Polyethylene Pellets Research Articles

Proton spin-lattice relaxation measurements on some high polymers of differing structure and morphology: Polyethylene, poly-4-methyl-pentene-1, poly- l-leucine, poly-phenyl- l-alanine, polystyrene and poly-α-methyl-styrene

Proton spin-lattice relaxation times (T 1 ) have been measured at a resonant frequency of 21.5 Mc/s in the temperature range from 77° to 520°K for polyethylene (Rigidex 35) crystals and poly-4-methyl-pentene-1 (P4MP1) crystals grown from dilute solution as well as for melt-formed samples. Exponential decay of magnetization was observed for the polyethylene pellets at all temperatures. On the other hand decay of magnetization describable by two exponentials (two T 1 s) was found at low temperatures for isothermally grown polyethylene crystals, polyethylene dendrites, P4MP1 crystals and melt-formed P4MP1. All three polyethylene samples showed a T 1 minimum at about 250°K which is believed to be related to the γ mechanical loss maximum found for partially crystalline samples, formerly attributed to motions of three or more CH 2 units in the amorphous regions. The position of another minimum in the melt was strongly dependent on sample history and is believed to be a consequence of the increased chain mobility in the crystalline areas. Two T 1 minima were observed for P4MP1, one at about 150°K and another weaker one at 430° to 470°K, the latter being dependent on the sample history. The 150°K minimum is attributed to methyl group reorientation while the higher temperature process is believed to be associated with the glass-rubber transition or possibly a combination of this and a change in the crystalline regions. Spin-lattice relaxation time measurements have also been made for two synthetic polypeptides, poly- l-leucine (PLL) and poly-phenyl- l-alanine (PPLA). For PLL two T 1 values were found at low temperatures. The shorter T 1 has a minimum value at 174°K similar to the P4MP1 crystals which is attributed to methyl group reorientation. A second minimum at about 290°K is attributed to isobutyl or isopropyl group motion. For PPLA a minimum at 200°K and another wide shallow one at 330° to 400°K are found. The first is believed to be due to phenyl motion while the second is believed to be caused by oscillations of the benzyl group with an amplitude which increases with increasing temperature. We have also studied polystyrene and poly-α-methyl-styrene. The higher temperature T 1 minimum is related to the glass-rubber transition. Polystyrene has a minimum at 260°K and a plateau in the T 1 temperature curve at 120° to 150°K. Poly-α-methyl-styrene has a broad minimum at 300° to 340°K due, at least partially, to methyl group reorientation. There is also an inflection in the T 1 temperature curve at 100° to 140°K. The possible relation of these relaxation processes to mechanical loss maxima at low temperatures is discussed.

Effects of high energy irradiation on polypropylene

AbstractIsotactic polypropylene (Profax) film samples irradiated in the Pennsylvania State University (PSU) reactor at ambient temperatures were examined by solvent extraction and infrared absorption techniques. At low irradiation doses, samples annealed (density = 0.909 g./cc. at 24°C.) prior to irradiation were found to have a lower gel content and a higher concentration of oxidation products than quenched samples (density = 0.899 g./cc. at 24°C.) irradiated under identical conditions. Similar gel fraction results were obtained for electron irradiated samples of polypropylene. These effects are believed to arise from the accumulation of a greater concentration of free macroradicals in the higher density samples. At higher reactor dosages the irradiation effects in the annealed and quenched films appear to be about the same. The gel content as a function of dose was found to be independent of the irradiation intensity level over a twofold range. Irradiations in the PSU reactor were also carried out on high density polyethylene (Marlex 50) pellets at two temperatures, viz., 40 ± 10°C. and 150°C. Irradiation at 150°C. led to a greater gel content at any given dose, a result in agreement with that reported by Lawton, Balwit, and Powell for electron‐irradiated specimens. Nuclear magnetic resonance spectra were obtained for molded annealed samples of partially isotactic polypropylene, irradiated in the Brookhaven reactor, at temperatures of 90–330°K. The observed shifts in the line narrowing process that occur in the 270–330°K. region are explained in terms of the decrease in crystallinity and the increasing density of crosslinks that accompany irradiation.