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Dielectric spectroscopy dynamic me-chanical thermal analysis styrene buta-diene rubber (SBR) carbon black EDM rubber relaxation processes linear and non-linear mechanical regime

The design of a new experimental setup that combines dielectric spectroscopy (DEA) with dynamic mechanical thermal analysis (DMTA) to perform simultane-ous dielectric and dynamic mechanical measurements is presented here for the first time. This allows the determination of dielectric material properties under varying mechanical compression loads in both linear and non-linear mechanical regime. The first results of this tech-nique applied to filled rubbers show the high potential of this combination. The measurements are performed simulta-neously, the sample being in exactly the same condition with and without static and dynamic mechanical load for each measurement.

Kombinierte dielektrische (DEA) und dynamisch-mecha-nische thermische Analyse (DMTA) in Kompression Dielektrische Spektroskopie dyna-misch-mechanische thermische Analyse Styrol-Butadien-Kautschuk (SBR) Ruß Ethylen-Propylen-Dien-Kautschuk (EP-DM) Relaxationsprozesse lineares und nichtlineares mechanisches Regime

DDie Entwicklung eines neuen experi-mentellen Aufbaus zur Kombination von dielektrischer Spektroskopie (DEA) mit der dynamisch-mechanischen thermi-schen Analyse (DMTA) zur simultanen Messung von dielektrischen und dyna-misch-mechanischen Eigenschaften wird hier zum ersten Mal vorgestellt. Der Aufbau ermöglicht die Bestimmung der dielektrischen Materialeigenschaf-ten bei variierender mechanischer Belas-tung in Kompression, sowohl im linear als auch im nichtlinear viskoelastischen Bereich. Diese Technologie wurde bei ge-füllten Kautschuken angewendet und erste Ergebnisse zeigen das große Poten-tial dieser Methode auf. Die gleichzeitige Durchführung beider Messungen ge-währleistet, dass diese Messungen im exakt gleichen Materialzustand mit oder ohne mechanische Vorlast erfolgen.

Figures and tables:By a kind approval of the authors.

The task of evaluating new materials and predicting their performance for specific applications is challenging for resear-chers and engineers. Rubber is a material that can be used in a wide variety of ap-plications leading to an enormous incre-ase in the commercial demand for rub-ber. For advanced applications, it is ne-cessary to have a thorough understan-ding of the structure, composition and physical/mechanical properties of rub-ber. The development of synthetic rubber has now reached a stage at which more precise methods of determining the che-mical composition and physical proper-ties of rubber materials are needed. This development will facilitate the economic design and manufacture of rubber pro-ducts to make goods with a low cost and/or high performance [1].

A great variety of experimental tech-niques have been applied for the study of unreinforced and filled rubber compo-sites, namely rheology [2-7], rheo-diel-ectric spectroscopy [8-12], Fourier trans-form infrared spectroscopy [13,14], ther-mogravimetry (TGA) and derivative ther-mogravimetry (DTG) methods [15,16], dynamic scanning calorimetry (DSC) and low field NMR [17,18], just to mention the most prominent techniques. Each technique provides different informati-on because of the different length and time scales of the measurements and some methods provide chemical infor-mation while others probe the mechani-cal response of the rubber. Results from different techniques help to understand the structure-property relationships pre-sent in these complex materials. Howe-ver, a deeper understanding of the ma-terial properties is possible when me-aningful techniques are combined to si-multaneous measurements that, for example, can reveal how the structure of the material changes under mechanical excitation.

The motivation for the development of a new and unique combined experi-mental technique is to gain a deeper in-sight into the dynamics of rubbers under

mechanical load based on the correlation of two powerful techniques, broadband dielectric spectroscopy and dynamic me-chanical thermal analysis (DMTA). Dyna-mic mechanical thermal analysis is a mechanical technique that measures the properties of materials as a function of frequency, amplitude and temperature as they are deformed under static or pe-riodic stress respective deformation. Spe-cifically, in DMTA, a variable sinusoidal stress can be applied usually under tensi-on or compression, and the resulting si-nusoidal strain is measured. The mecha-nical phase difference, together with the amplitudes of the stress and strain waves, is used to determine a variety of fundamental material parameters, inclu-ding the storage and loss Young’s moduli (E’, E”), tan δ, complex viscosity (η*), tran-sition temperatures (e.g. Tg) and relaxati-on times, as well as related performance attributes, such as the impact resistance [19, 20].

