Polymers dynamic mechanical methods

Sauer, J. A., and A. E. Woodward Transitions in polymers by nuclear magnetic resonance and dynamic mechanical methods. Rev. Mod. Phys. 32, 88-101 (1960). 

The influence of the cyclohexane ring on the glass transition temperature and sub-Tg transitions has been studied by dynamic mechanical methods [10,16,42]. These comparative studies have focused on the influence of the cyclohexyl substituent on observed thermal transitions. A systematic study of cycloalkyl substituents on alternating carbons of a polyethylene backbone was performed to study the influence of these substituents on the dynamic mechanical spectra of the polymers [42], These workers also prepared materials in which the cycloalkyl substituents were spaced away from the polymer backbone by successively longer methylene chains, testing the influence of ring proximity... 

Based on this, it is no surprise that dynamic mechanical methods may be employed to quantify the effects of plasticizers on the glass transition temperature and the compatibility of such plasticizers with the polymeric components. Such agents are common components of polymeric systems and are included to reduce the glass transition temperature of the polymer, frequently to temperatures below... 

Dynamic mechanical methods may be also used to characterize time-dependent changes in the elastic modulus during polymer curing and cross-linking, from which... 

Schaefer et al. (19) studied the interphase microstructure of ternary polymer composites consisting of polypropylene, ethylene-propylene-diene-terpolymer (EPDM), and different types of inorganic fillers (e.g., kaolin clay and barium sulfate). They used extraction and dynamic mechanical methods to relate the thickness of absorbed polymer coatings on filler particles to mechanical properties. The extraction of composite samples with xylene solvent for prolonged periods of time indicated that the bound polymer around filler particles increased from 3 to 12 nm thick between kaolin to barium sulfate filler types. Solid-state Nuclear Magnetic Resonance (NMR) analyses of the bound polymer layers indicated that EPDM was the main constituent adsorbed to the filler particles. Without doubt, the existence of an interphase microstructure was shown to exist and have a rather sizable thickness. They proceeded to use this interphase model to fit a modified van der Poel equation to compute the storage modulus G (T) and loss modulus G"(T) properties. 

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

To compare NMR and infrared spectroscopy as tools for studying polymers, it is first necessary to distinguish between two types of NMR spectroscopy. These are broad-band and high-resolution spectroscopy. Broad-band NMR spectroscopy can be used for solids and, therefore, polymeric materials are studied quite extensively by this technique. In contrast to high-resolution spectroscopy in the broad-band technique, spin-spin coupling or small chemical shifts are not measured. Instead, the parameters measured are line widths and line positions. Line widths can be related to amorphous and crystalline regions in a polymer thus, broad-band spectroscopy can be correlated with other physical measurements of crystallinity of polymers. It is possible to study crystallinity of polymers by infrared spectroscopy however, the methods are limited. It is probably better to compare broad-band NMR measurements with other physical measurements on polymers, such as dynamic-mechanical methods. 

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

Dynamic mechanical techniques for studying polymers are described in detail in Chapter 7. For the moment we will restrict ourselves to a simple outline of the method of DMTA as it is applied to the determination of Tg. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

All the macroscopic properties of polymers depend on a number of different factors prominent among them are the chemical structures as well as the arrangement of the macromolecules in a dense packing [1-6]. The relationships between the microscopic details and the macroscopic properties are the topics of interest here. In principle, computer simulation is a universal tool for deriving the macroscopic properties of materials from the microscopic input [7-14]. Starting from the chemical structure, quantum mechanical methods and spectroscopic information yield effective potentials that are used in Monte Carlo (MC) and molecular dynamics (MD) simulations in order to study the structure and dynamics of these materials on the relevant length scales and time scales, and to characterize the resulting thermal and mechanical proper-... 

Dynamic Mechanical Properties. The dynamic mechanical properties of branched and linear polyethylene have been studied in detail and molecular interpretation for various transitions have already been given, although not necessarily agreed upon in terras of molecular origin.(52-56) Transitions for conventional LDPE (prepared by free radical methods) when measured at low frequencies, are located around +70°C, -20°C and -120°C and are assigned to o, 5, and y transitions respectively. (53) Recently Tanaka et al. have reported the dynamic mechanical properties for a sample of HB which was also prepared by anionic polymerization, but contrary to our system the hydrogenation of the polybutadiene was carried out by a coordinate type catalyst.(12) The transitions reported for such a polymer at 35 Hz are very similar to those of LDPE.(12)... 

One tool for working toward this objective is molecular mechanics. In this approach, the bonds in a molecule are treated as classical objects, with continuous interaction potentials (sometimes called force fields) that can be developed empirically or calculated by quantum theory. This is a powerful method that allows the application of predictive theory to much larger systems if sufficiently accurate and robust force fields can be developed. Predicting the structures of proteins and polymers is an important objective, but at present this often requires prohibitively large calculations. Molecular mechanics with classical interaction potentials has been the principal tool in the development of molecular models of polymer dynamics. The ability to model isolated polymer molecules (in dilute solution) is well developed, but fundamental molecular mechanics models of dense systems of entangled polymers remains an important goal. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

An associated technique which links thermal properties with mechanical ones is dynamic mechanical analysis (DMA). In this, a bar of the sample is typically fixed into a frame by clamping at both ends. It is then oscillated by means of a ceramic shaft applied at the centre. The resonant frequency and the mechanical damping exhibited by the sample are sensitive measurements of the mechanical properties of a polymer which can be made over a wide range of temperatures. The effects of compositional changes and methods of preparation can be directly assessed. DMA is assuming a position of major importance in the study of the physico-chemical properties of polymers and composites. 

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