Influence of fluorine on thermal properties of lead oxyfluoride glass

Oxyfluoride glasses are the basic materials for obtaining transparent glass–ceramic (TGC) which can be used in a wide range of optoelectronics devices such as: amplifiers, up-conversion, telescopes, laser sources. Oxyfluoride TGC is obtained by the control heat treatment of the parent glass due to low phonon nanocrystalline phases. The oxyfluoride glasses from the sodium–lead–silica system were the object of investigation. The influence of fluoride content on the thermal properties of glasses was analyzed. Thermal characteristics of glasses like the transition temperature T _g, the temperature for the crystallization onset T _x, and the maximum crystallization temperature T _c, thermal stability parameter were determined by DTA/DSC method. The linear expansion coefficients of oxyfluoride glasses as a function of temperature were measured using a thermo-mechanical analyzer (TMA 7 Perkin-Elmer). The effect of crystallization on the thermal expansion coefficient and softening temperature T _s was found.     

Comparison of electrical conductivity and thermal properties of borosilicate glass with and without simulated radioactive waste

Electrical conductivity and percentage linear thermal expansion of the borosilicate glass (BSG) and simulated waste-loaded borosilicate glass (BSGW) were measured in the temperature range of 300–780 K and compared. Pronounced increase in electrical conductivity was observed around glass transition temperature (T _g) of BSG and BSGW. The activation energy (E _a) of electrical conduction determined from the measured data for BSG and BSGW is 0.961 ± 0.005 and 0.960 ± 0.005 eV, respectively. The % average linear thermal expansion of BSGW showed a slight decreasing trend compared with pristine BSG. The average coefficient of thermal expansion determined from dilatometry data is 12.87 ± 0.24 × 10^?6 and 11.94 ± 0.23 × 10^?6 K^?1 for BSG and BSGW, respectively. The T _g measured by dilatometry is 806 ± 24 K for BSG and 790 ± 23 K for BSGW, respectively. The T _g measured by DTA was found to be 820 ± 7 and 805 ± 5 K for BSG and BSGW, respectively, for heating cycle. The T _g values obtained from DSC measurements are 805 ± 5 and 803 ± 5 K for BSG and BSGW, respectively. The T _g of BSGW showed a slight decrease compared with that of BSG. The values obtained by DSC examination also showed the lowering of T _g values for the waste-loaded composition. The lowering of T _g may be attributed to the interaction of glass-forming agents and simulated waste elements.     

Pressure‐volume‐temperature and glass transition behavior of silica/polystyrene nanocomposite

The pressure‐volume‐temperature (PVT) behavior and glass transition behavior of a 10 wt % silica nanoparticle‐filled polystyrene (PS) nanocomposite sample are measured using a custom‐built pressurizable dilatometer. The PVT data are fitted to the Tait equation in both liquid and glassy states; the coefficient of thermal expansion α, bulk modulus K, and thermal pressure coefficient γ are examined as a function of pressure and compared to the values of neat PS. The glass transition temperature (T_g) is reported as a function of pressure, and the limiting fictive temperature (T_f′) from calorimetric measurements is reported as a function of cooling rate. Comparison with data for neat PS indicates that the nanocomposite has a slightly higher T_g at elevated pressures, higher bulk moduli at all pressures studied, and its relaxation dynamics are more sensitive to volume. The results for the glassy γ values suggest that thermal residual stresses would not be reduced for the nanocomposite sample studied. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1131–1138     

Ellipsometric study of the glass transition and thermal expansion coefficients of thin polymer films

The glass transition (T_g) of thin polystyrene films (ca. 3000 A?) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. T_g was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in T_g, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley & Sons, Inc.     

Coefficient of thermal expansion evolution for cryogenic preconditioned hybrid carbon fiber/glass fiber-reinforced polymeric composite materials

Polymeric matrix composites are susceptible to degradation and material properties changes if subjected to low-temperature environmental conditions. This paper attempts to present a study on effective coefficient of thermal expansion for various hybrid carbon fibers/glass fibers polymeric composite structures previously subjected to low-temperature environmental conditioning. The hybrid composite architectures were made from various layers of glass mat and/or glass woven embedded along with layers of unidirectional carbon fibers into a polymeric matrix. The samples were preconditioned to a low-temperature environment at a constant temperature of ?35 °C for 1-week long, 24 h/day. The instantaneous CTE and thermal strain fields were recorded with a DIL 402 PC/1 dilatometer from Netzsch GmbH (Germany) by setting a monotonically linear rise of temperature from 20 to 250 °C, at a rate of 1 °C min^?1. The experimentally retrieved data were compared with the values obtained by running a micromechanical-based approach simulation on a representative volume element.     

