Yijie Bian

Stereocomplex crystallite network in poly(D,L-lactide): formation, structure and the effect on shape memory behaviors and enzymatic hydrolysis of poly(D,L-lactide)

Stereocomplex crystallization is a very interesting crystal modification formed between enantiomeric polymers, such as poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). Herein, biodegradable poly(D,L-lactide) (PDLLA) and stereocomplex-poly(L- and D-lactide) (sc-PLA) blends were prepared by solution blending at various sc-PLA loadings ranging from 2.5 to 10 wt%. Wide-angle X-ray diffraction and differential scanning calorimetry results verified that complete stereocomplex crystallites without any evidence of the formation of homocrystallites in the PDLLA could be achieved. By a rheological approach, a transition from the liquid-like to solid-like viscoelastic behaviour was observed for the stereocomplex crystallites reserved PDLLA melt, and a frequency-independent loss tangent at low frequencies appeared at a sc-PLA concentration of 5 wt%, revealing the formation of stereocomplex crystallite network structure. By a delicately designed dissolution experiment, the structure of the formed network structure was explored. The results indicated that the network structure were not formed by stereocomplex crystallites connected directly with each other or by bridging molecules, but by the interparticle PDLLA chains which were significantly restrained by the crosslinking effect of sc-PLA. Accordingly, the mechanical properties of PDLLA were greatly enhanced after blending with sc-PLA. Moreover, the most intriguing result was that the shape memory behaviors of PDLLA had been improved obviously in the blends than in neat PDLLA, especially when a percolation network structure had formed, which may be of great use and importance for the wider practical application of PDLLA. Finally, it was found that the enzymatic hydrolytic degradation rates had been retarded in the blends than in neat PDLLA. The erosion mechanism of neat PDLLA and the blends was further discussed.

Bioresource-based blends of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) and stereocomplex polylactide with improved rheological and mechanical properties and enzymatic hydrolysis

Novel bioresource-based blends of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) and stereocomplex polylactide (sc-PLA) were prepared herein via a simple melt blending method at various sc-PLA loadings at the temperature above the melting points of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), and much lower than that of sc-PLA. Wide-angle X-ray diffraction and differential scanning calorimetry results verified that complete stereocomplex crystallites without any evidence of the formation of homocrystallites in the P34HB melt could be achieved. Scanning electron microscopy observation indicated that sc-PLA was nicely dispersed in the P34HB matrix as spherical particles; the dispersed size of the sc-PLA did not display a pronounced increase with an increase in the content of PLLA and PDLA. As solid fillers, sc-PLA could reinforce the P34HB matrix in a relatively wider temperature region. Accordingly, the rheological and mechanical properties of P34HB were greatly improved after blending with sc-PLA, particularly when a percolation network structure of spherical filler (a characteristic solid or gel-like structure) had formed in the blends. Moreover, the most intriguing result was that the enzymatic hydrolysis rates had been clearly enhanced in the P34HB/sc-PLA blends than that in the neat P34HB, which may be of significant use and importance for the wider practical application of biosourced P34HB. The erosion mechanism of the neat P34HB and the P34HB/sc-PLA blends was discussed further.

Toughening mechanism behind intriguing stress–strain curves in tensile tests of highly enhanced compatibilization of biodegradable poly(lactic acid)/poly(3-hydroxybutyrate-co-4-hydroxybutyrate) blends

Highly enhanced compatibilization of biosourced and biodegradable polylactide (PLA) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) blends were successfully prepared by reactive melt compounding. Large shifts towards each other in terms of glass transition temperatures, a considerable reduction in the dispersed phase particle size and a significant increase in the interfacial adhesion between the PLA and P(3HB-co-4HB) phases were observed after compatibilization. In addition, chain branches occurred during the branching reaction decreased the crystallization ability of PLA, while crosslinks formed in the crosslinking reaction enhanced the crystallization ability of PLA on a large scale. Moreover, the blends exhibited a remarkable improvement of rheological properties of melt state when compared with that of blank PLA/P(3HB-co-4HB) blends. Upon increasing the content of the crosslinking agent, dicumyl peroxide (DCP), the blends showed increased yield tensile strength, modulus, and elongation at break. However, when DCP cooperated with triallyl isocyanurate (TAIC), the elongation at break decreased because the crosslinking network limited the mobility of the polymer chains to deform under a tensile load. Most notably, two typical and different kinds of growth of stress–strain curves were observed, and for the first time we demonstrated the toughening mechanism behind it in detail. Furthermore, SEM images of the fracture surfaces of the blends confirmed the toughening mechanism and that plastic deformation of the matrix and a debonding process were the two important ways of induced energy dissipation leading to toughened blends.

