- Email: [email protected]
polycaprolactone blends to obtain shape memory fibers (SMF)
Accepted Manuscript Electrospinning of Cationically Polymerized Epoxy/Polycaprolactone Blends to Obtain Shape Memory Fibers (SMF) Alvaro Iregui, Lourdes Irusta, Oihane Llorente, Loli Martin, Tamara CalvoCorreas, Arantxa Eceiza, Alba González PII: DOI: Reference:
S0014-3057(17)30435-4 http://dx.doi.org/10.1016/j.eurpolymj.2017.07.026 EPJ 7979
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
13 March 2017 14 July 2017 17 July 2017
Please cite this article as: Iregui, A., Irusta, L., Llorente, O., Martin, L., Calvo-Correas, T., Eceiza, A., González, A., Electrospinning of Cationically Polymerized Epoxy/Polycaprolactone Blends to Obtain Shape Memory Fibers (SMF), European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj.2017.07.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrospinning of Cationically Polymerized Epoxy/Polycaprolactone Blends to Obtain Shape Memory Fibers (SMF) Alvaro Iregui, Lourdes Irusta*, Oihane Llorente, Loli Martin, Tamara CalvoCorreas, Arantxa Eceiza, Alba González. This is dedicated to Prof. Iñaki Eguiazabal
––––––––– A. Iregui, Prof. L. Irusta, O. Llorente, Dr. A. González. POLYMAT, Department of Polymer Science and Technology, University of the Basque Country UPV-EHU, PO Box 1072, 20080. Donostia/San Sebastian, Spain. Dr. L. Martin. Macrobehaviour-Mesostructure-Nanotechnology SGIker Service, Polytechnic School, University of the Basque Country UPV-EHU, Plaza Europa 1, 20018. Donostia/San Sebastian, Spain. T. Calvo-Correas, Prof. A. Eceiza. Group “Materials+Technologies”, Department of Chemical and Environment Engineering, University of the Basque Country UPVEHU, Plaza Europa 1, 20018. Donostia/San Sebastian, Spain. E-mail: [email protected] ––––––––– Abstract: A mat of fibers with shape memory effect is obtained using commercially available components and a simple electrospinning process. For obtaining this goal, a blend of diglycidyl ether of bisphenol A (DGEBA) and Polycaprolactone (PCL) is electrospun and the obtained mats are cured by UV radiation, avoiding the melting of the PCL component. The cationic photoinitiator based on iodonium salts, used for the first time in the electrospinning process, increases the solution conductivity and consequently the spinnability of the blend. The obtained mats show shape memory properties through several cycles, with shape fixity ratios that range from 95 to 99% and shape recovery ratios of between 88 and 100% respectively. Keywords: shape memory polymers, solution electrospinning, photopolymerization, smart materials, UV curing reactions
cationic
1. Introduction Shape memory polymers (SMP) are materials capable of reacting to external stimuli such as temperature, magnetic or electric fields and solvents, to return from a pre-programmed shape to their original form [1]. The SMPs most studied are those that can be preset using temperature as external stimuli [2–4]. Among the techniques used to process SMPs, electrospinning has recently attracted most attention as a tool for obtaining mats that could be applied in biomedical materials as scaffolds for tissue engineering or drug delivery [5]. Electrospun mats have shown an enhanced shape memory effect (SME) [6,7], and electrospinning is also a way of obtaining shape memory composites [8,9]. Highly oriented fibers can be recollected and, after being immersed in a proper resin, an anisotropic material with shape memory properties is obtained [10]. SMPs have also been obtained by electrospinning two polymers (dual-spun) simultaneously [11]. Polycaprolactone (PCL) based shape memory polymer blends have been the focus of recent research. These materials show sharp and rapid shape memory-recovery behavior as a consequence of the melting/recrystallization of the PCL component as switching transition [12–14]. Research has also been carried out into PCL based electrospun fibers. In literature it is possible to find these types of materials obtained by imbibing electrospun PCL fibers into an epoxy resin using amines as thermal curing agents [15] or by coaxial electrospinning of a solution of PCL and modified epoxy monomers [16]. The direct introduction of epoxy monomers in the electrospinning process is not possible due to their low molecular weight. Thus, the introduction of epoxy monomers in the fiber requires the use of a second component (this can be the high molecular weight PCL), and the curing process must be performed once the fibrous structure is formed. However, the curing process must be performed in mild conditions (low temperature) in order to avoid the PCL melting as it destroys the fibrous structure. In order to overcome this problem, the cationic photopolymerization of the epoxy is an interesting alternative [17,18]. In addition, the iodonium salts used as photo-initiators could greatly enhance electrospinning, favoring fibers over beads and smaller diameters [19]. Taking into account the advantages that cationic epoxy polymerization offers, our approach was to create, for the first time, a mat of nanofibers with SME using commercially available components and a simple spinning process. In the general process employed in this work, a blend of a semicrystalline thermoplastic with a photocurable monomer is used in a two-step process. The first step consists on the electrospinning process, and nanofibers are obtained due to the semicrystalline thermoplastic; the second step is the polymerization of the monomer to obtain a thermostable material in reaction conditions that maintain the morphology of the nanofibers.
In order to achieve our goal, a blend of diglycidyl ether of bisphenol A (DGEBA) and PCL was electrospun and the obtained mats were cured by UV radiation (avoiding the melting of the PCL component) thanks to the cationic initiators that were added to the solution and were used for the first time in electrospinning. The obtained mats present multiple future applications. As an example, they could be of potential use in the design of smart separation membranes, in which the change in pore size due to the recovery of the permanent shape changes the selective properties of the membrane [20].
