Solid-state NMR study of stereocomplexes formed by enantiomeric star-shaped PEG-PLA copolymers in water

Solid-state NMR was applied to samples obtained by freeze-drying hydrogels of 1:1 (PEG65-NHCO-PLLA13)8/(PEG65-NHCO-PDLA13)8 or (PEG65-NHCO-PDLA13)8 only star block copolymers (where PEG, PLLA, and PDLA stand for poly(ethylene glycol), poly(l-lactide), and poly(d-lactide), respectively) in order to get insight into the different structural and dynamic properties of stereocomplexed poly(lactide) (PLA) aggregates with respect to single enantiomer ones responsible for the improved mechanical and degradation properties of the corresponding hydrogels. 13C MAS NMR experiments together with 13C relaxation time measurements indicated that the PLA domains in (PEG65-NHCO-PLLA13)8/(PEG65-NHCO-PDLA13)8 were highly crystalline, whereas those in (PEG65-NHCO-PDLA13)8 were mainly amorphous. On the basis of 1H relaxation and spin-diffusion experiments, similar average dimensions were determined for the PLA aggregates in the two samples. PLA stereocomplexation was found to strongly affect the conformational behavior of PEG chains. Under the assumption that freeze-drying preserves the structure of at least the PLA aggregates, the results obtained are of value for understanding self-aggregation of PEG–PLA star block copolymers in water

Anisotropy and NMR spectroscopy

Abstract In this paper, different aspects concerning anisotropy in Nuclear Magnetic Resonance (NMR) spectroscopy have been reviewed. In particular, the relevant theory has been presented, showing how anisotropy stems from the dependence of internal nuclear spin interactions on the molecular orientation with respect to the external magnetic field direction. The consequences of anisotropy in the use of NMR spectroscopy have been critically discussed: on one side, the availability of very detailed structural and dynamic information, and on the other side, the loss of spectral resolution. The experiments used to measure the anisotropic properties in solid and soft materials, where, in contrast to liquids, such properties are not averaged out by the molecular tumbling, have been described. Such experiments can be based either on static low-resolution techniques or on one- and two-dimensional pulse sequences exploiting Magic Angle Spinning (MAS). Examples of applications of NMR spectroscopy have been shown, which exploit anisotropy to obtain important physico-chemical information on several categories of systems, including pharmaceuticals, inorganic materials, polymers, liquid crystals, and self-assembling amphiphiles in water. Solid-state NMR spectroscopy can be considered, nowadays, one of the most powerful characterization techniques for all kinds of solid, either amorphous or crystalline, and semi-solid systems for the obtainment of both structural and dynamic properties on a molecular and supra-molecular scale. Graphic abstrac

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry.

Dielectric spectroscopy and NMR relaxometry unveiled the PVB segmental dynamics across the glass transition temperature

Solid-State Nuclear Magnetic Resonance of Triple-Cation Mixed-Halide Perovskites

Mixed-cation lead mixed-halide perovskites are the best candidates for perovskite-based photovoltaics, thanks to their higher efficiency and stability compared to the single-cation single-halide parent compounds. TripleMix (Cs0.05MA0.14FA0.81PbI2.55Br0.45 with FA = formamidinium and MA = methylammonium) is one of the most efficient and stable mixed perovskites for single-junction solar cells. The microscopic reasons why triplecation perovskites perform so well are still under debate. In this work, we investigated the structure and dynamics of TripleMix by exploiting multinuclear solid-state nuclear magnetic resonance (SSNMR), which can provide this information at a level of detail not accessible by other techniques. 133Cs, 13C, 1 H, and 207Pb SSNMR spectra confirmed the inclusion of all ions in the perovskite, without phase segregation. Complementary measurements showed a peculiar longitudinal relaxation behavior for the 1 H and 207Pb nuclei in TripleMix with respect to single-cation single-halide perovskites, suggesting slower dynamics of both organic cations and halide anions, possibly related to the high photovoltaic performances

Self-assembly and photo-cross-linking of eight-armed PEG-PTMC star block copolymers

