Lijing Han

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.

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.

Thermal and mechanical properties of polylactide toughened with a butyl acrylate-ethyl acrylate-glycidyl methacrylate copolymer

In this work, a specific polylactide (PLA) 4032D was melt-mixed with a new toughener: butyl acrylate (BA), ethyl acrylate (EA) and glycidyl methacrylate (GMA) copolymer (BA-EA-GMA). DMA tests showed that PLA/BA-EA-GMA blends were partially miscible. The degree of crystallinity of PLA increased while the cold crystallization temperature shifted to higher temperatures with increasing BA-EA-GMA content. The SEM micrographs showed that PLA/BA-EA-GMA blends had a good dispersion and this phenomenon was in good agreement with their higher impact strength. The result showed that the adding of BA-EA-GMA has enhanced the flexibility of PLA/BA-EA-GMA blends as compared with pure PLA. The impact strength was changed from 3.4 kJ/m2 for pure PLA to 29.6 kJ/m2 for 80/20 PLA/BA-EA-GMA blend.

Enzymatic degradation and porous morphology of poly(L-lactide) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) blends

Fully biodegradable polymer blends based on biosourced polymers, namely poly(L-lactide) (PLLA) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) are prepared by melt compounding. The enzymatic degradation and porous morphology of PLLA/P(3HB-co-4HB) blends are investigated in detail. The lipase from Pseudomonas mendocina reveals preferred enzymatic degradation of P(3HB-co-4HB) but insignificant attack to PLLA in the blends. At the same time, proteinase K can degrade PLLA, but cannot degrade P(3HB-co-4HB). On account of the surface erosion mechanisms, the enzymatic degradation rates of both the P(3HB-co-4HB) and PLLA in the blends are improved because of the presence of the other component to increase the specific surface area. The results of the 1H NMR and GPC indicate that there is no more intermediate products formed during the enzymatic degradation of the PLLA and P(3HB-co-4HB). Due to the specificity of the degradation enzymes, selective enzymatic degradation is adopted to degrade and remove one component from the blends, and various porous morphologies are acquired.

Poly(ɛ-caprolactone) composites reinforced by biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fiber.

Biodegradable and biosourced poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) fiber was used as a reinforcing agent, and environment friendly poly(ɛ-caprolactone) (PCL) composites were prepared by melt compounding. The mechanical properties, rheological properties, and enzymatic degradation of the PCL composites were investigated in detail. With the addition of PHBV fibers, the PCL composites showed increased tensile yielding strength and modulus. Especially, the storage modulus from the results of dynamic mechanical analysis was increased significantly, suggesting that PCL was obviously reinforced by adding PHBV fibers. With increasing the PHBV fiber content, the complex viscosity and modulus of PCL increased, especially at low frequencies, indicating that a network structure was formed in the composites. The network structure resulted in evident solid-like response due to the restriction of the chain mobility of PCL matrix, which was further confirmed by the Han and Cole-Cole plots. The morphology, evaluated by scanning electron microscopy, indicated PCL and PHBV fiber were not highly incompatible and the interfacial adhesion was good, which was beneficial to the reinforcement effect. The biodegradability of the PCL was significantly promoted after composites preparation. Such studies are of great interest in the development of environment friendly composites from biodegradable polymers.

Thermal, mechanical, and rheological properties of poly(propylene carbonate) cross-linked with polyaryl polymethylene isocyanate

In this study, cross-linked poly(propylene carbonate) (PPC) was prepared using polyaryl polymethylene isocyanate (PAPI) as a cross-linking agent. The gel content, thermal behaviors, mechanical and rheological properties of the cross-linked PPC were investigated. FTIR results showed that the chemical reactions took place between PPC and PAPI and the interactions demonstrated that PPC may be cross-linked with the PAPI. The results of gel content revealed that PPC was partially cross-linked with the PAPI. The cross-linked PPC showed higher glass transition temperature and decomposition temperature compared with pure PPC. Accordingly, the melt flow index gradually decreased and complex viscosity increased with increasing PAPI content. Moreover, the mechanical properties proved also to be enhanced as evidenced by tensile tests. The introduction of small amount of cross-linkable moiety provides an efficient and convenient method to improve the properties of PPC and extend its application area.

