Abstract
More than a million miles of tire testing is generally required to prove the utility of a new elastomer. For example, such a program has recently demonstrated that an alkyllithium solution polymerized butadiene styrene copolymer (Duradene rubber) has improved abrasion resistance over that of emulsion SBR1. It is, thus, extremely important to find meaningful relationships between the structure of an elastomer and its commercial importance to further guide the polymer chemist toward the development of new general purpose or specific application elastomers from laboratory sample through to final tire evaluation. Butadiene homopolymers and butadiene styrene copolymers prepared by alkyllithium catalysis in hydrocarbon solution offer unique opportunities to relate basic polymer structure parameters to tire compound performance. Their mixed micro-structure (cis-1, 4, trans-1, 4, and vinyl) preclude crystalline transitions which would otherwise complicate the interpretation of structural behavior. Furthermore, this polymerization system permits the production of controlled and specific variations in molecular weight distribution and degree of branching. n-Butyllithium catalyzed 1, 4-polybutadiene with its mixed microstructure and high cis-1, 4-polybutadiene have both shown outstanding abrasion resistance, especially under conditions of severe tire service. There is as yet no generally clear molecular interpretation for the outstanding abrasion resistance of the 1, 4-polybutadienes. For reasons of both improved processing and increased wet traction, these 1, 4-polybutadienes are seldom used alone but are blended with either natural rubber or SBR plus large amounts of extending oils. As a start, using as few fundamental concepts as possible, attempts were made to relate the industrial processing, heat build up, traction, and abrasion resistance characteristics of amorphous elastomers to two features measurable in terms of molecular parameters. The macrostructure (molecular weight distribution and branching) of the polymer is especially related to processing behavior. The glass temperature, Tg, of a polymer characterizes both the temperature and rate of deformation conditions in which the polymer exhibits rubbery behavior. Hence, the polymer Tg is directly related to both heat build-up and failure characteristics of an elastomer. Thus we shall be mostly discussing polymer structure in just two terms (i) macrostructure and (ii) Tg.
EFFECTS OF MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION ON MECHANICAL PROPERTIES OF UNDRAWN FILAMENTS OF ISOTACTIC POLYPROPYLENE
