Glass Transition Process Research Articles

Laser light scattering studies of bulk polymers in the glass transition regime

AbstractThe application of laser light scattering techniques to characterize the structural and dynamical properties of bulk polymers near the glass transition temperature is reviewed. Preliminary results of a study of poly(n‐butyl methacrylate) are presented. As in an earlier study of atactic polystyrene, the results suggest the coexistence of two relaxation mechanisms near Tg: a fast, single exponential decay; a slow process characterized by a wide distribution of relaxation times. The slow relaxation mechanism is evidently the main‐chain glass transition process; it appears possible that the fast decay is a manifestation of side‐chain motion. However, in contrast to the polystyrene system where the fast mechanism is always of small amplitude in the temperature range amenable to study (Tg < T < Tg + 40°), the fast mechanism in the poly(n‐butyl methacrylate) system exhibits a sudden transition to high amplitude (∼50 percent of the total observable relaxation amplitude) at T ∼ Tg + 30°. At this point therefore, we suggest that a strong resonant coupling of the fast process with the slow relaxation is occurring.

LASER LIGHT SCATTERING STUDIES OF BULK POLYMERS IN THE GLASS TRANSITION REGIME

The application of laser light scattering techniques to characterize the structural and dynamical properties of bulk polymers near the glass transition temperature is reviewed. Preliminary results of a study of poly(n-butyl methacrylate) are presented. As in an earlier study of atactic polystyrene, the results suggest the coexistence of two relaxation mechanisms near Tg: a fast, single exponential decay; a slow process characterized by a wide distribution of relaxation times. The slow relaxation mechanism is evidently the main-chain glass transition process; it appears possible that the fast decay is a manifestation of side-chain motion. However, in contrast to the polystyrene system where the fast mechanism is always of small amplitude in the temperature range amenable to study (Tg < T < Tg + 40°), the fast mechanism in the poly(n-butyl methacrylate) system exhibits a sudden transition to high amplitude (∼50 percent of the total observable relaxation amplitude) at T ∼ Tg + 30°. At this point therefore, we suggest that a strong resonant coupling of the fast process with the slow relaxation is occurring.

Frequency dependence of dielectric loss peak temperature and determination of glass transition temperature

AbstractAmbiguities arising in the identification of the observed dielectric loss peak with the glass transition process are described, in the light of frequency dependence of the loss‐peak‐temperature. The usefulness of the WLF equation in solving these ambiguities is explored and illustrated with the help of data on four polymers. A simplified method is described, for evaluating the parameters, reference temperature of the WLF equation and thereby the glass transition temperature from the data at two frequencies.

Behavior of the drift mobility in the glass transition region of some hole-transporting amorphous organic films

Drift mobility in prototypical hole-transporting organic glasses is observed to undergo a relatively abrupt, but fully reversible, decrease in activation energy during rate heating near Tg. Sensitivity of charge transport to the dynamic phenomena peculiar to the glass transition process is therefore indicated. No significant hysteresis of the drift mobility is observed during rate heating and subsequent rate cooling through the Tg region. The latter contrasts sharply with the behavior observed in amorphous chalcogenides. It is suggested that absence of mobility hysteresis is a consequence of the fact that electonic states which control transport arise from incorporated chemical species and not, as in the chalcogenides, from structural defects.

Relaxation transitions in polyethylene

Eight relaxation processes have been investigated below the melting point in high-density polyethylene (HDPE) by relaxation spectrometry, and the nature of the processes has been examined. Two relaxation processes ( β and β 1) correspond to small-scale motion of the polymer chains in amorphous and crystalline phases. Three relaxation processes ( α, α 1 and α 2) correspond to segmental mobility of the chains respectively in amorphous phase (glass transition process, in crystal→amorphous phase transition layers, and in amorphous parts of spherulite fibrils. The three high-temperature processes ( λ 1, λ 2 and λ 3) relate to molecular mobility of elements of supersegmental structure of HDPE amorphous phase and are similar to those in noncrystalline polymers that are in the high-elastic state (elastomers).

