Investigation of chemi‐crystallization and free volume changes of high‐density polyethylene weathered in a subtropical humid zone

Abstract

The chemi-crystallization and free volume changes of high-density polyethylene (HDPE) exposed to subtropical humid climate of Guangzhou, China, were investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, wide-angle X-ray diffraction, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. An increase in content of carbonyl groups and significant chemi-crystallization were observed to occur during natural exposure. Chain scission accounted for the chemi-crystallization and would lead to greater crystallizability of the molecules. The reheating DSC run indicated that the crystallizability of the degraded HDPE molecules increased initially with exposure time and then decreased. Positron data showed the new crystals induced by chemi-crystallization indeed had more imperfect crystal structure in comparison with the pre-existing parent crystals, and the free volume located in amorphous regions decreased involving a shrinking of the free volume holes. The shrinkage of free volume holes was correlated with the loss of mobility of HDPE molecules, which was confirmed by the increase of glass transition temperature. The formation of new imperfect crystals might increase the amount of rigid amorphous fraction of HDPE materials, as well as the occurrence of crosslinking reactions of molecules located in the interior of HDPE materials, consequently decreasing the molecular mobility.

From macroplastics to microplastics: Role of water in the fragmentation of polyethylene

“…Photodegradation leads to modification of the polymer chemistry through the introduction of various oxygenated moeities. These molecular changes conduct to competitive reactions of chain scission and crosslinking (Xiong et al 2016). Chemical modification of the weathered In semi-crystalline polymers such as LDPE, oxidation mainly occurs in amorphous regions in which oxygen can readily diffuse.…”

Nanoscale Abrasive Wear of Polyethylene: A Novel Approach To Probe Nanoplastic Release at the Single Asperity Level

“…The friction of polymers can result from (1) internal viscoelastic dissipation from tip-polymer interaction affected by crystallinity and glass transition temperature ( T g ), (2) noncovalent surface chemical interactions, and (3) surface roughness relative to the size of the counterface. , First, a more crystalline LDPE would have reduced energy dissipation and friction force, opposite to what we observed. Similarly, a previous outdoor weathering study on HDPE revealed an increase in T g and a decrease in free volume, which would have also led to lower friction. Second, the enhanced polarity and hydrophilicity of the oxidized LDPE surface (Figure A, B) will induce a stronger dipole–dipole interaction between the tip and sample, leading to higher friction force .…”

“…81,82 First, a more crystalline LDPE would have reduced energy dissipation and friction force, 83 opposite to what we observed. Similarly, a previous outdoor weathering study on HDPE revealed an increase in T g and a decrease in free volume, 84 which would have also led to lower friction. Second, the enhanced polarity and hydrophilicity of the oxidized LDPE surface (Figure 1A, B) will induce a stronger dipole−dipole interaction between the tip and sample, leading to higher friction force.…”

Thermo‐oxidative degradation during sintering of polyethylene particles

“…Under this circumstance, sintering is also a solid‐state annealing process in which the following phenomena affect the crystallinity: (1) the melting (partial melting) and recrystallization of the crystals with a melting point below or near the sintering temperature (secondary crystals), (2) the attachment of the molecular chains from the amorphous phase to the existing (primary) crystals, or chemi‐crystallization, and (3) the solid‐state rearrangement of the molecular chains in the primary crystals 57–59 . For instance, the higher temperature of peak I (135.5°C) compared to the melting peak of the nascent powder (132.4°C) is due to chemi‐crystallization and the rearrangement of the molecular chains, both resulting in the thickening of the primary crystals.…”