Latex, IPN

Silicone co-polymer networks and IPNs have recently been reviewed.321 The development of IPNs is briefly described, and the definitions of the main (non-exclusive) classes of the IPNs are cited. Examples of latex IPNs, simultaneous and sequential IPNs, semi-IPNs, and thermoplastic IPNs are provided. The use of silicone-silicone IPNs in studies of model silicone networks is also illustrated. Networks in which siloxane and non-siloxane components are connected via chemical bonds are considered co-polymer networks, although some other names have been applied to such networks. Today, some of the examples in this category should, perhaps, be discussed as organic-inorganic hybrids, or nanocomposites. Silicone IPNs are discussed in almost all of the major references dealing with IPNs.322-324 Silicone IPNs are also briefly discussed in some other, previously cited, reviews.291,306... 

A few papers dealing with silicone latex IPNs have appeared. Frisch et al. [ 111 ] patented a process whereby two emulsions were prepared the first contained a hydroxyl-terminated PDMS and stannous octoate, the second a crosslinked polyurethane, poly(urethaneurea) or polyacrylate latex. The two lattices were blended and a film was cast and cured at 120 °C. The mechanical properties were found to be enhanced, especially in the case of the polyacrylate/PDMS IPN. Noteworthy is the fact that semi-IPNs such as the thermally stable... 

In a related patent (46) Amagi et al. synthesized a triple latex IPN. In brief, polymer 1 was a crosslinked SBR, polymer 2 was a crosslinked styrene-methyl methacrylate copolymer, and polymer 3 was a crosslinked poly (methyl methacrylate). All three were sequentially synthesized on the same latex particle. The latex material was then mechanically blended with linear poly (vinyl chloride). Also, Torvik (47) blended together four polymers that had different glass transition temperatures. 

Latex IPNs. Latex IPNs are the third type of IPNs and are manufactured according to the general schematic illustrated in Figure 3. Latex IPN synthesis involves the initial synthesis of a crosslinked seed polymer, usually in the form of an aqueous latex. The seed latex is then swollen with a second monomer/crosslinker/initiator system which is then polymerized "in situ" to form an aqueous IPN emulsion. Materials of this type are best suited to coating applications as illustrated by the development of "Silent Paint" by Sperling et al ( ). However, latex IPNs are limited to water emulsifiable monomer/polymer systems, most of which have fairly low service temperatures, less than 150 C. 

An early attempt to utilize the vibration absorbing effect of an IPN mixture was made by Sperling et al (4), who produced Silent Paint, of which one layer was an IPN. Hourston et al (5) illustrated typical IPN behavior in a 1 1 weight ratio Polyethylacrylate/Polyethylme-thacrylate latex IPN. A continued need for similar types of materials has prompted investigation of all polymeric materials known to be effective energy absorbers. 

Sound and vibration damping research with IPNs began in the early 1970 s and resulted in the formation of a constrained layer damping system with the inner damping layer a latex IPN paint (3 ). The constrained layer system results in a shearing effect within the IPN layer along with flexural and extensional motions as the composite panel vibrates. The added shear mechanism, not present in extensional applications, increases the amount of energy that is dissipated in each vibrational cycle. 

It is apparent from Figure 3 that the "Silent Paint" formulation enables useful damping over a temperature range from -20 to +50 C, as evidenced by a nearly constant percent critical damping. Transitions in the epoxy material immediately above the latex IPNs transitions increase the effective damping range to to +90 C. 

The submicroscopic emulsion polymerized form of IPN s would be expected to differ in mechanical properties from the counterpart bulk polymerized form in that (1) The latex particles are not crosslinked one to another allowing movement of one latex particle past another. (2) In bulk IPN s (10) it was shown that polymer I forms the continuous or more continuous phase while in latex IPN s polymer II tends to form the more continuous phase (1). 

All latex IPN s were synthesized by two-stage emulsion polymerization techniques (1 18) as follows To 300 ml of deionized deaerated stirre 3 waTer at 60°C were added 50 ml of a 10% (W/V) solution of sodium lauryl sulfate followed by 5 ml of a 5% (w/V) solution of potassium persulfate. The calculated quantity of comonomer was added at a rate of about 2 ml per minute. When the first monomer was fully added a minimum of one hour was allowed to elapse. Then a new portion of initiator was added but no new soap followed by the second charge of comonomers under similar reaction conditions. 

