Local Dynamics of Bismaleimide Adhesives in an Aggressive Environment

Molecular aspects of chemical and physical changes in bismaleimide (BMI) adhesive joints caused by absorbed moisture were investigated. The focus was on the early (pre-damage) stage that precedes the formation of voids and microcracks. Local dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and the changes in the chemical state of the matter were monitored by Fourier transform infrared spectroscopy (FTIR). Absorbed water interacts with the BMI network and gives rise to a fast relaxation process (termed γ*), characterized by an increase in the dielectric relaxation strength, an Arrhenius temperature dependence of the average relaxation time, and an activation energy of 50 kJ/mol. The γ* dynamics are slower than the relaxation of bulk liquid water because of the interactions between the absorbed water and various sites on the network (the ether oxygen, the hydroxyl group, the carbonyl group, and the tertiary amine nitrogen). One particularly significant finding is that the average relaxation time for the γ* process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as a non-destructive inspection (NDI) tool for adhesive joints. FTIR spectra reveal the presence of non hydrogen-bonded water and hydrogen-bonded water, the latter bonded to one and/or two sites on the BMI network. A good agreement was found between the calculated ratio of non hydrogen-bonded to total absorbed water from DRS and FTIR data.     

Local Dynamics of Adhesives in Aggressive Environment in the Pre-Damage Stage

Molecular aspects of chemical and physical changes in adhesive joints caused by absorbed moisture were investigated. The focus was on the pre-damage stage that precedes the formation of voids and microcracks. A model and a commercial epoxy-amine formulation were studied. Local dynamics were monitored by broad-band dielectric relaxation spectroscopy (DRS). One portion of the absorbed water does not form hydrogen bonds with the network and gives rise to a fast relaxation process (termed γ) with activation energy of 28 kJ/mol. The local β dynamics are slowed down by the interactions between water and various sites on the network that include the ether oxygen, the hydroxyl group and the tertiary amine nitrogen. One particularly significant finding is that the average relaxation time for the β process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high-precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as nondestructive inspection (NDI) tool for adhesive joints.     

Local Dynamics of Adhesives in Aggressive Environment in the Pre-Damage Stage

Molecular aspects of chemical and physical changes in adhesive joints caused by absorbed moisture were investigated. The focus was on the pre-damage stage that precedes the formation of voids and microcracks. A model and a commercial epoxy-amine formulation were studied. Local dynamics were monitored by broad-band dielectric relaxation spectroscopy (DRS). One portion of the absorbed water does not form hydrogen bonds with the network and gives rise to a fast relaxation process (termed γ) with activation energy of 28 kJ/mol. The local β dynamics are slowed down by the interactions between water and various sites on the network that include the ether oxygen, the hydroxyl group and the tertiary amine nitrogen. One particularly significant finding is that the average relaxation time for the β process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high-precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as nondestructive inspection (NDI) tool for adhesive joints.     

Dielectric spectroscopic results and chemical accessibility of sulfite pulps

The relation between the chemical reactivity of a dissolving pulp and the molecular mobility of cellulose was investigated by an unconventional method for these problems: dielectric relaxation spectroscopy (DRS). Carefully dried pulps show better reactivity in comparison to a mill dried sample. A specific deformation of the local polymeric backbone dynamics (β‐relaxation) was observed in the former samples. The intensity of the dielectric β‐relaxation correlates with the acetylation velocity in an excellent manner. A similar correlation was found between the relaxation strength and the water retention capacity of pulps. All experimental results support the explanation that hydroxyl groups of those anhydroglucose units which contribute to the low frequency unstructured underground relaxation (here named δ‐relaxation) outside of the real β‐peak in the dielectric spectrum are preferably substituted. The results of the investigation indicate that dielectric spectroscopy can provide a new, additional, alternative tool to characterize the accessibility and chemical reactivity of cellulose.     

