Other Thermal Transitions

Thermal transitions other than Tg and are sometimes observed in polymers. Some polymers possess more than one crystal form, so there will be an equilibrium temperature of transition from one to another. Similarly, second-order transitions below Tg occur in some materials Tg is then termed the a transition, the next lower is the /3 transition, and so forth). These are attributed to motions of groups of atoms smaller than those necessary to produce Tg (type 3 motions, Section 6.2). These transitions may strongly influence properties. For example, tough amorphous plastics (e.g., polycarbonate) have such a transition well below room temperature, while brittle amorphous plastics (e.g., PS and PMMA) do not. 

The existence of another transition above Tg has been claimed, but is still the subject of considerable controversy. This 7)/ liquid-liquid transition) presumably represents the boundary between type 1 and type 2 motions. It has been observed in a number of systems [6-8], and it has been suggested that T)/ 1-2 (in absolute temperature) for all polymers [6]. For each article that reports 7)/, however, it seems that there is another that claims that 7)/ results from impurities (traces of solvent or unreacted monomer) in the sample or is an artifact of the experimental or data-analysis technique [9,10]. 

2 Three DSC runs are made on a semicrystalline polymer sample starting at room temperature and passing through the glass-transition temperature and melting point. 

Three different heating rates are used 1, 5, and 20°C/min. Sketch qualitatively the expected DSC thermograms to show how you think the observed Tg and will vary with heating rate. 

3 Injection molding consists of squirting a molten polymer into a cold metal mold. When thick parts are molded from a crystallizable polymer (e.g., polypropylene), they sometimes exhibit sink marks, where the surface of the part has actually sunk away from the mold waU. 

In addition to the glass transition temperature Tg, amorphous polymers have other relaxation temperatures such as Tp, Ty, and Tip. As previously indicated, the molecular motions associated with Tg result from semicooperative actions involving torsional segmental oscillations and/or rotations around backbone bonds in a 

Covalent chemical bonds that occur between macromolecules are known as crosslinks. Their presence and density have a profound influence on both the chemical and mechanical properties of the materials in which they occur. 

As we have seen previously the presence of crosslinks between macromolecules influences the way in which these materials respond to heat. Uncrosslinked polymers will generally melt and flow at sufficiently high temperatures they are usually thermoplastic. By contrast, crosslinked polymers cannot melt because of the constraints on molecular motion introduced by the crosslinks. Instead, at temperatures well above those at which thermoplastics typically melt, they begin to undergo irreversible degradation. 

The mechanical properties of polymers also depend on the extent of crosslinking. Uncrosslinked or lightly crosslinked materials tend to be soft and reasonably flexible, particularly above the glass transition temperature. 

Heavily crosslinked polymers, by contrast, tend to be very brittle and, unlike thermoplastics, this brittleness cannot be altered much by heahng. Heavily crosslinked materials have a dense three-dimensional network of covalent bonds in them, with little freedom for motion by the individual segments of the molecules involved in such structures. Hence there is no mechanism available to allow the material to take up the stress, with the result that it fails catastrophically at a given load with minimal deformation. 

Again because of the crosslinks, such brittle behaviour occurs whatever the temperature unlike brittle materials based on linear polymers, there is no temperature at which molecular motion is suddenly freed. In other words, the Tg, if there is one, does not produce dramahc changes in mechanical properties so that the material is changed from one that undergoes brittle behaviour to one that exhibits so-called tough behaviour. 

Crosslinking can be introduced into an assembly of polymer molecules either as the polymerisation takes place or as a separate step after the initial macromolecule has been formed. Typical of the first category are polymers made by step processes, often condensations, in which monomers of functionality greater than 2 are present. The relative concentration of such higher functionality monomers then determines the density of crosslinks in the final material. 

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Some polymers undego other thermal transitions in addition to Ts and Tm. These include crystal-crystal transitions (i.e., transition from one crystalline form to another and crystalline-liquid crystal transitions. 

Thermal properties are the relationships between the polymer properties and temperature. We will discuss melting temperature, glass transition temperature, and other thermal transitions, as well as heat capacity, thermal conductivity, and dimensional changes due to temperature variation. 

Polymers may possess other thermal transitions besides the melt and glass-transition temperatures. Not all of these transitions can be observed... 

A melting process is also illustrated in Figure 1.41 for a PET polymer which is slowly heated through its melting temperature and two other thermal transitions besides. 

Differential scanning calorimetry (DSC) was employed for the measurement of the Tg s and the detection of any other thermal transitions. For this purpose, a TA Instrument Model DSC 2910 was used with a heating rate of 10°C min for samples weighing 5-15 mg. An Auto TGA 2950HR V5.4A thermogravimetric analyzer... 

It is interesting to note that for the pea storage protein, other thermal transitions can be induced, by changing the ionic conditions. Transitions such as these should be investigated... 

Outside the temperature range given above (and in a few cases also in this range) a more complex temperature behavior can take place (e.g helix-coil and other thermal transitions, cold and heat denaturation). The occurrence of complicated thermal transitions is not restricted to proteins, but is of special interest for nucleic acids, lipids and other biopolymers (cf [86D1]). 

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