Application of temperature modulation to FTIR spectroscopy: an analysis of equilibrium and non-equilibrium conformational transitions of poly(ethylene terephthalate) in glassy and liquid states

AbstractIn this paper, the application of a temperature modulation to the temperature-resolved FTIR analysis is reported. The advantage offered by the spectroscopic investigation, able to follow the micro-structural and conformational sample modification involved in sample thermal transformation, was merged to that of temperature modulation, related to the possibility to separate the reversing (in-equilibrium within the experimental condition) to the non-reversing (non-equilibrium) processes. The potentiality of the technique (modulated temperature FTIR, MTFTIR) is highlighted through the study of the thermal transitions of amorphous poly(ethylene terephthalate) from 50 °C to the cold-crystallization. After the presentation of the theoretical framework and the experimental conditions, a step-by-step description of acquired data elaboration is given. The total variation of a selected band intensity as function of mean temperature as well as its reversing and non-reversing components are obtained. The evolution of the bands at 1340 and 971 cm−1, assigned to the trans conformation of the ethylenic unit and to the all-trans conformation of the repeating unit, respectively, are investigated. As expected, the glass transition is observed in the reversing components meanwhile the recovery of the glass relaxation and cold crystallization in non-reversing ones. Particularly interesting resulted the behaviour of the sample in the supercooled liquid state, between the glass transition and the cold-crystallization onset, in which the results show that the ethylenic conformers are in-equilibrium while the all-trans sequences are not. MTFTIR is confirmed to be a technique particularly suitable for the characterization of non-equilibrium conformational states of polymers

Colorimetric transition of polydiacetylene/cyclodextrin supramolecular assemblies and implications as colorimetric sensors for food phenolic antioxidants

Molecular self-assembly has significant potential in the field of sensing. Polydiacetylenes (PDAs) are conjugated polymers possessing peculiar optical properties obtained by photopolymerization of self-assembled diacetylene monomers. Herein, the blue-to-red phase transition upon either thermal stimulus or interaction with cyclodextrins (CDs) of two PDAs, bearing either carboxylic (PCDA) or amino (PCDA-NH2) polar heads, is investigated to develop a colorimetric sensor for food phenolic antioxidants. The change in the PDA polar head does not affect significantly thermo-chromatic transition. Upon thermal stimulus, in both PDAs, color transition occurs straightforward between two distinct stable states and does not involve the disordering of the PDA crystal phase, as revealed by UV-vis spectroscopy and SAXS analysis. Contrarily, PDA/alpha-CD interaction is influenced by intermolecular forces among PDA polar heads and is more efficient for PCDA. alpha-CDs presumably cause changes in both PDA backbone conformation and local environment surrounding the individual PDA chains. The PCDA/alpha-CD assemblies are investigated as colorimetric sensors for the detection of Tyrosol (Ty) and caffeic acid (CAF), by using the principle of competitive inclusion complex formation. The system results to be more sensitive to CAF than Ty and may permit the determination of CAF in concentration ranges suitable for different food products

Thymol-functionalized hyaluronic acid as promising preservative biomaterial for the Inhibition of Candida albicans biofilm formation

Hyaluronic acid (HA) is a naturally occurring biopolymer that has been employed for a plethora of medicinal applications. Nevertheless, as HA is a natural polysaccharide, it can be a substrate able to promote microbial growth and proliferation. Biopolymer–drug conjugates have gained attention over the years to overcome drawbacks of each single component. Within this context, thymol (Thy), a phenolic compound occurring in essential oils (EOs) extracted from Thymus and Origanum, has been largely studied for its antimycotic applications. However, it is characterized by a low water solubility and moderate cytotoxicity. Herein, we report an innovative HA–thymol conjugate (HA-Thy) biomaterial to circumvent the drawbacks of free thymol use by providing the polymer conjugate with the beneficial properties of both components. Preliminary biological tests evidenced the decrease of thymol cytotoxicity for the HA-Thy conjugate, paired with a promising antibiofilm formation activity against Candida albicans, similar to pure thymol, highlighting its potential application as a preservative biomaterial in formulations

