Gamini Samaranayake

Novel cellulose derivatives. iv. Preparation and thermal analysis of waxy esters of cellulose

Cellulose esters with linear aliphatic acyl substituents ranging in size from C12 (lauric acid) to C20 (eicosanoic acid) were prepared in homogeneous solution (DMAc/LiCl) using a novel synthetic method based on the use of a mixed p-toluenesulfonic/carboxylic acid anhydride. The resulting waxy cellulose esters had a high degree of substitution (DS), between 2.8 and 2.9, and showed little degradation. Thermal analysis of these cellulose derivatives by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) revealed a series of transitions that represented motion by both ester substituents and cellulosic main chain. Broad crystallization and melting transitions attributed to side-chain crystallinity were observed in the range between −19 and +55°C; these side-chain Tm and Tc transition temperatures increased by 10°C per carbon atom of the ester substituent. The Tg of these derivatives increased linearly with increasing substituent size from 94°C for C12 (cellulose laurate) to 134°C for C20 (cellulose eicosanoate). Evidence of “main-chain” crystallization was not observed for these samples, except in the case of peracetylated C12 and C14 esters, which had Tm values of 96°C and 107°C, respectively.

Cellulose derivatives with low DS. I. A novel acylation system

The acylation of cellulose in homogeneous phase (2% concentration in N,N-DMAc/LiCl-9%) with acid anhydrides and carboxylic acids was accomplished using N,N-dicyclohexylcarbodiimide (DCC) and 4-pyrrolidinopyridine (PP). This activation mixture assures retention of homogeneous phase by keeping all components in solution. DCC serves as sacrificial activator in the reaction with acid anhydrides and carboxylic acids, making it possible to achieve efficient modification with only a modest excess of reagent (0 to approximately 50% based on DS). DCC may be recycled and regenerated. The method was used to prepare cellulose propionates to stearates with DS ranging between <0·1 and 2·5. Both type and degree of substitution was found to influence solubility. The OH groups in positions C6 and C2 were favored to an equal degree over those in C3, and this varied from around 1 (i.e. equal reactivity as C3) to 3 depending on DS.

Chitin derivatives. I. Kinetics of the heat-induced conversion of chitosan to chitin

The water-soluble solids comprised of the ionic complex between chitosan and acetic acid, chitosonium acetate, are converted into chitin by heating. The thermally-induced conversion of a water-soluble chitosonium acetate in film form into a water-insoluble chitin film was examined by thermal analysis (DMTA, TGA, DSC, and TMA) and by solid state 13C-NMR spectroscopy. Results indicate that tan δ-transitions occur at increasingly high temperatures, and over progressively wider temperature ranges, as the transformation progresses. Likewise, the storage modulus, log E′, increases as the chitosonium acetate film undergoes “cure” and converts to chitin. Cure kinetic parameters are obtained using the model proposed by Provder et al. modified for glass transition temperature (Tg). The results suggest the existence of two sequential first order reactions, an initial and a late cure reaction, having activation energies of approximately 15 and 21 kcal/mol, respectively.

Novel cellulose derivatives. V. Synthesis and thermal properties of esters with trifluoroethoxy acetic acid

Esters of cellulose with trifluoroethoxy acetic acid (TFAA) were prepared in homogeneous phase using a mixed anhydride with p-toluenesulfonic acid. Esters with low degree of substitution (DS), and with DS rising from 0 to 3, had hydrophobic character that prevented the usual association with moisture, which is otherwise typical of cellulose esters with low DS. Cellulose trifluoroethoxy acetate (CT) had T g s declining by about 40 °C per DS-unit (from 160 to 41 °C) as DS rose from 1 to 3. Mixed esters, cellulose derivatives with acetate and trifluoroethoxy acetate substituents (CAT), exhibited glass-to-rubber and melting transitions by DSC. A linear relationship between both T g and T m with respect to DS was recorded with the T g and T m separated by 30° to 40 °C. This is consistent with cellulose esters described elsewhere. Surprisingly, the T g s of CT and CAT were found to be identical when the DS was equivalent to the DS of the fluoro substituents (DS F ).

Cellulose derivatives with low DS. II. Analysis of alkanoates

The determination of the degree of substitution (DS) of cellulose alkanoates representing a wide range of ester types (acetate (C-2), to stearate (C-18), with a DS of 0 to 3) was performed by 1H-NMR spectroscopy and aminolysis/gas chromatography. Poor solubility combined with complex substituent signals preclude proton NMR spectroscopy as a method of analysis when the DS of alkanoates falls below one. This is true for all ester-derivative types, especially for the derivatives with bulky substituent groups. By contrast, the aminolysis/gas chromatography method is independent of both solubility and DS, and thus represents the method of choice for cellulose esters which have low degrees of substitution and bulky substituent groups.