Two Branched Silicone Resins with Different Reactive Groups: A

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Materials and Interfaces

Two branched silicone resins with different reactive groups: A comparative evaluation Xibing Zhan, Huijuan Liu, and Junying Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05172 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Industrial & Engineering Chemistry Research

Two branched silicone resins with different reactive groups: A comparative evaluation Xibing Zhana, Huijuan Liub, Junying Zhangb* a

College of Chemical and Material Engineering, Quzhou University, Zhejiang 324000, China

b

Lab of Adhesives and In-situ Polymerization Technology, Key Laboratory of Carbon Fiber and

Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China

ABSTRACT：：In this study, vinyl-terminated polysiloxane (abbreviated for VISR) and allyl-terminated polysilxoane (abbreviated for ALSR) were synthesized and characterized. The curing behaviors, viscoelastic and thermal properties of VISR/PHSR and ALSR/PHSR were comparatively investigated. DSC and in situ FT-IR spectroscopy can be adopted to study the influence of molecular structure on reactivity. The results prove that ALSR/PHSR shows higher reactivity including higher reaction rate constant and cure degree than VISR/PHSR. Moreover, it’ found that Šesták-Berggren equation can adequately depict cure kinetic model of silicone resin in hydrosilylation comparing the calculated results with experimental data. Additionally, DMA exhibits that the glass transition temperature and crosslinking density of ALSR/PHSR are much lower than those of VISR/PHSR, and TGA data reveal that they have similar thermal stability as well as high char yield, and the decomposition energy range from 100 kJ/mol to 270 kJ/mol with increment of degree of mass conversion (αd) (αd =0.15~0.85). KEYWORDS: branched silicone resin, curing kinetics, hydrosilylation, reactivity

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1. INTRODUCTION Silicone resins are known as one kind of highly branched three-dimensional structure polymers with Si-O-Si backbone chains and some pendent groups (R: e.g. methyl, vinyl, phenyl, hydride, etc.) connected to silicon atoms. At present silicone resins are of major interest to researchers and have been widely utilized as matrix resin of composites,1-3 adhesive and coatings,4-7 reinforcement materials or tackifier,8, 9

and the precursors of ablator10,

11

in the fields of aerospace, nuclear industry,

electronic and electrical industry and other emerging industries (like LED encapsulation materials),12-14 mainly due to their extraordinary properties such as low dielectric property, good resistance to high and low temperature, excellent resistance to short-wave irradiation and thermal aging, etc. 15,16 Now, the silicone materials have become desirable encapsulants and widely used for commercial high-power LED devices. In general, liquid silicone polymers can be solidified by the aid of three different curing mechanisms, namely, free radical coupling reaction, polycondensation and hydrosilylation.17-20 Hydrosilylation is one of the key curing ways for preparing organosilicon materials in both industry and academia because this curing process proceeds at low temperature (60～150oC) compared to the polycondensation type requiring for 200oC and even more, and can avoid the formation of bubbles and voids from volatile liberation that often occur in the conventional condensation process.18 Hydrosilylation crosslinking is an addition reaction between unsaturated bonds (e.g. CH=CH2) and silicon-hydrogen (Si-H) bonds derived from silicone polymers in the presence of platinum complex catalyst (Scheme 1). Scheme 1. Schematic diagram of polysiloxane network formation by hydrosilylation


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To the best of our knowledge, the curing behavior, crosslinking network and final physicochemical properties of silicone materials obtained by hydrosilylation were easily controlled by curing conditions like cure temperature and time, catalyst types and concentration,21, 22 as well as the ratio of Si-H to Si-Vi. 23 However, the special property of organosilicon polymers sometimes can’t be obtained by above methods so that we usually consider tailoring molecular architecture of polysiloxane to adjust their properties. To this date, some investigations were devoted to evaluate the effect of the molecular structure of organosiloxane on the combination properties of 3D network polymers in hydrosilylation and mainly focused on the effect of different structural polysiloxane with Si-H groups on reactivity and performance of silicone bulk.24-29 Nevertheless, Few literature is reported about the influence of different architectural polysiloxane with unsaturated double bond. Lin30 firstly prepared some vinyl silicone crosslinkers with various structures and then evaluated the effect of their molecular structure on the adhesion properties of pressure sensitive adhesive (short for PSA) and curing reactivity in bulk hydrosilylation. The observations were shown that the linear vinyl-terminated polydimethylsiloxane had much higher reactivity, but possessed worse adhesion and mechanical properties than other polyvinylsiloxane crosslinking agents. Bae and co-workers31 chosen two types of polyvinylsiloxane with various molecular structures, namely, linear oligosiloxane with vinyl as side group and branched vinyl oligosiloxane resins, and then made an investigation and comparison about the influence of their structure on reactivity in hydrosilyation and properties of cured silicone bulk. The findings disclosed that linear oligosiloxane was cured at a relatively lower temperature in the presence of a small amount of Pt catalyst and had a comprehensive performance suitable for LED