2. Experimental Section 2.1 Materials Bisphenol A diglycidyl ether (DGEBA, Mw= 340.41 g/mol), 2,2-dimethoxy-2phenyl-acetophenone, bis(4-tert-butylphenyl)iodonium hexafluorophosphate, acetone and dimethylformamide (DMF) were purchased from Sigma Aldrich. Linear PCL (Mw= 45 000 g/mol) was purchased from Perstrop. All materials were used as received. 2.2 Electrospinning Process and UV Curing Two different 20 wt% solutions were prepared maintaining PCL/DGEBA 50/50 weight ratio but changing the solvent. For the first solution (named A), 2.97 g of PCL and 2.97 g of DGEBA were added to 30 ml of acetone. Then the solution was heated to 50 ºC and stirred for 4 hours. When the solution was homogeneous, 30 mg of bis(4-tert-butylphenyl)iodonium hexafluorophosphate (photoinitiator) and 30 mg of 2,2-dimethoxy-2-phenyl-acetophenone (sensitizer) were added and the solution was kept in darkness. The other solution (A3DMF1) was prepared under the same procedure but using an acetone/DMF 3/1 as a volume mixture. For the electrospinning process, polymer solutions were placed into a syringe that was mounted on a syringe pump (Cole-Parmer) and randomly oriented fibers were electrospun by applying a voltage of 9 kV to the needle using a high voltage source (0-30 kV, CZE1000R, Spellman High Voltage Electronics Corp.). The collector was a stainless steel sheet and was placed at 15 cm from the needle tip. The flow rate was established at 0.30 ml/h. The samples were collected during 8 hours, and in order to cure the desired mats, the samples were separated from the collector and cured for both sides for 1 hour using a UV LED lamp (ThorLabs M365LP1), with an UV intensity of 2 mW/cm2 at 365 nm. The morphology of the electrospun mats was analyzed using a Hitachi S2700 SEM at 15 kV accelerating voltage, and diameter distribution was measured on 4 different images for each sample using ImageJ software [21].
2.3 Characterization In order to characterize the materials and the mats, Fourier Transform Infrared Spectroscopy (Nicolet 6700 FTIR) equipped with a single reflection ATR system (Specac Golden Gate), and Differential Scanning Calorimetry experiments (DSC TA Instruments Q2000) were employed. All DSC experiments were conducted at a heat rate of 20 ºC/min in N2 atmosphere. The curing reaction was analyzed using a Photocalorimetry accessory (Omnicure S2000 with a 200W mercury lamp) at a temperature of 25 ºC with a UV intensity between 1 and 2 mW/cm2. 2.4 Shape Memory Effect SME was characterized using a Universal Testing Machine (MTS Insight 10) with a load cell of 10 kN and thermostated at 60 ºC. Two samples of each solution were cut into rectangular shapes (20x5 mm), and the initial distance between the grips was established in 10 mm, εi. The tests were conducted applying a deformation of 2 mm/min up to 40% of deformation, the programmed elongation, εp. After two minutes to acclimatise the material, it was quickly cooled using an aerosol coolant and the fixed elongation was measured, εf. Then the mats were immersed in an oil bath at 60 ºC, and the recovery elongation recorded, εr. This cycle was repeated 5 times for each mat. Shape fixity (Rf) and shape recovery (Rr) ratios were calculated for each cycle N according to Equation (1) and (2) [22]. (1) (2)
3. Results and Discussion 3.1 Electrospinning
Figure 1. SEM images of the electrospun mats from solutions in acetone before (a) and after (b) UV irradiation, and from solutions in acetone/DMF before (c) and after (d) UV irradiation. (e) SEM image of electrospun solution without iodonium salt. Figure 1a and b show the SEM images for the electrospun mats obtained from the solution in acetone before and after UV curing. The morphology consists of a network structure of bead-free fibers with diameters of around one micrometer. PCL is the only polymer present in the solution, and as entanglements are required in the process of electrospinning [23], it is fair to assume that is the only component capable of being electrospun, and the others (epoxy monomer, initiator and sensitizer) have to imbibe the fibers. The SEM images also show that differences between the mats before and after curing are not appreciable and the fibrous structure is maintained. Figure 1c and d show the SEM images in the case of electrospun mats obtained from the solution in acetone and DMF before and after UV curing. The fibers obtained with this mixture of solvents present less coalescence than those from acetone, similar to the results of other groups [24,25]. These mats also maintain their morphology after UV curing.
The iodonium salt employed as a photo-initiator has a crucial role in the process; when iodonium salt was not added to the solutions (Figure 1e), electrospray takes place, and mainly beads with a poorly defined morphology are obtained. This is in agreement with literature, where it is reported than a little amount of salt in electrospinning solutions improves the spinnability due to the increase in conductivity [19]. This way, the photo-initiator employed not only is crucial for achieving a UV curing of the epoxy monomer, but it also has a positive effect in the process of electrospinning. To our knowledge, this is the first time that iodonium salts for cationic photo-polymerization have been used in electrospinning.
Figure 2. Diameter distribution for electrospun mats from A and A3DMF1 solutions. The diameter of the fibers and its distribution was calculated for samples obtained using acetone and acetone/DMF mixtures (Figure 2). As can be seen, for both samples the diameter of the fibers does not present a normal distribution, which is in accordance with other literature data [26] where it is shown that the normal distribution is not always suitable for the evaluation of the mat structure. The mats were characterized by the mean fiber diameter and standard deviation. Diameters of 0.92 and 1.04 m with standard deviations of 0.46 and 0.37 m were achieved for the samples obtained in acetone and acetone/DMF mixtures respectively. It is well known that in electrospinning process the solution conductivity has a strong effect on the diameter of the fibers. However, in our case, both samples have similar conductivity values because the iodonium salt concentration is the same. However, the lower viscosity and lower surface tension of acetone can be in the origin of the lower fiber diameter showed by the fibers obtained using this solvent. In addition, the fibers obtained using the acetone/DMF mixture presented lower standard deviation. This effect is probably due to the lower volatility of the DMF that allows a more continuous process with lower fluctuation jet, resulting in fibers with narrower diameter distribution [27].
3.2 UV Curing of the Mats
Figure 3. ATR-FTIR spectra before and after UV irradiation for electrospun mats from acetone (a) and acetone/DMF (b). Analyses were conducted with the objective of determining the extension of the epoxy curing reaction. The infrared spectra (Figure 3a and b) of the mats were obtained before and after UV irradiation using an ATR accessory. The decrease of the absorbance of the 915 cm-1 band, corresponding to the bending of the epoxy group, and the increase of the absorbance of the band around 1100 cm-1 corresponding to the stretching of the C–O–C bond indicate that epoxy polymerization has occurred [28].
Figure 4. DSC thermograms of A samples before (a) and after (b) curing. SEM images of the non cured (c) and cured (d) A samples after the DSC scan.
