Isothermal crystallization kinetics in bulk of olefin-based multiblock copolymers

Abstract

Isothermal crystallization kinetics of ethylene/1-octene (C2/C8) multiblock copolymers synthesized by chain shuttling technology is investigated. The samples are a reactor blend of segmented chains characterized by alternating crystalline and amorphous blocks with C8 content of 0.5 and 20 mol%, respectively, and statistical distribution of block number/chain and block length. The analysis is carried out after complete removal of a fraction (5–12 wt%), namely consisting of C8-rich blocks, through Kumagawa extraction with boiling diethyl ether. The resultant diethyl ether-insoluble fractions have similar average content of C8 units (≈13–14 mol%) and of crystalline blocks (≈23–27 wt%) but different molecular mass (the number average molecular mass M n is ≈ 60–70 kDa for the samples 1,2 and ≈38 and ≈21 kDa for the samples 3 and 4, respectively). An additional sample with M n ≈ 93 kDa, but a greater average content of C8 units (≈15 mol%) and a smaller content of crystalline blocks (15 wt%) is also analyzed. The crystallization half time of the samples increases with increase of M n and, for each sample, its logarithm increases linearly with a decrease of the undercooling by a factor of -0.155/°C, for the samples 1–4 and −0.031/°C, for the sample 5. Using the classic kinetic crystallization model by Lauritzen and Hoffman, values of energy barrier constant due to contributions from primary nucleation K N and crystal growth K G are extracted. The K N contribution is esteemed to amount to ≈34% of the total barrier assuming regime II for the sample 5 and regime III (or I) for the samples 1–4, to ≈34% for the sample 5 and 67% for the samples 1–4, assuming regime II for all the samples. In all the cases, regardless of the assumed regimes, the K N values of the sample 5 are lower than those of the samples 1–4. As a final remark, the implications of crystallization kinetics on the solid-state morphology are also discussed, considering that transmission electron microscopy (TEM) images present a partially mesophase separated morphology for the samples 1,2, and 5 and a classic lamellar morphology for the samples 3 and 4.