Showing papers on "Glass transition published in 1983"

Time–temperature–transformation (TTT) cure diagram: Modeling the cure behavior of thermosets

Abstract: The times to gelation and to vitrification for the isothermal cure of an amine-cured epoxy (Epon 828/PACM-20) have been measured on macroscopic and molecular levels by dynamic mechanical spectrometry (torsional braid analysis and Rheometrics dynamic spectrometer), infrared spectroscopy, and gel fraction experiments. The relationships between the extents of conversion at gelation and at vitrification and the isothermal cure temperature form the basis of a theoretical model of the time–temperature–transformation (TTT) cure diagram, in which the times to gelation and to vitrification during isothermal cure versus temperature are predicted. The model demonstrates that the “S” shape of the vitrification curve depends on the reaction kinetics, as well as on the physical parameters of the system, i.e., the glass transition temperatures of the uncured resin (Tg0), the fully cured resin (Tg∞), and the gel (gelTg). The bulk viscosity of a reactive system prior to gelation and/or vitrification is also described.

Molecular dynamics of solid polymers as revealed by deuteron NMR

Abstract: Pulsed deuteron NMR spectroscopy is described, which has recently been developed to become a powerful tool for studying molecular dynamics in solid polymers. It is shown that by analyzing the line shapes of2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. By applying these techniques to selectively deuterated polymers, motional mechanisms involving different segments of the monomer unit can be monitored. In addition, motional heterogeneities in glassy polymers can be detected. The information about polymer dynamics available now is illustrated by a number of experimental examples. The chain motion in the amorphous regions of linearpolyethylene is discussed in detail and it is shown that it can clearly be distinguished from the chain motion of an amorphous polymer above the glass transition, wherepolystyrene is used as an example. Localized motions in the glassy state are illustrated through the jump motion phenyl groups exhibit both in the main chain (polycarbonate) and as a side group (polystyrene). The latter polymers also serve as examples for detecting motional heterogeneity. Finally, the mobility in novel classes of systems,liquid crystalline polymers andpolymer model membranes as revealed by2H NMR are described.

Effect of the extent of cure on the modulus, glass transition, water absorption, and density of an amine-cured epoxy

Abstract: The modulus, density, glass transition temperature (Tg), and water absorption characteristics of an amine-cured resin [diglycidyl ether of bisphenol A (Epon 828)/diaminodiphenyl sulfone (DDS)] were studied as a function of extent of cure. The glass transition is a function of the extent of cure and reaches a maximum temperature, T, when it is completely cured; specimens with different extents of cure were formed by isothermal cure below T, for different times. After slowly cooling, the density at each extent of cure was obtained at room temperature. Moisture absorption was monitored gravimetrically at 25°C for 2 months at several humidity levels. The room temperature density and modulus decreased with increasing extent of conversion whereas the glass transition temperature and equilibrium water absorption increased. The equilibrium water absorption increased linearly with relative humidity, and the absorptivity increased linearly with specific volume. An interpretation of these anomalous results is made in terms of the nonequilibrium nature of the glassy state. The glass transition temperature increases as the extent of cure increases resulting in a material that is further from equilibrium at room temperature and therefore has more free volume and a greater propensity to absorb water.

Effects of annealing and prior history on enthalpy relaxation in glassy polymers. 3. Experimental and modeling studies of polystyrene

Abstract: The relaxation component of the heat capacity at constant heating rate (10 K min -1) of a polydisperse polystyrelie as a function of previous cooling rate, annealing time, and annealing temperature is accurately predicted by an adaptation of an algorithm due to Moynihan using parameter values determined from an analysis of the glass transition kinetics of a single thermal history. For this material, the annealing behavior below Tg is determined by the same kinetic parameters that describe the glass transition phenomenon. A similar analysis of published data for a monodisperse polystyrene indicates that the monodisperse material has a broader distribution of relaxation times and is more nonlinear. The parameter optimization method used appears to be sufficiently sensitive to permit routine characterization of the enthalpy relaxation of amorphous materials.

Depression of Glass Transition Temperatures of Polymer Networks by Diluents

Abstract: A classical thermodynamic theory is used to derive expressions for the depression of the glass transition temperature Tg of a polymer network by a diluent. The enhanced sensitivity of Tg in cross-linked systems to small amounts of diluent is explained. Predictions of the theory are in satisfactory agreement with the experimental values of a particular polymer network/diluent system.

Short time structural relaxation processes in liquids: Comparison of experimental and computer simulation glass transitions on picosecond time scales

Abstract: Recent Brillouin scattering studies of relaxation in the simple ion system Ca++–K+/NO−3 (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon ‘‘glass,’’ with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so‐called ‘‘ideal’’ glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca++–K+–NO−3, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far in...

