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.     

The degradation kinetics of poly(3-hydroxybutyrate) under non-aqueous and aqueous conditions

The degradation of poly(3-hydroxybutyrate), P(3HB), was determined in two conditions namely, a non-aqueous condition of chloroform-methanol mixture in the presence of either one of the two following catalysts, 4-toluenesulphonic acid and imidazole, and secondly in an aqueous condition of increasing pH. From our study, a random chain scission of PHB occurred in the non-aqueous condition while the degradation of PHB in the presence of water occurred through surface hydrolysis with no change in the molecular weight. In the surface hydrolysis of the polymer, the rate was increased with higher pH values.     

The thermal degradation of poly(diethyl fumarate)

The kinetics and mechanism of the thermal degradation of poly(diethyl fumarate) (PDEF) were studied by thermogravimetry, as well as by analysis of the thermolysis volatiles and polymer residue. The characteristic mass loss temperatures were determined, as were the overall thermal degradation activation energies of three PDEF samples of varying molar mass. Ethylene and ethanol were present in the thermolysis volatiles at degradation temperatures below 300 °C, while diethyl fumarate was also evidenced at higher degradation temperatures. The amount of monomer increased with increasing degradation temperature. The dependence of the molar mass of the residual polymer on the degradation time and temperature was established and the number of main-chain scissions per monomer unit, s/P₀, calculated. A thermal degradation mechanism including de-esterification and random main-chain scission is proposed. The thermal degradation of PDEF was compared to the thermolysis of poly(ethyl methacrylate) (PEMA), poly(diethyl itaconate) (PDEI) and poly(ethyl acrylate) (PEA).     

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.     

Thermal degradation behavior of poly(4-hydroxybutyric acid)

Thermal degradation behavior of poly(4-hydroxybutyric acid) (P(4HB)) was investigated by thermogravimetric and pyrolysis-gas chromatography mass spectrometric analyses under both isothermal and non-isothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes occurred at temperatures below and above 350 °C during the non-isothermal degradation of P(4HB) samples depending on both the molecular weight and the heating rate. From ¹H NMR analysis of the residual P(4HB) molecules after isothermal degradations at different temperatures, it was confirmed that the ω-hydroxyl chain-end was remained unchanged in the residual P(4HB) molecules at temperatures below 300 °C, while the ω-chain-end of P(4HB) molecules was converted to 3-butenoyl units at temperatures above 300 °C. In contrast, the majority of the volatile products evolved during thermal degradation of P(4HB) was γ-butyrolactone regardless of the degradation temperature. From these results, it is concluded that during the thermal degradation of P(4HB), the selective formation of γ-butyrolactone via unzipping reaction from the ω-hydroxyl chain-end predominantly occurs at temperatures below 300 °C. At temperatures above 300 °C, both the cis-elimination reaction of 4HB unit and the formation of cyclic macromolecules of P(4HB) via intramolecular transesterification take place in addition to unzipping reaction from the ω-hydroxyl chain-end. Finally, the primary reaction of thermal degradation of P(4HB) at temperatures above 350 °C progresses by the cyclic rupture via intramolecular transesterification of P(4HB) molecules with a release of γ-butyrolactone as volatile product. Moreover, we carried out the thermal degradation tests for copolymer of 93 mol% of 4HB with 7 mol% of 3-hydroxybutyric acid (3HB) to examine the effect of 3HB units on thermal stability of P(4HB).     

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.     

