Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
NMR Methodologies for the Detection and Quantification of Nanostructural Defects in Silicone Networks Jennifer N. Rodriguez, Cynthia T. Alviso, Christina A. Fox, Robert S. Maxwell, and James P. Lewicki* Materials Science Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States S Supporting Information *
ABSTRACT: We present and discuss a sensitive spectroscopic means of detecting and quantifying network defects within a series of polysiloxane elastomers through a novel application of 19 F solution state nuclear magnetic resonance (NMR). Polysiloxanes are the most utilized non-carbon polymeric material today. Their final network structure is complex, hierarchical, and often ill-defined due to modification. Characterization of these materials with respect to starting and age-dependent network structure is obfuscated by the intractable nature of polysiloxane network elastomers. We report a synthetic strategy for selectively tagging chain-end silanols with an organofluorine compound, which may then be conveniently and quantitatively measured as a function of structure and environment by means of 19F NMR. This study represents a new and sensitive means of directly quantifying aspects of network architecture in polysiloxane materials and has the potential to be a powerful new tool for the spectroscopic assessment of structural dynamic response in polysiloxane networks.
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polyhedralsilsequioxanes,15,16 carboranes,17,18 nanotubes,19 and platelets.20 On a molecular scale, this modification results in a complex, heterogeneous, intractable, and often ill-defined network structure. Because of this combination of structural complexity and intractability, polysiloxane elastomers are challenging to characterize by most spectroscopic methods. Today there is no single suitable spectroscopic method which can differentiate information received relating the network interactions within the resulting spectra due to bulk assessment of the structural markers.21 Furthermore, many network level functionalities (e.g., free chain ends, chemical cross-link points, and stress-induced chain scissions), which, despite having a significant impact on the dynamics and properties of polysiloxane elastomers, occur at the nano- to picomolar levels with respect to the total network and as such are often beyond the limits of resolution of common spectroscopic techniques. Some knowledge of the original structure and components of engineered silicones can be understood from destructive techniques, such as pyrolysis, which allows for the interpretation of the thermal degradation products of the engineered silicones.22 Other techniques such as solid state nuclear magnetic resonance (NMR), attenuation total reflectance Fourier transform infrared (ATR-FTIR), and Raman spectroscopy are limited to bulk or surface analysis which generates broadly averaged data21,23 and limited structural resolution. Of particular interest to many researchers and technology areas is the long-term stability of polysiloxane elastomers under
INTRODUCTION Polysiloxanes are a significant class of non-carbon backboned elastomeric material1,2 having the base repeating unit consisting of [−Si(R2)−O−]x where the R groups may be a range of moieties including methyl, ethyl, phenyl groups, and others. The most common polysiloxanes produced today are polydimethylsiloxane (PDMS). PDMS has attractive physical properties, such as low surface energy, high electrical breakdown/bulk resistance, and elasticity over a broad range of temperatures.3 In addition, these materials also tend to have good chemical stability, low toxicity, optical clarity, and oxidative resistance.4 Because of their versatility, PDMS polymers, typically in the form of high molar mass fluids or cross-linked network elastomers, have been used in many research and commercial applications, such as sealants, microfluidics, potting compounds, electronic encapsulants,5 microelectromechanical systems (MEMS),6 insulators, medical devices,7 and cushions.8 Unmodified polysiloxane networks generally possess poor mechanical properties, exhibiting low tensile strength and minimal elongation prior to breakage; they tend to be brittle and typically fail easily under shear or compression.9 In order to improve the mechanical properties and robustness of polysiloxane networks, commercial polysiloxane elastomers are “filled” with a heterogeneous secondary phase to reinforce the polymer matrix,4,10−12 forming a composite material. Further improvements in mechanical and physical properties are typically achieved through modification of network architecture13 (modality, cross-link functionality/density, free chain ends), the inclusion of copolymers within the network,14 and even the incorporation of nanomodifiers such as This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Received: October 20, 2017 Revised: January 30, 2018
A
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
a
2.0
2.0
1.0 × 10−4
1.0 × 10−3
1.0 × 10−2
1.0 × 10−1
1.0 × 100
2.0 × 100
3.0 × 100
4.0 × 100
5.0 × 100
1.0 × 100
8.0 × 10−3
9.9 × 10−2
9.8 × 10−1
9.7 × 100
9.3 × 100
1.8 × 102
2.5 × 102
3.2 × 102
3.9 × 102
6.4 × 102
B
f = number, corresponds to number of reactive groups.
