Celluloses glass transition temperatures

Orientation, wet stretching For plastics whose glass transition temperature (Tg) is above their decomposition temperature, orientation can be accomplished by swelling them temporarily with plasticizing liquids to lower their Tg of the total mass, particularly in solution processing. As an example, cellulose viscous films can be drawn during coagulation. Final removal of the solvent makes the orientation permanent. 

Glassy polymers with much higher glass transition temperatures and more rigid polymer chains than rubbery polymers have been extensively used as the continuous polymer matrices in the zeolite/polymer mixed-matrix membranes. Typical glassy polymers in the mixed-matrix membranes include cellulose acetate, polysul-fone, polyethersulfone, polyimides, polyetherimides, polyvinyl alcohol, Nafion , poly(4-methyl-2-pentyne), etc. 

Figure 5.17 Glass transition temperatures of simple fatty triesters and mixed acetic-fatty triesters of cellulose...

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

Polymer crystallization may also complicate the transition from the amorphous state to the intermediate liquid crystalhne state. Even small crystalline fractions in a polymer prevent free movement of macromolecules This is the case, for example, with cellulose. Cellulose fibres, obtained via cellulose xanthate (viscose), in the process of forming partially crystallize. Although the amorphous fraction in these fibres is large (up to 70-75 %), at a short-term heating of the fibres above the glass-transition temperature (240-260 °C) only slight self-elongation of fibres is observed, which can be attributed to the transition to the liquid crystalline state at the expense of the amorphous fraction... 

Other successful determinations of glass transition temperatures by inverse gas chromatography indude those of Perrault et al. 22) on poly(butadiene), Nakamura et al 26) on cellulose triacetate and Cahigaru et id. 27) on poly(viiq l acetate). 

Excipients can lose quality over time. Oils, paraffins, and flavors oxidize cellulose gums may lose viscosity. Polymeric materials used in film coating or to modify release from the dosage form can age due to changes in glass transition temperature. This can lead to changes in elasticity, permeability, and hydration rate and associated changes in release properties or appear-ance. " ... 

Resultant particles have a high glass transition temperature, which aids in stabilizing the amorphous state. These particles can be compressed into tablets. In one case, tablets containing 150 mg drug were pressed from a 1 1 mixture of the processed drug powder, microcrystalline cellulose and 5% carboxymethylcellulose. 

Sakellariou, P. Rowe, R.C. White, E.F.T. The thermo-mechanical properties and glass transition temperatures of some cellulose derivatives. Int. J. Pharm. 1985, 27,... 

Sakellariou P, Rowe RC, White EFT. The thermomechanical properties and glass transition temperatures of some cellulose derivatives used in film coating. Int J Pharm 1985 27 267-277. Callahan JG, Cleary GW, Elefant M, et al. Equilibrium moisture content of pharmaceutical excipients. Drug Dev Ind Pharm 1982 8 355-369. 

A continuous effect is the decrease in water content and void volume with Increasing temperature. Water is lost from the primary gel during annealing, both because of the formation of virtual crosslinks and because of the decrease in hydrogen bonding and cluster size in the water Itself. An example of a discontinuous effect is the dramatic increase in permselectivity (salt rejection) observed when cellulose acetate membranes are heated above the glass transition temperature 68.6 C. In fact, not one but two... 

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