Isothermal crystallization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Isothermal crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was investigated by means of differential scanning calorimetry and polarized optical microscopy (POM). The Avrami analysis can be used successfully to describe the isothermal crystallization kinetics of PHBV, which indicates that the Avrami exponent n=3 is good for all the temperatures investigated. The spherulitic growth rate, G, was determined by POM. The result shows that the G has a maximum value at about 353 K. Using the equilibrium melting temperature (448 K) determined by the Flory equation for melting point depression together with U∗=1500 cal mol⁻¹, T_∞=30 K and T_g=278 K, the nucleation parameter K_g was determined, which was found to be 3.14 ± 0.07 × 10⁵ (K²), lower than that for pure PHB. The surface-free energy σ=2.55×10⁻² J m⁻² and σ_e=2.70±0.06×10⁻² J m⁻² were estimated and the work of chain-folding (q=12.5±0.2 kJ mol⁻¹) was derived from σ_e, and found to be lower than that for PHB. This implies that the chains of PHBV are more flexible than that of PHB.     

Structural analysis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibers prepared by drawing and annealing processes

Structure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibers prepared by drawing and annealing processes has been analyzed through wide-angle X-ray diffraction (WAXD), density, infrared dichroism, and birefringence measurements. There are three different types of crystalline structure in these fibers: two of these are the two types of orthorhombic with different orientation modes (the ordinary c-axis orientation (c//Z), and the preferential orientation of c-axis to the direction perpendicular to the fiber axis (c⟂Z)); and the third is pseudohexagonal. The weight fractions of the three types of crystals and amorphous phase were analyzed combining the WAXD integrated intensity and density data. The relation between crystalline orientation factors obtained separately from the WAXD measurement and the infrared dichroic ratio is also discussed. The birefringence of these fibers shows negative and positive values, depending on drawing and annealing temperatures. Considering the intrinsic birefringence and weight fraction of the c//Z, c⟂Z, and pseudohexagonal crystals, birefringence of the amorphous phase was evaluated. The amorphous birefringence shows positive values and decreases with an increase in the annealing temperature. From the analyzed fiber structure, it was speculated that the c⟂Z and pseudohexagonal crystals are preferentially formed in the drawing process irrespective of the drawing temperature.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2471–2482, 1998     

Macroporous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) matrices for cartilage tissue engineering

Polymer scaffold systems consisting of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) were investigated for possible application as a matrix for the three-dimensional growth of chondrocyte culture. The PHBV scaffolds were fabricated by a compression moulding, thermal processing and salt particulate leaching method without using organic solvent. The porous structure of the scaffolds was investigated with an optical microscope (OM) and scanning electron microscope (SEM) and the porosity was calculated. Then, the chondrocytes were cultured on the PHBV scaffolds for lone time to investigate whether it can be applied to construct the cartilage tissue in vitro. The results showed that the chondrocytes maintained their activity, fully expressed their phenotype and produced the extracellular matrix after incubation in vitro on the scaffolds for 7 days. In addition, in the prolonged incubation time, the percent of chondrocytes in their nature round morphology increased with an increase in the incubation period and they could synthesize the type II collagen and cartilage-specific proteoglycans. All of these results showed that the PHBV scaffolds had the potential to be used as chondrocytes carrier for cartilage engineering.     

The influence of processing conditions on the properties and the degradation of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)

The influence of melt-extrusion on the degradation behavior of poly(3-hydroxybutyrate-co-7% 3-hydroxyvalerate) in a small-scale compost and in salt media with Aspergillus fumigatus has been studied. The degradation has been monitored by Size Exclusion Chromatography (SEC), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and X-ray analysis. During the first 9 weeks of composting the degradation proceeds through surface erosion without any significant change in molecular weight or weight crystallinity. After 10 months the weight average molecular weight, Mw, has decreased by approximately 15–50%. It is suggested that degradation mechanism is converted from enzymatic to chemical hydrolysis as the porosity of the samples increases which facilitates water penetration. In addition, the acidic degradation products would accelerate the chemical hydrolysis inside the sample due to lowering of the pH. During degradation in media containing A. fumigatus the sample processed at low temperature exhibited extensive surface degradation and a 12% reduction of the Mw. In contrast the sample processed at high temperature showed an homogeneous surface degradation and no reduction in molecular weight. The differences in degradation are attributed to variations in the initial morphology of the samples caused by the processing conditions. Further investigations are, however, required to separate effects attributed to local differences in the compost environment and the structure of the samples.     