2.0
2.0
2.0
2.0
2.0
2.0
5.0
1.0 × 101
2.0
mmol of DMS-V21, MW 6000 g/mol, divinyl PDMS, f = 2
% silanol defects added per gram
amount of silanol defects added to Model 13 per microgram
3.6 × 10−1 6.5 × 10−1 9.8 × 10−1 1.3 1.6 2.0 3.6
4.0 × 10−1 4.0 × 10−1 4.0 × 10−1 4.0 × 10−1 4.0 × 10−1 4.0 × 10−1
3.3 × 10−1
9.8 × 10−1
4.0 × 10−1
4.0 × 10−1
1.0
2.0
3.3 × 10−1
4.0 × 10−1 2.0
mmol of tetrakis(dimethylsiloxy)silane MW 328.73 g/mol, silane cross-linker, f = 4
mmol of DMS-V31, MW 28 000 g/mol, divinyl PDMS, f = 2
Table 1. Silicone Model Networks with Engineered Defectsa
7.0 × 103
4.0 × 103
3.0 × 103
2.0 × 103
1.0 × 103
7.0 × 102
7.0 × 101
7.0
2.0
4.0 × 10−1
μmol of VDS-1013, MW 600 g/mol, disilanolterminated PDMS, f = 1
6.4 × 102
3.9 × 102
3.2 × 102
2.5 × 102
1.8 × 102
9.3 × 101
9.7 × 100
9.8 × 10−1
9.9 × 10−2
8.0 × 10−3
amount of silanol defects added to the formulation, μmol normalized to mass 5.7 × 104 ± 2.0 × 104 1.2 × 105 ± 7.6 × 103 1.1 × 105 ± 4.7 × 103 7.6 × 104 ± 2.8 × 104 7.8 × 104 ± 2.2 × 104 3.7 × 105 ± 4.7 × 105 9.2 × 105 ± 4.0 × 104 7.0 × 105 ± 4.5 × 105 1.3 × 106 ± 5.9 × 105 2.5 × 106 ± 2.8 × 105 7.1 × 106 ± 5.8 × 106
av mol wt between crosslink density, M⟨x⟩
1.0 × 10−6 ± 3.7 × 10−7
1.4 × 10−7 ± 6 × 10−8
2.4 × 10−8 ± 8.1 × 10−9
1.4 × 10−8 ± 1.1 × 10−8
4.2 × 10−9 ± 2.1 × 10−9
2.4 × 10−9 ± 1.7 × 10−9
2.8 × 10−9 ± 3.0 × 10−10
no. of silanols per gram observed by 19F NMR, normalized to mass of sample
Macromolecules Article
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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a range of often harsh physical and chemical conditions.24−33 Polysiloxane networks undergo a range of degradation mechanisms, which include a broad spectrum of temporal and spatial scales ranging from the molecular, meso, and macro scale.21 Common causes of degradation within polysiloxane materials are environmental influences, such as irradiation, mechanical stresses, and thermal exposure. Degradation reactions within polysiloxane materials involve concurring competing processes of backbone chain breaking (scission, backbiting, and unzipping) and newly formed bonds (via free radical or silanol rearrangement cross-linking) mechanisms.24 In addition to the degradation mechanisms that occur within the bulk network, breaking and forming of bonds at the polymer−filler interface are concurrently occurring within many aging scenarios. Chain scission of the polysiloxane backbone is a key marker of stress-induced degradation and aging, common to a range of degradation processes, and results in the formation of two new chain end silanols [−Si−OH] for every chain scission event. Previously, nuclear magnetic resonance (NMR) has been used to characterize the structure of polysiloxane networks with respect to age and degradation.21 However, both pre- and postdegradation, isolation of the specific chemical moieties methyl [−CH3] groups on the backbone and hydrogen atoms attached to the silanol [−Si−OH] ends and silica filler functionality has been beyond the limits of 1H, 29Si, or 13C NMR spectroscopic techniques, making the direct quantitation at molar levels that are relevant to many aging and degradation processes, impossible. 19 F NMR is both highly sensitive and specific within the context of a material such as a polysiloxane where there is no native 19F bath to present a signal background or interference. It has been previously demonstrated34 that reactive organofluorine species may be utilized as a chemical tag for silica silanol and alcohol specieswith a demonstrated detectability down to 1.00 × 10−6 mol using solid state MAS NMR.35 The approach we report here is the modification of this basic strategy and its application to the detection and quantification of age-induced chain scission events within polysiloxane elastomer systems. Through use of chlorodimethyl-3,3,3trifluoropropylsilane as a reactive marker, we have been successful in chemically labeling or “tagging” network silanols within a range of polysiloxane networks and have demonstrated that through subsequent swelling of the tagged networks in deuterated solvents we have been able to apply high-resolution solution-state 19F NMR to quantify network silanols at concentrations as low as 2 × 10−9 mol/g in this study. These quantified levels of silanols have been correlated with both γradiation-induced damage to the native networks and also engineered levels of free chain ends within a range of network architectures.
