Polyisobutylene: A most unusual polymer

The influence of molecular weight, M, on the fragility and fast dynamics in polyisobutylene (PIB) was studied using dielectric and mechanical relaxation spectroscopies, calorimetry, and Raman spectroscopy. The measurements indicate a decrease in fragility with increasing M for shorter chains, in the range of M where T_g is M‐dependent. Such behavior is not observed for other polymers and is at odds with traditional theoretical models that predict an increase in fragility with chain length. These results confirm the unusual character of PIB, as evident in various properties including extremely low gas permeability, a low fragility, and a segmental relaxation spectrum much broader than expected for a low‐fragility material. The reason for this anomalous behavior remains unclear, but might be related to the symmetric structure of the PIB repeat unit, together with comparable flexibility of both structural components, the backbone and side groups. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1390–1399, 2008     

Glass‐formation kinetics of miscible blends of atactic polystyrene and poly(2,6‐dimethyl‐1,4‐phenylene oxide)

We present a detailed investigation of the kinetics associated with the glass transitions of miscible blends composed of atactic polystyrene (a‐PS) and poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO). According to both dynamic mechanical analysis and differential scanning calorimetry, relaxation times displayed an enhanced temperature dependence (i.e., more fragile or more cooperative behavior) for the blends compared with additive behavior based on the responses of neat a‐PS and PPO. This is consistent with the notion that specific interactions between the blend components heighten the intermolecular cooperativity. The compositional dependence of fragility provided insight into physical aging results for the properties of volume and enthalpy. The combination of our research and a previously reported pressure–volume–temperature study by Zoller and Hoehn (J Polym Sci Polym Phys Ed 1982, 20, 1385) provided evidence that the observation of increased glassy densities for the blends compared with those of the pure polymers was kinetic in origin and was not a feature of the thermodynamics of miscibility. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2118–2129, 2001     

Correlation between superheated liquid fragility and potential energy landscape in Gd-and Pr-based glass-forming alloys

The kinetic viscosities of superheated liquids on the Gd-based bulk glass-forming alloys are measured by an oscillating viscometer in a high vacuum atmosphere. According to the viscosity data, the parameters of superheated liquid fragility, M, are calculated. Based on the values of M in Gd-and Pr-based (cited from the literature) glass-forming alloys, we find that there is a linear correlation between M and the absolute value of mixing enthalpy, |ΔH _mix|, in an alloy system with the same base element, and the larger M, the smaller |ΔH _mix|. The alloy with larger M exhibits the larger height of energy barriers separating the minima on the potential energy landscape. Supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB613901), the National Natural Science Foundation of China (Grant No. 50231040), the Natural Science Foundation of Shandong Province of China (Grant No. Z2004F02), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050422024)     

Influence of the molecular mass on the segmental relaxation times of polystyrene determined by DSC

Journal of Thermal Analysis and Calorimetry - The segmental dynamics of narrow fractions (áMwñ/áMnñ ≈1.05) of polystyrene with molecular masses ranging from 4000 to 600000...     

Local segmental relaxation in bidisperse polystyrenes

Dynamic mechanical results are reported for segmental relaxation of monodisperse polystyrenes (PSs) with molecular weights of 0.7, 3, 18, and 104 kg/mol and bidisperse PSs created from blending pairs of these materials. The data for the monodisperse polymers confirm previous findings; namely, there is an increase in the glass‐transition temperature normalized temperature dependence of the segmental relaxation times (fragility) with increasing molecular weight, along with a breakdown of the correlation between the fragility and the breadth of the relaxation function. For both the monodisperse and bidisperse PSs, the glass‐transition temperature is a single function of the average number of chain ends, independent of the nature of the molecular weight distribution. It is also found that these materials exhibit fragilities that uniquely depend on the number‐average molecular weight, that is, on the concentration of chain ends. In blends with linear PS, cyclic PS with a low molecular weight behaves as a high polymer, similar to its neat behavior, reflecting the overriding importance of chain ends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2604–2611, 2004     

A novel method based on maximum likelihood estimation for the construction of seismic fragility curves using numerical simulations

Seismic fragility curves presenting some probability of failure or of a damage state exceedance versus seismic intensity can be established by engineering judgment, empirical or numerical approaches. This paper focuses on the latter issue. In recent studies, three popular methods based on numerical simulations, comprising scaled seismic intensity, maximum likelihood estimation and probabilistic seismic demand/capacity models, have been studied and compared. The results obtained show that the maximum likelihood estimation (MLE) method is in general better than other ones. However, previous publications also indicated the dependence of the MLE method on the ground excitation input. The objective of this paper is thus to propose a novel method improving the existing MLE one. Improvements are based on probabilistic ground motion information, which is taken into account in the proposed procedure. The validity of this new approach is verified by analytical tests and numerical examples.     

