Other Aromatic Polyesters

Botelho and co-workers [25] made a comparative study of the thermo-oxidative degradation of poly(ethylene naphthalate) (PEN) and poly(butylene naphthalate) (PBN) The mechanism of the degradation of model compounds for these two polymers was similar in many ways to that noted for the terephthalate equivalents [13]. 

GC-MS analysis of the oxidation of ethylene dinaphthalate revealed the following products (where Np denotes a naphthalene ring system)  

CO and CO2 were not detected, unlike with the terephthalate equivalents. 

The thermo-oxidation of PEN and PBN leads to yellowing, with the latter discolouring faster than the former. For both polymers, yellowing is related to oxygen uptake, indicating that this discoloration is due to 

Carboxylic acid end groups and anhydrides were detected in oxidised samples of these polymers, so it is proposed that the oxidation mechanism is very similar to that of the model compounds. 

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Several fundamental studies have shown the importance of monomer sequence distribution on mesophase behavior (26). Simply changing the direction of ester linkages in a chain affects the transition temperatures, the range of the mesophase stability and, in some cases, even the mesophase texture (2Z). Polyester chains are susceptible to transesterification, which raises the question of which sequence structure is actually responsible for the properties observed for a given polymer. A recent study of aromatic LC polymers by neutron scattering indicates that transesterification occurs in the mesophase at rates twice that in poly(ethylene terephthalate) (28). Such behavior has also been observed to occur in other aromatic polyesters where rapid sequence redistribution was detected by nmr, see for example, the chapters by Jin and Economy et al. The temperature dependence of this effect has not been fully explored, and it may not be as pronounced in those polymers which exhibit mesophase behavior at lower temperatures, for example, those with aliphatic spacers. 

Finally we mention that draw-induced mesophases have also been observed in some other semi-rigid chain polymers. Examples of such studies are cold drawing of glassy PEN [108,109] and oriented crystallization of PET/PEN copolymers [110, 111]. In all cases a smectic mesophase has been reported. In the absence of crystallization it can attain a degree of stability that resists decay from chain relaxation. This supports the view that the mesophase represents a thermodynamic state [101]. We expect that similar mesophases based on semi-rigid monomers can be observed in other polymers with intrinsic rigid building blocks, such as other aromatic polyesters and polyamides. 

Other aromatic polyesters which have been commercialised include poly(l,4-cyclohexylenedimethylene terephthalate) (PCT used in the production of circuit board components and automotive applications) and the clear, amorphous, polymer poly(l,4-cyclohexylenedimethylene terephthalate-co-isophthalate). 

Early studies of the thermal degradation of so-called polyarylates were covered by Neiman [100] and Ehlers and co-workers [101]. Since then, several highly aromatic and specifically liquid crystalline (mesogenic) polyesters have been examined in terms of their anaerobic thermal degradation characteristics. These inclnde homopolymers of hydroxybenzoic acids [102-105] copolymers of hydroxybenzoic acid with hydroxynaphthoic acid [105-108] polymers which are essentially copolymers of hydroxybenzoic acid and alkyene terephthalates [107-118] copolymers of hydroxybenzoic acid with other aromatic polyesters [119-122] phenolic and bisphenolic terephthalates [123-127] poly(oxynaphthoate)s [128] and liquid crystal polyesters (LCP) containing unsaturated acids as part of a copolyester chain [129-131]. 

In terms of tonnage and use, poly(ethylene terephthalate) is these days virtually a commodity plastic, with widespread, large volume, use in food packaging, beverage bottle and fibres. Other aromatic polyesters have increased in volume production over the last decade or so, such as poly( butylene terephthalate) and poly(ethylene naphthalate). There is also the promise of large quantity use of the most recently commercialised member of the family, poly (trimethylene terephthalate). 

Its flexibility and segmental mobility are reduced so much that the crystallization of a quenched, amorphous sample of molar mass 28,000 Da needs an induction time of almost 50 h at 460 K, the temperature of fastest growth, and the half-time of crystallization is only reached after about 7 days [43]. The heat capacity of the solid PC has been analyzed, and the heat capacity of the liquid PC was measured and compared to the other aromatic polyesters. 

