Article pubs.acs.org/Macromolecules
Melt Miscibility in Diblock Copolymers Containing Polyethylene and Substituted Hydrogenated Polynorbornenes William D. Mulhearn and Richard A. Register* Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: The thermodynamic interaction strengths between linear polyethylene (PE) and members of a family of hydrogenated polynorbornenes prepared by ring-opening metathesis polymerization can be tuned across a wide range via the choice of substituent appended to the polynorbornene backbone at the 5-position. Isopropyl and certain n-alkyl groups yield polynorbornenes that are highly miscible with PE, capable of producing symmetric diblock copolymers with homogeneous melts at molecular weights in excess of 100 kg/mol. In contrast, phenyl-substituted polynorbornenes are quite immiscible with PE, exhibiting microphase separation in the melt at diblock molecular weights as low as 10 kg/mol. Interaction strengths within this series of polymers do not quantitatively obey regular mixing; entropic contributions to the mixing energy arising from mismatches in free volume and chain stiffness cannot account for the observed deviations. Instead, the interactions can be satisfactorily described by an empirical mixing rule of the form X = (Δγ)1.5, where X is the interaction energy density and γ is a pure-component quantity, operationally analogous to a solubility parameter, with a distinct value for each polymer. These empirical γ parameters are obtained by regression against the entire set of experimental pair interaction energies.
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INTRODUCTION Although polyethylene (PE) is the world’s most widely produced synthetic polymer, very few polymer species have been identified with sufficiently weak repulsive interactions against PE, characterized by a low Flory interaction parameter χ, to be useful for preparing PE-containing block copolymers with disordered melts or PE-containing miscible blends at high molecular weights. Melt-miscible block copolymers have the advantage of improved processability due to substantially lower melt viscosity and elasticity relative to an analogous microphase-separated system.1 Moreover, most suitably miscible candidates tend to be chemically similar to PE, such as copolymers of ethylene with an α-olefin,2,3 and so have limited utility for solid-state property modification due to similar physical properties like the glass transition temperature (Tg). In this work, we investigate the mixing thermodynamics of perfectly linear PE and a family of polymers derived from substituted norbornene monomers, prepared via ring-opening metathesis polymerization (ROMP) followed by backbone hydrogenation. Hydrogenated ROMP polynorbornenes are attractive candidates for incorporation into PE-containing block copolymers due to their synthetic compatibility with PE4−6 prepared by living ROMP of cyclopentene followed by hydrogenation and the wide range of physical properties accessible by the choice of substituent on the norbornene monomer7−9 (the eight polynorbornene derivatives in this work span a Tg range of −40 to 115 °C). The chemical identity of the substituent, here all hydrocarbons, also has a strong influence on mixing behavior, both with PE and between pairs © 2017 American Chemical Society
of dissimilar polynorbornene derivatives. Since the polymers studied in this work consist purely of hydrocarbons, all thermodynamics are governed by nonspecific interactions. It may thus be possible to describe these interactions in terms of a simple mixing rule.2 When comparing across a range of polymer chemical structures, it is convenient to eliminate the arbitrary segment volume in the definition of χ, by defining the interaction energy density coefficient:
X≡
χRT Vref
(1)
where R is the gas constant, T is absolute temperature, and Vref is the reference volume. For a binary mixture, ΔHm,ideal = Xϕ1ϕ2, where ϕi is the volume fraction of component i. Principally, we compare the observed mixing thermodynamics against the regular solution model, in which the interaction energy follows Xreg = (δ1 − δ2)2, where δ is the Hildebrand solubility parameter,2 ideally the square root of the cohesive energy density. Although regular mixing accurately describes the interactions of a wide range of hydrocarbon polymers, deviations in both the positive direction (X > Xreg) and negative direction (X < Xreg) have been observed.10−12 A survey2 of more than 150 weakly interacting (X < 0.5 MPa) polyolefin blends3,12,13 revealed that ∼20% showed irregular mixing Received: June 17, 2017 Revised: July 18, 2017 Published: July 25, 2017 5830
DOI: 10.1021/acs.macromol.7b01295 Macromolecules 2017, 50, 5830−5838
Article
Macromolecules
(Wyatt miniDAWN TREOS detector, room temperature and λ = 658 nm). Weight-average molecular weights, Mw, were measured by light scattering using the specific refractive index increment, dn/dc, of the polymer (a weight-fraction-averaged dn/dc was used for diblock copolymers30). Homopolymer dn/dc values were measured on a Wyatt Optilab rEX detector (λ = 658 nm) in THF at 25 °C. The polymer dispersity, Đ, was measured from the elution time distribution obtained with the GPC refractive index detector, calibrated with narrow-distribution polystyrene standards. Number-average molecular weights were calculated as Mn = Mw/Đ. Several of the high-molecular-weight polymers containing a polycyclopentene block exhibited larger dispersities (Đ > 1.15), owing to the tendency of this chemistry to undergo acyclic metathesis side reactions.28 These samples were fractionated to remove the highand low-molecular-weight portions of the distribution. The polymer was dissolved in toluene at a concentration of 4 g/L along with 0.5 wt % to polymer of 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) to suppress oxidative degradation. The solution was heated to 40 °C, and methanol was added until the mixture reached its cloud point. The mixture was transferred to a separatory funnel and allowed to cool and phase-separate into a concentrated, dense phase containing higher-molecular-weight polymer and a dilute phase containing lower-molecular-weight polymer. These phases were collected, and the polymer precipitated into methanol. Hydrogenation. Polymers without aromatic groups were hydrogenated using a nonselective Pd0 catalyst. The polymer was dissolved in cyclohexane at a concentration of 4 g/L, along with 0.5 wt % BHT. The solution was added to a stirred 2 L Parr stainless steel reactor along with a 2:1 weight ratio to polymer of a heterogeneous palladium catalyst supported on calcium carbonate (Pd0/CaCO3, 5 wt % Pd0, Alfa Aesar). The reactor was pressurized with 500 psi (3.4 MPa) of H2 and heated to 130 °C. Hydrogenation proceeded for ∼30 h, until olefinic bonds were undetectable by either 1H NMR for amorphous polymers (>99.9% hydrogenated) or FTIR for crystalline polymers (>98% hydrogenated). The catalyst was removed by filtration, and the polymer was collected by precipitation into methanol. Polymers with aromatic groups were hydrogenated using the olefinselective diimide method.31 The polymer was dissolved in mixed xylenes at 4 g/L, along with 0.5 wt % BHT. The solution, along with 2 equiv each of p-toluenesulfonyl hydrazide (TSH, Sigma-Aldrich, 97%) and tri(n-propylamine) (TPA, Sigma-Aldrich, 98%) to olefinic sites in the polymer, was added to a round-bottom flask equipped with a magnetic stirrer and reflux condenser. The flask contents were purged with N2 and then heated under reflux (bp =140 °C) until the evolution of gas ceased after ∼1 h. Diimide hydrogenation exhaustively saturates polycyclopentene and polynorbornene backbones, although multiple TSH and TPA charges were often required. No hydrogenation of aromatic rings was measured within the detection limit of 1H NMR (
