G. Di Marco

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below T g , the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

Size effect of the alkali cation on the structure and the molecular mobility of solid polymeric electrolytes

Polymeric electrolytes of the poly(ethylene) oxide (PEO) potassium thiocyanate system have been studied by analyzing the temperature dependence of the low‐frequency (0.3–30 Hz) mechanical characteristics. It is found that the local and cooperative molecular motions characterizing the host polymer are strongly influenced by the addition of the salt, as a result of strong interactions between the polymeric chains and the alkaline salt molecules, which ensure the formation of homogeneous complexes. In the low‐concentration region (molar fraction X≤0.08) a relevant enhancement of the noncrystallinity and a considerable shift of the glass‐to‐rubber transition toward higher temperatures characterize the blends. Further salt addition stiffens the material, this peculiarity being connected to the gradual formation of a crystalline complex with a fixed stoichiometry. It is believed that the observed inversion in the structural trend arises from an increase of the interchain interactions via the polarizing effects ...

Anelastic relaxations and molecular motions in polymeric electrolytes

A study of the frequency (0.1–30 Hz) and temperature (120–373 K) behavior of the internal friction and of the dynamic modulus was carried out in polyethylene oxide (PEO)‐Ba(SCN)2 polymeric electrolytes. The mechanical spectra reveal the presence of the following anelastic relaxations: (a) the γ process due to localized movements within the main chain, (b) a process due to relaxing particles introduced by salt, and (c) the αa process associated to the glass‐rubber like transition. Both the γ‐ and αa ‐loss peaks are shifted to higher temperatures by the addition of salt. A careful analysis of the observed peculiarities permits us to gain an insight on the microscopic nature of the relaxation mechanisms and to associate the γ relaxation with localized segmental motion within the amorphous phase. In pure PEO and in the complex with the lowest salt content, the loss due to the melting transition of PEO crystals is also observed. The cited loss is lacking in the complexes with a salt molar fraction X>0.05, whic...

Low-temperature excess specific heat and fragility in polymers: Crystallinity dependence

A study of the mechanical characteristics and the low-temperature specific heat has been performed on a class of polymers whose crystallinity degree varies in a wide range, from a wholly amorphous to a highly crystalline structure. Typical features of the amorphous phase have been analyzed in order to shed further light on their correlations: the deviation from the exponentiality and the “fragility” obtained by modeling the mechanical αa-relaxation and the excess specific heat over the predictions of the Debye theory. It has been found that increasing crystallinity enhances the nonexponentiality of the αa-relaxation, and decreases both the fragility and the excess specific heat. This behavior is intrinsic to semicrystalline polymers. Consequently, the recently suggested correlation between the degree of fragility and the additional low-energy vibrations causing the low-temperature excess specific heat in glasses fails in semicrystalline polymers.

Structural relaxation processes in poly(ethylene glycol) methacrylate macromonomers

The investigation of the structural relaxation properties of poly(ethylene glycol) methacrylates are of practical interest, due to their ability to work as the inert backbone to which poly(ethylene oxide) oligomers can be attached to obtain highly amorphous polymeric matrices. Two poly(ethylene glycol) methacrylate, PEGMA, macromonomers, with different side chain lengths, are investigated by the dielectric technique, in the frequency range 0.3-300 kHz, and by Brillouin scattering. The analysis of the Brillouin spectra gives evidence for the existence of a relaxation process in both the systems. Furthermore, a comparison of the normalized absorption data with their corresponding classical values (deduced from the shear viscosity data) suggests that what the Brillouin scattering experiment detects is a dissipative relaxational process in which the shear viscosity plays the main role. The comparison of the Brillouin scattering results with the dielectric data shows that what we are observing, in both the systems, is a single relaxation process. The temperature dependences of the relaxation times, observed on more than 10 decades, fail to follow simple Arrhenian behaviours. Both the systems can be interpreted as intermediate between strong and fragile liquids, following the Angell classification, and appear characterized by the existence of a wide variety of local structural environments, triggered by some relaxation process. Such a situation is more clearly evidenced by the macromonomer with longer chain. From the whole body of experimental data, it can be deduced that shorter-side-chain PEGMA macromonomers are better candidates for the formation of highly amorphous comb-branched polymers.

Thermodynamic analysis of two‐dimensional crystal growth in nucleated isotactic polypropylene

A thermodynamic model is used to describe the two‐dimensional crystallization kinetics (as observed by means of a calorimeter) of polypropylene nucleated with small amounts of indigo. The customary expression of the free enthalpy associated to secondary nucleation is initially used to analyze the experimental data. A significant dependence of the apparent surface tension (either basal or lateral) of the nuclei on the concentration of indigo is pointed out. To account for this effect, a phenomenological correction to the free enthalpy of nucleation is considered afterwards, which is connected with an entropy contribution arising from the formation of the crystal–melt interface. A more detailed data analysis is then carried out in light of this model.

Fragility in amorphous blends of linear polymers

By analysing the specific heats and the mechanical characteristics of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous single-state blends in the glass transition region, it has been found that increasing PVDF content causes a small reduction of the fragility of these systems. The degree of fragility has been evaluated from ΔCp /Cp,l, the normalized change of the heat capacity at Tg, and by modelling the mechanical αa-relaxation. The observed behaviour has been ascribed to a more coherent structure of the blends due to strong specific interactions between PMMA and PVDF polymeric chains.

Anharmonicity and fragility in semi-interpenetrating polymer networks

Two series of semi-interpenetrating polymer networks (semi-IPN) based on the same linear polyurethane and two different heterocyclic polymer networks were characterized in terms of the complex dynamic modulus and the mechanical loss tangent, measured between 150 and 500 K at a frequency varying in the range between 0.3 and 30 Hz. The fragility and the deviations from exponentiality of the dynamics of the segmental motion of semi-IPNs in the glass transition interval have been obtained by modelling the mechanical a -relaxation. It was observed that the incorporation of linear polyurethane softens the heterocyclic polymer networks, lowers their resistance to the temperature-induced structural degradation, and increases the complexity of the long-range segmental dynamics. There has also been found a correlation between the fragility and the anharmonicity of the system, the most fragile polymer being characterized by the largest degree of anharmonicity. By extending the analysis to linear polymers, it was established that a growing anharmonicity is predictive of an increasing fragility.

Thermodynamic analysis of spherulitic growth in semicrystalline poly(ethylene oxide)

A thermodynamic model for the description of spherulitic growth in (semi‐) crystalline polymers is presented. Its validity is restricted to the free growth stage of crystallization. The relevance of the role played by surface tension in the early stage of the process is pointed out. With the due approximations the predicted growth rate is found to be consistent with the well‐known Avrami model. As an application, the melting temperatures Tm of poly(ethylene oxide) samples of different molecular weights are derived by fitting the appropriate crystallization thermograms carried out in isothermal conditions. A linear dependence of T−1m as a function of the inverse polimerization degree is found, which is consistent with a well‐known result of the lattice theory.