Lecture 7 Flashcards
Thermodynamics of Polymer Solutions and Determination of MW
Entropy of mixing equation
= k ln(omega)
define X(chi)
dimensionless, empirical, characterizes change in local free energy upon mixing
when X = 0
system is a thermal
when X> 0
system is endothermal, favors demixing
when X<0
system is exothermal, favors mixing
spinodal decomposition
region where spontaneous phase separation occurs and a system minimizes its energy
metastable region
where phase separation occurs via homogeneous nucleation and growth
Two types of techniques to determine MW
absolute and relative
absolute
- no calibration, only procedure
-osmometry
-light scattering
relative
-requires calibration
-SEC/GPC
-viscometry
most MW determination techniques require
polymers to be dissolved in solution
Techniques sensitive to number of polymer molecules determine _____ average MW
number
Techniques sensitive to mass of polymer molecules determines _____ average MW
weight
GPC and viscometry do not measure MW directly… why?
they probe hydrodynamic properties
Rh vs Rg
objects of different shapes can be approximated through a sphere with Rh
Rg for infinitely thin disk
Rg^2 = R^2 / 2
Rg for solid sphere
Rg^2 = 3/5 R^2
translation from Rh to Rg for gaussian coil
Rh = 0.66Rg
Static Light Scattering (SLS):
- Rg or RMS radius
- mass averaged distance from each point in a molecule from the center of gravity
Dynamic Light Scattering (DLS)
- Rh or hydrodynamic radius
- radius of a sphere with the same diffusion coefficient as the sample
Chain Expansion/Contraction and Solvent quality relationship
Rg ~ N^v
when v>0
good solvent
when v<0
poor solvent
when v = 0
theta solvent
T/F expanded chains in good solvents are Gaussian.
FALSE, they are not
For 2nd virial coeff, A>0, X<1/2
good solvent
For 2nd virial coeff, A<0, X>1/2
bad solvent
For 2nd virial coeff, A=0, X=1/2
theta conditions
A2 ~
(1-2X)
