Nowadays, polymers are of major practical importance, since they have low cost and show unique physical properties, such as viscoelasticity, toughness, and ability to form either glasses, semi-crystalline or elastic materials, that scale easily with minimal variation of either chemistry, and molecular weight [1]. They are found in many applications, from packaging to automotive and healthcare [2]. It is fundamental to understand the materials’ behavior, correlating their microscopic properties with their macroscopic performances in real applications and vice versa, in order to improve the final products, acting on either the polymer chemistry or the manufacturing parameters. Therefore, tools are needed to monitor the “state of the art” of the current materials and processes.

NMR (nuclear magnetic resonance) has always been one of the main characterization techniques for solid polymeric materials, since its first applications in the 1970s. Particularly, solid-state NMR allows to study polymers at a molecular level in almost all their states and with minimal sample preparation procedure, in a non-destructive manner [3,4,5]. The use of high field instruments gives insights into the chemical and structural composition of polymers, but these instruments are expensive and require great maintenance limiting their application in industrial contexts. A powerful alternative can come from low-field benchtop devices, operating in time domain. They are easy to use, even for relatively unspecialized personnel, therefore they allow to conduct measurements fast and without any special sample preparation. With appropriate calibration procedure, quantitative information is obtainable. In addition to that, their small size makes them easily implementable in every laboratory, and their footprint is low since they operate through permanent magnets, which do not require cryogenic gases. Low-field instruments are both in closed geometry and in open geometry, such as the NMR-MOUSE [6]. The former are valuable tools to characterize bulk properties, while the latter possesses highly inhomogeneous fields, which, through hampering FID (free induction decay) detection, enables to gain space-dependent information, varying the position of the instrument on the material and its distance to the surface, thus selecting a slice along the material’s thickness. The most industrially relevant topic is by far the NMR of proton (1H) atoms, ubiquitous in organic compounds, polymers, and natural materials.

Though 1H LF-TD-NMR does not provide chemical information, since after Fourier transform chemical shift dispersion is extremely limited and basically all the protons contribute to a single resonance, it has been proved to be useful to study the dynamical properties of polymer chains.

This information can be drawn from the relaxation of 1H spins. The restoration of a spin thermal equilibrium state with the lattice in an external static magnetic field, following a perturbation through sequences of electromagnetic pulses, is characterized by two relaxation times: a spin–spin (T2) and a spin–lattice (T1) relaxation time. Their magnitude is dependent upon how the interaction between spins and with the environment (the so-called lattice) is modulated by the molecular mobility. In fact, they show well-known dependence with the motional correlation time τc (Figure 1), described by the BPP equations (Bloembergen–Purcell–Pound) [7], in the hypothesis of two equal spins where the only relaxation channel is the dipolar interaction between them:

 

Therefore, their determination can be exploited to discriminate different dynamical regimes inside the material, depending on its structure and morphology. Motional correlation times affect, in turn, macroscopic dynamical behavior, related to viscoelasticity, mechanical response, and processing characteristics, which are, in the end, the main concerns for specific applications.

Some methods require additional equipment, such as fast-field cycling (FFC) and pulse field gradient (PFG).

FFC allows to measure T1 as function of the variation of the magnetic field strength, spanning on a wide range of frequencies. This can be obtained either with a single instrument, capable of fast electrical switching of the field, or physically moving the sample in instruments with different magnetic fields [8]. Referring to Equation (1), the analysis of the dispersion curve of T1 vs. ω0 (Larmor frequency, that is the frequency associated with the precession of the spin caused by the magnetic field) allows to distinguish different correlation times, which define different dynamics inside the polymeric material [9]. This can be useful to perform “molecular rheology” experiments.

Instead, PFG is a technique to monitor the molecular diffusion and requires a tunable magnetic field gradient. The NMR signal attenuation after the pulsed gradient is dependent on the diffusion coefficient of the molecule [10]. It can be exploited to study the dynamics of polymers in solution or inside porous materials [11,12]. Although interesting for polymer characterization, these two techniques will not be further addressed in this review, since we decided to deal with the basic equipment for LF-TD-NMR in the characterization of polymeric items.

Although in principle all nuclear spins can be studied with LF-TD-NMR, only abundant ½ spins can be measured with reasonable signal-to-noise ratio, furthermore, only spins with high gyromagnetic have significant Larmor frequencies at low field, basically only two systems satisfy these two requirements at the same time: 1H and 19F. In this review, we focus just on 1H, and we would like to highlight the main applications of pulse sequences that can be conducted on low-field NMR instruments and the relevant parameters that can be drawn out. Parameters that can help to understand polymeric materials in two different environments: at an academic level for the characterization of innovative polymeric materials or to highlight basic polymer physics, in the industry for R&D and quality control (QC). A special section has been dedicated to bioplastics. In fact, future perspectives are oriented towards the substitution of traditional plastics with raw materials coming from natural resources. Preliminary studies have been made to assess the physical properties of these new materials.

 

Therefore, their determination can be exploited to discriminate different dynamical regimes inside the material, depending on its structure and morphology. Motional correlation times affect, in turn, macroscopic dynamical behavior, related to viscoelasticity, mechanical response, and processing characteristics, which are, in the end, the main concerns for specific applications.

Some methods require additional equipment, such as fast-field cycling (FFC) and pulse field gradient (PFG).

FFC allows to measure T₁ as function of the variation of the magnetic field strength, spanning on a wide range of frequencies. This can be obtained either with a single instrument, capable of fast electrical switching of the field, or physically moving the sample in instruments with different magnetic fields [8]. Referring to Equation (1), the analysis of the dispersion curve of T₁ vs. ω₀ (Larmor frequency, that is the frequency associated with the precession of the spin caused by the magnetic field) allows to distinguish different correlation times, which define different dynamics inside the polymeric material [9]. This can be useful to perform “molecular rheology” experiments.

Instead, PFG is a technique to monitor the molecular diffusion and requires a tunable magnetic field gradient. The NMR signal attenuation after the pulsed gradient is dependent on the diffusion coefficient of the molecule [10]. It can be exploited to study the dynamics of polymers in solution or inside porous materials [11,12]. Although interesting for polymer characterization, these two techniques will not be further addressed in this review, since we decided to deal with the basic equipment for LF-TD-NMR in the characterization of polymeric items.

Although in principle all nuclear spins can be studied with LF-TD-NMR, only abundant ½ spins can be measured with reasonable signal-to-noise ratio, furthermore, only spins with high gyromagnetic have significant Larmor frequencies at low field, basically only two systems satisfy these two requirements at the same time: ¹H and ¹⁹F. In this review, we focus just on ¹H, and we would like to highlight the main applications of pulse sequences that can be conducted on low-field NMR instruments and the relevant parameters that can be drawn out. Parameters that can help to understand polymeric materials in two different environments: at an academic level for the characterization of innovative polymeric materials or to highlight basic polymer physics, in the industry for R&D and quality control (QC). A special section has been dedicated to bioplastics. In fact, future perspectives are oriented towards the substitution of traditional plastics with raw materials coming from natural resources. Preliminary studies have been made to assess the physical properties of these new materials.