Effect of crosslink density on the refractive index of a

Research Article Received: 27 September 2011

Revised: 16 March 2012

Accepted: 22 March 2012

Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/pi.4315

Effect of crosslink density on the refractive index of a polysiloxane network based on 2,4,6,8-tetramethyl-2,4,6, 8-tetravinylcyclotetrasiloxane Looi Yien Tyng,a Mohamad Riduwan Ramli,a Muhammad Bisyrul Hafi Othman,a Rafiza Ramli,a,c Zainal Arifin Mohd Ishaka,b and Zulkifli Ahmada∗ Abstract Crosslink density is one of the factors which dictate the optical and mechanical properties of silicone-based polymers. A series of hydrosilyl-terminated polydimethylsiloxane cured with 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane was prepared to form a crosslink network. They were clear gel forms whose elastomeric features were dependent on the endcapper concentration, 1,1,3,3-tetramethyldisiloxane, employed during their synthesis. The siloxane-based polymers displayed excellent thermal stability whose decomposition temperature was around 480 ◦ C. The refractive index was in the range 1.410–1.425. It was found that the refractive index is dependent on the crosslink density of the network. The effect of fractional free volume and ‘densification’ of the network was elucidated and found to contribute to this dependence. The Lorentz–Lorenz model was used to integrate these observations. c 2012 Society of Chemical Industry
Keywords: polysiloxane; crosslink network; refractive index; Lorentz–Lorenz relationship

INTRODUCTION Polysiloxanes have many outstanding mechanical, thermal and electro-optical properties, which have led to their wide use in various applications.1 – 7 Their use as a light emitting diode (LED) encapsulant demands good light transmission efficiency by exhibiting an excellent optical transparency, a high refractive index and UV radiation stability. A refractive index close to 1.70 is desirable in order to be compatible with the LED dies.8 The refractive index of a material can be expressed by the Lorentz–Lorenz relation given by9 RM =

n2 − 1 Mw n2 + 2 ρ

(1)

where RM is the molar refractivity, n is the refractive index, Mw is the mass-average molar mass and ρ is the density. The refractive index is the ratio of the velocity of light in a vacuum to the velocity of light in a material. This value is always greater than 1.0, because light slows down due to its interaction with atoms in a material, especially when they are non-hydrogen atoms. Several models have been used to account for and predict the refractive index of a given material. The empirical approach by Van Krevelen employed group contribution methods to build models for refractive index prediction.10 Another method is the theoretical approach based on the quantitative structure–property relationship (QSPR), which uses several descriptors.11 The QSPR method, as shown by Xu et al., proposed a reduction in the speed of light due to the closeness between two atoms, which pose ‘obstacles’ to the traversing electromagnetic radiation.12 As an example, besides

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bond polarizability, the π bond affects the closeness between two bonding atoms. Hence, introducing a phenyl ring increases the refractive index significantly. A quantitative treatment by Hougham et al. established that a reduction in free volume induced an increased refractive index.13 The decrease in free volume, as measured by positron annihilation lifetime spectroscopy, after UV treatment on a biopolymer was attributed to intense crosslink formation detected by XRD.14 On the basis of this information, it is envisaged that increased crosslink density in a polymeric system will lead to an increase in refractive index due to the effect of tightness and closeness between chains.14,15 2,4,6,8-Tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (D4 V) is an important cyclic precursor for use in the synthesis of many linear, branched and crosslinked polysiloxanes. The presence of the vinyl group in the structure affords a myriad of structural modifications, due to its high reactivity. Hydrosilylation is an important addition reaction, which involves the formation of a new bond between the vinyl group and the Si–H bond.16 Several studies

∗

Correspondence to: Zulkifli Ahmad, School of Materials and Minerals Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300 Pulau Pinang, Malaysia. E-mail: [email protected]

a School of Materials and Minerals Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300 Pulau Pinang, Malaysia b Engineering and Technology Research Platform, Universiti Sains Malaysia, Nibong Tebal, 14300 Pulau Pinang, Malaysia c Penchem Technologies Sdn. Bhd, Pulau Pinang, Malaysia

