Dynamic shear compliance of polymer melts and networks in dependence on crosslinking density

Abstract

Mechanical relaxation processes in polymer melts and networks are discussed. This is performed by decomposing master curves of the dynamic shear compliance into i) glass relaxation with its plateau compliance JeN; ii) shearband process with its relaxation strength AJB, which is reciprocal to the total crosslink density pc; and iii) flow relaxation AJF and viscous flow (for uncrosslinked melts only). Plateau compliance JeN is exponentially reduced only by effective crosslinks (pc ~-pd30). This behavior is understood on the level of a meander superstructure, which includes shearbands.The observed saturation inleN at higher dicumylperoxide (DCUP) crosslinking-which doesn't appear with radiation -can be explained by the lack of chemically induced effective crosslinks across the interfaces among meander cubes. This lack may be a consequence of DCUP molecules concentrating at the interfaces and thereby preventing the contact and radical recombination between chains at adjacent meander faces.Crosslink densities Pc (per monomer), determined from the reduction of shearb and relaxation strength, vary linearly with the crosslinking agent and read: pc ~ 2,4-10 -z Dose/MGy and Pc ~ 0.97 9 10 -z DCUP/phr for radiation and DCUP crosslinking, respectively. This implies, e. g., that a dose of 0.4 MGy (40 Mrad) is equivalent to 1 part DCUP phr in a crosslinking polyisoprene. From activation-curve analysis it follows that 3 rid stays constant, and esr (flee energy of formation of a segment-dislocation) and Qr -Qyo (activation energy for segmental jumps) vary with the square of pc, as does the glass temperatur Tg -Tgo from DSC measurements.

Detection of compatibility of some rubber blends by DSC

“…It can be seen in Fig. 5 that pure NBR and NR exhibit two relaxation processes namely, the glass process (that appears at 100 Hz and 10 -4 Hz for NBR and NR, respectively, at -21~ and a low frequency process is the shear-band process [10]. The glass relaxation process is associated with the microbrownian cooperative reorientation of the polymeric segments.…”

“…The dynamic mechanical measurements were carried out in the frequency range of 10-4-200 Hz using a double-sandwich sample holder over temperature range from -70~ to +70~ Figures 5a-b show the master curves for pure NR and NBR. Each master curve is composed of compliance J', J" curves by shifting them along both logarithmic scales [10]. It can be seen in Fig.…”

“…The measurements were carried out in the frequency range of 10-4-200 Hz at different temperatures [10].…”

“…The mechanical measurements were carried out for the NBR/NR blends using the mechanical spectrometer of the University of Ulm, Germany, using a double sandwich sample holder [10]. The spectrometer set-up contains a low noise generator, type HP203, which produces an alternating current in the range of 5x10 -s Hz to 60 Hz with an amplitude ranging from 0 and 15 V. The generator is controlled by 16-bit processor, type 21005, allowing measurements over 8 decades of frequency.…”

Rheological properties of ionically crosslinked poly(propylene)‐type thermoplastic elastomers

“…Increasing the degree of crosslinking results in an increase and flattening of the storage modulus of a maleated PP that is covalently crosslinked with ethylene glycol according to Ishigure et al 49 This relationship is also reported for various peroxide-crosslinked PEs, 50 -52 ethylene-hexene-diene terpolymer, 53 electron beam-crosslinked LLDPE, 54 silane-crosslinked polybutadiene, 55 siloxane-crosslinked polystyrene, 56 and PP/EPDM TPV dynamically crosslinked with a phenolic resin, 36 and is therefore considered to be a general fact. As for the rubbery plateau modulus, Pechhold et al 57 report that although the crosslinking of a polyisoprene continues to increase with increasing the radiation dose in the case of electron beam crosslinking, it tends to level off above a peroxide content in the case of peroxide crosslinking. The tendency that the storage modulus GЈ is higher than the loss modulus GЉ is also found for ionomers with high degrees of ionic bonding 15,58 and covalently highly crosslinked polymers.…”