Controlling silicone-saccharide interfaces: greening silicones

Benjamin Macphail and Michael A. Brook *
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St. W., Hamilton, ON L8S 4 M1, Canada. E-mail: mabrook@mcmaster.ca

Received 10th July 2017 , Accepted 9th August 2017

First published on 9th August 2017


Silicone elastomers, which are normally crosslinked using metal catalysts, are traditionally reinforced with mineral fillers. We report that renewable saccharides can instead be used to both crosslink and reinforce silicones. The grafting of boronic acids to silicone polymers gives materials that, when added to aqueous solutions of mono- or polysaccharides, without catalysts, generated elastomers via the boronic acid interaction with saccharides. The efficiency of crosslinking, as shown by Young's moduli, depended strongly on the specific saccharide and the density of boronic acid groups on the silicone. Simple silicones normally phase separate in water saccharide mixtures. However, pretreatment of silicone boronates with the saccharide phytoglycogen, followed by exposure to water, led to stable aqueous phytoglycogen/silicone dispersions (pastes). The different outcomes arising from the order of addition are attributed to better dispersion of the silicone and saccharide in the latter case. Rheological studies of the pastes showed that, unlike the elastomers, viscosities depended more on the fraction of silicone in saccharide; number of boronic acid contact points between the silicone and saccharide was only a minor contributor. The equilibrium concentration of sugar/boronate contacts, which stabilize the water/oil interfaces, remains high even at high concentrations of water and even when the specific binding constant for an individual saccharide is low.


Introduction

Silicone polymers are commercially important materials that impact daily life in advanced economies. Silicones do not often appear on the pages of this journal because silicones do not readily fit the ‘green’ mandate. However, their environmental behavior is excellent.1 Extensive studies have shown that silicones in virtually all environmental compartments readily decompose to sand, water and CO2 following environmentally and biologically-mediated depolymerization and then oxidation.2 However, silicone synthesis is not environmentally friendly, as the first, key step involves conversion of sand to silicon in a highly energy intensive and CO/CO2-generating process.3

One way to reduce the environmental impact of silicone elastomers is to dilute them with recyclable, biodegradable and renewable materials. A more positive perspective would be to recognize that composites of silicone could be improved by taking advantage of natural materials. Silicone rubbers are weak unless reinforced by fillers;4 expensive and heavy pyrogenic silica is most commonly used for this process. We reasoned that polysaccharides, which are emblematic green polymers, could simultaneously dilute the impact of silicones and silica on the environment while leading to elastomeric materials with enhanced properties.

Silicone polymers are, of course, known for exceptional hydrophobicity; surface energies are typically about 20 mN m−1.5 Most saccharides, particularly polysaccharides, are hygroscopic and so working with truly dry materials is problematic. Thus, the main synthetic challenge associated with creating silicone/saccharide composites is overcoming the hydrophobic silicone/hydrophilic saccharide interface.

Silicone interfaces can be controlled by organic modification. For example, grafting polyethers6,7 or zwitterions8 to a silicone surface decreases the water contact angle. Such surfaces will have greater physical affinity for (wet) polysaccharides, but at a level not expected to lead to stable composites.

In some cases, the direct bonding of silicones to saccharides is possible. For example, the use of silane coupling agents in wood can improve durability, dimensional stability by hydrophobization,9 and fire resistance;10 multistep processes can analogously lead to silicone-hydrophobed wood.11 The Piers–Rubinsztajn reaction has been used to directly graft silicone resins to cellulose or to another important natural polyphenolic polymer, lignin, while simultaneously blowing foams (Fig. 1).12,13 The resulting materials exhibited excellent adhesion between silicone and cellulose or lignin, respectively and, surprisingly in the latter case, led to silicone foams that were much better able to withstand exposure to flames than a commercial silicone foam, in spite of the fact that the foam contains an excellent fuel – lignin!.


image file: c7gc02088k-f1.tif
Fig. 1 The Piers–Rubinsztajn reaction grafts both saccharides and lignin to silicones.

