Polyisobutylene glass transition temperature

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

Symmetry also affects Tg. It might be expected that polyisobutylene, (—CH2 — C(CH3)2—) , would have a Tg greater than polypropylene, (—CH2 — CH(CH3)—) , as a consequence of the two —CH3 groups bonded to the backbone, which would tend to increase the chain rigidity. But this is not the case polyisobutylene is a rubber that has a Tg of —71°C, while polypropylene presents Tg = —23°C. Another example of how symmetry plays an important role in the Tg is seen if we compare poly (vinyl chloride) Tg = 81°C) and poly(vinylidene chloride) Tg = — 19°C) (31). As a general rule, it can be said that an increase in symmetry produces a decrease in the glass transition temperature. 

Demonstration of Instant Equilibration (Zf = 0). Data were gathered for several n-alkanes at various gas flow rates and column temperature of 100 C using a column coated with polyisobutylene (PIB). Under these conditions PIB is far above its glass transition temperature (Tg) and equilibration of probe and polymer is expected to be instantaneous. 

Before reviewing in detail the fundamental aspects of elastomer blends, it would be appropriate to first review the basic principles of polymer science. Polymers fall into three basic classes plastics, fibers, and elastomers. Elastomers are generally unsaturated (though can be saturated as in the case of ethylene-propylene copolymers or polyisobutylene) and operate above their glass transition temperature (Tg). The International Institute of Synthetic Rubber Producers has prepared a list of abbreviations for all elastomers [3], For example, BR denotes polybutadiene, IRis synthetic polyisoprene, and NBR is acrylonitrile-butadiene rubber (Table 4.1). There are also several definitions that merit discussion. The glass transition temperature (Tg) defines the temperature at which an elastomer undergoes a transition from a rubbery to a glassy state at the molecular level. This transition is due to a cessation of molecular motion as temperature drops. An increase in the Tg, also known as the second-order transition temperature, leads to an increase in compound hysteretic properties, and in tires to an improvement in tire traction... 

Common SS include polyethers, polyesters and polyalkyl glycols with glass transition temperatures in the range of -70°to -30°C. Commonly used macrodiols in the PUs synthesis are polyalkyl-diols, such as polyisobutylene diol [70], polybutadiene (PBU) [20, 71], or oligo-butadiene diols [72] as well as hydrogenated polybutadiene diol [20] polyether diols polytetrahydrofuran (PTHF or PTMO) [50-52], polyethylene glycol (PEG) or (PEO) [73], polypropyleneoxide (PPO) [73] or mixed blocks of them PEO-PPO-PEO [74] and PPO-THF [54] polyester diols poly(ethylene adipate) (PEA) [4,20], poly(butylene adipate) (PBA) [20, 73], and latterly polycaprolactone diol (PCL or PCD) [75], polyalkylcarbonate polyol [20] or mixed blocks of them, for example poly(carbonate-co-ester)diol [76], poly(hexamethylene-carbonate)diol [77], as well as poly(hexamethylene-carbonate-co-caprolactone)diol [78] and a mixed block copolymer of polyether and polyester blocks PCL-b-PTHF-b-PCL [79]. Examples schemes of macrodiols are shown in Eig. 1.9. 

The most well-studied polysiloxane viz., [Mc2SiO]n has a Tg of -123 °C [1]. This is one of the lowest values for any polymer. As mentioned earlier in Chapter 2 the glass-transition temperature of a polymer corresponds to a description of its amorphous state. The Tg of a polymer can be taken as a measure of the torsional freedom of polymer chain segments. Above its Tg a polymer has reorientational freedom of motion of its chain segments, while below its Tg this is frozen. Usually elastomeric materials have low glass-transition temperatures. For example, natural rubber has a glass-transition temperature of -72 °C. Similarly, polyisobutylene has a ass-transition temperature of -70 °C. Thus, poly(dimethylsiloxane) has consid-... 

Polymers of the isobutylene family (i.e., butyl and polyisobutylene) have very little tendency to crystallize and depend upon molecular entanglement or crosslinking for their strength, rather than upon crystallinity. The completely amorphous character of these polymers gives an internal mobility which imparts flexibility, permanent tack, and resistance to shock. Their low glass transition temperature, —60°C, in-... 

John D. Ferry is known as the leading figure in the history of polymer science on the subject of viscoelasticity. He graduated from Stanford University at the age of 19, as noted above. For his doctoral work with George Parks he studied the properties of polyisobutylene as a function of temperature. He found the glass transition temperature and characterized the viscoelastic properties (Fig. 5.6). 

The increase in heat capacity at the glass transition temperature is 5.4 cal deg mole or half of this amount per chain unit, which is close to average increase in heat capacity per bead of 41 glasses analyzed by Wunderlich (1960). Above the glass transition temperature, the heat capacity of polyisobutylene is about 20% higher and increases 20% faster than the heat capacity of polypropylene. One would expect this from the increased number of optical vibrations. 

Polyisobutylene and butyl mbber are two materials that are based upon the polymer made from the polymerization of isobutylene. Poly(isobutylene) is not reactive after manufacture and hence it has been used as a modifier for various types of adhesives and sealants. Butyl mbber is made by the addition of isoprene to the polymerization of isobutylene. This addition yields a small amount of unsaturation in the polymer, thus making this material crosslinkable. In addition, chlorinated and brominated versions of this material are also available. Poly(isobutylene)s have a low glass transition temperature of -60 °C and are therefore expected to have flexibility at low temperature. Because of their stmaure, poly (isobutylene) and butyl mbber are expected to have high impermeability to air and moisture transfer, making these materials ideal for sealant formulation. Poly(isobutylene) is manufactured in a wide range of molecular weights, ranging from 45 000 to 2110 000. 

The glass transition temperature, Tg, is the temperature below which the translational as well as long and short cooperative wriggling motions are frozen. In the robbery state, only the first kind of motion is frozen. The polymers that have their Tg values less than room temperature would be rubbery in nature, such as neoprene, polyisobutylene, or butyl rubbers. The factors that affect the glass transition temperatures are described in the following subsections. 

Ferguson and Prather (1944) found that with rapidly cooled samples upward temperature drifts occurred in the calorimeter in the glass transition range while the opposite effect, namely downward temperature drifts, was observed in the case of slowly cooled, or carefully annealed samples. In Furukawa and Reilly s (1956) investigation of polyisobutylene, the shock-cooled samples exhibited upward temperature drifts below T and downward temperature drifts above T , while the annealed sample exhibited only downward temperature drifts above T . Furukawa, McCoskey and Reilly (1955) found similar drifts in the case of some butadiene-styrene copolymers. 

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