The temperature and strain‐rate dependence of mechanical properties in polyoxymethylene

Abstract

The computer aided design approach used in current applications of semicrystalline polyoxymethylene (POM) requires high strain‐rate mechanical data. The primary aim of this work has been to measure the room temperature modulus and tensile strength of injection molded samples of POM of different molecular weights at cross‐head speeds of between 10−5 ms−1. We observe no major transition in bulk mechanical behavior in this range of test speeds, the Young's modulus E, in particular, showing little strain rate dependence. This is rationalized on the basis of tensile tests over a range of temperatures, these indicating room temperature to correspond to the plateau in the E(T) curves (Tg for these materials is taken to be −70°C, and the DSC melting onset occurs at ∼ 170°C). The tensile strength increases as ∼log(dϵ/dt) and the behavior is found to be highly nonlinear, strains to fail of the order of 1 being observed even at the highest strain rates, depending on the molecular weight. It is believed that the yield stress of th crystalline regions determines the tensile strength above Tg, the higher degree of crystallinity associated with lower molecular weights resulting in a slightly higher tensile strength. Nevertheless, failure is qualitatively brittle, with no necking and relatively little permanent deformation. This behavior is discussed in terms of morphological investigations of the fractured samples by optical and scanning electron microscopy (SEM). In attempting to relate ultimate failure to the molecular/crystalline structure of the samples, measurements of the critical stress intensity for crack initiation in mode I opening, KIC, as a function of crystallization temperature Tc have been carried out using compact tension specimens machined from injection molded and compression molded plaques. KIC increases with molecular weight and decreases with Tc at low test speeds (in spite of an increase in crystallinity with Tc). This is accounted for in terms of a crack shielding model for crack initiation and of molecular rearrangements occurring during crystallization which lead to a decrease in the effective entanglement density with Tc. The implications of this model are then compared with KIC results over a range of cross‐head speeds and temperatures.

Molecular composites of poly(p-phenylene terephthalamide) anion and poly(ethylene oxide): Mechanical properties

“…A somewhat similar behavior has been exhibited by another crystalline polymer, poly(oxy methylene). 27 For the composites containing 2, 4, or 6 wt % of the reinforcement, the effect of annealing is to increase both the modulus and the tensile strength. As Figure 12 shows, the maximum value of the tensile strength of the annealed samples is about double that of the PEO matrix, and it occurs at about 4 wt % of the PPTA-anion reinforcement.…”

Impact and dynamic fracture resistance of crystalline thermoplastics: Prediction from bulk properties

“…Using an ingenious ultra-high K rate tensile fracture test, BCguelin (15) has generated data that suggest that the impact G, of POM does indeed bottom out at G, , , , according to Eq 6 (91. There may be an argument, by analogy with the use of plane-strain KIc data for design in tough steels, for designing to G, .…”

“…Combining Eqs 2, 3, 9, and 10 yields E'BC' = 2aYi (14) so that the compliance at any crack length is given by (15) where Co is the uncracked specimen compliance. Hence the compliance factor is In terms of Y,, the impact rate factor is For a standard IS0 179/ leB Charpy specimen, for which $ = 5.458, the test-sensitive terms $W'/3u-z/3 in Eq 3 amount to a factor of 0.578 s2l3 m-ll3 for an impact speed of 2.9 m/s.…”

A model of traps for yield in amorphous glassy polymers

“…Observations show that the yield point depends on the rate of strain _ , [5,7,10,12,14,35,45,46,50,55,56,62,63,69,71], the program of loading and the strain state, [12,34,42,43,44,48,71,72], temperature T, [5, 7, 9, 15, 19, 20, 35, 55, 56, 71], and pressure p [7, 43, 57], as well as on the molecular weight, [40], orientation of chain molecules, [4,54], degree of crosslinking, [15,50], composition [52,69] and the annealing time, [8,33,39].…”