Crystallization of trans 1, 4 polyisoprene morphology and kinetics.
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A detailed electron microscopic study of the morphology and kinetics of crystallization of trans-1,4-polyisoprene with cis isoprene units varying from Of to 19.8 has been carried out. Two crystal structures of trans3., 4-polyisoprene (LIS and in p) were generally observed. In an unstrained film, lamellar single crystals which developed into hedrites, sheaves and spherulites of both crystal modifications were observed. The morphology attained was determined by the growth rate and the nucleation density of the crystals. Electron diffraction study suggests that the lamellar crystals have a chain-folded configuration. In a strained film, a row nucleation morphology was observed LIT and FIMP lamellar crystals were observed to nucleate from the same central 'backbones'. The row nucleation density of the lamellar crystals increases with strain. The growth habits of the lamellar crystals in, both strained and unstrained films were dependent on the film thinness, and on the proximity and the orientation of the neighbouring crystals. Models for the observed growth habits were discussed. The lamellar thickness data agrees with the kinetic theory of chain folding. The lamellar thickness of both crystal structures increases with decreasing degree of supercooling. The fold surface free energies estimated were 45.1 x 10-3 J/n2 and 60.1 x 10-3 J/m2 for the LIT and F ULF crystals of gotta percha respectively. The growth kinetics data of the lamellar crystals determined agrees with the existing theories of crystal growth. At the range of temperature from 400 to 560, the HMF crystal has a faster growth rate than the LMF crystallized at the same temperature. At the same degree of supercooling, the LMF crystal has a faster growth rate. The growth rates of both the LMF and HMF crystals decrease as the cis content on a trans polymer chain increases. With 66 cis units, the growth rate of the isomerized specimen decreased to a value of 1/6 to 1/8 of that for gutta percha (0f cis), whereas in cis-polyisoprene, the growth rate is retarded by 100 times with the inclusion of trans isoprene unit. This, difference in behaviour is discussed in terms of the possibility of including non-crystallizable units in the crystal lattice. The growth rate analysis showed the importance of determining the equilibrium melting temperature and glass transition temperature for each isomerized specimen to obtain meaningful estimates of
AuthorsLong, Ong Eng
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