Chapter 4 Polymer solutions

Transcription

1 Chapter 4 Polymer solutions 4.1 Introduction Solution: any phase containing more than one component.(gas, liquid or solid) Polymer solution is important: Classical analyses of polymers are conducted on dilute solutions size exclusion chromatography osmometry, viscometry light scattering. Application: adhesives and coatings. Electronic device process: LED, FET, PV, Memeory etc.

2 Solubility: Similar compound solves similar compounds Polar solvent e.g. (H 2 O) ----polar polymer Unpolar solvent ----unpolar polymer Crosslink Crystallinity Solubility Dissolving process: T Concentration Swelling Polymer release in solvent Some polymers take days to be dissolved Necessary thermodynamic condition for mixing two components: G mix = H mix - T S mix < 0

3 4.2 Entropy of binary mixing Mix A and B with no volume change shown in 2-D V A+B =V A +V B Lattice of a binary mixture of two low molar mass components. Volume fractions: φ A = V A VA + V B V B φb = = 1 φa VA + VB

4 4.5 More on the Flory-Huggins theory Homogeneous mixture: Uniform and all components of mixture are intermixed on a molecular scale. Heterogeneous mixture: Consists of several different phases (region with different compositions) 1. Regular solutions The free energy of mixing at 300 K for the following B-values: (a) B=0; (b) B=2000 J mol-1; (c) B=4000 J mol-1; (d) B=6000 J mol-1; (e) B=8000 J mol-1 Entropy dominated Enthalpy dominated

5 < 0 unstable spinodal points, > 0 Local stable binodal points have a common tangent in the Gmix vs. x 1 plot. Free energy as a function of composition (x1) showing binodal (B) and spinodal (C) concentrations.

6 B is set to 6000 J mol-1 The free energy of mixing at different temperatures is shown in the diagram. The binodal (B) and spinodal (S) points are indicated in the figure. At high T, homogeneous mixture.

7 Spinodal and binodal curves meet. UCST: upper critical solution temperature At T greater than the UCST miscibility occurs at all compositions Phase diagram showing the binodal (B) and spinodal (S) curves.

8 2. Polymer solutions 3. Limitation of the Flory-huggins theory Colligative properties are those properties of a solution which depend only upon the number of solute species present in a certain volume, and not on the nature of the solute species.

9 membrane permeable only to the solvent molecules and not to the polymer molecules. The osmotic pressure П is the pressure that must be applied to the solution to stop the flow of solvent molecules through the membrane. V1 is the molar volume of the solvent. can be obtained as the slope coefficient in a plot of Simple experimental set-up for osmosis Deviation from linearity is T-dependent

10 Modify1 e.g. PMMA in toluene, at 20 C modified Flory-Huggins

11 The co-existence of UCST and lower critical solution temperatures (LCST) Modify 2 θ is theta temperature, which is the UCST for a solution of a polymer of infinite molar mass and C p is the exchange heat capacity. Schematic phase diagram for the system cyclohexane-polystyrene (PS). With Cp<0, the presence of both UCST and LCST can be described by the Flory-Huggins equation.

12 3. Good solvent or poor solvent is the specific volume of the pure polymer, is the molar mass of the repeating unit of the polymer. second virial coefficient (A 2 ) Osmotic pressure (ΠRT/c) versus polymer concentration. The chain conformation is influenced by the "goodness" of the solvent

13 4.6 Polymer-polymer blends 1. Techniques assessing of polymer/polymer miscibility scanning electron microscope Transmission electron microscopy Small-angle neutron scattering Small-angle X-ray scattering Wide-angle X-ray scattering Nuclear magnetic resonance Infrared spectroscopy

14 2. Miscibility of polymer/polymer blends Miscibility in p/p blends can normally only be achieved when the heat of mixing is negative, which accomplished by specific interactions between different molecules. The most common type of specific interaction is the hydrogen bond. Another group of miscible blends involves polymers of great similarity without the potential of specific interaction.