Functionalization of hyaluronic acid with a chondroprotective molecule and preparation of sulfonated HA-based materials

In the work of thesis, several aspects of hyaluronic acid chemical modification have been explored. In particular, the polysaccharide has been functionalized with NAPA, an amino acidic derivative of D-glucosamine and N-Acetyl-L-phenylalanine with chondroprotective properties, by means of esterification of the glucuronic unit. Preliminary biological studies on human primary chondrocytes (HPCs) has also been presented. The procedure discussed so far represents an innovative and fascinating method for the linkage of a glucosamine derivative to hyaluronic acid that could be useful also in the case of other GlcN-based products. The new HA bioconjugates could gain high relevance as injectable treatments against osteoarthritic disorders. An insight on racemization mechanism that occurs for N-acetylated amino acid was also conducted. In addition, hyaluronic acid was subjected to sulfonation in water. Two sulfonating agents, such as Tau and Bes, were reacted in order to synthetized soluble HA-SO3H derivatives and 3D hydrogels enriched in polar sulfonic groups. Sulfonated polymer demonstrated to have tunable properties depending upon reaction conditions and showed to be promising HA-based materials for biomedical applications

Surface functionalized PLLA for scaffold preparation

Biocompatible and degradable poly(alpha-hydroxy acids) are among the more widely used materials in scaffolds for tissue engineering, although they often need surface modification to improve their interaction with the cells. In the present research Poly(L-lactide) 3D scaffold were prepared by salt-leaching method, with a porosity of 80 % and interconnected pores. In order to increase the hydrophilicity of the PLLA scaffolds surface, taurine was grafted through aminolysis reaction. The reaction enriched the surface with sulfonate groups increasing PLLA hydrophilicity and electrostatic interaction with collagen. In vitro biological tests with chondrocytes or fibroblasts showed that taurine grafting and collagen absorption improved cell viability and adhesion compared to the unmodified scaffold, suggesting that these modifications make PLLA substrate suitable for cartilage repair

Nano-organocatalytic Michael reactions promoted by Chiral Carbon Dots

Chiral Carbon Dots (CCDs) are carbon nanoparticles easily synthetized from small and readily available organic molecules, with an active surface which retains the molecular features of the starting materials, including chiral properites.1 Recent studies demonstrate the low toxicity of this promising and bio- compatible materials, underling widespread applications in chemistry.2 High solubility in water, in addition to others organic solvents, together with the possibility of an efficient functionalization of the active surface allow the use of CCD as nano-organocatalyst in several organic reactions.3 In fact, the presence of characteristic and tunable functional groups on the surface, paired with the chiral information, could lead to a strong interaction with substrates, aiming to promote stereoselective transformations. Herein, an innovative synthetic approach to activate Michael donors is reported, exploring the potentiality of CCDs as green catalysts with two different types of activations. The reaction between 3-Methyl-1- phenyl-2-pyrazoline-5-one (1) and β-Nitrostyrene (2) was investigated relying on a non-covalent activation by L-phenylalanine-derived CCDs, while L-proline-functionalised CCDs were prepared to promote the Michael reaction between cyclohexanone (4) and 2. The desired products were isolated with high yields and moderate to good stereoselectivity under operationally simple and sustainable conditions, allowing for the efficient recovering and recycle of the nano-catalysts

