Annealing relaxation dynamics

In this contribution, we employ local dielectric spectroscopy (LDS), a method recently used to characterize relaxation dynamics of ultrathin polymer films at nanometer scale [45 7], to study the evolution of irreversibly adsorbed layers in poly(vinyl acetate) films deposited on different solid substrates, i.e. aluminum, gold and silicon, under annealing above the Tg [30, 41, 48]. After describing the setup and sample preparation in Sect. 7.2, we present, in Sect. 7.3, our results on the growth of the layer as a function of the interfacial interaction. The influence of the adsorbed layer on the relaxation dynamics of nano-confined PVAc films and their capacity to absorb moisture in various degrees of relative humidity is studied in details. 

In order to clarify the influence of the adsorbed layer on the average relaxation dynamics of PVAc films the loss-tangent spectra of thick ( 226 nm) and thin ( 23 nm) films supported on the three different substrates were measured for different annealing times. Some representative results are reported in Fig. 7.10. 

The physics of ultrathin films is stiU a hot topic presenting several unresolved issues. In the recent years, it has been definitively shown that the development of irreversibly adsorbed layer affects many physical properties of ultrathin films, and likely can be at the origin of some of the not yet resolved problems. In this Chapter we reported clear evidences on the impact of the irreversibly adsorbed layer on the average relaxation dynamics of ultrathin film. This result, in agreement with several others in literature, can shed light in the observed evolution of relaxation time of ultrathin film with annealing above Tg. 

Among these three polybibenzoates, PTEB has a smectic mesophase stable during several days at any temperature below its isotropization point, although the transformation into a three-dimensional crystal can be attained by annealing at the appropriate temperatures, thus making it possible to analyze the effect of the thermal history on the dynamic mechanical relaxations of PTEB [27]. 

In conclusion, the different thermal histories imposed to PTEB have a minor effect on the /3 and y relaxations, while the a. transition is greatly dependent on the annealing of the samples, being considerably more intense and narrower for the specimen freshly quenched from the melt, which exhibits only a liquid crystalline order. The increase of the storage modulus produced by the aging process confirms the dynamic mechanical results obtained for PDEB [24], a polyester of the same series, as well as the micro-hardness increase [22] (a direct consequence of the modulus rise) with the aging time. 

Fig. 13 a Optical image (top view) of the sample geometry for a thin PS film of 239 nm after 4 hours at 180 °C in a pure nitrogen atmosphere ( 1 mm x 1 mm) b the same sample after 4 hours at 180 °C in air ( 1 mm x 1 mm) c the relaxation time of the dynamic glass transition vs. inverse temperature for a thin PS film of 63 nm after different annealing times at 180 °C in a pure nitrogen atmosphere and in air. Inlet, time dependence of the sample capacity at 180 °C and 0.1 MHz in a pure nitrogen atmosphere and in air... 

The dynamic mechanical behaviour of ultra high modulus polypropylene is shown in Fig. 32. As in LPE, the modulus is temperature dependent, rising to a value of 25 GPa at —140 °C, which is rather more than half (he value of 42 GPa obtained from crystal measurements. Although the a and y relaxations of the isotropic polymer can be seen in the highly drawn material, the -relaxation is undetectable. On annealing, the modulus at high temperatures is markedly reduced, and a P-relaxa-... 

For thin PS films of 63 nm, using the usual fitting procedure [1], the relaxation rate as a function of inverse temperature is extracted (Fig. 34), after different annealing steps in air and in pure nitrogen. While unchanged after 24 hours at 180°C in a nitrogen atmosphere, the dynamic glass transition becomes one decade faster when the sample is annealed in air. This corresponds to a shift to lower temperatures of the maximum position of the alpha relaxation peak (inset, Fig. 34). 

Deformation such as drawing, compression, annealing, strain, creep and stress relaxation of polymers including fibers may produce quite different orientational behavior, the results of which can be examined with solid-state NMR from both the static and dynamic viewpoints. The accurate model produced on the basis of atomic resolution of the local structure and the local dynamics can be built up in order to interpret the mechanical properties of polymers and the deformation mechanisms. 

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