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(VDS 1013, Gelest, Morrisville, PA) using the reported molar ratios (Table 1) for the vinyl PDMS monomers DMS V21 and DMS V31. The defect engineered model networks were prepared using the given proportions A:B. The components were combined using the appropriate molar ratios and catalyst, 4 μL/g of material made, and mixed at 3500 rpm using a (Flakteck) speed mixer for 60 s. After initial mixing, a minimum amount of 6:1 solution of toluene:acetone, approximately 5 mL/10 g of PDMS, was added to the polymer mix until phase separation was no longer observed after a second mixing of 60 s at 3500 rpm. The polymer and solvent mixture was sealed in 20 mL scintillation vials, and the resultant networks were initially cured for 12 h at 80 °C. After an initial cure of the polymer matrix the materials were demolded from the vials and postcured for an additional 24 h at 120 °C while under a 30 mmHg vacuum. For model networks with higher levels of defects (5−10% silanol defects), a higher temperature of 120 °C was required for the initial cure step. Solvent uptake studies were performed on all samples in this study. Approximately 100 mg samples were cut from the center of the volume of the cured polymers and their initial masses recorded. These samples were placed into 20 mL scintillation vials with a mixture of (5:6) toluene:acetone (spectroscopy grade). These vials were placed in a heated circulating bath at 29.5 °C for 24 h. After an initial 24 h, the solvent was removed from each vial, and the swollen samples were accuratley weighed and recorded. The vials were replenished with the same toluene:acetone solution and placed back into the 29.5 °C bath for a subsequent 24 h. The second swollen masses were recorded for each sample and were placed on glass slides. The samples were then placed in a vacuum oven at 80 °C while under 30 mmHg vacuum for 12 h. The final dried masses were recorded for each of the samples. The Flory−Huggins relationship was used to determine average crosslink density, average cross-link distance, and sol fraction; three samples were tested per concentration (n = 3) for all formulations.38 Irradiation of Commercial Polymers. Engineered silicones, unfilled, cured Sylgard 184 (Dow Corning, Auburn, MI), and silica filled SE1700 (Dow Corning, Auburn, MI) were prepared according to their respective specifications. Rectangular samples of approximately 1 × 2 × 6 mm rectangular dimension were cut and placed into nitrogen backfilled sealed NMR tubes. The samples were then exposed to a 60 Co radiation source at a dose rate of 0.5 kGy/h until the desired dosages were reached and ranged from 1 to 25 Mrad. Solvent uptake studies were performed postirradiation using the previously described method. 19 F Tagging of Monomers and Polymer Samples. All silicone materials, including model networks, irradiated Sylgard 184, and SE1700, were tagged with chlorodimethyl-3,3,3-trifluoropropylsilane as purchased from Sigma-Aldrich: All glassware was silanized prior to exposure to the fluorine tag using 1:10 dichlorodimethylsilane:hexane solution. All glassware was filled with silanizing solution, sealed, and allowed to react with the internal glass surface for 1 h prior to being removed. The glassware was then washed with methanol and dried in an oven at 110 °C for 1 h. Dried silicone samples from the previous solvent uptake study and 0.5 g of dried sodium carbonate (90 °C, 30 mmHg vacuum, and 12 h) were placed into silanized 20 mL scintillation vials having Teflon septas. The fluorine tag, chlorodimethyl-3,3,3-trifluoropropylsilane, was diluted into toluene (1:10). 20 mL of the fluorine tagging solution, a large molar excess of chlorosilane to silanol groups, was added to each of the sample vials. The vials were then topped off with an inert gas (dry nitrogen) and sealed. The samples in the vials were left to react with the chlorosilane tag for 48 h at room temperature. Neutral pH was maintained throughout the volume of the polymer during the course of the reaction (see Supporting Information). The samples were removed from the vials and toluene washed multiple times by allowing the elastomer sample to equilibrate in a large volumetric excess of toluene for 24 prior to removal of the sol., removing unreacted tag. The sample sorbed toluene and any residual free tag were removed by placing the samples in a vacuum oven, fitted with a cryogenic trap at 30 mmHg and a temperature of 90 °C for 24 h. Two linear bis-silanol-terminated PDMS polymers were also tagged for this work: DMS S27 (Gelest, PA), a linear PDMS end-capped with
EXPERIMENTAL SECTION
Polysiloxane model networks were prepared using bis-vinyl-terminated PDMS (DMS V31, Gelest, Morrisville, PA) and tetrakis(dimethylsiloxy)silane (Sigma-Aldrich) as the cross-linker (1:1.75, vinyl:silane) and was polymerized in the presence of a cyclovinylmethylsiloxanestabilized platinum complex to form an end-linked, addition-cured elastomer. These chemistries were previously shown to make welldefined model silicone networks convenient for analytical studies of structure−property relationships in silicones.36,37 These base network chemistries were modified to possess a known amount of silanol free chain ends or “defects” by the substitution of varying amounts of a silanol-terminated vinyl side chain-substituted polydimethylsiloxane C
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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peak was evaluated to determine the number of fluorine molecules in each sample, and that number was scaled to represent the number of silanols measured. There are three fluorine molecules for every tagged silanol within the samples, six fluorine molecules per engineered silanol defect, and three fluorine molecules for each TFT molecule. All spectra were normalized to the concentration of the internal standard and mass of each sample measured.
