ATRP原子转移自由基 氢氧化镁Preparation and characterization of PGMA grafted from Mg(OH)2 via SI-ATRP

Applied Surface Science 258 (2012) 6127–6135

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Applied Surface

Science

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u s

c

Preparation and characterization of poly(glycidyl methacrylate)grafted from magnesium hydroxide particles via SI-ATRP

Jianhui Liu a ,Na Feng a ,?,Suqin Chang b ,Hongliang Kang c

a

School of Textile and Materials Engineering,Dalian Polytechnic University,Dalian 116034,China b

China Leather and Footwear Industry Research Institute,Beijing 100015,China c

State Key Laboratory of Polymer Physics and Chemistry,Joint Laboratory of Polymer Science and Material,Beijing National Laboratory for Molecular Sciences (BNLMS),Institute of Chemistry,Chinese Academy of Sciences,Beijing 100080,China

a r t i c l e

i n f o

Article history:

Received 4September 2011

Received in revised form 30January 2012Accepted 3March 2012

Available online 11 March 2012

Keywords:

Surface modi?cation ATRP

Magnesium hydroxide particles Grafting from Polymers

a b s t r a c t

In order to improve the compatibility of magnesium hydroxide particles [Mg(OH)2]and polymer matrix,poly(glycidyl methacrylate)(PGMA)grafted from magnesium hydroxide particles were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP).In this work,two approaches for the immobilization of ATRP initiator on the magnesium hydroxide particles surface were compared and selected.The density of initiator was signi?cantly increased by the method of introducing more hydroxyl groups via ATRP of 2-hydroxyethyl methacrylate (HEMA)on the surface.The percentage of bromine atom for the initiator-functionalized magnesium hydroxide particles [Mg(OH)2-g-PHEMA–Br]reached to 1.75%,compared to 0.48%for Mg(OH)2–Br determined by XPS analysis.The surface-initiated ATRP of glycidyl methacrylate (GMA)can be conducted in a controlled manner,as revealed by the linear kinetic plot,linear increase of number average molecular weight (M n )with monomer conversions,and the rela-tively narrow molecular weight distributions (M w /M n ～1.4)of PGMA chains.The percentage of grafting PG (%)and the thickness of the grafted polymer layer increased with the increasing of polymerization time and reached to 116.6%and 197.6nm after 300min respectively.As for the polymerization with dif-ferent initial monomer concentration,the number average molecular weights (M n )and weight average molecular weights (M w )of PGMA increased with the increasing of initial monomer concentration.TGA indicated that the initial decomposition temperature of Mg(OH)2-g-PHEMA–PGMA composite particles (253?C)was much lower than that of unmodi?ed magnesium hydroxide particles (337?C).

? 2012 Elsevier B.V. All rights reserved.

1.Introduction

Organic/inorganic composite materials have both the advan-tages of organic polymers and inorganic materials,and have become a new hotspot in materials science [1,2].Inorganic com-pound magnesium hydroxide [Mg(OH)2]as a smokeless and non-toxic additive has been extensively used in halogen-free ?ame-retardant polymeric materials [3–6].However,the disadvan-tage of Mg(OH)2is the high ?lling levels (more than 60%,w/w)required to achieve the desired ?ame retardant effect,which leads the mechanical properties of a ?ame-retardant polymeric material to drop down sharply due to its agglomeration and poor compati-bility with polymer matrix [7,8].In order to overcome this obstacle,the surface treatment of Mg(OH)2is an important way to improve their compatibility.The traditional method of treating the ?ller surface with low-molecular-weight coupling agents or surfactants

?Corresponding author.Tel.:+8641186323683;fax:+8641186323683.E-mail address:fengna12@https://www.360docs.net/doc/8d14275077.html, (N.Feng).

has been found to be reasonably effective [9,10].However,low-molecular-weight compounds have a tendency to migrate out from the interface to the surface of the composites.As a consequence,there is degradation of the mechanical and physical properties of the composites.Also,an undesirable surface appearance occurs because of surface scorching or bulging.

