Effect of Poly(L-lactic acid) on Hydrolytic Degradation of Poly(glycolic acid)

The in-vitro degradation behavior of poly(glycolic acid) (PGA) rods and the composite rods containing poly(L-lactic acid) (PLLA) were investigated via mass loss, pH value change, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Since the degradation rate of PLLA is lower than that of PGA, PLLA/PGA composite rods exhibit a slower degradation rate in comparison with PGA. This finding indicated that it was possible to control the degradation rate of the composites by changing their composition. This result indicates that this kind of composite biomaterial may be applicable to devices for the need of prolonged degradation.     

生物降解材料聚(乳酸-乙醇酸)研究进展

生物降解材料聚(乳酸-乙醇酸)(PLGA)具有良好的生物相容性，在药物缓释材料、组织工程材料、手术缝合线等医用领域有广泛的应用。综述了PLGA的合成、性能与应用，尤其是详细介绍了PLGA类生物降解高分子的合成研究进展，指出简单易行的以乳酸、乙醇酸等单体为原料的直接缩聚法合成值得关注。     

Borassus powder-reinforced poly(lactic acid) composites with improved crystallization and mechanical properties

This article reports on the development of biocomposites based on polylactic acid (PLA) and borassus powder. Borassus powder was treated with alkali to remove hemicelluloses and lignin. The treated borassus improved the homogeneous mixing with PLA and increased the crystallinity of PLA. Dispersibility of the borassus was studied by scanning electron microscopy (SEM) and X-ray MicroCT. PLA/borassus composites were prepared by melt mixing of PLA with 5, 10, and 15 wt % treated/untreated borassus. Composites were examined for mechanical properties and crystallization. Composites showed enhanced tensile strength compared to neat PLA. The PLA/treated borassus powder composites displayed higher crystallinity than PLA. The isothermal cold crystallization study showed increase in the crystallization rate of PLA in the presence of treated borassus. The spherulitic growth was studied using polarized optical microscopy. The enhanced performance of the PLA-borassus composites was observed in the presence of borassus. This study demonstrates that the PLA-borassus composites show great promise for bioplastics applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47440.     

Bamboo fiber-reinforced polypropylene composites: Crystallization and interfacial morphology

Bamboo fiber-reinforced polypropylene (PP) composites were prepared. PP and two maleated polypropylenes (s-MAPP and m-MAPP) were used as matrices. Crystallization and interfacial morphology were studied by using differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and optical microscopy. It has been shown that the addition of bamboo fiber to any of the three polymers causes an increase in the overall crystallization rate. A considerable amount of β-form crystallinity was produced in the PP, s-MAPP, and m-MAPP by mixing with bamboo fiber; and all the bamboo fiber-filled samples contain both the α- and the β-forms. The relative amount of the β-form in the samples was calculated from WAXD data by the K value. There is no β-form observed in the pure PP, s-MAPP, and m-MAPP. Bamboo fiber acted as both a reinforcement and a β-nucleator. The nucleation density of both s-MAPP and m-MAPP at the fiber surface is remarkably higher than that of PP because an improved interfacial adhesion is reached in the case of s-MAPP and m-MAPP as matrices. The transcrystalline growth of s-MAPP and m-MAPP on the bamboo fiber surface was observed under optical microscope with crossed polars. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1267–1273, 1997     

Mechnical properties of thermoplastic film/metallic glass composites with improved interfacial strength

The mechanical properties of the uniaxial and continuous composite with high strain to fracture PP matrix reinforced with the Fe₄₀Ni₄₀B₂₀ (at%) and Metglas 2605SC metallic glass ribbons have been determined and compared with the rule of mixtures. The reinforcing ribbons were coated with PVA (polyvinlyl alcohol) to improve interfacial bonding. The experimental fracture strength results for both types of reinforcement deviate from the values predicted. The strength of composite at the fracture strain of ribbon determined experimentally obeys a modified rule of mixture (isostrain condition) for both types of reinforcement over the entire volume fraction range of reinforcement (up to 16%). The experimental composite eleastic modulus results for composite reinforced with relatively narrow Fe₄₀Ni₄₀B₂₀ ribbons are found to be generally near the values predicted by the rule of mixtures, but for the Metglas 2605SC wide the modified rule of mixtures, which takes into account the thermally induced tensile stresses at the interface resulting from the mismatch of the coefficients of thermal expansion.     

