Tuning High and Low Temperature Foaming Behavior of Linear and Long-Chain Branched Polypropylene via Partial and Complete Melting.

While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3).

Extensional Strain Hardening in Highly Entangled Molecular Bottlebrushes.

The highly strained conformation in dense molecular bottlebrushes (DB) has profound effects on the dynamics of these type of macromolecules. Understanding of such effects in both their linear and nonlinear viscoelastic responses is crucial for their design and processing. The nonlinear response of poly(1-octadecene), a highly entangled α-olefin DB with linear side chains sixteen carbons long, is studied here and compared to the nonlinear response of a linear polyolefin (polypropylene) with equivalent linear viscoelastic response.