Research Article Hybrid nanocomposites based on POSS and networks of methacrylated camelina oil and various PEG derivatives Brindusa Balanuca 1 *, Adriana Lungu 1 *, Ana‐Maria Hanganu 2 , Liane Raluca Stan 3 , Eugeniu Vasile 1 and Horia Iovu 1 1 Advanced Polymer Materials Group, Department of Bioresources and Polymer Science, University Politehnica of Bucharest, Bucharest, Romania 2 Center of Organic Chemistry “Costin D. Nenitescu”, Romanian Academy, Bucharest, Romania 3 Department of Organic Chemistry, University Politehnica of Bucharest, Bucharest, Romania Several photo‐curable hybrid systems based on methacrylate‐modified camelina oil (CO) were synthesized through a copolymerization reaction with hydrophilic dimethacrylated poly(ethylene glycol) macromonomers (PEG MW ¼ 300, 550, and 750 g/mol). In a first step, the epoxidation and subsequently the methacrylation reactions of CO were performed and monitorized using 1 H NMR and FTIR. Further, the polymerization reaction of the new synthesized oil‐based monomer under visible light was proved by FTIR. The chain length of methacrylate functionalized PEG was found to directly influence some key properties of the oil‐based networks. The curing performance of the systems was studied by GF measurements. Water uptake capacity tests and contact angle measurements were undertaken and it was found that the internal arrangement of components is strongly affecting the hydrophilicity of the materials. Furthermore, nanostructured polyhedral oligomeric silsesquioxane (POSS) compounds bearing one or eight methacrylated groups were selected in order to obtain innovative organic–inorganic nanocomposites. Mechanical and thermal properties were evaluated by compression tests, DMA, TGA, and also the morphology of the synthesized materials was investigated by SEM. Practical applications: Tailoring the copolymer composition and the reinforcing agent in the manufacturing process leads to a wide range of products with optimum properties suitable for use in a variety of industrial areas. Well‐defined oil‐derived systems with controllable compressive strength were prepared in this work covering the whole range, from a hard material (with short chain PEG) to a soft and flexible one (with long chain PEG). Keywords: Camelina oil / PEG dimethacrylate / Polyhedral oligomeric silsesquioxane / VIS curing Received: October 7, 2013 / Revised: December 13, 2013 / Accepted: December 16, 2013 DOI: 10.1002/ejlt.201300370 1 Introduction Nowadays, there is a growing interest to produce biopoly- mers. Recently, the use of plant oils as renewable feedstock to design new polymeric materials has received particular attention due to environmental concerns and to eventually substitute fossil and depleting feedstocks. Oil‐based biopoly- mers exhibit many advantages compared with polymers prepared from petroleum‐based monomers including biode- gradability and low cost. Recently biobased, thermosetting polymers from vegeta- ble oils such as soybean oil, linseed, castor oil, and palm oil have been synthesized and investigated [1–4]. A relatively These authors contributed equally to this work. Correspondence: Professor Horia Iovu, Advanced Polymer Materials Group, Department of Bioresources and Polymer Science, University Politehnica of Bucharest, 149 Calea Victoriei, Bucharest 010072, Romania E‐mail: [email protected]Fax: þ4021 311 17 96 Abbreviations: CA, static contact angle; CO, camelina oil; CQ, camphorquinone; DMA, dynamic mechanical analysis; ECO, epoxidized camelina oil; EDMAB, ethyl‐4‐dimethylaminobenzoate; FTIR, Fourier transform IR spectroscopy; GF, gel fraction; MCO, methacrylated camelina oil; MSD, maximum swelling degree; PEG, poly(ethylene glycol); PEG300, triethyleneglycol dimethacrylate, MW ¼ 300 g/mol; PEG550, poly(ethylene glycol) dimethacrylate, MW ¼ 550 g/mol; PEG750, poly(ethylene glycol) dimethacrylate, MW ¼ 750 g/mol; POSS, polyhedral oligomeric silses- quioxanes; POSS1, monofunctional POSS; POSS8, octafunctional POSS; SEM, scanning electron microscopy; TGA, thermogravimetric analysis Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000 1 ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com
Transcript
Research Article
Hybrid nanocomposites based on POSS and networks ofmethacrylated camelina oil and various PEG derivatives
1 Advanced Polymer Materials Group, Department of Bioresources and Polymer Science,University Politehnica of Bucharest, Bucharest, Romania
2 Center of Organic Chemistry “Costin D. Nenitescu”, Romanian Academy, Bucharest, Romania3 Department of Organic Chemistry, University Politehnica of Bucharest, Bucharest, Romania
Several photo‐curable hybrid systems based on methacrylate‐modified camelina oil (CO) weresynthesized through a copolymerization reaction with hydrophilic dimethacrylated poly(ethylene glycol)macromonomers (PEGMW¼ 300, 550, and 750 g/mol). In a first step, the epoxidation and subsequentlythe methacrylation reactions of COwere performed andmonitorized using 1HNMR and FTIR. Further,the polymerization reaction of the new synthesized oil‐based monomer under visible light was proved byFTIR. The chain length of methacrylate functionalized PEG was found to directly influence some keyproperties of the oil‐based networks. The curing performance of the systems was studied by GFmeasurements. Water uptake capacity tests and contact angle measurements were undertaken and it wasfound that the internal arrangement of components is strongly affecting the hydrophilicity of thematerials. Furthermore, nanostructured polyhedral oligomeric silsesquioxane (POSS) compoundsbearing one or eight methacrylated groups were selected in order to obtain innovative organic–inorganicnanocomposites. Mechanical and thermal properties were evaluated by compression tests, DMA, TGA,and also the morphology of the synthesized materials was investigated by SEM.
