Brillouin scattering and hypersonic relaxation in amorphous polymers

Mechanical relaxation at hypersonic frequencies is measured using Brillouin spectroscopy for polyisobutylene, atactic polypropylene, polydimethyl siloxane, and polyvinyl acetate. The temperatures of maximum loss determined in the gigahertz range are compared to the published transitions maps for the above polymers. It is found that the hypersonic relaxation data fall on an extrapolation of the secondary main chain glass–rubber relaxation line above the region where the primary and secondary lines merge.     

High-Frequency Dynamical Behavior of Poly(ethylene glycol)+H2O Mixtures by Brillouin Spectroscopy

Brillouin spectroscopy has been used to examine high-frequency dynamical behavior of aqueous solutions of poly(ethylene glycol) (Mw ≈ 400g/mol) at 298K in the entire concentration region. It was found that a relaxation process takes place in the experimental frequency window that significantly affects the shape of experimentally recorded spectrum of the density fluctuations (dynamical structure factor). The process detected was attributed to segmental motion of the flexible polymeric chain. The full spectrum analysis of Brillouin spectra has been performed taking advantage of the relaxation function previously used in describing a single relaxation process in dielectric examination of water solutions of PEG 400. The proposed data processing procedure permits a qualitative reproduction of concentration dependencies of the hypersonic wave velocity and absorption measured. The shapes of the concentration dependencies of the relaxation times obtained from the Brillouin and the dielectric spectroscopies are in good agreement over a very broad concentration range, although their absolute values are scaled by the factor of 3. This result indicate that the two processes revealed independently by dielectric and Brillouin spectroscopies, apparently separated in time-scale, are just the same relaxation process.     

Effects of petroleum resins on asphaltene aggregation and water-in-oil emulsion formation

Asphaltenes from four crude oils were fractionated by precipitation in mixtures of heptane and toluene. Solubility profiles generated in the presence of resins (1:1 mass ratio) indicated the onset of asphaltene precipitation occurred at lower toluene volume fractions (0.1–0.2) than without resins. Small-angle neutron scattering (SANS) was performed on solutions of asphaltene fractions in mixtures of heptane and toluene with added resins to determine aggregate sizes. Water-in-oil emulsions of asphaltene–resin solutions were prepared and separated by a centrifuge method to determine the vol.% water resolved. In general, the addition of resins to asphaltenes reduced the aggregate size by disrupting the π–π and polar bonding interactions between asphaltene monomers. Interaction of resins with asphaltenic aggregates rendered the aggregates less interfacially active and thus reduced emulsion stability. The smallest aggregate sizes observed and the weakest emulsion stability at high resin to asphaltene (R/A) ratios presumably corresponded to asphaltenic monomers or small oligomers strongly interacting with resin molecules. It was often observed that, in the absence of resins, the more polar or higher molecular weight asphaltenes were insoluble in solutions of heptane and toluene. The addition of resins dissolved these insolubles and aggregate size by SANS increased until the solubility limit was reached. This corresponded approximately to the point of maximum emulsion stability. Asphaltene chemistry plays a vital role in dictating emulsion stability. The most polar species typically required significantly higher resin concentrations to disrupt asphaltene interactions and completely destabilize emulsions. Aggregation and film formation are likely driven by polar heteroatom interactions, such as hydrogen bonding, which allow asphaltenes to absorb, consolidate, and form cohesive films at the oil–water interface.     

EPOXY RESINS TOUGHENED WITH CARBOXYL TERMINATED POLYETHERS

Carboxyl terminated polyethers, the adducts of hydroxyl terminated polytetrahydrofuran and maleic anhydride, were used as toughener for epoxy resins. The morphology of the toughened resins was investigated by means of turbidity measurement, dynamic mechanical testing and scanning electron microscope observation. It turned out that the molecular weight and the carboxyl content of the polyether and the cure conditions are important factors, which affect the particle size of the polyether-rich domains and, in turn, the mechanical properties of the cured resin. Carboxyl terminated polytetrahydrofurans have a low glass transition temperature, and in appropriate amount they do not affect the thermal resistance of the resin. These advantages make them preferable as toughener for epoxy resins.     

Simultaneous determination of elastic and optical properties of polymers by high performance Brillouin spectroscopy using different scattering geometries

General aspects of high performance Brillouin spectroscopy in polymers using special scattering geometries such as 90A-scattenng geometry are discussed. Technical improvements are reported resulting in absolute accuracies up to 0.05 % for sound velocity determination. A method of data analysis is presented delivering simultaneously the complete set of elastic stiffness constants. The influence of birefringence on the Brillouin line shifts in anisotropic polymeric systems is estimated and techniques to reduce this influence are proposed. The determination of the principal refractive indices by Brillouin spectroscopy is discussed. Furthermore, a quantityD ^X , which is sensitive to hypersonic relaxation processes, is introduced.Dedicated to Prof. Dr. H.-G. Kilian on the occasion of his 60th birthday.     