Combined Dielectric (DEA) and Dynamic Mechanical Thermal Analysis (DMTA) in Compression Mode

AuthorsRoxana Figuli, Lukas Schwab, Manfred Wilhelm, Karlsruhe, Jorge Lacayo-Pineda, Hannover, Horst Deckmann, Ahlden Corresponding Author:Manfred WilhelmInstitute for Chemical Technology and Polymer ChemistryKarlsruhe Institute of Technology (KIT), Karlsruhe, GermanyE-Mail: Manfred.Wilhelm@kit.edu

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Dielectric analysis is a very powerful technique for characterizing materials with multiple phases or relaxation pro-cesses that can be monitored by changes in the distribution of charged ions or molecular dipoles existing in the materi-al. In general, transport processes in so-lids, especially in complex, structured polymers, depend strongly on their mor-phology as well as on the influences of major physical variables [21, 22]. In filled rubbers, for example, the dielectric res-ponse is highly influenced by the size and spatial distribution of the filler, e.g. carbon black. One of the advantages of dielectric analysis is that it provides an enormous measurable frequency range from 10-2 Hz to typically 107 Hz for each temperature.

Though many results are available on either the electrical or mechanical pro-perties of styrene–butadiene rubber (SBR)-carbon black (CB) composites [23–27], to our knowledge no open publica-tions are available where these proper-ties are measured simultaneously, e.g. under linear and non-linear mechanical compression. This combined mechanical-dielectric technique is of special impor-tance for investigating complex samples like filled rubbers.

Filled rubbers represent technically important examples of materials be-cause their mechanical and dielectric properties change significantly under load when leaving the linear elastic range towards higher loads. This publi-cation presents: i) the design of a new experimental setup that combines die-lectric spectroscopy with DMTA to per-form simultaneous dielectric- and dy-namic mechanical measurements that allow the determination of dielectric material properties under varying me-chanical compression loads in both the linear and non-linear mechanical re-gime and ii) the first results of this technique applied to filled rubbers with the advantage, that the measure-ments are performed simultaneously so the sample is in exactly the same condition for each measurement. This is of special importance due to the complexity of the material and helps to achieve meaningful structure property relations.

The outcome of this combined tech-nique allows us to quantify and correlate important characteristics including how the molecular mobility, molecular trans-port and relaxation processes are influ-enced by mechanical deformation.

Experimental

MaterialsThe reference material for this study was 1,4-cis-polyisoprene (PI, uncrosslinked) obtained from Kashima Plant, Elastomer Divison, Chemical Company, Japan. The molecular weight was measured by GPC and was found to be 17 kg/mol.

The second material investigated was styrene-butadiene rubber (SBR). The samples were obtained from Conti-nental Reifen Deutschland GmbH. The recipe for the sample preparation was 100 phr (parts per hundred parts of rub-ber by weight ) SBR (solution polyme-rized, with a styrene content of 25 % (w/w) and a vinyl content of 35 % (w/w) with respect to the total BR content), 6 phr of different stabilizers and anti-oxidants (2.5 phr wax, 1.5 phr 6PPD (N-(1,3-Dimethylbutyl)-N`Phenyl-p-pheny-lendiamine), 1 phr TMQ (2,2,4-Trime-thyl-1,2-dihydro quinoline ), 1 phr DTPD (N-N´-(p-Phenylene)ditoluidine)), 5 phr activators (2 phr stearic acid, 3 phr ZnO), 0.7 phr TBBS (N-tert-butylbenzothiazo-le-2-sulphenamide - vulcanization acce-lerator), 1.65 phr sulfur and 20 phr car-bon black (CB) of different grades. Three CB fillers were chosen: N121, N339 and N660. The first character in the nomen-clature system for rubber-grade carbon blacks is a letter indicating the effect of the carbon black on the cure rate of a typical rubber compound containing the carbon black. The letter “N” is used to indicate a normal curing rate that is typical for furnace blacks that have re-ceived no special modification to alter their influence on the rubber cure rate. The second character in the system is a

digit to designate a measure of the ave-rage surface area of the carbon black as measured by nitrogen surface area ad-sorption where smaller numbers indica-te a higher surface area. The third and the fourth character in this system are via ASTM standard classification assig-ned digits but are of minor relevance for the presented investigation [28].

