Surface crosslinking

Paraformaldehyde fixes by crosslinking surface proteins. Ethanol and methanol work through precipitation of membrane components. Triton X-100 and Tween-20 permeabilize membranes. Other permeabilizers are saponin (0.1-10% [v/v] in PBSG), L-lysophosphatidylcholine, and n-octyl-P-D-glucopyranoside (1-10 pg/mL in water). 

There are a wide range of functional monomers which can be copolymerized with the principal monomers described in the previous section. These functional monomers are often used in very small amounts (typically 1-3% in a formulation) and provide reactive sites for crosslinking, surface modification, and post-polymerization processing of latex particles [21]. Examples of the roles that these functional monomers play during the interfacial crosslinking process are also given in this section. There are several major classes of these functional monomers, based on the type of reactive moiety which is introduced into the latex particle. These moieties include ... 

To a large extent the end-use requirements can be satisfied by the bulk polymer properties the other requirements are dictated by surface and colloidal properties. The bulk properties are usually determined by a handful of monomers as shown in Table 6.1. Crosslinking, surface and colloidal properties are governed by functional monomers which are described in Section 6.2.3. 

For example, in the earliest paper of Schonhorn and Hansen it was shown that the maximum tensile strength of the laminated structure Al-epoxy-He treated PE-epoxy Al is obtained after 5 s of plasma exposure, which corresponds to a crosslinked surface thickness of about 200 500 A. For untreated PE this appears to be the upper limit of the weak boundary layer [158]. 

Remove the substance by cleaning or via a liquid that dissolves it Crosslink surface, diffusion barrier Investigate penetration of water in the liquid Introduce adhesion promoters for stronger adhesion forces (organosilanes, organometallics)... 

If tire coupling to tire substrate is weak (physisorjDtion), as is tire case for alkylsiloxanes on a SiO surface in tire presence of a water layer, for example, tire packing may also be mainly driven by intennolecular forces. Stability in tliis system is provided by crosslinking between tire molecules (see below). 

Adechanical stahility. ChemisoriDtion to tire surface, intennolecular interactions and crosslinking between adjacent compounds—if possible—all contribute to tire resulting stability of tire monolayer film. Lateral force microscopy investigations revealed tliat tire mechanical stability towards lateral forces on tire nanometre scale is likely to be detennined by tire defect density and tire domain size on a nano- to micrometre scale [163, 1731. 

Coacrete can also be made water-repeUent by the polymerisation of vinyl monomers on the surface (85). Polymerisation can be iaitiated with peroxides, and polyfunctional methacrjiates can be used as crosslinking agents. These treatments have a tendency to produce changes ia color and gloss. 

Polymer-based, synthetic ion-exchangers known as resins are available commercially in gel type or truly porous forms. Gel-type resins are not porous in the usual sense of the word, since their structure depends upon swelhng in the solvent in which they are immersed. Removal of the solvent usually results in a collapse of the three-dimensional structure, and no significant surface area or pore diameter can be defined by the ordinaiy techniques available for truly porous materials. In their swollen state, gel-type resins approximate a true molecular-scale solution. Thus, we can identify an internal porosity p only in terms of the equilibrium uptake of water or other liquid. When crosslinked polymers are used as the support matrix, the internal porosity so defined varies in inverse proportion to the degree of crosslinkiug, with swelhng and therefore porosity typically being more... 

In the JKR experiments, a macroscopic spherical cap of a soft, elastic material is in contact with a planar surface. In these experiments, the contact radius is measured as a function of the applied load (a versus P) using an optical microscope, and the interfacial adhesion (W) is determined using Eqs. 11 and 16. In their original work, Johnson et al. [6] measured a versus P between a rubber-rubber interface, and the interface between crosslinked silicone rubber sphere and poly(methyl methacrylate) flat. The apparatus used for these measurements was fairly simple. The contact radius was measured using a simple optical microscope. This type of measurement is particularly suitable for soft elastic materials. 

Fig. 12. Schematic of a polymer-coated crosslinked PDMS cap in contact with a polymer-coated flat surface. The PDMS cap is oxidized in 02-plasma, and the polymer layer is coated by solvent casting. On flat surface, the polymer layer is spin coated.

Tirrell et al. [42,43] studied the role of interfacial chains in a more detailed fashion. Tirrell et al. [42,43] used a crosslinked PDMS cap in contact with a silicon wafer on to which a,o)-hydroxyl terminated PDMS chains are tethered by adsorption from a solution. The molecular weight of the narrow disperse PDMS samples was in the range of 20,000-700,000. The surface chain density was given by27 yj g e 0 is the volume fraction of PDMS in solution. 

Brown [46] continued the contact mechanics work on elastomers and interfacial chains in his studies on the effect of interfacial chains on friction. In these studies. Brown used a crosslinked PDMS spherical cap in contact with a layer of PDMS-PS block copolymer. The thickness, and hence the area density, of the PDMS-PS layer was varied. The thickness was varied from 1.2 nm (X = 0.007 chains per nm-) to 9.2 nm (X = 0.055 chains per nm-). It was found that the PDMS layer thickness was less than about 2.4 nm, the frictional force between the PDMS network and the flat surface layer was high, and it was also higher than the frictional force between the PDMS network and bare PS. When the PDMS layer thicknesses was 5.6 nm and above, the frictional force decreased dramatically well below the friction between PDMS and PS. Based on these data Brown [46] concluded that ... 

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%. 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

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