Characterization of the molecular weight distribution of high-density polyethylene by a new method using the turbidity at a lower critical solution temperature

The molecular weight distribution of linear high-density polyethylene (PE) has been measured by a new method using the turbidity at a lower critical solution temperature (LCST). The MW dependence of the LCST (T) is given by T = T sub infinity + BM exp --1/2 (eq 1), where T sub infinity is the LCST for an infinite MW. The constants T sub infinity and B, characteristic of a given polymer--solvent system, can be obtained from the LCSTs of samples with a narrow MW distribution. 2,4-dimethylpentane has been used as solvent. The technique consists of obtaining a thermogram which is the recording of the maxima (h sub i ) of the peaks of turbidity provoked in a solution by a succession of temperature increases Delta T sub i during phase separation. The elevation of temperature leads to the phase separation of the smaller and smaller MW between T sub 0 and T sub f , the temperature of the initial and final turbidity peaks. Typically, T sub f -T sub 0 ranges between 30-100K for samples whose polydispersity M sub W /M sub n lies between 1.1-10. Separation is effected, at each T sub i , by allowing the concentrated phase containing the higher MW to fall to the bottom of the tube. The simple relation m sub i = kh sub i (eq 2) is used with eq 1 for calculating the MW distribution, w(log M), from the set of h sub i , T sub i , and T values, the quantity m sub i being the concentration of polymer of MW M sub i , whose phase separation at T sub i creates a turbidity h sub i . The thermograms of ten PE samples of different polydispersity were obtained and their W(log M)s found in very good agreement with those obtained by the SEC technique. Verification of eq 2 is made by establishing a gravimetric and turbidimetric correlation of the amount of polymer in the concentrated phase between T sub 0 and T sub f . This method, using low-cost equipment and maintenance, can be applied to a variety of solvent--polymer (or copolymer) systems since the LCST is a general phenomena in polymer solutions. 34 ref.--AA