Compatibility and Thermal and Structural Properties of Poly(l-lactide)/Poly(l-co-d-lactide) Blends

Abstract

To explicate the interaction processes between poly­(l-lactide) (PLLA) and poly­(l-co-d-lactide) (PLDLA) in their blends, a series of high-molecular-weight PLDLA-m polymers (where m represents the percentage of l-lactate units) were prepared by ring-opening polymerization at different feed ratios of l- to d-lactides and blended with highly optically pure PLLA by solution blending. The compatibility and thermal and structural properties of the PLLA/PLDLA-m blends were investigated by differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FE-SEM) in detail. The PLLA/PLDLA-m blends with m = 60–20 wt % were only partially compatible because obviously double glass transitions were detected in DSC curves and multiphase structures were observed in SEM micrographs. The compatibility between PLLA and PLDLA-m decreased as the l-lactate unit content in PLDLA-m decreased. Crystallization of the PLLA phase in the PLLA/PLDLA-m blends was restricted by PLDLA-m at m = 95–40 wt % (the restriction was strongest at m = ∼70 wt %); however, it was enhanced as the l-lactate unit content decreased at m ≤ 40 wt %. The PLLA/PLDLA-m blends demonstrated weak-to-strong hydrogen-bonding interactions as the l-lactate unit content decreased at m ≤ 80 wt %. The PLLA/PLDLA-m blends can form not only a eutectic but also individual crystal structures of PLLA and PLDLA-m at m ≥ 80 wt %, while the formation of individual crystals of PLLA, PLDLA-m, and stereocomplexed polylactide (sc-PLA) is favored at m ≤ 15 wt %.

Competitive Mechanism of Stereocomplexes and Homocrystals in High-Performance Symmetric and Asymmetric Poly(lactic acid) Enantiomers: Qualitative Methods

“…Its thermal resistance is limited by a relatively low heat deflection temperature due to the slow crystallization and low crystallizability in practical processing such as injection molding. , It is worth mentioning that the presence of chiral carbon in the skeletal chain of PLA results in two stereoregular enantiomers, namely poly­( l -lactide) (PLLA)­and poly­( d -lactide) (PDLA). The PLLA /PDLA racemic blends lead to the formation of SCs with denser chain packing and stronger interchain interactions. − The chemical and physical properties of the stereocomplex poly­(lactic acid) (sc-PLA) strongly exceed those of its parent enantiomeric pure, homocrystalline polymers, such as higher mechanical strength, better thermal stability, lower thermodegradation, etc. − Meanwhile, this specific complex structure can effectively solve the above-mentioned limitations. Since this first report, the influences of the homopolymer molecular weight, blending ratio, blending conditions, and optical purity on the formation and properties of stereocomplexes have been well investigated. − …”

“… 7 − 10 The chemical and physical properties of the stereocomplex poly(lactic acid) (sc-PLA) strongly exceed those of its parent enantiomeric pure, homocrystalline polymers, such as higher mechanical strength, better thermal stability, lower thermodegradation, etc. 11 − 13 Meanwhile, this specific complex structure can effectively solve the above-mentioned limitations. Since this first report, the influences of the homopolymer molecular weight, blending ratio, blending conditions, and optical purity on the formation and properties of stereocomplexes have been well investigated.…”

New opportunities for sustainable bioplastic development: Tailorable polymorphic and three-phase crystallization of stereocomplex polylactide by layered double hydroxide

“…4 proposes a mechanism of the competing polymorphic crystallization in SC-PLA. On one hand, separation of racemic segments may occur due to their thermodynamic immiscibility [44][45][46], or due to the kinetically favored HC crystallization [47]. On the other hand, at the same time, due to thermal fluctuation, the racemic segments can be paired by hydrogen bonding that leans towards SC nucleation [42,48].…”

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

“…The chemical and physical properties of SC PLA strongly exceed those of its parent enantiomeric pure, homocrystalline polymers, such as higher mechanical strength, better thermal stability, lower thermodegradation, etc. 20,21 For example, the blending of PLLA and PDLA can result in the formation of SCs with a melting point of about 50 °C higher than that of homocrystals (HCs). 22−24 As a biocompatible and flexible polymer, poly(ethylene glycol) (PEG) is a good candidate for modification of PLA toughness through physical blending.…”

“…Since Y. Ikada proposed the stereocomplex (SC) crystallization of PLLA and PDLA in 1987, a large number of studies on the formation of the SC have been reported. The chemical and physical properties of SC PLA strongly exceed those of its parent enantiomeric pure, homocrystalline polymers, such as higher mechanical strength, better thermal stability, lower thermodegradation, etc. , For example, the blending of PLLA and PDLA can result in the formation of SCs with a melting point of about 50 °C higher than that of homocrystals (HCs). − …”