Positron annihilation lifetime studies of free volume in gelatin

The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Positronium annihilation in amine-cured epoxy polymers

Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between ?78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm³. Two important transition temperatures were found: one corresponds to the glass transition temperature T_g determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below T_g. The sub-T_g transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.     

A positron annihilation lifetime study of isotactic polypropylene

Positron lifetime studies were performed on well-characterized annealed and quenched samples of isotactic polypropylene. The positron experiments were conducted from ?20 to 110°C as a function of both heating and cooling. Of the three decaying exponential components resolved from the lifetime spectra, only the long-lifetime ortho-positronium (o-Ps) pickoff component was affected by the changes in temperature. The behavior of both the lifetime and intensity of the o-Ps component was interpreted with the aid of x-ray diffraction, densitometry, and optical microscopy examinations and results from previously reported investigations of the thermal transition behavior of polypropylene. The present experiments demonstrate that o-Ps lifetimes were similar for both the annealed and quenched samples, independent of thermal cycling, while the o-Ps component intensity was significantly larger for the quenched material during heating, with both sample types exhibiting a significant hysteresis upon cooling. These results suggest that the mean free-volume cavity size is independent of prior thermal treatment, while the density of free-volume sites is a sensitive function of structure and prior thermal history. The variations of lifetime and of intensity with temperature have provided insight into polypropylene's glass transition phenomena.     

Free-volume study of ethylene - vinyl alcohol copolymer evaluated through positronium lifetime measurement

The free-volume, of size ranging from 0.2 to 0.4 nm in radius, in an ethylene-vinyl alcohol copolymer was estimated using positronium lifetime measurement to elucidate the dependence of oxygen permeability on the free-volume size and fraction, on the ethylene content and on the crystallinity. The permeability and the free-volume fraction with varying the ethylene content were well related and the relation was interpreted based on the free-volume theory near below and above the glass transition temperature. On the other hand, the crystallinity significantly influenced the fraction of the amorphous region, where the free-volume hole exists, along with a slight change of the free-volume size. The variation of the permeability with the crystalline degree cannot be explained from the averaged free-volume fraction estimated by the whole volume of the polymer, but the permeability correlated with the free-volume size apparently.     

荧光光谱跟踪结冷胶水溶液的溶液-凝胶转变

将异硫氰酸荧光黄(FITC)标记在结冷胶分子链,用荧光光谱跟踪了结冷胶水溶液凝胶化过程中FITC荧光强度和各向异性比的变化.结果表明在结冷胶的凝胶化转变中,FITC的荧光相对强度和各向异性随温度降低而增大,在某一温度荧光相对强度和各向异性比对温度的曲线出现了明显的转折点,这个转折点的温度低于流变温度扫描曲线中G′=G″的温度.利用荧光的方法确定物理交联体系的关于重均聚合度和凝胶分数的相关临界指数γ和β.γ和β不符合Flory-Stockmayer和逾渗模型的预测.     

Thermal variation of free-volumes size distribution in polypropylenes. Probed by positron annihilation lifetime technique

Positron annihilation lifetime measurement was applied to the study of free-volume properties in three kinds of polypropylene as a function of temperature in the range of 25–180°C at thermal equilibrium. Positron lifetime data for polypropylenes were analyzed with a Laplace inversion technique in order to obtain continuous positron annihilation lifetime (PAL) distributions. At each temperature, four distinct PAL distributions were recognized. The distribution of the longest lived component was associated with a pick-off annihilation of ortho-positronium (o-Ps) trapped in free-volume of amorphous region, which grew bigger as the temperature increased. The hole radius distributions of free-volumes were estimated from the results of o-Ps lifetime distributions. A detailed analysis showed a mean radius of free volumes was 0.34 nm at room temperature and that was 0.42 nm near the melting point for each specimen. The distributions of hole radii of free volumes were found to be broader after thermal treatments. The relaxation of free volumes was attributed to the thermal equilibrium and the evacuation of included molecules in free volumes. © 1995 John Wiley & Sons, Inc.     

