A combined study of local relaxation and dynamic fragility is presented in a series of poly methylmethacrylates (PMMA) of varying molecular weight from oligomers to polymers. Rotational relaxation properties are measured using ESR spectroscopy, viscosity fragility and thermal parameters are obtained by shear rheology measurements. Significant variations of fragility with molecular weight are observed, with a decreasing trend from low masses to polymers that reaches a plateau for high polymers. The results also suggest a positive correlation between kinetic fragility and the change ΔCp (Tg) with mass. Changes in the rotational relaxation properties (dynamic crossovers) are observed as a function of temperature at Tc. A step-like behavior appears in the dependence of crossover temperatures on the the masses, separating oligomers and unentangled PMMA polymers. Decoupling phenomena and violations of the Debye Stokes Einstein relation characterize the rotational dynamics of the PMMA series, evidencing that chain length exerts a strong influence on local segmental relaxation properties. Different dynamics regimes and coupling degrees of the local relaxation to the viscous flow will be discussed in the present work as well as step-like features with mass. The latter is a recurrent result and discriminate in the rotational relaxation and in its dynamic parameters the typical molecular-like behavior followed by the oligomers from a more specific polymeric response. Particular attention has been devoted to the onset of the Rouse dynamics and to the polymer regime of PMMAs. The scaling law of the relaxation times with the mass are considered in the different temperature regions, above and below the dynamic crossover at Tc. It is found that in the high temperature region the results are in agreement with the forecasts of the Rouse theory. Below Tc, the scaling law exponent is nicely accounted for by the Guenza theory of the cooperative intra-chain dynamics of polymers (1). (1) M.G.Guenza, Phys. Rev. Lett. (2002) 88, 025901
Influence of chain length on relaxation and fragility: a study from oligomers to polymers of poly(methylmethacrylate).
Laura Andreozzi
Primo
;Marco Giordano;
2017-01-01
Abstract
A combined study of local relaxation and dynamic fragility is presented in a series of poly methylmethacrylates (PMMA) of varying molecular weight from oligomers to polymers. Rotational relaxation properties are measured using ESR spectroscopy, viscosity fragility and thermal parameters are obtained by shear rheology measurements. Significant variations of fragility with molecular weight are observed, with a decreasing trend from low masses to polymers that reaches a plateau for high polymers. The results also suggest a positive correlation between kinetic fragility and the change ΔCp (Tg) with mass. Changes in the rotational relaxation properties (dynamic crossovers) are observed as a function of temperature at Tc. A step-like behavior appears in the dependence of crossover temperatures on the the masses, separating oligomers and unentangled PMMA polymers. Decoupling phenomena and violations of the Debye Stokes Einstein relation characterize the rotational dynamics of the PMMA series, evidencing that chain length exerts a strong influence on local segmental relaxation properties. Different dynamics regimes and coupling degrees of the local relaxation to the viscous flow will be discussed in the present work as well as step-like features with mass. The latter is a recurrent result and discriminate in the rotational relaxation and in its dynamic parameters the typical molecular-like behavior followed by the oligomers from a more specific polymeric response. Particular attention has been devoted to the onset of the Rouse dynamics and to the polymer regime of PMMAs. The scaling law of the relaxation times with the mass are considered in the different temperature regions, above and below the dynamic crossover at Tc. It is found that in the high temperature region the results are in agreement with the forecasts of the Rouse theory. Below Tc, the scaling law exponent is nicely accounted for by the Guenza theory of the cooperative intra-chain dynamics of polymers (1). (1) M.G.Guenza, Phys. Rev. Lett. (2002) 88, 025901I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
