Showing papers on "Glass transition published in 1985"

Specific-Heat Spectroscopy of the Glass Transition

Abstract: The glass transition has historically been viewed as an anomaly involving the specific heat of supercooled liquids. It is also associated with a divergence of the relaxation times for liquid rearrangements. We have developed a new spectroscopy to study the frequency dependence of the specific heat of liquids which connects these two approaches. We report measurements as a function of temperature of the distribution of relaxation times which are directly responsible for the glass transition in glycerol.

Plasticization of glassy polymers by CO2

Abstract: A technique is described which uses differential scanning calorimetry to estimate the glass transition of polymers containing a dissolved gas. The technique is simple and appears to give reliable results. The effects of CO2 sorption at pressures up to 25 atm were examined in detail for poly(methyl methacrylate) and its blends with poly(vinylidene fluoride). Less extensive results for polystyrene, polycarbonate, poly(vinyl chloride), and poly(ethylene terephthalate) are also given. Reductions in Tg of up to 50°C are observed. A theoretical relation by Chow predicts results in reasonable agreement with the experimental data. These findings are relevant to various applications such as membrane separation processes for gases.

Chemistry of Glass

Abstract: Historical Development of Glass Chemistry. Freezing of a Melt to a Vitreous Solid. Structural Elements of Silicates. Classical Theories of Glass Structure. Methodology in Glass Research. Microphase Separation. Structure and Properties of Colorless Glasses. Structure and Properties of Colored Glasses. Crystallization of Glasses. Strength of Glass. Interaction Between High-Energy Radiation and Glass. Survey of the Physical Basis of Some Glass Properties.

Ionic conductivity and mobility in network polymers from poly(propylene oxide) containing lithium perchlorate.

Abstract: Ionic conductivity σ and mobility μ in the amorphous network polymers from poly(propylene oxide) (PPO) containing lithium perchlorate (LiClO4) at the concentration of [LiClO4]/[PO unit]=0.042 and 0.076 were investigated by means of complex impedance and time‐of‐flight methods. The σ values of the PPO–LiClO4 complexes reached 10−5 S cm−1 at 70 °C. The temperature dependence of σ deviated from a single Arrhenius behavior above a critical temperature (−1 °C and 11 °C) which approximately corresponded to the glass transition temperature Tg. The μ values were relatively high and changed from 10−6 to 10−5 cm2 V−1 s−1 in the temperature range of 40–100 °C. The Nernst–Einstein equation correlated μ with the ionic diffusion coefficient D. The Williams–Landel–Ferry equation with C1≂5 and C2≂30–50 held with a temperature dependence of D in the order of 10−8–10−7 cm2 s−1. The change in the number of ionic carriers n with temperature obeyed the Arrhenius equation with the activation energy of 0.26 and 0.34 eV. The deg...

Glass transition of poly(oxymethylene)

Abstract: Heat capacity data of semicrystalline poly(oxymethylene) samples. Delrin and Celcon, are analysed in order to discuss the glass transition behaviour of this polymer. There are two types of non-crystalline poly(oxymetheylene). the mobile and rigid amorphous parts. The glass transition of the former occurs in a rather wider range of temperature: it starts at 180 K and could end at 265 K. The latter, under restraint due to the crystallites, remains frozen up to the melting temperature.

Small-angle scattering analysis of hard-microdomain structure and microphase mixing in polyurethane elastomers

Abstract: The microdomain structure of a series of segmented polyurethane block copolymers is characterized by small-angle x-ray and neutron scattering analyses. The materials contain hard segments formed from 4,4′- diphenylmethane diisocyanate (MDI) and butanediol (BD), and range in hard-segment content from 20 to 80% by weight. The results provide evidence for a transition from discrete to continuous hard-microdomain morphology as the hard-segment content is increased above ca. 50%. The measured concentration dependences of the interdomain spacing, specific interfacial area, diffuse microphase boundary thickness, and scattering invariants are used to examine the validity of present models for hard-microdomain structure. The observed behavior corresponds well with the general predictions of a lamellar model wherein partially coiled hard-segment sequence configurations are allowed. The thickness of the hard microdomains extracted from the model corresponds to approximately four hard-segment repeat units. Scattering invariant calculations are used together with determinations of the soft-microphase glass transition temperatures to examine possible models for microdomain mixing. These calculations suggest that both the hard and soft microphases are phase mixed.

The significance of the glass transition of lignin in thermomechanical pulping

Abstract: Changing the temperature at which the thermomechanical pulping of wood is carried out may result in significant changes in pulp properties. Such changes are widely attributed to the effect of the glass transition of the lignin. Pulping literature references to this effect frequently appear to imply that this transition is manifested as an abrupt softening of the lignin either at a single temperature (the glass transition “point”) or over a relatively narrow temperature range. Application of the time temperature correspondence principle of polymer science and the Williams-Landel-Ferry equation to laboratory measurements of the transition suggests otherwise: in terms of the response of the lignin material to the very rapid cyclic deformation processes occurring within the refiner, the glass transition is predicted to occur over a temperature interval spanning the approximate temperature range 100–170°C within which most thermomechanical pulping is carried out. The consequences of this prediction are discussed in terms of the likely fracture processes occurring during the refining process.