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www.soci.org have been performed to affect a crosslinked network utilizing D4 V and silsesquioxane via a hydrosilylation reaction in either organic–inorganic systems or wholly inorganic systems.17 – 19 Most of these works studied the thermal and oxidative stability and gel formation of the network. Except for a few, e.g. Zhiyi Zhang et al., not much work has been done in exploring the quantitative basis of the optical properties of polysiloxanes.20 Because of interest in utilizing this material for photoelectronic device encapsulation, a series of organic–inorganic polysiloxane systems was designed to study the effect of crosslink density on refractive index. In this system, D4 V is utilized to act as the anchoring point to generate a crosslinked network via hydrosilylation with a linear hydrosilyl-terminated polysiloxane. The crosslink density can be monitored by varying the chain length of the polysiloxane. The optical properties will be discussed in relation to the crosslinked network with D4 V, based on the Lorentz–Lorenz relation.9

MATERIALS AND EXPERIMENTAL Materials The D4 V and octamethylcyclotetrasiloxane (D4 ) 99% were purchased from Aldrich (Singapore) and stored in CaH2 . 1,1,3,3Tetramethyldisiloxane 97% was also obtained from Aldrich and was freshly distilled before use. Karstedt’s platinum catalyst (2 wt% in xylene) and triflic acid 98% were obtained from Aldrich and used as received. Syntheses Syntheses were performed in two stages, namely cationic ring opening polymerization to afford hydrosilyl-terminated polydimethylsiloxane (HTP) prepolymer followed by hydrosilylation with the vinyl group of D4 V using a platinum catalyst. The molar mass of HTP was monitored by introducing and controlling the mass ratio of 1,1,3,3-tetramethyldisiloxane as the end-capper. Synthesis of HTP prepolymer Five different samples with different molar masses of HTP were synthesized using different mass ratios, 10, 20, 30, 40 and 50 %(v/w), of 1,1,3,3-tetramethyldisiloxane as the end-capper. The representative method used was as follows. D4 (20.02 g) and 1,1,3,3-tetramethyldisiloxane (3.54 mL, 0.02 mol) were charged in a two-neck round-bottom flask that was initially purged with nitrogen. The temperature was increased to 55 ◦ C and trifluoromethane sulfonic acid catalyst (0.13 mL, 0.65 wt% based on siloxane monomer) was slowly added via a syringe. The reaction was maintained at 55 ◦ C for 48 h. The acidic mixture was cooled to room temperature, and then neutralized by repeatedly extracting with sodium bicarbonate and left to settle for 24 h in a separating funnel. The viscous silicone layer was separated, diluted in diethyl ether and dried over anhydrous magnesium sulfate, with continuous stirring for 1 h. It was subsequently filtered using glass wool. Filtration was performed to remove magnesium sulfate left during the drying of the fluid product. Diethyl ether was evaporated under vacuum using a rotary evaporator at 110 ◦ C for 3 h. Hydrosilylation of HTP with D4 V HTP (1.50 mL) product, toluene (10.00 mL) and Karsted’s platinum catalyst (0.07 mL, 70 ppm) were charged into a two-neck reaction flask with a dropping funnel, a magnetic bar, a thermocouple and a nitrogen inlet. D4 V (0.50 mL) was charged into a dropping funnel. After the temperature was increased to 55 ◦ C, the D4 V was