silanols, and VDS 1013 (Gelest, PA), a linear PDMS molecule terminated with a silanol on each end and a vinyl reactive group located on the PDMS backbone. DMS S27 was used as a proof of concept for the initial trial of the tagging method, and VDS 1013 was tagged to determine the theoretical amount of silanol added to each reaction in the prepared model networks. DMS S27 and VDS 1013 were tagged in a modified manner compared to the cured PDMS networks. Approximately 0.5 g of the PDMS materials was added to 2 mL silanized scintillation vials, having Teflon septas, with 0.5 g of dried sodium bicarbonate blanketed with nitrogen. 1 mL of tag solution was added to the vials using a 1 mL syringe through the silicone septa. After 24 h of reaction with the fluorine tag, the tagged PDMS was removed from the vial and transferred to new vial. The samples were toluene washed multiple times (filtered with glass wool using gravity filtration); any residual free tag was removed by placing the samples in a vacuum oven, fitted with a cryogenic trap at 30 mmHg and a temperature of 90 °C for 12 h. NMR sample preparation of the tagged linear silanols involved suspension of the retrieved samples in clean toluene that was allowed to solvate overnight. The tagged PDMS was solvated with fresh toluene and placed on a roller overnight. This solution was added to the NMR tubes, and the toluene was driven off from the tubes using 90 °C while under 30 mmHg vacuum for overnight. The initial and final mass of the NMR vials were used to determine the mass of tagged silicone within the NMR sample. Because of a large molar excess of chlorosilane to silanol reactive groups within the initial tagging reaction, and considering the favored reactivity of chlorosilanes with silanol groups, full reaction of all available silanol is the expected reaction outcome. This was further confirmed by 19F NMR due to an absence of a separate representative peak of the free tag within the spectra of the tagged samples. NMR Sample Preparation. 500 MHz thin walled (0.38 mm), 5 mm o.d. quartz NMR tubes with 5 mm rubber septas (WilmadLabGlass, Vineland, NJ) were used for the NMR experiments. Like the glassware used in the tagging of the siloxane materials, all NMR tubes were silanized using the same protocol. A stock solution of deuterated toluene (Cambridge Isotope Laboratories, Inc., Tewksbury, MA) was prepared with an internal standard of 0.03% by volume α,α,αtrifluorotoluene on a balance, and the amount of each component was recorded for accurate molar concentrations. Tagged DMS S27 was prepared with deuterated chloroform that had been spiked with 0.04 vol % of hexafluorobenzene. Tagged VDS 1013 silanol-terminated PDMS was added to a silanized NMR tube, and 600 μL of the stock deuterated toluene was added to the vial. Thin strips of each of the tagged siloxane samples were weighed and ranged from between 0.04 and 0.07 g. The silicone samples were placed into the silanized NMR tube, and 600 μL of deuterated stock solution was added to each vial. The prepared NMR vials were stored in a freezer at −20 °C to prevent solvent evaporation and allowed to swell the polymer samples for at least 24 h prior to measurement. NMR Acquisition Parameters. All samples were measured using a 1D 1H-coupled 19F spectrum, Bruker zgflqn fluorine pulse sequence (see Supporting Information) on a 500 MHz Bruker NMR spectrometer, equipped with a Prodigy broad band output (BBO) cryoprobe tuned to 376.2 MHz, the resonant frequency of the 19F fluorine nucleus. For polymerized PDMS samples recycle delay (D1) 12.5 s, dead time (DE) 18.00 μs, pulse length (P1) 14.00 μs, and 90° flip angle were used. The spectrometer was locked onto deuterated toluene, tuned, and matched, and 4000 scans for each data set were acquired per sample. The data for the linear tagged silanols, typically having a significantly greater relative concentration than the swollen cross-linked samples, were aquired using 128−256 scans only. All other parameters remained the same. The resultant spectra were calibrated to trifluorotoluene (TFT) at −63.72 ppm. NMR Data Processing. All spectra were calibrated and normalized to the internal standard, phased, and background subtracted, and the relative intensities for each sample were obtained from Spectrus ACD laboratories. These spectra were then exported as text files to be further analyzed in Origin (OriginLab, Northampton, MA). Within Origin, integration for all peaks and separation of overlapping peaks were performed using peak analysis and reported. The area under each
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RESULTS AND DISCUSSION The linear silanol, DMS S27, was tagged, an NMR sample was prepared, and 19F spectra were acquired for the sample. In addition to the tagged linear silanol, three more samples were prepared for comparison: a sample of untagged DMS S27, a sample of the chlorodimethyl-3,3,3-trifluoropropylsilane tag, and a sample of the tag. Each sample was prepared, and NMR was performed accordingly. Figure 2A is a summary of the results of the 19F NMR scans. It was demonstrated that the linear silanol was tagged with the chlorodimethyl-3,3,3trifluoropropylsilane tag. Figure 2B is the 19F chemical shift exhibited by the chlorodimethyl-3,3,3-trifluoropropylsilane tag. From Figure 2B, it was shown that the tag itself can hydrolyze with time, which is indicated by the two smaller triplets slightly downfield from the main triplet. Figure 2C is the resultant spectra of the linear silanol and internal standard, TFT (0.035 vol %). In Figure 2D the 19F chemical shift of the tagged linear silanol is shown. This portion of the study was performed to demonstrate feasibility of the tagging scheme represented in Figure 1. In addition to the proof of concept, these data were useful in determining the respective chemical shifts of the tag, tagged PDMS, and approximate concentration of internal standard. The tagged linear silanol modifier, VDS 1013, was prepared in deuterated toluene that was spiked with TFT and analyzed via 19F NMR; the respective spectra are shown in Figure 3.
