Liquid‐Glassy Polymer Diffusion: Effects of Liquid Molecular Weight and Temperature

Abstract

We examine mechanistic aspects of the diffusion between a series of liquid polystyrenes (PS) and a glassy poly(phenylene oxide) (PPO) matrix, through the use of confocal Raman microspectroscopy. The results show that the diffusion kinetics has Fickean characteristics, similar to those found in liquid‐liquid polymer diffusion. No signatures of the linear regime typical of the case‐II diffusion mechanism were found. Overall, these observations are consistent with the claim that case‐II is unlikely to occur in liquid‐glassy polymer diffusion.magnified image

Liquid‐Glassy Polymer Interphases: Diffusion Kinetics in Conditions of Unlimited Liquid Supply

“…230 kJ Á mol À1 in a temperature interval between 15 and 150 8C below T g . [11,18] The independence of E df with the stage of the diffusion process appears consistent with the fact that PS concentration at the liquid side remains almost steady. The larger values for E df obtained from limited supply experiments (E df ¼ 106.6 and 130.8 kJ Á mol À1 ), correspond to later stages of the diffusion process that occur under lower F PS values at the plasticized layer (or higher local T g ).…”

“…unlimited supply, solid symbols), from which we obtained E df ¼ 93.6 AE 2.1 kJ Á mol À1 , in the range 120-180 8C, independently of the stage of the diffusion process. In Figure 5, we have also included data obtained from diffusion [9,11] experiments under conditions of limited liquid supply, on the same system and on a similar temperature range, taken from ref. [11] (hollow symbols).…”

“…In Figure 5, we have also included data obtained from diffusion [9,11] experiments under conditions of limited liquid supply, on the same system and on a similar temperature range, taken from ref. [11] (hollow symbols). In this setup, E df has shown to be sensitive to the stage of the diffusion process considered; for this reason, E df values were calculated and reported for specific values of PS concentration at the liquid side.…”

“…At this point, we refer to previous work carried out by other research groups with techniques with higher spatial resolution (few nanometers), which have shown that the liquid/glassy polymer interphases are indeed sharp, with a thickness on the order of the radius of gyration of the molecules of the glassy layer. [2,11] Overall, the absence of Fickean tails reflects the low diffusivities one would expect for penetration of large-sized molecules through a pathway of interstitial holes; [3,5] consequently the efficiency of these molecules in decreasing the glassy matrix yield stress will be also low, which in turn penalizes the eventual initiation of control by Case-II.…”

“…The solid symbols correspond to experiments carried out by Tomba et al using Confocal Raman Microspectroscopy (CRM, see below), for a sample held at 140 8C, well below the T g of the glassy matrix (200 8C). [11] The inset shows the depletion in the volume fraction of PS (F PS ) throughout the liquid, as measured during the diffusion process, from its original value, 0.7, to about 0.4. The solid lines illustrate the expected t 1/2 and t scaling relationships characteristic of Fickean and Case-II regimes, the latter of which appears with an upward curvature in the Fickean plot.…”