The Mechanical And Crystallization Of Polylactic Acid And Its Stereocomplexes

Polylactic acid(PLA)is a synthetic bio-based polymer that does not depend on oil resources,it is biodegradable,and it has excellent mechanical properties.These performances endow PLA attracting more and more attentions today.But the melting point of PLA does not exceed 180 °C,limiting its scope of usage.Blending L-configured polylactic acid(PLLA)with D-configured polylactic acid(PDLA)could obtain PLA stereocomplex,which has a higher melting point,faster crystallization rate than neat PLLA or PDLA and it becomes more potential in biodegradable fields.At present,the study of PLA stereocomplex and PLA alone are not systematic.In this paper,PDLA / PLLA blends with different molecular weights,different contents and different optical purity were designed.The ductile behavior of PLA and the crystal ization of PLA and the crystal ization behavior of stereocomplex were studied.(1)The pure P DLA crystal properties of the study found that the melting temperature of PDLA increases with molecular weight.With increasing optical purity of PLLA,the crystallization temperature,crystallization enthalpy,melting temperature and enthalpy increased gradually,the crystallization temperature range became wider.The crystalline structure of PLLA and PDLA did not c hanges with molecular weight and optical purity.(2)PLLA samples with lower optical purity exhibited higher elongation at break,and the elongation at break decreased as the optical purity increasing.When PDLA was added to PLLA,the elongation at break became smaller than that of pure PLLA.With the increment of optical purity,more homo and stereocomplex crystals(HC and SC)are formed in PLLA and PLLA / PDLA blends,which results in lower elongation at break,higher destruction temperatures and lower loss factors.(3)When PLLA with low optical purity was mixed with synthetic PDLA at different contents.The mechanica l properties of the PLLA/PDLA samples showed a significantly higher elongation at break(~350%)in samples with lower crystallinity(x 10%)after solution casting.As more microcrystals formed in the sample,the strain drops rapidly.DSC and WAXD spectra confirmed that the orientation of the amorphous molecular chains and segments induced the formation of PLA mesophase and the mesophase crystallized into PLA HC during heating.PLA SC is arranged in the stretching direction during the stretching process,but its structure does not change significantly,regardless of the toughness or brittleness of the specimen.(4)In the PLLA / PDLA blends with different optical purity,the crystallinity increased and the crystallization time shortened and the crystallizatio n rate accelerated with increasing of optical purity of PLLA and reduceing molecular weight of PDLA.