Ethylene Comonomer-Directed Epitaxial Nucleation and Growth of β-Nucleated Isotactic Polypropylene

Abstract

We present a molecular-level investigation of the epitaxial crystallization of β-nucleated isotactic polypropylene associated with the minor and local topographic modification at the interface, i.e., the inclusion of an ethylene comonomer. The experimental results show that the relative fraction of β-crystal (K β) decreases progressively with increasing the amount of ethylene comonomer, which originates from a simultaneous reduction in the β-nucleation ability of a β-nucleating agent (N,N′-dicyclohexyl­terephthalamide, DCHT) toward propylene–ethylene random copolymers (PERs) and the growth rate of the β-crystal relative to the α-crystal. With the aid of molecular mechanics simulation, we demonstrate that such a topographic modification weakens the intermolecular interactions at the nucleation and growth interfaces, thereby modifying the epitaxial nucleation of the β-crystal and the growth kinetics of β- and α-crystals. The study reveals that the polymorph selection of the PERs/DCHT system is not solely determined by the lattice match but also profoundly influenced by the topography characteristics dependence of molecular interaction at the crystalline interfaces. Our fundamental insight could have important implications in the control of the polymorphism of polymers through the careful design of the topography of the nucleation and growth interfaces.

Toward a Further Understanding of the Decreased β-Phase Content in β-Nucleated Polypropylene Random Copolymer: From Nucleation and Growth Kinetics

“…This temperature range is widely reported to be about 100−140 °C for PPH. 9,18,22,42 Despite what happens to T** being unclear to the authors' knowledge, T* has been reported to decrease in PPR. 19,20 Varga et al 21 have observed a lowering of T* and attributed it to a higher deceleration of G β than G α with increasing defect fraction.…”

“…16−18 For example, the introduction of N,N′-dicyclohexylterephthalamide (DCHT) can induce a large amount of β-phase in PPH with k β exceeding 0.8 15 while the values of k β decreased progressively to 0.70, 0.53, and 0.07 in PPR with ethylene contents of 0.34, 0.60, and 3.11 mol %, respectively. 18 Regarding the varying k β with the matrix chain structure, it is now widely accepted that the insertion of comonomer in PPR disrupts the chain regularity, leads to an unfavorable β-growth kinetically, and results in decreased k β . 19−21 Generally, the αand β-phases in iPP have different temperature-dependent growth rates.…”

“…This mechanism based on growth kinetics can well explain the decreased k β with comonomer inserted and has been widely accepted since its proposal. 18,20,21 Crystallization is a complex process that involves nucleation and growth, both of which significantly influence the final polymorphic composition. In addition to the adverse β-growth mentioned above, fewer β-nuclei at nucleation may also result in a decreased k β .…”

“…The larger the ratio, the more favored is the formation of β-nuclei. i β /i is calculated to be 91% for PPH, which is gradually decreased to 89% for PPR-l, to 85% for PPR-m, and to 78% for PPR-h. From the simulation results by Wang et al, 18 the inclusion of ethylene comonomer leads to a higher nucleation barrier for β-phase, making it difficult for DCHT to promote βnucleation in PPR. Considering that β-nucleation is the critical step of β-crystallization, the reduced β-nuclei with comonomer insertion must have a contribution to the decreased β-phase.…”

3D Morphology of Different Crystal Forms in β-Nucleated and Fiber-Sheared Polypropylene: α-Teardrops, α-Teeth, and β-Fans

“…Polymorphism in semicrystalline polymers has considerable influence on their physical properties. − Isotactic polypropylene ( i- PP) is a typical example with four common polymorphs: α-, β-, γ-, and δ-forms, ,− and a recently reported ε-form . All these crystal forms contain only one chain conformation, i.e., the threefold helix with a 6.5 Å repeat.…”