Polymeric networks, kinetics formation

Recently the polymeric network (gel) has become a very attractive research area combining at the same time fundamental and applied topics of great interest. Since the physical properties of polymeric networks strongly depend on the polymerization kinetics, an understanding of the kinetics of network formation is indispensable for designing network structure. Various models have been proposed for the kinetics of network formation since the pioneering work of Flory (1 ) and Stockmayer (2), but their predictions are, quite often unsatisfactory, especially for a free radical polymerization system. These systems are of significant conmercial interest. In order to account for the specific reaction scheme of free radical polymerization, it will be necessary to consider all of the important elementary reactions. 

Generalization of Flory s Theory for Vinyl/Divinyl Copolvmerization Using the Crosslinkinq Density Distribution. Flory s theory of network formation (1,11) consists of the consideration of the most probable combination of the chains, namely, it assumes an equilibrium system. For kinetically controlled systems such as free radical polymerization, modifications to Flory s theory are necessary in order for it to apply to a real system. Using the crosslinking density distribution as a function of the birth conversion of the primary molecule, it is possible to generalize Flory s theory for free radical polymerization. 

Kinetic investigations demonstrate that the order of the network formation is nearly unity (see Fig. 1). This result agrees with the polymerization kinetics [3], The formation of the network and the decrease of double bond follow the same kinetic law. 

In the case of network formation controlled by (irreversible) kinetics programmed polymerization regime (starved feed conditions, etc.). 

The mathematics again must be tailored to specific applications but, as an illustration, we consider the case that chains polymerize while being crosslinked by ABP, the latter forming tetrafunctional junctions (f = 4). We assume that binding of ABP occurs sufficiently fast that network formation is limited only by chain growth kinetics, so that the condition a>a implies (2A/4C) (-l), and (cf. Equation 10)... 

The chemistry described in this chapter is the same for the synthesis of both thermoplastic and thermosetting polymers. The transformations occurring during network formation may have a bearing either on the mechanisms (e.g., variation of the reactivity ratios along polymerization) or on the kinetics of network formation (e.g., decrease of reaction rate at the time of vitrification). These transformations and the effects they produce on the buildup of the polymer network will be discussed in the following chapters. 

A sustained drug release is favourable for drugs with short elimination half-life. It can be controlled by hydration and diffusion mechanisms or ionic interactions between the drug and the polymeric carrier. In the case of diffusion control the stability of the carrier system is essential, as its disintegration leads to a burst release. Therefore, the cohesiveness of the polymer network plays a crucial role in order to control the release over several hours. Due to the formation of disulphide bonds within the network thiomers offer adequate cohesive stability. Almost zero-order release kinetics could be shown for insulin embedded in thiolated polycarbophil matrices (Clausen and Bernkop-Schnurch 2001). In the case of peptide and protein drugs release can be controlled via ionic interactions. An anionic or cationic polymer has to be chosen depending... 

Other synthetic approaches to the kinetic problem have been taken. Variations in catalyst concentration for the formation of each component network from linear polyurethanes and acrylic copolymers have been used along with a rough measure of gelation time (5) to confirm the earlier (2-3.) results. Kim and coworkers have investigated IPNs formed from a polyurethane and poly(methyl methacrylate) (6) or polystyrene (7) by simultaneous thermal polymerization under varied pressure increasing pressure resulted in greater interpenetration and changes in phase continuity. In a polyurethane-polystyrene system in which the polyurethane was thermally polymerized followed by photopolymerization of the polystyrene at temperatures from 0 to 40 C, it was found (8.) that as the temperature decreased, the phase-... 

---