The Role of Density and Temperature in the Dynamics of Polymer Blends

Abstract

The relative effect of volume and thermal energy on the local segmental dynamics, as reflected in the ratio of the isochoric, EV, and isobaric, EP, activation enthalpies, is determined for blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and with poly(vinyl methyl ether) (PVME). We find that neat PPO near T g has the lowest value of EV/EP ) 0.25 ( 0.02 reported for any polymer, indicating volume-dominated dynamics. Addition of the lower Tg PS alleviates constraints on local motion, resulting in a weaker volume effect. The opposite situation prevails with PS/PVME blends. PS has a higher T g, and in blends segmental relaxation of the PVME becomes more controlled by volume than for neat PVME. We also show herein that the relaxation times for the PVME/PS blends measured at various T and P superpose when plotted vs Tυ γ , where υ is the specific volume and γ is a material constant. This scaling, which has previously been demonstrated for various neat glass-formers and is reported herein for the first time for a blend, enables E V/EP to be determined at Tg in the absence of actual measurements near Tg.

Defect diffusion and temperature vs. density effects for glass formers

“…(18) that E Ã V =H Ã should increase with molecular weight since the numerator will increase and the denominator will decrease. This behavior is in accord with experimental results for blends of poly(2,6-dimethyl-1,4-phenylene oxide) with polystyrene [28]. Of course, whether the ratio increases or decreases also depends on the behavior of (oT C /oP) T with molecular weight.…”

“…If Dv is negative (as we assume for a fragile liquid), then a pressure increase (or volume decrease) causes mobile single defects (MSDs) to cluster (and rigidity to increase) as the equilibrium in Eq. (28) shifts to the right. Large (negative) volume changes on defect pairing lead to large effects of increased pressure (or decreased volume) on clustering, and give rise to large values of o ln T C /oP.…”

Entropy basis for the thermodynamic scaling of the dynamics ofo-terphenyl

“…Eq. (1) has also been found to be valid for the normal mode in polymers [19,20] and for the component dynamics in polymer blends [21,22]. A breakdown of the scaling can be expected wherever the material itself changes (chemically) with T or P -an example being strongly hydrogen bonded materials such as water and glycols [16].…”

Volume effects on the glass transition dynamics

“…(An exceptional polymer is polyphenylene oxide, which, due to its flexible chain structure in combination with a high T g , has an unusually small E V /E P = 0.25 [18].) These values are for T $ T g ; that is, for relaxation times in the range from 1 to 100 s.…”