Secondary Relaxation Research Articles

Comparative study of Poly(n-butyl methacrylate) by thermo-stimulated currents and dynamic dielectric spectroscopy

The dielectric behaviour of Poly(n-butyl methacrylate) (PnBMA) has been studied by thermo-stimulated currents (TSC) and dynamic dielectric spectroscopy (DDS). The analysis of the TSC complex spectrum shows the existence of three dipolar relaxations around the glass transition. The first one, labelled β, near -25°, has been assigned to a secondary relaxation mode. The second one, α, at 30°, has been associated with the glass relaxation. The third one, above Tg at 90°, might be assigned to the dielectric manifestation of the liquid-liquid transition. The use of fractional polarizations allows us to determine the fine structure of these three modes. The relaxation times of the β and α modes are widely distributed and they follow an Arrhenius equation. We also report a compensation phenomenon for the α mode. As for the α' mode, the relaxation times are narrowly distributed and they obey a Vogel equation. By using a blocking electrode, the study by DDS exhibits three relaxational phenomena: the β and α modes which merge approximately at 104 Hz and lead to an (αβ) mode, and an additional α' phenomenon. By using the Havriliak-Negami parameters, we characterize the shift of their maxima with frequency and temperature. The consistent set of data obtained from TSC and DDS allows us to follow the relaxation modes of PnBMA in a very broad temperature and frequency range. They significantly improve the knowledge of molecular mobility/dynamic structure relationships.

Fracture and yielding behaviors of polystyrene/ethylene‐propylene rubber blends: Effects of interfacial agents

AbstractThe aim of this work is to investigate the effects of two triblock copolymers, used as coupling agents, on fracture and yielding behaviors of a blend of 80 volume % of polystyrene (PS) and 20 volume %of ethylene‐propylene rubber (EPR), over a large range of loading rates and temperatures. For this purpose, blends containing different concentrations of two triiblock copolymers were studied at various test conditions. The focus was put on the time‐temperature dependence of fracture performance of the blends. Addition of triblock copolymer makes the PS/EPR blend more ductile. The time‐temperature dependence of the brittle‐ductile transition in fracture performance of the blend is controlled by an energy activation process. The interfacial agent lowers the temperature at brittle‐ductile transition and reduces the energy barrier controlling the fracture process. This effect, however, is much more pronounced for the lower molecular weight interfacial agent. The correlation between temperature, loading rate and yield stress of the blends seems to be controlled by a molecular relaxation process according to the Ree‐Eyring theory. This model, based on the assumption of two relaxation processes (α and β) acting in parallel, allows prediction of yield stress at various loading rates and temperatures. Addition of the interfacial agents results in a reduction of the activation energy and an increase in the activation volume V* for both the α and β processes. Furthermore, the similarity of the value of the activation energy ΔHβ in the β yielding process and the energy barrier ΔH controlling the brittle‐ductile transition in fracture seems to suggest that a similar secondary relaxation mechanism controls the yielding and the fracture behavior of the blend.

Rheological behavior of cellulose/monohydrate of N-methylmorpholine N-oxide solutions. Part 2. Glass transition domain

Dynamic mechanical experiments performed with original conditions allowed the analysis of a solution containing 15% cellulose dissolved in a monohydrate of N-methylmorpholine N-oxide (NMMO) in the amorphous state. The glass transition zone is studied by dynamic tensile experiments, while dynamic torsion technique is used to determine the viscoelastic behavior in the glassy state. A master curve of the storage and loss modulus versus frequency can be deduced from the isochronal curves measured by both techniques. This work allows one to complete the corresponding master curve obtained for the ‘liquid’ state and presented in a previous work [Rheol. Acta 1998;37: 107]. The measurements below the glass transition temperature exhibit two secondary relaxations. A modeling of the overall viscoelastic behavior, using the Nowick and Berry approach and the quasi-point defect theory, is proposed. Physical parameters deduced from this modeling are then discussed.

The influence of the type of substituents on the polymer dynamics of different regioselective 2-O-esters of starch using dielectric spectroscopy

The dielectric relaxation spectra of different regioselective 2-O-acyl-esters of amylomaize starch with a degree of substitution DS=1 were measured in an extended temperature (−135 to 0°C) and frequency range (0.01Hz to 3MHz). For all starch esters, two relaxation processes were found, which are from the type of a secondary relaxation (constant activation energy). One process can be assigned to the local backbone motion of the polymer and the other one with the ester side group motion in position C2 at the anhydro glucose unit. The dynamic parameters of both relaxations were determined and compared. The double bond in the crotonoyl-ester side group has a little influence on both the segmental and the side group motion in relation to the butanoyl-ester. The bulky benzoyl-ester side group shows an inverse influence on the local chain dynamics than the alkyl-ester side group.