Alumina fiber-reinforced silica matrix composites with improved mechanical properties prepared by a novel DCC-HVCI method

A novel direct coagulation casting via controlled release of high valence counter ions (DCC-HVCI) method was applied to prepare the alumina fiber-reinforced silica matrix composites with improved mechanical properties. In this method, the silica suspension could be rapidly coagulated via controlled release of calcium ions from calcium iodate and pH shift by hydrolysis of glycerol diacetate (GDA) at an elevated temperature. The influence of tetramethylammonium hydroxide (TMAOH) dispersant amount, volume fraction and calcium iodate concentration on the rheological properties of suspensions was investigated. Additionally, the effect of alumina fiber contents on the mechanical properties of alumina fiber-reinforced silica matrix composites was studied systematically. It was found that the stable suspension of 50 vol% solid loading could be prepared by adding 2.5 wt% TMAOH at room temperature. The addition of 0–15 wt% alumina fibers had no obvious effect on the viscosity of the silica suspension. The controlled coagulation of the suspension could be achieved by adding 6.5 g L⁻¹ calcium iodate and 1.0 wt% GDA after treating at 70 °C for 30 min. Compressive strength of green bodies with homogeneous microstructure was in the range of 2.1–3.1 MPa. Due to the fiber pull-out and fracture behaviors, the mechanical properties of alumina fiber-reinforced composites improved remarkably. The flexural strength of the composite with 10 wt% alumina fibers sintered at 1350 °C was about 7 times of that without fibers. The results indicate that this approach could provide a promising route to prepare complex-shaped fiber-reinforced ceramic matrix composites with uniform microstructure and high mechanical properties.     

Fabrication by stereolithography of fiber-reinforced fused silica composites with reduced crack and improved mechanical properties

Chopped quartz fiber-reinforced fused silica (SiO_2f/SiO₂) composites were fabricated by stereolithography. The fiber orientation characteristics and crack distributions after the debinding process of the green bodies were investigated. The results showed that the distribution of fibers presented orientation characteristics; additionally, the number of cracks after debinding decreased as the fiber content increased and the cracks oriented along the fiber orientation. The mechanical properties of SiO_2f/SiO₂ ceramics with different fiber contents were also considered. As a result, a compressive strength of 51.2 MPa and flexural strength of 24.3 MPa were achieved for the SiO_2f/SiO₂ ceramic with 4 wt% fiber, and a sintered cambered structure with a size over 150 mm × 150 mm × 3 mm was fabricated successfully without cracking and deformation for the SiO_2f/SiO₂ composites with a fiber content of 4 wt% and 6 wt%.     

A slender-body theory for interfacial failure in unidirectional fiber-reinforced composites

Based on asymtotic techniques which have been recently developed for the mechanics of linear continuous media containing slender inclusions, a theory is presented for the nonlinear problem of interfacial failure in a fiber-reinforced composite. Based on the lowest order slender-body theory, and a correspondingly simple model of interfacial slip or plastic yielding, a constitutive equation is, derived for unidirectional, dilute-fiber composites. This equation provides a tensile stress-strain relation which exhibits microscopic, ductile yield arising from the microscopic failure process. Approximate formulae are proposed to account for fiber interactions among closely-spaced parallel fibers and for interfacial slip with sliding friction. It is shown that all the results can be represented in terms of a “reduced-variable” plot, which suggests that experirniental data for various fiber aspect ratios and concentrations might he reducible to a single curve, depending only on tine mode of interfacial failure.     

