Browsing by Author "Khan, Saad A."

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Anion-Specific Water Interactions with Nanochitin: Donnan and Osmotic Pressure Effects as Revealed by Quartz Microgravimetry
(2021-09-28) Jin, Soo Ah; Khan, Saad A.; Spontak, Richard J.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The development of new materials emphasizes greater use of sustainable and eco-friendly resources, including those that take advantage of the unique properties of nanopolysaccharides. Advances in this area, however, necessarily require a thorough understanding of interactions with water. Our contribution to this important topic pertains to the swelling behavior of partially deacetylated nanochitin (NCh), which has been studied here by quartz crystal microgravimetry. Ultrathin films of NCh supported on gold-coated resonators have been equilibrated in aqueous electrolyte solutions (containing NaF, NaCl, NaBr, NaNO3, Na2SO4, Na2SO3, or Na3PO4) at different ionic strengths. As anticipated, NCh displays contrasting swelling/deswelling responses, depending on the ionic affinities and valences of the counterions. The extent of water uptake induced by halide anions, for instance, follows a modified Hofmeister series with F- producing the highest swelling. In marked contrast, Cl- induces film dehydration. We conclude that larger anions promote deswelling such that water losses increase with increasing anion valence. Results such as the ones reported here are critical to ongoing efforts designed to dry chitin nanomaterials and develop bio-based and sustainable materials, including particles, films, coatings, and other nanostructured assemblies, for various devices and applications.
Associative structures formed from cellulose nanofibrils and nanochitins are pH-responsive and exhibit tunable rheology
(2021-04-15) Facchine, Emily G.; Bai, Long; Rojas, Orlando J.; Khan, Saad A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hypothesis: Nanocellulose and nanochitin are both biobased materials with complementary structures and properties. Both exhibit pH-dependent surface charges which are opposite in sign. Hence, it should be possible to manipulate them to form complexed structures via ionic bond formation at prescribed pH conditions. Experiment: Nanocellulose and nanochitin were mixed after exposure to acidic or neutral conditions to influence their ionization state. The heat of interaction during the introduction of nanochitin to nanocellulose was monitored via isothermal titration calorimetry. The strength and gel properties of the resulting structures were characterized via rheological measurement. Findings: The resultant gel properties in the designed hybrid systems were found to depend directly on the charge state of the starting materials, which was dictated by pH adjustment. Different interparticle interactions including ionic attraction, hydrophobic associations, and physical entanglement were identified in the systems and the influence of each was elucidated for different conditions of pH, concentration, and ratio of nanochitin to nanocellulose. Hydrophobic associations between neutralized nanochitin particles were found to contribute strongly to increased elastic modulus values. Ionic complex formation was found to provide enhanced stability under broader pH conditions, while physical entanglement of cellulose nanofibers was a substantial thickening mechanism in all systems.
Cellulose nanofibers and the film-formation dilemma: Drying temperature and tunable optical, mechanical and wetting properties of nanocomposite films composed of waterborne sulfopolyesters
(2021-09-15) Jin, Soo Ah; Facchine, Emily G.; Rojas, Orlando J.; Khan, Saad A.; Spontak, Richard J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hypothesis: Waterborne sulfopolyesters have gained considerable interest as coating materials due to their excellent film-forming and optical properties. Their commercial use has been limited, however, due to their fragile nature. Incorporating cellulose nanofiber (CNF), a sustainable biopolymer, into the polymer matrix is expected to enhance the mechanical integrity of the nanocomposite as these two components synergistically interact. Experiments: In this study, we have investigated the suspension and film characteristics of three sulfopolyesters varying in charge density, glass transition temperature and molecular weight, as well as their mixtures with CNF. We have performed steady-shear rheology on mixtures with different CNF loading levels, and resulting films have been subjected to quasistatic uniaxial tensile and water contact-angle tests to elucidate the effects of CNF on mechanical and surface properties. Findings: Addition of CNF to waterborne polyester promotes shear-thinning behavior that remains unaffected by the CNF content. Solid films cast from these suspensions possess enhanced mechanical properties, as well as tailorable surface hydrophilicity, depending on composition and film-drying temperature. Tensile tests reveal that films containing 10 wt% CNF display the greatest mechanical improvements, suggesting the existence of a previously unidentified Goldilocks composition window.
