PDMS/PEMS

Since there had not been any measurements of thermal diffusion and Soret coefficients in polymer blends, the first task was the investigation of the Soret effect in the model polymer blend poly(dimethyl siloxane) (PDMS) and poly(ethyl-methyl siloxane) (PEMS). This polymer system has been chosen because of its conveniently located lower miscibility gap with a critical temperature that can easily be adjusted within the experimentally interesting range between room temperature and 100 °C by a suitable choice of the molar masses [81, 82], Furthermore, extensive characterization work has already been done for PDMS/PEMS blends, including the determination of activation energies and Flory-Huggins interaction parameters [7, 8, 83, 84],... 

The measurements near the critical point have been performed with a PDMS/ PEMS blend with molar masses of Afw = 16.4kgmol 1 (PDMS, Mw/Mn= 1.10) and Afw = 22.8 kg mol 1 (PEMS, Afw/Mn =1.11). The corresponding degrees of polymerization are N = 219 and N = 257, respectively. The phase diagram shows a lower miscibility gap with a critical composition of cc = 0.548 (weight fractions... 

Fig. 2 Typical heterodyne diffraction signals measured for a critical PDMS/PEMS mixture for different distances T — Tc to the critical point. All curves have been normalized according to (15). The dashed line indicates the nearly constant initial slope of the concentration signal (an exponential function in the logarithmic plot) caused by the almost constant value of Dj- The inset shows, for comparison, a considerably smaller signal for an off critical PDMS/PEMS mixture (860 and 980g mol-1)...

Fig. 4 Arrhenius plot of the ratio /> /D- j. according to (11) for critical (16.4/22.8) and a number of off-critical PDMS/PEMS blends of various molar masses and concentrations c = 0.5 [97]. The legends give the PDMS and PEMS molar masses in kg mol-1. Also shown is a line corresponding to the activation energy of the viscosity according to [92]...

Experiments have shown that, at least for PDMS/PEMS blends of equal weight fraction, K(T) indeed depends only weakly on temperature and is independent of the molar mass of the constituents [97], Consequently, the different values of the Soret coefficient in the classical mean field regime are almost exclusively caused by the variation of the static structure factor. 

Fig. 6 Left Soret coefficient. S j for a number of PDMS/PEMS blends. The red bullets correspond to the critical blend with a critical temperature of Tc = 38.6 °C. Right Same data as left normalized to mean field static structure factor S(0). The legends give the PDMS and PEMS molar masses in kg mor1 [97]...

Fig. 7 Phase diagram of PDMS/PEMS (16.4/48.1). The cloud points that mark the binodal squares) have been obtained by turbidimetry. Pseudo-spinodal points are as explained in the text. The color encodes the modulus of the Soret coefficient. Figure from [100]. Copyright (2007) by The American Physical Society...

Pattern writing experiments have been performed with an almost symmetric PDMS/PEMS (16.4/15.9) blend having a critical composition cc = 0.48 and a convenient critical temperature Tc = 290.15K. It has been shown in [100] in detail that the parameterization of the transport coefficient determined for the higher PEMS molar mass still yields a good description for this blend too after adjusting the critical concentration and taking (T - Tsp)/T as dimensionless temperature. 

The experiments reported here have been performed with a PDMS/PEMS (16.4/ 48.1) blend. The mixture is a UCST mixture with a critical composition ofcc = 0.61. 

Fig. 16 Forced demixing of an initially homogeneous off-critical PDMS/PEMS blend for c = 0.3 (upper row) and c = 0.9 (lower row)...

Fig. 17 Phase diagram of a PDMS/PEMS (16.4/48.1) blend. The dashed lines are the binodal and the spinodal. The phase contrast micrographs show typical demixing patterns for spinodal decomposition and nucleation and growth in the respective regions. The bullets mark the initial sample positions. See text for details. Figure from [112]. Copyright (2007) by the American Chemical Society...

Due to the negative Soret coefficient of PDMS/PEMS, the composition in the center of the focus evolves towards higher PDMS concentrations and, hence, towards the two phase region. The mixture crosses the binodal after a time... 

In this section it will be demonstrated how spinodal decomposition patterns in the two phase region can locally be manipulated in a controlled way by heating a polymer blend PDMS/PEMS by a focused laser beam. It will also be shown that the essential spatial and temporal phenomena, as observed in the experiments, can only be reproduced in numerical simulations if thermodiffusion (Soret effect) is taken into account in the basic equations. 

The polymer blend PDMS/PEMS with molar masses of Mw = 16.4 and 22.3kgmol 1, respectively, is similar to the one which has previously been used for the investigation of transport properties in the critical regime [81]. A 515nm and 20 mW laser was used for local heating. The blend with a PDMS weight fraction of c = 0.536 is almost critical with a critical temperature of Tc = 37.7°C. A minute amount of an inert dye (quinizarin) was added for optical absorption at... 

To analyze this phenomenon further, 2D numerical simulations of (49) and (50) were performed using a central finite difference approximation of the spatial derivatives and a fourth order Runge-Kutta integration of the resulting ordinary differential equations in time. Details of the simulation technique can be found in [114, 119]. The material parameters of the polymer blend PDMS/PEMS were used and the spatial scale = (K/ b )ll2 and time scale r = 2/D were established from the experimental measurements of the structure factor evolution under a homogeneous temperature quench. 

WAL Walker, T.A., Colina, C.M., Gubbins, K.E., and Spontak, RL, Thermodynamics of poly(dimetltylsiloxane) ly(etltylmethylsiloxane) (PDMS/PEMS) blends in the presence of high-pressure CO2, Macromolecules, 37, 2588,2004. 

While the pressure induced increase of the two-phase boundary seems to be the normal case, there exist a few polymer blends which show an enhanced miscibihty with pressure. Such a behavior was observed in PEP/PDMS (poly(ethylenepropylene)/polydimethylsiloxane) and PDMS/ PEMS (polyethyl-methylsiloxane) blends [42,43]. hi both blends an increase of the enthalpic and entropic terms of the FH parameter with pressure, i.e., to... 

PPMSM-PS PPMSA f S(100MPa) PDMS/PEMS d-PSff VME... 

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