Boronic acids efficiently bind to saccharides.14,15 The binding constant is sufficiently dependent on saccharide structure that physical separation of different sugars based on affinity with boronic acid is the basis of an analytical technique.16 Previously, we prepared silicone-modified boronic acids (SiBA) (Fig. 3). Upon exposure to water the tartrate protecting groups hydrolyze to give crosslinked elastomer films.14,15 It was shown that the origin of crosslinking in the films was boronic acid dimers; crosslinks disappeared in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometric relationship as monofunctional boronic acids were added to the elastomer.17 However, these films could be disrupted if the aqueous solution also contained competitive ligands for the boronic acid, which include amines and sugars.18 We therefore reasoned that either the one-step14,15 physical or covalent modification of sugars by SiBAs could provide a means to (reversibly) control the silicone/saccharide interface (Fig. 2); the strength of the interaction should be a consequence of both their concentrations in water and the specific saccharides and boronic acids chosen. The preparation and characterization of SiBA/sugar composite materials is described below.


image file: c7gc02088k-f2.tif
Fig. 2 Possible binding motifs for SiBA/saccharides.

image file: c7gc02088k-f3.tif
Fig. 3 Preparation of SiBAs (two regioisomers form during hydrosilylation, only one of which is shown).

Results

SiBAs were prepared by the hydrosilylation of the appropriate SiH-containing silicone oils with tartrate-protected styrylboronic acid (Fig. 3). The process occurs in high yield to yield organosoluble polymers with tunable concentrations of boronic acids in a silicone matrix.19 After the protecting groups were removed by exposure to water – an extremely rapid and efficient process – the strength of the resulting elastomeric films (Young's modulus) was measured as a function of molecular weight of the silicone, and the boronic acid density. The telechelic SiBAs, which possess a lower boronic acid density, exhibited low Young's moduli 15–17 kPa that decreased with increasing silicone molecular weight (Fig. 4A, Table 1). The pendant SiBAs exhibited much higher moduli 220–2300 kPa as expected; the crosslink density should ultimately depend on the spacing between boronic acids rather than the overall silicone molecular weight. The modulus of a given SiBA could be tailored simply by mixing starting constituents as shown in Fig. 4B where telechelic silicone SiBA-17 was diluted with the pendant SiBA-P50, which has a higher boronic acid concentration.
image file: c7gc02088k-f4.tif
Fig. 4 Moduli of silicone boronic acid elastomers (A) homopolymers telechelic SiBA-17 and pendant SiBA-P50, (B) mixtures of telechelic SiBA-17 when diluted with pendant SiBA-P7.
Table 1 Young's moduli of SiBA-17 saccharide mixtures
Saccharidea (mol)[thin space (1/6-em)]:[thin space (1/6-em)]boronic acid (mol) Young's modulus (kPa)
a Saccharide concentrations in polysaccharides are based on monosaccharide constituents. In pullulan, only a fraction of the saccharide units in pullulan will be available. b Best ratio based on sorbitol. c Pullulan 35 mg: SiBA-17[thin space (1/6-em)]500 mg. Diluting this solution with water led to weaker composites.
Using telechelic SiBA-17
No saccharide, 1 BA/8 Me2SiO groups 171 ± 18
 
Sorbitol
1[thin space (1/6-em)]:[thin space (1/6-em)]4 111 ± 23
1[thin space (1/6-em)]:[thin space (1/6-em)]2 602 ± 51
3[thin space (1/6-em)]:[thin space (1/6-em)]4 353 ± 43
1[thin space (1/6-em)]:[thin space (1/6-em)]1 343 ± 95
 
Glucose
1[thin space (1/6-em)]:[thin space (1/6-em)]4 118 ± 51
1[thin space (1/6-em)]:[thin space (1/6-em)]2b 69 ± 27
 
Pullulan
1[thin space (1/6-em)]:[thin space (1/6-em)]4c 140 ± 30
1[thin space (1/6-em)]:[thin space (1/6-em)]2 106 ± 17
3[thin space (1/6-em)]:[thin space (1/6-em)]4 141 ± 60
1[thin space (1/6-em)]:[thin space (1/6-em)]1 108 ± 35