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dropped slowly into the reaction mixture, due to a highly exothermic reaction. After addition, the temperature was increased to 65 ◦ C and stirring was continued for a further 2 h. The mixture was cooled to room temperature. The toluene was removed by distillation at 100 ◦ C for 1 h followed by an increase in temperature to 120 ◦ C for 2 h, to afford a water clear product designated as DVPS. These procedures were repeated by using different end-capper percentages of HTP, i.e. 0, 20, 30, 40 and 50 %(v/w). Curing of DVPS Product DVPS (2.00 mL) with different end-capper mass ratios was cast into aluminium foil moulds (15 × 30 × 3 mm3 ), cured and aged at 120 ◦ C for 24 h in a vacuum oven under 40 MPa. The sample was then de-moulded after cooling. Characterization Fourier transform infrared (FTIR) spectra were recorded using a Perkin Elmer Spectrum GX0 Series FTIR spectrophotometer. 1 H NMR was performed using a Bruker 400 Ultra Shield 400 MHz spectrometer at 25 ◦ C and deuterated chloroform was used as a solvent. The density of the samples were measured using Cole-Parmer Pycnometer with freshly prepared deionised water as weighing media. Measurements were made in triplicate. Thermal analysis was performed using a Star SW 9.30 differential scanning calorimeter. Samples were cooled to–140 ◦ C from room temperature, held for 3 min and then heated continuously to 25 ◦ C at a 10 ◦ C min−1 heating rate, during the second scan. TGA was performed using a TGA 4000 Perkin Elmer, in the heating range 30–700 ◦ C with a 10 ◦ C min−1 heating rate under nitrogen. Refractive indices were determined using an Atago Abbe refractometer NAR-1T Solid. The laser radiation used was the low-power sodium D line (589 nm) at 25 ◦ C. The refractometer was calibrated using distilled water in accordance with the instrument instructions. Uncertainty on the refractive index values was ±0.0001 unit. The molar mass distributions (Mw /Mn ) were determined by gel permeation chromatography (GPC) on a Waters system equipped with a 501 RI detector. Hardness was tested using a Shore A Teclock Durometer GS-706G on a sample size of 15 mm × 40 mm × 4 mm, based on ASTM D2240 standard. XRD patterns were obtained using a Bruker Advanced X-ray Solution Diffractometer (USA) machine, with Cu Kα radiation of wavelength 1.5406 Å, a 2θ range of 6◦ –70◦ and a step scan mode of 0.051◦ /358 s.21 Crosslink density measurement Extraction/swelling experiments in toluene were used to estimate crosslink density.22 Samples were cut into 10×10 mm2 from a sheet 3 mm thick and weighed (W’o ). The samples were then immersed in freshly distilled toluene (10 mL) in bottles for 24 h and the toluene was refreshed every 24 h over a 120-h period. Next, the swelled samples were removed from the bottles. Excess solvent was removed using tissue paper, and masses (Wsw ) of the toluene-swollen samples were determined. The toluene-swollen samples were dried in a vacuum oven until a constant mass was reached (Wo ). The magnitude of the soluble fraction (S, %) was determined using S=

Wo − Wo × 100% Wo

(2)

The degree of swelling (dsw , %) was also calculated according to

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dsw =

Wsw − Wo × 100% Wo

(3)

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The crosslinking density (nFR ) was estimated using the Flory–Rehner equation nFR =

−[v2 + v22 χ 1 + ln(1 − v2 )] 1/ V1 (v2 3 − 0.5v2 )

(4)

where v2 corresponds to the polymeric volume fraction in the swollen mass, V1 is the solvent molar volume and χ 1 is the Flory–Huggins polymer–solvent interaction parameter. The χ 1 value used for this estimation was 0.48 for polydimethylsiloxane–toluene.23 The number-average molar mass between crosslinks was determined using the equation24 Mc = 1/nFR . The fractional free volume (FFV) was calculated according to FFV =

V − Vo V

(5)

where V = 1/ρ is the specific volume and Vo is the occupied volume of the polymer computed from the van der Waals volume Vvdw by the Bondi method,25 i.e. Vo = 1.3Vwdw ; the value of V is the total volume taken up by 1 mol of repeat units, including free and occupied volume.

40 and 50 %(v/w). It was established from GPC that the amount of end-capper used affects the molar mass of HTP (Table 1). An increasing amount of end-capper induces a shorter chain-length distribution and hence a lower molar mass, as it prevents further chain growth during the propagation step. The end-capper serves as the terminating agent during the propagation step. Thus once it becomes incorporated into the growing end-chain this group becomes inactive due to the absence of any newly formed electrophile to continue further propagation. Apparently the product at this stage is in the form of oligomers based on relatively low Mn values – hence its viscous fluid form rather than gel. The viscosities of the synthesized HTP decreased proportionally to the increase in the end-capper concentration. The prepolymer were clear in colour displaying a visually excellent transparency. They were stored at 5 ◦ C for further treatment. Step 2 of Scheme 1 shows the hydrosilylation process between the hydrosilyl group at the terminal of the HTP with the vinyl side group of D4 V using a platinum catalyst. The product of this reaction transformed the viscous HTP prepolymer into an elastomeric gel form, whose colour remained clear. With the presence of four vinyl groups per D4 V structure, they readily served as the crosslinking point with the HTP prepolymer to afford a highly crosslinked network through the two methylene spacers. The proposed crosslink network is shown in Fig. 1.