Polymer crystallization induced by sorption of CO2 gas

Abstract: Previous work has shown that sorption of CO2 at relatively high pressures by glassy polymers reduces their glass transition temperatures and may convert the glass into a rubber under certain conditions. It is shown here that this plasticization by a gas can induce crystallization just as sorption of vapors or liquids is known to do. This point is extensively explored for miscible blends of poly(vinylidene fluoride) and poly(methyl methacrylate) and to a lesser extent for poly(ethylene terephthalate). In some cases, this secondary crystallization process results in small crystals whose melting endotherms are just above the glass transition and are very similar to peaks resulting from heat capacity overshoots, or enthalpic relaxation, caused by sub-Tg annealing; however, by appropriate techniques peaks arising from these two separate mechanisms can be distinguished. For oriented materials, evidence is shown which demonstrates that the additional crystals formed on CO2 sorption have the same preferential orientation as the original material.

A new class of liquid crystal side chain polymers mesogenic groups laterally attached to the polymer backbone

Abstract: A new class of liquid crystal side chain polymers has been synthesized, where the mesogenic groups are laterally attached to the polymer backbone. The monomeric liquid crystals are laterally substituted with an alkyl chain having a methacrylic ester group in the ω-position. While the monomers exhibit a metastable nematic phase with respect to the crystalline state, the polymers are liquid crystalline in a broad temperature range above the glass transition temperature.

Heat capacity of semicrystalline, linear poly(oxymethylene) and poly(oxyethylene)†

Abstract: The heat capacities of 38 semicrystalline poly(oxymethylene)s and poly(oxyethylene)s were determined by differential scanning calorimetry from 205 K through the melting transition. By comparison with the well known limiting heat capacities of the supercooled liquids and the crystals of the macromolecules it was found that there are negative and positive deviations from additivity of the heat capacities with crystallinity between the glass transition and the melting transition. The negative deviations are linked with “rigid amorphous” material, and the positive deviations were previously linked to defect formation or early melting. The rigid amorphous fraction in poly(oxymethylene) is constant up to the melting region, in contrast to polypropylene, where it is decreasing with temperature. The proposed mesophase transition in poly(oxymethylene) is shown to be a minor effect. The poly(oxyethylene) heat capacity is governed by positive heat capacity deviations within the rather short temperature range between glass transition and melting.

Microporous polymeric films and process for their manufacture

Abstract: Microporous polymeric films having good water vapor transmission rates and hydrostatic resistance to the penetration of liquid water are obtained by stretching a composition consisting of a matrix of a thermoplastic orientable polymer having a hydrophobic surface, a glass transition temperature of less than 25° C., a Vicat softening point of more than about 25° C., an ultimate elongation between those two temperatures of at least 100%, and a flexural modulus of less than 200 MPa and a particulate filler having an aspect ratio of less than 3, an average diameter of 0.05-50 m, a negligible attraction to the matrix polymer, and a surface tension no higher than that of the matrix polymer at the latter's melting temperature. Such films are useful in applications involving separation of liquids and vapors, e.g., in making "breathing" waterproof garments or linings.

Nematogenic polymers having rigid chains. 1. Substituted poly(p-phenylene terephthalates)

Abstract: Influence des substituants Cl, Br, C 6 H 5 et C 6 H 13 sur la temperature de transition de ces polymeres

Low-temperature glass transition in proteins

Abstract: The viscoelastic properties of solid samples (crystals, amorphous films) of hen egg white lysozyme, bovine serum albumin, and sperm whale myoglobin were studied in the temperature range of 100–300 K at different hydration levels. Decreasing the temperature was shown to cause a steplike increase in the Young's modulus of highly hydrated protein samples (with water content exceeding 0.3 g/g dry weight of protein) in the temperature range of 237–251 K, followed by a large increase in the modulus in the broad temperature interval of 240–130 K, which we refer to as a mechanical glass transition. Soaking the samples in 50% glycerol solution completely removed the steplike transition without significantly affecting the glass transition. The apparent activation energy determined from the frequency dependence of the glass-transition temperature was found to be 18 kcal/mol for wet lysozyme crystals. Lowering the humidity causes both the change of the Young's modulus in response to the transition and the activation energy to decrease. The thermal expansion coefficient of amorphous protein films also indicates the glass transition at 150–170 K. The data presented suggest that the glass transition in hydrated samples is located in the surface layer of proteins and related to the immobilization of the protein groups and strongly bound water.

Small-Angle X-Ray Scattering from Bulk Block Polymers in Disordered State. Estimation of χ-Values from Accidental Thermal Fluctuations

Abstract: Small-angle X-ray scattering (SAXS) from accidental thermal concentration fluctuations of block polymers in bulk and in the disordered state was studied for a polystyrene–polyisoprene diblock polymer as a function of temperature. It was shown that the experimental SAXS profiles are predictable by Leibler’s theory based on the random phase approximation. Temperature dependence of χ-parameter between polystyrene and polyisoprene was estimated by fitting the experimental and theoretical SAXS profiles. An effect of glass transition on the thermal concentration fluctuations was also predicted.

