Polyurethane methacrylate/silicone interpenetrating ... - Springer Link

J Polym Res (2008) 15:89–96 DOI 10.1007/s10965-007-9147-1

Polyurethane methacrylate/silicone interpenetrating polymer networks synthesis, thermal and mechanical properties A. Vuillequez & J. Moreau & M. R. Garda & B. Youssef & J. M. Saiter

Received: 22 May 2007 / Accepted: 16 August 2007 / Published online: 6 September 2007 # Springer Science + Business Media B.V. 2007

Abstract The synthesis of an interpenetrating polymer network (IPN) combining a polyurethane methacrylate network (PUMA) and a silicone network is reported. The PUMA network is synthesized by UV-light cure. The silicone network is formed through a condensation between α, ω dihydroxy polydimethyl siloxane and γ-methacryloxypropyl trimethoxy silane (γ-MPS) as a cross-linking agent. The IPN is prepared by different mechanism: radical and condensation types. According to thermogravimetric analysis of the hybrid material, the thermal stability stayed unchanged but the kinetic of degradation changed. Tg decreased with increasing silica content. The thermal cure process under humid atmosphere influence properties just for PUMA/4.2%SiUV+T. Condensation between γ-MPS decreases the penetration depth from 158 to 82 μm and increases the mechanical glass transition temperature from 106 to 141 °C. Keywords Grafted-IPN . Polyurethane methacrylate . Silicone . TGA . TMA

Introduction An interpenetrating polymer network (IPN’s) is an intimate combination of two polymers both in the network form [1]. The entanglement of two cross-linked polymers leads to forced miscibility compared to usual blends and the resulting materials is expected with good dimensional stability. The aim of these types of polymer associations A. Vuillequez : J. Moreau : M. R. Garda : B. Youssef (*) : J. M. Saiter Laboratoire PBM, UMR 6522, LECAP Institut des Matériaux de Rouen, Université de Rouen, Faculté des Sciences, Avenue de l’Université BP 12, 76801 Saint Etienne du Rouvray, France e-mail: [email protected]

is to obtain materials with better mechanical properties, better resistance to degradation and improve their properties by the add of the properties of their components. GraftedIPNs differ from IPNs in that they are composed of two polymer networks of different nature linked by chemical bonds. Interpenetrating polymer networks are also one of the possibilities to form organic/inorganic composite materials [2–4]. Arakawa et al. [4] presented an inorganic/ organic hybrid materials of polycarbonate and silica. This hybrid material was superior to polycarbonate in terms of the morphological homogeneity, heat surface and surface hardness. In this work we will focus our attention in such organic/inorganic mixture made of polyurethane methacrylate and silicone. Urethane acrylate or methacrylate networks are known and widely used in various applications like furniture, aerospace industries… [5, 6]. Urethane methacrylate resin is formed through a di- or poly-isocyanate and an acrylate or methacrylate bearing hydroxy functions. The use of hexamethylene di-isocyanate trimer (HDT) thanks to isocyanurate functions introduces a high thermal and mechanical stability [7]. To modify properties of polyurethane methacrylate, they can be combined with other compounds, such as silica or epoxy resin [8–11]. Chen and Chen [9] explored the effects of epoxy resin content on water resistance and mechanical properties i.e. the elongation at break decreases with the increase of epoxy resin content. It was demonstrated by Wu et al. [10] that the introduction of functional silane improves some physical and chemical properties, i.e. very low surface energy, excellent gas and moisture permeability, good heat stability, low temperature flexibility and biocompatibility. The use of γ-methacryloxypropyl trimethoxy silane (γ-MPS) inserts the silane into the polyurethane methacrylate network due to methacrylate functions. The polymerisation is induced by UV light cure process leading to a fast kinetic of transformation, while

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Scheme 1 Synthesis of urethane methacrylate resin

the alkoxy silane groups –Si–(OMe)3– with α,ω dihydroxy polydimethyl siloxane polymerise by self condensation which is characterized by a low kinetic of transformation. This second reticulation is induced by thermal cure process under humid atmosphere [12, 13]. The Si-(OR)3 absorbs water from the atmosphere and undergoes hydrolysis and condensation reaction forming silicon network. In the present study, the synthetic pathway (in two steps) of grafted-IPNs composed of polyurethane methacrylate and silicone are presented. All components are first mixed together and the networks are then formed through independent reaction mechanisms. The formations of resin and networks have been investigated by FTIR spectroscopy. In order to qualify materials measured using thermogravimetric analysis, differential scanning calorimetry, thermomechanical analysis and surface hardness have been performed.

