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Controlling Properties of Acrylate/Epoxy Interpenetrating Polymer Networks by Premature Termination of Radical Polymerization of Acrylate.

INTRODUCTION

Functionally graded materials (FGMs) are materials with inhomogeneous properties designed to better serve the function of a part. FGMs are a common component of most biological systems and are becoming of industrial importance. For example, polymers with graded mechanical properties have been used to improve the life of artificial biomedical knee joints [1], and alloys with graded thermal properties have been used to improve the thermal barrier of the combustion chamber of engines [2]. As a result, building 3D printers that can print graded properties has recently attracted attention [3-7]. Interpenetrating polymer networks (IPNs) is one material system that can be used to make FGMs [7-9] and can be used to 3D print FGMs [3, 5, 7]. This article proposes a method to control properties of sequentially constructed IPNs in a manner that may have potential to printing FGMs.

IPNs are polymer systems that at each point include two or more distinct, but interpenetrating, networks. There are two main methods to synthesize IPNs based on the simultaneous or sequential construction of the individual networks. The IPNs resulting from using these methods are referred to as simultaneous or sequential IPNs [10]. Both methods can be used to construct FGMs [11]. A sequential method to construction IPNs is used here to control properties and grading of FGMs.

To construct sequential IPNs, one normally constructs a networked polymer system, swells it with monomers or oligomers of the second network and then cures the second network in the already formed first network. Final properties are typically controlled by methods that focus on the formation and characteristics of the second and later networks. This can be done by controlling the extent of swelling by using the swelling time, temperature, or pH [12-15]. This can also be done by controlling the degree of curing of the second and later networks [7, 12, 16], or by controlling the structure of the subsequent networks [17-19]. Unlike these methods that control the characteristics of the second and later networks, this article looks at controlling the final properties by using premature termination during the formation of the initial network. Photo polymerization was used to control the formation and termination of this network.

An acrylate/epoxy system was used to make the sequential IPNs. In this process, the initial network was the acrylate network, which was built by photo radical polymerization. The extent of this conversion was controlled using the ultra-violet (UV) exposure time. This partially formed acrylate network was then washed and used to construct a sequential acrylate/epoxy IPNs. Homogenous properties were obtained by uniform formation of the acrylate network and inhomogeneous (i.e., graded) properties were obtained by spatial variation of the local formation of the acrylate network.

The article is organized in two parts. The first part describes and studies the proposed method to obtain different mechanical properties of sequential acrylate/epoxy IPNs. The second part uses this to design and print homogeneous and inhomogeneous IPNs. The properties of the final IPNs were evaluated by nanoindentation, differential scanning calorimetry (DSC), and by mechanical tensile testing.

MATERIALS AND SYNTHESIS

The initial network of the IPNs is made from Bisphenol A ethoxylate dimethacrylate (acrylate) by photo radical polymerization using the initiator 2-hydroxy-2-methylpropiophenone. The second network is made from 3, 4-epoxycyclohexylmethyl 3, 4-epoxycyclohexanecarboxylate (ECH) by cationic polymerization using the initiator triarylsulfonium hexafluoroantimonate. These were purchased from Sigma-Aldrich[R], USA. The associated chemical structures are given in Fig. 1. The acrylate oligomer has the repeat length of m = n = 15, and its molecular weight is 1,700 g/mol.

The acrylate system was made from 0.5 wt. % radical initiator and the acrylate oligomer. The epoxy system was made from 2 wt. % cationic initiator and the ECH oligomer. Before polymerization, both mixtures were vacuumed and stirred at room temperature for 45 min, and then purged with nitrogen. The acrylate system was kept under an Argon environment for the remainder of the processing.

Fig. 2 shows the steps in the synthesis of the acrylate/epoxy system. First, the acrylate system is partially photo polymerized to form the initial network. This network is then washed in acetone to remove any uncured acrylate oligomers, swollen with the epoxy system, and then cured to make the IPNs. The initial photo polymerization of acrylate at 25[degrees]C and at 1 atm in Argon was done under a UV laser (322 nm; Opolette HE 355 LD, USA). The duration of the UV exposure was varied. The UV exposure was followed by sufficient time to allow completion of dark curing (i.e., continuation of network formation after termination of UV exposure). The resulting system was washed in acetone at room temperature for 40 to 50 min to remove uncured acrylate oligomers and then dried for 5 min in air to evaporate the acetone. The resulting acrylate network was swollen in the epoxy system at 50[degrees]C for 8 h in an Argon environment at 1 atm. The surface of the system was wiped and then exposed to polychromatic light (400 W, maximum light intensity of 375 mW/[cm.sup.2] at 365 nm) for 1 min to initiate the polymerization of the epoxy. The system was then thermally cured at 150[degrees]C for 1 h [20].

