Glass Transition Process Research Articles

Dielectrical properties of u.v. cured acrylated polysiloxane films

The dielectric properties of films obtained through u.v. curing of an acrylated polysiloxane resin were evaluated in the temperature range from −136 to 24‡C. Different relaxation processes (α, Β, γ, δ), characterized by decreasing dielectric relaxation intensities, were evidenced and quantitatively analysed: the α process was attributed to the glass transition process on the basis of the dynamic mechanical spectrum of the film and of its behaviour at different temperatures. The other processes (Β, γ, δ), weaker than the α process, could be attributed to local dipolar motions present, even at low temperatures, in the polymer network.

Dynamic behaviour of power-compensated differential scanning calorimeters. Part 3. The influence of material properties (an error evaluation)

Falsification (smearing) of measured DSC curves is caused by heat conduction phenomena from the DSC and from the sample itself. In a previous paper in this series, an algorithm is given to desmear measured heat flow-rate curves on the basis of the theory of linear response. In the case of a sample with low thermal conductivity and small changes of the material properties, the Green's function can be estimated from the measured curve itself. These conditions are valid for the glass transition process of polymers and one can obtain the Green's function for desmearing the DSC curves of this process from the glassy state or from the liquid state respectively. But every change of material properties during the thermal event causes an error in the desmearing procedure. This error can be evaluated from a model calculation.

Unconventional orientational glass transitions in symmetrical difluorotetrachloroethane

Frozen-in trans and gauche conformational disorder of symmetrical difluorotetra-chloroethane (DFTCE) produces the tendency of this plastic crystal to form an orientational glassy state on cooling below Tg=86 K. To throw light upon the freezing mechanism in DFTCE neutron diffraction and Brillouin scattering as well as infrared and dielectric measurements are employed. The overall picture of the results obtained can be characterized in terms of a classical glass-transition process with a relaxation frequency obeying the Vogel-Fulcher law. However, a more sophisticated picture of the freezing process is indicated by a quantitative analysis of the new Brillouin data, e.g. the temperature dependence of the sound velocity. The behaviour could be understood on the basis of a mode-coupling theory.

Transverse deuteron spin relaxation studies of a smectic liquid crystal polymer: Local motions, order director fluctuations, and the glass transition process

Molecular dynamics of a main chain thermotropic liquid crystal polymer in the smectic A phase has been investigated using multipulse dynamic nuclear magnetic resonance (NMR) techniques. Transverse deuteron spin relaxation times T2ECP from quadrupole echo pulse trains (modified Carr–Purcell–Meiboom–Gill sequence) measured for deuterons in the aromatic rings of the mesogenic units are obtained as a function of pulse spacing τ sample orientation θN, and temperature. Just below the nematic–smectic A phase transition, the relaxation times exhibit a linear dispersion regime T2ECP ∼ τ−1 consistent with smectic director fluctuations. At lower temperatures, the dispersion step gradually disappears, indicating that faster molecular motions are the dominant transverse relaxation process. The observed anisotropy in T2ECP, measured at short pulse spacings, approximately follows the (sin4θN)−1 dependence expected for axial diffusion in a highly ordered medium. Analysis of the experiments is achieved employing a density operator treatment based on the stochastic Liouville equation. The intramolecular motion is identified with phenyl ring flips and is the fastest process studied, with correlation times varying from 10−10 to 10−7 s over the temperature range investigated. Intermolecular (individual molecule) dynamics are somewhat slower and have been interpreted as rotational diffusion in an orienting potential. The correlation times for intermolecular motion exhibit non-Arrhenius behavior approaching the glass transition, following a temperature dependence described by the Williams–Landel–Ferry equation over six orders of magnitude. This result indicates a strong coupling of the intermolecular motion to the glass transition process. The slowest motion affecting transverse deuteron spin relaxation is assigned to smectic director fluctuations or undulation waves. Analysis of the T2ECP dispersion yields information concerning the viscoelastic properties of the polymer. At T=418 K, a splay elastic constant of K1=2×10−11N has been estimated. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, the root mean square fluctuation 〈θ20〉1/2 of the director is calculated to be 4°.

