High entanglement density polymers

J Craze-shear Deformation Transitions — High Entanglement Density Polymers... 

Some highly entangled hnear polymers, including high-density polyethylene (HDPE), hnear low-density polyethylene (LLDPE), 1,4-polybutadiene (PBDE), and polydimethylsiloxane (PDMS), exhibit an extrusion instabihty known as sharkskin, in which a small-amplitude, high-frequency disturbance appears on the extru-date surface at a wah shear stress that is typically on the order of the hnear viscoelastic plateau modulus (0.1 - 0.2 MPa for most polymers). Figure 12.10 shows... 

From the foregoing it will be clear that whenever entanglements and long chain branching are both present the dynamics in a polymer melt are highly co-operative. The orientational relaxation time of chain segments is exponentially dependent on both the contour distance to the nearest effective free end and on the effective entanglement density of its enviroiunent at all previous timescales. 

The extension ratios obtained at room temperature on a large number of homopolymers and copolymers have been analysed [19]. Polymers with high A.max, i.c. large Me or low entanglement density, ve, lead to crazes. In contrast, polymers with low A.max, and thus low Me and large ve, show SDZs. 

Fig. 11). It is, therefore, highly probable that the bulky filler particles impose geometrical hindrances (entropy constraints) for the chain dynamics at the time scale of the NMR experiment (of the order of 1 ms). This effect may be compared with the effect of transient chain entanglements on chain dynamics in polymer melts. It should be remarked that the entanglements density estimated for PDMS melts by NMR is close to its value fi om mechanical experiments [38]. Therefore, it can be assimied that topological hindrances from the filler particles can also be of importance in the stress-strain behavior of filled elastomers. 

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