For the experimental investigations the raw material used was vulcanized under a pressure of 5 bar at 180 °C for 30 min. The specimens were prepared as circular disks of 25 mm diameter and 2 mm thickness for the measurement of the dielectric/mechanical properties.

The third material investigated was an ethylene propylene diene monomer (M-class) rubber (EPDM). The EPDM sam-ple was provided from GABO Qualimeter.

Experimental methods

DMTAThe DTMA explores dynamic mechanical material properties by applying sinusoi-dal dynamic mechanical deformations or forces. The mechanical frequencies used usually range from 10-2 Hz to 102 Hz for this technique. In practice a static load is applied first and the dynamic load is su-perimposed. The sample’s response to the exciting force is its deformation which is determined experimentally by varying static load, frequency and ampli-tude. Sinusoidal forces usually lead to si-nusoidal sample deformations. The res-ponse is delayed in time respective to the excitation. This delay can be quantified as a phase shift δ (Fig. 1a). The force and corresponding strain allow the computa-tion of the sample stiffness. Both stiff-

Fig. 1: The basic principle of DMTA. a) The sample deforms in response to the excitation force. Sinusoidal forces lead to sinusoidal sample deformations (and vice versa). The res-ponse is delayed in time with respect to the excitation. The delay represents a phase shift. b) Typical parameters for E’’ and E’ for examples of viscous (oil, fluids), viscoelastic (polymer melt, elastomers) and elastic materials (metal). The geometrical interpretation of the ratio of E”/E’ in the complex plane as tan δ identifies the phase shift between the sample deformation and stress.

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ness and phase shift depend on the che-mistry of the material and its actual physical state (temperature, load, humi-dity, relaxation times etc.). Elastic mate-rials as, for example, a stainless steel spring respond to a mechanical deforma-tion by a sinusoidal force in phase (δ = 0°) with the excitation. Viscoelastic materi-als cause a constant time-delay (90° > δ > 0°) with respect to the external excitati-on. The manifold of viscoelastic materi-als covers a wide range of phase shifts (δ) as Fig. 1b indicates. The dynamic materi-al properties are represented and quanti-fied by the complex modulus E* = E’+ iE’’. The complex modulus E* depends on the structure and morphology of the samp-les. Rubber materials generally exhibit viscoelastic behaviour. Their mechanical characterization requires the experimen-tal determination of the complex modu-lus E* and the characteristic phase shift δ (Fig. 1a) between the sinusoidal force and deformation. The complex modulus E* consists of two components: the real part (on the x-axis) reflecting the purely elastic properties (called the storage mo-dulus, E’) and the imaginary part (on the y-axis) mirroring exclusively the viscous portion of the sample properties (called the loss-modulus, E”). Whereas E’ is a measure of the reversibly (=mechani-cally) stored energy, E” is a measure of the energy which is dissipated as heat (=lost) during the sample deformation. The geometrical interpretation of the ra-tio of E”/E’ in the complex plane is dis-played in Fig. 1 as tan δ identifies the

phase shift between sample excitation and response. Metal springs deform in a purely elastic way (ϕ = 0°), Newtonian fluids behave purely viscous (ϕ close to 90°) and most polymers show intermedi-ate phase angles (0° < ϕ < 90°) (Fig. 1b).

High power servomotors and strong electro dynamic shakers generate the time dependent sample load. Force and strain sensors record the raw signals. The EPLEXOR 150 N system is specifically de-signed to operate from very low to high static and dynamic loads for analyzing linear and non-linear material proper-ties.

