Rheology of the cellulose nanocrystals filled poly(ε-caprolactone) biocomposites

How surface modification of cellulose nanocrystals affects the crystallization process of poly (β-hydroxybutyrate)

Hydroxyl groups on the surface of cellulose nanocrystals (CNC) are modified by chemical methods, CNC and the modified CNC are used as fillers to prepare PHB/cellulose nanocomposites. The absorption peak of carbonyl group of the modified CNC (CNC-CL and CNC-LA) appears in the FT-IR spectra, which proves that the modifications are successful. Thermal stability of CNC-CL and CNC-LA is better than that of pure CNC. Pure CNC is beneficial to the nucleation of PHB, while CNC-CL and CNC-LA inhibit the nucleation of PHB. The spherulite size of PHB and its nanocomposites increases linearly over time, and the maximum growth rate of PHB spherulite exists at 90 °C. Rheological analysis shows that viscous deformation plays the dominant role in PHB, PHBC and PHBC-CL samples, while the elastic deformation is dominant in PHBC-LA. According to the rheological data, the dispersion of CNC-CL and CNC-LA in PHB is better than that of CNC. This work demonstrates the impact of modified CNC on the crystallization and viscoelastic properties of PHB. Moreover, the interface enhancement effect of modified CNC on PHB/CNC nanomaterials is revealed from the crystallization and rheology perspectives.

More than just a beer - Brewers' spent grain, spent hops, and spent yeast as potential functional fillers for polymer composites

Beer is among the most popular beverages in the world, with the production distributed uniformly between the biggest continents, so the utilization of brewing by-products is essential on a global scale. Among their potential recipients, the plastics industry offers extensive range of potential products. Herein, the presented study investigated the application of currently underutilized solid brewing by-products (brewers' spent grain, spent hops, spent yeast) as fillers for highly-filled poly(ε-caprolactone)-based composites, providing the first direct connection between spent hops or spent yeast and the polymer composites. Comprehensive by-product characterization revealed differences in chemical composition. The elemental C:O ratio, protein content, and Trolox equivalent antioxidant capacity varied from 1.40 to 1.89, 12.9 to 32.4 wt%, and 2.41 to 10.24 mg/g, respectively, which was mirrored in the composites' structure and performance. Morphological analysis pointed to the composition-driven hydrophilicity gap limiting interfacial adhesion for high shares of brewers' spent grain and spent hops, due to high hydrophilicity induced by carbohydrate content. Phytochemicals and other components of applied by-products stimulated composites' oxidative resistance, shifting oxidation onset temperature from 261 °C for matrix over 360 °C for high spent yeast shares. Simultaneously, spent yeast also provided compatibilizing effects for poly(ε-caprolactone)-based composites, reducing complex viscosity compared to other fillers and indicating its highest affinity to poly(ε-caprolactone)due to the lowest hydrophilicity gap. The presented results indicate that the proper selection of brewing by-products and adjustment of their shares creates an exciting possibility of engineering composites' structure and performance, which can be transferred to other polymers differing with hydrophilicity.

Effect of surface acetylation of chitin nanocrystals on the preparation and viscoelasticity of sunflower seed oil-in-water Pickering emulsions

Acetylated chitin nanocrystals (ChNCs) were used as stabilizer in this work to prepare sunflower seed oil-in-water emulsions for the morphological and rheological studies. The results revealed that the acetylation with moderate degree of substitution (0.38) reduced hydrophilicity and increased surface charge level of rod-like ChNCs, and as a result, significantly improved the emulsifying ability of ChNCs. At the same oil/water ratio and particle loading, the emulsions stabilized with the acetylated ChNCs had far smaller droplet size (∼3 μm) as compared to the emulsions stabilized with the pristine ChNCs (5-7 μm). The increased droplets numbers and improved surface coating level resulted in the enhanced viscous resistance and yield stress level, which improved the physical stability of the acetylated ChNC-stabilized emulsions as a result. In addition, the droplet clusters easily formed in this system, contributing to weak strain overshoot and decreased large-deformation sensitivity during dynamic shear flow. Therefore, the acetylated ChNC-stabilized system showed enhanced transient stress overshoot during startup flow and weakened thixotropy during cyclic ramp shear flow as compared to the pristine ChNC-stabilized system. The relationships between surface acetylation of ChNCs and flow behavior of emulsions were then established, which provide valuable information on the modulation of the ChNC-stabilized Pickering emulsions.