In recent years,grafting polymer brushes on inorganic par-ticles in order to elaborate organic/inorganic composites have lead to an increasing interest [11].Here the grafting of polymer chains onto the particles will serve to improve the compatibility between the particles (hydrophilic)and the polymer matrix (gen-erally hydrophobic).In literature,different strategies for attaching polymer brushes are described.They include the “grafting to”method,in which the end-functionalized polymers react with an appropriate surface [12]and the “grafting from”method,in which polymer chains are grown from the surface-attached initiator [13,14].The “grafting from”technique is a promising candidate to achieve both high stability of polymer layer and high graft density.Surface-initiated graft polymerization has been extensively studied [15–22].In our previous work [23–29],the

0169-4332/$–see front matter ? 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2012.03.017

6128J.Liu et al./Applied Surface Science258 (2012) 6127–6135

surface-initiated in situ polymerization of styrene(St)on the surface of Mg(OH)2was performed,and the effects of polystyrene-encapsulated magnesium hydroxide on varies properties of high impact polystyrene(HIPS)/magnesium hydroxide composites were investigated.However,the effects of molecular weight and molec-ular weight distribution of the grafted PS on?ame retardant properties were not investigated because molecular weight and its distribution of PS could not be controlled over the above method [16,17].

Living/controlled polymerization techniques were successfully applied to surface-initiated graft polymerization to prepare a dense polymer layer with controlled structures[18–22].Compared to conventional polymerizations,living/controlled processes are advantageous as they can provide control on molecular weight, molecular weight distribution,polymer architecture,and end func-tionality.Among the different living/controlled polymerization techniques,surface-initiated atom transfer radical polymerization (SI-ATRP)is one of the most investigated controlled radical poly-merization techniques and allows the synthesis of polymers at mild temperature and in aqueous or organic solvents[30–33].Previ-ous literatures on SI-ATRP mainly focus on nanoparticles(such as SiO2,TiO2,Fe3O4),?bers,peptide etc.[34–38].They have smaller diameter,larger speci?c surface area,so it is ef?cient to immobilize the ATRP initiator on the surface due to they have adequate active groups on the surface.

However,magnesium hydroxide particles used in this study have larger diameter(1.5?m)and smaller speci?c surface area (5.0m2/g),the amount of hydroxyl groups on Mg(OH)2particle sur-face was very limited,so it is dif?cult to introduce enough ATRP initiator on the Mg(OH)2surface.Glycidyl methacrylate(GMA)is a functional monomer to graft as the polymer incorporates a large number of epoxy groups on the surface that are capable to react with a variety of functional groups(hydroxyl,amino,carboxyl etc.)

[39].Further on,the reactivity of GMA in ATRP allows for polymer-ization at moderate temperatures in various solvents as reported in several studies[40–42].

There have been few reports about the surface modi?cation of magnesium hydroxide particles with glycidyl methacrylate(GMA) by surface-initiated ATRP.There are two research purposes of this article:on the one hand,to increase the amount of ATRP initiator on Mg(OH)2surface,as that can contribute to the polymerization of GMA effectively;on the other hand,to prepare the poly(glycidyl methacrylate)grafted from magnesium hydroxide particles.In this work,two approaches for the immobilization of ATRP initiator on the magnesium hydroxide particles surface were compared and selected to produce the poly(glycidyl methacrylate)grafted from Mg(OH)2particles[Mg(OH)2-g-PHEMA–PGMA]via SI-ATRP.The typical samples were characterized by FT-IR,SEM,TGA,XPS and GPC.The molecular weight,molecular weight distribution of the grafted PGMA,the percentage of grafting PG(%),thickness of the grafted polymer layer and the thermal properties of Mg(OH)2-g-PHEMA–PGMA composite particles were studied.

2.Experimental

2.1.Materials

Magnesium hydroxide,Mg(OH)2,was a high-purity untreated grade(Magni?n H5)supplied by Martinswerk GmbH.The average volume particle size was1.5?m and the speci?c sur-face area was5.0m2/g.?-Bromoisobutyryl bromide(BiBB,98%, Aldrich),N,N,N ,N, N -pentamethyldietylenetriamine(PMDETA, 99%,Aldrich),ethyl2-bromoisobutyrate(EBiB,98%,Yancheng Creator chemical Co.,Ltd,China)were used without any fur-ther puri?cation.3-Aminopropyltriethoxy silane(APTES)was purchased from Jingzhou Jianghan Fine Chemical Co.,Ltd.(China). In order to remove copper(II),copper(I)bromide(CuBr)(Tianjin Jinke Fine Chemical Co.,Ltd.,Tianjin,China)was puri?ed via stirring in acetic acid and washing with ethanol and then dried in vacuo. 2-Hydroxyethyl methacrylate(HEMA,98%)was obtained from Tianjin Chemical Reagent Research Institute.Glycidyl methacry-late(GMA,99%)was provided by Nanjing Chemical Co.,Ltd.HEMA and GMA were passed through a column of neutral alumina to remove inhibitor prior to use.Toluene,dichloromethane(DCM), N,N-dimethylformamide(DMF)were distilled on calcium hydride before use.All the other relevant chemicals were of analytical grade and dried by anhydrous magnesium sulfate.

