Morphology,deformation behavior and thermomechanical properties of polypropylene/maleic anhydride grafted polypropylene/layered silicate nanocomposites

Nanocomposites polypropylene (PP) with 3 and 7 wt % of clay were prepared by melt mixing. Four types of maleic anhydride grafted PP (MAPP) in broad range of MA groups content (0.3–4 wt %) and molecular weights (MW) were used as polar compatibilizers. The effect of the MAPP kind on both the clay dispersion and miscibility with PP was studied. The mixed intercalated/exfoliated morphologies of nanocomposites in the presence of all studied compatibilizers were revealed by XRD and TEM. The oligomer compatibilizer with 4 wt % of MA groups increases the intercalation ability of polymer into clay galleries but this one has limited miscibility with PP and worsens crystalline structure of polymer matrix. The MAPPs with 0.3–1.3% of MA are characterized by the lower intercalation ability but well cocrystallize with PP. Maximum reinforcing effect is attained using high MW MAPP with 0.6% MA and for nanocomposite with 7 wt % (3.8 vol %) of clay it averages almost 1.7 times relative to neat PP and 1.3 times relative to noncompatibilized composite. Dynamic storage moduli of nanocomposites compatibilized by MAPPs with 0.3–1.3% of MA containing 7 wt % of clay increase up to 1.4–1.5 around 30–75°C and over the whole temperature range remain higher compared with both neat PP and uncompatibilized composite. On the contrary, the oligomer MAPP with 4 wt % of MA groups decreases the thermal–mechanical stability of nanocomposite at high temperature compared with both PP and uncompatibilized composites. The study of nanocomposites flammability showed that creating complex composites containing both layered silicate and relatively small amount of magnesium hydroxide can be a successful approach to reduce the combustibility of PP‐based nanocomposites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of fiber/matrix chemical modification on the mechanical properties and water absorption of extruded flax/polypropylene composite

Compound of flax/polypropylene (PP) is characterized concerning the mechanical properties of stiffness, strength, and impact in addition to the water absorption behavior. Manufacturing takes place by twin‐screw extruder. The extruder screw layout is modified through different kneading elements to get high fiber aspect ratio. Sodium hydroxide solution was used as a washing solution for the flax fibers' surfaces. Both fiber and matrix are chemically modified. Selected groups of the fibers were further treated using trimethoxyvinylsilan TMVS and acrylic acid AA. The PP matrix is also treated with different coupling agents; namely, maleated PP MAPP, TMVS‐MAPP, and acrylic acid‐functionalized PP AAPP. The combinations of different fiber/matrix are extruder compounded, injection molded, and finally tested. Fiber modification seems to be positive with AA‐modified surface. AAPP matrix modification improves the stiffness four times that of the untreated flax/PP. Till 30 and 40 wt %, the more the fiber is the more the strength and stiffness, respectively. MAPP‐modified matrix improves the mechanical properties and keeps low water absorption values. AAPP‐modified matrix shows the best stiffness values. TMVS‐MAPP does not seem to have distinguished improvement compared with MAPP. NaOH‐TMVS/MAPP and NaOH‐TMVS/AAPP systems can serve as alternatives to the normal NaOH/MAPP treatment. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of fiber treatment on the performance of sisal–polypropylene composites

This article concerns the effectiveness of MAPP as a coupling agent in sisal–polypropylene composites. The fiber loading, MAPP concentration, and fiber treatment time influenced the mechanical properties of the composites. It was observed that the composites prepared at 21 volume percent of fibers with 1% MAPP concentration exhibits optimum mechanical strength. SEM investigations confirmed that the increase in properties is caused by improved fiber‐matrix adhesion. The viscoelastic properties of the treated and untreated composites were also studied. From the storage modulus versus temperature plots, an increase in the magnitude of the peaks was observed with the addition of MAPP and fiber reinforcement, thus showing an improvement in stiffness of the treated composites. The damping properties of the composites, however, decreased with the addition of the fibers and MAPP. The thermal properties of the composites were analyzed through DSC and TGA measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1336–1345, 2004     

Nanoscale characterization of interphase properties in maleated polypropylene‐treated natural fiber‐reinforced polymer composites