Crystallization behavior, thermal property and biodegradation of poly(3-hydroxybutyrate)/poly(ethylene glycol) grafting copolymer

The poly(3-hydroxybutyrate)(PHB)/poly(ethylene glycol)(PEG) grafting copolymer was successfully prepared by PHB and acrylate groups ended PEGM using AIBN as initiator. The crystallization behavior, thermal stability and environmental biodegradability of PHB/PEG grafting copolymers were investigated with differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and Biodegradation test in vitro. In the results, all the grafting copolymers were found to show the X-ray diffraction arising from the PHB crystal lattice, while none of the PEG crystallized peaks could be found even though the graft percent reached 20%. This result indicated that PEG molecules were randomly grafted onto PHB chain. The thermal properties measured by DSC showed that the melting temperature(T_m) and glass transition temperature (T_g) were both shifted to lower temperature with the graft percent increasing, and this broadened the narrow processability window of PHB. According to TGA results, the thermal stability of the grafting copolymers is not changed compared to pure PHB. From the biodegradation test, it could be concluded that degradation occurred gradually from the surface to the inside and that the degradation rate could be adjusted by the PEG grafting ratio. In another words, the biodegradation profiles of PHB/PEG grafting copolymer can be controlled. These properties make PHB/PEG grafting copolymer have promising potential applications especially in agriculture fields.     

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.     

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.     

Highly selective transformation of poly[(R)-3-hydroxybutyric acid] into trans-crotonic acid by catalytic thermal degradation

Highly selective transformation of poly[(R)-3-hydroxybutyric acid] (PHB) into trans-crotonic acid was achieved by thermal degradation using Mg compounds: MgO and Mg(OH)₂ as catalysts. Through catalytic action, not only the temperature and E_a value of degradation were lowered by 40-50 °C and 11-14 kJ mol⁻¹, respectively, but also significant changes in the selectivity of pyrolyzates were observed. Notably, Mg(OH)₂ showed nearly complete selectivity (∼100%) to trans-crotonic acid. Kinetic analysis of TG profiles revealed that the catalytic thermal degradation of PHB was initiated by some random degradation reactions, followed by the unzipping β-elimination from crotonate chain-ends as a main process. It was suggested that the Mg catalysts promote the totality of the β-elimination reactions by acting throughout the beginning and main processes, resulting in a lowering in the degradation temperature and the completely selective transformation of PHB.     

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.     

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.     

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.     

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.     

Thermal degradation kinetics of poly(methylvinylsilylene-co-styrene)

Thermal degradation kinetics of poly(methylvinylsilylene-co-styrene) copolymers, viz., PMVSS-I to PMVSS-V obtained by reacting methylvinyldichlorosilane (MVDCS) and styrene in 1:0.25, 1:0.5, 1:1, 1:3 and 1:7 mole ratios under dechlorination conditions, using sodium, was studied by thermogravimetry. The homopolymer, poly(methylvinylsilane) (PMVS), synthesized from MVDCS using sodium was also subjected to the above study for comparative evaluation. The kinetic parameters for thermal degradation, viz., activation energy (E) and pre-exponential factor (A) for the above polymers were estimated by non-isothermal kinetic methods such as Mac Callum-Tanner (M-T), Horowitz-Metzger (H-M), Madhusudhanan-Krishnan-Ninan (MKN) and Coats-Redfern (C-R). The order for thermal degradation of PMVS was found to be almost 0. In the case of the copolymers, the order was 1 for PMVSS-I and 2 for PMVSS-II to PMVSS-V. The observed difference in the order for thermal degradation of PMVSS-I when compared to the other copolymers is attributed to the presence of polysilyl linkages in PMVSS-I. It was found that the activation energy and pre-exponential factor showed an increase in trend with increase in concentration of styrene in the copolymer system.     

Racemization on thermal degradation of poly( -lactide) with calcium salt end structure

Poly( -lactide) with calcium salt end structure (PLLA-Ca) is a promising material for PLLA recycling because of the ease of lactide recovery through the unzipping depolymerization process. However, the pyrolysis of PLLA-Ca also causes meso-lactide to form. In this article, the racemization in PLLA-Ca pyrolysis was analyzed in detail with Py-MS, Py-GC/MS, and a glass tube oven. The results suggested that at a temperature lower than 250 °C, nucleophilic attack by a carboxylate anion end on an asymmetrical methyne carbon in a penultimate lactate unit occurred, resulting in the predominant formation of meso-lactide. On the other hand, also at temperatures over 320 °C, by-reactions, such as enolization reactions, caused the meso-lactide to form, but not dominantly. In the temperature range of 250–320 °C, -lactide was produced exclusively, because unzipping depolymerization proceeded as the main reaction. This is a very significant result for PLLA recycling, because PLLA-Ca is an easily recyclable material, which depolymerizes based on the 1st-order weight loss process.     

