Kinetics analysis and physical properties of photocured silicate-based thiol-ene nanocomposites: The effects of vinyl POSS ene on the polymerization kinetics and physical properties of thiol-triallyl ether networks.

Abstract The kinetics and thermal/physical properties of the trithiol-TAE (triallyl ether) system were measured with respect to increasing polyoligomeric silsesquioxane (POSS) concentrations in order to understand how the presence of POSS nanoparticles affects network formation at low loadings. Vinyl POSS monomer (vPOSS-B[u.sub.4]) with both vinyl and carboxylate pendant groups was synthesized via a thermally initiated, free-radical reaction to improve the compatibility of the inorganic particles with the trithiol and triallyl ether comomoners. Chemically modified vPOSS-B[u.sub.4] particles were incorporated into the trithiol-TAE polymer networks by a thiol-ene freeradical photopolymerization at molar concentrations of 0, 1, and 5 ene mol%. The polymerization rates were analyzed using real-time FTIR and photo-DSC. The polymerization rates showed no significant changes with increasing vPOSS-B[u.sub.4] concentration. Thermal analyses of the films by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) demonstrated that thermal stability improves without affecting [T.sub.g] as the POSS concentration increased. Additionally, scratch resistance increased and flame spread decreased markedly with increasing POSS concentration for concentrations up to 5 mol% vPOSS-B[u.sub.4].

Keywords Thiol-ene photopolymerization, Hybrid nanocomposite, Polyoligomeric silsesquioxane (POSS), Thermal stability

Introduction

The general class of polymeric nanocomposites is defined as any polymer-based material incorporating nanosized inorganic additives. (1), (2) Polymeric nanocomposites consist of a variety of systems including amorphous materials and materials having multidimensional frameworks. Typical inorganic additives are zeolites, clays, metal oxides, and silicate-based particles.

In recent years, research focused on silicate-based nanocomposite materials has grown since this class of inorganic additives imparts a wide range of improved thermal, mechanical, and physical properties to the parent polymer. Recent literature cites examples of many silicate-based polymeric nanocomposites, including hybrid materials incorporating polyoligomeric silsesquioxane (POSS) cages. (3-8) POSS cages are nanosized particles (1-3 nm) that offer a unique blend of organic and inorganic characteristics having an empirical structure ([(RSi[O.sub.1.5]).sub.n]) that varies both in cage size and types of organic pendant groups. POSS cage sizes may vary (n = 8, 10, 12, and 14) and can have reactive and/or nonreactive organic substituents attached at each corner of the cage. Reactive substituents can potentially enable POSS particles to be chemically incorporated into the polymer matrix either thermally or photochemically. Nonreactive substituents enhance the compatibility of POSS cages with the monomer mixture, thus improving the solubility of the inorganic cage. For this study, ester-modified vinyl POSS (vPOSS-B[u.sub.4]) particles are photochemically incorporated into a series of TriThiol-TAE (See Fig. 1) copolymer films keeping the ene molar concentrations equal to the total molar thiol group concentrations. The physical properties are compared to films (blends) containing equivalent weights of fully functionalized (vPOSS-B[u.sub.10]) particles (Fig. 1).

Thiol-ene cured films are a unique class of materials having a uniform crosslink matrix and many desirable properties such as flexibility and good adhesion. (9) Thiol-ene systems are combinations of multifunctional thiols and enes wherein the ene structures include vinyl ethers, allyl ethers, acrylates, and vinyl siloxanes. (9) The photopolymerization of thiol-enes is advantageous over the photopolymerizations of many traditional acrylate polymers for several reasons: (1) polymerization rates are fast, (9) (2) polymerization may be initiated with or without a photoinitiator present, (9-12) and (3) the polymerization proceeds with low oxygen inhibition. (10), (13), (14) The photopolymerization mechanism by which thiol-ene networks are formed make this system of great interest for studying the effects of the presence of POSS on the kinetics of the polymerization. Additionally, the TriThiol-TAE (triallyl ether) network is an excellent matrix for singularly analyzing the effects of POSS concentration on the physical properties since the POSS particles are incorporated directly into the structure of the thiol-ene network.

[FIGURE 1 OMITTED]

Experimental

Allyl Pentaerythritol (triallyl ether) was obtained from Perstorp Specialty Chemical Company, and vinyl POSS cage mixture (OL 1170) was obtained from Hybrid Plastics, Inc. Photoinitiator Darocur 1173 was obtained from Ciba Specialty Chemicals, Inc. Trimethylopropane tris (3-mercaptopropionate) (TriThiol), butyl 3-mercaptopropionate, and diethyl amine were purchased from Aldrich Chemical Company and used as received.

