Temperature Areas of Local Inelasticity in Polyoxymethylene

Abstract

The spectra of internal friction and temperature dependencies of the frequency of a free-damped oscillation process excited in the specimens of an amorphous-crystalline copolymer of polyoxymethylene with the co-monomer trioxane (POM-C) with a degree of crystallinity ~60% in the temperature range from -150 °C to +170 °C has been studied. It has been established that the spectra of internal friction show five local dissipative processes of varying intensity, manifested in different temperature ranges of the spectrum. An anomalous decrease in the frequency of the oscillatory process was detected in the temperature ranges where the most intense dissipative losses appear on the spectrum of internal friction. Based on phenomenological model representations of a standard linear solid, the physical-mechanical (shear modulus defect, temperature position of local regions of inelasticity) and physical-chemical (activation energy, discrete relaxation time, intensities of detected dissipative processes) characteristics of each local dissipative process were calculated. It was found that the intensities of dissipative processes remain virtually unchanged for both annealed and non-annealed samples. The maximum variation in the shear modulus defect is 0.06%. Additionally, according to computational data, small changes are also characteristic of the following parameters: the activation energy varies from 0.5 to 1.4 kJ/mol and the relaxation time changes from 0.002 to 0.007 s, depending on the presence or absence of annealing. As a result of annealing, there is a significant increase in the relaxation microinheterogenity of the polymer system across the entire temperature range (250% for the low-temperature region and 115% for the high-temperature region).

Keywords: dissipation mechanisms; internal friction spectra; local dissipative processes; polyoxymethylene; relaxation time; shear modulus defect; temperature–frequency dependencies.

Figure 1

Internal friction spectrum $\lambda =f\left(T\right)$ of HDPE.

Figure 2

A schematic diagram describing in general terms a free-damping oscillatory process excited in the material under study; (a) in isothermal mode $T=const$ by pulse action. Sweep of the time dependence of the twist angle $\phi \left(t\right)$ relative to the longitudinal axis $Z$ of the specimen—(b). The deformation of the sample—$\gamma \left(t\right)$ (c) and the corresponding shear stresses ${\sigma }_{ij}$ occurring in the sample—(d). $\beta$—damping coefficient of the oscillatory process; $\theta$— period of the vibration process. All other designations are defined below in the text of the article [53,54].

Figure 3

Internal friction spectra $\lambda =f\left(T\right)$ and temperature dependency of frequency $\nu =f\left(T\right)$ of the free-damped oscillatory process for original (a,c) and annealed (b,d) POM specimens.

Figure 4

Structure of specimens of amorphous–crystalline polyoxymethylene films: (a) Maltese cross; (b) dendritic spherulites. Resolution: 70 × 16.

Figure 5

DSC curves for both the original and annealed POM specimens.

Figure 6

Expansion of dissipative loss peaks using the Gaussian normal distribution for the original and annealed specimens.

Figure 7

Internal friction spectrum (a) and temperature–frequency dependence (b) for the original POM-27 specimen.

Figure 8

Generalized spectrum of internal friction (a) and temperature–frequency dependence (b) for POM-27 original and annealed specimens.