Two-Stage Crystallization Kinetics and Morphological Evolution with Stereocomplex Crystallite-Induced Enhancement for Long-Chain Branched Polylactide/Poly(D-lactic acid) Blends

Abstract

A series of long-chain branched polylactide (LCB PLA)/poly­(D-lactic acid) (PDLA) blends with different low amounts of PDLA were prepared by a solution blending method. The formation of stereocomplex (SC) crystallites in LCB PLA/PDLA blends during a quenching process was proved by differential scanning calorimetry (DSC) and dynamic rheological frequency sweep, and the content of SC crystallites increased as the PDLA composition increased. The morphological evolution of LCB PLA/PDLA blends during crystallization at 130 °C was traced by polarized optical microscopy (POM), disclosing the separate growths of SC crystallites and PLA homocrystallites. The crystallization kinetics of LCB PLA/PDLA blends was investigated by rheometry, which was accelerated dramatically with increasing PDLA composition mainly due to the nucleation density enhancement. In particular, a two-stage crystallization process was found for LCB PLA/PDLA blends with PDLA compositions of 5 and 10 wt %, evidenced by the two plateaus in the storage modulus–time curves, which were apparently related to the existence of an LCB structure. The DSC result further confirmed that both SC crystallites and homocrystallites of PLA coexisted in LCB PLA/PDLA blends, with homocrystallites appearing much later as compared to SC crystallites, even after the complete crystallization of SC crystallites.

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

“…It has been demonstrated that rheometry can be well applied to trace the crystallization kinetics of polymers, polymer blends, and polymer composites. , The changes in storage modulus, G ′, with time at certain isothermal crystallization temperatures for PDLA/MPEG- b -PLLA 6.7k - g -glucose blends can be used to trace the nucleation and crystal growth processes. The data regarding transformation from the storage modulus ( G ′)–time curves to relative crystallinity ( x ( t ))–time curves can be obtained from the previously reported papers.…”

“…The n hc values between 2.0 and 4.0 for the HC stage indicate that the crystallization in this HC stage for a few of the samples takes place through a heterogeneous nucleation mode while most of the samples proceed through a homogeneous nucleation mode. It also seems that the nucleating ability of SCs could act as a heterogeneous nucleating agent for the late formation of HCs in the blend system, accelerating the crystallization kinetics of HCs by enhancing the nucleation density (Figure b,c) . This is the reason that the samples proceed through a heterogeneous nucleation mode.…”

“…It is proposed that the formation of SCs in the SC stage could act as a heterogeneous nucleating agent for the late formation of HCs in the blend system, accelerating the crystallization kinetics of HCs by enhancing the nucleation density. 32 The crystallization rate of SCs in PDLA/3A-MPEGb-PLLA 6.7k -g-MG-5.0 blends is faster due to the higher mobility in the SC stage, while the nucleation density of SCs in PDLA/ 5A-MPEG-b-PLLA 6.7k -g-Glu-5.0 blends is more intensive due to the more branched architecture in the small range of the SC stage. Therefore, the mobility of SCs in the blend system with the branched MPEG-b-PLLA 6.7k -g-glucose and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a T c of 130 °C.…”

“…It also seems that the nucleating ability of SCs could act as a heterogeneous nucleating agent for the late formation of HCs in the blend system, accelerating the crystallization kinetics of HCs by enhancing the nucleation density (Figure 7b,c). 32 This is the reason that the samples proceed through a heterogeneous nucleation mode. Meanwhile, possibly due to the low melting point for the HCs, the formation of HCs for most of the samples proceeds through a homogeneous nucleation mode in accord with the sufficiently low undercooling for this HC stage.…”

“…One important point to be noted is that Xu and coworkers 32 revealed that a two-stage isothermal cold crystallization at T c = 130 °C is found uniquely for long-chain branched PLLA/PDLA blends with PDLA compositions of 5 and 10 wt %. Notably, the first stage is related to the formation of SCs and the second stage to that of HCs.…”