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Optical Properties of Photopolymerized Thiol-Ene Polymers Fabricated Using Various Multivinyl Monomers Collin C. McClain, Christopher G. Brown, Jasmine Flowers, Vinh Q. Nguyen, and Darryl A. Boyd Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00856 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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Industrial & Engineering Chemistry Research

Optical Properties of Photopolymerized Thiol-Ene Polymers Fabricated Using Various Multivinyl Monomers Collin C. McClain,1 Christopher G. Brown,1 Jasmine Flowers,2 Vinh Q. Nguyen,3 Darryl A. Boyd3* 1

University Research Foundation, 6411 Ivy Ln Ste 110, Greenbelt, MD, 20770, USA The Washington Center for Internships and Academic Seminars, 1333 16th St., NW, Washington, DC, 20036 3 Optical Sciences Division, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC, 20375 2

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic showing a transparent thiol-ene polymer lens.

Abstract A common tetrathiol was combined with various multivinyl monomers via photoinitiated thiol-ene chemistry reactions to determine the impact of divinyl comonomers on the optical properties of the resulting polymers in comparison to the use of tetravinyl comonomers. The optical properties, thermal properties and hydrophilic character of the resulting polymers were determined and compared to each other. The reactions resulted in transparent, hydrophilic materials that could be polymerized and molded. The results showed that divinyl comonomers polymerized with the common tetrathiol comonomer tended to have greater transmissive character, but lower refractive indices than tetravinyl comonomers polymerized with the common tetrathiol comonomer It was ultimately determined that the varying polymers could all be useful in optical applications that require transparent, soft and/or non-hydrophobic materials.

1. Introduction Durable, transparent materials are ubiquitous and play a necessary role in society. Contact lenses, electronic device displays, food packaging, and vehicle windows are just a few examples of applications in which such materials have become indispensable. Whereas in centuries past glass was the primary material used in materials applications that required transparency, polymers have become a common alternative to glass in the past 100 years. The ability to modify polymers and to impart comparable and/or superior properties to glass make polymers better suited in many daily functions. Thus research to better understand and ultimately improve polymers is a major focus of materials scientists. Polymers fabricated via thiol-ene chemistry have been heavily researched in the past decade,1-4 and have found use in applications ranging from shaped microfibers for biological purposes5,6 to polymeric purification columns.7,8 The simple nature by which thiol-ene polymers can be fabricated, in conjunction with the variety of attainable properties (e.g. flexibility, optical transparency), makes these polymers favorable candidates as useful materials in the area of optical polymers. This includes using thiol-ene chemistry in the development of high refractive index materials,9,10 production of holographic materials,11-


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waveguiding,17 LED encapsulation,18,19 and color tunable nanocomposites.20-22 The variety of applications is a direct result of the versatility of the chemistry involved. Depending on the monomers used and/or the polymerization methodology employed (e.g. thermal cure, photopolymerization), the materials properties can vary significantly. In the present work, the optical, thermal, and hydrophobic/hydrophilic tendencies of thiol-ene polymers (fabricated via UV initiated photopolymerization using various multivinyl comonomers) are used to determine the viability of using these polymers in applications such as optical coatings.