Solution Intercalation and Relaxation Properties of Maleated Polypropylene/Organoclay Nanocomposites

In this work we report the investigation of the dynamics of maleic anhydride grafted polypropylene (PPgMA) and PPgMA-based nanocomposites by means of dielectric relaxation spectroscopy (DRS) and dynamic mechanical thermal analysis (DMTA). After characterization of neat PPgMA, we study the effect of organically modified silicate filler on the dynamics of PPgMA with dynamic mechanical thermal analysis. Our results suggest that the β-relaxation process, corresponding to the glass transition of PPgMA, is affected by the clay loading. The glass-transition temperature of PPgMA increases in a composition with filler content of 30 wt.%, probably because of polymer-filler interactions, which reduces the polymer chain mobility.

In the nanocomposite materials a separate high temperature process due to Maxwell-Wagner-Sillars (MWS) polarization was observed with dielectric relaxation spectroscopy.     

Physical crosslinking effects in α,ω-dihydroxy terminated polybutadienes

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320-10?500 g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between T_g and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δ?_α(M_n), the fraction of associated hydroxy groups (f_bond) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity f_bond(M_n) in a modified Fox-Flory approach, the measured T_g(M_n) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.     

Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.     

Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.     

Dynamics of concentrated solutions of low molecular weight phenolics and poly(2-vinylpyridine): Role of intermolecular hydrogen bonding

The molecular dynamics of solutions of poly(2-vinylpyridine) (P2VPy) and a series of low molecular weight phenols containing from one to six hydroxyl groups were investigated using broadband dielectric spectroscopy (DRS). Dynamic mechanical analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, small-angle X-ray scattering and wide-angle X-ray diffraction were employed in a complementary role. Segmental relaxation times for the α processes of all solutions follow expectations from T_gs derived from DSC experiments. For three of the model mixtures at 30 and 50 mol% [i.e., those containing bis (4-hydroxyphenyl) methane, 2,6 dihydroxynaphthalene, and 2,2-methylenebis[6-(2-hydroxy-5-methylbenzyl)-p-cresol] significantly broadened dielectric α relaxation time distributions were observed, indicating dynamic heterogeneity. On the other hand, 4-ethyphenol-P2VPy solutions display dynamic homogeneity. P2VPy with 10 mol% 2,3,3,4,4,5-hexahydroxybenzophenone behaved differently than all mixtures investigated in this study: it displayed a T_g (and T_α) significantly higher than that of the neat components, a small SAXS scattering peak, and an additional dielectric relaxation that we propose originates from Maxwell-Wagner-Sillars interfacial polarization. We propose that this behavior is a result of a phase separation of different types of hydrogen-bonded complexes, one rich in P2VPy and the other involving the type of 2,3,3,4,4,5-hexahydroxybenzophenone hydrogen-bonded structures found in the neat state. Intermolecular hydrogen bonding in all of the P2VPy-phenol mixtures suppresses, in some cases completely, the local P2VPy β relaxation by decreasing the mobility of the pyridine side groups.     

Dielectric properties of nylon 6/clay nanocomposites from on-line process monitoring and off-line measurements

Nylon 6/clay nanocomposites were studied by dielectric relaxation spectroscopy (DRS) to correlate morphology and microstructure with relaxation behavior of the polymer matrix at the molecular level. Partially exfoliated clay microstructure was achieved by extruding nylon 6 with surfactant-treated montmorillonite clays. A new on-line dielectric slit die sensor was used to examine the melt state properties during extrusion compounding. Solid state properties were probed by off-line DRS over a temperature range from −50 to 180 °C in a frequency range from 10⁻³ to 10⁶ Hz. Using non-linear regression methods in conjunction with the temperature-frequency positions of relaxations observed in the dielectric loss data, the experimental data were fit with the Havriliak-Negami and Cole-Cole dielectric relaxation functions corrected for electrode polarization and DC conductivity. Characteristic frequency, relaxation strength, and DC conductivities were extracted from curves with overlapping relaxation modes. Two dielectric relaxations were observed in the composite melt: the α relaxation associated with molecular segmental motion, and a Maxwell-Wagner relaxation (MW) resulting from interfacial polarization at the resin/clay interface. Analysis of the solid-state data yielded a comprehensive master plot of dielectric relaxations attributed to segmental and local molecular dynamics and other relaxations resulting from water and Maxwell-Wagner interfacial polarization. The impact of clay fillers is seen in nearly all relaxation processes changing both characteristic frequency and strength of the relaxation.     

