Semi-crystalline polymers glass transition

In a DSC analysis of a semi-crystalline polymer, a jump in the specific heat curve, as shown in Fig. 2.22, becomes visible. The glass transition temperature, Tg, is determined at the inflection point of the specific heat curve. The release of residual stresses as a material s temperature is raised above the glass transition temperature is often observed in a DSC analysis. 

Brittleness is found with semi-crystalline polymers below their glass-rubber transition Tg. An example is PP, which becomes brittle at about T -10 °C. PE retains its ductile nature down to very low temperatures. Other polymers have a Tg of some tens of °C above room temperature, such as polyamides and thermoplastic polyesters. Various mechanisms are responsible for a reasonable impact strength at room temperature for polyamides this is, for instance, the absorption of water also secondary transitions in the glassy region may play a role. 

For semi-crystalline polymers with melting points of more than 100 °C above the glass transition temperature and for amorphous polymers far above the glass transition temperature Tg (at around T = Tg + 190°C), the shift factors obtained from time-temperature superposition can be plotted in the form of an Arrhenius plot for thermally activated processes ... 

The approximated lines at the right side of Fig. 3.14 correspond to an Arrhenius activation energy of approximately 120 kj/mol, which is significantly higher than the flow activation energy of the semi-crystalline polyolefin melts shown in Fig. 3.13. The temperature dependence of the melt viscosity for amorphous thermoplastics is substantially higher than that of semi-crystalline polymers and increases dramatically as the temperature approaches the glass transition temperature. 

FIG. 2.13 Extended glass transition in semi-crystalline polymers (reproduced from Struik, 1978). 

A second glass transition in semi-crystalline polymers... 

In some semi-crystalline polymers, two glass transitions can be distinguished a lower glass transition, Tg(L) and an upper glass transition, Tg(U). It may be assumed that Tg(L) arises from purely amorphous material, while Tg(U) arises from amorphous material which is under restraint due to the vicinity of crystallites. Frequently Tg(U) increases with the degree of crystallization. Some general rules are ... 

To semi-crystalline polymers with a glass transition temperature well below the reference temperature, Eq. (13.28) may directly be applied. 

Struik also (1987-1989) showed that his concept of physical ageing and its affects on the mechanical behaviour can be extended to semi-crystalline polymers the only additional assumption needed is that in semi-crystalline polymers the glass transition is broadened and extended towards the high temperature side. Fig. 13.55 illustrates this. 

We have seen already (Sect. 13.4.7) that every amorphous material (including that in semi-crystalline polymers) becomes brittle when cooled below the first secondary transition temperature (Tp) and becomes ductile when heated above the glass transition point (Tg). Between these two temperatures the behaviour - brittle or ductile - is mainly determined by the combination of temperature and rate of deformation. 

Those which do crystallise invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterised by their melting temperature (TJ. Many thermoplastics are, however, completely amorphous and incapable of crystallisation, even upon annealing. Amorphous polymers (and amorphous phases of semi-crystalline polymers) are characterised by their glass transition temperature (T), the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft). This transition corresponds to the onset of chain motion below T the polymer chains are unable to move and are frozen in position. Both T and T increase with increasing chain stiffness and increasing forces of intermolecular attraction. 

The heat performance of conventional ABS correlates in general with the glass transition temperature (Tg) of the rigid phase. Table 15.2 lists some typical rgs of amorphous polymers. Also listed are the crystalline melting points (Tm) for semi-crystalline polymers. Typically, the heat performance of a neat semicrystalline polymer under low load correlates with its Tm. 

The conditions under which a particular deformation mode (coarse shear banding or deformation in a diffuse shear zone) predominantes seem to depend mainly on the ambient temperature, T, as compared to the glass transition temperature, T, of the material. This hypothesis can be deduced from a diagram of shear modulus G vs. ratio T/T for the tested polymers (Fig. 32). The amorphous PS as well as the semi-crystalline polymers PP and PB-1 exhibit a tendency to formation of coarse shear bands when the ratio of T/T is distinctly smaller than 0.75. There exists a... 

Twombly, B. Determination of weak glass transitions in semi-crystalline polymers. Proc NATAS 1991, 20, 63. 

Figure 3.2. Typical output from a DSC when a semi-crystalline polymer is heated through the glass (Tg), crystallization (Tc) and melt (Tni) transitions. The exotherm on oxidation is also shown.

A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers like, for instance, polypropylene. Struik [11] measured for PP a continuously increasing dynamic stiffness at 20°C in combination with a decrease of the intensity of the glass-rubber (S) transition of PP (the temperature location of the S-transition did not change). Struik called this phenomenon an amorphous phase ageing effect a densification process of the amorphous PP phase due to a free volume relaxation effect. 

Solid state forming is normally understood as forming a semi-crystalline polymer above its glass transition temperature, T, but below its melting point T. Amorphous pofymers have been used but with less success. They are too stiff and too brittle below T, whereas above T the... 

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