Is the cross-over modulus a reliable measure of polymeric polydispersity?

✍ Scribed by Sudhir S. Bafna

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 88 KB
• Volume: 63
• Category: Article
• ISSN: 0021-8995
• DOI: 10.1002/(sici)1097-4628(19970103)63:1<111::aid-app11>3.0.co;2-8

✦ Synopsis

The answer to the question posed in the title of this paper is, ''Sometimes yes and sometimes no; it depends on the specific case.'' The use of the reciprocal of the crossover modulus (actually 100,000/cross-over modulus, where the cross-over modulus has units of Pascal) as a measure of polymeric polydispersity [as determined by the M w / M n ratio, e.g., by gel permeation chromatography (GPC)] was first proposed by Zeichner and Patel (Proceedings of the Second World Congress of Chemical Engineering, Montreal, Quebec, Canada, 1981, p. 333) specifically for polypropylenes made by Ziegler-Natta catalysis and degraded by chain scission. However, their correlation is now being indiscriminately used without ensuring the applicability of the assumptions underlying the work of Zeichner and Patel to the specific case being considered. While Zeichner and Patel's correlation is indeed true in certain specific cases, it is not true in all cases. It would do well to dispel the apparently widespread misconception, especially among industrial rheologists, that the reciprocal of the cross-over modulus can always be used indiscriminately as a measure of polymeric polydispersity, and to emphasize the strict limitations under which this can indeed be done.