Photothermal Control over the Mechanical and Physical Properties of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Photothermal Control over the Mechanical and Physical Properties of Polydimethylsiloxane R. Joseph Fortenbaugh, Sabrina A. Carrozzi, and Benjamin J. Lear* Department of Chemistry, Penn State University, University Park, Pennsylvania 16802, United States

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S Supporting Information *

ABSTRACT: Though it is known that the photothermal effect of nanoparticles can be used to greatly increase the rate of polymer curing, at present, little is known about how the parameters of photothermal curing affect the desirable chemical and physical properties of the cured polymer. We report the swelling, gel fraction, and Young’s modulus for the thermoset polydimethylsiloxane cured under a variety of photothermal and traditional conditions. We find that all of these properties can be tuned via the intensity of light and propose that the crosslink density within the thermoset decreases with increasing intensity of light during curing.

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INTRODUCTION Polymer chemistry has long focused on controlling physical and chemical properties of polymeric materials, motivated by a breadth of applications that lead to a similarly wide demand for properties such as softness, flexibility, chemical resistance, wear resistance, and coefficient of friction.1 Although changes in polymer properties can be realized by changing the underlying chemistry of the polymer, it is also desirable to tune polymer properties within any given polymer system. In this manner, physical properties can be tuned without the need to optimize fundamentally new chemical reactions. For thermoplastics, control over physical and chemical properties can be accomplished by changing parameters such as molecular weight, polydispersity index, or the degree of branching.2 For thermosets, the density of crosslinking provides the dominant parameter controlling their emergent physical and chemical properties.2 In order to continue to expand the utility of established polymeric systems, it is necessary to develop new means of controlling these parameters. Recent work from our laboratory3,4 and others5−7 has identified photothermally driven chemistry as a new way to generate polymers in an on-demand fashion. For thermosets, this approach can increase the rate of polymerization or crosslinking by up to a billion-fold. Such large rate increases are realized by supplying nanoscale heat with temperatures exceeding 1500 K. In brief, we disperse strongly lightabsorbing nanoparticles within a batch of polymer precursor. When light is shined on this reactive mixture, the nanoparticles thermalize the light, producing a set of well-dispersed heat sources within the mixture. Given the small size of the particles, it is possible to heat them by thousands of degrees, and return them to room temperature on the order of nanosecondsheating only cubic nanometers at a time.8−15 The tight control over heat distribution allows for the reaction to be run cleanly at extreme temperaturesand hence extreme rates. © XXXX American Chemical Society

In addition to generating polymers, photothermal heating has been shown to drive small molecule chemistry,16−20 effect phase changes,21,22 and heal physical polymer defects.23,24 Thus, it is clear that photothermal heating is generally useful for controlling the chemistry and structure of environments local to the photothermal agent. Because it is the local structure that ultimately controls the emergent physical properties of polymers, the prior work in photothermal heating suggests that such local heating may provide a means for ondemand control over the final physical properties of polymersfor instance, by controlling the cross-linking density.25,26 In addition, due to the need to disperse photothermal agents within the reactive system, this approach naturally generates nanocomposite materials, the nature of which can also be varied to tune physical properties.27−31 In total, photothermally driven transformations promise new means of controlling the physical properties of polymeric materials. In order to test the above hypothesis, we have studied the effects of photothermal curing on the final physical and chemical properties of bulk samples of polydimethylsiloxane (PDMS). PDMS is a thermoset widely used in applications spanning biomedical implants,32 protective coatings,33,34 resistance coatings,33 microfluidics,35 and textiles.36−38 Effective usage of PDMS across this range of applications requires tuning of the material’s final physical properties. Thus, PDMS offers a thermoset system that is commercially relevant, and for which there is a clear desire to fine-tune physical properties. Herein, we compare the swelling39,40 and tensile41,42 properties of a PDMS system (Sylgard 184) cured both conventionally and photothermally using gold nanoparticle (AuNP) photothermal agents. A schematic of the hydrosilation cross-linking Received: January 18, 2019 Revised: April 18, 2019

A

DOI: 10.1021/acs.macromol.9b00134 Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Experimental Conditions Employed in This Studya

reaction for this system is shown in Figure 1. We find that both of these measured properties respond to photothermal

Figure 1. Reaction scheme for curing of PDMS.

polymer

[AuNPs]/wt %

light/MW

Toven/°C

i ii iii iv v vi

PDMS PDMS PDMS PDMS PDMS PDMS

0 0 0.05 0.05 0.05 0.05

0 50 0 10 25 50

100 100 100 100 100 100

a

Provided is the condition number, the polymer used, the weight of AuNPs employed for the final composite, the fluence of light the samples were exposed to, and the temperature of the oven used for the final curing of the samples.

conditions, and use standard models2 of thermoset polymers to explain these changes in terms of changes to cross-link density.

