Enrique M. Valles

The Effect of Network Structure in the Equation of Rubber Elasticity

Abstract Though the stress, strain, and temperature dependence for an ideal rubber is fairly well established, the relation between network structural features like crosslinks, dangling ends, and entanglements and mechanical response is uncertain. The modulus-structure relations recently derived by Miller and Macosko for several types of networks are tested here with a model system: the hydrosilation crosslinking of vinyl-terminated polydimethylsiloxane chains with a tetra-functional silane. Results of shear modulus as a function of extent of reaction and of stoichiometric imbalance are in good agreement with the theory.

Thermogravimetric analysis of starch-based biodegradable blends

SummaryThermogravimetric analysis (TGA) has been shown to be a useful technique to determine the content of starch and other components that are usually present in blends of starch with synthetic polymers. However, some of these blends are composed of elements with an important superposition in their temperature degradation range. In these cases interpretation of the TGA results becomes quite difficult and usually important errors are committed in the determination of blend composition. We present here a method to improve the accuracy of the interpretation of TGA data. The analysis was developed to study blends containing starch, an ethylene-vinyl alcohol copolymer, and water plus glycerin as plasticizers. Using this procedure it is possible to predict blend composition with an error less than 3%, even when there is an appreciable superposition between the temperature degradation range of starch and poly(ethylene-co-vinyl alcohol). As the proposed method of analysis is quite general it may be extended to be used with other types of blends that give thermograms with similar characteristics to those described in this paper.

Small-strain modulus of model trifunctional polydimethylsiloxane networks

Abstract A number of studies to test quantitatively the molecular basis of the theory of rubber elasticity have been performed during the last few years on elastomeric networks of relatively well-known structure. Many of these studies have been done on end-linked networks, for which the number of chains between junctions, their average length, and the functionalities of the junctions could be calculated. Several studies have been with poly(dimethylsiloxane) (PDMS) networks. One of the major controversial points up to now has been the effect of topological interactions between network chains on mechanical properties. Two groups of researchers have come to quite different conclusions about the role of entanglements with respect to their contribution to the elastic modulus. The first group led by Flory considers that the entanglement contribution to the elastic equilibrium modulus is due to the restrictions that topological interactions impose on junction fluctuations among their mean equilibrium position. T...

Hyaluronan-Itaconic Acid-Glutaraldehyde Films for Biomedical Applications: Preliminary Studies.

New hyaluronic acid–itaconic acid films were synthesized as potential materials with biomedical applications. In this work, we explored the homogeneous cross-linking reactions of hyaluronic acid using glutaraldehyde in the presence of itaconic acid and triacetin as plasticizers. Biomechanical properties were assessed in terms of stability by measuring swelling in aqueous environments, investigating wettability using contact angle tests, and evaluating bioadhesive performance. The ductility of the materials was evaluated through stress-strain measurements and the morphology was explored by scanning electron microscopy. The results show that the incorporation of itaconic acid improved most of the desirable properties, increasing adhesiveness and reducing wettability and swelling. The use of triacetin enhanced the strength, bioadhesiveness, and ductility of the material.

THE EFFECT OF NETWORK STRUCTURE IN THE EQUATION OF RUBBER ELASTICITY. II. FURTHER RESULTS

Though the stress, strain and temperature dependence for an ideal rubber is fairly well established, the relation between network structural features like crosslinks, dangling ends and entanglements and mechanical response is still uncertain. Miller and Macosko have recently derived some modulus structure relations for several types of networks. These relations are tested here with a model system: the hydrosilation crosslinking of vinyl terminated polydimethylsiloxane chains with tetra- and tri-functional silane. Results of shear modulus versus extent of reaction and versus stoichiometric imbalance are in good agreement with the theory.

Copolymerization of a polydisperse oligomer with a monodisperse chain extender with intramolecular reaction allowed

We have modeled stepwise, irreversible copolymerizations with intramolecular reaction allowed where one of the reactants is a polydisperse oligomer. A 2 + B 2 systems are studied, where A 2 is the bifunctional chain extender and B 2 is the bifunctional oligomer. Two models are presented. They are both extensions of a previous one where B 2 was assumed to be monodisperse. In both cases B 2 is approximated as the sum ofN monodisperse fractions. In one model, it is necessary to write a system of kinetic equations to describe the A 2 + (B 2 ) 1 + (B 2 ) 2 +... + (B 2 ) N copolymerization where all the B-fractions are simultaneously available for reaction. In the second model, N different monodisperse systems are set up and made to react separate and simultaneously. The species from these N systems are then combined using probability theory. This results in an important reduction in the number of equations needed. The theoretical error involved in this second model is small. Our results indicate that disregard for both polydispersity and ring formation may result in sizable errors in the prediction of average molecular weights.