Practical applications: Tailoring the copolymer composition and the reinforcing agent in themanufacturing process leads to a wide range of products with optimum properties suitable for use in avariety of industrial areas. Well‐defined oil‐derived systems with controllable compressive strength wereprepared in this work covering the whole range, from a hard material (with short chain PEG) to a soft andflexible one (with long chain PEG).
Received: October 7, 2013 / Revised: December 13, 2013 / Accepted: December 16, 2013
DOI: 10.1002/ejlt.201300370
1 Introduction
Nowadays, there is a growing interest to produce biopoly-mers. Recently, the use of plant oils as renewable feedstock todesign new polymeric materials has received particularattention due to environmental concerns and to eventuallysubstitute fossil and depleting feedstocks. Oil‐based biopoly-mers exhibit many advantages compared with polymersprepared from petroleum‐based monomers including biode-gradability and low cost.
Recently biobased, thermosetting polymers from vegeta-ble oils such as soybean oil, linseed, castor oil, and palm oilhave been synthesized and investigated [1–4]. A relatively
�These authors contributed equally to this work.
Correspondence: Professor Horia Iovu, Advanced Polymer MaterialsGroup, Department of Bioresources and Polymer Science, UniversityPolitehnica of Bucharest, 149 Calea Victoriei, Bucharest 010072, RomaniaE‐mail: [email protected]: þ4021 311 17 96
new raw material of vegetable origin used in the industry offats and oils is camelina oil (CO) obtained by cold pressing ofthe oilseeds of Camelina sativa, a member of the Brassicaceae(mustard) family. In the recent years, CO received a great dealof attention as low‐cost feedstock for biolubricant andbiodiesel production for commercial and military aircrafts[5–8]. With an iodine value of 133–153g I2/100g, CO is asicative oil being subject to severe drying on contact withatmospheric oxygen. The fatty acids in CO are primarilyunsaturated (80–90%) with high content of omega‐3 andomega‐6 fatty acids, whichmakes it an excellent choice either asedible oil or in pharmaceutical and cosmetics industry [9–11].An advantage of CO against to the more investigated soybeanand linseed oils is both a convenient degree of unsaturation anda relatively high content in mono‐unsaturated acids, which areeasily to be functionalized in comparison with the polyunsatu-rated acids that exhibit a limited access to their reactivesites [12]. Thus, from the point of view of macromolecularchemistry, CO is a polyfunctional monomer that can beconverted to high‐molecular weight products through theintroduction of more reactive functional groups.
In the recent years, polymers reinforced with well‐definednanosized inorganic Si–O clusters/core have attracted atremendous amount of interest because of their versatility;among these systems polyhedral oligomeric silsesquioxanes(POSS) compounds, which possess unique cage‐like structureand nanoscale dimensions, are of particular interest [13, 14].Recently, our research team published several studies concern-ing the influence of organic substituents grafted onto the POSScage on the properties of some innovative systems [15–18].
In this context, our aim was to formulate innovativecopolymer networks by VIS‐initiated free‐radical polymeri-zation of modified CO with different types of poly(ethyleneglycol) (PEG) co‐monomers of various spacer arm lengthsactivated at both ends with specific functional groups(methacrylate). Thus methacrylated CO was firstly synthe-sized by grafting new polymerizable moieties on the longaliphatic CO chains according to several experimentalprocedures described in literature for other types of plantsoils [19–21]. As far as we know, these modifications have notbeen reported elsewhere for CO. The structure of methacry-lated CO was proved using 1H NMR and FTIR. The newlysynthesized oil‐based monomer was subsequently combinedwith different dimethacrylated‐terminated PEG derivativesand subjected further to photopolymerization to producehighly crosslinked polymer networks.
Furthermore, to develop novel photo‐cured hybridmaterials with enhanced performances, POSS containingmethacrylic groups were loaded within bio‐based polymericmatrix in order to achieve multiple crosslinking points withinthe polymeric host. For the current study, two types of POSSwere selected: octafunctional POSS substituted with eightmethacrylate groups (POSS8) and monofunctional POSS(POSS1) with one methacrylate group and heptasubstitutedwith isobutyl unreactive groups. POSS compounds may be
used both as reinforcing agents due to the Si–O–Si nanocagesand as co‐monomer because of the polymerizable bearinggroups. The unreactive substituents may improve thecompatibility of POSS molecules with the organic matrix.The resulting materials with various crosslinking density andtheir POSS‐based nanocomposites were investigated in termsof wettability (contact angle and water uptake), structural(FTIR), mechanical (compression tests), thermal (DMA andTGA) and morphological (SEM) features.
To the best of our knowledge, this is the first example ofVIS photo‐cured hybrid materials synthesized by incorpo-ration of POSS compounds within an organic host based onfunctionalized CO and PEG hydrophilic derivatives. Thenovel photo‐cured hybrid materials developed in the currentstudy with good hydrophilicity and mechanical propertiesmay lead to apply them as biomaterials in many biologicalapplications.