2. Experimental Materials. Pentaerythritol tetrakis (3-mercaptopropionate) (PETMP), pentaerythritol tetraacrylate (PETA), tetravinyltin (TVSn), tetravinylsilane (TVSi), diethylene glycol divinyl ether (DEGDVE), polyethylene glycol divinyl ether [average Mn 250] (PEGDVE), and 2,2-dimethoxy-2-phenylacetophenone (DMPA) were all purchased from Sigma Aldrich and used as received. Food coloring dye was obtained from McCormick & CO., Inc. Polymer Fabrication. In a glass vial, 1 mole equivalent of PETMP was combined with stoichiometric amounts of each of the multivinyl components based on the number of vinyl groups in the various multivinyl materials (i.e. 1 thiol per 1 vinyl). Once the multivinyl was added to PETMP in the glass vial, the components were mixed by vigorous agitation for ~20 seconds. DMPA photoinitiator (0.5 mol %) was then added to the vial components by vigorous agitation/sonication until completely dissolved. The prepolymers were then poured into molds and photopolymerized under UV irradiation (365 nm; 200 mW/cm2) for ~10 seconds. Final polymerized products were then carefully removed from the polymer molds. The polymers used for optical analyses were 1.0 mm in thickness. In order to dye the polymers, the monomers that make up the prepolymer materials were added to a vial along with 0.5 mol % DMPA photoinitiator. ~150 µL of food coloring dye was then added to the same vial and the vial was subjected to vigorous agitation for ~1 minute. The dyed components were allowed to polymerize under ambient light conditions for >72 hours. Spectroscopic Data Collection: UV-visible-NIR transmission data was obtained using an Agilent Technologies Cary 7000 Universal Measurement Spectrophotometer in the range of 250 – 2300 nm. Differential Scanning Calorimetry Analysis (DSC). DSC was performed on a TA instruments Q200. The reference/control material used for this analysis was an empty aluminum DSC cup, and nitrogen was flowed at 50 ml/min over the sealed cups. The temperature was ramped from -80 to 125 °C at a rate of 5 °C/minute. The glass transition temperatures (Tg) are given in Table 1. Thermogravimetric Analysis (TGA). TGA was performed on a TA Instruments 2960 SDT. The samples were heated from room temperature to 400 °C at 10 °C/minute under a nitrogen gas purge. The degradation temperature values given in Table 1 represent the temperature at which 90% of the sample remained. Refractive Index Determination. Refractive indices were measured using a Metricon 2010/M Prism Coupler refractometer. Laser sources used in this experiment were at 636.4 nm, 983.6 nm, and 1548.4 nm. The instrument utilized a gadolinium gallium garnet prism whose index was calibrated at each



respective wavelength using a reference Hi-Index glass (Schott N-LaF3) provided by Metricon. The recorded index values were averaged over five instrument scans. Water Contact Angle Measurements. Water contact angle (CA) measurements were taken using a RaméHart instrument, model #290-U1. Deionized water droplets (10 µL) at room temperature were placed on the surface of smooth, flat thiol-ene polymers. CA values were taken from the left and right side of the water droplets, and an average value taken from these data points (CA uniformity was assumed along the entire surface of each polymer).

3. Results and Discussion

Figure 1. Structure and synthesis route for each thiol-ene polymer.

Polymer Fabrication. The thiol-ene polymers fabricated for the study each had PETMP as the common monomer (Figure 1). While DEGDVE, PEGDVE, and PETA were readily miscible with PETMP, both TVSi and TVSn required longer agitation time to mix well with PETMP. Following the addition of DMPA photoinitiator, each of the polymers was synthesized via UV irradiation in various polymer molds (Figure S1). UV irradiation lasted ~10 seconds for each of the polymers except for polymer 5, which required ~30 seconds to fully polymerize. No post-curing procedures were performed on the polymers. Qualitatively, all of the polymers were transparent (Figure 2). Polymer 5, though transparent, had a slight yellow tint following photopolymerization. It is worth noting that polymer 5 was also the only polymer that contained a metal, while polymer 4 was the only one that contained a semi-metal. Each of the other polymers were completely composed of non-metal elements.


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Figure 2. Digital images showing (a) the various thiol-ene polymers, (b) the curvature of a lens made of polymer 3, and (c) showing the transparency of the polymer 3 lens. Scale bars: 2 cm.

Qualitatively, polymer 3 was the easiest to mold into various shapes due to its apparent hardness. This included fabricating transparent lenses made from polymer 3 (Figure 2b). Polymer 4 was the most difficult to mold, even more-so than the very similar polymer 5. This result was surprising because it was expected that each of the polymers fabricated using the TVSi and TVSn tetravinyls would be stiffer and more durable than the polymers fabricated using the divinyls due to the relative small size of the tetravinyls in polymers 4 and 5. Quantitative mechanical analysis of these polymers will be performed in future work. An additional experiment was conducted in which water-based food coloring dye was added to each of the prepolymer solutions to see if the polymers could be dyed in this fashion. The prepolymers for 4 and 5 rejected the inclusion of the dye despite vigorous agitation. However, the prepolymers 1, 2 and 3 could accommodate the dye. Interestingly, upon exposure to UV irradiation, each of these polymers ‘expelled’ the dye to the outer edges of the newly formed polymer. Because the DMPA photoinitiator functions not only in the UV range, but also in the visible range of the electromagnetic spectrum,23 it was possible to ‘trap’ the dye in polymers 1 and 2 by (much slower) photopolymerization under ambient light (Figure S2). Polymer 3 did not maintain the dyed color under any of the polymerization conditions attempted. That said, as time passed, the dye color in polymer 1 diminished while polymer 2 maintained its color. The fact that polymers 1, 2, 3 could accommodate the dye at all, while polymers 4 and 5 could not, may be due to hydrogen bonding intermolecular forces acting between the water in the dye and the oxygens in polymers 1, 2 and 3. Polymer 2, having more oxygens in it than polymer 1, is likely able to maintain the dye color due to many more intermolecular hydrogen bonding interactions than polymer 1.