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condition

Sylgard 184 (7.3 cal1/2 cm−3/2),44 which is expected to facilitate swelling measurements. Swelling percentage is calculated as (mswell/mdry) × 100%, where mswell is the mass of the disc swollen with solvent and mdry is the final mass of the dry disc post swelling. In this way, we compare the swollen mass to the mass after any uncrosslinked PDMS oligomers were removed. Figure 2 and Table 2 show the average percent swelling, which ranges from 167 to 303%, and standard deviation observed for the samples. The swelling we observed for pure PDMS is slightly higher than reported in prior work using Sylgard 184 in hexane.44 Although the use of the Soxhlet apparatus means that the temperature of the swelled polymer is above room temperature, the degree of swelling is expected to decrease with increasing temperature for alkanes in PDMS.45 Thus, it is likely that this difference, compared to prior work, may be a result of different ratios of elastomer to crosslinking agent or differences in the efficiency of mixing before curing. From the swelling experiments, we can also arrive at an approximate determination of the Young’s modulus (E) for the polymer. This is carried out by using the Flory−Rehner equation46 ÄÅ É v ÑÑ Å − [ln(1 − v2) + v2 + X1v2 2] = n·V1ÅÅÅÅv21/3 − 2 ÑÑÑÑ ÅÇ (1) 2 ÑÖ

EXPERIMENTAL SECTION

Sylgard 184 silicone elastomer base and curing agent (Dow Corning, Midland, MI) were used in all experiments. All experimental conditions used a 10:1 ratio of elastomer to curing agent. AuNPs were synthesized based on a procedure by Ryu et al.43 Hydrogen tetrachloroaurate(III) hydrate (Alfa Aesar, Haverhill, MA) was added to ultrapure (type 1) water from a Labconco WaterPro BT. The aqueous gold solution was mixed with the PDMS elastomer to form 0.05% wt Au samples. Samples were mixed for about 10 min until the gold salt was homogeneously distributed throughout the prepolymer, then placed in a desiccator for an hour to remove air bubbles, and then heated in an oven at 100 °C, where reduction of the gold-salt to nanoparticles occurred over three to 4 h. Reduction of nanoparticles was accompanied by a color change from yellow (Au-salt) to ruby red (AuNPs). For all experimental conditions, samples were irradiated with 532 nm light (Quanta Ray 130 Nd:YAG). The laser generated 8 ns pulses at a 10 Hz repetition rate and was operated at fluencies of 10, 25, and 50 MW•cm-2. Swelling was measured using PDMS discs with a diameter and height of 1 cm. These were cast in molds using either pure PDMS or the AuNP/PDMS elastomer composite. The samples were either left unirradiated or irradiated for 4 min. After these 4 min, the discs were placed in an oven at 100 °C to ensure full curing of PDMS samples. After curing, samples were weighed and placed in a Soxhlet extractor. The Soxhlet was charged with hexane and samples were run for at least 24 h. After removing the samples from the solvent, they were quickly weighed, dried in an oven, and then weighed once more. From these measurements, the percent swelling of the discs in hexane and their mass loss was determined. To perform tensile testing of the samples, micro-tensile dog-bone molds were created based on the ASTM Standards D1708-1341 and D638-14.42 Molds were either exposed to irradiation for 4 min or not irradiated. For samples exposed to laser irradiation, the laser was aimed at the center of the wishbone. All samples were cured in an oven at 100 °C. After curing, the samples were subject to mechanical testing using an Instron 5966 (1 kN) instrument with a crosshead speed of 500 mm/min. The Young’s modulus was calculated using the linear region (initial 10%) of the stress−strain curve.

where v2 is the polymer volume fraction in the swelled state, X1 is the dimensionless solvent parameter for solvent/polymer pairs (X1 = 0.39 for hexane/PDMS),47 V1 is the molar volume of the solvent (0.000132 m3/mol for hexane), and n is the crosslink density. Using the swelling results, we can solve for n, which in turn can be used to calculate the Young’s modulus using the relationship E ≈ 3nRT

(2)

where R is the gas constant and T is temperature. Thus, the Young’s modulus is directly related to the crosslink density. Values of E determined in this manner usually yield values within a factor of 2 or 3 of those determined by mechanical measurements.2 The moduli we obtained from our swelling data (Eswell) are given in Table 2 and shown in Figure 3. Using the mass of the PDMS disc before swelling (minitial), we can also determine the mass of uncrosslinked oligomers removed from the cured PDMS or the gel fraction, calculated as 1 − (minitial − mdry)/minitial. However, due to the high-degree of curing, it is easier to represent the fraction of mass lost during the swelling experiment, which is equal to 1 − (gel fraction) = mass of uncrosslinked oligomers. This value is another indication of the efficiency of forming crosslinks between polymer chains. Figure 4 shows the average and standard deviation of this value, which varies from 4.35 to

