Heterogeneous Nucleation of High-Density Polyethylene Crystals on Graphene within Microdomains

Abstract

In polymer processes, nucleating agents are often used to control the kinetics of crystallization, but their application remains largely a matter of trial and error. Thermodynamically, the efficiency of a nucleating agent can be quantified by the difference in substrate/crystal, crystal/melt, and substrate/melt interfacial energies, Δσ. In this work, the efficiency of graphene nanoplatelets (GNP) as nucleating agents for high-density polyethylene (HDPE) is investigated. HDPE nucleates and crystallizes rapidly, so Δσ can be especially difficult to measure experimentally. To overcome this difficulty, blends of HDPE+GNP are confined to microdomains so that crystallization becomes nucleation limited. Two methods of microdomain formation are employed. In the first, HDPE+GNP is melt-blended with an immiscible matrix of polystyrene (PS), and crystallization is characterized using differential scanning calorimetry (DSC); in the second, dispersions of HDPE+GNP in toluene are sprayed onto PS substrates, and crystallization is characterized with polarized optical microscopy (POM). Heterogeneous nucleation rates at several crystallization temperatures and for several GNP loadings were measured by these two methods and found to give excellent agreement across GNP loadings. The value of Δσ for HDPE+GNP is calculated to be 0.83 ± 0.18 erg/cm 2 . This value is only 2.8 times larger than that reported for HDPE nucleated heterogeneously on a HDPE fiber, a nearly ideal nucleating agent for HDPE, and much smaller than many of the best nucleating agents reported for other polymers. We conclude that GNP is an efficient nucleating agent for HDPE.

Directional Epitaxy Intensifying Intermolecular =C–H···O=C Hydrogen Bonding to Expedite Bulk Crystallization of Biobased Poly(butylene 2,5-furandicarboxylate)

“…For pure PBF, the Avrami index n is 2.50 and 2.59 for its crystallization at 130 and 140 °C, respectively, close to the past reported 2.40 at 135–142.5 °C, but it decreases to 2.16 at 150 °C, perhaps meaning the dimensional transition from spherulites to axialites at high T c . With NA, the n reduces to ∼2 and ∼2.3 at 130 and 140 °C, respectively, which was generally attributed to the formation of a large number of nuclei at the polymer/filler interface. , It is then reasonable that the n of ∼2 implies a high possibility of two-dimensional nucleation of PBF upon the NA surface. This point is more convincible by the fact that the n value fitted in the X t range of 3–20% is much closer to 2 (Table S1), for the more initial crystals formed in this primary stage should be nucleated by the NA.…”

“…To improve material performances, increase processing efficiency, and/or endow desired functions for end-use products, introducing fillers as nucleating agents (NAs) is an effective strategy that has been frequently utilized to manipulate both the crystallization kinetics and crystalline structures of polymers because of the low cost, high efficiency, and convenience of this method. − However, determining the effectiveness of a NA remains largely a matter of empiricism. − It is therefore of high interest and industrial importance to develop highly efficient NAs and unveil the intrinsic nucleation mechanism for controlling polymer crystallization.…”

“…Moreover, the important effect of secondary nucleation upon epitaxial crystallization between polymers was also highlighted . Very recently, Tong and co-workers have approved that lattice matching is indeed required but insufficient for successful epitaxy and the critical nuclei size dominates the epitaxial growth. , Despite the extensive studies with diverse systems over the past decades, regulating polymer crystallization via epitaxy is nowadays hardly desirable. ,,− More importantly, due to the very short range of interfacial epitaxial interaction, the past works involve polymer epitaxy focused on very thin films with the main evidence of crystal morphologies and orientation relationships obtained by atomic force microscopy, transmission electron microscopy, and electron diffraction, ,− while polymer epitaxy from bulk crystallization is seldom concerned and the featured evidence is essentially missing . Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. ,− ,, Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. − , …”