Photo-differential Scanning Calorimetry Research Articles

In Situ Blue-Light-Induced Photocurable and Weavable Hydrogel Filament.

A self-lubricating hydrogel filament was achieved by establishing an in situ photocuring system and using camphorquinone/diphenyl iodonium hexafluorophosphate (CQ/DPI) as the blue-light photoinitiators, acrylamide (AM) and N,N-dimethylacrylamide (DMAA) as the monomers, polyethylene glycol diacrylate (PEGDA) as the cross-linker, and lecithin as the lipid lubricant. The blue-light photopolymerization efficiency and the photorheological properties of the hydrogel precursor were investigated by photodifferential scanning calorimetry and a photorheological system. With the increase of DMAA, the photopolymerization efficiency of the precursor improved, while the elasticity of poly(DMAA/AM) decreased accordingly. The physical cross-linking effect between lecithin and the poly(DMAA/AM) network led to improved polymerization properties and elasticity. The lipid-based boundary layer at the hydrogel surface endowed the self-lubrication of the hydrogel filament. The extruded hydrogel filaments exhibited excellent mechanical properties and weavability, which were expected to play a realistic role in soft robots and bioengineering.

Compositional effects on cure kinetics, mechanical properties and printability of dual-cure epoxy/acrylate resins for DIW additive manufacturing

Interest in 3D printing of thermoset resins has increased significantly in recent years. One approach to additive manufacturing of thermoset resins is printing dual-cure resins with direct ink write (DIW). Dual-cure resins are multi-component resins which employ an in situ curable constituent to enable net-shape fabrication while a second constituent and cure mechanism contribute to the final mechanical properties of the printed materials. In this work, the cure kinetics, green strength, printability, and print fidelity of dual-cure epoxy/acrylate thermoset resins are investigated. Resin properties are evaluated as a function of acrylate concentration and in situ UV exposure conditions. The acrylate cure kinetics are probed using photo-differential scanning calorimetry and the impacts of resin composition and UV cure profile on the acrylate extent of conversion are presented. Continuous and pulsed UV cure profiles are shown to affect total conversion due to variances in radical efficiency at different UV intensities and acrylate concentrations. The effects of acrylate concentration on the kinetics of the epoxy thermal cure and the final mechanical properties are also investigated using dynamic mechanical analysis and three-point bend measurements. The glass transition temperature is dependent on formulation, with increasing acrylate content decreasing the Tg. However, the room temperature shear moduli, flexural moduli, strength, strain-to-failure, and toughness values are relatively independent of resin composition. The similarity of the final properties allows for greater flexibility in resin formulation and in situ cure parameters, which can enable the printing of complex parts that require high green strength. We found that the in situ UV print intensities and exposure profiles that are necessary to achieve the best print quality are not, in most cases, the conditions that maximize conversion of the acrylate network. This highlights the importance of developing optimized resin compositions which enable complete cure of the acrylate network by promoting acrylate dark cure or thermal cure.

Photocurable Epoxy Acrylate Coatings Preparation by Dual Cationic and Radical Photocrosslinking

In this work, epoxy acrylate resin (EA) based on the industrial-grade bisphenol A-based epoxy resin (Ep6) and acrylic acid (AA) has been synthesized in order to develop hybrid resin comprising both epoxide group and reactive, terminal unsaturation. Obtained epoxy acrylate prepolymer was employed to formulate photocurable coating compositions containing, besides the EA binder, also cationic or radical photoinitiators. Hence, when cationic photoinitiators were applied, polyether-type polymer chains with pending acrylate groups were formed. In the case of free radical polymerization, epoxy acrylates certainly formed a polyacrylate backbone with pending epoxy groups. Owing to the presence of both epoxy and double carbon–carbon pendant groups, the reaction product exhibits photocrosslinking via two distinct mechanisms: (i) cationic ring-opening polymerization and (ii) free radical polymerization. Therefore, photopolymerization behavior of synthetized hybrid resin with various photoinitiators was determined via photo-differential scanning calorimetry (photo-DSC) and real-time infrared spectroscopy (RT-IR) methods, and properties of cured coatings were investigated. The performance of the following type of photoinitiators was tested in the cationic photopolymerization: diaryliodonium cations or triarylsulfonium cations, and the following type of photoinitiators were used to induce free radical photopolymerization: α-hydroxyketones, acylphosphine oxides, and their mixtures. Lastly, the basic physicomechanical properties of cured coatings, such as tack-free time, hardness, adhesion, gloss, and yellowness index, were evaluated. Some structural factors and parameters of cationic and radical photoinitiators and photopolymerization mechanisms affecting the epoxy acrylate hybrid coatings performance are discussed.