An examination of the ability of saccharides to displace boronic acid dimers as a crosslinking mode was undertaken in a of variety ways. Initially, this was expediently done by directly mixing – under high shear – a saturated aqueous solution of the (poly)saccharide into the protected SiBA. Young's moduli were then measured of the resulting silicone elastomers that were doped and crosslinked with saccharides. Survey experiments with different concentrations of sorbitol, a good binder for boronic acids (BA) (Keq = 440 M−1),20 were used to gauge the binding ratios that led to the best crosslinking. The Young's modulus went through a maximum at a 1[thin space (1/6-em)]:[thin space (1/6-em)]2 ratio of [monosaccharide]/[boronic acid unit] (Table 1), suggesting that 2 boronic acids bind to 1 sorbitol to give a crosslink. At low levels of sorbitol, boronic acid dimerization remains the main type of crosslink. With too much sorbitol, on the other hand, each BA is modified by a single saccharide and the (weaker) crosslinking that results is a consequence of the physical separation of the sorbitol in the hydrophobic silicone matrix rather than covalent crosslinking. This mimics the more classic reinforcement of silicones with relatively large mineral structures, except that the saccharide domains are in this case highly hydrated. It is counterintuitive to observe reinforcement of hydrophobic polymer by a water swollen saccharide.

The Young's modulus of silicone/glucose composites at the same concentrations was significantly lower (30–60 kPa, Table 1), consistent both with a weaker binding constant (Keq = 5)20 and with glucose forming only a single ester with boronic acids and, therefore, not being able to act as a chemical crosslinker.

The ability of a long chain, branched saccharide pullulan (MW 200[thin space (1/6-em)]000) to act as a crosslinker was also examined. This polysaccharide is surprisingly soluble in water, with saturation concentrations above 30 wt%. As it is comprised entirely of glucose monomers (note: the fraction of saccharide units available for binding to BAs is unknown) it is not possible to deconvolute crosslinking due to specific binding interactions from simple phase segregation.

A selection of concentrations of pullulan in water was utilized in the attempt to find a concentration where the polysaccharide would best reinforce the silicone. It can be seen that the efficiency of reinforcement was enhanced compared to the boronic acid dimers themselves at the same water concentration, indicative of a reinforcement of the silicone by the pullulan (Table 1). However, the reinforcement was not appreciably different from glucose itself.

To further probe the nature of these interactions, SiBA was first mixed with different concentrations of phytoglycogen, a 35 nm spherical, monodisperse nanoparticle that swells to 70 nm diameter in water (GlyNP, MW 15 million according to the manufacturer); the silicone saccharide was then dispersed in water at the same concentrations as the elastomers (phytoglycogen + SiBA 500 mg/1 g H2O; the ratio of SiBA to GlyNP was varied, using SiBA-17 or SiBA-P7). In the absence of the saccharide, silicone films formed at the air water interface, or formed an elastomeric ball that floated in water. Surprisingly, unlike the elastomers formed co-mixing of silicone/saccharide/water, premixing the saccharide with silicones and then adding water led to stable white pastes of differing viscosity. These were shown, by drying the material and then imaging with SEM, to be coarse emulsions in which the saccharide/silicone interface is stabilized by the boronate/saccharide interaction (ESI). By contrast, if any of the phytoglycogen solutions was mixed with normal silicone oil of any viscosity, immediate gross phase separation occurred (Fig. 5).


image file: c7gc02088k-f5.tif
Fig. 5 (A) Phase separated mixture of PDMS and phytoglycogen-in-water (with blue food color). (B) 200 μm thick film cast from paste mixture of SiBA-17 (1) and phytoglycogen-in-water. The different color intensities reflect layers of different thickness. (C) Cartoon of the interface of an oil-in-water SiBA/saccharide dispersion. (D) Viscosity of silicone-phytoglycogen pastes changes with relative silicone and phytoglycogen concentrations.

