Neat Glass Research Articles

Secondary dynamics in glass formers: Relation with the structural dynamics and the glass transition

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari–Goldstein process, originating by a local motion of the whole molecule.

Effects of initial saturation on properties modification and displacement of tetrachloroethene with aqueous isobutanol

Packed column experiments were conducted to study effects of initial saturation of tetrachloroethene (PCE) in the range of 1.0–14% pore volume (PV) on mobilization and downward migration of the non-aqueous phase liquid (NAPL) product upon contact with aqueous isobutanol (∼ 10 vol.%). This study focused on the consequences of swelling beyond residual saturation. Columns were packed with mixtures of neat PCE, water and glass beads and waterflooded to establish a desired homogeneous residual saturation, and then flooded with aqueous isobutanol under controlled hydraulic conditions. Results showed a critical saturation of approximately 8% PV for these packed column experimental conditions. At low initial PCE saturations (< 8% PV), experimental results showed reduced risk of NAPL-product migration upon contact with aqueous isobutanol. At higher initial PCE saturations (> 8% PV), results showed NAPL-product mobilization and downward migration which was attributed to interfacial tension (IFT) reduction, swelling of the NAPL-product, and reduced density modification. Packed column results were compared with good agreement to theoretical predictions of NAPL-product mobilization using the total trapping number, N T. In addition to the packed column study, preliminary batch experiments were conducted to study the effects of PCE volumetric fraction in the range of 0.5–20% on density, viscosity, and IFT modification as a function of time following contact with aqueous isobutanol (∼ 10 vol.%). Modified NAPL-product fluid properties approached equilibrium within approximately 2 h of contact for density and viscosity. IFT reduction occurred immediately as expected. Measured fluid properties were compared with good agreement to theoretical equilibrium predictions based on UNIQUAC. Overall, this study demonstrates the importance of initial DNAPL saturation, and the associated risk of downward NAPL-product migration, in applying alcohol flooding for remediation of DNAPL contaminated ground water sites.

UHMWPE as interface toughening agent in glass particle filled composites

Abstract UHMWPE was adsorbed from dilute solution onto glass beads surface modified with a copolymer of 0.5–1.2 mol% vinyltrimethoxysilane-ethylene (PE-silane). Functional methacrylate groups were introduced through treatment of the UHMWPE with octadecyltrimethoxysilane (OMS) followed by reaction with 3-methacryloxypropyltrichlorosilane (Cl-MPS). Composites were prepared with 20 vol% PE-silanated/UHMWPE glass and BisGMA/TEGDMA (60/40 wt/wt); Cl-MPS treatment did not affect the results. Toughness increased 40% compared with neat or glass-filled resin, consistent with the addition of the high toughness UHMWPE; moduli were intermediate between the neat resin and glass filled composites, consistent with the relative moduli of glass and UHMWPE; flexural strengths decreased in the order: neat resin > resin + glass beads > silanated/UHMWPE coated glass beads, suggesting that each interface negatively affected the flexural strength.

On the dielectric susceptibility spectra of supercooled o-terphenyl

Supercooled o-terphenyl has been the subject of many investigations including dielectric relaxation spectroscopy. Due to the low dielectric strength and the tendency to crystallize at elevated temperatures, a detailed shape analysis of the loss profile from the glass transition temperature Tg to approximately 1.2 Tg is not available for the neat glass former. Assessing the origin of the different temperature dependencies of translational and rotational motions in supercooled liquids and its possible connection to heterogeneity requires this knowledge regarding the possible changes in the relaxation-time distribution across the 100 s-100 ns relaxation-time range. This note provides this information for o-terphenyl on the basis of a master curve representation: time-temperature superposition applies with a constant stretching exponent of beta=0.5 in the range of interest.

