The polycondensation of resorcinol with formaldehyde under alkaline conditions results in the formation of surface functionalized polymer “clusters”. The covalent crosslinking of these “clusters” produces gels which are processed under supercritical conditions to obtain low density, organic aerogels ( ⩽ 0.1 g cm−3). The aerogels are transparent, dark red in colour, and consist of interconnected colloidal-like particles with diameters of approximately 10 nm. The polymerization mechanism, structure and properties of the resorcinol-formaldehyde aerogels are similar to the sol-gel processing of silica.

Organic aerogels from the polycondensation of resorcinol with formaldehyde

Bulk polymers are generally regarded as thermal insulators, and typically have thermal conductivities on the order of 0.1 W m(-1) K(-1). However, recent work suggests that individual chains of polyethylene--the simplest and most widely used polymer--can have extremely high thermal conductivity. Practical applications of these polymers may also require that the individual chains form fibres or films. Here, we report the fabrication of high-quality ultra-drawn polyethylene nanofibres with diameters of 50-500 nm and lengths up to tens of millimetres. The thermal conductivity of the nanofibres was found to be as high as approximately 104 W m(-1) K(-1), which is larger than the conductivities of about half of the pure metals. The high thermal conductivity is attributed to the restructuring of the polymer chains by stretching, which improves the fibre quality toward an 'ideal' single crystalline fibre. Such thermally conductive polymers are potentially useful as heat spreaders and could supplement conventional metallic heat-transfer materials, which are used in applications such as solar hot-water collectors, heat exchangers and electronic packaging.

http://www.ase.gatech.edu/wp-content/uploads/2015/05/Sheng-nnano.2010.27.pdf

Polyethylene nanofibres with very high thermal conductivities.

The role Of matrix ductility on the toughenability and toughening mechanism of elastomer-modified, diglycidyl ether of bisphenol A (DGEBA)-based epoxies is investigated Matrix ductility is varied by using epoxide resins of varying epoxide monomer molecular weights These epoxide resins are cured using 4,4′ diaminodiphenyl sulphone (DDS) and, in some cases, modified with 10 vol% carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) Fracture toughness values for the neat epoxies are found to be almost independent of the monomer molecular weight of the epoxide resin used However, the fracture toughness of the elastomer-modified epoxies is found to be very dependent upon the epoxide monomer molecular weight Tensile dilatometry indicates that the toughening mechanism, when present, is similar to the mechanism found for piperidine cured, elastomer-modified epoxies studied previously Scanning electron microscopy and optical microscopy techniques corroborate this finding

/pdf/toughening-mechanisms-in-elastomer-modified-epoxies-5775eqdi9l.pdf

Toughening mechanisms in elastomer-modified epoxies

This paper deals with ultra-high-strength monofilaments of linear polyethylene that are produced by solution spinning and subsequent hot drawing at 120° C. The influence of the draw ratio on the mechanical and thermal properties of the fibres was investigated. Some salient features of a polyethylene filament with a draw ratio of 31.7 are: tensile strength at break=3.0 GPa, Young's modulus=90 GPa and DSC-melting point at a scan speed of 10° C min−1=145.5° C. The modulus was found to depend linearly on the draw ratio. The tensile strength tended, by contrast, to approach an upper limit at high draw ratios. Additional morphological and X-ray studies revealed an extremely good orientation of the macromolecules in the fibre direction of the highly drawn polyethylene monofilaments.

/pdf/ultra-high-strength-polyethylene-filaments-by-solution-56klgvl41c.pdf

Ultra-high-strength polyethylene filaments by solution spinning/drawing

THE formation of highly oriented structures such as single crystals, single-domain liquid crystals and systems comprising uniaxially oriented crystallites is important in many applications of thin films and interfaces, ranging from materials reinforcement to molecular electronics. Of the methods that exist for forming such oriented structures, however, few have sufficient generality to make them applicable to materials of differing chemical composition or physical properties. Here we present a simple and surprisingly versatile method1 for orienting a wide variety of crystalline and liquid-crystalline materials, including polymers, monomers and small organic and inorganic molecules. In our technique, a thin, single-crystal-like film of poly(tetrafluoroethylene) (PTFE) is deposited mechanically on a smooth substrate such as glass. Materials grown on this coated surface from solution, melt or vapour phases show a remarkable degree of alignment.