NMR analysis and triad sequence distributions of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Polyhydroxyalkanoates (PHAs) are considered promising “green” alternatives to synthetic polymers because they are bio-derived, biodegradable and biocompatible. The properties of bacterial PHA copolymers depend on their microstructures, which can be modified with the use of different fermentation processes and feed materials. Thus, it is desirable to have an improved testing method for the determination of PHA microstructures. In this work, a detailed NMR analysis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) microstructure was made. Previously only two of the hydroxyvalerate ¹³C NMR peaks have been assigned at the triad level. In this work, three of the ¹³C hydroxyvalerate peaks and two of the hydroxybutyrate peaks were found to be split into four peaks each due to comonomer sequence effects. Using eight copolymer samples with a wide compositional range, we were able to assign all these peaks to B-centered and V-centered triad sequences. Through curve deconvolution, the triad intensities were determined. These triad sequence intensities can then be analyzed via both the first-order Markovian and two-component Bernoullian models to obtain more in-depth information on copolymer composition and comonomer reactivities.     

Effect of multi-walled carbon nanotubes on crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The effect of multi-walled carbon nanotubes on the crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) has been investigated. The results have shown that carbon nanotubes (CNTs) act as an effective heterogeneous nucleation agent, inducing an increase in crystallinity and crystallite sizes. Comparing with the double melting peaks in pure PHBV, there is only one peak in the melting curves of nanocomposites. The isothermal crystallization kinetics of PHBV and its nanocomposite containing 0.5% CNTs were examined based on Avrami equation, indicating that the crystallization half-time decreases while the overall crystallization rate k increases dramatically with CNTs addition. The spherulitic nucleation and growth kinetics were also discussed grounded on Lauritzen–Hoffman equation. It is found that there is a spherulitic growth rates (G) maximum within selected temperature range in our study. Also, the temperatures corresponding to G maximum shift to a high level with addition of CNTs. The parameters of the equilibrium melting temperature T_m⁰ T_m^0 , the nucleation parameter K _g , the lateral surface free energy σ, the fold surface free energy σ _e , and the work of chain folding q of PHBV and its composite containing 0.5% CNTs were all calculated. The reductions of K _g , σ _e and q values of nanocomposite are in agreement with the fact that the crystallization rate of PHBV increases greatly by addition of CNTs.     

Assessing the thermoformability of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(acid lactic) blends compatibilized with diisocyanates

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a renewable alternative to conventional barrier packaging polymers due to its thermoplastic properties, biodegradability and gas barrier performance but its potential industrial applications are limited by its high price and difficult processability. A thorough study concerning the thermoforming ability of PHBV, and blends with poly(lactic acid) (PLA) incorporating three different diisocyanates as compatibilizers (hexamethylene diisocyanate, poly(hexamethylene) diisocyanate and 1,4-phenylene diisocyanate) is herein presented after component melt blending. A straightforward universal qualitative method is proposed to assess the thermoformability, based on a visual inspection of a thermoformed specimen and the ability to reproduce the mold shape, and the thermoforming window of the material. The results reveal a significant improvement in the thermoforming capacity and a widening of the thermoforming windows as the correct amounts of diisocyanates are incorporated. The barrier properties and the biodisintegrability of the blends was also studied, confirming a predictable slight decrease of the barrier performance when PLA is added, but without negatively affecting the disintegrability under composting conditions with respect to pristine PHBV.     

Sphaerotilus natans isolated from activated sludge and its production of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)

Sphaerotilus natans is a sheathed bacterium existing in the activated sludge of wastewater treatment plants. IT is one of the filamentous bacteria causing the bulking and foaming difficulties of activated sludge. Isolating the strain and culturing it in an axenic environment could not only provide the metabolic knowledge of the strains that would be useful in the development of wastewater treatment methods, but also could enable us to gain an understanding of the mechanism by which poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (poly[3-HB-co-3-HV]) is produced by this strain. This article reports the screening and isolation of the strain from the activated sludge using the Nile blue staining method together with Fourier transform infrared analysis. We investigated the ability of the selected strain to produce poly(3-HB-co-3-HV) copolymer using glucose and peptone, or by adding valeric acid or sodium propionate as precursor. Proper precursor feeding could dramatically enhance its 3HV content in the copolymer P(3HB-co-3HV). By controlling the different feeding times in fed-batch fermentation, different desired copolymers were obtained with 15, 40, and 70% 3HV mole fraction of the copolymer. Polymer properties were analyzed by gas chromatography, differential scanning calorimetry, thermo-gravimetry, and nuclear magnetic resonance analysis. Open laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis The University Grants Committee Area of Excellence Scheme (Hong Kong).     

