Crystallization kinetics of isotactic polypropylene

Abstract

Crystallization kinetics of Hercules polypropylenes were studied between 133 and 155^°C. using dilatometers. The crystallization rates of these samples became too rapid below 133^°C. for accurate dilatometric measurements. One sample (A) was a commercial sample containing stabilizer and 9% extractable polymer (with boiling n‐heptane). B was an experimental extracted sample containing no stabilizer. A portion of B was thermally degraded under vacuum at 350^°C. for 7 hr. (C). The DP_n′s of A, B, and C were 2300, 7300, and 1200, respectively. Their melting points were determined dilatometrically, under conditions approaching equilibrium, and were 175, 176, and 173^°C., respectively. 95% of the crystallization can be described by an Avramitype equation with an n value of 3, indicating a three‐dimensional growth with residual nucleation or a two‐dimensional growth with spontaneous nucleation. The heterogeneities may result from catalyst residues. The crystallization rate of A was lower than that of B or C at a given temperature, possibly due to the presence of atactic polymer. At a given temperature C crystallized faster than B, which may be an effect of the difference in molecular weight. Plotting the rate as a function of the degree of undercooling (ΔT) increases the difference between B and C.