Photo-differential Scanning Calorimetry Research Articles

Kinetics of UV‐initiated RAFT crosslinking polymerization of dimethacrylates

AbstractThe UV‐initiated RAFT polymerizations of a series of poly(ethylene glycol) dimethacrylates (PEGDMA) were investigated using differential scanning photocalorimetry (DPC) at room temperature. The rate of the RAFT system was much lower than that of a conventional free radical polymerization. A mild autoacceleration occurred as the addition reaction became diffusion controlled. The influence of the spacer length (CH2CH2O)x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate of PEGDMA decreased with an increased x value from 4 to 9, but it increased with a further increased x value from 9 to 14. Mechanical properties of the resulting polymers were also examined by dynamic mechanical analysis (DMA). It was concluded that the presence of the RAFT agent during polymerization of multifunctional monomers did not have an effect on the heterogeneity of the polymer network. In comparison with three different PEGDMAs, the PEGDMA with the longest spacer formed the most homogeneous networks with a lower crosslinking density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

The effect of an aminoalcohol on the polymerization of acrylates with type II initiators

In this study, the authors report the use of an aliphatic amine and an aromatic aminoalcohol as synergists with various Type II initiators by using Photo-DSC (Photo-differential scanning calorimetry) technique. The photopolymerization of triacrylate and diacrylate was performed under nitrogen atmosphere in the presence of an amine or aminoalcohol with type II initiators, which allowed us to compare the efficiency of the initiating radicals. The oxygen scavenging role of the amine was left out.

Synthesis and photopolymerization kinetics of linear alicyclic urethane acrylate macromonomer in presence of reactive diluents

This study deals with the photopolymerization of a macromonomer in the presence of reactive diluents using Photo Differential Scanning Calorimetry or Photo DSC. The kinetic profiles of these systems showed that the rate of photopolymerization rapidly increases at very early stages of the reaction. The rate of reaction was further found to increase with the addition of crosslinking agents. The addition of trifunctional crosslinking agent to the macromonomeric formulation resulted in a higher polymerization rate and conversion than that of a difunctional crosslinking agent. From the heat flow profiles, the kinetic parameters such as induction time, time to attain peak maximum, rate of maximum polymerization, and final conversion were noted for all the formulations. The initiation of photopolymerization was found to depend on the functionality while the in situ viscosity controlled the time scale for reaction diffusion which resulted in varying levels of conversions. Other parameters such as effect of temperature and concentration of photoinitiator on photocuring kinetics are also discussed.

Characterization of surface plasmon energy transduction in gold nanoparticle/polymer composites by photo-DSC

In this study, we investigate the formulation and optimization of stimulus-responsive composites consisting of gold nanoparticles in polyethylene glycol diacrylate (PEGDA) matrices, which can be remotely heated through localized surface plasmon resonance (SPR). In these materials, laser radiation is absorbed by the nanoparticles and transduced into thermal energy. Optothermal properties of the polymer/nanoparticle composites are characterized using an adaptation of photo-differential scanning calorimetry (photo-DSC), in which a sample is characterized in isothermal mode in the presence and absence of optical illumination. Au/PEGDA composite samples are determined by photo-DSC to transduce energy from a 532 nm optical source with high efficiencies (>80%). UV/Vis/NIR spectrophotometry is used to characterize the optical properties of the samples. Nanoparticle dispersion and size within composite polymer matrices are characterized using transmission electron microscopy (TEM). It is shown that the magnitude and rate of energy transduction can be tuned by varying both nanoparticle concentration and dispersion.

Investigation of the photopolymerization-induced phase separation process in polymer dispersed liquid crystal

A novel experimental method involving simultaneous resistivity and transmittance measurements was used to examine the polymerization-induced phase separation process. Measuring the resistivity and transmittance of a polymer dispersed liquid crystal (PDLC) cell allows the polymerization and phase separation to be monitored during the PDLC formation process. To verify the utility of this method, resistivity data were compared with conversion data obtained by photo differential scanning calorimetry (DSC) and FTIR analysis using NOA65 and an NOA65-E7 mixture. The resistivity data of NOA65 (a neat prepolymer without liquid crystals) were identical to the conversion data obtained by photo-DSC and FTIR analysis. However, the resistivity data of the NOA65-E7 mixture (a prepolymer with liquid crystals) differed from the conversion data because the resistivity of the NOA65-E7 mixture was affected by the degree of liquid crystal phase separation and monomer conversion. The photopolymerization was slower and the degree of conversion was higher for the NOA65-E7 mixture than for the neat prepolymer due to dilution and plasticization effects, respectively.

