Hai-Mu Ye

Biosynthesis and characterization of poly(3-hydroxydodecanoate) by β-oxidation inhibited mutant of Pseudomonas entomophila L48.

A medium-chain-length (MCL) polyhydroxyalkanoates (PHAs) producer Pseudomonas entomophila L48 was investigated for microbial production of 3-hydroxydodecanote homopolymer. Pseudomonas entomophila L48 was found to produce MCL PHA consisting of 3-hydroxyhexanoate (3HHx), 3-hydroxyoctanoate (3HO), 3-hydroxydecanoate (3HD), and 3-hydroxydodecanoate (3HDD) from related carbon sources fatty acids. In this study, some of the genes encoding key enzymes in β-oxidation cycle of P. entomophila such as 3-hydroxyacyl-CoA dehydrogenase, 3-ketoacyl-CoA thiolase, and acetyl-CoA acetyltransferase were deleted to study the relationship between β-oxidation and PHA synthesis in P. entomophila. Among the mutants constructed, P. entomophila LAC26 accumulated over 90 wt % PHA consisting of 99 mol % 3HDD. A fed-batch fermentation process carried out in a 6 L automatic fermentor produced 7.3 g L(-1) PHA consisting of over 97 mol % 3HDD fraction. Properties of MCL PHA were significantly improved along with increasing 3HDD contents. P(2.1 mol % 3HD-co-97.9 mol % 3HDD) produced by P. entomophila LAC25 had the widest temperature range between T(g) and T(m), which were -49.3 and 82.4 °C, respectively, in all MCL PHA reported so far. The new type of PHA also represented high crystallinity caused by side-chain crystallization compared with short side chain PHA. For the first time, P(3HDD) homopolymers were obtained.

Anisotropic surface effects on the formation of chiral morphologies of nanomaterials

Nanomaterials with chiral morphologies hold promise for a wide diversity of technologically important applications in such fields as micro/nano-electromechanical systems and medical engineering. Understanding the mechanisms underlying the formation of chiral morphologies of natural and synthesized materials remains an issue of crucial significance. In this study, a refined Kirchhoff rod model taking into account anisotropic surface effects is employed to describe quasi-one-dimensional nanomaterials with complicated spatial morphologies. It is shown that anisotropic surface stresses can induce the formation of rich morphologies of nanomaterials. A general shape equation of nanowires is derived by the variational method of energy. Thereby, the effects of anisotropic surface properties, bulk elastic properties and cross-sectional sizes on the chiral morphologies of nanomaterials are quantitatively investigated, and the conditions for the formation of binormal nanohelices are given. The physical mechanism addressed in this study is verified by our recent experiments on tuning the twisting chirality of polymer lamellae via surface treatments. Our analysis suggests that one can design and adjust the morphology of synthesized nanohelices by tailoring or functionalizing their surfaces during fabrication. This study is also helpful in interpreting the formation of such artificial and biological chiral materials as the flagella of bacterial and self-assembled helical ribbons.

Improve the thermal and mechanical properties of poly(L-lactide) by forming nanocomposites with pristine vermiculite

Poly(L-lactide) (PLLA)/pristine vermiculite nanocomposites were prepared by melt blending in a twin-screw extruder, and the detailed information of vermiculite dispersion state and effect of vermiculite on thermal and mechanical properties were systematically studied. The results show that the dispersion of vermiculite in the matrix is quite well when the loading content does not exceed 3 wt%. Pristine vermiculite can obviously improve the melt-crystallization temperature during the nonisothermal crystallization. Both crystallization time span and spherulitic size of PLLA decrease with the increasing amount of vermiculite under isothermal crystallization condition by enhancing the primary nucleation of PLLA. And the adding vermiculite can also improve the tensile modulus and Izod impact strength of PLLA. The intrinsic mechanism for the nucleating effect of vermiculite on PLLA is proposed to be the epitaxial crystallization and specific interaction between vermiculite and PLLA.

