Glassy polymers temperature dependence

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

In this contribution we present results obtained with tetra-ethyleneglycol diacrylate (TEGDA). This compound was chosen since its polymer shows an easily discernible maximum in the mechanical losses as represented by tan 5 or loss modulus E" versus temperature when it is prepared as a thin film on a metallic substrate. When photopolymerized at room temperature it forms a densely crosslinked, glassy polymer, just as required in several applications. Isothermal vitrification implies that the ultimate conversion of the reactive double bonds is restricted by the diffusion-limited character of the polymerization in the final stage of the reaction. Therefore, the ultimate conversion depends strongly on the temperature of the reaction and so does the glass transition. 

Spin-spin relaxation times (T2) in polymer systems range from about 10-5 s for the rigid lattice (glassy polymers) to a value greater than 10-3 s for the rubbery or viscoelastic state. In the temperature region below the glass transition, T2 is temperature independent and not sensitive to the motional processes, because of the static dipolar interactions. The temperature dependence of T2 above Tg and its sensitivity to low-frequency motions, which are strongly affected by the network formation, make spin-spin relaxation studies suitable for polymer network studies. 

A detailed insight into the freezing-in process is given by optical investigations. As described in 2.3.1.4 for nematics, 2.3.2.3 for cholesterics and 2.3.3.3 for smectics, the optical uniaxial character of the polymers in the liquid crystalline state has been proved by birefringence measurements and the state of order was calculated from these measurements. This method also provides information about the glassy state. For conventional l.c s it has been demonstrated, that the temperature dependence of... 

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