Nucleation roles of cellulose nanocrystals and chitin nanocrystals in poly(ε-caprolactone) nanocomposites

Using polysaccharide nanocrystals such as chitin nanocrystals (ChNCs) or cellulose nanocrystals (CNCs) as fillers of biodegradable aliphatic polyesters is an attractive approach of fabricating completely biodegradable nanocomposites. Most aliphatic polyesters are semi-crystalline and hence to reveal the effect of nanocrystals on their crystallization behaviors is key to regulate final properties of the nanocomposites. In this work, poly(ε-caprolactone) (PCL) nanocomposites filled with ChNCs and CNCs were prepared as templates for the study. It is intriguing that these two nanocrystals play completely different roles towards crystallization of PCL. CNCs are nucleating agent, promoting nucleation of PCL and accelerating subsequent crystal growth; while ChNCs are anti-nucleation agent, retarding nucleation of PCL and depressing whole process of PCL crystallization. This difference arises from different particle-polymer affinities in the nanocomposites, which is confirmed by the thermodynamic and rheological tests. This work provides valuable information around tuning the thermal properties of polysaccharide nanocrystals filled polymeric nanocomposites.

Cellulosic nanofibers filled poly(β-hydroxybutyrate): Relations between viscoelasticity of composites and aspect ratios of nanofibers

Dispersion states are vital for fibrous nanocelluloses to be used as reinforcements for polymers, which is highly dependent on geometry of nanocelluloses. Three types of nanocelluloses with various fiber aspect ratios were used to prepare target composite samples with poly(β-hydroxybutyrate) in this work. Viscoelasticity/elastoplasticity were used as probes to detect the flexibility-morphology relations of nanocelluloses in polymer. Cellulose nanocrystals (aspect ratio = 8) were rigid in polymer, retaining their rod-like shape, whereas bacterial celluloses (aspect ratio = 600) fully flexible, forming closely networked structure, and cellulose nanofibers (aspect ratio = 70) semi-flexible, dispersing as loosely flocculated clusters. Owing to these differences, the viscoelastic flow and elastoplastic deformation of three kinds of composites differed from one another. The strain-scaling and hysteresis work-scaling behaviors were then used to establish relaxation scale-structure correlations of target samples. This work provides interesting information around regulating the dispersion of nanocelluloses in polymer composites by tailoring aspect ratios of nanocelluloses.

Biopolymer blends of polyhydroxybutyrate and polylactic acid reinforced with cellulose nanofibrils

Poly(lactic acid) (PLA) was used in an effort to enhance the mechanical properties of poly(hydroxybutyrate) (PHB) and the blends were reinforced with cellulose nanofibrils (CNF). The conventional and dynamic mechanical, morphological, thermal and rheological properties of the obtained composite blends were determined. The results showed that the mechanical properties of neat PHB noticeably increased attributable to the good interaction between the biopolymers and CNF from the scanning electron microscopy (SEM) characterization. Thermal stability of the neat PHB was improved by adding PLA, however differential scanning calorimetry results showed that PLA created enhanced thermal properties while adding CNFs did not provide any change in the composite thermal properties. Dynamic mechanical and rheological properties of the neat PHB generally improved with both PLA and CNFs, however, it decreased at high loadings of CNFs attributed to fiber aggregations and fiber pull-out in comparison to the low loading level of CNF.

Stiffening, strengthening, and toughening of biodegradable poly(butylene adipate-co-terephthalate) with a low nanoinclusion usage

