Dynamic Mechanical Analysis, composites

Pandey et al. have used ultrasonic velocity measurement to study compatibility of EPDM and acrylonitrile-butadiene rubber (NBR) blends at various blend ratios and in the presence of compa-tibilizers, namely chloro-sulfonated polyethylene (CSM) and chlorinated polyethylene (CM) [22]. They used an ultrasonic interferometer to measure sound velocity in solutions of the mbbers and then-blends. A plot of ultrasonic velocity versus composition of the blends is given in Eigure 11.1. Whereas the solution of the neat blends exhibits a wavy curve (with rise and fall), the curves for blends with compatibihzers (CSM and CM) are hnear. They resemble the curves for free energy change versus composition, where sinusoidal curves in the middle represent immiscibility and upper and lower curves stand for miscibihty. Similar curves are obtained for solutions containing 2 and 5 wt% of the blends. These results were confirmed by measurements with atomic force microscopy (AEM) and dynamic mechanical analysis as shown in Eigures 11.2 and 11.3. Substantial earher work on binary and ternary blends, particularly using EPDM and nitrile mbber, has been reported. 

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. 

Figure 1. Dynamic mechanical analysis of T300/934 composite. (Reproduced from reference 7.)...

Shown in Fig. 2 is the dynamic mechanical analysis of a representative copolymer series containing, in this case, varying aryl ether phenylquinoxaline compositions [44]. Two transitions were observed indicative of a microphase-sepa-rated morphology. For this copolymer series, the first transition was observed at... 

Thomason, J.L. (1990). Investigation of composite interphase using dynamic mechanical analysis artifacts and reality. Polym. Composites 11. 105-113. 

A collection of polyolefin foams with closed cell structure and different chemical compositions and densities was studied by using SEM, DSC and dynamic mechanical analysis. Deformation mechanisms were also studied. 26 refs. 

The complex sorption behavior of the water in amine-epoxy thermosets is discussed and related to depression of the mechanical properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental vapor and liquid sorption tests, differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis. In particular, two different types of epoxy materials have been chosen low-performance systems of diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, and high-performance formulations based on aromatic amine-cured tetraglycidyldiamino diphenylmethane (TGDDM) which are commonly used as matrices for carbon fiber composites. 

One of most popular techniques for dynamic mechanical analysis is the torsion pendulum method. In a modification of this method designed to follow curing processes, a torsion bar is manufactured from a braid of fibers impregnated with the composition to be studied this is the so-called torsional braid analysis (TBA) method.61 62,148 The forced harmonic oscillation method has been also used and has proven to be valuable. This method employs various types of rheogoniometers and vibroreometers,1 9,150 which measure the absolute values of the viscoelastic properties of the system under study these properties can be measured at any stage of the process. The use of computers further contributes to improvements in dynamic mechanical analysis methods for rheokinetic measurements. As will be seen below, new possibilities are opened up by applying computer methods to results of dynamic measurements. 

The y transition observed from dynamic mechanical analysis at 1 Hz, is centred around - 150 °C. However, the temperature range available experimentally does not permit observation of the whole y relaxation. The temperature position is independent of the chemical composition of the xTy -y copolyamides. 

The estimation was made by putting different data together, which included thermal transition data obtained by dynamic mechanical analysis and spectroscopic data quantified mainly by solid-state NMR techniques [43-48], The binary compositions situated in an enclosed area, denoted by connection of dotted lines in the scale list, can be assumed to be a highly compatible state of mixing. a Obtained by hydrolysis treatment of MC/P4VPy... 

In addition, Seferis and Wedgewood have pointed out the many pitfalls that should be avoided when using dynamic mechanical analysis (DMA) to determine thermal properties in epoxy systems [134]. However, Sanz, et al. have investigated Tg of epoxy systems via DMA for a myriad of epoxy compositions and compiled large amounts of reasonable data using this technique [ 135]. Zukas has done the same using torsional braid analysis (TBA) on many epoxy systems and produced similar conclusions to Sanz [129]. 

Fig. 15. Dynamic mechanical analysis of 100 minutes-aged Thornel 300/Fiberite 934 composite showing the loss tangent and the dynamic storage modulus...

Giraldo et al. (38) reported polyamide 6 nanocomposites in which the crystallization temperature of the polymer was observed to increase with the addition of 2 wt% CNTs. The temperature was 185°C for the pure polyamide which subsequently increased to 190 °C. The authors suggested that the nanotubes might serve as the nucleation sites for the polymer crystals to grow which was also confirmed by the reduction of the chain mobility by dynamic mechanical analysis. The thermal stability of the composites was reported to enhance after the incorporation of nanotubes. 

The main experimental methodology used is to directly characterize the tensile properties of CNTs/polymer composites by conventional pull tests (e.g. with Instron tensile testers). Similarly, dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA) were also applied to investigate the tensile strength and tensile modulus. With these tensile tests, the ultimate tensile strength, tensile modulus and elongation to break of composites can be determined from the tensile strain-stress curve. 

In conclusion, it has been shown that the predicted order of miscibility in composite latex particle systems is not necessarily bourne out when the extent of miscibility is guaged by dynamic mechanical analysis, and, very recently, by the same authors using solid-state NMR spectroscopy. Control over particle morphology, and, hence, over damping behaviour can be exercised by the differences in hydrophilicity between the polymer pair in question, by the degree of crosslinking in the first network and by whether or not the first-formed polymer is above or below its Tg when the second monomer is polymerised. 

Table III shows the composition of IPNs prepared for dynamic mechanical analysis.

By tuning the relative composition and degree of polymerization (DP) of the two segments, phase-separated microstructures were formed in thin films of the copolymer. Specifically, dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM) observations revealed that, for a small molar ratio of p(MA-POSS)/pBA (DP = 6/481/6), no evidence of microphase separation was evident while a large ratio (1 2 1) revealed strong microphase separation (Fig. 8) [122]. 

While TMA is one of the older and simpler forms of thermal analysis, its importance is in no way diminished by its age. Advances in DSC technology and the appearance of dynamic mechanical analysis (DMA) as a common analytical tool have decreased the use of it for measuring glass transitions, but nothing else allows the measurement of CTE as readily as TMA. In addition, the ability to run standardized material test methods at elevated temperatures easily makes TMA a reasonable alternative to larger mechanical testers. As the electronic, biomedical, and aerospace industries continue to push the operating limits of polymers and their composites, this information will become even more important. During the last 5 years a major renewed interest in dilatometry and volumetric expansion has been seen. Other thermomechanical techniques will also likely be developed or modernized as new problems arise. 

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