Mechanical spectrometry of alpha relaxations of high‐density polyethylene

Abstract

By using an automated low‐frequency apparatus, dynamic mechanical experiments are performed on bulk‐crystallized high‐density polyethylene in the temperature range of the α relaxation. In order to characterize the key morphological features governing the presence of multiple α relaxations, we have developed a simple model from calorimetric data to assess the crystallite size distribution of samples with different thermal histories. The morphological characterizations are completed by wide‐angle x‐ray diffraction measurements. Isochronal spectrometry and frequency scans performed under isothermal conditions both exhibit two α relaxations designated α1 and α2, with increasing temperature (or increasing frequency). These two relaxations are frequency dependent but they are not thermorheologically simple processes. Some analogy is found between tan ϕ versus temperature or frequency and the biomodal lamellar size distribution curves determined from calorimetric data. Moreover, both the temperature of α2 peak and the most probable lamellar thickness of the larger lamellae depend on the thermal history of the sample: with increasing thickness of the larger lamellae, the α2 peak temperature is shifted toward higher temperature. In contrast, both the temperature of the a peak and the most probable lamellar thickness of the thinner lamellae seem to be independent of thermal history: the thinner lamellae should be formed on cooling from the remaining uncrystallized fraction. From thege findings, it is proposed that the α, and α2 relaxations have the same origins and that they could arise from defect diffusion within the thinner and thicker crystallites, respectively, with some influence of the amorphous matter in the interfacial regions.

Crystal thickness distributions from melting homopolymers or random copolymers

“…A more complex situation exists for random copolymers that have nonequilibrium distributions of crystal thickness, because the relation between melting temperature T and crystal thickness depends additionally on the composition of the melt, which is not known. The approximate g 2 …”

“…Notice that the desired crystal thickness distribution is related to the DSC power P(T) times the square of the temperature difference (T m 0 Ϫ T). A functionally similar expression was derived by Alberola et al 2 It is not necessary to use the calculated normalization constant K, because the product 2 gives a function proportional to g(l), which can be integrated numerically to establish the normalization factor. The relation between g(l) and melting is illustrated in Figure 1.…”

Non-fluorinated proton-exchange membranes based on melt extruded SEBS/HDPE blends

“…One relaxation peak was seen in the DMTA curves of HDPE around 70 • C, corresponding to the ␣ relaxation [35]. This relaxation is found in all samples containing a certain degree of crystallinity, and is related to the lamellar thickness; more specifically this relaxation is due to the motion of the chains within the crystalline lamellae [35,36]. Glass transition temperatures, defined as the position of tan ı peaks are summarized in Fig.…”

Morphology of chemically crosslinked ultrahigh molecular weight polyethylene

“…The values used for calculation were as follows 11 : T°m = 418.15 K; ␦ e = 93.0 ergs/cm 2 ; ⌬H m = 288 J/g; and c = 1.005 g/cm 3 . Using the Thomson-Gibbs equation, Alberola et al 12 developed a model to express the lamellar thickness distribution from DSC endotherms as follows:…”