Detection of free volumes in polyaniline complexes with various acids by using positron lifetime spectroscopy

The positron lifetime spectroscopy (PLS), a non-destructive characterization method, utilizes positronium (Ps; an electron–positron bound state) as a probe and measures its lifetime in polymer free volumes. For the first time the free volumes have been estimated by PLS in polyaniline (PANI) complexes with various inorganic and organic acids. It was found that the o-Ps lifetime increases and the intensity decreases with increasing ionic radius of the counter-ions in PANI complexes. Obviously, larger counter anions result in enhanced mean size of the voids corresponding to the free volume in the bulk polymer.Electrical conductivity has been measured by conventional four-probe technique. The glass transition temperature and temperature of removal of the absorbed water have been determined by using differential scanning calorimetry. It was established fairly well correlation of the mentioned polymer parameters with the o-Ps lifetime and the free volume of PANI complexes, respectively. The greater free-volume results in a decrease of conductivity, glass transition temperature and temperature of removal of the absorbed water.     

Some remarks on the relation between free-volume fractions and ortho-positronium lifetimes in amorphous polymers

Unique information about the properties of free-volume sites in polymers is gained from Positron Annihilation Lifetime (PAL) measurements. After calibration with data from other techniques the method may be used to determine free-volume fractions. From pressure–volume–temperature (PVT) and PAL (ortho-Ps lifetime τ₃) data, measured on identical amorphous poly(methyl methacrylate) (PMMA) samples with controlled thermal histories, we find a linear relationship between free-volume fractions, derived from PVT measurements, the Simha–Somcynsky equation-of-state theory and the mean subnanometer free-volume size both below and above the glass transition temperature.     

Crystallinity dependence of free-volume size and its distribution of ethylene-vinyl alcohol copolymer evaluated by positronium lifetime measurement

The size distribution of free-volume (<~0.1 nm³) of ethylene-vinyl alcohol copolymer with various crystalline degrees was estimated by positronium lifetime measurement. With increasing degree of crystallinity, the size distribution significantly narrowed and the intensity of positronium decreased. This indicates that the inhomogeneity reduces with the increase of the degree of crystallinity. It is found that the free-volume fraction estimated is reflected by the fraction of the amorphous region.     

A free-volume model for photopolymerizable/cross-linkable dry-film systems

A photopolymerizable/cross-linkable dry-film system is generally a compatible mixture of three main ingredients: a preformed polymeric binder, a multifunctional polymerizable monomer, and a photoinitiating system. The photographic speed of systems sensitized with certain free-radical sensitizers which undergo intermolecular hydrogen-abstraction (e.g., benzophenone/Michler's ketone) is proportional to the excess fractional free volume (V_f ? V_fg), of the dry coating. Free-volume dependence appears to be determined by the molecular size or configuration of the actual initiating species. We used as a working model the copolymer system poly[4,4′-isopropylidenediphenylene-1,1,3-trimethyl-3-phenylindan-5,4′-dicarboxylate:azelate] and tri(6-acryloyloxyhexyl)-1,3,5-benzenetricarboxylate as the polymerizable/cross-linkable monomer. A mathematical model based on experimental data and the free-volume theory was developed. The photographic speed of a photopolymerizable/cross-linkable dry-film system suffering from free-volume dependence can be correlated with the glass transition temperatures of the polymeric binder and the monomer, the volume fraction of the monomer, and the temperature of exposure.     

Positron annihilation spectroscopy study of moisture absorption in protective epoxy coatings

Position annihilation spectroscopy (PAS) was used to measure the relative free-volume fraction of protective epoxy coatings before and after exposure to liquid water at room temperature. The relative free volume fraction determined before water exposure was compared to the equilibrium water uptake of each coating and a correlation was found. The relative free-volume fraction of the epoxy coatings decreased slightly after water exposure. This decrease is contrary to the free volume theory of plasticization, but is consistent with the antiplasticization process. Larger decreases in the relative free volume fraction were sought by repeating the water uptake experiments with nitrobenzene which in the bulk, liquid form quenches ortho-positronium (o-Ps). Since the o-Ps lifetime remained approximately constant and the o-Ps intensity decreased after nitrobenzene absorption, nitrcbenzene is believed to be inhibiting the formation of o-Ps in the epoxy free volume cavities. Larger decreases in the relative free volume fractions were found after nitrobenzene exposure than after water exposure. These larger decreases are due to the fact that nitrobenzene is a better inhibitor of o-Ps formation than water in the epoxy free volume cavities. Larger volume fractions of nitrobenzene were absorbed by the coatings than water and were interpreted to be due to interactions between nitrobenzene and the epoxies. © 1993 John Wiley & Sons, Inc.     

