Surface-block copolymers

Excellent substrate compatibility Properly compounded adhesives, sealants, and pressure sensitive materials will adhere to almost any surface. Block copolymers have exhibited superior pressure sensitive adhesion to polyolefin and other low energy surfaces. 

Gaines [13] has reported on dimethylsiloxane-containing block copolymers. Interestingly, if the organic block would not in itself spread, the area of the block polymer was simply proportional to the siloxane content, indicating that the organic blocks did not occupy any surface area. If the organic block was separately spreadable, then it contributed, but nonadditively, to the surface area of the block copolymer. 

Annis B K, Noid D W, Sumpter B G, Reffner J R and Wunderlich B 1992 Application of atomic force microscopy (AFM) to a block copolymer and an extended chain polyethylene Makromol. Chem., Rapid. Commun. 13 169 Annis B K, Schwark D W, Reffner J R, Thomas E L and Wunderlich B 1992 Determination of surface morphology of diblock copolymers of styrene and butadiene by atomic force microscopy Makromol. Chem. 193 2589... 

The first case concerns particles with polymer chains attached to their surfaces. This can be done using chemically (end-)grafted chains, as is often done in the study of model colloids. Alternatively, a block copolymer can be used, of which one of the blocks (the anchor group) adsorbs strongly to the particles. The polymer chains may vary from short alkane chains to high molecular weight polymers (see also section C2.6.2). The interactions between such... 

Finally, we briefly mention interactions due to adsorbing polymers. Block copolymers, witli one block strongly adsorbing to tire particles, have already been mentioned above. Flere, we focus on homopolymers tliat adsorb moderately strongly to tire particles. If tliis can be done such tliat a high surface coverage is achieved, tire adsorbed polymer layer may again produce a steric stabilization between tire particles. 

In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

Certain block copolymers have also found appHcation as surfactants (88). Eor example, AB or ABA block copolymers in which one block is hydrophilic and one block is hydrophobic have proven useful for emulsifying aqueous and non-aqueous substances and for wetting the surface of materials. Examples of such surfactants are the poly(propylene oxide- /oi / -ethylene oxide) materials, known as Pluronics (BASC Wyandotte Co.). 

Special grades of styrenic block copolymers are usehil modifiers for sheet mol ding compounds (SMC) based on thermoset polyesters. They improve surface appearance, impact resistance, and hot strength. 

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 ... 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

Fig. I. Block copolymers tend to organise at an interface so that the two blocks, shown here as solid and dashed lines, are on either side of the interface. If one of the blocks is deuterated then chain pull-out can be distinguished from chain scission by the location of the deuterium on the fracture surface.

As with block copolymers, the important parameters are the surface density and length of the copolymer chains. Toughening of the interface may occurs as a result of pull-out or scission of the connector chains, or of fibril or craze formation in matrix. This last mechanism gives the highest fracture toughness, F, and tends to occur at high surface density of chains. 

Most types of PSAs have found some application in the label industry. Block copolymer-based adhesives are perhaps the most popular because of their high adhesion to a variety of surfaces, their low cost, their good performance over a range of temperatures and peel rates, and their ease of processing. For applications where high temperature performance is required, block copolymers have been formulated with high T end block associating resins or polymers. 

For particularly difficult to adhere substrates such as metallized or fluorocarbon treated surfaces the same principles that are applied in freezer grade adhesives are used. For especially difficult situations, styrenic block copolymers are added to... 

The chain arrangement of this morphology was schematically proposed as in Fig. 10. The cell of the microsphere has a hexagonal surface, and the AB diblock copolymers form a bilayer between the microspheres. From this schematic arrangement, the optimal blend ratio of the AB block copolymer in this system was calculated as 0.46. This value was very close to the blend ratio of the AB type block copolymer 0.5 at which the blend showed the hexagonal packed honeycomb-like structure. 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

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