Charakterisierung hochdynamischer Relaxationsvorgänge in gefüllten Elastomeren

Dynamische Relaxationsvorgänge und insbesondere der damit verbundene dynamische Glasübergang sind wichtige Größen zur Bewertung der Leistungsfähigkeit von Elastomermaterialien, wie sie in Reifen eingesetzt werden. Hierfür kann das Frequenz- und Temperaturverhalten der Relaxationsvorgänge im Elastomer einen Beitrag liefern. Diese Arbeit beschäftigt sich mit der Charakterisierung von Relaxationsvorgängen und dem dynamischen Glasübergang in einem Frequenzbereich von über 10 Dekaden, sowie die Beeinflussung dieser Größen durch die Harze poly-(α-Methylstyrol) (AMS) und poly-(Inden-Cumaron) (IC). Diese Harze haben eine ähnliche Aromatizität, unterscheiden sich jedoch in ihrer molekularen Rigidität. Die Rigidität beeinflusst der Mischbarkeit der Harze in den Elastomermischungen. Als Elastomermischungen werden Silica-gefüllter Styren-Butadien-Kautschuk (SBR) und Butadien-Kautschuk (BR) verwendet. Im Transmissionselektronenmikroskop (TEM) zeigt sich bei beiden Elastomermischungen eine Phasenseparation bei einem Gehalt von 80 phr IC, die bei AMS nicht auftritt. Die Kühlratenabhängigkeit des Glasübergangs ist in der SBR-Mischung stärker, wenn AMS statt IC als Harz verwendet wird. Die Krümmung des Vogel-Fulcher-Tammann-Hesse- (VFTH-) Verlaufs des dynamischen Glasübergangs, die sogenannte Fragilität, ist bei hohen Gehalten an Harz stärker durch AMS als durch IC beeinflusst. Die Messung der Relaxationsvorgänge erfolgt durch breitbandige dielektrische Spektroskopie (BDS) mit Frequenzen von bis zu 2 MHz. Die dielektrisch ermittelte Dynamik des Glasübergangs wird durch Chip-kalorimetrische (FDSC) Messungen mit der Kinetik des Glasübergangs gemäß der Frenkel-Kobeko-Reiner- (FKR-) Hypothese korreliert. Diese Relation zwischen der thermischen Verglasung und der dielektrischen Relaxation wurde erstmalig an Elastomeren untersucht und konnte durch eine einzige Vogel-Fulcher-Tammann-Hesse- (VFTH-) Gleichung beschrieben werden. Die hierfür notwendige Präparation bei Kryo-Mikrotomie und die Kalibration mittels Adamantan für FDSC-Messungen mit Kühlraten bis 1500 K/s wird ebenfalls ausführlich beschrieben

Kinetics of the Glass Transition of Silica-Filled Styrene–Butadiene Rubber: The Effect of Resins

Resins are important for enhancing both the processability and performance of rubber. Their efficient utilization requires knowledge about their influence on the dynamic glass transition and their miscibility behavior in the specific rubber compound. The resins investigated, poly-(α-methylstyrene) (AMS) and indene-coumarone (IC), differ in molecular rigidity but have a similar aromaticity degree and glass transition temperature. Transmission electron microscopy (TEM) investigations show an accumulation of IC around the silanized silica in styrene–butadiene rubber (SBR) at high contents, while AMS does not show this effect. This higher affinity between IC and the silica surface leads to an increased compactness of the filler network, as determined by dynamic mechanical analysis (DMA). The influence of the resin content on the glass transition of the rubber compounds is evaluated in the sense of the Gordon–Taylor equation and suggests a rigid amorphous fraction for the accumulated IC. Broadband dielectric spectroscopy (BDS) and fast differential scanning calorimetry (FDSC) are applied for the characterization of the dielectric and thermal relaxations as well as for the corresponding vitrification kinetics. The cooling rate dependence of the vitrification process is combined with the thermal and dielectric relaxation time by one single Vogel–Fulcher–Tammann–Hesse equation, showing an increased fragility of the rubber containing AMS