Simultaneous stiffening, strengthening, and toughening of biodegradable polymers, such as poly(butylene adipate-co-terephthalate) (PBAT) and others, is necessary for their use in packaging and agriculture applications. However, a high content of nanoinclusions is usually required, leading to a tradeoff between composite toughness and strength or stiffness in the reinforcement. Herein, we report an iterative reinforcement strategy that uses one nanocomposite to reinforce PBAT. An in-situ grafting polymerized cellulose nanocrystal (CNC)/PBAT (CNC-g-PBAT) nanocomposite consisting of ungrafted/free PBAT (PBAT_f) was used as an inclusion directly to reinforce a commercial PBAT. At an exceptionally low CNC usage of 0.02 wt.%, we achieved a simultaneous enhancement of the Young's modulus by 26 %, tensile strength by 27 %, elongation at break by 37 %, and toughness by 56 % over those for PBAT. To the best of our knowledge, such reinforcement efficiency is the highest among similar biodegradable polymer nanocomposites reported in the literature. The rheology, differential scanning calorimetry, and wide-angle X-ray diffraction measurements confirmed the mechanical reinforcement attributed to a synergistic contribution from PBAT_f and CNC-g-PBAT. In particular, the use of PBAT_f enhanced both stiffness and toughness of the composites, while the CNC-g-PBAT interacted within the polymer matrix and increased the crystallinity of the polymer matrix, leading to the strengthening and toughening effect. The strategy proposed here is greatly beneficial to producing high-performance biodegradable polymer nanocomposite films for packaging and agricultural applications using a very low amount of nanoinclusion.

Poly(lactic Acid)-Biochar Biocomposites: Effect of Processing and Filler Content on Rheological, Thermal, and Mechanical Properties

Biocomposites based on poly(lactic acid) (PLA) and biochar (BC) particles derived from spent ground coffee were prepared using two different processing routes, namely melt mixing and solvent casting. The formulated biocomposites were characterized through rheological, thermal, and mechanical analyses, aiming at evaluating the effects of the filler content and of the processing method on their final properties. The rheological characterization demonstrated the effectiveness of both exploited strategies in achieving a good level of filler dispersion within the matrix, notwithstanding the occurrence of a remarkable decrease of the PLA molar mass during the processing at high temperature. Nevertheless, significant alterations of the PLA rheological behavior were observed in the composites obtained by melt mixing. Differential scanning calorimetry (DSC) measurements indicated a remarkable influence of the processing method on the thermal behavior of biocomposites. More specifically, melt mixing caused the appearance of two melting peaks, though the structure of the materials remained almost amorphous; conversely, a significant increase of the crystalline phase content was observed for solvent cast biocomposites containing low amounts of filler that acted as nucleating agents. Finally, thermogravimetric analyses suggested a catalytic effect of BC particles on the degradation of PLA; its biocomposites showed decreased thermal stability as compared with the neat PLA matrix.

Ultralight Programmable Bioinspired Aerogels with an Integrated Multifunctional Surface for Self-Cleaning, Oil Absorption, and Thermal Insulation via Coassembly

Creating a configurable and controllable surface for structure-integrated multifunctionality of ultralight aerogels is of significance but remains a huge challenge because of the critical limitations of mechanical vulnerability and structural processability. Herein, inspired by Salvinia minima, the facile and one-step coassembly approach is developed to allow the structured aerogels to spontaneously replicate Salvinia-like textures for function-adaptable surfaces morphologically. The in situ superimposed construction of bioinspired topography and intrinsic topology is for the first time performed for programmable binary architectures with multifunctionality without engendering structural vulnerability and functional disruption. By introducing the binding groups for hydrophobicity tailoring, functionalized nanocellulose (f-NC) is prepared via mechanochemistry as a structural, functional, and topographical modifier for a multitasking role. The self-generated bioinspired surface with f-NC greatly maintains the structural unity and mechanical robustness, which enable self-adaptability and self-supporting of surface configurations. With fine-tuning of nucleation-driving, the binary microstructures can be controllably diversified for structure-adaptable multifunctionalities. The resulting ultralight S. minima-inspired aerogels (e.g., 0.054 g cm^-3) presented outstanding temperature-endured elasticity (e.g., 90.7% high-temperature compress-recovery after multiple cycles), durable superhydrophobicity, anti-icing properties, oil absorbency efficiency (e.g., 60.2 g g^-1), and thermal insulating (e.g., 0.075 W mK^-1), which are superior to these reported on the overall performance. This coassembly strategy offers the opportunities for the design of ultralight materials with topography- and function-tailorable features to meet the increasing demands in many fields such as smart surfaces and self-cleaning coatings.