Water in epoxy resins Part II. Diffusion mechanism

Abstract: A model is proposed for the diffusion mechanism of water in glassy epoxy resins. The polymer network is assumed to consist of two regions in which water molecules possess different mobilities. By considering the distribution of water molecules among these regions it is possible to describe the concentration dependency of the diffusion coefficient in the sorption and resorption processes. The diffusion coefficient becomes constant when the sorption temperature is close to the effective glass transition temperature of the epoxy-water binary mixture. An explanation of this effect is also provided by the model.

Dielectric Properties of Some Side Chain Liquid Crystalline Polymers

Abstract: An experimental setup to determine the anisotropic dielectric constants for liquid crystalline side chain polymers as function of the temperature and the frequency in the range between 100 Hz and 10 MHz is described. From the anisotropic dielectric data some more specific informations can be obtained comparing with the results obtained from unoriented samples. - The anisotropic dielectric properties of two liquid crystalline side chain polymers containing the same polyacrylic main chain and slightly modified side chains are described in the glassy, nematic as well as in the isotropic state as function of temperature and frequency. For these compounds, three different relaxation processes are described. A process was found with the main component in ϵ∥ due to the relaxation of the mesogenic side chain around the main chain back bone. A broad relaxation process, near above the glass temperature, with the main component in ϵ⟂ is interpreted as due to diffusive motion of different dipoles. Another broad relaxation process occurs in the glassy state, containing parallel and perpendicular dipole components of the mesogenic ester group. In the isotropic state, the relaxation process is dominated by the parallel relaxation in the nematic state.

Structural properties of lead-iron phosphate glasses☆

Abstract: The effects of iron on the structure and chemical stability of several lead-iron phosphate glasses ((1 − x) Pb(PO3)2 + (x) Fe2O3, O < x < 0.25) were investigated using Mossbauer, electron paramagnetic resonance, Raman, and infrared spectroscopies. Dilatometer measurements were used to determine the glass transition temperature, softening point, and thermal expansion coefficient of glasses where the fusion temperature, soak time, or iron concentration had been varied. The major conclusion of this study is that iron improves the chemical durability of these glasses by strengthening the cross bonding between the polyphosphate chains in the glass structure.

Molecular relaxations in a rigid molecular glassy crystal

Abstract: The dielectric permittivity and loss of 1-cyanoadamantane have been measured under isothermal conditions in the frequency range, 100-2*105 Hz and the temperature range, 80-350K in the glassy crystal, supercooled (metastable) and stable (disordered) crystals and partially ordered crystalline states. Two relaxation regions were observed, one above the glass-like transition temperature (170K) and the other below it. (The latter finding is contrary to the observations of Amoureux et al., for which reasons are given.) The characteristics of the two relaxations are remarkably similar to those found in other isotropic and anisotropic glasses and glassy crystals. The implications of this similarity for concepts of the glass transition and configurational states are pointed out. The static permittivity of 1-cyanoadamantane shows that it is only partially disordered even at high temperatures and that its disorder-order transformation occurs over a wide temperature range. Isothermal measurements in this range show no time dependence of the ordering process.

Phase behavior of polyarylate blends

Abstract: Melt mixtures of a polyarylate based on bisphenol A and tere/isophthalates were made with poly(ethylene terephthalate), several cyclohexane dimethanol-based polyesters, polycarbonate, and the poly(hydroxy ether) of bisphenol A. The phase behavior was determined using classical methods. With minimum time and temperature exposure, polyarylate exhibits phase separation with poly(ethylene terephthalate) (PET) at >30 wt % PET. With moderate time and temperature exposure, adequate ester exchange occurs with polyarylate/PET blends to yield single-phase behavior. The activation energy of the ester-exchange reaction was determined to be 37.0 kcal/mole. Under minimum time and temperature exposure conditions, miscibility of polyarylate with three different cyclohexane dimethanol-based polyesters was observed. A polyarylate-polycarbonate 50:50 mixture was shown to be phase separated under minimum mixing conditions but capable of exchange reactions to yield single-phase behavior with proper time and temperature exposure. Likewise, a 70:30 polyarylate-poly(hydroxy ether of bisphenol A) blend was phase separated as mixed, but with further elevated temperature exposure, a cross-linked single-phase system resulted. The density versus composition of the polyarylate-PET blends was linear with the phase-separated systems but exhibited a slight densification with the miscible systems produced by higher temperature exposure. The glass transition of the miscible polyarylate-polyester blends exhibited a significant deviation (lower) than predicted by a linear or Fox equation prediction. This was attributed to the low value of ΔCp (specific heat difference between the glass and rubber states) of polyarylate as noted by the Couchman equation to be a major factor in the Tg versus composition relationship. The optical characteristics of the blends paralleled the observed phase behavior as single-phase blends were all transparent (in the amorphous state) whereas phase-separated blends were translucent to opaque. These results clearly demonstrate the importance of ester-exchange or transesterification reactions in the phase behavior of blends of polymers capable of these reactions.