Poly(dimethylsiloxane) sealant with improved properties by blending with vinyl polymers

Novel adhesive sealants with markedly improved mechanical and durability properties have been prepared by blending poly(dimethylsiloxane) with PVC or (VAc-VC) copolymer. The effect of blending with the vinyl-type polymer, the influence of three types of substrates (California redwood, aluminum, or Portland cement mortar), and the effect of weathering on the properties of the PDS or the resulting products were assessed by stress-strain testing and toughness determinations. The morphology of unblended PDS and the PDS-based blends was studied by SEM observation in conjunction with EDXA and DSC measurements. Results have indicated considerable improvement in mechanical properties and enhanced weathering resistance in some of the blends used in conjunction with either wood or aluminum substrates.     

Poly(lactic acid) mass transfer properties

Poly(lactic acid) (PLA), a biodegradable and compostable polymer, is gaining market acceptance and has been extensively investigated. The versatility of PLA has led to its broad and different applications in medical, agriculture, and food packaging fields. Similar to other polymers, PLA is permeable to gases, vapors and organic compounds. Thus, the mass transfer properties of PLA can influence its suitability for end-use applications. Here, we present a comprehensive, systematic, and critical review of more than 300 papers published since 1990 reporting the mass transfer properties of PLA, which include permeability, diffusion and solubility to gases, water vapor and organic vapors, along with migration of chemical compounds from PLA. Overall, PLA provides moderate barrier to gases, water vapor, and organic compounds. Barrier enhancement can be achieved through modifications such as blending with other polymers and formation of composite structures. Most of the mass transfer parameters reported in the literature are based on two-phase mobile amorphous and crystalline fractions, omitting the role of the restricted amorphous fraction, which can lead to unclear comprehension of PLA barrier properties as well as what affects those properties. Additional research is needed to address this shortcoming. This review provides an in-depth analysis of PLA mass transfer and a foundation for future research and commercial development.     

Structural Characterization of Poly-l-lactic Acid (P(L)LA) and Poly(glycolic acid)(PGA) Oligomers

Structural characterization of poly-l-lactic acid (P(L)LA) and poly(glycolic acid) (PGA) oligomers containing three units was carried out with an atomistic approach. Oligomer structures were first optimized through quantum chemical calculations, using density functional theory (DFT); rotational barriers concerning dihedral angles along the chain were then investigated. Diffusion coefficients of l-lactic acid and glycolic acid in pure water were estimated through molecular dynamic (MD) simulations. Monomer structures were obtained with quantum chemical computation in implicit water using DFT method; atomic charges were fitted with Restrained Electrostatic Potentials (RESP) formalism, starting from electrostatic potentials calculated with quantum chemistry. MD simulations were carried out in explicit water, in order to take into account solvent presence.     

Improved interfacial properties of carbon fiber-reinforced epoxy composites with Fe2O3/graphene nanosheets using a magnetic field

The performance of carbon fiber-reinforced composites largely depends on the properties of the fiber-matrix interface. Here, to improve the interfacial strength properties of carbon fiber/epoxy composites, we doped different concentrations of Fe₂O₃/graphene nanosheets onto the interfacial region of the carbon fiber composites by nano-coating technology. With the aid of the magnetic field, the arrangement of nanosheets could be controlled in the interface. The nanosheets can be arranged on the carbon fiber surface parallel or perpendicularly with different concentrations. The tensile strength and interfacial shear strength of the modified fiber microcomposites had increased by 22.1 and 44.4% respectively with 1.0 mg/mL Fe₂O₃/graphene nanosheets. The results indicated that the Fe₂O₃/graphene nanosheets have an important influence on the carbon fibers and carbon fibers composites.     

