13C and 1H amorphous linewidths in semicrystalline poly(4-hydroxybutyrate) and poly(3-co-4-hydroxybutyrate) above Tg by high resolution and solid-state NMR experiments

The high-resolution ¹³C and ¹H nuclear magnetic resonance (NMR) linewidths of semi-crystalline poly(4-hydroxybutyrate), P4HB, and poly(3-hydroxybutyrate-co-4-hydroxybutyrate), (P3/4HB-18, 18% 4HB units) in the amorphous phase and in the melt are studied as a function of temperature and magnetic field strength. Measurements of the ¹³C spin-spin relaxation times under the same experimental conditions show that the natural line-width is a minor contributor to the line-broadening observed in the ¹³C spectra of the solid polymers. A variety of coherent averaging solid-state NMR methods are used to examine possible contributions from various line-broadening mechanisms. It is shown that magnetic susceptibility and chemical shift dispersion are the major factors for the broadening of the proton and carbon resonances of P4HB in the amorphous phase and the melt, respectively. Incomplete motional narrowing due to a slow motional mode restricted in amplitude by the presence of crystallites and/or chain constraints was found to be the major line-broadening factor for P3/4HB-18 in the amorphous phase. Correlations between crystalline morphology, physical and mechanical properties, and polymer chain dynamics are discussed, along with the way these factors affect the NMR linewidth data presented. © 1996 John Wiley & Sons, Inc.     

Environmental Degradation of Microbial Polyhydroxyalkanoates and Oil Palm-Based Composites

This paper investigates the degradation of polyhydroxyalkanoates and its biofiber composites in both soil and lake environment. Time-dependent changes in the weight loss of films were monitored. The rate of degradation of poly(3-hydroxybutyrate) [P(3HB)], poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-23?mol% 4HB)] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxybutyrate) [P(3HB-co-9?mol% 3HV-co-19?mol% 4HB)] were investigated. The rate of degradation in the lake is higher compared to that in the soil. The highest rate of degradation in lake environment (15.6?% w/w week^?1) was observed with P(3HB-co-3HV-co-4HB) terpolymer. Additionally, the rate of degradation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-38?mol% 3HV)] was compared to PHBV biofiber composites containing compatibilizers and empty fruit bunch (EFB). Here, composites with 30?% EFB displayed the highest rate of degradation both in the lake (25.6?% w/w week^?1) and soil (15.6?% w/w week^?1) environment.     

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.     

Development of a closed-loop control system for production of medium-chain-length poly(3-hydroxyalkanoates) (mcl-PHAs) from bacteria

Large scale availability of bacterial polyhydroxyalkanoates (PHAs) is still limited to a few types of short-chain-length PHAs, namely poly(3-hydroxybutyrate) (PHB) and its copolymer Biopol™, consisting of 3-hydroxybutyrate and 3-hydroxyvalerate repeating units. In order to increase the number of available medium-chain-length PHA (mcl-PHA) copolymers a flexible high-cell-density fed-batch process was developed. Continuous process monitoring and substrate control were achieved by coupling on-line gaschromatography (on-line GC) to a software-based Proportional Integral (PI) substrate controller. System development time and continuous system upgrading were considerably shortened by using LABView™, a powerful graphical programming environment. The control of octanoic acid and 10-undecenoic acid at 1.5 and 0.5 gL⁻¹ respectively, enabled the production of high levels of biomass (30 gL⁻¹) and mcl-PHA (10.5 gL⁻¹) by avoiding substrate limitations or toxicities. The resulting mcl-PHA was an amorphous copolyester consisting of 37 mol% unsaturated monomers. The present system represents a valuable tool for the production of tailor-made mcl-PHAs, where the desired monomer composition is determined by the ratio of added cosubstrates.     