Nonresonant dielectric hole burning in neat and binary organic glass formers

Binary mixtures of the molecular glass former 2-picoline in oligostyrene, in which the dielectric response of 2-picoline exhibits a particularly broad distribution of correlation times, are investigated by nonresonant dielectric hole-burning (NDHB) spectroscopy and the results are compared with NDHB in neat systems, in particular, glycerol. It turns out that in both substance classes spectral selectivity is achieved, which indicates that dynamics is heterogeneous, i.e., slow and fast responses coexist in the material. However, in binary systems the position of the spectral modifications is completely determined by the spectral density of the pump field, and thus shifts linearly with burn frequency as expected, also at pump frequencies around the alpha-relaxation maximum. It is shown that in binary systems the lifetime tau(rec) of the spectral modifications is determined by the burn frequency omega(p) and exceeds its inverse by about one order of magnitude, indicating long-lived dynamic heterogeneity. The data are described in terms of a previously suggested model of dynamically selective heating, which was extended to include intrinsic nonexponential relaxation. It turns out that the spectral broadening in binary mixtures is not only due to pronounced dynamic heterogeneity, but partially also due to intrinsic broadening of the relaxation function.

Large angle jumps of small molecules in amorphous matrices analyzed by 2D exchange NMR

The reorientation dynamics of deuterated benzene and hexamethyl benzene as additives to the glass former oligostyrene is studied below the glass transition temperature T g. By means of 2D 2H NMR, analyzed in the frequency and in the time domain, it is shown that the dynamics of the small molecules is governed by an isotropic large angle reorientation process, which is close to the random jump model. Furthermore, the dynamics is characterized by a broad distribution of correlation times. Even 65 K below T g, a fraction of small molecules reorients on the timescale of 100 ms. In contrast, small angle reorientation dominates in the neat glass former polystyrene near T g. As a consequence of the presence of large angle jumps, the 2D spectra can be described by an additive superposition of two sub-spectra—a ridge along the diagonal and a complete exchange pattern—where the weighting factor W( t m) is directly given by the reorientational correlation function F 2( t m). Additionally, for a sample with very low benzene concentration ( c≈0.5%), the 1D spectra indicate that the same dynamic scenario is present in the single particle limit. Tentatively, we assume that the large angle reorientation of the small molecules is associated with a translational diffusion process of the small molecules within the amorphous matrix.

A 2H NMR and dielelectric spectroscopy study of the slow β-process in organic glass formers

We study the slow β-process of several organic compounds by recording dielectric and 2H NMR spectra below the glass transition temperature T g. For the neat systems toluene, polybutadiene, cis-decalin and ethanol as well as for the binary glasses chlorobenzene/ cis-decalin and toluene/2-picoline, the β-process manifests itself in very similar changes of the 2H NMR spectrum when varying the spatial resolution by increasing the interpulse delay t p in the solid-echo pulse sequence. These findings indicate that for all considered glasses, in particular, for both components of the binary mixtures, molecular dynamics involved in the β-process are highly comparable. A line-shape analysis reveals that the secondary relaxation is mainly caused by a highly restricted, step-wise reorientation of essentially all molecules.For the neat glasses 2-picoline, glycerol and polystyrene (PS) which do not show a β-peak in dielectric spectroscopy, there is no evidence for molecular reorientation in the 2H NMR spectra at T< T g.

Fabrication of Various Ordered Films of Oxotitanium(IV) Phthalocyanine by Vacuum Deposition and Their Spectroscopic Behavior

Different types of ordered solid films of oxotitanium(IV) phthalocyanine (OTiPc) were grown in 50−500 nm thickness by vacuum deposition onto such conventional substrates as metal- and ITO-coated glass and neat glass at a rate of ∼0.05 nm s-1. Under 10-3−10-4 Pa isotropic films with short-range molecular organization and preferential molecular orientation were grown on the substrates kept at ∼25 °C but ordered α (phase II) or β (phase I) crystal films on the substrates heated at 150 °C. The deposition onto surface-oxidized Al, Ti, and Cu/glass, ITO/glass, and neat glass at ∼25 or 150 °C selectively gave ordered isotropic or α-crystal films which have preferential molecular alignments with “standing” orientations of the molecular plane with respect to the substrate surface. On the other hand, another type of ordered isotropic or β-crystal films with preferential “lying” molecular orientations was formed by the deposition onto Au, Ag, and unoxidized Cu on glass at ∼25 or 150 °C. In the case of 150 °C-heated Pt/glass, the α-crystal film with “standing” molecular orientations was grown under 10-3−10-4 Pa, but another type of α-crystal film with “lying” orientations under 2.7 × 10-5 Pa. Scanning electron microscopic observations of the ordered crystalline films indicated that single-crystal-like domains are grown from the bottom to the top of the film and are densely packed to give monolithic morphologies with little grain boundary. Visible−near-IR absorption and fluorescence spectra of the ordered solid films were recorded, showing unique dependences on the molecular alignments involving anisotropic behavior in the polarized absorption spectra of the α-crystal films with different molecular alignments.

