Semicrystalline polymers, creep

You have developed a new semicrystalline polymer, which has a typical activation energy for relaxation of Erei = 120 kJ/mol. You wish to know the creep compliance for 10 years at 27°C. You know that, in principle, you can obtain the same information in a much shorter period of time by conducting your compliance tests at a temperature above 27°C. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Components of switches, relays, and connectors use glass fiber reinforced semicrystalline polymers, such as PA, PPS, PBT, and PET. The following requirements are important in these applications dimensional stability and precision, low moisture absorption, strength, resistance to creep, electrical insulating properties, and resistance to high working temperatures ( 85°C). 

The restraining influence of the crystallite alters the mechanical behavior by raising the relaxation time T and changing the distribution of relaxation and retardation times in the sample. Consequently, there is an effective loss of short T, causing both the modulus and yield point to increase. The creep behavior is also curtailed and stress relaxation takes place over much longer periods. Semicrystalline polymers are also observed to maintain a relatively higher modulus over a wider temperamre range than an amorphous sample. 

Struik, L. C. E., Mechanical behavior and physical aging of semicrystalline polymers 3, Prediction of long-term creep from short-time tests. Polymer, 30, 799—814 (1989a). 

The time dependent response of high-density polyethylene was represented by a one-dimensional integral including recoverable viscoelastic strain and irrecoverable viscoplastic strain. The response was represented by an effective time concept. Creep, recovery, two-step creep, and constant stress loading and unloading rates. The concept can also model preconditioning of semicrystalline polymers. 

Increasing crystallinity of a semicrystalline polymer is accompanied by increases in modulus, stiffiiess, density, yield stress, chemical resistance, melting point, glass-transition temperature, abrasion resistance, creep resistance, and dimensional stabihty, and by reduction in impact resistance, elongation, thermal expansion, permeabihty, and swelling. 

Similar to many semicrystalline polymers, PLA exhibits both Tg and 7j . The Tg of PLA can range from 35 to 60°C depending on molecular weight, the presence of plasticizers, physical aging, polymer architecture, and degree of crystallinity [14]. Above Tg PLA is rubbery, while below Tg it becomes a glass, but it is still able to creep until it is cooled to its 3 transition temperature at approximately —45°C, below which it behaves like a brittle polymer [15]. In applications where mechanical properties are important, such as packaging... 

Hong K, Rastogi A, Strobl G (2004b) Model treatment of tensile deformation of semicrystalline polymers static elastic moduli and creep parameters derived for a sample of polyethylene. Macromolecules 37 10174... 

This is different in the case of semicrystalline polymers which form a ductile and a brittle branch of the stress-lifetime curve. There it can be said that two different mechanisms are active, of which the creep crack initiation has the apparently smaller activation energy (181 kJ/mol) and activation volume (1.8 nm). The fact that chain breakages in PE are rarely observed even at high stresses and low temperatures in highly oriented samples makes it very unlikely that the mechanism of creep crack initiation involves chain breakages. 

Not only do the creep properties of crystalline polymers change rapidly with temperature, but in some cases at a given temperature a crystalline type will creep more with time than will the rigid amorphous or cross-linked types. However, a crystalline type above its Tg creeps very little, compared to the others. Thus, crystalline types tend to have an even broader distribution of retardation times than do the amorphous types. (Remember that the term crystalline refers to polymers that are actually semicrystalline.)... 

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