The rheological properties of these pastes were measured to better understand the nature of the boronic acid/saccharide interactions (Fig. 5D). Viscosities of the white pastes increased with boronic acid concentration, but this was a less significant factor than the total quantity of available silicone. At 0.1% strain (shear rate of 0.1 s−1) viscosities for pastes containing telechelic silicone boronic acids ranged between ∼9 kPa s–33 kPa s, whereas those containing pendant silicone boronic acids were slightly higher, ranging from between ∼11 kPa s and 44 kPa s. A 5% (w/w) addition of either pendant or telechelic SiBA was enough to increase the viscosity above that of a phytoglycogen paste containing no added SiBA.

SiBA binding affects phytoglycogen nanoparticle diameter: DLS

In dimethylsulfoxide (DMSO) solution, the phytoglycogen nanoparticles used in these experiments have a diameter of about 66 nm; the particles are rather monodisperse; note that commercially available phytoglycogen samples are typically polydisperse.21 SiBA appear to dissolve in DMSO, but dynamic light scattering (DLS) data shows instead that they form aggregates, with telechelic SiBA-17 forming ∼82 nm aggregates, and pendant SiBA-P7 creating larger aggregates ∼143 nm diameter. These silicone aggregates were not monodisperse and some very large (micron sized) particles existed prior to the addition of phytoglycogen. It is inferred that adventitious water leads to tartrate deprotection and boronic acid crosslinking.17

The ability of the silicone and SiBA to interact was tested by titrating phytoglycogen/DMSO into silicone/DMSO dispersions (Fig. 6). Immediately upon addition of phytoglycogen the large SiBA aggregates broke up; the resulting smaller particles shown reflect SiBA-modified phytoglycogen and the presence of much smaller SiBA aggregates. As the phytoglycogen concentration increased, the average particle size increased to slightly larger than that of the native phytoglycogen. We interpret these data to indicate that rapid loss of the SiBA aggregates occurs once a compatible glycogen surface is present (Fig. 6A); the silicone becomes grafted to the surface of the GlyNP.


image file: c7gc02088k-f6.tif
Fig. 6 (A) Cartoon showing breakup of SiBA aggregates and subsequent adsorption onto GlyNP. (B) Particle diameter upon titration of SiBA with GlyNP in DMSO.

Discussion

Boronic acids reversibly form boronic esters with diols, of which saccharides are emblematic as a class (Fig. 2).22 The equilibrium constant for ester formation is very sensitive to structure, particular of the diol. As noted above, the binding equilibrium constant for the monosaccharide sorbitol is two orders of magnitude larger than that for glucose. Other factors also influence the location of the equilibrium, including higher pH and the presence of Lewis base ligands for boron, both of which favor the ester side of the equation. Dilution with water will also move the equilibrium towards starting materials.

In the absence of ligands for the boronic acids, including saccharides, SiBA formed dimeric structures that crosslink the silicones (note that secondary crosslinking arises from Lewis acid/base interactions between boron and oxygen atoms on the silicone backbone).18 The higher the boronic acid and therefore crosslink density – the shorter the distance between crosslink sites – the higher the modulus (Fig. 4A). More robust elastomers arose from pendant functional silicones that can generate more crosslinks per unit volume. It is possible to tune the modulus simply by mixing structurally different boronic acids (e.g., pendant + telechelic) to tune the crosslink density and adding sufficient water to deprotect the tartrates (Fig. 4B).

If the SiBA was wet with aqueous saccharide solutions a product elastomer rapidly formed that was phase separated from water. Here, the nature of the saccharide played an important role in the physical properties of the elastomer. In the case of sorbitol, the outcome is consistent with insertion of the sorbitol into the boronic acid dimer to give a new type of covalent, bifunctional crosslinker sorbitol. The strongest elastomers formed at the optimal 2[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio of sorbitol[thin space (1/6-em)]:[thin space (1/6-em)]boronic acid (Fig. 5C I). At higher concentrations of sorbitol, or in the case of saccharides that have poorer binding constants, or possibly have difficulty in achieving a proper geometry to bind more than once,23 the less effective crosslinking likely arises through physical interactions with individual polysaccharides or aggregates of (poly)saccharides (Fig. 5C II). The strength of the latter interaction was weaker than the former, at least at an optimal stoichiometric balance (Table 1). At lower saccharide concentrations, the net crosslinking can be ascribed to a composite of boronic acid dimers and boronic acid/saccharide complexes.