2.2.Characterization

Fourier transform infrared(FT-IR)spectra were recorded on a Tensor27FT-IR spectrometer(Bruker,Germany)by KBr disk in the range4000–400cm?1with a spectral resolution of4cm?1.Thermo-gravimetric analysis(TGA)was performed on a Perkin-Elmer Pyris 1TGA with a heating rate of10?C/min from room temperature to 700?C under a nitrogen atmosphere.Particle morphologies were studied using a mode S-4800scanning electron microscope(SEM) (Hitachi Co.,Ltd.)with an accelerating voltage of5.0kV.All samples were coated with a thin layer of gold prior to analysis.Molecular weights and molecular weight distributions of the free polymers produced in solution were determined by a gel permeation chro-matograph(GPC)equipped with a Waters1515Isocratic HPLC Pump,a Waters2414refractive index detector and a set of Waters Styragel columns(HR3,HR4).THF was used as an eluent at a?ow rate of1.0ml/min and the narrow molecular weight polystyrenes were used as the standard.All the measurements were carried out at35?C.X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer from VG Scienti?c using 300W Al K?radiation.The base pressure was about3×10?9mbar. The binding energies were referenced to the C1s line at284.8eV from adventitious carbon.

The weight loss of the grafted PGMA on Mg(OH)2surface was calculated by TGA using the following Eq.(1),

W Mg(OH)

2

–Br,700?C

100?W Mg(OH)

2

–Br,700?C

=

W Mg(OH)

2-g-PGMA,700

?C?X

100?W Mg(OH)

2-g-PGMA,700

?C

(1)

where W Mg(OH)

2

–Br,700?C,the weight loss at700?C after graft-ing initiator,W Mg(OH)

2-g-PGMA,700

?C,the weight loss at700?C after grafting PGMA and the“X”is the weight loss of grafted PGMA on Mg(OH)2surface.

The conversion of monomer C(%)and the percentage of grafting PG(%)were calculated according to the following relationships:

C(%)=

PGMA grafted(g)

Monomer used(g)

×100%(2) PG(%)=

PGMA grafted(g)

Mg(OH)2charged(g)

×100%(3)

The thickness of the polymer layer(T)on the magnesium hydroxide particles were calculated by Eq.(4),assuming that the PGMA could be coated evenly on the surface of every particle of Mg(OH)2and that the density of the polymer on the Mg(OH)2par-ticles was the same as in the bulk[43].

T=

W

S(nm)(4) where W is the weight loss of grafted PGMA on per gram magne-sium hydroxide particles, is the density of PGMA( =1.18g/cm3), and the S is the speci?c surface area of magnesium hydroxide par-ticles(S=5m2/g).

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Scheme1.Synthetic route of poly(glycidyl methacrylate)(PGMA)grafted from magnesium hydroxide particles via SI-ATRP.

2.3.Synthesis of amino-functionalized Mg(OH)2particles

Typically,5g of magnesium hydroxide particles(dried at 120?C for6h before use),100ml of toluene and20ml of3-aminopropyltriethoxy silane(APTES)were placed in a250ml ?ask equipped with a magnetic stir bar.The?ask was immersed in an oil bath at115?C.The reaction mixture was re?uxed for 12h,and then the solution was cooled to room temperature.The Mg(OH)2particles were isolated by centrifugation and washed with abundant toluene and acetone in turn to remove excess APTES,then dried under vacuum at room temperature for24h. [Mg(OH)2NH2].2.4.Synthesis of ATRP initiator-functionalized Mg(OH)2particles

In this work,two approaches for the immobilization of ATRP initiator on the magnesium hydroxide particles surface were used to produce the ATRP initiator-functionalized Mg(OH)2particles [Mg(OH)2–Br and Mg(OH)2-g-PHEMA–Br]as shown in Scheme1.