encapsulant compared to branched vinyl oligosilxoane resin. In brief, the different molecular structures of polysiloxane containing vinyl groups make a great impact on reactivity in hydrosilylation. Generally, the segment structures of R2ViSiO1/2 (abbreviated for Mvi) and RViSiO2/2 (abbreviated for Dvi) in polysiloxane have higher reactivity than ViSiO3/2 (abbreviated for Tvi) so that the polysiloxane with Tvi segment possesses the highest curing temperature (up to 100oC) owing to the inductive and conjugative effects, and steric hindrance effect of substituent group (R) connected to silicon. In order to obtain highly reactive silicone resin and better understand the influence of R group on cure kinetics and performance of cured samples, a novel strategy was utilized merely changing the R substituent group of RSiO3/2 segment and two different kinds of silicone resin containing vinyl groups or allyl groups were designed and synthesized. And then the curing behaviors

including reactivity and curing kinetics, thermal dynamic performance for vinyl silicone resin (short for VISR) and allyl silicone resin (short for ALSR) were made a comparative study. Anyhow, our present investigations can clarify the effect of molecular architecture of polysiloxane on curing temperature and physicochemical properties of cured silicone bulk. Meanwhile, this research may be valuable and meritorious to the further design, development and application of novel silicone resin, in the thoughts and foundation data.

2. EXPERIMENTAL SECTION 2.1 Materials Vinyltriethoxysilane, Allytriethoxysilane, Cyclohexane and Hydrochloric acid (36%–38%) were bought from Aladdin Reagent (Shanghai) Co., Ltd. (China). The Pt complex catalyst (Pt content: ～ 3000ppm) was purchased from AB Specialty silicones NanTong Co., Ltd. The cross-linking agent (hereafter shorted as PHSR) was obtained from Pingdingshan Shengmei technology Co., Ltd. (China). The molecular structure was shown below and the active hydrogen (Si-H) content of PHSR was 0.46 wt%.


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2.2 Instruments and testing methods 1

H NMR and 29Si NMR spectra were recorded on a Bruker AV600MHz nuclear

magnetic resonance spectrometer at room temperature using CDCl3 as solvent. FT-IR spectra were collected on Alpha-T spectrometer (Bruker, Germany), in which the sample was thinly coated on the surface of KBr pellets. The range of the measurement was from 4000 to 400 cm-1 with a resolution of 4 cm-1. The mixture of VISR (or ALSR), cross-linking agents (PHSR) and a certain amount of Pt catalyst (～15ppm) were blended homogeneously, in which the molar ratio of Si-Vi to Si-H was around 1:1.05. The samples from above mixture were added to aluminum pans for dynamic DSC scans on differential scanning calorimeter (Netzsch 204F1, Germany) at the heating rate of 5oC/min, 8oC/min, 11oC/min, 14oC/min in N2 atmosphere from 25oC up to 240oC, respectively. The rest mixture was cast into a preheated Teflon mold, degassed in a vacuum and then put into an oven for cure at 85 oC for 1 h and 150 oC for 2 h. Dynamic

mechanical

analysis

of

the

cured

silicone

specimens

(30mm×6mm×2mm) were carried out on Rheometric Scientific DMTA V analyzer in the temperature from -80 oC to 150 oC at 5 oC/min with a constant frequency of 1Hz in N2 atmosphere. A thermal gravimetric analysis of silicone bulks was performed on TG analyzer (NETZSCH TG209C, Germany) in N2 to evaluate thermal decomposition behavior from 25 oC to 800 oC at a heating rate of 15min/oC, 20min/oC and 30min/oC, respectively. For non-isothermal degradation, the apparent activation energies for VISR/PHSR and ALSR/PHSR systems were calculated by Ozawa-Flynn-Wall (OFW) method.32, 33



lg β = lg

AEd E − 2.315 − 0.4567 d − R ln(1 − α d ) RT

αd =

W0 -Wt W0 -W f

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(1)

(2)

In Eq (1), β, T and Ed represent heating rate, decomposition temperature and apparent activation energy at a given degree of mass conversion (αd), respectively. The activation energy can be derived from the plot of lgβ versus 1/T at a given αd. In Eq (2), W0, Wt and Wf stand for the initial sample weight, sample mass at different time and final mass at the end of degradation, respectively (taken at 800oC).

