Polyolefin semi-crystalline

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

The approximated lines at the right side of Fig. 3.14 correspond to an Arrhenius activation energy of approximately 120 kj/mol, which is significantly higher than the flow activation energy of the semi-crystalline polyolefin melts shown in Fig. 3.13. The temperature dependence of the melt viscosity for amorphous thermoplastics is substantially higher than that of semi-crystalline polymers and increases dramatically as the temperature approaches the glass transition temperature. 

If the orientation process in semi-crystalline fibres is carried out well below the melting point (Tm), the thread does not become thinner gradually, but rather suddenly, over a short distance the neck. The so-called draw ratio (A) is the ratio of the length of the drawn to that of the undrawn filament it is about 4-5 for many polymers, but may be as high as 40 for linear polyolefins and as low as 2 in the case of regenerated cellulose. 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

Figure 3.2 shows a typical DSC curve for a semi-crystalline polymer such as a polyolefin as it is heated from sub-ambient temperatures until it decomposes. 

In previous sections we have shown that the redistribution of additives at the spherulite boundaries during polymer crystallization leads to the additives uneven distribution, whose form is determined by the kinetics of the growth rejection process. In time, this initial dynamic distribution should relax to an equilibrium form in which the noncrystalline polymer is uniformly permeated by the additive, whose distribution reflects that of the noncrystalline polymer. The relevanoe of these observations to oxidative degradation processes in semi-crystalline polyolefins is discussed in this section. 

The control of biodegradation rate is of critical importance for many applications of degradable polymers. Amorphous polyesters absorb water and hydrolyse much more rapidly than crystalline materials. Consequently, in partially crystalline polymers, hydrolysis occurs initially in the amorphous phase and continues more slowly in the crystalline phase. This selective degradation leads to an increase in crystallinity by chemicrystallisation. A very similar selective abiotic oxidation process occurs in the semi-crystalline polyolefins which fragment rapidly due to failure at the crystallite boundaries. 

In semi-crystalline polymers such as polyolefins, initial oxidation occurs in the amorphous tie molecules between crystallites and sometimes even in the inter-lamellar amorphous chains within crystallites. This allows a relatively low levels scission to cause a disproportionately large changes in mechanical properties due to brittle failure in the amorphous regions (Celina, 2013). In PP for instance, beginning stages of bulk embrittlement corresponded to only 0.01% of oxidation (Fayolle et al., 2004). 

Polyketone resins n. A new and unique family of aliphatic polymers composed of carbon monoxide, ethylene and minor amounts of other alpha olefins. This family of semi-crystalline resins exhibits many of the properties of engineering resins while processing similarly to polyolefins. 

A structured film of a semi-crystalline polyolefin contains a P-nucleating agent. The P-nucleating masterbatch was obtained from the Mayzo Corporation, under the trade designation MPM 1114. ... 

The materials used for the microporous polymer membranes are semi-crystalline polyolefin materials, like polyethylene (PE), polypropylene (PP), and their blends PE-PP. The preparations of the membranes can be classified into dry process and wet process [16]. 

Most polyolefins are semi-crystalline materials with melting temperatures above 100°C. They are resistant to most solvents and, indeed, do not dissolve at ambient temperature. Most polyolefins dissolve only at temperatures above their melting temperatures and require specific high-boiling point solvents. The full dissolution of polyolefins is usually achieved at temperatures between 130 and 160 C. Various solvents for the dissolution of polyolefins were proposed, among them tetrachloroethylene, decalin, a-chloronaphthalene, 1,2-dichlorobenzene, TCB, methylcyclohexane [81], cyclohexane [82-84] and TCB being most frequently used for SEC of polyolefins. Typical stationary phases are polymeric materials based on cross-linked polystyrene (PS-DVB gels). Various dissolution procedures... 

The contour plots from 2D-LC differ from those obtained by TREF-SEC because the separation principles are different. While TREE is based on crystallization-dissolution, interactive HPLC is based on the selective adsorption and desorption of the macromolecules. As a result, high-temperature 2D-LC enables selective separation of both semi-crystalline and amorphous polyolefins, whereas TREF-SEC cannot distinguish amorphous components. 

The classical techniques for chemical composition analysis of polyolefins are based on crystallization behaviour of different components of these materials. These techniques are only apphcable for the crystalline part of the sample and the amorphous part is obtained as a bulk fraction. Nevertheless, these techniques are stiU the analytical workhorse in most polyolefin research laboratories. The reason behind this is that most of the commercially important polyolefin materials are semi-crystalline. There has been a number of recent advances in these techniques that have enabled a reduction in analysis time, better resolution and mathematical modelling etc. The most fascinating innovation in this regard is the development of CEF. CEF combines the separation powers of both TREF and CRYSTAF, resulting in better separation of fractions along with considerable reduction in analysis time. CEF has the promise and potential to be the major technique in crystallization analysis in future. 

In semi-crystalline polyolefins, failure is also often by embrittlement, but for different reasons. The main mechanism is chain scission, with lowering of molecular weight and of Tg. Failure occurs at very low levels of oxidation the molecular weight typically falls by less than a factor of two and there is less than one oxygen molecule per hundred carbon atoms. The reason for this sensitivity lies in the complex morphology of many polymers. 

Hydrolysis can occur only if the polymer has functional groups capable of undergoing reaction with water and if the water can gain access. Hydrocarbon polymers are very resistant to hydrolysis because they are not wetted by water and contain no hydrolyzable groups. In the polyolefins, water access is also restricted by the semi-crystalline nature of the polymers. In complete contrast, many natural polymers, especially the polysaccharides (cellulose, starch etc.) are quite readily hydrolysed at appropriate pH, because they are water absorbing and contain readily hydrolyzable links. 

For this technique to be successful the sample must be readily soluble in an organic solvent. It has a wide application in the analysis of plastic samples. It is possible to obtain molecular weight data on both amorphous plastics and semi-crystalline plastics. In the amorphous case (e.g., polystyrene), the system used is set around ambient temperature (30 °C) and solvents such as THF, chloroform and toluene are used. With semi-crystalline plastics (e.g., polyolefins), more aggressive solvents such as ortho-dichlorobenzene and meta-ctesol are used at elevated (140 °C) temperatures. 

DSC is used extensively in the analysis of plastics, particularly those that are semi-crystalline - polyolefins, nylons, polyesters, etc. It is sensitive enough to... 

---