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RESULTS Samples of PDMS were prepared and exposed to six different curing conditions that explored the permutations of AuNPs and light, as well as various intensities of irradiation. Prior work showed that the rate of curing depended on the laser fluence, and so we hypothesized that the cross-link density (and emergent physical properties) would as well. The conditions explored in this study are given in Table 1. All individual values of our measurements are recorded in Table S1. Averages and standard deviations of these values are collected in Table 2. Swelling Experiments. Swelling and mass loss experiments were performed in hexane. Hexane was chosen because its solubility parameter (7.3 cal1/2 cm−3/2) matched that of B

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Table 2. Average Value ± Standard Deviations of the Measured and Extracted Physical Parameters: 1 − Gel Fraction, % Swell, Eswell, and Emecha condition i ii iii iv v vi

1 − gel fraction 4.59 4.35 4.90 4.70 5.42 5.77

± ± ± ± ± ±

0.26 0.10 0.04 0.04 0.48 0.67

Eswell/MPa

% swell 167.63 178.15 191.43 185.93 249.26 303.12

± ± ± ± ± ±

8.38 0.93 1.12 1.51 9.46 11.23

1.81 1.46 1.15 1.27 0.52 0.30

± ± ± ± ± ±

0.09 0.01 0.01 0.01 0.02 0.01

Emech/MPa 4.85 4.07 1.57 1.21 1.68 0.83

± ± ± ± ± ±

0.28 0.41 0.14 0.13 0.12 0.18

The value, 1 − gel fraction is the same as the mass fraction of unreacted oligomers.

a

5.77%. The value of mass loss for the pure PDMS is consistent with prior reports for Sylgard 184 in pentane.44 Stress−Strain Measurements. We recorded stress strain curves for samples cured under all six conditions. Representative curves for conditions (i) and (iii) are shown in Figure 5. These, and all other measurements (Figure S1), produced curves consistent with an elastomer.

Figure 2. Percent swell for all conditions investigated. The height of the bar is the average of the measured values, and the error bars mark ± one standard deviation.

Figure 5. Stress−strain curves obtained for samples cured under both condition (i) and condition (iii). Curves are shown only to the point of breakage. These measurements used the ASTM Standards D17081341 and D638-14.42

From the tensile measurements, we extracted the Young’s modulus (Emech), the percent elongation at break, the tensile strength, and the toughness (area under the stress−strain curve). Figures 6, S2, S3, and S4 show the average and standard deviation of these properties, respectively. The main focus in this paper is on the Young’s modulus, which varied from 4.85 to 0.83 MPa and is recorded in Table 2 for all conditions.

Figure 3. Young’s modulus determined from swelling measurements for all experimental conditions. The height of the bar is the average of the measured values, and the error bars mark ± one standard deviation.

Figure 4. 1 − gel fraction measured for all experimental conditions. This value is equal to the mass fraction of uncrosslinked oligomers within the polymer. The height of the bar is the average of the measured values, and the error bars mark ± one standard deviation.

Figure 6. Young’s modulus determined from tensile measurements for all experimental conditions. These measurements used the ASTM Standards D1708-1341 and D638-14.42 The height of the bar is the average of the measured values, and the error bars mark ± one standard deviation. C

