Crystallisation polymers

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Mold temperature can vary widely. Typical temperatures range from 40 to 150°C. Higher mold temperatures favor polymer crystallisation and result ia more dimensionally stable parts. Crystallinity can be developed ia parts molded ia cold molds by annealing at approximately 200°C. 

Fig. 22.8. (a) The volume change when a simple melt (like a liquid metal) crystallises defines the melting point, T (b) the spread of molecular weights blurs the melting point when polymers crystallise ( ) when a polymer solidifies to a gloss the melting point disappears completely, but a new temperature at which the free volume disappears (the gloss temperature, TJ con be defined and measured. 

SHARPLES, A., Introduction to Polymer Crystallisation, Arnold, London (1966)... 

The commercial appearance of phenolic resins fibres in 1969 is, at first consideration, one of the more unlikelier developments in polymer technology. By their very nature the phenolic resins are amorphous whilst the capability of crystallisation is commonly taken as a prerequisite of an organic polymer. Crystallisability is not, however, essential with all fibres. Glass fibre, carbon fibre and even polyacrylonitrile fibres do not show conventional crystallinity. Strength is obtained via other mechanisms. In the case of phenolic resins it is obtained by cross-linking. 

The fringed micelle theory has been less favoured recently following research on the subject of polymer single crystals. This work has led to the suggestion that polymer crystallisation takes place by single molecules folding themselves at intervals of about 10 nm to form lamellae as shown in Figure 3.3b. These lamellae appear to be the fundamental structures of crystalline polymers. 

It has proved difficult to decide which of these two theories of polymer crystallisation is correct, since both are consistent with the observed effects of crystallinity in polymers. These effects include increased density, increased stiffness, and higher softening point. However, the balance of opinion among those working with crystalline polymers favours the latter theory, based on lamellae formed by the folding of single molecules. 

In high polymers crystallisation means the formation of areas of regularity in chain aggregation rather than the formation of discrete crystals, as in simple chemical compounds. Crystallite... 

Significantly, for macromolecular materials the rate of polymer crystallisation can be extremely slow and polymers that can potentially crystallise are often isolated in a kinetically stable, amorphous state. A potentially crystal-lisable polymer that is in an amorphous state can show an exothermic crystallisation transition T at elevated temperatures. The thermal transitions of a polymer are commonly investigated by the technique of differential scarming... 

It is not intended to present a comprehensive review of the extensive literature on polyethylene morphology. Several themes have been selected on the basis of novelty and importance in the author s eyes. The historical review, which is part of Sect. 1, presents the old discoveries in a brief form and it also serves as an introduction to the more specialised themes presented in the subsequent sections. The link to the theory of polymer crystallisation— historically closely related to discoveries made within the polyethylene morphology field—is briefly discussed. 

At high crystallisation temperatures, the high molar mass polymer crystallised alone. Data for the fold surface free energy obtained from linear growth rate data supported the view that the nature of the fold surface of the dominant lamellae was related only to the molar mass of the crystallising component and was not affected by the composition of the melt. 

The properties of a given polymer very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. A polymer crystallised under conditions of high nucleation/growth ratios, with smaller structures, is generally more transparent. 

Since polymers cannot be completely crystalline (i.e. cannot have a perfectly regular crystal lattice) the concept "crystallinity" has been introduced. The meaning of this concept is still disputed (see Chap. 2). According to the original micellar theory of polymer crystallisation the polymeric material consists of numerous small crystallites (ordered regions) randomly distributed and linked by intervening amorphous areas. The polymeric molecules are part of several crystallites and of amorphous regions. 

As a model of the nucleus in polymer crystallisation one often takes a rectangular prism. A breakthrough in this respect was the discovery and exploration of polymer single crystals (Schlesinger (1953) and Keller (1957)) which are indeed small prisms, platelets of polymeric chains, folded back and forth in a direction perpendicular to the basal plane (see Fig. 19.1)... 

Table 19.2 gives a survey of the morphology of polymer crystallisation. The survey is self-explanatory it demonstrates an almost continuous transition from the pure folded chain to the pure extended-chain crystallite. 

We shall now discuss, successively, the four main modes of polymer crystallisation ... 

Most polymers crystallise at measurable rates over a range of temperatures that is characteristic of each polymer. It may extend from about 30 °C above the glass temperature (Tg) to about 10 °C below the melting point (Tm). 

TABLE 19.5 Comparison of experimental and estimated (predicted) data in polymer crystallisation... 

A completely different approach to polymer crystallisation in extended-chain conformation became possible with the coming of a new class of polymers the para-para type aromatic polymers. These polymers possess inherently rigid molecular chains in an extended conformation (Preston, 1975 Magat, 1980 Northolt, 1974, 1980, 1985 Dobb, 1985). Theoretically they should give rise to high orientation in fibre form without the necessity of subjecting the as spun filaments to the conventional drawing process. 

Polymer crystallisation is an old and tumultuous field that is experiencing a revival of interest, particularly with the advent of time-resolved X-ray scattering studies of structure development in the early stages of crystallisation. Older discussions revolved around establishing chain folding as the... 

The ot-L-Ara/-(l->5)-arabinan, oiDP 45-80, covalently attached to rhamnose residues, can be cleaved and isolated, and forms reasonably well-defined crystallites, which, however, cannot be aligned for fibre X-ray. Recent powder X-ray work has indicated that the polymer crystallises in a monoclinic unit cell with one chain per unit cell. It adopts a single two-fold helix with the... 

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