Figure 1. Overall diagram of proposed tagging scheme including sources of degradation within silicone networks, tagging of the resultant silanols within the silicone networks, and measuring the amount of silanols present with 19F NMR. Overall schematic of the tagging methodology: (A) ideal silicone network; (B) damage to the network due to exposure to radiation, mechanical compression, or harsh chemical environments; (C) resulting chain scission represented within the network; (D) tagging of the silicone network with trifluorine tag; (E) resulting tagged network; (F) perform NMR measurements of the sample; (G) resultant spectra from tagged silicone network. D
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. NMR demonstration of proof of concept for use of the chlorodimethyl-3,3,3-trifluoropropylsilane with a tagged linear PDMS disilanol, DMS S27. The samples were prepared with deuterated toluene and trifluorotoluene (−63.72 ppm) as the internal standard where (A) is the overall waterfall plot of all 19F NMR spectra, (B) is the 19F spectra of the chlorodimethyl-3,3,3-trifluoropropylsilane tag, (C) is the 19F spectra of the linear PDMS end-capped silanol with internal standard, and (D) is the 19F spectra of the tagged linear silanol.
From the concentration of the internal standard it was determined that 1.96 × 10−4 mol of silanol was added per gram of VDS 1013 macromonomer added to the networks. The synthesis of model networks with engineered defects was accomplished using the reported values in Table 1. The resultant model networks (given the sample code “Model-13” networks) with defects were swollen in toluene to perform solvent uptake studies. The results of the average molecular weight between cross-links are summarized in Figure 4A and Table 1. Solvent-extracted samples from the model networks having defects ranging from 0 to 10 wt % were prepared and measured using the same 19F NMR techniques described previously. Figure 4B is a waterfall plot summarizing the results of silanols tagged within the networks. Unmodified Model 13 was tagged and measured to have a baseline amount of residual silanols within the networks, 2.82 × 10−9 ± 2.95 × 10−10 silanols per gramassumed to be present as low-level synthetic residues and chain defects from the commercial starting materials. It should be noted that this study went beyond the previously reported detection levels by Skutnik et al. in 2004, who reported micromolar detection levels in fluorine-tagged elastomers using 19F NMR.35 We have demonstrated in this study detection limits within siloxane elastomers down to the
Figure 3. 19F NMR spectra of tagged PDMS disilanol defect for model silicones, with a molecular weight of 550−650 g per mole. Trifluorotoluene was used as the internal standard at −63.72 ppm.
E
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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nanomolar range due our use of solution state methods via network swelling and advancements in the hardware, specifically the use of a cryoprobe in this study. Silanol content per unit mass was varied by incorporating from 0.0001 to 10 wt % of the silanol containing PDMS macromonomer the Model 13 series. Figure 4C is a plot of the average moles of silanols measured within samples versus the average theoretical number of silanols determined from the chemistry of the starting materials. In the low range of silanol defects, 0.001−0.01%, the amount of silanols observed was greater than the theoretical quantity of silanols that should have been observed, if we assume perfect network formation and incorporation of the disilanol defect within the network. In contrast at higher incorporation levels (1 and 10 mass %) we observe less than the expected theoretical level of silanols per gram. However, the difference was within an order of magnitude. In the midrange of samples prepared (0.1 mass % silanol defects), theoretical and actual levels were significantly closer in value. This nonlinear relationship across a broad concentration range suggests that the actual mol/g of silanols in the macromonomer may be greater than what is reported, resulting in the increase of the expected value at low concentrations. In addition, it was observed that the reversal of this trend at high macromonomer concentrations was accompanied by the requirement for increased thermal cure profiles in order for the networks to be thermally driven to completion. On the basis of