Glass Transition Study

Because electron micrographs and glass transition studies as such cannot prove interpenetration, some modeling of the data is necessary to arrive at a coherent picture. While some of the quantitative aspects of the interpenetration and phase continuity were discussed in Chapter 4 on homo-IPNs, a qualitative review of the evidence is nevertheless instructive. 

A. J. Curtius, M. J. Covitch, D. A. Thomas, and L. H. Sperling, Polybutadiene/Polystyrene Interpenetrating Polymer Networks, Polym. Eng. Sci. 12(2), 101 (1972). Polybutadiene/Polystyrene Network. Interpenetrating polymer network. Impact resistance and glass transition studies. 

Glass transition Glass transition studies have been reported on amorphous PS [343], araldite 2011, adhesive-metal bonds [344], thin PS films [345], and PEG entrapped into polylactic acid [346]. 

Molecular weight, thermogravimetric (TGA), differential thermal (DTA), and glass transition studies have also been run on both fractionated and unfractionated copolymers (Table 9.1). For a fractionated copolymer, with anhydride in the range of 7-13 mol %, the following relationship between number-average molecular weight and intrinsic viscosity holds ... 

Glass transition studies have also been reported on thin polyethylene films [20], and PEG entrapped into poly lactic acid [17]. 

How is the glass transition studied A common method is to observe the variation of some thermodynamic property with T, for example, the specific volume, as shown in Fig. 8.1. Note that the slope of the o vs. T plot increases above the glass transition temperature. 

A semi-IPN type of synthesis was carried out by Patel et on castor oil prepolymers reacted with oxalic, malonic, succinic, glutaric, adipic, suberic and sebacic acids. The highly viscous prepolymers were dissolved in acetone, and acrylamide and benzoyl peroxide were added. After reaction, the semi-IPN was precipitated and washed. Glass transition studies via DSC resulted in two glass transitions, one near 75 °C and one near 290 ""C. 

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). 

---