Revisitation of the molecular mobility of the amorphous solid 4,4′-methylenebis(N,N-diglycidylaniline) (MBDA): New contributions from the TSDC technique

In the present work we revisit the previously published study by Thermally Stimulated Depolarisation Currents (TSDC) of the slow molecular mobility in the amorphous solid state of 4,4′-methylenebis(N,N-diglycidylaniline), MBDA (H. P. Diogo, J. J. Moura Ramos, J. Mol. Liq. 129(2006)138–146) in order to add two important points dealing with recent uses of the TSDC technique. One of them concerns a new method to account for nonexponentiality in Thermally Stimulated Depolarisation Currents (TSDC) data treatment which affects the analysis of the alpha relaxation peak and the determination of the fragility index of the glass-forming system. The other point concerns the analysis of the secondary relaxations in MBDA. It is shown that two kinds of secondary relaxations are differently influenced by aging. The faster components of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes show a significant dependence on aging and correspond to the genuine Johari-Goldstein β-relaxation.     

The density and pressure of helium in bubbles in metals

Abstract

The energy shift of the He 1¹S₀?2¹P₁ transition, ΔE(n), can be used to determine the density, n, of He in bubbles in metals. A self-consistent band structure calculation for solid fcc He yields a linear relationship ΔE=C.n with C _th=22 × 10^?3 eV nm³. Systematic electron energy loss spectroscopy and transmission electron microscopy studies of He bubbles in Al for various He doses and temperatures result in C_exp=(24±8).10^?3 eV nm³ in agreement with theory. The analysis is consistent with the assumption that dislocation loop punching is the dominant bubble growth mechanism during high-dose room temperature implantation. The application to He bubbles in Ni indicates a maximum He density of n=0.2 × 10³ nm^?3 for which He should be solid at room temperature.     

Crystallization of Poly(Ethylene Oxide) in Thin Films

Crystallization of poly(ethylene oxide) (PEO) in thin films was studied using hot-stage polarized optical microscopy. Isothermal linear crystal growth rates were measured for various film thicknesses at various degrees of undercooling. At a given crystallization temperature, the linear crystal growth rate decreased exponentially with decreasing film thickness below a film thickness of 80 nm. Films showed similar spherulitic morphology down to a film thickness of 30 nm. Control experiments on hydrophilic and hydrophobic surfaces showed that surface chemistry affects stability of the polymer films and causes a competition between crystallization and dewetting.     

The determination of the glass transition temperature by thermally stimulated depolarization currents. Comparison with the performance of other techniques

Several experimental techniques are currently used for the determination of the glass transition temperature, T_g. Thermally stimulated depolarization currents (TSDC) is a thermal analysis technique whose experimental results display a very clean glass transition signature and that, nevertheless, is seldom used as a technique for T_g determination. In the present work we explain how to get the glass transition temperature from TSDC data, and we compare the values obtained for a vast number of glass forming systems (with T_gs in a wide range between ?145 and +180 °C and fragilities between m = 15 and m = 100), with the values obtained by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). We conclude that the T_g determination by TSDC is direct, accurate and reproducible and that the obtained values correlate very well with those obtained by DSC and DRS. This general survey thus suggests TSDC as a valuable alternative technique for determining T_g.     

Light-induced metastable defects in a-Se90X10 (X = Sb,In and Ag) thin films

Thin films of chalcogenide glasses in a binary system of Se₉₀X₁₀ (X = Sb, In and Ag) have been prepared by the vacuum evaporation technique. Thermally stimulated current measurements have been made to find out the trap density in these materials. To study light-induced defects in these materials, white light of intensity 1200 lux is shown on the amorphous films in vacuum for different exposure times. It is observed that the density of traps increases with exposure time, indicating the appearance of light-induced metastable defects in these materials.     

