Thermal Analysis of Polymeric Materials
.pdf
16 1 Atoms, Small, and Large Molecules
__________________________________________________________________
Fig. 1.14
butyraldehyde. The source-based name does not tell the structure. The rather complicated structure-based name can be deduced from the structure in the figure.
The chain ends can be identified also as shown in Fig. 1.14. This naming is needed in case the properties of a macromolecule are influenced by its end groups or the molecule is short enough to change its overall composition due to the ends. The left chain end is preceded by the first letter of the Greek alphabet ( ) and the right end, by the last ( ).
Besides the sourceand structure-based names, trade names are in common usage because of their convenient brevity. Unfortunately, commercial polymers may have been changed by copolymerization (see below), contain additives to enhance handling or performance, and may even change in composition with time as improvements are made in the product. When using trade names, always identify them as such with a trademark sign: ™ or ®, then give the trade name holder, as well as the IUPAC name of the polymer and list deviations from the pure polymer, if they are known.
Often polymer names are cumbersome and one may want to use an abbreviated letter code, as are listed in Table 1.1. The shorter the letter code, the easier are accidental duplications possible. It is, thus, again necessary to identify a letter code with the IUPAC name. For example, the full statement that should identify the polymer PTFE in a publication is PTFE = polytetrafluoroethylene (source-based name), poly(difluoromethylene) (structure-based name), Teflon® (trade name of the DuPont Company).
With this simple set of rules of nomenclature one can name a large number of macromolecules. To make this chore more interesting, one should not only learn names, but connect the various polymers to their well-known uses. Perhaps the most difficult polymer to find an application for when looking through handbooks is a
1.2 Nomenclature |
17 |
__________________________________________________________________
polymer you see almost every day. It is poly(vinyl butyral), used as the inner layer in safety glass (automobile windshields) and used as the complex example of naming in Fig. 1.14. Its structure is also given in Fig. 1.17, below.
Table 1.1. List of frequently used abbreviations of names of polymers
__________________________________________________________________
ABS |
acrylonitrile/buta- |
PMI |
poly(methacrylimide) |
|
diene/styrene rubber |
PMMA |
poly(methyl methacrylate) |
CPE |
chlorinated polyethylene |
PMP |
poly(4-methyl pentene-1) |
CTA |
cellulose triacetate |
POB |
polyoxybenzoate |
EPS |
expanded polystyrene (foam) |
POM |
poly(oxymethylene) |
HIPS |
high-impact polystyrene |
PP |
polypropylene |
HDPE |
high-density (linear) |
PPO |
poly(phenylene oxide) |
|
polyethylene |
PPOX |
poly(propylene oxide) |
LDPE |
low-density (branched) |
PPP |
poly-p-phenylene |
|
polyethylene |
PPS |
poly(phenylene sulfide) |
LLDPE |
linear, low-density |
PPT |
poly(propylene terephthalate) |
|
polyethylene |
PPX |
poly-p-xylylene |
PAA |
poly(acrylic acid) |
PS |
polystyrene |
PAN |
polyacrylonitrile (fiber) |
PTFE |
polytetrafluoroethylene |
PB |
poly(1-butene) |
P3FE |
polytrifluoroethylene |
PBI |
polybenzimidazole |
PVAC |
poly(vinyl acetate) |
PBS |
butadiene/styrene copolymer |
PVAL |
poly(vinyl alcohol) |
PBTP |
poly(butylene terephthalate) |
PVB |
poly(vinyl butyral) |
PC |
polycarbonate |
PVC |
poly(vinyl chloride) |
PDMS |
polydimethylsiloxane |
PVF |
poly(vinyl fluoride) |
PE |
polyethylene |
PVF2 |
poly(vinylidene fluoride) |
PEG |
poly(ethylene glycol) |
SAN |
styrene/acrylonitrile |
PEO |
poly(ethylene oxide) |
|
copolymer |
PET |
poly(ethylene terephthalate) |
SBR |
styrene/butadiene rubber |
PI |
polyimide |
UHMPE |
ultrahigh molar mass |
PIB |
polyisobutene |
|
polyethylene |
PMA |
poly(methyl acrylate) |
XLPE |
cross-linked polyethylene |
__________________________________________________________________
Figures 1.15 to 1.18 show a standard frame for name and structure information. The first example is polyethylene. Note, that in this case source-based and structurebased names have different CRUs, causing problems in assessing molar quantities. The subsequent 19 structures are for other, frequently used polymers. Structure and nomenclature should be studied. At the same time, one should find out where one has heard about this particular macromolecule.