Photo-DSC method for liquid samples used in vat photopolymerization

The photo differential scanning calorimetry (photo-DSC) is an appropriate method to characterize photopolymers used in additive manufacturing (AM). Important process parameters such as optimal ultraviolet (UV) exposure time and reaction heat can be attained by this method. However, achieving reliable and meaningful results from photo-DSC experiments requires careful sample preparation, i.e. the selection of a suitable sample shape, sample mass and sample holder (crucible). The sample shapes drop and spread with 1.0 mg and 2.8 mg sample masses were investigated in this study. Three different times from sample preparation until the start of the measurement (0, 4 and 7 h) were tested, in order to investigate different surface effects such as oxygen-diffusion, prior UV-curing through ambient radiation and evaporation of volatile components. While the 1.0 mg spread sample shape offers the thinnest film thickness (40 μm) and thus the closest comparability to high resolution print jobs, the 2.8 mg drop shape offers a more robust sample preparation with minimized surface effects. To further reduce time-dependent surface effects, this study shows how a preexisting test protocol was shortened from 42 min to 24 min without losing measuring accuracy. Furthermore, to reduce evaporation, different covers were placed on different crucibles, which were tested over time in the device’s automated sample changer (ASC) that enables automated and consecutive measurements. The combination of a cold pressed 85 μL crucible covered with a 300 μL Al2O3 crucible, which is removed shortly before the actual measurement, provides the best sample preparation for the ASC since mass loss remains below 1% for up to 10 h. Finally, two two-part resin systems, namely a methacrylate-urethane and an acrylate-epoxy based resin that are used in Digital Light Synthesis (DLS) are characterized part by part as well as in mixed state. Together with the investigation of different temperatures and atmospheres, it was possible to identify not only the part with the photoinitiator and the type of system (radical or cationic), but also a difference between methacrylates and acrylates with the aid of the photo-DSC method.

Influence of Acrylic Acid on Kinetics of UV-Induced Cotelomerization Process and Properties of Obtained Pressure-Sensitive Adhesives.

A new environmentally friendly method of photoreactive pressure-sensitive adhesives (PSAs) preparation was demonstrated. PSAs based on n-butyl acrylate (BA), acrylic acid (AA) and 4-acryloyloxy benxophenone (ABP) were prepared via the UV-induced cotelomerization process in the presence of a radical photoinitiator (acylphosphine oxide) and telogen (tetrabromomethane). Hydroxyterminated polybutadiene was used as a crosslinking agent. Influence of AA concentration (0–10 wt %) on kinetics of the cotelomerization process was investigated using a photodifferential scanning calorimetry method, selected physicochemical features of obtained photoreactive BA/AA/ABP cotelomers (molecular masses, polydispersity, monomers conversion and dynamic viscosity) and self-adhesive properties of obtained PSAs (adhesion, tack and cohesion) were studied, as well. It turned out that AA content is the important factor that influences monomers conversion (thereby the volatile parts content in prepolymer) and PSAs’ properties. As the acrylic acid content increases, the reaction rate increases, but the total monomers conversion and the solid content of the prepolymer decreases. Additionally, the adhesion and cohesion of PSAs were grown up, and their tackiness decreased. However, the AA content has no effect on molecular weights (Mw and Mn) and polydispersity (c.a. 1.5) of photoreactive cotelomers. The optimal AA content necessary to obtain a prepolymer with low volatile parts content and good PSA properties was determined.

Curing Behavior, Rheological, and Thermal Properties of DGEBA Modified with Synthesized BPA/PEG Hyperbranched Epoxy after Their Photo-Initiated Cationic Polymerization.

This paper investigates the photo-initiated cationic polymerization of diglycidyl ether of bisphenol A (DGEBA) modified with bisphenol A (BPA)/polyethylene glycol (PEG) hyperbranched epoxy resin. The relationship between curing behavior, rheological, and thermal properties of the modified DGEBA is investigated using photo-differential scanning calorimetry (DSC) and photo-rheometer techniques. It is seen that the addition of the hyperbranched epoxy resin can increase UV conversion (αUV) and reduce gelation time (tgel). After photo-initiation polymerization (dark reaction) occurred, a second exothermic peak in the DSC thermogram takes place: namely, the occurrence of curing reaction owing to the activated monomer (AM) mechanism. Consequently, the glass transition temperature decreased, and at the same time, UV intensity increased which was due to the molecular weight between crosslinking points (Mc). Furthermore, the radius of gyration (Rg) of the network segment is determined via small-angle X-ray scattering (SAXS). It is noted that the higher the Mc, the larger the radius of gyration proves to be, resulting in low glass transition temperature.

