Interface copolymer

Hesselink et a/. (1971) have corrected the error that crept into Meier s theory for steric stabilization by isolated identical tails. They have also extended the theory to encompass stabilization by identical loops, and by homopolymers where every segment of the chain has a priori an equal chance of being adsorbed at the interface. Copolymers with anchor groups randomly distributed along the chains have also been treated. The following discussion will be restricted to loops and tails. This limitation is adopted because... 

In a reactively grafted interface copolymers are effectively created in situ at... 

Figure 12.13 Segregation in a tricomponent thin layer on silica system Polystyrene, DP = 6000, poly(2-vinyl pyridine), DP= 6000, and a dPS-PVP diblock copolymer, DP= 391-68. The polystyrene migrates to the air interface. The polymer-polymer interface copolymer excess, zf, corresponds to the hatched area.

Fig. 20 Copolymer distribution for a dPS-b-P2VP 391-68 diblock copolymer added at constant concentration to the top PS layer (overlaid onto a P2VP layer) following by an 8-h annealing at 178°C. For this sample, the final equilibrium copolymer concentration in the PS phase was 2.1%. The interface copolymer excess, z(, corresponding to the shaded area, was equal to 100...

Figure 6.7 Self-assembly of arborescent G1 polystyrene-gra/t-poly (ethylene oxide) copolymers at the air/water interface Copolymers with PEO contents of (a) 15%, (b) 31%, and (c) 74% by weight. The width of each picture is 1.5 pm. (Adapted with permission from G.N. Njikang, L. Cao and M. Gauthier, Self-assembly of arborescent polystyrene-gra/r-poly(ethylene oxide) copolymers at the air/ water interface, Macromolecular Chemistry and Physics, 2008, 209, 907-918. Wiley-VCH Verlag GmbH Co. KGaA.) (Colour version of this figure is available on the book companion web site.)...

Most of the experimental information concerning copolymer microstructure has been obtained by physical methods based on modern instrumental methods. Techniques such as ultraviolet (UV), visible, and infrared (IR) spectroscopy, NMR spectroscopy, and mass spectroscopy have all been used to good advantage in this type of research. Advances in instrumentation and computer interfacing combine to make these physical methods particularly suitable to answer the question we pose With what frequency do particular sequences of repeat units occur in a copolymer. 

Studies of the particle—epoxy interface and particle composition have been helphil in understanding the mbber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in mbber-modifted epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and -butyl acrylate—acryhc acid copolymers (309), have been used for toughening epoxy resins. 

In the suspension polymerization of PVC, droplets of monomer 30—150 p.m in diameter are dispersed in water by agitation. A thin membrane is formed at the water—monomer interface by dispersants such as poly(vinyl alcohol) or methyl cellulose. This membrane, isolated by dissolving the PVC in tetrahydrofuran and measured at 0.01—0.02-p.m thick, has been found to be a graft copolymer of polyvinyl chloride and poly(vinyl alcohol) (4,5). Early in the polymerization, particles of PVC deposit onto the membrane from both the monomer and the water sides, forming a skin 0.5—5-p.m thick that can be observed on grains sectioned after polymerization (4,6). Primary particles, 1 p.m in diameter, deposit onto the membrane from the monomer side (Pig. 1), whereas water-phase polymer, 0.1 p.m in diameter, deposits onto the skin from the water side of the membrane (Pig. 2) (4). These domain-sized water-phase particles may be one source of the observed domain stmcture (7). 

Polyisobutylene and similar copolymers appear to "pack" well (density of 0.917 g/cm ) (86) and have fractional free volumes of 0.026 (vs 0.071 for polydimethylsiloxane). The efficient packing in PIB is attributed to the unoccupied volume in the system being largely at the intermolecular interfaces, and thus a polymer chain surface phenomenon. The thicker cross section of PIB chains results in less surface area per carbon atom. 

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%. 

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.

The main results of this miero-mechanical model in the quasi-static regime have been compared with experimental results obtained by placing polystyrene (PS)-polyvinyl pyridine (PVP) diblock copolymers with a short PVP block between PS and PVP homopolymers. The fracture toughness was found to increase linearly with E from that of the bare PS/PVP interface, while the slope of the line increased with the degree of polymerization of the block being pulled out. If the data for the different copolymers were plotted as AG vs. (where... 

Fig. 6. Variation of interface toughne.ss with area den.sity of copolymer for a range of different molecular weight PS-PMMA copoly mens between PMMA and PPO (or PPE) [39J.

The model has also been found to work well in describing the mechanics of the interface between the semicrystalline polymers polyamide 6 and polypropylene coupled by the in-situ formation of a diblock copolymer at the interface. The toughness in this system was found to vary as E- where E was measured after the sample was fractured (see Fig. 8). The model probably applied to this system because the failure occurred by the formation and breakdown of a primary craze in the polypropylene [14],... 

The toughness of interfaces between immiscible amorphous polymers without any coupling agent has been the subject of a number of recent studies [15-18]. The width of a polymer/polymer interface is known to be controlled by the Flory-Huggins interaction parameter x between the two polymers. The value of x between a random copolymer and a homopolymer can be adjusted by changing the copolymer composition, so the main experimental protocol has been to measure the interface toughness between a copolymer and a homopolymer as a function of copolymer composition. In addition, the interface width has been measured by neutron reflection. Four different experimental systems have been used, all containing styrene. Schnell et al. studied PS joined to random copolymers of styrene with bromostyrene and styrene with paramethyl styrene [17,18]. Benkoski et al. joined polystyrene to a random copolymer of styrene with vinyl pyridine (PS/PS-r-PVP) [16], whilst Brown joined PMMA to a random copolymer of styrene with methacrylate (PMMA/PS-r-PMMA) [15]. The results of the latter study are shown in Fig. 9. 

Creton, C., Kramer, E.J., Hui, C.-Y. and Brown, H.R., Failure mechanisms of polymer interfaces reinforced with block copolymers. Macromolecules, 25, 3075-3088 (1992). Boucher et al., E., Effects of the formation of copolymer on the interfacial adhesion between semicrystalline polymers. Macromolecules, 29, 774-782 (1996). 

Benkoski, J.J., Fredrickson, G.H. and Kramer, E.J., The effect of composition drift on the effectiveness of random copolymer reinforcement at polymer-polymer interfaces. Macromolecules (2001, in press). 

More extensive roughening of an interface between incompatible polymers can be obtained by use of various types of copolymer, introduced at the interface as putative compatibilisers. The interface may be strengthened, as a result of interdiffusion and roughening on a nanoscale. Many elegant experiments have been done in this area. 

These effects have been found by Creton et al. [79] who laminated sheets of incompatible polymers, PMMA and PPO, and studied the adhesion using a double cantilever beam test to evaluate fracture toughness Fc. For the original laminate Fc was only 2 J/m, but when interface reinforced with increasing amounts of a symmetrical P.M.M.A.-P.S. diblock copolymer of high degree of polymerisation (A > A e), the fracture toughness increased to around 170 J/m, and then fell to a steady value of 70 J/m (Fig. 9). 

Fig. 10. Schematic representation of a random copolymer at the interface between two incompatible homopolymers. Incompatibility increases in the order (a), (b), (e).

Toughening of a polymer-polymer interface with random copolymers can sometimes be more effective than with diblocks, providing the polymers are not too incompatible [80]. This is of industrial, as well as of scientific, interest as random copolymers are usually cheaper to produce. 

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. 

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