Random type copolymers

The chain microstructure of random copolymers can be represented by p = Aa. the mole fraction of the major crystallizing repeating unit. Studies of the overall crystallization kinetics of random copolymers are of particular importance since as the comonomer content increases the spherulitic structure becomes poorly developed 

The overall crystallization kinetics of polyethylenes that contain long chain branches follow a similar pattern, as is illustrated in Fig. 10.2.(2) In this case, the methine carbons, to 

Analysis of experimental data for some copolyesters (4,5,6) and syndiotactic poly(propylene)(6a) indicates a superposition of isotherms. However, in these examples the crystallization was conducted at relatively large undercoolings. Superposition is to be expected under these circumstances. 

The crystallization rate is also dependent on the co-unit content at a fixed molecular weight. This point is illustrated in Fig. 10.4 for random ethylene copolymers, where the co-unit concentration varies, but the molecular weight is held fixed at about M = 5 X10 . (3) The major features of copolymer crystallization are observed again. However, these features are accentuated with increasing co-unit content. For 

Based on these considerations the kinetic equations appropriate to the crystallization of a copolymer composed of A and B units arranged in random sequence distribution has been investigated.(lO) In this analysis only the A units are allowed to enter the crystal lattice. It is convenient to characterize the extent of the transformation at time t by a parameter 9 = [I — X(t)]/[1 — A(oo)], where A,(oo) is the fraction of the noncrystalline material at the completion of the transformation and X(t) is the corresponding quantity at time t. With these conventions, the steady-state nucleation rate per untransformed unit volvune, at the conversion 6, can be expressed as 

---

The very important question then arises, as to what happens when the co-unit concentration is increased further. It is well known that for crystallization from the pure melt an increase in co-unit content lowers the melting temperature and level of crystallinity of random type copolymers. The level of crystallinity can become very small and eventually vanishes.24.25 Ye are then concerned with the question as to whether gels can form at high co-unit content and if so, are the crystallites still lamellar in character. The companion question is whether the gel mechanism remains that of an overlapping super-molecular structure for higher co-unit copolymers. 

Fig. 5.1 Theoretical plot, according to Eq. (5.19) of the fraction of crystalline A units, Wc, as a function of temperature for random type copolymers of different compositions. Short vertical arrows indicate melting temperature Tm of copolymer. For case illustrated, T = 400 K, AHu = 10 cal mol-, and In = -1.

Experimental results random type copolymers 5.4.1 Course of fusion... 

Despite the lack of quantitative agreement between theory and experiment, much of which can be attributed to experimental shortcomings and inaccessibility of the very long sequences, the data in Figs. 5.2 and 5.3 qualitatively show all of the major characteristics of the theoretical fusion curves. They can be expected to be typical of the fusion of random type copolymers, irrespective of the chemical nature and structure of the noncrystallizing chain units. Random ethylene copolymers, prepared by a completely different method, display similar fusion characteristics.(21) It is important, however, to assess the generality of the conclusions with other copolymers, rather than just depending on the results of ethylene copolymers. 

Experimental results clearly indicate that stereo-irregular polymers do indeed crystalhze as though they were copolymers. For example, specific volume-temperature curves for isotactic poly(propylene) display all the characteristics expected for a random type copolymer. The results of such a study by Newman (44)... 

A different type of pseudo-phase diagram based on the liquidus, and involving ethylene, is found in ethylene-vinyl alcohol random type copolymers.(96) These copolymers are prepared by the saponification of ethylene-vinyl acetate copolymers. Since the latter are in random sequence distribution the ethylene-vinyl alcohol copolymers have the same distribution. However, the crystallinity levels and melting points between the two are quite different. The level of crystallinity of the ethylene-vinyl acetate copolymer decreases continuously with co-unit content, as was illustrated in Fig. 5.11. The crystalline phase remains pure for this copolymer. The copolymer becomes completely noncrystalline at ambient temperature, when the co-unit content reaches about 20 mol %. The ethylene-vinyl alcohol copolymer, on the other hand, gives quite different results as is shown in Fig. 5.13.(96) This rather unusual diagram for a random type copolymer requires a more detailed... 

When a block copolymer is dissolved in a solvent that is a good one for one set of units and a poor one for the other a micellar structure forms.( 183,284) The ability to form micelles is a distinguishing feature of block and graft copolymers. Homopolymers and random type copolymers do not form micellar structures in solution. A micelle usually consists of a swollen core of the insoluble block connected to and surrounded by the soluble blocks. As the copolymer concentration is increased the micelles aggregate and organize into structures that have been termed mesomorphic gels. It is from this organized structure, where the chains themselves are in nonordered conformation, that crystallization takes place. 

The melting temperatures are sensitive to the quantity p, particularly at low comonomer composition. For example, the melting temperatures of two ethylene-butene random-type copolymers prepared by using similar catalysts differ by about 5 °C for 0.5 mol % of side groups and the difference increases to 10 °C with about 3 mol % of side groups [31]. These differences in melting temperature for chemically identical copolymers at the same composition can be attributed to differences in their respective sequence-propagation probabilities. 

According to equilibrium theory the melting temperature-composition relations of ideal random-type copolymers should obey Eq. (4.6). The functional form of Eq. (4.6) is usually obeyed even when directly observed non-equilibrium melting temperatures are used. However, the A//u values that are deduced are usually much... 

Random type copolymers nucleation rate is then given by... 

---