A. Charlesby

Identification of luminescence centres in low density polyethylene

Abstract Using fluorescence, phosphorescence and thermoluminescence techniques it is established that there are several types of luminescence centres, most of which are unattached to the polymer main chain and distributed in different structural regions. The principal centres in the amorphous regions are free molecules of anthracene and a derivative of phenanthrene with possibly molecules of a naphthalene derivative attached to the polymer chain. In the crystalline region the luminescence centres are almost entirely benzoic acid with some alkyl-aryl ketone, the latter being formed from benzoic acid by oxidation. The presence of other luminescence centres is demonstrated, but identification is only tentative.

Effects of radiation and chain ends on fatigue behaviour of polystyrene

Abstract Fatigue lifetimes, under a given alternating stress amplitude, have been determined for a series of linear and branched polystyrenes. The branched polymers were obtained by a crosslinking reaction using γ-irradiation from a Co60 source. By control of irradiation time, a series of branched samples of progressively increasing weight average molecular weight ( M w), with little change in number average molecular weight ( M n, were obtained. From comparison of fatigue data for these irradiated and branched samples with fatigue data obtained on a series of linear polystyrenes of increasing molecular weight, it may by concluded that appreciable increases in fatigue endurance can be achieved by increase in M n and reduction in chain end density. For the irradiated samples, whether irradiated in air or in vacuum, fatigue lifetimes were comparable to or less than lifetimes to fracture for the unirradiated polymer, even though significant increases in M w had occurred. It is suggested that the improved fatigue performance with increase of M n is a consequence of increased craze stability resulting from the greater degree of chain entanglement and the smaller proportion of chain end defects.

Effect of previous history on the NMR relaxation of polyethylene

Abstract The proton spin-spin ( T 2 ) relaxation of linear polyethylene in the melt is shown to be dependent on the previous history of the specimen. Differences are observed between the granular (as supplied) and solution crystallized forms of the same material and they persist even after long periods in the melt. Possibilities such as molecular weight segregation and memory effects are investigated.

Pulsed NMR of cis-polyisoprene solutions T1 and T2 relaxations, free volume, viscosity relationships

Abstract Previous pulsed NMR studies of polyisoprene have largely been concerned with entangled or crosslinked networks. This paper deals with (i) the relaxation of high molecular weight entangled; (ii) cross-linked; (iii) monodisperse low molecular weight; and (iv) high molecular weight polymer in the presence of tetrachloroethylene which, by increasing molecular mobility, can be expected to influence the NMR relaxation. For all four types of polyisoprene, the spin-lattice T 1 , relaxation shows a minimum with position depending only on the free volume, as influenced by changes in temperature T and polymer concentration v 1 ,. For monodisperse polyisoprene of molecular weight 7200, insufficient to form an entangled network, the spin-spin relaxation decay constant T 2L is quantitatively related to the free volume 1 by two parameters A ′ and B ″ when the free volume is altered by a change in temperature, or in polymer concentration (10–100/). This can also be expressed in the form where the parameter T ∞ at 100% concentration agrees with the value used to describe rheological properties. At other concentrations of polymer, T ∞ and B ′ can be derived quantitatively from the coefficients of volume expansion of polymer and solvent. The variation of T 2L with molecular weight ( T 2L ∝ M −0.5 ) occurs via the A ′ parameter. It is concluded that T 2L can be quantitatively related to the free volume available for molecular motion (as influenced by temperature and solvent concentration) as well as to molecular weight. Furthermore T 2L is simply related to viscosity n , over a wide range of temperatures and concentrations. T 2 can be used to analyse the molecular motions involved in theology.

Anomalous crosslinking in acrylic acid and acrylic acid-polyethylene systems by γ-irradiation

Abstract Studies have been carried out on irradiated systems of acrylic acid in aqueous solution and acrylic acid-polyethylene systems in order to discover the cause of the enhanced crosslinking mentioned in previous literature from this laboratory. Theories which have been investigated include: free radical mechanisms, anhydride mechanisms, ionic mechanism, entanglements, hydrogen bonding. It seems that hydrogen bonding is the most plausible.

