Crosslink network

Mangipudi et al. [63,88] reported some initial measurements of adhesion strength between semicrystalline PE surfaces. These measurements were done using the SFA as a function of contact time. Interestingly, these data (see Fig. 22) show that the normalized pull-off energy, a measure of intrinsic adhesion strength is increased with time of contact. They suggested the amorphous domains in PE could interdiffuse across the interface and thereby increase the adhesion of the interface. Falsafi et al. [37] also used the JKR technique to study the effect of composition on the adhesion of elastomeric acrylic pressure-sensitive adhesives. The model PSA they used was a crosslinked network of random copolymers of acrylates and acrylic acid, with an acrylic acid content between 2 and 10%. 

It may also be possible to crosslink the acrylic PSA with the help of multifunctional acrylates or methacrylates [87], These monomers can simply be copolymerized with the balance of the other monomers to form a covalently crosslinked network in one step. Since the resulting polymer is no longer soluble, this typ)e of crosslinking is typically limited to bulk reactions carried out as an adhesive coating directly on the article or in emulsion polymerizations where the crosslinked particles can be dried to a PSA film. 

Addition cure silicones can be delivered from solvent, waterborne emulsions, or 100% solids systems. The solvent free versions employ base polymers of intermediate molecular weight to achieve processable viscosity. These base polymers can have reactive moieties in terminal and/or pendant positions. These lower molecular weight, more functional systems result in a tighter crosslink network which feels rubbery to the hand. Low amounts of high molecular weight additives are included in some formulations to provide a more slippery feel [51,52]. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

The side group R is an ether of varying chain length and end group. Crosslinked networks can easily be prepared by incorporating unsaturated centers. A number of network structures of varying complexity... 

PF.O)l, crosslinked network, t (Polyin), various polymers, t Current fraction. 

N number of molecular chains between crosslinks (network strands) in... 

Amino resins are those polymers prepared by reaction of either urea or melamine with formaldehyde. In both cases the product that results from the reaction has a well crosslinked network structure, and hence is a thermoset polymer. The structures of the two parent amino compounds are shown in Figure 1.1. 

In conclusion, the lesson learned from the research carried out to date on the subject of polycarbosilanes is that the general rule that linear, noncrosslinked polymers are not suitable preceramic polymers applies here as well. Crosslinked network-type polymers are needed. Such structures can be generated in more than one way, but in the case of the polycarbosilanes they have, to date, been obtained mainly by thermolytic routes thermal treatment (with or without other chemical additives) in the case of the Yajima polycarbosilanes and the thermolysis of tetramethylsilane in the case of the Bayer process-derived polycarbosilane. 

NA Peppas, EW Merrill. Poly(vinyl alcohol) hydrogels Reinforcement of radia-tion-crosslinked networks by crystallization. J Polym Sci Polym Chem 14 441-444, 1976. 

Physical Crosslinking. A crosslinked network can also be achieved through physical aggregation of the hard segments of... 

Inhomogeneities in a real network may occur either because of a continuous distribution of molecular weight between crosslinks or due to the regions of different average molecular weights (as may be the case in randomly crosslinked networks). 

Two types of networks were prepared (i) randomly crosslinked polybutadiene, and (ii) model urethane networks, (a) polybutadiene based, and (b) poly(e-caprolactone) based. The randomly crosslinked networks were prepared from polybutadiene (Duragen 1203 obtained from General Tire and Rubber Co.) crosslinked with di-cumyl peroxide. Specifications of the as obtained polybutadiene are given in Table I. Polybutadiene was purified by dissolving in benzene and precipitating in methanol. Precipitated polybutadiene was redissolved in benzene. Seven different weights of dicumyl... 

If an inert good solvent is used in solution polymerization, the gel thus obtained will have a supercoiled (expanded) structure (Gel B). Gel B swells in good solvents much more than Gel A which is synthesized in bulk. If the amount of the crosslinking divinyl monomer in the reaction mixture is increased while the amount of solvent remains constant, highly crosslinked networks are formed that cannot absorb all solvent molecules present in the reaction mixture and a heterogeneous structure results (Gel C). A part of the solvent separates from the gel phase during polymerization and the formed Gel C consists of two continuous phases, a gel and a solvent phase. If the amount of solvent is further increased, a... 

---