Dynamics of single-walled carbon nanotube (SWNT)/polyisoprene (PI) nanocomposites in electric and mechanical fields

Relaxation dynamics of single-walled carbon nanotube (SWNT)/polyisoprene (PI) nanocomposites were examined by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS) over a wide range of frequency and temperature. Both functionalized (SWNT-f) and pristine (SWNT-p) nanotubes were used and their effect on dynamics compared. Functionalized (PISF) nanocomposites were characterized by an increase in the time scale of the normal mode process as a consequence of the strong surface interactions between the polymer matrix and the nanotubes. The exact opposite is seen in pristine (PISP) nanocomposites where a decrease in the time scale of the normal mode relaxation is observed and attributed to weaker surface interactions and the effect of confinement on dynamics. The segmental process in PISF or PISP is not affected by the presence of nanotubes. The temperature dependence of the average relaxation time for normal and segmental modes is of the Vogel-Fulcher-Tammann (VFT) type. A good agreement is observed in the time scale of processes measured by DRS and DMS in PISF nanocomposites. In PISP nanocomposites, however, the time scales obtained from DRS and DMS measurements are not in consistently good agreement and an explanation is offered in terms of confinement.     

Determination of Water Diffusion Coefficients and Dynamics in Adhesive/Carbon Fiber-Reinforced Phenolic Resin Composite Joints

Energy-dispersive X-ray spectroscopy analysis (EDX) is an easy and exact method for determination of water diffusion coefficients and dynamics. Here we have calculated the water diffusion coefficients and dynamics in adhesive/carbon fiber-reinforced phenolic resin composite joints subjected to different surface treatments with both EDX and elemental analysis. The water diffusion coefficients and dynamics in the adhesive joints determined with EDX analysis are almost the same as those determined with elemental analysis. The durability of the adhesive joints with carbon fiber-reinforced phenolic resin composites subjected to silane coupling agent treatment is better than those subjected to sandpaper burnishing and chemical oxidation treatment.     

Determination of Water Diffusion Coefficients and Dynamics in Adhesive/Carbon Fiber–Reinforced Phenolic Resin Composite Joints

Energy-dispersive X-ray spectroscopy analysis (EDX) is an easy and exact method for determination of water diffusion coefficients and dynamics. Here we have calculated the water diffusion coefficients and dynamics in adhesive/carbon fiber–reinforced phenolic resin composite joints subjected to different surface treatments with both EDX and elemental analysis. The water diffusion coefficients and dynamics in the adhesive joints determined with EDX analysis are almost the same as those determined with elemental analysis. The durability of the adhesive joints with carbon fiber–reinforced phenolic resin composites subjected to silane coupling agent treatment is better than those subjected to sandpaper burnishing and chemical oxidation treatment.     

Dielectric and thermal behavior of liquid crystalline comb-like polybutadiene-diols with mesogenic groups in side chains

Ordered polybutadiene-diols (LCPBDs) with the comb-like architecture were prepared by radical reaction of a 5-(4-{[4-(octyloxy)phenyl]azo}phenoxy)pentane-1-thiol with double bonds of telechelic HO-terminated polybutadiene (PBD); several polymers with various initial molar ratios of thiol to double bonds of PBD, R₀, in the range from 0 to 1 were prepared. DSC, polarizing microscopy, WAXS and dielectric relaxation spectroscopy (DRS) were employed to investigate their thermal and dielectric behavior in relation to morphology. DRS of the LCPBDs have revealed both collective and individual dynamic motions of molecules. Secondary β- and segmental α-relaxation were observed in unmodified PBD. In the LCPBDs, two secondary γ- and β- and two high-temperature α- and δ-relaxations were observed and assigned to specific molecular motions; all relaxations were analyzed and discussed in terms of time scale (Arrhenius diagram), magnitude (relaxation strength) and shape of the response.     