CHARACTERIZATION METHODS

The curing kinetics of free radical photo polymerization of the acrylate network was monitored using a rapid scan Fourier Transform Infrared Spectroscopy (FTIR) spectrometer (Cary 670, Agilent technologies, USA). The acrylate network was cured on an ATR (PIKE, USA) connected to the FTIR. The spectral range of the FTIR was set to cover from 400 to 4000 [cm.sup.-1] with a spectral resolution of 4 [cm.sup.-1], and the measuring rate of 1 spectra/s. Figure 3 shows a schematic of the experimental setup. The acrylate was put in a 200 [+ or -] 2 [micro]m thick reference mold made in the shape of a hollow cylinder and placed on the diamond of the ATR. The mixture was sealed by placing a sheet of fused silica on the mold. The temperature of the ATR was set to 25[degrees]C. The acrylate network was cured using a tunable pulsed laser (Opolette HE 355 LD, USA). The laser was set to produce a 322 nm wavelength beam at a frequency of 20 Hz and a pulse duration of 5 ns. The laser beam was transported to the ATR using a UV optic fiber connected to a vertical collimator that was placed above the ATR. The light power intensity on the top surface of the acrylate was a uniform 4.6 mW/[cm.sup.2] measured using a beam profile camera (Ophir, Israel). The FTIR measurement occurred on the surface of the ATR diamond under the curing acrylate. Due to light attenuation as it traveled through the acrylate system, the power intensity of the light at the point of measurement was calculated, using the known coefficient of attenuation [7], to be 4.48 mW/[cm.sup.2]. This intensity decreases with curing of the acrylate. A shutter was used to control the light exposure time of the system. The testing environment was isolated using a plastic film hood that was purged with Argon.

The conversion ratio of the acrylate oligomer into an acrylate network was calculated from the FTIR spectrum. Figure 4 shows the initial, intermediate, and final spectrum. The absorbance peak at the wavenumber of 1637 [cm.sup.-1] is associated with the acrylate double bond. The conversion ratio was calculated using the peak height method [22] based on the ratio of the peak of the acrylate bond at 1637 [cm.sup.-1] to the reference peak at 1608 [cm.sup.-1] [7].

The material loss due to washing was characterized by weighing. After partially polymerizing acrylate, the network was washed in acetone at room temperature to remove the uncured portion of the acrylate. The weight loss during washing was recorded using a scale (Mettler Toledo AT201). A 5 min wait in air was used before the final weight measurement to allow evaporation of acetone. The percentage weight loss fw was calculated using the equation

[f.sub.w] = [W.sub.o] - [W.sub.t]/[W.sub.o] x 100% (1)

where [W.sub.o] is the initial weight of the cured acrylate system before washing and [W.sub.t] is the weight after washing. A similar method was used to characterize the weight gain after swelling. The percent weight gain [f.sub.s] of the acrylate network in the swollen system is given by the equation

[mathematical expression not reproducible] (2)

where [W.sub.s-o] and [W.sub.s] are, respectively, the weight of the samples before and after swelling in the epoxy resin. During washing and swelling, the volume of the acrylate network was monitored by measuring the diameter and thickness of the sample. The percentage of volume change, fv, was calculated by the equation

[f.sub.v] = V - [V.sub.o]/[V.sub.o] x 100%, (3)

where [V.sub.o] is the volume of the sample before washing, and V is the volume after washing/swelling.

The network structure characteristics were evaluated by fast scanning calorimetry (FSC) using a Flash DSC 1 apparatus with a MultiSTAR UFS 1 MEMS chip sensor (Mettler-Toledo[R]). The FSC measurement was done using a Huber TC100 immersion cooler with a 20 mL/min Nitrogen purge. Samples of about 200 ng were taken from the central portion of the parts. So as to be representative, the FSC samples were large enough to include the entire cross-section. The FSC apparatus was set to first cool the sample to -80[degrees]C, equilibrated the sample for 10 min, and then heat the sample at 500[degrees]C/s from -80[degrees]C to 200[degrees]C.

