On the role of inter- and intra-molecular potentials in the simulation of vitrification with the bond fluctuation model

The glass transition of linear chain polymers was simulated by means of the bond fluctuation model, employing a Lennard-Jones inter-molecular potential and a bond-length intra-molecular potential. The influence of the balance between inter- and intra-molecular potentials on the model prediction was studied and an appropriate range was determined in order to simulate physical ageing. The relationship between molecular mobility and the dynamically accessible volume was shown in comparison with other definitions of specific volume or free volume. The temperature dependence of the dynamically accessible volume shows the vitrification of the material analogously to the temperature dependence of the lattice energy. Dynamically accessible volume has been proposed as a universal parameter to explain glass transition, but in the bond fluctuation model it does not give the same value for all simulated glass transitions. This fact could be corrected by including the Metropolis criterion in the dynamically accessible volume in order to extend the concept to thermal systems.     

The relationship between diffusion and dynamically accessible volume in polymer chain dynamics simulated with the bond fluctuation model

The dependence of the diffusion coefficient, D, of polymer systems on the Dynamically Accessible Volume (DAV) was calculated in the framework of the Bond Fluctuation Model. The quadratic relationship between dynamically accessible volume and diffusion coefficient, which has been observed for some other models, has been confirmed for high values of DAV, whereas smaller diffusion coefficients than those predicted by the model were shown for DAV values lower than 0.1 independently of the chain length. This critical value is related to the loss of connectivity of the holes of the system and the consequent diminution of the diffusion of the system. The influence of the chain length on this behaviour has been established, showing that there is a linear relationship between the γ parameter (the proportionality constant between D and the squared DAV) and the reciprocal of the chain length.     

Entropic model for the relaxation in vitreous systems. Estimation of uncertainty in the calculation of the conformational relaxation times

Differential scanning calorimetry experiments were conducted on a thermotropic polyester. The experimental results consists of a series of thermograms measured on heating after different thermal histories that contained (or not) an isothermal stage at a temperature T_a below the glass transition with a duration t_a. The thermogram showed the characteristic peak in the region of the glass transition. The calculation of the relaxation times of the co-operative conformational rearrangements related to the glass transition and structural relaxation phenomena has been conducted by curve fitting using a phenomenological model. The curve fitting procedure was conducted simultaneously on a fixed number n of experimental thermograms. The aim of the paper is to present a method to estimate the uncertainty in the calculation of the model parameter. The results show how the computer simulated thermograms agree quite well to the experimental data. The uncertainty in the model parameters and through them the uncertainty in the calculated relaxation times is quite important when the number of experimental curves n is small but rapidly decreases as n increases and if more than five curves are used simultaneously in the fitting routine both the values of the model parameters and their uncertainty become independent on the number and thermal histories of the experimental thermograms.     

Effect of confinement on the relaxation behavior of poly(ethylene oxide)

It is widely thought that confinement of an amorphous polymer alters the chain mobility, which affects the temperature and intensity of the glass transition. The present study sought to determine whether the same effects extend to semicrystalline polymers. Confinement was achieved by forced assembly of hundreds of alternating layers of poly(ethylene oxide) (PEO) with either poly(ethylene-co-acrylic acid) or polystyrene. The confinement gradually reduced the intensity of the PEO β-relaxation as the layer thickness decreased from the microscale to the nanoscale. By considering the changes in crystalline morphology that accompanied layer confinement, it was possible to completely account for the reduction in relaxation intensity using standard mechanical models. The viscoelastic behavior of the amorphous phase was satisfactorily represented by a modified standard linear solid (SLS). The amorphous and crystalline contributions were combined using a combination of parallel and series coupling in accordance with the Takayanagi model. No adjustment in the viscoelastic parameters of the modified SLS was required, indicating that there was no significant change in amorphous chain dynamics even in layers as thin as 45 nm.     