2 Materials and methods
2.1 Materials
CO extracted in a cold‐pressing process was kindly suppliedby University of Agricultural Sciences and VeterinaryMedicine Bucharest. Triethyleneglycol dimethacrylate(PEG300 with MW¼300 g/mol; n¼ 3), PEG dimethacrylate(PEG550 with MW¼ 550 g/mol; repetitive units n� 9) andrespectively PEG750 (with MW¼ 750 g/mol; n� 14) werepurchased from Sigma–Aldrich. The dimethacrylate mono-mers were used as received, without further purification. Twodistinct POSS derivatives commercially available wereselected for this study and received from Sigma–AldrichChemicals: POSS8 (octamethacryl‐POSS cage mixture) andPOSS1 (POSS‐(1‐propylmethacrylate)‐heptaisobutyl substi-tuted. The chemical structures of the used POSS compoundsare presented in Fig. 1.
The photoinitiating system consisting of camphorquinone(CQ) and ethyl‐4‐dimethylaminobenzoate (EDMAB) wasalso purchased from Sigma–Aldrich. All other solvents andreagents were supplied by Sigma–Aldrich and used asreceived. The samples were irradiated with a halogen lightcuring lamp (CE Smaco model SLC‐III). This sourceconsisted of a 75‐W halogen reflector bulb, which emitsradiation between 380 and 510nm (visible light) and has themaximum peak at 470 nm.
2.2 Characterization
2.2.1 Nuclear magnetic resonance analysis
1H NMR spectra were recorded on a Bruker Avance DRX400 spectrometer, operating to 400.13MHz for the 1Hnucleus, equipped with a direct detection four nuclei probe‐head and field gradients on z‐axis. Samples were analyzed in
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5mm NMR tubes (Wilmad 507). The NMR samples wereprepared by dissolving 0.5mL oil in 0.5mL CDCl3. Thechemical shifts are reported in ppm, using the tetramethylsi-lane (TMS) as internal standard.
2.2.2 Gas chromatography–mass spectroscopyanalysis (GC–MS)
Gas‐chromatograms of the FAMEmixtures were recorded onan Agilent Technologies 6890N instrument with FID.Separation into components was done on a capillary columnspecially designed for the FAME analysis (Supelco SPTM2560: 100m length, 0.25mm inner diameter, and 0.2mm filmthickness). Fatty acids identification was made by comparingthe retention time for each peak with a commercially availablestandard (mixture of 37 FAME provided by SupelcoTM). Thecalibration of the signals was made by taking into account theconcentration of each component of the standard mixture,correlated with the detector response.
2.2.3 FTIR spectra
Fourier transform IR (FTIR) spectra were registered on aVertex 70 Brucker FTIR spectrometer equipped with anattenuated total reflectance (ATR) accessory in order todetermine the chemical structure for the studied specimens.All FTIR measurements were performed in the ATR‐FTIRcell on Ge crystal, at RT. The FTIR spectra were recordedusing 32 scans in 600–4000 cm�1 wave number region.
2.2.4 Gel fraction analysis (GF)
The total weight (solþ gel), w0, was firstly measured and theneach cured sample was Soxhlet extracted with THF for 24 h todetermine the insoluble part in the samples. The remaininggels were completely dried in a vacuum oven at 40°C for 24 h,and their masses (w1) were recorded.
After extraction, the gel fraction percentages (GF %) inthe irradiated samples were calculated using Eq. (1):
GF% ¼ w1
w0� 100 ð1Þ
where w0 and w1 represent the initial weights of irradiatedsamples (before Soxhlet extraction) and the insoluble part(after Soxhlet extraction).
The experiment was performed in triplicate. After extrac-tion, no shape deformation was observed for any specimen.
2.2.5 Static contact angle (CA)
Static CA values were visually determined at RT using a KSVCAM200 apparatus. Ultrapure water droplets were used witha drop volume of approximately 20mL. The measurement ofeach CA was done within 10 s of the drop contact with thesurface. The contact angles reported represent the mean ofthree determinations. Smaller contact angles correspond toincreased wettability.
2.2.6 Water uptake behavior
Water swelling extents were obtained by immersing each ofthe dried samples (10� 10mm2, thickness of about 2mm) ina large excess of double distilled water (ddw) at ambienttemperature. At predefined periods of time t, the sampleswere taken from water, the excess water was gently removedby blotting the surfaces of the samples and the weights of theswollen samples were then recorded. After measuring,samples were reimmersed in ddw.
The swelling degree (SD) was determined according tothe Eq. (2) [22, 23]:
SD ð%Þ ¼ wt � w0
w0� 100 ð2Þ
where wt and w0 represent the weights of swollen and driedstate samples, respectively.
The maximum swelling degree (MSD) was estimated asthe equilibrium value of SD. Each sample type was run intriplicates; the reported values are the mean values, and theassociated errors are the SDs.
2.2.7 Dynamic mechanical analysis
Thermo‐mechanical properties of the samples after curingwere measured using dynamic mechanical analysis (DMA).
Figure 1. Chemical structures of the POSS derivatives.
Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000 Methacrylated camelina oil, PEG derivatives and POSS compounds 3
Rectangular specimens with approximate dimensions of10mm wide, 2mm thick, 20mm long were tested on aTritec 2000 (Triton Technology) instrument operated insingle cantilever bending mode. The samples were tested at0.316Hz with displacement amplitude of 0.05mm whileramping the temperature from �100 to 150°C or 250°Cdepending on the sample at a heating rate of 5°C/min. Threedifferent ramp experiments were run for each sample.
