Abstract
Poly(l-lactic acid) with high molecular weight was used to prepare PLLA films by means of the solvent casting technique. Poly(d-lactic acid) (PDLA) and poly(d-lactic acid-co-glucose) copolymer (PDLAG) with a low molecular weight were synthesized from d-lactic acid and glucose through melt polycondensation. PLLA films were immersed in PDLA or PDLAG solution to prepare surface-modified PLLA films. The modified PLLA film presented stereocomplex crystal (SC) on its surface and homogeneous crystals (HC) in its bulk. The HC structure and surface morphology of modified PLLA films were obviously damaged by PDLA or PDLAG solution. With increasing immersion time, the PLLA films modified by PDLA decreased both the HC and SC structure, while the PLLA films modified by PDLAG increased the SC structure and decreased the HC structure. Hydrophilic glucose residues of PDLAG on the surface would improve the hydrophilicity of surface-modified PLLA films. Moreover, the hydrophilicity of glucose residues and the interaction of glucose residues with lactic acid units could retard HC destruction and SC crystallization, so that PLLA films modified by PDLAG possessed lower melting temperatures of HC and SC, the crystallinity of SC and the water contact angle, compared with PDLAG-modified PLLA films. The SC structure could improve the heat resistance of modified PLLA film, but glucose residues could block crystallization to promote the thermal degradation of PLA materials. The surface modification of PLLA films will improve the thermal stability, hydrophilicity and crystallization properties of PLA materials, which is essential in order to obtain PLA-based biomaterials.
Highlights
Poly(lactic acid) (PLA) can replace traditional plastic in food packaging [1,2,3,4,5,6], bioengineering materials [7,8,9], composites [10,11,12] and other fields due to its good biodegradability, biocompatibility, and processability
The fc,homogeneous crystals (HC) of m−poly(L-lactic acid) (PLLA) was much lower than the crystallinities modification time, while the sum of f c,HC and f c,stereocomplex crystal (SC) was far less than the crystallinities of PLLA and Poly(D-lactic acid) (PDLA). These results indicate that surface modification by immersing PLLA films in PDLA dissolution could destroy the imperfect HC structure of PLLA and form imperfect SC structure, which resulted in higher Tm,HC and lower f c,HC
The f c,HC of m-PLAG and the sum of f c,HC and f c,SC of m-PLAG were much lower than the crystallinities of PLLA and poly(D-lactic acid-co-glucose) copolymer (PDLAG), and decreased gradually with increasing modification time, while f c,SC of m-PLAG increased slightly. These results indicate that surface modification by immersing PLLA films in PDLA and PDLAG dissolutions could destroy the imperfect HC structure of PLLA and form an imperfect SC structure, which results in higher Tm,HC and lower f c,HC for both m-PLLA and m-PLAG compared with the neat PLLA
Summary
Poly(lactic acid) (PLA) can replace traditional plastic in food packaging [1,2,3,4,5,6], bioengineering materials [7,8,9], composites [10,11,12] and other fields due to its good biodegradability, biocompatibility, and processability. In 1987, Ikada [20] first blended poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) in solutions to obtain the stereocomplex of PLA (sc-PLA) with a melting point about 50 ◦C higher than that of PLLA or PDLA, which can effectively improve PLA heat resistance, thereby reducing PLA processing difficulty and expanding PLA application fields. Ajiro [21] controlled the mobility of PDLA and PLLA chains to conduct stereocomplex crystallization owing to the good chain mobility of PDLA and PLLA chains. Tretinnikov [22] found that PDLA could be selectively adsorbed on the surface of PLLA via stereocomplexation between PLLA and PDLA
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