Lecture 1 - University of Reading



Polymers 3/PH/AM

Lecture 2: Meet the Molecules

The Centipede was happy quite,

Until the Toad, in fun,

Said “Pray, which leg comes after which?”

This raised her doubts to such a pitch,

She fell exhausted in a ditch,

Not knowing how to run.

(1) The Macromolecule as a Collection of Atoms

(2) The Macromolecule as a Physical Object

(3) How can we Observe Macromolecules?

(1) The Macromolecule as a Collection of Atoms

Configuration and Conformation.

Configuration refers to different ways of sticking the same atoms together. You cannot change the configuration of a molecule without breaking and remaking bonds.

Conformation refers to different ways of bending the same molecule, without breaking bonds.

Configuration (1)

|[pic] |This is a Glucose Molecule. |

| |In order to switch between the two configurations, it is |

| |necessary to break bonds at the right-hand carbon atom. |

Starch and Cellulose

|[pic] |These are two polymers of glucose, differing only in being |

| |joined by β- (cellulose) or α-(starch) glycoside bonds. |

| |These small chemical differences, however, make the polymers |

| |very different in practice. Cellulose is a strong insoluble |

| |crystalline structural material in plants, while starch is |

| |soluble, meltable (in baking) material which is easily |

| |digested and sued by plants as a food store. Cows, etc., |

| |need bacteria to break down cellulose in their food.. |

Configuration (2)

The most important configurational difference – tacticity.

|[pic] |These are molecular models of polyvinyl chloride (PVC). If one stretches out the |

| |main chain: |

| |isotactic PVC has all the chlorine atoms coming of the same side |

| |Syndiotactic PVC has the chlorine atoms coming out on alternate sides. |

This is a linear (unbranched) Polyethylene chain, in two views.

[pic]

[pic]

Isotactic Polypropylene has methyl groups coming off every second chain carbon atom, all on the same side.

[pic]

[pic]

Syndiotactic Polypropylene with methyl groups alternating

[pic]

[pic]

Atactic Polypropylene has methyl groups coming off randomly.

[pic]

These three polypropylenes are different polymers.

Conformation (1)

So far, we have drawn all our polymer chains stretched out straight in a zig-zag. Is this a realistic picture?

[pic]

The left hand picture: the all-trans conformation of a polyethylene chain: this is the minimum energy conformation. This would be the equilibrium conformation at Absolute Zero. At T>0 deviations from the minimum-energy conformation are possible due to thermal motion. According to Boltzmann’s law the probability of realization of the conformation with the excess energy U over the minimum energy conformation is

[pic]

Since there are so vastly many more conformations like the one on the right, one may ask:

Are straight chains ever found?

In fact, long straight all-trans segments are commonly found in crystals. This is because crystallization releases energy to the environment, and the increase in multiplicity of states for the rest of the universe more than offsets the reduced multiplicity of states for the polymer chain.

Conformation (2)

Polypropylene – 3-fold helix

|[pic] |The helix represents the folding up of one of the chain |

| |patterns below (I can’t tell you which!). The folding |

| |moves the methyl group apart so they do not interfere with|

| |each other. |

| |[pic] |

| |[pic] |

Amino Acids – Polypeptides and Proteins

For the structure of these, I would recommend going to books. You can also check out the following two websites.



stingray.bio.cmu.edu/~web/bc1/pstruc/pstruc.htm

Conformation (3)

|[pic] |Here are an ethane molecule and a butane molecule. The latter can act as |

| |a simple model for the chains of polyethylene and all vinyl polymers. |

| |The central bond of the butane molecule is taken as fixed, but the carbon |

| |atoms at each end of this bond are capable of rotation. In butane the |

| |groups are methyl, but they serve as models for chains extending onwards. |

|[pic] |As one group rotates relative to the other, it goes between |

|The black blobs represent methyl groups (butane) or chain extensions |staggered (,120,240) and eclipsed (60,180,300) conformations. When |

|(PE), while the white blobs represent hydrogen atoms. |eclipsed the hydrogen atoms and the big groups all interfere, this |

| |being unfavourable energetically. When staggered the interference |

| |is much less, especially in the trans conformation. |

|[pic] |This shows the energy associated with different rotations of the |

| |butane molecule (solid) and ethane molecule (dotted). The end on |

| |views are staggered. |

| |The eclipsed conformations are quite unlikely at room temperature, |

| |but since |

| |at room temperature, RT ~ 2.4 kJ.mole-1 |

| |there will be considerable a population in the gauche states. |

(2) The Macromolecule as a Physical Object

The next overheads are the lecture notes to:

Introduction to polymer science by Prof. A.R.Khokhlov, (Moscow State University) Lecture 2



(3) How can we Observe Macromolecules?

Sometimes we can actually see them under the electron microscope. DNA is a good example of this.

[pic]

Scattering Methods

Light Scattering

Neutron Scattering

|Atomic Nucleus |Scattering Length (fm) |

|1H |- 3.741 |

|2D |+ 6.671 |

|C |+ 6.646 |

|N |+ 9.362 |

|O |+ 5.803 |

Direct Measurement

|Size exclusion chromatography. |

|[pic] |

|MALDI Mass Spectroscopy. |

|[pic] |

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