The β relaxation behavior of bisphenol-type resins

The β relaxation of a phenoxy resin and a number of related uncured and cured bisphenol-type epoxide resins was studied using a torsion pendulum. Two molecular mechanisms are proposed to contribute to the β relaxation in the range from ?55 to ?80°C. One is the motion of the hydroxyether group and the other is related to motions of the crosslink itself. The β-relaxation peak temperature was found to increase with the concentration of hydroxyether groups in the matrix. Motion of the crosslinks in the matrix gives rise to a contribution to the β peak at ?80°C. In addition, motion of the diphenyl propane group is proposed to result in a relaxation process at ca. ?110°C.     

Ultrasonic and hypersonic dispersion in polysiloxanes

We report ultrasonic attenuation and velocity measurements on poly(dimethylsiloxane) (PDMS), poly(phenylmethyl siloxane) (PPMS), and copolymer poly(dimethyl phenylmethyl siloxane) in the temperature range of 10–50°C and frequency 0.3–45 MHz. The present data complement previously reported Brillouin spectra at hypersonic frequencies. Whereas the ultrasonic velocity u₀ is virtually independent of frequency, the ultrasonic absorption exhibits strong dispersion which can be ascribed to the viscoelastic normal mode relaxation. The ultrasonic attenuation data for PPMS at low temperatures display an additional relaxation process related to localized segmental motion. This mode is also responsible for the relatively large dispersion of the sound velocity and attenuation in the gigahertz frequency range accessible to the Brillouin scattering experiment. The extended information, which can be extracted by studying hypersonic dispersion, is discussed in detail.     

Structural relaxation processes in polyethylene glycol/CCl4 solutions by Brillouin scattering

We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentration dependencies of the hypersound velocity and normalized absorption are compared against the indications from several models proposed in the literature. The concentration evolution of the system is described in terms of two distinct regimes. At high polymer content, the system is dominated by the structure of the dense polymer, where polymer-polymer interactions, together with excluded volume effects, induce the existence of a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the average value of the relaxation time following a simple Arrhenius temperature dependence. As the concentration decreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number of different local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiple relaxation. At low concentrations, the experimental data are well fitted assuming a Vogel-Fulker-Tammon behavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for the refractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarization effects, it can develop local hetero-associated structures via electrostatic interaction with the O-H end groups of the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior of the hypersonic velocity to a recent three-component model, suitable to describe the concentration dependence of sound velocity in moderately interacting fluids. The indication of the model furnishes a very high value for the association constant of the PEG600, confirming the literature indication that, in polymeric systems capable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitive to the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogen bonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive only to the density fluctuations of the local segmental motions.     

Studies of molecular relaxation of poly(propylene oxide) solutions by dielectric relaxation and brillouin scattering

Dielectric relaxation and Brillouin scattering are jointly used in studying molecular relaxation in poly(propylene oxide) (PPO) and its solutions in methylcyclohexane. The dielectric method was applied to the more concentrated (100%, 80%, 60%, by volume) solutions over a wide temperature and frequency range (30 Hz to 8 GHz) in order that the variation in activation energy characteristic of a glass-forming substance could be delineated. The present work extends previous work on the undiluted polymer to higher frequencies so that range of 12 decades in the dielectric loss maximum f_max as a function of temperature is now available. The “Antoine” equation is found to represent the behavior of log f_max, of the bulk concentrated solutions very well. The more dilute (40%, 20%) solutions were studied only in the high-frequency (GHz) region since phase separation occurred at low temperatures. Both the temperature and dilution effects were interpreted in terms of free-volume theory. Brillouin scattering spectra were obtained at several scattering angles and a wide range of temperatures. A maximum in the curve of hypersonic attenuation versus temperature was observed in each polymer solution. The attenuation maximum shifts toward lower temperature upon dilution, in agreement with the dielectric relaxation result. The Brillouin scattering follows different activation parameters and evidences a more rapid process than does the dielectric relaxation. It is speculated that it monitors a secondary or subglass relaxation, due perhaps, to damped torsional oscillations.     

Temperature dependence of vibrational relaxation in carbon tetrachloride

The temperature dependence of vibrational relaxation in carbon tetrachloride has been investigated from near the melting point to near the boiling point. The method of spontaneous Brillouin scattering has been used to determine the hypersonic frequencies and velocities as functions of temperature. The dispersion in the velocities are then compared with theoretical predictions in order to investigate the character of the relaxation.     