Development of poly(vinyl alcohol) porous scaffold with high strength and well ciprofloxacin release efficiency

Hydrophilic porous polymer scaffolds have shown great application in drug controlled release, while their mechanical properties and release efficiency still need further improvement. In the current study, the porous scaffolds of polyvinyl alcohol (PVA) prepared by quenching in liquid nitrogen and freeze drying method from different original concentration aqueous solutions were fabricated. Among different PVA scaffolds, the scaffold stemming from 18wt.% PVA aqueous solution exhibited the best mechanical properties, 10.5 and 1.54MPa tensile strengths for the dry and hydrogel states respectively. The inner morphology of such PVA scaffold was unidirectional honeycomb-like structure with average microchannel section of 0.5μm, and the scaffold showed porosity of 71% and rather low ciprofloxacin (Cip) release efficiency of 54.5%. Then poly(ethylene glycol) (PEG) was incorporated to enhance the Cip release efficiency. The release efficiency reached 89.3% after introducing 10wt.% PEG, and the mechanical properties of scaffold decreased slightly. Various characterization methods demonstrated that, adding PEG could help to enlarge the microchannel, create extra holes on the channel walls, weaken the interaction between PVA chains and Cip, and miniaturize the crystal size of Cip. All these effects benefit the dissolution and diffusion of Cip from scaffold, increasing its release capability. Moreover, based on biocompatible material composition, PVA/PEG scaffold is a non-cytotoxicity and have been verified that it can promote cell growth. And PVA/PEG scaffolds loaded with Cip can completely inhibit the growth of microorganism because of Cip sustaining release. The PVA scaffold would have a good potential application in tissue engineering, demanding high strength and well drug release capability.

Novel polymorphism behavior of poly(butylene adipate) in its nanocomposites with carbon nanofibers

Nanocomposites formed between poly(1,4-butylene adipate) (PBA) and carbon nanofibers (CNF) were prepared and characterized. The results showed that CNF had significant α-form crystal nucleating ability on PBA, and the α-form crystals could appear at rather low crystallization temperatures in PBA/CNF composites, for example 0 °C. Furthermore, a novel plateau content phenomenon of α-form crystals was discovered, and the value of the plateau content almost increased linearly with increasing CNF loading content. Microscopic morphology data suggested that PBA chains could wrap (or coat) around the nanofillers after the solution mixing process to form an interaction region. And in situ FTIR spectra proved the existence of multiple-weak CH–π interactions between CH2 groups on PBA chains and sp2 structures on the surface of CNF in composites, which helped to induce some precursory structures in the interaction region containing a chain conformation close to that in α-form crystals. Thus, the wrapping (or coating) behavior and precursory structures are responsible to the formation of α-form crystals at low temperature and the appearance of a novel plateau content phenomenon of α-form crystals. The change of PBA polymorphism behavior in nanocomposites provides a pathway for evaluating the interaction between polymer chains and nanofillers more intuitively.

Fractionated crystallization, polymorphism and crystal transformation of poly(butylene adipate) confined in electrospun immiscible blend fibers with polystyrene

Utilizing electrospun immiscible blend fibers of poly(butylene adipate) (PBA) and polystyrene (PS) and following coating by the high glass transition temperature poly(4-tert-butylstyrene) (P4tBS), confined PBA specimens in nanometer space were effectively prepared. The effect of a physical confinement environment on the PBA crystallization behavior, polymorphism and crystal transformation was adequately investigated. For non-isothermal crystallization, the crystallization ability and melting of subsequent heating was obviously suppressed in the nanospace compared with in the bulk state. Especially, the confined PBA could show fractionated crystallization due to the existence of different sizes of nanospace and the crystallization mechanism was either 1D crystal growth after heterogeneous nucleation or homogeneous nucleation dependent on the domain size. The temperature-dependent polymorphism and crystal transformation is also affected by the nanometer confinement. The formation temperature of the pure β crystal shifts slightly toward a high temperature and the β → α crystal transformation becomes easier for confined PBA compared with bulk PBA. More interestingly, the 2D IR correlation spectra revealed that the β → α crystal transformation in the nanospace takes the melt-recrystallization path, which is quite different from the usually solid–solid transformation process.

The effect of polymer-substrate interaction on the nucleation property: Comparing study of graphene and hexagonal boron nitride Nanosheets

Hexagonal boron nitride nanosheets (BNNSs) can work as a more efficient nucleating agent for two polyesters compared to graphene. Studies on the crystallization and dewetting processes of two polyesters, poly(butylene succinate) and poly(butylene adipate), on the two substrate surfaces prove that the interaction between BNNSs and the polyesters is stronger than that between graphene and the polyesters. This strong interaction induces the pre-ordered conformation of molten PBA which has been identified by the in situ FTIR spectra. Thus BNNSs possess higher nucleation property than graphene. Finally, a new polymer-substrate interaction induced nucleation mechanism was proposed to explain the nucleation efficiency difference between graphene and BNNSs.

