Interpenetrating polymer network Glass transition temperatures

Polyurethane-acrylic coatings with interpenetrating polymer networks (IPNs) were synthesized from a two-component polyurethane (PU) and an unsaturated urethane-modified acrylic copolymer. The two-component PU was prepared from hydroxyethylacrylate-butylmethacrylate copolymer with or without reacting with c-caprolactonc and cured with an aliphatic polyisocyanate. The unsaturated acrylic copolymer was made from the same hydroxy-functional acrylic copolymer modified with isocyanatoethyl methacrylate. IPNs were prepared simultaneously from the two-polymer systems at various ratios. The IPNs were characterized by their mechanical properties and glass transition temperatures. 

When the blend is prepared with a linear polymer and a crosslinkable monomer, a semi-interpenetrated polymer (semi-IPN) network is obtained and this concept has been applied to a linear heterocyclic polymer and a crosslinkable thermostable oligomer [111]. The linear heterocyclic polymers exhibit high glass transition temperatures, good fracture toughness (Table 6), but the high viscosity above Tg make them difficult to process. 

A random co-polymer or a blend of compatible polymers will have a single glass transition temperature intermediate between those of the two homopolymers. An example is shown in Figure 14 for nitrile-butadiene-rubber (22). The specific weight percents shown are those of commercial interest for NBR. In contrast, most polymer blends, graft and block copolymers, and interpenetrating polymer networks (IPN s) are phase separated (5) and exhibit two separate glass transitions from the two separate phases. Phase separated systems will not be considered here. 

For simultaneous interpenetrating networks (SINs), two independent, non-interfering reactions are required. Thus, a chain and a step polymerization have been the method of choice for many such polymerizations. Typical examples have involved PS and polyurethanes [Hourston and Schafer, 1996 Mishra et ai, 1995], and PMMA. A key factor in the kinetics of such polymerizations is to keep the system above the glass transition temperature of both components. If the glass transition of either the polymer network I or polymer network II rich phase vitrifies, the polymerization in that phase may slow dramatically. 

Characterisation of Glass Transition Behaviour in Interpenetrating Polymer Networks The multi-phase nature of IPNs results in complicated glass transition behaviour [101]. Figure 3.46 shows that heat capacity changes with temperature for a series 60 40 polyurethane (PU)/ polystyrene (PS) IPNs (see Table 3.5 for the compositional details) [131,132]. It is, however, not possible to obtain much detailed information from these heat capacity signals. 

In this case, an apparent activation energy is determined, and it has higher values than secondary relaxations 100-300 kJ/mol for urethane-soybean oil networks (Cristea et al. 2013), 200-300 kJ/mol for polyurethane-epoxy interpenetrating polymer networks (Cristea et al. 2009), more than 400 kJ/mol for semicrystalline poly(ethylene terephtalate) (Cristea et al. 2010), and more than 600 kJ/mol for polyimides (Cristea et al. 2008, 2011). The glass transition temperature is the most appropriate reference temperature when applying the time-temperature correspondence in a multifrequency experiment. The procedure allows estimation of the viscoelastic behavior of a polymer in time, in certain conditions, and is based on the fact that the viscoelastic properties at a certain tanperature can be shifted along the frequency scale to obtain the variation on an extended time scale (Brostow 2007 Williams et al. 1955). The shift factor is described by the Williams-Landell-Ferry (WLF) equation ... 

D. Klempner, H. K. Yoon, K. C. Frisch, and H. L. Frisch, Polyurethane-Polyacrylate Pseudo-Interpenetrating Networks Chemical Properties of Crosslinked Polymers, in Chemistry and Properties of Crosslinked Polymers, S. S. Labana, ed.. Academic, New York (1977). SINs of urethane rubbers. Morphological and mechanical properties. Glass temperature, transitions and polymer morphology of urethane rubber/acrylic copolymer SINs. 

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