Figure 3. (a) UV-vis-nIR comparison plots for the various thiol-ene polymers. (b) Refractive index comparison plots of the various thiol-ene polymers at three wavelengths. Legends indicate the comonomer used along with PETMP to form the final polymer analyzed.

Optical Analyses of Thiol-ene Polymers. All of the polymer transmission plots had absorbances that arose due to the common PETMP comonomer. These features were evident at ~1170-1225 nm, 1380-1440 nm, and 1700-1740 nm, with the strongest absorbance (1700-1740 nm) resulting from the carbonyls present in PETMP (Figure 3a). Qualitatively, each of the polymers appeared transparent, suggesting a high percentage of transmission. UV-Vis-NIR analysis demonstrated this to be true for polymers 1, 2, 3, with each polymer returning transmission values greater than 75% from the visible and into the near infrared (Figure 3a). However, polymers 4 and 5, which contained the tetravinyls TVSi and TVSn, respectively, were far less transmissive (Figure 3a). It is not immediately clear why the transmission for these polymers is so low. Yet, repeat analysis of freshly made polymers using the same compositions for polymers 4 and 5 returned similar results. Perhaps the low transmission for the polymers containing TVSi and TVSn can be attributed to the fact that TVSi contains the semi-metal silicon and TVSn contains the metal tin, while polymers 1, 2 and 3 only contain the non-metals carbon, oxygen and hydrogen in addition to the nonmetal sulfur imparted by PETMP to all of the polymers. With refractive indices ranging from 1.51 to 1.56 at all wavelengths measured, each of the polymers qualifies as a high-refractive index polymer.24,25 Because refractive index (n) increases with the addition of atoms with higher polarizability,9 it would be expected that polymers 4 and 5 would have the greatest n values. Indeed, polymer 5, which contained tin, had a significantly greater n than all the other polymers at each wavelength measured. Although polymer 4 had the next greatest n at 636 nm, its n decreased much more than all of the other polymers at both wavelengths taken in the NIR. Only polymer 2 had n less than that of polymer 4 at each wavelength. That said, the overall high-refractive index nature of all of these polymers is consistent with other similar polymers previously reported.9 Thermal Analyses of Thiol-ene Polymers. The thermal properties of thiol-ene polymers have been wellreported in recent years.1 Numerous reports consistently indicate favorable degradation temperatures exceeding 300 °C,18,19 indicating overall strong thermal stability for this family of polymers. However, thiol-ene polymer glass transition temperatures vary greatly, being often found far below room temperature. Rationales for the variability in the often low Tg values reported include poor conversion of the functional groups (thiol and/or vinyl), inadequate curing times, thiol-ene monomer combinations


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utilized and the use of photoinitiators (which have been reported to weaken the thermal and mechanical properties of polymers).9,26-29

Table 1. Thiol-ene polymer thermal analysis data.

Polymer

Tg (°C)

90% Decomposition Temp. (°C)