2.2.8 Mechanical properties
The compression mechanical tests were done on a universalmechanical tester (Instron, Model 3382, USA). The testspeed was 0.5mm/min on dry samples at RT and relativehumidity was 45–50%. Rectangle samples of 5mm� 5mm� 2mm were prepared. A minimum of three specimens weretested for each sample and the average values of compressivestress (MPa) at 20% strain are reported.
2.2.9 Thermogravimetric analysis
The thermogravimetric analysis (TGA) results wereachieved on a Q500 TA instrument. A sample of about2.5mg was placed in a platinum crucible and heated from20 to 600°C at a heating rate of 10°C/min under aconstant N2 flow rate (balance flow 10mL/min, oven flow90mL/min).
2.2.10 Scanning electron microscopy
Information on the morphological structure of the POSS‐based specimens was expected from scanning electronmicroscopy (SEM) using a Quanta Inspect F SEM deviceequipped with a field emission gun (FEG) with a resolution of1.2 nm. The specimens were firstly immersed in liquidnitrogen and then manually broken allowing cross‐sectionalanalysis. Prior to imaging the samples were sputtered with athin layer of gold to enhance the surface conductivity.
2.3 Synthesis
2.3.1 Synthesis of epoxidized camelina oil (ECO)
Prior to functionalization, a detailed characterization ofthe crude vegetable CO was performed using 1H NMR andFTIR spectroscopy and subsequently the oil was chemicallymodified through a two‐step reaction.
1H NMR (CDCl3, TMS, d in ppm): 5.22–5.45(m, CH–O–CO, glycerol b‐psn., –HC––CH– from unsatu-rated acids), 4.1–4.3 (m, CH2–O–CO, glycerol, a‐psn.),2.8 (–CH2 between two double bonds), 2.35 (m,CH2–COO acyl group), 2.04 (m, –CH2–CH––CH2), 1.60(m, CH2–CH2–COO), 1.29–1.25 (m, CH2, all alkyl chains),0.97 (t, terminal CH3 from linolenic acid), 0.88 (t, terminalCH3 from all fatty acids except linolenic acid).
).Based on fatty acids composition preliminary determined
by 1H NMR and GC–MS, the average molecular weight andnumber of double bonds of the CO was calculated. Theaverage molecular weight of CO is M¼ 875 g/mol and theratio of double bond is 5.1mol C––C/mol CO.
The oil modification was performed according to aprocedure described in the literature for other types of oil(linseed, castor, and soybean oil) with minor modifica-tions [19–21, 24]. CO was epoxidized with H2O2/glacialacetic acid, with toluene as diluent for the organic phase and50% H2SO4 as catalyst, using a molar ratio H2O2/glacialacetic acid/CO unsaturation of 10/2/1. H2O2 30% was addeddropwise in themixture of oil, toluene, and acetic acid, kept atRT. The temperature was raised up to 60°C along withconstant magnetic stirring; the reaction was allowed to occurfor 22 h. The oily phase was separated and washed severaltimes with water and then with saturated NaHCO3 solution,until a neutral pH was attained. The obtained product (ECO)was further washed with saturated NaCl solution and driedover MgSO4. Finally, the solvent was removed undervacuum. The ECO product was obtained in 92% yield as alight yellow liquid.
1H NMR (CDCl3, TMS, d in ppm): 5.2–5.5(m, CH–OCO, glycerol b‐psn., unreacted –HC––CH– fromunsaturated acids), 4.1–4.3 (m, CH2–O–CO, glycerol,a‐psn.), 3.18 (m, CH, internal protons of epoxy ring),2.99 (m, CH, marginal protons of epoxi ring), 2.35(m, CH2–COO acyl group), 1.78 (m, CH2 between twoepoxi ring), 1.69–1.42 (m, CH2–CH2–COO), 1.29–1.25(m, CH2, all alkyl chains), 1.05–0.97 (m, terminal CH3 fromlinolenic acid), 0.88 (t, terminal CH3 from all fatty acidsexcept linolenic acid).
FTIR (ATR, cm�1): 2925, 2857 (nC–H asim, sim.); 1742(nC––O); 1453, 1376 (dCH from CH2 and CH3); 1236, 1158,1105 (nC–O); 830 (nC–O–C from epoxi ring); 728 (rCH2
).The epoxidation degree was calculated to be 0.932 from
the 1H NMR spectrum, using the integrals for the residualsignals assigned to the vinyl protons unmodified by epoxida-tion reaction (d¼ 5.2–5.5 ppm) and the peaks for the glycerolbackbone as internal standard (d¼4.16 ppm, dd, 2H andd¼ 4.28 ppm, dd, 2H).
2.3.2 Synthesis of methacrylated camelina oil (MCO)
ECO was mixed in a stoichiometric amount with methacrylicacid (MA) in order to open totally the newly introducedoxirane rings. Experimental protocols described in literaturefor soybean oil [19–21] were slightly adjusted and developedfor CO. Hydroquinone was added as a free‐radical inhibitorin a fixed amount (0.07wt%), while as a catalyst 10wt%triethylamine (TEA) was used. Reaction mixture was heated
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at 75°C, under nitrogen and kept with constant stirring 5 h.The reaction product was purified using 0.1NHCl to removethe catalyst. The product was extracted in diethylether toremove freeMA and then the final productMCOwas washedwith water and saturated NaHCO3 solution. Finally, themodified oil was dried under vacuum to yield a slightlyviscous oily liquid (h¼ 96%). The methodology described inthis work was carried out several times to gain a goodreproductibility. It was good agreement between experimen-tal and theoretical values of methacrylate content of MCO,which indicated that the selected synthetic conditions wereappropriate.
