Architectural characteristics of stochastic honeycombs fabricated from varying melt strength polypropylenes

Abstract

Stochastic honeycombs are a cellular polymer fabricated through a simple melt-stretching process. In this study, five polypropylenes (PP) with varying rheological properties were used to fabricate stochastic honeycombs over a range of densities. Rheology tests were performed to determine the melt behavior, while micro-CT scans were used as a nondestructive characterization technique to determine the internal architecture. The sandwich structure consists of parallel skins separated by a network of interconnected webs. The cross-sectional area decreased from unit area at the skins to a plateau over the middle third of the honeycomb height where the cross-sectional area and total edge length of the webs were relatively constant. The rheological properties of the polymer were found to determine the web length per unit area, while the starting areal density of the polymer controlled the mid-height cross-sectional area. Other features, such as archways and buttresses, act as secondary structures that support the webs. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40074.

Architecture–process relationships in stochastic honeycombs

“…In Stage IV, the stress reaches a nearly constant value as the final web pattern has been formed and the webs stretch at the plateau extensional value (σ ext ). Some of the webs undergo ductile failure as the sample is stretched to its final height, t final , and partial webs and archways are formed . Note that previous work showed that there was some small increase in the crystallinity of the PP in the webs compared to the PP with no thermal history, implying that some strain‐induced crystallinity occurs as a result of melt stretching .…”

“…While the process to fabricate stochastic honeycombs is simple, their internal architecture is not. Previous work analyzed the three‐dimensional architecture of stochastic honeycombs through X‐ray tomography using area fractions and Minkowski functionals (surface area, volume, and Euler number) . The cross‐sectional area fraction (area of the skin or webs/area of the sample) was shown to decrease from 1 at the upper and lower skins to a plateau over the central third of the sample height, which varied with the relative density of the sample .…”

“…Previous work analyzed the three‐dimensional architecture of stochastic honeycombs through X‐ray tomography using area fractions and Minkowski functionals (surface area, volume, and Euler number) . The cross‐sectional area fraction (area of the skin or webs/area of the sample) was shown to decrease from 1 at the upper and lower skins to a plateau over the central third of the sample height, which varied with the relative density of the sample . Surface area and volume, two of the Minkowski functionals in three‐dimensions, were combined into an internal surface area to volume ratio, which was found to decrease as the core density increased, and could be used to obtain a value for the effective web thickness .…”

“…Some of the webs undergo ductile failure as the sample is stretched to its final height, t final , and partial webs and archways are formed . Note that previous work showed that there was some small increase in the crystallinity of the PP in the webs compared to the PP with no thermal history, implying that some strain‐induced crystallinity occurs as a result of melt stretching . The temperature at each stage as a function of the elongation rate is detailed in Table .…”

“…Previous work has examined how the internal architecture changes with properties of the polymer melt, such as the melt flow index (MFI) and the zero‐shear viscosity (η o ) . The presence of long‐chain branches increases the entanglement density with branches acting as entanglement points with primary bonds as opposed to secondary bonds; branched PPs formed stochastic honeycombs with many more webs and web branch nodes than linear PP, implying a greater number of activated entanglement points. The melt flow properties of PP were thus previously shown to affect the architecture of stochastic honeycombs.…”