These observations support the conclusion that the stabilization of the silicone/water interface requires saccharides to be present in the aqueous phase (Fig. 5C). The strength of that interaction depends on the concentration of sugars in water, the number of available boronic acid sites/number of available saccharides and, in some cases, the binding equilibrium constant. It is not possible to tease out the relative impact of each of these. Although the Young's moduli were lower in the presence of saccharides with low binding constants, it nevertheless demonstrates that the saccharides remain bound to the SiBA. There is a statistical advantage here. Even though the boronic acid/sugar interactions are equilibria, multiple contact points on the saccharides favor net binding; there will always be a sufficient population of bound sugars such that the SiBA are not released from the sugars. That is, the fact that water rich silicone elastomers are stable demonstrates it is the boronic acid that stabilizes the silicone at the water interface. This is even more evident with the pastes (see below).

When ‘dry’ SiBA was exposed to a dry saccharide in the absence of water, and then water was added in a subsequent step, stable oil-in-water dispersions were formed as pastes. The viscosities of the pastes were directly related to the relative quantities of silicone/saccharide; the density of boronic acid groups was less important (Fig. 5B and D). The binding constant of boronic acids for glucose or glucose-based polymers is relatively low. Thus, it is expected that the interaction between silicone and (poly)saccharide in this case is physical, although it is not possible to discount boronate esters in equilibrium with such structures (Fig. 5C III vs. IV). Irrespective, the binding reflects an equilibrium; the pastes were stable over weeks (so far). We infer that the build in viscosity is associated with silicone domains on adjacent saccharide surfaces that associate through a hydrophobic effect to reduce the exposure of silicone to water (Fig. 6).24 The more silicone on saccharides, the ‘stickier’ will be the particles to each other in water. This hypothesis is supported by the solution studies in DMSO, which show competitive binding of SiBA to a saccharide surface, when compared to the formation of a separate silicone phase.

Dimethylsilicones have little affinity for water. The solubility of the most soluble silicone materials D4 (Me2SiO)4 is about 50 ppb (literature reports range from ∼20–900 μg L−1 H2O); higher molecular weight materials are less soluble. The presence of a sugar in the aqueous phase does nothing to change this (Fig. 5A). However, boronic acid anchor groups can be used to link silicones to saccharides. If this is done in the absence of water pastes are produced, while if done in the presence of water, sugar-crosslinked, phase separated elastomers are formed. That is, while specific sugar/SiBA interaction will be similar in both cases, the efficiency with which they are established depends on the availability of silicone-phobic water during synthesis. Irrespective of the mode of synthesis, it is possible to choose how those interactions lead to the formation of a given material.

In recent years, there has been some pushback against silicones, particularly in the cosmetics industry, because they are ‘not green.’ As noted above, that is not correct from a degradation point of view, but the degree to which silicones are ‘green’ is challenged by their energy input cost. They possess very interesting properties that are not matched by organic materials and they fulfill many needs in developed economies. The study here shows that useful silicone-based materials, with enhanced interfacial properties, may be prepared by taking advantage of readily available (poly)saccharides that can be loaded at levels up to 50 wt%. The properties of the resulting products are readily tuned by the nature of the SiBA and, more importantly, the saccharide. In the latter case, factors including molecular weight and particularly nature of the saccharide and its binding constant with the boronic acid can affect the nature of the product. The use of boronic acids provides a mechanism to control the properties of silicones by doping them with saccharides, thereby improving the environmental profile of silicones.

Experimental

Materials

Sorbitol (VWR), glucose (BDH), pullulan (Hayashibara Company), phytoglycogen nanoparticles (PhytoSpherix, Mirexus Biotechnologies) and DMSO (Caledon Laboratories Ltd, anhydrous) were used as received. Synthesis of silicone boronate dimethyl tartrate esters followed literature procedures.19 Distilled water was used throughout.

