Crystallization behavior and biodegradation of poly(3-hydroxybutyrate) and poly(ethylene glycol) multiblock copolymers

Block copolymerization by using isocyanates is an effective method for incorporating PHB and PEG because it can prepare copolymers with good properties, such as toughness, strength, and so on. In this study, we adopted soil suspension system to estimate the biodegradability of a series of PHB/PEG multiblock copolymers with different compositions and block lengths. In the degradation process, the changes in weight loss, molecular weight, and tensile strength were periodically measured to determine the biodegradability, and the surface morphology was also observed by SEM. In contrast to pure PHB, the weight loss of the copolymer was relatively lower. On the other hand, the tensile strength and molecular weight experienced apparent decrease, and for BHG1000-3-1, they reached 46.7% and 77.7% of the initial value, respectively. SEM observation showed that the surface was covered with numerous erosion pits. All these indicate that the degradation indeed took place and long-chain molecules have been hydrolyzed into shorter ones. The crystallization behavior was also investigated by DSC and WAXD. The results showed that both the segments, PEG and PHB, can form crystalline phases at lower PHB contents ranging from 29% to 44%, and when PHB component was more than 60%, only PHB phase can crystallize.     

Influence of poly (ethylene glycol) on the thermal, mechanical, morphological, physical-chemical and biodegradation properties of poly (3-hydroxybutyrate)

Blends of poly (3-hydroxybutyrate) (PHB) with poly (ethylene glycol) (PEG), (PHB/PEG), in different proportions of 100/0, 98/2, 95/5, 90/10, 80/20 and 60/40 wt%, respectively, were investigated for their thermal properties (using differential scanning calorimetry and thermogravimetric analysis), tensile properties, water vapor transmission rate, enzymatic biodegradation (using light microscopy) and mass retention. The addition of plasticizer did not alter the thermal stability of the blends, although an increase in the PEG content reduced the tensile strength and increased the elongation at break of pure PHB.     

Crystallization of photo-chain extended poly(ethylene glycol)

Coumarin-functionalized poly(ethylene glycol) (PEG) monols and diols were isothermally crystallized at temperatures between 20 and 35 °C before and after exposure to approximately 110 J cm⁻² of ultra-violet A (λ > 300 nm, UVA) irradiation. Irradiation dimerized the coumarin groups and chain-extended the coumarin-functionalized PEG oligomers. The higher molecular weights reduced the crystal growth rate by as much as 50% compared to the non-irradiated coumarin-functionalized PEG oligomers under ambient crystallization conditions. Hoffman’s kinetic nucleation theory was utilized to evaluate the types of nucleation that occurred for the coumarin-functionalized PEG diols (COU-PEG-COU). Crystallization regimes II and III were observed for the coumarin-modified PEG oligomers before and after exposure to UVA light.     

Preparation and characterization of biodegradable nanoparticles based on amphiphilic poly(3-hydroxybutyrate)-poly(ethylene glycol)-poly(3-hydroxybutyrate) triblock copolymer

Amphiphilic triblock copolymers of poly(3-hydroxybutyrate)-poly(ethylene glycol)-poly(3-hydroxybutyrate) (PHB-PEG-PHB) were directly synthesized by the ring-opening copolymerization of β-butyrolactone monomer using PEG as macroinitiator. Their structure, thermal properties and crystallization were investigated by ¹H NMR, differential scanning calorimetry (DSC) and X-ray diffraction. It was found that both PHB and PEG blocks were miscible. With the increase in the PHB block length, the triblock copolymers became amorphous because amorphous PHB block remarkably depressed the crystallization of the PEG block. Biodegradable nanoparticles with core-shell structure were prepared in aqueous solution from the amphiphilic triblock copolymers, and characterized by ¹H NMR, SEM and fluorescence. The hydrophobic PHB segments formed the central solid-like core, and stabilized by the hydrophilic PEG block. The nanoparticle size was close related to the initial concentrations of the nanoparticle dispersions and the compositions of the triblock copolymers. Moreover, the PHB-PEG-PHB nanoparticles also showed good drug loading properties, which suggested that they were very suitable as delivery vehicles for hydrophobic drugs.     

Nonisothermal crystallization and melting behavior of poly(3-hydroxybutyrate) and maleated poly(3-hydroxybutyrate)

Nonisothermal crystallization and melting behavior of poly(3-hydroxybutyrate) (PHB) and maleated PHB were investigated by differential scanning calorimetry using various cooling rates. The results show that the crystallization behavior of maleated PHB from the melt greatly depends on cooling rates and its degree of grafting. With the increase in cooling rate, the crystallization process for PHB and maleated PHB begins at lower temperature. For maleated PHB, the introduction of maleic anhydride group hinders its crystallization, causing crystallization and nucleation rates to decrease, and crystallite size distribution becomes wider. The Avrami analysis, modified by Jeziorny, was used to describe the nonisothermal crystallization of PHB and maleated PHB. Double melting peaks for maleated PHB were observed, which was caused by recrystallization during the heating process.     