Expanding the upper limits of robustness of cellulose nanocrystal aerogels: Outstanding mechanical performance and associated pore compression response of chiral-nematic architectures
(2019-07-07) Tripathi, Anurodh; Tardy, Blaise L.; Khan, Saad A.; Liebner, Falk; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Control over the nanoscale architecture of a material enables fine tuning of its physical characteristics and associated functions. Depending on the performance demands, properties such as active surface area, density, optical response, transport characteristics and mechanical resilience can be tailored by nanostructuring. Herein, we exploit the liquid crystalline phase transitions in aqueous dispersions of highly anisometric, nanoscaled and high strength (EA > 150 GPa) cellulose nanocrystals (CNCs) to afford chiral-nematic ordered aerogels with controlled meso- and microstructures. Unprecedented levels of specific strength and toughness were achieved by controlling CNC assembly and derived architectures. We determined that the specific strength, and toughness, of CNC aerogels are improved by up to 137% and 60%, respectively, compared with the highest reported values for aerogels formed solely from cellulose nanofibrils or nanocrystals. Our results demonstrate that chiral-nematic ordered aerogels with controlled meso- and microstructures replicate the liquid crystalline phase transitions of CNCs in aqueous dispersions. The obtained architectures are evaluated systematically by varying the long-range order of the aqueous CNC dispersion from mostly isotropic to completely anisotropic. The resulting aerogels display a strong relationship between the mesopore fraction and selective light reflection (iridescence) as a function of mechanical load. Specifically, we find that the mechanical performance associated with pore compression under load is greatly enhanced by chiral-nematic ordering. The new limits in the mechanical properties of CNC-based aerogels point to new structural considerations for the synthesis of next generation porous constructs that exploit the inherent long-range order of such building blocks.
Experimental and Predictive Description of the Morphology of Wet-Spun Fibers
(2019-06) Tripathi, Anurodh; Rutkevicius, Marius; Bose, Arnab; Rojas, Orlando J.; Khan, Saad A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The prediction of the morphology of wet-spun fibers has so far been only possible by complex and experimentally intensive approaches that include the construction of ternary phase diagrams. Ultimately, however, the available models give approximate information. Here we propose an alternative predictive approach that uses design principles based on the combination of (1) relative energy difference (RED) of Hansen solubility and (2) a kinetic parameter "T" that considers mass transfer effects. Such a model is applied and experimentally validated for a priori determination of the diameter and internal morphology of wet-spun fibers. Remarkably, only three. variables relevant to wet-spinning are needed, namely, the choice of polymer, solvent, and nonsolvent types. A combination of systems is tested, and the morphology of the obtained fibers is determined via electron microscopy. Aspects related to demixing, internal specific surface area (BET), and layer formation on the fibers are described qualitatively. The facile implementation of the design parameters is further confirmed through comparison with data published on the subject. Our proposed model is expected to accelerate future developments in nanomaterials, especially in the context of ongoing efforts related to fiber spinning with biopolymers.
Featherlight, Mechanically Robust Cellulose Ester Aerogels for Environmental Remediation
(2017-08) Tripathi, Anurodh; Parsons, Gregory N.; Rojas, Orlando J.; Khan, Saad A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A unique combination of well-established synthesis procedures involving chemical cross-linking, careful solvent exchange to water, and subsequent freeze drying is used to produce ultralight (4.3 mg/mL) and highly porous (99.7%) cellulose diacetate (CDA) aerogels with honeycomb morphology. This versatile synthesis approach is extended to other nonaqueous polymers with hydroxyl functionalities such as cellulose acetate propionate and cellulose acetate butyrate to produce a single component polymer aerogel. These aerogels demonstrate a maximum water and oil uptake of up to 92 and 112 g/g, respectively. The honeycomb morphology provides a maximum compression strain of 92% without failure and reaches a compressive stress of 350 kPa, for 4 w/v % CDA aerogels (4%), which is higher than that reported for cellulosic aerogels. The 4% CDA aerogel were rendered hydrophobic and oleophilic via chemical vapor deposition with organosilane. The modified CDA aerogel surpasses their counterparts in maintaining their mechanical integrity for fast oil cleanup and efficient oil retention from aqueous media under marine conditions. These aerogels are identified to be reusable and durable for a long period.
Heterogeneous Acetylation of Plant Fibers into Micro- and Nanocelluloses for the Synthesis of Highly Stretchable, Tough, and Water-Resistant Co-continuous Filaments via Wet-Spinning
(2018-12-26) Tripathi, Anurodh; Ago, Mariko; Khan, Saad A.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Heterogeneous acetylation of wood fibers is proposed for weakening their interfibrillar hydrogen bonding, which facilitates their processing into micro- and nanocelluloses that can be further used to synthesize filaments via wet-spinning. The structural (SEM, WAXD), molecular (SEC), and chemical (FTIR, titration) properties of the system are used to propose the associated reaction mechanism. Unlike the homogeneous acetylation, this method does not alter the main morphological features of cellulose fibrils. Thus, we show for the first time, the exploitation of synergies of compositions simultaneously comprising dissolved cellulose esters and suspended cellulose micro- and nanofibrils. Such colloidal suspension forms a co-continuous assembly with a matrix that interacts strongly with the micro- and nanofibrils in the dispersed phase. This facilitates uninterrupted and defect-free wet-spinning. Upon contact with an antisolvent (water), filaments are easily formed and display a set of properties that set them apart from those reported so far for nanocelluloses: a remarkable stretchability (30% strain) and ultrahigh toughness (33 MJ/m3), both surpassing the values of all reported nanocellulose-based filaments. All the while, they also exhibit competitive stiffness and strength (6 GPa and 143 MPa, respectively). Most remarkably, they retain 90% of these properties after long-term immersion in water, solving the main challenge of the lack of wet strength that is otherwise observed for filaments synthesized from nanocelluloses.
