Preparation and Properties of Stereocomplex of Poly(lactic acid) and Its Amphiphilic Copolymers Containing Glucose Groups

Abstract

The stereocomplex of poly(lactic acid) containing glucose groups (sc-PLAG) was prepared by solution blending from equal amounts of poly(l-lactic acid) (PLLA) and poly(d-lactic acid-co-glucose) (PDLAG), which were synthesized from l- and d-lactic acid and glucose by melt polycondensation. The methods, including 1H nuclear magnetic resonance spectroscopy (1H NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), polarizing microscope (POM), scanning electron microscope (SEM), transmission electron microscope (TEM), and contact angle were used to determine the effects of the stereocomplexation of enantiomeric poly(lactic acid) (PLA) units, the amphiphilicity due to glucose residues and lactic acid units, and the interaction of glucose residues with lactic units on the crystallization performance, hydrophilicity, thermal stability, and morphology of samples. The results showed PDLAG was multi-armed, and partial OH groups of glucose residues in PDLAG might remain unreacted. The molecular weight (Mw), dispersity (Ɖ), and glucose proportion in the chain of PDLAG thereby had significant effects on sc-PLAG. There were the stereocomplexation of enantiomeric lactic units and the amphiphilic self-assembly of PDLAG in sc-PLAG, which resulted in glucose groups mainly in the surface phase and lactic units in the bulk phase. The sc-PLAG only possessed the stereocomplex crystal owing to the interaction between nearly equimolar of l-lactic units of PLLA and d-lactic units of PDLAG, and had no homo-crystallites of l- or d-lactic units, which improved the melting temperature (Tm) of sc-PLAG about 50 °C higher than that of PLLA. Glucose groups in sc-PLAG played an important role by forming heterogeneous nucleation, promoting amphiphilic self-assembly, and affecting the ordered arrangement of lactic units. The glass transition temperature (Tg), the melting temperature (Tm), crystallinity, crystallization rate, and water absorption of sc-PLAG showed similar changes with the increased glucose content in feeding. All these parameters increased at first, and the maximum appeared as glucose content in feeding about 2%, such as the maximum crystallinity of 48.8% and the maximum water absorption ratio being 11.7%. When glucose content in feeding continued increasing, all these performances showed a downward trend due to the decrease of arrangement regularity of lactic acid chains caused by glucose groups. Moreover, the contact angle of sc-PLAG decreased gradually with the increased glucose content in feeding to obtain the minimum 77.5° as the glucose content in feeding being 5%, while that of PLLA was 85.0°. The sc-PLAG possessed a regular microsphere structure, and its microspheres with a diameter of about 200 nm could be observed. In conclusion, sc-PLAG containing proper glucose amount could effectively enhance the crystallinity, hydrophilicity, and thermal stability of PLA material, which is useful for drug delivery, a scaffold for tissue engineering, and other applications of biomedicine.

Surface Modification of Poly(l-lactic acid) through Stereocomplexation with Enantiomeric Poly(d-lactic acid) and Its Copolymer

“…This result indicates that the SC structure formed on the PLLA surface owing to the interaction of PLLA chains with PDLA blocks of PDLAG, while the HC structure in the bulk of the polymer was affected due to the penetration of the polymer dissolutions into the bulk of the film. Moreover, the intensity of the SC peak of m-PLAG was weaker than that of m-PLLA with the same modification time, because the glucose residues in PDLAG chains may interact with PLLA and PDLA blocks, retard crystallization, and result in imperfect HC and SC structures [ 32 , 37 ].…”

“…The synthetic route of poly( d - lactic acid) is shown in Scheme 1 via direct melt polymerization [ 32 ]. A certain amount of d - LA was added into a three-necked flask and heated up to 150 °C under normal pressure for 1 h. Then, the catalyst stannous octoate with 0.5 wt% of the total mass of d -LA was added to the flask.…”

“…The separated precipitates were dried under vacuum at 50 °C for 10 h to obtain PDLAG samples. The obtained PDLAG were multi-arm amphiphilic copolymers with the M w of 15,600 and the PDI of 1.90 determined by GPC [ 32 ].…”

“…The average molecular weights ( M w ) of PLLAG and PDLAG were about 15,600–20,600 with the PDI being about 1.90. PLLAG and PDLAG were blended to obtain sugar-containing stereocomplexes of PLA (sc-PLAG) [ 31 , 32 ], which resulted in higher heat-resistance and better hydrophilicity of sc-PLAG compared with PLLAG and PDLAG. Due to low M w usually reducing PLA properties, in this paper, we utilized a PLLA with high molecular weight ( M w ) to prepare PLLA film, and surface-modified the film with low- M w PDLA and PDLAG through stereocomplexation of enantiomeric PLA chains with amphiphilicity due to glucose residues and lactic acid segments.…”

“…The stereocomplex of PLLA with PDLA or PDLAG would form on the surface of modified PLLA films, while glucose residues of PDLAG sticking to the surface would improve the hydrophilicity of these films. Stereocomplex formation between enantiomeric sections of PLLA with PDLA or PDLAG is related to the increase in the T m of PLLA [ 31 , 32 ]. Therefore, the surface modification of PLLA films would improve the thermostability and hydrophilicity, and change the surface structure of PLA materials, which is essential in order to obtain PLA-based biomaterials.…”