Surface Roughness Enhances Self-Nucleation of High-Density Polyethylene Droplets Dispersed within Immiscible Blends

Abstract

Highly linear or high-density polyethylenes (HDPEs) have an intrinsically high nucleation density compared to other polyolefins. Enhancing their nucleation density by self-nucleation is therefore difficult, leading to a narrow self-nucleation Domain (i.e., the so-called Domain II or the temperature Domain where self-nuclei can be injected into the material without the occurrence of annealing). In this work, we report that when HDPE is blended (up to 50%) with immiscible matrices, such as atactic polystyrene (PS) or Nylon 6, its self-nucleation capacity can be greatly increased. In addition, temperatures higher than the equilibrium melting temperature of the HDPE phase are needed to erase the significantly enhanced crystalline memory in the blends. Morphological evidence gathered by Scanning and Transmission Electron Microscopies (SEM and TEM) indicates that these unexpected results can be explained by the modification of the interface between blend components. The filling of the solid HDPE surface asperities by the low viscosity polystyrene during heating to the self-nucleation temperature, or the crystallization of the matrix in the case of Nylon 6, enhances the interface roughness between the two polymers in the blends. Such rougher interfaces can remarkably increase the self-nucleation capacity of the HDPE phase via surface nucleation.

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

“…The sample without GNPs shows no crystallization until approximately 80 °C, which can be traced to heterogeneous nucleation at the interface between the HDPE and PS phases, consistent with reports in the literature for other PS/HDPE systems. 29,43 By contrast, samples containing GNP exhibit a strong exothermic peak near 120 °C, also consistent with prior reports for HDPE+GNP systems. 4,45,47 The volume fraction of droplets containing GNPs was calculated by integrating the two crystallization peaks and expressing as a fraction of the total enthalpy.…”

“…In most cases, polymers crystallize through the processes of nucleation and growth, by which small stable crystalline clusters, called nuclei, form first, and these nuclei subsequently grow by accretion of polymer from the surrounding melt or solution. The kinetics of nucleation and growth can be manipulated by changes in temperature, pressure, flow-induced melt structure, or the introduction of nucleating agents (NA), which alter the rate of nucleus formation or may promote the formation of one crystal polymorph over another. , NAs can be crystalline powders of small molecules − or other polymers. − Self-nucleation occurs on crystalline residuals from an incompletely melted polymer. − NAs thus provide a mechanism by which to tailor macroscopic properties for a desired application . However, determining the “effectiveness” of a NA is still largely a matter of empiricism …”

“…36 Santana and Muller subsequently extended this approach to the study of homogeneous nucleation of isotactic polypropylene (iPP) blended with polystyrene (PS). 37 More recently, heterogeneous nucleation of iPP blended with additives, 27 high-density polyethylene (HDPE) at the interface with iPP, 24 and selfnucleation of HDPE 29 have been investigated using microdomains in an immiscible matrix. In addition to measuring nucleation rates, the mechanisms of epitaxy (with a crystalline matrix) and graphoepitaxy due to roughness of the interface with the surrounding amorphous or crystalline matrix have been observed in these two-phase mixtures.…”

Morphological Evolution and Structural Orientation of Poly(ε-caprolactone)/Poly(lactic acid) Biodegradable Blend Films under Complex Coupling Flow

“…Nevertheless, most polymers are immiscible, and spatial phase separation of polymer blends caused by the dissimilar backbone structure of the polymers warrants consideration. 14,15 In fact, the scale and architecture of reinforcing elements in biological systems 16−18 have been proven to contribute to the unique behaviors of materials such as bone, tendon, and nacre. Hence, the design and control of these elements by flow induction, including the secondary phase morphology from spherical 19 to fibrillar, 20−22 ribbon, 23,24 or lamella matrices 25 and the hybrid architecture 26,27 from molecular to atomic scale, facilitates the phase interface interactions of polymer blends.…”

“…Analogous to bulk materials, blending provides an efficient means of tailoring the properties of biodegradable polymers. , For example, fabricated biodegradable polymer blends of poly­(ε-caprolactone) (PCL) and poly­(lactic acid) (PLA) exhibit complementary properties. − PCL has good flexibility and processability due to its freely rotating functional group, while PLA offers robust mechanical strength with limited ductility. Nevertheless, most polymers are immiscible, and spatial phase separation of polymer blends caused by the dissimilar backbone structure of the polymers warrants consideration. , …”