Glass Transition Behavior and Dynamic Fragility of PMMA-SAN Miscible Blend-Clay Nanocomposites

An investigation of the segmental dynamics and glass transition behavior of a miscible polymer blend composed of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) and its melt intercalated nanocomposite by dynamic mechanical analysis is presented. The principle goal was to address the effect of intercalation on local molecular structure and dynamics. The results showed that the intercalation of polymer chains in the galleries of organoclay (Cloisite 30B) led to a lower temperature dependence of the relaxation time (fragility) and activation energy of α-relaxation. Moreover, calculation of the distribution of the segmental dispersion showed a narrower dispersion in the glass transition region so that the Kohlrausch-Williams-Watts (KWW) distribution parameter (β_KWW) increased from 0.21 for neat PMMA to 0.34 for the 50/50 PMMA/SAN blend nanocomposite containing 3 wt% organoclay. Furthermore, the relaxation behavior of the blends showed a negative deviation from mixture law predictions based on the responses of the neat PMMA and SAN. These behaviors were attributed to the lack of specific interactions between the blend components (PMMA, SAN, and nanoclay layers) and the less cooperative behavior, i.e., less constraint for segmental relaxation, of the intercalated chains.     

Simulating local mobility and mechanical properties of thermostable polyimides with different dianhydride fragments

The dynamic and mechanical properties of the three thermoplastic polyimides—crystallizable polyimide BPDA‐P3 and amorphous polyimides ODPA‐P3 and aBPDA‐P3—have been simulated using the atomistic molecular dynamics technique. The three simulated polyimides differ in the chemical structure of their corresponding dianhydride fragments. Analyzing the local orientational mobility of different phenylene rings, it has been established that the increase of the glass‐transition temperature (T_g) in the ordered set is caused by the slowing down of the phthalimide rings relaxation in the corresponding dianhydride fragments. It has been observed that rather poor mechanical characteristics upon aBPDA‐P3 stretching in the strain‐hardening regime are also due to the low orientational mobility of the phthalimide rings. The correlation between the dynamic fragility and the polyimides strain‐hardening moduli has been observed; the increase of the dynamic fragility leads to the increase of the strain‐hardening modulus. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 375–382     

Enthalpy relaxation and fragility in polychlorinated biphenyls

We employ temperature modulated DSC (TMDSC) to determine the dependence of the fictive temperature on cooling rate for a series of polychlorinated biphenyls (PCB). From the slopes of semi-logarithmic plots of cooling rate vs. fictive temperature, the latter normalized by the fictive temperature for an arbitrary cooling rate, we determine the enthalpic fragilities. Despite significant differences in glass transition temperature and chemical structure (specifically chlorine content), the PCB have the same fragility. The value of the fragility determined using TMDSC is consistent with the fragility previously determined using dielectric relaxation spectroscopy.     

Thermodynamic and kinetic behaviour of salicylsalicylic acid

The thermal behaviour of salicylsalicylic acid (CAS number 552-94-3) was studied by differential scanning calorimetry (DSC). The endothermic melting peak and the fingerprint of the glass transition were characterised at a heating rate of 10°C min^-1. The melting peak showed an onset at T _on = 144°C (417 K) and a maximum intensity at T _max = 152°C (425 K), while the onset of the glass transition signal was at T _on = 6°C. The melting enthalpy was found to be Δ_mH = 28.9±0.3 kJ mol^-1, and the heat capacity jump at the glass transition was ΔC _P = 108.1±0.1 J K^-1mol^-1. The study of the influence of the heating rate on the temperature location of the glass transition signal by DSC, allowed the determination of the activation energy at the glass transition temperature (245 kJ mol^-1), and the calculation of the fragility index of salicyl salicylate (m = 45). Finally, the standard molar enthalpy of formation of crystalline monoclinic salicylsalicylic acid at T = 298.15 K, was determined as Δ_fH_m ^o(C₁₄H₁₀O₅, cr) = - (837.6±3.3) kJ mol^-1, by combustion calorimetry. This revised version was published online in August 2006 with corrections to the Cover Date.