Origin of Lamellar Stack Structures in Dilute Semicrystalline Polymers: Role of Entanglements and Tie-Molecules

To achieve better performance and processability of semicrystalline polymers [especially with ultrahigh molecular weight (MW)], miscible oligomers or solvents are usually involved to disentangle polymer melt and reduce its viscosity. Entanglements, crystallization temperature, and possible phase segregation behavior make it complicated to predict the morphologies of polymer crystals containing low MW fractions. In our recent work [Proc. Natl. Acad. Sci. U. S. A. 2023, 120 (27), e2217363120], by blending high-MW polycaprolactone with its unentangled oligomers, we elucidated that at a relatively low crystallization temperature (Tc), the thickness of amorphous regions is controlled by entanglement strand density, while the lamellar thickness remains nearly identical. The present work further investigates the morphology of semicrystalline polymers containing a certain amount of molten short oligomer chains at higher Tc (53 °C) as determined by solid-state nuclear magnetic resonance free-induction decay and double quantum measurements. The amorphous layer thickness detected from small-angle X-ray scattering experiments is found following a similar trend as that crystallized at 35 °C even with a strong molecular segregation effect. The inner part fraction of an amorphous region obeys a similar power law for polymer concentration regardless of the value of Tc, which is the same as the plateau modulus from the melt rheological measurements. On the other hand, the thickness of the boundary part of the amorphous region at the vicinity of crystals increases with the increase of Tc. Our results strongly suggest that long polymer chains tend to form crystalline lamellar stacks, which are connected by intercrystalline links (e.g., tie chains and entanglements), while oligomer chains are expelled into larger spaces in between.