Carbon Tetrachloride as Thermoporometry Liquid-probe to Study the Cross linking of Styrene Copolymer Networks

Mesh Size Distributions (MSDs) of swollen cross linked styrene copolymer networks have been measured by thermoporometry using CCl4 as a probe liquid. All numerical relationships required for the calculation of the MSD were established for both the liquid-to-solid and the solid-to-solid thermal transitions of CCl4 and successfully validated on test samples. It was found that the polymer network, for both thermally and photo-cross linked materials, was completely built in about four hours of exposure. A clear correlation was established between the average mesh size of the swollen polymer network on the one hand and the benzoyl peroxide groups content and swelling ratio on the other hand

Quantum versus classical descriptions of sub-Poissonian light generation in three-wave mixing

Sub-Poissonian light generation in the non-degenerate three-wave mixing is studied numerically and analytically within quantum and classical approaches. Husimi Q-functions and their classical trajectory simulations are analysed to reveal a special regime corresponding to the time-stable sub-Poissonian photocount statistics of the sum-frequency mode. Conditions for observation of this regime are discussed. Theoretical predictions of the Fano factor and explanation of the extraordinary stabilization of the sub-Poissonian photocount behavior are obtained analytically by applying the classical trajectories. Scaling laws for the maximum sub-Poissonian behavior are found. Noise suppression levels in the non-degenerate vs degenerate three-wave mixing are discussed on different time scales compared to the revival times. It is shown that the non-degenerate conversion offers much better stabilization of the suppressed noise in comparison to that of degenerate process.Comment: 9 pages, 12 figures, to be published in J. Optics

Lattice worldline representation of correlators in a background field

We use a discrete worldline representation in order to study the continuum limit of the one-loop expectation value of dimension two and four local operators in a background field. We illustrate this technique in the case of a scalar field coupled to a non-Abelian background gauge field. The first two coefficients of the expansion in powers of the lattice spacing can be expressed as sums over random walks on a d-dimensional cubic lattice. Using combinatorial identities for the distribution of the areas of closed random walks on a lattice, these coefficients can be turned into simple integrals. Our results are valid for an anisotropic lattice, with arbitrary lattice spacings in each direction.Comment: 54 pages, 14 figure

Photo-stabilizing efficiency of N-substituted hindered amines in polypropylene : Effects of processing conditions and exposure to a protonic acid

The effects of exposure to processing conditions and strongly acidic gases on the photo-stability of polypropylene films stabilized with hindered amine light stabilizers (HALS) have been studied. A series of monofunctional hindered amines based on tetramethylpiperidine containing \u3009NH\u3009NO\u3009NCOCH\u2083 and \u3009N--OC(==O0CH\u2083 substituents in the piperidyl ring was compared with a bifunctional secondary amine. Exposure to heat and shear in a Brabender Plastograph caused loss by volatilization and decomposition of the \u3009N O C(==O)CH\u2083 additive. All of the HALS derivatives formed salts upon exposure to gaseous HCl and their photo-stabilizing ability was greatly impaired although the monofunctional secondary amine was superior to the bifunctional amine under these conditions. The \u3009N-COCH\u2083 additive was extensively decomposed by the HCl exposure.Peer reviewed: YesNRC publication: Ye

The oxidative degradation of ethylene/vinyl alcohol copolymers

The oxidative behavior of ethylene~vinyl alcohol copolymers has been examined after initiation by \u3b3-irradiation. Oxidation products were identified and quantified by infrared spectroscopy after the use of selective derivatization reactions which allowed the resolution of the various alcohol, hydroperoxide and carbonyl species produced. Extensive preferential attack at the H--C--OH sites was observed to give backbone ketone and probably hydrogen peroxide. The latter was not unambiguously identified or quantified because of its volatility. The derivatization techniques failed at vinyl alcohol levels above 20 wt%, probably because of the low gas permeability of these films. Implications of the observed products to the oxidation of polyethylene are discussed.Peer reviewed: YesNRC publication: Ye

Spectral Properties of Probes Containing Benzothioxanthene Chromophore linked with Hindered Amine in Solution and in polymer Matrices

International audienceAbsorption and emission spectroscopy as well as laser flash photolysis was employed in order to characterize the spectral properties of novel probes based on benzothioxantheneimide chromophore covalently linked with different types of sterically hindered amines. These were chosen as 2-(2,2,6,6-tetramethyl-4- piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINH), the equivalent stable nitroxyl radical, i.e. 2-(1-oxo-2,2,6,6-tetramethyl-4- piperidyl)thioxantheno[2,1,9dej]isoquinoline 1,3-dione (BTXINO) and the alkoxy derivative 2-(1-(1'-phenylethoxy)-2,2,6,6-tetramethyl-4-piperidyl)- thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINOR). Spectral properties, in solutions and in various polymer matrices such as polystyrene, polymethyl methacrylate, polyvinyl chloride and polypropylene, were compared with the compound 2-(1-dodecyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXID) taken in the present study as a reference compound. By means of the fluorescence decay and in the contrary to three other probes, BTXINO probe clearly showed a biexponential decay while the three other probes led to monoexponential decay. Two different singlet excited states with lifetimes of about 0.4 and 5 ns were proposed. They correspond to two dispositions of the nitroxyl radical chain above and along the fluorescent moiety of the molecule. Such behaviour depends on the surrounding media. Moreover, an efficient intramolecular quenching of the fluorescence emission was only observed with the short lived singlet excited state. The ratio BTXID/BTXINO was found equal to about 4 and 9 in solutions and polymer matrices respectively. Laser flash photolysis indicated that the novel probes as well as the model compound yielded transient absorption with maximum at 530 nm, corresponding to the triplet states. The intermolecular quenching of such species by molecular oxygen and by free N-oxyl, such as 1-oxy-2,2,6,6-teramethylpiperidine (TEMPO) and 1-oxy-2,2,6,6-teramethyl-4- hydroxypiperidine (TEMPOL), and the intramolecular quenching was not efficient

Oxidation of solid polyethylene films : Effects of backbone branching

The oxidation rates and product distribution of a series of linear and branched polyethylene films have been studied when initiated by \u3b3-irradiation. Branch groups in laboratory-synthesized samples ranged from ethyl to C\u2081\u2086H\u2083\u2083\u2014and were at levels comparable with those found in commercial linear-low and low-density polyethylenes. In the complete absence of stabilizers and at the controlled rates of radical formation possible with \u3b3-irradiation, all of the polyethylene samples oxidized at essentially the same rate to give the same mixture and distribution of products. The differences in oxidation rates previously attributed to the presence of tert. C---H groups at branch sites more probably originate from the effects of impurities, especially on photo- and thermal initiation processes.Peer reviewed: YesNRC publication: Ye

Stabilization of polypropylene to \u3b3-initiated oxidation

The chemical and physical property changes produced in polypropylene samples by \u3b3-irradiation in air have been measured for a range of potential stabilizers. Impact properties of post-irradiation aged samples were compared with the yields of tertiary and secondary hydroperoxides, peroxyl radicals and ketones. Some additives markedly increased the initial rate of peroxyl radical decay, but this effect did not correlate with subsequent stability. Retention of impact strength was found for additives which minimized the initial formation of hydroperoxides and ketonic, backbone scission products. A piperidyl additive gave good protection but was especially effective when used in combination with some benzophenone derivatives. The latter were largely ineffective radiation stabilizers when used alone. None of the stabilizers appeared to reduce the initial yield of peroxyl radicals produced in the \u3b3-irradiated polymer and the piperidyl compound was only converted to its nitroxide during the post-irradiation period.Peer reviewed: YesNRC publication: Ye