DEADielectric analysis (DEA) explores the electric charge distribution and change with respect to time of electrical dipole moments in a molecule in a material after applying a sinusoidal electrical field via two plate-plate electrodes (electrical capacitor) [21]. An oscillating electrical field causes an oscillating electrical current through the sample. This results in a phase shifted signal to the applied electrical field with respect to the current across the capacitor. The current and phase depend on the mate-rial and its condition. The evaluation of the experimentally recorded signals al-lows for the calculation of the complex dielectric function, in direct analogy to the mechanical case via ε* = ε’– iε’’. The fundamental mathematics for mechani-cal and dielectric spectroscopy are very similar [21]. As electric currents consist

of an active current (real part) and a blind current (imaginary part), the diel-ectric function (ε*) also consists of a real (ε’) and an imaginary (ε’’) part. The real and imaginary components, hence the complex dielectric function, are deter-mined by electric measurements using the above mentioned plate type capaci-tor arrangement where the investigated material is placed between the plates. The dielectric function ε* describes the dielectric behavior of the material and contains information about molecular transport and relaxation processes by monitoring the charged ions and elec-tric dipoles existing in the molecule, re-spective material. In general, transport processes in solids, especially in com-plex structured polymers, depend on their internal morphology as well as on the influence of important physical pro-perties. The dielectric function ε* de-pends on several variables such as the temperature, frequency, the molecular mobility within the material, the macro-scopic orientation of the polymer chains, electromagnetic fields and the applied mechanical loads including pressure and tensile stresses. Fig. 2 illus-trates the influence of the sample tem-perature on the dielectric loss function for a very simple, non filled 1,4-cis- poly-isoprene (PI) model sample. The 1,4-cis PI has a molecular dipole that is parallel to the chain backbone so that the end-to-end fluctuation of the PI chains re-sults in the slowest dielectric relaxation within the molecule, which is called the

Fig. 2: a) The temperature and frequency dependence of the dielectric loss ε’’ for 1,4-cis polyisoprene sample shows a relaxation process that is characteristic for a normal-mode process, which corresponds to reptation motion of the entire chain caused by dipole components parallel to the backbone. It can be clearly seen that higher temperatures increase the molecular mobility for linear homopolymers. The diel-ectric measurement was performed during mechanical measurements in the linear regime (ω/2π = 1 Hz and static load of 1 N) where the mechanical load did not affect the dielectric measurement. b) Activation plot or also named Arrhenius plot: logarithm of the relaxation rate 1/τ vs 1000/T for the normal mode of 1,4-cis polyisoprene. The six data points constitute an activated process (Arrhenius dependence) with an activation energy of 14 kJ/mol.

2

CH2 CH2

CH3H

C C

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normal mode. Fig. 2a displays the corre-sponding normal mode dispersions in the dielectric loss as a function of tem-perature. The dielectric normal mode represents, in this specific case, the rep-tation motion for the 1, 4-cis PI chains. Fig. 2b shows the temperature depen-dence of the relaxation times of the normal mode corresponding to the fluc-tuation of the dipole components paral-lel to the chain contour of polyisoprene.

Simultaneous Dielectric (DEA) and Dynamic Mechanical Thermal Analy-ses (DMTA)

Dielectric and dynamic mechanical thermal analysis measurements were carried out simultaneously on filled SBR samples (compression mode) to deter-mine the dependence of the dielectric field at a varying mechanical load. DEA polarizes the sample by periodically va-rying the applied electrical field and monitors the load induced changes of the charge distributions. When the po-lar bonds in polymer molecules are ex-posed to electric fields, the interatomic distances are changed. Mechanical de-formations also affect the interatomic distances in a material. A periodic, low frequency excitation, which is typical for dynamic mechanical thermal analy-sis, can induce similar polymer chain movements to those caused by periodic electrical fields (DEA) in the polar bonds of dielectric materials [29, 30]. Carbon black, a common filler for technical rub-bers especially for tire applications, is difficult to disperse in a rubber matrix. While driving, tires are mechanically deformed. These movements can break the relatively weak filler-rubber bonds over time. This potential damage to the filler-rubber interface caused by mecha-nical loading can be monitored by measuring the changes in the dielectric spectrum of the sample, which is sensi-tive to changes in the molecular distan-ces and carbon black distribution. Me-chanical loads created by DMTA are well defined and large enough to mimic real applications in a lab experiment. DEA supplements the mechanically acquired material properties with information about changes of the internal structure of the sample under the same load. Therefore, DEA provides an option to trace internal structural changes of the samples. Therefore, the simultaneous use of DMTA and DEA results in a tech-nique that correlates mechanical pro-perties (DMTA) and the changes in the

