Melt viscoelasticity of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers

The rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHxs), with varying comonomer (HHx) content, were systematically investigated. Shear viscosities show dependence on the rate of deformation, temperature, molecular weight, and copolymer compositions. The zero-shear viscosity η₀ follows the classical power-law relationship with the weight average molecular weight M_w. The characteristic relaxation time λ, which indicates the onset of shear thinning, ranges from 0.02 to 0.2 s for different PHB-HHxs and is roughly linearly related to η₀. The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow E_a are obtained from the slope of the natural logarithm of the shift factor α_T plotted against the inverse of temperature curve, and the values for PHB-HHxs are found to be in the range of 27-36 kJ/mol E_a decreases with HHx content in the copolymer, a trend that can be related to the difference in chemical structure between HHx and HB, according to the method of Vankrevelen and Hoftyzer. A Generalized Maxwell model models the viscoelastic behavior of the PHB-HHx melt well. The value of the plateau modulus obtained suggests a highly entangled configuration. The molecular weight between entanglements M_e decreases from 11,600 to 9400 as HHx content increases from 3.8 to 10.0 mol%. Our results suggest that the presence of propyl groups in HHx increases the steric hindrance of the PHB-HHx chains, thus resulting in increased segmental friction and entanglement density. As a result, viscoelastic parameters for PHB-HHx copolymers, such as η₀ and , are readily tunable by varying the HHx content, making them attractive as “green” substitutes for non-degradable thermoplastics.     

Processing and characterization of electrospun poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) nanofibrous membranes

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) nanofibrous membranes were first fabricated via electrospinning from chloroform (CHCl₃) or CHCl₃/dimethylformamide (DMF) polymer solutions. The electrospinning conditions such as the polymer concentration, the solvent composition, and the applied voltage were optimized in order to get smooth and nano-sized fibers. The crystalline structure, the melting behaviors and the mechanical properties of the obtained nanofibrous membranes were characterized. With pure CHCl₃ as the solvent in the electrospinning process, the finest smooth PHBHHx fibers were about 1 μm in diameter. When DMF is added to CHCl₃ as a co-solvent, the conductivity and volatility of the solution increased and reduced, respectively, and the electrospinnability of the polymer solution increased as a result. The averaged diameters of PHBHHx fibers could be reduced down to 300-500 nm when the polymer concentration was kept at 3 wt%, the ratio of DMF/CHCl₃ was maintained at 20/80 (wt%), and the applied voltage was fixed at 15 kV during electrospinning. WAXD and DSC results indicated that the crystallization of the PHBHHx nanofibers was restricted to specific crystalline planes due to the molecular orientation along the axial direction of the fibers. The crystallization behaviors of the electrospun nanofibers were significantly different from that of the cast membranes because of the rapid solidification and the one-dimensional fiber size effect in the electrospinning process. Mechanically, the electrospun PHBHHx nanofibrous membranes were soft but tough, and their elongation at break averaged 240-300% and could be up to 450% in some cases. This study demonstrated how the size of electrospun PHBHHx fibers could be reduced by adding DMF in the solvent and gave a clue of the presence of oriented molecular chain packing in the crystalline phase of the electrospun PHBHHx fibers.     

Different thermal behaviors of microbial polyesters poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Various methods were employed to study the thermal behaviors of a novel microbial polyhydroxyalkanoate (PHA) terpolyester, namely, poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (PHBVHHx) compared with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx). PHBVHHx showed higher crystallization rate and degree of crystallinity. PHBVHHx exhibited also different multiple melting behaviors from PHBHHx. The WAXD results demonstrated that the crystal lattice of PHBVHHx was more compact than that of PHBHHx, suggesting stronger interaction between chain stems. DSC and in-situ heating WAXD studies revealed that PHBVHHx showed a partial melting-lamellar thickening-remelting process during heating, while PHBHHx demonstrated a melting-rapid formation of new crystals-remelting process. It is proposed that the simultaneous introduction of 3-hydroxyvalerate and 3-hydroxyhexanoate monomers into poly(3-hydroxybutyrate) improves the mobility of chain stems along the chain direction, leading to easier intralamellar slip during heating or drawing, further resulting in improvement of mechanical properties, which was supported by the DMA tests. Consequently, we establish a relationship between the thermal behavior and the mechanical properties of biodegradable plastics, which we believe is applicable to other polymers as well.     

Crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) studied by 2D IR correlation spectroscopy

Crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx)) (HHx=12 mol%) was studied by means of two-dimensional infrared (2D IR) correlation spectroscopy. Three types of crystallization; the gradual cooling from the melt, the isothermal crystallization of the supercooled melt, and the isothermal crystallization of the solution-cast film were investigated. The order of crystal growth steps taking place during the three different types of crystallization processes was analyzed in detail. It was revealed by the asynchronous 2D correlation spectra generated from the dynamic IR spectra in the CO stretching band region that the development of the highly ordered crystals occurs prior to that of the less ordered crystals for the gradual cooling crystallization. On the other hand, for the supercooled melt and solution-cast film crystallization, the formation of the less ordered crystals takes place before that of the highly ordered crystals. The transition from the amorphous phase to the less ordered crystals is a simultaneous process for all three types of crystallization.     

Comonomer-unit compositional distribution and its effect on thermal and crystallization behavior of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

In order to clarify the relationships between comonomer-unit compositional distribution and physical properties of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)], the comonomer-unit composition and its distribution as well as their effects on the physical properties of P(3HB-co-3HHx) have been studied. A series of fractions with very different comonomer-unit composition but very narrow distribution were obtained by repeated fractionation and re-fractionation. It was found the as-bacterially synthesized P(3HB-co-3HHx) had very broad comonomer-unit compositional distribution. The solvent/non-solvent fractionation of P(3HB-co-3HHx) was found to be firstly regulated by the comonomer-unit composition and then by the molecular weight. Significant effect of the comonomer-unit compositional distribution on thermal and crystallization behavior as well as crystalline morphology of P(3HB-co-3HHx) was observed. It was concluded that as-bacterially synthesized P(3HB-co-3HHx) samples should be considered as polymer blends of component polymers with different 3HHx unit content, whose physical properties considerably depend on the comonomer-unit composition as well as comonomer-unit compositional distribution.     

Effect of comonomer-unit composition and its distribution of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) on miscibility and physical properties of its blend with poly(ethylene oxide)

In order to investigate the effect of comonomer-unit composition and its distribution of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] on the miscibility of its blend with poly(ethylene oxide) (PEO), as-bacterially synthesized and well-fractionated P(3HB-co-3HHx)s with different 3HHx unit content were blended with PEO at different content. It is found that the miscibility between P(3HB-co-3HHx) and PEO decreases dramatically with increasing both 3HHx unit content of P(3HB-co-3HHx) and PEO content in the blend. The miscibility is also found to be significantly affected by the comonomer-unit compositional distribution of P(3HB-co-3HHx). Strongly depending on the comonomer-unit composition of P(3HB-co-3HHx) and PEO content in the blend, thermal stability and mechanical properties of P(3HB-co-3HHx) can be regulated by blending with PEO. Moreover, PEO exhibits good dispersion in P(3HB-co-3HHx) matrix. It is concluded that the P(3HB-co-3HHx)/PEO binary blend can exhibit diverse properties by varying the comonomer-unit composition and comonomer-unit compositional distribution of P(3HB-co-3HHx) as well as the PEO content in the blend.     

Preparation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) particles in O/W emulsion

In the present paper, we investigate the preparation of polymeric particles based on the biodegradable polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). A new technique for PHBV particle preparation has been developed. This method utilizes the thermoreversible gelation of PHBV in toluene. Particles have been obtained by the secondary dispersion technique in a three-step procedure: (a) preparation of PHBV solution in toluene; (b) preparation of O/W emulsion by ultrasound followed by the gel formation in toluene/PHBV droplets; and (c) toluene extraction. In the present study we investigated the influence of the stabilizer type and its concentration in the aqueous phase, ultrasound power, and PHBV concentration in toluene on the size and stability of the formed droplets as well as the final PHBV particles. It has been found that PEO/PS block copolymers are the best stabilizers for the present system as compared to conventional tensides such as SDS or CTAB. It has been found that PEO/PS block copolymers allow obtaining PHBV particles with a regular shape and controlled dimensions after toluene extraction. The minimal size of the PHBV particles obtained by this technique was ca. 100 nm. The obtained particles exhibit a relatively broad particle size distribution and the particle shape is strongly affected by the block copolymer composition, ultrasound power and the way of toluene extraction.     