Evolution of concentric spherulites in crystalline-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-b-poly(ethylene glycol) copolymers

Crystalline-crystalline poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly (ethylene glycol) (PEG) block copolymers (PHEGs) were synthesized by telechelic hydroxylated PHBV2000 and PEG with different number-average molecular weights. The synthesized PHEGs were analyzed using gel permeation chromatography and proton nuclear magnetic resonance. The cooling curves of the differential scanning calorimetry showed that the range of the melt-crystallization temperature of the PEG and PHBV blocks in the PHEGs partially overlapped. The spherulite of each PEG block and each PHBV block in the PHEGs crystallizes individually, but nucleated in the same site to form a concentric spherulite. The observations of the hot-stage polarized microscope (HSPM) showed that the first spherulite growth acted as a template for the later spherulite growth in the PHBV-b-PEG concentric spherulite. The spherulite growth rate of individual spherulite from the PEG block and PHBV block in PHEGs depends on the crystallization environment. The evolution of concentric spherulites in PHEGs at different crystallization conditions was studied in this study.     

聚羟基丁酸-戊酸的非等温热分解反应动力学

用非等温TG-DTA技术, 在5.0、10.0、15.0和20.0 K•min^-1线性升温条件下, 研究聚羟基丁酸-戊酸(PHBV)的热分解反应动力学. 结果表明, 分解过程分三个阶段：分解初期、分解中期和分解后期. 分解初期的机理函数为Avrami-Erofeev方程(n=1/2), 对应随机成核和随后生长机理, 表观活化能E_a(β→0)为69.44 kJ•mol^-1, 指前因子A(β→0)为106.27 s^-1；分解中期的机理函数为Avrami-Erofeev方程(n =2/5), 对应随机成核和随后生长机理, 表观活化能Ea(β→0)为117.64 kJ•mol^-1, 指前因子A(β→0)为10^11.48 s^-1；分解后期的机理函数为Mampel Power法则(n=1/3), 对应机理为幂函数法则, 表观活化能Ea(β→0)为116.64 kJ•mol^-1, 指前因子A(β→0)为10^8.68 s^-1.     

Thermal and mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) plasticized by biodegradable soybean oils

The effects of soybean oil (SO) and epoxidized soybean oil(ESO) as biodegradable plasticizers for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were studied using thermal and mechanical analyses. PHBV/SO and PHBV/ESO blends were prepared by evaporating solvent from blend solutions. The levels of additive in the blend varied from 5% to 30%. As a plasticizer for PHBV, ESO was more effective than SO in depression of the glass transition temperature as well as in increasing the elongation at break and the impact strength of the films with increasing levels of additive. Biodegradation of the plasticized PHBV films was carried out by accelerated compost method. The degradation rates of the blend films with SO or ESO were found to be faster than that of PHBV film. From the thermogravimetric analysis, it was found that the thermal reaction between the epoxide groups of ESO and PHBV fragments with carboxylic chain ends, occurred during the degradation of PHBV/ESO blends.     

Synergistic effect of compatibilizer and sepiolite on the morphology of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(butylene succinate) blends

Blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(butylene succinate) (PBS) with different PHBV/PBS weight ratios (100/0, 75/25, 50/50 and 0/100) were elaborated by melt mixing. The morphological investigation of the different samples, in comparison with that of neat PHBV and neat PBS, pointed out that PHBV/PBS blends form a biphasic system over the whole composition range. Low amount of compatibilizing agent (5 wt%), obtained by grafting maleic anhydride (MA) onto PHBV, i.e. PHBV-g-MA, was used for improving the miscibility between the two components of the blend. The incorporation of a fibrous filler as the sepiolite, easily dispersible in a polymer matrix, was also investigated. The morphology of the different blends as well as the evolution of their material properties were discussed in terms of the sepiolite and compatibilizing agent contents. The dispersion of PBS in the PHBV matrix markedly became finer with incorporation of sepiolite and PHBV-g-MA, due to enhanced interactions between the components. This paper highlighted a synergistic effect induced by the presence of both compatibilizer and sepiolite leading to an improved miscibility of the two blend components. The resulting properties were correlated with the morphology observed for the different blends.     

Synthesis and properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/chitin nanocrystals composite scaffolds for tissue engineering

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/chitin nanocrystals (CNC) composite scaffolds were synthesized by the salt leaching and thermally induced phase separation (TIPS) technique. The scaffolds have porous structures with macro-pores (100-300 mm in diameters) and micro-pores (10 mm). The surface characteristics of the scaffolds were characterized by X-ray photoelectron spectroscopy (XPS) and static water contact angle measurement, and the mechanical properties were investigated by a compression test. Human adipose-derived stem cells (hADSCs) were seeded onto the PHBV/CNC scaffolds and in vitro cell culture results showed that the composite scaffolds enhanced the hADSCs adhesion, which implies that the material may have potential application in tissue engineering.     