Diffusion-Limited Aggregates of Poly(Ethylene Oxide) on Alkali Halides

Poly(ethylene oxide) (PEO) was crystallized on alkali halides, such as NaCl, KCl, and KBr, from solution in isopentyl acetate. PEO dendrites first formed on alkali halide surfaces and grew to sheet the whole surface. Individual fibrils consisting of dendrites were arranged with the preferred orientation in the <110> direction on KBr and KCl, and mainly in the <100> direction on NaCl. Tetragonal lamellae grew not on the substrate directly but on the sheet platelet only, establishing a homoepitaxial relationship to the dendrites. Because the dendritic sheet that intervened between them controlled the orientation of the lamellae, tetragonal lamellae formed with a preferred orientation on alkali halide surfaces. The dendrites had morphological features of fractals, i.e., diffusion-limited aggregates (DLA). Their morphology was controlled by the diffusion process of polymer molecules on the substrate surface, as well as by epitaxial interaction. The fractal dimension (D) was obtained by analyzing the dendrites by taking the autocorrelation function of density: D ranges from 1.6 to 1.8, depending on the crystallization conditions, such as the kinds of substrate and crystallization temperature.     

Investigation of Pendant Group and Chain Segment Motions in P(MMA/BA) Copolymers by Thermally Stimulated Depolarization Current

Poly(methyl methacrylate/butyl acrylate) [P(MMA/BA)] copolymers (M_η ～2×10⁵) with different mass percentages of MMA were synthesized by the method of solution polymerization. Thermally stimulated depolarization current (TSDC) technique was used to investigate the effect of copolymerization on pendant group and chain segment motions. Three TSDC peaks were observed over the temperature range from 310 to 400 K. The highest temperature, ρ peak originates from the detrapping of trapped charge carriers. The lower temperature, α peak corresponds to the glass transition. The activation energy of the α relaxation decreases from 1.2 eV for PMMA to 0.98 eV for MMA(75)/BA(25). In the fitting process, another peak, β′, is separated on the low temperature side. The apparent energy barrier of the β′ for PMMA is 0.80 eV. The β′ relaxation is thought to correspond to the motion of pendant groups including intra‐ and inter‐molecular interactions. All three peaks move to lower temperatures with an increase in BA component, and the activation energy for the α and β′ relaxations also decreases with the increase of BA component in copolymers, indicating that the flexible side groups of BA have an effect of plasticization on the glass transition and motion of pendent groups. The temperatures of the α and β′ peaks of P(MMA/BA) copolymers follow the Fox equation. Fitting results gives the α peak at 238 K and β′ peak at 225 K for polybutyl acrylate (PBA).     

Transient and thermally stimulated depolarization currents in pure and iodine doped polyvinyl formal (PVF) films

Transient currents, measured with pure and iodine doped polyvinyl formal (PVF) films as a function of poling field (15–100 kV/cm) and temperature (30–95°C), have been found to follow Curie-von Schweidler law characterized with two slopes in short and long time regions. The isochronals (i.e. current/temperature plots at constant times) have been found to give rise to a peak located at 75°C. The order of current has been found to increase with increase in poling field, temperature and iodine mixing. The comparative studies of the isochronals with the thermally stimulated discharge current (TSDC) indicated the strong resemblance between the two studies. It is suggested that both the dipolar orientation due to molecular mechanism of motions with the side chains and space charge due to trapping of injected charge carriers in energetically distributed traps may be responsible for the observed currents. The dependence of current and activation energy on iodine mixing is explained on the basis of a charge transfer type of interaction.     

Complexation of Molecular Iodine by Linear Poly (Ethylene Glycol)

Abstract

Linear poly (ethylene glycol) (PEG) is able to complex with neutral molecular Iodine in the solution state and in solid state, to produce I^? ₃ ions detectable by conventional spectroscopy and by photoacoustic spectroscopy (PAS). This behavior of PEG is another instance of its ability to mimic the complexation features of the macrocyclic polyether 18-crown-6.     

Study on the Molecular Mobility in the Polyamide/SrFeO Composites by In Situ Infrared Spectroscopy

Polyamide 6(PA6) and its composite with strontium ferrite (SrFeO) magnetic powders at different concentration levels (33%, 80%) are in situ investigated by temperature-dependent Fourier transform infrared spectroscopy (FT-IR). It is found that the intensity changes of the “free” (nonhydrogen bonded) N?H stretching vibration as a function of temperature can be used to determine the glass transition (T _g) and melting temperature (T _m); whereas the hydrogen-bonded N?H stretching vibration is only sensitive to T _m. Our results reveal that high SrFeO contents increase the T _g and T _m of PA6. These observations suggest that the molecular chains of PA6 are confined with increasing content of SrFeO.     

