Influence of Chain Primary Structure and Topology (Branching) on Crystallization and Thermal Properties: The Case of Polysulfides

• Published in: Macromolecules 2019-03, Vol.52 (5), p.2093-2104
• Main Authors: Pérez-Camargo, Ricardo A, d’Arcy, Richard, Iturrospe, Amaia, Arbe, Arantxa, Tirelli, Nicola, Müller, Alejandro J
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.8b02659

We have investigated how variations in primary structure (distribution of monomeric units, chain length) and degree of branching affect the crystallization behavior of polysulfides. In particular, we have used copolymers of crystallizable (ethylene sulfide, ES) and non-crystallizable (propylene sulfide, PS) units, prepared via anionic ring-opening mechanism. PS units interrupt the crystallization of ES sequences; therefore, strong differences in thermal properties are expected between copolymers with a gradient (single addition of ES/PS mixtures) or a semirandom structure (sequential additions, which yield a more constant composition along the chain). Additionally, we prepared different chain topologies: linear, 4- and 8-armed stars, and combs with 10, 15, and 20 arms, in a comparison where each arm had a gradient primary structure and degree of polymerization (DP) of 10, 20, and 30. The influence of these variables (topology, number of arms, and DP) on the thermal properties was studied by polarized light optical microscopy (PLOM), standard and advanced DSC techniques (i.e., self-nucleation and successive self-nucleation and annealing (SSA)), and small and wide angle X-ray scattering (WAXS and SAXS). First, we have confirmed the much higher order obtained in gradient polymers, in comparison to the semirandom ones. Second, we have seen that the type of crowding of polymer chains affected the level of order achievable. In star polymers, an increasing number of arms increased topological restrictions, which in turn decreased crystallization and melting temperatures, crystallinity, and lamellar thickness. For combs, which are characterized by a more parallel than convergent crowding of the chains, the increase in arms number did not produce a significant decrease in crystalline order; on the contrary, the higher density of chains caused moderate increases in crystallization and melting temperatures, crystallinity, and lamellar thickness.