Broadband dielectric spectroscopy of the inter- and intramolecular dynamics of a series of random polyester copolymers

Broadband dielectric spectroscopy (1–106 Hz, 183–423 K) and differential scanning calorimetry are employed to analyze the inter- and intramolecular dynamics of a series of random copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylene dimethylene terephthalate). In addition to an interfacial relaxation (α*-process), three dielectric relaxation processes are observed: The α-relaxation (“dynamic glass transition”) and two secondary relaxations (β- and β*-relaxations). The α-relaxation depends sensitively on the composition of the copolymer and shows a rapid slowing down with increasing content of cyclohexylene dimethylene (CHDM) linkages. Besides the β-relaxation, attributed to local motion of the ester group, an additional process (β*-relaxation) is observed on introducing the CHDM linkages. Increasing the content of the latter reduces the strength of the β-relaxation strongly and increases its activation energy by more than 30%. This proves that owing to interactions between the cylohexylene rings and the ester group the β-relaxation no longer has local character only.

Forced compatibility in poly(methyl acrylate)/poly(methyl methacrylate) sequential interpenetrating polymer networks

The aim of this work is to study the miscibility of poly(methyl acrylate)/poly(methyl methacrylate), (PMA/PMMA), sequential interpenetrating networks, (IPNs), as a function of the crosslink density using dielectric and dynamic-mechanical techniques. The PMA/PMMA system is immiscible and so, for low crosslink densities, phase separation appears, as detected by the occurrence of two clearly differentiated main dynamic-mechanical relaxation processes corresponding to the two components. If crosslink density is high enough, a homogeneous IPN can be obtained, achieving a forced compatibilization of both networks. The IPN crosslinked with 10% ethyleneglycol dimethacrylate shows a single main dynamic-mechanical relaxation process. Only the α main relaxation process appears in the PMA networks within the temperature range (−60 to 200°C) of the experiments conducted in this work. The dielectric relaxation spectrum of PMMA networks shows the secondary β relaxation followed by a small α relaxation partially overlapped with it. In the IPNs, both the main relaxation processes tend to merge into a single one and the dielectric spectrum shows a single peak that mainly corresponds to the secondary relaxation of the PMMA.

Novel formulations of high‐performance epoxy–amine networks based on the use of nanoscale phase‐separated antiplasticizers

AbstractThe addition of an antiplasticizing agent in epoxy–amine resin formulations was revisited. The choice, as the antiplasticizer, of a chemical species which is fully miscible with the mixture of monomers but gives rise to nanoscale phase separation along the network construction was shown to be greatly favorable. Small domains, enriched in additive molecules, segregate in the polymer matrix, which is itself plasticized by residual additive molecules. Use of the additives did not change markedly the cure cycle and the total extent of reaction after full cure. The main effects of this special morphology on network properties were the depression of the glass transition temperature, Tg, of the matrix and the appearance of some damping (so‐called μ‐relaxation) in the temperature range intermediate between the secondary relaxation and the main mechanical relaxation. Networks prepared in this way were shown (1) to save a sufficiently high value of Tg in the view of the applications, (2) to present a higher modulus and higher toughness around room temperature than those of the conventional systems without an additive, and (3) to exhibit a lower water uptake at equilibrium than that of their regular homolog. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 759–774, 2001

Dynamics of glasses

The dynamics of glasses is characterized by disorder-specific, more or lesslocalized modes: tunnelling states, soft vibrations and classical relaxation.These modes coexist and interact with the sound waves, giving rise toanomalies in the thermal properties of glasses at low temperatures. At highertemperatures, the low-frequency dynamics is dominated by the classicalsecondary relaxation, which crosses over to the primary relaxation at theglass transition.The present article describes first the low-temperature glass anomalies in theheat capacity and in the thermal conductivity. Then, three closely relatedmodels are reviewed: the tunnelling model, the soft-potential model and theGilroy-Phillips model. The latter allows a joint quantitative description ofprimary and secondary relaxation, giving a new view on the glass transition.Finally, recent inelastic x-ray, light and neutron scattering experiments onthe mixture of sound waves and other modes in glasses are discussed.

