Liquid crystalline properties of symmetric and asymmetric end-grafted cellulose nanocrystals

The hydrophilic polymer poly[2-(2-(2-methoxy ethoxy)ethoxy)ethylacrylate] (POEG3A) was grafted onto the reducing end-groups (REGs) of cellulose nanocrystal (CNC) allomorphs, and their liquid crystalline properties were investigated. The REGs on CNCs extracted from cellulose I (CNC-I) are exclusively located at one end of the crystallite, whereas CNCs extracted from cellulose II (CNC-II) feature REGs at both ends of the crystallite, so that grafting from the REGs affords asymmetrically and symmetrically decorated CNCs, respectively. To confirm the REG modification, several complementary analytical techniques were applied. The grafting of POEG3A onto the CNC REGs was evidenced by Fourier transform infrared spectroscopy, atomic force microscopy, and the coil–globule conformational transition of this polymer above 60 °C, i.e., its lower critical solution temperature. Furthermore, we investigated the self-assembly of end-tethered CNC-hybrids into chiral nematic liquid crystalline phases. Above a critical concentration, both end-grafted CNC allomorphs form chiral nematic tactoids. The introduction of POEG3A to CNC-I does not disturb the surface of the CNCs along the rods, allowing the modified CNCs to approach each other and form helicoidal textures. End-grafted CNC-II formed chiral nematic tactoids with a pitch observable by polarized optical microscopy. This is likely due to their increase in hydrodynamic radius or the introduced steric stabilization of the end-grafted polymerPeer ReviewedPostprint (author's final draft

Challenges in synthesis and analysis of asymmetrically grafted cellulose nanocrystals via atom transfer radical polymerization

When cellulose nanocrystals (CNCs) are isolated from cellulose microfibrils, the parallel arrangement of the cellulose chains in the crystalline domains is retained so that all reducing end-groups (REGs) point to one crystallite end. This permits the selective chemical modification of one end of the CNCs. In this study, two reaction pathways are compared to selectively attach atom-transfer radical polymerization (ATRP) initiators to the REGs of CNCs, using reductive amination. This modification further enabled the site-specific grafting of the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) from the CNCs. Different analytical methods, including colorimetry and solution-state NMR analysis, were combined to confirm the REG-modification with ATRP-initiators and PSS. The achieved grafting yield was low due to either a limited conversion of the CNC REGs or side reactions on the polymerization initiator during the reductive amination. The end-tethered CNCs were easy to redisperse in water after freeze-drying, and the shear birefringence of colloidal suspensions is maintained after this process.Peer ReviewedPostprint (author's final draft

Patience is a virtue: self-assembly and physico-chemical properties of cellulose nanocrystal allomorphs

Cellulose nanocrystals (CNCs) are bio-based rod-like nanoparticles with a quickly expanding market. Despite the fact that a variety of production routes and starting cellulose sources are employed, all industrially produced CNCs consist of cellulose I (CNC-I), the native crystalline allomorph of cellulose. Here a comparative study of the physico-chemical properties and liquid crystalline behavior of CNCs produced from cellulose II (CNC-II) and typical CNC-I is reported. CNC-I and CNC-II are isolated by sulfuric acid hydrolysis of cotton and mercerized cotton, respectively. The two allomorphs display similar surface charge densities and ¿-potentials and both have a right-handed twist, but CNC-II have a slightly smaller average length and aspect ratio, and are less hygroscopic. Interestingly, the self-assembly behavior of CNC-I and CNC-II in water is different. Whilst CNC-I forms a chiral nematic phase, CNC-II initially phase separates into an upper isotropic and a lower nematic liquid crystalline phase, before a slow reorganization into a large-pitch chiral nematic texture occurs. This is potentially caused by a combination of factors, including the inferred faster rotational diffusion of CNC-II and the different crystal structures of CNC-I and CNC-II, which are responsible for the presence and absence of a giant dipole moment, respectively.Peer ReviewedPostprint (published version

Patience is a virtue: self-assembly and physico-chemical properties of cellulose nanocrystal allomorphs

Cellulose nanocrystals (CNCs) are bio-based rod-like nanoparticles with a quickly expanding market. Despite the fact that a variety of production routes and starting cellulose sources are employed, all industrially produced CNCs consist of cellulose I (CNC-I), the native crystalline allomorph of cellulose. Here a comparative study of the physico-chemical properties and liquid crystalline behavior of CNCs produced from cellulose II (CNC-II) and typical CNC-I is reported. CNC-I and CNC-II are isolated by sulfuric acid hydrolysis of cotton and mercerized cotton, respectively. The two allomorphs display similar surface charge densities and ζ-potentials and both have a right-handed twist, but CNC-II have a slightly smaller average length and aspect ratio, and are less hygroscopic. Interestingly, the self-assembly behavior of CNC-I and CNC-II in water is different. Whilst CNC-I forms a chiral nematic phase, CNC-II initially phase separates into an upper isotropic and a lower nematic liquid crystalline phase, before a slow reorganization into a large-pitch chiral nematic texture occurs. This is potentially caused by a combination of factors, including the inferred faster rotational diffusion of CNC-II and the different crystal structures of CNC-I and CNC-II, which are responsible for the presence and absence of a giant dipole moment, respectively.status: publishe

Grafting Polymers from Cellulose Nanocrystals: Synthesis, Properties, and Applications

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Thermally switchable liquid crystals based on cellulose nanocrystals with patchy polymer grafts

A thermally “switchable” liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). “Patchy” polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the “patchy” PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the “brush” PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the “patchy” PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.Peer ReviewedPostprint (updated version

Thermally Switchable Liquid Crystals Based on Cellulose Nanocrystals with Patchy Polymer Grafts

A thermally “switchable” liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). “Patchy” polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the “patchy” PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the “brush” PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the “patchy” PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.Peer ReviewedPostprint (updated version

Grafting Polymers from Cellulose Nanocrystals: Synthesis, Properties, and Applications

Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface- modified CNCs, which range from building blocks in nano- composites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macro- molecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the design of nanoscale building blocks whose readily tunable properties are useful for the formation of both colloidal dispersions and solid state materials. In this Perspective, we provide an overview of the morphology and surface chemistry of CNCs and detail various techniques to manipulate their surface chemistry via polymer grafting from approaches. Moreover, we explore the most common polymerization techniques that are used to graft polymers from the surface and reducing end groups of CNCs, including surface-initiated ring-opening polymerization (SI-ROP), surface- initiated free (SI-FRP), and controlled (SI-CRP) radical polymerization. Finally, we provide insights into some of the emerging applications and conclude with an outlook of future work that would benefit the field.status: publishe

Grafting Polymers <i>from</i> Cellulose Nanocrystals: Synthesis, Properties, and Applications

Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface-modified CNCs, which range from building blocks in nanocomposites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macromolecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the design of nanoscale building blocks whose readily tunable properties are useful for the formation of both colloidal dispersions and solid state materials. In this Perspective, we provide an overview of the morphology and surface chemistry of CNCs and detail various techniques to manipulate their surface chemistry via polymer grafting <i>from</i> approaches. Moreover, we explore the most common polymerization techniques that are used to graft polymers from the surface and reducing end groups of CNCs, including surface-initiated ring-opening polymerization (SI-ROP), surface-initiated free (SI-FRP), and controlled (SI-CRP) radical polymerization. Finally, we provide insights into some of the emerging applications and conclude with an outlook of future work that would benefit the field