Impact of Exposure Conditions on the Morphology of Polyethylene by Compact NMR

Abstract

Today many polyethylene (PE) products are employed in long‐term applications that require the use of elevated temperatures and/or contact with different solvents. Both the exposure to heat and solvents can result in morphological changes, compromising the mechanical stability of the material. Disentangling morphological changes induced by either type of exposure and understanding their combined effect is crucial for establishing reliable structure‐property relationships. This, in turn, requires sensitive analytical methods that can probe the complex morphology of PE both in the laboratory and on site. Among existing analytical methods, NMR with compact devices have recently become a key analytical tool for the detailed morphological characterization of a variety of solid polymers including PE. This current work reviews the potential of compact NMR for monitoring and quantifying morphological changes in PE exposed to the aforementioned external conditions. To prove the reliability of compact proton NMR relaxation measurements, the results are compared with data from conventional high‐field proton wide‐line NMR spectroscopy, DSC, and FTIR spectroscopy. It could be shown that simple, static proton relaxation measurements garner detailed information about the exposure‐induced morphological changes through the quantification of phase composition and chain dynamics. Moreover, the NMR method has a key advantage over other methods, because the chain mobility of the soft amorphous phase is a very sensitive microscopic parameter under exposure conditions. The various presented examples and the good agreement of the NMR results with those of other analytical methods show that low‐field NMR is a promising option for in‐situ aging studies.

Time Domain NMR in Polymer Science: From the Laboratory to the Industry

“…Similar behavior can be observed after exposure to solvents, due to increased mobility of chains, but in the longer time the morphological changes are less strong than the reorganization attained with temperature. The impact of PE grade was also taken into consideration, showing almost no composition change for LLDPE and HDPE and instead wider thermal evolution for LDPE (Figure 18c), because of its lower melting temperature, lower crystallinity, and tertiary carbons easily subjected to attack by free-radicals [107]. contribution of chemical reactions, such as oxidation, causes a reduction, at short times, in the overall mobility, because of a higher confinement of the mobile-amorphous part and at longer exposure times, mobility increases because of oxidative chain cleavage [104,106].…”

“…The improvements caused by changes in the formulation can also be studied, for example, the impact of different crosslinkers [66], stabilizers [107] or plasticizers. The introduction of new plasticizers had been the focus of Patel et al [118] in a common resin employed as adhesive, such as hexamethylenetetramine (HMTA).…”

“…Copyright 2016 Elsevier). (c) Temperature-induced morphology changes, quantified with the proton effective relaxation time, for different PE grades (Reprinted with permission from reference[107]. Copyright 2018 Wiley-VCH).…”

Aging of polymeric materials by stray‐field NMR relaxometry with the NMR‐MOUSE

“…In many cases the polymer material hardens with increasing age due to decreasing free volume and mounting rotational barriers for chemical groups resulting from these processes unless chain‐scission dominates . These changes in molecular mobility can be followed with remarkable sensitivity by NMR relaxation measurements and in particular nondestructively by mobile stray‐field NMR …”

“…19 These changes in molecular mobility can be followed with remarkable sensitivity by NMR relaxation measurements 5,[19][20][21][22] and in particular nondestructively by mobile stray-field NMR. 17,23,24 In semi-crystalline polymers three morphological domains can be discriminated based on differences in the molecular mobility and traced by transverse 1 H NMR relaxation. These are the mobile amorphous domains with a long relaxation time T 2 , the disordered but rigid chain segments at the interface between amorphous and crystalline domains with an intermediate T 2 , and the crystalline domains where the chain segments are ordered and immobile with a short T 2 .…”