Increase In Degree Of Crystallinity Research Articles

Gutta. II. Interconversion of alpha and beta forms

AbstractThe gutta isolated from chicle has been demonstrated to be structurally identical with trans‐polyisoprene from gutta percha and balata but has a molecular weight of 16,000–18,000, about one‐half that of the latter two polymers. Its ease of purification and marked crystalline behaviour suggested its use in a study of gutta polymorphism.The α‐and β‐forms of chicle gutta were prepared and their identities were established by means of x‐ray diffraction measurements and melting properties. The degree of crystallinity and internal molecular strain were shown to affect the melting points of the two forms of gutta through wide ranges. Interconversions of α‐and β‐guttas were effected by means of heat conditioning, solution and reprecipitation, melting and resolidification, and treatment of the gutta with nonsolvents. Dilatometric data were used to describe the change of specific volume when β‐gutta was converted to α‐gutta in a conditioning bath as the temperature was increased stepwise from 30°C. to 80°C. at the rate of 1°C. per hour. The discontinuous, saw‐toothed nature of the specific volume vs. time curve was rationalized on a basis of reactions involving changes in degree of molecular order and relief of strain within the gutta. The curve suggested the following gross reactions: 30–36°C., thermal expansion of β‐gutta of low crystallinity; 36–52°C., increase in degree of crystallinity of β‐gutta; 52–60°C., melting of β‐gutta; 60–64°C., transition of β‐ to α‐gutta; 64–72°C., melting of α‐gutta; 72–80°C., thermal expansion of molten gutta. α‐Gutta, in the melted condition, was shown to undergo a transition, above 72°C., to a form whose stereo identity has not yet been established. The rate of transition from β‐to α‐gutta at temperatures at which α‐is the stable form, appeared to be hastened in the presence of certain liquids.

Pseudostructures in Electron Microscope Specimens

Pseudostructures, which are not real properties of electron microscope specimens but are introduced by the action of the electron beam, are described. Examples are given from samples of tetracopper calcium oxychloride, precipitates from slowly hydrolized ferric chloride solutions, and tungsten oxide to illustrate some of the effects which may occur. The polymorphism of these substances, particularly of tungsten oxide, is discussed. The major effects which contribute to the pseudostructure formation are melting, evaporation, change of crystalline states, increase in degree of crystallinity, and migration of material. Reasons are given for concluding that the primary cause of specimen changes is to be found in the heating effect of the beam. The possibilities of secondary causes are also considered. Artifacts may also be introduced into bacterial specimens. Observed changes in such samples are described briefly and examples are given.