Monomer synthesis

The POSS monomer (vPOSS-B[u.sub.4]) with both vinyl and ester groups and vPOSS-B[u.sub.10] with ester groups was synthesized by a free-radical reaction of aliphatic thiols to vinyl groups of the vinyl POSS structure initiated by thermal decomposition of peroxides present in the solvent (Scheme 1). A 20-g vinyl POSS cage mixture was charged into a 250-mL flask with 60-mg diethyl amine and 25 mL THF. The solution was slowly added to 11 and 27 g of butyl 3-mercaptopropionate for synthesis of vPOSS-B[u.sub.4] and vPOSS-B [u.sub.10], respectively. The solution was stirred for at least 6 h (40[degrees]C) before the solvent THF was removed by rotovaporization. After complete evaporation of THF oil remained that contained the product. Complete disappearance of the chemical shift at 1.6 ppm (thiol peak) indicates that each product contained no unreached butyl 3-mercaptopropionate by proton NMR analysis. The absence of the vinyl peaks (5.9 and 6.1 ppm) indicates the total consumption of vinyl groups for vPOSS-B[u.sub.10].

Polymer synthesis

POSS/thiol-ene copolymers containing 0, 1, and 5 mol% vPOSS-[Bu.sub.4] nanoparticles and blends containing 0, 2.2, and 10.2 wt% vPOSS-[Bu.sub.10] (equivalent weights of the ene molar concentrations) were photocured on a Fusion Epiq 6000 UV cure line using a 400-W D-bulb lamp as the light source. Reaction mixtures contained equal moles of thiol and ene functional groups in addition to 2 wt% photoinitiator. For the series of copolymers and blends, reaction mixtures were maintained at 50:50 thiol:ene molar ratios, and the molar concentration of each ene was varied for copolymer formulations. Each film was cast onto a glass substrate as a polymer solution using a 20-mil drawn down bar and photocured at a light intensity of 2.23 W/[cm.sup.2] and a line speed of 12.19 m/min.

[ILLUSTRATION OMITTED]

Instrumenntaion

Photo-differential scanning calorimetry (photo-DSC) was set up using a Perkin-Elmer DSC-7 modified by adding quartz windows to the sample head cover. A medium-pressure mercury lamp was used as the light source. The light intensity of the full are was 49.3 mW/[cm.sup.2] approximately, with lower light intensities obtained by using a neutral density filter. The actual light intensity used herein was ~4.93 mW/[cm.sup.2] obtained by incorporating a 1.0 neutral density filter. Exotherms give direct information about photopolymerization rates.

Real-time Fourier Transform IR (real-time FTIR) was performed on an IFS-88 Bruker spectrometer. From analysis of the appropriate peaks, it was possible to obtain actual conversions of thiol and ene groups vs time. Samples were placed between NaCl plates and irradiated by the filtered UV light with a 1.0 neutral density filter. The light intensity of the full arc was 187 mW/[cm.sup.2], whereas the actual light intensity used was 18.7 mW/[cm.sup.2]. The peak assignments for the thiol, allyl ether, and vinyl groups were 2570, 1670, and 1640 [cm.sup.-1], respectively.

Visible transmittance of the TriThiol-TAE systems containing 0, 1, and 5 mol% vPOSS-[Bu.sub.4] was measured on a Cary 500 UV-Vis-NIR spectrophotometer from 400 to 800 nm. Each film sample was placed within the sample holder and %T was measured directly by the detector beam. The values for %T were reported for a specified wavelength of 450 nm.

Analysis of thermal properties of TriThiol-TAE/vPOSS-[Bu.sub.4] was measured by a DSC Q1000 analyzer (TA Instruments) from -90 to 40[degrees]C at a heating rate of 10[degrees]C/min. Glass transition temperatures ([T.sub.g]) were obtained at the inflection point of the DSC curve in the glass transition region. Samples were approximately 3-10 mg.

A Hysitron TriboIndenter was used in a closed loop/feedback displacement method to measure surface hardness. A Berkovich diamond tip with an included angle of 142.3[degrees] with an average radius curvature of 100-200 nm used as the probe for nanoindentation. Indents were made using a lift height of 100 nm followed by a displacement of 2500 nm into the surface of the samples at a rate of 500 nm/s on a 4 x 4 grid having a separation of 15 [micro]m between indents. Drift measurements were done over 20 s with the analysis of drift being measured over the final 10 s of the test with a preload force of 0.5 [micro]N. Films were obtained with a 20 mil drawdown bar.