Investigation of Structure and Crystallization Behavior of Poly(butylene succinate) by Fourier Transform Infrared Spectroscopy

The detailed structure and crystallization behavior of poly(butylene succinate) (PBS) have been investigated by Fourier transform infrared (FTIR) and other methods systematically. For the first time, we confirmed that the C═O stretching modes of PBS can respond to three distinguish absorption bands in the FTIR spectrum, at around 1736, 1720, and 1714 cm-1 respectively. The 1736 cm-1 band is adopted as the stretching mode of C═O groups in free amorphous fraction (MAF); the 1714 cm-1 band which is relevant to more stable structure, displays more anisotropic in polarized FTIR spectra, and has been confirmed as stretching vibrations of hydrogen-bonded C═O groups in the crystalline phase. The 1720 cm-1 band is linked to crystallization but comes from less ordered structure. Moreover, the 1720 cm-1 band can be destroyed prior to 1714 cm-1 band during heating and constructed behind 1714 cm-1 band during cooling. Thus, the 1720 cm-1 band is reasonably ascribed to the C═O groups in rigid amorphous fraction (RAF) or intermediate phase which locates between MAF and crystalline phase. The corresponding investigation by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) further supports that the three particular C═O absorption bands indeed reveal the typical three-phase structure for PBS. More important, the FTIR spectrum of PBS is very sensitive to sample preparation process and measurement mode. The relative content of each band depends on the crystallization temperature (Tc) and measured thickness. The higher Tc, the more RAF content appears when measured at room temperature; the thinner penetration thickness of FTIR measurement, the less RAF content can be detected, and the penetration thickness-dependent behavior is suggested as the result of higher mobility of chains in the air/bulk surface. Additionally, the particular three absorption bands of C═O groups in PBS force us to carefully reconsider previous reports on structure and interaction state obtained by FTIR spectroscopy in PBS and its composites.

Regulating the polymorphism behaviour and crystal transformation of poly(butylene adipate) by incorporating butylene fumarate units into the crystal lattice

Poly(butylene adipate-co-butylene fumarate) (PBAF) copolyesters with different butylene fumarate (BF) contents were synthesized, and the effect of BF units on the polymorphism, crystallization behavior and crystal transformation were studied using various methods. BF units are found to cocrystallize into both α- and β-form crystal lattices, and the β-form lattice shows a better accommodation degree for BF units than the α-form. Incorporation of BF units alters the formation temperature ranges for polymorphism behavior, and facilitates the β-form crystal at high temperature due to their stronger restriction effect on primary and secondary nucleation of the α-form crystal than the β-form crystal. Furthermore, BF units retard β to α solid–solid crystal transformation, and a novel triple melting phenomenon emerges in the DSC curve of β-form PBAF, which is reasonably attributed to the melting of the original β-form crystal and two types of newly-formed α-form crystals during heating. The β to α transformation takes place through both solid–solid and solid–liquid–solid (melt–recrystallization) routes. A special hydrogen-bonding interaction between neighbouring chain stems in the crystal phase is suggested to be responsible for the restriction of the solid–solid route and the induction of the solid–liquid–solid route for β to α transformation.

Combined effect of cellulose nanocrystals and poly(butylene succinate) on poly(lactic acid) crystallization: The role of interfacial affinity

Poly (lactic acid) (PLA)/cellulose nanocrystals (CNC), poly(butylene succinate) (PBS)/CNC and PLA/PBS/CNC composite films were prepared using a solution-casting technique. CNCs can be used to enhance the crystallization of PLA by offering more nucleation sites, and PBS can increase spherulite growth rate of PLA by providing flexible chains. However, CNCs and PBS together tend to interfere with each other and thus enhancement in the crystallization of PLA is lost. FTIR, contact-angle measurements, and dissolution experiments were used to characterize the materials. It was found that the interfacial affinity was greater in the CNC-PBS system than the CNC-PLA system. It was therefore concluded that the PBS chains occupy most of the CNC surfaces in the molten state before cooling. Consequently, PLA was mainly blocked from the CNCs and the nucleation effect was greatly weakened. The binary and ternary composite systems are discussed in terms of their crystallization processes.