Flow-induced chain scission

Flow-Induced Chain Scission. If the chains are overstretched, that is, e increases along the plateau in Figure 2, they will eventually break 12, 13). The molecular weight of the fracture products can then be determined by the method just outlined. Thus, through our technique we can both break chains in a controlled manner and analyze the resulting fracture products within the same apparatus, this double capacity enabling a systematic study of the flow-induced chain fracture. The principal results, to be of consequence later, are as follows ... 

Buchholz BA, Zahn JM, Kenward M, Slater GW, Barron AE (2004) Flow-induced chain scission as a physical route to narrowly distributed, high molar mass polymers. Polymer 45 1223-1234... 

Horn, A. F. and Merrill, E. W., Extensional flow-induced chain scission of macromolecules in dilute solution Effect of solvent power in a mixed solvent system, Polym. Commun. 25 172 (1987). 

Sim HG, Khomami B, Sureshknmar R (2007) How-induced chain scission in dilute polymta-solutions algorithm development and results for scission dynamics in elongational flow. JRheol 51 1223-1251... 

Mechanical transduction in these systems can be at the molecular, supramolecular or the micro/macro levels. For instance, chemical and structural changes caused by mechanical stimuli, such as flow- and ultrasound-induced chain scission, are interesting phenomena that can be taken into account in the development of mechano-responsive materials. In the following section, we will look closely at the types of response to mechanical stimuli and the underlying mechanisms involved either in mechano-chemical or physical transformations. 

These radical species then induce chain scission, radical transfer, oxidation and disproportionation to generate functional surface groups such as hydroxyl and carboxyl moieties (Fig. 1). For some materials, such as PDMS, these activated surfaces can be bonded to fabricate channels, as shown in Fig. 2. Plasma treatment of microchannels can also be used to control surface properties by incorporating reactive molecules or by increasing the hydrophilicity of the surfaces that eases the flow of fluids and minimizes the formation of bubbles. 

Shen and co-workers [29] used a twin-screw extrnder to devnlcanise waste tyre rubber and then blended the resnlting DR into high-density polyethylene (HDPE) to produce thermoplastic elastomers (TPE). They found that the high shear conditions during the extrusion process induced chain scission and oxidative degradation of the GTR. The reduction in crosslink density was apparent in a decrease in the gel content of the rubber/HDPE blends and an increase in the melt flow of the materials. 

In flow-induced degradation, K is strongly dependent on the chain length and on the fluid strain-rate (e). According to the rate theory of molecular fracture (Eqs. 70 and 73), the scission rate constant K can be described by the following equation [155]... 

Flow-induced degradation in solution is a complex function of polymer concentration which can alter the rate of chain scission in several respects. It can modify ... 

In order to obtain solutions with the desired flow properties, shear-induced degradation should be avoided. From mechanical degradation experiments it has been shown that chain scission occurs when all coupling points are loose and the discrete chains are subjected to the velocity field. Simple considerations lead to the assumption that this is obtained when y) is equal to T sp(c-[r ]) (Fig. 18). The critical shear rate can then easily be evaluated [22]. 

The G value for crosslinking may also be estimated from the sol-gel data. In the present study the crosslinking rate of stressed samples was essentially the same as for unstressed samples, but the rate of scission in stressed samples was zero compared with 0.013 in the absence of stress. (If one were to envisage the stress plus radiation-biased creep as being caused by radiation scission followed by stress-induced flow of the scissioned chain segments, one could expect a higher rate of perma-... 

Fig. 9 Schematic drawing of flow-induced birefringence as a function of strain rate during CST and chain scission. (Adopted with permission from Odell and Keller [28], Copyright 1986 John Wiley Sons)...

Hsieh CC, Park SJ, Larson RG (2005) Brownian dynamics modeling of flow-induced birefringence and chain scission in dilute polymer solutions in a planar cross-slot flow. Macromolecules 38 1456-1468... 

Mechanical or flow-induced degradation of polymers may occur under extreme shear or extrusion flow conditions giving rise to unfolding of macromolecules in extensional flow with subsequent chain scission. 

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