Molecular-Weight Dependence of Enthalpy Relaxation of PMMA

Abstract

The effects of the molecular weight on the physical aging of polymers were investigated by means of differential scanning calorimetry (DSC) experiments in five PMMA samples with mass values across the entanglement mass M e. The Tool−Narayanaswamy−Moynihan (TNM) model and a recent configurational entropy approach were systematically compared with the experiments. For the samples with the lowest molecular weights, the two approaches exhibited similar agreement with experiments; however, in the systems with higher molecular weights, the TNM model met major difficulties, and a clear improvement was obtained with the other model. The additional parameter introduced by the entropic model showed a strong molecular-weight dependence, with a sharp increase at the entanglements mass of PMMA. These results evidence the role of the chain entanglements on the physical aging of polymers. Finally, the influence of the molecular weight on the fragility was investigated and related to the recent literature debates on this topic.

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

“…The inset to fig.4 shows the change of the heat capacity at the glass transition ΔC P , versus M w , including some literature data. 57,61,62,63 For low molecular weights (<10 3 g/mol), the heat capacity jump at the glass transition is constant within the limit experimental error, ΔC P = 0.35 ± 0. …”

“…, equations. The inset shows the change with molecular weight of the heat capacity at the glass transition from measurements herein and literature data, 57,61,62,63 ; the dotted line is to guide the eyes. Table 3.…”

The Effect of Annealing on the Stability of Amorphous Solids: Chemical Stability of Freeze-Dried Moxalactam

“…9,10 Below the glass transition temperature (T g ), vitrification of the melt occurs thus forming the ''glassy'' state, 9,11 accompanied by an increase in viscosity to very high values (typically >10 12 Pa.s). [11][12][13] The material is now regarded as a super-cooled liquid-a metastable state-that retains much of the disorder in the previous melt.…”

Dynamics, fragility, and glass transition of low-molecular-weight linear homopolymers

“…In Table 2, the mass dependence of D and m can be observed for PEA samples and literature PMMA series [11]. It is interesting to observe how these two series fulfil Eq.…”

“…In this sense, the case of PMMA is emblematic. In fact, even if most authors agree that for PMMA m(M ) increases with the mass, very different (often contradicting) values of m(M ) are found in the literature [1,11,17,22, and references therein]. Source of further ambiguity comes from the comparison of m indexes calculated for different relaxation times, because of their possible different temperature dependence [8,23].…”