Читать книгу Poly(lactic acid) - Группа авторов - Страница 114

The formation of stereoblock copolymers by chain extension is another strategy well explored by researchers for tailoring the properties of PLA. In a study reported by our group, the ROP of D‐ and L‐lactides was performed to yield mono‐maleimide‐terminated PDLA (M‐PDLA) and mono‐anthracene‐terminated PLLA (A‐PLLA), respectively [52]. This was followed by the reaction of A‐PLLA with hexamethylene diisocyanate (HMDI) to dimerize and form di‐anthracene‐terminated two‐armed PLLA (A‐PLLA‐A). The stereo di‐ and tri‐block copolymers were readily formed by the terminal Diels‐Alder coupling reaction between A‐PLLA/M‐PDLA and A‐PLLA‐A/M‐PDLA, respectively. This process resulted in the development of copolymers with improved thermomechanical and thermal properties due to the easy sc crystal formation. Extending the possibilities of applying Diels–Alder reactions to the sb‐PLA formation, the isocyanate coupling of maleimide‐terminated PLLA (M‐PLLA) and furan‐terminated PDLA (F‐PDLA) was conducted to yield bis‐maleimide‐terminated PLLA (M‐PLLA‐M) and bis‐furan‐terminated PDLA (F‐PDLA‐F), which were mixed in 1 : 1 ratio in solution. The resulting solution was electrospun where the sc‐PLA was formed by the terminal Diels–Alder coupling between the respective enantiomeric polymers [53]. The chain extension reaction was ascertained by the molecular weight of the electrospun fibers, which was found to increase from 10 to 45 kDa after the electrospinning and annealing. The fibers (as spun) were converted from the amorphous or semicrystalline state to the fully crystalline state by thermal annealing with the formation of sc crystals [53]. Chain extension by HMDI has also been used to develop multiblock copolymers with significantly improved mechanical properties. In line with this, tri‐sb copolymers synthesized by two‐step ROP have been subjected to chain extension by using HMDI to develop multiblock sb‐PLA (multi‐sb) copolymers. For example, tri‐sb‐PLA copolymers having equivalent composition of PLLA and PDLA enantiomers were reacted with HMDI to form multi‐sb‐PLA copolymers with controlled block sequences, which were reported to exclusively form sc crystallites by suppressing the hc crystallization [54]. The multi‐sb copolymers having longer PLLA/PDLA blocks, upon annealing, showed improved thermo‐mechanical properties along with higher sc crystallinity. It was remarked that the thermal properties of multi‐sb copolymers can be tailored by controlling the block lengths of tri‐sb‐PLA [54]. The sc crystallization in multiblock copolymers by dynamic Monte Carlo simulations has been reported by Qiu et al. who identified the effect of block numbers and the crystallization temperature on the sc formation [55]. Several systems with alternating sequences of A and B were used, namely A/B blend, A–B diblock, tetrablock, octablock, and sixteen‐block copolymers. In multiblock copolymers with low block numbers, sc formation was found to increase with increasing crystallization temperature. The effect of crystallization temperature was not detected for the multiblock copolymers with relatively high block numbers. The miscibility between the different blocks, the block length, and the size of the crystal thickness were reported to be the governing parameters for sc formation in the multiblock copolymers.