A data analysis algorithm for programmed field-flow fractionation.

An algorithm that employs numerical integration for analysis of field-flow fractionation (FFF) data is presented. The algorithm utilizes detector response, field strength, and channel flow rate data, monitored at discrete time intervals during sample elution to generate a distribution of sample components according to particle size or molecular weight. The field strength and channel flow rate may either be held constant or programmed as functions of time, and it is not necessary for these programs to follow specific mathematical functions. If experimental conditions are monitored during a run, the algorithm can account for any deviation from nominal set conditions. The algorithm also allows calculation of fractionating power for the actual conditions as monitored during the run. The method provides greatly increased flexibility in the application of the FFF family of techniques. It removes the limitations on experimental conditions incurred by adherence to analytically available solutions to FFF theory, allowing ad hoc variation of field strength and other experimental parameters as necessary to increase sensitivity and specificity of the method. An implementation of the algorithm is described that is independent of the FFF technique (i.e., independent of field type) and mode of operation. To reduce computation time, it uses mathematical techniques to reduce the required number of numerical integrations. This is of particular importance when the perturbations to ideal FFF theory, such as those due to the effects of hydrodynamic lift forces, particle-wall or particle-particle interactions, and secondary relaxation, necessitate relatively lengthy numerical calculations.

Dielectric relaxations of poly(2-norbornyl methacrylate) and poly(3-methyl-2-norbornyl methacrylate)

Dielectric behaviour of poly(2-norbornyl methacrylate) (P2NBM) and poly(3-methyl-2-norbornyl methacrylate) (P3M2NBM) was studied. The effect of the methyl substituent on the norbornyl ring is to decrease the temperature of the dynamic glass transition about 50K. Moreover, weak secondary relaxations are observed in the two polymers under study. The secondary process of P3M2NBM has been conveniently analysed in terms of the empirical Fuoss–Kirkwood (F–K) equation.

Molecular analysis of the plastic deformation of amorphous semi-aromatic polyamides

The plastic behavior of a series of amorphous semi-aromatic polyamides (SAPA-A) was investigated in compression mode at temperatures ranging from −100°C to their glass transition temperatures. Data analysis was mainly based on the inspection of the temperature dependence of yield stress, σy, plastic flow stress, σpf, and strain softening, SSA=σy−σpf. The activation volumes, V0, and the index of non-elastic behavior, I, were also determined, to account for the sensitivity of plastic deformation to strain rate and for the relative easiness of elastic and non-elastic processes, respectively. Some connections have been established between the nature of the molecular motions and the values of σy, σpf, SSA, V0, and I, thanks to the knowledge of the relaxation behavior of these materials. The role played by the secondary relaxation motions (and especially by those presenting a cooperative character) was highlighted, in agreement with our conclusions in earlier reports on the subject. In addition, the molecular analysis proposed for describing the plastic deformation of the SAPA-A materials was shown to hold for other polymers also bearing phenylene rings in the main chain, namely some semi-aromatic polyamides of another series (SAPA-R) and aromatic polycarbonates. Interestingly, the line of reasoning proposed here proved to be suitable also to account for the plastic deformation characteristics of various vinyl polymers and copolymers.

Temperature dependence of the photoluminescence in poly(ethylene terephthalate) films

Temperature effects on the photoluminescence spectrum, phosphorescence lifetime and phosphorescence intensity of poly(ethylene terephthalate) have been investigated in the range −185 to +30°C. Both the fluorescence and phosphorescence emission spectra are composed of a monomeric-like component and a red-shifted emission attributed to the formation of ground state dimers. Monomeric emissions are enhanced at low temperature. The transient phosphorescence signals at excitation switch-on and switch-off are clearly not exponential, but could be suitably fitted to a stretched-exponential function. The temperature dependence of the related parameters is discussed. The kinetic analyses of temperature effects on steady-state phosphorescence indicated that phosphorescence quenching is controlled by oxygen diffusion. A correlation between the phosphorescence decay and the secondary relaxation of PET as probed by dielectric spectroscopy has been established.