Measurement of size‐dependent glass transition temperature in electrospun polymer fibers using AFM nanomechanical testing

The glass transition temperature (T_g) of individual electrospun polymer polyvinyl alcohol fibers of varying diameter was measured using atomic force microscopy (AFM) based nanomechanical thermal analysis. Indentation and bending of individual electrospun fibers using AFM allowed the calculation of the elastic modulus of the polyvinyl alcohol (PVA) fibers across a range of different temperatures. The elastic modulus of electrospun PVA fibers was observed to decrease significantly when passing through T_g, which allowed accurate determination of T_g. The T_g of electrospun PVA fibers was shown to decrease for smaller fiber diameters especially for fiber diameters below 250 nm. This size‐dependent glass transition behavior of electrospun PVA fibers is indicated as being due to polymer chain confinement. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The extent of the glass transition from molecular simulation revealing an overcrank effect

A deep understanding of the transition between rubber and amorphous state characterized by a glass transition temperature, T_g, is still a source of discussions. In this work, we highlight the role of molecular simulation in revealing explicitly this temperature dependent behavior. By reporting the specific volume, the thermal expansion coefficient and the heat capacity versus the temperature, we actually show that the glass transition domain extends to a greater range of temperature, compared with experiments. This significant enlargement width is due to the fast cooling rate, and actually explains the difficulty to locate T_g. This result is the manifestation of an overcranking effect used by high‐speed cameras to reveal slow‐motion. Accordingly, atomistic simulation offers the significant opportunity to show that the transition from the rubber state to the glass phase should be detailed in terms of the degrees of freedom freeze. © 2017 Wiley Periodicals, Inc.     

Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene,high-vinyl polybutadiene,and their miscible blends

Measurements of average free volume hole sizes, 〈v_f〉, and the fractional free volumes, f_ps, in vulcanized cis-polyisoprene (CPI), high-vinyl polybutadiene (HVBD), and their 50 : 50 blend were made via determination of orthopositronium annihilation lifetimes. The results are compared to corresponding data on the uncured materials. On crosslinking, 〈v_f〉 decreases in the rubbery state but remains essentially unchanged in the glass. This is consistent with the expectation that the crosslinks greatly restrict the thermal expansion of the chains above the glass transition temperature (T_g) but have less influence on the packing density in the glass. Scaling relationships between 〈v_f〉, f_ps, the thermal expansion coefficient α_f = df_ps/dt, and T_g are examined. We find that 〈v_f〉_g, the hole volume at T_g, and f_ps,g, the fractional free volume at T_g, each increase significantly with increasing T_g. This behavior is consistent with previous observations reported in the literature and has been interpreted as a manifestation of the kinetic character of the glass transition. High-T_g polymers need a larger free volume to pass into the liquid state. The change in expansion coefficient on passing from the glass to the liquid, Δα_f = α_f,l − α_f,g, increases slowly with T_g, as predicted by free volume theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2754–2770, 1999     

Thermal properties of anhydride-cured bio-based epoxy blends

Epoxidized palm oil (EPO) (0–12 wt%) was added into petrochemical-based epoxy blends (diglycidyl ether of bisphenol-A (DGEBA)/cycloaliphatic epoxide resin/epoxy novolac resin) to develop a thermal curable bio-based epoxy system. The thermal behaviors of the EPO, epoxy blends (EB), and bio-based epoxy blends (EB/EPO) were characterized using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMT) and thermo-mechanical analysis (TM). The glass transition temperature (T _g) and storage modulus (E′) of the EB/EPO system was reduced with the increasing of the EPO loading. This is attributed to the plasticizing effect of the EPO. It was found that epoxy blends with higher loading of EPO possessed higher coefficient of thermal expansion (CTE) and tanδ value. This is due to the increase of the free volume and chain flexibility in the three-dimensional network of the epoxy blends. The internal thermal stresses of the EB/EPO were decreased as the increasing loading of EPO, owing to the reduction of crosslink density, modulus of elasticity, and T _g in the epoxy blends.     