Hygrothermal ageing of adhesive joints with nanoreinforced adhesives and different surface treatments of carbon fibre/epoxy substrates

The effect of water absorption on the strength of single lap adhesive joints subjected to accelerated hygrothermal ageing (55 °C, 95% relative humidity, 800 h) was analysed. Two different variables were studied: the surface treatment of the carbon fibre/epoxy laminates (peel ply, grit blasting and atmospheric pressure plasma) and the addition of carbon nanofillers (0.5 wt% nanofibres and 0.25 wt% nanotubes) to the epoxy adhesive. The joint strength and the failure mode of the joints were investigated. Furthermore, the amount of water absorbed by the adhesive was determined.Adhesive joints with peel ply-treated laminates exhibit an increase in their strength, which is attributed to a relaxation of stresses in the adhesive/laminate interface; with grit blasting, this property remains almost constant. Plasma treatment provides the worst ageing behaviour because this treatment results in a surface with a higher surface free energy, which is more susceptible to environmental attack. The nanoreinforcement of the adhesive has a beneficial effect: it decreases the amount of absorbed water.     

Glassy dynamics of hydrogen-bonded heteroditopic molecules

A self-complementary heteroditopic molecule composed of thymine and diamidopyridine end groups and a flexible aliphatic interconnecting chain has been synthesized. The glassy dynamics of this hydrogen-bonded supramolecule have been investigated by using dielectric and rheological measurements, in combination with infra-red spectroscopy and solid-state ¹³C NMR experiments. Decoupling of main dielectric relaxation from viscosity has been found in the vicinity of the glass transition and the temperature dependence of viscosity appears to be stronger than that of dielectric relaxation. The unusual dynamic decoupling phenomenon is ascribed to the chemical/dynamic heterogeneity and formation of hydrogen bonds in the supramolecules.     

Physical properties and dielectric behavior of the poly(urethane-urea) based on o-dianisidine and renewable cross-linkers

The novelty of the poly(urethane-urea) series consists in inclusion of o-dianisidine units in the main chain and cross-linking with renewable biomaterials, unused compounds so far in the synthesis of the poly(urethane-urea) (Tween 20, Span 20, Phloroglucinol). The effects of these components on the structure, surface, thermo-mechanical properties and dielectric behavior of the obtained poly(urethane-urea) were investigated by Fourier transform infrared (FTIR) spectroscopy, thermo-gravimetric analysis, static contact angles, broadband dielectric spectroscopy, and mechanical testing. The FTIR spectra showed that the urethane hydrogen bonds decreased with the increase of o-dianisidine content. Such that, at the increase of the o-dianisidine content, decreased the thermo-mechanical properties, and increased strongly the water contact angle from 83 to 108°. By dielectric relaxation spectroscopy was studied the molecular dynamics within the polymeric matrices with identical soft segments but different structure of the hard domains. These poly(urethane-urea) materials exhibit two secondary relaxations (β and γ) and a relaxation process α, corresponding to the segmental movements in the soft phase, which occurs around the temperature of −50°C independent of the measurement frequency. o-Dianisidine prevents the formation of all the urethane hydrogen bonds and so increases the chains mobility and dipoles polarization of polymer matrix, thus increasing the dielectric constants.     