Rotations of nitroxide radicals in condensed polymeric systems

An analytical expression was developed to explain the effect of molecular weight (M w ) upon the effective rotational frequencies (v eff) of different nitroxide spin probes incorporated in natural rubber. It was found that the effective activation energy (E) of probe rotation decreases with increasingM w and approaches the true activation energy (E 0) of segmental relaxations of polymer chains. Thev eff values of different probe radicals can be reduced to a single master curve by means of the equation: $$\frac{{v_{eff} }}{{exp\left( {\frac{{E_0 e^{ - M_W /M_S } }}{{RT}}} \right)}} = v_0 \exp \left( { - \frac{{E_0 }}{{RT}}} \right)$$ whereE 0 = 7.8 kcal mol−1,v 0 = 2 x 1013 s −1 andM s = 114 g mol−1. The value ofM s was interpreted as the smallest rotating unit of polymer chain which corresponds to a segment of about 1.7 monomer units. This leads to the interpretation that the glass transition process involves a coordinated motion of a group of about 30 short polymer segments. As a consequence of increasing intramolecular mobility of large radicals they can be regarded as flexible chains of more or less freely rotating links. Since the intermolecular coupling of the radical rotations with segmental rotations of polymer chains increases with increasingM w it is proposed that large flexible nitroxide probes describe best the segmental relaxations of polymers.

Thermal properties and relaxation transitions in copolymers of dimethylsiloxane with methyltrifluoropropylsiloxane

The linear thermal expansion coefficient, heat conductivity and dynamic and mechanical characteristics have been investigated for a number of copolymers of dimethylsiloxane with methyltrifluoropropyl in relation to the ratio of copolymer components over a wide range of temperatures. Relaxation transitions in these polymers have been examined by means of the radiothermoluminescence method. No effects related to processes of crystallization and melting were detected for the copolymers under study. The main β relaxation transition is displaced towards positive temperatures as the content of methyltrifluoropropyl units in the copolymer rises, besides which there is also a secondary γ transition that makes a major contribution to the glass transition process in the region of 150–160°K. A modified Simha-Boyer equation is presented to describe the multiple character of the glass transition process.

Investigation of viscoelastic behaviour and the concept of iso-free volume during glass transition as applied to systems containing a polymer and polymeric filler of like nature

The method of deformation and harmonic analysis provided a basis for investigation of filled epoxy systems containing ED-5 and polyethylenepolyamine to determined the temperature and frequency dependences of the complex shear modulus and the phase angle between stress and deformation. The filler was a powder of the same ED-5 polymer, which made it possible for samples to be prepared with variations in their physical structure, while the chemical compositions of the samples remained identical. Experimental results were analyzed by the method of Williams-Landell-Ferry applying the concept of free volume. It was found that the free volume fraction of the polymer is not a constant at the glass transition temperature, and is a function of the physical structure of the polymer. It is seen that in the case of rigid network polymers the free volume theory with respect to glass transition processes retains its importance solely from a qualitative standpoint, but is unacceptable from a quantitative point of view.

Thermomechanical behavior of amorphous tactic methacrylate polymers

Dynamic mechanical spectra of amorphous stereoregular poly(methyl methacrylate)s and poly(t-butyl methacrylate)s with assigned microtacticities are presented and discussed. An intermolecular argument is invoked to account for the higher glass transition temperature of syndiotactic vis a vis isotactic PMMA, in spite of the higher density of the latter at 30 C. An argument is presented to show that the ratio of glassy-region relaxation temperature to glass transition temperature is not only a measure of the degree of coupling of the beta and glass transition processes, but also of the degree to which intermolecular factors influence these processes. The greater extent of the low-temperature irreversibilities observed in the thermomechanical spectra of poly(t-butyl methacrylate)s is attributed to the brittle character induced by the bulky side groups which presumably weaken cohesive forces.

