Characterization of aged nitrile rubber elastomers by NMR spectroscopy and microimaging

¹H spin-lattice and spin-spin NMR relaxation times change markedly during the aging of nitrile rubber elastomers. NMR microimaging reveals heterogeneous changes in the aged samples. Aging proceeds mainly via additional chain cross-linking, and scission of the polymer chains does not appear to contribute. T₁ imaging shows that there is no spatial distribution of spin-lattice relaxation times.     

Phase-mixing and molecular dynamics in poly(urethane urea) elastomers by solid-state NMR

The dynamical heterogeneity in a series of 4,4′-dicyclohexylmethane diisocyanate-diethyltoluenediamine-poly(tetramethylene oxide) based poly(urethane urea) (PUU) elastomers was studied by solid-state nuclear magnetic resonance (NMR) methods. Extensive phase mixing was evidenced by the ¹H wideline signal, which can be approximately fitted by a single exponential model. ¹³C T₁ relaxation time measurements indicate that the hard segments (HS) exhibit some small-amplitude mobility, likely “activated” by neighboring soft segments (SS). Fitting of the time-domain wideline separation (TD-WISE) data was employed to characterize the extent of phase mixing, which revealed that a PUU elastomer contains four fractions: rigid-HS, mobile-HS, rigid-SS, and mobile-SS regions. For a variety of SS MWs, the dynamics and relative portions of rigid vs. mobile fractions among HS were substantially similar, while those for the SS exhibit large contrast. Furthermore, the dynamics in the rigid-SS fraction is at least an order of magnitude slower than that in mobile-SS for all PUUs. Greater phase-mixing substantially lowers the SS mobility, facilitating SS to undergo glass transition at high strain rates, thus can be key to enhancing dynamic mechanical strengthening.     

Preparation and performance of high‐impact polystyrene (HIPS)/nano‐TiO2 nanocomposites

High‐impact polystyrene (HIPS)/nano‐TiO₂ nanocomposites were prepared by surface pretreatment of nano‐TiO₂ with special structure dispersing agent (TAS) and master batch manufacturing technology. The results show that when the nano‐TiO₂ content is 2%, the notched impact strength, tensile strength, and elastic modulus of HIPS/nano‐TiO₂ nanocomposites increased to a maximum. This result indicates that nano‐TiO₂ has both toughening and reinforcing effects on HIPS. The heat‐deflection temperature and flame‐retardance of HIPS/nano‐TiO₂ nanocomposites are also obviously improved as the nano‐TiO₂ content is increased. The nanocomposites manufactured by the two‐step method have better mechanical properties than that made by a one‐step method. HIPS/nano‐TiO₂ nanocomposites are also non‐Newtonian and pseudoplastic fluids. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 381–385, 2003     

Micromechanics of uniaxial tensile deformation and failure in high impact polystyrene (HIPS)

While it is recognized that the heterogeneous particles in HIPS play the dual role of providing multiple sites for craze initiation in the polystyrene (PS) matrix and allow the stabilization of the crazing process through cavitation/fibrillation in the PB phase within the particle, the precise role of particle morphology is not well understood or quantified. This work probes the micromechanics of uniaxial tensile deformation and failure in rubber-toughened PS through axi-symmetric finite element representative volume element (RVE) models that can guide the development of blends of optimal toughness. The RVE models reveal the effect on craze morphology and toughness by various factors such as particle compliance, particle morphology, particle fibrillation and particle volume fraction. The principal result of our study is that fibrillation/cavitation of PB domains within the heterogeneous particle provides the basic key ingredient to account for the micro- and macro-mechanics of HIPS.     