1H NMR (CDCl3, TMS, d in ppm): 6.25, 5.68 (s, CH2––
FTIR (ATR, cm�1): 2925, 2857 (nC–H asim, sim.); 1743,1708 (nC––O); 1633 (nC––C from methacrylic group); 1453,1376 (dCH from CH2 and CH3); 1301, 1009, 946, 820 (dC–H
and d––C–H); 1165, 1104 (nC–O); 728 (rCH2).
2.3.3 Polymerization procedure
Copolymerization reactions were performed with methacry-lated CO and dimethacrylated ethylene glycol derivatives.Three distinct ethylene glycol derivatives of defined lengthfunctionalized with methacrylate moieties (PEG300,PEG550, and PEG750) were further used in the copolymeri-zation reaction with MCO previously synthesized. THF wasemployed as reaction medium to ensure a good miscibility ofthe two components. Firstly, the required amount of ethyleneglycol derivative was dissolved in THF into a round‐bottomflask equipped with a magnetic stirrer and a reflux condenserand kept at 60°C and subsequently a fixed amount ofMCO inTHF was added dropwise. The temperature was then raisedto 65°C for 30min. Finally, the solvent was removed byvacuum evaporation.
The copolymers of MCO with PEG derivativeco‐monomer in THF were synthesized using a 1:2.5molar ratio (approximately 2:1 monomer double bonds/difunctional PEG double bonds). Table 1 summarizes thefeed composition for the synthesized formulations. Thenetworks were prepared by dissolving the photoinitiatorsystem (CQ 0.2% mass fraction and EDMAB 0.8% massfraction, respectively) into the co‐monomer mixtures.
The obtained homogeneous mixtures were poured slowlyinto a clean Teflon mold (10mm wide, 2mm thick, and20mm long) that was covered with a microscope slide (about1mm thick) in order to overcome the inhibiting effect ofoxygen and to obtain a smooth surface. The VIS‐radiationsource of the lamp was fixed on the surface of the glass slide in
order to avoid the decrease of the intensity as the distancefrom the source increases. Subsequently, the systems wereirradiated with VIS‐light for 4min. Crosslinked materialswere stored at 5°C prior to characterization. Photopolymer-ized neat MCO (without PEG) was also obtained and used asa reference.
2.3.4 Synthesis of POSS reinforced nanocomposites
A certain amount (5wt% of total co‐monomers mixture) ofdifferent polymerizable POSS compounds bearing one(POSS1) or eight methacrylic groups (POSS8) was used toreinforce the organic matrix and to obtain polymer/POSSnanocomposites. In order to disperse the inorganic com-pounds within the organic host, the mixture was sonicated for30min using an ultrasound bath. No obvious phase‐separation was observed. CQ and EDMAB were furtheradded in the POSS/organic mixture and the obtainedformulations were subjected to photocuring reaction.
The main steps of the protocol and also the chemicalreactions involved are illustrated in Fig. 2. Curing of theinnovative formulations produced from flexible to toughertranslucent materials.
3 Results and discussion
Structural and compositional data for the starting material,CO, were obtained using 1H NMR and GC–MS techniques.CO FAME composition was established by GC–MS for theFAME mixture obtained by oil transesterification, usingNaOH–MeOH complex as catalyst, according to literaturedata [19], using appropriate capillary column and acommercially available FAME mixture standard of 37FAME. The results are presented into the Table 2.
Structural information was obtained using a set ofchemometric equations described in the literature [25, 26]that use the integrals values and signal intensities from the 1HNMR spectrum of CO. The results are in good agreementwith GC–MS data: tri‐unsaturated (linolenic acid: 31%), di‐unsaturated (linoleic acid: 21%), mono‐unsaturated (39%),and saturated (9%) fatty acids.
Table 1. The synthesized samples of various compositions
The synthesis of vegetable oil‐based monomer (MCO)involve a two‐step reaction, where the first stage transformsthe unsaturation sites of CO into epoxy groups by H2O2/glacial acetic acid and finally producing MCO via epoxy ringopening with methacrylic acid.
Schematic representations of the used chemical pathwaysare outlined in Fig. 3.
The functionalization procedures were followed using 1HNMR and FTIR spectroscopy. In the overlapped 1H NMRspectra (Fig. 4) the specific signals of the CO, ECO, andMCO were assigned. CO sample shows the specific peakrelated to the protons of double bonds CH––CH from the fattyacid chains at 5.3 ppm. After epoxidation, the peaks assignedto the protons from CH––CH double bonds strongly diminishand new peaks related to the oxirane group appear at 2.9 and
3.1 ppm assigned to –CH internal andmarginal protons of theepoxy ring, 1.7 ppm to –CH2 protons between two epoxyrings. The peaks from 5.6 and 6.2 ppm represent two protonsfrom –CH2 of the methacrylate group after methacrylationprocess occurs. Also, another peak was found at 1.9 ppmcorresponding to the –CH3 from the methacrylic groups.
