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M e ), following the well-established concept that in solution M e sol varies as (1) M e melt represents the molecular weight between entanglements of macromolecular chains in the undiluted melt, and Φ is the polymer volume fraction in the solution. Invoking classical rubber elasticity theory, max (i.e., final/initial sample length) of a permanent network varies with the molecular weight between entanglements as (2) (3) max melt is the maximum draw ratio of a melt-quenched solid, and as above, Φ is the polymer volume fraction in the solution that is employed to produce the fibers or films by the gel-processing route. In the basic paper on the mechanism of the solution-spinning/drawing process (1) —which became to be known as “gel spinning”—to create ultrahigh modulus and ultrahigh strength polyethylene fibers and films, Booij et al. (2) advanced the concept that the limiting factor in achieving by tensile deformation ultimate levels of extension and uniaxial alignment of weakly interacting flexible macromolecules and the, therewith, associated limiting mechanical properties is not the shape, size, or order of crystalline entities in a (semi)crystalline solid polymer such as polyethylene, but the degree at which the constituent macromolecules are entangled in it. In their experimental studies, that degree was varied by dissolving ultrahigh molecular weight polyethylene (UHMW PE) at different concentrations and consolidation through gelation the spacing between macromolecular entanglements (), following the well-established concept that in solutionvaries as (3) Hererepresents the molecular weight between entanglements of macromolecular chains in the undiluted melt, and Φ is the polymer volume fraction in the solution. Invoking classical rubber elasticity theory, (4) and considering entanglements trapped during the gelation event in the solid polymer to act as (semi)permanent cross-links, it follows that the maximum draw ratio λ(i.e., final/initial sample length) of a permanent network varies with the molecular weight between entanglements asleading Booij et al. (2) to propose the following expression for the polymer-volume-fraction dependence of the maximum draw ratio of solution-gelled UHMW PE:Here λis the maximum draw ratio of a melt-quenched solid, and as above, Φ is the polymer volume fraction in the solution that is employed to produce the fibers or films by the gel-processing route.

max , of UHMW PE (typical weight-average molecular weight M̅ w ≥ 3 × 106) from a value of λ max ≈ 6 for melt-crystallized material to a value of, for instance, a molecular draw ratio of about 50—commonly required to generate levels of stiffness in excess of 150 GPa and tensile strengths exceeding 2 GPa, a polymer solution volume fraction as low as Φ ≈ 0.03 would be required Thus, in order to enhance the “drawability”, i.e. increase λ, of UHMW PE (typical weight-average molecular weight≥ 3 × 10) from a value of λ≈ 6 for melt-crystallized material to a value of, for instance, a molecular draw ratio of about 50—commonly required to generate levels of stiffness in excess of 150 GPa and tensile strengths exceeding 2 GPa, a polymer solution volume fraction as low as Φ ≈ 0.03 would be required (5, 6) —this assuming that all entanglements present in the melt or solution are trapped.

Despite this seemingly unattractive and uneconomical process, nonetheless, ultrahigh modulus and strength UHMW PE fibers have been successfully developed on a significant commercial scale, albeit of a somewhat lower level of mechanical properties due to the higher polymer solution concentrations employed (typically ∼10% v/v). These materials, which are sold under various trade names, including Dyneema and Spectra, today are widely employed in a broad spectrum of applications ranging from bullet-proof vests to sails and ropes as well as surgical sutures (see e.g. refs 7 and 8 ).

M e = 1250) in order to impede their formation. This was achieved by employing low polymerization temperatures, low monomer pressure, and/or low catalyst activity. Recognizing the above-described concentration issue with the solution-spinning/drawing process, Rotzinger et al. (9-11) subsequently set out to control the entanglement density in solid UHMW PE by physicochemical means during the synthesis of the polymer, rather than the above referred semidilute solution concentration route. In their approach, conditions of the polymerization of ethylene were selected to reduce the length of the growing macromolecular chain in the fluid phase to that well below the spacing between chain entanglements found in an unperturbed polyethylene melt (12) = 1250) in order to impede their formation. This was achieved by employing low polymerization temperatures, low monomer pressure, and/or low catalyst activity. (9-11)

The polymers thus produced were compressed in the solid state, i.e., below their melting temperature—in order to maintain their low entanglement density (hence the connotation “virgin”)—into coherent films, which were subsequently drawn into ultrahigh modulus and strength fibers and films applying the usual tensile deformation techniques. Today, more than two decades after its invention, (13) also this solvent-free process apparently is being commercialized in the form of the production of high-performance films. (14)

While both the “semidilute solution” and “virgin polymer” processing routes have been developed, as mentioned above, and to a certain extent have been a practical and economic success, cumbersome issues with these two technologies remain:

In the “gel-processing” method, which on the one hand is relatively forgiving from a production-control point of view, on the other hand has the significant drawback that large amounts of flammable solvent (typically 90 kg per 10 kg fiber) need to be recovered and purified, such as decalin by evaporation, (15) or mineral oil extracted with, for instance, hexane or fluorocarbon compounds. (16) In addition, it has been reported that the solvents and extraction means employed carry significant health hazards (ref 17 and references therein).

Although the “virgin polymer” processing route has the advantage that no solvent is employed in the film or fiber production process, it requires delicate control of both the polymerization—involving nontrivial catalyst systems—as well as of the production of solid-state compressed precursor films and relatively low rates of tensile deformation.

Accordingly, technologically attractive production of ultrahigh modulus and strength polyethylene fibers and films leaves ample room for improvement, especially as the number of beneficial applications, as well as the market as a whole, continues to expand.

In the present study, we attempted to address the above issues by exploring the concept of controlling the density of chain entanglements in solution-processed, solid UHMW PE not only simply through the volume fraction of the polymer in solution, as in the above “gel-processing” technique, but in addition by optimization of the “quality” of the solvent. As will be demonstrated below, this approach permitted us to dramatically reduce the amount of diluent required and trivialized its recovery and purification, while maintaining the flexibility of solution processing and providing an environmentally more sound approach.

N.B. It should be mentioned, of course, that exploration of diluents other than, for instance, decalin and mineral oil have been reported. For instance Motooka et al. (18) described use of aliphatic carboxylic acids, alcohols, acid amides, carboxylic acid esters, aliphatic mercaptans, aliphatic aldehydes, and aliphatic ketones to gel-process UHMW PE from solutions of relatively high polymer concentrations, but yielding disappointingly low mechanical properties of the resulting fibers, with Young’s moduli invariably below 65 GPa. In addition, extraction means such as hexane, heptane, hot ethanol, chloroform, and benzene were employed with their well-known, above-referred environmental and health issues.