Glasses have been the subject of a great deal of interest in the last fifty years. Models of structure recovery have provided an adequate understanding of the metastable behavior of glasses. However, these models are known to provide a very incomplete picture making glasses an area where understanding still lacks severely. Most of the work in this area has centered on following the recovery towards equilibrium of macroscopic properties such as volume and enthalpy after various thermal histories leading the samples to the glass phase. Recent studies have shown that the effect of size on the glass transition can be followed up to the nanometer scale.(1) During my master’s and doctorate work, we developed a methodology using infrared spectroscopy and molecular probes to quantitatively evaluate the presence of rigid amorphous regions in semi-crystalline polymers. Changes in thermal history can lead to variations in the relative phase composition of the sample. This implies that the evolution of glasses could possibly be followed using molecular probes. We use infrared imaging techniques to follow the changes in molecular probes during the structural recovery of glasses. Variations in probe position as well as conformational changes in the probe could lead to better understanding of the glass transition. This should lead to improved models of structure recovery and better predictive ability.
1. McKenna G. B. Journal de physique IV, 2002, 10 (P7), 53.
Epoxy resins have a wide range of applications including adhesives, coatings and encapsulation of electronic compounds. Contributing to their versatility is the ability to cure the low viscosity reactants to a desired form on site. In order to optimize the properties as well as the processing time, knowledge of the effect of the cure schedule on the final properties is essential. For example, it was shown that residual stresses could be avoided by reducing the heterogeneity of the temperature field during cure (1). The properties of epoxy resins are related to the network morphology which is itself determined by the mechanism and kinetics of cure. Thermal analysis methods can directly measure the heat evolved during reaction. The challenge then consists in linking this information to the detailed reaction mechanism and kinetics. Molecular spectroscopy, particularly in the infrared region of the spectrum, has been used successfully to determine the structure of epoxy resins. Very few studies, however, use in vivo determination of the resins structure as it cures. In fact, the handful of studies using hybrid instruments combining differential thermal analysis with infrared spectroscopy has mostly centered on inorganic components. The recent development of nanocalorimetry (2) makes it possible to precisely measure and control cure history while in a standard infrared spectrometer. I plan to study structure-property relationships in epoxy resins by using a novel version of hybrid FTIR-DSC involving a nanocalorimeter. The cure kinetics will be monitored simultaneously with the structural or chemical changes. The degree to which the resin structure are sensitive to the cure history as well as to polymer type and impurity levels such as water content will be investigated.
1. Rozenberg B. A. Fibre Science and Technology, 1983, 19, 77.
2. Merzlyakov M. and Shick C. “Integrated circuit thermopile as a new type of temperature modulated calorimeter”, Proceedings of the 30th North American Thermal Analysis Society,September 23-25, 2000, Pittsburgh, PA, p 714
All biopolymers such as starch and cellulose have structural and physical properties that are strongly related to their water content. Although the role of water is still not well understood, it is thought that it may modify the mobility of polymer chains and influence the degree of entanglement.(1) In addition, the crystalline content of the polymer as well as the molecular conformations of the chains are related to the pH and hence the amount of water. Since in addition to the quantity of water, the origin of the biopolymer has a bearing on the chain morphology, it is difficult to make a comprehensive study of the effect of water content. Studies that have approached the problem have mostly been centered on the relationship between physical properties and water content, without addressing the biopolymer water interaction. During my doctoral research, I have studied the phase content of amylose, a major component of starch, using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). We found that although the water content was important in determining the phase content, of more significance was the fact that the way the water was absorbed by the sample had an even greater influence on the phase content. However, the water-biopolymer interactions were not further explored. We investigate these interactions using FTIR spectroscopy combined with DSC and thermogravimetric analysis to measure the water-related weight loss. Using these methods, for a systematic sampling of starches and cellulose from different sources, the chemical interactions between water and polymer chains will be investigated. Since it is expected that a major effect of water molecules on the polymer chains will be the changes in molecular conformations the comprehensive study of the water-biopolymer interactions will play an important role in determining the phase composition of starches and cellulose and by the same way, their physical properties. determination of these will yield improvements in the usefulness of these materials.
1. S. H. D. Hulleman, F. H. P. Janssen and H. Feil, Polymer, 1998, 39(10), 2043.
Recently, a great deal of interest has been invested in the production of biodegradable polymers. Although the general consensus is that the wide use of such polymers would be beneficial to all, the present production of biodegradable polymers is still low compared to overall polymer production. This is because other commodity plastics are both inexpensive and familiar to the consumer. To make biopolymers competitive, it is imperative that they have physical properties similar to those of the synthetic polymers to be replaced. (1) A major point of research interest is the control of the overall strength of biodegradable polymers. This can be attempted in numerous ways, either by mixing biodegradable material with lower molecular weight synthetic polymers, stiffening the polymer chain by substituting some aliphatic diacid building blocks with more rigid aromatic diacids or by increasing inter-chain bonding for example with hydrogen bonds or covalent bonds. These methodologies have encountered some degree of success but due to a lack of understanding of the structure-property relationships, are far from competitively threatening synthetic polymers. In recent investigations we have studied the thermal and rheological properties of blends and polymers as a function of entanglement concentration. We have shown that although the glass transition did not depend on degree of entanglement, physical properties varied with many other factors including the molecular weight and thermal history of the polymer, making predictive determinations difficult. I wish to investigate the structure-property relationships of biodegradable polymers, including blends of starch and synthetic-polymer with an aim of predicting polymer strength. Investigation of the physical interaction between the polymeric chains will be done using thermal degradation (TGA), component analysis (FTIR) and dynamic mechanical analysis (DMA). My goal is to develop a methodology that would enable the prediction of the overall strength of a biodegradable polymer from the blend components and the thermal history.
1. D. L. Kaplan, E. Thomas and C. Ching, Biodegradable Materials and Packaging, Technomic Press, Lancaster, PA, 1993, pp 411.