Interfacial Contributions in Nanodiamond-Reinforced Polymeric Fibers
(2021-09-16) Adhikari, Prajesh; Jani, Pallav K.; Hsiao, Lilian C.; Rojas, Orlando J.; Khan, Saad A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the interfacial energy parameters that explain the reinforcement of polymers with nanodiamond (ND) and the development of mechanical strength of electrospun ND-reinforced composites. Thermodynamic parameters such as the wettability ratio, work of spreading and dispersion/aggregation transition are used to derive a criterion to predict the dispersibility of carboxylated ND (cND) in polymeric matrices. Such a criterion for dispersion (Dc) is applied to electrospun cND-containing poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), and polystyrene (PS) fiber composites. The shifts in glass transition temperature (ΔTg), used as a measure of polymer/cND interfacial interactions and hence the reinforcement capability of cNDs, reveal a direct correlation with the thermodynamic parameter Dc in the order of PAN < PS < PVA. Contrary to expectation, however, the tensile strength of the electrospun fibers correlates with the Dc and ΔTg only for semicrystalline polymers (PAN < PVA) while the amorphous PS displays a maximum reinforcement with cND. Such conflicting results reveal a synergy that is not captured by thermodynamic considerations alone but also factor in the contributions of polymer/cND interface stress transfer efficiency. Our findings open the possibility for tailoring the interfacial interactions in polymer-ND fiber composites to achieve maximum mechanical reinforcement.
Mesophase characteristics of cellulose nanocrystal films prepared from electrolyte suspensions
(2021-10) Jin, Soo Ah; Facchine, Emily G.; Khan, Saad A.; Rojas, Orlando J.; Spontak, Richard J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Cellulose nanocrystals (CNCs) exhibit a cholesteric mesophase above a critical concentration in aqueous suspensions. Above this concentration, CNCs self-organize into left-handed helicoidal structures that can be preserved in dried, stratified films. In this systematic study, we have prepared optically-active CNC films cast from different electrolyte suspensions and investigated, via circular dichroism and other techniques, the effects of counterion type (six mono/divalent salts, including those responsible for promoting “salting-out” and “salting-in” in the Hofmeister series) and ionic strength on mesomorphic behavior and cholesteric arrangement. The presence of electrolytes influences CNC colloidal stability by compressing the electric double layer and altering interactions among neighboring CNCs and water, thereby affecting the extent to which the CNCs form a mesophase. Interestingly, mesomorphic behavior and CNC alignment appear to be sensitive to cationic radius and charge valence, in which case the optical properties of CNC films can be adjusted for targeted sustainable applications. Such heuristic rules can be valuable for predicting the stability and characteristics of CNC microstructure in designer coatings and thin films prepared by introducing suitable cations prior to film formation.
Quantitative Calorimetric Studies of the Chiral Nematic Mesophase in Aqueous Cellulose Nanocrystal Suspensions
(2020-09-15) Facchine, Emily G.; Jin, Soo Ah; Spontak, Richard J.; Khan, Saad A.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Aqueous suspensions of cellulose nanocrystals (CNCs) can spontaneously form a chiral nematic mesophase at a critical concentration (c*). Unfortunately, no current analytical technique permits rapid detection of c*. Herein, we introduce a facile and accurate approach to assess c* rapidly (<2 h) from a small sample volume and compare our results with those obtained by conventional methods. Our strategy employs isothermal titration calorimetry (ITC) to measure the heat associated with interactions in the suspension, which can identify the onset of mesophase formation as the heat signature is sensitive to the suspension viscosity and thus capable of detecting small changes in the suspension environment. We measure c* for CNC samples differing in surface charge and aspect ratio, and find that both lower aspect ratios and higher surface charges increase c*. Our ITC results reveal the role of CNC interactions prior to the visual observation of mesophase formation and elucidate mesomorphic effects related to nanocrystals and their suspensions.
Synthesis of organic aerogels with tailorable morphology and strength by controlled solvent swelling following Hansen solubility
(2018-12-01) Tripathi, Anurodh; Parsons, Gregory N.; Khan, Saad A.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We introduce a generalized approach to synthesize aerogels that allows remarkable control over its mechanical properties. The Hansen solubility parameters are used to predict and regulate the swelling properties of the precursor gels and, consequently, to achieve aerogels with tailored density and mechanical properties. As a demonstration, crosslinked organogels were synthesized from cellulose esters to generate aerogels. By determination of Hansen's Relative Energy Difference, it was possible to overcome the limitations of current approaches that solely rely on the choice of precursor polymer concentration to achieve a set of aerogel properties. Hence, from a given concentration, aerogels were produced in a range of mass densities, from 25 to 113 mg/cm3. Consequently, it was possible to tailor the stiffness, toughness and compressive strength of the aerogels, in the ranges between 14-340, 4-103 and 22-373 kPa, respectively. Additionally, unidirectional freeze-drying introduced pore alignment in aerogels with honeycomb morphologies and anisotropy. Interestingly, when the swelling of the polymeric gel was arrested in a non-equilibrium state, it was possible to gain additional control of the property space. The proposed method is a novel and generic solution to achieving full control of aerogel development, which up to now has been an intractable challenge.