Glassy region

An additional check is the almost coincidence of the linear thermal expansion coefficients of the composite in the glassy region. Theory yields acl = 48.20 x 10-6 °C whereas experiment gives ac, = 48.00x 10 6 °C 1. This coincidence does not hold beyond glass transition. Indeed it was found that ot = 122.90 x 10-6 °C, whereas the experiment gave a 2 = 158 x 10"6 °C 1. 

The polymer may show a secondary transition in the glassy region as a result of small movements of parts of the chain or side groups (example PC). 

With an amorphous thermoplastic the brittleness temperature is about Tg, unless a secondary transition temperature Tgec occurs in the glassy region in that case the brittleness temperature may be in the neighbourhood of Tstc-... 

In the glassy region, the polymer is below its glass transition temperature, Tg, and typically has a modulus of 1010 dynes/cm2. The transition region includes the Tg, which is taken as the point of inflection of the modulus or the maximum in the damping curve. The modulus drops by a factor of 1000 in this region. The... 

This picture is oversimplified strictly speaking, curve b should be drawn at a somewhat higher level than a in the glassy region the stiffness of the crystal exceeds that of the glass. 

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. 

Amorphous, glassy polymers, used far below their Tg, are cold-brittle if no other mechanisms are active an example is PS. If a polymer has been improved in impact strength by the addition of a rubbery phase (high-impact PS or PVC, ABS etc.), then the cold-brittleness temperature is related to the Tg of the added rubber. If the polymer shows a secondary transition in the glassy region (such as PC), then this governs the brittleness temperature. 

It is clear that the determination of such a modulus temperature curve takes an awful lot of time. Moreover, the transitions in the glassy region are difficult to determine, because the time needed for such a transition will be very small it may be of the order of or even much faster than the time in practice to apply an instantaneous deformation. For that reason in general use is made of dynamic mechanical measurements as a function of frequency to elucidate the modulus temperature curves, in particular in the glassy region. An additional advantage is that elastic and viscous forces are separated in this kind of measurements. 

As already reported by several authors, the addition of carbon nanotubes did not affect the storage modulus in the glassy region, nevertheless a strong increase with the filler content is observed in the rubbery region. In conventional composites, this increase is mainly attributed to interfacial interactions leading to introduction of additional cross-links into the network by the filler. These interfacial interactions contribute to the formation of an adsorption layer whose thickness has been estimated around 2 or 3 nm and where... 

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