Long Chain Branching 358 INDEX

Two analytical methods, size exclusion chromatography and rheology, provide a long chain branching index. The application of size exclusion chromatography to long chain branching analysis was described in Section 5.2.3.1. 

Grinshpun, V. Rudin, A. Russell, K.E. Scammell, M.V. Long-chain branching indexes from size-exclusion chromatography of polyethy-lenes. J. Polym. Sci. B Polym. Phys. 1986, 24 (5), 1171-1176. 

Shroff, R.N. Mavridis, H. Long-chain branching index for essentially linear polyethylenes. Macromolecules 1999, 32, 8454-8464. 

Both the Goto et al. [35] and the Lorenzini et al. [34] models are well suited to fit the experimental long-chain branching index /lcb (number of branches per 1000 chain carbon atoms), the measured Afw, and also the full MWD. In LDPE modeling, the scheme of Goto et al., eqs (4.6-9) and (4.6-10), is frequently favored because of a reduced stiffness of the differential equations associated with this approach. If the type and the concentration of branched macroradicals need to be specified, as is desirable in copolymerization modeling, the scheme of Lorenzini et al. [34] appears to be more suitable. 

Figure 4.6-7 Weight average, M, of the MWD and long-chain branching index, /lcb. calculated for polyethylenes produced at 200 °C and 2000 bar in a CSTR at different conversions. The rate coefficients used for this simulations are A ,p = 256 LmoP s and kp = 1976 s . Values for kp and kt are taken from Schweer [24] and ka,M from Reference [38].

Fig. 18.9 Slice long-chain branching index (SLCBI) as a function of a slice MW determined by SEC for two polymers in a favorable blend (Lue and Kwalk 2005)...

Then they propose that the average number of branches per 1000 carbon atoms, which they call the long chain branching index LCBI, is given by ... 

With the availability of the higher density polymers the value of the melt flow index as a measure of molecular weight diminishes. For example, it has been found that with two polymers of the same weight average molecular weight (4.2 X 10 ), the branched polymer (density = 0.92 g/cm ) had only 1/50 the viscosity of the more or less unbranched polymer (density = 0.96 g/cm ). This is due to long chain branches as explained above. 

FIGURE 16.17 Nonbranched/long chain branched glucans of potato starch dissolved in hot water-steam and 0.1 M NaOH 1.2 ml of the 18-mg/ml solution was separated on Sephacryl S-1000 (95 X 1.6 cm) 3-ml fractions were collected for further analysis normalized (area = 1.0) eluogram profiles (ev) constructed from an off-line determined mass of carbohydrates for each of the fractions branching index ( ) determined from iodine-complexing potential of individual 3-ml fractions flow rate 0.40 ml/ min V ,i = 75 ml, Vtot = 162 ml eluent 0.005 tA NaOH. 

Linear low-density polyethylene (LLDPE)440-442 is a copolymer of ethylene and a terminal alkene with improved physical properties as compared to LDPE. The practically most important copolymer is made with propylene, but 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are also employed.440 LLDPE is characterized by linear chains without long-chain branches. Short-chain branches result from the terminal alkene comonomer. Copolymer content and distribution as well as branch length introduced permit to control the properties of the copolymer formed. Improvement of certain physical properties (toughness, tensile strength, melt index, elongation characteristics) directly connected to the type of terminal alkene used can be achieved with copolymerization.442... 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

ScHREiBER, H. P., and E. B. Bagley The Newtonian melt viscosity of polyethylene An index on long-chain branching. J. Polymer Sci. 58, 29 (1962). 

Another index developed to detect long-chain branching is the Dow rheology index (DRI). It was specifically derived for polyethylenes with similar, narrow molecular weight distributions 2)... 

The CG catalysts produce highly processable polyolefins with a unique combination of narrow MWD and long chain branches. Ethylene-octene copolymers produced with CG catalysts have useful properties across a range of densities and melting indexes. These novel copolymer families are called polyolefin plastomers (POP) and polyolefin elastomers (POE). POPs possess plastic and elastic properties while POEs containing greater than 20 wt% octene comonomer units have higher elasticity. 

Metallocene LLDPEs are relatively difficult to process because of narrow molecular weight distribution (MWD) when compared to conventional Ziegler LLDPEs. Metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to process better as a result of their long-chain branched (LCB) structure, referred to as Dow Rheology Index (DRI) numbers.The LCB is also responsible for improved melt strength in mLLDPEs.Exxon has also addressed the processibility issue with advanced performance terpolymers.LCB bimodal mLLDPE resins are commercially available from BP Chemicals.Such mLLDPEs are produced by BP s proprietary gas phase fluid bed technology called Innovene technology. 

As a result of the controlled long-chain branching in otherwise linear polymers, the processability of these resins is claimed to be significantly enhanced compared to the standard LLDPE and EP rubber materials. Envircmmental stress crack resistance of metallocene polyolefin is also claimed to be significantly better. A wide range of densities (0.86-0.93), flexural modulus (10-100 MPa), melting points (60-120 °C), and melt flow index (0.5-125) is available in the commercial metallocene-based ethylene copolymers. 

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