Temperature Dependence Of Free Volume Research Articles

The temperature dependence of free volume in phenyl salicylate and its relation to structural dynamics: A positron annihilation lifetime and pressure-volume-temperature study

Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean <v(h)> and standard deviation sigma(h) was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density N(h)('), the specific volume V(f), and the fraction of holes f(h) in the amorphous state. From comparison with dielectric measurements in the temperature range above T(B)=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume V(f) would predict, on the basis of the Cohen-Turnbull (CT) free volume theory. CT plots can be linearized by replacing V(f) of the CT theory by (V(f)-DeltaV), where DeltaV is a volume correction term. This was interpreted as indication that the lower wing of the hole size distribution contains holes too small to show a liquidlike behavior in their surroundings. Peculiarities of the relaxation behavior below T(B)=265 K and the possible validity of the Cohen-Grest free volume model are discussed.

Temperature Dependence of Free Volume in Epoxy/Rectorite Nanocomposites by Positron Annihilation

Recently polymer nanocomposites[1,2] have been receiving special attention because of its advantages and unique properties in comparison to the traditional polymer composites.The structural transformation of Epoxy (cured by polyamide) and epoxy/Rectorite nanocomposite(with 0.5% rectorite content) is studied by PALS as a function of temperature from 30K to 490K. For Epoxy and Epoxy/Rectorite nanocomposite, the glass transition temperature (Tg) is observed to be 270K and 260K, the second transition temperature (Tγ) is 160K and 120K, the viscous flow temperature (Tf) is the same as 380K. The result shows that properties of the free-volume in nanocomposite have been changed by the rectorite platelets fully separated in material at low temperature. At the same time, we find that the intensity of o-Ps decreases with increase of the temperature at the softening temperature above the Te.

Investigation of the Temperature Dependence of Free Volume in Polymethylpentene by Positron Annihilation Method

Positron lifetime measurements in polymethylpentene were performed in a wide temperature range. A conventional fast{slow coincidence lifetime spectrometer with plastic scintillators was used. All the measurements were made in vacuum. Additional measurements (X-ray difiraction, difierential scanning calorimetry, and mechanical spectroscopy) were also performed to establish properties of samples. Mean free volume radii in the investigated samples were estimated from the positron lifetimes measurements results. An increase in I3 with the elapsed time of measurements was observed.

Atomistic Simulation of the Water Influence on the Local Structure of Polyamide 6,6

To understand, at a molecular scale, the effect of water on the structure of the amorphous region of polyamide 6,6 (PA6,6), atomistic molecular dynamics simulations have been carried out. Our results concerning the very local water organization relative to PA moieties agree qualitatively with a two-step sorption model. The first sorption mode seems to be saturated well below the lowest water concentration studied (5% relative to the amorphous phase). Moreover, above this saturation, the overall water organization displays at 300 K larger clusters than the 2−3 molecules usually assumed in the literature. The temperature dependence of free volume, hole size, and hydrogen bonding has also been investigated. It shows a transition between plasticized and antiplasticized behavior.

The temperature dependence of free volume in polymethylpentene by positron annihilation

Positron lifetime measurements in polymethylpentene were performed in the temperature range 8.3–299 K. A conventional fast–slow coincidence lifetime spectrometer with plastic scintillators were used. All the measurements were made in vacuum. Additional measurements (X-ray diffraction, differential scanning calorimetry and mechanical spectroscopy) were also performed to establish properties of samples. Mean free volume radii and fractional free volume in the investigated samples were estimated from the positron lifetimes.

Determination of the Binodal and Spinodal Phase Separation Temperatures in the Blend System PαMS0.5-co-AN0.5 (Luran)/PMMA0.95MA0.05 (Lucryl) with Positron Annihilation Lifetime Spectroscopy

The temperature dependence of free volume in the blend system of commercial PαMS0.5-co-AN0.5 (Luran KR 2556)/PMMA0.95MA0.05 (Lucryl G77) has been investigated by means of positron annihilation lifetime spectroscopy (PALS). The PALS measurements show characteristic changes in free volume parameters at temperatures that coincide with the cloud point and spinodal temperatures of the system. Hence, PALS seems to be a powerful method to detect in-situ both binodal (cloud point) and spinodal phase separation temperatures in polymers. The binodal can be detected at a very early stage.