Physical characterization of poly(ω‐pentadecalactone) synthesized by lipase‐catalyzed ring‐opening polymerization

AbstractThe Candida antarctica lipase B (Novozyme‐435)‐catalyzed ring‐opening polymerization of ω‐pentadecalactone in toluene was performed. Poly(ω‐pentadecalactone) [poly(PDL)] was obtained in a 93% isolated yield in 4 h with a number‐average molecular weight of 64.5 × 103 g/mol and a polydispersity index of 2.0. The solid‐state properties of poly(PDL) were investigated by thermogravimetric analysis (TGA) coupled with mass spectrometry, differential scanning calorimetry (DSC), stress–strain measurements, wide‐angle X‐ray diffraction, and dynamic mechanical and dielectric spectroscopies. Poly(PDL) is a crystalline polymer that melts around 100 °C. The polyester shows good thermal stability, with a main TGA weight loss centered at 425 °C. Because of the high degree of poly(PDL) crystallinity, the glass transition (−27 °C) is revealed by relaxation techniques such as dynamic mechanical and dielectric spectroscopies, rather than by DSC. In addition to the glass transition, the viscoelastic spectrum of poly(PDL) also shows two low‐temperature secondary relaxations centered at −130 (γ) and −90 °C (β). They are attributed to local motions of the long methylene sequence (γ) and complex units involving water associated with the ester groups (β). The mechanical properties of poly(PDL) are typical of a hard, tough material, with an elastic modulus and yield parameters comparable to those of low‐density polyethylene. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1721–1729, 2001

Glass transition of an epoxy resin. A wideband dielectric investigation

The dynamics of an epoxy resin, poly(phenyl glycidyl ether-co-formaldehyde), has been investigated in the supercooled and glassy phases by wideband dielectric spectroscopy (10/sup -2/ to 3/spl times/10/sup 9/ Hz) and compared with that of the previously investigated epoxy resin, diglycidyl ether of bisphenol-A. The temperature evolution of the dynamics of the system is monitored through the characteristic parameters of the relaxations, namely the relaxation times, the relaxation strengths and the shape parameters. Two transition regions are revealed: the glass transition and the split between structural and fast secondary relaxation, where the onset of the structural relaxation is located also.

Relaxation phenomena and morphology in polymer blends based on polyurethanes investigated by various thermal analysis techniques

The thermal, thermomechanical and thermal-dielectric properties of polyurethane blends based on branched polymer polyol (PUR) and styrene–acrylonitrile (SAN) copolymer were studied by differential scanning calorimetry (DSC), thermomechanical analysis (TMA), thermally stimulated depolarization currents (TSDC) techniques and dielectric relaxation spectroscopy (DRS). Several molecular mobility mechanisms associated with secondary local relaxations, primary (main) relaxations due to the dynamic glass transition and conductivity effects were observed and studied in detail. The use of several molecular mobility techniques, characterized by various spatial and time scales, allowed for several intercomparisons, in particular with respect to the determination of glass transition temperatures and of the temperature dependence of relaxation times which indicate the advantages and the limits of each particular technique. The results by the different techniques suggests in agreement to each other, that on addition of SAN the microphase separation between hard segment (HS) microdomains and soft segment (SS) microphase of PUR is improved.

Dynamic mechanical behavior of oriented semicrystalline polyethylene terephthalate

AbstractThe dynamic mechanical behavior of molecularly oriented semicrystalline polyethylene terephthalate (PET) induced via the equal‐channel angular extrusion (ECAE) process was investigated. Dynamic mechanical analyses in both torsional mode and bending mode were utilized. The results indicate that the ECAE‐oriented PET has a higher dynamic storage modulus above the glass‐transition temperature than that of the reference (control sample). The combined effect of molecular orientation and crystallinity is responsible for the changes in the primary and secondary relaxations of PET. Further analyses show that the shifting and broadening of the primary and secondary peak positions in oriented PET are mainly due to the amorphous‐phase orientation because the crystallinity of PET decreases upon the ECAE processing. A good correlation was found between the structural anisotropy and the dynamic mechanical properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1394–1403, 2001

Slow β process in simple organic glass formers studied by one- and two-dimensional H2 nuclear magnetic resonance. I

We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) H2 nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-d5 and polybutadiene-d6 (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-d5 and polystyrene-d3 (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D 2H NMR spectra and in the orientational correlation functions of 2D 2H NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature Tg, the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene molecules and PB monomeric units is characterized by an amplitude χ<10°. This amplitude is approximately unchanged below Tg, but strongly increases above the glass transition. Closer investigating the 1D 2H NMR line shape for large interpulse delays we moreover demonstrate that the reorientation involved in the β process takes place step-by-step via many elementary rotational jumps. On the other hand, for glycerol and PS, hardly any effects are observed in 1D and 2D 2H NMR experiments below Tg when applying comparable experimental parameters. We conclude that reorientations with an amplitude χ>1° do not occur on a time scale of μs−ms for the majority of molecules in glassy glycerol and PS.