Eight-armed poly(ethylene glycol)-poly(trimethylene carbonate) star block copolymers (PEG-(PTMC)8) linked by a carbamate group between the PEG core and the PTMC blocks were synthesized by the metal-free, HCl-catalyzed ring-opening polymerization of trimethylene carbonate using an amine-terminated eight-armed star PEG in dichloromethane. Although dye solubilization experiments, nuclear magnetic resonance spectroscopy, and dynamic light scattering clearly indicated the presence of aggregates in aqueous dispersions of the copolymers, no physical gelation was observed up to high concentrations. PEG-(PTMC9)8 was end-group-functionalized using acryloyl chloride and photopolymerized in the presence of Irgacure 2959. When dilute aqueous dispersions of PEG-(PTMC9)8-Acr were UV irradiated, chemically cross-linked PEG-PTMC nanoparticles were obtained, whereas irradiation of more concentrated PEG-(PTMC9)8-Acr dispersions resulted in the formation of photo-cross-linked hydrogels. Their good mechanical properties and high stability against hydrolytic degradation make photo-cross-linked PEG-PTMC hydrogels interesting for biomedical applications such as matrices for tissue engineering and controlled drug delivery systems

Unravelling Main- and Side-Chain Motions in Polymers with NMR Spectroscopy and Relaxometry: The Case of Polyvinyl Butyral

Polyvinyl butyral (PVB) is an amorphous polymer employed in many technological applications. In order to highlight the relationships between macroscopic properties and dynamics at a microscopic level, motions of the main-chain and of the propyl side-chains were investigated between Tg − 288 °C and Tg + 55 °C, with Tg indicating the glass transition temperature. To this aim, a combination of solid state Nuclear Magnetic Resonance (NMR) methods was applied to two purposely synthesized PVB isotopomers: one fully protonated and the other perdeuterated on the side-chains. 1H time domain NMR and 1H field cycling NMR relaxometry experiments, performed across and above Tg, revealed that the dynamics of the main-chain corresponds to the α-relaxation associated to the glass transition, which was previously characterized by dielectric spectroscopy. A faster secondary relaxation was observed for the first time and ascribed to side-chains. The geometry and rate of motions of the different groups in the side-chains were characterized below Tg by 2H NMR spectroscopy

Influence of Process Parameters on the Hydrothermal Carbonization of Olive Tree Trimmings: A 13C Solid-State NMR Study

Chars obtained from the hydrothermal carbonization (HTC) of agricultural wastes are increasingly being employed as solid biofuels. Their properties are strongly dependent on HTC process parameters. In this study, 13C solid-state NMR spectroscopy was applied to semiquantitatively investigate carbon functionalities present in olive tree trimming feedstock and in the corresponding hydrochar samples. Hydrochars were obtained by HTC under different conditions, that is, at two different temperatures (180 and 250 °C), with two different biomass/water ratios (B/W of 7 and 25% w/w) and with reaction times at peak temperatures of 30, 60, and 180 min. The NMR analysis was complemented by infrared spectroscopy experiments. A detailed analysis of carbon functionalities and their evolution during HTC allowed the transformation of feedstock into hydrochar to be followed and the structure of hydrochars to be correlated to the different reactions occurring during HTC in dependence on reaction time, temperature, and B/W ratio, as well as to the hydrochar properties fundamental for their application as solid biofuel reported in previous studies. 13C solid-state NMR spectroscopy revealed a powerful tool for explaining hydrochar properties as a function of HTC parameters

Orientational ordering studies of fluorinated thermotropic liquid crystals by NMR spectroscopy

Fluorinated calamitic thermotropic liquid crystals represent an important class of materials for high-tech applications, especially in the field of liquid crystal displays. The investigation of orientational ordering in these systems is fundamental owing to the dependence of their applications on the anisotropic nature of macroscopic optical, dielectric, and visco-elastic properties. NMR spectroscopy is the most powerful technique for studying orientational order in liquid crystalline systems at a molecular level thanks to the possibility of exploiting different anisotropic observables (chemical shift, dipolar couplings, and quadrupolar coupling) and nuclei (2H, 13C, and 19F). In this paper, the basic theory and NMR experiments useful for the investigation of orientational order on fluorinated calamitic liquid crystals are reported, and a review of the literature pub- lished on this subject is given. Finally, orientational order parameters determined by NMR data are discussed in comparison to those obtained by optical and dielectric anisotropy measurements