8) dropped and subsequently jumped up above Tg correspondingly to the glass‐to‐rubber and rubber‐to‐crystal transitions, respectively. In the G ′ curve of PLA with low L contents (90 and 72.5), only a drop was shown due to the simple glass‐to‐rubber transition. Wide‐angle X‐ray diffraction (WAXD) of the former L‐rich PLA supported the responsibility of the crystallization for the jump‐up of G ′ above Tg. The tensile strength of these glassy PLA specimens decreased with their decreased OP, while the tensile modulus was almost identical irrespectively of OP. On the other hand, the density of the PLA specimens was inversely proportional to OP, being in contrast to the above Tg change. The opposite relationship between Tg and density changes can reasonably be explained by a packing model in which the partial helices are formed along the L‐sequenced segments of the PLA chain and randomly packed in solid state to afford a large free volume in between the helices.

Properties and Biodegradability of Polymer Blends of Poly(L-lactide)s with Different Optical Purity of the Lactate Units

“…[12] The weight-average molecular weight (M -w ) and polydispersity index M -w /M -n were determined by gel permeation chromatography (GPC). The analyzer was composed of a Shimadzu LC-9A pump, a Shimadzu RID-6A RI detector, and a Shimadzu C-R4A Chromatopac data processor.…”

“…[16] In the present case, however, all of the specimens are glassy or amorphous, and the difference in degradation rate cannot be explained by the crystalline/ amorphous ratio of the specimens, but by other structural difference. In the former paper, [12] we proposed a random packing model of the local helices of the l-sequenced chains for the l-rich PLA samples in the amorphous state as well as an ordinary random coil model for the racemicrich samples. These models can also be effective for explaining the different degradation rate of the PLA specimens with different l-content of the lactate units.…”

Mechanical and Thermal Properties of Poly(L‐lactide) Incorporating Various Inorganic Fillers with Particle and Whisker Shapes

“…This recovery of G 0 is attributed to the crystallisation of the amorphous PLLA. [2,19] The storage moduli (G 0 ) of the ifR-PLA composites are almost constant at 3-4 Â 10 9 Pa below T g of PLLA, dropping to 1 Â 10 7 Pa at the glass to rubber transition, and then increasing again to 4-6 Â 10 8 Pa by crystallisation. This behaviour is similar to that of the unfilled PLLA, but G 0 of ifR-PLA samples is always higher than that of PLLA.…”

“…PLLA is a semi-crystalline polymer whose thermal and mechanical properties are superior to those of the other biodegradable aliphatic polyesters, e.g., poly(butylene succinate) (PBS), poly(3-hydroxybutyrate) (PHB), and poly(e-caprolactone) (PCL). [1] Simple injection-moulding of PLLA, however, generally affords amorphous specimens [2] that are transparent and brittle and likely deform above its glass transition temperature (T g % 60 8C). It is therefore desirable to improve the thermal and mechanical properties of the injection-moulded specimens of PLLA, particularly for engineering purposes.…”

Microstructure and Thermal Properties of Polylactides with Different L‐ and D‐Unit Sequences: Importance of the Helical Nature of the L‐Sequenced Segments

“…[5] Recently, we reported the thermo-mechanical properties of injection-molded specimens of PLLA with different L-content and proposed a new micro-structural model for the glassy state of PLLA in which the L-sequenced segments with helical nature are randomly packed to form a large free volume between the chains. [6] This model structure can reasonably explain the opposite dependences of glass transition temperature (T g ) and density on the L-content of PLA [6] and the difference in biodegradability among the PLLA with different L-content. [7] All these studies may suggest that the helical nature of the L-sequenced segments of PLLA may give a profound effect on the understanding of the structure-property relationship of PLA.…”