Preliminary thermal characterization of natural resins from different botanical sources and geological environments

The preliminary studies on thermal behavior of differently aged natural resins from Russia (Khatanga), Dominican Republic (El Valle), Colombia and Poland (Jantar) were performed. Thermal stability and behavior under elevated temperature were investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC), while the differences in the structure and composition by FT-IR spectroscopy. Analyzed resins show different thermal effects during heating suggesting that possible post-reactions and structural changes occurred. TG results indicated that Dominican, Russian and Colombian resins present relatively high thermal stability under air conditions in the range of 228–300 °C, whereas the mass loss of 5mass% at about 217 °C was observed for Baltic amber. During DSC experiments, the analyzed resins expose thermal events which make impossible determination of glass transition temperature in a raw sample. The results indicate that both TG and DSC cannot be considered as methods for age dating of natural resins and more advanced techniques should be applied. Careful analysis of FT-IR data in the carbonyl region may provide additional information about the composition and history of the natural resin.

     

Mechanical relaxation mechanism of epoxide resins cured with acid anhydrides. II. Effect of the chemical structure of the anhydrides on the β relaxation mechanism

The mechanism of low-temperature mechanical relaxation of acid-anhydride-cured epoxide resins has been investigated in detail. One mechanical relaxation, denoted as the β relaxation, is observed from ?80 to ?50°C for all epoxide resin systems cured with aromatic, alicyclic, and aliphatic anhydrides. The β relaxation increases in peak height and shifts to higher temperature with increasing molecular volume of the diester segments formed by the reaction of acid anhydrides. From these results, it is concluded that the β relaxation for anhydridecured systems is due to the motion of the diester segment, and that the intensity and peak position of the β relaxation depend on the molecular volume of this segment. Moreover, it was shown that the tensile impact strength of the anhydride-cured systems is governed by the intensity of the β relaxation of these systems when the parameters T_g and v of these systems are nearly constant.     

Thermal degradation of lignin–phenol–formaldehyde and phenol–formaldehyde resol resins

Resol resins are used in many industrial applications as adhesives and coatings, but few studies have examined their thermal degradation. In this work, the thermal stability and thermal degradation kinetics of phenol–formaldehyde (PF) and lignin–phenol–formaldehyde (LPF) resol resins were studied using thermogravimetric analysis (TG) in air and nitrogen atmospheres in order to understand the steps of degradation and to improve their stabilities in industrial applications. The thermal stability of samples was estimated by measuring the degradation temperature (T _d), which was calculated according to the maximum reaction rate criterion. In addition, the ash content was determined at 800 °C in order to compare the thermal stability of the resol resin samples. The results indicate that 30 wt% ammonium lignin sulfonate (lignin derivative) as filler in the formulation of LPF resin improves the thermal stability in comparison with PF commercial resin. The activation energies of degradation of two resol resins show a difference in dependence on mass loss, which allows these resins to be distinguished. In addition, the structural changes of both resins during thermal degradation were studied by Fourier transform infrared spectroscopy (FTIR), with the results indicating that PF resin collapses at 300 °C whereas the LPF resin collapses at 500 °C.     

Spectroscopic and photopolymerization studies of benzyl methacrylate/poly(benzyl methacrylate) two‐component system

The influence of the viscosity of a two‐component system on its molecular dynamics (on the basis of hypersonic wave velocity and attenuation coefficient) and photopolymerization kinetics was studied. The system investigated represented the solution of poly(benzyl methacrylate), PBzMA (MW = 70000) in its monomer, benzyl methacrylate (BzMA). The viscosity of the system was varied by adding various amounts of the polymer to the monomer (10–50 wt %). The molecular dynamics in the neat BzMA was studied by the proton Nuclear Magnetic Resonance (NMR) spin‐lattice relaxation time measurements and the wide‐line ¹H NMR spectroscopy in a wide range of temperature. Information on the local dynamics in liquid BzMA above its melting temperature was gained from the high‐resolution ¹H and ¹³C NMR spectra. The hypersonic wave velocity and the attenuation coefficient were investigated in the appropriate temperature range related to a viscoelastic relaxation process by the Brillouin light scattering method. The kinetic measurements have demonstrated that the photopolymerization rate rapidly increases and the monomer conversion decreases with increasing polymer‐to‐monomer ratio; this effect has been noted in the whole range of polymer concentration and reaction temperature studied. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1336–1348, 2010     