Structure and properties of ethylene—methacrylic acid copolymers and their sodium salts: Dielectric and proton magnetic relaxation studies

Dielectric and proton magnetic relaxation data have been obtained for an ethylene-methacrylic acid copolymer (containing about 4 mole% methacrylic acid units) and its 53% ionised sodium salt. The degrees of crystallinity and percentage ionisation of the samples investigated were estimated by infra-red methods. The dielectric results were obtained principally in the frequency range 100 Hz to 10 kHz and at temperatures ranging from 80° to – 120°. A few results are also reported at frequencies down to 0·1 Hz and up to 100 MHz. For the acid copolymer, two dielectric loss regions are observed and these are correlated with the reported mechanical β′- and γa-processes respectively. The partly ionised copolymer exhibits three dielectric relaxation regions which correlate with the mechanical α-, β- and γa-relaxations respectively. In addition, a dielectric peak appears at about —40° in the presence of absorbed water, a result similar to that found in the polyamides. The proton magnetic relaxation results were obtained by pulse methods which yielded the spin-lattice relaxation times T₁ (at 30 MHz) and T_1p (at kilohertz frequencies) as a function of temperature from ?180° to 100°. Two components were generally observed for T_1p. For the acid copolymer the β′- and γ-processes have been observed from these results, as well as a lower-temperature (δ) process which has not been detected by the mechanical or dielectric methods. For the sodium salt the γ- and δ-processes are also found, in addition to a high-temperature process in the region of the merged α- and β-processes. The present data are consistent with previous assignments for the β′-, α-, β- and γ-processes. The ‘water’ relaxation appears to involve some rotation of water molecules, or of ionic segments to which water molecules are attached, in the proposed ionic domains. The δ-process is ascribed to the rotations of methyl groups present in the methacrylic acid units.     

Effect of side chain architecture on dielectric relaxation in polyhedral oligomeric silsesquioxane/polypropylene oxide nanocomposites

Segmental and normal mode dynamics in polyhedral oligomeric silsesquioxane (POSS)/poly(propylene oxide) (PPO) non-reactive and reactive nanocomposites were investigated using a broadband dielectric relaxation spectroscopy (DRS) over wide ranges of frequency and temperature. Three POSS reagents with varying side chain architecture were selected for the study: OctaGlycidyldimethylsilyl (OG), TrisGlycidylEthyl (TG) and MonoGlycidylEthyl (MG). Spectra of OG and TG show a segmental (α) process at lower frequency and a local (β) relaxation at higher frequency, while MG displays only a local relaxation. Neat PPO has both segmental and normal mode (α_N) process. In POSS/PPO non-reactive nanocomposites, the presence of OG and TG causes a decrease in the time scale of α_N and α relaxation, while MG has no impact on the dynamics of PPO. Chemical reactions in POSS/PPO reactive nanocomposites lead to the formation of nanonetworks. Prior to the onset of reaction, POSS nanoparticles promote the motions of PPO chains, decrease the time scale of relaxation and give rise to thermodielectrically simple spectra. During the reaction, however, the network formation leads to spectral broadening and a gradual increase in the time scale of both segmental (α) and normal mode (α_N) relaxation. A detailed account of the effects of structure, concentration and dispersion of POSS in the matrix, molecular weight of PPO, extent of reaction and temperature on the molecular origin, temperature dependence and spectral characteristics of relaxation processes in POSS/PPO nanocomposites is provided.     

Characterizing Molecular Structure of Water Adsorbed by Cellulose Nanofiber Film Using in situ Micro-FTIR Spectroscopy

In situ micro-Fourier transform infrared (FTIR) spectra of a typical cellulose nanofiber (CNF) film (i.e., CNF with cellulose I structure) were collected within the full relative humidity (RH) range from 0% to 90%. Red shifts of two peaks at 1036 and 1204 cm^?1 and the variation of different spectra indicated the chemical adsorption sites for adsorbed water. From component band analysis of the spectral range of 2900–3700 cm^?1, three component peaks at 3293, 3397, and 3543 cm^?1 were due to strongly, moderately, and weakly hydrogen-bonded water. The signatures of these three types of hydrogen-bonded water were then observed to rise with an increase in RH. Based on growth regularities of these three types of hydrogen-bonded water, the water adsorption process of CNF film was divided into three stages; most of the water absorbed in these three stages was demonstrated to be CNF???HOH???CNF, WATER????HOH????WATER, and five-molecule tetrahedral structure, respectively.