Cooperativity in stereoregular PMMAs observed by molecular simulation

The motion correlation along the backbone and between the side-chain and the backbone for the two PMMA chains of opposite tacticity, are investigated using molecular dynamics simulations. According to the coupling model (CM), the index of fragility, which can be considered as a measure of the cooperativity, can be related to the Kohlrausch-Williams-Watts (KWW) exponent. This exponent actually stems from the computation of the correlation time of different bonds inside the polymer chains. From suitably selected bonds, its behavior with temperature can thus unveil cooperativity along the backbone and between the side-chain and the backbone. Compared with experimental data and present theories, these simulation data show that at a relative temperature above the glass transition temperature, T_g, both configurations exhibit the same behavior for the backbone, but a different coupling is clearly observed between the side-chain and the backbone. Such a behavior tends to explain the difference in T_gs between the two configurations in PMMA.     

Thermal characterization on the relaxation transition of the phenylphosphonate-modified epoxy copolymers by thermally stimulated current technique and relaxation mapping analysis

TSC/RMA study on the depolarization relaxation transitions of the phenylphosphonate-modified epoxy copolymers with thiodiphenol (P2-ETP) or bis-phenol A (P2-EBPA) segments were conducted. P2-ETP resin shows a lower T_g than that of P2-EBPA, indicating that divalent thio-linkage (-S-) is flexible than isopropyl (-C(CH₃)₂-) group. Thermal windowing technique of RMA was used to obtain thermokinetic data in which ΔS of P2-ETP is higher than P2-EBPA, thus confirms the easier motion in P2-ETP which then leads to the lowering of the glass transition (T_g). Methods used for the confirmation on the T_g temperature were illustrated from thermokinetic data obtained by RMA measurement.     

Origin of the divergence of the timescales for volume and enthalpy recovery

Although the relationship between the relaxation timescales of thermodynamic, mechanical, viscoelastic, and dielectric properties in amorphous materials has been studied extensively, no general consensus has been reached. In this work, we examine the relationship between the timescales of volume and enthalpy relaxation for polystyrene using the cooling rate dependence of the glass transition temperature (T_g) obtained from capillary dilatometry and differential scanning calorimetry (DSC). Our analysis suggests that both volume and enthalpy exhibit similar relaxation timescales at temperatures above and below T_g. The divergence of the times required to reach equilibrium noted in the literature at temperatures several degrees below the nominal T_g is attributed to the effects of nonlinearity. The relationship between nonlinearity and dynamic heterogeneity is discussed.     

Glass transition dynamics and structural relaxation of PLLA studied by DSC: Influence of crystallinity

Poly(l-lactic acid), PLLA, in amorphous and semi-crystalline forms were studied by DSC, in order to investigate both molecular dynamics and structural relaxation features, and to understand the influence of the crystalline confinement on the segmental mobility of the intra-spherulitic amorphous phase. Experimental data were generated after submitting the materials to different thermal histories below T_g and were treated with a model based on the configurational entropy concept, allowing to extract a series of physical-meaningful parameters and to obtain the temperature dependence of the relaxation times. The main features of the relaxation process in the glassy state (activation energy of the glassy state and x) and the fragility index were found to be apparently insensitive to crystallinity. Significant differences between the two materials were detected in both the position of the glass transition temperature and the width of the distribution of relaxation times. In the framework of the Adam and Gibbs theory, it is suggested that for the semi-cristalline PLLA the mobility of the amorphous chains is more or less restricted, depending on their distance to the rigid lamellar walls or on the intra-lamellar thickness; this will imply that their conformational motions will take place at different temperatures, typically above the glass transition temperature of the (unconfined) bulk amorphous phase.     

Confinement, composition, and spin-coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene-containing random copolymers: Sensing by intrinsic fluorescence

