In contrast to most diblock copolymers which exhibit the classical upper critical ordering transition (UCOT), polystyrene-b-poly n-butyl methacrylatePS-b-PBMAhas 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.
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.
Earlier experimental investigations performed on a family of block copolymers formed from styrene and a homologous series of n-alkyl methacrylates revealed a strong dependence of thermodynamic compatibility between the two blocks on the length of the alkyl side chain of the methacrylate. Here we report the effect of hydrostatic pressure on the phase behavior of the same series of block copolymers, as determined by in situ small-angle neutron scattering. We find that hydrostatic pressure is a very effective means of driving styrene/n-alkyl methacrylate block copolymers with intermediate side chains from the highly viscous ordered state to the fluid disordered state of the copolymer. Hence, for n ranging from 2 to 6 (ethyl to hexyl methacrylate), pressure induces mixing with an absolute value of the pressure coefficient of the order/disorder transition, dT ODT/dP, of up to 1.5°C/MPa (150°C/kbar). Similar results are obtained when the methacrylate block consists of a random sequence of short and long alkyl side chains with carefully chosen and predictable composition. In contrast, pressure suppresses mixing when the methacrylate block is composed of either very short (n ) 1) or very long (n > 8) side chains. In terms of rheological properties, these results indicate that pressure applied at a constant temperature can be used to induce flow in some copolymers of this series. The ability to design such "baroplastic" behavior into commercially relevant thermoplastic elastomers would be highly advantageous from a processing standpoint.
Branch contents in sparsely short-chain branched polyethylenes (
The influence of compressed carbon dioxide sorption on the phase behavior of polymer blends and diblock copolymers exhibiting lower critical solution temperatures (LCSTs) and lower disorder-toorder transitions (LDOTs), respectively, was studied using in situ high-pressure small-angle neutron scattering. Homogeneous blends of poly(deuterated styrene) and poly(vinyl methyl ether) phase separate at temperatures more than 115°C below the ambient pressure LCST upon sorption of less than 3.3 wt % CO 2. The LDOTs in symmetric poly(deuterated styrene)-block-poly(n-butyl methacrylate) copolymers having total molecular weights of 78 000 and 32 000 g/mol are depressed by as much as 250°C upon exposure to supercritical CO2 at modest fluid-phase densities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.