D. Chatain

Multiple transitions in isotactic polypropylene around and above the glass transition

Abstract Gas chromatography, thermal expansion, dynamic mechanical relaxation and thermally stimulated current measurements were used to study multiple transitions in isotactic polypropylene. The crystalline phase relaxation Tαc was decomposed into two components, Tαc1 and Tαc1, in order of decreasing temperature. The activation energy of the Tαc mode was found to be lower than that of the relaxation observed around the glass transition temperature. Thermally stimulated current (TSC) measurements revealed the existence of an apparent double liquid-liquid transition at 39°C and 68 °C in undrawn polypropylene and at 28°C and 55°C in drawn polypropylene. These TSC peaks correspond to a relaxation time following a Vogel equation whose critical temperatures let us predict an apparent double glass transition in polypropylene.

Experimental resolution of the retardation time spectrum in polymeric solids by a new method: Thermostimulated creep

Abstract We propose a new method for the investigation of molecular motion in polymeric solids. In a mechanical step-function experiment, we thermally stimulate the response to a constant stress. The high resolving power of this technique permits detailed study of the complex retardation modes observed in polymers. By using “fractional” loading programs it is possible to differentiate a discrete and a continuous distribution of retardation time. This technique allows us to predict the complex compliance in a very wide frequency range: 104-10−12 Hz for experiments performed between liquid nitrogen temperature and 500°K. In low-density polyethylene, we have shown the existence of a discrete spectrum of mechanical retardation times which has the same fine structure as the spectrum of dielectric relaxation times obtained from the study of depolarization thermocurrent on the same sample. The predicted variation versus temperature and frequency of the loss compliance is compared with that of the dielectric loss...

Thermally stimulated current and creep of solid amylose

Abstract Dielectric and viscoelastic properties of solid amylose were investigated by thermally stimulated current and creep techniques in order to clarify the relation between its structure and properties, and also the different binding modes of water molecules. Several current peaks were observed, and these were decomposed into elementary processes by the fractional polarization technique. A peak at about —110[ddot]C was attributed to the rotational motion around the C5-C6 axis of methylol groups attached to the C5 atom in the glucose residue. A peak at about — 50dGC was attributed to bound water: This peak was eliminated by drying the specimen. A peak at about —230[ddot]C was attributed to another kind of bound water. A peak at about 30[ddot]C was attributed to dehydration during the experiment. Thus, three different relaxation modes of bound water were separated by the present work.

Thermally stimulated current study of relaxation in glassy polycarbonate

The transitions/relaxations of polycarbonate around its glass transition temperature, Tg, have been investigated by differential scanning calorimetry and thermally stimulated current spectroscopy. A sub-glass transition/relaxation has been observed with higher intensity in the quenched sample. This precursor of Tg is characterized by relaxation times following a Fulcher-Vogel equation. It might be explained by the diffusion of conformational defects all along the chain. The glass transition/relaxation is constituted of two components: (i) The lower-temperature component is associated with a distribution of relaxation times obeying a compensation law. It corresponds to cooperative movements of sequences of various length in the ‘true’ amorphous regions. (ii) The upper-temperature component is enhanced by annealing above Tg. It has been attributed to the ‘constrained’ amorphous regions. Note that such domains with high local order have been previously observed by electron microscopy and X-ray diffraction.

Thermally Stimulated Current Studies of Transitions in Amorphous Polymers

Thermally Stimulated Current (TSC) studies allow us to investigate the transition spectra of amorphous polymers. The relaxation modes observed around and above the glass transition (T g ) have common features: (1) The TSC peak isolated around T g corresponds to a distribution of relaxation times following an Arrhenius equation. The width of the distribution characterizes the distribution of the order parameter. (2) The TSC peak observed some 50° above T g is well described by a Fulcher-Vogel equation. This mode, which can also be distributed, has been associated with the dielectric manifestation of the liquid-liquid transition (T ll ).

Thermostimulated creep in amorphous polyolefins. II

Abstract The thermostimulated creep of two amorphous polyolefins having the repeating unit ─(CH2)mC(CH3)2─, where m = 2 and 3, was investigated from 77° to 350°. Two relaxation processes are distinguished: a secondary relaxation is observed at 138 and 113°, respectively, for m = 2 and m = 3; a primary relaxation is found around the glass transition. These relaxations have been resolved in their elementary components. From the data acquired, the mechanical losses have been calculated and compared with data from an inverted torsional pendulum. The activation energy found for the secondary relaxation—0.26 eV at 138° K for the amorphous polyolefin with m = 2 and 0.19 eV at 113° K for the polyolefin with m = 3—confirms that this relaxation mode is associated with restricted backbone motion.

Thermally stimulated current and creep in poly(ethylene terephthalate)

Two thermal stimulation techniques (thermally stimulated current and creep) have been used to study the dielectric and mechanical retardation modes observed in poly(ethylene terephthalate) between liquid‐nitrogen temperature and the glass transition temperature. From the data acquired, the dielectric and mechanical energy losses have been calculated: they are found to be in good agreement with those derived from other dielectric and mechanical techniques. The high resolving power of the thermal stimulation methods has allowed us to distinguish three modes in both bioriented and amorphous PET. In the bioriented sample, the three modes correspond to a discrete distribution of retardation time τ, while in the amorphous sample, they correspond to a continuous distribution of τ. In amorphous PET, the distributions of dielectric and mechanical retardation times are reduced to a single τ—5×10−4 and 7×10−3 sec—for a converging temperature of 233 and 277 K, respectively.

Thermally stimulated currents in poly(vinyl chloride): Tacticity and molecular weight influence

Abstract Thermally stimulated currents (TSC) have been measured in several samples of poly(vinyl chloride) differing in tacticity and molecular weight as a result of polymerizing them at different temperatures. This has allowed us to characterize the relaxation behavior of PVC. No dielectric relaxation can be observed by this experimental technique at temperatures between liquid helium and liquid nitrogen. The β relaxation is observed around 173°K, with similar parameters in all samples studied. Around the glass transition the relaxation times isolated in the α peak follow a compensation law. Molecular weight and tacticity have a strong influence on the temperature of the maximum and the intensity of this relaxation, respectively.

Thermally stimulated current and creep in amorphous poly(ethylene terephthalate)

Abstract Two thermally stimulated techniques were used to perform a detailed study of the retardation mode found in amorphous poly(ethylene terephthalate) below the glass-rubber transition. The complex retardation process observed has been resolved in its elementary components. From the data acquired, the dielectric and mechanical losses have been calculated and compared with data from conventional mechanical and dielectric techniques.

Thermally stimulated creep

Abstract We propose a new method for the investigation of molecular movements in solids: in a mechanical step-function experiment, we thermally stimulate the response to a given frozen-in stress (or strain). The high resolving power of this technique enables a detailed study of the complex retardation (or relaxation) modes observed in polymers. Results on thermally stimulated creep of polyethylene are presented as an example.