嵌段共聚物OSM1-PCLA-PEG-PCLA-OSM1的合成及其pH和温度敏感性

采用开环聚合法和自由基聚合法合成了生物可降解嵌段共聚物OSM1-PCLA-PEG-PCLA-OSM1, 并对其进行了结构表征. 采用荧光分光光度计和激光粒度仪对共聚物溶液临界胶束浓度(CMC)和粒径大小及分布进行了考察, 研究了温度和pH对共聚物胶束形成的影响. 相转变过程研究结果表明, 共聚物溶液具有pH和温度双重敏感性. 共聚物溶液在一定温度和pH条件下可发生溶液-凝胶相转变.     

Effect of ortho-methoxycinnamic acid on the sol-gel transition of methylcellulose solutions in the presence of cetyltrimethylammonium bromide

The sol-gel transition of methylcellulose (MC) solutions in the presence of ortho-methoxycinnamic acid (OMCA) or cetyltrimethylammonium bromide (CTAB) and in the coexistence of OMCA and CTAB was determined by the rheological measurement. It has been found that the sol-gel transition temperature of MC solutions increases linearly with the concentration of either OMCA or CTAB in solution, respectively. However, in the coexistence of OMCA and CTAB, the sol-gel transition temperature of MC solutions remains invariable, independent of the concentration of CTAB in solution. The experimental results show that OMCA has priority to adsorb on the methyl group of MC chains to form polymer-bound aggregates. In particular, these aggregates inhibit the hydrophobic interaction between CTAB and the methyl group of MC chains completely. Taking into account the fact that OMCA is almost insoluble in MC-free solutions but dissolves very well in aqueous MC solutions, we propose the formation of the core-shell architecture prompted by OMCA and the methyl group of MC chains, with the methyl group of MC chains serving as the core and the self-assembly of OMCA molecules serving as the shell. Obviously, the formation of the core-shell structure increases the solubility of OMCA, improves the stability of methyl groups of MC chains at high temperatures and inhibits the hydrophobic interaction between CTAB and the methyl group of MC chains in solution. The abnormal behavior relating to the sol-gel transition of MC solutions in the presence of OMCA or in the coexistence of OMCA and CTAB is therefore explained. Upon UV irradiation, the sol-gel transition temperature of MC solutions in the presence of OMCA, or in the coexistence of OMCA and CTAB, decreases notably. However, the dependence of the sol-gel transition temperature of MC solutions as a function of OMCA concentration, or CTAB concentration in the presence of OMCA, does not change after UV irradiation.     

The polyurethane membranes with temperature sensitivity for water vapor permeation

The size and shape of free-volume holes available in membrane materials control the rate of gas diffusion and its permeability. Based on this principle, two segmented thermo-sensitive polyurethane (TSPU) membranes with functional gates, i.e. the ability to sense and respond to external thermo-stimuli, were synthesized and used for water vapor controllable permeation. Differential scanning calorimetry (DSC), positron annihilation lifetimes (PAL), water swelling and water vapor permeability (WVP) were used to evaluate how the structure of the polyurethane (PU) and the temperature influence the free-volume holes size and the water vapor permeability (WVP) of the PU membranes. DSC study reveals that TSPU with a glass transition or a crystalline transition reversible phase shows an obvious phase-separated structure and a phase transition temperature (defined as switch temperature, T_s). PAL study indicates that the free-volume holes size of TSPU is closely related to the T_s. When the temperature is higher than the T_s, the ortho-positronium (o-Ps) lifetime (τ₃) and the average radius (R) of free-volume holes of TSPU membrane increase dramatically. As a result, the WVP of TSPU membrane shows a dramatic increase. Additionally, the water swelling and the WVP of TSPU membrane are found to depend on the inner structure of the polymer, and they also give different responses to temperature variation. When the temperature is higher than the T_s, there is a significant increase of WVP from 3.80 kg/m² day to 7.63 kg/m² day for TSPU(a) and from 4.30 kg/m² day to 8.58 kg/m² day for TSPU(b), respectively. Phase transition accompanying significant changes in free-volume holes size and WVP can be used to develop “smart membranes” with functional gates and controllable gas permeation.     

