Crystalline polymers polyethylene

For a polymer where crystallinity dominates its relaxation behavior, the situation is quite different. Figure 4-3 shows the 10-second modulus vs. temperature curve for such a crystalline polymer, polyethylene (PE). Included also in this figure is the modulus-temperature curve for polyvinyl chloride... 

An example of the effect polymer chain conformation can have on high-resolution solid-state C CPMAS/DD NMR spectra is presented for the semi-crystalline polymer polyethylene (PE) in Figure 5.11. These spectra [20] were recorded with Torchia s CF-T pulse sequence [21], as illustrated, which... 

We shall begin with a brief and simplified discussion of the main features of experimental observations and proceed to consider the interpretation of these features. Three polymers are selected as paradigms PET, which can exist in the wholly amorphous state but also as a partially crystalline polymer polyethylene, which is a high crystalline polymer and a liquid crystalline polymer, the thermotropic copolyester whose mechanical anisotropy was discussed in Section 7.5.4 above. 

The applications of these techniques is illustrated by data on a variety of polymers, although the bulk of the discussion concerns a typical amorphous polymer, atactic polystyrene and a crystalline polymer, polyethylene oxide. A number of results are quoted which seem to indicate that phenyl group rotations or oscillations are not an important mechanism in the relaxation of polystyrene. Other materials mentioned are polypropylene oxide, starch and ionized copoly-... 

Choy, C. L., and Grieg, D., The Low Temperature Thermal Conductivity of a Semi-crystalline Polymer, Polyethylene Terephthalate, /. Phys. C Solid State Phys. 8, 3121,1975. [Mylar]... 

The methods used for the crystallinity determination can also be used to determine the ratios of the different polymorphs that are often present in some polymers. In the case of PA6 shown in Figure 2.6a, the scan is resolved into the contribution from its two polymorphs, a and y, along with the amorphous halo [47]. Relative areas of the various peaks are used to calculate the relative amounts of the a and y components as well as the total crystallinity. The method can also be extended to determine when more than one polymer is present in the sample, such as polymer blends [48,53], Figure 2.6b shows an example of a mixture of amorphous poly(2,6-dimethyl-p-phenylene ether) (PPE) and PA6. The amorphous templates of PPE and PA6 were obtained from the scans of the homopolymers as discussed in Section 2.5.1. These templates were used as constraints in least squares fitting the data from the blend. Such analyses were useful in demonstrating that crystallinity and crystallite sizes of the PA6 were smaller in an alloy of the two polymers than in a blend [48]. Similar analyses have been carried out in a blend of two crystalline polymers, polyethylene and polypropylene [53]. 

Ionic polymerizations are almost exclusively solution processes. Many Zieg-ler-Natta polymerizations are also. They can be run under conditions such that the polymer product stays in solution, as in the production of stereospecific rubbers. The crystalline polymers polyethylene and isotactic polypropylene are commonly produced at temperatures sufliciently below T so that the polymer product is a solid that grows on the catalyst particles as in gas-phase polymerizations. Such processes are known as slurry polymerizations. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

Fig. 22.6. A schematic drawing of a largely crystalline polymer like high-density polyethylene. At the top the polymer has melted and the chain-folded segments hove unwound.

If a polymer molecule has a sufficiently regular structure it may be capable of some degree of crystallisation. The factors affecting regularity will be discussed in the next chapter but it may be said that crystallisation is limited to certain linear or slightly branched polymers with a high structural regularity. Well-known examples of crystalline polymers are polyethylene, acetal resins and polytetrafluoroethylene. 

In the case of commercial crystalline polymers wider differences are to be noted. Many polyethylenes have a yield strength below 20001bf/in (14 MPa) whilst the nylons may have a value of 12 000 Ibf/in (83 MPa). In these polymers the intermolecular attraction, the molecular weight and the type and amount of crystalline structure all influence the mechanical properties. 

In the case of crystalline polymers better results are obtained using an amorphous density which can be extrapolated from data above the melting point, or from other sources. In the case of polyethylene the apparent amorphous density is in the range 0.84-0.86 at 25°C. This gives a calculated value of about 8.1 for the solubility parameter which is still slightly higher than observed values obtained by swelling experiments. 

By the use of a moderately crystalline polymer with a Tg well below the expected service temperature (e.g. polyethylene). 

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. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

Dow ABS Nylon 6/6 Polycarbonate Polyethylene, HDPE, LDPE, LLDPE, ULDPE Polypropylene HPPP, CPPP Polystyrene HIPS, GPPS, Recycled, Advanced Styrenic Resin SAN Polyurethane Elastomers Polyolefin Plastomer PC/ABS Crystalline Polymer ABS/TPU... 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

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