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

Abstract

SynopsisLiterature data for the dynamic viscoelastic properties of binary blends of nearly monodisperse polybutadienes, polystyrenes, and poly(methy1 methacry1ate)s was analyzed using logarithmic plots of dynamic storage modulus G' versus loss modulus G", based on a recent theoretical study by Han and Jhon.% It has been found that for binary blends of monodisperse polymers with molecular weights M much greater than the entanglement molecular weight Me, the value of G' in log G'-log G" plots becomes independent of molecular weight, increases sharply as small amounts of a high-molecular-weight component are added to a low-molecular-weight component, and passes through a maximum G,!,,,, at a critical blend composition (C$2)mm, and that G,!,,,, becomes larger and (C$2)m,, becomes smaller as the ratio of component molecular weights increases. However, as the molecular weight distribution of the constituent components becomes broader, the effect of blend composition on G' in log G'-log G" plots becomes less pronounced. This observation has enabled us to explain why log G'-log G" plots of binary blends of commercial polymers, namely, blends of two low-density polyethylenes, blends of poly( c-caprolactone) and poly(styrene-co-acrylonitrile), and blends of poly(methy1 methacrylate) and poly(viny1idene fluorids), all having broad molecular weight distributions, give rise to values of G' between those of the constituent components. When one of the constituent components has molecular weight smaller than Me, while the other has molecular weight larger, and as small amounts of the high-molecular-weight component are added to the low-molecular-weight component, log G'-log G" plots of binary blends give rise to values of G' larger than those of the constituent components a t low values of G", but approaches the value of G' for the low-molecular-weight component as the value of G" is increased. However, as the amount of the high-molecular-weight component is increased above a certain critical composition, binary blends give rise to values of

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“…In other words, the two normal stress differences of ( 11 Ϫ 22 ) and ( 11 Ϫ 33 ) were almost identical, and the normal stress difference ( 11 Ϫ 33 ) was proportional to the square root of 12 . Thus, their results were consistent with a linear viscoelastic theory such as the Maxwell model in which the plot of ( 11 Ϫ 33 ) versus 12 gives rise to the slope of factor 2 in log-log scale as the Cole-Cole plot of GЈ vs GЉ (44,45). Consequently, ͗n 11 Ϫ n 33 ͘ is proportional to the square root of 12 and n 12 ϰ ͌ |͗n 11 Ϫ n 33 ͘| for a linear viscoelastic material.…”

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“…All the data measured at various temperatures and molecular weights can be described by a single master curve in the log(G 0 ) versus log(G 00 ) plot. [23] Experimental investigations have also shown that the SCB would not affect the master curve. However, any change in chain topology, such as LCBing, and PDI would lead to a significant deviation of data points from the master curve.…”

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