Contact resonance force microscopy has been used to evaluate the effect of maleated polypropylene (MAPP) concentration on interphase thickness as well as the spatial distribution of mechanical properties within the interphase of cellulose fiber‐reinforced polypropylene composites. The average interphase thickness values ranged from 25 nm to 104 nm for different concentrations of MAPP. The interphase region showed a gradient in the elastic modulus, with a gradual decrease in modulus from fiber to matrix. The interphase region in the specimen containing 0% MAPP showed a narrow interphase with steep gradient in modulus from fiber to matrix, whereas the use of MAPP significantly increased the interphase thickness which resulted in a more gradual change in modulus from fiber to matrix. POLYM. ENG. SCI., 2013. © Society of Plastics Engineers     

Kudzu fiber‐reinforced polypropylene composite

Kudzu fiber‐reinforced polypropylene composites were prepared, and their mechanical and thermal properties were determined. To enhance the adhesion between the kudzu fiber and the polypropylene matrix, maleic anhydride‐grafted polypropylene (MAPP) was used as a compatibilizer. A continuous improvement in both tensile modulus and tensile strength was observed up to a MAPP concentration of 35 wt %. Increases of 24 and 54% were obtained for tensile modulus and tensile strength, respectively. Scanning electron microscopy (SEM) showed improved dispersion and adhesion with MAPP. Fourier transform infrared (FTIR) spectroscopy showed an increase in hydrogen bonding with an increase in MAPP content. Differential scanning calorimetry (DSC) analysis indicated little change in the melting temperature of the composites with changes in MAPP content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1961–1969, 2002     

Influence of interfacial adhesion on the structural and mechanical behavior of PP‐banana/glass hybrid composites

Hybrid composites of polypropylene (PP), reinforced with short banana and glass fibers were fabricated using Haake torque rheocord followed by compression molding with and without the presence maleic anhydride grafted polypropylene (MAPP) as a coupling agent. Incorporation of both fibers into PP matrix resulted in increase of tensile strength, flexural strength, and impact strength upto 30 wt% with an optimum strength observed at 2 wt% MAPP treated 15 wt% banana and 15 wt% glass fiber. The rate of water absorption for the hybrid composites was decreased due to the presence of glass fiber and coupling agent. The effect of fiber loading in presence of coupling agent on the dynamic mechanical properties has been analyzed to investigate the interfacial properties. An increase in storage modulus (E′) of the treated‐composite indicates higher stiffness. The loss tangent (tan δ) spectra confirms a strong influence of fiber loading and coupling agent concentration on the α and β relaxation process of PP. The nature of fiber matrix adhesion was examined through scanning electron microscopy (SEM) of the tensile fractured specimen. Thermal measurements were carried out through differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA), indicated an increase in the crystallization temperature and thermal stability of PP with the incorporation of MAPP‐treated banana and glass fiber. POLYM. COMPOS., 31:1247–1257, 2010. © 2009 Society of Plastics Engineers     

Banana/Glass Fiber-Reinforced Polypropylene Hybrid Composites: Fabrication and Performance Evaluation

Hybrid composites of Polypropylene (PP) reinforced with intimately mixed short banana and glass fibers were fabricated using Haake twin screw extruder followed by compression molding with and without the presence maleic anhydride grafted polypropylene (MAPP) as a coupling agent. Incorporation of both the fibers into PP matrix resulted in an increase in tensile, flexural and impact strength with an increasing level of fiber content upto 30 wt% at banana: glass fiber ratio of 15:15 wt% and 2 wt% of MAPP. The rate of water absorption for the hybrid composites decreased due to the presence of glass fiber and coupling agent. The effect of fiber loading in presence of coupling agent on the dynamic mechanical properties has also been analyzed to investigate the interfacial properties. An increase in the storage modulus (E′) of the treated composite indicates higher stiffness. The tan δ spectra confirms a strong influence of fiber contents and coupling agent on the α and β relaxation processes of PP. The nature of fiber matrix adhesion was examined through scanning electron microscopy (SEM) of the tensile fractured specimen. Thermal measurements were carried out employing differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA) which indicated a decrease in the crystallization temperature and thermal stability of PP with the incorporation of MAPP treated banana and Glass fiber.     