The physical properties of poly(l-lactide) and functionalized eggshell powder composites

Aiming at improved crystallization performance and simultaneously enhanced solid-state properties of poly(l-lactide) such as mechanical properties and enzymatic hydrolysis. A novel functionalized eggshell powder decorated with calcium phenylphosphonic acid (NES) was synthesized via the chemical reaction between phenylphosphonic acid and calcium ion on the surface of eggshell powder to form effective nucleating surface for poly(l-lactide). The resultant NES was incorporated into PLLA matrix to form fully biodegradable composites by melt blending, which exhibited superior crystallization, mechanical properties, and enzymatic hydrolysis. Upon the addition of 20 wt% NES, the crystallization half-time of a PLLA/NES composite decreased from 27.09 to 0.69 min at 130°C, compared to that of neat PLLA. The storage and tensile moduli of the composites increased with increasing NES loadings. Even with 20 wt% NES, the composite still exhibited good mechanical properties with tensile strength of 53.4 MPa, tensile modulus of 2460MPa and elongation at break of 2.5%, respectively. Moreover, it was interesting to find that the enzymatic hydrolytic degradation rates had been enhanced pronouncedly in the PLLA/NES composites than in neat PLLA. Such high performance biocomposites have great potential in expanding the utilization of eggshell powder from sustainable resources and practical application as PLLA-based bioplastic.

Enhancing the crystallization of poly(L-lactide) using a montmorillonitic substrate favoring nucleation

Montmorillonite (MMT) generally has weak nucleating ability or even retarded crystallization for poly(L-lactide) (PLLA) depending on the dispersion morphology in the matrix. A novel MMT with a nucleating surface (NMMT) chemically supported by calcium phenylphosphonic acid (PPCa) (an effective nucleant of PLLA), was prepared through the chemical reaction between phenylphosphonic acid (PPOA) and a calcium ion on the surface of MMT for the first time. Differential scanning calorimetry, infrared spectroscopy and wide-angle X-ray diffraction confirm the reaction between PPOA and Ca-montmorillonite and the formation of PPCa on the surface of MMT. Then, NMMT was introduced into PLLA via simple melt blending. The most intriguing result is that the crystallization rate of PLLA greatly increases after incorporation of NMMT, and the crystallization rate of PLLA increases with increasing NMMT fraction and decreasing MMT/PPOA mass ratio. The nucleation density of PLLA increases and the spherulite size decreases significantly in the presence of NMMT. Epitaxy is the possible mechanism to explain the nucleation phenomenon of the PLLA/NMMT system. Moreover, the tensile test results show that NMMT has a strengthening effect on the amorphous PLLA. Through a short time annealing procedure, the mechanical properties such as the tensile modulus and storage modulus of PLLA are improved by the addition of NMMT.

Intriguing crystallization behavior and rheological properties of radical-based crosslinked biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

A series of branched/crosslinked poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] with changing gel fractions were obtained by adding small amounts of crosslinking agents dicumyl peroxide (DCP) and triallyl isocyanurate (TAIC). The thermal and rheological properties of the samples were investigated. The chain branches formed by adding a certain amount of DCP bring in not only excess free volume which enhanced the cold crystallization ability but also defective crystals which decreased the melting temperature. Additionally, the rheological properties of branched samples were improved compared with those of neat P(3HB-co-4HB). The most intriguing result was the crystallization behavior of crosslinked P(3HB-co-4HB). The crosslinks, acting as favorable nucleation sites, can enhance the crystallization nucleation rate markedly. However, too many crosslinks could impede the transportation of macromolecular chain segments during the crystallization, resulting in a decreased crystallization rate, and the final crystallinity of crosslinked P(3HB-co-4HB) was independent of the degree of crosslinking. Furthermore, due to the formation of a gel network, crosslinked biodegradable P(3HB-co-4HB) exhibited remarkable improvement in rheological properties than branched samples, extending its processing methods, like foaming and film blowing. Accordingly, the practical applications of this biosourced and biocompatible polymer can be widely achieved.