Blue Light Induced Photopolymerization and Cross-Linking Kinetics of Poly(acrylamide) Hydrogels.

Blue light induced photopolymerization and photo-cross-linking kinetics of acrylamide (AM), with camphorquinone/diphenyl iodonium hexafluorophosphate (CQ/DPI) as photoinitiators, were investigated. The effects of a number of parameters, including mass fraction of CQ, DPI, and AM (wCQ, wDPI, and wAM) and light intensity (I), on photopolymerization efficiency and photogelation process were systematically studied by photo-differential scanning calorimetry (DSC) and photo-rheometry. Photo-DSC indicated that the maximum photopolymerization rate (Rp,max) was proportional to wCQ0.5, wDPI0.5, I0.5, and wAM, while Photo-Rheometry showed linear relationships between gel time tgel and wCQ and I, respectively, and power law relationships between tgel and wDPI and wAM, respectively. In addition, both peak cross-linking rate Rc,max, and delay time td, which were both linearly proportional to wCQ0.5, wDPI0.5, and I0.5, showed power law relationships with wAM. Furthermore, exponential patterns were observed between all these factors, wCQ, wDPI, wAM, and I and plateau modulus G'∞. Combining such correlations obtained from experimental data, an empirical model was established describing the projected mechanical properties of poly(acrylamide) hydrogels from blue light initiated photopolymerization and photo-cross-linking.

Magnetically gradient-distributed microcapsule/epoxy composites: Low capsule load and highly targeted self-healing performance

One-component self-healing epoxy-matrix composites are needed urgently in the electronics industry because of their ability to repair surface damage autonomously. However, their practical applications are limited by unsuitable external stimuli and heavy microcapsule loads. Here, we report a magnetic-field and ultraviolet dual-responsive microcapsule to endow epoxy composites with a highly targeted self-healing capability under low microcapsule loads. Specifically, 5 wt% Fe3O4@SiO2 nanoparticles that were incorporated into the core material served as magnetic targets and helped the microcapsules to navigate to the near-surface region of a high-failure probability, to form magnetically gradient-distributed microcapsule/epoxy composites. This resulted in a more than fourfold increase in the local microcapsule concentration. The successful targeted self-healing with a microcapsule doping percentage as low as 4 wt% was confirmed by laser-scanning fluorescence confocal microscopy and scanning electron microscopy observations, and electrochemical testing. In addition, a kinetic model based on isothermal photo-differential scanning calorimetry measurements and a molecular-dynamics simulation model were both established to analyze the self-healing mechanism. Because of the facility and universality of this magnetic control method, these magnetic microcapsules could be guided to concentrate in vulnerable parts of other electrical insulators as desired, which highlights the good potential of this targeted self-healing method in energy fields.

Comparative study of the impact of additives against oxygen inhibition on pendulum hardness and abrasion resistance for UV-curable wood finishes

The multiple advantages of UV-curable coatings, such as high curing speeds and cross-linking densities, low VOC and energy consumption led to a widespread interest for a wide range of applications. Free-radical photopolymerization constitutes one of the most used methods of polymerization. However, oxygen present in the atmosphere is able to inhibit radical formation and propagation, which is considered a major drawback. Among all the strategies developed for reducing the oxygen inhibition, the addition of reactive additives is often preferred. This method does not require major changes in the production lines of the industries or in the formulations compositions. The efficiency of reactive additives against oxygen inhibition has been investigated in multiple studies, but in different experimental conditions. The addition of the reactive additives can also influence coatings hardness and abrasion resistance which are considered important mechanical properties. The aim of this study is to evaluate additives overall performances. Eleven of the chosen additives were previously described in the literature. The four remaining are commercial products never used for that application, like a polyhedral oligomeric silsesquioxane bearing a thiol function, which later showed the best results. Reduction of oxygen inhibition was analyzed by photo differential scanning calorimetry (Photo-DSC) and Confocal Raman Micro-spectroscopy (CRM). Pendulum hardness and abrasion resistance measurements were used to determine the influence of the additives on the mechanical properties of the coatings. The results obtained show that most of the additives tested had reducing effect on the pendulum hardness, but helped in improving the abrasion resistance.