RESULTS AND DISCUSSION Synthesis considerations Crosslinked DVPS samples were prepared following a two-step synthesis, as shown in Scheme 1. The first step was the preparation of HTP prepolymer from D4 and 1,1,3,3-tetramethyldisiloxane as the end-capper. This involved an acid-catalysed ring opening equilibration polymerization using trifluoromethane sulfonic acid as a catalyst. The acidic catalyst was preferred to a basic catalyst due to the sensitivity of hydride (Si–H) groups towards the latter which might lead to O-silylation.26 Molar masses of HTP were varied using 1,1,3,3tetramethyldisiloxane as the end-capping agent at 10, 20, 30,

FTIR spectroscopy Figure 2 shows the FTIR spectra before and after hydrosilylation of the HTP with D4 V, as well as for the pure D4 V. Both spectra show the C–H bond that arose from the stretching vibration of the methyl group in the region 2830–2970 cm−1 and the characteristic broad and strong peak at 1010 cm−1 corresponding to the Si–O bond of the polysiloxane chain backbone. The asymmetric C–Si stretching vibration band appeared near 800 cm−1 in both spectra. Compared with before hydrosilylation, the spectrum shows a marked reduction of the peak at 2150 cm−1 after hydrosilylation. This peak represents the characteristic Si–H stretching peak.27

Scheme 1. Procedure for preparation of the crosslinked network of the polysiloxane DVPS.

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Table 1. Molar mass and polydispersity of HTP samples with different end-capper percentages based on GPC Percentage of end-capper %(v/w)

Mw (g mol−1 )

Polydispersity

4400 4100 3900 3500 3300

1.24 1.35 1.21 1.25 1.26

10 20 30 40 50

Figure 1. Proposed crosslinked network (not to scale) between DVPS and HTP. The bold square structure refers to D4 V acting as an anchorage for the crosslinked network of HTP linear chains.

The decrease in peak intensity indicates that an almost complete hydrosilylation process has occurred. A small peak still remains at this position after hydrosilylation, indicating the presence of some residual Si–H bonds. This is similarly observed during the ageing of the analogous structure at room temperature, or even at 600 ◦ C for 24 h.27 There was also an absence of peaks at 3020 cm−1 and 1598 cm−1 , which corresponds to the disappearance of the vinyl group of D4 V.28 Their absence indicates complete hydrosilylation of the vinyl group of D4 V with the hydrogen of the hydrosilyl group. The methylene rocking vibration as a spacer supposedly appears at 720 cm−1 but could not be detected in the spectra of DVPS (after hydrosilylation). The presence of this peak could otherwise further substantiate the crosslinking of the vinyl group of HTP onto the D4 V. This is because there were only two methylene repeat units in the present system, which is very small for any detection by FTIR.29 Nevertheless, the presence of peaks in the region 2930–2850 and at 1420 and 1380 cm−1 , corresponding to the C–H bond, would sufficiently prove that the addition reaction did occur between D4 V and HTP. Nuclear magnetic resonance 1 H NMR was performed to confirm the structure of the synthesized HTP. Figure 3 shows the representative spectrum at 30%(v/w). The signals at 0.10 and 0.20 ppm correspond to the inner and terminal methyl protons of Si–CH3 respectively, whose multiplicity represents various tacticities of these methyl groups attached to the silicone nucleus. The peak at 4.70 ppm corresponds to the