The domain structure and mobility of semi-crystalline poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate): A solid-state NMR study

Solid-state NMR techniques have been employed to investigate the domain structure and mobility of the bacterial biopolymeric metabolites such as poly(3-hydroxybutyrate) (PHB) and its copolymers poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) containing 2.7 mol% (PHBV2.7) and 6.5 mol% (PHBV6.5) 3-hydroxyvalerate. Both single-pulse excitation with magic-angle spinning (SPEMAS) and cross-polarization magic-angle spinning (CPMAS) ¹³C NMR results showed that these biopolymers were composed of amorphous and crystalline regions having distinct molecular dynamics. Under magic-angle spinning, ¹H T_1ρ and ¹³C T₁ showed two processes for each carbon. Proton relaxation-induced spectral editing (PRISE) techniques allowed the neat separation of the ¹³C resonances in the crystalline regions from those in the amorphous ones. The proton spin-lattice relaxation time in the tilted rotating frame, , measured using the Lee-Goldburg sequence with frequency modulation (LGFM) as the spin-locking scheme, was also double exponential and significantly longer than ¹H T_1ρ. The difference between for the amorphous and crystalline domains was greater than that of ¹H T_1ρ. Our results showed that the differences could be exploited in LGFM-CPMAS experiments to separate the signals from two distinct regions. ¹H spin-diffusion results showed that the domain size of the mobile components in PHB, PHBV2.7 and PHBV6.5 were about 13, 24 and 36 nm whereas the ordered domain sizes were smaller than 76, 65 and 55 nm, respectively. The results indicated that the introduction of 3-hydroxyvalerate into PHB led to marked molecular mobility enhancement in the biopolymers.     

Investigation of water diffusion in poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) by generalized two-dimensional correlation ATR-FTIR spectroscopy

Water diffusion process in biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx, HHx = 12 mol%) was investigated by generalized 2D correlation time-resolved ATR-FTIR spectroscopy based on the analysis of v(OH) stretching and δ(OH) bending bands of water as well as v(CO) and v(C-O-C) stretching bands of PHBHHx. Three states of water were figured out during water diffusion process. They are bulk water, bound water and free water. The water diffusion mechanism was suggested as: water molecules firstly diffuse into the micro-voids in bulk water form or are dispersed on the surface in free water form, and then penetrate into the polymer matrix in hydrogen bound water with the hydrophilic groups of PHBHHx. Moreover, water molecules diffuse into the loose amorphous phase and then into compact crystalline phase. Water diffusion coefficient in PHBHHx was thus evaluated as 7.8 ± 0.7 × 10⁻⁸ cm² s⁻¹ for the PHBHHx with crystallinity of 16.2 ± 0.3% at 293 K.     

Effect of hydrogen bonding on the crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/silica hybrid composites

Biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-HHx)/hydrophobically modified silica hybrid composites were prepared using simple melt compounding and the effect of hydrogen bonding on their crystallization behavior was observed. The intermolecular hydrogen bonding between PHB-HHx and silica increased gradually with the increase of silica content of the hybrid composites. However, the extent of intermolecular hydrogen bonding was not directly proportional to the silica content. Although, the crystallization rates of the PHB-HHx/silica hybrids decreased as the strength of intermolecular hydrogen bonding increased, the constant value of the Avrami exponent indicates that the presence of silica does not alter the nucleation mechanism or the geometry of the crystal growth of the PHB-HHx hybrids. The calculated crystallization activation energy increased with the addition of silica, suggesting that silica retards the overall crystallization rate of the PHB-HHx hybrid composites as a result of the existence of intermolecular hydrogen bonding. The relationship between the extent of intermolecular hydrogen bond and crystallization rate is described by the empirical second-order equation.     