Chiral Carbon Dots as nano-organocatalysts to promote Michael Reactions

Chiral Carbon Dots (CCDs) are carbon nanoparticles easily synthetized from small and readily available organic molecules, with an active surface which retains the molecular features of the starting materials, including chiral properites.1 Recent studies demonstrate the low toxicity of this promising and bio-compatible materials, underling widespread applications in chemistry.2 High solubility in water, in addition to others organic solvents, together with the possibility of an efficient functionalization of the active surface allow the use of CCD as nano-organocatalyst in several organic reactions.3 In fact, the presence of characteristic and tunable functional groups on the surface, paired with the chiral informations, could lead to a strong interaction with substrates, aiming to promote stereoselective transformations. Herein, an innovative synthetic approach to activate Michael donors is reported, exploring the potentiality of CCDs as green catalysts with two different types of activations. The reaction between 3-Methyl-1-phenyl-2-pyrazoline-5-one (1) and β-Nitrostyrene (3) was investigated relying on a non-covalent activation by L-phenylalanine-derived CCDs, while L-proline-functionalised CCDs were prepared to promote the Michael reaction between cyclohexanone (4) and 2. The desired products were isolated with high yields and moderate to good stereoselectivity under operationally simple and sustainable conditions, allowing for the efficient recovering and recycle of the nano-catalysts

Isotactic polypropylene reversible crystallization investigated by modulated temperature and quasi‐isothermal FTIR

Modulated temperature techniques allow to separatethe reversing and non-reversing contributions of material transitions. To investigate reversible crystallization and melting of isotactic polypropylene (iPP) at microstructural level, in this research, modulated temperature Fourier transform infrared (MTFTIR) and quasi-isothermal FTIR (QIFTIR) analyses are used.By following the intensity variation of iPP regularity bands, associated with 31 helix structures of different lengths (n repeating units), MTFTIR evidences that, independently from helix length, a reversing coil–helix transition takes place few degrees below the non-reversing crystallization onset. By comparing spectroscopicand differential scanning calorimetry experiments performed inquasi-isothermal conditions, the reversing transition was found to be associated with the reversible melting-crystallization phenomenon. Moreover, QIFTIR evidences that helices of different lengths contribute differently to the reversible transition: the helices composed of n=10 and n= 12 are active into all the explored temperature range (30–130°C) whereas the shortest (n= 6) and the longest (n> 15) helices contribute to reversibility atT>100°C

Isotactic polypropylene reversible crystallization investigated by modulated temperature and quasi‐isothermal FTIR

Modulated temperature techniques allow to separatethe reversing and non-reversing contributions of material transitions. To investigate reversible crystallization and melting of isotactic polypropylene (iPP) at microstructural level, in this research, modulated temperature Fourier transform infrared (MTFTIR) and quasi-isothermal FTIR (QIFTIR) analyses are used.By following the intensity variation of iPP regularity bands, associated with 31 helix structures of different lengths (n repeating units), MTFTIR evidences that, independently from helix length, a reversing coil–helix transition takes place few degrees below the non-reversing crystallization onset. By comparing spectroscopicand differential scanning calorimetry experiments performed inquasi-isothermal conditions, the reversing transition was found to be associated with the reversible melting-crystallization phenomenon. Moreover, QIFTIR evidences that helices of different lengths contribute differently to the reversible transition: the helices composed of n=10 and n= 12 are active into all the explored temperature range (30–130°C) whereas the shortest (n= 6) and the longest (n> 15) helices contribute to reversibility atT>100°C

Effects of annealing above Tg on the physical aging of quenched PLLA studied by modulated temperature FTIR

The effect of the annealing few degrees above the glass transition temperature (Tda = 62 C) on the physical aging (Tpa = 51 C) of amorphous quenched poly(L-lactide) is investigated by an implementation of variable temperature Fourier transform infrared (FTIR). By using a temperature program composed of a linear heating ramp superimposed to a temperature modulation (modulated temperature FTIR), the reversing and nonreversing intensity variation of selected bands, related to highenergy gg and low-energy gt conformers, is investigated. It is observed that the annealing above Tg changes irreversibly the conformation distribution of the liquid polymer. The glasses obtained from annealed and nonannealed liquids behave differently, evolving in the physical aging toward their own liquid state and retaining the memory of their original condition before the vitrification. The recovery through Tg of the relaxation occurred in the physical aging depends not only from aging conditions but also by the thermal history of the sample above the Tg