In this era of portability and rapid technological advances, polymers are more than ever under pressure to be cheap and offer tailored property profiles. Often, the key lies in designing blends and alloys carefully structured at the appropriate scale (preferably less than a micrometre) from existing polymers. Block copolymers - two or more different polymer chains linked together - have long been thought to offer the solution. Local segregation of the different polymer blocks yields molecular-scale aggregates of nanometre size. Recent progress in synthetic chemistry has unveiled unprecedented opportunities to prepare tailored block copolymers at reasonable cost. Over twenty years of intense academic research and the advent of powerful statistical theories and computational methods should help predict the equilibrium and even non-equilibrium behaviour of copolymers and their blends with other polymers. The gap between block copolymer self-assembly and affordable nanostructured plastics endowed with still-unexplored combinations of properties is getting narrower.

In contrast to most diblock copolymers which exhibit the classical upper critical ordering transition (UCOT), polystyrene-b-poly n-butyl methacrylatePS-b-PBMAhas been shown to undergo ordering upon heating through a lower critical ordering transition (LCOT). Here we report the phase behavior of a family of diblock copolymers formed from styrene and a homologous series of n-alkyl methacrylates, as determined by combined dynamic rheological testing and small-angle neutron scattering (SANS). It is shown that the shortest side chain methacrylates, with the exception of methyl methacrylate, exhibit the LCOT, while for side chains longer than n-butyl, the copolymers exhibit the classical UCOT behavior. Combined group contribution/lattice fluid model calculations of the solubility parameter and specific volume of the corresponding homopolymers qualitatively support these observations. The same calculations were further employed to molecularly design LCOT behavior into a new diblock material consisting of styrene and a random copolymer of methyl and lauryl methacrylate, denoted PS-b-P(MMA-r-LMA). The success of this approach suggests a simple semiquantitative method for predicting and designing the phase behavior of weakly interacting polymer pairs.

Self-assembly of high molecular weight polydisperse acrylic block copolymers and their blends is presented under conditions as close as possible to thermodynamic equilibrium. Di- and triblock copolymers comprising a poly(butyl acrylate) (PBA) first or middle block, and poly(methyl methacrylate) (PMMA) second or outer blocks, denoted MBA and MBAM, respectively, are prepared by nitroxide-mediated polymerization (NMP). Their particularity is that the acrylic block is controlled while the methacrylate block is polymerized via an uncontrolled radical process under the synthesis conditions used. Overall composition and molecular weight polydispersities are large. Molecular disorder does not yield macrophase separation, and TEM on solvent cast films reveals lamellar and poorly ordered bicontinuous, cylindrical, or spherical morphologies. Except for the lamellar phase, clear multiple orders of diffraction are not visible in SAXS, and scattering profiles instead indicate a liquidlike order of microdomains. More importantly, morphology boundaries are strongly shifted compared to those commonly accepted for model monodisperse block copolymers. Hence, symmetric copolymers adopt morphologies with highly curved interfaces while lamellae are displaced to PMMA-rich compositions. These results suggest that unbalanced polydispersity between the two blocks can induce interfacial curvature toward to broadest molecular weight distribution, thereby releasing stretching energy of the whole chain. This effect is expected to be encountered in radical or hybrid block copolymer syntheses whenever control cannot be optimized for all blocks.

The effect of hydrostatic pressure on the lower critical ordering transition (LCOT) was investigated by in situ small angle neutron scattering on symmetric and asymmetric diblock copolymers of perdeuterated polystyrene and poly(n-butyl methacrylate). These systems exhibit a transition from the disordered to ordered state upon heating. Similar to the lower critical solution transition (LCST) in polymer mixtures, the LCOT is entropically driven and is accompanied by an increase in volume on demixing of the copolymer blocks. As a consequence, application of hydrostatic pressure markedly increases the temperature at which the transition from the disordered to the ordered state occurs. Small angle neutron scattering studies as a function of temperature and pressure show that the pressure dependence of the LCOT, ΔT LCOT/ΔP, is up to +147 °C/kbar (1.45 ± 0.07 °C/MPa), roughly 1 order of magnitude greater than that seen at elevated pressures for diblock copolymers exhibiting an upper critical ordering transition (UCOT). Additionally, SANS data obtained at various pressures were superimposed to generate master curves for the peak intensity, peak position, and full width at half-maximum (fwhm). This suggests an equivalence between temperature and pressure of the thermodynamic behavior of systems that exhibit the LCOT.