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terminal hydrosilyl proton Si–H. The peak at 1.60 ppm could be due to trapped moisture. 1 H NMR analysis of DVPS could not be carried out since it was insoluble in most solvents and too fragile for any solid 29 Si NMR. Both the FTIR and 1 H NMR analyses as well as the swelling test proved unambiguously that hydrosilylation did occur, resulting in a highly crosslinked network. The molar mass of HTP (stage 1) is shown in Table 1. The molar mass proportionally decreases with increase in the proportion of end-capper. Meanwhile, the polydispersities obtained were very low representing a homogeneous molar mass distribution. Thermal properties The cured product DVPS displays a high thermal stability which undergoes 10% degradation in the region 406–478 ◦ C. This corresponds to the onset of the mineralization process involving the cleavage of Si–C and C–H bonds, leading to the evolution of hydrocarbons and hydrogen.30 The residual mass initially increased with an increasing content of end-capper. However, it reached its optimum at 40%(v/w) end-capper. The residual mass at 600 ◦ C for 30 and 40%(v/w) end-capper showed high values in the range 60%–70%, while the sample with 10%(v/w) end-capper showed the lowest value. It has been established that residual values are indirectly proportional to the crosslink density in a polysiloxane system. A linear uncrosslinked poly(dimethylsiloxane) heated in an argon atmosphere was completely degraded at ca 600 ◦ C with no residue.31 The result obtained here confirms that samples derived from 30 and 40%(v/w) end-capper attained an optimum level of crosslink density compared with the 10, 20 and 50%(v/w) DVPS samples. This is consistent with the swelling test result. Degree of crosslink network Table 2 shows the values of the soluble fraction, the degree of swelling and the crosslinking density. During the swelling test, a system having a high crosslink density would deter efficient diffusion of solvent into the matrix and thus reduce the values of the soluble fraction and the degree of swelling. The sample with 50%(v/w) DVPS displayed the highest swelling degree compared with the other samples. Samples with 40 and 30%(v/w) end-capper were lower in swelling degree and did not show any significant difference from each other. The crosslink density of 30%(v/w) is the highest while those of 40 and 50%(v/w) are lower, both with comparable values. The fact that the sample with 40%(v/w) has a lower swelling degree but comparable crosslink density to the 50%(v/w) sample can be explained by envisaging that at 40%(v/w) the optimum crosslink density has been achieved. A further increase in end-capper amount would result in excess hydrosilyl end-chain with respect to the vinyl group of D4 V. This free hydrosilyl end-chain contributes to a rather loose crosslinked network which explains the higher swelling ratio in the 50%(v/w) sample. Thus the values of Mc for both 40 and 50%(v/w) samples are consistently similar.22 The swelling behaviour of the samples in toluene is represented in Fig. 4. The 10 and 20%(v/w) samples could not be measured as they became homogenized with the solvent during the course of measurement. This further substantiated the low level of crosslink density in these systems. These samples have higher molar mass as the result of using lower end-capper ratio during reaction in stage 1. They have longer chains with effectively lower end-capper concentration. The number of D4 V involved during hydrosilylation with the Si–H group will be reduced per

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Figure 2. FTIR spectra of D4 V (upper), HTP (middle) and the hydrosilylation product DVPS.

application requirements.32 In this study, the refractive index of synthesized DVPS was measured using an Abbe refractometer (as shown in Table 3). The Mossotti–Clausius equation relates the dielectric constant ε to the density:33 4 ε−1M = πNα ε+2 ρ 3

(6)

where N is Avogadro’s number, ρ is density and α is the polarizability of the molecule. The left-hand side of the Mossotti–Clausius equation is called the molar polarization (P) of the medium. According to Maxwell’s law, at optical wavelengths the dielectric constant is related to the refractive index of the medium as follows: ε = n2

(7)

Incorporating the two equations and rearranging gives Figure 3. 1 H NMR (in CDCl3 ) spectrum of HTP.