Preparation and properties of CaAlSiON oxynitride glasses

Abstract: The effect of preparation condition on the melting behavior and the effect of nitrogen content on the properties were investigated for CaAlSiON OXYNITRIDE GLASSES. It was found that melting at lower temperatures produces more homogeneous and clearer glasses than melting at higher temperatures when the glass is melted at temperatures between 1530°C and 1750°C. The density, glass transition and softening temperatures, Vickers hardness and Young's modulus increased markedly with increasing nitrogen content. The observed values of densities of nitrogen-containing glasses was found to agree well with the additivity calculation based on the assumption that each nitrogen atom in the glass structure is connected with three silicon atoms.

Quantitative DSC evaluation of phase segregation rate in linear segmented polyurethanes and polyurethaneureas

Abstract: Linearly segmented polyurethanes and polyurethaneureas are known to be phase separated; one phase is the soft phase, the other, the hard domain. Quantitative evaluation of the degree of phase separation was obtained by DSC. An estimation of the dispersed soft segments outside the soft phase can be made by ΔCp measurements and the increase in glass transition (Tg) can be attributed to the presence of hard blocks in the soft phase. The segregation rate depends on the nature of the soft segments which decrease from hydrogenated polybutadiene to polybutadiene to polyether. It also depends on the nature of the chain extender from aliphatic to aromatic diol and from aliphatic to aromatic diamine. The presence of soft segments in hard domains can be related to the thermoplastic behavior of the polyurethanes and polyurethaneureas.

Glass transition of polystyrene versus molecular weight

Abstract: Glass transition temperature of polystyrene has been studied by DSC Glass transition, well known for macromolecules, has been observed up to the monomer Measurements of the change in heat capacity ΔCp at Tg were carried out and a relationship between \(\frac{{\Delta Cp}}{T}\) and \(\overline {Dp}\) has been found The results have been interpreted as a modification of interaction occuring at \(\overline {Dp}\)≃25

Mechanisms of photodarkening in amorphous chalcogenides

Abstract: A comparative study on the reversible photodarkening in several chalcogenides at normalized temperatures reveals that the characteristics are governed with the species of chalcogen atoms. This observations can be accounted for with a structural model, in which the atom shifts between stable and quasi-stable twisted positions. Covalent and van-der-Waals bonding plays an essential role in the dynamics. The model also explains the acoustic attenuation observed below the glass transition temperature.

The transition in ethylene copolymers: The β-transition

Abstract: The well-established data which gives a complex composition dependence for the relaxation of ethylene copolymers has been reexamined. Emphasis is placed on the β-transition. In the reanalysis of the data two new experimentally determined concepts are introduced. These are that this transition is definitely found in the homopolymer linear polyethylene, and that it is a reflection of segmental motions which take place within the interfacial region. With these principles it is possible to separate the β-transition from the glass temperature and thus explain the very unique composition relations.

Study on local structure in amorphous SbS films by Raman scattering

Abstract: A stoichiometric Sb 40 S 60 film contains SbSb and SS bonds in addition to SbS bonds in an SbS 3 unit. The amount of the SbSb bond rapidly decreases with an increase in the S content beyond the stoichiometric value. On the other hand, as the S content decrease beyond the stoichiometric value, the SbS bond decreases and the SbSb bond increases. Annealing at temperatures near the glass transition decreases the fraction of the SbSb bond in the Sb 40 S 60 film.

Dielectric behavior and ferroelectric transition of copolymers of vinylidene fluoride and trifluoroethylene.

Abstract: Dielectric behavior of copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) with VDF contents of 52 to 78 mol % has been investigated over wide frequency and temperature ranges with particular reference to the ferroelectric transition. Some related physical properties such as the dynamic tensile modulus, the thermal expansion, and the enthalpy of transition were also studied. The copolymers exhibited three dielectric relaxations γ, β, and Tt in the increasing order of temperature. The γ process is the local mode relaxation of part of frozen molecular chains below the glass transition. The β relaxation is related with micro-Brownian motions of molecular chain backbone in amorphous regions. The Tt relaxation which is featured with a rather narrow distribution of relaxation times is associated with cooperative motions of molecular chain in crystalline regions in the vicinity of the ferroelectric transition. Dielectric measurements under hydrostatic pressure revealed that the anomalou...

Vibration-damping constrained-layer constructions

Abstract: A constrained-layer construction comprising a stiff layer or substrate and a layer of a viscoelastic polymer which is the reaction product of (a) 25 to 75 weight percent of an acryloyl or methacryloyl derivative of at least one oligomer, said oligomer having a glass transition temperature of less than 25° C. and a molecular weight per oligomer of 600 to 20,000, and (b) 75 to 25 weight percent of a copolymerizable monomer whose homopolymer has a glass transition temperature of at least 50° C., the copolymer being suitable for damping vibrations at relatively high temperatures, e.g., 50°-150° C., is disclosed. Also included is a process for damping vibrations in component parts of devices used in automotive, aerospace, and computer industries.

