Local Mechanism of Phenyl Ring π-Flips in Glassy Polycarbonate

Dynamics and mechanism of flame retardants in polymer matrixes: experiment and simulation

We investigate the dynamics and the mechanism of flame retardants in polycarbonate matrixes to explore for a way of designing efficient and environment-friendly flame retardants. The high phosphorus content of organic phosphates has been considered as a requirement for efficient flame retardants. We show, however, that one can enhance the efficiency of flame retardants even with a relatively low phosphorus content by tuning the dynamics and the intermolecular interactions of flame retardants. This would enable one to design bulkier flame retardants that should be less volatile and less harmful in indoor environments. UL94 flammability tests indicate that even though the phosphorus content of 2,4-di-tert-butylphenyl diphenyl phosphate (DDP) is much smaller with two bulky tertiary butyl groups than that of triphenyl phosphate (TPP), DDP should be as efficient of a flame retardant as TPP, which is a widely used flame retardant. On the other hand, the 2-tert-butylphenyl diphenyl phosphate (2-tBuDP), with a lower phosphorus content than TPP but with a greater phosphorus content than DDP, is less efficient as a flame retardant than both DDP and TPP. Dynamic secondary ion mass spectrometry and molecular dynamics simulations reveal that the diffusion of DDP is slower by an order of magnitude at low temperature than that of TPP but becomes comparable to that of TPP at the ignition temperature. This implies that DDP should be much less volatile than TPP at low temperature, which is confirmed by thermogravimetric analysis. We also find from Fourier transform infrared spectroscopy that Fries rearrangement and char formation are suppressed more by DDP than by TPP. The low volatility and the suppressed char formation of DDP suggest that the enhanced flame retardancy of DDP should be attributed to its slow diffusivity at room temperature and yet sufficiently high diffusivity at high temperature.

Effect of stretching on the sub-Tg phenylene-ring dynamics of polycarbonate by neutron scattering

We have investigated the effect of cold drawing on the motion of phenylene rings in bisphenol-A polycarbonate by neutron scattering. The intensity scattered by isotropic and stretched polycarbonate is different, the quasielastic broadening being larger for the isotropic sample. This difference can be well accounted for by considering that preferentially oriented rings in the stretched polymer have their motion sterically hindered. The extent of the effect of stretching on the phenylene-ring motion obtained from this neutron-scattering investigation is in good agreement with that obtained when studying the effect of cold drawing on the gamma relaxation by dielectric spectroscopy.

Phenylene ring dynamics in phenoxy and the effect of intramolecular linkages on the dynamics of some engineering thermoplastics below the glass transition temperature

We have investigated the dynamics of phenylene rings in the engineering thermoplastic bisphenol-A poly(hydroxyether) -- phenoxy -- below its glass transition temperature by means of neutron scattering techniques. A relatively wide dynamic range has been covered thanks to the combination of two different types of neutron spectrometers, time of flight and backscattering. Partially deuterated samples have been used in order to isolate the phenylene ring dynamics. The resulting neutron scattering signal of phenoxy has been described by a model that considers pi flips and oscillation motions for phenylene rings. The associated time scales are broadly distributed with mean activation energies equal to 0.41 and 0.21eV , respectively. Finally, a comparative study with the literature shows that the dielectric and mechanical gamma relaxation in phenoxy exhibit good correlation with the characteristic times of the aliphatic chain published elsewhere and with the characteristic times observed for the motion of phenylene rings by neutron scattering. These findings are discussed in a more general framework that considers, in addition, previous results on other polymers, which also contain the bisphenol-A unit.

Phenylene ring dynamics in bisphenol-A-polysulfone by neutron scattering

We have investigated the dynamics of phenylene rings in a glassy polysulfone (bisphenol-A-polysulfone) by means of quasielastic neutron scattering. Nowadays it is well known that these molecular motions are directly connected with the mechanical properties of engineering thermoplastics in general. The particular system investigated by us has the advantage that by selective deuteration of the methyl groups, the neutron scattering measured is dominated by the incoherent contribution from the protons in the phenylene rings. In this way, the dynamics of such molecular groups can be experimentally isolated. Two different types of neutron spectrometers: time of flight and backscattering, were used in order to cover a wide dynamic range, which extends from microscopic (10(-13) s) to mesoscopic (10(-9) s) times. Moreover, neutron diffraction experiments with polarization analysis were also carried out in order to characterize the structural features of the sample investigated. Fast oscillations of increasing amplitude with temperature and pi-flips are identified for phenylene rings motions. Due to the structural disorder characteristic of the amorphous state, both molecular motions display a broad distribution of relaxation times, which spreads over several orders of magnitude. Based on the results obtained, we propose a model for phenylene rings dynamics, which combines the two kinds of molecular motions identified. This model nicely describes the neutron scattering results in the whole dynamic range investigated.

Compensating for pulse imperfections in REDOR

Rotational-echo double resonance (REDOR) is a magic-angle spinning technique for measuring heteronuclear dipolar couplings. Rotor-synchronized pi pulses recouple the dipolar interaction. The accuracy of a REDOR determination of distance or orientation depends totally on the quality of the dephased (recoupled) and full-echo spectra. We present a scheme for measuring and compensating for the effects of pulse imperfections in REDOR experiments. No assumptions are made about the quality of the pi pulses, and no pulses are added or taken away in implementing the compensation for incomplete REDOR dephasing by imperfect pi pulses.

Two-dimensional transferred-echo double resonance study of molecular motion in a fluorinated polycarbonate

A two-dimensional version of the transferred-echo double resonance (TEDOR) experiment is introduced to measure strong heteronuclear magnetic dipolar interactions between rare spins. A quantitative description of this 2D-TEDOR experiment is given for an I-S pair of spins undergoing relative motion. 2D-TEDOR calculations are consistent with data for a calibration model: the directly bound 13C-31P pair in [3-13C]glyphosate, HO31(3)P13CH2NHCH2COOH. Analysis shows that the indirect spin-spin interaction and the dipole-dipole interaction have opposite signs in this system. The 2D-TEDOR experiment is then used to measure heteronuclear 13C-19F interactions in a fluorinated polycarbonate in which an aromatic proton is replaced by a fluorine on every fourth ring. Results from the 2D-TEDOR experiment show that the aromatic ring flips that occur in ordinary polycarbonate are blocked for a fluorinated ring.