internal structure as determined by DEA, for a filled rubber system. This type of measurement was possible using a modified version of the EPLEXOR 150 N (Fig. 3a) instrument. The EPLEXOR 150 N (Dynamic Mechanical Thermal Analy-zer) system serves as the host instru-ment for the dielectric sensor system (Novocontrol Alpha Analyzer) and the data acquisition unit. A new combined dielectric and DMTA sample holder (Fig. 3b) was designed to operate in com-pression mode and can hold cylindri-cally shaped samples with a thickness of ideally 1 to 2 mm and a typical dia-meter of 25 mm. The new sample hol-der was specially designed for the com-bined system and can withstand large forces, up to 500 N. The basic technical

drawings are presented in Fig. 4. With the new designated holder, the new Di-PLEXOR system was released by GABO Qualimeter and offers the option of si-multaneous dielectric and mechanical measurements.

DiscussionThe present system under investigation consists -beside the already discussed 1,4-cis PI- of an insulating SBR matrix containing embedded conductive carbon black fillers of different types. The dielec-tric properties of a polymer composite depend on four major factors: 1) the properties of the constituent phases, such as their electric permittivity and electric conductivity; 2) their relative vo-lume fractions; 3) the morphology and the state of dispersion of the particles inside the matrix material and 4) additi-onally the relative orientation of the ag-gregates with respect to the electrical field. As these parameters will influence the dielectric spectrum, dielectric spect-roscopy can monitor any changes to the-se factors over time. The dielectric loss factor (ε’’) is related to the dielectric rela-xation phenomenon and is a measure of the decay in the electrical polarization as a function of time [31], respectively fre-quency.

The electrical resistance, capacitance, and dissipation factor (tan δ) values were directly measured from our new setup. From the values of the capacitance and dissipation factor, the dielectric loss fac-tor of the samples is calculated by the relation

ε’’/ ε’= tan δdiel (1)

Fig. 3: Setup of the combined Dielectric (DEA) and Dynamic Mechanical Thermal Analysis (DMTA) technique: a) Simultaneous mechanical and dielectric measurements allow for the determination of dielectric material properties at varying mechanical loads. b) The sample holder is designed for compression mode and can hold cylindrical shaped flat samples with a thickness of typically 1 to 2 mm. The dielectric frequency range is from 10-2 to 107 Hz and the maximum applied mechanical force is in this case 150 N.

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Fig. 4: Sketch of a DMTA-DEA sample hol-der. The base consists of steel followed by a ceramic isolator and a brass electro-de with a diameter of 25 mm. The elect-rode is connected to a Novocontrol Alpha Analyzer input via a BNC connector.

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Where tan δdiel is the dielectric dissipati-on factor of the composites. Fig. 5 shows the dependence of the dielectric loss ε’’ as a function of the frequency of the ap-plied electrical field for the vulcanized samples N121 (small particle size), N339 (medium particle size) and N660 (large particle size). The dielectric measure-ments were performed under a simulta-neous mechanical load, where the ap-plied force was 30 N (48 kPa) – (square symbols) or 120 N (192 kPa) – (round symbols) for all three samples. It can be observed that the dielectric loss factor ε” increases with increasing frequency. At low frequencies, ε’’ attains low values and then increases with increasing fre-quency for all three samples. This behavi-or is typical for interfacial polarization of

the carbon black aggregates, which is known as the Maxwell-Wagner-Sillars effect (MWS) [32, 33]. Maxwell-Wagner-Sillars polarization occurs in dielectrically heterogeneous system because of the accumulation of charges at the interface. In this system, the internal interfaces are mainly related to interfacial regions bet-ween the dispersed carbon black and the rubber matrix. The free carriers moving in the different phases of the composite are blocked at the interface between the two phases, which have different con-ductivities and permittivities. These im-mobile charges are unable to discharge freely or accumulate at an electrode and therefore result in a significant increase in the system capacitance and, conse-quently, the appearance of an interfacial

polarization. The frequency dependence of this phenomena can be explained be-cause the dipoles have less time to orient themselves in the direction of the alter-nating field as the frequency is increased [34, 35].

A maximum in ε” was found at a hig-her frequency range. This result is an evidence for the presence of superimpo-sed processes due to the orientation of the main chain and its related motions, but could also be caused by disaggregati-on of the fillers due to mobility of the main polymer chain. One important fin-ding is that the highest dielectric loss was observed for the samples containing N121, which indicates that these samp-les have the most interfacial area and/or highest particle mobility and is consis-tent with N121 having the smallest pri-mary particle size.