Effect of stacking sequence on mechanical properties of hybrid flax/jute fibers reinforced thermoplastic composites

In this study, the hybrid composites were prepared by stacking jute/PP nonwoven and flax/MAPP woven fabrics in defined sequences. Polypropylene (PP) and maleic anhydride grafted polypropylene (MAPP) were used as matrix materials. Jute and flax fibers were treated with alkali solution in order to improve the interface properties of the resultant composites. The mechanical properties of these hybrid composites were analyzed by means of tensile, flexural, and drop‐weight impact tests. The effect of fabric stacking sequence on the mechanical properties of the composites was investigated. The stacking of nonwovens at the top and in alternate layers has resulted in maximum flexural strength, flexural stiffness, and impact force. It was also shown that hybrid composites have improved tensile, flexural, and impact properties in comparison to neat PP matrix. POLYM. COMPOS., 36:2167–2173, 2015. © 2014 Society of Plastics Engineers     

Processing and properties of mineral‐interfaced cellulose fibre composites

Polypropylene (PP) or, in some cases, poly (lactic acid) (PLA) were compounded with cellulosic fibres. The amount of fibres used was in the range 10–30 vol % and, in case of PP, a series of compounds was prepared with a minor amount of maleated PP as a compatibiliser. This matrix was denoted MAPP. Before compounding the polymers and the fibres, undelaminated bentonite (industrial scale) or delaminated clay (nanoclay) was deposited on the fibres in different amounts to improve the dispersion of the fibres in the polymer matrix, i. e., to avoid detrimental fibre bundles. The PP‐based compounds were either extruded or injection moulded, whereas the PLA‐compounds were only injection moulded. The mechanical properties were primarily evaluated for the injection moulded specimens. In general, the fibres had a strong effect on the mechanical behaviour of the materials, especially in the case of PLA and MAPP. Treating the fibres with undelaminated clay or nanoclay improved to some extent the dispersion of the fibres and the mechanical performance of the composites, but further optimization of the function of the mineral in this respect is probably required. The combination of the mineral treatment with a debonding agent appeared to be an interesting route here. With such a combination, a visually very good dispersion of the fibres in the PP‐based matrix could be obtained, partly at the expense of the mechanical performance. The compounding of the cellulosic fibres with PP led in this case to a marked decrease in the fibre length. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

The impact of polypropylene‐graft‐maleic anhydride on the crystallization and dynamic mechanical properties of isotactic polypropylene

Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to identify the mechanisms that lead to differences in the mechanical behavior of formulations of polypropylene blended with maleated polypropylene (MAPP) copolymers. MAPP lowered the melting temperature of PP indicating that less stable crystals were formed possibly because of cocrystallization of PP and MAPP. Crystallization kinetics revealed that copolymers do not change the rate of crystal growth, but may retard nucleation leading to a more spherulitic morphology. The dynamic storage modulus slightly increased in the glassy region with the small addition amounts of MAPP, while mechanical dampening systematically decreased with MAPP addition. An analysis of the viscoelastic behavior did not reveal any real differences in molecular coupling around the β‐transition of PP with the addition of the MAPP copolymer. At low addition levels, MAPP does not appear to have a significant impact on the viscoelastic properties of the polymer blend. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation and characterization of maleic anhydride‐g‐polypropylene/diamine‐modified clay nanocomposites

Polypropylene (PP)/montmorillonite (MMT) nanocomposites were prepared by compounding maleic anhydride‐g‐polypropylene (MAPP) with MMT modified with α,ω‐diaminododecane. Structural characterization confirmed the formation of characteristic amide linkages and the intercalation of MAPP between the silicate layers. In particular, X‐ray diffraction patterns of the modified clay and MAPP/MMT composites showed 001 basal spacing enlargement as much as 1.49 nm. Thermogravimetric analysis revealed that the thermal decomposition of the composite took place at a slightly higher temperature than that of MAPP. The heat of fusion of the MAPP phase decreased, indicating that the crystallization of MAPP was suppressed by the clay layers. PP/MAPP/MMT composites showed a 20–35% higher tensile modulus and tensile strength compared to those corresponding to PP/MAPP. However, the elongation at break decreased drastically, even when the content of MMT was as low as 1.25–5 wt %. The relatively short chain length and loop structure of MAPP bound to the clay layers made the penetration of MAPP molecules into the PP homopolymer phase implausible and is thought to be responsible for the decreased elongation at break. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 307–311, 2005     