Photon correlation spectroscopy of bulk polymers

Abstract: The use of photon correlation spectroscopy to study the dynamics of concentration fluctuations in polymer solutions and gels is now well established. In bulk polymers near the glass transition there will be slowly relaxing fluctuations in density and optical anisotropy which can also be studied by this technique. In this article we review the development of the field of photon correlation spectroscopy from bulk polymers. The theory of dynamic light scattering from pure liquids is presented and applied to polymers. The important experimental considerations involved in the collection and analysis of this type of data are discussed. Most of the article focuses on the dynamics of fluctuations near the glass transition in polymers. All the published work in this area is reviewed and the results are critically discussed. The current state of the field is summarized and many suggestions for further work are presented.

Impurity effects of some metals on electrical properties of amorphous As2Se1Te2 films

Abstract: Amorphous As2Se1Te2 (a-As2Se1Te2) films, into which Cu or Cd ions were doped by thermal diffusion below the glass transition temperature exhibited the increase of conductivity by several orders of magnitude. In addition, these samples exhibited enhanced photoconductivity and the activation type conduction. From the measurements of the thermoelectric power, it was found that Cu doped samples were p-type and Cd doped samples were n-type. The impurities was also studied by SIMS.

The influence of thermal properties on the glass transition temperature in styrene/divinylbenzene network–diluent systems

Abstract: The depression of glass transition temperature in polystyrene and styrene/divinylbenzene networks, by the addition of ethylbenzene, ethyl acetate, and m-diethylbenzene, respectively, has been investigated by differential scanning calorimetry. The predictive quality of the Couchman–Karasz treatment for the composition dependence of Tg in compatible systems was found to provide excellent agreement with experimental results for the depression of Tg by ethylbenzene, provided certain modifications were made. Qualitatively, the theory predicts that for a given diluent, the depression of Tg is particularly sensitive to the incremental change in heat capacity of the pure polymer or network. The behavior of ethylbenzene provided good quantitative agreement with the theory, provided that the incremental change in heat capacity of the diluent at its Tg was approximated by ΔCp = constant/T and that the incremental change in heat capacity of the network at Tg represents only those units capable of thermal activation. The anomalous behavior observed on plasticization by ethyl acetate and m-diethylbenzene could in part be explained by a reduction in solvent quality or syneresis of diluent in the highly crosslinked samples.

Batch and semicontinuous emulsion copolymerization of vinyl acetate–butyl acrylate. II. Morphological and mechanical properties of copolymer latex films

Abstract: Vinyl acetate/(VAc)-butyl acrylate/(BuA) copolymer latex films of various copolymer compositions were investigated for their morphological properties by electron microscopy techniques, and for their mechanical properties by dynamic mechanical spectroscopy (DMS), differential scanning calorimetry (DSC), and tensile strength measurements. Batch copolymer latex films showed domains of PBuA dispersed in PVAc matrix; the domain sizes were increased with increased BuA content. Semicontinuous latex films were homogeneous in composition. Glass transition temperatures Tg determined from DMS and DSC indicated the presence of two, low and high, transition temperatures for batch latex films. The two temperatures approached the individual homopolymers, with increased PBuA content up to 51 mol %. Semicontinuous latex films showed only one single Tg. Tensile properties of the batch copolymer films showed a higher ultimate tensile strength, higher Young's modulus, and lower percent elongation to break compared to semicontinuous latex films. These differences were found to reflect the effect of mode of monomer addition during the emulsion copolymerization process on the particle morphology, and confirmed earlier data on bulk, colloidal, and surface properties of the same copolymer latexes.

Measurement of stresses generated in cured polyimide films

Abstract: We have measured ‘‘in situ’’ the stresses generated in polyimide films produced from the ethyl ester of benzophenone tetracarbolic acid and methylene dianiline during the curing cycle The room temperature stress in a fully cured film (to 400 °C) is approximately 70 MPa The stresses at room temperature vary with the cure temperature and are due to coefficient of thermal expansion mismatch between film and substrate The in situ measurements show that the glass transition temperature is the same as the maximum cure temperature that the film experiences The stress measurements were made using an x‐ray double crystal lattice curvature technique

Polyisophthalates and copolymers thereof having high barrier properties

Abstract: Polyisophthalates, for example, poly(ethyleneisophtha- late), or copolymers thereof, have low glass transition temperatures, even lower than poly(ethylene terephthalate). Unexpectedly, such polyisophthalates have very good barrier properties with regard to oxygen and carbon dioxide penetration. The polyisophthalates can be utilized as packaging materials, for example as films or containers such as bottles for beverages such as intoxicating beverages such as beer, and the like.