Glass transition temperature of polyethylene

FIGURE 10-64 The approximate glass transition temperatures of polyethylene, atactic polypropylene and atactic polystyrene. 

The y relaxation of polyethylene, expressed in terms of tan 5, is centered in the vicinity of — 120°C at 1 Hz. This relaxation is believed to be caused by molecular motions occurring in the amorphous phase as indicated by the fact that the relaxation is very weak in highly crystalline polyethylene crystallized from dilute solutions (39,40). The relatively high intensity and universality of the y relaxation in polyethylenes, independent of whether they do or do not have branches in their structure, seems to suggest that the y relaxation may be associated with the glass transition. According to this interpretation, the glass transition temperature of polyethylene would be located in the vicinity of — 120°C. 

The heterogeneity of the crystalline polymer solid is accentuated still more in the case of mechanical properties by the enormous mechanical anisotropy of the crystals and the large difference in the elastic moduli of the crystalline and amorphous components. With polyethylene, the elastic modulus of the crystals is 3452 or 2403 X 1010 dynes/cm2 in the chain direction (E ) and 4 X 1010 dynes/cm2 in the lateral direction (E ) (2, 3). The elastic modulus of the amorphous component (Ea) of polyethylene is 109-1010 dynes/cm2 (4). This is significantly less than Eu and Ebut at least 10 times the elastic modulus of a rubber that has about five monomers in the chain segments between the crosslinks. This is quite surprising since room temperature is far above the glass transition temperature of polyethylene (Tg is either —20°C or — 120°C), and therefore one would expect a fully developed rubbery... 

THE DETERMINATION OF THE GLASS TRANSITION TEMPERATURE OF POLYETHYLENE BY X-RAY DIFFRACTION. 

Fakirov S and Krasteva B (2000) On the glass transition temperature of polyethylene as revealed by microhardness measurements, J Macromol Sci Phys B39 297-301. 

Considerable confusion exists in the literature concerning the real glass transition temperature of polymethylene, i.e. of ideal linear polyethylene. Values between 140 and 340 K have been reported (see Boyer, 1973,1975). In agreement with Boyer we are convinced, for a variety of reasons (see Boyer, 1973), that the correct Tg of amorphous polymethylene is 195 10 K. This gives for the basic contribution of -CH2- to Yg ... 

Polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), and PUR are the most common foams however, PP foams can provide favorable properties at a lower material cost. PP is stiffer than PE and can perform better in load bearing or structural applications. The low glass transition temperature of PP compared to PS provides increased flexibility and impact strength. Use of PP foams include packaging, automotive, insulation, and protection of underground pipe. 

Kanemitsu and Einami (1990) investigated the role of the polymer on hole transport in a series of 2-(p-dipropylaminophenyl)-4-(p-dimethylaminophenyl)-5-(o-chlorophenyl)-l,3-oxazole (OX) doped polymers. The polymers were a polyarylate (PA), bisphenol-A polycarbonate (PC), poly(methyl methacrylate) (PMMA), poly(styrene) PS, poly(vinyl chloride) (PVC), polyethylene terephthalate) (PET), and poly(vinyl butyral) (PVB). The glass transition temperatures of the polymers range from 322 (PVB) to 448 K (PA). The temporal features of the photocurrent transients were strongly dependent on the polymer. Figure 76 shows the results. The field was 4.0 x H)5 V/cm and the temperature 295 K. The transients were near rectangular for PS, PET, PA, and PMMA, and highly dispersive for PVC land PVB. This was attributed to the fact... 

Polyethylene glycol also lowers the glass transition temperature of PVA and acts as a plasticizer. 

Example 13.6 Both polyoxymethylene (DuPont Delrin 550) and polyethylene (Calanese Fortiflex A70) show similar mechanical behavior but the glass transition temperature of polyoxymethylene is about 50°C higher than that of polyethylene. Explain. 

Two processes may potentially take place in the presence of plasticizers increased mobility may induce crystallization or the solvating power of the plasticizer may affect crystalline phase or interfere with crystallization. Before these two potential results are discussed the effect of the plasticizer on chain mobility is reviewed. Figure 10.55 shows that the glass transition temperature of plasticized PVC is a linear function of plasticizer concentration. Figure 10.56 shows that introduction of the plasticizer increases a free volume. Figure 10.57 shows that the free volume linearly increases in plasticized polyethylene oxide. " These three graphs are characteristic of the main functions of plasticizers which are to make polymers more elastic and increase their tolerance to lowering the temperature. In fact, addition of the plasticizer and temperature increase have a very similar effect on the resultant flexibility and mobility of chain (plasticization and plastification, respectively). 

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