Effect of polyethylene glycol on mechanical properties of bamboo fiber-reinforced polylactic acid composites

The bamboo fiber (BF)-reinforced polylactic acid (PLA) composites were prepared using the twin-screw extruder and injection molding. Thermal gravimetric analyzer results indicated the thermal stability of BF/PLA composites decreased with increasing BF content. Differential scanning calorimeter and X-ray diffraction curves showed that BF played a role as a nucleating agent, but the crystallinity of composite materials decreased with the increasing BF content. The melt flow rate of composites reduced with the increase in BF content, resulting in a poorer processing property. The processability of the composites was improved with the addition of high molecular polyethylene glycol (PEG). Mechanics performance test showed that tensile strength and bending strength of composites increased at low loading with the BF content increased then decreased when the loading continued to increase. The tensile strength of the composite materials reached 65.46 MPa when alkali-treated BF (ABF) content was 20 wt %. The flexural strength of the composites reached 97.94 MPa when ABF content was 10 wt %. Impact performance has also been improved. PEG-20000 was the best plasticizer among the PEG-6000,PEG-10000, and PEG-20000. When the component of PEG was 10 wt %, the elongation increased by 56%. The scanning electron microscopy (SEM) result showed that the fracture of the composites was smooth, most ABF were wrapped in matrix and distribution of ABF in PLA matrix was more uniform. It means that interfacial compatibility of bamboo fiber and PLA improved after BF modified by alkali. High molecular weight PEG enhance melt flow ability of polymer, result in fibers were further enclosed in the PLA matrix and increase properties of composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47709.     

Poly(lactic acid) formulations with improved toughness by physical blending with thermoplastic starch

This work focuses on poly(lactic acid) (PLA) formulations with improved toughness by physical blending with thermoplastic maize starch (TPS) plasticized with aliphatic–aromatic copolyester up to 30 wt %. A noticeable increase in toughness is observed, due to the finely dispersed spherical TPS domains in the PLA matrix. It is worth to note the remarkable increase in the elongation at break that changes from 7% (neat PLA) up to 21.5% for PLA with 30 wt % TPS. The impact‐absorbed energy is markedly improved from the relatively low values of neat PLA (1.6 J m^?2) up to more than three times. Although TPS is less thermally stable than PLA due to its plasticizer content, in general, PLA/TPS blends offer good balanced thermal stability. The morphology reveals high immiscibility in PLA/TPS blends, with TPS‐rich domains with an average size of 1 μm, finely dispersed which, in turn, is responsible for the improved toughness. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45751.     

Poly(2-acrylamido glycolic acid): A water-soluble polymer with ability to interact with metal ions in homogenous phase

A water-soluble polymer, poly(2-acrylamido glycolic acid) was obtained by radical polymerization and characterized by FT-IR, ¹H NMR, and ¹³C NMR spectroscopy. The metal ion retention properties were investigated through the liquid-phase polymer based retention (LPR) technique at different pHs and filtration factor Z. The affinity of the ligand groups for the metal ions depends strongly on the pH. At lower pH, the retention is lower than 50%, which increased as the pH increased. At pH 5, the polymer showed a high affinity and selectivity for Pb(II), and at pH 7 the P(AGA) formed stable complexes with Cu(II), Co(II), Ni(II), Cd(II), and Pb(II). Of the three potential ligand groups, amide, hydroxyl, and carboxylate groups, the carboxylate groups form the more stable complexes with the metal ions.     

Effect of fiber surface treatment on mechanical,interfacial, and moisture absorption properties of cattail fiber-reinforced composites

Surface treatment of cattail, a lignocellulosic renewable fiber, was investigated to determine the conditions that would reduce moisture absorption while maximizing the properties of cattail fiber-reinforced unsaturated polyester composites. Surface modification of cattail fiber was studied by treating them with 2.5, 5, and 10% of 1,6-diisocyanatohexane (DIH) and 2-hydroxyethyl acrylate (HEA) for three different immersion times (10, 20, and 30 min). DIH-HEA treated fibers were preformed into a non-woven mat and impregnated with unsaturated polyester resin to manufacture composite. The existence of covalent bonds on the treated fibers via N H and C N groups was confirmed by FTIR spectroscopy. The 10% DIH-HEA resulted in the best results; while the mean diameter of the treated fiber decreased by ~37%, the modulus and the strength of it increased by ~267 and ~151%, respectively. Equilibrium moisture regain of the treated fibers and their composites decreased by ~43% and ~40%, respectively. The tensile modulus of the composites increased by ~171%. Enhancement in tensile strength is observed but could not be quantified due to the difference in V_f and scatter in the data. SEM examination confirmed the enhancement in fiber–matrix bonding due to surface treatment.     