these observations we assert that the efficiency of the incorporation of the silanol containing monomer into the bulk networks is decreased at high relative concentration and would suggest a decrease in the efficiency of incorporation of the silanol containing macromonomer with respect to the bulk network. In addition to studying Model 13 with engineered defects, the resin reinforced commercial silicone elastomer, Sylgard 184, was tagged and analyzed as a function of γ irradiation using our 19 F NMR methodology. The resulting spectra are shown in Figure 5A, quantities of silanols observed per sample are summarized in Figure 5B, and the effects of irradiation on unfilled siloxane materials are summarized in Scheme 1. The nonirradiated Sylgard 184 had 2.45 × 10−9 ± 6.21 × 10−10 mol of silanol present within the network, a level comparable to the quantity of silanols determined to be present in the unmodified Model 13 networks and suggestive of a common residual silanol content in addition cured PDMS systems. Upon irradiation, there was a small yet statistically significant decrease in the amount of silanols present within the sample at the 5 Mrad dose,39 followed by linear increase in average silanol content between 5 and 15 Mrad to levels ∼120% above the pristine value.40 However, between 15 and 25 Mrad cumulative dose, the quantity of silanol detected declined to a level comparable to that of the pristine, nonirradiated material, which is consistent with previous results which demonstrated increased cross-linking of siloxane materials in the presence of increasing dosages of γ irradiation.39,41 Miller et al. demonstrated crosslinking of linear PDMS varied do to temperature and dose, suggesting that the competing chain scission and cross-linking reactions are complex.39 We interpret these trends as the combined and competing influences of cross-linking and scission at various dosages of irradiation; an overall scheme for these cumulative processes is given in Scheme 1. As observed in Scheme 1, these data suggest that there is a native residual silanol population within the network that may be consumed at low doses of γ irradiation. At intermediate
Figure 4. Characterization of Model 13 silicone networks with engineered silanol defects. (A) Average molecular weight between cross-links of Model 13 networks with engineered defects determined by solvent uptake study. (B) All 19F NMR spectra from tagged Model 13 networks representing 0 to 10 mass % addition of silanol defect VDS 1013, where the signal intensity was normalized to the internal standard. (C) Average moles of silanols measured within samples, after normalization to mass versus the average theoretical number of silanols from the chemistry of the materials; gray diagonal line indicates a linear trend. F
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population. At high doses, we observe a progressive decline in silanol population, and this is attributed to the increasing influence of irradiative cross-linking (radiation hardening).39 Such observations are consistent with other reports in the literature;25 however, this is the first direct spectroscopic observation of the combined scissioning/cross-linking interplay as a function of dose within a complex silicone elastomer. SE1700, a silica-filled engineered siloxane network, was also irradiated, tagged, and assessed by the same 19F NMR protocol. Analysis of this elastomer was intended to investigate the ability of the methodology to discriminate between polymer associated network silanols and silica silanols present on both the surface and accessible internal structure of the filler phase. The resultant spectra and relative peak areas of each sample of SE1700 are summarized in Figure 6A,B. With these filled
Figure 5. Results of irradiated Sylgard 184. (A) Waterfall plot of all 19F NMR spectra of irradiated Sylgard, where the signal intensity was normalized to the internal standard. (B) Observed moles of silanols versus irradiation dose for Sylgard 184 normalized to mass of each sample.
Scheme 1. Representative Occurrences within Unfilled Siloxane Networks Degraded by γ Irradiation
Figure 6. Results of irradiated silica filled SE1700 siloxane elastomer. (A) Waterfall plot of all 19F NMR spectra of irradiated SE1700, where the signal intensity was normalized to the internal standard. (B) Total observed moles of silanols versus irradiation dose for SE1700 normalized to the mass of each sample.