Simultaneous time-resolved measurement of the reaction rates and the refractive index of photopolymerization processes

We explore the use of imaging surface plasmon resonance (iSPR) to simultaneously measure the refractive index and reaction rates of the commercially available Ormocore photosensitive resist during photopolymerization. To this end, we adapted a commercially available iSPR device. We demonstrate good accuracy in the measurement of the refractive index determined independently of the thickness of the polymerized film. Furthermore, we demonstrate that the refractive index is proportional to the degree of cure (double bond conversion) of the resist. This allows the determination of the reaction rates of the polymerization processes, which show reasonable agreement with photodifferential scanning calorimetry measurements.

Photopolymerization of biocompatible phosphorus-containing vinyl esters and vinyl carbamates

Phosphorus-containing vinyl esters and vinyl carbamates were synthesized as new biocompatible and degradable photopolymers. Reactivity of the monomers with one, two, and three polymerizable double bonds was evaluated by photo-differential scanning calorimetry. With respect to their potential application in the biomedical field, studies on cytotoxicity, mechanical stability, and hydrolytic erosion behavior of the poly(vinyl alcohol)-based derivatives were performed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2916–2924, 2010

PHOTOINITIATED RAFT CROSSLINKING POLYMERIZATION OF POLYETHYLENE GLYCOL DIMETHACRYLATES

A reversible addition-fragmentation chain transfer (RAFT) agent,S-1-dodecyl-S'-(α,α'-dimethyl-α″-aceticacid) trithioncarbonate was synthesized. The UV-initiated RAFT polymerization of polyethylene glycol dimethacrylates ( PEGDMA ) at room temperature was investigated by using differential scanning photocalorimetry (DPC) and dynamic mechanical analysis (DMA). The photopolymerization rate of the RAFT system was much lower than that in conventional free radical polymerization,a mild autoacceleration occurred as the addition reaction became diffusion controlled. In addition the influence of different spacer lengths between the methacrylate groups on RAFT polymerization rate and final conversion was also explored. The results show that the final conversion increases slowly and the polymerization rate decreases at first and then increases as the number of ethylene glycol units between the methacrylate groups increaseing. Comparison with three different PEGDMAs,PEGDMA with longer middle segments yields more homogeneous networks and lower crosslinking density.

Synthesis and photopolymerization of phosphonic acid monomers for applications in compomer materials

AbstractNovel methacrylate monomers bearing phosphonic acid groups 1 and 2 as well as new sulfur methacrylates 9 and 10 have been prepared in good yields from thiophenol. They have been fully characterized by 1H‐NMR, 13C‐NMR, 31P‐NMR, and HRMS. Their copolymerization with a bis‐GMA : TEGDMA (1 : 1) blend has been investigated with photodifferential scanning calorimetry at 50°C with camphorquinone as a photoinitiator and ethyl 4‐(dimethylamino)benzoate (EDAB) as a coinitiator. The higher the content of acidic monomer 1 or 2 incorporated in the bis‐GMA : TEGDMA (1 : 1) blend, the lower the mixtures reactivity. The phosphonic acid group has been proved to be responsible for this drop of reactivity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Non-extractable photoinitiators based on thiol-functionalized benzophenones and thioxanthones

