On the β Transition in High Density Polyethylene: the Effect of Transcrystallinity

Abstract

Summary: Transcrystallinity in UHMWPE fiber‐reinforced HDPE composites promotes a significant β transition that is untypical of high‐density polyethylene. Surface profiling by atomic force microscopy identifies two distinct morphologies in the composite without a boundary phase between them, which coincide with the transcrystalline layer and with the bulk spherulitic matrix. As a result, the claim that attributes this transition to loose chain folds at the lamella surface is favored.Atomic force microscopy scan of the transcrystalline layer above the fiber with the impression of the fiber in the center.magnified imageAtomic force microscopy scan of the transcrystalline layer above the fiber with the impression of the fiber in the center.

Amorphous and crystalline phase interaction during the Brill transition in nylon 66

“…It is well known that aramid fiber reinforced nylon 66 composites transcrystallize in such a way that a transcrystalline layer invades the whole space between the fibers and replaces most of the bulk crystallinity [23]. The amplification of the α′ peak can then be associated with the transcrystalline structure, similar to the β peak in HDPE [5]. Two examples of samples that exhibit an apparent α′ peak are shown in Figure 4.…”

“…An appropriate example from our own research is of high density polyethylene (HDPE), which in its pristine form exhibits a single second order transition, namely, T g . However, after undergoing a certain solid state crystallization stage, which results in crystalline restructuring, the same HDPE exhibits an untypically immense β transition in the amorphous phase [4,5]. In the present study we deal with yet another first order transition, namely the Brill transition in polyamides (e.g., nylon 66), for which a counterpart transition in the amorphous phase is explored.…”

New Insights into Lamellar Twisting in Transcrystalline Polyethylene

“…This behavior was also observed by Flores et al [17] It is suggested that this very gentle twist may be due to residual chain fold ordering at the fold surface, [12,13] perhaps due to a high concentration of loose folds at the lamellae surface as was found in ref. [4] We can compare our measured value of the pitch with values found for spherulitic PE [13] and with the value (12.68/mm) reported previously. [5,6] In that case, an epitaxial relationship between axially aligned orthorhombic crystals at the PE fiber surface and the crystalline lamellae in the TC layer was assumed as will be discussed below.…”

“…[16] Upon cooling, the fiber skin re-crystallizes to form an unoriented lamellar structure with evidence of cocrystallization with the matrix. [4] This last observation, as well as more recent X-ray diffraction data, [17] has put into question the validity of our previous premise that the epitaxial nucleation of the transcrystalline lamellae occurs such that their c-axes are aligned parallel to the axis of the fiber.…”

“…On the basis of the experimental evidence it was concluded that the prominent b relaxation in transcrystalline HDPE results from amorphous material, possibly loose chain folds, which is associated with the lamellar surfaces within the transcrystalline layer. [4] In their turn, the lamellae within the transcrystalline layer, which nucleate on the fiber surface and grow in the direction of their crystallographic b-axes, exhibit twisting. [5,6] The twisting of lamellar crystals about their growth Summary: The morphology of the transcrystalline layer grown by nucleating high density polyethylene on fibers of ultra high molecular weight polyethylene was investigated by microbeam synchrotron X-ray diffraction.…”