Chan I. Chung

Melt rheological and thermodynamic properties of polyethylene homopolymers and poly(ethylene/α-olefin) copolymers with respect to molecular composition and structure

Various types of polyethylene homopolymers and copolymers, including linear high-density polyethylene (HDPE), branched low-density polyethylene (BLDPE), poly(ethylene vinyl acetate) copolymer (EVA), heterogeneous linear poly(ethylene/α-olefin) copolymer (het-LEAO) or commonly known as linear low-density polyethylene, homogeneous linear poly(ethylene/α-olefin) copolymer (hom-LEAO), and homogeneous branched poly(ethylene/α-olefin) copolymer (hom-BEAO), were evaluated for their melt rheological and thermodynamic properties with emphasis on their molecular structure. Short-chain branching (SCB) mainly controls the density, but it has little effect on the melt rheological properties. Long-chain branching (LCB) has little effect on the density and thermodynamic properties, but it has drastic effects on the melt rheological properties. LCB increases the pseudo-plasticity and the flow activation energy for both the polyethylene homopolymer and copolymer. Compared at a same melt index and a similar density, hom-LEAO has the highest viscosity in processing among all polymers due to its linear molecular structure and very narrow molecular weight distribution. Small amounts of LCB in hom-BEAO very effectively reduce the average viscosity and also improve the flow stability. Both hom-LEAO and hom-BEAO, unlike het-LEAO, have thermodynamic properties similar to BLDPE.

Retarded Cross-linking in ZnO-low-density Polyethylene Nanocomposites

ZnO nanoparticles were mixed with branched low-density polyethylene and were found to increase the resistance of the polymer to thermal degradation without changing other thermal properties. Submicron-size ZnO particles were mixed with low-density polyethylene for comparison, and it was found that the increased thermal stability of the nanocomposite was due to the surface properties of nanoparticles smaller than approximately 100 nm in diameter.

Low temperature mechanical relaxation in poly-p-xylylenes☆

Abstract The mechanical relaxation behaviour of poly- p -xylylene, poly(chloro- p -xylylene) and poly(dichloro- p -xylylene) has been investigated by means of an inverted torsion pendulum apparatus. Values are given for the shear modulus and loss for each of these three polymers at various temperatures from 80 K to room temperature. It is found that the unsubstituted poly- p -xylylene, even though it contains only two CH 2 sequences in its chain between phenylene units, exhibits a strong, low temperature relaxation process with a loss maximum at 159 K (0.54 Hz). The mono-substituted polymer, poly(chloro- p -xylylene), shows no loss peak in the 150 K region but does exhibit a damping maximum at 254 K (0.40 Hz). For the di-substituted polymer, poly(dichloro- p -xylylene), the loss modulus peak is again found in the low temperature region, being situated at 150 K (0.34 Hz). The data, together with other experimental observations, suggest that local reorientational motions of the phenyl, or substituted phenyl units, are involved in these secondary relaxation processes.

A Scientific Approach to Screw Design

Screw design technology is considered to be empirical and secretive. The production rate of single-screw extruders is often limited by the melting capacity. The melting capacity of a screw depends on the polymer properties, the processing conditions and the particular geometry of the screw. Once the melting capacity is predicted, the screw can be designed to match the melting capacity. This paper presents a scientific approach to screw design based on an analytical melting model, with an example.

The Effects of Molecular Structure on the Melt Rheology of Low Density Polyethylene

A rapid method for obtaining information on average molecular weights, molecular weight distribution, molecular size, and long chain branching (LCB) characteristics of low density polyethylene (LDPE) from gel-permeation chromatography (GPC) and intrinsic viscosity (IV) was recently reported. This investigation is primarily concerned with the effect of molecular weight and molecular structure, obtained by the use of the GPC-IV method, on the melt rheology of whole LDPE. Some of the assumptions involved in the GPC-IV method are also examined. It is found that the melt viscosity of whole LDPE depends not only on weight average molecular weight, but also strongly on the LCB characteristics. The combined influence of these two molecular parameters on melt viscosity can be described through the effect which each has on the weight-average mean square radius of gyration of the polymer coil, (S2)w. The experimental data indicate that the dependence of zero shear viscosity on (S2)w is substantially greater for LDPE than for linear polymers. The apparent flow activation energy at zero shear is found to be about 12 kcal/mole for all whole LDPE samples of differing LCB characteristics studied in this investigation. Our calculations based on various assumptions in the GPC-IV method suggest that the polydisperse model of the Zimm and Stockmayer equation, which relates molecular weight and branching frequency to the branching parameter (g), and the exponent of b = 1/2 in the equation, gb = [ƞ]B/[ƞ]L are the best choices for whole LDPE. [ƞ]B is the IV of branched molecule and [ƞ]L is the IV of linear molecule of the same molecular weight.

Frictional Behavior of Polyethylenes with Respect to Density and Melting Characteristics

The frictional behavior of 12 polyethylene samples with densities ranging from 0.963 down to 0.870 g/cc on a metal surface was studied. The samples with densities higher than 0.908 g/cc behaved as rigid plastics, sliding on the metal surface. The samples with densities lower than 0.905 g/cc behaved as elastomers, strongly adhering on the metal surface and tearing within the polymer. Melting occurred when the metal was heated to a temperature above the melting range of the sample. The frictional behavior of a polyethylene can be understood in terms of the density and melting temperature range of the sample.