Influence of Stiffness, Monomer Structure, and Energetic Asymmetries on Polymer Blend Miscibilities:  Applications to Polyolefins

Abstract

The generalization of the lattice cluster theory (LCT) to include explicit trans-gauche energy differences is applied to study the combined influences of chain stiffness disparities, monomer molecular structures, energetic asymmetries, and nonrandom mixing on the miscibilities of binary polymer blends. The combination of all these relevant physical features within a single theory enables testing various divergent suggestions concerning the dominant physical factors governing the miscibility of polyolefin blends. Thus, tests are presented of models ascribing the observed miscibility patterns in polyolefin blends solely to entropic factors (stiffness disparities) or solely to enthalpic factors (solubility parameter models). The LCT computations demonstrate the combined importance of both factors, as well as several others arising from monomer molecular structures and compressibility. An important and highly nontrivial ingredient in these tests is the novel computation of the mean square radius of gyration for structured monomer chains. The LCT also provides partial tests of a model in which thermodynamically equivalent semiflexible linear chains replace real polyolefin chains. In addition, we extend to semiflexible chains and to asymmetric polymerization indices a remarkable correlation between the binary blend critical temperature and a structural parameter that depends on the fractions of tri-and tetrafunctional united atom groups in the component chains (for model blends in which all van der Waals interactions are equal). Several comparisons with experiment for polyolefin blends serve to explain the molecular origins of observed deviations from solubility parameter models for the phase behavior of blends containing poly(isobutylene), as well as for the observed very weak variation of the critical temperature with molecular weights observed in some experimental blends.

New patterns of polymer blend miscibility associated with monomer shape and size asymmetry

“…͑PS/ PVME blends are classified as type IV blends, based on fits to blend scattering data and the resulting conditions bϽ0, and a,c 0.͒ Other aspects of non-FH-type critical behavior of PS/PVME blends are described in our prior studies of this system. 8,9,34 Measurements 27,62 and LCT computations 50 for PS/PVME mixtures over a limited range of M indicate a weak M -dependence of the correlation length amplitude o ͑typical values 63 of o are on the order of 10 Å͒, and the theta temperature of PS dispersed at a low concentration in PVME has been estimated 60 as T (PVME) ϭ147°C, a value remarkably close to the critical temperature values found by Han et al, 27 T c ϭ145Ϯ5°C. A similar insensitivity of T c to M is reported 29 for binary blends of PIB with several other polyolefins, which provide additional examples of LCST systems and class IV blends.…”

“…The limiting high pressure ͑incompressible͒, high molecular weight LCT can readily be extended to include chain semiflexibility 9 ͑and thereby model chain tacticity͒, but only at the expense of introducing ''bending'' energies E b that reflect the conformational energy differences in the actual polymers. 9 This extension 31 merely renders the two basic counting indices r i and p i as dependent on temperature T, whereupon the a, b, and c in Eqs. ͑8͒ and ͑10͒ then become functions of T. The same Eqs.…”

“…The problem of understanding the interrelation between molecular monomer structure and blend miscibility has been considered by several complementary methods. The lattice cluster theory, developed by Freed and co-workers, [6][7][8][9][10] directly addresses the effects of blend compressibility, chain semiflexibility, and differences in monomer shapes, sizes, and interactions on the blend critical behavior. ͓FH theory, which neglects these effects, is the leading order contribution in the lattice cluster theory ͑LCT͒.͔ The LCT generalization of FH theory provided the first explanation for the origin of the ''entropic'' portion a of the Flory interaction parameter in terms of differences in monomer shapes and sizes between the blend components.…”

Search for a model polymer blend

“…Most of the recent attempts at refining the Flory-Huggins theory (Bates et al, 1988;Bidkar and Sanchez, 1995;Foreman and Freed, 1997;Schweizer, 1993) lead to a composition-dependent , y parameter that does not vanish in any reasonable limit. It is clear that none of these theories are applicable to the PM/PE system.…”

Block Copolymers by Living Free Radical Polymerization

“…For example, the synthesis of graft copolymers consisting of poly(ethylene-co-vinyl alcohol) as backbone polymer and PEO as side chains has been performed via anionic polymerization of EO, where the hydroxyl groups of the poly(ethylene-co-vinyl alcohol) backbone were ionized and used as initiation sites for the polymerization of EO. 70 Gohy et al 71 reported the synthesis of amphiphilic PB-gpoly(sodium methacrylate) copolymers via anionic polymerization. Alternatively, anionic polymerization has also been applied in the surface modification of PE with poly(acrylic acid) grafts.…”