Methods

Young's modulus measurement. Modulus data was collected within 96-well plates using a MACH-1 micromechanical testing instrument equipped with a 0.5 mm hemispherical indenter using a Poisson ratio of 0.3, 24 h after elastomer formation.
Rheometry. The viscosity of the pastes were measured using a TA instruments Discovery HR-3 Hybrid Rheometer equipped with a 40 mm parallel Peltier steel plate geometry. Trios software (TA instruments) was used for data acquisition. The first experiment was to find the linear viscoelastic zone by strain-sweep testing between 0.1%–100% strain. Angular frequency was set at 10 rad s−1. The strain for the linear region was used in subsequent frequency-sweep experiments. Frequency-sweep measurements were performed at a frequency range between 0.05 s−1 to 10.0 s−1. All experiments were performed at 25 °C.
DLS. Phytoglycogen nanoparticle size was characterized using a Malvern Zetasizer Nano dynamic light scattering unit at 25 °C.

Elastomer formation

General procedure for preparing silicone boronic acid elastomers by immersion in H2O followed by agitation, no saccharide. SiBA (500 mg) was weighed into a 2 dram glass vial to which distilled H2O (1 mL) was added. The solution was agitated for one minute using a vortex mixer. The resulting elastomers, which formed a ‘puck’ on the base of the vial were transferred to a 96-well plate using a spatula.
General procedure for preparing silicone boronic acid elastomers by immersion in H2O followed by agitation, mixtures of pendant and telechelic SiBAs (shown for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 telechelic SiBA-17[thin space (1/6-em)]:[thin space (1/6-em)]SiBA-P7). SiBA-17 (500 mg) and SiBA-P7 (500 mg) weighed into a 2 dram vial; the mixture was agitated for one minute using a vortex mixer. Distilled H2O (1 mL) was added to the solution which then agitated again for one minute using a vortex mixer. The resulting elastomer was transferred to a 96-well plate using a spatula and allowed to cure overnight.
General procedure for preparing silicone boronic acid elastomers by immersion in H2O followed by agitation, saccharide reinforced elastomers. 1 M solutions of sorbitol and glucose were prepared by adding sorbitol (182 mg, 1 mmol) or glucose (180 mg, 1 mmol), respectively, to a 2dram glass vial; distilled H2O (1 mL) was then added to each using an Eppendorf pipette and the solution was sonicated for 1 min to ensure complete dissolution. Solutions of pullulan (1016 mg, MW ∼ 1[thin space (1/6-em)]811[thin space (1/6-em)]800, 56 nmol in distilled H2O mL, converts to ∼5.7 mmol glucose equiv.). Elastomers were formed by adding the appropriate SiBA (500 mg, 0.139 mmol) and saccharide solution/H2O (70 μL–350 μL, 0.07 mmol–0.35 mmol) to a polypropylene mixing cup (Flaktek, size 10). The mixture was then mixed under high shear at 3000 rpm for 2 min using a Flaktek Speedmixer (model: DAC 150.1 FVZ-K). The resulting elastomer was transferred to a 96-well plate using a spatula.

Stoichiometric saccharide addition

500 mg (0.139 mmol) SiBA-17 was added to a speed mixing polypropylene cup for all experiments. 0.5 mol, 1 mol, 1.5 mol, 2 mol, and 2.5 mol equivalents of saccharide were added per boronic acid by pipetting 69.9 μL (0.0699 mmol), 139.9 μL (0.139 mmol), 209.8 μL (0.2098 mmol), 279.8 μL (0.2798 mmol), 349.7 μL (0.3497 mmol) of saccharide solution, respectively, into Flaktek mixing cups. The SiBA-17-saccharide solutions were then mixed at 3000 rpm for 2 min using a Flaktek Speedmixer (model: DAC 150.1 FVZ-K). The resulting elastomers were transferred to a polypropylene 96-well plate using a spatula and allowed to cure (a few minutes).

