Premium
Mechanical relaxations in heat‐aged polycarbonate. Part I: Comparison between two molecular weights
Author(s) -
LeeSullivan Pearl,
Dykeman Donna,
Shao Qing
Publication year - 2003
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.10031
Subject(s) - materials science , polycarbonate , relaxation (psychology) , intermolecular force , cooperativity , glass transition , thermodynamics , stress relaxation , embrittlement , volume (thermodynamics) , polymer chemistry , composite material , polymer , molecule , chemistry , creep , organic chemistry , physics , psychology , social psychology , biochemistry
As part of a wider research program related to polycarbonate embrittlement, the effects of heat‐aging on mechanical relaxation behavior have been studied by examining the relationship between secondary transitions and stress relaxation behavior. In this Part I, the differences in response between two molecular weight polycarbonates (PC) are compared for injection molded samples. Dynamic mechanical spectra showed that the presence of an intermediate β transition (∼ 80°C) is strongly dependent on molecular weight and heat‐aging. However, the β 1 (35°C) and the γ (‐100°C) peaks are generally insensitive to either effect. The study also attempted to interpret the similarities and differences in relaxation response using free volume and conformational change arguments, which have been subject to much scrutiny. Using the KWW stretched exponential to characterize stress relaxation, it appeared that bulk free volume recovery concepts could explain differences in stress relaxation response but not the corresponding losses in toughness. Hence, it is proposed that changes in relaxation response are most likely due to an interplay of relatively large scale molecular volume and molecular conformation processes that affect inter molecular cooperativity. These high‐activation processes are related to the broad β region (β 1 < T < T g ).