Geoffrey S. Park

Transport Principles—Solution, Diffusion and Permeation in Polymer Membranes

The concept of the diffusion coefficient, D, and Fick’s Laws are considered. Measurements of D and of solubility, s, of gases in polymers are outlined. The factors influencing D and s of gases in polymers are discussed. Methods for obtaining D and s of organic vapours in elastomeric polymers are given. The effects of concentration, temperature, diffusant size and polymer properties on D are correlated using free volume ideas. Equilibrium vapour uptake is discussed. Vapour sorption anomalies in glassy polymers and the reasons for these are outlined. D, s, and permeability, P, of water in both hydrophobic and hydrophilic polymers are considered and interpreted in terms of hydrogen bonding to polymer sites and water clustering by hydrogen bonding. D, s, and P in non-uniform polymers are considered and discussed in terms of the effects of fillers, crystallites and layer structures.

Thermoreversible gelation in polymer systems—I: The sol-gel transition in dilute poly(vinyl chloride) gels☆

Abstract The gel to sol transition temperature (Tm) has been obtained at several concentrations between 25 g l−1 and 140 g l−1 for six different samples of poly(vinyl chloride) in dioxan and at concentrations of 63 g l−1 and 92 g l−1 for a particular sample of poly(vinyl chloride) in several solvents. For the dioxan solutions, the Ferry and Eldridge relationships In M W =Const + ΔH m RT m , and In C= Const + ΔH m RT m approximate to the data and give junction point energies (ΔHm) of about 35 kJ mol−1. Two “regular” polymers, made at low temperatures, give higher values of Tm as would be expected for gel networks linked by “fringed micelle” crystallites; ΔHm values of only about 27 kJ mol−1 for these two polymers suggest other complications such as microsyneresis. Changes in Tm with solvent type are related to the Flory-Huggins interaction parameter χ and the solvent molar value V by a form of the Flory depressed melting point expression: 1 T m − 1 T c ° = R(1−χ) V ΔH ν The low values of 438 K for T°c, the ultimate melting point of pure poly(vinyl chloride), and of 73 J cm−3 for ΔHv, the enthalpy of melting per unit volume of pure poly(vinyl chloride), given by this expression are attributed to the minuteness of the crystallites that have to melt at the sol-gel transition temperature.

The autoxidation of poly(propylene oxide)s

Abstract The autoxidation of poly(propylene oxide)s of molecular weights 398, 1020 and 2030 at 95, 110, 125 and 140° follows the same kinetic path independent of the molecular weight of the polymer indicating random hydroperoxidation along the polymer chain. When the OH end-groups are replaced by OCONHC2H5 groups the kinetic path is unchanged confirming random hydroperoxidation and showing similarity of polymer breakdown in simple polyether and in the derived urethane elastomers. The volatile oxidation products produced account for only ∼2% of the absorbed oxygen, the major part of the rest leads to water (45%) and to carbonyl groups in the structures at the ends of newly formed chains produced by scission of the polymer molecules. Vapour phase osmometry indicates at least 0.48 chain scissions for each oxygen molecule absorbed. A reaction scheme based on three routes for hydroperoxide decay is presented to rationalize the findings.

Diffusion of additives and plasticizers in poly(vinyl chloride)—II: The self-diffusion of the stabilizers dibutyltin dilaurate and dibutyltin bismonobutylmaleate in plasticized poly(vinyl chloride)

Abstract The self-diffusion coefficient, D, of dibutyltin dilaurate and dibutyltin bismonobutylmaleate have been obtained at 35, 45 and 55° in samples of poly(vinyl chloride) plasticized with 34, 60 and 100 phr of di(2-ethylhexyl)phthalate. D at 2 phr of the laurate is 3–5 times larger than for the smaller maleate molecule. In all cases, D increases with increasing plasticizer concentration, an effect interpreted in terms of the free volume theory of diffusion. D for the laurate increases by a factor of about 2.7 when the laurate diffusant concentration is increased from 0 to 4 phr. The activation energies for diffusion, ED, lie between 50 and 90 kJ mol−1. They increase with increasing plasticizer concentration but become constant at higher plasticizer concentrations (60–100 phr). It is impossible to correlate all the known data on diffusion in plasticized PVC with an equation of the form log D0 = C1 + C2 ED/RT

Characterization of the short-chain branches in poly(vinyl chloride)s by γ irradiation of the reduced polymers☆

