Phase Separation in Polymer Film Blends

Introduction

The study of phase separation in thin films of binary mixtures is important because polymer blends often exhibit more desirable characteristics than individual homopolymers.   Unfortunately, most blend components are also highly incompatible with each other and will demix.  The degree of separation in blends will greatly affect the resulting morphology, which can have adverse affects on the mechanical and electrical properties of the resulting film.  Conversely, a phase separated film could exhibit more desirable characteristics, and being able to tune the resulting morphology with a tailored preparation technique could prove beneficial. Therefore, understanding the mechanisms that affect demixing morphology and to what degree the kinetics and dynamics are influenced are issues of considerable commercial interest.  From a basic science perspective, systematic investigations of these processes is important since a full understanding of blend separation processes such as nucleation and growth, spinodal decomposition and Ostwald ripening are still lacking for polymer thin films.

Why do polymer blends separate?

Polymer blends are driven to phase separate by the systemís desire to minimize free energy.  For a symmetric system of A and B (letting NA=NB, MwA=MwB) the Gibbs free energy of mixing is given by Florry-Huggins theory:

The first two terms are the entropy of mixing and the last term is the heat of mixing (enthalpy) where monomer-monomer interactions are summarized in c, the Flory-Huggins interaction parameter, which is generally a function of temperature (aT-1+bT-2 with a,b constants).   Because polymers are chains of individual monomers, the increase in entropy from mixing the two components together is drastically reduced in polymers due to the decrease of the number of possible monomer configurations.  Therefore, increasing the number of repeat units in the polymer (increasing molecular weight) will decrease the entropic contribution to the free energy, allowing the quadratic enthalpy term to dominate and create two minimums in the Gibbs free energy. The basic Flory-Huggins equation above yields only a qualitative understanding of why polymers tend to phase separate, more detailed theories include concentration gradients and hydrodynamics.  Plots of the Gibbs F.E. for different temperatures are below:

Plotted below the free energy curves are corresponding binodal and spinodal curves.  On the binodal, or coexistance curve, the system is at a local thermodynamic minimum and concentration fluctuations cease.  The spinodal curve defines regions of stability, so if a system is below the spinodal curve the system is unstable and will spontaneously phase separate, a phenomenon known as spinodal decomposition.
    Polymer blend films pose even more intricate questions due to the introduction of two surfaces, the preparation substrate and the vacuum-film interface.  Substrate energy and surface tension now become dominant mechanmisms in the separation process.

Sample Preparation

We have investigated phase separation in polymer blends primarily with a system of 50/50 PS/PMMA.  Films were spun cast from toluene solution onto silicon substrates.  These samples were then annealed in vacuum for various times and then transferred to TEM grids and imaged with Scanning Transmission X-ray Microscopy.  We perform compositional mapping with the STXM data to produce PS and PMMA projected thickness maps and can be summed to form total thickness maps similiar to Atomic Force Microcsopy topographs.

Results

One sequence of samples we investigated are shown below.  Samples were low molecular weight (27k/27k) spun onto silicon and annealed at 180C for the time indicated.  Initial spin casting produces a PMMA continuum with PS domains which, upon annealing (2 minutes) subsequently rearrange into domains much smaller than those originally formed.  To understand the nature of this phenomenon better we prepared an identical as-spun sample (not presented here) and washed it with cyclohexane to remove the PS.  Quantitative compositional analysis with STXM of this sample revealed that 20% of the PS was retained after washing indicating that the PS domains in the as-spun samples were not completely phase separated.    As PS regions become larger, they begin to experience lateral jaggedness of the PS-PMMA interface.  These fluctuations are due to the interfacial tension between phases and occur when the wavelength of capillary waves at the interface become approximately the same size as the domains [W.Zhao, PRLí93]. Complicated polymer-polymer interfaces persist even in the later stages that are explained in terms of the geometric constraints of a thin film and the dependance of polymer viscosity on film thickness.

      50/50 PS/PMMA (27k/27k) spun onto silicon surfaces, annealed in vacuum at 180 C

Viscosity differences

The viscosity in polymer blends can be adjusted by changing the molecular weight..  The following figures are from a 50/50  PS/PMMA system annealed at 165 C with higher molecular weight PS (Mw=90k).  Note that spin casting now forms PMMA domains within a PS matrix due to the toluene solubility differences of the polymers.   These rounded PMMA domains remain late into the separation process due to the increased viscosity slowing its recession to the substrate.  These smaller domains appear to be the material which formed the PMMA spikes seen in the previous system as well.

       50/50 PS/PMMA (90k/27k), spun onto silicon surfaces, annealed in vacuum at 165 C

Time-Temperature Superposition

Below are compositional maps from three distinct series of samples which individually represent qualitatively similiar regimes in the phase separation process.   The differences in film preparation produce vastly different initial morphologies, however the dynamics exhibit striking similiarities.  Initial movement of the phase separated regions is not a simple coalescence into larger, separated domains but instead a "bursting" into smaller regions.  The bursting effect is more pronounced with higher temperature.   Later stages yield intricate PS-PMMA interfacial fluctuations due to resonant capillary wave effects.  In the latest stages, all three systems exhibit coarsening of the domains as the system is minimizing the high energy PS-PMMA interface by becoming more rounded.


All materials here copyright 1999 by Dr. Harald Ade and the Polymer X-ray Spectromicroscopy Research Group.  Reproduction without express permission from Dr. Harald Ade is prohibited (single print-out excepted).