Comonomer, Crystaf

Rytter et al. reported polymerizations with the dual precatalyst system 14/15 in presence of MAO [30]. Under ethylene-hexene copolymerization conditions, 14/MAO produced a polymer with 0.7 mol% hexene, while the 15/MAO gave a copolymer with ca. 5 mol% hexene. In the mixed catalyst system, the activity and comonomer incorporation were approximate averages of what would be expected for the two catalysts. Using crystallization analysis fractionation (CRYSTAF) and differential scanning calorimetry (DSC) analysis, it was concluded in a later paper by Rytter that the material was a blend containing no block copolymer [31],... 

In experimental practice, a straight-line correlation between temperature and comonomer composition has been obtained by various authors with TREF [61,62], DSC [63], and CRYSTAF [64]. These correlations are practically independent of molar mass. 

The same solvents, same IR detector and similar calculation parameters to those presented in Sect. 4.1.2 for TREF are applicable for CRYSTAF analysis. The calibration of temperature to comonomer content can be performed by using narrow composition standards (metallocene-type resins) of the same comonomer type, with similar results to TRFF as discussed in Sect. 4.1.2. Octene and hexene copolymers follow the same calibration curve [92]. 

The CCD is the second most important microstructural distribution in polyolefins. Differently from the MWD, the CCD carmot be determined directly only the distribution of crystallization temperatures (CTD) in solution can be measured and one can try to relate this distribution to the CCD using a calibration curve. Two techniques are commonly used to determine the CTD or CCD of polyolefins TREF and Crystaf. Both operate based on the same principle chains with more defects (more comonomer molecules or stereo-and/or regioirregularities) have lower crystallization temperatures than chains with fewer defects. Figure 2.11 compares the TREF and Crystaf profiles of an ethylene/1-butene copolymer made with a heterogeneous Ziegler-Natta catalyst. Notice that they have very similar shapes the Crystaf curve is shifted toward lower temperatures because it is measured as the polymer chains crystallize, while the TREF curve is determined as the polymer chains dissolve (melt) and are eluted from the TREF column, as explained in the next few paragraphs. 

Similarly to GPC, the amount of information obtained with TREF and Crystaf can be increased by adding more detectors to the system. For instance, LS and VlSC detectors have been used to determine molecular weight averages as a function of crystallization/elution temperature or comonomer content in the copolymer. The analytical results shown in Figure 2.4, for instance, were measured with a TREF/IR-LS system. Another TREF/IR-LS profile is depicted in Figure 2.12 for a rather complex trimodal polyolefin resin. 

Crystallization analysis fractionation (Crystaf) fractionates polymer chains according to differences in crystallizability. Crystaf can be used to fractionate polymers due to differences in chemical composition, comonomer sequence length, and tacticity. It may also respond to long-chain branching, provided that the polymer is branched enough to affect its crystallinity. The fractionation principle operative in Crystaf was discussed in the section on batch fractionation for the case of slowly cooling (or warming) solutions of semicrystalline polymers. 

One of the main difficulties for the quantification of Crystaf is the nonimiver-sality of its calibration curves. Even for a series of ethylene/a-olefin copolymers, the calibration curves will vary as a function of comonomer type, as illustrated in Figure 15. The general rule of thumb for these copolymers (from propene to 1-octene) is, the longer the a-olefin, the lower the crystallization temperature for a given a-olefin molar fraction. This has been explained by several authors on the basis of the difference in the degree of inclusion of the a-olefin in the crystalline lattice shorter a-olefins are more likely to cocrystallize with ethylene and therefore depress the crystallization temperature to a lesser extent. 

For ethylene/1-olefin copolymers, chain crystaUizabihty is mainly controlled by the fraction of noncrystalhzable comonomer imits in the chain. Consequently, the differential Crystaf profile shown in Fig. 1, together with an appropriate cahbration curve, can be used to estimate the copolymer chemical composition distribution (CCD), also called the short-chain branch distribution. The CCD of a copolymer describes the distribution of the... 

In the case of stereoregular polymers, such as isotactic and syndiotac-tic polypropylene, chain tacticity is the main factor affecting crystallizability. Crystaf can also be used to measure the distribution of tacticity. Since the distribution of tacticity is often modeled with pseudo binary copolymerization models (i.e. the meso and racemic insertions stand for the comonomer type in the case of a copolymer), the following discussion for copolymers can be easily modified to describe the tacticity distribution of stereoregular polymers. 

All microstructural features impacting chain crystallizability can potentially influence the Crystaf fractionation process. The main microstructural properties of interest are (1) number average molecular weight, (2) CC, and (3) comonomer type. Each of these factors will be discussed below. 

Sarzotti et al. [58] investigated the effect of comonomer content on Crystaf profiles using a series of ethylene/l-hexene copolymers with different comonomer contents but approximately the same molecular weight, effectively eliminating any possible misinterpretations that might arise because of molecular weight effects (Fig. 34) [58]. As expected, Crystaf peak temperatures are dramatically influenced by the CC of the copolymer chains. Moreover, the Crystaf profiles become broader with an increase in comonomer content. 

Fig. 34 Effect of comonomer content on Crystaf profiles. These samples are ethylene/ 1-hexene copolymers synthesized using a single-site-type catalyst. All samples have similar molecular weights [58]...

The effect of comonomer type was studied by Brull et al. [59] using propylene/1-olefin copolymers with several comonomer types (1-octene, 1-decene, 1-tetradecene, and 1-octadecene). They reported that, for their set of samples, not only Crystaf peak temperatures but also melting and crystallization temperatures measured by DSC were independent of comonomer type but depended strongly on comonomer content. 

