Polyacrylonitrile glass transition temperature

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

This combination of monomers is unique in that the two are very different chemically, and in thek character in a polymer. Polybutadiene homopolymer has a low glass-transition temperature, remaining mbbery as low as —85° C, and is a very nonpolar substance with Htde resistance to hydrocarbon fluids such as oil or gasoline. Polyacrylonitrile, on the other hand, has a glass temperature of about 110°C, and is very polar and resistant to hydrocarbon fluids (see Acrylonitrile polymers). As a result, copolymerization of the two monomers at different ratios provides a wide choice of combinations of properties. In addition to providing the mbbery nature to the copolymer, butadiene also provides residual unsaturation, both in the main chain in the case of 1,4, or in a side chain in the case of 1,2 polymerization. This residual unsaturation is useful as a cure site for vulcanization by sulfur or by peroxides, but is also a weak point for chemical attack, such as oxidation, especially at elevated temperatures. As a result, all commercial NBR products contain small amounts ( 0.5-2.5%) of antioxidant to protect the polymer during its manufacture, storage, and use. 

Because the polymer degrades before melting, polyacrylonitrile is commonly formed into fibers via a wet spinning process. The precursor is actually a copolymer of acrylonitrile and other monomer(s) which are added to control the oxidation rate and lower the glass transition temperature of the material. Common copolymers include vinyl acetate, methyl acrylate, methyl methacrylate, acrylic acid, itaconic acid, and methacrylic acid [1,2]. 

Howard, W. H., A. T. Watson and T. W. Dewitt The glass transition temperature of polyacrylonitrile. Paper presented at the International Symposium on Macromolecules. Wiesbaden 1959. 

The glass transition temperatures of polyacrylonitrile at +90°C and of polybutadiene at -90°C differ considerably therefore, with an increasing amount of acrylonitrile in the polymer, the Tg temperature of NBR rises together with its brittleness temperature. The comonomer ratio is the single most important recipe variable for the production of acrylonitrile-butadiene rubbers. 

Stretch blow molding holds the parison above its glass transition temperature (7 ) and stretch-orients it to increase modulus, strength, impact resistance, transparency, and impermeability. This is most important for PET, and is also used for PVC, polypropylene, and polyacrylonitrile. 

Below the glass transition temperature (Tg ca 100 °C) the depicted mechanism is assumed to be the main response of polyacrylonitrile fibers to stress. It may account for an elongation of the fiber of up to W%. As it may be expected, the extension is almost entirely reversible, if the stress is removed, and the kinetic conditions are provided permitting the macromolecules to restore the state of lowest energy (e. g. by steam treatment above Tg). 

Nevertheless, the migration of water into the fiber is sufficient to produce the above-mentioned plasticization effect. The chain mobility is inaeased, as indicated by a decrease of the glass transition temperature by 35—50 C This is a very fortunate fact, because dyeing of the fibers is possible only above the Tg, where the increased polymer segment mobility permits dye diffusion within the fiber. For the commercial polyacrylonitrile copolymer fibers, the Tg in water hes in the r on of= 80 so that dyeing at, or slightly below, the boiling point of water becomes... 

The polyether polyols based exclusively on ACN are commercialised because of their high glass transition temperature (T ) of the polyacrylonitrile solid fraction. However, they are not used for production of slabstock foams, but for PU elastomers (microcellular elastomers for shoe soles) and integral skin foams. 

FIGURE 12.16 Melting and glass transition temperatures of polyacrylonitrile gels plotted against the volume fraction of polymer. (From Krigbaum, W.R. and Tokita, N., J. Polym. Sci. 43, 467, 1960.)... 

Polyacrylonitrile is most commonly used in fiber form. Since it softens only slightly below its thermal degradation temperature, it must be processed by wet or dry spinning rather than melt spinning. Some typical properties are Glass Transition Temperature, Tg = 85 C Melting Temperature, Tm = 317 C Amorphous density at 25 C = 1.184 g/cc Molecular weight of repeat unit = 53.06 g/mole. 

