CELL 180 |
| The process of nucleation is essential to development of ordered or crystalline domains in poloymeric materials. With thermo-plastic ploymers, nucleation occurs after the melt is cooled down below the melting temperature. When the polymer is coagulated from solution, the nucleation occurs as soon as the polymer is precipitated from solution. Cellulose nucleates in the cellulose II form when regenerated at ambient temperatures; it has therefore been assumed to be the most stable form. Our observations, however, show that when regeneration is carried out at elevated temperatures, cellulose also nucleates in the cellulose I form. We therefore believe that cellulose I is indeed the more stable form. In past observations we have reported that the Raman spectra of cellulose IV, which is the form precipitated at elevated temperatures, are almost identical to linear combinations of the spectra of celluloses I and II. We previously interpreted our observations as indicating cellulose IV to be a mixed crystal with both cellulose I and cellulose II conformations coexisting in the same lattice. We now believe cellulose IV to consist of nanoscale nuclei of both celluloses I and II coexisting as would the nanoscale nuclei of block copolymers. We will present Raman spectra of cellulose IV prepared in the classical manner, that is, heating cellulose II in glycerol to 250 degrees C, that show it to consist of nanodomains of celluloses I and II. Based on recent analyses of structures of native celluloses we will show that the difficulties in regenerating cellulose I from solution are associated with the long period (300 to 2400 nm) helical structure of native celluloses. Furthermore, the spectra of the cellulose forms prepared in the classical manner clearly indicate that cellulose II cannot have an anti-parallel structure. |
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Structure and Properties of Cellulosic Polymers, Assemblies, and Nanocomposites
8:00 AM-11:45 AM, Tuesday, April 8, 2008 Morial Convention Center -- Rm. R07, Oral
Division of Cellulose & Renewable Materials |