More recent work by the same research group [60] has investigated the effect of comonomer type using a series of ethylene/1-olefin copolymers (1-decene, 1-tetradecene, and 1-octadecene). Notice that ethylene instead of propylene was used in this particular study. Once more, they reported that Crystaf peak temperatures were practically independent of comonomer type (Fig. 35). 

Fig. 35 Effect of comonomer type on Crystaf peak temperature and cooling and melting DSC peak temperatures [60]...

In our recent work [67], we investigated the effect of comonomer type on CO crystallization using a series of ethylene/1-olefin copolymers with four comonomer types propylene, 1-hexene, 1-octene, and 1-dodecene. Four blends, one for each copolymer type, were prepared such that they crystallized at the same temperature range and had similar ATq to ehminate the effect of similarity of chain crystalHzabihties. The Crystaf results of these blends indicated that the comonomer type of the parent samples did not appreciably influence their cocrystalHzation behavior, as illustrated in Fig. 39. 

Two methods for preparing the calibration curve have been reported. Both methods were done by performing Crystaf analysis in a series of narrow-CCD copolymer samples with known comonomer contents with crystallizabilities covering a broad range of crystallization temperatures. The only difference between these two methods is the type of samples used in the calibration. The first method uses a series of polymer samples synthesized with single-site-type catalysts [58,68], while the second method uses a series of fractions from broad-CCD Ziegler-Natta copolymers obtained with P-Tref [1,49]. After the whole series of samples has been analyzed, the relationship between Crystaf peak temperature and CC is used as the cahbration curve. 

Using a careful factorial experimental design, the effect of polymerization temperature, polymerization pressure, amount of hydrogen, and the comonomer-to-monomer feed ratio on Crystaf profiles can be identified [78]. 

Although these Monte Carlo models can explain qualitatively the effects of molecular weight and comonomer content on Crystaf profiles, they are unable to take into account crystallization kinetics and cocrystaUization effects that have recently been reported as significant factors affecting Crystaf profiles [29,67]. More work is required to quantify these important effects. Such a model, if developed, would be invaluable to obtain a imiversal calibration curve for Crystaf and Tref. 

The amount of the polymer crystallizing at each temperature can be obtained by differentiation of the integral CRYSTAF profile at each temperature. The plot of amount of polymer crystallized as a function of temperature is the most common and the clearest reporting method of CRYSTAF results. Both types of plots for a blend of HOPE and PP are shown in Fig. 8. Similar to TREF, there are several factors which can affect CRYSTAF profiles and the reliability of results including the molar mass of the polymer, the comonomer type and content, the cooling rate (CR), co-crystallization effects. Soares et al. wrote two comprehensive reviews about crystallization based techniques in 2005 [13, 51]. In this review we focus on the work that has been published more recently. 

The chemical composition distribution of polyolefins is measured (indirectly) by either temperature rising elution fractionation (Tref) or crystallization analysis fractionation (Crystaf). These two techniques provide similar information on the chemical composition distribution of polyolefins and can be used interchangeably in the vast majority of cases. Both methods are based on the fact that the crys-tallizability of HOPE and LLDPE depends strongly on the fraction of a-olefin comonomer incorporated into the polymer chains, that is, chains with an increased a-olefin fraction have a decreased ciystallizability. A similar statement can be made for polypropylene and other polyolefin resins that are made with prochiral monomers resins with high stereoregularity and regioregularity have higher crystalliz-abilities than atactic resins. 

The use of NMR makes it possible to probe details of molecular features not accessible using other techniques. TREF and CRYSTAF are techniques based on crystaUizability in solution and are essential in the determination of the level and distribution of the short-chain branches resulting from the introduction of a comonomer. Much of the material in this chapter is presented in more detail by Sperling [101] and in the book edited by Pethrick and Dawkins [102],... 

Crystallization Analysis Fractionation (CRYSTAF) It is based on the segregation of crystals of different morphology or comonomer content by crystallization, In CRYSTAF, the separation and analysis are performed in a single step (the crystallization cycle), where concentration of the polymer solution is intermittently sampled and analyzed as the temperature goes down. The temperature-concentration data which are obtained correspond, in the cases of branched poly(olefins) or their copolymers with a-olefins, to the cumulative curve of the side chain branching distribution (SCBD). The last point at the lowest temperature of the experiment is the soluble or noncrystallizable fiaction. 

Anantawaraskul and co-workers investigated the effect of the molecular mass and the comonomer content of ethylene-1-hexene copolymers on the CRYSTAF profile. Figure 22 shows that the distribution of peak in the CRYSTAF profile becomes narrower with increasing molecular masses while the peak maximum simultaneously shifts to higher crystallization temperatures. Figure 23 presents the effect of the deaeasing comonomer content the peak narrows and shifts to higher crystallization temperatures as the composition of the copolymer approaches that of the pure polyethylene. 

Figure 23 CRYSTAF profiles of ethylene-1-hexene copolymers of different 1-hexene comonomer contents. Reprinted from Satzotti, D. M. Soares, J. B. P. Penlidis, A. J. Polym. Sci., Part B Polym. Phys. 2002, 40,2595. Copyright 2002, with permission from Elsevier.

Figure 25 Melting temperature, crystallization temperature from melt (DSC), and crystallization temperature from solution (CRYSTAF) of propene-higher a-olefin copolymers as function of the comonomer content. Reproduced with permission from Brull, R. Pasch, H. Raubenheimer, H. G. etal. Macromol. Chem. Phys. 2001,202,1281. Copyright Wiley-VCH Verlag GmbH Co. KGaA.

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