DSC was also conducted to confirm the curing reaction and how it affects the crystallinity of the PCL, crucial in the shape memory effect. In the uncured mats (Figure 4a) three significant signals can be observed: at -21 ºC, assigned to the Tg of DGEBA, and at 34 ºC and 47 ºC two endothermic signals assigned to the Tm of the DGEBA and PCL respectively. In the case of the mats after UV irradiation (Figure 4b), the signals of the DGEBA monomer disappear as expected due to photo-curing, and only the endothermic peak of PCL melting is observed at 58 ºC. Regarding the crystallinity of the PCL, the value of Tm before UV curing (47 ºC) decreases from the value of pure PCL (around 56 ºC) indicating that DGEBA monomer in the blend is partially miscible with the PCL [29]. After the photocuring reaction, the melting point of the PCL returns to its base value as the polymerization of the epoxy induces a phase separation.
These mats studied in DSC were removed from the aluminum pans and observed by SEM. As can be seen in Figure 4c and d, the cured mats show few defects, confirming the UV curing reaction, whereas the uncured mats are melted and the integrity of the fibers is lost.
Figure 5. Conversion (a) and rate of polymerization (b) calculated from photoDSC. DSC scans for the photocured mats of samples A (c) and A3DMF1 (d).
Photocalorimetry studies were conducted in order to estimate the conversion of the curing reaction. Conversion and polymerization rate were calculated using the method described in the bibliography [30]. The results in Figure 5a and b show that conversion and rate of polymerization are very similar for both samples, and therefore it can be concluded that, as expected, the solvent employed in electrospinning has no effect in the photocuring reaction. In both cases, the conversion is around 25-30% after 45 minutes of irradiation. The polymerization rate reaches a maximum in the first few minutes and then decreases as the chains are immobilised due to the advance of the curing reaction [31]. In post-DSC experiments (Figure 5c and d), both samples present an exothermic signal in the first scan that can be correlated to a post curing reaction, as polymerization continues due to high temperatures [32]. This peak disappears in the second scan, supporting this assignment. These all results indicate that the epoxy polymerization that is achieved using UV curing at room temperature is not complete [33], but enough to maintain the integrity of the fibers when the temperature is higher than the Tm of the PCL.
3.3 Shape Memory Effect The results obtained for the shape memory effect are displayed in the Figure 6a and b. The shape fixity ratio ranges from 95 to 99%, and shape recovery ratio from 88 to 100%, maintaining the shape memory properties after five consecutive cycles. Fejos et al. studied SMP composites of electrospun PCL imbibed with epoxy resin, and obtained similar values [34]. This indicates that the method employed in this study make it possible to obtain a great shape memory effect maintaining the fibrous structure that is electrospun. There is no significant difference in the shape memory effect among the mats obtained from different solvents.
Figure 6. Shape fixity ratio (a) and recovery ratio (b) for A, A3DMF1 and casting film. For electrospun A3DMF1 mat, photographs of initial shape (c), fixed shape after folding the mat three times in half at 60 ºC (d) and recovery shape after heating again to 60 ºC (e). Figure 6c to e show photographs of a qualitative shape memory test, manifesting than these electrospun mats can adopt compact shapes and return to its extended form when needed, heating to 60 ºC. A video of the recovering process can be found in the supplementary material.
Figure 7. SEM images for A3DMF1 electrospun mat during a shape memory test in the (a) original shape, (b) temporary shape and (c) recovery shape.
Regarding the morphological stability of the nanofibers, in Figure 4d we have shown that it is maintained at temperatures higher than the PCL melting. In the shape memory process, not only temperature but also force is applied to the nanofibers, and therefore the morphology of the mats was analyzed for samples in the temporary shape and after the recovery process. As can be seen in Figure 7 the nanofibers show the same morphology during all the shape memory test. This means that the process is non-destructive. Employing a different semicrystalline polymer instead of PCL, this shape memory effect could be achieved at other temperature, allowing its applications in fields such as tissue engineering if body temperature is employed to recover the desired shape.
4. Conclusions Electrospun mats from PCL/DGEBA mixtures were obtained adding an iodonium salt to the solutions. This UV initiator not only was necessary for curing the DGEBA after electrospinning, but also improved the spinnability allowing beadfree fibers to be obtained. The epoxy polymerization characterized by FTIR and Photo-DSC allowed the integrity of the material and the fiber morphology to be maintained when the mats were heated up to the melting point of the PCL. These mats showed great shape memory effect during several cycles.
Acknowledgements: The authors acknowledge the University of the Basque Country UPV/EHU (UFI 11/56) and the Basque Government (PhD scholarship) for the funding received to develop this work. Technical and human support provided by Macro-behaviour-Mesostructure Nanotechnology SGIker Service of UPV/EHU is also gratefully acknowledged.
5. References [1]
Q. Meng, J. Hu, A review of shape memory polymer composites and blends, Compos. Part A. 40 (2009) 1661–1672.
[2]
R. Chang, Y. Huang, G. Shan, Y. Bao, X. Yun, T. Dong, P. Pan, Alternating poly(lactic acid)/poly(ethylene-co-butylene) supramolecular multiblock copolymers with tunable shape memory and self-healing properties, Polym. Chem. 6 (2015) 5899–5910.
[3]
X. Xiao, D. Kong, X. Qiu, W. Zhang, F. Zhang, L. Liu, Y. Liu, S. Zhang, Y. Hu, J. Leng, Shape-memory polymers with adjustable high glass transition temperatures, Macromolecules. 48 (2015) 3582–3589.
[4]
L. Gu, B. Cui, Q.Y. Wu, H. Yu, Bio-based polyurethanes with shape memory behavior at body temperature: effect of different chain extenders, RSC Adv. 6 (2016) 17888–17895.
[5]
F. Zhang, Z. Zhang, T. Zhou, Y. Liu, J. Leng, Shape memory polymer nanofibers and their composites: electrospinning, structure, performance, and applications, Front. Mater. 2 (2015) 62.
[6]
H. Chen, X. Cao, J. Zhang, J. Zhang, Y. Ma, G. Shi, Y. Ke, D. Tong, L. Jiang, Electrospun shape memory film with reversible fibrous structure, J. Mater. Chem. 22 (2012) 22387–22391.
[7]
J.N. Zhang, Y.M. Ma, J.J. Zhang, D. Xu, Q.L. Yang, J.G. Guan, X.Y. Cao, L. Jiang, Microfiber SMPU film affords quicker shape recovery than the bulk one, Mater. Lett. 65 (2011) 3639–3642.