systems, there are two overlapping peaks, which are observed in each sample regardless of dosea sharp peak associated with polymer-based, tagged silanol and a broad peak associated with those silanols in the silica environment. Deconvolution of the two representative peaks for the irradiated SE1700 samples is summarized in Figure 7A. Figure 7B summarizes the amount of tagged silanols for each of the deconvoluted peaks, the
dosages, however, the cumulative effects of chain scission are observed through the significant increase in total silanol G
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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Table 2. Summary of Silanols Detected via 19F NMR
material
irradiation dose, Mrad
Sylgard 184
0
Sylgard 184
5
Sylgard 184
10
Sylgard 184
15
Sylgard 184
25
SE1700
0
SE1700
1
SE1700
5
SE1700
10
total no. of silanols per gram observed by 19F NMR 2.5 × 10−9 ± 6.2 × 10−10 1.1 × 10−9 ± 2.6 × 10−10 3.15 × 10−9 ± 3.9 × 10−10 5.7 × 10−9 ± 7.8 × 10−10 2.2 × 10−9 ± 8.0 × 10−12 7.6 × 10−9 ± 3.9 × 10−9 1.9 × 10−8 ± 2.3 × 10−9 2.4 × 10−8 ± 1.4 × 10−9 4.0 × 10−8 ± 2.7 × 10−9
mol of silanol attached to polymer, mol
mol of silanol attached to silica particles, mol
1.7 × 10−9
1.2 × 10−8
5.6 × 10−9
1.9 × 10−8
3.5 × 10−9
3.1 × 10−8
4.6 × 10−9
6.1 × 10−8
Charlsbey and others.42 (2) γ irradiation induces a significant increase in the levels of silica silanol species within a filled silicone rubber. It is known that the SE1700 elastomer is formulated with an organically modified grade of fumed silica which has a large proportion of its accessible silanol sites “passivated” with hexamethyldisilazine to increase its hydrophobicity.43 It therefore seems likely that the increase in silica silanol population is a result of radiolytic cleavage of the passivation layer and subsequent regeneration of the silica silanols. As both the quantity and interactions of silica silanols within a network have been shown to be implicated in a range of network rearrangement44−46 and age related cross-linking phenomena47,48 in polysiloxane networks, these data therefore suggest that even passivated silica elastomers may become susceptible to such secondary aging pathways in response to irradiation. These results are in agreement with Roggero et al., where the influence of inorganic silica on the radiation induced aging of PDMS was studied; it was observed that with increasing irradiation there was an increase in one of the silica populations measured by 29Si NMR, and this increase was contributed to a radiation induced change of the filler interface increasing the amount of SiO4 in the system.49
Figure 7. (A) Plots of irradiated SE1700, where silanols tagged on the network have been isolated from silanols generated on the silica particles, where the signal intensity was normalized to the internal standard and the mass of each sample. (B) Observed moles of tagged silanols for silanols located on silica and networks of both SE1700 and Sylgard versus irradiation dosage.
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CONCLUSIONS We have clearly demonstrated a new and sensitive methodology for the direct quantification of trace levels of network and silica silanols within polysiloxane elastomer networks. Through the use of a tag molecule having three fluorine atoms to every silanol it reacts with, we have been able to chemically label the population of silanol species present within a range of polysiloxane networks, as a result of starting chemistry, silica filler content, and radiation-induced damage. Through the application of solution state 19F NMR on tagged, solvent swollen networks, we have demonstrated quantitation of network silanols down to the nanomolar range, and in unfilled, unassaulted silicone networks (both commercial and custom synthesized) we observed residual silanols in the range of (2− 3) × 10−9 mol of silanols per gram of network elastomer due to the commonality in the starting purity of the network feedstocks and the propensity for polysiloxane materials to
combined fit of the two peaks, and the resulting amount of silanols. The amount of tagged silanols for irradiated Sylgard is also overlaid on the graph for reference. Table 2 summarizes the relative quantity of silanols for both SE1700 and Sylgard 184. With increasing irradiation dose, there is an increase in the peak area and spectral width of the silica silanol peak and an initial increase in the area of the network silanol peak which overlaps the broad peak at 1 Mrad irradiation. An overall decrease in the area of the network peak with increasing irradiation dosage beyond 1 Mrad was observed. From these data, a number of observations and assertions may be made: (1) Radiation-induced network silanols in the silica filled SE1700 elastomer are formed at a similar rate as a function of dose to those in the nonsilica filled Sylgard 184 material. This suggests a common damage mechanism to the polymer network which is at least initially dominated by chain scission in response to γ irradiation, which agrees with that suggested by H
DOI: 10.1021/acs.macromol.7b02197 Macromolecules XXXX, XXX, XXX−XXX
