Stress relaxation mode, dynamic mechanical

There are three fundamental test methods for characterization of the viscoelastic behavior of polymers creep, stress relaxation, and dynamic mechanical analysis. Although the primary focus for this chapter is DMA, it is useful first to discuss the fundamentals of creep and stress relaxation, not only because they are conceptually simpler but because most DMA instruments also are capable of operating in either a creep or stress relaxation mode. All three of the methods are related, and numerical techniques are available for calculating creep and stress relaxation data from dynamic mechanical data (Ferry 1980). 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Dynamic Mechanical Analysis and Stress Relaxation Behavior. Samples were compression molded into bars of the dimensions 38.xl2.5x0.78 0.007 mm and 65.x9.7xl.7 0.007 mm in a Carver laboratory hot press model C. A TA Instruments 983 DMA, which was operated in the fixed frequency mode, was used to characterize the storage and loss moduli as a function of temperature. Samples were scanned at fi-equencies from 0.05 to 10.0 Hz over a temperature range from -150 C to above the glass transition temperature. The displacement was 0.4 - 0.6 mm. Stress relaxation curves were determined for the same size samples at a constant strain. The sample was displaced for 10.0 minutes and then allowed to recover for 10.0 minutes. The stress data were taken in five degree increments. A microprocessor controlled Liquid Nitrogen Cooling Accessory (LNCA) was used for sub-ambient operations. 

Dynamic mechanical analysis (DMA) is a sensitive method for glass transition temperature measurement, for detection of side-chain and main-chain motions, and for local mode relaxation measurements. Loeal mode relaxation can not be measured by DSC. DMA can give information about the crosslinking process of modified phenolic prepolymer [218] and about the erosslinked material [132]. During DMA measurements, sinusoidally varying stress of frequency is applied to the sample. Frequency and the stress are connected by equation 57, where is the maximum stress amplitude and is the phase angle at which the stress proceeds the strain. 

EXTAR 6000 Dynamic Mechanical Spectrometer This instrument applies various deformations, such as bending, tension, compression, and shear, to a solid sample and operates in the oscillatory mode as well as the static mode for stress relaxation and creep. For dynamic measurements, a new synthetic oscillation mode has been added to the existing high-precision sine wave oscillation mode. The synthetic oscillation mode can measure multiple frequencies at an extremely fast rate, which allows the instrument to measure samples with extremely rapid elastic modulus transformations. Measurements from -150 °C are fully automatic using the automatic gas cooling unit. 

Dynamic mechanical analysis (DMA) was performed to determine the influence of the polymer constitution on tensile modulus and mechanical relaxation behavior. For this purpose, a Perkin Elmer DMA-7 was run in tensile mode at an oscillation frequency of 1 Hz with a static stress level of 5 x lO Pa and a superposed oscillatory stress of 4 x 10 Pa. With this stress controlled instrument, the strain and phase difference between stress and strain are the measured outputs. Typically, the resulting strain levels ranged from 0.05% to 0.2% when the sample dimensions were 8 mm x 2 mm x 0.1 mm. A gaseous helium purge and a heating rate of 3°C min" were employed. The temperature scale was calibrated with indium, and the force and compliance calibrations were performed according to conventional methods. 

At the University of Wisconsin since 19 6, studies of viscoelasticity have evolved from concentrated polymer solutions to undiluted amorphous polymers, dilute solutions, lightly cross-linked rubbers, glassy polymers, blends of different molecular weights, copolymers, cross-linked rubbers with controlled network structures, and so forth. It became evident that each type of system required a different approach. Moreover, in amorphous polymers, the terminal, plateau, and transition zones had to be described separately. Both dynamic (sinusoidal) and transient measurements such as creep and stress relaxation have been utilized. The inderlying theme of this work is the relation of macromolecTilar dynamics—modes of motion of polymer molecules— to mechanical and other physical properties. 

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