Thermogravimetric analysis (TGA), performed on a TA Instrument TGA 2050 Thermogravimetric Analyzer, was used to determine the mass loss (char) as a function of temperature by heating 10 mg samples to 500[degrees]C at a rate of 10[degrees]C/min in air. Sample thickness was ~200-250 [micro]m.

Physical property analysis

Scratch resistance of each Trithiol-TAE film was determined by using a pencil hardness test with a series of pencils of varying hardness according to ASTM method D 3363 00. The range of pencil lead hardness is graded from softest (9B-B) to mid-grade (F and HB) to hardest (H-9H) (15). Burn tests were performed on free-standing thin films using ASTM method D 0565 56 T (16). The films, which were cut in geometrical dimension of 6 mm (length) x 1 mm (width), were ignited by a flame source, and the time recorded from the start of flame ignition to the extinction of the flame and/or total consumption of the film by the flame.

Results and discussion

All of the components used for the photopolymerization investigation are shown in Fig. 1 along with their acronyms. In the next two sections, we will describe the thiol-ene photopolymerization kinetics of TriThiol with combinations of TAE and vPOSS-[Bu.sub.4] keeping a 1:1 molar ratio of total thiol functional groups (TriTthiol) to total ene groups (TAE + vPOSS-[Bu.sub.4]). Since neither TAE nor vPOSS-[Bu.sub.4] enes homopolymerize to an appreciable extent, the thiol-ene polymerization process in Scheme 1 predominates. A brief analysis of film physical/thermal properties will conclude the investigation.

Kinetic analysis

The kinetic analyses of various TriThiol-TAE/vPOSS-[Bu.sub.4] polymerizations, as measured by real-time FTIR (RTIR) and photo-DSC, were performed to understand the concentration effects of ene vPOSS-[Bu.sub.4] concentration on thiol-ene polymerization keeping the ratio of TriThiol to total ene groups constant. The polymerization rates and molar conversions of the TriThiol-TAE/vPOSS-[Bu.sub.4] systems at low concentrations of the silicate nanoparticle ([vPOSS-[Bu.sub.4] = 1 and 5 mol%) were compared to the rate and molar conversions of the pure TriThiol-TAE system ([vPOSS-[Bu.sub.4] = 0 mol%). Throughout the series of nanocomposites, no changes in polymerization rate and functional group conversions were observed when the concentration of vPOSS-[Bu.sub.4] increases from 0 and 5 mol%. For the pure TriThiol-TAE system, RTIR-based conversion (Fig. 2) shows that thiol and allyl ether groups completely polymerize after approximately 20-30 s exposure. For the sample with [vPOSS-[Bu.sub.4]] = 1 mol% of the total ene concentration, the total molar conversion of each functional group was unaffected by the presence of vPOSS-[Bu.sub.4] (Fig. 3). The combined sum of the total relative mole conversions of both ene groups has a molar quantity equal to the total thiol relative mole conversion at a given exposure time. Note that we assume that there is 100 total moles present in the sample and, hence, there are 50 relative moles (i.e. 50 mol%) of thiol groups and 50 relative moles (i.e. 50 mol%) of ene groups (49.5 moles are allyl ether groups on TAE and 0.5 moles are vinyl groups on the vPOSS-[Bu.sub.4]). At a concentration of [vPOSS-[Bu.sub.4]] = 5 mol% of total ene, no observable decrease in the total molar conversions of the thiol and ene groups was found indicating that the presence of electron-deficient groups on vPOSS-B[u.sub.4] has no effect on the reaction rate between thiol and ene groups (Fig. 4). Note that the total ene conversion (allyl ether on TAE + vinyl on vPOSS-B[u.sub.4]) is essentially equal to the total thiol conversion for a given exposure time.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

A comparison of the exotherms for polymerizations (Fig. 5) at [vPOSS-B[u.sub.4]] = 0, 1, and 5 mol% show that the maximum rates of polymerization changes minimally (decreases) as the concentration of v-POSS increases. The peak height of each curve, which corresponds to the maximum polymerization rate of each mixture, decreased in the order 84, 79, and 62 mW for [vPOSS-B[u.sub.4]] = 0, 1, and 5 mol%, respectively. This corresponds to the small changes in the slopes of the RTIR-based conversion curves (Fig. 4) for [vPOSS-B[u.sub.4]] = 1-5 mol%. In the case of the exotherm curves, the reduction in rate results from a total reduction in polymerizable ene and thiol groups in the mixture due to the high molecular weight per double bond of the vPOSS-B[u.sub.4] enes compared to the allyl ether enes.

