Abstract
AbstractThe effect of plastic deformation on the molecular dynamics of atactic polystyrene (a‐PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier‐transform infrared spectroscopy (FTIR) and polarized‐light microscopy. Sheets of a‐PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass‐transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α‐ and γ(I)‐relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation‐enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G‐ratio); and (ii) processes III and IV were identified as helix‐inversion processes of T2G2 helices as reported earlier for syndiotactic‐rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized‐light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.
Highlights
One of the most fascinating subjects in condensed matter science is arguably the state of a glass
McKenna suggests that deformation above the yield point in the polymer leads to a polyamorphic phase transition and not to rejuvenation, based on the fact that the yield stress does not reach the same value after thermal rejuvenation and aging as compared to mechanical cycling and aging.[8]
By the results presented above, yet unknown dynamics of cold-rolled atactic polystyrene (a-PS), a polymer widely used in industry and broadly investigated in academia, have been uncovered
Summary
One of the most fascinating subjects in condensed matter science is arguably the state of a glass. A recent investigation on polymer glasses concluded that the thermally and mechanically rejuvenated glasses are in different states, and that mechanical deformation leads to an amorphous-amorphous phase transition.[4] Their results are confirmed by positron-annihilation lifetime spectroscopy experiments on a-PS and polycarbonate (PC) based on the fact that the free volume follows a completely different time-evolution for mechanically rejuvated films compared to the thermally rejuvenated ones This idea has been supported theoretically by other studies.[5,6,7] In his thorough review on the subject of mechanical rejuvenation, McKenna[8] discussed the possible interpretations of the erasure of history of glassy materials with deformation below and above the yield point. Supporting FTIR spectroscopy and polarizedlight microscopy studies connect the molecular dynamics with changes in the molecular structure of a-PS
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