Influence of block sequence and molecular weight on morphological, rheological and dielectric properties of weakly and strongly segregated styrene-isoprene triblock copolymers

Abstract

In this work, morphological, thermal, viscoelastic and dielectric properties of triblock copolymers consisting of styrene (S) and isoprene (I) blocks are discussed. SIS and ISI triblock copolymers were synthesized by sequential anionic polymerization, three of them exhibiting molecular weights below the entanglement molecular weight M e , three of them exhibiting molecular weights in the order of M e and two of them exhibiting molecular weights above M e . The objective of this work is the investigation of the influence of molecular weight and block sequence on relaxation of the block copolymer chains. Morphological investigations using small angle X-ray scattering and transmission electron microscopy studies reveal that a larger molecular weight is associated with a more pronounced microphase separation. The presence of two glass transition temperatures reveals microphase separation of the PI and PS blocks. The Fox-Flory equation was applied in order to describe the molecular weight dependence of the glass transition temperature of the polyisoprene and the polystyrene blocks. The analysis of rheological data reveals a Maxwell fluid behaviour for the weakly segregated block copolymers, whereas for the strongly segregated block copolymers a pronounced elastic behaviour at low frequencies of small amplitude shear oscillations was observed.In the intermediate regime of molecular weight, a complex viscoelastic behaviour appears. The plateau modulus G N o is influenced by the sequence of the PS and the PI blocks (SIS or ISI). Our analysis of segmental and normal mode relaxation in dielectric spectroscopy experiments show that the relaxation processes are strongly influenced by the block sequence (PI blocks tethered at one or 1 both ends) and the molecular weight. As a result, the block sequence in triblock copolymers influences dynamical properties in the glassy state and in the melt.

Supramolecular Networks from Block Copolymers Based on Styrene and Isoprene Using Hydrogen Bonding Motifs—Part 2: Dynamic Mechanical Analysis

“…The observed results correspond to the calculated entanglement molecular weight M e , which showed a decrease after modification (see Table 1 ). The M e of unfunctionalized I 5 S 90 I 5 62 is 21.1 kg/mol, which is in good agreement with the literature [ 41 ]. For comparison, the M e of PS is about 17 kg/mol, and that of pure PI is about 6 kg/mol [ 44 ].…”

“…For microphase-separated polymers, G ′ shows at low frequencies a plateau arising from the increased elasticity, with PI acting as a softener (with T g,PI << T g,PS ). For I 10 S 83 I 7 67 , which was investigated by Georgopanos et al [ 41 ]—with similar composition compared to the presented block copolymers of this work, but with different microstructures of PI—the terminal flow regime is clearly visible at a temperature of 180 °C, indicating a disordered state (at room temperature, microphase separation was observed with small angle X-ray scattering). Shabbir et al [ 52 ] observed for poly( n -butyl methacrylate) similar deviations with slopes of 1 and 0.5 for G ′ and G ′′, respectively, which were explained by the presence of hydrogen bonds.…”

“…For a better overview of the data, the Boltzmann time-temperature superposition (TTS) principle was used to create master curves for the angular-frequency-dependent measurements with a reference temperature T ref of 140 °C. Entanglement molecular weights M e were calculated via the plateau modulus (Equation 2) [ 41 , 42 , 43 ], with density ρ , the universal gas constant R (8.314 J/molK), and the temperature T ( T ref = 413.15 K). …”

“…To compare the calculated values, and with respect to its dominating weight fraction of at least 90 wt %, ρ was defined as the density of pure PS at 140 °C (1.00 g/cm³) [ 44 , 45 ]. is associated with the value of G ′ at the minimum of tan δ [ 41 , 42 ]. The calculated M e of all modified and unmodified block copolymers are also listed in Table 1 .…”

Selektive Katalyse für Polymerisationen mit Monomermischungen: Verwendung eines kommerziellen Cr‐Salen‐Katalysators zur Herstellung von ABA‐Blockpolyestern

“…Ähnliche Effekte wurden für Poly(styrol‐ b ‐isopren‐ b ‐styrol) beobachtet: bei niedrigem M n wurde eine Glasübergangstemperatur beobachtet ( M n =11 000 g mol −1 , T g =38 °C), bei höheren M n ‐Werten traten zwei dazwischenliegende Glasübergangstemperaturen auf ( M n =16 000 g mol −1 , T g,1 =−48 °C, T g,2 =43 °C) und bei großem M n wurden zwei Glasübergangstemperaturen, ähnlich zu den Homopolymeren, detektiert ( M n =161 000 g mol −1 , T g,1 =−65 °C, T g,2 =105 °C) …”