1

-33.43

336.2

2

-40.7

336.1

3

13.17

358.5

4

-45.79

340.6

5

-15.53

293.3

Polymers 1, 2, 3, and 4 returned degradation temperatures ranging from 335 to 360 °C (Table 1). Even though polymer 5, which contained tin in its structure, had a significantly lesser degradation temperature of 293 °C, it still qualifies as a thermally stable material for most optical applications that utilize polymers instead of glass. Whilst their degradation temperatures were nearly identical, polymers 1 and 2 (which had similar polymer structures) had notably different Tg values (Table 1), though both were far below 0 °C. When compared to polymer 1, the inclusion of many more units of ethylene glycol in polymer 2 appears to result in decreased Tg, likely due to the increased length of each polymer unit. Even more dramatic was the difference in both the degradation temperatures and the Tg values for polymers 4 and 5 despite the fact that the only difference in their construction was the presence of silicon (from TVSi) in place of tin (from TVSn) in their backbone (Table 1). This difference in thermal character between polymers 4 and 5 may be attributed to a number of factors, including the much larger atomic size of tin in comparison to silicon. Among all of the thiol-ene polymers studied, polymer 3, which was composed of two similar monomer components (PETA and PETMP), had both the greatest Tg and decomposition temperature by a large margin. The comparatively high Tg and degradation temperature for polymer 3 may arise due to the similarity in both size and composition of its monomer components. These similarities could allow for greater polymer unit stacking and overall polymer density, thus resulting in the greater thermal values.



Figure 4. Photograph images of 10 µL water droplets on the surface of each of the thiol-ene polymers. Labels indicate the polymer on which the water droplet is resting. Table 2. Thiol-ene polymer water contact angle data.

Polymer

Left Angle (°)

Right Angle (°)

Average (°)

1

54.4

55.3

54.9

2

60.4

66.9

63.6

3

65.0

68.7

66.9

4

78.4

82.3

80.3

5

70.8

68.2

69.5

Water Contact Angle Analyses of Thiol-ene Polymers. In order to better evaluate the value of these polymers for use in certain applications, surface water contact angle analysis was employed (Figure 4). With all of the polymers containing oxygen in their structure as a result of the common comonomer PETMP, it was expected that each of the polymers would exhibit a notable degree of hydrophilicity. Furthermore, it was conjectured that polymers 1, 2 and 3 would be more hydrophilic than polymers 4 and 5 because the comonomers used to fabricate them added even more oxygens to the polymer structures. These hypotheses proved to be correct, as polymers 4 and 5 were indeed less hydrophilic than the others, with polymer 4 being the most hydrophobic of them all (Table 2). These data suggests that the polymers in this study could be useful in applications where water-repellency is not a requirement.

4. Conclusions An analysis of the optical, thermal and hydrophobic character of various thiol-ene polymers, that shared a common comonomer, was performed. All of the polymers synthesized were transparent, high in refractive index, possessed significant thermal stability and were hydrophilic in character; yet, there was a range of polymer characteristics within the group. In general, the thiol-ene polymers fabricated using divinyl comonomers tended to have greater optical transmission but lesser refractive index than the thiolene polymers that contained tetravinyl comonomers. The divinyl comonomers also resulted in thiol-ene polymers with very low Tg. The varying optical and thermal properties indicate that certain comonomer combinations should be used for targeted applications. For example, polymer 3 (composed of PETMP and PETA) would be the best polymer for applications that require a highly transmissive, high refractive index,


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and high thermal stability material. In general, given their properties, polymers like the ones fabricated for this study may find use as thermoplastic elastomers,30 LED encapsulating materials,18,19 or in soft imprint lithography.31

ASSOCIATED CONTENT Supporting Information Images of the polymer molds, images of molded and dyed polymers, and thermal analysis plots are included. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The author declares no competing ﬁnancial interest. ACKNOWLEDGMENTS This work was supported by The Washington Center for Internships and Academic Seminars HBCU-MI Summer Internship Program. The views are those of the authors and do not represent the opinion or policy of the US Navy or Department of Defense.

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Table of Contents Graphic showing a transparent thiol-ene polymer lens. 129x91mm (150 x 150 DPI)


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Figure 1. Structure and synthesis route for each thiol-ene polymer. 245x181mm (150 x 150 DPI)



Figure 2. Digital images showing (a) the various thiol-ene polymers, (b) the curvature of a lens made of polymer 3, and (c) showing the transparency of the polymer 3 lens. Scale bars: 2 cm. 248x64mm (150 x 150 DPI)


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Figure 3. (a) UV-vis-nIR comparison plots for the various thiol-ene polymers. (b) Refractive index comparison plots of the various thiol-ene polymers at three wavelengths. Legends indicate the comonomer used along with PETMP to form the final polymer analyzed. 245x88mm (150 x 150 DPI)



Figure 4. Photograph images of 10 µL water droplets on the surface of each of the thiol-ene polymers. Labels indicate the polymer on which the water droplet is resting. 198x50mm (124 x 140 DPI)


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