High‐performance aromatic thermosetting resins with hydroxyl and ethynyl groups

A novel hydroxyl‐ethynyl‐arene (HEA) resin was synthesized via Aldol condensation and Sonogashira reaction. The structure of the obtained resin was confirmed by the techniques of mass spectroscopy (MS), gel permeation chromatography (GPC), proton nuclear magnetic resonance spectroscopy (¹H‐NMR), Fourier transform infrared spectroscopy, (FT‐IR) and elemental analysis (EA). Differential scanning calorimetry (DSC) results showed an exotherm at the temperature range of 187°C–245°C, attributable to crosslinking reaction of the acetylene groups. After thermal cure, the obtained cured resin possessed excellent thermal stability. Thermal gravimetric analysis (TGA) in nitrogen showed the Td₅ (temperature of 5% weight loss) was about 400°C, and the char yield in nitrogen was about 78% at 900°C. The laminate composite of HEA resin was prepared and its mechanical and thermal properties were determined. The usefulness of the HEA resin as matrix for ablative composite was evaluated. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of nanoparticles on structural morphology, thermal and flammability properties of two epoxy resins with different functionalities

The effect of layered silicate nanoclays, nano-silica and double-walled carbon nanotubes (DWNTs) on the thermal stability and fire reaction properties of two aerospace grade epoxy resins (a high temperature curing tetra-functional and a low temperature curing bi-functional resin) has been investigated using thermal analysis, cone calorimetry, LOI and UL-94 techniques. The morphology of the polymer-clay nanocomposites, determined by X-ray diffraction and transmission electron microscopy indicated intercalated structures. The addition of nanoclays (5-wt%) to both resins had a thermal destabilisation effect in the low temperature regime (<400 °C), but led to higher char yield at higher temperatures. The inclusion of nano-silica at 30-wt% significantly improved the thermal stability of the resins while DWNTs had an adverse effect due to their poor dispersion in the matrix. The nanoclays and carbon nanotubes significantly increased the fire resistance of the tetra-functional epoxy resin while a minimal effect was observed for the bi-functional resin.     

Mechanical and dielectric relaxations of epoxide resins containing spiro-ring structure

Low-temperature relaxation behavior was investigated for a bisphenol-A and three spiroring type epoxide resins cured with aromatic and spiro-ring-type diamines. A new well-defined relaxation denoted here as the β′ relaxation was observed from 50 to 100°C for the spiro-ring-type resin systems in both mechanical and dielectric measurements. The peak height and the activation energy of the β′ relaxation were entirely independent of the degree of curing. It is concluded that the β′ relaxation is due to the motion of p-phenylene group adjacent to the spiro ring. The β′ relaxation was not observed in the bisphenol-A-type resin system.     

Evidence for the γ‐relaxation in the light scattering spectra of poly (n‐hexyl methacrylate) near the glass transition

The full range of relaxation processes present in optically pure poly‐(n‐hexyl methacrylate) (PHMA) was studied using Rayleigh–Brillouin and photon correlation spectroscopy (PCS). Brillouin shifts, linewidths, and Landau–Placzek ratios (LPR) were measured over the temperature range from ?11 to 21 °C. The Brillouin splitting and linewidth were consistent with previous studies of PHMA, but the LPR was much lower, indicating that the scattered light primarily comes from intrinsic density fluctuations. Relaxation functions of the same PHMA sample were measured using PCS over the temperature range 0.5–52.5 °C. The average relaxation times calculated from a Williams–Watts fit follow a VFT temperature dependence, with the stretching parameter β decreasing with decreasing temperature. The distribution of relaxation times reveals a merging of the α and β‐relaxations over this temperature range, and the temperature dependent width confirms that there are at least two processes with separate temperature dependences. Furthermore, there appears a process at short times in the correlation function window at low temperatures. This upturn at the fastest relaxation times is attributed to the γ‐relaxation present in higher order methacrylate polymers. The effect of the γ‐relaxation is discussed in terms of the dynamic behavior over 12 decades in time. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1504–1519, 2005     

Prediction of Heat Shield Performance in Terms of Epoxy Resin Structure

Abstract

High-strength, readily processable, char-forming, insulative materials are being sought for application in ablative heat shields for re-entry vehicles. Toward this end, the family of epoxy resins has been evaluated. The structure of epoxy resins and curing agents in terms of their functionality, aromaticity, and chemical nature is discussed in relation to its effect on ablative properties. The concept of controlled, constructive thermal degradation is extremely important in ablative epoxide compositions. Bridged Diels-Alder adducts based upon cyclic dienes and maleic anhydride perform well as epoxy resin curing agents in this respect. The mechanism of thermal ablative degradation of these systems is discussed in terms of in situ thermal control and char-forming reactions. The position of attachment of glycidyl groups, as well as the nature and position of other sub-stituents around the aromatic nucleus, has little effect in general upon the ablative properties of epoxy resins. Thermal and ablative data of both benzene and naphthalene derivatives are given. A new epoxy resin based upon 2-nitro resorcinol has been synthesized. This resin gives significant char increases, both quantitatively and qualitatively, over conventional epoxides. The unusual mechanism of polymerization and thermal degradation of this resin is discussed.     

Nanoparticle composition of a ferrofluid and its effects on the magnetic properties

Experiments were carried out on a water-based ferrofluid (gamma-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.