2.4.1.Synthesis of Mg(OH)2–Br

The ATRP initiator-functionalized Mg(OH)2particles [Mg(OH)2–Br]were prepared by the reaction of the amide groups of the Mg(OH)2–NH2surfaces with2-bromoisobutyryl bro-mide.Typically,2.0g of Mg(OH)2–NH2,40ml of dichloromethane

6130J.Liu et al./Applied Surface Science258 (2012) 6127–6135

(DCM)and dry pyridine(6.0ml)were added into a dried100ml ?ask,which was then immersed in an ice/water bath.The solu-tion was stirred and a solution of?-bromoisobutyryl bromide (5ml)in dichloromethane(10ml)was added dropwise under nitrogen atmosphere.The reaction mixture was stirred at0?C for1h and then at room temperature for24h,The product was isolated by centrifugation and washed with abundant dichloromethane,acetone,acetone–water solution(1/1,v/v)in turn,?nally the2-bromoisobutyrate-functionalized Mg(OH)2 particles were dried under vacuum at room temperature for24h. [Mg(OH)2–Br].

2.4.2.Synthesis of Mg(OH)2-g-PHEMA–Br

In order to increase the density of ATRP initiator on Mg(OH)2 surface,the ATRP of2-hydroxyethyl methacrylate(HEMA)from Mg(OH)2–Br surface was carried out to increase the amount of hydroxyl groups on the surface.The procedure was as follows,1.5g of2-bromoisobutyrate-functionalized magnesium hydroxide par-ticles Mg(OH)2–Br,methanol(10ml),HEMA(5.0ml,41mmol)and a magnetic stir bar were placed in a dried?ask which was?tted with a rubber septum.After bubbling with a nitrogen stream for30min, CuBr(57.4mg,0.4mmol)was added under protection of N2?ow, the?ask was then degassed with three evacuate-?lling nitrogen cycles under an ice/water bath.PMDETA(83.0?L,0.4mmol)was deoxygenated with bubbling dry nitrogen before injection into the reaction system via a microsyringe.Then,the?ask was immersed into an oil bath at50?C for5h.The mixture was diluted with ethanol at a solution/ethanol volume ratio of1:5.The?nal product was isolated by centrifugation and washed with methanol and acetone in turn,then dried under vacuum at room temperature for24h. [Mg(OH)2-g-PHEMA]

The subsequent esteri?cation reaction of pendent hydroxyl groups of PHEMA segment with2-bromoisobutyryl bromide was carried out as same as the synthesis of Mg(OH)2–Br,which grafting one bromine moiety on each monomer repeating unit within the PHEMA segment.[Mg(OH)2-g-PHEMA–Br].

2.5.Synthesis of Mg(OH)2-g-PHEMA–PGMA via SI-ATRP

A typical procedure for surface-initiated ATRP of GMA was described below.The ATRP initiator-functionalized Mg(OH)2par-ticles[Mg(OH)2–Br or Mg(OH)2-g-PHEMA–Br](0.3g)were added into a?ask that contained DMF(5.0ml),GMA(5.0ml,35mmol), the solution was degassed with nitrogen for30min,CuBr(43.0mg, 0.3mmol)and free initiator ethyl2-bromoisobutyrate(8.8mg, 0.06mmol)was added under protection of N2?ow,the?ask was then degassed with three evacuate-?lling nitrogen cycles under an ice/water bath.PMDETA(63.0?L,0.3mmol)was deoxygenated with bubbling dry nitrogen before injection into the reaction sys-tem via a microsyringe.Then,the?ask was immersed into an oil bath at30?C.After the polymerization proceeded for a period time, the?ask was removed from the oil bath and opened to air.The mixture was diluted with dichloromethane DCM(15ml).The par-ticles were isolated by centrifugation,and the supernatant was passed through a short column of activated neutral aluminum oxide three times.The particles were dispersed in DCM and separated by centrifugation.This process was repeated three times,and then the particles was dried in a vacuum at30?C for24h for thermo-gravimetric analysis and X-ray photoelectron spectroscopy(XPS) analyses.The free polymer formed in the solution was precipitated in hexanes and analyzed by GPC relative to polystyrene standards. [Mg(OH)2-g-PGMA,Mg(OH)2

-g-PHEMA–PGMA].Fig.1.FT-IR spectra of(a)bare Mg(OH)2,(b)Mg(OH)2–NH2,(c)Mg(OH)2–Br,(d) Mg(OH)2-g-PHEMA–Br and(e)Mg(OH)2-g-PHEMA–PGMA.