[8]
X. Luo, P.T. Mather, Triple-shape polymeric composites (TSPCs), Adv. Funct. Mater. 20 (2010) 2649–2656.
[9]
A. Shirole, J. Sapkota, E.J. Foster, C. Weder, Shape memory composites based on electrospun poly(vinyl alcohol) fibers and a thermoplastic polyether block amide elastomer., ACS Appl. Mater. Interfaces. 8 (2016) 6701–6708.
[10]
E.D. Rodriguez, D.C. Weed, P.T. Mather, Anisotropic shape-memory elastomeric composites: Fabrication and testing, Macromol. Chem. Phys. 214 (2013) 1247–1257.
[11]
J.M. Robertson, H. Birjandi Nejad, P.T. Mather, Dual-spun shape memory elastomeric composites, ACS Macro Lett. 4 (2015) 436–440.
[12]
T. Tsujimoto, T. Takayama, H. Uyama, Biodegradable shape memory polymeric material from epoxidized soybean oil and polycaprolactone, Polymers. 7 (2015) 2165–2174.
[13]
L. Peponi, I. Navarro-Baena, A. Sonseca, E. Gimenez, A. MarcosFernandez, J.M. Kenny, Synthesis and characterization of PCL-PLLA polyurethane with shape memory behavior, Eur. Polym. J. 49 (2013) 893– 903.
[14]
A. Cipitria, A. Skelton, T.R. Dargaville, P.D. Dalton, D.W. Hutmacher, Design, fabrication and characterization of PCL electrospun scaffolds—a review, J. Mater. Chem. 21 (2011) 9419–9453.
[15]
Y. Yao, J. Wang, H. Lu, B. Xu, Y. Fu, Y. Liu, J. Leng, Thermosetting epoxy resin/thermoplastic system with combined shape memory and self-healing properties, Smart Mater. Struct. 25 (2016) 15021.
[16]
F. Zhang, Z. Zhang, Y. Liu, W. Cheng, Y. Huang, J. Leng, Thermosetting epoxy reinforced shape memory composite microfiber membranes: Fabrication, structure and properties, Compos. Part A. 76 (2015) 54–61.
[17]
U. Bulut, J. V. Crivello, Investigation of the Reactivity of Epoxide Monomers in Photoinitiated Cationic Polymerization, Macromolecules. 38 (2005) 3584–3595.
[18]
J. V. Crivello, Radical-promoted visible light photoinitiated cationic polymerization of epoxides, J. Macromol. Sci. Part A. 46 (2009) 474–483.
[19]
W. Klairutsamee, P. Supaphol, I. Jangchud, Electrospinnability of poly(butylene succinate): Effects of solvents and organic salt on the fiber size and morphology, J. Appl. Polym. Sci. 132 (2015) 42716–42727.
[20]
J.S. Ahn, W.R. Yu, J.H. Youk, H.Y. Ryu, In situ temperature tunable pores of shape memory polyurethane membranes, Smart Mater. Struct. 20 (2011) 105024–105033.
[21]
N.A. Hotaling, K. Bharti, H. Kriel, C.G. Simon, DiameterJ: A validated open source nanofiber diameter measurement tool, Biomaterials. 61 (2015) 327– 338
[22]
Y.M. Kim, L. Kris Kostanski, J.F. MacGregor, Kinetic studies of cationic photopolymerizations of cycloaliphatic epoxide, triethyleneglycol methyl vinyl ether, and cyclohexene oxide, Polym. Eng. Sci. 45 (2005) 1546–1555.
[23]
S.L. Shenoy, W.D. Bates, H.L. Frisch, G.E. Wnek, Role of chain entanglements on fiber formation during electrospinning of polymer solutions: Good solvent, non-specific polymer-polymer interaction limit, Polymer. 46 (2005) 3372–3384.
[24]
K.H. Lee, H.Y. Kim, M.S. Khil, Y.M. Ra, D.R. Lee, Characterization of nano-structured poly(ε-caprolactone) nonwoven mats via electrospinning, Polymer. 44 (2003) 1287–1294.
[25]
K.H. Lee, H.Y. Kim, H.J. Bang, Y.H. Jung, S.G. Lee, The change of bead morphology formed on electrospun polystyrene fibers, Polymer. 44 (2003) 4029–4034.
[26]
J. Malašauskienė, R. Milašius, E. Kuchanauskaitė, Possibilities for the estimation of electrospun nanofibre diameter distribution by normal (Gaussian) distribution, Fibres Text. East. Eur. 24 (2016) 23–28.
[27]
X. Yan, M. Gevelber, Investigation of electrospun fiber diameter distribution and process variations, J. Electrostat. 68 (2010) 458–464.
[28]
X. Fernandez-Francos, J.M. Salla, G. Perez, A. Mantecon, A. Serra, X. Ramis, New thermosets obtained by thermal and UV-induced cationic copolymerization of DGEBA with 4-phenyl-γ-butyrolactone, Macromol. Chem. Phys. 210 (2009) 1450–1460.
[29]
Q. Guo, C. Harrats, G. Groeninckx, H. Reynaers, M.H.J. Koch, Miscibility, crystallization and real-time small-angle X-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends, Polymer. 42 (2001) 6031–6041.
[30]
R. Harikrishna, Inhibition effect of N,N-diglycidyl-4-glycidyloxy aniline on photosensitized cationic polymerization of formulations involving resorcinol diglycidyl ether and poly(propylene glycol)diglycidyl ether, J. Photochem. Photobiol. A Chem. 303-304 (2015) 17–27.
[31]
V.Y. Voytekunas, F.L. Ng, M.J.M. Abadie, Kinetics study of the UVinitiated cationic polymerization of cycloaliphatic diepoxide resins, Eur. Polym. J. 44 (2008) 3640–3649.
[32]
B. Golaz, V. Michaud, Y. Leterrier, J.A.E. Manson, UV intensity, temperature and dark-curing effects in cationic photo-polymerization of a cycloaliphatic epoxy resin, Polymer. 53 (2012) 2038–2048.
[33]
G. Liu, X. Zhu, B. Xu, X. Qian, G. Song, J. Nie, Cationic photopolymerization of bisphenol A diglycidyl ether epoxy under 385 nm, J. Appl. Polym. Sci. 130 (2013) 3698–3703.
[34]
M. Fejos, K. Molnar, J. Karger-Kocsis, Epoxy/polycaprolactone systems with triple-shape memory effect: Electrospun nanoweb with and without graphene versus co-continuous morphology, Materials. 6 (2013) 4489–4504.