A number of the displayed polymers can be constructed of diradicals that represent different isomers (Gk. = equal, = part). From the structure of polypropylene, for example, it can be seen that the methyl group can be located in front of the screen (d, as drawn) or in back (l). As long as the end-groups of the molecule can be neglected, the two possible polymer molecules can be superimposed by rotation about an axis vertical within the screen, i.e., the two molecules are not true isomers. If the configurations change along the chain, special problems arise with nomenclature and properties (see also Fig. 1.20, below).
18 1 Atoms, Small, and Large Molecules
__________________________________________________________________
Fig. 1.15
1.2 Nomenclature |
19 |
__________________________________________________________________
Fig. 1.16
20 1 Atoms, Small, and Large Molecules
__________________________________________________________________
Fig. 1.17
1.2 Nomenclature |
21 |
__________________________________________________________________
Fig. 1.18
22 1 Atoms, Small, and Large Molecules
__________________________________________________________________
Figure 1.15 shows polyisobutylene, a vinylidene polymer with symmetric substitution, and thus without stereoisomers. Cis and trans isomers are possible in butenylene polymers. Two examples are at the bottom of Fig. 1.15. They are not interconvertable by rotating of the molecule. Shown in the figures are the trans isomers ( \=\ ). In the cis isomers the backbone chain continues on the same side of the double bond ( \=/ ). In Figs. 1.16 and 1.17 a series of vinyl and vinylidene polymers are shown. The above-mentioned PTFE, poly(vinyl butyral), and poly(methyl methacrylate) are given, starting in Fig. 1.17. Polyoxides are drawn at the bottom of Fig. 1.17, and the top of Fig. 1.18. Poly(ethylene terephthalate) and two aliphatic polyamides (nylon 6,6 and nylon 6) round out Fig. 1.18. The 20 polymers just looked at should serve as an initial list that must be extended many-fold during the course of study of thermal analysis of polymeric materials.
1.2.2 Copolymers and Isomers
If a linear macromolecule is made up of more than one repeating unit, one calls the molecule a copolymer, in contrast to homopolymers with only one CRU. The repeating units of polymers may be fixed during synthesis, or they may still change after polymerization. Examples of the former are most vinyl polymers, examples for the latter, polyesters and polyamides. The vinyl polymers usually decompose irreversibly on heating, while the polyesters and polyamides can interchange chain segments under proper conditions (ester and amide interchange). For full description, it is thus advantageous to know the reaction kinetics of the different monomer units (see Chap. 3).
Copolymers can be distinguished as random or regular as illustrated in Fig. 1.19. The nomenclature requires simply the syllable -co- between the names of the two
Fig. 1.19
1.2 Nomenclature |
23 |
__________________________________________________________________
repeating units (source or structure based). If a random copolymer does not deviate from randomness within any molecule and between molecules, full characterization is possible by giving just the mole fractions of the components, if not, characterization is very difficult. Often copolymers are compared erroneously to alloys. There exists, however, a big difference, the possible permanence of the arrangement along the chain of the two or more components of copolymers. This leads to a much larger multiplicity of copolymers than alloys. The different sequences will also give special effects in the thermodynamics (see Chap. 7).
The regular copolymers require additional structure information. Three simple examples are listed with their nomenclature in Fig. 1.19. While alternating copolymers also need only information on the concentration for full characterization, more detail is needed for block and graft copolymers. The number and length of the blocks or grafted chains and their possible distributions within and between the molecules must be known for full structural characterization.
Many different regular arrangements of two or more components within a molecule can be envisioned. In fact, the multiplicity is practically unlimited and a systematic study of all possible molecules is impossible. A goal for the material scientist is thus to understand copolymer behavior to such a degree that an optimized structure can be predicted before synthesis is attempted.
The number of possible polymers is increased further by the many isomers. Isomers have the same number of atoms in each of their repeating units, but arranged in different structures. A simple type of isomer is the positional isomer, observed in most vinyl polymers as indicated at the top of Fig. 1.20. Fortunately the reaction mechanism prefers often one of the isomers (usually the head-to-tail isomer) to such a degree that the concentration of the other one is small. Uncontrolled isomerization causes a loss of regularity, similar to that in random copolymers. The irregular
Fig. 1.20
24 1 Atoms, Small, and Large Molecules
__________________________________________________________________
molecules have often a low crystallinity and with it comes a loss of mechanical properties (see Chap. 5).