Multiple melting behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) investigated by differential scanning calorimetry and infrared spectroscopy

The multiple melting behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx)) (HHx = 12 mol%) isothermally crystallized from the melt state has been characterized by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The influence of different experimental variables (such as crystallization temperature, time, and heating rate) on the multiple melting behavior of P(HB-co-HHx) was investigated by using DSC. Moreover, it has been further examined by monitoring intensity changes of the characteristic IR bands during the subsequent heating process. For the isothermally crystallized P(HB-co-HHx) samples, triple melting peaks were observed upon heating. The weak lowest-temperature DSC endotherm I always appears at the position just above the crystallization temperature, and shifts to a higher temperature linearly with the logarithm of the crystallization time. The combination of DSC and IR results suggested that the occurrence of peak I was a result of the melting of crystals formed upon long-time annealing. As for the other two main melting endothermic peaks, endotherm II corresponds to the melting of crystals formed during the primary crystallization, and endotherm III is ascribed to the melting peak of the crystals formed by recrystallization during the heating process.     

A novel biodegradable nanocomposite based on poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) and silylated kaolinite/silica core–shell nanoparticles

A novel biodegradable nanocomposite was fabricated based on poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) and silane-modified kaolinite/silica core–shell nanoparticles (SMKS), via solution-casting method using chloroform as solvent. Unmodified (ORK) and glycidoxypropyltrimethoxysilane (G-silane) surface-modified kaolin (SMK) were also introduced into PHBHHx matrix for comparison. SMKS significantly improved the mechanical properties of the PHBHHx compared to that of ORK and SMK. The composite filled with a low loading of SMKS increased the tensile strength and toughness from 18.2 to 23.5 MPa and 2.6 to 4.2 MPa, respectively. Transmission electron microscopy (TEM) indicated that SMKS was finely distributed in the PHBHHx matrix compared with the other two kinds of fillers. These significant improvements in these mechanical properties were attributed to the fine dispersion of SMKS into the polymer and covalent interaction between polymer chains and the surface of SMKS.     

Fully biodegradable blends of poly(l-lactide) and poly(ethylene succinate): Miscibility, crystallization, and mechanical properties

Both poly(l-lactide) (PLLA) and poly(ethylene succinate) (PES) are biodegradable semicrystalline polyesters. The disadvantages of poor mechanical properties and slow crystallization rate of PLLA limit its wide application. Fully biodegradable polymer blends were prepared by blending PLLA with PES. Miscibility, crystallization behavior, and mechanical properties of PLLA/PES blends were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and tensile tests in this work. Experimental results indicated that PLLA was immiscible with PES. Crystallization of PLLA/PES blends was studied by DSC using two-step crystallization condition and analyzed by the Avrami equation. The crystallization rate of PLLA at 100 °C was accelerated with the increase of PES in the blends while the crystallization mechanism did not change. In the case of the isothermal crystallization of PES at 67.5 °C, the crystallization mechanism did not change, and the crystallization rate decreased with the increase of PLLA. The mechanical properties of PLLA/PES blends were examined by tensile testing. The elongation at break of PLLA was improved significantly in the blends, while its considerably high Young's modulus was still kept. SEM images of fracture surfaces indicated that the fracture behavior of PLLA/PES blends changed from brittle fracture to ductile fracture behavior in the blends.     

The effect of copolymer composition on the spatial structural hierarchy developed in injection molded bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) parts

