Relaxations and chain dynamics of sequential full interpenetrating polymer networks based on natural rubber and poly(methyl methacrylate)

Abstract

The relaxations of natural rubber (NR)/poly(methyl methacrylate) (PMMA) interpenetrating polymer networks (IPNs) were studied using dynamic mechanical analysis, electron spin resonance (ESR) and solid state NMR spectroscopy. Samples with a lower concentration of PMMA in IPNs (25 wt%) showed only one relaxation, which corresponds to NR with a slight shift to higher temperature. IPNs with 35 wt% of PMMA showed very broad transitions arising from βand α-relaxations in PMMA, with the β-relaxation slightly shifted to lower temperature. These compositions also showed a higher modulus at all temperatures. Highly phase separated IPNs showed a complete drop of modulus at 423 K. Higher crosslinking in the NR phase increases the miscibility and decreases the temperature difference between transitions, while in PMMA it increases the phase separation and does not affect the β-relaxation of the PMMA chains. The ESR results showed that PMMA chains located in the PMMA-rich and NR-rich domains have different motional characteristics. The strong interaction between PMMA and NR chains was also observed by carbonyl relaxation in solid state NMR spectra. It was found that medium level crosslinking is needed for better interpenetration between phases. EXPERIMENTAL MaterialsHigh molecular weight NR (M w ≈ 500 000-900 000) was supplied by Rubber Research Institute of India, Kottayam, Kerala. Dicumyl peroxide (99%), the crosslinker for NR, was purchased from Aldrich and used as received. Methyl methacrylate (MMA, Aldrich) and ethylene glycol dimethylacrylate (EGDMA, Aldrich), the crosslinker for MMA, were distilled under vacuum prior to use. Azobisisobutyronitrile (AIBN, Aldrich), the initiator for MMA polymerization, was purified by recrystallization from methanol. Preparation of IPNsSheets of crosslinked NR (1-2 mm thick) were weighed and kept immersed in a homogeneous mixture of MMA, EGDMA and AIBN (0.7 g per 100 g of MMA). The NR sheets were swollen for various time intervals to obtain different weight percentages of PMMA. The swollen samples were kept for a few hours at 273 K to achieve an equilibrium distribution of the MMA monomer in the matrix. These swollen networks were heated at 353 K for 6 h and at 373 K for 2 h in an atmosphere of MMA to complete the polymerization and crosslinking of MMA. Four IPN samples were made with the following ratios of NR and PMMA: 75:25 (NIM 25 ), 65:35 (NIM 35 ), 50:50 (NIM 50 ) and 40:60 (NIM 60 ). Concentrations of crosslinker in weight percentage per 100 g for the NR and PMMA phases are denoted as superscripts before ( x ) and after ( y ) the sample's symbol, respectively. Thus, a general symbol for all the IPN samples is x NIM y z where x stands for wt% of crosslinker for the NR phase, y stands for wt% of crosslinker for the PMMA phase and z stands for wt% of the PMMA phase in the IPN sample. The obtained IPNs were kept in vacuum to eliminate residual unreacted MMA. The composition of the IPN samples was determined on wileyonlinelibrary.com

Presence of vacuoles in natural rubber–Cloisite 15A nanocomposites

“…In the pristine NR spectra [Figure (a)], the fast component was first observed in the spectrum recorded at 253 K. The greatest differences between the spectra of the unfilled and filled samples were visible around 273 K: for pristine NR, the slow component was still more intense than the fast one, whereas it had already almost vanished for the filled samples. At 293 K, the slow component was absent for the filled samples, yet it was still present for the pristine NR, even at 353 K. Such spectra have already been observed for NR and crosslinked polyisoprene at similar temperatures. Finally, at 403 K, the spectra of all of the samples contained only fast components.…”

“…The lowest value of τ R (slow) (rotational correlation time of the slow probes) was found in NC15A‐5 (∼20 ns). For comparison, the values of τ R (slow) and τ R (fast) (rotational correlation time of the fast probes) of a TEMPOL spin probe (4‐hydroxyl‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl, similar in volume to the TEMPONE probe used here) incorporated at the same temperature in an uncrosslinked NR were reported as 10 and 1.8 ns, respectively …”

“…Local chain dynamics, environment, interactions, and free volume variations on the timescale from 10 −11 s to 10 −7 s were investigated with conventional electron spin resonance (ESR) spectroscopy, either with the spin‐label or spin‐probe method. In the spin‐label method, stable free radicals are covalently bonded to the site of interest.…”

Phase Behavior of NR/PMMA Semi-IPNs and Development of Porous Structures

“…The viscoelastic properties were studied using the DMA analysis (Rheometrics MK II DMA, Piscataway, NJ, USA) [ 23 ]. Measurements were taken in a tensile mode with frequency of 1 Hz and at a heating rate of 3 °C/min in the temperature interval from −85 to 170 °C.…”

“…A better understanding of polymer mixing on molecular level and phase morphology of produced blend can be done by studying the viscoelastic properties and polymer chain dynamics using electron spin resonance (ESR). In the literature, it was reported that ESR spin probes follow different environments in a given sample and are, therefore, associated to the dynamics of the host polymer [ 23 , 24 , 25 , 26 , 27 , 28 , 29 ].…”