Novel self‐initiating UV‐curable acrylate monomers

AbstractBased on the addition reaction of hydroxyl groups and the isocyanate groups in the bi‐functional monomer acroyloxyethyl isocyanate (AOI), two novel self‐initiating ultraviolet (UV)‐curable acrylate monomers were synthesized from benzoin and 2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone (commercial‐available Irgacure® 2959), and the products were denoted as Benzoin‐AOI and 2959‐AOI, respectively. The chemical structures of the products were confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The UV–visible absorption spectrum showed that the self‐initiating acrylates exhibited similar maximum absorption wavelengths to both benzoin (250 nm) and Irgacure® 2959 (270 nm). When the products were continually exposed to a mercury lamp, the maximum absorption peak gradually blue shifted, showing that the products decomposed under the UV radiation. The photo differential scanning calorimetry (Photo‐DSC) analysis showed that the self‐initiating monomers could initiate the polymerization of 1,6‐hexanediol diacrylate (HDDA) as quickly as the original photo‐initiator, and the efficiency of Benzoin‐AOI was slightly lower than that of benzoin, while for 2959‐AOI, the efficiency remained stable after being linked to the AOI moiety. The cytotoxicity assay against fibroblasts cell line L929 showed that 2959‐AOI exhibited lower cytotoxicity at low dosages but higher cytotoxicity at high dosages compared to Irgacure® 2959 after the introduction of the AOI structure.

Photosensitive Formulation for Additive Manufacturing-3D Printing

UV curing is a photochemical process in which high-intensity ultraviolet light is used to instantly cure or ��dry�� coatings, inks, adhesives and thin film technology. It has been around as a coating for wood, paper and as a clear coating via photolithography process on printed circuit boards PCBs or integrated circuit boards ICBs for years. It is fast becoming one of the most popular techniques in the paint and coatings industry. Most of the formulation use multifunctional acrylate monomers or oligomers or a mixture of them that crosslink under exposure to UV/EB radiations in a free radical process. We briefly present the advantages of EB vs. UV. A new type of formulation based on multifuctional monomer of dicyclopentadiene epoxy derivative with additional diluent as co-reactive solvent will be described and evaluated. This formulation differs from the acrylate one by the use of a cationic photoinitiator. The final product presents all the advantages of epoxy resins viz. better adherence, mechanical and thermal properties, compared to acrylate systems. Formulation has been optimized thanks to the differential scanning photocalorimetry DPC. This type of formulation is developed for additive digital manufacturing - 3D Printing (building layer by layer).

Multifunctional biphenyl derivatives as photosensitisers in various types of photopolymerization processes, including IPN formation, 3D printing of photocurable multiwalled carbon nanotubes (MWCNTs) fluorescent composites.

A series of 2-(diethylamino)-4-(1-ethylpropyl)-6-phenyl-benzene-1,3-dicarbonitrile derivatives were investigated in terms of photosensitisation in various photopolymerization processes in UV-A and vis light conditions. A full spectroscopic analysis of the tested compounds was performed. In addition to excellent spectroscopic properties, these compounds enable highly efficient photopolymerization processes, including free-radical, cationic and hybrid photopolymerization. As proven by a real-time FTIR study, these photosensitisers allow the formation of both thin and thick layers from different monomers. Finally, the investigated 2-(diethylamino)-4-(1-ethylpropyl)-6-phenyl-benzene-1,3-dicarbonitrile derivatives were used to obtain multiwalled carbon nanotubes (MWCNTs) composites for which the degree of conversion was determined using real-time FT-IR and Photo-Differential Scanning Calorimetry (Photo-DSC). Selected derivatives were applied as photosensitisers in two-component photoinitiating systems, operating according to the mechanism of photo-oxidation and photo-reduction, for the preparation of photo-cured MWCNTs composites. The importance of the quantity of multiwalled carbon nanotubes (MWCNTs) added to the polymeric matrix on the curing degree is also discussed in this study. The structures of the MWCNTs composites were analysed using an optical and fluorescence microscope. Moreover, this study also examines the applicability of new photoinitiator systems for printing nanocomposites by vat photopolymerization, which has gained increasing attention in recent years. Therefore, photocurable nanocomposite resin based on methacrylates was used for 3D printing in room temperature and atmospheric conditions, under a visible LED with emission at 405 nm, in order to obtain fluorescent photocurable patterns.

