Glass transition effect amorphous polymer

For cross-linking by y rays, see Section 21.2.1. Inspection of the Gr values shows that, although styrene can be graft-copolymerized onto poly(vinyl chloride), vinyl chloride cannot be grafted onto poly(styrene). In the first case, so many radicals are produced on the polymer chain that practically no homopolymerization occurs. Above the glass transition temperature (amorphous polymers) or the melting point (crystalline polymers), the Gr values of the polymer increase because of increased chain mobility. Thus different effects can be observed by varying the temperature. 

Figure 8.2 Volume as a function of time after memory effect thermal history. Curve 1 is a pure quench. Curves 2 to 4 correspond to quenches and annealing at 10, 15, and 25°C, respectively. (From Kovacs, A.J., Glass transition in amorphous polymers a phenomenological study, Adv. Polym. Sci., 3, 394,1963. With permission.)...

Two distinct models have been used for interpreting the influence of features such as chemical stmcture, molecular mass, cross-linking and plasticisers on the glass transition in amorphous polymers. The first approach considers changes in moleeular flexibility, which modify the ease with which conformational changes can take place. The alternative approach relates all these effects to the amount of free volume, which is assumed to attain a critical value at the glass transition. 

The incorporated SP exhibited photochromism in both of the immobilized bilayer complexes with montmorillonite and PSS. Kinetic measurements of the thermal isomerization (decoloration) were carried out for the annealed film. The decoloration reaction rate is dependent on the mobility of the surroundings and, in polymer matrices, is influenced by the glass transition. It was found that the reaction rates abruptly increased near the gel-to-liquid-crystal phase-transition temperature (54°C) of the immobilized bilayer due to increased matrix mobility in this system. The film prepared with montmorillonite gives more homogeneous reaction environments for the chromophore than those with the linear polymer (PSS). This leads to drastic changes in the reaction rate at the crystal-to-liquid-crystal phase transition of the bilayer, showing the effect of the phase transition of immobilized bilayers to be more pronounced than that of the glass transition of amorphous polymer matrices. 

The crystallization of blends tends to depend on the level of mutual miscibility of the components. In miscible blends, the general result is that suppression or otherwise of crystallization with miscibility is dependent on the relative glass transition temperatures of both phases [33, 34]. For example, in a blend of an amorphous and semicrystalline polymer, if the amorphous material has the higher Tg, the miscible blend will also have a higher Tg than that of the semicrystalline homopolymer and, at a given temperature, the mobility and thus the efficacy of the semicrystalline phase molecules to crystallize is reduced. The converse is often true if the amorphous phase has a lower glass transition. Effects such as chemical interactions and other thermodynamic considerations also play a role and the depression of the melting point in a miscible blend can be used to determine the Flory interaction parameter x [40]. 

The effect of the amorphous state and glass transition temperature on polymer properties... 

The glass transition of amorphous polymeric films was investigated by SPM imder various conditions. We established, using lateral force measurements, that the pressure exerted by the tip does not have an effect similar to a hydrostatic pressure on the properties of the polymer. Instead, an apparent transition is observed due to the viscoelastic nature of the sanq)le. We confirm the existence of a critical load and scan speed, which need to be determined to obtain accurate glass transition temperature measurements. 

On comparison of the yield strengths and elastic moduli of amorphous polymers well below their glass transition temperature it is observed that the differences between polymers are quite small. Yield strengths are of the order of 8000 Ibf/in (55 MPa) and tension modulus values are of the order of 500 000 Ibf/in (3450 MPa). In the molecular weight range in which these materials are used differences in molecular weight have little effect. 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

This difference in spatial characteristics has a profound effect upon the polymer s physical and chemical properties. In thermoplastic polymers, application of heat causes a change from a solid or glassy (amorphous) state to a flowable liquid. In thermosetting polymers, the change of state occurs from a rigid solid to a soft, rubbery composition. The glass transition temperature, Tg, ... 

Plasticizers and Copolymerization also shift the glass transition responses of the amorphous phase of crystalline polymers. In addition, the degree of Crystallinity and melting point are lowered. The resulting effects on the... 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

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