Gamma Irradiation Effects on Mechanical and Thermal Properties and Biodegradation of Poly(3-hydroxybutyrate) Based Films

Summary: Their biodegradable properties make polyhydroxyalkanoates (PHAs) ideal candidates for innovative applications. Many studies have been primarily oriented to poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-valerate) (PHBV) and afterwards to blends of PHAs with synthetic biodegradable polymers, such as poly(ε-caprolactone) (PCL). Medical and pharmaceutical devices require sterilization and γ irradiation could provide a proper alternative since it assures storage stability and microbiological safety. This contribution presents the effect of γ irradiation on the mechanical and thermal properties and on the biodegradation of PHB, PHBV and a commercial PHB/PCL blend. Samples, prepared by compression moulding, were irradiated in air at a constant dose rate of 10 kGy/h, from 10 to 179 kGy. Polymer chain scission was assessed by changes in the molecular weight, thermal properties and tensile behaviour. The correlation between absorbed dose and changes in the mechanical properties and biodegradation is discussed in detail. The optimum dose to guarantee microbiological sterilization without damage of the structure or meaningful loss of the mechanical properties is also reported.     

The physicochemical properties of polyhydroxyalkanoates with different chemical structures

A set of polyhydroxyalkanoates are synthesized, and a comparative study of their physicochemical properties is performed. The molecular masses and polydispersities of polyhydroxyalkanoates are found to be independent of their chemical structures. It is shown that the temperature characteristics and degrees of crystallinity of polyhydroxyalkanoates are affected by the chemical compositions of the monomers and their quantitative contents in the polymers. The incorporation of 4-hydroxybutyrate, 3-hydroxyvalerate, and 3-hydroxyhexanoate units into the chain of poly(3-hydroxybutyrate) decreases its melting point and thermal degradation temperature relative to these parameters of a homogeneous poly(3-hydroxybutyrate) sample (175 ± 5°C and 275 ± 5°C, respectively). The higher the content of the second monomer units in the poly(3-hydroxybutirate) chain, the greater the changes. The degrees of crystallinity of polyhydroxyalkanoate copolymers are generally lower than that of poly(3-hydroxybutyrate) (75 ± 5%). The effect on the ratio of the amorphous and crystalline phases of the copolymer samples becomes more pronounced in the series 3-hydroxy-valerate-3-hydroxyhexanoate-4-hydroxybutyrate. The prepared samples exhibit different properties ranging from rigid thermoplastic materials to engineering elastomers.     

β-Butyrolactone polymerization initiated with tetrabutylammonium carboxylates: A novel approach to biomimetic polyester synthesis

The anionic ring-opening polymerizations of (R,S)-β-butyrolactone and (S)-β-butyrolactone initiated with tetrabutylammonium salts of carboxylic acids as initiators were investigated. Poly[(R,S)-3-hydroxybutyrate] and poly[(R)-3-hydroxybutyrate], with relatively high molecular weights (170,000 and 35,000, respectively) and low molecular weight distributions, were synthesized. Moreover, biomimetic poly(3-hydroxybutyrate)s with well-defined microstructures and predictable iso-dyad contents were obtained. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2184–2189, 2002     

Effects of the architecture and environment on polymeric molecular assemblies of novel amphiphilic diblock copolynorbornenes with narrow polydispersity via living ring‐opening metathesis polymerization

Diblock copolymers of 5‐(methylphthalimide)bicyclo[2.2.1]hept‐2‐ene (NBMPI) and 1,5‐cyclooctadiene were synthesized by living ring‐opening metathesis polymerization with a well‐defined catalyst {RuCl₂(CHPh)[P(C₆H₁₁)₃]₂}. Unhydrogenated diblock copolymers showed two glass transitions due to poly(NBMPI) and polybutadiene segments, such as two glass‐transition temperatures at ?86.5 and 115.3 °C for poly 1a and ?87.2 and 115.3 °C for poly 1b . However, only one melting temperature could be observed for hydrogenated copolymers, such as 119.8 °C for poly 2a and 121.7 °C for poly 2b . The unhydrogenated diblock copolymer with the longer poly(NBMPI) chain (poly 1a ; temperature at 10% mass loss = 400 °C) exhibited better thermal stability than the one with the shorter poly(NBMPI) chain (poly 1b ; temperature at 10% mass loss = 385 °C). Two kinds of hydrogenated diblock copolymers, poly 2a and poly 2b , exhibited relatively poor solubility but better thermal stability than unhydrogenated diblock copolymers because of the polyethylene segments. Poly[(hydrochloride quaternized 2‐norbornene‐5‐methyleneamine)‐b‐butadiene]‐1 (poly 3a ) was obtained after the hydrolysis and quaternization of poly 1a . Dynamic light scattering measurements indicated that the hydrodynamic diameters of the cationic copolymer (poly 3a ) in water (hydrodynamic diameter = 1580 nm without salt), methanol/water (4/96 v/v; hydrodynamic diameter = 1500 nm without salt), and tetrahydrofuran/water (4/96 v/v; hydrodynamic diameter = 1200 nm without salt) decreased with increasing salt (NaCl) concentration. The effect of temperature on the hydrodynamic diameter of hydrophobically modified poly 3a was also studied. The inflection point of the hydrodynamic diameter of poly 3a was observed at various polymer concentrations around 30 °C. The critical micelle concentration of hydrophobically modified poly 3a was observed at 0.018 g dL^?1. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2901–2911, 2006     

