Polymer Structure

Polymers are macromolecules consisting of repeating chains of smaller molecules, the monomers, connected by chemical bonds. Any molecule having two or more reacting groups (bi — or poly-functional) is a good candidate for a monomer. In addition to the variety of monomers available for polymerisation, it is further possible to control the polymerisation reactions in many ways. When a given monomer is polymerised (by one or a sequence of chemical reactions), different output variables are to be considered:

1. Chain length: The number of monomers that form one chain is called the degree of polymerisation; its value depends on the way the polymerisation proceeds.

2. Chain topology: The topology of polymer chains can be controlled during polymerisation: either linear or branched chains can be produced. Branches are sub-chains connected to the linear main chain, at some places called branch points.

Branching can give rise to major changes in properties and performance of polymers. Polyethylene is one of the best examples: short-chain and long-chain branches strongly affect material properties and material functions in different ways.

It is not always possible to difference main chain and branches in macromolecules; if so, macromolecules look as a set of linear sub-chains assembled in some given way. Some examples are (see Fig.10.1):

• Ladder polymers consist of two parallel chains interconnected by smaller sub-chains in regular intervals.

• Comb polymers are constituted by an array of sub-chains grafted in one main chain.

• Tree polymers are also constituted by a main chain with a digit number of grafted sub-chains; in tree polymers, the grafted sub-chains are themselves branched chains (as in a tree). This feature enables the distinction between tree polymers and branched polymers.

• Star polymers are constituted by a digit number of sub-chains, the “arms”, connected to a common core.

• Dendrimers may be considered as star polymers where the arms are constituted by tree polymers. Dendrimers are obtained when star polymers (first-order branching), with multifunctional ends on the arms, add additional sub-chains (second-order branching). If the sub-chains have also multifunctional ends, they may add additional sub-chains (third-order branching). By this way, different orders of branching can be produced; the number of chain ends (terminal groups) increases in an exponential way with the maximum order of branching.

• H-polymers are constituted by five sub-chains bound at two points, as in uppercase H.

3. Crystallinity: On cooling, chain monomers may become spatially organised in a regular way at given places, and crystallisation proceeds; otherwise, a glassy polymer is formed. In both cases, the final morphology depends on the way cooling was done. The volume fraction of crystallised material is called the degree of crystallinity; its value is manageable in several cases.

4. Cross-linking: Polymer chains (or sub-chains) can also form networks of cross — linked chains. The mesh size of the network is related to the degree of cross­linking (the number of cross-links per chain), to the cross-linking density (the number of cross-links per unit volume) or to the sub-chain length, given by the molecular weight between consecutive cross-links; the three quantities are related one another. The mesh size of the network increases as the number of monomers between consecutive cross-links decreases. Vulcanised rubber is an example of a low mesh, loose polymer network (low degree of cross-linking, many monomers between consecutive cross-links). Thermoset polymers (after cure) are an example of very tight cross-linked chains (high degree of cross­linking, very few monomers between consecutive cross-links).

Homopolymers are constituted by identical monomers. Nevertheless, their properties can be modified within wide limits by changes in the degree of

Fig. 10.1 Chain topology of polymers: (a) linear chain; (b) long chain branches; (c) short chain branches; (d) network of crosslinked chains; (e) ladder polymer; (f) comb-like polymer; (g) tree­like polymer; (h) 4-arms star polymer; (i) dendrimer; (j) H-polymer. Full circles: branching points. Open circles: end chains

polymerisation, changes in the degree of crystallinity, changes of the amount of branching and changes of the length of branches of the main chain. Such changes make it possible to adjust the properties (“to set the grade”) of the resulting homopolymer material to a wide range of uses and requirements. Polyethylene is one such example.

Polymerisation may also involve more than one kind of monomers.

Copolymers are polymer chains produced by polymerisation of two or more different monomers. The order and arrangement of the different monomers that make up a copolymer can also be set as follows:

• Alternating copolymers—monomers A and B are arranged alternately in the chain: ABABABABABABABAB.

• Block copolymers—monomers A, B and C are organised into two or more blocks of identical monomers: di-blocks like AA(.. ,)AA-BB(.. .)BB, tri-blocks like AA(.. ,)AA-BB(.. ,)BB-CC(.. ,)CC and so on.

Example: triblock copolymers of styrene-butadiene-styrene (SBS)

• Random copolymers—monomers are randomly arranged in the chain. For instance,

AABABBBBAABAAAA.

Example: ethylene-propylene-diene copolymers (EPDM).

With few exceptions, polymers cannot be processed just as obtained at the reactor exit (“fluffy polymers”). Fluffy polymers need to be combined with other substances before processing. The designation of such procedures varies with industrial sector: thermoplastic and thermosetting polymers are compounded; rubbers, adhesives and inks are formulated; textiles are finished. Compounded thermoplastic and thermosetting polymers (before cure) are called resins. Resin is, in some sense, a utilitarian name based on the fact that molten thermoplastics before processing, thermosetting polymers before cure and coniferous resins look and flow in a similar way.

Compounding may include mixing with:

• Additives (pigments or dyes, flame retardants, heat stabilisers, lubricants, antioxidants, anti-static agents, etc.). They are used in small proportions (<5 %) for changing specific properties other than mechanical properties.

• Modifiers are added in greater proportions and do affect mechanical properties. Examples are fillers and/or reinforcements, such as mineral particles and glass fibres.

• Other polymers: The particular designation of such mixing is material depen­dent. Thermoplastics are blended with thermoplastics, toughened (impact — modified) with rubbers and reinforced with fibres. Rubbers are mixed with fibres and blended with other rubbers. Thermosets are reinforced with fibres. Polymer alloys are special polymer blends, looking homogeneous and with controlled morphology in a reproducible way.

In general, different polymers are not thermodynamically miscible: few exceptions are known. Here “thermodynamically miscible” or “miscible in thermodynamic sense” means that, after mixing, one single homogeneous phase in thermodynamic equilibrium is produced. Low-molecular-weight substances are miscible quite often; for example, water and ethanol (ethyl alcohol) are thermodynamically miscible. Egg and olive oil are not thermodynamically miscible: if they are put together, three phases are obtained: olive oil, egg white and egg yolk. Nonetheless, by mechanical action (“blender”…) and by inducing changes in surface tension (egg yolk lecithin and added mustard….!), it is possible to get a product homogeneous to the naked eye: it is the emulsion called “mayonnaise”. With the aid of a microscope, one can easily check that immiscibility remains at microscopic level.

Polymer blends may be divided into:

• Miscible blends (thermodynamically miscible)

• Immiscible blends (thermodynamically immiscible)

• Polymer alloys (thermodynamically immiscible, visually homogeneous, with controllable and reproducible morphology)

The difference between immiscible blends and polymer alloys is that polymer alloys look visually homogeneous, and their morphology can be controlled in a reproducible way. Morphology is set up by adding one or several “compatibilisers”; “compatibilisers” act like surfactants reducing the interfacial tension and improving

Continuous phase

Solid

Liquid

Gas

Dispersed phase

Solid

Solid sols

Suspension

Smoke

Liquid

Sol

Emulsion

Fog

Gas

Solid foam

Foam (liquid)

Table 10.1 Dispersed systems

the dispersion. “Compatible blends” is a utilitarian term for immiscible blends that look transparent to the naked eye.

Polymer blends and alloys are examples of dispersed systems, not necessarily composed of liquids, in which one component (the “dispersed phase”) is dispersed in the “continuous” phase formed by the other component.

A general classification of dispersed systems is presented in Table 10.1.

Updated: 31 августа, 2015 — 11:32 дп