Polymer network polymerization

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

Equation (5.47) is of considerable practical utility in view of the commercial importance of three-dimensional polymer networks. Some reactions of the sort we have considered are carried out on a very large scale Imagine the consequences of having a polymer preparation solidify in a large and expensive reaction vessel because the polymerization reaction went a little too far Considering this kind of application, we might actually be relieved to know that Eq. (5.47) errs in the direction of underestimating the extent of reaction at... 

Gels are viscoelastic bodies that have intercoimected pores of submicrometric dimensions. A gel typically consists of at least two phases, a soHd network that entraps a Hquid phase. The term gel embraces numerous combinations of substances, which can be classified into the following categories (2) (/) weU-ordered lamellar stmctures (2) covalent polymeric networks that are completely disordered (2) polymer networks formed through physical aggregation that are predominantly disordered and (4) particular disordered stmctures. 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

To understand the global mechanical and statistical properties of polymeric systems as well as studying the conformational relaxation of melts and amorphous systems, it is important to go beyond the atomistic level. One of the central questions of the physics of polymer melts and networks throughout the last 20 years or so dealt with the role of chain topology for melt dynamics and the elastic modulus of polymer networks. The fact that the different polymer strands cannot cut through each other in the... 

Sheu and coworkers [111] produced polysty-rene-polydivinylbenzene latex interpenetrating polymer networks by the seeded emulsion polymerization of styrene-divinylbenzene in the crosslinked uniform polystyrene particles. In this study, a series of uniform polystyrene latexes with different sizes between 0.6 and 8.1... 

To prepare an interpenetrating polymer network (IPN) structure, PU networks having ACPA units were immersed with MMA and polymerized. PU-PMMA semi-lPN thus formed was given improved interfacial strength between PU and PMMA phases and showed flexibility with enforced tear strength [65,66]. 

The polymerization of nonconjugated diene monomers might be expected to afford polymer chains with pendant unsaturation and ultimately, on further reaction of these groups, crosslinked insoluble polymer networks. Thus, the finding by Butler et a .,, 03, n5 that polymerizations of diallylammonium salts, of general structure 8 [e.g. diallyldimethylammonium chloride (9)] gave linear saturated polymers, was initially considered surprising. 

Chemical analyses reveal that measurable amounts of uranyl ion are actually present in Pu(IV) polymers grown in mixtures of Pu(IV) and uranyl nitrate suggesting that uranyl ion is being taken up in the polymer network and consequently hampers the growth through a chain termination process as suggested in Fig. 3. The uranyl serves to terminate active sites because it does not typically form extensive polymeric aggregates as does Pu(IV) instead it tends only to dimerize and, at most, tri-merize (4). 

Electro-optic materials can be made using liquid crystal polymer combinations. In these applications, termed polymer-stabilized liquid crystals [83,86], the hquid crystal is not removed after polymerization of the monomer and the resulting polymer network stabilizes the liquid crystal orientation. 

Under UV irradiation, the photoinitiator cleaves into radical fragments that react with the vinyl double bond and thus initiate the polymerization of the monomer. If the latter molecule contains at least two reactive sites, the polymerization will develop in three dimensions to yield a highly crosslinked polymer network. 

J Hasa, J Janacek. Effect of diluent content during polymerization on equilibrium deformational behavior and structural parameters of polymer network. J Polym Sci Part C 16 317-328, 1967. 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

This process involves the suspension of the biocatalyst in a monomer solution which is polymerized, and the enzymes are entrapped within the polymer lattice during the crosslinking process. This method differs from the covalent binding that the enzyme itself does not bind to the gel matrix. Due to the size of the biomolecule it will not diffuse out of the polymer network but small substrate or product molecules can transfer across or within it to ensure the continuous transformation. For sensing purposes, the polymer matrix can be formed directly on the surface of the fiber, or polymerized onto a transparent support (for instance, glass) that is then coupled to the fiber. The most popular matrices include polyacrylamide (Figure 5), silicone rubber, poly(vinyl alcohol), starch and polyurethane. 

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

In this work, the kinetics of these reactions are closely examined by monitoring photopolymerizations initiated by a two-component system consisting of a conventional photoinitiator, such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and TED. By examining the polymerization kinetics in detail, further understanding of the complex initiation and termination reactions can be achieved. The monomers discussed in this manuscript are 2-hydroxyethyl methacrylate (HEMA), which forms a linear polymer upon polymerization, and diethylene glycol dimethacrylate (DEGDMA), which forms a crosslinked network upon polymerization. 

The polymerization rate was essentially zero in each of the systems (even with unreacted double bonds present and continued initiation) after 9 minutes of exposure to UV light. The maximum functional group conversion reached in each system was 96% (1 wt% 1651), 87% (0.5 wt% 1651), and 83% (0.1 wt% 1651). If equal reactivity of the double bonds is assumed, only between 0.16 to 2.89% of unreacted monomer will be present at these total double bond conversions. Unreacted monomer can affectively plasticize the polymer network rendering it more pliable and decreasing its mechanical properties, and unreacted monomer may compromise the biocompatible nature of the system if the monomer leaches to a toxic level. Therefore, it is desirable to identify polymerization conditions which maximize the conversion of monomer. 

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