Morphological Characteristics of β-Nucleating Agents Governing the Formation of the Crystalline Structure of Isotactic Polypropylene

Abstract

The effect of the morphological characteristics of N,N′-dicyclohexylterephthalamide (DCHT), an aromatic bisamide β-nucleating agent (β-NA) showing a dual nucleation capability for the β- and α-crystals of isotactic polypropylene (iPP), on the polymorphic composition of iPP and the underlying mechanism were systematically investigated. DCHT crystals differing in their size and distribution state in the iPP melt and recrystallized morphology were obtained by modulating the final heating temperature (T f), subsequent cooling rate (V c), and content of DCHT. The crystalline structures formed on the different surfaces of needle-like DCHT crystals were clearly identified. For the iPP/0.1 wt % DCHT samples, the relative fraction of β-crystal (K β value) in the iPP sample nucleated with small-sized needle-like DCHT crystals is much higher than that in the iPP sample nucleated with large-sized needle-like DCHT crystals. This increase is primarily because there is less space for the crystallization of the iPP melt into α-spherulites arising from a more homogeneous distribution of small DCHT crystals in the iPP matrix. Upon cooling at the same V c, dendritic DCHT crystals tend to induce more β-crystals than needle-like ones, especially at a high V c, which is predominantly due to an increase in the relative density of β-nuclei to α-nuclei (N β/N α). Interestingly, it was found that DCHT crystals crystallize into microcrystalline agglomerates in the subsequent cooling process when completely dissolved in the iPP/0.05 wt % DCHT melt. More interestingly, the K β value in the iPP sample reaches as high as ∼1.0 during isothermal crystallization at 135 °C, indicating that DCHT microcrystalline agglomerates unexpectedly exhibit a specific nucleation selectivity for β-crystal. The present investigation elucidates the significant impact of the morphological characteristics of DCHT on the polymorphic composition of iPP, providing a promising way to tune the microstructure and macroscopic performance of iPP materials.

Temperature Effects on the Crystalline Structure of iPP Containing Different Solvent-Treated TMB-5 Nucleating Agents

“…The polymorphic phenomenon is commonly found in semicrystalline polymers resulted from the different conformations and/or packing modes of molecular chains in the crystal cells [ 1 ]. The final crystalline modifications and crystalline structures depend on both the structure of molecular chains and the external crystallization conditions (e.g., the thermal conditions, the mechanical conditions, and the addition of nucleating agent, etc.).…”

“…Due to the better toughness, improved heat resistance [ 1 ], high impact and tear strength [ 7 ] and improved elongation at break [ 8 ], β -iPP has attracted much attention in both academic and industrial research. Some researchers found that the Izod impact strength, flexural modulus and heat distortion temperature of an injection molded polypropylene increased with the β -crystal content within a certain range [ 9 ].…”

Highly Efficient Supramolecular Nucleating Agents for Poly(3-hexylthiophene)

“…Heterogeneous nucleation, in general, is highly desirable because it controls the nucleation density during polymer processing. , As a result, nucleation reduces the average crystal size and size distribution and, in exceptional cases, allows to induce a specific crystal modification. Hence, heterogeneous nucleation can significantly alter the final property profile, including mechanical, electronic, and optical properties. ,− …”

The influence of melt status and beta-nucleation agent distribution on the crystallization of isotactic polypropylene

“…24 Some agents were even synthesized specifically to further increase β-nucleating efficiency. [25][26][27][28] Nucleating agents with higher β-nucleation efficiency were reported constantly, such as calcium pimelate, 26,27 N,N′-dicyclohexylterephthalamide (DCHT), [28][29][30] WBG-II, 31 and TMB-5. 16 With these nucleating agents, the content of β-iPP can be increased significantly.…”

“…The dispersion of β-NA becomes a key issue affecting β-nucleating efficiency. It was found that with the increase of melting temperature, the morphology of β-NA changes constantly from granular, needle-like to dendritic, [28][29][30]32 leading to inhomogeneous dispersion of β-NA, which probably affects nucleation efficiency. Some researchers found that adding some undissolved agents such as TMB-5, 33 graphene oxide, 34 and ultrafine full-vulcanized powder rubber 35 can prevent the dissolved β-NA from selfassembling into large aggregates effectively.…”