The volatile products from the γ irradiation of samples of poly(vinyl chloride) prepared under different conditions and reduced to the corresponding polyethylenes have been measured quantitatively and compared with those for low- and high-density polyethylenes and copolymers of ethylene with small amounts of α-olefins. The presence of methyl branches is clearly demonstrated and there is also evidence for ethyl and butyl branches, although these had not been considered significant in previous [13C] NMR studies. The method is shown to be extremely sensitive to small quantities of residual trialkyl tin hydride reductant occluded in, and possibly also reacted with, the polymer, and to residual chlorine (<0.5%). The yields of alkanes are higher than expected from the branch frequencies determined by i.r. and [13C] NMR and results for ethylene-α-olefin copolymers. This difference is apparently due to sensitisation of the radiation degradation by residual chlorine and reductant. Quantitative determinations of branch frequencies by the radiolytic method are unlikely to be obtained until these problems are overcome.

The Site of Initiation in Vinyl Chloride Polymerization

Abstract Doubly labeled (3H and 14C) benzoyl peroxide has been used to study the relative incorporation of phenyl and benzoyloxy initiating radicals in PVC polymerized in bulk and over a range of monomer concentrations in dichloroethane solutions. From the results it is concluded that (a) transfer occurs between phenyl radicals and the solvent, (b) that vinyl chloride is a relatively unreactive monomer, and (c) that the main site of initiation in the earlier stages of the bulk polymerization is the continuous monomer phase. Explanations are suggested for the very high ratio of benzoyloxy to phenyl incorporation in polymers formed by polymerization of monomer dissolved in preformed PVC films and for the decreased benzoyloxy to phenyl ratio obtained with increasing initiator concentration.

The Stabilizing Effect of Allyl Chloride Copolymer Units on the Thermal Degradation of Poly(vinyl Chloride)

Abstract The rate of dehydrochlorination at 190°C of vinyl chloride/allyl chloride copolymers decreases with increasing content of allyl chloride. In agreement with the hypothesis that this is due to the blocking action of the CH2CI side groups, copolymer samples degraded at 170°C give a UV spectrum showing a greater preponderance of short polyene sequences than is found for the vinyl chloride homopolymer.

Vinyl chloride studies—IV. Transfer to trichloroethylene in vinyl chloride polymerization

Abstract Molecular weights have been used to obtain transfer constants to trichloroethylene and deuterotrichloroethylene in vinyl chloride polymerization at temperatures between 20° and 55°. In the homogeneous polymerization in chlorobenzene, there is very little difference between the two transfer agents indicating that the CH bond on the trichloroethylene is not involved in the transfer process. Measurements of the incorporation of 14C-labelled transfer agent into the polymer show that copolymerization of trichloroethylene occurs; from activation energy considerations, it is concluded that transfer occurs via copolymerization: . A kinetic isotope effect of 1·75 is observed for transfer in the heterogeneous polymerization; although copolymerization occurs, the activation energies do not rule out direct transfer in this case, i.e. .

A radiotracer technique for estimating double bonds in polymers: the double bond content of poly(vinyl chloride) produced by free radical polymerization in chloroethane solvents

Abstract A radiotracer technique based on the addition of 125I Cl has been used for estimating double bonds in poly(vinyl chloride) formed by polymerization under homogeneous conditions in 1,2,-dichloroethane,1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane. From the data on polymer made at a 5 M monomer concentration and neglecting mutual termination the transfer constants to these three solvents increase approximately in the ratio 1:2:6 as the chlorine content increases. From the increase in the number of double bonds per monomer unit in the polymer as the monomer concentration is decreased in the polymerization it is concluded that mutual termination involves some disproportionation. In dichloroethane disproportionation appears to be the main mechanism of mutual termination.

Termination and other processes in the polymerization of p-methoxystyrene and 2,2-dideutero-p-methoxystyrene

Abstract The uncatalysed thermally initiated free radical bulk polymerization of ϱ-methoxystyrene and 2,2-dideutero-ϱ-methoxystyrene (65% deuterated) have been studied at 80, 100 and 120°. Both the rate of polymerization and the degree of polymerization are increased by deuteration, as would be expected from the deuterium kinetic isotope effect if disproportionation plays a part in the radical termination process. The effect of deuteration is least at the highest temperature, but it is shown that this does not mean that the extent of disproportionation decreases with increasing temperature. From a comparison of the effect of deuteration on the polymerization rate with its effect on the degree of polymerization, it is concluded that the transfer constant to monomer is less for the deuterated material than for the undeuterated material. It is suggested that the transfer process probably involves the donation of a deuterium or hydrogen atom from the growing radical to the monomer.