Molecular Weight Dependence of Primary Nucleation Rate of Poly(Ethylene Succinate)

The molecular weight (MW) dependence of the primary nucleation rate (I) was studied for poly(ethylene succinate). The primary nucleation rate was measured in a wide range of temperatures. It has a bell shape with a maximum nucleation rate (I _max ) that showed a remarkable MW dependence. I _max decreased first with MW to about 3000 and then increased. The MW of about 3000 was attributed to the transition from intermolecular nucleation to intramolecular nucleation. The MW dependence of I _max was expressed as I _max ∝MW^α. The value of α was negative unity for intermolecular nucleation and positive unity for intramolecular nucleation.     

Influence of Blend Composition on Crystallization of Poly(L-Lactic Acid)/Poly (Ethylene Oxide) Crystalline/Crystalline Blends

The effect of blend composition on crystallization morphology and behavior of a crystalline/crystalline blend, poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO), during slow, non-isothermal crystallization was studied by polarized light microscopy (PLM) connected with a hot-stage and differential scanning calorimetry (DSC). The results showed that all of the PLLA/PEO blends produced spherulites which gradually became bigger and looser, as well as coarser, with the increment of the PEO content, indicating that the PEO crystals was resided in the interlamellar or interfibrillar (between clusters of commonly oriented lamellae) regions of the PLLA spherulites. In the (25/75) and (10/90) blends, the nucleation and growth processes of the PEO spherulites could be clearly observed in the pre-existing PLLA spherulites. The onset crystallization temperature and the melting point of one component decreased with increasing the content of the other one owing to the good miscibility of the two components in the non-crystalline state and the interaction between their macromolecules, indicating that the crystallization of each component was influenced by the other one.     

Miscibility and Isothermal Crystallization Behavior of Poly (Butylene Succinate-co-Adipate) (PBSA)/Poly (Trimethylene Carbonate) (PTMC) Blends

Poly(butylene succinate-co-adipate) (PBSA)/poly (trimethylene carbonate) (PTMC) blend samples with different weight ratios were prepared by solution blending. The morphologies after isothermal crystallization and in the melt were observed by optical microscopy (OM). Differential scanning calorimetry (DSC) was used to characterize the isothermal crystallization kinetics and melting behaviors. According to the OM image before and after melting, it was found that the blends formed heterogenous morphologies. When the PTMC content was low (20%), PBSA formed the continuous phase, while when the PTMC contents was high (40%), PBSA formed the dispersed phase. The glass transition temperatures (T_g) of the blends were determined by DSC and the differences of the T_g values were smaller than the difference between those of pure PBSA and PTMC. In addition, the equilibrium melting points were depressed in the blends. According to these results, the PBSA/PTMC blends were determined as being partially miscible blends. The crystallization kinetics was investigated according to the Avrami equation. It was found that the incorporation of PTMC did not change the crystallization mechanism of PBSA. However, the crystallization rate decreased with the increase of PTMC contents. The change of crystallization kinetics is related with the existences of amorphous PTMC, the partial miscibility between PLLA and PTMC, and the changes of phase structures.     

聚环氧乙烷在水和叔丁醇溶液中分子构型变化的研究

在聚合物溶液中,聚合物单体和溶剂分子之间的相互作用会影响聚合物高分子的尺寸和结构。本文使用动态光散射实验系统研究了聚环氧乙烷在水+TBA二元混合溶液中的分子尺寸变化,实验结果表明聚环氧乙烷分子会呈现出收缩-膨胀的奇异性构型变化。本文也系统分析了产生该现象的原因,依次分析了在二元混合溶液中可能对聚合物分子尺寸产生影响三个原因:溶剂非理想性影响(Shultz-Flory理论)、临界涨落影响以及水+醇溶液所呈现出来的奇异性性质。结合这三个因素,本文较好地解释了实验所观测到的奇异现象。     

Fluorescence Dynamics of DNA Condensed by the Molecular Crowding Agent Poly(Ethylene Glycol)

Condensation of extended DNA into compact structures is encountered in a variety of situations, both natural and artificial. While condensation of DNA has been routinely carried out by the use of multivalent cations, cationic lipids, detergents, and polyvalent cationic polymers, the use of molecular crowding agents in condensing DNA is rather striking. In this work, we have studied the dynamics of plasmid DNA condensed in the presence of a molecular crowding agent, polyethylene glycol (PEG). Steady-state and time-resolved fluorescence of the recently established condensation-indicating DNA binder, YOYO-1 [G. Krishnamoorthy, G. Duportail, and Y. Mely (2002), Biochemistry 41, 15277–15287] was used in inferring the dynamic aspects of DNA condensates. It is shown that DNA condensed by PEG is more flexible and less compact when compared to DNA condensed by binding agents such as polyethyleneimine. The relevance of such differences in dynamics toward functional aspects of condensed DNA is discussed.     