The effects of copolymer composition on structure development in injection molded bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBHV) parts were investigated. The increase of hydroxyvalerate co-monomer content lowers the melting temperature as it disrupts the crystalline order as a result, depth variation of melting behavior in the injection molded samples were found to depend primarily on the co monomer composition. At lower HV concentrations, the skin regions were found to exhibit a single melting peak that is also higher than those in the interior of the parts where generally bimodal melting behavior is observed. At higher HV content bimodal melting prevails throughout the injection molded parts including the skin and shear regions. This unusual behavior was attributed to the flow induced crystallization in extended chain formation at the skin and ease of inclusion of HV defects in the HB crystals that formed at slower cooling conditions in the interior creating thinner HV rich crystals with lower melting and thicker HV poor crystals with higher melting peaks.Depth profiling micro beam wide angle X-ray diffraction studies revealed that these polymers exhibit two distinct orientation behaviors depending on the distance from the surface. At the skin, invariably the chain axes are oriented along the flow direction. Beyond the transition layer located between the shear layer and core, the orientation does not disappear as expected from fast crystallizing polymers but rather preferential orientation of crystals with their a-axes along the flow direction was observed. At low HV content, the materials exhibit unusually high preferred orientation behavior throughout the thickness even for thick moldings, resembling the orientation behavior of polymers with low orientation relaxation behavior such as thermotropic liquid crystalline polymers. This is partly attributed to the unusually low injection melt temperature employed in these materials to avoid thermal degradation. The increase of HV content in the copolymers was found to change this c+a type orientation gradient across the thickness to gradual decrease of c-axis oriented crystals. This change was attributed to the decrease of crystallizability with the addition of HV and increasing melt fraction in the melt stream as the overall melting temperature decreases with the increase of HV content.     

Solid-state NMR study on the structure and mobility of the noncrystalline region of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The noncrystalline structures of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers were studied by variable temperature solid-state wide-line ¹H NMR and solid-state high-resolution ¹³C NMR spectroscopy. It is found that at room temperature there exists a rich and rigid component in the noncrystalline region of PHB and PHBV. The content of this component decreases with the increase in 3-hydroxyvalerate content in PHBV and with the increase in temperature. The brittleness of PHB may be partly attributed to the rigidness of the noncrystalline region at room temperature, while the improvement of the properties of PHBV may come from the enhanced mobility of the noncrystalline region.     

Segregation morphology of poly(3-hydroxybutyrate)/poly(vinyl acetate) and poly(3-hydroxybutyrate-co-10% 3-hydroxyvalerate)/poly(vinyl acetate) blends as studied via small angle X-ray scattering

Segregation morphology of poly(3-hydroxybutyrate) (PHB)/poly(vinyl acetate) (PVAc) and poly(3-hydroxybutyrate-co-10% 3-hydroxyvalerate) (P(HB-co-10% HV)/PVAc blends crystallized at 70 °C have been investigated by means of small angle X-ray scattering (SAXS). Morphological parameters including the crystal thickness (l_c) and the amorphous layer thickness (l_a) were deduced from the one-dimensional correlation function (γ(z)). Blending with PVAc thickened the PHB crystals but not the P(HB-co-10% HV) crystals. On the basis of the composition variation of l_a, and the volume fraction of lamellar stacks (?_s) revealed that PHB/PVAc blends created the interlamellar segregation morphology when the weight fraction of PVAc (w_PVAc)≤0.2 and the interlamellar and interfibrillar segregation coexisted when w_PVAc>0.2, while P(HB-co-10% HV)/PVAc blends yielded the interfibrillar segregation morphology at all blend compositions. For both PHB/PVAc and P(HB-co-10% HV)/PVAc blends, the distance of PVAc segregation was promoted by increasing PVAc composition and the distance of PVAc segregation in P(HB-co-10% HV)/PVAc blends was greater than in PHB/PVAc at a given PVAc composition. The crystal growth rate played a key role in controlling the segregation of PVAc.     

Synthesis and characterization of poly(ethylene glycol)-b-poly (l-lactide)-b-poly(l-glutamic acid) triblock copolymer

A biodegradable amphiphilic triblock copolymer of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(l-glutamic acid) (PEG-b-PLLA-b-PLGA) was obtained by catalytic hydrogenation of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(γ-benzyl-l-glutamic acid) (PEG-b-PLLA-b-PBLGA) synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG-NCA) with amino-terminated MPEG-b-PLLA-NH₂ as a macroinitiator. MPEG-b-PLLA-NH₂ converted from MPEG-b-PLLA-OH first reacted with tert-Butoxycarbonyl-l-phenylalanine (Phe-^NBOC) and dicyclohexylcarbodiimide (DCC) and then deprotected the tert-butoxycarbonyl group. MPEG-b-PLLA-OH was prepared by ROP of l-lactide with monomethoxy poly(ethylene glycol) in the presence of stannous octoate. The triblock copolymer and its diblock precursors were characterized by ¹H NMR, FTIR, GPC and DSA (drop shape analysis) measurements. The lengths of each block polymers could be tailored by molecular design and the ratios of feeding monomers. The triblock polymer PEG-b-PLLA-b-PLGA containing carboxyl groups showed obviously improved hydrophilic properties and could be a good potential candidate as a drug delivery carrier.     

