Branching melting temperature

High-density polyethylene is characterized by a higher crystallinity and higher melting temperature than LDPE due to the absence of branching. 

Due to both kinds of branching leading to chain irregularities, the crystallisation of radical chain-polymerised polyethylene is strongly hindered. Its maximum degree of crystallinity is limited to about 50%, its melting temperature ranges from 80°C to 115°C and its density remains low ( 0.92). From this latter property, it received the name of low-density polyethylene (LDPE). 

Figure 19 (a) Peak melting temperature as a function of the branch content in ethylene-octene copolymers (labelled -O, and symbol —B (symbol, ) and -P (symbol, A) are for ethylene-butene and ethylene-propylene copolymers, respectively) and obtained from homogeneous metallocene catalysts show a linear profile, (b) Ziegler-Natta ethylene-octene copolymers do not show a linear relationship between peak melting point and branch content [125]. Reproduced from Kim and Phillips [125]. Reprinted with permission of John Wiley Sons, Inc. 

Figure 20 Linear dependence of peak melting temperature with branch content in terms of comonomer (left hexene right octene) content from the work of Mirabella and Crist [126]. Reproduced from Mirabella and Crist [126]. Copyright 2004, John Wiley Sons, Inc. Reprinted with permission of Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc.

A similar continuity in the Tj s through the melting temperature was previously reported for linear polyethylene. (17) We have now investigated the temperature dependence of this quantity, for this polymer, in more detail and have also studied a low density (branched) polyethylene. The results for the poly-ethylenes are summarized in Fig. 8. The new data reported here substantiate the conclusion previously reached for linear polyethylene. A similar conclusion can now be reached for the baclc-bone carbons of low density (branched) polyethylene. The melting temperature for this particular sample, under the crystallization conditions studied, is less than 110°C. (33) Thus, the spin-lattice relaxation parameters for the bac)cbone carbons are the same for both the linear and branched polymers over the temperature range studied here. Changes that occur in Tq as the temperature is reduced below 0°C involve other considerations and will be discussed in detail elsewhere. (22)... 

The star-branched and two-armed systems appear to have enhanced entanglements or crosslinked networks that allow them to maintain their forms at temperatures above the linear nylon 6 melt temperature. Both the star-branched and two-armed species have melt temperatures slightly below that of single-armed nylon 6. The di-armed nylon appears to have properties similar to both the singlearmed and the star species. 

Ionic surfactants actually only form micelles when their hydrocarbon chains are sufficiently fluid, that is at temperatures above their chain melting temperature. Below a specific temperature for a given surfactant, the Krafft temperature, the surfactant becomes insoluble rather than self-assembles. For CTAB this temperature is around 20 °C and only above this temperature are micelles formed. In general, the longer the hydrocarbon chain length, the higher the Krafft temperature. For this reason, shorter-chain-length surfactants or branched chain soaps... 

Teyssie and coworkers [86] studied the effect of macromolecular architecture on the lamellar structure of the poly(ethylene oxide) crystallizable arms in (poly tert-butyl styrene)(poly(ethylene oxide))2 [PtBuS(PEO)2] miktoarm stars by using SAXS and differential scanning calorimetry (DSC). The results were compared with the ones obtained on poly(tBuS-fe-EO) materials. At the same total molecular weight and composition the melting temperature, the degree of crystallinity and the number of folds of PEO chains were found to be lower for the branched samples. 

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