The glass transition temperatures (T_gs) of polystyrene (PS) and styrene/methyl methacrylate (S/MMA) random copolymer films are characterized by intrinsic fluorescence, i.e., monomer fluorescence from an excited-state phenyl ring and excimer fluorescence from an excited-state dimer of two phenyl rings. The T_g is determined from the intersection of the rubbery- and glassy-state temperature dependences of the integrated fluorescence intensity measured upon cooling from an equilibrated state. With PS, the effects of nanoconfinement on T_g and the transition strength agree with results from studies using probe fluorescence and ellipsometry. The T_g-nanoconfinement effect is “tuned” by copolymer composition. As S-content is reduced from 100 mol% to 22 mol%, the confinement effect changes from a reduction to an enhancement of T_g relative to bulk T_g. Intrinsic fluorescence is also a powerful tool for characterizing relaxation of residual stresses. Stresses induced by spin coating affect local conformations, which in turn affect excimer and monomer fluorescence and thereby integrated intensity. The heating protocol needed to achieve apparently equilibrated local conformations is determined by equivalence in the integrated intensities obtained upon heating and subsequent cooling. While partial stress relaxation occurs upon heating in the glassy state, full relaxation of local conformations requires that a film be heated above T_g for times that are long relative to the average cooperative segmental relaxation time. For example, in thin and ultrathin films, equilibration is achieved by heating slowly (∼1 K/min) to 15-20 K above T_g. Dilute solution fluorescence of PS and S/MMA copolymers is also characterized and compared to reports in the literature.     

Thermal relaxation behaviors of phenylphosphonate-containing TDI-based polyurethane elastomers studied by thermally stimulated current technique with relaxation mapping analysis

TSC/RMA study on the thermal behavior of the phenylphosphonate-containing TDI-based polyurethane elastomers, reveals that the bulky phenylphosphonate moiety, which was sandwiched in the middle of the PPG soft chain, untightens the matrix and facilitates the cooperative motion at T_g transition, as indicated by the lowering of the T_g temperature. An additional relaxation transition, the T_global transition, in associated with the hard TDI-MOCA segment was detected and evaluated. Thermal windowing technique in RMA reveals the coexistence of two amorphous regions with one dominated by the urethanic segment and the other is the global/mechanical property associated with the TDI-MOCA hard segment. DSC thermogram showed only one T_g transition without detection of the crystalline domain, implicating that domain was not formed by the hard segment; The detection by TSC/RMA on the relaxation transition of the hard segment existed in the amorphous region may be significant and provide a tool to study the thermal behavior of the hard segment in the polyurethane matrix.     

A study of the bond degradation of rebar due to cathodic protection current

Bond degradation of rebar embedded in concrete due to impressed cathodic protection current was studied and is reported in this paper. Different mix designs of concrete are found to have influence on the degradation percentage of bond strength. More specifically, bond strength degradation percentage with higher water-to-cement (w/c) ratio is found to be larger than that with lower w/c ratio. The microstructure scanning electron microscope (SEM) photos of the concrete near the rebar-concrete interface showed that a loose structure with larger microvoids existed in the interface zone. Further, microhardness tests on the concrete near the interface and chemical titration to determine contents of potassium and sodium ions were performed to ensure that the main cause of bond degradation is the softening effect of concrete. A unified parameter, which combined the effects of cathodic current density and polarization time, is used to build up the relationship between experimental data. This concept allows engineers to quickly obtain design information from the experimental data of accelerated tests.     

电流波动对后期隔膜电解槽的影响

河南省实行电力峰谷分时电价。为减少电耗，新乡树脂厂在每日16：00-22：00将隔膜碱电解槽运行电流密度由1438A/m^2下调为1250A/m^2。运行1周后，电解槽各项指标有不同程度的恶化，有的不得不除槽。分析得知，发生这种情况是由普通隔膜电解槽的自身特点决定的。     

Correlation of the glass transition temperature of plasticized PVC using a lattice fluid model

A model has been developed to estimate the glass transition temperature of polymer+plasticizer mixtures (up to 30 wt% plasticizer). The model is based on the Sanchez-Lacombe equation of state and the Gibbs-DiMarzio criterion, which states that the entropy of a mixture is zero at the glass transition. The polymers studied included polystyrene and poly(vinyl chloride). The plasticizers studied included a wide range of chemicals from methyl acetate to di-undecyl phthalate. The model qualitatively accounted for the effect of different plasticizers on the mixture glass transition temperature.     