Temperature Dependence of Free Volume in Cross-linked Polyurethane Studied by Positrons

Positron annihilation lifetime spectroscopyhas been used to study the free volume properties of the cross-linkedpolyurethane network as a function of temperature in the range from 15 Kto 300 K. The glass transition of polyurethane was observed and thethermal expansion coefficient of the free volume was calculated. Analysisof the ortho-positronium (o-Ps) lifetime distribution shows that thedistribution of the free volume in polyurethane is slightly broadened withthe increasing temperature in the glassy state, and then significantlybroadened with the increasing temperature above the glass transition. PACS: 78. 70. Bj, 64. 70. Pf, 65. 70. +y

Temperature dependence of free volume of polyacrylamide gels studied by positron lifetime measurements

Changes of positronium (Ps) cavity radii in polyacrylamide and poly( N-isopropylacrylamide) gels were studied from 120 to 300 K by positron lifetime technique and it has been shown that the Ps cavity radius in the hydrogels changes by three or four stages. Temperature dependence of the Ps cavity radius exhibits variations similar to common polymers around the glass transition temperature. Hydrophilicity of the polymer chains significantly affects the Ps cavity radius just below 273 K. These results suggest an important role of free volume on the state of water in hydrogels.

Compositional and temperature dependence of free volume in segmented copolymer PET/PEO studied by positron annihilation lifetime measurement

Positron lifetime measurements were performed for segmented copolymer poly(ethylene oxide)–poly(ethylene terephthalate) (PEO/PET) as a function of the hard segment content (PET) and the temperature, which is used to study the free volume properties, structural transition and miscibility behaviour of segmented copolymer PEO/PET. From the variation of the ortho-positronium ( o-Ps) lifetimes with changes in the hard segment content, we find that the free volume decreases with increase of the hard segment, this fact means that the hard segment microdomains act as physical cross-links that suppress movement of the chains and that the interactions between the hard segment (PET) and soft segment (PEO) also plays an important role. On the other hand, the temperature dependence of positron lifetimes reveals the existence of two glass transition temperatures. This experimental results show that the hard segment PET is immiscibility with soft segment PEO in segmented copolymer PEO/PET (PEO molecular weight 4000, PET content 30 wt.%). The maximum entropy lifetime method (MELT) is used to detect the distributions of the free volume holes. Large differences in width of the free volume hole distributions as a function of temperature are observed.

Positron Annihilation Lifetime Measurements of Free Volume in Wholly Aromatic Copolyesters and Blends

The temperature dependence of free volume in random copolyesters of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) has been studied from -50 to +350 °C by measuring the average lifetime, τ 3 , and intensity, I 3 , of ortho-positronium (o-Ps) annihilation. These parameters have also been used to determine the fractional free volume, h ps . Significant changes in the temperature coefficients of τ 3 , I 3 , and h ps are observed at the α-transition temperature, T α , and at the melt transition temperature, T m . The results indicate that both the average size and the number of free volume cavities occupied by positronium increase at T α , continue to increase uniformly with temperature up to T m , and then level off. A miscible blend of 75/25 and 30/70 copoly(HBA/HNA) with overall monomer ratio 60/40 exhibits a single melt transition well below those of the component copolymers or a random copolymer of the same composition, suggesting that the chains are less ordered in the blend. Comparison of PALS data for these systems indicates that the blend has a larger h ps below T m , which is primarily due to a larger number of cavities accessible to o-Ps, and a substantially larger temperature coefficient of h ps between T α and T m . In the nematic melt, the free volume of the blend decreases to that of the pure copolyesters, indicating that h ps in the nematic phase is independent of composition. Compared to other amorphous or semicrystalline polymers, the HBA/HNA copolymers have both fewer detectable free volume cavities and smaller average cavity sizes, presumably due to the quenched nematic morphology of the noncrystalline regions.

Use of a theoretical equation of state to interpret time-dependent free volume in polymer glasses

Many physical properties of polymer glasses change spontaneously during isothermal aging by a process commonly modeled as collapse of free volume. The model has not been verified rigorously because free volume cannot be unambiguously measured. In the present investigation we tentatively identify the free-volume fraction with the fraction of empty sites in the equation of state of Simha and Somcynsky. With this theory, volume recovery measurements can be analyzed to yield directly the time-dependent, free-volume fraction. Using this approach, recent volume measurements on poly(methyl methacrylate) are analyzed. The resulting free-volume fractions are then used in the Doolittle equation to predict the shift in stress relaxation curves at 23 °C. These predicted shift factors agree with the experimental measurements of Cizmecioglu et al. In addition, it is shown that previous assumptions concerning temperature dependence of free volume are inconsistent with the theory.