Slow molecular dynamics in binary organic glass formers

We investigate systems benzene in polystyrene as model for binary glass formers by applying multidimensional 2H NMR experiments. It is demonstrated that the molecular larger component of these systems essentially behaves as a neat glass former, whereas the small benzene molecules show a completely different scenario. Their 6-fold axes still reorientate isotropically below the glass transition of the mixed system. This dynamics is characterised by a very broad distribution of correlation times. Within the distribution an exchange between fast and slow reorientating molecules is detected which takes place on the same time scale as the reorientation itself.

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

The influence of glass fibers content and testing conditions (frequency and mean load) on the fatigue crack propagation rate in injection molded neat and fiber reinforced polypropylene has been investigated. The resistance to fatigue crack propagation increases as the fiber volume fraction increases. The fatigue resistance of either neat and fiber reinforced polypropylene is tremendously dependent on the crack propagation direction, with higher values for crack propagating transversely to the melt flow direction in the molded plaques. A dramatic increase of the crack growth rate per cycle by decreasing frequency, at any given crack length was found for all the materials investigated. Analysis of data obtained at various frequencies and applied mean loads suggests that crack propagation is determined mostly by viscoelastic creep processes at the crack tip, the role of pure fatigue appearing quite secondary.

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

Microwave irradiation of nadic-end-capped polyimide resin (RP-46) and glass-graphite-RP-46 composites: cure and process studies

Microwave energy was investigated to cure nadic-end-capped polyimide precursors (RP-46 resin) using a Cober Electronics Model LBM 1.2A/7703 microwave oven at a frequency of 2.45 GHz. Both neat resin samples and glass cloth and hybrid glass cloth–graphite cloth–RP-46 resin composites were studied. For the resin studies, the effect of various parameters, such as power level, sample size, processing temperature, time, and graphite fiber absorber, were investigated. The variables investigated with the composite study were the power level, mold material, vacuum, and low pressure. The results showed that microwave energy was effective in curing both neat resin samples and composite specimens. The presence of a small quantity of absorber (chopped carbon fiber) accelerates the cure dramatically. Moreover, soapstone mold material was found to be an efficient absorber for glass and glass–graphite composite processing, causing an effective cure in less than 1 h. Glass and glass–graphite hybrid composites with flexural strengths of 372–588 MPa (54–85 ksi) and moduli of 28.7–31.7 GPa (4.2–4.6 Msi) have been fabricated. This is equivalent to 50 to 80% of the properties of composites fabricated by conventional means. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2391–2411, 1999

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

AbstractThis paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP‐blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF‐PP with different amounts of maleic anhydride grafted PP (PP‐g‐MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF‐blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF‐PPs with varying PP‐g‐MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.

Dynamical disorder in glasses studied by 2H and 1H NMR

AbstractWe review 2H and also some 1H nuclear magnetic resonance (NMR) results on mobile guest molecules dissolved in organic high and low molecular weight glasses. The guest molecules such as benzene exhibit high symmetry and perform a rotational jump process between equivalent orientations in the solid state. Characteristic two‐phase NMR spectra and a pronounced non‐exponential 2H spin‐lattice relaxation are observed. The 1H spin‐lattice relaxation is exponential, and shows a non‐Arrhenius temperature dependence. All these features can be well described over a large temperature range (10–200K) by assuming a broad distribution of activation energies and entropies. The distribution of activation energies displays significant differences for the various guest/host systems. We think the approach chosen here can also be applied to describe β‐processes or internal processes in neat glasses.

Effect of heat treatment on low cycle fatigue properties of zircaloy-4: Xiao, L. and Gu, H.C. Xiyou Jinshu Cailiao yu Gongcheng (Rare Metal Materials and Engineering) (Oct. 1993) 22(5), 61–66 (in Chinese)

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.