Investigation of the influence of exposure time on the dual-curing reaction of RPU 70 during the DLS process and the resulting mechanical part properties

Photo-differential scanning calorimetry (Photo-DSC), standard DSC and tensile testing are used in order to investigate the influence of different exposure times on the dual-curing reaction and mechanical properties of RPU 70 specimens produced by the additive manufacturing technology Digital Light Synthesis (DLS). A comparison of reaction enthalpies measured with a Photo-DSC of different exposure times followed by thermal curing and DSC experiments with different exposed DLS green products shows that the degree of thermal cross-linking of the two-component resin RPU 70 is determined by the preceding cross-linking reaction during photo-polymerization. A shorter exposure time of UV-light enhances the subsequent thermal cross-linking, while increased exposure reduces the effect of thermal curing in the second process step.Tensile testing results of specimen produced with custom-made print profiles on the DLS Carbon M2 with different exposure times show that mechanical characteristics such as tensile strength and elongation at break are strongly dependent on the degree of UV-cross-linking. The mechanical characteristics of DLS parts produced with the RPU 70 resin system can therefore be adapted to specific requirements and applications using these customized print profiles with different exposure times.

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal.

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such as on-demand switchable adhesion technologies, actuators, molecular clutches, and nano-/microscopic mass transporters. Recently it was found that through a special solid-liquid transition, rheological properties can exhibit significant changes, thus providing suitable smart viscoelastic materials. However, designing materials with such a property is complex, and forward and backward switching times are usually long. Therefore, it is important to explore new working mechanisms to realize solid-liquid transitions, shorten the switching time, and enhance the contrast of rheological properties during switching. Here, a light-induced crystal-liquid phase transition is observed, which is characterized by means of polarizing light microscopy (POM), photorheometry, photo-differential scanning calorimetry (photo-DSC), and X-ray diffraction (XRD). The light-induced crystal-liquid phase transition presents key features such as (1) fast switching of crystal-liquid phases for both forward and backward reactions and (2) a high contrast ratio of viscoelasticity. In the characterization, POM is advantageous in offering information on the spatial distribution of LC molecule orientations, determining the type of liquid crystalline phases appearing in the material, and studying the orientation of LCs. Photorheometry allows measurement of a material's rheological properties under light stimuli and can reveal the photorheological switching properties of materials. Photo-DSC is a technique to investigate thermodynamic information of materials in darkness and under light irradiation. Lastly, XRD allows studying of microscopic structures of materials. The goal of this article is to clearly present how to prepare and measure the discussed properties of a photorheological material.

The Synthesis of New Type II Polymeric Photoinitiator (thioxantone) via Atom Transfer Radical Polymerization and Their Curing and Migration Studies

Polymeric photoinitiators are widely used in ultraviolet (UV)-curable printing inks because of their low migration behavior. In this study, a new phenylphosphine oxide-polystyrene-thioxanthone (PPO-PSt-TX) polymeric photoinitiator was synthesized. Bis[(4-hydroxy)phenyl]phenyl phosphine oxide (BHPPO) that was synthesized by Grignard technique, was functionalized with 2-bromopropionyl bromide (atom transfer radical polymerization (ATRP) initiator) and then used in styrene polymerization. The bromine end-capped polystyrene was then reacted with 2-thioxanthone-thioacetic acid and final polymeric photoinitiator PPO-PSt-TX was obtained. Proton nuclear magnetic resonance (1H NMR), attenuated total reflectance- Fourier transform infrared spectroscopy (ATR-FTIR), gel permeation chromatography (GPC) and ultraviolet-visible spectroscopy (UV-Vis) confirmed the obtained structure. The curing characteristic of PPO-PSt-TX was compared with a standard flexographic printing varnish formulation containing thioxanthone (TX). The photopolymerization kinetics were determined by photo differential scanning calorimetry (Photo-DSC). The conversion of methylmethacrylate polymerization by using macrophotoinitiator is 78%. The migration behavior of PPO-PSt-TX was identified with liquid chromatography-mass spectrometry (LC-MS). It was shown that PPOPSt- TX macro photoinitiator is suitable for flexographic varnish and the migration level of photoinitiator is reduced by using polymeric photoinitiator.

Stimuli-responsive poly(hydroxyethyl methacrylate) hydrogels from carboxylic acid-functionalized crosslinkers.

