Light-Scattering and TEM Analyses of Virtual Upper Critical Solution Temperature Behavior in PCL/SAN Blends

Abstract

It has been known that blends of poly(6-caprolactone) (PCL) and poly(styrene-ron-acrylonitrile) (SAN) show a miscibility window and, at the edges, lower critical solution temperature (LCST) behavior occurs. The presence of a virtual upper critical solution temperature (UCST) of PCL/SAN blends was found at lower temperatures as follows. Time-resolved light scattering under Vv and Hv optical alignments was used in order to distinguish two competitive processes-crystallization and liquid-liquid phase separation of blends quenched below the melting point. Three types of measurements were carried out in order to obtain the information: (1) kinetics of phase separation vs crystallization by using the invariant, (2) the most probable wavenumber of concentration fluctuation qm having the highest growth rate, (3) the apparent diffusion coefficient. The positions of the virtual UCST obtained by different methods were in good agreement with each other. In total, the LCST's and UCST's form a closed miscibility area for PCL/SAN blends in the temperature-copolymer composition plane. This behavior is interpreted in terms of an equation-of-state theory. TEM micrographs were taken in order to confirm the difference in morphology between the blends quenched below and above the virtual UCST. The photographs showed the familiar lamellar crystal-amorphous structure for a system crystallized above the UCST and a morphology segregated to PCL-rich and SAN-rich phases for the same blend crystallized below the UCST.

Characterization of Phase Behavior in Polymer Blends by Light Scattering

“…The D app for phase dissolution was found to have values between 3 and 11 Â 10 À17 m 2 s À1 over the temperature range of 50 to 115 C. Closer to LCST, the values decrease; they are also low in the range of 50-65 C. The diffusion should be slower at a lower temperature due to a lower mobility. In the present case, the presence of a virtual UCST of about 40 C, as was found earlier for similar systems [29], should cause the decrease in D app . Comparable values of D app for phase separation (but of opposite sign, i.e., negative values) were found in the temperature range of 125 to 150 C. By further increasing the temperature the SD would proceed much more quickly, an example being value of D app at 180 C which was more than 10-fold higher than the fastest phase dissolution.…”

“…It is reasonable that the curve has a minimum because, according to thermodynamic calculations, the miscibility window might be a "closed" area under some circumstances [34]. An experimentally obtained LCST border and a calculated miscibility loop are also shown in Figure 5.46 [29].…”

Isothermal Crystallization of Poly(ε‐caprolactone) in Blend with Poly(styrene‐co‐acrylonitrile): Influence of Phase Separation Process

“…[14] Numerous studies have been carried out to modify PCL by blending or co-polymerization with other polymers. [15][16][17][18][19][20][21][22][23][24][25] In the case of PCL/SAN blend, the miscibility of this system and the influence of acrylonitrile (AN) content in the copolymer upon blend miscibility have been studied by Chiu et al [20] They found that SAN and PCL are miscible when the AN content in SAN ranges from 8 to 28 wt.-%. Savoboda et al [21,22] studied the morphology of PCL/SAN blend via competition with spinodal decomposition.…”

Biomedical Applications Polymer Blends

“…Poly(ether sulfone) is a chemically inert and highly thermostable polymer showing a hydrophobic surface which leads to high fibrinogen adsorption and subsequent thromboembolization [111]. This effect is generally avoided by non-desirable heparin doses.…”