The living anionic copolymerization of isoprene and styrene in cyclohexane affords tapered block copolymers due to the highly disparate reactivity ratios of r I = 12.8 and r S = 0.051. Repeated addition of a mixture of these monomers was exploited to generate tapered multiblock copolymer architectures of the (AB) n type with up to 10 blocks (1 ≤ n ≤ 5), thereby subdividing the polymer chains in alternating flexible polyisoprene (PI) and rigid polystyrene (PS) segments. Three series of well-defined tapered multiblock copolymers with approximate molecular weights of 80, 240, and 400 kg/mol were prepared on the 100 g scale. Via this synthetic strategy polymer chains were divided in di-, tetra-, hexa-, octa-, and decablock tapered multiblock structures. Because of the living nature of the polymerization, low dispersities in the range 1.06–1.28 (decablock) were obtained. To ensure full monomer conversion prior to the addition of the isoprene/styrene mixture, kinetic Monte Carlo simulation was employed, permitting to simulate chain growth in silico by employing the known polymerization rates and rate constants k p. The synthesized tapered multiblock copolymers were characterized via SEC and selected samples via oxidative degradation of the polyisoprene block in solution, confirming the well-defined nature of the PS segments. Subsequently, the question was addressed, to which extent the tapered multiblock copolymers are capable of forming ordered nanosegregated morphologies. Detailed thermal, structural, and rheological investigations showed that the tapered multiblock copolymers with a molecular weight of 240 kg/mol formed ordered phases with the expected lamellar morphology. However, X-ray scattering data and transmission electron microscopy (TEM) images of the octablock and decablock copolymers reflect weakly ordered structures at ambient temperature. The domain spacing, d, was found to scale as d ∼ N 0.62, where N is the total degree of polymerization, suggesting stretching of chains and nonideal configurations. Following the structure factor, S(q), as a function of temperature revealed that the tapered multiblock copolymers undergo a fluctuation-induced first-order transition at the respective order-to-disorder transition temperature, T ODT. The viscoelastic response of the tapered copolymers was controlled by the nanodomain structure, the degree of segregation, nanodomain-bridging configurations of blocks, and also the proximity to the glass temperature of the vitrified PS domains. Tapered hexablock copolymers were found to best combine structural integrity and mechanical toughness, while maintaining a large strain at break (>900%).

The synthesis of tapered multiblock copolymers by statistical living anionic copolymerization of a mixture of isoprene (I) and 4-methylstyrene (4MS) in cyclohexane is based on vastly different reactivity ratios of I and 4MS (r I = 25.4 and r 4MS = 0.007). A library of tapered multiblock copolymers was prepared with different molecular weights (approximate molecular weights of 80, 240, and 400 kg/mol) and number of blocks (P(I-co-4MS) n with 1 ≤ n ≤ 5), and their thermomechanical properties were investigated by differential scanning calorimetry, rheology, and tensile testing in relation to their nanodomain structure, the latter investigated by small-angle X-ray scattering. The interaction parameter between I and 4MS segments was obtained based on the order-to-disorder transition temperatures of a series of PI-b-P4MS diblock copolymers prepared by sequential addition of monomers. The obtained χ(T) dependencies (χ MFT = 23.2/T − 0.024 and χ FH = 36.0/T − 0.041) are weaker than in the corresponding PI-b-PS system, revealing that the different reactivity ratios of the monomers is not the sole factor that controls the miscibility of the segments in the tapered multiblock copolymers. The latter is controlled by the value of the interaction parameter, the width of the tapered interfaces, and the number of blocks and total molecular weight. Tapered multiblock copolymers undergo a fluctuation-induced first-order transition from the ordered to the disordered state. The domain spacing scales as d ∼ n −0.83±0.02 when compared under a fixed total molecular weight, reflecting the conformational properties of the middle blocks. In addition, the domain spacing depends on molecular weight, as d ∼ N 0.55 , revealing stretching of chains and nonideal configurations. These structural features of the tapered multiblock copolymer affected their mechanical properties. Tensile tests showed a dramatic enhancement of the strain at break with a concomitant increase in toughness. These mechanical properties can be fine-tuned by the judicious selection of molecular weight and number of blocks. The state of order (ordered, weakly ordered vs disordered) and proximity to the glass temperature of the hard phase are additional parameters that affect the mechanical response. The improved mechanical properties reflect the enhanced interfacial strength, the latter provided by the configurations of the middle blocks in the copolymers. The influence of methyl group substitution in the para position of styrene is discussed by comparing the self-assembly and thermomechanical properties of the current P(I-co-4MS) n with the P(Ico-PS) n system. We found that the shorter tapered interface in the former is counterbalanced by its lower effective interaction parameter resulting in similar domain spacings.

High molar mass copolymers with a tapered interface are mechanically tough materials with an accessible order-to-disorder transition temperature and hence processability. We report the first ordering kinetics for a tapered tetrablock copolymer in comparison to a conventional diblock copolymer made sequentially. We show that tapered copolymers belong to the Brazovskii “universality class,” where fluctuations play a dominant role. Consequently, the order-to-disorder transition has a very weak, fluctuation-induced first-order character. The ordering kinetics of the lamellar phase from the supercooled disordered melt revealed several distinct differences associated with the range of metastability (increased), the timescales (bimodal), and the exact mechanism of ordering. The results are discussed in terms of the reduced interaction parameter and the introduction of structural defects within the lamellar grains.

Although several phase diagrams of block copolymers prepared by sequential monomer addition are known today, the phase diagrams of the corresponding tapered copolymers have not been reported in detail and this despite the industrial importance of the latter. A phase diagram based on a series of tapered diblock copolymers is reported, generated by the sec‐butyllithium initiated statistical anionic copolymerization of styrene and isoprene in cyclohexane. This affords copolymers with polyisoprene volume fractions, f, in the range of 0.43 < f < 0.82 and total molar mass in the range of 46–160 kg mol–1. The phase diagram consists of lamellae, hexagonally packed cylinders, weakly ordered hexagonally packed cylinders (CYL), and perforated (PL) as well as irregular bicontinuous morphologies. The phase state in the tapered copolymers bears some similarities and several distinct differences in comparison to the respective copolymers prepared by conventional sequential monomer addition. It is shown that the weakly ordered CYL/PL morphologies comprise a large part of the phase diagram that is significantly extended in comparison to diblock copolymers prepared sequentially. On the other hand, the composition range for the bicontinuous morphologies is similar in the two systems (0.64 < fPI < 0.68).

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