Rheology of Polydisperse Star Polymer Melts: Extension of the Parameter-Free Tube Model of Milner and McLeish to Arbitrary Arm-Length Polydispersity

This paper considers the extension of the parameter-free tube model of Milner and McLeish for stress relaxation in melts of monodisperse star polymers to star polymers whose arms have a continuous molecular weight distribution such as the Flory distribution in the case of star-nylons and star-polyesters. Exact expressions are derived for the relaxation spectrum and the relaxation modulus for star polymers having an arbitrary continuous arm-length distribution. For a Flory distribution a comparison is made with results of dynamic measurements on a melt of 8-arm poly(-caprolactone) (PCL) stars. An excellent quantitative agreement over a large frequency range is found, however, only if one treats, in contrast with the original parameter-free tube model approach, the entanglement molecular weight that determines the relaxation spectrum as a fitting parameter independent of the entanglement molecular weight of the linear PCL. This discrepancy is not in anyway related to the polydispersity in arm-length, but a consequence of the thermorheological complexity of the PCL stars. A similar discrepancy has been observed for hydrogenated polybutadiene stars, as described by Levine and Milner

Compositions and methods for high-temperature jetting of viscous thermosets to create solid articles via additive fabrication

Described herein are methods and compositions for forming three-dimensional objects via material jetting processes, the methods including the repeated steps of selectively depositing a liquid thermoset material onto a surface from a nozzle of at least one jetting head in a first specified direction and exposing at least a portion of the liquid thermoset material to a source of actinic radiationin order to form a three-dimensional object from the cured thermoset material, wherein the jetting head is configured to eject droplets of o the liquid thermoset material from the nozzle at prescribed elevated operating temperatures, and wherein the liquid thermoset material is chosen so as to possessing prescribed viscosity and rheological characteristics.<br/

Analysis of Cell-Stabilizing Additives in Low-Density Polyethylene Foams Using Low-Frequency Dielectric Spectroscopy

Addition of low amounts of additives is necessary to guarantee dimensional stability of low-density flexible polyethylene foam after extrusion. It is expected that the additives form a layer on the foam cell walls. However, the physical phenomena responsible for the stability improvement are not well understood. Using low-frequency dielectric spectroscopy, we show the occurrence of interfacial polarization in these low-density polyethylene foams. We hypothesize and prove that a stabilized low-density, closed cell polyethylene foam can be regarded as a three-phase system with a polyethylene matrix forming the cell walls, a filler being the gas phase, and an intermediate additive layer. At elevated temperatures the additive layer will become conductive resulting in interfacial polarization, thereby excluding the entire filler volume from the electrical field. This is reflected in an increase of the dielectric constant of the foam from initially 1.0 to about 300 at the melting point of the additive. The frequency-independent position of the peaks suggests that the additive layer is crystalline