SynopsisThe traditional melt index test (ASTM D 1238) is widely used in the plastics industry to characterize polymer processability. The data from this measurement must be interpreted cautiously, however, because polymers are usually processed under conditions that are far removed from those of the melt index test. In this study, melt index ( M I ) values from a series of polystyrene, polypropylene, linear low-density polyethylenes (butene and octene copolymers), and high-density polyethylenes were measured and related to molecular weight distributions of these materials. It was found that a simple relationship between 1/MI versus (where z = 3.4-3.7) was followed for the linear polymers with similar polydispersities. For branched polymers, the best correlation was that of -In ( MI) versus In ( Mu). A general relation for shear modifiable polymers like polyethylenes cannot be obtained unless the rheological state of the material can also be defined.

SYNOPSISIn this article we consider characterization of the branching distribution of LLDPE by analytical TREF and by DSC techniques. Qualitatively, both methods yield parallel information, provided that the DSC samples are prepared at very slow cooling rates. Analytical TREF provides more quantitative information, however, in that the mass fraction of material with different branch levels can be estimated. Full characterization of a polyethylene requires the use of a preparative TREF technique, in which fractions dissolved at preselected temperatures are investigated for branching with high resolution 13C NMR and for molecular weight distribution, with high temperature SEC analyses.

SYNOPSISThe effect of particle size in high-impact polystyrene (HIPS) is difficult to determine because of a size polydispersity and changes in particle morphology during the HIPS synthesis process. In this study, poly ( n -butyl acrylate) rubber core/polystyrene shell particles were made by emulsion polymerization methods such that the only difference was in particle diameter, which ranged from 0.4 to 6.2 pm. The latexes were subsequently incorporated into a polystyrene matrix to form a toughened composite that acted as a simple model for HIPS. Charpy impact energies (notched and unnotched) of the composites showed that there was no toughening for particle sizes less than 2 p m in diameter. The optimal impact energy was obtained with particle diameters in the region of 2-3 pm at 8 w t % rubber loading. The results imply that craze stabilization is the most important aspect of the toughening process. A simple toughening model based on the crack opening displacement of craze breakdown between adjacent rubber particles is suggested, with interparticle distance as the most important variable.

In a recent article,' we pointed out that a simple relation can be found for families of commercial polymers with similar polydispersities. An equation was developed to show that this simple relationship is to be expected, more or less. These additional comments are intended to emphasize the limitations of the experimental correlations. These ideas are implied in the cited article, but we did not give them the prominence they deserve.In the article, an approximate relationship was formulated between melt flow index (MFI) and the weight average molecular weight, M,: where x = 3.4-3.7. In the development of this equation, the correlation between the zero shear viscosity, v0, and viscosity, 1 , at the shear rate of the melt indexer was given as:Use of this relation to derive eq. ( 1 ) implies that the constant K is the same for all polymers of a particular type, i.e., the ratio between the zero shear viscosity and the viscosity at the melt index shear stress is constant for all polymers of a given family. Therefore, there exists a family of viscosity-shear stress curves for different MFIs as shown in Figure 1.It should be noted, however, that this is not generally true because of differences in long chain branching, polydispersity, comonomer content (where applicable), shear history, and so on. Thus, somewhat different relations were noted between ethylene-butene and ethylene-octene LLDPE copolymers that are made by different processes. LDPEs were not included in this general correlation be- cause a common relation is not expected between MFI and M , for polymers that vary widely in branching and polydispersity. Equation ( 2 ) , then, summarizes the key requirement for MFI of a series of polymers to correlate with a single average molecular weight.A two-point capillary flow curve should suffice in most cases to verify that a particular polymer falls into a family for which a particular relation between MFI and M , has been established. One point should be at the MFI apparent shear stress and the other at a significantly lower stress.

Low‐density polyethylene (LDPE) and also linear low‐density polyethylene (LLDPE) resins can be characterized by the degree of strain hardening and down‐gaging during elongation. A new method for the determination of the apparent elongational flow characteristics is presented. In a small scale apparatus, a molten monofilament is stretched under nonisothermal conditions similar to those found in tubular film extrusion. Measurement of resistance to elongational flow and apparent elongational strain rates permit the comparison of the process‐ability of different resins under specified conditions. The effect of melt temperature and extension ratio are examined. The importance of the molecular structure of both LDPE and LLDPE resins on these properties is also outlined.