As the primary particle size is increa-sed from N339 to N660, ε’’ decreases. This reflects either a decrease in the in-terfacial area or an increase in the rigidi-ty of the particles with size. The effect of mechanical loading on the dielectric spectrum also depends on the type of filler used. For example, for the samples containing N121 (smallest primary par-ticle size), it is seen (Figure 5) that there was a 25 % increase in the ε’’ module when the applied mechanical force was increased from 30 N (48 kPa) to 120 N (192 kPa). It is interesting to note that this result was not seen for samples con-taining N339 or N660 perhaps because the force was insufficient to affect the interfacial region between the filler and rubber sufficiently, when the particle size is larger.

Fig. 5: Frequency dependence of the dielectric loss ε’’ at room temperature for two diffe-rent mechanical loads (square symbols- 30 N mechanical load and round symbols- 120 N mechanical load) resulting in a pressure of 48 kPa and 192 kPa for SBR rubbers filled with 20phr carbon black of different grades CB (N121 small size particle, N339 medium size and N660 large particle size). The small particle size sample N121 shows a ~ 550 % increase of the dielectric loss compared to the N660 sample at 50 Hz and 120 N mechanical load.

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Fig. 6: a) Dielectric tan δdiel as a function of dielectric frequency at room temperature for EPDM rubber sample at different mechanical loads ranging from 10 N to 112 N and respective pressure 16 kPa to 180 kPa. b) The effect of the applied force on the relaxation time: logarithm of the relaxation rate 1/τ vs pressure.

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The dielectric loss ε’’ shows an increa-se of ~550 % between N121 (the small particle size sample) and N660 (big par-ticle size sample) with the applied force. Therefore it is directly proven that the mechanical force affects the dielectric response.

As a second application of the me-thod, simultaneous mechanical and diel-ectric measurements were performed on an EPDM rubber sample. The effect of the mechanical force on the dielectric spectra as determined via tan δdiel can be seen in Fig. 6 for the EPDM rubber samp-le. Increasing the applied mechanical force from 10 N (16 kPa) to 110 N (176 kPa) caused an unexpected and very sub-stantial shift in the ε’’(ω) by a factor of 100 towards slower dynamics due to re-duced mobility of the polymer chains. Therefore, it can be clearly concluded from Fig. 6b that mechanical loading has a strong influence on the dielectric rela-xation.

The new setup is designed for com-bined measurements; dielectric and mechanical results can be acquired si-multaneously. Dynamic mechanical thermal analyses together with dielec-tric analysis allow the correlation of pa-rameters accessible by dielectric spect-roscopy, such as relaxation time, disper-sion quality, particle size, with mechani-cal parameters. A sensitive way to investigate the effects of mechanical loading on the dielectric spectrum opens. As both mechanical spectroscopy and dielectric spectroscopy are highly influenced by the polymer dynamics, e.g. Tg filler content and the state of dis-persion, this new technique provides an unique access to study the molecular dynamic and filler interaction in rubber materials.

ConclusionA new combination of mechanical and dielectric analyses (DTMA/DEA) for large and oscillatory forces is introduced and applied for investigations of carbon black filled rubber systems, in the linear and non-linear mechanical regime. Therefore by coupling DMTA and DEA together re-sults a technique that for the first time can relate the applied mechanical load with internal structural changes revealed by DEA.

The essential part is that the measu-rements are performed at the same time so the sample is in exactly the same sta-te. By applying this combined method to different SBR samples with varying filler

particle size systematical results were obtained. A factor 5 increase of the diel-ectric loss for the sample with small particle size (N121) was observed com-pared to the sample with big particle size (N660) at a pressure of 192 kPa. An incre-ase of 50 % if the pressure is increased from 50 kPa to 200 kPa could be obser-ved.

For the EPDM rubber sample the diel-ectric behavior was strongly influenced by the mechanical load. A factor 100 could be determined if the pressure in-creases from 16 to 180 kPa.

The results achieved enable us to re-commend the combined technique as a powerful tool that allows to quantify and correlate important characteristics including how the molecular mobility, molecular transport and relaxation pro-cesses are influenced by mechanical de-formation.

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