Non-extractable photoinitiators could be advantageous for use in biological, electronic, and food packaging applications. Therefore, thioxanthone and benzophenone derivatives were synthesized from 1,6-hexane dithiol and chlorinated benzophenone or thioxanthone. The efficiency of thiol-functionalized photoinitiators in combination with amine co-initiators was compared to benzophenone and isopropylthioxanthone with amine co-initiators, and the cleavage of photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one in acrylic resins. The reaction kinetics were analyzed using photo-differential scanning calorimetry and real-time FTIR. Coating physical properties were evaluated by pendulum and pencil hardness, steel–wool scratch and mandrel bend tests. The non-extractable photoinitiators had higher absorbance than their benzophenone or isopropylthioxanthone counterparts due to the sulfide substitution on the phenyl ring, and the free thiol groups reacted with the acrylate by either an amine catalyzed Michael addition or a free-radical chain process. The combination of thiol-functionalized photoinitiators with secondary amines provides an efficient photoinitiator system that is locked into the photopolymerized network and cannot be extracted with typical solvents.

Synthesis and characterization of triptycene type cross-linker and its use in photoinduced curing applications

A novel triptycene type diacrylate cross-linker, triptycene hydroquinone diacrylate (THDA) was synthesized from the reaction of triptycene hydroquinone with acryloyl chloride and characterized. The photocuring behaviour and the reaction kinetics of the synthesized cross-linker were investigated by means of photo-differential scanning calorimetry (photo-DSC) experiments. Formulations containing monofunctional (meth)acrylate monomers, namely glycidyl methacrylate (GMA), 2-hydroxyethyl acrylate (HEA), 2-hydroxyethyl methacrylate (HEMA), and 2-ethylhexyl methacrylate (EHMA), cross-linker, THDA and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator were irradiated. A conventional cross-linker without triptycene unit such as hydroquinone diacrylate (HDA) was used under the same conditions for comparison. The effects of the structure of the monofunctional monomer and triptycene moiety on the photopolymerization kinetics were evaluated and discussed.

Time-intensity superposition for photoinitiated polymerization of fluorinated and hyperbranched acrylate nanocomposites

The validity of the time-intensity superposition principle for the photoinitiated polymerization of nanocomposites based on a monofunctional fluorinated acrylate and on a multifunctional hyperbranched polyether acrylate was investigated in this work. Master curves were obtained for the conversion as a function of time, measured by photo differential scanning calorimetry, by shifting on the time axis the curves obtained at different intensities. A power-law dependence of the shift factor on the intensity was found for all materials, with exponents equal to 0.45 ± 0.03 for the fluorinated acrylates and to 0.71 ± 0.05 for the hyperbranched polyether acrylates. Consequently it is inferred that the radiant exposure reciprocity law, implying linear dependence of the shift factor on intensity, does not apply to the studied compositions. The kinetics of the photopolymerization of materials based on the fluorinated acrylate was analyzed with the autocatalytic model. The final conversion was independent on intensity and filler content. The rate constants showed for all materials a power-law dependency on intensity, with exponents similar to those of the corresponding shift factors.

Kinetic studies of UV-waterborne nanocomposite formulations with nanoalumina and nanosilica

UV-waterborne coatings are gaining in popularity because of their environment friendly advantages. Most of the mechanical properties of such coatings are lower in comparison with high solids content UV coatings. For these reasons a nanocomposite-based approach was followed and seems promising in order to promote the use of UV-waterborne coatings in the wood products industry. The kinetic study of coatings gives interesting information regarding the curing process and photodifferential scanning calorimetry (photo-DSC) has proved its efficiency. The aim of this work was to study the photopolymerization kinetics by photo-DSC of a typical UV-waterborne coating formulation containing different amounts of nanoparticles. Formulations were mainly based on a polyurethane–acrylate (PUA) resin and a photoinitiator. Nanoalumina and functionalized nanosilica were added to the formulation at three concentrations: 1, 3 and 5 wt%. UV-waterborne formulations were dried during 10 min at 80 °C before curing under a UV source (47 mW/cm 2). DSC-exothermic curves demonstrated that photopolymerization was less efficient when nanoparticles were introduced but the UV-curing of coatings based on functionalized nanosilica were better than those containing nanoalumina. Finally the kinetic profile of UV-waterborne coating formulations were characterized using an autocatalytic model and kinetic parameters k, m and n were determined.

