Concepts of Nucleation in Polymer Crystallization

Abstract

Nucleation plays a vital role in polymer crystallization, in which chain connectivity and thus the multiple length and time scales make crystal nucleation of polymer chains an interesting but complex subject. Though the topic has been intensively studied in the past decades, there are still many open questions to answer. The final properties of semicrystalline polymer materials are affected by all of the following: the starting melt, paths of nucleation, organization of lamellar crystals and evolution of the final crystalline structures. In this viewpoint, we attempt to discuss some of the remaining open questions and corresponding concepts: non-equilibrated polymers, self-induced nucleation, microscopic kinetics of different processes, metastability of polymer lamellar crystals, hierarchical order and cooperativity involved in nucleation, etc. Addressing these open questions through a combination of novel concepts, new theories and advanced approaches provides a deeper understanding of the multifaceted process of crystal nucleation of polymers.

Self-Seeding Procedure for Obtaining Stacked Block Copolymer Lamellar Crystals in Solution

“…For a constant T C , we can assume that the mean thickness of the platelets h = η•h 0 consisting on average of a number η of lamellar crystals with a thickness h 0 each, was constant, i.e., independent of T SS . Thus, the mass (m C ) of an individual platelet is proportional to its volume and the density of the polymer (ρ ≈ 1 g/cm 3 ). Therefore, the amount of polymer in solution above the solubility limit, deduced from the total volume per area of all the crystals obtained for various conditions, can be approximated by V C ≈ N•L 2 •η•h 0 , where η represents the average number of lamellae in a stack.…”

“…However, at temperatures close to the melting point this process is extremely slow and is often competing with heterogonous nucleation through foreign substances (nucleating agents, surfaces, or “dirt”) [ 1 , 2 ]. Furthermore, homogeneous nucleation is a statistical process, which continuously initiates (with a decreasing probability in time) the growth of additional crystals [ 3 ]. Correspondingly, after a given crystallization time, the resulting crystals will have a distribution in size.…”

“…At concentrations below the solubility limit, the polymer solution is homogenous and no crystalline structures form [ 15 ]. Above the solubility limit, polymer–polymer interactions become more frequent allowing the formation of ordered structures, which, however, typically is accompanied by a nucleation barrier [ 3 ]. Crystals only form when this energy barrier for nucleation is overcome.…”

Influence of crystal orientation on stretching induced void formation in poly(4‐methyl‐1‐pentene) investigated by in‐situ small‐angle and wide‐angle X‐ ray scattering

“…“Memory effect” refers to the phenomenon that the polymer melt containing clusters of molecular segments with a more ordered conformation than the equilibrated random coil will crystallize faster due to a depression in free energy change for nucleation 43,44 . When the semicrystalline polymer is heated to a temperature close to its nominal melting temperature or well below the equilibrium melting point, the “memory effect” will be triggered 45,46 . As T h was higher than 236°C, f H began to decrease and reached 0 when T h was 250°C due to that the majority part of the previously existed crystals was destroyed and lost the orientation nature, although the residual part can still serve as nuclei to increase the crystallinity of the sample (see Figure 4C).…”

Homogeneous Nucleation of Sheared Liquids: Advances and Insights from Simulations and Theory

“…Where the microscopic motions show a tendency towards colloidal aggregation or secondary structure, the CNT model fails. In particular, conventional CNT typically cannot be applied without further extensions to glassy liquids and polymer systems, which are prone to ageing and 'slow' rearrangement of molecules or chains [76,77].…”