Polyethylene, crystallinity

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

Figure 8.3. Unit cell of crystalline polyethylene, adapted from a figure by Keller 1968.

The actual experimental moduli of the polymer materials are usually about only % of their theoretical values [1], while the calculated theoretical moduli of many polymer materials are comparable to that of metal or fiber reinforced composites, for instance, the crystalline polyethylene (PE) and polyvinyl alcohol have their calculated Young s moduli in the range of 200-300 GPa, surpassing the normal steel modulus of 200 GPa. This has been attributed to the limitations of the folded-chain structures, the disordered alignment of molecular chains, and other defects existing in crystalline polymers under normal processing conditions. 

Thus, two factors may be pointed out that determine the possibility of obtaining high yields of crystalline polyethylene on a solid catalyst with no diffusional restriction (1) the splitting up of the catalyst into small particles (< 1000 A), possible when using supports with low resistance to breaking (2) the formation of polymer grains with polydispersed porosity. 

Multiblock polyethylene-polydimethylsiloxane copolymers were obtained by the reaction of silane terminated PDMS and hydroxyl terminated polyethylene oligomers in the presence of stannous octoate as the catalyst 254). The reactions were conducted in refluxing xylene for 24 hours. PDMS block size was kept constant at 3,200 g/mole, whereas polyethylene segment molecular weights were varied between 1,200 and 6,500 g/mole. Thermal analysis and dynamic mechanical studies of the copolymers showed the formation of two-phase structures with crystalline polyethylene segments. 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Elenga, R., Seguela, R. and Rietsch, F., Thermal and mechanical behaviour of crystalline polyethylene terephthalate) effects of high temperature annealing and tensile drawing, Polymer, 32, 11, 1975-1981 (1991). 

FIGURE 5.4 Space-filling structure of a portion of a linear crystalline polyethylene (PE) region. 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

The block copolymers shown in both Table V and VI were hydrogenated. The B-lU block produced polyethylene and the polyisoprene block produced ethylene propylene alternating copolymer. The physical properties of this copolymer, composed of crystalline polyethylene block and a soft elastomeric segment made of an EPR block, is tabulated in Table VII. The data in this table illustrate the fact that a diblock of hydrogenated polybutadi ene-polyisoprene gave excellent physical properties. This is a further illustration of the new concept of soft chain interpenetrating the crystalli zable polyethylene chain via chain folding. 

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

Table 6. Character Table and Selection Rules for Crystalline Polyethylene...

One aspect of the spectrum that has received considerable attention but is as yet not completely understood is the splitting in the a fundamentals of a single chain which gives rise to the BlM and B2u fundamentals of the crystal. From studies on polyethylene and n-paraffins it was concluded [Stein and Sutherland (207, 208)] that this splitting arises from interaction between the two chains in the unit cell. As we have seen, such a splitting is predicted from a group theory analysis of the spectrum of crystalline polyethylene, and the predicted dichroic properties of the components are verified by studies on n-paraffin single crystals [Krimm... 

There is good agreement between some x-ray estimates of the crystallinity of Marlex-50 polyethylene and the calorimetric value of 93%. Smith (1956) quoted 93%, as did Rempel, Weaver, Sands and Miller (1957) who determined the crystallinity according to the method of Matthews, Peiser and Richards (1949). It should be pointed out, however, that this method has been criticized by Vonk (1959) whose crystallinity-density equation yields a value of wc equal only to 0.71 at a density of 0.96 g/cm3. On extrapolating his equation to 100% crystallinity, the density obtained is 1.016 g/cm3. In their survey of x-ray data Charlesby and Callaghan (1958) adopted 0.9898 as the density of 100% crystalline polyethylene. 

ABA ABS ABS-PC ABS-PVC ACM ACS AES AMMA AN APET APP ASA BR BS CA CAB CAP CN CP CPE CPET CPP CPVC CR CTA DAM DAP DMT ECTFE EEA EMA EMAA EMAC EMPP EnBA EP EPM ESI EVA(C) EVOH FEP HDI HDPE HIPS HMDI IPI LDPE LLDPE MBS Acrylonitrile-butadiene-acrylate Acrylonitrile-butadiene-styrene copolymer Acrylonitrile-butadiene-styrene-polycarbonate alloy Acrylonitrile-butadiene-styrene-poly(vinyl chloride) alloy Acrylic acid ester rubber Acrylonitrile-chlorinated pe-styrene Acrylonitrile-ethylene-propylene-styrene Acrylonitrile-methyl methacrylate Acrylonitrile Amorphous polyethylene terephthalate Atactic polypropylene Acrylic-styrene-acrylonitrile Butadiene rubber Butadiene styrene rubber Cellulose acetate Cellulose acetate-butyrate Cellulose acetate-propionate Cellulose nitrate Cellulose propionate Chlorinated polyethylene Crystalline polyethylene terephthalate Cast polypropylene Chlorinated polyvinyl chloride Chloroprene rubber Cellulose triacetate Diallyl maleate Diallyl phthalate Terephthalic acid, dimethyl ester Ethylene-chlorotrifluoroethylene copolymer Ethylene-ethyl acrylate Ethylene-methyl acrylate Ethylene methacrylic acid Ethylene-methyl acrylate copolymer Elastomer modified polypropylene Ethylene normal butyl acrylate Epoxy resin, also ethylene-propylene Ethylene-propylene rubber Ethylene-styrene copolymers Polyethylene-vinyl acetate Polyethylene-vinyl alcohol copolymers Fluorinated ethylene-propylene copolymers Hexamethylene diisocyanate High-density polyethylene High-impact polystyrene Diisocyanato dicyclohexylmethane Isophorone diisocyanate Low-density polyethylene Linear low-density polyethylene Methacrylate-butadiene-styrene... 

Until now we have been concerned mostly with crystalline polyethylene. In this section we consider the solid-state structure of poly(tetramethylene oxide) [22]. Since the melting temperature of this polymer is 42 °C, we examined the structure at temperatures below room temperature. The sample was prepared by ringopening polymerization of tetrahydrofuran by using triethyloxonium hexa-... 

Fig. 44. The SFM amplitude (a) and force modulation (b) maps of a cryogenic faced impact copolymer (ICP) composed of a polypropylene (PP) matrix with high ethylene (60 wt. %) ethylene-propylene copolymer (EP). Crystalline polyethylene (PE) phases are seen in the EP domains, which are surrounded by the PP matrix. Modulus contrast in the force modulation (drive amplitude 100 mV) image associated with the three polymers the stiff PP matrix is dark, the soft EP domains are light. The crystalline PE regions have modulus between the PP and the EP,thus an intermediate shade of grey is observed for the PE domains [128]...

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