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APSU CHEM 3610 - Physical Properties of Macromolecules/Polymers

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Physical Properties of Macromolecules/Polymers Ron Robertson rr c:\files\courses\3620\macromolecules.docPhysical Properties of Macromolecules/Polymers Slide 1 Overview Natural – polysaccharides, proteins, nucleic acids Synthetic – condensation and addition, derivatives of natural polymers Colloids – aggregates of molecules that can also behave like macromolecules (See previous notes for extended outline.)Physical Properties of Macromolecules/Polymers Slide 2 Polydispersity of macromolecules Heterogeneity/Polydispersity Some macromolecular solutions are not uniform in size, that is there are varying size molecules in the solution. Theses macromolecules are called heterogeneous or polydisperse. Synthetic polymers are a good example of this. On the other hand many natural polymers such as proteins are rather uniform in size – there is no variation. Such polymers are said to be homogeneous or monodisperse. How do we determine the polydispersity? Various techniques that we use to determine the properties ofPhysical Properties of Macromolecules/Polymers Slide 3 macromolecular solutions can give us the average molar mass of the macromolecule. It is interesting that the different techniques give different numbers for the average molar mass. This is because there are several ways to define the molar mass. There are 4 common types of molar masses – the number average <M>n, the mass average <M>w, the z – average <M>z, and the viscosity average. Each gives a different number for the average molar mass if the solution is polydisperse. We will examine each equation in detail. (see notes from board) The ratio of the mass average to the number average is called the Polydispersity or Heterogeneity Index.Physical Properties of Macromolecules/Polymers Slide 4 Thus for proteins <M>w/<M>n = 1. Synthetic polymers are said to be polydisperse if the ratio is greater than 1.1. A common value is around 5 and it may be as high as 30.Physical Properties of Macromolecules/Polymers Slide 5 Techniques for property determination Osmometry For an ideal solution π = [P] RT [P] = c/M c is the mass concentration M is the molar mass For an non ideal solution π = [P] RT(1 + B [P] + . . . ) using a series expansion This is similar to the virial equation for non-ideal gases π = (c/M) RT{1 + B (c/M) + . . . } π/c = RT/M{1 + B (c/M) + . . . } If we neglect terms past the 2nd in the series as being small, then graphing π/c vs. c gives a molar mass fromPhysical Properties of Macromolecules/Polymers Slide 6 the intercept. The molar mass is a number average molar mass <M>n because osmometry is a colligative property. Colligative properties depend on the # of particles in solution and not the type.Physical Properties of Macromolecules/Polymers Slide 7 Sedimentation In a gravitational field, heavy particles settle towards the bottom of a column of solution. The rate depends on the strength of the field and the masses and shapes of the particles. Heavy particles sediment faster than lighter ones and spherical particles sediment much faster than extended molecules. This technique can be used to determine the z-average molar mass, <M>z. as well as the mass average, <M>w, and the number average, <M>n. An ultracentrifuge accelerates the rate of sedimentation and runs at about 105 times the acceleration due to gravity.Physical Properties of Macromolecules/Polymers Slide 8 Electrophoresis Electrolyte macromolecules move in an electric field according to mass and shape. This technique generates a mass average <M>w molar mass. Gel filtration Beads of a porous polymeric material capture molecules according to size; the elution time can be used to determine the molar mass, a mass average <M>w. Size exclusion and gel permeation chromatography make use of this technique. Recent advances also allow the determination of the number average <M>n.Physical Properties of Macromolecules/Polymers Slide 9 Viscosity The resistance to flow of a solution increases in the presence of macromolecules. At low concentrations η = η* (1 + [η] c + . . . ) η [=] g/cm-s or kg/m-s η* is the viscosity of the pure solvent [η] is called the intrinsic viscosity [η] [=] l/concentration The intrinsic viscosity is defined as [η] = limc→0 [(η/η*) –1]/c so a graph of [(η/η*) –1]/c vs c can be extrapolated back to zero concentration to give the intrinsic viscosityPhysical Properties of Macromolecules/Polymers Slide 10 The Ostwald viscometer can be used to measure time for a solution to flow through a capillary. This is compared to a standard sample and the viscosity coefficient determined as in lab. The ratio of η/η* can be used to obtain the intrinsic viscosity. At a temperature called the Flory temperature the solution is ideal and obeys the Mark–Houwink equation [η] = K Ma M is the viscosity average molar mass (<M>v) “K” and “a” depend on the solvent and macromoleculePhysical Properties of Macromolecules/Polymers Slide 11 Light scattering When light falls on an object it causes excitation of the electrons in the object. This excitation is usually much more complicated that the simple Bohr model predicts. The excitation can be modeled as the light energy driving the oscillation of a spring. If the medium is homogeneous (perfect crystal or a dispersement that is down to individual atoms, molecules or ions) there is no scattered light, only light along the direction of the light ray. If the medium is not homogeneous, light is scattered in all directions (including 90°).Physical Properties of Macromolecules/Polymers Slide 12 Light scattering by particles smaller than the wavelength of light is called Rayleigh scattering. This occurs in a gas or in liquid solutions of the colloidal phase. The intensity of this scattering depends on the 4th power of the frequency (and thus 1/λ4) and thus the scattering is more intense for shorter wavelengths (the blue/violet end of the spectrum). Since our eyes are much more sensitive to blue than violet we see the scattering as blue. Quantitatively for unpolarized light I = A I0 c M (1 + cos2θ) (see diagram) c is the concentration of the solute M is the molar mass (<M>w), a mass average A is a constant that relates distance, refractive index of the solution and the wavelengthPhysical Properties of Macromolecules/Polymers Slide 13 Turbidity Scattering of light reduces the transmitted intensity It = I0 exp(-τ l) τ is called the turbidity and has units


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