Chain pull-out

Viscoelastic polymers essentially dominate the multi-billion dollar adhesives market, therefore an understanding of their adhesion behavior is very important. Adhesion of these materials involves quite a few chemical and physical phenomena. As with elastic materials, the chemical interactions and affinities in the interface provide the fundamental link for transmission of stress between the contacting bodies. This intrinsic resistance to detachment is usually augmented several folds by dissipation processes available to the viscoelastic media. The dissipation processes can have either a thermodynamic origin such as recoiling of the stretched polymeric chains upon detachment, or a dynamic and rate-sensitive nature as in chain pull-out, chain disentanglement and deformation-related rheological losses in the bulk of materials and in the vicinity of interface. 

The techniques described above have been used to demonstrate that failure can occur by (1) simple chain pull-out, (2) chain scission close to the interface, or (3) chain scission within one of the blocks (typically PS). The transition from chain pull-out to scission is essentially controlled by molecular weight whilst the location of the scission seems to depend on the entanglement density. Fig. 2 shows the transition between (2) and (3) as E is increased. 

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.

Models of chain pull-out in glassy polymers without crazing... 

Fig. 4. The effect of chain pull-out of PVP on for three PS-PVP diblocks whose PVP block varied from degree of polymerisation 49 to 220 [4].

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Brown, H.R., Effects of chain pull-out on the adhesion of elastomers. Macromolecules, 23, 1666-1670(1993). 

Creton, C., Brown, H.R. and Shull, K.R., Molecular weight effects in chain pull-out. Macromolecules, 27, 3174-3183 (1994). 

Alle the deformation zones contain a finite and equal number of extended chains in their most highly stretched strands. This surprising conformity of the deformation zones may well be the consequence of the imposed plane-strain fracture condition which impedes lateral contraction of the material. However, no quantitative explanation has been presented as yet. A plausible explanation would be to assume that due to the hindered lateral contraction additional tensile stresses are transferred to the most extended strand with each additional chain pulled out of the matrix [112]. 

On the other hand, if either or both of the copolymer blocks are unentangled (MAc or MBc < A/e), chain pull-out may be preferred (Creton et al. 1992 Dai et al. 1996, 1997). The stress required to cause chain pull-out is dependent on the frictional forces between the homopolymer and the copolymer block being pulled out and is also linearly dependent on the copolymer areal density. It is given by (Dai et al. 1997 Kramer et al. 1994)... 

Keywords. Polymers, Interfaces, Grafted chains, Adsorbed chains, Polymer brushes, Adhesion, Friction, Chains pull out, Slip at the wall... 

Let <5 denote the length of the connector chain pulled out from the polymer by a force / as shown in Fig. 8. When 6=1, where l is the total connector chain length, the chain is completely pulled out and the force/vanishes. Assuming that the chains are pulled out normal to the interface so that the tangential component of the pullout force is zero, then a, the traction stress component normal to the planar interface, is related to the force/and 2, the number of chains crossing a unit area of the interface, by ct=/2. a is related to the rate of chain pullout <5(f, x) and the remaining chain length l - by ... 

During drawing at 120°C chains pulled out from polypropylene crystals posses much higher mobility and may form assemblies ready to crystallize into well ordered a form crystals. 

Figure 7.8. Failure transitions in block copolymer reinforced interfaces. The critical stress versus the areal chain density is sketched for chain pull-out (dotted lines) and chain scission (solid). The dashed lines show the crazing stress, which is independent of the areal chain density. For large N (a) there is a transition from failure by chain scission to failure by crazing at a critical areal chain density whereas for a smaller value of N (b) chains are pulled out before they break and there is a transition from failure by chain scission to failure by crazing at a critical areal chain density After Kramer et al. (1994).

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