To confirm the NMR findings, Fig. 5a–c shows the FTIRspectra of functionalized oil (epoxidized and methacrylated,respectively) with respect to unmodified oil. The bandsobserved at 3010 and 1654 cm�1 confirm the presence of highproportion of PUFA (especially linolenic and linoleic acids).After the first step of reaction (epoxidation), the spectralchanges were followed and none of these peaks was detected,which confirms the modification of double bonds fromunsaturated fatty acids. Moreover, a representative band
Figure 2. Chemical reactions involved to produce oil‐based hybrids with PEG derivatives and reinforced with POSS. Inset: photo‐crosslinked structure formed between MCO and bi‐functional PEG.
appears at 833 cm�1 assigned to C–O–C stretching vibrationfrom oxirane ring. The 1232 cm�1 band, also specific forepoxy rings usually is overlayed with nC–O vibration fromesters [27]. Compared to ECO,MCO spectrum contains newspecific absorption bands at 1634 and 946 cm�1 that could beassigned to nC––C and dC–H frommethacrylic groups [28]. ThenC––O stretching vibration is typical of the ester/acid carbonylgroups at �1747 cm�1.
Changes in vibrational mode amplitudes are also moni-tored by measuring the absorption bands intensities in FTIRspectra after irradiation (Fig. 5d). The photopolymerizationof methacrylate groups is clearly confirmed in the irradiatedsamples: photo‐sensitive absorption bands centered at 1634and 946 cm�1 significantly reduce their intensities. A smallamount of methacrylate groups may remain unreacted afterphotopolymerization due to some residual monomers thatcould be entrapped in the formed three‐dimensional polymermatrix.
3.1 Gel fraction determination
The amount of GF (%) was estimated through extraction bymeasuring the insoluble fractions, such as crosslinked ornetwork polymers in order to check the efficiency of thenetwork formation. The GF values of oil‐based systems as afunction of PEG chain length were calculated based ongravimetric measurements and using Eq. (1). The resultsindicated that GF values are ranging from 30� 1% for MCOself‐crosslinked to 53� 2% for MCO‐PEG300, 62� 3%
Figure 3. Idealized structure for CO (a) and its derivatives:epoxidized (b), and methacrylated (c) CO, respectively (includingthree distinct fatty acids as linolenic, linoleic, and oleic derivatives).
Figure 4. 1H NMR spectra of (a) CO, (b) ECO, and (c) MCO.
Figure 5. FTIR spectra of raw material CO (a), ECO (b), MCO (c), and corresponding photo‐cured MCO (d).
Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000 Methacrylated camelina oil, PEG derivatives and POSS compounds 7
for MCO‐PEG550, and 70� 4% for MCO‐PEG750,respectively. GF sharply increased when PEG reagent wasused due to the fact that the formation of the network isassisted by the PEG chains. The highest GF (%) was obtainedin case of PEG750 with long flexible spacer arm, whichenhances the overall molecular mobility of the oil‐basedsystem. The results obtained from the GF (%) confirm thatemployment of PEG as co‐monomer helps to the success ofthe network formation through the described synthesisprocedure. When long chain monomer is used in the reactionwith the oil‐based monomers the formation of a tightly cross‐linked network is disturbed and therefore higher GF wasachieved. This could be due to an enhanced molecularmobility that favors higher conversion as Bowman previouslyreported [29]. Hence, the amount of remaining C––C bondsdecreased with increasing number of ethylene glycol units inthe PEG co‐monomer. As a result, the studied samplesexhibit different chemical conversions ofmethacrylic moietiesprobably due to different viscosity, mobility, and chain lengthof di‐functional PEG. Also, Scherzer [30] reported that thepenetration degree of light radiations strongly increases as thechain length of monomer increases.
3.2 Surface hydrophilicity and water uptake behavior
To determine the “wettability” of the newly obtained oil‐based networks, static contact angle (CA)measurements, andwater swelling extents (according Eq. 2) were employed.Figure 6 shows the values of the CA of water on oil‐basednetworks surfaces and the obtained MSD after 48 h ofsamples immersed in water. The error bars represent the SDof three measurements.
The obtained results indicate that the CA values slightlydecrease with the increasing of PEGmolecular weight used inthe reaction with the functionalized oil‐based monomer.Contact angles were found to be 70� 1.9° for MCO (self‐crosslinked), 66� 1.8° for MCO‐PEG300, 57� 1.2° forMCO‐PEG550 and respectively 49� 0.8° for MCO‐
PEG750. These data show that the addition of PEGs
determines a moderate increase of the material hydrophilicitybecause of its hydrophilic polymer main chain.
Figure 6 suggests that MCO homopolymer is the mosthydrophobic material among all the synthesized networks, itsswelling capacity being very low. The other three systemsexhibit considerably higher capacity to absorb water incomparisonwithMCOwithout PEG, being dependent on thetype of PEG used in the initial polymerization mixture. Thus,the association of oil‐based monomer with an ethylene glycoldimethacrylate co‐monomer extends the swelling of thematerials.
These results followed the expectation that the hydrophi-licity would increase as the spacer arm length of PEGincreases. Additionally, the data reported in this sectionprovided an indication that water‐soluble polymer is presentboth at the surface and inside of the materials. This may beattributed to the distribution of the hydrophilic domains,that is, the PEG chains, which may easily distributed out orwithin polymeric matrix. The percentage of water absorptionand CA values do not suffer significant changes uponreinforcement with POSS compounds. When POSS withmethacrylate moieties was incorporated, the crosslinkingdensity of the synthesized networks increases due to theformation of multiple crosslinking points. This behavior leadsto the decrease of the water uptake capacity and consequentlyto higher values for water CA especially when POSS8 wasused (around 2–4° above the corresponding unreinforcedmatrix). The most significant influence on the POSSreinforced systems could be observed in the case of MCO‐
PEG750_POSS8. Due to its long chain, PEG750 leads to alower crosslinking density in the formed oil‐based networkand therefore POSS incorporation helps to create a morecompact material. Hence,MSD decreases with 3%, while CAincreases to 54�0.6°.