Kinetic behavior of photo-polymerization of UV-curable resins with carboxylic acid and amino groups

Photo-polymerization behaviors of bisphenol-A epoxy diacrylate (EPA) and six kinds of EPA-derived resins containing different amounts of carboxylic acid, urethane, amide, and imide groups were studied by a photo differential scanning calorimetry. The dark polymerization was performed and pseudo-steady state assumption of growing radicals was made to obtain the kinetic constants for propagation, bimolecular termination, monomolecular termination, and the concentration of growing radicals of different resins as a function of extent of reaction. Compared with EPA, it was found that the rate of polymerization and kinetic constants of the six resins were relatively small because the mobility of reacting species in resins was restricted by carboxylic acid, urethane, amide, and imide groups. Finally, three different photo-initiators were used to initiate the polymerization, and their kinetic behaviors were compared. The effect of tertiary amine group of photo-initiator on the rate of polymerization of resins having carboxylic acid group and the initiator efficiency were discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Development of reactive phosphonated methacrylates

AbstractFive novel phosphonated mono‐ and dimethacrylate monomers have been synthesized by two different routes. Monomers 1 and 2 were synthesized by reactions of methacryloyl chloride with diethyl (2‐hydroxyphenyl) phosphonate or tetraethyl (2,5‐dihydroxy‐1,4‐phenylene) bisphosphonate; monomers 3 and 4 by reactions of α‐(chloromethyl)acryloyl chloride (CMAC) first with dimethyl (2‐hydroxyethyl) phosphonate and then with benzoic or formic acids. The reaction of CMAC with two moles of dimethyl (2‐hydroxyethyl) phosphonate gave monomer 5. Thermal homopolymerization of monomers 1, 3, 4, and 5 and copolymerization of monomer 1 with methyl methacrylate (MMA) were investigated using azobisisobutyronitrile (AIBN) at 60 °C. Glass transition temperatures were observed for poly‐1, poly(MMA‐co‐1) (50:50), poly(MMA‐co‐1) (90:10), PMMA, poly‐3, and poly‐5 at 52, 90, 99, 129, 50, and 70 °C, respectively. TGA analysis of these polymers indicated formation of char on combustion. Homo‐ and/or copolymerization behavior of the synthesized monomers with 2,2‐bis[4‐(2‐hydroxy‐3‐methacryloyloxy propyloxy) phenyl] propane (Bis‐GMA) were investigated with photodifferential scanning calorimetry. The maximum rate of polymerizations decreased in the following order: Bis‐GMA∼3 > 1 > 4 > 5. The conversions of monomers 1, 3, 4, and 5 (73.9, 85.9, 98.2, and 62.2%) were very high compared with Bis‐GMA (40.5%). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5737–5746, 2009

Copolymers Obtained by ε-Caprolactam and Methyl Methacrylate Polymerization in the Presence of Polyimides

New copolymers have been synthesized by anionic or radical photopolymerization of ε-caprolactam or methyl methacrylate (MMA) in the presence of dissolved polyimides bearing hexafluoroisopropylidene, fluorene or other groups. Using diimides as models it was shown that the kinetics of anionic polymerization of ε-caprolactam depends on the nature of spacer between the imide cycles. The mechanical and tribological properties of copolymers, their water absorbance and the microstructure of copolymer films were studied. It was found that upon the selection of polyimide activator it was possible to gain the desirable control over the polymer properties, namely, the gel fraction content, phase composition, compression modulus, notched Izod impact strength, friction coefficient and temperature of frictional contact. The kinetics of radical polymerization of MMA in the presence of polyimide and model diimide has been studied by differential scanning photocalorimetry and infrared spectroscopy. From the results of the reaction kinetics and the study of polymer structures by Fourier transform infrared spectroscopy, nuclear magnetic resonance, size-exclusion chromatography and thermogravimetric analysis it has been established that radical photopolymerization of MMA in the presence of polyimide leads to the formation of copolymers owing to chain transfer reactions and/or chain termination by the relevant condensation polymer. It has been established that the imide cycles play a significant role in the formation of both copolymers with ε-caprolactam and MMA, respectively.