Kinetics of thermal degradation and thermal oxidative degradation of poly(p-dioxanone)

The kinetics of the thermal degradation and thermal oxidative degradation of poly(p-dioxanone) (PPDO) were investigated by thermogravimetric analysis. Kissinger method, Friedman method, Flynn-Wall-Ozawa method and Coats-Redfern method have been used to determine the activation energies of PPDO degradation. The results showed that the thermal stability of PPDO in pure nitrogen is higher than that in air atmosphere. The analyses of the solid-state processes mechanism of PPDO by Coats-Redfern method and Criado et al. method showed: the thermal degradation process of PPDO goes to a mechanism involving random nucleation with one nucleus on the individual particle (F₁ mechanism); otherwise, the thermal oxidative degradation process of PPDO is corresponding to a nucleation and growth mechanism (A₂ mechanism).     

Thermal degradation of poly(vinylpyridine)s

The thermal stability and the temperature at which maximum degradation yields are detected were quite similar for both poly(2-vinylpyridine) (P2VP) and poly(4-vinylpyridine) (P4VP). However, considerable differences among the thermal degradation products of both polymers were detected indicating a correlation between the polymer structure and the degradation mechanism. Direct pyrolysis mass spectrometry analyses revealed that P2VP degrades via a complex degradation mechanism, yielding mainly pyridine, monomer, and protonated oligomers, whereas depolymerization of P4VP takes place in accordance with the general thermal behaviour of vinyl polymers. The complex thermal degradation behaviour for P2VP is associated with the position of the nitrogen atom in the pyridine ring, with σ-effect.     

Synthesis and degradation behavior of poly(ethyl cyanoacrylate)

Poly(ethyl cyanoacrylate) was synthesized using N,N′-dimethyl-p-toluidine as an initiator through an anionic/zwitterionic pathway. The degradability and the degradation mechanism of the prepared polymer were examined from various viewpoints. A combination of TGA and GPC analysis allowed us to confirm that the thermal degradation of this polymer was predominantly due to an unzipping depolymerization process initiated from the polymer chain terminus. The polymer was inherently unstable and exhibited interesting degradation behavior in solution with basic reagents. The degradation in solution was also found to be attributed to the unzipping of the monomer from the chain end. However, the degradation behavior of the polymer could be controlled by changing solvents, temperatures, and additives. These findings give an insight into the degradation behavior of poly(alkyl cyanoacrylate)s, which is a crucial point in utilizing these polymer homologues for various applications.     

Effect of layered double hydroxides on the thermal degradation behavior of biodegradable poly(l-lactide) nanocomposites

This study elucidates the thermal degradation behavior of biodegradable poly(l-lactide) (PLLA)/layered double hydroxide (LDH) nanocomposites was explored using thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) in an inert atmosphere. PLLA/LDH nanocomposites were fabricated using PLLA and organically-modified magnesium/aluminum layered double hydroxide (P-LDH) in tetrahydrofuran solution. According to the TGA results, the thermal stability of PLLA/P-LDH nanocomposites was significantly lower than that of pure PLLA matrix, perhaps because P-LDH provides thermal acceleration of the degradation of the underlying polymer from the heat source. The identification of the thermal degradation products by Py-GC/MS evidently shows that introducing P-LDH into PLLA leads to a remarkable change during the thermal degradation process. The main reaction route of neat PLLA was through inter- and intra-transesterification to generate lactides and oligomer. The primary volatile products obtained from PLLA/P-LDH nanocomposites were lactides regardless of the temperature of degradation. These results suggest that the thermal degradation behavior of PLLA/P-LDH nanocomposites is governed by the preferential formation of lactide by the unzipping depolymerization reaction, which is catalyzed by Mg and Al components in P-LDH.