Highlights Polycaprolactone/epoxy blend is electrospun Epoxy is cationically polymerized using a iodonium salt as latent photo-initiator The fibers show shape memory: Fixity ratios 95-99% and Recovery ratios 88 100%
S0014-3057(17)30435-4 http://dx.doi.org/10.1016/j.eurpolymj.2017.07.026 EPJ 7979
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
13 March 2017 14 July 2017 17 July 2017
Please cite this article as: Iregui, A., Irusta, L., Llorente, O., Martin, L., Calvo-Correas, T., Eceiza, A., González, A., Electrospinning of Cationically Polymerized Epoxy/Polycaprolactone Blends to Obtain Shape Memory Fibers (SMF), European Polymer Journal (2017), doi: http://dx.doi.org/10.1016/j.eurpolymj.2017.07.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrospinning of Cationically Polymerized Epoxy/Polycaprolactone Blends to Obtain Shape Memory Fibers (SMF) Alvaro Iregui, Lourdes Irusta*, Oihane Llorente, Loli Martin, Tamara CalvoCorreas, Arantxa Eceiza, Alba González. This is dedicated to Prof. Iñaki Eguiazabal
––––––––– A. Iregui, Prof. L. Irusta, O. Llorente, Dr. A. González. POLYMAT, Department of Polymer Science and Technology, University of the Basque Country UPV-EHU, PO Box 1072, 20080. Donostia/San Sebastian, Spain. Dr. L. Martin. Macrobehaviour-Mesostructure-Nanotechnology SGIker Service, Polytechnic School, University of the Basque Country UPV-EHU, Plaza Europa 1, 20018. Donostia/San Sebastian, Spain. T. Calvo-Correas, Prof. A. Eceiza. Group “Materials+Technologies”, Department of Chemical and Environment Engineering, University of the Basque Country UPVEHU, Plaza Europa 1, 20018. Donostia/San Sebastian, Spain. E-mail: [email protected] ––––––––– Abstract: A mat of fibers with shape memory effect is obtained using commercially available components and a simple electrospinning process. For obtaining this goal, a blend of diglycidyl ether of bisphenol A (DGEBA) and Polycaprolactone (PCL) is electrospun and the obtained mats are cured by UV radiation, avoiding the melting of the PCL component. The cationic photoinitiator based on iodonium salts, used for the first time in the electrospinning process, increases the solution conductivity and consequently the spinnability of the blend. The obtained mats show shape memory properties through several cycles, with shape fixity ratios that range from 95 to 99% and shape recovery ratios of between 88 and 100% respectively. Keywords: shape memory polymers, solution electrospinning, photopolymerization, smart materials, UV curing reactions
cationic
1. Introduction Shape memory polymers (SMP) are materials capable of reacting to external stimuli such as temperature, magnetic or electric fields and solvents, to return from a pre-programmed shape to their original form [1]. The SMPs most studied are those that can be preset using temperature as external stimuli [2–4]. Among the techniques used to process SMPs, electrospinning has recently attracted most attention as a tool for obtaining mats that could be applied in biomedical materials as scaffolds for tissue engineering or drug delivery [5]. Electrospun mats have shown an enhanced shape memory effect (SME) [6,7], and electrospinning is also a way of obtaining shape memory composites [8,9]. Highly oriented fibers can be recollected and, after being immersed in a proper resin, an anisotropic material with shape memory properties is obtained [10]. SMPs have also been obtained by electrospinning two polymers (dual-spun) simultaneously [11]. Polycaprolactone (PCL) based shape memory polymer blends have been the focus of recent research. These materials show sharp and rapid shape memory-recovery behavior as a consequence of the melting/recrystallization of the PCL component as switching transition [12–14]. Research has also been carried out into PCL based electrospun fibers. In literature it is possible to find these types of materials obtained by imbibing electrospun PCL fibers into an epoxy resin using amines as thermal curing agents [15] or by coaxial electrospinning of a solution of PCL and modified epoxy monomers [16]. The direct introduction of epoxy monomers in the electrospinning process is not possible due to their low molecular weight. Thus, the introduction of epoxy monomers in the fiber requires the use of a second component (this can be the high molecular weight PCL), and the curing process must be performed once the fibrous structure is formed. However, the curing process must be performed in mild conditions (low temperature) in order to avoid the PCL melting as it destroys the fibrous structure. In order to overcome this problem, the cationic photopolymerization of the epoxy is an interesting alternative [17,18]. In addition, the iodonium salts used as photo-initiators could greatly enhance electrospinning, favoring fibers over beads and smaller diameters [19]. Taking into account the advantages that cationic epoxy polymerization offers, our approach was to create, for the first time, a mat of nanofibers with SME using commercially available components and a simple spinning process. In the general process employed in this work, a blend of a semicrystalline thermoplastic with a photocurable monomer is used in a two-step process. The first step consists on the electrospinning process, and nanofibers are obtained due to the semicrystalline thermoplastic; the second step is the polymerization of the monomer to obtain a thermostable material in reaction conditions that maintain the morphology of the nanofibers.
In order to achieve our goal, a blend of diglycidyl ether of bisphenol A (DGEBA) and PCL was electrospun and the obtained mats were cured by UV radiation (avoiding the melting of the PCL component) thanks to the cationic initiators that were added to the solution and were used for the first time in electrospinning. The obtained mats present multiple future applications. As an example, they could be of potential use in the design of smart separation membranes, in which the change in pore size due to the recovery of the permanent shape changes the selective properties of the membrane [20].