unit volume resulting in a low crosslink density such that the solvent is able to homogenize these networks. However, in the 30, 40, 50 %(v/w) samples, the chain length was much shorter due to the used of higher end-capper ratio. The number of D4 V involved in hydrosilylation with the terminal Si–H group effectively increased per unit volume. This result in the samples having higher crosslink density which remains insoluble in the solvent. Refractive index Polysiloxanes are uniquely suitable for optical applications because they allow high flexibility in optical integration, by providing a wide range of refractive indices. A number of optical properties are controllable and can be tuned depending on the Polym Int (2012)

n2 − 1 1 =r n2 + 2 ρ

(8)

where r is the specific refraction of the medium and is also a constant. The above equations indicate that the refractive index is dependent on polarizability as well as density. A plot of (n2 − 1)/(n2 + 2) against ρ should give a linear relationship with gradient corresponding to the specific refraction.34 Such a plot is given in Fig. 5. A systematic linear dependence of the quantities plotted in Fig. 5 confirms the Lorentz–Lorenz relationship between refractive index and density for the polysiloxanes studied. According to this plot the specific refraction (r) is 0.2533 cm3 g−1 . The refractive index of the 40%(v/w) end-capper sample displays the optimum value in this system, which is 1.4204, whilst the lowest is that of

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Table 2.

Thermal and swelling behaviour of DVPS samples with different proportions of end-capper

Percentage of end-capper %(v/w)

T10% ( ◦ C)

10 20 30 40 50 a

LY Tyng et al.

478.0 474.4 453.3 406.8 409.0

Residual mass (%)

Soluble fraction (%)

Degree of swelling (%)

Crosslinking density (nFR ) (mol cm−3 )

Mc

28.1 38.1 59.0 67.5 49.6

a

a

a

a

a

a

a

a

30.5 33.4 39.6

846.4 910.8 1126.5

0.0163 0.0159 0.0158

61.4 63.0 63.4

Undetectable due to fluid state.

Figure 4. Crosslink density and swelling behaviour of the DVPS samples in toluene.

Table 3. Refractive index (at 25 ◦ C), FFV and dominant d-spacing (Å) for DVPS samples with different proportions of end-capper Percentage of endRefractive Density, ρ Vvdw capper %(v/w) index, n (g cm−3 ) (cm3 g−1 ) 10 20 30 40 50 a

1.4174 1.4178 1.4194 1.4204 1.4180

0.9938 0.9948 0.9971 0.9985 0.9950

0.5985 0.6000 0.6162 0.6165 0.6169

FFV 0.2268 0.2241 0.2013 0.1998 0.2020

Dominant d-spacing (Å) a a

3.30 2.96 3.57

Not detectable due to fluid state.

Figure 5. Plot of (n2 − 1)/(n2 + 2) versus density for DVPS samples.

the 10%(v/w) sample. The result displayed in the present system is consistent with the respective values of crosslink density. At high crosslink density, there will be a maximum reduction in the molar free volume since the chains will be closely aligned

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affecting densification of the network. Several studies based on positron annihilation lifetime spectroscopy have established that the formation of a crosslinked network effects a decrease in free volume in a polymeric system.14,15 Fractional free volume In order to quantify the free volume changes in refractive index, FFV was calculated based on FFV =

V − Vo = 1 − 1.3Vvdw ρ V

(9)

where V is the specific volume given by the inverse of the bulk density 1/ρ and Vo is the occupied volume given by 1.3Vvdw .35 Vvdw was derived using the method of Bondi.25 Vvdw for the Si–O unit was calculated by Herskowitz and Gottlieb, yielding a value of 16.687 cm3 mol−1 .36 The repeat unit for the polymeric network was assumed to be as shown in Fig. 6. The calculated FFV values for all samples with different endcapper concentrations are shown in Table 3. Samples with 30 and 40 wt% both display a lower FFV compared with the others which correspondingly showed higher refractive indices. As proposed by Xu et al.,12 a high crosslink density will affect the ‘closeness’ between chains. The particles pose considerable obstacles, thus reducing the speed of the traversing light radiation. The observation from this work that the refractive index is directly related to crosslink density corroborates well with this proposal. X-ray diffraction In order to determine the effect of densification on the crystalline phase during crosslink formation, the XRD patterns were obtained as shown in Fig. 7. It is noted that the densification effect occurred on the bulk of the polymer structure based on the FFV results.