IPN s of 54/46 poly(methyl methacrylate)/poly(ethyl acrylate) were prepared by both the latex and bulk (10) routes. Both IPN s contained 0,4% (W/V) tetraethylene glycol dimethacrylate (TEGDM) crosslinking agent in each polymer. Samples of the latex IPN were film formed on glass petri dishes, All samples were vacuum dried at 60°C to constant weight. 

The loss moduli (E") of the latex IPN s shown in Figures 4, 7 and 10 are lower than that of PEMA with its broad secondary loss maximum. A summary of the loss modulus behavior of all the materials is given in Table IV in the form of the temperature bandwidths and the temperature bandwidth constants, as determined by equation 1. The value of E was assumed to be the same for all the materials because of the unavailability of accurate E data and because the calculation of the temperature bandwidth constant is very sensitive to the selection of E . 

Latex IPN. The polymers are made in the form of latexes, each particle constimting a micro-IPN. Depending on the rates of monomer addition relative to the rates of polymerization, various degrees of interpenetration and/or core/shell morphologies may develop. There are several kinds of latex IPNs, see Section 6.4. 

Latex IPNs offer unique synthetic opportunities. Since an IPN double network, ideally, is contained in each sub-microscopic latex particle, special effects are possible. The simplest case involves a crosshnked seed latex particle that is polymerized first. Then, monomer mix II is added. There are two subclasses. First, all of the monomer mix II can be added at once, or at least far more rapidly than the polymerization takes place. In that case, the monomer will first swell the latex particle, and then the excess monomer forms a shell around the swollen core. If the monomer mix II is added slowly, or more slowly than the initiator can polymerize the material, little monomer can swell into the particle, and a better defined core/sheU structure develops. 

The volume fraction of the two polymers also plays critical roles, see Figure 6.6 [Hsieh et al, 1996]. This behavior is similar to many polymer blends, where the intensity of the transition depends on the volume fraction. In Figure 6.6, an inward shift may also be observed. The glass transition temperature of three-polymer latex IPNs will be treated in Section 6.4.4. 

Latex IPNs. A crosslinked seed latex of polymer 1 is synthesized first. Then, monomer 2, plus crosslinker and initiator are added, usually without new surfactant. If the monomer 2 mix is added either all at once or rapidly, then swelling of polymer 1 by monomer 2 is encouraged, with subsequent greater interpenetration. 

As defined in Section 6.1 and above, the compositions described in Table 6.5 are all latex lENs, where two latexes are blended, then crosslinked. Latex IPNs are described in Section 6.4.7. 

Liu et al. also studied LIPN systems for damping control in coating applications [Liu et al, 1995]. A polystyrene (PS)/polyacrylate (PAcr) latex IPN was synthesized in a two-stage emulsion polymerization. Crosslinked PS was synthesized first as the seed polymer by a semi-continuous process,... 

Langmuir-Blodgett teehnique Latex blending Latex blends Latex IPN (LIPN)... 

The structure of the latex IPN s is imagined to be complex, especially in light of the Williams shell-core work discussed in Sections 3.1.2.2 and 13.4. 

Figure 8.30. Model of predicted latex IPN morphology, showing cellular structures, fine structures, and a shell-core morphology. (Sperling et aU 1972.)...

Other types of IPN s exist, of course. For example, Johnson and Labana (1972) recently synthesized a modified type of latex IPN as follows A crosslinked polymer network I prepared by emulsion polymerization served as a seed latex to linear polymer II. The resulting semi-IPN exhibited the usual core-shell morphology. After suitable coagulation and molding steps, polymer II was selectively crosslinked to form a macroscopic network, resulting in a thermoset material. The topology of this IPN therefore involves microscopic network islands of polymer I embedded in a continuous network of polymer II. 

IPN s have two continuous networks. In the case of latex IPN s, each particle ideally is composed of two network molecules. In bulk-prepared materials, the two networks are often presumed to be continuous on a... 

The characteristic damping behavior of a semicompatible mechanical blend (Mizumachi, 1969, 1970) is illustrated in Figure 13.12. This behavior may be compared with the behavior of the semicompatible latex IPN already discussed in Section 8.8.2. 

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