Continuous and gradual photo-activation methods: influence on degree of conversion and crosslink density of composite resins

Thermal properties and degree of conversion (DC%) of two composite resins (microhybrid and nanocomposite) and two photo-activation methods (continuous and gradual) displayed by the light-emitting diode (LED) light-curing units (LCUs) were investigated in this study. Differential scanning calorimetry (DSC) thermal analysis technique was used to investigate the glass transition temperature (T _g) and degradation temperature. The DC% was determined by Fourier transform infrared spectroscopy (FT-IR). The results showed that the microhybrid composite resin presented the highest T _g and degradation temperature values, i.e., the best thermal stability. Gradual photo-activation methods showed higher or similar T _g and degradation temperature values when compared to continuous method. The Elipar Freelight 2^TM LCU showed the lowest T _g values. With respect to the DC%, the photo-activation method did not influence the final conversion of composite resins. However, Elipar Freelight 2^TM LCU and microhybrid resin showed the lowest DC% values. Thus, the presented results suggest that gradual method photo-activation with LED LCUs provides adequate degree of conversion without promoting changes in the polymer chain of composite resins. However, the thermal properties and final conversion of composite resins can be influenced by the kind of composite resin and LCU.     

Theoretical calculations of glass transition temperatures of plasticized polymers

Based on free volume, an equation has been derived enabling calculation of glass transition temperature (T_g) of plasticized polymers, knowing the plasticizer content, the thermal expansion coefficients and values of T_g of the polymer and plasticizer. Determined and calculated T_g are in satisfactory agreement up to the plasticizer concentration which dissolves the polymer.     

Use of thermal analysis techniques for examining blow-molded pet bottle specimens

Eight samples from different areas of stretch-blow-molded poly(ethylene terephthalate) [PET] bottles, including a PET resin control, were tested by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The glass transition temperature (T _g) was found to linearly decrease about 6C from zero to 45 percent initial crystallinity. Measurements ofT _c (crystallization temperature, DSC) and film tension modulus (TMA) were related to crystallization rate during stretch-blow-molding. The TMA linear coefficients of thermal expansion and shrinkage were shown to be important for blow-molding temperature control.     

An analysis of the thermal aging behaviour in high-performance energetic composites through the glass transition temperature

Differential scanning calorimetry (DSC) was used to analyze the thermal aging behaviour in energetic composite materials where a hydroxyl-terminated polybutadiene (HTPB)/isophorone diisocyanate elastomer is the polymeric matrix. Different parameters from the analysis of the glass transition, such as the glass transition temperature (T_g), were used in order to monitor this isothermal aging at 65 °C during a total time of 3000 h, finding an increasing and broadening T_g. In addition, the accelerated aging behaviour of these materials was also studied by a classical method, based on the change of mechanical properties such as those of Young's modulus or strain at break. The correlation between both methodologies was examined, demonstrating that an analytical technique such as DSC allows the evaluation of the actual state of composite solid propellants with a small sample and a straightforward measurement.     

Influence of thermal treatment on the glass transition temperature of thermosetting epoxy laminate

The influence of accelerated thermal treatment of thermosetting epoxy laminate on its glass transition temperature was studied. Lamplex^® FR-4 glass fibre-reinforced epoxy laminate (used for printed circuit board manufacturing) was used in these experiments. The composite was exposed to thermal treatments at temperatures ranging from 170 °C to 200 °C for times ranging from 10 to 480 h. The glass transition temperature (T_g) was analysed via dynamic mechanical analysis (DMA). It has been proven that the glass transition temperature rapidly decreases in reaction to thermal stress. The obtained T_g data were used for Arrhenius plots for different critical temperatures (T_g-crit. = 105–120 °C). From their slopes (?E_a/R), the activation energy of the thermal degradation process was calculated as 75.5 kJ/mol. In addition to this main relaxation mechanism, DMA also recorded one smaller relaxation process in the most aged samples. Microscopic analysis of the sample structure showed the presence of pronounced small regions of degradation both on the surface and in the inner structure, which are probably the causes of microscopic delamination and the smaller relaxation process.     

Effect of montmorillonite content and the type of its modifier on the thermal properties and flammability of polyimideamide nanocomposite fibers

The effects of montmorillonite (MMT) content (1, 3, 5%) and the type of its modifiers on the thermal properties and flammability of PIA nanocomposite fibers have been assessed. Sodium montmorrilonite was modified with aminododecane acid and octadecylamine. Samples of PIA nanocomposite containing commercial MMT: Nanomer PGW from Nanocor were also included in the comparative analysis. It has been found that the glass transition temperature (T _g) of the fibers under investigation depends on the type of MMT’s modifier. On the other hand this parameter does not affect the thermal stability of fibers defined with T ₅ and T ₅₀ indicators since the thermal decomposition of modifiers takes place at lower temperatures.     