Diffusion in blends of poly(methyl methacrylate) and poly(styrene- co-acrylonitrile)

Forward recoil spectrometry was used to obtain the tracer (D*) and mutual (D) diffusion coefficients in a miscible polymer blend of poly(methyl methacrylate) (PMMA, Tg=136°C) and poly(styrene-co-acrylonitrile) with ∼23 wt% acrylonitrile content (SAN, Tg=112°C). For blends with SAN weight fraction w of 0.2 and 0.5, the temperature dependence of D* for both species was nearly identical. Tracer diffusion coefficients D*PMMA and D*SAN were determined for matrices consisting of 176 000 molecular weight SAN (w=0.5) and various molecular weights of PMMA ranging from 27 000 to 840 000. The results were consistent with those expected from the mechanisms of reptation and constraint release. Analysis of the tracer diffusion coefficients D* showed that SAN has a monomer friction coefficient, ζ0,PMMA, about five times smaller than that of PMMA (ζ0,PMMA) in a matrix of pure PMMA, but the difference decreased monotonically as w increased, so that the ζ0 values were nearly equal when w=1. Corresponding to this relative change in ζ0, the glass transition process is broad for PMMA-rich blends and narrow for SAN-rich ones, raising the possibility that the difference in ζ0 in the PMMA-rich blends is due to the existence of two local glass transitions, one for each species in the blend. For blends at the composition of w=0.5, the D was measured as a function of PMMA molecular weight. The data followed closely the predictions of the fast theory, the result expected if D is ultimately controlled by the diffusion of the faster moving species. From the measurements of D and D* at 187°C, the composition dependence of the Flory interaction parameter was also obtained, which showed good agreement with the recent small angle neutron scattering results by Hahn et al.

Coupled relaxations in a blend of PMMA and a polycarbonate

Proton lineshape and proton spin-lattice relaxation, T 1 ρ , measurements were made on blends of bisphenol chloral polycarbonate and perdeutero poly(methyl methacrylate) (PMMA). The solution-cast blends are optically clear and display a single thermal glass transition temperature, T g, as reported. The proton lineshape and proton relaxation data are used to follow the dynamics of the polycarbonate in the blend. The glass transition process is monitored by lineshape collapse as a function of temperature and the sub-glass transition process associated with π flips of the phenylene group is monitored by the proton T 1 ρ values as a function of temperature. The glass transition as viewed from the polycarbonate remains similar in character to the behaviour observed in pure polycarbonate. Below T g, the lineshape consists of a broadened Pake doublet and above T g a narrow Lorentzian grows in on top of the Pake pattern. The bimodal behaviour and the growth of the narrow line with temperature can be described by the Vogel-Tamman-Fulcher equation with T 0 values which decrease with PMMA concentration. The proton T 1 ρ data show antiplasticization in the form of suppression of the π flip process as PMMA is added. In addition, a new minimum associated with the motion of the polycarbonate is observed at the temperature and time scale of the ester group rotation of the PMMA. This apparent coupling of the sub- T g relaxation processes is related to the behaviour of polycarbonate upon addition of low-molecular-weight ester diluents. The level of antiplasticization and the amount of coupling is underestimated by a lattice model and an assumption of random mixing. On the very local scale of the π flip motion and at the higher concentrations of PMMA, there are apparently fewer PMMA-polycarbonate contacts than a random mixing assumption indicates.

Experimental study of the nature of the glass transition process in monohydroxy alcohols

The nature of the glass transition process in six monohydroxy alcohols, viz., the isomers of butanol, 2-propanol, and 3-methyl–1-butanol had been studied in detail using dielectric spectroscopy (frequency range 10+6–10−3 Hz) and differential scanning calorimetry (DSC). Measurements were made down to a temperature of 77 K in dielectric experiments and a temperature of 95 K in DSC. These results together with the published microwave data on these liquids reveal the existence of two glass transition processes—one due to the hydrogen bonded network which is very dominant and the other one due to the nonhydrogen bonded molecules, which is much weaker. Associated with the latter process, there is a secondary (or β-) relaxation process whose activation energy varies from 4.8 to 6.7 kcal/mol which is dependent on the shape and size of the liquid molecules. In addition to the β process, all the alcohols revealed yet another sub-Tg (γ) process whose activation energy varies from 2.1 to 2.6 kcal/mol and is found to be little dependent on the molecular shape. These results perhaps indicate that the β process is somehow connected to the relaxation of the free monomeric molecule and the γ process to that of hydroxyl group rotation, respectively.

Elastic and viscous behavior of an amorphous Zr 33Y 27Al 15Ni 25 alloy with a two-stage glass transition

A Zr 33Y 27Al 15Ni 25 amorphous alloy was found to exhibit a two-stage glass transition and crystallization processes because of the insoluble nature between Zr and Y. The first-stage exothermic reaction is due to the precipitation of the nanoscale Y-rich phase from the amorphous matrix and the precipitates cause the suppression of the decrease in viscosity and elasticity in the supercooled liquid region.

Determination of the parameters of the relaxation spectrum of polymers from the results of calorimetric measurements

AbstractA method is suggested which allows the determination of a complex of relaxation characteristics describing the process of glass transition of amorphous polymers from the results of calorimetric measurements in one single experiment.