Nitrogen and helium diffusion in polystyrene

Abstract A study was made of the temperature dependence of the rate of nitrogen and helium desorption from gas-filled polystyrene films in the range 100–410°K. Considerable differences in the desorption regularities for nitrogen and helium were observed. In the range 190–380°K the diffusion of nitrogen is described by the Arrhenius law with the parameters D 0 =21 × 10 −5 cm 2 /sec and E =5·3 kcal/mole. At temperatures above 380°K there is a marked change in the diffusion parameters owing to the process of glass transition. The regularities of helium desorption are only slightly dependent on film thickness in the range 45–320 μm. This is explained by permeation of the gas through defects with dimensions comparable to those of helium atoms.

Effect of the type of state of stress on the characteristics of the mechanical glass transition process in polymers

The effect of the type of state of stress on the activation energy and relaxation time is investigated with reference to the mechanical glass transition (softening) process in polymers. An expression relating the mechanical glass transition temperature with the structural glass transition temperature, the mean stress, and the stress intensity is obtained for isotropic homogeneous polymers. Experimental data obtained for polymethyl methacrylate in uniaxial tension and compression, pure bending, and shear are presented.

The Actual Contact Area and Friction Properties of Elastomers under Frictional Contact with Solid Surfaces

Abstract Using a specially designed apparatus simultaneous measurements were carried out of the area of contact and the friction between elastomers and solid surfaces. For a wide range of load, the friction force is proportional to the actual contact area and the friction constant depends only on the nature of the rubbing solids. In the range of low sliding velocities the actual contact area remains almost constant. At high sliding velocities increase of the friction force is associated with the process of the mechanical glass-transition of the surface layer of the crosslinked rubber which causes the actual contact area to decrease.

The actual contact area and friction properties of elastomers under frictional contact with solid surfaces

Using a specially designed apparatus simultaneous measurements were carried out of the area of contact and the friction between elastomers and solid surfaces. For a wide range of load, the friction force is proportional to the actual contact area and the friction constant depends only on the nature of the rubbing solids. In the range of low sliding velocities the actual contact area remains almost constant. At high sliding velocities increase of the friction force is associated with the process of the mechanical glass-transition of the surface layer of the cross-linked rubber which causes the actual contact area to decrease.

Nuclear magnetic resonance study of low molecular weight polychlorotrifluoroethylene

AbstractThe molecular motion of low molecular weight polychlorotrifluroethylene of different molecular weights between 500 and 1300 was investigated by means of broad‐line nuclear magnetic resonance measurements. The line width and second moment of the resonant absorption lines were obtained at temperatures from 77 to 300°K. The line narrowing for low molecular weight samples takes place in one step. In samples higher than 900 in molecular weight, however, there appears to be a glass transition process, and the line narrowing tends to occur in three steps. The line narrowing due to local molecular motion becomes observable with increasing molecular weight.

Dielectric studies of mobility of polar molecules in polystyrene

Dielectric measurements (5Hz–8.5 GHz) at temperatures from –30 to 100°C have been made on polar solutes molecularly dispersed in a polystyrene matrix. Rigid molecules of varying size and shape (camphor, anthrone, cholest-4-en-3-one, tetraphenyl cyclopentadienone) and β-naphthol are considered. By 80° all the molecules show their full dipole moments in polystyrene : the apparent (Eyring) activation energies ΔH*E for molecular dipole relaxations vary from 0.9 kcal mole–1 for camphor to 130 kcal mole–1 for tetracyclone. The apparent ΔH*E over limited temperature ranges themselves reflect changes in the polymer matrix relaxations, and the 130 kcal mole–1 value may well be derived from the glass-transition process. The ΔH*E and ΔS*E correlate linearly and the former is dependent upon the molecular rotational volume. The “whole molecule” and hydroxyl group rotations are well resolved for β-naphthol with ΔH*E values of 11.2 and 0.5 kcal mole–1 respectively.