Mechanical properties were evaluated by nanoindentation using a Hysitron TI-950 with a Ti-0093 conical tip. The sample was sandwiched in an epoxy glue, cured, and then cut with a microtome to expose the cross-section. The modulus was measured using two to three 5 [micro]m x 5 [micro]m grids (3 X 3) going from the edge to the center of the cross-section. The displacement mode was used for the indentation with a maximum displacement of 800 nm. The elastic modulus was calculated using the equation

1/[E.sub.r] = 1 - [v.sup.2.sub.s]/[E.sub.s] + 1 - [v.sup.2.sub.i]/[E.sub.i], (4)

where [E.sub.s] is the sample modulus, [E.sub.r] is the measured reduced modulus, and [E.sub.i] is the indenter modulus (1050 GPa, diamond), and [v.sub.s]. is the sample Poisson's ratio (estimated as 0.4) and [v.sub.i] is the Poisson's ratio of the indenter (0.2, diamond) [23].

RESULTS FOR UNIFORM IPNS

Uniform IPNs with different properties were manufactured by altering the initial acrylate illumination times. Partial polymerization of acrylate was achieved by using 2 s, 4 s, 6 s, and 8 s UV exposure followed by 30 min of dark curing. This was done on the ATR to allow characterization with FTIR. Figure 5 shows the curing kinetics of polymerization of these acrylate systems measured using the FTIR spectra. As can be seen, the extent of UV exposure controls the final extent of network formation. For 2 s, 4 s, 6 s, and 8 s exposure times, the final conversions were, respectively, 36%, 51%, 70%, and 85%. The measurement also indicates that the acrylate system continues to cure after turning off the light source (dark cure). Dark curing continues due to unconsumed radicals in the system and gradually stops as they are all consumed [24]. Termination is observed in all systems. The conversion in the last 10 min of the dark cure for all systems was only 2% to 3%, which is close to the noise of the measurement. The figure clearly indicates that the initial UV exposure controls the extent of conversion of the acrylate network and shows the expected conversions after dark curing for the different UV exposures.

The weight loss of the partially cured acrylate system during washing in acetone was calculated using Eq. 1, and is plotted in Fig. 6a. In each case, the washing was complete, as indicated by the termination of the weight loss. Figure 6b presents the wash ratio as a function of the conversion ratio. The repeatability of these measurements were checked by three repetitions. The difference between the unconverted fraction and weight loss during washing is due to partially connected molecules that cannot be washed out of the system [24].

Fig. 7 presents the relative volume change after washing and after swelling using Eq. 3 and measured relative to the initial volume before washing. As can be seen, there is little volume change due to washing. However, excluding fully cured acrylate, there are large volume changes (40%-100%) due to swelling in the epoxy resin.

Figure 8 shows the proportion of acrylate in the swollen acrylate/epoxy system evaluated using Eq. 2. As shown, the acrylate content varies from 22.3% to 68.8%, depending on the initial acrylate UV exposure times.

The network structure of the final systems after initiation and thermal curing of the epoxy were evaluated using FSC. Figure 9 shows the heat flow signals, which have been shifted and scaled to magnify the glass transition signatures ([T.sub.g]). Pure acrylate (Tg = -30[degrees]C) and pure epoxy (Tg = 135[degrees]C) are included as references 20. In general, the FSC signals are not well defined, but show a gradual appearance of the acrylate Tg as the exposure time is increased. The broadening and shifting of this transition suggests the development of structural heterogeneity brought by the epoxy curing in the acrylate network [25]. The occasional appearance of two distinct transitions indicates some local separation of phases [26, 27], while the observation of a single glass transition is the signature of locally homogeneous IPNs [28]. The broad and diffuse transition of the pure epoxy limits clear identification of these characteristics.

Table 1 shows elastic moduli measured by nanoindentation of pure acrylate and pure epoxy and for the IPNs samples as calculated using Eq. 4. The moduli were measured at different locations in the cross-section, included in the table by the noted distance from the edge. As can be seen, the modulus variation in the cross-section is small, which indicates that samples were uniform.

Figure 10a shows the elastic modulus as a function of acrylate content. As shown, the modulus varies from 16.7 MPa to 715 MPa with decreasing UV exposure of the acrylate system. Exposures below 2 s resulted in a weak acrylate network that would fragment during washing and thus indicated a minimum exposure time needed to produce a coherent network. In general, other than the 8 s UV exposure, the variation of the indentation measurement was low over the indentation grid, indicating fairly homogeneous samples. The 8 s UV exposure had a large variation of moduli over the measurement grid indicating an inhomogeneous material, which is consistent with the observation of the two distinct transitions in the FSC. IPNs can also be manufactured by simultaneously curing both the acrylate and the epoxy systems, which is known as simultaneous IPNs. For comparison, Fig. 10b shows the modulus of simultaneous IPNs manufactured under a polychromatic lamp [20]. As can be seen, the trends were similar, but the values of the sequentially made IPNs are smaller.