3.3 Thermo‐mechanical properties
In order to obtain additional relevant information concerningthe methacrylated oil/PEG hybrids, thermo‐mechanicalproperties by DMA and compression tests, respectively,and also thermal degradation using TGA were furtherevaluated.
The influence of the flexible PEG chain length and also ofthe POSS type on the thermal and mechanical properties ofthe obtained systems were investigated and the data aresummarized in Table 3.
To experimentally confirm the structural changes thataccompanied variations both in the length of the PEG chainused in the copolymerization reaction with oil‐basedmonomer MCO as well as the type of POSS nanostructuredcompounds, the dynamic mechanical behavior was furtherconsidered (Fig. 7). All the specimens were run in DMA tostudy the glass transition temperature (Tg) and moduli fordetermining if a single‐phase morphology occurs in oil‐basedPOSS nanocomposites.
Figure 6. Maximum swelling degree (MSD) at 48 h and watercontact angles for the oil‐based networks.
8 B. Balanuca et al. Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000
Figure 7a depicts the tan d and storage modulus (E0) as afunction of temperature starting from �100 to 200°C for thePEG/oil‐based copolymers. It can be observed that the usageof PEG with shorter chain length (PEG300) in thecombination with oil‐based monomer leads to a Tg of about113°C. If PEGwith higher chain length (PEG750) is used as anetwork former within the vegetable oil‐based matrix, the Tg
values exhibit a significant decrease below RT up to 4°C. Thisbehavior could be easily explained through the flexible PEGchain length. Besides the shift of maxima, an importantdecrease in height and a broadening of the transitiontemperature were observed if PEG with lower molecularmass was used. This suggests higher limitations on freedom ofchain mobility in the samples due to an increased crosslinkingdensity [31]. Also, the DMA curves for the samples includinglower PEG length exhibit another mechanical relaxation
process at low temperatures (approximately�50°C) assignedto secondary (or b) relaxation associated with side chainmotions.
To describe the dynamic mechanical behavior of the oil‐based networks copolymerized with PEG derivatives, visco-elastic measurements were employed further using storagemodulus (E0) as an indicative of the elastic properties. Theelastic modulus E0 depends primarily on the tightness of thenetwork structure. Moreover, E0 at temperature above Tg is agood empirical method of characterizing cross‐linked materi-als. The tan d versus temperature curves previously presentedshowed that samples become less rigid as PEG with longerchain was used in reaction with MCO. This fact is furtherconfirmed when storage moduli in the rubbery region arecompared. Due to the fact that the oil‐based networks containthe same weight percentage of PEG derivative, increasing the
Table 3. Thermal and mechanical properties of oil‐based polymers and hybrid networks
No. Samples Tg (°C) Compressive stress (MPa) T5% loss (°C) Tmax (°C) Mass residues (%)
Tg¼ glass transition temperature (maximum of tan d); compressive stress at 20% strain; T5% loss¼ temperature of the 5% weight loss;Tmax¼ temperature of the maximum weight loss rate; mass residues at 600°C.
Figure 7. The dependence of tan d and storage modulus (E0) versus temperature for (a) oil‐based/PEG copolymers; (b) MCO‐PEG750system reinforced with POSS.
Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000 Methacrylated camelina oil, PEG derivatives and POSS compounds 9
molecular weight of PEG‐DMA leads to lower crosslinkdensities and lower elastic storage moduli in the rubberyregion.
Concerning the dynamic mechanical properties of thePOSS reinforced systems, it was noticed that monofunctionalPOSS does not significantly affect the Tg values. On theopposite if octafunctional POSS is loaded the Tg increaseswith 10–12°C in comparison with unreinforced systems(Table 3; Fig. 7b – representative example forMCO‐PEG750system reinforced with two types of POSS). An improvementof the storage moduli in the rubbery region was also observedin comparison with the unreinforced network. This behavioris probably due to the formation of a highly crosslinkedsystem, which increases the stiffness of the network structure.The reinforcing effect of the POSS was thus confirmed andalso a good interfacial adhesion between polymeric matrixand POSS compounds was obtained in the tested composites.
Due to the fact that crosslinking density is one of the mostimportant structural parameters, which control the physicalproperties of cured material, the influence of the flexiblePEG dimethacrylate chain length as well as the effect ofreinforcement with different types of POSS on themechanicalproperties of the synthesized materials were also investigatedby compression tests (Table 3) in the following section.
After the mechanical tests were carried out, it was noticedthat the maximum value for the compressive stress wasobtained for MCO‐PEG300 (14.1� 1.87MPa), the follow-ing value (1.76�0.54MPa) corresponding to MCO‐
PEG550 samples, while the lowest values (0.43� 0.19MPa)are for the samples copolymerized with long chain PEG750.These results indicated that the increase of PEG molecularmass leads to more soft and flexible materials while theintroduction of short linear chains, PEG300, into the oil‐based polymer makes the network stiffer. To conclude,significant changes in the mechanical behavior of oil‐basedmaterials may be generated by minor modification of thechain length of the PEG co‐monomer.
The thermal stability of the synthesized materials wasevaluated by TGA under nitrogen atmosphere and theobtained data are listed in Table 3. All the thermogramsshowed similar profiles (Fig. 8).