Novel block copolymers of atactic PHB with natural PHA for cardiovascular engineering: Synthesis and characterization

Low-molecular weight macroinitiators derived from natural poly(3-hydroxyalkanoates) (PHAs), which contain olefinic and activated by 18-crown-6 ether carboxylic end groups, were used in anionic ring opening polymerization (ROP) of racemic β-butyrolactone and new diblock copolymers of selected PHAs (PHB, PHBV, PHO) with atactic poly[(R,S)-3-hydroxybutyrate] (a-PHB) were obtained. These novel copolymers were characterized using ¹H NMR, GPC and DSC. Hydrolytic degradation studies of selected copolymers were also performed. Finally, the suitability of these polymeric materials for cardiovascular engineering and as blend compatibilizers was demonstrated.     

Brush Type Copolymers of Poly(3-hydroxybutyrate) and Poly(3-hydroxyoctanoate) with Same Vinyl Monomers via “Grafting From” Technique by Using Atom Transfer Radical Polymerization Method

Brush type graft copolymers of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyoctanoate) (PHO) with methylmethacrylate, (MMA), styrene, (S), and n-butylmethacrylate, (n-BuMA) were obtained by using Atom Transfer Radical Polymerization Method, (ATRP), via “grafting from” technique. Firstly PHB and PHO were chlorinated by passing chlorine gas through their solution in CHCl₃/CCl₄ (75/25 v/v) mixture and CCl₄, respectively, in order to prepare chlorinated PHB, PHB-Cl, and chlorinated PHO, PHO-Cl, with different chlorine contents. The determination of the chlorine content in chlorinated poly(3-hydroxyalkanoate) (PHA-Cl) was performed by the Volhard Method. Then ATRP of vinyl monomers was initiated by using PHA-Cl as macroinitiators in the presence of cuprous chloride (CuCl)/2,2′-bipyridine complex as catalyst, at 90 °C in order to obtain brushes containing PHAs. The polymer brushes were fractionated by fractional precipitation methods and the γ values calculated from the ratio of the volume of nonsolvent (methanol) and the volume of solvent (chloroform) of brushes varied between 0.82 and 6.50 depending on the composition of brushes. The polymer products were characterized by gel permeation chromatography (GPC), ¹H NMR, FTIR, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques.     

Branching and cross-linking of poly(ethylene terephthalate) and its foaming properties

The branching and cross-linking of poly(ethylene terephthalate) were investigated using two chain extenders: glycidyl methacrylate-styrene copolymer (GS) and poly(butylene terephthalate)-GS (PBT-GS) in order to improve the melt viscosity and melt strength of poly(ethylene terephthalate). An obvious increase in torque evolution associated with chain extending, branching and cross-linking was observed during the process. The properties of modified poly(ethylene terephthalate) were characterized by intrinsic viscosity and insoluble content measurements, rheological and thermal analysis. The intrinsic viscosity and rheological properties of modified PET were improved significantly when using PBT-GS, indicating that PBT-GS should be a better chain extender. Good foaming of poly(ethylene terephthalate) materials were obtained using supercritical CO₂ as blowing agent. The average cell diameter and cell density were 61 μm and 1.8 × 10⁸ cells/cm³, respectively.     

Condensation polymers containing silicon and germanium in the main chain

Poly(carbonates), poly(thiocarbonates) and poly(esteres) containing silicon and/or germanium in the main chain were obtained under phase transfer conditions. Polymers were synthesized in a biphasic system NaOH/CH₂Cl₂ at 20°C using several phase transfer catalysts, and characterized by IR and ¹H and ¹³C NMR. The results were evaluated by the yields and the inherent viscosity values. The process was effective observing an increase of both parameters in comparison with the essays without catalyst. The increases depended of the nature of the polymer and the catalyst. In poly(ester) synthesis there was an increase of these parameters when the NaOH concentration was increased due to a salting out effect of the diphenolate from the aqueous phase to the organic one. Also poly(amides) containing silicon or germanium were synthesized by solution polymerization.     