Experimental Chemicals The reagents (hydroxy-2-ethyl methacrylate, dibutyl tin dilaurate, 2,2-dimethyl-2-hydroxyacetophenone, α,ω dihydroxy Scheme 2 Synthesis of polyurethane methacrylate network

polydimethyl siloxane and γ-methacryloxypropyl trimethoxy silane) were purchased from Aldrich except for hexamethylene diisocyanate trimer (HDT) which was supplied by Rhône Poulenc (now Rhodia; NCO=7.088.10−3 meq.g−1). All products were used without further purification. Synthesis Synthesis of urethane methacrylate resin (UMA) A 22.26 g sample of HDT and 20.22 g of hydroxy-2-ethyl methacrylate were placed in a 50 cm3 three-necked flask equipped with a condenser, a dropping funnel and a device for nitrogen flow. Thereafter, 0.1 g of dibutyl tin dilaurate (DBTDL) as catalyst was added to the initial mixture. The solution was stirred at room temperature for 20 min until the monomer was obtained in quantitative yield (Scheme 1). The synthesis of this resin has been already largely described elsewhere by Burel et al. [7]. The condensation reaction was monitored by FTIR through the disappearance of the characteristic isocyanate (–N=C=O) absorption band at 2270 cm−1. Isocyanate groups are engaged in the condensation process and must disappear when the reaction is completed.

PUMA/silicone IPN synthesis, thermal and mechanical properties

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Table 1 The composition of this materials in UMA resin, siloxane and γ-MPS and the origin of silica percentage contain on this materials is giving

UMA (%) (with Darocure 1173) Siloxane (%) %SiSiloxane γ-MPS (%) %Siγ-MPS

PUMA/ 2.8%Si

PUMA/ 3.6%Si

PUMA/ 4.2%Si

91.1 6.5 2.55 2,4 0.25

82.6 5.9 2.3 11.5 1.3

76.8 5.5 2.15 17.7 2.05

Synthesis of polyurethane methacrylate network (PUMA) Polyurethane methacrylate network was obtained by photochemical curing reaction. The photo initiator 2,2-dimethyl2-hydroxyacetophenone (0.4 g of Darocure 1173) was dissolved in the resin under stirring at room temperature. A part of this sample was irradiated for 1 min at room temperature with a UV-light (400 mW.cm−2) to obtain the PUMA (Scheme 2). The radical photo-polymerisation of resin was monitored by FTIR through the disappearance of the characteristic acrylate absorption band at 1,639 cm−1 because these acrylate groups must react under UV light by radical polymerisation. Synthesis of polyurethane methacrylate/Silica (UV) networks (PUMA/SiUV) After the addition of Darocure 1173, α,ω dihydroxy polydimethyl siloxane (Siloxane) and γ-methacryloxypropyl trimethoxy silane (γ-MPS) were dissolved in the resin under stirring at room temperature. Three samples with different content of siloxane and γ-MPS have been prepared. The weight composition of the samples was reported in Table 1. These entire samples were irradiated for 1 min at room temperature with a UV-light to obtain the PUMA/SiUV (Scheme 3).

Scheme 4 Reaction of Si–O–CH3 condensation

Measurements Fourier transform infrared (FTIR) spectroscopic analysis was carried out under ambient conditions using a PerkingElmer Spectrophotometer in the spectral range 4,000 to 600 cm−1. Each signal has been normalized to the HDT isocyanurate absorption peak at 1,450 cm−1. This signal was used as the internal standard to compare the band absorption relative to reacting groups because the molecular HDT group remains invariant during synthesize. Thermogravimetric measurements were performed on a Netzsch TG209 thermo balance under nitrogen atmosphere (flow rate: 20 ml.min−1). Sample pellets with a mass of 5 to 10 mg, put in aluminium crucible, were heated at 5 °C.min-1 from 30 up to 750 °C. Differential scanning calorimetry (DSC) investigations were performed on a Perking-Elmer series 7 calorimeter. The calorimeter was calibrated in energy and temperature with Indium as standard, under nitrogen atmosphere (flow rate: 10 ml.min−1) and with a heating rate equal to 10 °C. min−1. The sample mass was between 10 and 20 mg. Thermomechanical analyses (TMA) were performed using a DMA Q800 TA instruments modified system under penetration mode. The TMA was calibrated in electronic, force and dynamic parameters and the penetration clamp was calibrated in mass and compliance. Disks (2 mm thick and 15 mm of diameter) were heated from 30 to 180 °C with a heating rate of 5 °C.min−1. Four static loads 1, 5, 10 and 15 N respectively were used. The surface hardness was measured according to Vickers standard method, using a durometer. The measurements were repeated three times for each sample, and the average values were taken for the evaluation of hardness.