Nonisothermal Crystallization Kinetics of Poly(Vinylidene Fluoride) in a Poly(Vinylidene Fluoride)/Poly(Methyl Methacrylate)/Dipropylene Glycol Dibenzoate System via Thermally Induced Phase Separation

The nonisothermal crystallization kinetics of poly(vinylidene fluoride) (PVDF) in PVDF/polymethyl methacrylate (PMMA)/dipropylene glycol dibenzoate (DPGDB) blends, where DPGDB served as a diluent, via solid–liquid (S-L) phase separation during a thermally induced phase separation process was investigated through differential scanning calorimetry (DSC) measurements. It was found that the Ozawa model could only describe the nonisothermal crystallization behavior of PVDF/PMMA/DPGDB system to some extent. The influence of the cooling rate and PMMA/PVDF weight ratio in the PVDF/PMMA/DPGDB system on the crystallization mechanism was also examined based on the Avrami–Jeziorny method and Mo method. Primary crystallization and secondary crystallization were observed in the Avrami–Jeziorny analysis. The analysis by the Avrami–Jeziorny and Mo models indicated that the increase of PMMA/PVDF weight ratio decreased the crystallization rate during the primary crystallization stage. The results showed that the Mo method could well explain the kinetics of the primary PVDF crystallization. The Avrami–Jeziorny method, however, could not well describe the nonisothermal crystallization process of PVDF in the primary crystallization stage. The activation energy, determined by the Kissinger method, was not suitable to reflect the PVDF crystallization process in the PVDF/PMMA/DPGDB system.     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below T_g carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.     

交叉极化时间对分子间交叉极化的影响的研究

以C₆₀和聚氧乙烯(PEO)复合物为研究对象,系统地研究了交叉极化时间对分子间交叉极化实验的影响. 结果表明,在使用分子间交叉极化方法研究复合体系相结构和界面结构时,必须考虑交叉极化时间的影响才能获得更可靠的结论. 同时,系统地改变交叉极化时间也能为分子间交叉极化实验的设计引入一个新的维度,从而可能得出一些在单个交叉极化时间下难以获得的结论. 例如,本实验发现C₆₀}的分布并不完全局限于PEO的非晶区,至少部分C₆₀和PEO的晶区在空间上比较接近.     

Influence of Shearing on Crystallization Behavior of Polyoxymethylene/Poly(Ethylene Oxide) Crystalline/Crystalline Blends

The effect of shearing on crystallization behavior of a crystalline/crystalline blend, polyoxymethylene [POM]/poly(ethylene oxide) [PEO], was investigated using polarized light microscopy connected with a CSS450 shearing hot-stage, scanning electron microscopy, differential scanning calorimetry [DSC], and x-ray diffraction [XRD]. The experimental results indicated that the shearing made POM and PEO disperse more evenly and increased the inclusion and entanglement effects between the molecular chains of POM and PEO and therefore enhanced the influence of PEO on the crystallization of POM. As a result, the blend sheared at a shear rate of 20 s^?1 for 10 min at 160°C formed shish–kebab crystals and produced more interlamellar structures compared with the formation of perforated spherulites in the unsheared blend. Moreover, a more obvious shoulder melting peak of POM appeared in the DSC heating trace and a new diffraction peak occurred at 2θ = 31.7° in the XRD pattern for the sheared POM/PEO [50/50] blend.     

Measurements of the 10B (n,He) reaction rate in zppr

Direct measurements of the ¹⁰B(n, He) reaction rate have been made in ZPPR. The measurements were made by isotope-dilution mass spectrometric analysis of stainless-steel encapsulated samples of enriched ¹⁰B. The results demonstrate that direct measurements of the ¹⁰B reaction rate in ZPPR are obtainable to an accuracy of ～1%. Comparisons with calculations give a predicted-to-experiment value of ～0.93. This value is consistent with earlier Rockwell measurements in benchmark neutron fields and further supports a need for revision in the ENDF/B cross section for ¹⁰B above ～0.1 MeV.