Paste formation and rheology

Pastes were prepared by combining silicone and phytoglycogen in a 2-dram vial. The relative amounts of silicone and phytoglycogen were varied to create a suite of formulations by adding 5 mg, 10 mg, 25 mg, 50 mg, 125 mg, and 250 mg of SiBA-17, respectively, to separate vials then adding enough phytoglycogen to each vial to give a total of 500 mg; a control formulation consisting solely of phytoglycogen and H2O was also made. Distilled H2O (1 g) was then added to each vial and the dispersion was mixed using a vortex mixer for 2 min. Films (200 μm thick) of these pastes were cast onto Teflon sheets using a stainless-steel film applicator (Elcometer 3540).

DLS characterization of GlyNP/SiBA solutions

Phytoglycogen solutions (1 wt%) were prepared by weighing 1 g phytoglycogen in a 10 mL volumetric flask, dissolving in dimethylsulfoxide (DMSO), and the performing a serial dilution in DMSO. SiBA-17 (0.58 mM) and SiBA-P7 (0.55 mM) solutions were prepared by weighing 100 mg of the silicone and diluting into DMSO (10 mL, followed by 2 serial dilutions). All solutions were prepared immediately prior to measurement. Particle sizes were collected for each of these solutions; 7 sequential scans were run in triplicate. Varying amounts of phytoglycogen solution (0.1[thin space (1/6-em)]:[thin space (1/6-em)]1000, 0.5[thin space (1/6-em)]:[thin space (1/6-em)]1000, 1[thin space (1/6-em)]:[thin space (1/6-em)]1000, 5[thin space (1/6-em)]:[thin space (1/6-em)]1000, 10[thin space (1/6-em)]:[thin space (1/6-em)]1000 phytoglycogen[thin space (1/6-em)]:[thin space (1/6-em)]boronic acid) were added to 10 mL of pendant silicone boronic acid solutions and telechelic silicone boronic acid solutions respectively. The particle sizes of these solutions were collected for each of these solutions; 7 sequential scans were run in triplicate. The opposite addition sequence was performed by starting with 10 mL solutions of phytoglycogen and adding varying amounts of pendant silicone boronic acid solution and telechelic silicone boronic acid solution respectively (0.1[thin space (1/6-em)]:[thin space (1/6-em)]1000, 0.5[thin space (1/6-em)]:[thin space (1/6-em)]1000, 1[thin space (1/6-em)]:[thin space (1/6-em)]1000, 5[thin space (1/6-em)]:[thin space (1/6-em)]1000, 10[thin space (1/6-em)]:[thin space (1/6-em)]1000 phytoglycogen[thin space (1/6-em)]:[thin space (1/6-em)]boronic acid). All solutions made were mixed for two minutes using a vortex mixer prior to particle size measurement.

Conclusions

Boronic acids, while bound to silicone polymers, maintain their ability to bind saccharides. Effective boronic acid dimerization provides one mechanism of crosslinking, which is supplemented by boronic acid-saccharide complexation or boronate ester formation. Although the binding constant with glucose units is low, polysaccharides based on glucose lead to reinforced silicones or, depending on relative concentration, reinforced phytoglycogen materials because of statistical efficiency of holding more than zero linkage between the two materials at any given time. Even under the stress of higher water concentration, it is difficult to separate the two materials. As a consequence, depending on volume ratio, the copolymers behave as hydrophobically modified natural materials or natural material-reinforced silicone elastomers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge with gratitude the financial support of the Natural Sciences and Engineering Research Council of Canada, the Ontario Centres of Excellence, and Mirexus Inc. We would further like to thank Dr Phil Whiting, Mirexus for helpful discussions, access to instruments and provision of their uniquely monodisperse form of phytoglycogen, PhytoSpherix.

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Footnote

Electronic supplementary information (ESI) available: SEM image of the 50% SiBA/phytoglycogen paste; table showing moduli of silicon boronate/saccharide elastomers. See DOI: 10.1039/c7gc02088k

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