Besides different positional placement of monomers, it is possible to grossly change the structure on polymerization. Three structural isomers are given for the butadiene polymerization in Fig. 1.20. The 1,2-polybutadiene can, as a vinyl polymer, also have head-to-tail and head-to-head positional isomers. The 1,4- polybutadiene allows, in addition, cis and trans isomers, as discussed along with the names of the polymers in Fig. 1.15. Even the cyclopolymerized polybutadiene may have several structural isomers of different conformations.
Stereoisomers are drawn in Fig. 1.20 for the two possible isotactic forms of polypropylene. The term isotactic indicates that all repeating units are identical (Gk.
= equal, = ordered). Each second chain atom of polypropylene is a chiral center. Writing this center: L HC*CH3 R, one can see that the tetrahedrally coordinated carbon C* has four different substituents as long as the left and right continuations of the chain (L and R) are different. The two isomers are mirror images of each other with the mirror plane parallel to the screen (see Fig. A-13.2). As was pointed out above, for long homopolymers, L and R are sufficiently equal that the two isomers are superimposable by rotation. Changing the chirality randomly along the chain gives rise to an atactic isomer (Gk. = privative, i.e., atactic = not tactic). This disordered polymer is, again, unable to crystallize. In addition, an unlimited number of specific, longer, regular sequences are possible. In Fig. 1.20 only the syndiotactic isomer is indicated (Gk. = together). Although interesting molecules may be among the more involved, regular isomers, they are difficult to make.
1.2.3. Branched, Ladder, and Network Polymers
Branched, ladder, and network polymers deviate from the linear macromolecules discussed up-to now. In order to remain fusible and plastic, the molecule must contain sufficient segments that are flexible and linear. Even without branches, linear molecules may have insufficient flexibility to melt. The linear poly(p-phenylene), for example, is a rigid macromolecule (class 3 of Sect. 1.1.3), because rotation about its bonds does not change the molecular shape. One must thus watch in such molecules that sufficient flexibility exists for plastics applications.
The question of nomenclature can be solved only in the simplest cases. In Fig. 1.21 a possible shape for a branched polyethylene is drawn. The branches are introduced during free-radical polymerization (see Chap. 3). By hydrogen abstraction, one chain is terminated, and somewhere else a new radical is introduced that can add a new run of monomers. A specially favorable configuration is the ring configuration of six atoms at the end of a growing molecule that leads by back-biting to short branches of four carbon atoms. To characterize the structure, the number, length, and position of the branches need to be known. For most molecules this information cannot be included in the nomenclature. Branched polyethylene is usually only named by recognition of its low density after crystallization (low-density polyethylene). For characterization of low-density polyethylene, the distribution of branches must be identified within and between the molecules.
1.2 Nomenclature |
25 |
__________________________________________________________________
Fig. 1.21
The regularly-branched polymers separate at fixed points and present special shapes that can be used to modify surfaces. The three-dimensional dendrimers [6] form a sphere with an interesting space-filling character which depends on the chemical structure of the chains and branch points.
The ladder, sheet, and some of the space network polymers shown are listed under letters B, C, and D in Fig. 1.21. They are often rigid, and are thus class 3 macromolecules (see Sect. 1.1.3). The flexible molecules of interest, however, are difficult to make with a specific structure so that they can be named (for a ladder polymer, see the isomers of polybutadiene in Fig. 1.20). Even epoxies and rubbers are usually so poorly characterized, that precise naming is impossible.
1.2.4 Funny Polymers
The funny attribute of the molecules in this group is some unusual feature that can produce shapes with special properties. The following listing is by no means complete and should serve only as an inducement to think about other possible funny polymers. Together with linear macromolecules and branched, ladder and network polymers they make the building blocks for an unlimited number of shapes which have barely been studied. Figure 1.22 illustrates some complicated structures. The first example illustrates a rotaxane [7]. It consists of a normal polymer backbone on which unconnected, mobile rings are threaded (example: cyclic molecules of oxyethylene repeating units). The properties of such polymers should be linked to the restrictions induced by the two components on each other and the ease of slippage of the rings along the chain.
The mesogens that are introduced in the main-chain and side-chain of the molecules shown as items 2 and 3 are rigid, elongated or disc-like groups. They are