Low Molecular Weight Linear Polyethylene Research Articles

Polyethylene and Semiconducting Polymer Blends for the Fabrication of Organic Field-Effect Transistors: Balancing Charge Transport and Stretchability

Polyethylene is amongst the most used polymers, finding a plethora of applications in our lives owing to its high impact resistance, non-corrosive nature, light weight, cost effectiveness, and easy processing into various shapes from different sizes. Despite these outstanding features, the commodity polymer has been underexplored in the field of organic electronics. This work focuses on the development of new polymer blends based on a low molecular weight linear polyethylene (LPE) derivative with a high-performance diketopyrrolopyrrole-based semiconducting polymer. Physical blending of the polyethylene with semiconducting polymers was performed at ratios varying from 0 to 75 wt.%, and the resulting blends were carefully characterized to reveal their electronic and solid-state properties. The new polymer blends were also characterized to reveal the influence of polyethylene on the mechanical robustness and stretchability of the semiconducting polymer. Overall, the introduction of LPE was shown to have little to no effect on the solid-state properties of the materials, despite some influence on solid-state morphology through phase separation. Organic field-effect transistors prepared from the new blends showed good device characteristics, even at higher ratios of polyethylene, with an average mobility of 0.151 cm2 V−1 s−1 at a 25 wt.% blend ratio. The addition of polyethylene was shown to have a plasticizing effect on the semiconducting polymers, helping to reduce crack width upon strain and contributing to devices accommodating more strain without suffering from decreased performance. The new blends presented in this work provide a novel platform from which to access more mechanically robust organic electronics and show promising features for the utilization of polyethylene for the solution processing of advanced semiconducting materials toward novel soft electronics and sensors.

Processing and Properties of Melt Processable UHMW‐PE Based Fibers Using Low Molecular Weight Linear Polyethylene's

AbstractThe rheology, solid state drawability, morphology, and mechanical properties of polyethylene blends containing ultrahigh molecular weight polyethylene (UHMW‐PE) and linear‐low molecular weight polyethylene waxes (PEwax) are explored. Addition of PEwax enables melt processing of UHMW‐PE and improves solid state drawability, providing opportunities in recycling of UHMW‐PE waste. Small angle X‐ray scattering results show that both PEwax and UHMW‐PE align fully in the drawing direction, irrespective whether the PEwax has an Mn below or above the critical molecular weight at which entanglements can form (Mc). Tensile moduli of drawn specimen are in accordance to the Irvine–Smith model confirming that both UHMW‐PE and PEwax align in the drawing direction and no chain slip occurs toward zero strain and both UHMW‐PE and PEwax fractions, irrespective of their molecular weight, contribute fully to the modulus. Tensile strength of the blends scales according to the rule of mixtures where PEwax below Mc scale toward zero and those above Mc do contribute to the tensile strength. Modulus hence can be regarded as insensitive to the molecular weight of the PEwax used, whereas strength does show to be sensitive to the molecular weight of the PEwax.

Phase Separation of Several Kinds of Polyethylene Solution under the Gelation/Crystallization Process

The gelation mechanism of polyethylene solutions was investigated by using light scattering and X-ray diffraction techniques in terms of the liquid-liquid phase separation. Three kinds of polyethylene, ultrahigh molecular weight polyethylene (UHMWPE), low molecular weight linear polyethylene (L-LMWPE), and low molecular weight branched polyethylene (B-LMWPE), were used as test specimens. When an incident beam of He-Ne gas laser was directed to the UHMWPE and B-LMWPE solutions quenched to a desired temperature, the logarithm of scattered intensity increased linearly in the initial stage and tended to deviate from this linear relationship in the latter stage. If the linear increase in the initial stage can be analyzed within the framework of the linear theory of spinodal decomposition proposed by Cahn, the gelation is obviously attributed to the phase separation leading to the concentration fluctuation of the solution. Furthermore, in the later stage showing the deviation from the linear relationship, light scattering from the UHMWPE solution with 0.5% concentration under Hv polarization condition showed an X-type pattern, indicating the appearance of optically anisotropic rods. On the other hand, the scattering from the B-LMWPE solution with 3% concentration showed a four-leaf clover pattern, indicating the appearance of optically anisotropic spherulites. No crystallite was confirmed by the X-ray diffraction measurements, when the rods and spherulites appeared. With further lapse of time, the Hv patterns became clearer and the corresponding X-ray diffraction intensity curves showed a very small diffraction peak from the (110) plane. In contrast, for the L-LMWPE solutions, the logarithm of scattered intensity against time showed a rapid increase and tended to level off because of rapid change from slightly transparent to whiten gels. In such a process, any quantitative analysis of the scattered light intensity was impossible. The corresponding X-ray diffraction revealed the strong reflections from the (110) and (200) planes, indicating the rapid formation of stable crystallites. Through a series of experiments for the three kinds of polyethylene solution, it turned out that gelation mechanism of polyethylene solutions is strongly affected by molecular weight and the degree of branching.