Effect of poly(vinyl acetate) (PVAc) on thermal behavior and mechanical properties of poly(3-hydroxybutyrate)/poly(propylene carbonate) (PHB/PPC) blends

To assess the compatibility of blends of synthetic poly(propylene carbonate) (PPC), with a natural bacterial poly(3-hydroxybutyrate) (PHB), a simple casting procedure of blend was used. poly(3-hydroxybutyrate)/poly(propylene carbonate) blends are found to be incompatible according to DSC and DMA analysis. In order to improve the compatibility and mechanical properties of PHB/PPC blends, poly(vinyl acetate) (PVAc) was added as a compatibilizer. The effects of PVAc on the thermal behavior, morphology, and mechanical properties of 70PHB/30PPC blend were investigated. The results show that the melting point and the crystallization temperature of PHB in blends decrease with the increase of PVAc content in blends, the loss factor changes from two separate peaks of 70PHB/30PPC blend to one peak of 70PHB/30PPC/12PVAc blend. It is also found that adding PVAc into 70PHB/30PPC blend can decrease the size of dispersed phase from morphology analysis. The result of tensile properties shows that PVAc can increase the tensile strength and Young’s modulus of 70PHB/30PPC blend, and both the elongation at break and the tensile toughness increase significantly with PVAc added into 70PHB/30PPC.     

Miscibility,Crystallization, and Rheological Behavior of Solution Casting Poly(3-hydroxybutyrate)/poly(ethylene succinate) Blends Probed by Differential Scanning Calorimetry,Rheology, and Optical Microscope Techniques

The miscibility and crystallization of solution casting biodegradable poly(3-hydroxybuty-rate)/poly(ethylene succinate) (PHB/PES) blends was investigated by differential scanning calorimetry, rheology, and optical microscopy. The blends showed two glass transition temperatures and a depression of melting temperature of PHB with compositions in phase diagram, which indicated that the blend was partially miscible. The morphology observation supported this result. It was found that the PHB and PES can crystallize simultaneously or upon stepwise depending on the crystallization temperatures and compositions. The spherulite growth rate of PHB increased with increasing of PES content. The influence of compositions on the spherulitic growth rate for the partially miscible polymer blends was discussed.     

Miscibility and crystallinity of poly(3-hydroxybutyrate)/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) blends

With the objective of developing new biodegradable materials, the miscibility and the crystallinity of blends of poly(3-hydroxybutyrate), P(3HB), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate), P(3HB-co-3HV), have been studied. P(3HB) (300 kg mol⁻¹)/P(3HB-co-3HV)–10% 3HV (340 kg mol⁻¹) blends were prepared by casting in a wide range of proportions, and characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The experimental values for the glass transition temperatures (T_g) are in good agreement with the values provided by the Fox equation, showing that the blends are miscible. It was observed that the T_g and the melting temperature (T_m) decreases with the increase in the P(3HB-co-3HV)–10% 3HV content, while the crystallization temperature (T_c) increases. FT-IR analyses confirmed the decrease on the crystallinity of P(3HB)/P(3HB-co-3HV)–10% 3HV blends with higher copolymer contents. Bands related to the crystallinity were changed, due to the copolymer content that produced miscible and less crystalline blends.     

Effect of ethylene glycol on the end group structure of poly(3-hydroxybutyrate)

Ralstonia eutropha was cultivated in a culture medium supplemented with ethylene glycol (EG), which is known to act as a chain transfer agent in the production of poly(3-hydroxybutyrate) (PHB). The PHB extracted from the bacterial cells was analyzed by ¹H and ³¹P NMR spectroscopies and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The addition of EG exerted a remarkable influence on the mass of production and molecular weight of PHB, and then was found to result in chain transfer and termination reactions. MALDI-TOF MS of the partially hydrolyzed PHB revealed that either succinate or glutarate combined with the hydroxyl terminals of PHB as polymerization starters. From ³¹P NMR analysis, the carboxyl groups of the succinyls and glutaryls held in the terminals of the isolated PHB were found to be capped with EG, giving the telechelic polyester with hydroxy functionalities. Based on these results, we propose a plausible mechanism of enzymatic polymerization in the microbial PHB synthesis in the presence of EG.     

Crystallization and melting behavior of poly(ethylene oxide)/poly(n-butyl methacrylate) blends

The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly(n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ₁₂=0, indicating a relatively low interaction strength.     