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Figure 6. Repeat unit for the polysiloxane network used to determine FFV, where n = 15, 16, 22, 18 and 14 for 10, 20, 30, 40 and 50%(v/w) end-capper respectively.

However, the XRD pattern shows some changes in lattice spacing of the crystalline phase. The samples are essentially amorphous in nature with several broad halos. A broad prominent peak in the range 2θ = 22◦ –30◦ possibly represents the stacking arrangement of the cyclotetrasiloxane backbone with a calculated interplanar distance of about 3.5 Å.21 The 50 %(v/w) sample showed the dominant d-spacing at a lower 2θ value than for the 30 and 40 %(v/w) samples. It represents an increase in interplanar distance. This could be due to the longer molecular chains of the latters which are able to reorient and aligned better between chains and affecting closeness and crystallinity. The molecular chains of the 50 %(v/w) is shorter, which restrict the possibility for good rearrangement thus resulting in a higher interplanar distance. These results suggest that densification occurred in the crystalline phase as a result of a higher crosslinked network. Peaks occurring at 2θ = 14◦ could possibly represent the lateral spacing of the cyclotetrasiloxane backbone. A significant appearance of peaks at 2θ = 8◦ for samples of 30 and 40%(v/w) was observed which began to diminish in the 50%(v/w) sample. They indicate generation of a new interplanar spacing with a larger lattice distance. This could be due to the ability for the longer chain repeat unit in the 30 and 40%(v/w) samples to reorient and rearrange to form local crystalline domains. This further induced a densification effect. The peak positions and shapes for all samples are similar from which it can be assumed that there is no significant change in crystal system upon change in crosslink density. Hardness As shown in Fig. 8, 30%(v/w) end-capper showed an optimum value of hardness, while the 10 and 20%(v/w) samples showed lower hardness. This result reflects the level of crosslink density in these samples, as established from the preceding discussions. Samples with a high crosslink density displayed an optimum hardness; they induce a rigid framework, which is better able to withstand deformation. This result is consistent with the hardness (Shore A) for a series of medical grade silicones, which showed a proportional increase with the level of crosslink density.37 The sample with 50 wt% displayed a lower hardness despite showing comparable crosslink density with the sample with 40%(v/w) end-capper, as shown in Table 2. As discussed above, the lower hardness in the sample with 50%(v/w) can be explained on the basis of the loose crosslink network due to excess of the hydrosilyl end-chain group. Hence, the latter network is more susceptible to failure under deformation and becomes more brittle. Polym Int (2012)

Figure 7. XRD data for DVPS samples with different proportions of endcapper.

Figure 8. Shore A hardness of DVPS samples with different proportions of end-capper.

CONCLUSIONS HTP, with a controlled molar mass, was prepared via the cationic ring opening polymerization of D4 , followed by hydrosilylation with D4 V to obtain a highly crosslinked network. The thermal stability, swelling behaviour and hardness of DVPS are closely related to the level of crosslink density. The refractive index was increased with the formation of a highly crosslinked network through a ‘densification’ effect. This could be attributed to the reduction in free volume, which affects the closeness in chain distances. The effect of crosslink density on refractive index was successfully verified based on the Lorentz–Lorenz model.

ACKNOWLEDGEMENT The authors gratefully acknowledge the USM Fellowship 814069 and USM Short Term Grant 6035286 for financial support.

REFERENCES 1 Zhang ZJ, Zhou N, Xu CH and Xie ZM, Chinese J Polym Sci 19:7–11 (2001). 2 Bogdan M and Julian C, Progress in Organosilicon Chemistry. Gordon and Breach Science Publishers, New Hampshire, USA p. 592 (1995). 3 Riffle JS, Yilgor I, Banthia AK, Wilkes GL and McGrath JE, Org Coat Appl Polym Sci 46:379–400 (1981). 4 Yilgor I, Steckle JWP, Yilgor E, Freelin RG and Riffle JS, J Polym Sci Polym Chem 27:3673–3690 (1989). 5 Kuo ACM, Silicone Release Coatings for the Pressure Sensitive Industry – OverviewandTrends. Dow Corning Corporation, Michigan, USA (2010).

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