SCHM-503/703 Structure & Properties of Polymeric Materials 603-2.1

Polymer Structure

The structure of polymeric material is rather complex. The juxtaposition of the various structural elements that make up the morphology of the bulk material depends on chemical composition, thermal and mechanical history, and the “length scale” being considered. What looks homogeneous at one magnification may appear quite heterogeneous at another.

Scanning electron micrograph of an ion etched fracture surface from an 85/15 wt% isotactic polypropylene/low density polyethylene blend. Pd/Au coated (M. Kojima).

603-2.2

Polymer Composites

603-2.3

Multicomponent Polymeric Materials

OsO4 – stained SBS Triblock Copolymer

603-2.4

Interpenetrating Polymer Networks (IPN)

603-2.5

Structure Hierarchy in Semi-Crystalline Polymers

Polypropylene Fibrils @ 3500x Polypropylene Sheet @ 100x

Polypropylene Spherulites @ 500x

Lamellar Structures of Polyethylene @ 2000x

603-2.6

Polymer Morphology

The morphology of a piece of polymeric material is a description of the spatial arrangement of the various microstructural domains in the bulk sample. The crystalline domains are characterized by an ordered arrangement of the polymer chains from which they are composed. The crystalline domains will become disordered when the material is heated above its melt temperature, Tm. The melt transition is associated with an abrupt decrease in bonding (increase in enthalpy) and increase in disorder (increase in entropy).

The amorphous domains contain disordered chains. These chains begin to move in a coordinated fashion at the glass transition temperature, Tg. If the amorphous polymer is held a temperatures below Tg there is insufficient thermal energy to overcome the activation energy for chain motion. This glassy state can be described as a structural liquid, yet a dynamic solid.

603-2.7

The Development of Morphology

The morphology of a polymer, to a great extent, determines its properties and, in turn, its potential uses. The development of morphology is governed by the thermodynamics and kinetics inherent to the processing technology being employed. We say that these processes are thermodynamically driven and kinetically controlled. The thermodynamic driving force is the reduction of the free energy of the material. The change in free energy is given by the Gibbs-Helmholtz relationship:

∆G = ∆H - T∆S

where G, H and S are free energy, enthalpy and entropy, respectively. In condensed phases, the dominant contribution to the enthalpy is the bonding energy. Bonds are significantly weakened when stretched beyond ~ 6 Å. Consequently, optimum packing, in the form of crystals, is paramount to lowering the enthalpy of the material. The elasticity of a crystal is dominated by enthalpic effects. Entropy is a measure of spatial and energetic randomness. By increasing the amount of space, or the number of available energy levels, the entropy is increased. Because entropy depends on the total (whole) system, rather than just being the sum of the individual (local) parts; processes dominated by entropy are called global phenomena. The elasticity of a rubber band is dominated by entropic effects. More often than not, thermodynamics only gives a partial description of polymer behavior. Because polymers are large they tend to move slowly. Consequently, they often get trapped in structures that only represent local minima in G, rather than global minima. These are called metastable states.

603-1.8

Dynamics in Bulk Materials As mentioned earlier, crystals are considered to be structural solids whereas glasses are dynamic solids, yet structural liquids. Throughout the course we need to be aware of the distinction. A structural liquid exhibits short-range disorder, this includes disorder in the arrangement of the polymer chains and in the arrangement of voids. The difference between the void space in a glass and in the corresponding crystal is referred to as the free volume. Because the repeat units of a polymer chain are interconnected, they can’t move independently. We say the motion the units is correlated. Correlated motion requires room.

The glass transition of a polymer will not occur unless sufficient free volume is available to enable 30 – 60 backbone bonds to move in concert.

Some of the required free volume is created by the short-

ranged motions of backbone and side-group rotations. It takes a huge number of coordinated local motions to give rise to the segment motion associated with the glass transition. Consequently, the “time scale” for local rotations is much shorter than for segment motion.

The time scale for local rotational motion depends on the ratio

of the energy barriers associated with the rotation (determined by chemical structure and intermolecular bonding) and the available thermal energy. The ratio ∆E/RT can be modified by:

1) Changing the temperature – raising the

temperture increases the rate of rotations. 2) Changing ∆E – for a given chain structure,

lowering intermolecular bonding generally increases the rate of chain motions.

603-2.9

Factors That Control Morphology The morphology of polymeric material can be controlled by selectively enabling one rate process over another.

Polyethylene Single Crystal that was slowly crystallized from solution Crystallization was faster than precipitation

Polyethylene that was precipitated as a gel Precipitation was faster than crystallization

603-2.10

Effect of Composition

Polyethylene and 1,2,4,5 Tetrachlorobenzene Mixtures

Precipitation from a Eutectic Mixture

Precipitation from a Non-eutectic Mixture

603-2.11

Effect of Polymer Chain Structure

Polyethylene 98% Branched

Polyethylene 80% Branched

603-2.12

Getting to Know the Players Although there is an enormous variety in types and grades of commercial polymers, we have seen that only a few are mass-produced. A listing on the most commonly used monomers is given below. These are reacted to make polymers by either

1) Addition or Chain-Growth Polymerization

Or

2) Condensation of Step-Growth Polymerization In addition polymerization monomers are added one at a time to the end of a growing chain, sometimes completing a 10,000-bond chain in a matter of seconds. In step-growth polymerization polymers grow by adding successively larger pieces together throughout the overall polymerization. Common Monomers Monomer Polymer Recycle # Ethylene Polyethylene 2, 4 Propylene Polypropylene 5 Vinyl Chloride Poly(vinyl chloride) 3 Terephthalic Acid Poly(ethylene terephthalate) 1 Styrene Polystyrene 6 Formaldehyde Urea/Phenol Resins Ethylene Glycol Poly(ethylene terephthalate) 1 Ethylene Oxide Poly(ethylene oxide) Phenol Phenolic Resins Butadiene Synthetic rubbers Acrylonitrile Poly(acrylonitrile), Acrylics Vinyl Acetate Poly(vinyl acetate), Poly(vinyl alcohol) Adipic Acid Nylon 6,6 Caprolactam Nylon 6