Effect of thermal degradation on rheological properties for poly(3-hydroxybutyrate)

Thermal degradation at processing temperature and the effect on the rheological properties for poly(3-hydroxybutyrate) have been studied by means of oscillatory shear modulus and capillary extrusion properties, with the aid of molecular weight measurements. Thermal history at processing temperature depresses the viscosity because of random chain scission. As a result, gross melt fracture hardly takes place with increasing the residence time in a capillary rheometer. Moreover, it was also found that the molecular weight distribution is independent of the residence time, whereas the inverse of the average molecular weight is proportional to the residence time. Prediction of average molecular weight with a constant molecular weight distribution makes it possible to calculate the flow curve following generalized Newtonian fluid equation proposed by Carreau as a function of temperature as well as the residence time.     

Determination of multiple thermal degradation mechanisms of poly(3-hydroxybutyrate)

The thermal degradation of poly(3-hydroxybutyrate) (PHB) was investigated by kinetic analyses in detail to clarify its complex degradation behavior, resulting in a finding of mixed mechanisms comprising at least a thermal random degradation with subsequent auto-accelerated transesterification, and a kinetically favored chain reaction from crotonate chain ends. The thermal degradation behavior of PHB varied with changes in time and/or temperature. From the kinetic analysis of changes in molecular weight, it was found that a non-auto-catalytic random degradation proceeding in the initial period was followed by an auto-accelerated reaction in the middle period. From the kinetic analysis of weight loss behavior, it is proposed that there are some kinetically favored scissions occurring at the chain ends, where the degradation proceeded by a 0th-order weight loss process in the middle stage. The observed 0th-order weight loss process was assumed to be an unzipping reaction occurring at ester groups neighboring the crotonate end groups.     

Dielectric relaxation study of atactic poly(epichlorohydrin)/poly(3-hydroxybutyrate) blends

Binary blends of atactic poly(epichlorohydrin) (aPECH) and poly(3-hydroxybutyrate) (PHB) were investigated as a function of blend composition and crystallization conditions by dielectric relaxation spectroscopy. The quenched samples were found to be miscible in the whole composition range by detecting only one glass transition relaxation, for each composition, which could be closely described by the Gorden-Taylor equation. The cold-crystallized blends displayed two glass transition relaxations at all blend ratios indicating the coexisting of two amorphous populations: a pure aPECH phase dispersed mainly in the interfibrillar zones and a mixed amorphous phase held between crystal lamellae. The interlamellar trapping of aPECH was small and decreases with increasing the overall PHB content in the blend. At high crystallization temperatures the aPECH molecules was found to reside mainly in the interfibrillar regions due to its high mobility relative to the crystal growth rate of PHB. Our results suggest that because the intersegmental interaction in aPECH/PHB blends is weak, the mobility of the amorphous component at a given crystallization temperature decides diluent segregation.     

聚谷氨酸苄酯-聚乙二醇-聚谷氨酸苄酯嵌段共聚物的圆二色光谱分析

用三乙胺和双端氨基聚乙二醇分别引发经酯化、环化等处理的谷氨酸开环聚合制备聚谷氨酸苄酯（PBLG）和聚谷氨酸苄酯-聚乙二醇-聚谷氨酸苄酯嵌段共聚物（PBLG-PEG-PBLG,GEG）。采用圆二色光谱对聚合物溶液的旋光性进行分析,以确定共聚物中PBLG嵌段的构型和含量。结果表明,均聚物和共聚物中的PBLG嵌段都以α-螺旋构型存在,中间的PEG不扰乱其构型,通过聚合物的圆二色性（circular dichroism,CD）计算出的PBLG嵌段含量与核磁共振（nuclear magnetic resonance,NMR）所得结果基本一致。     

Crystallization behavior and thermal property of biodegradable poly(butylene succinate)/functional multi-walled carbon nanotubes nanocomposite

Biodegradable poly(butylene succinate) (PBSU)/functional multi-walled carbon nanotubes (f-MWNTs) nanocomposite were prepared by melt compounding. Nonisothermal crystallization and subsequent melting behavior, isothermal crystallization kinetics, spherulitic morphology, and crystal structure of neat PBSU and its nanocomposite were studied by differential scanning calorimetry, optical microscopy and wide angle X-ray diffraction in detail. The presence of f-MWNTs has a significant heterogeneous nucleation effect on the crystallization and morphology of PBSU, resulting in that the crystallization is enhanced during both nonisothermal and isothermal crystallization in the nanocomposite. Moreover, the crystal structure of PBSU is not modified by f-MWNTs in the nanocomposite. The thermogravimetric analysis illustrates an improvement in thermal stability of PBSU by around 10 °C in the presence of f-MWNTs compared with that of neat PBSU.     