Glass formation in organic binary liquids studied using differential scanning calorimetry

Differential scanning calorimetric (DSC) studies have been performed on binary mixtures of organic liquids at temperatures down to 100 K over the entire concentration range. The liquids investigated are simple liquids such as chlorobenzene and bromobenzene in cis-decalin and o-terphenyl. All the binary liquids exhibited a simple eutectic phase diagram. Glass formation was studied in detail by varying the cooling rate, degree of crystallinity and concentration. In liquids where the components are bad glass-formers on their own, the glasses formed are mixed glasses and the probability of glass formation is maximum for intermediate concentrations.Over a wide range of concentration, the liquid mixtures freeze into a glassy state around the same temperature. The dominant freezing in the liquid mixtures occurs at the eutectic concentration. This is supported by studying the freezing and associated enthalpy for the eutectic mixture. For faster cooling rates almost all the binary liquids exhibit two or more glass-transition processes, and these are discussed on the basis of the phase diagram. The nature of glass formation in binary liquids whose components are good glass formers is also discussed.

Dielectric analysis of poly(vinyl chloride) films doped with cobalt(II) chloride

The dielectric behaviour of solution-grown thin films of poly(vinyl chloride) containing cobalt(II) chloride as dopant was investigated within the temperature range 20–120°C and a 1–100 kHz frequency band. When correction was made for a conduction effect at low frequency, a high-temperature glass-transition relaxation process was observed in addition to the ordinary glass-transition process of pure PVC. The former was thus considered to be a cluster-ionic transition process which might be due to a phase-separation behaviour. The effect of moisture uptake by the doped films on the electrical properties is discussed.

Amorphism and physicochemical stability of spray-dried frusemide.

The physicochemical properties of amorphous forms of frusemide, prepared by spray-drying at 50 or 150 degrees C, and their hygroscopic stability at temperatures of 25 and 40 degrees C, and at 0 and 75% relative humidity were investigated. The glass transition temperature of the amorphous form A was 44.2 degrees C as measured by differential scanning calorimetry, while that of the amorphous form B was 54.4 degrees C. The activation energies for glass transition and crystallization processes were calculated from the differential scanning calorimetry thermograms of amorphous forms A and B, respectively. Stability determined by X-ray diffraction at 0% relative humidity, 25 and 40 degrees C suggested that form B was more stable than form A. However, the stability of form A at 75% relative humidity and 25 and 40 degrees C was similar to that of form B.

Relaxation time spectra and the peculiarities of the process of boron anhydride glass transition

For a boron anhydride glass, the pre-exponential coefficient, B α , and the activation energy, u α , of the Boltzmann-Arrhenius equation are estimated by temperature-frequency dependence of the α -process. The temperature dependence of activation energy obeys the well-known Vogel-Fulcher-Tammann equation. A standard glass transition temperature, T g , is equal to 220°C, and the corresponding activation energy is equal to 110 kJ/mol. From these data, it can be concluded that glassy B 2 O 3 , by contrast with fused silica, is a linear polymer. Its viscosity and the process of glass transition, as in organic linear polymers, is determined with the overcoming of intermolecular bonds and the mobility of boron-oxygen chains rather than the rupture of the chemical B O bonds. A method of calculation of the continuous relaxation time spectra is proposed based on the Kohlrausch function, commonly used to describe the relaxation characteristics in the glass transition region. The parameter b , characterizing a relaxation time spectrum width at temperatures lower than a critical one, T c = 300°C, is b = 0.5. At higher temperatures, b increases, approaching a maximum possible value b = 1, and the relaxation time spectrum transfers to a δ -function and degenerates into the spectrum with a single relaxation time. This change is explained by the fact that structural micro-inhomogeneity of the B 2 O 3 liquid gradually disappears as the temperature increases. A method of evaluation of the degree of micro-inhomogeneity of inorganic glasses by b is proposed.

Ultrasonic properties in the glassy state of MNA(methylnadic anhydride) cured epoxy resin.

Ultrasonic properties in the MHz region were studied for a series of epoxy resins cured with methylnadic anhydride. The longitudinal and shear wave velocities in the glassy state resins decreased with crosslinking. The apparent modulus calculated from veloc ities, and longitudinal wave attenuation also showed that the elasticity in the glassy state resins vary inversely with the crosslinking density, whereas the elasticity in the rubbery resins was expected to vary directly with the crosslinking density. Those differences in elasticity between the glassy state resins and the rubbery state resins can be explained by a hindering effect of crosslinks during glass transition process.

