Relationship between crystallization state and degradation behavior of poly(l‐lactide)/four‐armed poly(d,l‐lactide)‐block‐poly(d‐lactide) blends with different poly(d‐lactide) block lengths

Abstract

A series of four-armed poly(D,L-lactide)-block-poly(D-lactide) (4-DL-D) copolymers were synthesized by ring-opening polymerization. By fixing the poly(D,L-lactide) (PDLLA) block length (1 kg mol −1 ) and changing the poly(D-lactide) (PDLA) block length (M n,PDLA = 0, 0.5, 1.1, 1.3, 1.8 and 2.6 kg mol −1 ), the crystallization and alkaline degradation of the PLLA/4-DL-D blends were investigated. The four-armed PDLLA core of the copolymer inhibited the crystallization of PLLA, while the outer PDLA block could affect the crystallization differently when its length changed. If M n,PDLA was 0 or 0.5 kg mol −1 , the crystallization of PLLA in the PLLA/4-DL-D blend was retarded markedly and the degradation rate of the blend films was much faster than that of neat PLLA film. Interestingly, when M n,PDLA was 1.1 kg mol −1 or higher, stereocomplex (SC) crystallites with different morphologies were formed, and the degradation rate of the PLLA/4-DL-D blend decreased gradually with increasing M n,PDLA . In the PLLA/4-DL-D1.1 blend, the SC crystallites acted as nucleators for PLLA homocrystallites, while in the PLLA/4-DL-D1.3 blend, small isolated SC crystallites were observed inside the PLLA homospherulites. When M n,PDLA was 1.8 or 2.6 kg mol −1 , a network structure of SC crystallites was formed and the degradation resistance of the films was markedly enhanced. A possible isothermal crystallization mechanism was proposed for the PLLA/4-DL-D blends, and the relationship between the crystallization state and degradation behavior was explored. This work revealed that the crystallization state, which was controlled by the PDLA block length, had a significant effect on the degradation behavior of PLLA/4-DL-D blend films.

Crystallization Studies on the Stereocomplexation of Poly(ʟ‐lactide) / 4‐Armed Poly(ϵ‐caprolactone‐co‐d,ʟ‐lactide)‐ block‐poly(d‐lactide) Blends

“…For the morphology of 20 % 4a‐CDL‐D1.5 and 20 % 4a‐CDL‐D2 blend in Figure 7(e) and 7(f), the PLLA homo crystallites were restricted by the surrounding SC crystallites, which should be formed during the cooling process. This result was similar to that of the 20 % 4a‐CDL‐D2 blend in Figure 5(a), and this morphology has also been reported in the asymmetric PDLA/PLLA blends [23,24] . The spherulites growth rates were shown in Figure 8.…”

“…Figure 6 showed the WAXD curves and the calculated crystallinities of SC and homo crystals for all the samples. The diffraction peaks at 2 θ ≈12.5°, 14.8°, 16.9°, 19.1°and 22.5° corresponded to the α ‐form of PLLA crystals, while the diffraction peaks at 2 θ ≈12.1°, 21.0° and 24.0° corresponded to the SC crystals [23] . Figure 6(a) confirmed again that SC crystallites could be formed when M n,PDLA was higher than 1 kg/mol.…”

“…This result was similar to that of the 20 % 4a-CDL-D2 blend in Figure 5(a), and this morphology has also been reported in the asymmetric PDLA/PLLA blends. [23,24] The spherulites growth rates were shown in Figure 8. All the blends exhibited higher spherulite growth rates than neat-PLLA due to enhancement of the PLLA chain mobilities as was shown in Figure 3(b).…”

“…It is thus necessary to adjust these parameters to optimize the effect of SC crystallites on PLLA. In our earlier studies, star‐shaped PDLA containing copolymers were blended with PLLA, [21–23] and we found that the molecular weight of PDLA blocks ( M n,PDLA ) had remarkable effect on SC crystallization and resulted in significant difference on final properties.…”

In-Situ Isothermal Crystallization of Poly(l-lactide)

“…Typically, PLLA is a semi-crystalline polymer, and consists of both amorphous regions and crystalline domains. Generally, upon degradation, the chains in the amorphous region start to degrade initially, followed by those in crystalline part [ 4 , 5 , 6 ]. Therefore, the degradation behavior of PLLA is closely dependent on the morphology and structure of the polymer matrix, which is in turn determined by the crystallization process leading to the formation of the material [ 7 , 8 ].…”