Preparation and physical properties of wood/polypropylene/clay nanocomposites

Wood plastic composites (WPCs) are attracting a lot of interests because they are economic, environmentally friendly, and show fairly good performance. To improve the performance of a wood/polypropylene (PP) composite, an organoclay was incorporated as a nanosize filler in this work. WPCs were prepared by melt blending followed by compression molding, and their performance was investigated by universal testing machine, izod impact tester, dynamic mechanical analyzer, thermal mechanical analyzer, differential scanning calorimetry, and TGA. Maleic anhydride polypropylene copolymer (MAPP) was used to increase compatibility between the PP matrix and wood particles and also improve the dispersion and exfoliation of the organoclay in the PP matrix. XRD analysis showed that the matrix of the WPCs with organoclay had intercalated structure. The SEM images of the WPCs with MAPP showed improved interfacial adhesion between the matrix and wood particles. The degree of water absorption increased with immersion time, but it could be restrained by incorporating MAPP. The performance of the WPCs was improved by the incorporation of the organoclay. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical properties of polypropylene/clay nanocomposites: Effect of clay content,polymer/clay compatibility,and processing conditions

In this work, polypropylene/clay nanocomposites with 0.5, 1, 3, and 5 wt % of montmorillonite (MMT) (unmodified clay) were prepared by intensive mixing at 50 rpm and 10 min of mixing. For the highest clay content (5 wt %), the initial materials or the processing conditions were changed to study their independent effect. On one hand, 10 wt % of PP‐graft‐MA (PP‐g‐MA) was incorporated or MMT was replaced by organomodified clays (C10A and C30B). On the other side, for the initial system, the speed of rotation (100 and 150 rpm) and the mixing time (5 and 15 min) were altered. In all cases, the state of the clay inside the matrix (DRX), the degree of dispersion in the micro (SEM) and nano (TEM) scales, and the rheological and mechanical properties were analyzed. It was found that the stiffness increased with clay content, whereas tensile and impact strength did not significantly change. Although intercalated structures were observed in the composites with unmodified clay, in the composites with modified clay or PP‐g‐MA, improved dispersion of clay in PP was found. The mechanical properties increased accordingly. The degree of dispersion of the filler in the matrix appeared to be unaffected by the changes in the processing conditions introduced. Finally, the elastic modulus was modeled by using an effective filler‐parameter model based on Halpin–Tsai equations, which also allowed estimating the relative degree of dispersion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Prospect of nanoscale interphase evaluation to predict composite properties

For meeting the requirements of lightweight and improved mechanical properties, composites could be tailor-made for specific applications if the adhesion strength which plays a key role for improved properties can be predicted. The relationship between wettability and adhesion strength has been discussed. The microstructure of interphases and adhesion strength can be significantly altered by different surface modifications of the reinforcing fibers, since the specific properties of the interphase result from nucleation, thermal and/or intrinsic stresses, sizing used, interdiffusion, and roughness. The experimental results could not confirm a simple and direct correlation between wettability and adhesion strength for different model systems. The main objective of the work was to identify the interphases for different fiber/polymer matrix systems. By using phase imaging and nanoindentation tests based on atomic force microscopy (AFM), a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites was conducted. As model sizings for PP composites, γ-aminopropyltriethoxysilane (APS) and either polyurethane (PU) or polypropylene (PP) film former on glass fibers were investigated. The EP-matrix was combined with either unsized glass fibers or glass fibers treated with APS/PU sizing. It was found that phase imaging AFM was a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficiently small indentation force was found to be sufficient for measuring actual interphase properties within a 100-nm region close to the fiber surface. Subsequently, it also indicated a different gradient in the modulus across the interphase region due to different sizings. The possibilities of controlling bond strength between fiber surface and polymer matrix are discussed in terms of elastic moduli of the interphases compared with surface stiffness of sized glass fibers, micromechanical results, and the mechanical properties of real composites.     