2.3 Synthesis and characterization of VISR and ALSR resins 1.5wt%

HCl

solution

(5.48g)

was added

dropwise to

mixtures of

allyltriethoxysilane (20.4g) and cyclohexane (25.2g) under vigorous stirring at 65oC for 8 hours. The ethanol and water generated in reaction process were removed by using Dean-Stark separator at elevated temperature until no liquid can be distilled. The resulting solution was washed with deionized water until pH=7 and separated the organic phase. The organic phase was dried with anhydrous MgSO4. The solvent was then quickly evaporated in vacuum and then the colorless transparent viscous liquid can be obtained. (Note: the content of unsaturated double bond of ALSR: 24.2 wt% 34) The synthetic method of VISR resin was similar to that of ALSR resin. (Note: vinyl content of VISR: 24.98 wt% 34)

Scheme 2. Synthetic route to VISR resin and ALSR resin


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FT-IR, 1HNMR and 29SiNMR spectra of VISR and ALSR resins are presented in Figure S1 (See Supporting Information) and the spectra data of VISR and ALSR resins are as follows.

FTIR data ( cm-1): For VISR resin: 3413 (νSi-OH), 1602 (νsC=C), 1409 (δC=C), 3023 (νsC=CH), 3062 (νasC=CH), 1119,1068 (νasSi-O-Si), 2979 (νasCH2CH3), 2890 (νsCH2CH3), 901 (δ=C-H), 763 (νsSi-C) ; For ALSR resin: 3415 (νSi-OH), 1634 (νsC=C), 1419 (δC=C), 3079 (νasC=CH), 1119,1075 (νasSi-O-Si), 2977 (νasCH2CH3), 2891 (νsCH2CH3), 760 (νsSi-C) 1

HNMR data (400MHz, CO(CD3)2, δ, ppm):

For

VISR

resin:

5.90~6.15

(Si-CH=CH2)(integral

area:

4.58),

3.80~3.87

(Si-O-CH2-CH3) (integral area: 0.64),1.18~1.26 (Si-O-CH2-CH3) (integral area: 1);

For

ALSR

resin:

5.7~5.8

(Si-CH2-CH=CH2) (integral

area:

1),

4.7~5.0

(Si-CH2-CH=CH2) (integral area: 2.01), 3.80 (Si-O-CH2-CH3) (integral area: 0.52), 1.65 (Si-CH2-CH=CH2) (integral area:

2.09), 1.24 (Si-O-CH2-CH3) (integral area: 0.83)



29

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SiNMR (600MHz, CDCl3, δ, ppm):

For VISR resin: -77~-81 (CH2=CH)SiO3/2), -69~-75 (-OSi(CH=CH2)(OEt)O-), -66~-69 (Si(CH=CH2)(OEt)2O-); For ALSR resin: -68~-75(CH2=CHCH2)SiO3/2),-59~-67 (-OSi(CH2CH=CH2)(OEt)O-), -56~-59 (Si(CH2CH=CH2)(OEt)2O-) In

NMR

notation,

Tn

denotes

Si

from

(CH2=CH)Si(OEt)3

and

(CH2=CHCH2)Si(OEt)3, where n stands for the number of siloxane bonds attached to Si atom. According to the peak area of

29

Si NMR spectra in Figure S1(c) (See

Supporting Information) and Eq (3), the calculated degree of condensation (DOC) of VISR and ALSR resins were 91.6% and 87.5%, respectively.

DOC =

T1 + 2T2 + 3T3 3(T1 + T2 + T3 )

×100

(3)

Degree of branching (DB) is a very important parameter to evaluate the branching density of branched polymers. The molecular structure of branched silicone polymer are made up of linear (L) (defined as Si (Ⅱ)) and dendritic (D) (defined as Si(Ⅲ)) repeat units (Scheme 2). The calculated DB values of VISR and ALSR are estimated to be 0.84 and 0.76 on the basis of Frey’s equation (Eq 4). 35 The mole fraction of L and D units in Eq 4 derived from the integration values of peak area of 29

Si NMR spectra. It’s found that DB values of VISR and ALSR are slightly higher

than one of the traditional hyperbranched polymer.20, 36, 37 This phenomenon may be resulted from the intramolecular cyclization of silanols in the process of random condensation polymerization so that it seems to increase DB value.

DB=

2D 2D + L

(4)

3. RESULTS AND DISCUSSION 3.1 Curing dynamic models for VISR/PHSR and ALSR/PHSR systems DSC has become an effective and convenient tool to monitor the crosslinking reaction of polymer and can help us to determine the cure kinetics parameter, establish cure kinetics model and understand the cure mechanism. The non-isothermal


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DSC curves of VISR/PHSR and ALSR/PHSR systems were depicted in Figure 1 and it’s clearly shown that the broader exothermic peak shifted to a higher temperature with increment of heating rates from 5 to 14 oC min-1. These phenomena are caused by the thermal lag at different heating rates.

o

o

5 C/min o 8 C/min o 11 C/min o 14 C/min


(b) ALSR/PHSR Heat flow/(mW/mg)

(a) VISR/PHSR Heat flow/(mW/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Exo

Exo

50

75

100

125

150

175

200

60

225

70

o

80

90

100 110 120 130 140 o

Temperature/ C

Temperature/ C

Figure 1. Non-isothermal DSC curves of VISR/PHSR (a) and ALSR/PHSR (b) Nowadays, many kinetic models for thermal analysis have been put forward.38, 39 Among of them, a thermal analysis method for stimulating probable mechanism function proposed by M á lek was very vital for kinetic study.