Secondary relaxations in poly(allyl alcohol), PAA, and poly(vinyl alcohol), PVA. II. Dielectric relaxations compared with dielectric behaviour of amorphous dried and hydrated cellulose and dextran

This work deals with the characterisation of the dielectric secondary relaxations of poly(allyl alcohol) (PAA) and poly(vinyl alcohol) (PVA) and its comparison with the cellulose and dextran dielectric behaviour. Cellulose and dextran are two polysaccharides made of glucosyl repeat units. Cellulose has two hydroxyl groups (OH) and one hydroxymethyl group (CH 2OH) per glucose ring, while dextran has three OH groups and no CH 2OH group. PAA and PVA are the two simplest vinyl polymers containing CH 2OH and OH lateral groups, respectively. They exhibit only one broad dielectric secondary relaxation with characteristics in between those of the γ and β mechanical relaxations in dried PAA. Each of these apparently unique relaxations is successfully explained and modelled by the overlaps of two relaxation processes, γ′ and β′. The γ′ relaxation, related to the rotation of side groups (CH 2OH for PAA and OH for PVA), has characteristics close to those of the γ mechanical relaxation for dried PAA. The β′ relaxation corresponds to more or less cooperative motions of main-chain segments. Furthermore, the influence of moisture on the secondary dielectric relaxations of PAA and PVA is discussed on the basis of a competition between γ′ and β′, where the intensity of β′ (γ′) increases (decreases) with increasing water content.

Influence of vinyl ester/styrene network structure on thermal and mechanical behavior

AbstractThe influence of vinyl ester/styrene network structure on thermal and mechanical properties was investigated. The crosslink density of the resins was altered by changing the molecular weight of the vinyl ester oligomer and by varying the amount of styrene used during the crosslinking reaction leading to variations in both the physical network structure and the chemical composition of the polymeric networks. The glass transition temperatures of the network polymers were found to increase systematically with increasing crosslink density without the additional influence of the chemical composition as determined from both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The breadth of the glass transition regions increased with crosslink density for the DSC data, but the breadth assessed from the DMA data did not vary significantly for the network materials. A secondary relaxation was observed for the materials using DMA, and this relaxation did not appear to be significantly affected by changes in either the crosslink density or the composition of the network. Cooperativity studies involving time–temperature scaling of dynamic mechanical data in the glass formation temperature region were also conducted. The degree of segmental cooperativity at Tg appeared to be primarily influenced by the chemical composition of the networks. These issues dealing with the structure of the networks provided insight into the associated fracture properties in the glassy state (ambient temperature). Specifically, an empirically based linear correlation was found between the fracture toughness of the networks and the cooperative domain size at the glass transition temperature normalized by the crosslink density. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 917–927, 2001

Study of the nature of glass transitions in the plastic crystalline phases of cyclo-octanol, cycloheptanol, cyanoadamantane and cis-1,2-dimethylcyclohexane

Cycloheptanol, cyclo-octanol, cyanoadamantane and cis-1,2-dimethylcyclohexane are known to form plastic crystals which can be supercooled to show a glass transition at a temperature Tg. The molecular dynamics in different plastic phases is studied in their supercooled states using dielectric spectroscopy (frequency range: 106 Hz–10−3 Hz) and differential scanning calorimetry (DSC) over a wide temperature range. The kinetic freezing of the various dielectric processes have been critically examined in relation to the Tg found in the DSC experiments. The plastic phase I of cyclo-octanol shows two Tg’s: one at 148.5 K and the other at 164 K, the former of which is not found in well annealed phase I. The dielectric α-modes correspond to the latter. Cycloheptanol exhibits many Tg’s for the different plastic phases. Unlike the cyclic alcohols, the dielectric spectra of cyanoadamantane and cis-1,2-dimethylcyclohexane is clearly found to follow the Havriliak–Negami equation, both of which interestingly show very little molecular mobility, often referred to as the secondary relaxation in their glassy states. In addition, the temperature dependence of the relaxation rates and the dielectric strengths are critically examined for various supercooled phases to gain an insight into the nature of the molecular mobility in those phases.