Charles E. Carraher, Jr. University of South Dakota Vermillion, south Dakota 57069
The topics of polymers and polymer chemistry are being included to larger degrees in undergraduate and high school chemistry courses. A basic problem is to present to students concepts concerning polymers-what they are, how they behave and why, and how their behavior is different from other molecules and why. A child's "pop-it-bead'' set or polystyrene spheres or corks with holes drilled through them and connected with a shoestring can be used to illustrate some relationships of structure to polymer properties. Such sets can be used to illustrate the general difference in volatility between polymers and smaller molecules such as carbon tetrachloride. First a beaker is filled with spheres and placed on a shaker. The beaker is covered and shaking begun. Shaking is increased until the spheres arein random motion throughout the beaker. The initial random motion in the bottom of the beaker represents the liquid state while the thorough random motion represents the gaseous state. This can be repeated with spheres connected in a linear manner to represent a polymer chain. Random motion, throughout the beaker only occurs a t much higher shaking rates than in the previous situation. I n actuality polymers do not volatilize but break apart into smaller units before they volatilize. The diierence in flow nature between small molecules and polymers can be demonstrated by observing and comparing the change in position of a specially marked sphere and spheres connected to represent a polymer chain (normally a chain of 5-10 spheres is sufficient to 1 BILLMEYER, F.,"Textbook of Polymer Science," John Wiley & Sons, Ino., New York, 1962. 2 A ~ T., ~ "Mechenical ~ ~ ~ Behavior , of High Polymers," Interscience (division of John Wiley & Sons, Inc.), New York,
1948.
Polymer Models
represent a polymer chain) as shaking occurs. The lone sphere is more mobile. The viscosity of molecules is a measure of this mobility or flow tendency. Polymers are experimentally found to be more L'viscous" than are smaller molecules. For instance a 1% by weight m-cresol solution of Nylon 6-10 with a numberaverage molecular weight of 22,000 has a viscosity three times as great as micresol itself. The sets can be easily modified to present polymers with branching and cross-linking present by drilling a second set of holes through the spheres, corks, or beads and securing appropriate spheres to the additional set of holes. A better model to illustrate cross-linking consists of connecting pieces of wire or s t i i cord together in a three dimensional array. This has the added advantage of illustrating the three dimensionality in most cross-linked polymers. Such a model also can be used to illustrate the "memory" of a rubber (see F. Billmeyer' or AlfreyZ for a discussion on rubber memory) since a given segment can be pulled away (not disconnected) from the array but remains connected to the array through cross-linking so the segment which was pulled away is referred to as having a "memory" since its general location in the array is retained. Polymer solubility properties are generally different from the solubility properties of smaller molecules. Polymers are generally solubilized more slowly. For instance, poly [oxy-l,4-phenyleneoxy(pheny1phosphinylidene)], a polypbosphonate ester, (weight-average molecular weight greater than 10') is only completely solubilized in dimethylsulfoxide (1% polymer by weight) after several days, with solubilization increasing with time (dimethylsulfoxide is considered a good solvent for this polymer). Benzoic acid is rapidly dissolved in ether until a saturated solution is reached, after which solubility remains constant with time.
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Also polymers generally have a more limited solubility range than do analogous smaller molecules. The above properties can be explained by considering that solubility depends on both energy (attractive and repulsive terms) and probability consideration. Solvation is encouraged by a higher attractive force existing between solute and solvent molecules than exists between solute molecules alone and solvent molecules alone. Net attractive force for solution = attractive forces between solute and solvent molecules minus attractive forces present m pure solvent and solute = AH,.,
The AH,, terms for both small and large molecules which are similar in electronic nature are of the same general value. (Even when AH,,&is positive solution may occur if the probability term is positive and large enough to counteract the energy term.) Solubilization generally occurs with a positive probability term, i.e., the final solution represents a more random arrangement of molecules than does the pure solute and solvent. It is the probability net probability term = randomness of solution minus rsndomness of solute and solvent = AS,.z
term, ASrns which is responsible for the difference in solubility behavior between large and small molecules. This can be demonstrated by first adding two colors of spheres to a heaker. The spheres represent the small molecule-solvent situation. Any sphere can he moved independently of any other sphere resultinginagenerally large incease in randomness and a large ASmI. The polymer-solvent situation can be represented by adding a rope material (or a chain of styrene balls) to a large beaker of spheres. While many d i e r e n t arrangements can be shown for the spheres and rope, the number of arrangements is limited since the individual segments of the rope do not move independently of one another. This results in a generally lower ASsoI value for polymers than for smaller molecules. This is one explanation for the limited solubility range of polymers as compared with analogous small molecules since a smaller positive ASmL value will counterbalance a smaller positive AH,,value. Free energy of solution = AH,.r - TAS.,, at T = constant
As the rope is initially added to the heaker of spheres only a portion of the rope comes in contact with the spheres. As the beaker is shaken more of the rope becomes exposed to the spheres until finally most of the rope is exposed to the spheres. This illustrates why polymer solubility is slow and increases with time (the shaking represents molecular motion).
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Another solubility property of polymers can be illustrated by adding a shorter rope to the heaker of spheres and noting the amount of contact initially made with the spheres and then noting the number of shakes required to achieve good rope-sphere mixing. These are compared to the initial contact and number of shakes required to achieve good mixing for the longer rope. This illustrates that polymer solubility time for a given polymer-solvent system generally increases as polymer molecular weight increases. Excluded volume is volume occupied by a molecule, and because of the occupancy it is excluded from occupancy by another molecule. Polymer chains exclude a volume which is greater than the volume which would be occupied by the uncombined segments. This can he demonstrated by first adding spheres to a heaker and measuring the height of the balls. Then some balls are taken out and combined to form a chain and again added to the heaker. The heaker is shaken and the height of balls measured. The second height should be greater than the first. This increase in excluded volume exhibited by polymer chains is due a t least to angle restrictions between chain segments and to the crossing of chains forming "pockets" which are too small or isolated for solvent molecules to occupy. One of the most widely and easily used models is rope (or rope-like material) which can be used not only to illustrate the unique "unidimensional" primary bonding connecting segments in a polymer chain but also by gathering the rope together and tossing it into the air several times, a good representation can be shown of the general randomness exhibited by many polymers in solution, melts, and amorphous solids. I n addition a "random-walk" experiment can be conducted by tossing the rope into the air and measuring the distance and between the ends of the rope (end-toend distance). After repeated tossings and measurings the data can he treated in a manner analogous to the treatment describing the distribution of molecular velocities (c.f., for instance Barrowa for a discussion pertaining to the distribution of molecular velocities and Flory4 for a discussion of the distribution of polymer chain ends). Such an experiment should result in a Gaussian distribution of end-to-end distances for the polymer chain. Polymers actually generally have a larger end-to-end distance than that represented by the rope experiment due a t least to steric and angle restrictions. I n solution there is usually an additional tendency for the chain to lengthen out to take advantage of more contact with solvent molecules. BARROW, G . M., "Physicd Chemistry," (1st Ed.) McGrawHill Book Co., New York, 1961, pp. 3 6 4 0 . "LORY, P. J., "Principles of Polymer Chemistry," (1st Ed.) Cornell University Press, Ithaca, 1953, pp. 299431.
