DSC Characterization of Short Chain Branching Distribution in Polyethylene

Introduction [1 – 9] 

The following acronyms are used in this article:

• MMD: Molar Mass Distribution 
• SCBD: Short Chain Branching Distribution 
• SCB: Short Chain Branching
  
• TREF: Temperature Rising Elution Fractionation 
• SIST: Stepwise Isothermal Segregation Technique 
• SSA: Successive Self-nucleation / Annealing

The properties of the copolymers of ethylene and higher α-olefines (1-butene, 1-hexene, 1-octene, 4-methyl-pentene, etc.) are closely related to the molar mass distribution (MMD) and the short chain branching distribution (SCBD) within the macromolecule. The MMD and the type and content of monomers (or branching) can be determined by Size Exclusion Chromatography (SEC) and Nuclear Magnetic Resonance (NMR). The determination of the SCBD requires the use of fractionation techniques.

For instance, the monomer distribution along the molecular chains can the determination of the characteristics of molecular structures. Although TREF has been successfully applied, the main disadvantages of this fractionation technique are the relatively high costs involved and the time needed to perform an experiment. Several research groups have therefore tried to develop simpler and more rapid methods for the analysis of polyethylenes and α-olefin copolymers using Differential Scanning Calorimetry, DSC, even though the results of these methods are not as accurate as those from TREF analysis.

This study compares two DSC methods for estimating the SCBD of be determined by Temperature Rising Elution Fractionation (TREF). This technique fractionates the heterogeneous polyethylene (PE) by stepwise crystallization in solution onto an inert support. The support temperature follows the crystallization temperatures of the different molecular structures of the PE chains. The method involves dissolving the polymer in a suitable solvent at high temperature at 130 °C to 140 °C. The dilute polymer solution is then cooled slowly to room temperature. Afterward the temperature is increased in steps. Fractions are thereby formed that can then be analyzed with the standard methods used for copolymers following appropriate thermal treatment. The first method is known as the Stepwise Isothermal Segregation Technique (SIST).

Conclusions

The results of the work presented in this article show that the SIST or SSA thermal treatments of PE with different SCB contents followed by DSC measurements are able to provide qualitative information on the molecular architecture of these copolymers. Of the different experimental conditions used in this work, the SSA treatment gives the best fractionation on crystallization. In any case, SIST and SSA analyses with DSC allow polymers to be more rapidly characterized than TREF.

_{DSC characterization of short chain branching distribution in polyethylene | Thermal Analysis Application No. UC 135 | Application published in METTLER TOLEDO Thermal Analysis UserCom 13}