The thermal properties and crystallization of biodegradable and optically active poly[(S)-2-hydroxybutyrate] [P(S-2HB)], poly(l-lactide) (PLLA), poly(d-lactide) (PDLA) and their blends were investigated. The results of differential scanning calorimetry, wide-angle X-ray scattering (WAXS), and polarized optical microscopy first indicated heterostereocomplexation between biodegradable and optically active polyesters having different chemical structures and opposite configurations, that is, P(S-2HB) and PDLA. The melting temperature of the heterostereocomplex was higher than those of pure polymers. Such cocrystallization was not observed for P(S-2HB)/PLLA blends having identical configurations. The WAXS profile of P(S-2HB)/PDLA heterostereocomplex was very similar to those of the PLLA/PDLA and P(S-2HB)/P(R-2HB) homostereocomplexes and each crystalline diffraction peak of the heterostereocomplex was located between those of the homostereocomplexes. The present study strongly suggests that heterostereocomplexation will provide a novel versatile method for preparing biodegradable polyester materials with a wide range of physical properties and biodegradability.
Accelerated crystallization of poly(L-lactide) (PLLA) homo-crystallites occurred in the presence of, and the mixture of P(L-2HB) and. The accelerating effect of incorporated polymers decreased in the following order: , for cooling. The P(L-2HB)/P(D-2HB) homo-stereocomplex (HMSC) crystallites, P(D-2HB)/PLLA heterostereocomplex (HTSC) crystallites, and P(L-2HB) or P(D-2HB) homo-crystallites are found to be promising biodegradable nucleating agents for PLLA homo-crystallization.The P(L-2HB)/P(D-2HB) HMSC crystallites are most effective during isothermal crystallization and nonisothermal crystallization with heating, whereas the P(D-2HB)/PLLA HTSC crystallites are most effective during nonisothermal crystallization with cooling from the melt. In addition to the nucleating effect, the plasticizing effect of free P(2HB) chains increases both G and the PLLA spherulite number per unit mass. These effects result in accelerated crystallization of PLLA homo-crystallites.
The maximum radial growth rate of spherulites of the novel stereocomplexationable blend of poly(L-2-hydroxybutyrate) (P(L-2HB)) and poly(D-2-hydroxybutyrate) (P(D-2HB)) was observed to be substantially higher than those of pure P(L-2HB) and P(D-2HB). The hydrolytic degradation rate of the P(L-2HB)/P(D-2HB) blend traced by gravimetry and gel permeation chromatography was significantly lower than those of pure P(L-2HB) and P(D-2HB); this indicated that the blend had higher resistance to hydrolytic degradation. Further, the thermal degradation rate of the P(L-2HB)/P(D-2HB) blend was retarded as compared with those of pure P(L-2HB) and P(D-2HB). The results obtained in the present study indicate that the intermolecular interaction between P(L-2HB) and P(D-2HB) chains having opposite configurations in the amorphous regions or in the molten state was higher than that between P(L-2HB) or P(D-2HB) chains with the same configurations. The information obtained in the present study should be very useful for designing and processing pure, biodegradable materials of P(L-2HB), P(D-2HB) and their blends for biomedical, pharmaceutical and environmental applications. Keywords: crystallization; hydrolytic degradation; poly(hydroxybutanoic acid); poly(hydroxybutyric acid); stereocomplex; thermal degradation INTRODUCTION Poly(L-lactide) (that is, poly(L-lactic acid) or PLLA) is a biodegradable polymer produced from plant-derived renewable resources and is now used for biomedical, pharmaceutical, environmental, industrial and commercial applications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Stereocomplexation between PLLA and its enantiomer poly(D-lactide) (that is, poly(D-lactic acid) or PDLA) can yield biodegradable materials having superior mechanical performance and resistance to hydrolytic and thermal degradation relative to pure PLLA and PDLA. [16][17][18][19][20] Poly(2-hydroxybutyrate) (that is, poly(2-hydroxybutanoic acid) or P(2HB)) is a biodegradable polymer with the structure of a poly(lactide) (that is, poly(lactic acid) or PLA) in which methyl groups are substituted with ethyl groups. A stereocomplex can also be formed by blending substituted enantiomeric PLAs, that is, poly(L-2-hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) (P (D-2HB)). 21 The melting temperature (T m ) of the P(L-2HB)/P(D-2HB) stereocomplex (ca. 200 1C) is higher than those of pure P(L-2HB) and P(D-2HB) (ca. 100 1C). The spherulite growth or crystallization of the P(L-2HB)/ P(D-2HB) stereocomplex is completed in a substantially shorter period of time as compared with those of pure P(L-2HB) and P(D-2HB). From the results reported for PLLA/PDLA blends, [16][17][18][19][20] it is expected that the resistance of P(L-2HB)/P(D-2HB) blends to hydrolytic and
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