2. Experimental Section 2.1 Materials Bisphenol A diglycidyl ether (DGEBA, Mw= 340.41 g/mol), 2,2-dimethoxy-2phenyl-acetophenone, bis(4-tert-butylphenyl)iodonium hexafluorophosphate, acetone and dimethylformamide (DMF) were purchased from Sigma Aldrich. Linear PCL (Mw= 45 000 g/mol) was purchased from Perstrop. All materials were used as received. 2.2 Electrospinning Process and UV Curing Two different 20 wt% solutions were prepared maintaining PCL/DGEBA 50/50 weight ratio but changing the solvent. For the first solution (named A), 2.97 g of PCL and 2.97 g of DGEBA were added to 30 ml of acetone. Then the solution was heated to 50 ºC and stirred for 4 hours. When the solution was homogeneous, 30 mg of bis(4-tert-butylphenyl)iodonium hexafluorophosphate (photoinitiator) and 30 mg of 2,2-dimethoxy-2-phenyl-acetophenone (sensitizer) were added and the solution was kept in darkness. The other solution (A3DMF1) was prepared under the same procedure but using an acetone/DMF 3/1 as a volume mixture. For the electrospinning process, polymer solutions were placed into a syringe that was mounted on a syringe pump (Cole-Parmer) and randomly oriented fibers were electrospun by applying a voltage of 9 kV to the needle using a high voltage source (0-30 kV, CZE1000R, Spellman High Voltage Electronics Corp.). The collector was a stainless steel sheet and was placed at 15 cm from the needle tip. The flow rate was established at 0.30 ml/h. The samples were collected during 8 hours, and in order to cure the desired mats, the samples were separated from the collector and cured for both sides for 1 hour using a UV LED lamp (ThorLabs M365LP1), with an UV intensity of 2 mW/cm2 at 365 nm. The morphology of the electrospun mats was analyzed using a Hitachi S2700 SEM at 15 kV accelerating voltage, and diameter distribution was measured on 4 different images for each sample using ImageJ software [21].
2.3 Characterization In order to characterize the materials and the mats, Fourier Transform Infrared Spectroscopy (Nicolet 6700 FTIR) equipped with a single reflection ATR system (Specac Golden Gate), and Differential Scanning Calorimetry experiments (DSC TA Instruments Q2000) were employed. All DSC experiments were conducted at a heat rate of 20 ºC/min in N2 atmosphere. The curing reaction was analyzed using a Photocalorimetry accessory (Omnicure S2000 with a 200W mercury lamp) at a temperature of 25 ºC with a UV intensity between 1 and 2 mW/cm2. 2.4 Shape Memory Effect SME was characterized using a Universal Testing Machine (MTS Insight 10) with a load cell of 10 kN and thermostated at 60 ºC. Two samples of each solution were cut into rectangular shapes (20x5 mm), and the initial distance between the grips was established in 10 mm, εi. The tests were conducted applying a deformation of 2 mm/min up to 40% of deformation, the programmed elongation, εp. After two minutes to acclimatise the material, it was quickly cooled using an aerosol coolant and the fixed elongation was measured, εf. Then the mats were immersed in an oil bath at 60 ºC, and the recovery elongation recorded, εr. This cycle was repeated 5 times for each mat. Shape fixity (Rf) and shape recovery (Rr) ratios were calculated for each cycle N according to Equation (1) and (2) [22]. (1) (2)
3. Results and Discussion 3.1 Electrospinning
Figure 1. SEM images of the electrospun mats from solutions in acetone before (a) and after (b) UV irradiation, and from solutions in acetone/DMF before (c) and after (d) UV irradiation. (e) SEM image of electrospun solution without iodonium salt. Figure 1a and b show the SEM images for the electrospun mats obtained from the solution in acetone before and after UV curing. The morphology consists of a network structure of bead-free fibers with diameters of around one micrometer. PCL is the only polymer present in the solution, and as entanglements are required in the process of electrospinning [23], it is fair to assume that is the only component capable of being electrospun, and the others (epoxy monomer, initiator and sensitizer) have to imbibe the fibers. The SEM images also show that differences between the mats before and after curing are not appreciable and the fibrous structure is maintained. Figure 1c and d show the SEM images in the case of electrospun mats obtained from the solution in acetone and DMF before and after UV curing. The fibers obtained with this mixture of solvents present less coalescence than those from acetone, similar to the results of other groups [24,25]. These mats also maintain their morphology after UV curing.
The iodonium salt employed as a photo-initiator has a crucial role in the process; when iodonium salt was not added to the solutions (Figure 1e), electrospray takes place, and mainly beads with a poorly defined morphology are obtained. This is in agreement with literature, where it is reported than a little amount of salt in electrospinning solutions improves the spinnability due to the increase in conductivity [19]. This way, the photo-initiator employed not only is crucial for achieving a UV curing of the epoxy monomer, but it also has a positive effect in the process of electrospinning. To our knowledge, this is the first time that iodonium salts for cationic photo-polymerization have been used in electrospinning.
Figure 2. Diameter distribution for electrospun mats from A and A3DMF1 solutions. The diameter of the fibers and its distribution was calculated for samples obtained using acetone and acetone/DMF mixtures (Figure 2). As can be seen, for both samples the diameter of the fibers does not present a normal distribution, which is in accordance with other literature data [26] where it is shown that the normal distribution is not always suitable for the evaluation of the mat structure. The mats were characterized by the mean fiber diameter and standard deviation. Diameters of 0.92 and 1.04 m with standard deviations of 0.46 and 0.37 m were achieved for the samples obtained in acetone and acetone/DMF mixtures respectively. It is well known that in electrospinning process the solution conductivity has a strong effect on the diameter of the fibers. However, in our case, both samples have similar conductivity values because the iodonium salt concentration is the same. However, the lower viscosity and lower surface tension of acetone can be in the origin of the lower fiber diameter showed by the fibers obtained using this solvent. In addition, the fibers obtained using the acetone/DMF mixture presented lower standard deviation. This effect is probably due to the lower volatility of the DMF that allows a more continuous process with lower fluctuation jet, resulting in fibers with narrower diameter distribution [27].
3.2 UV Curing of the Mats
Figure 3. ATR-FTIR spectra before and after UV irradiation for electrospun mats from acetone (a) and acetone/DMF (b). Analyses were conducted with the objective of determining the extension of the epoxy curing reaction. The infrared spectra (Figure 3a and b) of the mats were obtained before and after UV irradiation using an ATR accessory. The decrease of the absorbance of the 915 cm-1 band, corresponding to the bending of the epoxy group, and the increase of the absorbance of the band around 1100 cm-1 corresponding to the stretching of the C–O–C bond indicate that epoxy polymerization has occurred [28].
Figure 4. DSC thermograms of A samples before (a) and after (b) curing. SEM images of the non cured (c) and cured (d) A samples after the DSC scan.
DSC was also conducted to confirm the curing reaction and how it affects the crystallinity of the PCL, crucial in the shape memory effect. In the uncured mats (Figure 4a) three significant signals can be observed: at -21 ºC, assigned to the Tg of DGEBA, and at 34 ºC and 47 ºC two endothermic signals assigned to the Tm of the DGEBA and PCL respectively. In the case of the mats after UV irradiation (Figure 4b), the signals of the DGEBA monomer disappear as expected due to photo-curing, and only the endothermic peak of PCL melting is observed at 58 ºC. Regarding the crystallinity of the PCL, the value of Tm before UV curing (47 ºC) decreases from the value of pure PCL (around 56 ºC) indicating that DGEBA monomer in the blend is partially miscible with the PCL [29]. After the photocuring reaction, the melting point of the PCL returns to its base value as the polymerization of the epoxy induces a phase separation.