Photopolymerization of Vinyl Ether Networks Using an Iodonium Initiator: Effect of Radiation Intensity and Iodonium Concentration

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the photocuring kinetics of visible light initiated cationic photopolymerization of triethylene glycol divinyl ether (TEGDVE) with a diphenyl iodonium salt initiator and the three photosensitizers: 1-chloro-4-propoxy-9H-thioxanthen-9-one (CPTXO), camphorquinone (CQ), and acridine orange (AO). The effects of water, a tertiary amine, and a radical scavenger were used to probe the photocuring mechanism. The rate of photopolymerization was approximately proportional to the incident light intensity, and in each system, the final conversions, as measured by photo-DSC, also increased with raised incident light intensity. The power-law dependence of the reaction rate on the concentration of iodonium salt with CPTXO and AO was ∼1.5, while the order is nearly unity with CQ as the photosensitizer, suggesting that these photosensitizers do not follow the same sensitization mechanism. For the three photosensitizers, the final conversions all increased with higher iodonium salt concentrations. After cessation of irradiation, a dark reaction was observed—although the rate of this process decreased rapidly with time (as shown by photo-DSC), the polymerization continued very slowly over a long period of time.

Conversion and shrinkage analysis of acrylated hyperbranched polymer nanocomposites

AbstractThe photo‐curing behavior of composites containing nanosized SiO2 in an acrylated hyperbranched polymer matrix was investigated by means of photo differential scanning calorimetry. The chemical conversion data were analyzed using an autocatalytic model, paying close attention to the influence of composition and UV intensity. It was shown that the reaction order and the autocatalytic exponent were independent of UV intensity and filler fraction, whereas the rate constant showed strong intensity dependence, but weak filler dependence. Maximum conversion was independent of UV intensity, but was reduced when a filler was present. The dispersion state influenced the gel‐point of the composites, but had no influence on the overall cure kinetics. Cure shrinkage reduction of ∼ 33% could be achieved by adding 20 vol% of filler. This was attributed to the reduced double bond conversion of the matrix due to the presence of the filler. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Kinetic Analysis of UV-Curable Epoxy Resins for Micromachining Applications

Multifunctional epoxy resin series Epiclon HP-7200 is classified under the high performance series and is well known for its application in electrical encapsulation as it exhibits excellent mechanical and electrical properties. In addition, they have an advantage of an ultra low moisture absorption compared to other conventional cresol novolac (ECN) and biphenyl epoxy resins, which potentially makes it the next new mainstream of epoxy resins in electronics application. In this paper, kinetic study of various concentration of Epiclon HP-7200 (40-70 wt %) with divinyl ether (DVE) as the reactive solvent has been performed by Photo-Differential Scanning Calorimetry (DPC). The thermal stability of the Epiclon HP-7200 solutions has been analyzed by DSC and TGA.

UV curable sulfonated hybrid materials and their performance as proton exchange membranes

In this study 2-acrylamido-2-methylpropanesulfonic acid (AMPS) containing UV curable nanocomposite membranes were prepared by using the sol–gel method. Tetraethylorthosilicate (TEOS), and 3-(methacryloyloxy)propyl trimethoxysilane (MAPTMS) were used, respectively as an inorganic precursor and coupling agent. Cross linking agents such as poly(ethylene glycol diacrylate) (PEGMA) and ethylene glycol dimethacrylate (EGDMA) were used to arrange the mechanical and physical properties of the resulting hybrid membrane. The hybrid formulation polymerized under UV irradiation and the gel percentage, water uptake of the membranes were calculated. The polymerization conversion of the organic part was investigated by using photo-differential scanning calorimetry (photo-DSC). The thermal and mechanical properties of the membranes indicated good stability. The morphological structure of membranes was investigated by scanning electron microscopy (SEM). In addition proton conductivity and methanol selectivity measurements were performed. The proton conductivity of the AMPS20–SOLGEL30 nanocomposite membrane is about 0.138 S cm −1 at 50 °C. Selectivity toward methanol for the same membrane is very low with a selectivity factor of α = 0.032, which satisfies the requirements for DMFC applications.