α-Hydroxy acid and α-amino acid-derived poly(l-lactic acid-co-l-alanine)s and poly(d-lactic acid-co-d-alanine)s with different alanine unit contents were synthesized and their stereocomplex crystallization and homo-crystallization were investigated.
The ternary stereocomplex (TSC) crystallizability of ternary substituted and unsubstituted poly(lactic acid) blends composed of poly(D-2-hydroxy-3-methylbutanoic acid) [P(D-2H3MB)], poly(L-2-hydroxy-3-methylbutanoic acid) [P(L-2H3MB)], and poly(L-2-hydroxybutanoic acid) [P(L-2HB)] or poly(L-lactic acid) (PLLA), together with heterostereocomplex (HTSC) crystallizability of binary blends composed of P(D-2H3MB) and PLLA, were investigated for solvent evaporated and precipitated samples. For the solvent evaporated P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) (mol/mol/mol) blend, formation of TSC crystallites with a very small amount of P(D-2H3MB) and/or P(L-2H3MB) homocrystallites was observed, whereas in the precipitated P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blend, P(D-2H3MB)/P(L-2HB) HTSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and P(L-2HB) homocrystallites were formed without formation of TSC crystallites. This is the first report for TSC crystallization of all substituted PLAs with linear and branched side chains. In contrast, in both solvent evaporated and precipitated P(D-2H3MB)/P(L-2H3MB)/PLLA (50/25/25) (mol/mol/ mol) blends, P(D-2H3MB)/P(L-2H3MB) homostereocomplex crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and PLLA homocrystallites were formed without crystallization of TSC crystallites. It was confirmed that HTSC between P(D-2H3MB) and PLLA is not formed in both solvent evaporated and precipitated P(D-2H3MB)/PLLA (50/50) (mol/mol) blends. Based on reported and present results, we proposed the rule for TSC and HTSC crystallization of, respectively, binary and ternary substituted and unsubstituted poly(lactic acid)s, wherein all the optically active polymer components are included in the same SC crystalline lattice. The difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is one.
Stereocomplex (SC) crystallization, homocrystallization, and polymorphism of poly(Llactic acid-co-L-alanine) [P(LLA-LAL)] and poly(D-lactic acid-co-D-alanine) [P(DLA-DAL)] copolymers with wide alanine unit content ranges from 0 to 21 and 22 mol% are investigated for melt-crystallization. P(LLA-LAL)/P(DLA-DAL) blends crystallize for wide alanine unit content ranges compared to unblended P(LLA-LAL) and P(DLA-DAL) samples, due to facile SC crystallization compared to homocrystallization. The phase diagrams of the unblended samples [α-and δ-form homocrystallites] and the blend samples (SC and homocrystallites) are drawn. The transition crystallization temperature of unblended samples from α-form to δ-form decreases with an increase in alanine unit content. In the unblended and blend samples, alanine units are correspondingly incorporated in α-form homocrystalline regions and excluded from SC crystalline regions. The maximum radial growth rate values of spherulites are higher for the blend samples than for the unblended samples. The experimental crystallization half time [t c (1/2)(exp)] values of the unblended samples increase with an increase in alanine unit content, whereas the t c (1/2)(exp) values of the blend samples with alanine unit contents of 3 and 4 mol% and 6 and 6 mol% and of 12 and 13 mol % are respectively higher than and similar to those of poly(L-lactic acid)/poly(D-lactic acid) blend.
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