Rheology, mechanical properties and crystallization behavior of glycidyl methacrylate grafted poly(ethylene octene) toughened poly(lactic acid) blends

Poly(lactic acid) (PLA)/poly(ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA denoted as GPOE) blends were prepared via simple melt compounding method at GPOE loadings from 5 to 20wt%. GPOE can significantly affect the physical properties of PLA. Compared to neat PLA, the elongation at break and impact strength of the blends were significantly improved. Scanning electron micrograph analysis revealed large numbers of cavities in the fracture surface of the blends, and the size of the cavities increased along with the increase of GPOE content in the PLA/GPOE blends. Furthermore, the overall crystallization rates were faster in the PLA/GPOE blends than that in neat PLA. However, the crystallization mechanism and crystal structure of these blends remained unchanged despite the presence of GPOE. The addition of GPOE decreased the degree of crystallinity of PLA. The toughened PLA could be of great use and importance for wider practical applications.

Effect of diameter of poly(lactic acid) fiber on the physical properties of poly(ɛ-caprolactone).

Biodegradable polymer composites based on poly(ɛ-caprolactone) (PCL) and poly(lactic acid) (PLA) fibers with diameters of 18, 26, 180 μm were prepared by melt compounding. The PLA fiber content in the composites was constant at 20% by weight. The effects of fibers with different diameters on the physical properties and enzymatic degradation of PCL were investigated. The morphological analysis indicated good interfacial adhesion between PCL and PLA fiber, which was beneficial to improve the physical properties of PCL. With increasing PLA fiber diameter, the complex viscosity and modulus of PCL were significantly increased, especially at low frequencies, indicating that the hindered effect of the fiber on the mobility of the PCL molecular chains was more obvious when PLA fiber diameter was thicker. However, as for the mechanical properties, the reinforcement was more obvious to PCL with the smaller PLA fiber diameter. This was because increasing efficient load transfer may be appeared due to the larger surface area and better interface bonding force of the fiber with thinner diameters. The enzymatic degradation of PCL was accelerated with the addition of large PLA fiber diameter of 26 and 180 μm, and hardly changed with the small PLA fiber diameter of 18 μm.

Effect of crystallinity on the thermal conductivity of poly(3-hydroxybutyrate)/BN composites

In this research, boron nitride (BN) acting as both nucleating agent and thermally conductive filler was melt-mixed with poly(3-hydroxybutyrate) (PHB). It was assumed that the introduction of BN not only formed thermally conductive pathways to increase the thermal conductivity of PHB, but also reduced the interfacial thermal resistance due to the interaction between BN and PHB. The assumption was confirmed by density test, morphological observation, and rheological tests. Besides, the introduction of BN improved the thermal stability of PHB, as well. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3,4HB) was introduced as a comparison with PHB to illustrate the effect of crystallinity on the thermal conductivity of PHB/BN composites. According to the DSC tests, BN was proved to be a good nucleating agent for both PHB and P3,4HB. As the wide-angle X-ray diffraction analysis results showed that the crystal structure of PHB/BN and P3,4HB/BN composites was the same, the reason that the thermal conductivity of PHB/BN composites was higher than that of P3,4HB/BN composites at all BN levels was mainly because of the difference of the degree of crystallinity.

Porous poly(l-lactic acid) sheet prepared by stretching with starch particles as filler for tissue engineering

Porous poly(L-lactic acid) (PLLA) sheets were prepared by uniaxial stretching PLLA sheets containing starch filler. Here, the starch filler content, stretching ratio, stretching rate and stretching temperature are important factors to influence the structure of the porous PLLA sheets, therefore, they have been investigated in detail. The pore size distribution and tortuosity were characterized by Mercury Intrusion Porosimetry. The results revealed that the porosity and pore size enlarged with the increase of the starch filler content and stretching ratio, while shrank with the rise of stretching temperature. On the other hand, the pore structure almost had no changes with the stretching rate ranging between 5 and 40 mm/min. In order to test and verify that the porous PLLA sheet was suitable for the tissue engineering, the starch particles were removed by selective enzymatic degradation and its in vitro biocompatibility to osteoblast-like MC3T3-E1 cells was investigated.