Physical and thermal analysis 

Overall, the thin films (200-250 [micron]m) in this nanocom-posite series were optically clear (Fig. 6) for concentrations of vPOSS-B[u.sub.4] up to 5 mol% exhibiting good scratch resistance and thermal stability. Visible transmittance decreased monotonically from 89% T to 86% T from zero to 5 mol% (Table 1). Scratch resistance, as measured by a pencil hardness test, was analyzed with respect to varying ene molar concentration. A modest but observable change in scratch resistance of the film series was found to be strongly influenced by the concentration of the incorporated silicate particles. The results in Table 2 show that scratch resistance of the base TriThiol-TAE film increases from 2H to 4H upon the chemical incorporation of 5 mol% vPOSS-B[u.sub.4] into the network, and, in contrast, significantly decreases from 2H to F when vPOSSB[u.sub.10] particles are blended into the network matrix at a weight percentage (10.2 wt%) equivalent to 5 ene mol%. Additional measurements of surface hardness analyzed by nano-indentation (Table 2) show an increase of modulus commensurate with an increased amount of vPOSS-B[u.sub.4] particles.

Table 1: Visible transmittance of light

Ene mol% vPOSS-B[u.sub.4]   % T, 450 nm

0                              88.8
1                              88.2
5                              87.3

Measurement of optical clarity of TriThiol-TAE systems containing 0, 1,
10, 5 one mol% vPOSS-B[u.sub.4]

Table 2: Pencil hardness test

Ene mol%           Pencil    Surface       Wt%       Pencil
vPOSS-B[u.sub.4]  hardness  hardness,    vPOSS-     hardness
                             [mu] Pa   B[u.sub.10]

0                    2H       42.4         0          2H
1                    2H       52.1         2.2        H
5                    4H       55.2         10.2       F

Measurement of scratch resistance of TriThiol-TAE systems containing 0,
1, 10, 5 one mol% VPOSS-B[u.sub.4]

An investigation of the thermal properties for the TriThiol-TAE/vPOSS-B[u.sub.4] films at 0, 1, and 5 mol%, as measured by TGA and differential scanning calorimetry (DSC) show that low loadings (1 and 5 mol%) of chemically incorporated vPOSS-B[u.sub.4] particles improves the thermal stability of the base TriThiol-TAE film without significantly affecting the network structure. TGA and DSC results in Table 3 show an increase of char formation after thermal degradation (char wt%) and relatively constant [T.sub.g] values with increases of vPOSS-B[u.sub.4] concentration up to 5 mol%. Burn tests for the nanocomposite POSS film series provided evidence for a significant reduction in flame spread. The burn rates of the film series decrease significantly as [vPOSS-B[u.sub.4]] increased from 0 to 5 mol% (Table 3). Remarkably, only 1 mol% (of total ene) of vPOSS-B[u.sub.4] reduces the rate of flame spread by < 33%.

[FIGURE 6 OMITTED]

Table 3: Measurement of flame spread (ASTM burn tests), [T.sub.g]
(DSC), and char wt% (TGA) of TriThiol-TAE systems containing o, 1, and
5 ene mil% vPOSS-B[u.sub.4]

Ene Mol%          Burn rates,   DSC [T.sub.g']    TGA,
vPOSS-B[u.sub.4]  [in.sup.2]/s    [degree].C    Char wt%

0                    11.2          -15.2          2.96
1                     6.24         -10.9          7.15
5                     4.63         -15.5         17.9

Conclusions

The effects of POSS concentration on the kinetics and physical properties of thiol-ene/POSS nanocomposites were studied and were shown to have some influence on physical properties when inorganic POSS particles were incorporated chemically into the thiol and allyl ether based thiol-ene networks. At low POSS loadings (1 and 5 mol%), the polymerization rates of the TriThiol-TAE/vPOSS-B[u.sub.4] photopolymerization are unaffected by increasing vPOSS-B[u.sub.4] concentrations up to 5 mol% vPOSS-B[u.sub.4]. Also, the incorporation of vPOSS-B[u.sub.4] has no significant affect on network formation as demonstrated by the minimal changes in [T.sub.g]. Thermal stability and film hardness were improved, and the burn rate reduced upon the chemical incorporation of vPOSS-B[u.sub.4] into the TriThiol-TAE network.

T. S. Clark, C. E. Hoyle . S. Nazarenko Department of Polymer Science, University of Southern Mississippi. Box 10076, Hattiesburg, MS 39406-0076. USA e-mail: [e-mail]

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[C] FSCT and OCCA 2008

T.S. Clark, C.E. Hoyle, S. Nazarenko Department of Polymer Science, University of Southern Mississippi, Box 10076, Hattiesburg, MS 39406-0076, USA e-mail: [e-mail]