Crustallization and structure of microbial polyesters, carbohydrate-based polyamides and n-polyurethanes

Resumen

This thesis describes a detailed investigation on three different clases of polymers known as polyesters, polyamides and polyurethanes. The main focus is the study of their crystallization behavior and solid state structure. Poly(B,L-malic acid) (PMLA) is a polyester composed of malic acid units that are linked by an ester bond formed between the hydroxyl group and the carboxyl group located at the beta position. This polymer is highly water-soluble, biodegradable and nontoxic. The first part of this thesis (chapters 1 and 2) is focus on the study of poly (a-methyl, B-malate) of different molecular weights prepared by methylation of the poliacid with diazomethane under different conditions. Their synthesis and molecular characterization is described, as well as a detailed study of their thermal properties and isothermal and non-isothermal crystallization. Its crystal structure was preliminary examined. Two crystal forms were identified by X-ray diffraction, their occurrence being dependent on crystallization conditions. Carbohydrate-based linear aliphatic polyamides PA-nSu (chapter 4 and 5) are the second class of polymer studied. In the chapter 4, three series of sugar-derived polyamides (PA-nSu) made from the L-arabinaric, D-mannaric and galactaric acids, respectively, and a,w-alkanediamines containing from 6 to 12 methylene units were analized. 10 different polyamides were studied by DSC, TGA and polarized optical microscopy under isothermal and non-isothermal crystallization from the melt. Crystallization half-time indicated that "crystallizability" of PA-nSu increases with the number of methylenes in the diamine unit and decreases with the length of the carbohydrate-derived unit. The relation between thermal data and the configuration of the sugar moiety present in the PA-nSu was discussed. In the chapter 5, the pair of stereoisomeric polyamides PA-6DAr and PA-6LAr, as well as the racemic stereocopolyamide PA-6DLAr were synthesized from hexamethylenediamine and 2,3,4-tri-O-methyl-arabinaric acid. A detailed study of the crystallization and crystal structure was carried out by X-ray, polarized optical microscopy, TEM and DSC. The all three polyamides adopted the same crystal structure, which consists of a rhombic lattice with the chains in a highly contracted conformation capable of accommodating efficiently the sugar moiety in the space. The Avrami kinetics analysis revealed that copolyamide PA-6DLAr crystallized isothermally much slower than the optically homogeneous polyamides. The last chapters (6, 7, 8 and 9) of this thesis are dedicated to the study of the n-polyuretanes. Linear polyurethanes (PUR) constitute a versatile class of polymers of great industrial interest and with a broad range of applications. The chemical structure of polyurethanes is intermediate between those of polyamides and polyesters but their properties are significantly different. In the chapter 6 a detailed crystallization study of the linear n-polyurethane (n-PUR) family for n ranging from 5 to 12 was carried out by DSC supported by polarizing optical microscopy. The study embraces crystallization of all the n-PUR under both nonisothermal and isothermal conditions. The odd and even series of n-PUR defined by the parity of the number of methylenes (n) contained in the polymer repeating unit are considered and separately analyzed. All the members of the two series showed a thermal behavior consistent with their chemical constitution. Isothermal crystallization data were analyzed by the kinetics Avrami approach which revealed that the "crystallizability'' of n-PUR increases steadily with the flexibility of the polyurethane chain. In the chapter 7, the crystal structure and morphology of a series of n-PUR have been investigated by using TEM combined with X-ray and electron diffraction analysis. A crystal structure similar to that described for the layered a-form of nylons seems to be commonly adopted by these polyurethanes. It essentially consists of a triclinic lattice made of hydrogen-bonded sheets with chains in fully extended conformation. In chapter 8, a comparative study of the nylon 12 and 10-polyurethane were performed based on their crystal structure and morphology. Beside their similar chemical structure nylon 12 crystallized in either a-form or y-form. On the contrary, 10-polyurethane invariably crystallized in the a-form. Finally, in chapter 9, an overview on the synthesis, structure and crystallization of 12-polyurethane was presented.