Thermal behavior of the maleic anhydride modified poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate), PHB has been structurally modified through reaction with maleic anhydride, MA. Transesterification reaction was carried out fixing the PHB and MA and besides time and temperature the concentration of the triethylamine (used as catalyst) was changed. Glass transition, melting and crystallization temperature obtained from DSC curves and thermal degradation temperatures obtained from TG traces were used to evaluate the influence of the reaction conditions on the modification of PHB according to factorial design. On the base of the results the optimum conditions are to perform the PHB modification reaction with MA reaction at 110°C for 1 h with 5% v/v triethylamine.     

Crystallization kinetics and morphology of poly(vinylidene fluoride)/poly(ethylene adipate) blends

The miscibility, isothermal crystallization kinetics and morphology of the poly(vinylidene fluoride)(PVDF)/poly(ethylene adipate)(PEA) blends have been studied by differential scanning calorimetry(DSC), optical microscopy(OM) and scanning electron microscopy(SEM). A depression of the equilibrium melting point of PVDF was observed. From the melting point data of PVDF, a negative but quite small value of the interaction parameter ?PVDF-PEA is derived using the Flory-Huggins equation, implying that PVDF shows miscibility with PEA to some extent. Nonisothermal and isothermal crystallization kinetics suggest that the crystallization rate of PVDF decreases with increasing the amount of PEA, and a contrary trend was found when PEA crystallizes with the increase of the amount of PVDF. It was further disclosed that the blend ratio and crystallization temperature affect the texture of PVDF spherulites greatly, which determines the subsequent crystallization of PEA. At high temperatures, e.g. 150 ℃, the band spacing of PVDF spherulites increases with the addition of PEA content and the spherulitic structure becomes more open. In this case, spherulitic crystallization of PEA is not observed for all blend compositions. At low temperatures, e.g. 130 ℃, for the PEA-rich blends, the interpenetrated structures are eventually formed by the penetration of the spherulites of PEA growing within the pre-existing PVDF spherulites.     

基于聚乙二醇结构调控高分子量聚乳酸立构复合体系的结晶行为

设计合成了梳形聚(聚乙二醇甲醚丙烯酸酯)(PPEGA)及其与聚乙二醇(PEG)的嵌段共聚物(PEG-b-PPEGA).通过与高分子量左旋聚乳酸(PLLA)和右旋聚乳酸(PDLA)共混探究了PEG不同的结构对PLA立构复合体系(sc-PLA)结晶的影响.结果 表明线形PEGA和PEG能与sc-PLA完全相容,两者均能促进...     

Polycaprolactone–poly(ethylene glycol) block copolymer,I: synthesis and degradability in vitro

A new type of biodegradable polymer material, poly(caprolactone)–poly(ethylene glycol) block copolymer (PCL-b-PEG), was synthesized by means of direct copolycondensation of ε-caprolactone with poly(ethylene glycol) in the presence of a Ti(OBu)₄ catalyst. The degradability of the polycaprolactone was improved by introducing a PEG component into it. The degradation of PCL-b-PEG copolymer increase with a decreasing crystallinity of the copolymer, and can be controlled by adjusting the component ratio of the copolymer.     

New approach on the development of plasticized polylactide (PLA): Grafting of poly(ethylene glycol) (PEG) via reactive extrusion

In this work, new ways of plasticizing polylactide (PLA) with low molecular poly(ethylene glycol) (PEG) were developed to improve the ductility of PLA while maintaining the plasticizer content at maximum 20 wt.% PLA. To this end, a reactive blending of anhydride-grafted PLA (MAG-PLA) copolymer with PEG, with chains terminated with hydroxyl groups, was performed. During the melt-processing, a fraction of PEG was grafted into the anhydride-functionalized PLA chains. The role of the grafted fraction was to improve the compatibility between PLA and PEG. Reactive extrusion and melt-blending of neat and modified PLA with PEG did not induce any dramatic drop of PLA molecular weight. The in situ reactive grafting of PEG into the modified PLA in PLA/PEG blends showed a clear effect on the thermal properties of PLA. It was demonstrated by DSC that the mobility gained by PLA chains in the plasticized blends yielded crystallization. The grafting of a fraction of PEG into PLA did not affect this process. However, DSC results obtained after the second heating showed an interesting effect on the T_g when 20 wt.% PEG were melt blended with neat PLA or 10 wt.% MAG-PLA. In the latter case, the T_g displayed by the reactive blend was shifted to even lower temperatures at around 14 °C, while the T_g of neat PLA and PLA blended with 20 wt.% PEG was around 60 and 23 °C, respectively. Regarding viscoelastic and viscoplastic properties, the presence of MAG-PLA does not significantly influence the behavior of plasticized PLA. Indeed, with or without MAG-PLA, elastic modulus and yield stress decrease, while ultimate strain increases with the addition of PEG into PLA.