A comparison of the effects of α and γ irradiation on the crystallinity of polyethylene

A comparison has been made of the effectiveness of Po-210 α-radiation and Co-60 γ-radiation in destroying crystallinity of “low-density” polyethylene, as measured by the infra-red absorption at 731 cm−1. At room temperature high doses of the order 103 Mrad were necessary to induce significant changes. In the absence of oxygen, α-radiation was found to be approximately twice as effective as γ-radiation in destroying crystallinity. This difference is ascribed to the greater density of radical radical interactions such as cross-linking within the alpha particle track. The possibility of local melting in the track due to the high rate of energy deposition is also discussed. In the presence of oxygen it was found that the effectiveness of α-radiation in destroying crystallinity was greatly increased and, since the crystalline regions of the polymer are relatively impermeable to oxygen, disordering of the close-packed chains is envisaged as taking place progressively from the crystallite surfaces as a result of the migration of radicals from the interior. The enhancing effect of oxygen is less marked in the case of α-irradiation and it is suggested that radical reactions within the track, by reducing the number of radicals free to migrate, are responsible for the diminished effect. At higher dose rates the effect of oxygen is negligible. Measurements over a range of alpha particle dose rates show that the loss of crystallinity increases with rate of energy input, but the effect, ascribed to gross heating of the specimen, is small. It is concluded that in spite of the very high rate of energy deposition by an alpha particle, the temperature within the track does not rise high enough, or long enough, to allow melting.

Dilute solution properties of randomly branched polymers—II: Light scattering from the solution of randomly cross-linked polystyrene

Abstract Cross-linked polystyrenes were prepared from linear polystyrenes of narrow molecular weight distribution by γ-ray irradiation, and their dilute solution properties were studied by light-scattering. The two-parameter theory was used for the analysis of the data; for this purpose the theory was modified with the use of the theory of cascade processes. It is found that the effect of excluded volume becomes smaller with increase of cross-linking and will be negligible in the critically branched state (very near to gel) as seen in the critically branched polycondensates. The depression of the theta temperature and the increase of the unperturbed dimension were discussed at least qualitatively with the aid of the theories of Orofino and Flory, Benoit et al ., and Vrij and Casassa.

Enhanced radiation crosslinking of acrylic acid and methacrylic acid to polymers: alkanes as model compounds for polyethylene

Abstract The crosslinking of polyethylene by irradiation is enhanced in the presence of acrylic acid. This process has been studied for model compounds; for 14C-labelled hexadecane, each molecule of polyacrylic acid is linked to an average of 47 hexadecane molecules; at the radiation dose used, this result implies a hitherto unexpected chain reaction. Evidence from ESR spectra indicates an ionic intermediate.

ESR spectra of irradiated polyacrylic acid

Abstract In view of the interest in acrylic acid co-polymerized with other polymers by radiation to promote enhanced crosslinking, a knowledge of the intermediate reaction products is highly desirable. ESR techniques have been used to study the nature and stability of the radicals formed. Acrylic acid was polymerized by exposure to gamma radiation; the nature of the radicals, their decay at various temperatures, and the effect of inhibitors were investigated. The ESR spectrum at room temperature is similar to that described by earlier workers; its dependence on temperature and impurities, as well as its decay, were investigated. From this work, there is reason to believe that the described triplet also contains a central singlet of Δ H ms 19 G; the addition of impurities to the acrylic acid causes the triplet to decay more rapidly, thus leaving the predominant singlet mentioned above. It was also found that the lines in the outer “wings” of the irradiated specimen disappear on heating; at the same time a new line, possibly a singlet of Δ H ms 131 G, either became more intense or more obvious. Some tentative explanations of this behaviour are advanced.

An investigation of the polymerization of styrene initiated by glow discharge

Abstract An electrical glow discharge has been used to initiate the bulk polymerization of liquid styrene monomer. Very efficient stirring was found to be an important requirement for high yields of polymer. The kinetics of the system have been investigated using solutions of styrene in chlorinated hydrocarbon solvents, at various temperatures. Molecular weight determinations have been made in order to determine initiation efficiency. It was found that this efficiency could be increased by the presence of various sensitizer gases. Potential copolymerizing monomers have been subjected to a glow discharge together with styrene, to distinguish between radical and ionic mechanisms of polymerization.