SynopsisThe mechanism of the low profile behavior in certain single-phase polyester systems has been examined. It appears that as polymerization occurs, crosslinked beads (
SynopsisThe mechanism by which low profile character is achieved in unsaturated polyester systems has been investigated. Using techniques of microscopy, data are developed that support a model in which micro stress cracking is induced to relieve stress promoted by polymerization shrinkage. Thus the strain is accommodated internally in a molded part, rather than through macroscopic shrinkage of the article.
Solution and solid‐state proton decoupled 13C‐NMR spectra were determined on two diimides derived from 4, 4′‐oxydiphthalic anhydride. Comparison of the individual diimide spectra to that of a mixture of the two diimides indicates that ordering of these materials occurs in the solid state via charge transfer complex formation. A similar study was conducted using two isomeric dianhydrides, 4, 4′‐isophthaloyldiphthalic anhydride (IDPA) and 4, 4′‐terephthaloyldiphthalic anhydride (TDPA). The solution spectra of these compounds are similar and are those which would be expected for these compounds. However, their solid state spectra differ from each other. The solid‐state spectrum of TDPA resembles its solution spectrum, whereas, that of IDPA differs greatly from its solution spectrum and indicates charge transfer complex formation occurs with this molecule. This difference is explained in terms of the stereochemistry of the two isomeric dianhydrides.
The interatomic distances for crystalline pyromellitic dianhydride (PMDA) were calculated from x-ray crystallographic data. The CP-MAS I3C NMR spectrum of PMDA was found to he consistent with and accountable for in terms of these distances after literature assignment errors were corrected. An anomalous peak was observed in the differential scanning calorimetry thermogram of PMDA. The entropy change associated with this transition indicates that a two-fold change in symmetry occurs approximately 60°C below the melting point of PMDA. Attempts to observe this transition by CP-MAS "C NMR were unsuccessful.
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