Rigidity of plasticizers and their miscibility in silica-filled polybutadiene rubber by broadband dielectric spectroscopy

An efficient use of plasticizers in rubber compounds requires an understanding of their miscibility behavior. Besides the chemical properties of both rubber and plasticizer, the rigidity of the plasticizer plays an important role for their miscibility. The miscibility is investigated here using the glass transition measured by differential scanning calorimetry and broadband dielectric spectroscopy (BDS). Additionally, the interfacial relaxation and phase separation measured by BDS are confirmed by transmission electron microscopy. While the flexible plasticizer, poly-(α-methylstyrene), stays miscible in a silica-filled polybutadiene rubber compound, the more rigid plasticizer, indene-coumarone (IC), shows a phase separation at high concentrations. The phase-separated IC tends to accumulate at the silica surface

Kinetics of the glass transition of styrene-butadiene-rubber : Dielectric spectroscopy and fast differential scanning calorimetry

The glass transition is relevant for performance definition in rubber products. For extrapolation to high-frequency behavior, time–temperature superposition is usually assumed, although most complex rubber compounds might be outside of its area of validity. Fast differential scanning calorimetry (FDSC) with cooling rates up to 1500 K/s and broadband dielectric spectroscopy (BDS) with frequencies up to 20 MHz are applied here to directly access both kinetics and dynamics of glass formation in a wide frequency range. For the first-time, the relation between the thermal vitrification and the dielectric relaxation is studied on vulcanized styrene-butadiene rubber, showing that both cooling rate and frequency dependence of its glass transition can be described by one single Vogel-Fulcher-Tammann-Hesse equation. The results indicate the validity of the Frenkel-Kobeko-Reiner equation. Another focus is the sample preparation of vulcanized elastomers for FDSC and BDS as well as the temperature calibration below 0°C. © 2020 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC

Kinetics of the Glass Transition of Silica-Filled Styrene–Butadiene Rubber: The Effect of Resins

Resins are important for enhancing both the processability and performance of rubber. Their efficient utilization requires knowledge about their influence on the dynamic glass transition and their miscibility behavior in the specific rubber compound. The resins investigated, poly-(α-methylstyrene) (AMS) and indene-coumarone (IC), differ in molecular rigidity but have a similar aromaticity degree and glass transition temperature. Transmission electron microscopy (TEM) investigations show an accumulation of IC around the silanized silica in styrene–butadiene rubber (SBR) at high contents, while AMS does not show this effect. This higher affinity between IC and the silica surface leads to an increased compactness of the filler network, as determined by dynamic mechanical analysis (DMA). The influence of the resin content on the glass transition of the rubber compounds is evaluated in the sense of the Gordon–Taylor equation and suggests a rigid amorphous fraction for the accumulated IC. Broadband dielectric spectroscopy (BDS) and fast differential scanning calorimetry (FDSC) are applied for the characterization of the dielectric and thermal relaxations as well as for the corresponding vitrification kinetics. The cooling rate dependence of the vitrification process is combined with the thermal and dielectric relaxation time by one single Vogel–Fulcher–Tammann–Hesse equation, showing an increased fragility of the rubber containing AMS

Piezoelectric shear rheometry:Further developments in experimental implementation and data extraction

The Piezo-electric Shear Gauge (PSG) [Christensen & Olsen, Rev. Sci. Instrum. 66, 5019, 1995] is a rheometric technique developed to measure the complex shear modulus of viscous liquids near their glass transition temperature. We report recent advances to the PSG technique: 1) The data extraction procedure is optimized which extends the upper limit of the frequency range of the method to between 50 and 70 kHz. 2) The measuring cell is simplified to use only one piezo-electric ceramic disc instead of three. We present an implementation of this design intended for liquid samples. Data obtained with this design revealed that a soft extra spacer is necessary to allow for thermal contraction of the sample in the axial direction. Model calculations show that flow in the radial direction is hindered by the confined geometry of the cell when the liquid becomes viscous upon cooling. The method is especially well-suited for -- but not limited to -- glassy materials.Comment: 21 pages, 15 figures. Final revision before publicatio