Herrero and Acosta (80) investigated the microstmcture of poly(ethylene oxide)-poly[(octafluoropentoxy)(trifluoroethoxy)phosphazene] blends. Limited miscibility of both components was inferred, based on the observed shift of the components glass-transition temperatures. Wycisk and co-workers (81) prepared membranes from blends of sulfonated poly[bis(3-methylphenoxy)phosphazene] with polyimides, polyacrylonitrile, and Kynar FLEX PVDF. Morphology, electrochemical performance, and methanol permeabilities of the membranes were then evaluated as part of a program to investigate such blends in direct methanol fuel cells. The polymers were immiscible and a domain-type structure was observed. The best compatibility resulted when the tetrabutylammonium or sodium salt of the polyphosphazene was used (82). 

Several thermoplastics, both of the commodities kind [polystyrene (PS), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polypropylene (PP), polyvinylchloride (PVC) etc.] and engineering pol)uners [polyamides (PA), polyesters (PE), polycarbonates (PC), polyimides (PI), polysulfones (PSF), polyoxymethylene (POM), polyphenylene oxide (PPO) etc.] exhibit glass transition temperatures (Tg) higher than or close to room temperature (R.T.). As a consequence they show, at R.T. or below it, the shortcoming of brittle impact behaviour, which limits their commercial end-uses. 

Stronger intermolecular attractive forces pull the chains together and hinder relative motions of segments of different macromolecules. Polar polymers and those in which hydrogen bonding or other specific interactions are important therefore have high Tg. Glass transition temperatures are in this order polyacrylonitrile > poly(vinyl alcohol) > poly(vinyl acetate) > polypropylene. 

First, studying different polar polymers [258,263-265] such as poly(ethylene oxide) (PEO), polyvinyUdene fluoride (PVdF), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and polyvinyl chloride (PVC) in order to enhance the ionic conductivity of the SPEs. PEO has been foimd to be the most successful host material for SPEs due to its low glass transition temperature (-60 °C) [266]. Second, increasing the niunber of charge carriers by use of highly dissociable salts, and increasing the salt concentration. Third, suppressing the crystallization of the polymer chains reduces the conductivity at room temperature ([Pg.1101]

Up to now, poly(methyl methacrylate) and methyl methacrylate copolymers e.g. with styrene, butyl acrylate and dodecyl methacrylate) have been the most widely used acrylic polymers for nanocomposite preparation by emulsion and suspension polymerization. Less research has been based on other acrylic polymers, such as polyacrylonitrile, poly(butyl acrylate), " poly(butyl methacrylate), poly(2-ethylhexyl acrylate), poly(2-hydroxyethyl methacrylate), polyacrylamide, poly(lauryl acrylate)," poly(butyl acrylate-co-styrene)," " poly(acrylonitrile-co-styrene), poly(acrylonitrile-co-meth-acrylate)," poly(ethyl acrylate-co-2-ethylhexyl acrylate)" and poly(2-ethylhexyl acrylate-co-acrylic acid)," and sometimes small amounts of hydophilic acrylic monomers, such as hydroxyethyl methacrylate, methacrylic acid and acrylic acid, have been used as comonomers. " Therefore, it may be stated that, so far, the preparation of acrylic-clay nanocomposites has been based mainly on high glass transition temperature polymers, although nanocomposite materials with lower glass transition temperatures with improved or novel properties, which exhibit a balance of previous antagonistic properties, can also be achieved and are very desirable. Regarding nanocomposites of low glass transition temperature polymers, such as poly(butyl acrylate), poly(ethyl acrylate) and poly(2-ethylhexyl acrylate), which have been utilized as the main components of acrylic pressure-sensitive adhesives, little information is available. 

The reason for this change in with composition is that the glass transitions of the two components, as homopolymers, are — 87 C for polybutadiene and 106 °C for polyacrylonitrile. The glass transitions of the copolymers lie in between these two extremes. This is both the anticipated and the observed behaviour. The glass transition of a copolymer poly AB will lie at a temperature between the glass temperatures of the two homopolymers poly A, and poly B. For example, the dependence of on composition for the styrene-butadiene copolymers is shown in Fig. 3.9 the styrene contents of the commercial SBR rubbers fall in the range 10 to 40 per cent. 

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