Construction of recombinant Escherichia coli strais for production of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate)

Plastic wastes constitute a worldwide environmental problem, and the demand for biodegradable plastics has become high. One of the most important characteristics of microbial polyesters is that they are thermoplastic with environmentally degradable properties. In this study, pUC 19/PHA was cloned and transformed into three different Escherichia coli strains. Among the three strains that were successfully expressed in the production of polyhydroxyalkanoates (PHA), E. coli HMS174 had the highest yield in the production of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (P[HB-HV]). The cell dry weight and PHA content of recombinant HMS174 reached as high as 10.27 g/L and 43% (w/w), respectively, in fed-batch fermentor culture. The copolymer of PHA, P(HB-HV), was found in the cells, and the biopolymers accumulated were identified and analyzed by gas chromatography, proton nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. We demonstrated clearly that the E. coli host for PHA production has to be carefully selected to obtain a high yield. The results obtained indicated that a superior E. coli with high PHA production can be constructed with a desirable ratio of P(HB-HV), which has potential applications in industry and medicine.     

Hydrolytic degradation of poly(ethylene oxide)-block-polycaprolactone worm micelles

Spherical micelles and nanoparticles made with degradable polymers have been of great interest for therapeutic application, but degradation-induced changes in a spherical morphology can be subtle and mechanism/kinetics appears poorly understood. Here, we report the first preparation of giant and flexible worm micelles self-assembled from degradable copolymer poly(ethylene oxide)-block-polycaprolactone. Such worm micelles spontaneously shorten to generate spherical micelles, triggered by polycaprolactone hydrolysis, with distinct mechanism and kinetics from that which occurs in bulk material.     

Hydrolytic Degradation Behaviors of Poly（p-dioxanone） in Ambient Environments

The effects of temperature and relative humidity on the hydrolytic degradation of poly（p-dioxanone）（PPDO） were investigated. The hydrolytic degradation behaviors were monitored by tracing the changes of water absorption, mechanical and crystalline properties, molecular weight and its distribution, surface morphologies, as well as infrared absorption peaks and hydrogen chemical shifts during the degradation. It is found that the water absorption increases whilst the intrinsic viscosity, tensile strength and elongation at break decrease as the temperature or relative humidity increases. With degradation time growing, the molecular weight drops and its distribution broadens. The crystallinity of PPDO has a tendency to increase at first and then to decrease, while the crystalline structure is not significantly changed. At the same time, some cracks are observed on the surface and keep growing and deepening. All results show that temperature plays more significant roles than relative humidity during the degradation. The analyses of Fourier transform infrared spectroscopy and hydrogen nuclear magnetic resonance spectroscopy reveal that the degradation of PPDO is a predominant hydrolysis of ester linkages.     

Valorization of olive husk flour as a filler for biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate): Effects of silane treatment

The paper deals with the influence of surface treatment of olive husk flour (OHF) by trimethoxyoctadecylsilane (TMOS) on the morphology and physical properties of bio-composites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Biocomposites based on PHBV/OHF: 80/20 (w/w) before and after filler treatment were prepared by melt compounding, while the modification of OHF surface by TMOS was performed by thermo-chemical vapor deposition. Scanning electron microscopic observations showed that modification of OHF by TMOS led to homogeneous and finer dispersion of the filler particles in PHBV matrix, indicating improved compatibility between the two components. Further, TGA data revealed an increase in thermal stability of treated PHBV/OHF biocomposites. An increase in the crystalline index determined by DSC was also observed due to the nucleating effect of OHF in the polymer matrix, however more pronounced for the treated biocomposites. The latter exhibited better tensile properties, as confirmed by DMA results, showing higher reinforcing effect of OHF for the treated biocomposites. This study highlighted significant improvements of the properties of PHBV/OHF biocomposites through silane treatment of OHF.     

Recent Advances and the Application of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as Tissue Engineering Materials

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), commonly referred to PHBV, are promising materials for tissue engineering applications because they are biodegradable, non-toxic and biocompatible. The surface modified PHBV and hybrid PHBV allow favorable mechanical properties, biocompatibility, and degradation times within desirable time frames under specific physiological conditions. We will shortly summarize what has been achieved in the PHBV tissue engineering area, namely, the surface modification reactions including functionalization and grafting reactions, as well as blending or compositing with other materials to improve the mechanical, thermal and hydrophilic properties, and the influence on cell-material interactions is also overviewed in the recent 5 years (from 2008 to 2012).     

Enzymatic degradation of poly(L-lactide) and poly(epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer. By means of differential scanning calorimetry and wide-angle X-ray diffraction, the crystallinity and orientation changes in the PLLA and PCL fibers during the enzymatic degradation were investigated, respectively.