40

The dynamic

parameters containing activation energy and reaction order were gained and curing dynamic model was deduced on the basis of non-isothermal DSC curves at different heating rates. The

apparent

activation

energy

(Eα)

was

calculated

by

means

of

Flynn-Wall-Ozawa method and the rate equation expression (Eq 5) was as follows.

lnβ = Const −

1.052 Eα RTα

(5)

In Eq 5, β and Tα stand for heating rate and temperature at given heating rate (β) and degree of conversion (α), respectively. The curing degree (α) can be determined by the following equation:

α=

Ht Hu

(0≤ α ≤1)

Where Ht is the reaction heat which is proportional to the area of the exothermic peak of DSC curves within time t. Hu is the total heat of reaction which is directly proportional to the total area of the exothermic peak of DSC curves. Additionally, the



area of exothermic peak of DSC curves was integrated by means of Origin 8.5 software. The relationship between degree of conversion and curing temperature for VISR/PHSR and ALSR/PHSR systems are illustrated in Figure 2. Eα is determined by the slope of lnβ versus 1/T plot at any given curing degree (α). From Figure 3, it can be seen that the average value of Eα for VISR/PHSR and ALSR/PHSR systems is about 86.9 kJ/mol and 94.3 kJ/mol, respectively.

100

100

Degree of conversion (α)/%

(a) VISR/PHSR

Degree of conversion (α)/%

80 60 40 o

5 C/min 8 oC/min 11 oC/min 14 oC/min

20 0 75

100

125

150

175

200

(b) ALSR/PHSR

80 60 40

5 oC/min 8 oC/min 11 oC/min 14 oC/min

20 0

225

60

70

80

90

100

110

120

130

140

o

Temprature/ C

o

Temprature/ C

Figure 2. Degree of conversion (α) versus temperature for VISR/PHSR (a) and ALSR/PHSR (b) systems at different heating rates

140

ALSR/PHSR VISR/PHSR

120 100

-1 Ea /kJ.mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0.0

0.1

0.2

0.3

0.4

0.5 α

0.6

0.7

0.8

0.9

1.0

Figure 3. Variation of Eα versus degree of conversion (α) for VISR/PHSR and ALSR/PHSR Two characteristic functions y (α) (Eq 6) and Z (α) (Eq 7) which can reflect the


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trends of dynamic models and can help us to judge an appropriate kinetic mechanism function during the reaction by means of the shape and maximum value of y (α) and Z (α) functions had been proposed by Málek to establish an appropriate kinetic model. The two equations can be defined as follows:

y (α ) =

E dα u e ；u = a dt RT

(6)

 dα  T Z (α ) = π ( u )    dt  β

(7)

Where β is the heating rate, π (u) is the integration of temperature and can be approximately calculated by Senum-Yang equation Eq 8 41 π (u ) =

(u

(u

4

3

+ 18u 2 + 88u + 96 )

(8)

+ 20u 3 + 120u 2 + 240u + 120 )

The αM and αp∞ represent the curing degree at the maximum value of y (α)

(Figure 4) and Z (α) (Figure 5). Table S1 (See Supporting Information) presents the values of αM and αp∞. 1.2

1.2 αM

1.0

(b) ALSR/PHSR

αM

(a) VISR/PHSR 1.0

o


0.6

Normalized y(α)

Normalized y(α)

o


0.8

0.4

0.0 0.0

0.8 0.6 0.4 0.2

0.2

0.2

0.4

0.6

0.8

1.0

0.0 0.0

0.2

0.4

0.6

α

0.8

1.0

α

Figure. 4 Dependence of y (α) on degree of conversion (α) for VISR/PHSR (a) and ALSR/PHSR (b)

o


(b) ALSR/PHSR

0.8

0.8 0.6 0.4

0.6 0.4

o


0.2

0.2 0.0 0.0

α∞

1.0

(a) VISR/PHSR

α∞

Normalized Z()))) α

Normalized Z(α)

1.0

p

1.2

p

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.4

α

0.6

0.8

1.0

0.0 0.0

0.2


0.4

0.6 α

0.8

1.0


Figure. 5 Dependence of Z (α) on degree of conversion (α) for VISR/PHSR (a) and ALSR/PHSR (b) It’s observed that the Šesták-Berggren (SB) model can describe the curing kinetic behavior of this system according to the Málek’s criteria (0 < αM

Two Branched Silicone Resins with Different Reactive Groups: A

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