These mats studied in DSC were removed from the aluminum pans and observed by SEM. As can be seen in Figure 4c and d, the cured mats show few defects, confirming the UV curing reaction, whereas the uncured mats are melted and the integrity of the fibers is lost.
Figure 5. Conversion (a) and rate of polymerization (b) calculated from photoDSC. DSC scans for the photocured mats of samples A (c) and A3DMF1 (d).
Photocalorimetry studies were conducted in order to estimate the conversion of the curing reaction. Conversion and polymerization rate were calculated using the method described in the bibliography [30]. The results in Figure 5a and b show that conversion and rate of polymerization are very similar for both samples, and therefore it can be concluded that, as expected, the solvent employed in electrospinning has no effect in the photocuring reaction. In both cases, the conversion is around 25-30% after 45 minutes of irradiation. The polymerization rate reaches a maximum in the first few minutes and then decreases as the chains are immobilised due to the advance of the curing reaction [31]. In post-DSC experiments (Figure 5c and d), both samples present an exothermic signal in the first scan that can be correlated to a post curing reaction, as polymerization continues due to high temperatures [32]. This peak disappears in the second scan, supporting this assignment. These all results indicate that the epoxy polymerization that is achieved using UV curing at room temperature is not complete [33], but enough to maintain the integrity of the fibers when the temperature is higher than the Tm of the PCL.
3.3 Shape Memory Effect The results obtained for the shape memory effect are displayed in the Figure 6a and b. The shape fixity ratio ranges from 95 to 99%, and shape recovery ratio from 88 to 100%, maintaining the shape memory properties after five consecutive cycles. Fejos et al. studied SMP composites of electrospun PCL imbibed with epoxy resin, and obtained similar values [34]. This indicates that the method employed in this study make it possible to obtain a great shape memory effect maintaining the fibrous structure that is electrospun. There is no significant difference in the shape memory effect among the mats obtained from different solvents.
Figure 6. Shape fixity ratio (a) and recovery ratio (b) for A, A3DMF1 and casting film. For electrospun A3DMF1 mat, photographs of initial shape (c), fixed shape after folding the mat three times in half at 60 ºC (d) and recovery shape after heating again to 60 ºC (e). Figure 6c to e show photographs of a qualitative shape memory test, manifesting than these electrospun mats can adopt compact shapes and return to its extended form when needed, heating to 60 ºC. A video of the recovering process can be found in the supplementary material.
Figure 7. SEM images for A3DMF1 electrospun mat during a shape memory test in the (a) original shape, (b) temporary shape and (c) recovery shape.
Regarding the morphological stability of the nanofibers, in Figure 4d we have shown that it is maintained at temperatures higher than the PCL melting. In the shape memory process, not only temperature but also force is applied to the nanofibers, and therefore the morphology of the mats was analyzed for samples in the temporary shape and after the recovery process. As can be seen in Figure 7 the nanofibers show the same morphology during all the shape memory test. This means that the process is non-destructive. Employing a different semicrystalline polymer instead of PCL, this shape memory effect could be achieved at other temperature, allowing its applications in fields such as tissue engineering if body temperature is employed to recover the desired shape.
4. Conclusions Electrospun mats from PCL/DGEBA mixtures were obtained adding an iodonium salt to the solutions. This UV initiator not only was necessary for curing the DGEBA after electrospinning, but also improved the spinnability allowing beadfree fibers to be obtained. The epoxy polymerization characterized by FTIR and Photo-DSC allowed the integrity of the material and the fiber morphology to be maintained when the mats were heated up to the melting point of the PCL. These mats showed great shape memory effect during several cycles.
Acknowledgements: The authors acknowledge the University of the Basque Country UPV/EHU (UFI 11/56) and the Basque Government (PhD scholarship) for the funding received to develop this work. Technical and human support provided by Macro-behaviour-Mesostructure Nanotechnology SGIker Service of UPV/EHU is also gratefully acknowledged.
5. References [1]
Q. Meng, J. Hu, A review of shape memory polymer composites and blends, Compos. Part A. 40 (2009) 1661–1672.
[2]
R. Chang, Y. Huang, G. Shan, Y. Bao, X. Yun, T. Dong, P. Pan, Alternating poly(lactic acid)/poly(ethylene-co-butylene) supramolecular multiblock copolymers with tunable shape memory and self-healing properties, Polym. Chem. 6 (2015) 5899–5910.
[3]
X. Xiao, D. Kong, X. Qiu, W. Zhang, F. Zhang, L. Liu, Y. Liu, S. Zhang, Y. Hu, J. Leng, Shape-memory polymers with adjustable high glass transition temperatures, Macromolecules. 48 (2015) 3582–3589.
[4]
L. Gu, B. Cui, Q.Y. Wu, H. Yu, Bio-based polyurethanes with shape memory behavior at body temperature: effect of different chain extenders, RSC Adv. 6 (2016) 17888–17895.
[5]
F. Zhang, Z. Zhang, T. Zhou, Y. Liu, J. Leng, Shape memory polymer nanofibers and their composites: electrospinning, structure, performance, and applications, Front. Mater. 2 (2015) 62.
[6]
H. Chen, X. Cao, J. Zhang, J. Zhang, Y. Ma, G. Shi, Y. Ke, D. Tong, L. Jiang, Electrospun shape memory film with reversible fibrous structure, J. Mater. Chem. 22 (2012) 22387–22391.
[7]
J.N. Zhang, Y.M. Ma, J.J. Zhang, D. Xu, Q.L. Yang, J.G. Guan, X.Y. Cao, L. Jiang, Microfiber SMPU film affords quicker shape recovery than the bulk one, Mater. Lett. 65 (2011) 3639–3642.
[8]
X. Luo, P.T. Mather, Triple-shape polymeric composites (TSPCs), Adv. Funct. Mater. 20 (2010) 2649–2656.
[9]
A. Shirole, J. Sapkota, E.J. Foster, C. Weder, Shape memory composites based on electrospun poly(vinyl alcohol) fibers and a thermoplastic polyether block amide elastomer., ACS Appl. Mater. Interfaces. 8 (2016) 6701–6708.