APPLICATION TO PRINTING

The proposed synthesis method was applied in a printer to manufacture uniform and graded IPNs. Unlike the uniform beam profile used on the ATR, the laser beam of the printer head has a conical power intensity profile, shown in Fig. 11a. As a result, different points on the material surface under the beam are exposed to different power intensities. As the printer head moves, shown in Fig. 11d, the material under the head is exposed to different amount of energy based on the position of the material relative to the centerline of the beam path. The power intensity profile along these relative positions are shown in Fig. 11b for a stationary beam. Figure 11c shows the resulting total energy density that different materials experience when the beam travels over them. As can be seen, the material under the centerline of the beam undergoes a longer exposure time and experiences higher power while both decrease as the distance of the material from the centerline increases. Overlapping of the exposure is also an important factor in printing. As shown in Fig. 1 If, the material located in the overlapping region undergoes two exposures with different exposure times and exposure profiles. The peak power intensity, the diameter of the beam spot, the percentage of overlapping, and the scanning speed are the main parameters that control the printing. These parameters will be selected to make the different acrylate/epoxy IPNs.

IPNs were made in the printer as follows. As shown in Fig. 1 Id, a thin layer of the acrylate system was placed on a hot plate set to 25[degrees]C and the beam was focused in the middle of the layer thickness. The extent of acrylate network formation was controlled using the scanning speed of the printer head and the beam overlap. This was selected based on the estimated relation between UV exposure and acrylate conversion. After printing, the acrylate system was allowed to dark cure for 30 min. The system was then washed in acetone for 1 h at room temperature. It was then swollen with the epoxy system at 50[degrees]C for 8 h in an Argon environment at 1 atm. After wiping the surface, the acrylate/epoxy system was exposed to polychromatic light for 1 min and then thermally cured at 150[degrees]C for 1 h.

Uniform and graded IPNs were printed by selecting the appropriate beam speed and scan overlap. As shown in Fig. lie, the relative location of a point to the centerline of the beam path determines the relative scale of the exposure and the speed determines the total amount of exposure. While points outside the overlapping get a single exposure, point in the overlapping region are exposed twice. As will be shown, both the profile and the number of exposures affect the conversion of the acrylate system.

As the curing kinetics of multiple UV exposures is not well studied, a set of experiments with multiple UV exposures were designed and done using the FTIR-ATR system described in Fig. 3. These multiple exposure experiments included: (a) equal exposures (1 s and 2 s) separated by a fixed delay (2 min), (b) unequal exposures separated by a fixed delay, and (c) equal exposures (2 s) but separated by varying delays (0 s, 25 s, 2 min, 5 min, and 10 min). These experiments were done using a 0.633 [+ or -] 0.005 mm layer of the acrylate system curing on the ATR. The power intensity on the top surface of the sample was set at 10 mW/[cm.sup.2], which due to attenuation of the system results in 6.06 mW/[cm.sup.2] exposure at the point the measurement was done.

Figures 12-14 show comparisons of different curing conditions. Figure 12a presents the conversion for a 2 s exposure followed by dark curing, and for comparison, it also shows two 1 s exposures separated by a 2 min delay and then followed by dark curing. As can be seen, the two 1 s exposures with a delay of 2 min result in higher conversion in comparison to a single 2 s exposure. The same can be seen in Fig. 12b for two 2 s exposures separated by a delay and for a single 4 s exposure. Figure 13 shows two unequal exposure times separated by a 1 min delay. As can be seen in Fig. 13a, an initial 0.67 s exposure followed by a 0.24 s exposure results in a larger conversion compared to switching the order. A similar result can be seen in Fig. 13b using 1.35 s and 0.48 s exposures. Figure 14 presents the effect of the delay time between equal UV exposures. As can be seen, the extent of acrylate polymerization increases with the delay time, but this effect is diminished for intervals greater than 2 min.

