Onset Of Segmental Motion Research Articles

Dielectric Spectroscopy of Polystyrene/Poly(ethylene oxide) Composites

Dielectric spectra of composites of poly(ethylene oxide), PEO, in polystyrene, PS, have been obtained, as a function of frequency and temperature. The dispersion of a high-dielectric constant, low-glass transition temperature, semicrystalline polymer (PEO) in an amorphous, lower dielectric constant, high-Tg continuum (PS) has enabled the dielectric observation of four molecular relaxation processes in PEO. They are the α relaxation associated with the crystalline melt of PEO; two WLF type relaxations, β and β‘, which we speculate to be respectively associated with the onset of segmental motion and free-volume expansion of unrestricted amorphous PEO chains and with amorphous PEO chain segments tethered in crystallites; and a sub-Tg, γ relaxation associated with localized motions in the crystalline chain segments. The relaxations associated with the α and β‘ processes have not been previously observed dielectrically. In pure, neat PEO, the α and β‘ relaxations are obscured by dc losses. The dispersion of PEO as micron-sized and submicron phases in PS enables its full relaxation spectrum to be observed.

A Broad-Line NMR Investigation of Isoprene Rubber Filled with Carbon Black

Abstract The results of NMR measurements on IR filled with various carbon blacks lead to the following conclusions. The rubber matrix is a heterogeneous system characterized by two rubber phases of different chain mobilities. One phase is almost similar to unfilled IR at the temperature of measurement, while the properties of the other are nearly identical to those of pure IR at a lower temperature. The following observations apply to the phase with restricted mobility : 1. The quantities of rubber and carbon black in this phase are proportional below a critical loading, which supports the assumption that immobilization is limited to the immediate surroundings of the filler aggregates. The critical loading varies with structure and defines the packing density above which the formation of a coherent gel becomes important. 2. The rubber content decreases with rising temperature, showing that the activation energy for the onset of segmental motion varies within the phase. The mobility of the rubber chains directly bound to the surface is restricted even far above the glass transition temperature. 3. The degree of immobilization varies with temperature but is mostly independent of carbon black type and loading. 4. The actual distribution of immobilized rubber within an aggregate is not completely known, but some evidence exists for the immobilization of a thin surface layer as well as parts of the occluded rubber. A considerable immobilized rubber fraction is independent of surface area and structure.

Electron spin resonance studies of irradiated polypropylene

AbstractPolypropylene has been irradiated at −196°C. and its electron spin resonance spectra studied at temperatures between −196°C. and room temperature. The intensity of the eight‐line spectrum observed at −196°C. corresponded to a G for radical production of 3.3. The spectrum could be reconstructed from theoretical curves assuming equal concentrations of radicals with four‐ and eight‐line spectra. On warming to room temperature the radical concentration was reduced to 30% of the initial concentration. In experiments in which the sample was warmed to −100°C. no change in radical concentration was observed. Annealing experiments at temperatures between −100°C. and the glass transition temperature showed that at each temperature there was a rapid decay of radicals after which no further change in radical concentration took place. Above the glass transition temperature a continuous decay occurred. The onset of decay above −100°C. may be associated with the onset of segmental motion in the amorphous regions of the polymer. The rapid decay of the radicals suggests that they are formed near each other as the results of hot hydrogen atom reactions.

Molecular motions in polyethylene terephthalate

The molecular motions in polyethylene terephthalate samples of different crystallinities have been investigated by dynamic mechanical measurements at a frequency of 1 cycle/sec. between −180° and +260°C., and are discussed in connection with nuclear magnetic resonance, dielectric, and X-ray results of other authors. The variations of the α-loss modulus peak, indicating the onset of segmental motion within the amorphous regions with increasing crystallinity, are discussed in connection with the morphological state and its variation with the thermal prehistory of the specimens. Especially, it is found that the glass temperature T g with the corresponding loss peak temperature T max goes through a maximum value with increasing crystallinity. A low-temperature loss modulus peak, occurring at 1 cycle/sec. and ca. −65°C., seems to be a very complex one. It is shown that probably two or three different relaxation processes are superimposed at this loss peak. The low-temperature component consists in the onset of hindered rotation of the CH 2-groups. Its high-temperature component obviously includes motions of the COO-groups, as is seen by dielectric measurements. The influence of crystallinity, of the addition of low molecular weight solvents, and of an enlargement of the number of CH 2-groups in the repeating unit, respectively, upon the low-temperature relaxation processes are in agreement with the above conclusions.