Polymers in the melt

Figure 25.24. Difference in behaviour between liquid crystal polymers and conventional crystalline polymers in the melt at rest, during shear and when cooled after shearing...

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

C. The high melting point of pure ethene/CO (265 °C) gives decomposition and formation of brown, fiiranised polymers in the melt, which makes processing in extruders impossible. 

Most important, however, was the discovery by Simha et al. [152, 153], de Gennes [4] and des Cloizeaux [154] that the overlap concentration is a suitable parameter for the formulation of universal laws by which semi-dilute solutions can be described. Semi-dilute solutions have already many similarities to polymers in the melt. Their understanding has to be considered as the first essential step for an interpretation of materials properties in terms of molecular parameters. Here now the necessity of the dilute solution properties becomes evident. These molecular solution parameters are not universal, but they allow a definition of the overlap concentration, and with this a universal picture of behavior can be designed. This approach was very successful in the field of linear macromolecules. The following outline will demonstrate the utility of this approach also for branched polymers in the semi-dilute regime. 

It has long been reaUsed that the key physics determining the rheology of high molecular weight polymers in the melt state arises from the topological interactions between the molecules [1,2]. This is deduced from observations on many different monodisperse materials that ... 

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

The viscoelastic properties of long-branched polymers in the melt are understood even less well than their solution properties the former are profoundly affected by entanglements, unless the polymer is of low DP, and it is intuitively obvious that entanglements involving branched molecules may be more difficult to unravel than those of linear molecules, especially those involving segments between two branch points but to treat this quantitatively would be difficult. 

Following the procedure used with JeR (Section 5), po data on these and other polymers were correlated in terms of cM/qMc, with results which are shown in Fig. 8.13. The parallel in behavior between the po and JeR master correlations is unmistakable. Even the relative positions of polymers on the master correlations are similar note for example the data on JeR and p0 for polybutadiene. Published data on relatively narrow distribution polyethylene (210,326) have not been included in Fig. 8.13 because departures from tj0 were too small to define P0 with accuracy. However, estimates of po from the data provided suggest that polyethylene may follow a different pattern than other polymers. Departures from rj0 seem to appear at anomalously low shear rates (326). Aside from tj0 values, viscoelastic data on well characterized crystallizable polymers in the melt state are rather scarce. Although not especially anticipated, it is certainly conceivable that erystallizability confers some unusual features to the flow behavior. 

Immiscibility of polymers in the melt is a common phenomenon, typically leading to a two-phase random morphology. If the phase separation occurs by a spinodal decomposition process, it is possible to control the kinetics in a manner that leads to multiphase polymeric materials with a variety of co-continuous structures. Common morphologies of polymer blends include droplet, fiber, lamellar (layered) and co-continuous microstructures. The distinguishing feature of co-continuous morphologies is the mutual interpenetration of the two phases and an image analysis technique using TEM has been described for co-continuous evaluation.25... 

Reactive chains can be obtained by anionic polymerization, followed by attachment of a reactive end-group. This route yields nearly monodisperse polymers with functional groups at their ends, polymers that are very well suited for systematic studies. The synthesis can be quite elaborate, and, for longer chains, the completeness of the end-functionalization is difficult to verify. If the mono-dispersity and the control of the molecular weight of the polymer are not crucial or not possible, more common techniques can be used. They involve either the free-radical synthesis of a polymer incorporating a small fraction of reactive comonomers that will then be distributed along the chain, or the random functionalization of the polymer in the melt (using a free-radical initiator) after the... 

Because of the high temperatures experienced by the polymer in the melt process, initial attempts at producing melt BPA-PC resulted in highly colored product due to thermal oxidative reactions occurring during... 

There is evidence that polymer-solvent interactions can cause die E of a polymer in solution to remain significantly dependent on Mn up to higher values of Mn than the ET OQ of the same polymer in the melt [16,17]. The same evidence [16,17] also suggests that the rules-of-mixture describing the behavior of the ET)oq of a solution as a function of Epp, EpS and d>p accurately will probably need to include terms related to the compatibility between the solvent and the polymer instead of having very simple forms such as equations 12.19 or 12.21. There is, presently, no method to predict the effects of such complicating features. 

The experiments described above indicate the existence of a Brownian behaviour for atactic polystyrenes in the two following physical situations polymers in the melt or in dilute solutions at the Flory temperature Tv. 

Polymer chains are strongly entangled in the melt but despite this they behave in a way that is thermodynamically ideal. This surprising fact was first reported by P. J. Flory in 1949, but may be readily understood. If we consider the repulsion potential, U, experienced by a monomer unit of a polymer in the melt, we can divide U into two terms, one due to repulsion by other monomer units of the same molecule, U, and the other due to repulsion by monomer units in different polymer mcdecules, f/,i, i.e. 

The viscoelastic behavior in the melt state of end-fiinctionalized polyisoprenes was also investigated (30). The results can be compared with predictions based on the star model for the aggregates. It is well known that the viscoelastic properties of star polymers in the melt state depend on arm molecular weight and they are insensitive to their functionality (31). 

Some explanation of terms is appropriate here, including the concept of viscosity. As stated above, the polymers in the melt exist in coiled form. As flow occurs, a... 

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