Molecular dynamics in glassy polyelectrolytes: a NMR study

The segmental dynamics of a special “ionene” polyelectrolyte, poly[(dimethylimino) decamethylene-tetrafluoroborate] ([—(CH 2) 10—N +(CH 3) 2—] n · nBF − 4), have been investigated by applying one-and two-dimensional deuteron NMR techniques. Selective deuteration in the methyl groups and in the α-, β- and γ-positions of the alkyl chain allowed for investigation of the molecular mobility of each site. Due to strong Coulombic interactions between the cationic centres, the ammonium groups are essentially static below the glass transition, whereas the alkyl chains undergo trans-gauche isomerizations. At T g the ammonium groups become mobile, and we discuss here the coupling of this glass transition process to the local methylene unit dynamics.

Segmental Dynamics in a Glassy Polyelectrolyte: Solid‐State 2H‐NMR

AbstractThe segmental dynamics of a special “ionene” polyelectrolyte, poly[(dimethylimino)decamethylene‐tetra‐fluoroborate]‐[–(CH2)10–N+(CH3)2–]n*BF4−, has been investigated by applying one‐and two‐dimensional deuteron NMR techniques. Selective deuteration in the methyl groups and in the α‐, β‐ and γ‐positions of the methylene chain allow for investigation of the molecular mobility of each site. Below the glass transition the ammonium groups are essentially static, due to strong Coulombic interactions between the cationic centres, whereas the methylene chains undergo trans‐gauche isomerizations. At Tg the ammonium groups become mobile, and we discuss the coupling of this glass transition process to the local methylene dynamics.

Relaxation properties of an epoxy polymer plastified with maleic anhydride

AbstractThe relaxation properties of the epoxy dian polymer ED‐16 plastified with maleic anhydride were studied in the wide range of temperature from −150°C to + 500°C. The experimentally found great number of relaxation transitions are connected with the complex structure of the polymer and with the existence of a large number of different forms of molecular motion. The observed relaxations are classified and their characteristic constants (activation energy and preexponential factor) are given. A nonsegmental mechanism of the process of glass transition in highly crosslinked polymers is suggested.

Glass transition and enthalpy relaxation process in coal tar pitches

Coal tar pitches generally have very complex chemical compositions and structures. Their physico-chemical properties depend on the mutual molecular interactions of their various components. In order to characterize these materials further the properties related to their glass transition behaviour have been investigated. As for polymeric materials, enthalpy relaxation can provide useful information on the interactions occurring in a glassy-type structure. Glass transition characteristics have been determined by means of differential scanning calorimetry. The kinetics of enthalpy relaxation have been analysed with the model developed by Moynihan et al. Mean molecular size and molecular size distribution have been determined for a coal tar and different types of pitches. The glass transition characteristics have been compared in some cases to results obtained by other techniques. It is shown that the investigation of tars and pitches by means of their glass transition properties leads to a good description of their structure and changes occurring during thermal treatment.

The Effect of Crosslinks on Ultrasonic Properties in Glassy Epoxy Resin

Ultrasonic properties in epoxy resins cured with three different curing agents, methylnadic anhydride (MNA) and two kinds of aliphatic diamine, were investigated in the MHz region. The apparent moduli calculated from ultrasonic velocities and longitudinal absorptions in glassy resins show a decrease of elasticity with crosslinking in the MNA cured system, whereas the elasticity of rubbery resins has been reported to be proportional to the amount of crosslinks. The elasticity in glassy resins depends on the glass transition process, and we observed a similar phenomenon in the diamine system cured at high temperature.

Calorimetric study of the glass transition in gelatin

The temperature variation of the absolute heat capacity values of crystalline and amorphous gelatin at various water concentrations was studied by the DSC method in a broad temperature range (from -50 to 130°C). At all concentrations of bound water (5–30%), with amorphous gelatins a heat capacity jump is observed, caused by devitrification (softening). The value of this jump was determined to amount to 0.50±0.05 J/g K. With increasing content of bound water, the glass temperature decreases. The appearance of free water in the system substantially affects the process of devitrification (softening). In contrast to bound water, free water does not act as plasticizer, but on the contrary, it forms a rigid ice matrix which prevents the appearance of a glass transition. It was determined how the glass-transition process is manifested in the course of heating of crystalline gelatins, of various heats of melting. In gelatins with small heats of melting, ≤ J/g, the glass-transition process practically does not differ from the process in amorphous gelatins of the same water content. In gelatins with greater heats of melting (≈ 30 j/g), softening only leads to the appearance of an additional small maximum on the linear heat-capacity-temperature plot, a heat-capacity jump is not observed.