Crystallisation copolymers

Fig. 2.22 SAXS curve of a crystallizable copolymers of 1-lactide, e-caprolactone and glycolide. Wavelength of X-rays are 1.4 A. Reprinted from figure 9a in Polymer 46 (17), 6411-6428 W. Channuan, J. Siripitayananon, R. Molloy, M. Sriyai, F.J. Davis and G.R. Mitchell, The structure of crystallisable copolymers of 1-lactide, e-caprolactone and glycolide. Copyright (2005) with permission from Elsevier...

The polyamide copolymer of dodecanoic acid with methylenedi(cyclohexylamine) (MDCHA, PACM) was sold as continuous filament yam fiber under the tradename QIANA. As late as 1981, over 145,000 t was produced using high percentages, typically 80%, of trans, trans MDCHA isomer. The low melting raffinate coproduct left after t,t isomer separation by fractional crystallisation was phosgenated to produce a Hquid aUphatic diisocyanate marketed by Du Pont as Hylene W. Upon terrnination of their QIANA commitment, Du Pont sold the urethane intermediate product rights to Mobay, who now markets the 20% trans, trans—50% cis, trans—30% cis, cis diisocyanate isomer mixture as Desmodur W. In addition to its use in polyamides and as an isocyanate precursor, methylenedi (cyclohexyl amine) is used directiy as an epoxy curative. The Hquid diamine mixture identified historically as PACM-20 is marketed as AMICURE PACM by Anchor Chemical for performance epoxies. 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Pecrystallisation of a copolymer having 15 wt % VC has been found to be nucleated by material that survives the melting process plus new nuclei (74). The maximum crystallisation rate occurred at 373 K the maximum nucleation rate at 283 K. Attempts to melt all the polymer led to degradation that interfered with recrystallisation. 

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

The regular structure of the alternating copolymer with its absence of side chains enables the polymer to crystallise with close molecular packing and with interchain attraction augmented by the carbonyl groups. As a result these polymers exhibit the following characteristics ... 

Because of the extensive crystallisation, even in the copolymers, high strengths are achieved even though the molecular weights are quite low (-20000-50000). A typical 85 15 copolymer plasticised with diphenyl ethyl ether has a melting point of about 170°C, a glass temperature of about - 17°C and a maximum rate of crystallisation at approximately 90°C. ... 

Since some properties of the vinylidene chloride-vinyl chloride copolymers are greatly dependent on crystallisation and orientation it is convenient to consider the applications of these copolymers and then to discuss the properties of the products. 

As has been mentioned earlier, a number of copolymers such as nylon 66/610/6 are available. Sueh a copolymer has an irregular structure and thus interchain bonding and crystallisation are limited. As a consequence the copolymer is soluble in alcohols and many other common polar solvents. 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

The copolymers are prepared using a mixture of dimethyl terephthalate and dimethyl naphthalate. Published data indicates a reasonably linear relationship between and copolymer composition on the lines discussed in Section 4.2, e.g. Tg for a 50 50 copolymer is about 100°C which is about mid-way between Tg figures for the two homopolymers. In line with most other copolymers there is no such linearity in the crystalline melting point (T, ). As comonomer levels are introduced drops from the values for both homopolymers and indeed crystallisation only readily occurs where one of the components is dominant, i.e. 80%. Thus commercial copolymers are usually classified into two types ... 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

Thermoplastic polyester rubbers are also block copolymers of polyethers and polyesters. The polyester groups are capable of crystallisation and the crystal structures act like cross-links. These materials have good hydrocarbon resistance. Similar thermoplastic polyamide rubbers are also now available. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

With increase in length of Aliphatic chain there occurs a decrease in Melting point and ease of processing. Copolymers have reduced ability to crystallise and have lower Melting points and tensile strengths. Nylons are soluble in a few solvents such as acetic acid, Phenol, etc. at room temperature. They swell in alcohol. They are resistant to oils and fuels. Concentrated mineral acids attack nylon rapidly. Nylon get affected by alkalis and oxidising agents. 

By rapid expansion of supercritical propane solution (RESS), and isobaric crystallisation (ICSS), isotactic polypropylene and ethylene-butylene copolymers were precipitated from the supercritical solution. The RESS process produced microfibres with a trace of microparticles, while the ICSS process produced microcellular products. Improvement in thermal stability was achieved by first synthesising a thermoplastic vulcanisate from polypropylene and ethylene-propylene-diene terpolymer from a supercritical propane solution, followed by RESS. 28 refs. 

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

Also, a series of well-defined polyolefins, including perfectly branched polyethylene and ethylene/a-olefin copolymers, have been synthesised via acyclic diene metathesis polycondensation [scheme (28)] [47] these well-defined polyolefins have been designed to model the crystallisation of polyethylene and olefin copolymers. 

However, replacement strategies are not straightforward with compromises on either performance and cost having to be addressed. An excellent example of this centres around the chlorine issue. Chlorine imparts a unique set of properties to addition polymers, such as the ability to crystallise on film formation. Vinylidene chloride copolymers have exeellent gas, odour and moisture barrier properties impossible to achieve from non-chlorinated waterborne film forming materials. In the barrier packaging area it has a unique position, with proposed replacements being unable to fully match its overall... 

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