Role of the Branched PEG-b-PLLA Block Chain in Stereocomplex Crystallization and Crystallization Kinetics for PDLA/MPEG-b-PLLA-g-glucose Blends with Different Architectures

Abstract

The isothermal crystallization behavior and corresponding morphology evolution of poly­(d-lactic acid) (PDLA) blends with PLLA6.7k or MPEG-b-PLLA6.7k-g-glucose with different architectures and different PLLA-grafted copolymer contents were investigated. The formation of stereocomplexes (SCs) in between the chain branched structure of MPEG-b-PLLA6.7k-g-glucose and PDLA chains acting as the physical crosslinking points slows down the motion of PDLA chains, but the SCs could act as a heterogeneous nucleating agent for the late formation of homocrystals (HCs) in the blend system, accelerating the crystallization kinetics of HCs through enhancing the nucleation density. For PDLA/MPEG-b-PLLA6.7k-g-glucose blends, the mobility of SCs in the blend system and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a T c of 130 °C. The evolution of the crystal growth and structure during the isothermal crystallization process by rheometry has revealed the interesting role of the branched chains of MPEG-b-PLLA6.7k-g-glucose in the mechanism of the crystallization in PDLA blends.

Designing of the Branched PDLA-Grafted Copolymers with Different Architectures through Cellulose Acetate as the Node for High-Performance Polylactide-Based Enantiomers

“…More interestingly, the branched PLLA/CA-rB-PDLA blends exhibit different behaviors, where the χ c,hc value increased from 17 to 22.2% with 2.5 wt % CA-rB-PDLA fillers added. The result describes that more branched structure leads to the formation of multiple heterogeneous nucleation spots, where the SCs formed at higher temperature during the cooling process can be used as the heterogeneous nucleating agent to accelerate the crystallization of HCs through enhancing the nucleation density; the crystallization of the polymer from melt disorder to order is reduced in difficulty . Subsequently, the χ c,hc value decreased to 11% due to the competitive effect in HC and SC crystallization.…”

“…Notably, PLLA/PDLA-rB and PLLA/CA-rB-PDLA blends exhibited a higher glass-transition temperature ( T g ) than neat PLLA. This contributes to the SC crystallization with the role of physical cross-linking points formed in between PLLA and the PDLA-rB block chains; the increased content of PDLA-rB chain segments results in more entanglement between the enantiomeric chains, which hinders segment migration in the amorphous region. , The curve of PLLA featured a broad and intense exothermal peak at 100–130 °C corresponding to the transformation of α′-to-α crystals during cold crystallization of PLLA ( T cc2 ). Upon heating, this peak shifted to higher temperatures ( T cc2 = 127.9 °C) with increasing PDLA-rB chain loading than neat PLLA (Figure A and Table ), which was ascribed that the SC act as physical cross-linking points, between PLLA and PDLA-grafted copolymer chains, to restrict the motion of segments between PLLA and PDLA chains in the amorphous chain segments of α′-HC crystallization yet .…”

“…The crystallization rate of SCs in PLLA/CA-rB-PDLA blends is faster than that of PLLA/PDLA-rB, which is due to the higher mobility in the SC stage. This contributed to the more branched chains in the SC stage and promoted the formation of SCs, which can act as a heterogeneous nucleating agent to accelerate the crystallization kinetics of HCs by enhancing the nucleation density. , Therefore, the nucleation density of SCs in PLLA/CA-rB-PDLA blends is higher, and the size of the crystals formed in PLLA/CA-rB-PDLA is smaller than that of PLLA/PDLA-rB blends. In conclusion, although the addition of a certain amount of PDLA-rB can promote the crystallization of the blends according to DSC, when the content of PDLA-rB is too high (>7.5 wt %), it can act as a diluent, leading to a slower crystallization rate in the SC stage.…”

“…The result describes that more branched structure leads to the formation of multiple heterogeneous nucleation spots, where the SCs formed at higher temperature during the cooling process can be used as the heterogeneous nucleating agent to accelerate the crystallization of HCs through enhancing the nucleation density; the crystallization of the polymer from melt disorder to order is reduced in difficulty. 42 Subsequently, the χ c,hc value the PLLA/CA-rB-PDLA blends attained an imperfect crystallization with the introduction of a large number of branched chains. As shown in Figure 5, the crystal structures of linear and branched blends were further analyzed by WAXD.…”

Role of the branched PEG-b-PLLA chain in morphological structures and thermodynamics for PEG-b-PLLA-g-glucose copolymers with different architectures

“…It was seen that all the copolymers had one endothermic peak higher than 100 1C, which was assigned to PLLA crystallites. 45 When the weight fraction of PLLA is low, the PEG segment as the diluting agent increases the flexibility and chain diffusion ability of PEG-b-PLLA-g-glucose copolymers 46,47 and meanwhile, also improves the formation of the a-form for the extensive melting process of the PLLA matrix during second subsequent heating. In addition, the crystallinity of PLLA (X PLLA ) was relatively low, which indicated fewer crystallites and more disordered lattices than other PEG-b-PLLA-g-glucose grafted copolymers (Fig.…”