ABSENCE OF ISOTHERMAL LAMELLAR THICKENING IN A LINEAR POLYETHYLENE BLEND

Melting and lamellar morphology of a polyethylene blend were studied. Two linear polyethylene (LPE) samples were used. A commercial LPE and a low molecular weight LPE fraction (M n ≈ 2015) were solution blended. The pure LPE and a blend (30% commercial LPE content) were held in the melt at 126° C for up to 48 h, above the equilibrium melting point of the fraction, but below the crystallization temperature of the commercial LPE. The melting behavior of both materials as a function of storage time was studied using differential scanning calorimetry (DSC), in addition to transmission electron microscopy (TEM) of chlorosulfonated samples. Results showed that, although the LPE lamellae grew at the same temperature, those in the pure LPE were thicker than in the blend. Correspondingly, isothermally grown lamellae in pure LPE melt at higher temperatures.

“Nematocrystalline”Chain Packing: Three-Dimensional Structure of Low Molecular Weight Linear Polyethylene

The three-dimensional orthorhombic crystal structure of low molecular weight linear polyethylene PE600 (polydispersity: 1.11) has been determined in space group Pnam from experimental electron diffraction intensities. Although a bridged “lamellar” structure was proposed in a preliminary investigation based on projected diffraction data from annealed samples, it is now clear that a true lamellar separation, with attendant lateral shifts of sequential chain layers, can never occur. In the most ordered form of this polymer, a superlattice exists for the extended chain packing with periodic clustering of nonunitary methylene group occupancies to account for the observed layer repeat. The apparent longitudinal spacing of chain layers is 57.77 ± 0.12 A, comparable to the true lamellar repeat of a C40-chain paraffin. Accordingly, because of the small amount of high molecular weight component, there is only a“nematocrystalline”, ordering of the polymethylene chains.

Lateral habits of single crystals of metallocene-catalyzed low molecular weight short chain branched polyethylene from the melt

The lateral habits of low molecular weight short chain branched polyethylene single crystals from the melt were studied. Three crystallization temperatures (102, 104 and 106°C) were selected for single crystal growth. It was found that the lateral habits of single crystals were asymmetric at all the crystallization temperatures selected. The electron diffraction patterns and tilting series experiments evidenced that there existed chain tilting in all the lamellae. It was the chain tilting that lead to the asymmetry of the growth rate and of lateral habits of the single crystals about the b-axis. The lateral habits substantially changed from the growth at 102°C where the truncated lozenge single crystals formed with straight (110) faces to the growth at 104°C where the lenticular single crystals appeared. This change occurred at 20°C lower than that in a low molecular weight linear polyethylene with the same molecular weight. Furthermore, kinetics theory analysis evidenced that the change of lateral habits from truncated lozenge to lenticular shape resulted from the transition of growth regime. The results were the same as that of high molecular weight linear polyethylene but different to that of low molecular weight linear polyethylene. It may be attributed by the existence of short branched chains.

Crystallography of Real Waxes: Branched Chain Packing in Microcrystalline Petroleum Wax Studied by Electron Diffraction

Single-crystal electron diffraction patterns have been obtained from oriented samples of two commercial microcrystalline waxes. Their crystal structures, which are also expressed in oriented samples of pure methyl-branched n-paraffins, reveal a bridged lamellar structure reminiscent of low molecular weight linear polyethylene. An interdigitated lamellar packing is proposed that accommodates the methyl branches and nearby vacancies. A parallel study of n-alkyl derivatives of benzene and cyclohexane, on the other hand, reveal that naphthenic components adapt an oblique layer structure unlike the rectangular layer packing found for the petroleum waxes and the branched alkanes.

