Junjia Bian

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.

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.

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.

Hydrophobic modification of polypropylene/starch blend foams through tailoring cell diameter for oil-spill cleanup

Frequent oil spillages and industrial discard of many organic solvents have created severe environmental and ecological problems. Therefore, it is imperative to find effective absorbent materials with high performance. Simultaneously, a green process of preparing such absorbent materials should be developed. Herein we present a facile approach to prepare open-cell polypropylene (PP)/starch blend foams with low density by twin-screw extrusion using water as a physical blowing agent and starch as an effective water carrier. The cell diameter of the prepared PP/starch blend foams was controlled by using different nucleating agents and changing the die geometry. Foams with mean cell diameter in the range of 0.4–4.5 mm and open-cell content larger than 90% were successfully obtained. Moreover, a remarkable improvement of hydrophobicity of the foams was obtained when decreasing the cell diameters. Consequently, the water contact angle and oil recovery efficiency were increased up to 142.2° and 98.4%, respectively, when the mean cell diameter was reduced to 0.4 mm. These characteristics make this foam a promising candidate absorbent material for use in oil-spill cleanup.

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.

Effect of Epoxy Resin on the Thermal, Mechanical and Rheological Properties of Polybutylene Terephthalate/Glycidyl Methacrylate Functionalized Methyl Methacrylate-butadiene Blend

Epoxy resin was used to modify polybutylene terephthalate(PBT) and glycidyl methacrylate functionalized methyl methacrylate-butadiene(MB-g-GMA) blend. Results show that MB-g-GMA dispersed in PBT matrix uniformly and PBT/MB-g-GMA/epoxy blends reveal good compatibility. However, the added epoxy resin restricted the mobility of PBT macromolecular chains during the growth process of the crystal, which reduced the final crystallinity of PBT. The PBT/MB-g-GMA blend containing 1%(mass fraction) epoxy resin exhibited good mechanical properties. For example, the notched impact strength of the PBT/MB-g-GMA blend with 1%(mass fraction) epoxy resin was about 2 times that of PBT/MB-g-GMA blend. Sanning electron microscope(SEM) results show that the shear yielding of the PBT matrix and the cavitations of rubber particles were the major toughening mechanisms. The chemical reaction between PBT and epoxy resin induced the high complex viscosity and storage modulus of PBT/MB-g-GMA blend.