Comparative analysis of phase behavior of ethylene/alk-1-ene statistical multiblock copolymers

The phase behavior of ethylene/alk-1-ene statistical multiblock copolymers (OBCs), synthesized via chain shuttling copolymerization, is investigated. These copolymers consist in the alternation of amorphous (soft) blocks with high alk-1-ene content and crystalline (hard) blocks with low alk-1-ene content and are characterized by significant inter- and intra-molecular constitutional heterogeneity. The alk-1-ene content in the soft and hard blocks is ≈ 20 and less than 0.5 mol%, respectively. Samples incorporating various comonomers (hex-1-ene, oct-1-ene, 4-methyl-pent-1-ene, hexadec-1-ene) and with different soft-to-hard block weight ratios (80/20, 65/35, 50/50) exhibit diverse solid-state morphologies, influenced by hard block crystallization. Rheological analysis in the melt reveals mesophase separation through time-temperature superposition (TTS) principle failure at low frequencies. Six key descriptors are identified to characterize phase behavior: (i) average number molecular mass (Mn); (ii) molecular mass of the longest hard-soft units; (iii) crystallization temperature (Tc); (iv) segregation strength (χN), given by the product of the interaction parameter χ of Flory and the number of monomers in the long hard-soft units N; (v) degree of morphological heterogeneity (Dh); (vi) the exponent (n) marking the low-frequency behavior of elastic modulus. These parameters are visualized in a Kiviat's diagram to assess mesophase separation, domain structure, and enable quantitative comparison across different OBC compositions. The approach can be more general, as it can be extended to other complex systems showing tendency toward mesophase separation, allowing for finding useful information to study structure-properties relationships.