Crystallinity and crystalline phase orientation of poly(1,4-cis-isoprene) fromHevea brasiliensisandTaraxacum kok-saghyz

Abstract

Crystallization is studied for poly(isoprene-1,4-cis) from Hevea brasiliensis (natural rubber [NR]) and from taraxacum kok-saghyz, mainly by collecting wide-angle X-ray diffraction patterns after processing and stretching. Although rubber samples before stretching are generally fully amorphous, crystallization can be induced in NR samples by processing at room temperature under moderate pressure. This phenomenon is possibly associated with nucleation by saturated fatty acid components. For rubber samples being fully amorphous in the undeformed state, straininduced crystallization occurs only at high strain ratios (α > 4), leading to high degrees of crystalline phase orientation (f c > 0.9 for α = 5). Rubber samples presenting some crystallinity already in the unstretched state, on the contrary, reach much lower degrees of axial orientation, even for high strain ratios (f c < 0.7 for α = 5). These differences in crystallinity and in crystalline phase orientations produce large differences in stress-strain behavior of the rubber. By room temperature processing, the considered NR samples can also develop an unreported disordered crystalline modification, with low intensity of 120 and 121 reflections. This disordered crystalline modification, which is also maintained after axial stretching procedures, can rationalized by a structural disorder along the b axis, possibly associated with statistical sequences of A + TA À or A À T A + conformations for poly(isoprene-1,4-cis) chains. Copyright © 2016 John Wiley & Sons, Ltd.Keywords: poly(isoprene-1,4-cis); crystallinity; strain-induced crystallization INTRODUCTIONNatural rubber (NR) is the regular head-to-tail polymer essentially made by isoprene-1,4-cis units. [1][2][3][4] It is the most important rubber, with increasing worldwide consumption, that amounts at present to more than 12 million t/year.[5] At the origin of such a large commercial success are the NR outstanding properties: great strength [6,7] and tack [8][9][10] in the uncrosslinked state and in the crosslinked state, very high tensile strength [11,12] and crack growth resistance, both in static [13][14][15][16][17] and in fatigue [18][19][20][21][22][23][24] loading conditions. Beneficial for such NR properties is definitely the great mobility of the polymer chains that are usually amorphous at rest. However, also the ability of NR to crystallize plays a fundamental role. NR undergoes strain-induced crystallization (SIC) that is the rapid development of crystallization under straining. Many aspects of SIC have been reviewed, [25][26][27] and further works are continuously appearing, focused on the study of models, [28,29] crystallite structure, [30,31] segmental dynamics, [32] local strain response, [33] mechanism of deformation, [34] morphology, [35] orientation and deformation mechanisms, [36] crystallite melting temperature, [37] temperature dependence of the mechanical properties, [38] kinetics [36,39,40] and time [41] of crystallization, fatigue behavior, [42,43] effect of entanglements and endlinking n...

Characteristics of mechanical properties of sulphur cross-linked guayule and dandelion natural rubbers

“…The stress relaxation phenomenon also was observed in the SIC behaviour of uncross-linked dandelion NR investigated by wideangle X-ray diffraction (WAXD) during slow deformation. 38 Recently, Ikeda et al described the SIC behaviours of sulphur cross-linked (vulcanized) guayule, dandelion and Hevea NRs by using quick time-resolved simultaneous synchrotron WAXD measurement during tensile deformation in situ. 39 The SIC behaviours of cross-linked dandelion NR were comparable to cross-linked Hevea NR.…”

Development of novel processes for the aqueous extraction of natural rubber from Taraxacum kok‐saghyz (TK)

“…It is being developed by multiple international industry/university research consortia to address issues associated with NR supply. Field grown TK roots contain from 5% to 22% NR on a dry weight basis, and this NR has molecular characteristics and composition similar to NR from Hevea brasiliensis (Hevea) . Like Hevea NR, TK NR develops strain‐induced crystallization (SIC) critical to rubber product performance and exhibits excellent properties for a variety of applications …”

“…Field grown TK roots contain from 5% to 22% NR on a dry weight basis, 2,5,6 and this NR has molecular characteristics and composition similar to NR from Hevea brasiliensis (Hevea). 7 Like Hevea NR, TK NR develops strain-induced crystallization (SIC) critical to rubber product performance and exhibits excellent properties for a variety of applications. [7][8][9] Rubber in TK is produced in the cytoplasm of laticifer cells, as cis-1,4-polyisoprene.…”

“…7 Like Hevea NR, TK NR develops strain-induced crystallization (SIC) critical to rubber product performance and exhibits excellent properties for a variety of applications. [7][8][9] Rubber in TK is produced in the cytoplasm of laticifer cells, as cis-1,4-polyisoprene. It exists in both latex and solid forms 5,10 making extraction of latex impractical since it would leave much NR behind and, because latex extraction can only be achieved using fresh roots, require processing facilities that could only operate during times of harvest and sit idle most of the year.…”

Preparation of Butadiene-Isoprene Copolymer with High Vinyl Contents by Al(OPhCH3)(i-Bu)2/MoO2Cl2∙TNPP

“…Rubber materials have aroused the intensive interest and attention from academia and industry due to many outstanding performances such as superior wet skid resistance, excellent fatigue resistance and low rolling resistance [1,2,3,4]. Among them, polybutadiene (PBd) and polyisoprene (PI) are regarded as two types of significant materials in the rubber industry, especially for tire manufacturing [5,6,7,8]. PI possesses superb aging resistance, high wet skid resistance, low heat build-up, and can be applied as the rubber material for high-performance tire tread, accounting for why there exists more 1,2- and 3,4-structures, thereby resulting in the decrease in main-chain double bonds and in the increase of vinyl group content [9,10,11,12].…”