Dielectric relaxation and glass transition temperature of copolymers

Dielectric relaxation measurements were made on methyl methacrylate—styrene and methyl methacrylate–p-chlorostyrene copolymers at temperatures higher than the glass transition temperature T_g. It was found that the temperature dependence of the relaxation time can be described satisfactorily by an expression derived recently for chain motion in amorphous polymers. The temperature T_g obtained from the expression agrees well with that determined by differential thermal analysis.     

Thermo-mechanical characterisation of in-plane properties for CSM E-glass epoxy polymer composite materials – Part 1: Thermal and chemical strain

The in-plane thermo-mechanical properties and residual stresses of a CSM E-glass/Epoxy material are characterised through the use of DSC and TMA. The measured data is used to generate material models which describe the mechanical behaviour as a function of conversion and temperature. The in-plane thermal expansion coefficient (α) of the composite material decreases above the glass transition temperature (T_g), which is compensated by a higher out of plane deformation above T_g. Comparison of α and chemical shrinkage measurements suggests that chemical bonds between the polymer matrix and the glass fibres are formed prior to shrinkage of the epoxy matrix, i.e., at an early processing stage. This suggests that production of composites with low residual stresses requires focus on reactivity between the matrix and the sizing rather than the matrix cure properties. As a consequence, residual stresses in the composite material are mainly a result of restricted cure shrinkage rather than mismatch between thermal expansion coefficients.     

Effect of thermoplastic toughening agent on glass transition temperature and cure kinetics of an epoxy prepreg

The isothermal time–temperature-transformation (TTT) cure diagram is developed in this article to investigate the effect of thermoplastic toughening agent on glass transition temperature (T _g) and cure kinetics of an epoxy carbon fiber prepreg, Cycom 977-2 unidirectional (UD) tape. The glass transition temperature was measured using differential scanning calorimetry (DSC) over a wide range of isothermal cure temperatures from 140 to 200 °C. Times to gelation and vitrification were measured using shear rheometry. The glass transition temperature master curve was obtained from the experimental data and the corresponding shift factors were used to calculate the activation energy. The kinetic rate model was utilized to construct iso-T _g contours using the calculated activation energy. It was observed that the iso-T _g contours did not follow the behavior of the neat epoxy resin, since they deviated from the gel time curve. This deviation was believed to be the effect of the thermoplastic toughening agent. The behavior of the neat epoxy resin in 977-2 was shown by constructing the iso-T _g contours using the activation energy obtained from gel time modeling.     

A study on the glass transition behavior and morphology of semi-interpenetrating polymer networks

The epoxy resin/polyurethane semi-IPN was prepared and the glass transition behavior of the semi-IPN was discussed with DSC and DMA methods. The results show that the two glass transition temperatures (T_g) referring to epoxy resin and polyurethane respectively get closer. Between the two T_g there exists another T_g related to the interface between the two polymers. SEM indicates that this semi-IPN has a two-phase continuous structure which changes with different weight compositions. This is also proved by testing the Young's modulus. It is found that the IPN system has a cellular structure. Comparatively, the compatibility between the two polymers is the best when the weight ratio of EP/PU is 70/30. © 1996 John Wiley & Sons, Inc.     

Thermal expansion,free volume,and molecular mobility in a carbon black-filled elastomer

The thermal expansion of a butadiene–styrene copolymer filled with carbon blacks differing tenfold in mean particle size (HAF and MT) was investigated. The glass transition was unaffected by MT and was raised only 0.2°C for every 10 parts per hundred by weight of polymer of HAF black added. The coefficient of expansion of the polymer component of the composite in the rubbery region was substantially unaffected by either carbon black, but decreased markedly with increasing black loading in the glassy state. These results suggest that free volume is not altered appreciably by the presence of the filler in the rubbery state, but expands with decreasing temperature below T_g. The latter effect is explained by dilatation due to stresses set up around filler particles, arising from differences in the expansion coefficients of filler and polymer, which are not relieved in the glassy state. The near invariability of T_g and of the rubbery fected by adsorption of polymer segments on the carbon black surface. A conservative rough estimate indicates that restriction of segmental motion is confined to a 30 Å layer around the particles in which T_g is elevated by only 10°C.