Secondary Glass Transition

A secondary glass transition is, in general, important for the impact strength of a polymer it creates the possibility to dissipate energy in situations of shock loading, so that the polymer is less brittle. 

Consequently, for low-temperature transitions with a low energy of activation, such as secondary glass transitions, the transition temperature is stronger time dependent than, e.g. the main glass-rubber transition. 

Figure 3.5. Effect of the nature of the side group on the secondary glass transition (J. Heijboer, Mechanical properties of glassy polymers containing saturated rings, dissertation RU-Leiden, Waltman, Delft, 1972).

We studied the effect of high surface area carbon black and fused silica fillers on the large, secondary glass transition of non-crystallizable SBR (31.6% styrene) copolymer which is associated with small main-chain motions of frans-polybutadiene units (25). Figure 4 shows plots... 

Secondary glass transition temperature for an amorphous polymer. 

Strain-rate Dependence. In the dispersion region of primary or secondary glass transitions, there is a viscoelastic strain-rate dependence of mechanical parameters its magnitude is related to the relaxation strength. Viscoelastic relaxation processes are no longer effective, at least below 30 K. 

Plots of loss modulus or tan 5 vs temperature for polymers give peaks at energy absorbing transitions, such as the glass transition and low temperature secondary transitions (Fig. 20). Such plots are useful for characterizing polymers and products made from them. 

As is commonly the case with crystalline polymers the glass transition temperature is of only secondary significance with the aliphatic polyamide homopolymers. There is even considerable uncertainty as to the numerical values. Rigorously dried polymers appear to have TgS of about 50°C, these figures dropping towards 0°C as water is absorbed. At room temperature nylon 66 containing the usual amounts of absorbed water appears to be slightly above the T ... 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

Most polymers show small secondary glas.s transitions below the main glass transition (3..37,71 -76). These secondary transitions can be important in determining such properties as toughness and impact strength. These transitions are discussed in more detail in later chapters. 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

Chapter 4 deals with the local dynamics of polymer melts and the glass transition. NSE results on the self- and the pair correlation function relating to the primary and secondary relaxation will be discussed. We will show that the macroscopic flow manifests itself on the nearest neighbour scale and relate the secondary relaxations to intrachain dynamics. The question of the spatial heterogeneity of the a-process will be another important issue. NSE observations demonstrate a subhnear diffusion regime underlying the atomic motions during the structural a-relaxation. 

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