Glass formation from low molecular weight organic melts
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Glass formation from the melt of organic monomers was studied for a variety of different organic molecular structures with Tg near ambient temperature. Crystallization is suppressed by one or more of the molecular properties, hydrogen bonding, interlocking, dipolar, and hydrogen bonding, combined with hindered rotational isomerism. Examples of materials in each category are presented for illustration. The viscosity of undercooled liquids was characterized by the Vogel-Tammon-Fulcher (VTF) equation, r) = Aexp[DT0/(T - To)], where A, D, and To are experimentally determined parameters. Our experimental D values are discussed in relation to the molecular structure and glass formation mechanism. The insight provided by our interpretation is intended to assist in the design of new molecular structures with controlled viscosity-temperature characteristics, as well as glass-forming ability by cooling from melts.
I. INTRODUCTION Glass formation from cooling the melt is a wellestablished technique in the area of inorganic composition. This technique involves melting of appropriate compositions and cooling them with little or no crystallization. Many theoretical treatments were given to the glass formation from the melt. The kinetic approach was proposed both by Turnbull and Uhlmann.1'2 Turnbull suggests that a glass is formed by avoiding a single nucleation event because the nucleation process precedes the crystal growth.1 Uhlmann suggested that a glass is formed by avoiding crystallization less than the allowable amount of crystallinity.2 Although this is an arbitrary amount, the volume fraction of 10~6 has been considered as an allowable amount. While Turnbull's model is a more restricted model than Uhlmann's. Uhlmann considered both nucleation and growth on cooling melts. Several structural models have been proposed to describe glass formation. One model is the random network model of inorganic silicate glass. This was well reviewed in the textbooks.3-4 The structural models of glass-forming organic monomeric melts were also proposed previously. Magill and Ubbelohde suggested a high concentration in the melt of clusters of interlocked molecules whose structure precludes the orientation required for crystallization.5 More recently, Molaire and Johnson and Alba et al. suggested a physical mixture of isomers of monomers to avoid crystallization.67 The purpose of this study was to identify the mechanism allowing formation of a glass from cooling organic monomer melt, and to study the effect of a diluent on the glass transition temperature and undercooled liquid viscosity. In this note, we present glass-forming properties of monomeric organics with various types of structures. 2128
J. Mater. Res., Vol. 10, No. 8, Aug 1995
http://journals.cambridge.org
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Our results are discussed in terms of crystallization kinetics theory. Examples are given to show features of different mechanisms that allow glass formation. These include network and a new mechanism of effective mixtures due to hindered rotational i
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