Cross-linked rubber relaxation

An effect of network morphology is illustrated by the work of Shen and Tobolsky (ISO). They cross-linked rubbers in the presence of inert diluents such polymerizations tend to promote intramolecular chain loops rather than interchain cross-links. Their polymers had very low stress-relaxation... 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

A great number of experiments in soft cross-linked rubbers are made at substantial finite deformations. There are experimental grounds suggesting that the relaxation stress can be factored into a function of time and a function of strain (39,40),... 

Figure 9,26, Results sfio wing the increase in hysteresis for raw natural rubber over cross-linked rubber compared with the theory based on crack stopping caused by higher relaxation constant.

The behavior of weakly cross-linked rubber was described in Section 11.1 as entropy-elastic. If this material is deformed, the chains are displaced from their equilibrium positions and brought into a state which is en-tropically less favorable. Because of the weak cross-linking, the chains are unable to slip past one another. On relaxation, the chains return from the ordered position to a disordered one the entropy increases. The phenomenon can be described in various ways. Seen thermodynamically, the rubber elasticity is related to a lowering of entropy on deformation. From the molecular point of view, the molecular particles are forced to adopt an... 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

It is now well-recognized that pendant chains make a significant contribution to the long-term relaxation behavior of cross-linked rubbers as seen in stress relaxation and creep experiments. The molecular mechanism accountable for this long-term process is the diffusion of pendant chains in the presence of entanglements. 

In a stress-relaxation test the deformation is held constant, and the resulting stress is measiued as a function of time. Deformation produces an initial stress that decays with time in the case of viscoelastic materials. In an experiment the sample is rapidly deformed, and the resulting force measiued as a function of time. The data may be plotted as stress, stress/initial stress, or modulus. A stress-relaxation ciuve for a lightly cross-linked rubber is shown in Figure 29 (290). 

The stress-relaxation technique is an extension of the use of equilibrium modulus measurements to measure the concentration of stress-bearing chains. A sample of cross-linked rubber is stressed in tension to a constant extension and the stress required to maintain this constant extension is monitored, often continuously. If chain scission occurs so the stress required to maintain the extension will drop. 

At the University of Wisconsin since 19 6, studies of viscoelasticity have evolved from concentrated polymer solutions to undiluted amorphous polymers, dilute solutions, lightly cross-linked rubbers, glassy polymers, blends of different molecular weights, copolymers, cross-linked rubbers with controlled network structures, and so forth. It became evident that each type of system required a different approach. Moreover, in amorphous polymers, the terminal, plateau, and transition zones had to be described separately. Both dynamic (sinusoidal) and transient measurements such as creep and stress relaxation have been utilized. The inderlying theme of this work is the relation of macromolecTilar dynamics—modes of motion of polymer molecules— to mechanical and other physical properties. 

The earliest published analysis of stress relaxation in cross-linked rubbers was performed by Chasset and Thirion [43], They established the following empirical equation ... 

The experiments on the PDMS cross-linked rubber were also performed using the TA Instruments ARES with the same transducer. The PDMS was a Wacker Silicones ELASTOSIL RT 601 two-component PDMS rubber. The rubber was mixed 9 parts A 1 part B by weight. The components were mixed and then vacuum devolatilized until no air bubbles remained. Parallel plate geometry with 25 and 50 mm platen diameters were used. Each sample was cured for 1 h at 40°C then the temperature was increased to 100°C. The samples were then cured at 100°C over night ( 10 hrs). The dynamic mechanical and stress relaxation experiments were performed at 25°C. 

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