Effect of Strain-Induced Crystallization

When a sample of elastomer is stretched, it becomes anisotropic in that the network chains tend to orient themselves more in the direction of stretch than in the lateral directions. The more ordered chains favor the formation of crystallites. These crystallites will tie together a number of neighboring network chains, thereby exerting an additional crosslinking effect. This increase in the degree of crosslinking will in turn cause a rise in the elastic stress. The reason that these crystallites in fact act as crosslinks is attributable 

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Figure 2.42 Stress-temperature data on inorganic rubber, in which the upturns in the stress at low temperatures illustrate the reinforcing effects of strain-induced crystallization.73 Reproduced by permission of Pergamon Press.

Figure 3.17 Mooney-Rivlin plots [Eq. (3.38)] showing the effect of strain-induced crystallization on the elastic behavior of highly crystallizable poly(cw-1,4-butadiene) networks at selected temperatures (17,18). The illed circles and the vertical lines have the same meaning as in Fig. 3.16. (From Ref. 15.)...

Rapid strain-induced crystallization has been associated with the outstanding mechanical properties characteristic of crosslinked NR compared to its synthetic analogue, i.e. crosslinked IR. This may be due in part to a reinforcing effect of strain-induced crystals on the properties of NR as a filler or physical crosslinking junctions. In fact, tensile and tear strengths of crosslinked NR are practically higher than those of crosslinked IR, at the high-speed limit of the tear test." Therefore, it is important to control the rate of crystallization, in order to control the mechanical properties of not only NR itself, but also the NR blends. 

Previous studies on PDB networks, which were reputed to be well-characterized, are therefore vitiated by the non-quantitative chemistry of the networks, the effect of strain-induced crystallization, and the question of equilibrium attainment during elastic measurements. Departures from theory can not be attributed to the highly speculative contributions from interchain entanglements. PBD of high cis-1,4... 

Some relevant results on the effects of strain-induced crystallization on ultimate properties have been obtained for 1,4-polybutadiene networks [112]. As has already been mentioned, the higher the temperature, the lower the extent of crystallization and, correspondingly, the lower the ultimate properties. The effects of increasing swelling parallel those for increasing temperature, since diluent also suppresses crystallization of the network. For noncrystallizable networks such as those of PDMS, however, neither change is found to be very important [118]. 

Mark, J.E. (1979) Effect of strain-induced crystallization on the ultimate properties of an elastomeric polymer network. Polymer Eng. ScL, 19, 6,409. 

Su T-K, Mark JE. The effect of strain-induced crystallization on the elastomeric properties of c 5-1,4-polybutadiene networks. Macromolecules 1977 10 120-5. 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

It has been observed that the crystallization behavior of polymers is modified when the material is strained. This behavior has been found in rubbers and in thermoplastics [14]. In thermoplastics, the effect of strain on crystallization behavior has been studied quite extensively in solutions, in melt, and in solid state. For instance, it has been found that flowing polymer melts can crystallize at temperatures that are substantially above the crystallization temperature of the same material in a quiescent state. This strain-induced crystallization is generally explained in terms of thermodynamic processes. During a phase change, the Gibbs free energy is ... 

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

We have mentioned above that if reaction occurs at defects in the structure this may lead to nontopochemical products. Such an effect may be important under certain circumstances thus, the monomer molecule and its arrangement in the crystal must be such that the excitation energy will be transferred from the ordered crystal to defects at a high rate. Inter alia this requires that the long-wavelength absorption band of the monomer be an intense one, so that this effect could be significant in the anthracenes, for example. Further, the defect population of most crystals is extremely small thus, for a finite amount of the defect dimer to be formed, additional defects must be created, for example as a result of strain-induced slip, as the reaction proceeds. 

Tg before testing, but comment here is limited to pre-strains below 250% where no strain induced crystallization occurs. AWiou weak signals ( 0 to 8 x lO spins/g) were recorded in uncross linked material beyond yield, this radical concentration was two orders of magnitude smaller than that obtained on even lightly crosslinked specimens. F re 17 shows the effect of cross4ink density for specimens pre-extended by 200% on the radical concentration after a further 200% test strain and at various temperatures of test. The effects of temperature and strain will be discussed later. 

The effects of HAF black on the stress relaxation of natural rubber vulcanizates was studied by Gent (178). In unfilled networks the relaxation rate was independent of strain up to 200% extension and then increased with the development of strain induced crystallinity. In the filled rubber the relaxation rate was greatly increased, corresponding to rates attained in the gum at much higher extensions. The results can be explained qualitatively in terms of the strain amplification effect In SBR, which does not crystallize under strain and in cis-polybutadiene, vulcanizates of which crystallize only at very high strains, the large increase in relaxation rate due to carbon black is not found (150). 

The most important parameter controlling the final properties is the draw ratio, X. There is a direct relationship between X and the mechanical properties, in particular the modulus. The maximum draw ratio is generally limited to about 5 for PET and most PA s, about 10 for PP whereas values of 15-20 are common for HDPE. Crystallinity also has a strong effect on final properties. Strain-induced crystallization develops during the orientation process as the molecular order progressively increases upon drawing. 

However, when compared with pure copolymer, the highly stretched nanocomposite exhibited a higher amount of unoriented crystals, a lower degree of crystal orientation, and a higher amount of 7-crystals. This behavior indicated that polymer crystals in the filled nanocomposite experienced a reduced load, suggesting an effective load transfer from the matrix to MCNF. At elevated temperatures, the presence of MCNF resulted in a thermally stable physically cross-linked network, which facilitated strain-induced crystallization and led to a remarkable improvement in the mechanical properties. For example, the toughness of the 10 wt% nanocomposite was found to increase by a factor of 150 times at 55°C. Although nanofillers... 

The Gough Joule effect, shown as an increase in modulus with an increase in temperature and the retraction of stressed rubber on heating An ability of some elastomers to undergo strain-induced crystallization The susceptibility of un.saturated rubbers to ozone attack and subsequent cracking in the stretched state... 

Andrews et al., 1971). The latter, secondary crystallization, can be quite slow (Mitchell and Meier, 1968), and its rate is unaffected by strain (Andrews et al 1971). However, the row nucleation rate is greatly enhanced by orientation, effecting rapid initial crystallization (Krigbaum and Roe, 1964 Ziabicki and Jarecki, 1978 Elyashevich, 1982 Roland and Sonnenschein, 1991). The time for strain-induced crystallization of NR is less than 60 ms at RT (Mitchell and Meier, 1968), so that synchrotron X-ray sources (Toki et al., 2003,2011) are required for on-the-fly measurements. 

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