Thermal analysis of single-site polymers in binary blends of low-molecular-weight linear polyethylene and high-molecular-weight branched polyethylene

The cocrystallization phenomenon has been studied by blending a low-molecular-weight linear polyethylene (LPE) with a high-molecular-weight lightly branched ethylene–hexene copolymer, as a function of blend composition and cooling rate. The materials are based on single-site catalysts. Differential scanning calorimetry indicates that the upper limit on the degree of branches for cocrystallization with LPE is lower than observed in blends with Ziegler–Natta based materials, in accordance with results obtained by other authors on different single-site blend systems. The effect of the cooling rate on blends with low LPE content is probed, and the observed behaviour is discussed in light of earlier observations on similar blends.

Bridged Lamellae: Crystal Structure(s) of Low Molecular Weight Linear Polyethylene

The crystal structure of a low molecular weight polyethylene (Mw = 595) with low polydispersity (1.11) was determined from electron diffraction intensity data collected from samples epitaxially oriented on benzoic acid and subsequently annealed. A distribution of local microcrystalline structures was observed, somewhat resembling orthorhombic n-paraffins: n-C37H76 to n-C44H90. However, a quantitative structure determination based on the 0kl single-crystal intensities reveals that the lamellae are not totally separated in the microcrystalline array, apparently due to residual bridging molecules that span adjacent chain layers. An atomic model for a frequently observed structure, comparable to n-C41H84, packs in space group A21am with a = 7.42, b = 4.96, and c = 109.80 Å and corresponds well to the observed diffraction amplitudes (R = 0.21).

Melt crystalfization and crystal morphology of two low molecular weight linear polyethylene fractions

Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G b) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17°C (at 117°C and 120°C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122°C and 124°C), at which the cusps of the G b data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments, it is found that within a 2°C temperature region, the G b change is accompanied by a sharp long period increase and a drastic change in single-crystal morphology from a truncated lozenge with curved (200) and (110) planes to a lenticular-shape crystal. The morphological change may result from a sudden increase in the G b coupled with a smaller change in the growth rate along the a axis with undercooling. This implies that, within this temperature region (2°C), the crystals may undergo substantial changes in the geometry of the (110) and (200) crystal growth fronts and chain folding behavior.

Morphology of binary blends of linear and branched polyethylene: transmission electron microscopy

Binary mixture of low molecular weight linear polyethylene (PE) ( M w = 2500, M w M n = 1.1 ) and higher molecular weight branched PE ( M w = 65 000–146 000, ethyl- butyl and hexyl branches, 0−1.3 mol%), crystallized at 387–389 K at which both components of the blends crystallize, have been studied by transmission electron microscopy (TEM). TEM on thin sections treated with chlorosulphonic acid and uranyl acetate provides direct evidence for cocrystallization of the components. The absence of white unstained spots and the uniform lamellar structure (i.e. thickness of amorphous layer and local crystallinity) constitute the experimental evidence in favour of this view. TEM on the pure branched PE samples further supports this view. The correspondence between the crystallinity values obtained by TEM and d.s.c. indicates that the structural features obtained by TEM are representative of the samples studied.

Phase transitions and defect structure in the lamellar surface of polyethylene and n‐alkane crystallites. Magic angle spinning 13C NMR studies

AbstractVariable temperature magic angle spinning 13C NMR experiments on linear alkanes and low molecular weight polyethylene allowed the observation of up to six carbon atoms at the chain ends near the surface of the lamellar crystallites. Conformational order‐disorder and intermolecular packing effects are reflected by the spectra. The effects are discussed with regard to the peculiar phase behaviour of the n‐alkanes and the fringed micella model for low molecular weight polyethylene. The solid‐solid transitions of nonadecane and hexatriacontane appear to be correlated with the occurrence of specific non‐liquid‐like conformational disordering. Thus, conformational defects were observed for the “rotator phase” which depend on the chain length and which vary along the chain. Whilst the center part of the chain remains all‐trans ordered, significant gauche defects were observed for the bonds at the chain ends. For a polydisperse low molecular weight linear polyethylene as well as for monodisperse octahexacontahectane reversible liquid‐like conformational disordering of the terminal chain segments could be detected when the melting point was approached. Hence, in this case the NMR spectra indicate pre‐melting of the lamella surface, while in case of the shorter n‐alkanes the occurrence of end‐gauche defects is correlated with solid‐solid transitions.