Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

Abstract

produce all-polymer composite by means of flow-induced crystallization during classical injection molding employed in the state-of-the-art processing of commodity and engineering plastics. Today, progress made in all-polymer composite technology holds great promise for upgrading and diversifying existing thermoplastics. Especially, with respect to converting commodity polymer such as polyolefins into high performance materials. As reviewed by Karger-Kocsis and coworkers, one-step (in situ) and multistep (ex situ) processes have been pioneered to produce all-polymer composites via selfreinforcement resulting from oriented polymer crystallization during polymer processing. [1,2] In one-step strategies, flow-induced crystallization is achieved in extrusion and injection molding processes either by redesigning machines or by employing special precisely controlled processing conditions such as high pressure, processing temperature close to polymer melting temperature, and high frequency. [3,4] However, most one-step processes prolong cycle times, require high investment costs, and are highly incompatible with the existing cost-efficient processing technology established for molding of commodity plastics. In multistep processes, oriented polymer crystallization results from either drawing of fibers or stretching of tapes, respectively, followed by lamination such as hot compaction in the subsequent process step. Again, such processes involving lamination of fibers, stacking of stretched films massively impair both throughput and cost efficiency typical for classical injection molding. [5,6] Flow-induced oriented polymer crystallization and in situ formation of extended-chain polymer microand nanofibers represent the key prerequisite for producing all-polymer composites in one-step processes. The concept of flow-induced polymer coil-stretch transition was pioneered by de Gennes for dilute solutions and by Keller and Kolnaar for elongation flow in polymer melts. [7,8] Only when exceeding a critical polymer molar mass flow-induced crystallization produces shish-kebab structures, in which extended-chain polymers form shish, which nucleate the crystallization of low molecular weight polymer forming kebab. Below the critical mass polymer, chains rapidly relax to the coiled state and fail Nanostructure Composites All-polyethylene composites exhibiting substantially improved toughness/ stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow-induced crystallization affords extended-chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish-kebab structures as reinforcing phases. This is unparalleled by melt compounding micron-sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C aff...

The Entanglements of Macromolecules and Their Influence on the Properties of Polymers

“…During injection molding the flow-induced crystallization of UHMWPE produced extended-chain nanofibers which efficiently reinforced the HDPE matrix. [157] The above examples show that disentangled polymers have mechanical properties that are interesting for practical applications. The majority of the studies of the mechanical properties concerned materials tested in tension.…”

“…Recently, UHMWPE with reduced entanglements was used to formation of all polymer composite with HDPE. During injection molding the flow‐induced crystallization of UHMWPE produced extended‐chain nanofibers which efficiently reinforced the HDPE matrix …”

Tailoring Hydrocarbon Polymers and All‐Hydrocarbon Composites for Circular Economy

“…Instead of tailoring all‐hydrocarbon composites by ethylene polymerization on three‐site catalysts, all‐hydrocarbon composites are also produced by melt compounding of HDPE with ultrabroad bimodal polyethylene comprising nanometer‐scaled unentangled aligned UHMWPE dispersed in HDPE wax as masterbatch (see Figure ) . Upon compounding the content of HDPE is readily varied as a function of HDPE addition in melt compounding whereas the UHMWPE/HDPE wax ratio is constant and set by the molar ratio of the catalytic sites in ethylene polymerization on two‐site catalysts during bimodal masterbatch formation.…”

“…The processing window is much wider with respect to hot compaction and does not require special injection molding machines or tedious adjustment of processing parameters. However, the processing temperature should not exceed 250 °C since stretch‐coil transition destroys the reinforcing phases . Typically the mechanical properties increase with decreasing processing temperature.…”

Thermally Assisted Electrohydrodynamic Jet High‐Resolution Printing of High‐Molecular Weight Biopolymer 3D Structures

“…Numerous fabrication techniques have been used to process high‐molecular weight biopolymers to form biostructures, such as gas foaming, injection molding, and freeze‐drying . Among the fabrication techniques, in particular, printing has been paid more attention due to its additive manufacture capabilities on demand, fast fabrication of complex structures, and no need of mold .…”