Unusual change in molecular weight of polyhydroxyalkanoate (PHA) during cultivation of PHA-accumulating Escherichia coli

This study aimed to investigate the factors affecting molecular weight of poly[(R)-3-hydroxybutyrate] [P(3HB)] when polyhydroxyalkanoate (PHA) synthase (PhaRC_Bsp) from Bacillus sp. INT005 was used for P(3HB) synthesis in Escherichia coli JM109. It was found that the molecular weight of P(3HB) decreased with time in mid- and late-phase of culture and was strongly affected by culture temperature. At 37 °C culture temperature, the molecular weight of P(3HB) rapidly decreased from 4.4 × 10⁵ to 4.8 × 10⁴ with culture time, whereas it was almost unchanged at 25 °C. Kinetic analysis suggested that the decrease in molecular weight of P(3HB) was due to random scission of the polymer chain. The decrease in molecular weight of P(3HB) was not observed when PHA synthases other than PhaRC_Bsp were expressed. This study sheds light on the unique behaviour in molecular weight change of P(3HB) that is synthesized by E. coli expressing PhaRC_Bsp.     

Influence of the structure and morphology of ultrathin poly(3-hydroxybutyrate) fibers on the diffusion kinetics and transport of drugs

Ultrathin fibers of a biodegradable polymer poly(3-hydroxybutyrate) with an encapsulated drug (dipyridamole, 0–5% of poly(3-hydroxybutyrate) mass) are obtained by electrospinning. Introduction of the drug substantially affects the geometric shape and crystallinity of individual filaments as well as the total porosity of the fibrillar film on their basis. As follows from the SEM data, in the absence of the drug or at its low concentration (<3%), poly(3-hydroxybutyrate) fibers appear as ellipse-like fragments alternating with cylindrical ones. At a higher content of the drug (3–5%), the abnormal ellipse-like structures are practically absent and the fiber acquires the cylindrical shape. A set of morphological and crystallinity characteristics of the fibers determines the absorption of water and the rate of the diffusion transport of the drug as well as the corresponding profiles of its controlled release. A simplified model of drug desorption from the fibrillar film is advanced which considers two sequential stages of the process: (i) diffusion of the drug in the polymer fiber with coefficient D _f ~ 10^–12 cm²/s and dimeter φ_f ~ 2–4 μm and (ii) transport of the drug in the interfibrillar porous space filled by solvent with diffusion coefficient D _w = 5.5 × 10^–6 cm²/s. Using the characteristics of porosity, crystallinity, and geometry of the fibers and diffusion effective coefficients D _eff calculated from the profile of drug release, it is shown that the limiting stage of the transport of the drug is its diffusion in the volume of the cylindrical fiber. The model makes it possible to turn from the experimental values of D _eff to partial diffusion coefficients D _f and to calculate the kinetic profile of drug release with allowance made for the above-listed factors.     

Characterization of surface modified flax fibers and their biocomposites with PHB

Flax fiber was treated with acetic anhydride or with ethylene plasma to improve adhesion with poly(3-hydroxybutyrate) (PHB). The flax fiber surface became hydrophobic by the surface treatments. The ethylene plasma treated flax fiber has better thermal resistance and shows higher interfacial adhesion strength in flax/PHB biocomposite than the chemically modified flax fiber with acetic anhydride. Optimum conditions of the ethylene plasma treatment were found to be 0.5 cm³/sec of the ethylene flow rate and 50 W of the plasma power with 5 min of the treatment time.     

Crystallization behaviors and morphology of biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

Crystallization behaviors and spherulitic morphology of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] with different 4-hydroxybutyrate (4HB) molar fraction were investigated by differential scanning calorimetry and polarized optical microscopy. Crystallization behaviors of P(3HB-co-4HB) are significantly affected by 4HB molar fraction. The melting temperature (T _m), glass transition temperature (T _g), and crystallinity (X _c) decrease with the increase of 4HB molar fraction. Banded spherulites are observed in poly (3-hydroxybutyrate) (PHB) and P(3HB-co-4HB) copolymers. The band spacing decreases with the increase of 4HB molar fraction. The morphology and growth rate of the spherulites strongly depend on 4HB molar fraction and the crystallization temperatures. The introduction of 4HB unit can inhibit the emergence of cracks in PHB spherulites.     

