The molecular weight and molecular weight distribution of a polymer can markedly affect its mechanical properties. While this has been recognized for a long time, quantitative studies have been relatively difficult to perform for a number of reasons. A major problem has been the characterization of the complete molecular weight distribution curve by fractionation methods because fraction collection and the determination of the various molecular weight averages of polymers is time consuming. Additionally, measurement of the number-average molecular weight, [Mbar]n, by classical osmometry procedure is often unreliable.

Mechanical Properties of Polymers: The Influence of Molecular Weight and Molecular Weight Distribution

A short survey is given of the correlation between mechanical properties and molecular model of homopolymer solids in rubbery, glassy and crystalline state. Particular emphasis is given to the plastic deformation of crystalline solids and the mechanical properties of the fibrous structure obtained by such deformation. The main difference between the spherulitic and the fibrous morphology, which explains the enormous difference in elastic modulus and strength, is not the crystallinity, crystal structure and orientation, but the fact that the basic element of the former structure is the extremely thin and easily deformable crystal lamella and, in the latter case, the very long and strong microfibril. The deformation of the fibrous structure is only possible by the sliding motion of microfibrils which is opposed by the autoadhesion forces between adjacent microfibrils over their full length or by fracture of microfibrils; which demands the rupture of the intrafibrillar tie molecules constituting a much large...

Mechanical Properties of Polymeric Solids

Creep rupture of polypropylene sutures as a function of diameter, radiation dose and temperature.

The controlled failure of polypropylene (PP) sutures is studied via creep rupture tests. From plots of log time (tB) vs. stress (sigma), linear relationships are generated over the failure times of 1-1000 h. Results show that as a function of stress, the time dependence varies with irradiation dose (15, 20, 25, and 50 Mrad), irradiation atmosphere (air and vacuum), suture diameter (7-0, 6-0, 5-0, and 4-0), and test temperature (26 and 37 degrees C). For a given stress, the time to failure is least for the greatest dose in the presence of air and at the highest temperature. When suture loops are wrapped around a small wire sheave, however, failure occurs in the largest suture as much as two decades sooner than the smallest suture studied. Within the limitations stated herein, they are independent of test method, loop diameter, aging, and humidity. Consequently, after irradiation in vacuum and postirradiation heat treatment, the processed material may be stored at room temperature for at least 1 month. Such materials are advocated when the time release of a dental or medical device is required, for example, in the self-activating cleft palate appliance.

Creep rupture behavior of polypropylene suture material and its applications as a time-release mechanism

The composite structure consisting of extremely thin crystal lamellae or blocks imbedded in a pseudo-amorphous matrix is the main factor determining the mechanical properties of the crystalline polymer solid, much more important than the crystal perfection. In the spherulitic (slowly cooled) or microspherulitic (quenched) structure of the unoriented material the stacks of parallel lamellae interconnected by a few tie molecules are the basic structural element. The stacks themselves are not particularly strong. As a consequence of rejected impurity concentration on the boundaries between adjacent stacks and between spherulites which are bridged by very few tie molecules they can be easily sheared, rotated and by lamella destruction transformed into the fibrous structure. The highly oriented fibrous material has as the basic element the extremely long and narrow microfibril consisting of alternating crystal blocks and amorphous layers bridged by a great many rather taut tie molecules. Hence the microfibril acts in the fibrous structure as a very strong axial connection. The frictional forces on the extremely large surface transmit the load efficiently over long distances. The material connection in the axial direction by tie molecules is interrupted at the ends of microfibrils which act as point defects of the microfibrillar lattice. Upon loading they open as microcracks which by radial or axial coalescence, or both, grow to critical size leading to catastrophic crack propagation and sample failure.

Morphology and mechanical properties of crystalline polymers

Tensile stress rupture is an important engineering property of plastics required in desiging parts subject to continuous or intermittent stress over long periods of time, especially in corrosive environments. This paper presents tensile stress rupture results on polypropylene and some of their applications in a program conducted over the past several years. Among the structural and compositonal paramaters studied in this work are the effects of molecular weight, molecular weight distribution, isotactic content, polyblends, and fillers on the shape and slope of the stress rupture-time curve. Data are presented showing the interrelationships between these variables, and the agreement with current stress rupture theories is discussed.

Stress rupture of polypropylene

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Stress rupture of polypropylene