[10]
E.D. Rodriguez, D.C. Weed, P.T. Mather, Anisotropic shape-memory elastomeric composites: Fabrication and testing, Macromol. Chem. Phys. 214 (2013) 1247–1257.
[11]
J.M. Robertson, H. Birjandi Nejad, P.T. Mather, Dual-spun shape memory elastomeric composites, ACS Macro Lett. 4 (2015) 436–440.
[12]
T. Tsujimoto, T. Takayama, H. Uyama, Biodegradable shape memory polymeric material from epoxidized soybean oil and polycaprolactone, Polymers. 7 (2015) 2165–2174.
[13]
L. Peponi, I. Navarro-Baena, A. Sonseca, E. Gimenez, A. MarcosFernandez, J.M. Kenny, Synthesis and characterization of PCL-PLLA polyurethane with shape memory behavior, Eur. Polym. J. 49 (2013) 893– 903.
[14]
A. Cipitria, A. Skelton, T.R. Dargaville, P.D. Dalton, D.W. Hutmacher, Design, fabrication and characterization of PCL electrospun scaffolds—a review, J. Mater. Chem. 21 (2011) 9419–9453.
[15]
Y. Yao, J. Wang, H. Lu, B. Xu, Y. Fu, Y. Liu, J. Leng, Thermosetting epoxy resin/thermoplastic system with combined shape memory and self-healing properties, Smart Mater. Struct. 25 (2016) 15021.
[16]
F. Zhang, Z. Zhang, Y. Liu, W. Cheng, Y. Huang, J. Leng, Thermosetting epoxy reinforced shape memory composite microfiber membranes: Fabrication, structure and properties, Compos. Part A. 76 (2015) 54–61.
[17]
U. Bulut, J. V. Crivello, Investigation of the Reactivity of Epoxide Monomers in Photoinitiated Cationic Polymerization, Macromolecules. 38 (2005) 3584–3595.
[18]
J. V. Crivello, Radical-promoted visible light photoinitiated cationic polymerization of epoxides, J. Macromol. Sci. Part A. 46 (2009) 474–483.
[19]
W. Klairutsamee, P. Supaphol, I. Jangchud, Electrospinnability of poly(butylene succinate): Effects of solvents and organic salt on the fiber size and morphology, J. Appl. Polym. Sci. 132 (2015) 42716–42727.
[20]
J.S. Ahn, W.R. Yu, J.H. Youk, H.Y. Ryu, In situ temperature tunable pores of shape memory polyurethane membranes, Smart Mater. Struct. 20 (2011) 105024–105033.
[21]
N.A. Hotaling, K. Bharti, H. Kriel, C.G. Simon, DiameterJ: A validated open source nanofiber diameter measurement tool, Biomaterials. 61 (2015) 327– 338
[22]
Y.M. Kim, L. Kris Kostanski, J.F. MacGregor, Kinetic studies of cationic photopolymerizations of cycloaliphatic epoxide, triethyleneglycol methyl vinyl ether, and cyclohexene oxide, Polym. Eng. Sci. 45 (2005) 1546–1555.
[23]
S.L. Shenoy, W.D. Bates, H.L. Frisch, G.E. Wnek, Role of chain entanglements on fiber formation during electrospinning of polymer solutions: Good solvent, non-specific polymer-polymer interaction limit, Polymer. 46 (2005) 3372–3384.
[24]
K.H. Lee, H.Y. Kim, M.S. Khil, Y.M. Ra, D.R. Lee, Characterization of nano-structured poly(ε-caprolactone) nonwoven mats via electrospinning, Polymer. 44 (2003) 1287–1294.
[25]
K.H. Lee, H.Y. Kim, H.J. Bang, Y.H. Jung, S.G. Lee, The change of bead morphology formed on electrospun polystyrene fibers, Polymer. 44 (2003) 4029–4034.
[26]
J. Malašauskienė, R. Milašius, E. Kuchanauskaitė, Possibilities for the estimation of electrospun nanofibre diameter distribution by normal (Gaussian) distribution, Fibres Text. East. Eur. 24 (2016) 23–28.
[27]
X. Yan, M. Gevelber, Investigation of electrospun fiber diameter distribution and process variations, J. Electrostat. 68 (2010) 458–464.
[28]
X. Fernandez-Francos, J.M. Salla, G. Perez, A. Mantecon, A. Serra, X. Ramis, New thermosets obtained by thermal and UV-induced cationic copolymerization of DGEBA with 4-phenyl-γ-butyrolactone, Macromol. Chem. Phys. 210 (2009) 1450–1460.
[29]
Q. Guo, C. Harrats, G. Groeninckx, H. Reynaers, M.H.J. Koch, Miscibility, crystallization and real-time small-angle X-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends, Polymer. 42 (2001) 6031–6041.
[30]
R. Harikrishna, Inhibition effect of N,N-diglycidyl-4-glycidyloxy aniline on photosensitized cationic polymerization of formulations involving resorcinol diglycidyl ether and poly(propylene glycol)diglycidyl ether, J. Photochem. Photobiol. A Chem. 303-304 (2015) 17–27.
[31]
V.Y. Voytekunas, F.L. Ng, M.J.M. Abadie, Kinetics study of the UVinitiated cationic polymerization of cycloaliphatic diepoxide resins, Eur. Polym. J. 44 (2008) 3640–3649.
[32]
B. Golaz, V. Michaud, Y. Leterrier, J.A.E. Manson, UV intensity, temperature and dark-curing effects in cationic photo-polymerization of a cycloaliphatic epoxy resin, Polymer. 53 (2012) 2038–2048.
[33]
G. Liu, X. Zhu, B. Xu, X. Qian, G. Song, J. Nie, Cationic photopolymerization of bisphenol A diglycidyl ether epoxy under 385 nm, J. Appl. Polym. Sci. 130 (2013) 3698–3703.
[34]
M. Fejos, K. Molnar, J. Karger-Kocsis, Epoxy/polycaprolactone systems with triple-shape memory effect: Electrospun nanoweb with and without graphene versus co-continuous morphology, Materials. 6 (2013) 4489–4504.
Highlights Polycaprolactone/epoxy blend is electrospun Epoxy is cationically polymerized using a iodonium salt as latent photo-initiator The fibers show shape memory: Fixity ratios 95-99% and Recovery ratios 88 100%