Glass transition temperature studies

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. 

U Wiesner, N. Reynolds, C. Boeffel, and H. W. Spiess, Photoinduced reorientation in liquid-crystalline polymers below the glass transition temperature studied by time-dependent infrared spectroscopy, Makromol. Chem., Rapid Commun. 12, 457-464 (1991). 

How is the glass transition temperature studied A common method is to observe the variation of some thermodynamic property with T, for example, the specific volume, as shown in Figure 6.1. As temperature rises, the polymer expands, with a change to a higher slope in the V versus T plot above the glass transition temperature. 

Several important problems involving polymer systems depend on a characterization of the interpenetration of chain molecules. These concern (a) the compatibility of polymers in bulk, (b) the entanglement of chain molecules in semidilute solution and (c) the interdiffusion of polymers above the glass transition temperature. Studies carried out over the last few years have shown that emission spectra from systems containing polymers tagged with two fluorescent labels can yield information concerning each of these problems. 

Cangialosi, D., Wubbenhorst, M., Groenewold, J., Mendes, E., Picken, S. Difliision mechanism for physical aging of polycarbonate far below the glass transition temperature studied by means of dielectric spectroscopy. J. Non-Cryst. Solids 351(33-36), 2605-2610 (2005)... 

The polymers compared in Table 2.3 were not all studied at the same temperature instead, each was measured at a temperature 100°C above its respective glass transition temperature Tg. We shall discuss the latter in considerable detail... 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

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