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(8) Duoss, E. B.; Weisgraber, T. H.; Hearon, K.; Zhu, C.; Small, W.; Metz, T. R.; Vericella, J. J.; Barth, H. D.; Kuntz, J. D.; Maxwell, R. S.; Spadaccini, C. M.; Wilson, T. S. Three-Dimensional Printing of Elastomeric, Cellular Architectures with Negative Stiffness. Adv. Funct. Mater. 2014, 24 (31), 4905−4913. (9) Mark, J. E.; Allcock, H. R.; West, R. Inorganic Polymers, 2nd ed.; Oxford University Press: New York, 2005. (10) Boonstra, B. B. Role of Particulate Fillers in Elastomer Reinforcement - Review. Polymer 1979, 20 (6), 691−704. (11) Boonstra, B. B.; Cochrane, H.; Dannenberg, E. M. Reinforcement of Silicone-Rubber by Particulate Silica. Kautschuk Gummi Kunststoffe 1976, 29 (1), 29−39. (12) Boonstra, B. B.; Cochrane, H.; Dánnenberg, E. M. Reinforcement of Silicone Rubber by Particulate Silica. Rubber Chem. Technol. 1975, 48 (4), 558−576. (13) Llorente, M. A.; Andrady, A. L.; Mark, J. E. Model Networks of End-Linked Polydimethylsiloxane Chains 11. Use of Very Short Network Chains to Improve Ultimate Properties. J. Polym. Sci., Polym. Phys. Ed. 1981, 19 (4), 621−630. (14) Grassie, N.; Beattie, S. R. The Thermal-Degradation of Polysiloxanes. 6. Products of Degradation of Poly(tetramethyl-parasilphenylene siloxane) and copolymers with dimethylsiloxane. Polym. Degrad. Stab. 1984, 7 (4), 231−250. (15) Kuo, S. W.; Chang, F. C. POSS related polymer nanocomposites. Prog. Polym. Sci. 2011, 36 (12), 1649−1696. (16) Lewicki, J. P.; Patel, M.; Morrell, P.; Liggat, J.; Murphy, J.; Pethrick, R. The stability of polysiloxanes incorporating nano-scale physical property modifiers. Sci. Technol. Adv. Mater. 2008, 9 (2), 024403. (17) Lewicki, J. P.; Maxwell, R. S.; Patel, M.; Herberg, J. L.; Swain, A. C.; Liggat, J. J.; Pethrick, R. A. Effect of meta-Carborane on Segmental Dynamics in a Bimodal Poly(dimethylsiloxane) Network. Macromolecules 2008, 41 (23), 9179−9186. (18) Lewicki, J. P.; Beavis, P. W.; Robinson, M. W. C.; Maxwell, R. S. A dielectric relaxometry study of segmental dynamics in PDMS/boron composite and hybrid elastomers. Polymer 2014, 55 (7), 1763−1768. (19) Lewicki, J. P.; Worsley, M. A.; Albo, R. L. F.; Finnie, J. A.; Ashmore, M.; Mason, H. E.; Baumann, T. F.; Maxwell, R. S. The effects of highly structured low density carbon nanotube networks on the thermal degradation behaviour of polysiloxanes. Polym. Degrad. Stab. 2014, 102 (0), 25−32. (20) Wang, S. J.; Long, C. F.; Wang, X. Y.; Li, Q.; Qi, Z. N. Synthesis and properties of silicone rubber organomontmorillonite hybrid nanocomposites. J. Appl. Polym. Sci. 1998, 69 (8), 1557−1561. (21) Lewicki, J. P.; Maxwell, R. S.; Mayer, B. P.; Maiti, A.; Harley, S. J. The Development and Application of NMR Methodologies for the Study of Degradation in Complex Silicones. In Concise Encyclopedia of High Performance Silicones; John Wiley & Sons, Inc.: 2014; pp 151− 176. (22) Lewicki, J. P.; Albo, R. L. F.; Alviso, C. T.; Maxwell, R. S. Pyrolysis-gas chromatography/mass spectrometry for the forensic fingerprinting of silicone engineering elastomers. J. Anal. Appl. Pyrolysis 2013, 99, 85−91. (23) Lewicki, J. P.; Maxwell, R. S. Degradative Thermal Analysis of Engineering Silicones. In Concise Encyclopedia of High Performance Silicones; John Wiley & Sons, Inc.: 2014; pp 191−210. (24) Chinn, S. C.; Alviso, C. T.; Berman, E. S. F.; Harvey, C. A.; Maxwell, R. S.; Wilson, T. S.; Cohenour, R.; Saalwächter, K.; Chassé, W. MQ NMR and SPME Analysis of Nonlinearity in the Degradation of a Filled Silicone Elastomer. J. Phys. Chem. B 2010, 114 (30), 9729− 9736. (25) Maxwell, R. S.; Chinn, S. C.; Alviso, C. T.; Harvey, C. A.; Giuliani, J. R.; Wilson, T. S.; Cohenour, R. Quantification of radiation induced crosslinking in a commercial, toughened silicone rubber, TR55 by H-1 MQ-NMR. Polym. Degrad. Stab. 2009, 94 (3), 456−464. (26) Lewicki, J. P.; Liggat, J. J.; Hayward, D.; Pethrick, R. A.; Patel, M. Degradative Thermal Analysis and Dielectric Spectroscopy Studies of Aging in Polysiloxane Nanocomposites. In Polymer Degradation and
undergo a continual and low level of chain rearrangement, even under ambient conditions.50−52 We have demonstrated the ability to discriminate between polymeric network silanols and those associated with silica filler phases. And we have demonstrated the utility of this method for tracking ageinduced chain scission processes in both the bulk network and the filler phase within a single, complex elastomer system. Through the combination of our synthetic tagging protocol reported here and the application of modern, sensitive 19F NMR methodologies, it is for the first time possible to directly observe and quantify silanol species in the nanomolar range formed as a result of network chemistry or age included degradation. This method therefore has far reaching implications for the spectroscopic assessment, elucidation, and quantitation of aging processes within polysiloxane materials and has the potential to open up new avenues of fundamental research into the long-term dynamics and stability of this increasingly relevant class of inorganic, polymeric material.
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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.7b02197. Verification of neutral pH of polymer throughout tagging process and 19F NMR pulse program details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(J.P.L.) E-mail
[email protected]. ORCID
James P. Lewicki: 0000-0002-2467-702X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM Release No.: LLNL-JRNL-737499.
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REFERENCES
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