Dependence of mechanical properties on molecular motion in polymers

Abstract

Considerable progress has been made in the past 20 years in the synthesis, characterization and fabrication of plastics. Previous SPE Award winners, such as Mark, Natta, and Marvel dealt largely with synthesis; Flory with characterization; Alfrey and Du Bois with fabrication. One of the still unsolved problems lies in the realm of relating mechanical properties, such as impact strenght and creep to molecular structure. The design enginner who wishes to use a plastic part is concerned primarily with how some property such as impact strenght varies with temperature, speed of test, test method, etc. The polymer scientist must know why. Through knowing why, he may be able to design better plastics. This paper attempts to survey some of the world‐wide progress made in this area in the past 10 years. The ultimate goal is to understand these mechanical properties in terms of internal molecular motions which occur in solid polymers. Internal motion can be detected by electrical, electromagnetic and dynamic mechanical measurements. When these three methods are applied on a given polymer over a range of temperatures, insights can be gained as to the variation of impact strength and other properties with temperature and frequency for that same polymer. These three fundamental methods, which require very small samples (less than 50 grams), can provide insight into the practical behavior of plastic materials over the wide range of temperatures and frequencies encountered in the real world.

Comportamento dinâmico-mecânico e relaxações em polímeros e blendas poliméricas

“…Os estudos que buscam correlacionar o comportamento dinâmi-co-mecânico e a resistência ao impacto tiveram início por volta de 1950 41 . A questão a ser considerada era se havia correlação entre as propriedades mecânicas de polímeros e o comportamento dinâmico-mecânico na região de temperatura de solicitação do material que, em geral, para polímeros vítreos está abaixo da temperatura de transição vítrea.…”

“…41) acreditavam que a existência de uma relaxação secundária abaixo da temperatura de transição vítrea poderia ser a responsável pela tenacidade encontrada em alguns polímeros, tomando como principais exemplos para isto o policarbonato (PC) e o poliestireno de alto impacto (HIPS). Nestes dois materiais há a ocorrência de uma relaxação à baixa temperatura, sendo o caso do PC já citado, enquanto que no HIPS essa relaxação é relacionada à transição ví-trea da fase borracha e sua temperatura pode variar de -100 a -20 o C, dependendo principalmente do tipo da borracha utilizada.…”

“…Vincent (apud ref. 41) acreditava não haver uma correlação entre a transição frágil-dúctil e as relaxações moleculares obtidas por ensaios dinâmi-co-mecânicos.…”

Chemical influence on β-relaxations and the formation of molecule-like metallic glasses

“…In addition to the primary (a-) relaxations that are responsible for the viscous flow, a kind of secondary relaxation, called Johari-Goldstein or b-relaxation [7][8][9] , often exists at high frequencies or low temperatures. Recent studies have demonstrated that b-relaxations are an intrinsic and universal feature of supercooled liquids and glasses 1,8,10 , and they are of significance to the understanding of many key issues in glassy physics and material sciences 1 , ranging from glass transition phenomenon [10][11][12][13] , diffusions [14][15][16][17] , physical aging 18,19 to mechanical properties 20,21 and mechanisms of plastic deformations [21][22][23] , as well as the stabilities and crystallization of glassy materials [24][25][26] .…”

“…On the other hand, to understand the dominating factors that govern the behaviours of b-relaxation are of practical importance. For example, polymers and MGs with pronounced b-relaxations at relatively low temperatures (for example, below or near room temperature) usually exhibit good ductility and they are favoured for mechanical applications 20,21 . However, for the pharmaceuticals industry, b-relaxations are sometimes undesired and should be suppressed as they cause the devitrification and destabilization of glassy medicines, which have better solubility and bioavailability than their crystalline counterparts 1,10,24,25 .…”