Synthesis of polyurethane methacrylate/Silicon (UV+T) networks (PUMA/SiUV+T) Results and discussion Some of these samples were heated at 150 °C during 24 h under humidity condition to obtain PUMA/SiUV+T (Scheme 4). The condensation of silica was monitored by FTIR through the disappearance of the characteristic Si–O–CH3 absorption band at 1,100 and 800 cm−1.

Scheme 3 Synthesis of PUMA/ SiUV

The formation of urethane methacrylate resin (UMA) from the initial reaction mixture seems to be possible with regard to equation presented in Scheme 1. However, in the present case, isocyanate groups and water (used for the condensa-

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Scheme 5 Possible reaction between isocyanate groups and water

tion of silica) can react leading to the following sidesreactions (Scheme 5). These reactions are not wished and must be avoided. In regard to these reactions, we have to propose to favour the UMA formation by using dibutyl tin dilaurate (DBTDL). It was demonstrated by Ni et al. [15] that the DBTDL is an organotin compounds known to be suitable catalysts for the isocyanate–hydroxyle reaction, contrary to amine compounds which promote the isocyanate–water reaction. Building of the UMA resin was observed by the disappearance of the –NCO stretching vibration at 2,270 cm−1 on the FT-IR spectra. After 20 min under stirring we observed the total disappearance of –NCO groups on the FT-IR spectra displayed on Fig. 1. We may conclude that our experimental protocol lead to the maximum efficiency. The PUMA network is obtained from UMA resin by curing under UV light. The PUMA network formation was monitored by FT-IR spectroscopy from the absorption band at 1639 cm-1, characteristic of the C═C stretching. The double bond C═C polymerises under UV light as shown in Scheme 2. On Fig. 2 the FT-IR spectra obtained for untreated resin and resin after 1 min of UV light treatment are presented. The absorption band at 1,639 cm−1, characteristic of the C═C stretching disappeared after 1 min under UV light. This disappearance points to the efficiency of the curing. This network will be used as reference material to control the influence of silica. Using the same protocol, we have characterized PUMA/ SiUV and PUMA/SiUV + T formation. On Fig. 3 the FT-IR spectra obtained for PUMA/SiUV and PUMA/SiUV+T are given. After 1 min under UV light we observed the disappearance of C═C groups at 1,639 cm-1 which prove the PUMA network formation after cure for PUMA/SiUV and PUMA/SiUV+T. After thermal cure under humidity condition

we observed the decrease of bands at ∼1,100 and 800 cm−1 assigned to the stretching vibrations of Si–O–R linkage for PUMA/SiUV+T. Consequently thermal cure process under humidity condition leads to the condensation of Si–(OR)3. After 24 h of thermal cure process under humidity condition the FT-IR spectra stays unchanged. Consequently the maximum efficiency is obtained to synthesis of silicone network after 24 h of curing. Thus from FT-IR spectroscopy, we have shown that the protocol proposed in this work leads to a maximum efficiency to synthesize PUMA network and Silicone network. The final material obtained is an hybrid organic/inorganic grafted-IPNs. The thermal stability in classical range of temperature (30 to 750 °C) is checked by thermogravimetric analysis. All the thermal quantities determined by TGA are reported in Table 2. The curves of degradation obtained are displayed on Fig. 4. Networks show any mass loss up to 250 °C. Then the degradation is revealed by two mass losses occurring in the same temperature domain (from 300 to 500 °C). This first mass loss is of the order of 31% for PUMA and 35% for PUMA/SiUV. For PUMA it was already mentioned that CO2 and CO vapours are mainly produced during the first step of decomposition which proceeds successively by chain breaking of the carbonyl groups located in the surrounding

Fig. 1 Infrared spectra of (grey dots) HDT and (black dots) UMA resin. Characteristic isocyanate absorption band at 2,270 cm−1 disappearance

Fig. 3 Infrared spectra of resin components (grey dots) PUMA/SiUV and (black dots) PUMA/SiUV+T. Characteristic Si–O–CH3 absorption band at 1,100 and 800 cm−1. Each signal was normalised to the HDT peak at 1,450 cm−1

Fig. 2 Infrared spectra of (grey dots) UMA resin and (black dots) PUMA network. Characteristic acrylate absorption band at 1,639 cm−1 disappearance. Each signal was normalised to the HDT peak at 1,450 cm−1


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Table 2 Thermogravimetric data measured on PUMA and PUMA/ 4.2%Si for different cure (UV cure, UV+T dual cure)