Effects of processing induced microstructure on the fatigue crack propagation of unfilled and short fibre-reinforced PA-6

The microstructure-related fatigue performance of injection-moulded neat and short glass fibre-reinforced (20 and 30 wt%) polyamide ( pa-6) was investigated. Change in the microstructure both of the neat and reinforced pa-6 was achieved by alterations in processing variables, ie, mould temperature ( T M) and injection speed ( v inj), and studied by light and scanning electron microscopy ( sem). The microstructure of the composites was characterized by a reinforcing effectiveness parameter, R C. Fatigue crack propagation ( fcp) curves indicated the validity of the Paris power law. The fcp response both of the neat and reinforced PA-6 was discussed by the microstructural efficiency ( M) approach. An attempt was made to define a reinforcing effectiveness parameter for the matrix, R M, comprising the morphology and thus correlate the fcp response with the microstructure.

Crosslinking of an epoxy with a mixed amine as a function of stoichiometry. II. Final properties via dynamic mechanical spectroscopy

AbstractVariation of the epoxy/curing agent ratio for a system containing a diglycidyl ether of bisphenol‐A (DGEBA) and a mixed aromatic amine was found to have a significant effect on the cure kinetics and final dynamic mechanical properties of both the neat resin and glass reinforced epoxy. For a partial B‐stage cure to an ultimate C‐stage cure, optimum cure temperatures were obtained as a function of stoichiometry. For the ultimately cured (C‐stage) resin, the molecular weight between crosslinks (Mc) passed through a minimum, and both the glass transition temperature (Tg) and a secondary (sub‐Tg) transition passed through a maximum at the stoichiometric ratio. In addition, the activation energy of the ultimately cured resin passed through a maximum at or near the stoichiometric ratio for both the Tg and sub‐Tg transitions.

Effect of styrene on properties of vinyl ester resins, I

AbstractThe paper describes the effect of styrene on the properties of bis(methacryloxy) derivatives of epoxy resins (vinyl ester resins). The styrene concentration in the resin was systematically varied between 20–60 wt.‐%. The curing characteristics of vinyl ester resins changed only marginally on dilution with styrene. Dilution with styrene resulted in a decrease in tensile modulus and increase in elongation of neat resin castings and glass fibre reinforced laminates. The glass transition temperature of laminates was determined by dynamic mechanical analysis and was found to decrease with increasing styrene content.

Electron trapping in alcohol clusters in γ-irradiated n-propanol–hydrocarbon glasses at 77 K

A study has been made of trapped electrons in n-propanol–hydrocarbon mixture glasses at 77 K by the optical absorption method. The yields of electrons trapped in alcohol clusters (ealc−) were measured for several γ-irradiated n-propanol–hydrocarbon glasses, where the yields of trapped electrons in these neat hydrocarbon glasses at 77 K are known to range from 0.015 to 1.1 G units. The results show that the yields of ealc– in the mixture glasses do not depend on the trapped electron yields in neat hydrocarbon glasses. Thus, it is concluded that electron migration from relaxed hydrocarbon matrix traps into alcohol clusters is not the major process for ealc− formation in the systems studied here. The major process for ealc− formation may be attributed to quasi-free electron scavenging by alcohol clusters and electron migration from unrelaxed hydrocarbon traps into alcohol clusters. Both ir-induced and isothermal changes in the ealc− yield have also been investigated. These investigations indicate that there is a correlation between the observed change in the ealc− yield and the matrix viscosity.

Neat and mixed crystal and glass optical spectra of the anthracene-trinitrobenzene complex

Temperature dependent optical and Raman spectra of the anthracene-sym-trinitrobenzene complex in neat and mixed crystals as well as glasses are reported. The charge-transfer absorption and fluorescence spectra are similar in all media and characterized by large Stokes shifts (2000–4000 cm −1) and exceedingly broad bands even at 4.2 K. These results are discussed in terms of exciton-phonon and electron-phonon coupling theories. Optical and Raman data are presented which indicate that there is probably a continuous phase transition occurring in the anthracene-trinitrobenzene crystals which onsets at T = 150 K.