Effect of Clay Types on the Mechanical,Dynamic Mechanical and Morphological Properties of Polypropylene Nanocomposites

In the present investigation Polypropylene–Maleic anhydride grafted polypropylene–organically modified MMT (PP-MAPP-OMMT) nanocomposites were prepared by melt mixing in a twin screw extruder followed by injection molding. The effect of clay chemistry and compatibilizer on the properties of the nanocomposites has been studied. Sodium montmorillonite has been organically modified using quaternary and alkyl amine intercalants. A comparative account with commercial quaternary ammonium modified clays i.e Cloisite 20A, Cloisite 15A and Cloisite 30B has been presented. Storage modulus of PP matrix also increased in the nanocomposites, indicating an increase in the stiffness of the matrix polymer with the addition of organically modified nanoclays. The morphology of the nanocomposites has been examined using wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Morphological findings revealed efficient dispersion of organically modified nanoclays within the PP matrix. MAPP compatibilized PP/Cloisite 15A nanocomposites displayed finely dispersed exfoliated nanomorphology as compared with other systems.     

Phenolic resin‐crosslinked natural rubber/clay nanocomposites: Influence of clay loading and interfacial adhesion on strain‐induced crystallization behavior

Nanocomposites of natural rubber (NR) and pristine clay (clay) were prepared by latex mixing, then crosslinked with phenolic resin (PhOH). For comparative study, the PhOH‐crosslinked neat NR was also prepared. Influence of clay loading (i.e., 1, 3, 5, and 10 phr) on mechanical properties and structural change of PhOH‐crosslinked NR/clay nanocomposites was studied through X‐ray diffraction (XRD), transmission electron microscopic (TEM), wide‐angle X‐ray diffraction (WAXD), tensile property measurement, and Fourier transform infrared spectroscopy (FTIR). XRD and TEM showed that the clay was partly intercalated and aggregated, and that the dispersion state of clay was non‐uniform at higher clay loading (>5 phr). From tensile test measurement, it was found that the pronounced upturn of tensile stress was observed when the clay loading was increased and a maximum tensile strength of the PhOH‐crosslinked NR/clay nanocomposites was obtained at 5 phr clay. WAXD observations showed that an increased addition of clay induced more orientation and alignment of NR chains, thereby lowering onset strain of strain‐induced crystallization and promoting crystallinity of the NR matrix during tensile deformation. FTIR investigation indicated a strong interfacial adhesion between NR matrix and clay filler through a phenolic resin bridge. This suggested that the PhOH did not only act as curative agent for crosslinking of NR, but it also worked as coupling agent for promoting interfacial reaction between NR and clay. The presence of strong interfacial adhesion was found to play an important role in the crystallization process, leading to promotion of mechanical properties of the PhOH‐crosslinked NR/clay nanocomposites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43214.     

Optimal performances of ultrasound treated kenaf fiber reinforced recycled polypropylene composites as demonstrated by response surface method

Kenaf fiber (KF) reinforced recycled polypropylene (RPP) composites were produced by melt cast method. To improve interfacial adhesion between fiber and RPP matrix, fiber surface modification was carried out by means of ultrasound treatment. Maleic anhydride grafted polypropylene (MAPP) was used as a coupling agent. Composites were examined by mechanical test, melt flow indexing test, scanning electron microscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Water uptake analysis and accelerated weathering test were carried out to find the suitability of the composites in outdoor application. Among the raw fiber contents ranging 10?50 wt % in the composites, the maximum tensile strength (TS) was observed at 40 wt % KF loading without using MAPP. Treated KF‐based composite with MAPP promotes this maximum TS value, which is 57% higher than that of raw KF‐based composite. TGA and DSC analyses exhibit an enhancement of thermal stability in treated KF‐reinforced RPP composites with MAPP. Incorporation of MAPP in the composites shows higher activation energy, suggesting improved interfacial bonding between fibers and matrix. Response surface method was employed to demonstrate the optimal treatment parameters for TS, showing excellent agreement with the observed values. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Valorization of pineapple peel waste and sisal fiber: Study of cellulose nanocrystals on polypropylene nanocomposites