Phase morphologies and mechanical properties of high‐impact polystyrene (HIPS) and polycarbonate blends compatibilized with polystyrene and polyarylate block copolymer

Phasemorphology and mechanical properties of blends of high‐impact polystyrene (HIPS) and polycarbonate (PC) blends compatibilized with a polystyrene (PS) and polyarylate (PAr) (PS–PAr) block copolymer were investigated. Over a broad range of composition from 50/50 through 30/70, HIPS/PC blends formed cocontinuous structures induced by the flow during the extrusion or injection‐molding processes. These cocontinuous phases had heterogeneity between the parallel and perpendicular directions to the flow. The micromorphology in the parallel direction to the flow consisted of stringlike phases, which were highly elongated along the flow. Their longitudinal size was long enough to be longer than 180 μm, while their lateral size was shorter than 5 μm, whereas that in the perpendicular direction to the flow showed a cocontinuous phase with regular spacing due to interconnection or blanching among the stringlike phases. The PS–PAr block copolymer was found to successfully compatibilize the HIPS/PC blends. The lateral size of the stringlike phases could be controlled both by the amount of the PS–PAr block copolymer added and by the shear rate during the extrusion or injection‐molding process without changing their longitudinal size. The HIPS/PC blend compatibilized with 3 wt % of the PS–PAr block copolymer under an average shear rate of 675 s^?1 showed a stringlike phase whose lateral size was reduced almost equal to the rubber particle size in HIPS. The tensile modulus and yield stress of the HIPS/PC blends could be explained by the addition rule of each component, while the elongation at break was almost equal to that of PC. These mechanical properties of the HIPS/PC blends can be explained by a parallel connection model independent of the HIPS and PC phases. On the other hand, the toughness factor of the HIPS/PC blends strongly depended on the lateral size of the stringlike phases and the rubber particle size in the HIPS. It was found that the size of the string phases and the rubber particle should be smaller than 1.0 μm to attain a reasonable energy absorbency by blending HIPS and PC. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2347–2360, 2001     

Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR)

Nuclear magnetic resonance (NMR) has been widely used in petrophysical characterization of sandstones and carbonates, but little attention has been paid in the use of this technique to study petrophysical properties of coals, which is essential for evaluating coalbed methane reservoir. In this study, two sets of NMR experiments were designed to study the pore types, pore structures, porosity and permeability of coals. Results show that NMR transverse relaxation (T₂) distributions strongly relate to the coal pore structure and coal rank. Three T₂ spectrum peaks identified by the relaxation time at 0.5-2.5 ms, 20-50 ms and >100 ms correspond to pores of <0.1 μm, >0.1 μm and cleats, respectively, which is consistent with results from computed tomography scan and mercury intrusion porosimetry. Based on calculated producible and irreducible porosities through a T₂ cutoff time method, we propose a new NMR-based permeability model that better estimates the permeability of coals. In combination with mercury intrusion porosimetry, we also propose a NMR-based pore structure model that efficiently estimates the pore size distribution of coals. The new experiments and modeling prove the applicability of NMR in petrophysical characterization of intact coal samples, which has potential applications for NMR well logging in coalbed methane exploration.     

Electrical properties of nanostructured interpenetrating polymer networks based on natural rubber (NR)/polystyrene (PS)

The electrical properties of nanostructured sequential interpenetrating polymer networks from natural rubber (NR) and polystyrene (PS) have been studied in the frequency range of 10²–10⁷ Hz. The permittivity, volume resistivity, dielectric loss factor, and dissipation factor were analyzed as a function of frequency, blend composition, crosslinking level, and initiating system. It was found that the volume resistivity and dissipation factor first increase, reach a maximum around 10³–10⁵ Hz, and then decrease gradually with the increase of frequency. As the NR content is increased from 30 to 70%, the dissipation factor (tan δ) increases. On increasing the extent of the PS phase crosslinking, the permittivity increases. However, at higher levels of PS crosslinking, the permittivity values decrease due to the agglomeration of PS phase arising from excessive crosslinking. The morphology studies using a scanning electron microscope confirmed the agglomeration of PS phase at high crosslinking level. The permittivity values are maximum at 4% of crosslinking content. The influence of initiating system on the dielectric properties was not very significant. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2017–2026, 2005     

NMR characterization of absorbed water in equilibrium swollen hydrogel P(AM‐NaA)