TM1/°C TM2/°C Δm/%1 Δm/%2 Δm/%high temperature

PUMA

PUMA/4.2% SiUV

PUMA/4.2% SiUV+T

324 439 31 65 4

350–383 478 35 53 12

356–381 481 34 53 13

Fig. 5 Derivative thermogravimetric curves obtained on PUMA (grey line) and PUMA/4,2%Si for different cure (black triangles UV cure, black diamonds UV+T dual cure). Different kinetic aspect

of the cross linking points and by decomposition of the acrylate groups [7]. As better evidenced on the derivative loss curves displayed in Fig. 5, PUMA and PUMA/Si exhibit differences in the magnitude of the first degradation stage. In regard to the composition of each material, the acrylate part concerns 30 and 34% mass/mass of the total material mass for PUMA and PUMA/Si respectively. At this point it is remarkable to notice that these ratios are exactly those experimentally found for the mass loss on the thermogravimetric curves. The temperatures characteristic of degradations for the first mass loss, measured at the peak minima observed on the derivative curves, are 324, 350–383 and 356–381 °C respectively. For the second mass loss occurs respectively at 439, 478 and 481 °C (Fig. 5). These peaks correspond to HDT groups and attest the breaking of the isocyanurate bonds at high temperature as shown by Burel et al. [7]. The full polyurethane methacrylate network is degraded at high temperature as already observed by Ledru et al. [14]. Non insignificant final mass for PUMA/SiUV were observed. The Fig. 6 shows that the final mass increases with increasing the silica content. These results lead to the conclusion that the final mass is partially due to the existence of ashes mainly composed of silica (inorganic). Although the PUMA network contains Silica, the start of degradation for all samples appeared to begin at about the same temperature is that around 250 °C. Nevertheless, kinetic aspect of the first degradation step depends on the compound of networks. The Fig. 5 shows that the kinetic of

the first step is faster for PUMA than for PUMA/Si which presents two successive kinetics. This difference is due to their own structures: for PUMA the acrylate part comes from hydroxy-2-ethyl methacrylate while for PUMA/Si the acrylate part comes from hydroxy-2-ethyl methacrylate and γ-MPS. The two kinetics of degradation are separated when γ-MPS content increases as shown in Fig. 7. DSC and TMA were used to determine the glass transition temperature domain of these materials. By means of DSC measurement, no significant thermal transition was observed on the temperature domain scanned (from 30 to 200 °C). This disappointing result suggests however a restricted mobility of PUMA networks due to the interpenetration of cross-linked polymer chains and the configuration of HDT. This is not a surprising result. Indeed the observation of the glass transition phenomena by means of DSC require the existence of a large enough value of ΔCp (Cpliquid–Cpglass) at the glass transition to be detected. In practical thermoplastic materials have a ΔCp(Tg) between 0.1 and 0.7 J.g−1.K−1. Generally thermosetting or three dimensional network exhibit values of ΔCp(Tg) lower than 0.1 J.g−1.K−1 over a large domain of temperature which makes the endothermic Tg signal very difficult to detect. Since DSC did not give quantitative results, TMA measurements were further undertaken. Figure 8 shows the results obtain for PUMA/2.8%SiUV in thermo mechanical analysis. We have to point out that the values of thermal parameters depend on the applied static

Fig. 4 Thermogravimetric curves obtained on PUMA (grey line) and PUMA/4,2%Si for different cure (black triangles UV cure, black dots UV+T dual cure). The degradation occurs as a multi step phenomenon. The silica slows the kinetic of decomposition

Fig. 6 Final mass obtained at 700 °C on (triangle) PUMA/SiUV and (square) PUMA/SiUV+T. The final mass increases with the increasing of silica content

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.

. .

. . .

Fig. 7 Derivative thermogravimetric curves during the first step of decomposition obtained on (squares) PUMA/2.8%SiUV, (triangles) PUMA/3.6%SiUV and (dots) PUMA/4.2%SiUV. Appearance of two kinetic with increasing γ-MPS content

load F. We note that the curve shaped with the increase of the static load. Consequently, to have comparative and shaped values, we chose to present results for a static load of 15 N. By TMA, different thermal parameters can be evaluated as temperature of the glass transition (Tg) characterized by the start of the penetration and the magnitude of the penetration (ΔL) measured between the start platform and the end platform. Figure 9 shows the results obtain for PUMA network and PUMA/SiUV. On the one hand we observed that the silica presence has an effect on thermal mechanical properties of PUMA network, on the other hand the increase of silica concentration shapes this phenomena. On the opposite from ΔL, data reported in Table 3 show that Tg decreases as the silica concentration increases [8] which is in accordance with the surface hardness. It follows differences in the rigidity of the materials. This is clearly observed with the mechanical probe acting as an identitor for which, at a given value, the penetration