Influences of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile solutions around the sol-gel threshold

Viscoelastic experiments were performed to study the influence of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile (PAN) solutions around the sol-gel threshold. The dynamic critical parameters around the sol-gel threshold were determined using dynamic rheometer. The sol-gel transition takes place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ holds, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. Although the gel points of PAN solutions increase with increasing H₂O content, the results show that the scaling exponent n at the gel point is found to be universal for all ternary PAN solutions, which is independent of temperature and H₂O content, indicating the similarity of the fractal structure in the critical PAN gels. The gelation of ternary PAN solutions induced by adding a nonsolvent and by decreasing the temperature is demonstrated to be a thermoreversible process, which implies that the PAN gels are physical gels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2637–2643, 2008     

Free volumes in nematic and smectic liquid-crystalline polymers probed by positron annihilation

Free volumes in thermotropic side-chain liquid-crystalline polymers were probed by positron annihilation technique. Lifetime spectra of positrons were measured in the temperature range between 130 and −60°C in cooling. For a nematic liquid-crystalline polymer (polyacrylate), the lifetime of ortho-positronium (τ₃) was decreased with decreasing temperature above the glass transition temperature (T_g, 21°C) with larger temperature coefficient than that below T_g. The intensity of ortho-positronium (I₃) was constant above T_g. These facts mean that the size of the free-volume holes decreased with the decreasing the temperature but the concentration was almost constant in nematic phase. For a smectic liquid-crystalline polymer (poly(p-methylstyrene) derivative), a discontinuous decrease in the value of τ₃ and that of I₃ were observed at 107°C, which was the transition temperature from smectic to crystalline phase. Such discontinuous changes were not observed for the polyacrylate specimen. This difference was considered to be attributed to the higher-ordered structure of the smectic phase. © 1996 John Wiley & Sons, Inc.     

A study of time dependence of ortho-positronium annihilation in a poly(butadiene) at different temperatures: a meaning of I3 parameter

Positron annihilation measurements as a function of temperature and time have been carried out on a poly(butadiene). The measurements were performed at several temperature points from 14 to 225 K. The measurement time was several hours to four days. The analysis of data shows the following features:

(i) the value of τ₃ does not depend on the rate of cooling or time,

(ii) the value of I₃ depends on the rate of cooling and the history of thermal treatment,

(iii) the dependence of I₃ on time can be described by Debye function. But the rise in I₃ is observed at very low temperatures,

(iv) the I₃ decays to value of I₃ observed during very slow cooling.

聚酯聚氨酯透气性的正电子研究

用正电子湮没寿命谱(PALS)研究了温度和PEG含量对以聚己二酸丁二醇酯(PBA)为软段的聚酯聚氨酯的自由体积特性和透气性的影响.实验结果表明,自由体积空洞的体积随着温度的升高而增加,分布变宽,导致透气性增大.不同PEG含量的聚酯聚氨酯PBA-10,PBA-15和PBA-20的水汽渗透系数(WVP)基本相同.结果表明,在这类聚氨酯中,影响透气性的主要因素不仅是自由体积,而且与材料的亲水性有关.     

Correlations between gas permeation and free-volume hole properties of polyurethane membranes

A series of polyurethane films based on hard segments consisting of toluene diisocyanate and 1,4-butanediol and different soft segments consisting of hydroxyl terminated polybutadiene, hydroxyl terminated polybutadiene/styrene and hydroxyl terminated polybutadiene/acrylonitrile were synthesized by solution polymerization separately. Positron annihilation lifetimes were measured at room temperature for all samples studied. We found that both the free volume size and fractional free-volume decreased with the increase of hard segment content. On the other hand, direct relationship between the gas permeability and the free-volume has been established based on the free-volume parameters and gas diffusivity measured. Experimental results revealed that the free-volume plays an important role in determining the gas permeability.