Thermal and mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/cellulose nanowhiskers composites

Bacterial polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was reinforced with cellulose nanowhiskers (CNW) in 1-5 wt.% concentrations using a solvent casting method. The CNW was prepared from microcrystalline cellulose (MCC) using sulfuric acid hydrolysis. The influence of CNW on the PHBV crystallization, thermal, dynamic mechanical and mechanical properties were evaluated using polarized optical microscope (POM), differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), tensile and bulge tests, respectively. POM test results demonstrated that CNW was an effective PHBV nucleation agent. Tensile strength, Young’s modulus and toughness of PHBV increased with the increasing concentration of CNW. DMA results showed an increased tan δ peak temperature and broadened transition peak, indicating restrained PHBV molecular mobility in the vicinity of the CNW surface. Storage modulus of the PHBV also increased with the addition of CNW, especially at the temperatures higher than the PHBV glass transition temperature. These results indicated that the CNW could substantially increase the mechanical properties of PHBV and this increase could be attributed to the strong interactions between these two phases.     

Effect of chemical compositional distribution on solid-state structures and properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Solid-state structures and crystallization kinetics were compared between poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHB-HV] and PHB/PHB-HV blends exhibiting the cocrystallization. As cocrystallizable blends, both the blends showing complete cocrystallization, i.e. the PHB content in the crystalline phase is the same as that of the whole blends, and the blends forming a PHB-rich crystalline phase were used. The PHB and HV content in the cocrystalline phase were determined by high-resolution solid-state ¹³C NMR spectroscopy. In order to determine these contents with a minimum experimental error, site-specific ¹³C-labeled PHB/PHB-HV blends and PHB-HV copolymers were used. The crystallinity, lamellar structures, spherulite growth rate, and melting behavior were analyzed by wide-angle X-ray diffraction, small-angle X-ray scattering, polarized microscope, and differential scanning calorimetry, respectively. In these data, no difference was observed between the complete-cocrystallizable PHB/PHB-HV blends and the PHB-HV copolymers with the same overall HV content. On the other hand, the PHB/PHB-HV blends forming a PHB-rich crystalline phase has the amorphous layers thicker than that of the PHB-HV copolymers with the same overall HV content. Based on the collected data, the similarity and differences in the solid-state structures and properties between PHB-HV copolymers and cocrystallizable PHB/PHB-HV blends were discussed.     

Synergetic toughness and morphology of poly(propylene)/nylon 11/maleated ethylene‐propylene diene copolymer blends

Polypropylene (PP)/nylon 11/maleated ethylene‐propylene‐diene rubber (EPDM‐g‐MAH) ternary polymer blends were prepared via melt blending in a corotating twin‐screw extruder. The effect of nylon 11 and EPDM‐g‐MAH on the phase morphology and mechanical properties was investigated. Scanning electron microscopy observation revealed that there was apparent phase separation for PP/EPDM‐g‐MAH binary blends at the level of 10 wt % maleated elastomer. For the PP/nylon 11/EPDM‐g‐MAH ternary blends, the dispersed phase morphology of the maleated elastomer was hardly affected by the addition of nylon 11, whereas the reduced dispersed phase domains of nylon 11 were observed with the increasing maleated elastomer loading. Furthermore, a core‐shell structure, in which nylon 11 as a rigid core was surrounded by a soft EPDM‐g‐MAH shell, was formed in the case of 10 wt % nylon 11 and higher EPDM‐g‐MAH concentration. In general, the results of mechanical property measurement showed that the ternary blends exhibited inferior tensile strength in comparison with the PP matrix, but superior toughness. Especially low‐temperature impact strength was obtained. The toughening mechanism was discussed with reference to the phase morphology. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008