The behavior of absorbed water in equilibrium‐swollen poly(acrylamide‐co‐sodium acrylate) [P(Am‐NaA)] hydrogel is studied with ¹H nuclear magnetic resonance (¹H‐NMR). The observed non‐exponential decay of the spin–spin relaxation data manifests at least two distinguishable environmental states of the absorbed water in all samples. The component, characterized by the relatively shorter T₂, is associated with the more tightly bound water; whereas the other, characterized by longer T₂, might be a combination of near normal and loosely bound water. Attention is directed to the way in which relaxation times and the corresponding fraction of each type of water behave as a function of the crosslink density at 25°C. There is evidence of a sudden change of the crosslink state in P(Am‐NaA) hydrogel at crosslink density of about 1.0%. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1203–1207, 1999     

Chemometric solid-state C NMR analysis of morphology and dynamics in irradiated crosslinked polyolefin cable insulation

The relative concentrations and carbon spin-lattice relaxation constants (T_1,C) of the amorphous, intermediate, and crystalline phases of unaged crosslinked polyolefin cable insulation (ultimate elongation, e=310%), ⁶⁰Co γ-irradiated (e=22%), and irradiated+annealed (e=220%) samples were determined by chemometric analyses of directly polarized solid-state ¹³C NMR spectra. The T_1,C relaxation curves of the intermediate and amorphous components were found to be mono-exponential. The intermediate component contains 23±5% of the CH₂ segments in the unaged sample and has an T_1,C relaxation constant of 1.4±0.3 s. γ-Irradiation caused a slight decrease in the amount of intermediate component to 19±5% and an increase of the relaxation constant to 1.8±0.3 s. The subsequent annealing of the irradiated sample resulted in an additional increase of the relaxation constant to 2.1 s and a slight loss of crystallinity. The amorphous T_1,C relaxation constants were found to be identical in all three samples and have a value of 0.38±0.03 s. At ambient temperature, the crystalline phase was found to relax via chain diffusion from the intermediate component. The rate of helical jumps was twice as fast in the irradiated and irradiated+annealed samples compared with the unaged material.     

Behaviour of the spin-spin relaxation time for natural rubber vulcanizates under large deformation

The spin-spin relaxation time, T₂, for DCP-cured natural rubber with various crosslink densities, v_e, has been measured under various deformation. T₂ is separated into two components: one is the long T₂ component, T_2L, for the mobility of amorphous network chains, the other is the short one, T_2S, for that of the strain-induced crystalline chains. T_2L decreased exponentially with increasing extension ratio,α, and the decreasing rate was more remarkable with increasing v_e. The α and v_e dependence of T_2L has been quantitatively explained by the equation experimentally derived by Nishi et al.T_2L under various extension increased and became almost constant with increasing temperature, while the corrected fraction of T_2S, T_2S (%), gradually decreased. The apparent melting point, T_m, at which the corrected T_2S (%) was zero under various deformation was determined. The α dependence of T_m, has been discussed by using Flory's equation.     

Mixing effects on the mid-kilohertz mobility of polystyrene in glassy polystyrene-diluent blends

Low molecular weight additive effects on the mid-kilohertz mobility of atactic polystyrene (PS) at 25°C via n.m.r. spin-lattice relaxation experiments in the rotating frame (T_1?) are analogous to the results of a previous study of diluents in glassy bisphenol-A polycarbonate. The diluent with a relatively low glass transition temperature (T_g), dioctylphthalate, increases the spectral density of thermal motions of the chain backbone and the pendant group in PS on the order of 38.5 kHz. Of the ¹³C nuclei in the glassy polymer, the relaxation behaviour of which can be differentiated by high-resolution, solid-state n.m.r., the T_1?'s of the aromatic carbons in the side group are affected most by dioctylphthalate. In contrast, the styrene oligomer, which has a higher T_g than that of dioctylphthalate, does not significantly alter the ambient temperature mobility of polystyrene at 38.5 kHz. The PS-styrene oligomer and PS-dioctylphthalate blends are examples of athermal and non-athermal mixtures, respectively. However, the effect of the enthalpy of mixing on the T_1?'s of the polymer is probably obscured by differences in blend mobility due to different blend T_g's.     

