The Brittle-Ductile Transition in linear Polyethylene

LEO MANDELKERN; FL SMITH; MARCELO D. FAILLA; MARY KENNEDY; ANDREW PEACOCK

JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS

JOHN WILEY & SONS INC

Lugar: New York; Año: 1993 vol. 31 p. 491 - 493

0887-6266

The ductile, or plastic, deformation of crystalline polymersis a well-known phenomenon and has been extensivelystudied.'.' Crystalline polymers can also undergo brittlefracture. It is of interest, therefore, to develop an understandingof the molecular factors (i.e., the molecular constitutionand the structural and morphological featuresinherent to the crystalline state) that govern the natureof the deformation and the transition from one type toanother. Some of the key factors that define and describethe crystalline state are now kn0wn.3.~S ince these factorscan be varied over wide limits it is of interest to ascertainwhich of these features control the specific type of deformation.In this communication we report the initial results ofan investigation that is directed to the aforementionedobjective. We have studied the draw ratio after break, XB,at ambient temperature for a set of linear polyethylenesamples of varying molecular weights, and molecularweight distributions. The procedure used to determine XBhas previously been described in detail? We define a brittlesample as one having XB 1. For all the specimens studiedhere yield is observed and the brittle failure takes placejust beyond this point. A ductile sample is one for whichXB is greater than unity, for a given molecular weight, andis independent of structural and morphological feature^.'.^However, XB depends on the weight-average molecularweight2z5 and on the rate of deformation.'*6 There must bea transition between the two types of deformation. Determiningsome of the factors that control this transitionare the major concern of this research. For this purposewe have limited this initial report to the results obtainedat ambient temperature (23-26°C). This temperature iswell-above the glass temperature of linear polyethylene.