Fig. 9 Thermomechanical behaviours measured in penetration on PUMA and PUMA/Si for different percentage of Silica (thin grey line PUMA, black PUMA/2.8%SiUV, medium black PUMA/3.6%SiUV, thick grey line PUMA/4.2%SiUV) for a static load of 15 N. Variation of ΔL and Tg with the silica concentration

depth ΔL in the glassy state is always greater for PUMA/Si than for PUMA. By UV light cure the γ-MPS was introduced in PUMA network leads to the increase of free volume. Consequently, when Tg and surface hardness decrease, movements are easier. For the present materials, surface hardness (Table 3) and Tg decrease with increasing silica content as also observed by Bonilla et al. [8]. Despite the thermal cure process under humidity atmosphere, the degradation of PUMA/SiUV and PUMA/SiUV+T start about the same degradation with same kinetic aspect and final mass (Figs. 4 and 5). In the same way, in thermomechanical analysis the penetration depth ΔL in the glassy state and the Tg did not advance (Fig. 10a). Nevertheless for PUMA/4.2%SiUV and PUMA/4.2%SiUV+T (Fig. 10b) we observed two different mechanical probes. After condensation the penetration depth decreases from 158 to 82 μm and the mechanical glass transition temperature increases from 106 to 141 °C. In accordance with this result, the surface hardness for PUMA/4.2%SiUV and PUMA/4.2%SiUV+T increase after condensation (Table 3). This evolution may be explained by their own structures. As shown in Scheme 6 for PUMA/ 2.8–3.6%SiUV+T links between γ-MPS are performed by siloxane while for PUMA/4.2%SiUV+T links between γ-

Table 3 Thermomechanical data measured under a static load of 15 N on PUMA and PUMA/%Si for different cure (UV cure, UV+T dual cure)

Fig. 8 Thermomechanical behaviours measured in penetration for different static load F (light grey 1N, medium grey 5 N, black dots 10 N and black triangles 15 N). Thermal parameters all the more shaped with increasing the load

PUMA PUMA/2.8%SiUV PUMA/3.6%SiUV PUMA/3.6%SiUV+T PUMA/4.2%SiUV PUMA/4.2%SiUV+T

Tg/°C

ΔL/μm

Hardness

102 136 115 112 106 141

46 95 85 84 158 82

74.2 51.5 37.8 37.8 27.6 32.1


a

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MPS are performed by siloxane and γ-MPS themselves. γMPS due their poor quantity in PUMA/2.8–3.6%SiUV+T do not make contact, so Si–O–Si links engaged siloxane (Scheme 6a) contrary due their great quantities in PUMA/ 4.2%SiUV+T Si–O–Si links can be created between γ-MPS directly (Scheme 6b). When Si–O–Si bonds engaged siloxane and γ-MPS, the long chain of siloxane gives some mobility by increasing free volume (decreasing the Tg), contrary to Si–O–Si bonds between γ-MPS blocks the mobility (increasing the Tg of 35 °C).

.

.

b .

. Fig. 10 Thermomechanical behaviours measured in penetration on PUMA/Si networks for different percentage of Silica (PUMA/3.6%Si and PUMA/4.2%Si) and cure (black UV cure, medium black UV+T dual cure). (a) Same probe displacement. (b) Different probe displacement Scheme 6 Structures expected for PUMA/SiUV+T (2.8%Si, 3.6%Si, 4.2%Si). Thin line PUMA network, thick line silicon network, triangle the link agent γ-MPS. (a) PUMA/2.8–3.6% SiUV+T Condensation of γ-MPS by siloxane. (b) PUMA/4.2% SiUV+T Condensation of γ-MPS by siloxane or itself

Conclusion These grafted-IPNs were prepared by two steps: UV light cure and thermal cure under humidity atmosphere. PUMA/ Si hybrid materials were successfully prepared in situ by a cross-linking agent: the γ-MPS. For the present materials, the silica presence increases the thermal stability. Moreover surface hardness and Tg decrease with increasing silica content. The thermal cure process under humidity atmosphere does not influence the kinetic of degradation and the TMA results to a poor quantity in γ-MPS. Nevertheless, for PUMA/4.2%Si we observe that ΔL and Tg advance after

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condensation. This change is characterised by a hardening of silicone network. Consequently, mechanical and thermal properties may be modulated by introduction of silica and their condensation. The add of this inorganic compound via the γ-MPS gives many possibilities for further applications. Acknowledgement The authors wish to thank E. Donzdorf to manufacture the penetration clamp and L. Delbreilh for assistance for penetration measurement.

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