Glass transition temperature crosslink density

Apparently, annealing was not impeded by crosslinks (Fig. 5.1). The density effects observed agree with the results of the glass transition temperature measurements (Sect. 4.2). There, the Tg of the annealed (and therefore denser) sample was consistently higher by about 2 K than the Tg of the quenched polymer. 

It is anticipated that the possibility of producing elastomeric lignin-containing PU s strongly depends on whether crosslink density can be held on a low level or not. It is also necessary to include soft segments, so that the glass transition temperature (Ta) of the PU falls below room temperature. 

Cyclopentene is readily available as a byproduct in the ethylene production. Norbornene 2-ethylhexyl carboxylate is obtained by the Diels-Alder reaction of 2-ethylhexyl acrylate with cyclopenta-diene (5). Norbornene isobornyl carboxylate, norbornene phen-oxyethyl carboxylate, and other related monomers are synthesized according to the same route. Polymers obtained from these esters exhibit excellent properties in terms of controlling the crosslinking density, the associated product modulus, and the glass transition temperature (Tg), thus allowing tailoring the properties of elastomers, plastics and composites. Other suitable monomers are summarized in Table 1.1 and sketched in Figure 1.2. 

Table 11.2 Crosslink density, glass transition temperature, and Young s modulus (Et from tensile test at 10 3, s 1 strain rate, Eu from ultrasonic propagation at 5 MHz frequency) for triglycidyl aminophenol-diaminodyphenylmethane-aniline (TGAP-DDM-AN) networks. (After Morel et a ., 1989.)... 

The Tg of linear polymers such as polyimides is heavily dependent on, and increases with, the molecular weight. The equivalent statement can also be said about crosslinked networks, where the glass transition temperature is dependent on the network crosslink density [111-117]. 

When chain segments can move relatively freely in cured polymers, it is most likely due to low crosslink density or the mobility of the molecular chain structure. The glass transition temperature is a measure of the mobility of the molecular chains in the polymer network as a function of temperature. The glass transition is the reversible change in a polymer from (or to) a rubbery condition to (or from) a hard and relatively glassy state condition (Fig. 3.14). This transition occurs at a temperature called the glass transition temperature or Tg. It is... 

The reduction in crosslink density increases the flexibility (elongation) of the resulting molecule, but at the expense of a lowering of the glass transition temperature. This, in turn, generally results in a significant decrease in tensile and shear strength as weh as a decrease in other performance properties, such as chemical and heat resistance. 

The main reason for the greater flexibility is due to long-chain difunctional materials that upon cure become part of the epoxy matrix. The result is a single-phase, flexible system. The disadvantage of this approach is a reduction in the crosslink density and consequently reductions in glass transition temperatures as well as the heat and chemical resistance of the system. 

The most significant differences in performance properties between a two-component, room temperature curing epoxy adhesive and a one-component, heat-curing type are the heat and chemical resistance. These differences are due primarily to the lower crosslink density or glass transition temperature of the room temperature curing types. 

Fig. 4. Glass transition temperature (T,) vs. crosslinking density (p(E )) for resins cured with various curing agents...

It is known that in the glassy state below the glass transition temperature the physical properties of epoxy resin as well as other amorphous polymers are generally little dependent on temperature and structure 1,28). Also, the modulus of elasticity (E) does only weakly depend on the crosslinking density. 

Control of the EB cured film properties through formulation is the key to meeting specific end use properties. A wide range of hardness and flexibility can be produced by adjustment of crosslink density and glass transition temperature. Studies on the effects of formulation variables on the tensile elongation properties of free films have proven especially valuable in development of coatings for flexible substrates. 

---