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DISCUSSION

Effects of Illumination of Pure PDMS. Comparing the results of conditions (i) and (ii) allow for comment on the impact of illumination on pure PDMS. Within error of experimental measurements, both swelling and gel fraction of pure PDMS is insensitive to illumination. The Young’s modulus determined by swelling (Eswell) or mechanical (Emech) measurements decreased by 19 and 16%, respectively, upon illumination. Thus, it seems that illumination reduces the number of active chains in the elastomer. However, the fact that we do not see meaningful changes in the gel fraction suggests that we are decreasing the number of crosslinks per chain, but not to the point of significantly increasing the number of chains without a single crosslink. Addition of AuNPs to PDMS. Comparing the results of conditions (i) and (iii) allow for comment on the impact of incorporating AuNPs into the polymer matrix. Addition of AuNPs results in an increase in swelling (14% increase over pure PDMS) and fraction of uncrosslinked oligomers (6.8% increase compared to pure PDMS), provided that the samples are not illuminated. This suggests that the crosslink formation has decreased significantly, producing a lower crosslink density and fewer chains without a single crosslink. Consistent with the loss of crosslinks, the Young’s modulus decreases dramatically upon incorporation of AuNPs. Eswell decreases by 36% and Emech decreases by 68% compared to pure PDMS. Although it is expected that composite materials will have moduli that differ from their parent materials, incorporation of a hard material into a polymer is expected to increase the modulus. We believe the decrease in crosslinking efficiency arises due to in situ formation of AuNPs using a gold-chloride salt. HCl is an expected byproduct, which is known to interfere with the hydrosilation crosslinking reaction. In addition, AuNP formation is thought to use the SiH sites (needed for crosslinking) for reduction of the gold-salt, which would reduce the number of available crosslinking sites. Both of these effects should produce a decrease in density of crosslinks. It is possible that future work could identify other photothermal agents that can be incorporated into the PDMS without decreasing crosslinking. Effects of Photothermal Curing. Because illumination of pure PDMS resulted in a decrease in crosslink density, it might be expected that the illumination of the AuNP/PDMS composite will result in a further change to this property. Indeed, we find that swelling, gel fraction, and Young’s modulus all show clear dependencies on illumination intensity. These dependencies are shown in Figure 7. Because synthesis of AuNPs within the polymer resulted in such large changes in crosslinks, it is best to compare the effects of illumination to the unilluminated composite (condition (iii)). Illumination of the composite during curing produces up to a 58% increase in swelling, a 18% increase uncrosslinked oligomers, and a 74 and 47% decrease in Eswell and Emech, respectively. All of these measurements indicate a reduced crosslinking density resulting from photothermal curing. An alternative hypothesis to a decrease in crosslink density explaining the change in physical properties is that the photothermal curing leads to morphological changes in the PDMS, and these morphological changes are responsible for the observed changes in physical properties. Past work on photothermally cured polymers has shown striking differences in morphology between photothermally and traditionally cured

Figure 7. Dependence of various physical properties of PDMS on the fluence of light used during curing for both PDMS without AuNPs (blue) and those with AuNPs (orange). Shown are values for all individual measurements of (top) percent swelling, (middle) 1 − gel fraction, which is equivalent to the mass fraction of uncrosslinked oligomers, and (bottom) Young’s modulus determined from swelling measurements (circles, dashed lines) and mechanical measurements (diamonds, solid lines). All lines are least squares fits to the data.

polymers.7 In order to test this hypothesis for our work, we obtained SEM images of polymers generated under condition (iii) and condition (iv). Examples of typical images found for these polymers can be seen in the Supporting Information. We do observe some qualitative differences between these polymers. Namely, the surface of the photothermally cured polymers seem to possess less large features, although both polymer samples show similar striations in the film. Quantifying these changes, and searching for a more definitive mechanism for our observed physical properties will be the subject of future work. Regardless of the exact mechanism of action, it is clear thatin terms of relative changes to the measured propertiesphotothermal conditions produce a stronger lightdependence than experienced by pure PDMS. This indicates that the photothermal conditions provide an increased ability to tune the final properties of the cured thermoset. For the system we have examined, photothermal curing appears to result in a decrease in crosslink density. The underlying mechanism for the decrease in crosslinking under photothermal curing remains unknown; however, the temperatures D

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the photothermal agents are thought to reach are well above the devitrification temperature and char temperature of PDMS. Regardless of the exact mechanism, this work clearly shows that photothermal curing provides access to a wide range of properties for the final cured PDMS.

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CONCLUSIONS

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00134. Bar charts for elongation, tensile strength, and toughness, stress−strain curves for all samples, table of measured values of swelling, mass loss, elongation, tensile strength, and Young’s modulus for all samples, SEM images of thermally of photothermally cured polymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (814) 867-4625. ORCID

Benjamin J. Lear: 0000-0001-5624-7991 Notes

The authors declare the following competing financial interest(s): Professor Lear owns equity in Actinic, which has an interest in this project. Dr. Lears ownership in this company has beenreviewed by the Universitys Individual Conflict of Interest Committee and is currently being managed by the University.

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REFERENCES

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We have demonstrated that photothermal curing provides a way to control and tune the physical properties of cured PDMS. The data indicates that increasing light intensity results in greater swelling, increased mass of uncrosslinked oligomers, and reduced Young’s modulus. These changes remain despite curing in an oven after exposure. Thus, the changes in physical properties arising from illumination appear to result from permanent alteration of the polymer network. It is worth noting that, although this has been achieved for thermally activated polymer reactions, light was used to induce it. This suggests that one can control the physical properties of thermally cured systems with the precision afforded by our control over light. In other words, this work suggests that one can attain the same level of control over physical properties of thermally cured systems, as is currently enjoyed by optically cured systems. The exact mechanism producing these permanent changes remains unknown, and further work is needed to fully understand the chemical changes photothermal curing produces, as well as the full range of physical properties which can be achieved using this approach.

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ACKNOWLEDGMENTS

The authors thank Dr. Jian Yang, professor of Biomedical Engineering at the Material Research Institute at The Pennsylvania State University, for allowing the use of their tensile testing devices. E

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