Molecular weight distribution dependence of the viscoelastic properties of linear polymers: the coupling of reptation and tube-renewal effects

Abstract

Considering that the terminal relaxation of every chain in a linear melt is dominated by reptational and constraint release motions, we propose an expression for the relaxation modulus G(t) of entangled polymers as a function of the molecular weight distribution (MWD). The expressions of zero-shear viscosity rj0 and steady-state compliance =/e°d erived from G(t) describe correctly the experimental behavior: 0 scales with average molecular weight as 0 Mw3•4"3•8 for a constant polydispersity P; Je°i ncreases strongly with P and varies widely with the shape of MWD. We also predict a decrease of viscosity with polydispersity; this is a possible explanation of some experimental variations of viscosity, which do not follow the classical power law with Mw. The behavior expected for strictly monodisperse samples shows that it is necessary to consider a relaxation times distribution in the terminal region.

Influence of molecular weight distribution on the linear viscoelastic properties of polymer blends

“…Indeed, the dependence of J:b on blend composition, depicted in Figure 1, has been observed in a number of experimental investigation^.^-^.'. lo, [14][15][16][17][18]21,22 The Rouse theory for a polydisperse binary blend may be expressed asz9 where p is the density, R is the universal gas constant, T is the absolute temperature, and a, and a=+ are the z-th and ( z + 1)th moments of the molecular weight distribution. Equation (7) indicates that J,4, is very sensitive to the higher averages of molecular weight.…”

Polydispersity index from linear viscoelastic data: unimodal and bimodal linear polymer melts

“…where k E is the constant of proportionality and generally lies between 3.3 and 3.7, except for extremely broad blends, for which higher values are observed [13][14][15][16]. is often taken as 3.4, since this is the value usually obtained for most linear polymers investigated.…”

Effect of Polydispersity on the Phase Behavior of Polystyrene (PS)/Poly (Vinyl Methyl Ether) (PVME)

“…These short molecular chains could disentangle rapidly during phase separation and act as an internal plasticizer for other longer chains, which can greatly shorten the relaxation time of the whole blend. [19][20][21] As a result, the time required for molecular chains to move from a nonequilibrium state to the equilibrium state is shortened, and the corresponding increase of cloud point temperature is less.…”

“…[19] Also, the relaxation time of the shortest chains increases with the concentration of long chains. [21] Available research about the effect of polydispersity on the phase behavior of polymer systems is mainly focused on polymer solutions, [15,17,[22][23][24][25] polymer blends, [26][27][28][29] and block copolymers. [30][31][32] Because a clear understanding of the effect of polydispersity on the phase behavior of polymer solutions is essential to investigate the effect of polydispersity on the phase behavior of polymer blends, the polydisperse polymer solution systems have been researched abundantly.…”