Delayed action heat seal adhesives. I: Dielectric relaxation studies of mixtures of dicyclohexyl phthalate with poly(vinyl acetate) and polystyrene

The design of delayed action heat seal adhesives depends on the physical properties of the polymer, plasticiser and their resultant mixtures. This paper explores the relationships between various molecular interactions and the performance of the adhesives. Dielectric relaxation measurements of mixtures of poly(vinyl acetate) or polystyrene and a plasticiser, dicyclohexyl phthalate, were performed to characterise the molecular dynamics of the system. Binary mixtures over the entire composition range were examined from 244 to 408 K and over a frequency range from 10^?1 to 6 × 10⁴ Hz, and allowed the nature of the interaction between plasticiser and polymer to be quantified. Dielectric studies are compared with measurements of the glass transition temperature obtained using thermal and mechanical analysis, and indicate that over certain composition ranges segregation of the components occurs at a molecular level. These observations are discussed in relation to the design of a delayed action heat seal formulation.     

Characterization of Structure and Transport in Porous Media by Pulsed Field Gradient (PFG) NMR Technique. Part I: Master Curve and Characteristic Inner Length

Systematic experimental studies focusing on the practical application of observation time dependent pulsed‐field‐gradient (PFG) NMR were performed. The objective was to provide engineering scientists with a reliable experimental tool for characterizing the structure and transport in fluid‐filled porous media. Observation time dependent self‐diffusion in glass bead packs as model systems was investigated, where the diffusing species (molecules of the solvent or dissolved particles) served as probes for the confining geometry in the porous medium. First, the basic question whether the obtainable structure information is independent of the actual mobility of the diffusing probe particles was examined experimentally. It could be demonstrated that plotting the normalized time‐dependent diffusion coefficient D(t)/D₀ versus the actual migration length l_D(t) during a given observation time t indeed yields a characteristic “master curve” that is independent of the mobility of the diffusing species, thus reflecting, as desired for a reliable method, solely the effects of the confining geometry of the porous system of interest. It was further shown that from the master curve a new quantity, i.e., a “characteristic inner length” or “correlation length” ξ_D can be derived that corresponds to a path length in the porous medium, after which particles in the pore fluid experience an averaged restricting geometry and diffuse with an effective diffusion coefficient D_eff. It turned out that ξ_D is surprisingly short, that is, in the order of the bead radius. Moreover, it was demonstrated that normalization of this migration length with the bead radius yields a common master curve for all monodisperse bead packs used and thus, it is obviously possible to derive and record master curves for different kinds of packs, beds or other porous media as references that can be used to characterize or certify the kind of the porous matrix of interest.     

糠醇树脂碳化过程的核磁共振研究

引用质子的连续波和核磁共振研究糠醇树脂在６０～８６０℃温度范围的碳化过程。并按热处理时发生的结构变化和化学变化解释该结果。加热到２４０℃时分子量和交联度增大;３２０℃时产品充分交联,形成坚硬的结构;３８０℃以上放出氢气并产生顺磁中心,实验证明温度是生产的关键。     

An investigation of 1H and 13C longitudinal relaxation and relaxation in the rotating frame in solid poly(N‐vinylcarbazole)

¹H and ¹³C longitudinal relaxation times (T₁) and relaxation times in the rotating frame (T_1ρ) have been measured for poly(N‐vinylcarbazole) in the solid state in air and nitrogen atmospheres in an attempt to elucidate molecular motions. In air, the T₁ relaxation of both ¹H and ¹³C was dominated by interaction with absorbed paramagnetic oxygen. In nitrogen, the ¹³C T₁ relaxation times were long (>300 s) and were averaged by ¹³C–¹³C spin diffusion. The ¹³C T_1ρ relaxation times showed an exponential dependence on the strength of the rotating ¹³C magnetic field and were thus controlled by spin–spin processes rather than spin–lattice processes. © 2001 Society of Chemical Industry     

Effect of the crystallization conditions on the phase behavior of syndiotactic polystyrene/poly(phenylene oxide) blends

Syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, miscible in the melt state, were crystallized from the melt and the quenched state at different temperatures. The effect of the crystallization temperature on the phase behavior of the blends and the polymorphic changes in sPS was investigated by dynamic mechanical analysis (DMA), wide‐angle X‐ray diffraction (WAXD), and density measurements. In most blends, the crystallization of sPS induced segregation into two homogeneous amorphous phases of different compositions. The temperatures of the DMA relaxations of the neat homopolymers and crystallized blends were fit by the Gordon–Taylor relation to calculate the compositions of these phases. In melt‐crystallized blends, with slower crystallization, the major amorphous phase became sPS‐rich, whereas the minor phase became PPO‐rich. These major and minor amorphous phases could be tentatively assigned to interfibrillar and interlamellar regions, respectively. In cold‐crystallized blends, slower crystallization decreased the sPS concentration in both phases, and the scale of segregation was much smaller. WAXD studies and density measurements indicated a complex polymorphic behavior of sPS after it was blended with PPO. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1975–1983, 2003     

Combination of Time‐Dependent Self‐Diffusion Measurements (Dynamic Imaging) with Magnetic Resonance Imaging

The new NMR method, demonstrated in the present work, allows within local small volumes the determination of integral structural parameters like the surface‐to‐pore volume ratio or the tortuosity of a fluid‐filled porous medium. A combination of conventional imaging (MRI) with measurements of observation‐time dependent self‐diffusion (dynamic imaging) was used.     

Dielectric relaxation behaviour of poly(cyclobutyl methacrylate)s

Dielectric behaviour of two poly(methacrylate)s with cyclobutyl groups in the side‐chains has been studied. This investigation was performed by determining the dielectric permittivity and loss in terms of frequency and temperature for poly(cyclobutyl methacrylate) (PCBuM) and poly(cyclobutylmethyl methacrylate) (PCBuMM). Dynamic dielectric measurements show sub‐glass relaxations, probably due to motions in the lateral chain, including the cyclobutyl ring. The effect of the insertion of a flexible spacer group into the side‐chain is also analyzed. The α relaxations were analyzed in terms of the Havriliak–Negami equation and the free volume theory tested according to the Vogel–Fulcher–Tamman–Hesse equation. The conductive and interfacial phenomena were studied by hopping and MacDonald–Coelho models. © 2002 Society of Chemical Industry     

Application of Magnetic Resonance Imaging (MRI) to Determine the Influence of Fluid Dynamics on Desulfurization in Bench Scale Reactors

The influence of fluid dynamics on the hydrodesulfurization (HDS) reactions of a diesel oil in bench‐scale reactors was evaluated. The porosities and liquid saturations of catalyst beds were quantified by using the MRI technique. The gas‐liquid systems used in the experiments were nitrogen diesel and hydrogen diesel. An apparatus was especially constructed, allowing in situ measurements of gas and liquid distributions in packed beds at elevated pressure and temperature up to 20 bar and 200 °C, respectively. The reactor itself had a length of 500 mm and an internal diameter of 19 mm. The packed beds used in this MRI study consisted of: (1) 2 mm diameter nonporous spherical glass beads and (2) 1.3 mm diameter porous Al₂O₃ trilobes having the same size as the original trilobe catalyst used in HDS bench‐scale experiments. The superficial gas and liquid velocities were set within the range of trickle flow, e.g., u_0G = 20–500 mm/s and u_0L = 0.1–6 mm/s. In parallel with the MRI experiments, the hydrodesulfurization of a gas oil was investigated in a bench‐scale plant. Its reactor had the same dimensions of the trickle‐bed column used in the MRI experiments and was filled with original trilobe catalyst. These catalytic experiments were carried out at a wide range of operating conditions (p = 30–80 bar, T = 300–380 °C, LHSV = 1–4 h^–1). The results of both fluid dynamic and catalytic reaction experiments were then combined for developing a simulation model to predict the HDS performance by accounting for fluid dynamic nonidealities.     

Enthalpy relaxation in poly(ethylene terephthalate) and related polyesters

Enthalpic relaxation data are presented on poly(ethylene terephthalate), poly(ethylene naphthalate) and their copolymers. Analysis of these data allows the determination of the amount of energy absorbed at the glass transition, Q_t, and the location of the enthalpic recovery peak, T_max, as a function of the time of ageing of the samples. Ageing measurements were carried out for periods of up to 2016 h and at temperatures between 40 °C and 110 °C, depending upon the chemical composition of the system being investigated. The enthalpic relaxation rates are influenced by the chemical structure and reflect the effects of local order pinning the chains and influencing the rate of enthalpic recovery. © 2000 Society of Chemical Industry