There are also many reports on the thermal decomposition behaviors of other aromatic polyesters, which are not necessarily TLCPs. It is necessary to introduce these studies here, because these polymers have almost the same chemical structures subsequently, they have very similar thermal degradation behaviors as the aromatic polyester TLCPs [20,70-72]. 

Historically, PET was probably the first polymer that was considered as a model system for the whole group of semirigid semicrystalline polymers including other aromatic polyesters and polyamides. The semicrystalline structure of this polymer has been extensively studied for more than 50 years employing various experimental techniques such as SAXS/WAXS, electron diffraction, TEM, " smaU-angle light scattering,... 

Another family of aromatic polyesters. However, unlike LCP (the other aromatic polyester), they are only partly aromatic and are based on a completely different chemistry. The chemistry is closer to that of polycarbonate, in that the PAR molecules are linked together by ester groups (polycarbonate utilizes an ester made from carbonic acid). 

In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Most polyesters (qv) are based on phthalates. They are referred to as aromatic-aHphatic or aromatic according to the copolymerized diol. Thus poly(ethylene terephthalate) [25038-59-9] (PET), poly(butyelene terephthalate) [24968-12-5] (PBT), and related polymers are termed aromatic-aHphatic polyester resins, whereas poly(bisphenol A phthalate)s are called aromatic polyester resins or polyarylates PET and PBT resins are the largest volume aromatic-aHphatic products. Other aromatic-aHphatic polyesters (65) include Eastman Kodak s Kodar resin, which is a PET resin modified with isophthalate and dimethylolcyclohexane. Polyarylate resins are lower volume specialty resins for high temperature (HDT) end uses (see HeaT-RESISTANT POLYAffiRS). 

As the author pointed out in the first edition of this book, the likelihood of discovering new important general purpose materials was remote but special purpose materials could be expected to continue to be introduced. To date this prediction has proved correct and the 1960s saw the introduction of the polysulphones, the PPO-type materials, aromatic polyesters and polyamides, the ionomers and so on. In the 1970s the new plastics were even more specialised in their uses. On the other hand in the related fields of rubbers and fibres important new materials appeared, such as the aramid fibres and the various thermoplastic rubbers. Indeed the division between rubbers and plastics became more difficult to draw, with rubbery materials being handled on standard thermoplastics-processing equipment. 

The successful development of polyfethylene terephthalate) fibres such as Dacron and Terylene stimulated extensive research into other polymers containing p-phenylene groups in the main chain. This led to not only the now well-established polycarbonates (see Chapter 20) but also to a wide range of other materials. These include the aromatic polyamides (already considered in Chapter 18), the polyphenylene ethers, the polyphenylene sulphides, the polysulphones and a range of linear aromatic polyesters. 

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

Liquid crystalline aromatic polyesters are a class of thermoplastic polymers that exhibit a highly ordered structure in both the melt and solid states. They can be used to replace such materials as metals, ceramics, composites and other plastics... 

Aliphatic polyesters are the most economically competitive of the biodegradable polymers moreover, synthetic polyesters are expected to be degraded nonspecifi-cally by lipases. Although these polyesters are biodegradable, they often lack good thermal and mechanical properties. On the other hand, aromatic polyesters - such as... 

Scheme 1 shows the reaction which occurs when an aromatic polyester such as [I] is subjected to UV irradiation (5). The polymer first undergoes main-chain cleavage with subsequent rearrangement to polymer [II] which is photostable and can be used as a thin coating to protect efficiently other substrates which are normally photodegradable. 

Compared with other polymeric materials. LCPs have very high unidirectional properties. Iei/nt7 1 (Celanese Corp.t resins are primarily aromatic polyesters based on p-hydroxybenzoic acid and hydroxynaphthoic acid monomers. Xytlar " (Celanese Carp.) injection molding resins are polyesters based on terephthalic acid. />. p -dihydruxybiphenyl and p-hydroxybenzoic acid Differences in monomers are primarily responsible for the differences in specific properties and end uses. The fibrous nature of the polymers imparls good impact strengths. 

Such a route has been applied to other chemistries aromatic polyesters (PET or PEN), polyamides, polysulfones, polyphenylene sulfide, etc. 

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