Crosslinking in elastomers

Characterization of the chemical structure of highly cross-linked polymers, and of the chemical changes that accompany degradation processes, relies on spectroscopic methods. Solid-state nuclear magnetic resonance techniques have the potential to allow a more detailed characterization than before possible of the chemical environment and structure of chemical crosslinks in elastomers and thermoset epoxies. Degradation processes in cross-linked systems have been studied by using infrared spectroscopy, solid-state NMR, and electron spin resonance. 

AMba, M., Hashim, A.S. Vulcanization and crosslinking in elastomers. Prog. Polym. Sci. 22... 

Apart from hairpins, other types of defect can be present in main-chain polymers (see Fig. 4). First, we note that chain ends represent a source of the local distortion of the director field [39]. Furthermore, a certain number of hairpins could become entangled. In contrast to standard hairpins, these kinds of defect cannot be removed by applying mechanical stress. Such entangled hairpins can easily suppress chain reptation and thus represent a source of (physical) crosslinking in the polymer matrix. Although not being quenched, as crosslinks in elastomer networks they introduce local sources of random orientational disorder in the director field. 

Baba M, George SC, Gardette JL, Lacoste J. Evaluation of crosslinking in elastomers using ther-moporometry, densimetry and differential scanning calorimetry analysis. Rubber Chem Tech 2002 75(1) 143-154. 

The degree of polymerization of the subchain is n. If the degree of polymerization of the molecule as a whole is n, then there are n/n subchains per molecule. We symbolize the number of subchains per molecule as N. Other properties of the subchain-which, incidentally, should not be confused with the chains between crosslink points in elastomers-will also have the subscript s as they emerge. 

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

W.C. Endstra and C.T.J. Wressmann, Peroxide Crosslinking of EPDM Rubbers, in Elastomer Technology Handbook, N.P. Cheremisinoff and Pn. Cheremisinoff, Eds., CRC Press, New Jersey, 1993. 

CTBN type elastomers (carboxyl-terminated butadiene acrylonitrile) used to toughen this epoxy system. The Tg peak of the elastomer (-30°Q was shifted to higher temperatures suggesting that electron interaction resulted in crosslinking. The fact that the Tg peak disappeared at 10 rads suggests that crosslinking in the elastomer was extensive at high dose levels. 

Tfrom thermodynamic considerations (2) supported by microscopic ob-servations (5), two different high molecular weight polymers when blended exist in a heterogeneous state. In the case of elastomers capable of crosslinking these separate phases may crosslink in the presence of one another. However, the question arises, does bonding exist across the interfaces ... 

At temperatures well below Tg, when entropic motions are frozen and only elastic bond deformations are possible, polymers exhibit a relatively high modulus, called the glassy modulus (Eg) which is on the order of 3 Gpa. As the temperature is increased through Tg the stiffness drops dramatically, by perhaps two orders of magnitude, to a value called rubbery modulus Er. In elastomers that have been permanently crosslinked by sulphur vulcanization or other means, the values of Er, is determined primarily by the crosslink density the kinetics theory of rubber elasticity gives the relation as... 

A powerful technique for the study of orientation and dynamics in viscoelastic media is line shape analysis in deuteron NMR spectroscopy [1]. For example, the average orientation of chain segments in elastomer networks upon macroscopic strain can be determined by this technique [22-31]. For a non-deformed rubber, a single resonance line in the deuterium NMR spectrum is observed [26] while the spectrum splits into a well-defined doublet structure under uniaxial deformation. It was shown that the usual network constraint on the end-to-end vector determines the deuterium line shape under deformation, while the interchain (excluded volume) interactions lead to splitting [26-31]. Deuterium NMR is thus able to monitor the average segmental orientation due to the crosslinks and mean field separately [31]. 

Multidimensional NMR spectroscopy proves to be a powerful method to reveal structural and dynamical information at the molecular level in elastomers. Residual dipolar couplings can be measured site-selectively and correlated with the crosslink density and mechanical stress. The local segmental order and information on local molecular motions can be also obtained with newly developed 2D NMR methods. The information at the molecular level can be correlated with macroscopic properties of elastomers and provides the basis for a better design of material properties for specific applications. 

Fig. 11. Normalised enhanced adhesive strength wyw as a function of the surface density, a, for two PDMS elastomers in contact with silicon wafers covered with irreversibly adsorbed chains. Wis the thermodynamic work of adhesion, W=2y, with ythe surface tension of PDMS, 7=21.6 mN m"1 at 25 °C. The filled symbols correspond to a molecular weight between crosslinks in the elastomer Mc=24.2 kg mol-1 while Mc=10.2 kg mol-1 for the open symbols. The adhesive strength, G, has been measured by peel tests performed at a very low velocity of the propagation of fracture, 0.17 im/s. The molecular weight of the surface anchored chains is Mw=242 kg mol-1...

The adhesion between PDMS chains irreversibly adsorbed on silica and crosslinked PDMS elastomers has been investigated in a systematic way in our group [118-120] over a wider range of surface densities than in the early work by... 

---