The present study focuses on the isolation and characterization of the cellulose nanocrystals (CNCs) from the pineapple peel waste (PPW) (agro-waste) and sisal fiber (SiF) (natural fiber) employing the acid hydrolysis method, and its comparison with the commercially available CNCs (CNC-C). Furthermore, the CNCs from PPW, SiF, and CNC-C were subjected to transmission electron microscopy (TEM), Fourier transform electron microscopy, X-ray diffraction, particle size distribution, and zeta potential analysis. The studied properties of the isolated CNCs are considerably different from the PPW and SiF. The CNCs so formed have been estimated by TEM to be around 10–40 nm wide and length of several micrometers. Fourier transform electron microscopy studies described the removal of the noncellulosic components like lignin, hemicellulose, and pectin substances from the base materials in both the cases by employing acid hydrolysis method. Additionally, nanocomposites of CNC isolated from PPW along with polypropylene (PP) matrix were fabricated using melt blending method followed by injection molding. Maleic anhydride grafted PP (MAPP) acts as a compatibilizer for improving the dispersibility of hydrophilic CNC within the nonpolar PP matrix. The addition of CNC (3 wt%) along with MAPP at 5 wt% showed optimum tensile strength and modulus to the tune of 10.39 and 25.53%, respectively, when compared to their counterparts without MAPP. Dynamic mechanical analysis revealed an increased stiffness of PP in its nanocomposite system due to the addition of CNC. Scanning electron microscopy studies revealed uniform distribution of CNC within the nonpolar PP matrix in the presence of MAPP.     

Morphology and thermal properties of maleic anhydride grafted polypropylene/ethylene–vinyl acetate copolymer/wood powder blend composites

Maleic anhydride grafted polypropylene (MAPP) was blended with ethylene–vinyl acetate (EVA) copolymer to form MAPP/EVA polymer blends. Wood powder (WP) was mixed into these blends at different weight fractions to form MAPP/EVA/WP blend composites. Differential scanning calorimetry (DSC) analysis of the blends showed small melting peaks between those of EVA and MAPP, which indicated interaction and cocrystallization of fractions of EVA and MAPP. The presence of MAPP influenced the EVA crystallization behavior, whereas the MAPP crystallization was not affected by the presence of EVA. Scanning electron microscopy, Fourier transform infrared spectroscopy, and DSC results show that the WP particles in the MAPP/EVA blend were in contact with both the MAPP and EVA phases and that there seemed to be chemical interaction between the different functional groups. This influenced the crystallization behavior, especially of the MAPP phase. The thermogravimetric analysis results show that the MAPP/EVA blend had two degradation steps. An increase in the WP content in the blend composite led to an increase in the onset of the second degradation step but a decrease in onset of the first degradation step. The presence of WP in the blend led to an increase in the modulus but had almost no influence on the tensile strength of the blend. The dynamic mechanical analysis results confirm the interaction between EVA and MAPP and show that the presence of WP only slightly influenced the dynamic mechanical properties. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) nanocomposites with optimal mechanical properties

With the ultimate goal to design renewable polymer nanocomposites with optimal mechanical properties, this study reports an investigation of structure–property relationships for a model system – silica/poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx) nanocomposites. Two molecular weights of PHBHx (M_w = 903,000 g/mol and M_w = 633,000 g/mol) and two types of silica nanoparticles (nominally spheres and fibers according to the manufacturer) were used to prepare the nanocomposites. Small-angle X-ray scattering shows that the sphere and fiber nanoparticles had similar surface areas and primary particle size, but differed in degree of aggregation of the primary particles. The thermal stability of the PHBHx matrix was slightly improved by the addition of nanofillers. Simultaneous improvement of both stiffness and toughness was observed at 1-wt% loading for the higher molecular weight matrix. The more highly aggregated SiO₂ fibers had a greater toughening effect than the SiO₂ spheres. Compared to the unfilled polymer matrix, a 30% increase in Young's modulus and 34% increase in toughness were obtained for the 1-wt% SiO₂ fiber/PHBHx072 nanocomposite. The addition of SiO₂ spheres to PHBHx072 resulted in the same increase in Young's modulus (30%) but a smaller increase (11%) in toughness. The dramatic increases in modulus for PHBHx072 cannot be explained on the basis of two-component micromechanical models. Apparently the filler alters the character of the semicrystalline matrix. When the loading was 3 wt% and above, Young's modulus continued to increase, but the strain at break and toughness decreased. The ultimate strength did not change compared with the unfilled polymer. In order to understand the mechanical properties observed, the thermal behavior, spherulitic morphology and the deformation mechanisms of the nanocomposites and the dispersion state of the nanofillers were studied. We found that a high molecular weight of the polymer matrix, weak interfacial adhesion and a good dispersion of the nanofillers are necessary to improve toughness and stiffness simultaneously.