Cellulose nanofibers reinforced biodegradable polyester blends: Ternary biocomposites with balanced mechanical properties

Blending two biodegradable aliphatic polyesters with complementary bulk properties is an easy way of tuning their final properties. In this work, the ductile poly(butylene succinate) was mixed with polylactide, and as expectable, the blends show improved toughness with sharply reduced strengths. The pristine cellulose nanofibers were then used as the reinforcement for the blends. It is found that most nanofibers are dispersed in the polylactide phase because polylactide has better affinity to nanofibers, and the lower viscosity level of polylactide also favors driving nanofibers into the continuous polylactide phase during melting mixing. In this case, the strength and rigidity losses resulted from the presence of soft poly(butylene succinate) phase are compensated to some extent. To further improve mechanical properties, a two-step approach (reactive processing of blends, followed by the incorporation with nanofibers) was developed. This work provides an interesting way of fabricating fully biodegradable composites with well-balanced mechanical performance.

Structure-Property Relationships in Polyethylene-Based Composites Filled with Biochar Derived from Waste Coffee Grounds

In this work, biochar (BC) derived from spent coffee grounds has been incorporated into high density polyethylene (PE) through melt mixing. The influence of the filler content on the rheological and thermal behavior of the obtained composites was assessed. In particular, a rheological study was performed systematically using different flow fields, including linear and nonlinear dynamic shear flow, revealing that the dynamics of PE macromolecules in the composite materials are slowed down because of the confinement of the polymer chains onto the filler surface and/or within the BC porous structure. Oscillatory amplitude sweep tests indicated that composites show weak strain overshoot behavior in the nonlinear regime: This finding clearly proves the formation of weak structural complexes, which cause a retardation of the macromolecular chains dynamics. Furthermore, the embedded BC particles were able to improve the thermo-oxidative stability of PE-based composites, remarkably increasing the PE decomposition temperatures.

Relaxation Dynamics in Polyethylene Glycol/Modified Hydrotalcite Nanocomposites

Polyethylene glycol-based nanocomposites containing an organo-modified hydrotalcite with loadings ranging from 0.5 to 5 wt.% were prepared by melt mixing performed just above the melting point of the polymer matrix. In these conditions, the dispersion of the nanofiller within the polymer matrix was quite homogeneous as revealed by TEM analyses. The effect of various thermal treatments and filler loadings was thoroughly investigated by means of rheological, morphological and gas chromatography-mass spectrometry, hyphenated to thermogravimetry analysis tests. Unfilled polyethylene glycol exhibited a continuous decrease in complex viscosity upon heating. In contrast, the complex viscosity of nanocomposites containing nanofiller loadings higher than 1 wt.% showed first a decrease, followed by an increase in the complex viscosity as the temperature increases, exhibiting a minimum between 130 and 140 °C. Annealing at 180 °C for different times further increased the viscosity of the system. This unusual behavior was explained by the occurrence of grafting reactions between the ⁻OH terminal groups of the polyethylene glycol chains and the hydroxyl groups of the organo-modified filler, thus remarkably affecting the relaxation dynamics of the system.

Effect of cellulose nanocrystals on crystallization kinetics of polycaprolactone as probed by Rheo-Raman

The development of biocompatible polymer nano-composites that enhance mechanical properties while maintaining thermoplastic processability is a longstanding goal in sustainable materials. When the matrix is semi-crystalline, the nanoparticles may induce significant changes to crystallization kinetics and morphology due to their ability to act as nucleating agents. To fully model this behavior in a process line, an understanding of the relationship between crystallinity and modulus is required. Here, we introduce a scalable model system consisting of surface-compatibilized cellulose nanocrystals (CNC) dispersed into poly(ε-caprolactone) (PCL) and study the effects of nanoparticle concentration on isothermal crystallization kinetics. The dispersion is accomplished by exchange of the Na⁺ of sulfated cellulose nanocrystals by tetra-butyl ammonium cations (Bu₄N⁺) followed by melt mixing via twin-screw extrusion. Crystallization kinetics are measured through the recently developed rheo-Raman instrument which extracts the relationship between the growth of the transient mechanical modulus and that of crystallinity. With extrusion and increasing CNC content, we find the expected enhancement of crystallization rate, but we moreover find a significant change in the relative kinetics of increase in modulus versus crystallinity. We analyze this via generalized effective medium theory which allows computation of a critical percolation threshold ξ _c and discuss the results in terms of a change in nucleation density and a change in the anisotropy of crystallization.