Due to the overlapping of exposure during printing, the material in the overlap region is exposed twice. This changes the curing kinetics depending on the exposure times and magnitudes/profiles. As described in Figs. 12-14, the extent of curing depends on the exposure times, delays, and exposure magnitudes/profiles. If one ignores the effects of multiple exposure and optimizes the beam overlap to produce a close to uniform total exposure, one arrives at the exposure profile and overlap described in Fig. 15a. Using results from Figs. 12-14, one can estimate the conversion profile associated with this overlap. Figure 15a also provides the estimated conversion based on this estimation. As can be seen, even though the exposure between beam centerlines is uniform, the conversion is not. On the other hand, one may use the results from multiple exposure studies to estimate the optimal overlap to produce a close to uniform conversion. Figure 15b shows this profile. Even though the variation in the conversion is less, the suggested overlapping results in both a nonuniform power exposure between the beam centerlines, and a nonuniform conversion. Unfortunately, the estimated optimal conversion given in Fig. 15b has low conversion dips that sometimes resulted in sample fragmentation during washing. To avoid this problem, a compromise overlap of 45% between the optimal uniform energy exposure and the optimal conversion was used when printing samples.

PRINTING UNIFORM PLATES

Uniform plates were printed using scanning speeds of 1.85, 2, 2.5, 3, 3.8, and 4.5 mm/s with a beam overlap of 45% as suggested earlier. A 0.43 mm thick layer of the acrylate system was placed on a heating plate set to 25[degrees]C. The peak power intensity of the UV beam was 18.5 mW/[cm.sup.2] and the diameter at the focal point was 1.16 mm. The distance between the centerlines of adjacent scans was set to 0.7 mm (45% overlap). Samples were printed with an approximate size of 15 mm x 8.5 mm. After printing, the acrylate samples were processed as shown in Fig. 2 and described earlier to create the uniform plates shown in Fig. 16.

The mechanical responses of the printed plates were characterized in uniaxial tension at room temperature with a loading rate of 0.001 [s.sup.-1]. Figure 17 provides the stress-strain response, elastic modulus, Poisson's ratio, and ultimate strain of these samples. As shown, the modulus increases from 62.9 to 808.2 MPa as the printing speed was increased, while the ultimate failure strain decreases.

PRINTING GRADED IPNS

Graded IPNs were manufactured in the printer by varying the scan speed. For this printing, the peak power intensity of the conical beam profile was 6.5 mW/[cm.sup.2] with a beam focal diameter of 1.25 mm. The acrylate layer used was 0.264 mm thick with the hot plate set to 25[degrees]C. The scanning path was a single line on which the printer head moved at different constant scanning speeds going from 0.5 to 0.0325 mm/s in steps of 0.0425 mm/s. The head traveled 1.25 mm at each scanning speed. After printing, the system was processed using the same synthesis method described earlier.

The distribution of modulus along the sample was characterized using nanoindentation. Before nanoindentation, 40% from the thickness of the sample was removed (sanded, 1000 grit, -18 [micro]m) and polished (liquid emulsion polishing compound, [Al.sub.2][O.sub.3], 1 [micro]m). Nanoindentation was done at 1-5 mm intervals along the centerline of the beam path in displacement mode with maximum displacement of 1000 nm. Each measurement was done on a 2 x 2 grid of size 5 [micro]m x 5 [micro]m.

Figure 18 shows the variation of modulus along the centerline of the beam path as a function of location along the scan direction. As can be seen, the modulus varied from 30 MPa to 1 GPa as the scan speed of the laser varied from 0.5 to 0.0325 mm/s. The strong grading in the sample appeared in the region between 10 and 20 mm corresponding to the approximate scanning speed of 0.245 to 0.0325 mm/s. Before this range, the scanning speed is too fast to form a strong acrylate network, thus resulting in a material response that is characteristic of the pure epoxy. After this point, the fraction of the acrylate network is large enough to dominate the properties of the acrylate/epoxy IPNs.

CONCLUSION

A method of sequential construction of IPNs is introduced and used to print uniform and graded samples. This method uses the extent of formation of the first network to control the swelling stage and, thus, the properties of the final IPNs. This process of control is different from the traditional method, which typically uses the swelling time to control the system [12].

Uniform acrylate/epoxy IPNs with different properties were synthesized by controlling the extent of UV exposure during formation of the initial acrylate network. This was done on an ATR-FTIR system to allow evaluation of the extent of partial polymerization of the acrylate. The IPNs constructed had elastic moduli varying from 16.7 to 715 MPa, which were characterized by nanoindentation. Uniform plates and a graded acrylate/epoxy system were printed using this control strategy. The scanning speed of the printer head was used to control the partial curing of the acrylate system. The plates made in this way had moduli ranging from 62.9 to 808.2 MPa, and the graded sample had modulus variation from 30 MPa to 1 GPa. The network structures of the IPNs were characterized using fast scanning calorimetry.

