Chain disentangling

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. 

Intuitively the toughness of an interface would be expected to be related to the depth of interpenetration of the chains. Wool [32] argues that the fracture energy, r, for chain disentanglement at least, is proportional to the square of the interface thickness, which, via Eqs. 36 and 37, gives ... 

How do highly interpenetrated random coil chains disentangle to cause fracture Disentanglement is considered to occur as shown in Fig. 14, where we depict the response of an entangled chain to a constant (step function) draw ratio X as follows ... 

A5. Chain disentangling prior to and after introduction of chemical cross-links in the strained state is negligible. 

Re A5. The experiment shows that no chain disentangling takes place after introduction of chemical cross-links in the strained state. Chain disentangling may take place prior to cross-linking if the relaxation period is long enough. 

Aryl-aliphatic copolyamide Bisphenol A polycarbonate Chain disentanglement craze N-Cyclohexyl maleimide unit... 

Under conditions where chain mobility is high enough, typically at high temperature and low strain rate, the loss of entanglement in the active layer at the craze-bulk interface can occur by chain disentanglement, resulting in chain disentanglement craze (CDC). 

As a direct consequence, the polymer MW enters in the chain disentanglement mechanism. More precisely, the relevant quantity is the ratio Coc(M/Me), which takes into account Me of the considered polymer. 

At higher mobility of polymer chains within the fibrils, another mechanism for fibril breakdown can happen chain slippage allowing chain disentanglement and fibril creep till rupture. Such a mechanism emphasises the time... 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

In order to understand fracture behaviour, it is important to analyse the types of deformation micromechanisms undergone under strain chain scission craze (CSC), shear deformation zone (SDZ), chain disentanglement craze (CDC) and the temperature range over which each one occurs. Furthermore, it is worth wondering whether these micromechanisms are related to /i transition motions. 

In the case of BPA-PC, the crazes occurring at high temperatures and showing a MW dependence have led the authors [57] to introduce the mechanism of craze formation by chain disentanglement (CDC). 

Furthermore, as chain disentanglements are involved in this regime, it is more appropriate to consider (T - Ta) instead of T for examining the temperature dependence of toughness. 

Several theoretical studies have been carried out on a phenomenological basis to justify the idea that a dynamic slip can indeed occur [43-45]. New experimental techniques and theoretical treatments must be developed to reveal the molecular origin of dynamic slip and characteristic time scales governing the dynamic processes in the melt/wall interfacial region. Our own experiments on polyethylenes [27-29] have persistently indicated that slip, either through chain desorption on weakly interacting surfaces or interfacial chain disentanglement... 

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

The mechanisms of drug release from dextran matrix occur in the early stage by polymer swelling, and the tablet thickness increases. Soon thereafter, polymer (and drug) dissolution starts occurring. The polymer dissolves because of chain disentanglement. Thus, there is a slow diminution of the thickness because of erosion until, finally, the tablet disappears (time > 480min). 

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