Supermolecular structures, cellulosics

By forming intramolecular and intermolecular hydrogen bonds between OH groups within the same cellulose chain and the surrounding cellulose chains, the chains tend to be arranged in parallel and form a crystalline supermolecular stracture. Then, bundles of linear cellulose chains (in the longitudinal direction) form a microfibril which is oriented in the cell wall structure. Cellulose is insoluble in most solvents and has a low accessibility to acid and enzymatic hydrolysis (Demirbas, 2008b). 

Morphological or supermolecular structure is the most easily changed property of cellulosic fibers. Interactions of selected monomer solutions with fibers can yield grafted products... 

Cellulose is one of the most important aixl most abundant natural polymers. This circumstance and certain specific features of its molecular and supermolecular structures (the linear, strictly regular structure of the macromolecules, the great rigidity dF tte polymeric chains, the possibility of formation elements of the supermolecular structure havii a high d ree of ordering, i.e. crystallites, etc.) are the factors that have stimulated extensive work on the chemistry, physicochemistry and technology of cellulose and its derivatives. 

The supermolecular structures of cellulose have been investigated extensively by many techniques including x-ray and electron diffrac-tometry, electron microscopy, IR and Raman spectroscopy, broad-line proton NHR and solid-state c-uuR. Nevertheless, many questions remain concerning the solid-state structures. 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Several models are presently used to describe the chiral nematic structure and its temperature dependence for cellulosics. The following behavior has been observed for their supermolecular structure... 

Ethoxypropyl cellulose [59], an ethyl ether of HPC forms excellent thermotropic and lyotropic mesophases, the lyotropic ones with acetonitrile, dioxane, and methanol. Both thermotropic and lyotropic systems exhibit cholesteric phases with a right-handed helicoidal supermolecular structure,... 

Yachi, T., Hayashi, J., Takai, M., Shimizu, Y.J. Supermolecular structure of cellulose stepwise decrease in LODP and particle size of cellulose hydrolyzed after chemical treatment. Appl. Polym. Sci. Appl. Polym. Symp. 37, 325-343 (1983)... 

Recently, a more detailed model of the supermolecular structure of natural cellulose has been developed and proposed (loelovich et al., 2010 loelovich, 2014b Pakzad et al., 2012). According to this model, the elementary nanofibril of cellulose is built from orientated nanocrystaUites and noncrystalline nanodomains (NCD) arranged along the fibril also, a thin paracrystalline layer (PCL) is located on the surface of the crystalline core (CRC), while the crystallites can contain local defects (DEF), for example, vacancies, caused by ends of the chains (Figure 9.6). 

This chapter first gives an overview of cellulose raw materials and their molecular and supermolecular structures. The principles of shaping cellulose into fibres, films, and nonwovens by means of solution techniques are then outlined followed by a section on properties and market applications of these materials. Derivatives of cellulose are presented with special emphasis on thermoplastic cellulose esters, typical plasticizers, and promising reinforcing materials. Finally, recent developments and future prospects of cellulose materials are reviewed as far as the above applications are concerned. This book does not cover the important applications of cellulose and ligno cellulose fibres for reinforcing thermoplastics, like wood plastic composites (WPC) and natural fibre reinforced plastics (NFRP), since in these cases cellulose does not substitute a thermoplastic. 

The supermolecular structure of cellulose derivatives - in the present case cellulose esters -is strongly influenced by the degree of substitution and the substitution pattern. With the exception of cellulose triacetate (CTA, tully substituted, DS = 3), most commercially used cellulose esters have a DS of less than three or, in the case of mixed esters, do not have a regular substitution pattern. Therefore, a proper crystallization with well defined X-ray interferences is normally not observed and the materials appear amorphous. 

Ding Jiadong, Feng Jun, and Yang Yuliang. Sinusoidal supermolecular structure of band textures in a presheared hydroxypropyl cellulose film. Polym. J. 27 no. 11 (1995) 1132-1138. 

Typical CP/MAS C-NMR spectra recorded on isolated cellulose I are made up of four signal clusters originating from the six carbons in the anhydroglucose unit. The Cl cluster centered around lOS ppm, the C4 cluster centered around 87 ppm the C2, C3 and CS clusters centered around 73 ppm and the C6 cluster centered around 63 ppm. The fine structure of each signal cluster is due to the supermolecular structure of cellulose I. All chemical shifts reported here are determined by using glycine as an external standard. 

In cellulose I systems widi large lateral dimensions of fibrils and fibril aggregates tihe relative intensity of the AS and IS signals are small, presenting a signal-to-noise problem, riiich makes estimates uncertain. The mathematical formulation of the SPAM (77) also becomes less sensitive at large lateral dimensions. Practical limitations sets an upper bound on the sizes that can be determined by the SPAM at about 50-100 nm. Hence, CP/MAS C-NMR spectroscopy can be considered a tool selective with respect to supermolecular structure. 

To characterize the supermolecular structure of cellulose, the primary structural parameters (type of crystalline allomorph, crystallinity, paracrystallinity and amorphicity, and orientation of nanofibrils, nanocrystallites, and nanoscale non-crystalline domains, as well as porosity of cellulose) should be determined. These structural parameters can affect physicochemical, chemical, biochemical, physical and mechanical properties of cellulose materials. 

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