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.     

Characterization and properties of biodegradable poly(hydroxyalkanoates) and 4,4-dihydroxydiphenylpropane blends: Intermolecular hydrogen bonds, miscibility and crystallization

Intermolecular hydrogen bonds, miscibility, crystallization and thermal stability of the blends of biodegradable poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-3HHx)] with 4,4-dihydroxydiphenylpropane (DOH₂) were investigated by FTIR, ¹³C solid state NMR, DSC, WAXD and TGA. Intermolecular hydrogen bonds were found in both blend systems, which resulted from the carbonyl groups in the amorphous phase of both polyesters and the hydroxyl groups of DOH₂. The intermolecular interaction between P(3HB-3HHx) and DOH₂ is weaker than that between PHB and DOH₂ owing to the steric hindrance of longer 3HHx side chains. Because of the effect of the hydrogen bonds, the chain mobility of both PHB and P(3HB-3HHx) components was limited after blending with DOH₂ molecules. Single glass transition temperature depending on the composition was observed in all blends, indicating that those blends were miscible in the melt. The addition of DOH₂ suppressed the crystallization of PHB and P(3HB-3HHx) components. Moreover, the crystallinity of PHB and P(3HB-3HHx) components also decreased with increasing DOH₂ content in the blends. However, the crystal structures of the crystallizable components were not affected. The existence of DOH₂ favors to thermal decomposition of PHB and P(3HB-3HHx) components, resulting in the decrease in thermal decomposition temperature.     

Preparation and characterization of optically active polymers containing pendent and terminal chiral units via atom transfer radical polymerization

Optically active homopolymers and copolymers, bearing chiral units at the side chain and end chain, were prepared via atom transfer radical polymerization (ATRP) techniques. The well‐defined optically active polymers were obtained via the ATRP of pregnenolone methacrylate (PR‐MA), β‐cholestanol acrylate (CH‐A), and 20‐(hydroxymethyl)‐pregna‐1,4‐dien‐3‐one acrylate (HPD‐A) with ethyl 2‐bromopropionate as the initiator and CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as the catalytic system. The experimental results showed that the polymerizations of PR‐MA, CH‐A, and HPD‐A proceeded in a living fashion, providing pendent chiral group polymers with low molecular weight distributions and predetermined molecular weights that increased linearly with the monomer conversion. Furthermore, the copolymers poly(pregnenolone methacrylate)‐b‐poly[(dimethylamino)ethyl methacrylate] and poly(pregnenolone methacrylate‐co‐methyl methacrylate) were synthesized and characterized with ¹H NMR, transmission electron microscopy, and polarimetric analysis. In addition, when optically active initiators estrone 2‐bromopropionate and 20‐(hydroxymethyl)‐pregna‐1,4‐dien‐3‐one 2‐bromopropionate were used for ATRPs of methyl methacrylate and styrene, terminal optically active poly(methyl methacrylate) and polystyrene were obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1502–1513, 2006     

Isolation and characterization of poly(butylene succinate)-degrading fungi

We isolated 12 poly(butylene succinate) (PBSu)-degrading fungi from various soil environments. Among the isolates, the NKCM1706 strain exhibited the fastest degradation rate for the PBSu film (10.5 μg cm⁻² h⁻¹). Phylogenetic analysis revealed that this strain is closely related to Aspergillus fumigatus (internal transcribed spacer (ITS) identity, 100%). Further, this strain exhibited PBSu-hydrolytic activity in the presence of poly(?-caprolactone) (PCL), PBSu, and poly(butylene succinate-co-adipate) (PBSA). On adding this strain into the soil sample, the PBSu degradation rate accelerated approximately sixfold, suggesting that this strain plays a crucial role in PBSu degradation in actual soil environments. In addition to PBSu, the NKCM1706 strain could degrade PBSA, poly(ethylene succinate) (PESu), poly(3-hydroxybutyrate) (P(3HB)), and PCL.