Printer parameters to print different IPNs were selected based on a study of the curing kinetics of the acrylate system. These included the effect of single and multiple UV exposure of the acrylate and an estimation of the associated effects on conversion under overlapping laser scans.

ACKNOWLEDGMENT

The authors would like to thank Mr. Mark Gode and Mr. Evan Schwahn for designing and constructing the 3D printer, Mr. Kevin Lefebvre for preliminary work on this project, and Mr. Walker Dimon and Mr. Bienvenu Atawa for the assistance to complete the Flash-DSC.

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Wenlong Li, (1) Nicolas Delpouve, (2) Steven Araujo, (2) Florian Batteux, (2) Emilie Bobo, (2,3) Jean-Marc Saiter, (3) Li Tan, (1) Mehrdad Negahban (iD) (1)

(1) Mechanical & Materials Engineering, University of Nebraska-Lincoln, Nebraska 68588-0526

(2) Groupe de Physique des Materiaux, Normandie Univ, UNIROUEN Normandie, INSA Rouen, CNRS, 76000 Rouen, France

(3) UNIROUEN, Lab SMS, Normandie Univ, 76000 Rouen, France

Correspondence to: M. Negahban; e-mail: mnegahban@unl.edu

DOI 10.1002/pen.25173

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Chemical structures of (a) Bisphenol a ethoxylate dimethacrylate, (b) 2-Hydroxy-2-methylpropiophenone, (c) 3, 4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, and (d) Triarylsulfonium hexafluoroantimonate salts.

Caption: FIG. 2. Schematic representation of the synthesis process for making sequential acrylate/epoxy IPNs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Schematic of the system for evaluating the curing kinetics of free radical photo polymerization of the acrylate [21]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Typical FTIR spectra from 1,400 to 1,700 [cm.sup.-1] during radical polymerization of the acrylate. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Curing kinetics of acrylate polymerization for different UV exposure times followed by dark curing. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Removal of uncured acrylate oligomers. Wash ratio: (a) as a function of washing time in acetone and, (b) as a function of measured acrylate conversion. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Volume change of acrylate systems after washing and after swelling. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Fast scanning calorimetry of pure acrylate and epoxy, and acrylate/epoxy IPNs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Elastic modulus against acrylate content of the (a) sequential acrylate/epoxy IPNs, and (b) previously constructed simultaneous IPNs from the same systems [[20]]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Laser beam profile, total exposure, overlap, and experimental setup of the printer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 12. Comparison of acrylate network conversion for a single long exposure as opposed to two short exposures separated by a delay. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 13. Comparison of acrylate network conversion when changing the order of the UV exposures during unequal exposures separated by a delay. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 14. The effect of delay time on acrylate network conversion during multiple UV exposures. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 15. Total exposure of the material with overlapping beam paths and the associated anticipated conversion of the acrylate network formation. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 16. Acrylate/epoxy plates manufactured in the printer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 17. Stress response and the mechanical properties of the printed plates. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 18. Distribution of modulus along the centerline of the graded IPNs. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Distribution of modulus in the cross-section
(distance measured from edge).

                Thickness   Distance (mm)/elastic
Samples           (mm)      modulus (MPa)

Pure epoxy         --       -12,503.2
2 s UV            0.23      0.019/679.3; 0.04/716.3;
                              0.059/769; 0.099/697.1
4 s UV            0.21      0.054/125.8; 0.101/167.7
6 s UV            0.25      0.046/29.8; 0.09/32
8 s UV            0.24      0.043/17.5; 0.102/15.9
Pure acrylate      --       -/20.2

FIG. 8. Fraction of acrylate in the swollen acrylate/epoxy system
after swelling. [Color figure can be viewed at
wileyonlinelibrary.com]

              Fully cured   8s      6s    4s      2s
              acrylate

Fully cured
acrylate        68.8%

8s                          57.3%

6s                                  48%

4s                                        39.6%

2s                                                22.3%

Note: Table made from bar graph.
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Author:Li, Wenlong; Delpouve, Nicolas; Araujo, Steven; Batteux, Florian; Bobo, Emilie; Saiter, Jean-Marc; T
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2019
Words:5670
Previous Article:Preparation and Characterization of Hydrophilic Temperature-Dependent Polyurethane Containing the Grafted Poly(N-Isopropylacrylamide).
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