In the synthesis of low-density polyethylene there is a direct relationship between the synthesis conditions, molecular structure, and technical properties of the product. The evaluation of the molecular structure of the polymer is therefore important. The most important structure parameters in low-density polyethylene are molecular weight distribution (MWD), degree of long-chain branching (LCB), and short-chain branching distribution (SCBD). Through chain transfer to polymer it is possible to get composite molecules made up of chains with different amounts of short-chain branching. By using temperature rising elution fractionation (TREF) and plotting the methyl contents or the DTA melt temperatures of the fractions as functions of the elution temperature, the SCBD within composite molecules can be evaluated. The deviations from linear relationships are caused by such SCBD within composite molecules. In order to demonstrate the buildup of composite molecules, samples from different parts of a reactor were investigated.
EVALUATION OF SHORT-CHAIN BRANCHING DISTRIBUTION (SCBD) IN LDPE
Analytical and physical methods can be used to investigate the degree of short-chain branching (SCB) in LDPE. SCB increases the amount of methyl groups in the polymer, and these can be analyzed with an IR spectrophotometer. The crystalline structure in LDPE is disrupted by SCB, and this enables physical methods to be used for measuring the amount and distribution of SCB. Early investigations of SCBD in LDPE were made by using temperature rising elution fractionation (TREF) and measuring the methyl contents of the fractions. The weight distribution of different methyl contents was taken as a measure of SCBD.',2
Temperature rising elution fractionation can be performed continuously using the fractionator seen in Figure .2 The 5-liter column is filled with Chromosorb and loaded with a 4-g polymer sample as a 1% xylene solution. The temperature of the column is 120°C when loaded. Then, the column is cooled down quite quickly to 90°C and after that, a cooling rate of 1°C/hr is used until 40°C is reached. This slow cooling rate is necessary in order to achieve complete crystallization and good separation when fractionating. Elution with xylene (20 ml/min) is made while the temperature is rasied (4"C/hr) from 50" to 90°C; every hour, four fractions are collected. The polymer in the fractions is precipitated with ethanol and filtrated. After drying the polymer is weighed.