The thermogravimetric plots showed that all the materialshave good thermal stabilities. The onset of degradation (5%weight loss) is distinctly visible after 100–150°C. If PEG with750 g/mol molecular weight was used the thermostability ofthe obtained systems significantly increases (�330°C) morethan double compared to the MCO‐PEG300 (�140°C).
Due to the fact that the selected POSS compounds exhibitquite different thermal stabilities (T5% loss¼ 295°C for POSS1and T5% loss¼ 395°C for POSS8, respectively), the POSSincorporation within polymeric matrices leads to distinctthermostabilities of the reinforced systems. Thus, theincorporation of POSS8, into the oil‐based polymericnetworks leads to the increase of thermostability due tomultiple crosslinking points formed by the octafunctional
POSS with the methacrylate chains from the matrix. On thecontrary, if POSS1 is incorporated the obtained hybridsdecompose at slightly lower temperatures. Minor perturba-tions were noted on the onset temperature (5%) regarding theoil‐based networks formed in the reaction with PEG300 andreinforced with POSS1. The 5% weight loss temperature ofMCO‐PEG750_POSS1 was 238°C, which is considerablylower than those of the MCO‐PEG750 (332°C) probablyto the agglomeration of POSS1 molecules, which do notestablish too many bonds with the matrix.
Also, the TGA results confirmed that the char yield ofoctafunctional POSS‐based nanocomposites increased incomparison with their neat matrices. The incorporationof monofunctional POSS into the organic polymer matrixleads to lower residual mass in comparison withmatrices reinforced with octafunctionalized POSS. Thisbehavior can be explained through the fact that the usedPOSS compounds functionalized with eight methacrylategroups are crosslinked in the vegetable oil matrix at amolecular level rather than POSS1 with single polymerizablemoiety.
The DTG curve of the oil‐based polymers shows one‐stepdegradation that exhibits a broad peak at around 400°C.If PEG derivatives with high‐molecular weight are employed,the maximum decomposition rate is shifted to highertemperatures (406°C for MCO‐PEG550 and 416°C forMCO‐PEG750, respectively).
SEM provided valuable information about morphologyof the obtained oil‐based systems reinforced with POSS.For comparison, SEM images of neat networks MCO,MCO‐PEG300, MCO‐PEG550, and MCO‐PEG750 arealso shown (Fig. 9). To examine the morphology amagnification of 20 000� was chosen.
It is clearly seen that the most important factors, whichinfluence the morphology of the samples are both the type ofsynthetic co‐monomer and also the type of POSS nano-particles that are loaded.
Figure 8. (a) TGA and (b) DTG plots (10°C/min) of PEG/oil‐basedcopolymers.
10 B. Balanuca et al. Eur. J. Lipid Sci. Technol. 2014, 116, 0000–0000
SEM micrographs revealed that the morphology of theMCO was smooth and dense, with no evidence of phaseseparation if POSS was loaded. If PEG compounds withdifferent chain length were added the morphology seems toshow a compact structure mostly homogenous but withisolated domains of less‐ordered structures. The influence ofPOSS components on MCO‐PEG_POSS structures issignificantly pointed out by the SEM images. Thus, thePOSS compound with high number of methacrylic groupswill lead to a great homogeneity of the structures (MCO‐
PEG750_POSS8), which is less evidenced for POSS withonly one methacrylic groups. This is probably due to thecontribution of the POSS methacrylic moieties to the overallprocess of polymerization, therefore allowing more bonds tobe achieved between MCO, PEG, and POSS components.
4 Conclusions
CO was epoxidized and further methacrylated (MCO) inorder to be suitable for a copolymerization reaction withmethacrylated PEGs. Thus, various systems were successfullyprepared and characterized in this work using VIS radiation.
The curing reaction was demonstrated by FTIR spectros-copy, not all the monomer being consumed due to theentrapment of small monomer quantities within the obtainedpolymer network. The curing performance of the oil‐basedsystems was studied further by GF determination. The extentof curing was found to be strongly dependent on the numberof ethylene glycol units in the PEG co‐monomer.
The final products exhibit a significant increase of theswelling capacity and a high hydrophilicity due to the PEGchains. Greater is the crosslinking density lower will be theswelling rate. If the PEG/oil‐based network is furtherreinforced with POSS compounds, no important changesin the water uptake are observed.
The PEG chain length exhibits a significant influence onthe Tg of final networks. Thus, the use of a higher PEG chainlength leads to a quite low Tg value (�4°C); the overallmolecular mobility is increased and subsequently a depres-sion of the Tg was noticed. On the other hand, if the PEGchain length is lower (PEG300) the value of Tg becomeshigher due to the strong network formed, which hinders themobility of polymer chains. POSS compounds were used toreinforce the obtained oil‐based networks. The DMA resultsshowed that the incorporation of POSS compounds withinthe oil‐based matrix lead to an increase of the storagemodulus E0, which confirms the POSS reinforcement role.The number of methacrylic groups from the POSS cage aswell as the PEG chain length significantly influences themechanical properties, e.g., compression strength. Thus,lower PEG chain length is required to achieve highercompression strength for the final product.
The structure of POSS molecules used to reinforce theoil‐based products plays a key role for a good thermostability.Therefore, if POSS compounds bearing more methacrylicgroups are employed a higher thermostability is achieved tothe high number of strong covalent bonds formed betweenPOSS methacrylic groups and still unreacted methacrylicgroups from the polymer network.
The work has been funded by the Sectorial Operational ProgrammeHuman Resources Development of the Romanian Ministry ofLabour, Family and Social Protection.
The authors have declared no conflict of interest.
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