Thermal degradation products of poly(styrenesulfides) investigated by direct pyrolysis�mass spectrometry and flash pyrolysis�gas chromatography/mass spectrometry

The thermal degradation products of two sulfur polymers, poly(styrenedisulfide) (PSD) and poly(styrenetetrasulfide) (PST), were investigated in parallel by direct pyrolysis-mass spectrometry (DPMS) and by flash pyrolysis-GC/MS (Py-GC/MS). The time-scale of the two pyrolysis techniques is quite different, and therefore they were able to detect significantly different products in the pyrolysis of PSD and PST because of the thermal lability of sulfur-containing compounds. However, the results obtained are not contradictory, and satisfactory mechanisms for the thermal degradation of PSD and PST have been derived from the overall evidence available. Pyrolysis compounds containing sulfur, styrene, and a number of cyclic styrene sulfides and diphenyldithianes have been observed by DPMS. However, in flash pyrolysis-GC/MS, styrene, sulfur, only one cyclic styrene sulfide, and two isomers of diphenylthiophene have been detected. These thiophene derivatives were indeed absent among the compounds obtained by DPMS because they were the terminal (most thermally stable) species arising from further decomposition of the cyclic styrene sulfides formed in the primary thermal degradation processes of PSD and PST.

Reversible shear-induced crystallization above equilibrium freezing temperature in a lyotropic surfactant system

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature (T-K(o)) in weakly swollen isotropic (L-i) and lamellar (L-alpha) mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e., on stopping the shear, the nonequilibrium crystalline phase L-c melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below T-K(o), which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the L-i phase to an L-alpha phase induced by shear flow, before the nucleation of the Lc phase. Shear diagram of the L-i phase constructed in the parameter space of shear rate ((gamma)) over dot vs. temperature exhibits L-i -> L-c and L-i -> L-alpha transitions above the equilibrium crystallization temperature (T-K(o)), in addition to the irreversible shear-driven nucleation of L-c in the L-i phase below T-K(o). In addition to revealing a unique class of nonequilibrium phase transition, the present study urges a unique approach toward understanding shear-induced phenomena in concentrated mesophases of mixed amphiphilic systems.