Melting parameters of poly(glycolic acid)

Equilibrium melting temperature T_m⁰, heat of fusion ΔH_f, and entropy of fusion ΔS_f of poly(glycolic acid) (PGA) was determined by using Clapeyron-Clausius equation. Equilibrium melting temperature T_m⁰ was 504.6 K which was determined by Hoffman-Weeks plots. The pressure dependence of T_m⁰ was determined by high pressure DTA up to 150 MPa. Volume change ΔV_f at melting was determined by using dilatometer. Heat of fusion in PGA was 183.2 (J g⁻¹), which is very close to the value reported by Chujo et al. who determined it by using T_m depression in copolymer with poly(lactic acid). ΔS_f of PGA was 0.363 (J g⁻¹ K⁻¹), which is about twice that of PLA, and the reason was discussed on the basis of the elastic modulus below T_m.     

Mechanical properties of pineapple leaf fiber-reinforced polyester composites

Pineapple leaf fiber (PALF) which is rich in cellulose, relatively inexpensive, and abundantly available has the potential for polymer reinforcement. The present study investigated the tensile, flexural, and impact behavior of PALF-reinforced polyester composites as a function of fiber loading, fiber length, and fiber surface modification. The tensile strength and Young's modulus of the composites were found to increase with fiber content in accordance with the rule of mixtures. The elongation at break of the composites exhibits an increase by the introduction of fiber. The mechanical properties are optimum at a fiber length of 30 mm. The flexural stiffness and flexural strength of the composites with a 30% fiber weight fraction are 2.76 GPa and 80.2 MPa, respectively. The specific flexural stiffness of the composite is about 2.3 times greater than that of neat polyester resin. The work of fracture (impact strength) of the composite with 30% fiber content was found to be 24 kJ m⁻². Significant improvement in the tensile strength was observed for composites with silane A172-treated fibers. Scanning electron microscopic studies were carried out to understand the fiber-matrix adhesion, fiber breakage, and failure topography. The PALF polyester composites possess superior mechanical properties compared to other cellulose-based natural fiber composites. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1739–1748, 1997     

Mechanical properties of ultrahigh-molecular-weight polyethylene fiber-reinforced PE composites

Three types of high-strength polyethylene (PE) fiber-reinforced composite sheets were made by compression molding at the vicinity of melting point of the fiber. Sheet I was molded from only PE fibers. Sheets II and III were prepared by the compression molding of PE fiber with conventional high- and low-density polyethylene films, respectively. The mechanical properties, thermal behavior, and morphologies of the sheets have been investigated and compared with each other. The tensile strength and elastic modulus of sheet III are 660 MPa and 14 GPa, respectively, which were 60 and 30 times higher than those of typical low-density PE film. Although the elastic modulus of sheet III is 6 GPa less than that of sheet II, the tensile strength of 660 MPa is highest in the three types of sheets prepared in this study. The mechanical properties of sheets II and III were about half of predicted theoretical ones. It was concluded that the interfacial adhesion between PE fiber and PE matrix was an important factor to improve the mechanical properties of this PE sheet. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1431–1439, 1998     

Tensile properties of short sisal fiber-reinforced polyethylene composites

Sisal fibers (Agave-Veracruz) have been used as reinforcements in low-density polyethylene (LDPE). The influence of the processing method and the effect of fiber content, fiber length, and orientation on tensile properties of the composites have been evaluated. The fiber damage that normally occurs during blending of fiber and polyethylene by the meltmixing method is avoided by adopting a solution-mixing procedure. The tensile properties of the composites thus prepared show a gradual increase with fiber content. The properties also increased with fiber length, to a maximum at a fiber length of about 6 mm. Unidirectional alignment of the short fibers achieved by an extrusion process enhanced the tensile strength and modulus of the composites along the axis of fiber alignment by more than twofold compared to randomly oriented fiber composites. © 1993 John Wiley & Sons, Inc.