Tailoring hydrogel properties by modifications of the crosslinker structure is a good method for the design of hydrogels with a wide range of properties. In this study, two novel carboxylic acid-functionalized dimethacrylate crosslinkers (1a and 2a) are synthesized by the reaction of poly(ethylene glycol) or 2-hydroxyethyl disulfide with tert-butyl α-bromomethacrylate followed by cleavage of tert-butyl groups using trifluoroacetic acid. Their copolymerization reactivity with 2-hydroxyethyl methacrylate (HEMA) investigated by photopolymerization studies performed on photo-differential scanning calorimetry shows higher reactivity of 2a compared to 1a. These crosslinkers are then used at different ratios for fabrication of pH- and redox-responsive poly(2-hydroxyethyl methacrylate)-based hydrogels. The swelling behavior of the hydrogels is found to be dependent on the structure of the crosslinker, degree of crosslinking, pH, and CaCl2 concentration. The redox-responsive behavior is demonstrated by degradation of the hydrogel upon exposure to 1,4-dithiothreitol. The dye Rhodamine 6G and the drug resorcinol are used as models to demonstrate the pH and redox dependent release of loaded compounds from the hydrogels. The electrostatic interactions between the carboxylate groups and the positively charged R6G are found to govern the release profile in DTT and counteract the diffusion of dye molecules and significant amount of release (79% in 120 hr) occurs only at highly acidic conditions. The degradation mediated release in DTT is observed better in case of resorcinol (around 88% in 5 hr). Overall, these hydrogels can be regarded as good candidates for several applications, such as matrices for controlled release, tissue repair, and regeneration.

Photo-differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5 seconds and a low intensity of 13.80 mW/cm2.

Dispersion Characteristics and Curing Behaviour of Waterborne UV Crosslinkable Polyurethanes Based on Renewable Dimer Fatty Acid Polyesters

In the last few years there has been a great deal of interest in modifying the methodologies for obtaining polyurethanes using more sustainable strategies. Following this tendency, in the present work, waterborne UV-curable polyurethanes were synthesized using commercially available dimer fatty acid based polyols obtained from renewable resources (Priplast from Croda). The polyols, characterized by proton nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC) and size exclusion chromatography (GPC–SEC), were semicrystalline and presented very broad melting related with their complex molecular weight distribution. Different polyurethane dispersions were obtained by changing the percentage of the hard segment. Comparing with waterborne polyurethane dispersions obtained from non-renewable resources, the dispersions showed higher particle size even using similar or higher amounts of internal emulsifier. The curing behaviour was characterized by photo-differential scanning calorimetry (Photo-DSC) and the results showed that the conversion and polymerization rate decreased with the hard segment content and temperature. The obtained products displayed good thermal characteristics with phase-separated structures.

Photothermal effect in solid-state MWCNT: Possible signatures of thermal anisotropy

The photothermal effect of multi-walled carbon nanotubes (MWCNTs) in solid-state was investigated by employing a sensitive heat detection technique, i.e., photo-Differential Scanning Calorimetry using an incoherent, broadband, continuous wave UV-visible light source of the range 250–450 nm. MWCNTs showed considerable photothermal activity, with the highest value recorded being ∼116.1 W/g arising from ∼ 0.2 mg of the sample under an illumination intensity of ∼ 706.8 mW/cm2. Moreover, a time-dependent two-step response in the heat output was obtained as soon as the irradiation source was turned ON or OFF. In view of the huge anisotropy in thermal conductivity as was reported in the case of MWCNTs, we attribute their origin to the quick detection of heat generated in the outer layer and a second gradual, delayed increase in heat output combining the contributions from the inner tubes as well, which gets conveyed to the exterior layer after a certain time-interval owing to the weak van der Waals interaction between the concentric tubes.

Evaluation of the Ultraviolet-Curing Kinetics of Ultraviolet-Polymerized Oligomers Cured Using Poly (Ethylene Glycol) Dimethacrylate

Ultraviolet (UV)-curable oligomers are increasingly being used in various industries because they can be applied rapidly and have excellent physical properties. Ultraviolet polymerization is used for manufacturing such oligomers. Reactive diluents, which are employed during the secondary curing of UV-curable oligomers, can help elucidate the curing behaviors of these oligomers. In this study, poly (ethylene glycol) dimethacrylate (PEGDMA) was used as the reactive diluent for UV-curable oligomers. Photodifferential scanning calorimetry (photo-DSC) and shrinkage measurements revealed that the curing behavior of the polymers was dependent on the size and number of molecules of PEGDMA. The effect of the small-size PEGDMA on curing behavior was greater than that of the larger molecules. Further, in most cases, the use of a larger amount of PEGDMA resulted in lower reactivity.