Highly oriented thin films of poly(tetrafluoroethylene) as a substrate for oriented growth of materials

Ultradrawing of films of high-molecular-weight polyethylene (Mw = 15 × 106) produced by gelation crystallization from solution is discussed The influence of the initial polymer volume fraction (ϕ) on the maximum draw ratio (λmax) of the dried films is examined in the temperature region from 90–130°C The results can be described very well by the relation λmax = λ ϕ−1/2 where λ is the (temperature-dependent) maximum draw ratio of the melt-crystallized film An attempt is made to discuss the marked influence of the initial polymer volume fraction on λmax in terms of the deformation of a network with entanglements acting as semipermanent crosslinks

/pdf/ultradrawing-of-high-molecular-weight-polyethylene-cast-from-4lrt8n7rxh.pdf

Ultradrawing of high-molecular-weight polyethylene cast from solution. II.Influence of initial polymer concentration

High-modulus structures of linear polyethylene with Young’s moduli of 70 GPa may be generated, for example, by drawing meltspun fibers to very high draw ratios’), solid-state extrusion (e. g., cf.’)) and by the solution-crystallization technique referred to as surface growth3). The tensile strength of the drawn and extruded materials is usually found below 1 GPa, whereas the longitudinal crystals of Zwijnenburg and Pennings may have a strength as high as 3 GPa. The superior strength of the latter filaments, which have comparable Young’s moduli, is due to the high molecular weight of the polymer sample used4*’). Previous attempts to draw or extrude polyethylene of similar molecular weight to high draw ratios only succeeded at relatively high temperatures, in fact above the melting point, where the effectiveness of the chain-extension process is known to be low (see6)). The structures thus produced, therefore, exhibited but moderately improved mechanical properties6). In a wide-ranging study on the processability of high molecular weight polymers it has been found that the effective drawability of these materials is drastically enhanced by spinning” or casting from dilute solutions. This effect will be illustrated in the present paper for polyethylene, and this improved drawability will be discussed in terms of a favourable intermolecular topology of the polymer.

/pdf/ultrahigh-strength-polyethylene-filaments-by-solution-2f552jfnnc.pdf

Ultrahigh-strength polyethylene filaments by solution spinning/drawing, 2. Influence of solvent on the drawability†

The past decade has seen a rapidly growing interest in highmodulus and high-strength polymeric fibers (see e.g. CIFERBI, WARD 1979). The methods to prepare these filaments are based on cold/ hot drawing (CAPACCI0, WARD 1974), two-stage drawing (CLARK, SCOTT 1974), hydrostatic (GI~SON et al. 1974) and direct (SOUTHERN, POR~ER 1970; TAKAYANAGI et al. 1966) extrusion, elongational flow (PENNINGS et al. 1972; ZWIJNENBURG, PENNINGS 1976) and on the intrinsic rigidity of particular macromolecules in solution (e.g. CIFERRI 1975). High-modulus polyethylene filaments with Young's moduli of about 70 GPa have been produced by CAPACCI0 and WARD (1974) through drawing. PORTER et al. (1970) performed solid-state extrusion to obtain polyethylene structures with similar mechanical properties. The tensile strength of these materials is usually found below I GPa (CAPACCI0, WARD 1975; KOJIMA, PORTER 1978). Longitudinal crystals of polyethylene with a Young's modulus of 100-120 GPa and a tensile strength of 3-4 GPa have been obtained from solution by ZWIJNEN~JRG and P~NNINGS (PENNINGS 1977) in a Couette-type apparatus employing a crystallization technique referred to as surface growth. This short communication describes some preliminary results on alternative routes to produce high-strength and high-modulus polyethylene filaments with a uni-axial Young's modulus of 90 GPa and a tensile strength of 3 GPa. The method is based on a simple solution-spinning and drawing process that can be performed continuously.

/pdf/ultrahigh-strength-polyethylene-filaments-by-solution-45owyb7cie.pdf

Ultrahigh-strength polyethylene filaments by solution spinning and hot drawing

The morphology and structure of gels produced by quenching semidilute solutions of high molecular weight polyethylene\((\bar M_w = 1.5 - 3.5 \cdot 10^6 )\) in decalin is discussed. The drawing behavior of the dried gel films at room temperature and at 130 °C and the structural features of films drawn to ratios as high as 130 were examined with various microscopic techniques and wide-angle X-ray scattering.

https://pure.tue.nl/ws/files/1841493/619806.pdf

PJ Piet Lemstra

Papers

Ultra-high-strength polyethylene filaments by solution spinning/drawing

Ultradrawing of high-molecular-weight polyethylene cast from solution. II.Influence of initial polymer concentration

Ultrahigh-strength polyethylene filaments by solution spinning/drawing, 2. Influence of solvent on the drawability†

Ultrahigh-strength polyethylene filaments by solution spinning and hot drawing

Ultra-drawing of high molecular weight polyethylene cast from solution