Annealing effects on thickness of polystyrene thin films as studied by neutron reflectivity

We performed neutron reflectivity measurements on deuterated polystyrene thin films supported on silicon substrate as a function of temperature. In order to see effects of annealing on the thickness, the films were annealed at 80 °C for 12 h and 135 °C for 12 h, termed weakly and strongly annealed films, respectively. One of the main purpose of this study is to see if the negative expansivity reported for very thin films [Phys. Rev. Lett. 71 (1993) 867] is caused by unrelaxed structure due to lack of annealing. It was found that the weakly annealed films show negative expansivity in the glassy state and it disappears for the strongly annealed films with thickness above about 90 Å. This suggests that the negative expansivity is due to the unrelaxed structure. In addition to this relaxation process, the thickness difference between the heating process and the cooling process suggests that there is another very slow relaxation process in thin films detectable at around 135 °C or about 32 °C above the glass transition temperature T_g. As a candidate for this slow process a sliding motion proposed by de Gennes is discussed.     

Characterization of polypyrrole degradation by the conformational relaxation model

Electrogenerated films of polypyrrole were degraded (overoxidized) by consecutive short anodic polarizations at 900 mV. The degree of degradation was quantified by means of the percentage of anodic voltammetric charge lost after each consecutive degradation processes. The charge consumed to compact 1 mol of polymeric segments (molar compaction charge) was obtained for every degraded state from experimental series of chronoamperograms, as indicated by the electrochemically stimulated conformational relaxation model. The molar magnitude decreases linearly with the change on the degradation percentage indicating the presence of shorter ideal lineal chains on a progressively degraded polymer. The correlation between these magnitudes opens the way to attain new molecular magnitudes of the polymeric chains by electrochemical measurements, as predicted by the model.     

Fibrous particle deposition in human nasal passage: The influence of particle length, flow rate, and geometry of nasal airway

Man-made vitreous fibers (MMVFs) have been used as a substitute for asbestos in industrial and residential applications. This shift has raised the concerns of the potential hazards associated with inhalation of these fibers. The human nose is an important protective organ that captures harmful particles and then clears them from human respiratory tract. However, studies have shown that some or even most of the inhalable fibrous particles can penetrate human nose and deposit into the deep lung. The understanding of fibrous particle deposition in the human nasal passage has important occupational health and possible drug delivery applications. To study the deposition pattern and influential factors, three realistic human nasal models were used and a dielectrophoretic classifier was applied to generate test aerosol of glass fibers with a narrow length distribution. These models were made by using stereolithography based on MRI data from two human subjects. Regional and total deposition efficiencies were measured for five different flow rates: 4, 8, 12, 15, and 18 Lpm and four different fiber length ranges: 10–19, 20–29, 30–39, and 40–. This study found that deposition of glass fibers (with about diameter) in human nasal passage is mainly due to inertial impaction and these fibers orientated themselves normal to the flow direction before deposition occurs. An effective aerodynamic diameter is defined such that the deposition efficiencies of glass fibers are comparable with those of spherical particles. Non-dimensional parameters were defined and an empirical model based on the experimental results is proposed to calculate fibrous particle deposition efficiency in human nose. Empirical expressions were also developed to estimate the pressure drop across the nasal model. Thus, empirical equations are now available for the prediction of total deposition in the human nasal tract for the fibrous particles under constant inspiring flow rates. In addition, this study suggested that these equations can also be used to predict the deposition of spherical particles.     

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

Cooperative behavior of poly(vinyl alcohol) and water as revealed by molecular dynamics simulations

An aqueous poly(vinyl alcohol) (PVA) model has been extensively studied by using the molecular dynamics (MD) simulation method. The employed molecular and force field models are validated against the available data in the literature. In particular, the glass transition temperature (T_g) is determined from the specific volume versus temperature, which compares well with the experimental observations. The diffusion coefficients of water (H₂O) through the PVA matrix follow the Arrhenius equations at both temperature regions separated by T_g, indicating the existence of free and bound water defined by hydrogen bonds (HBs). It has also been confirmed that HBs occur between PVA and H₂O, between PVA and PVA, between H₂O and H₂O, and all of them play the key roles in the glass transition. The local dynamics suggested by the decorrelations of various bond vectors can be well described by the Williams-Landel-Ferry (WLF) equation. This work demonstrates the cooperative behavior of PVA and H₂O which is responsible for the glass transition of the whole binary system.     

氯气管道积水引起压力波动的规律及处理

分析氯气管道积水引起压力波动的原因,根据压力波动规律总结积水部位的判断方法,并结合实例,证明该方法简洁实用.