3.Results and discussion

3.1.Characterization of Mg(OH)2–Br macroinitiator

3.1.1.Analysis of FT-IR

Fig.1shows the FT-IR spectra of(a)bare Mg(OH)2particles(b) Mg(OH)2–NH2,(c)Mg(OH)2–Br,(d)Mg(OH)2-g-PHEMA–Br and(e) Mg(OH)2-g-PHEMA–PGMA.In Fig.1a,the characteristic adsorption peak at3700cm?1was associated with O H stretching vibrations of hydroxyl group on the surface of Mg(OH)2particles,the band at 1632cm?1was due to the residual water on the surface of Mg(OH)2 particles.In Fig.1b,for Mg(OH)2NH2,the characteristic absorption peaks at2930cm?1and2850cm?1correspond to C H stretching vibrations of alkyl occurs,the peaks at1121cm?1and1044cm?1 were the absorption bands of Si O stretching vibration due to the presence of an APTES layer on the surface.The content of APTES on the surface of Mg(OH)2particles calculated in accordance to TGA(Fig.2b)was2.43wt%.In Fig.1c,for Mg(OH)2–Br,the char-acteristic absorption peaks at1690cm?1corresponds to the C O stretching vibrations of amide group on Mg(OH)2–Br.Although a large excess of2-bromoisobutyryl bromide was used,however,the peak intensity of amide group is very little,suggesting that the density of ATRP initiator on the surface was limited.According

to Fig.2.TGA curves of(a)bare Mg(OH)2,(b)Mg(OH)2–NH2,(c)Mg(OH)2–Br and (d)Mg(OH)2-g-PHEMA–Br.TGA was performed in nitrogen at a heating rate of 10?C/min.

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Fig.3.XPS spectra of(a)Mg(OH)2–Br and(b)Mg(OH)2-g-PHEMA–Br. TGA(Fig.2c),the grafting percentage of2-bromoisobutyryl bro-mide was only2.32wt%.While in Fig.1d,a new band at1730cm?1 appears,which is a characteristic peak of the C O stretching vibra-tions of PHEMA and the esteri?cation reaction of pendent hydroxyl groups of PHEMA segment with2-bromoisobutyryl bromide,this demonstrated that PHEMA has grafted from the surface of Mg(OH)2 particles and the initiator was immobilized on Mg(OH)2surface. The weight fraction of organic moieties of Mg(OH)2-g-PHEMA–Br was10.44wt%calculated by TGA(Fig.2d).

3.1.2.Analysis of XPS

To further investigate the ATRP initiator content of Mg(OH)2–Br and Mg(OH)2-g-PHEMA–Br,XPS measurement was used.The X-ray photoelectron survey spectra of Mg(OH)2–Br(Fig.3a)and Mg(OH)2-g-PHEMA–Br(Fig.3b)show peaks corresponding to magnesium(49.8eV,Mg(2p);89.0eV,Mg(2s)),oxygen(531.6eV, O(1s)),nitrogen(399.3eV,N(1s)),carbon(284.8eV,C(1s))and bromine(69.9eV,Br(3d);189.3eV,Br(3p))atoms in agreement with the expectations.The bromine signal Br(3d)is notice-ably enhanced compared to Mg(OH)2–Br(I),which indicated that more initiators have been immobilized on the surface of mag-nesium hydroxide particles.Table1summarizes the surface elemental compositions(in atom%)of Mg(OH)2–Br and Mg(OH)2-g-PHEMA–Br determined by XPS analysis.The immobilization of ATRP initiator on Mg(OH)2surface for the second time leaded to Table1

Elemental compositions(in atom%)on the surface of Mg(OH)2–Br and Mg(OH)2-g-PHEMA–Br determined by XPS analysis.

Sample C N O Br Mg

Mg(OH)2–Br22.9 2.2149.90.4824.51 Mg(OH)2-g-PHEMA–Br54.350.6434.7 1.758.57

Table2

Reaction conditions and results for ATRP of HEMA on the surface of Mg(OH)2 particles.

Sample R wt a R mol b Temp./?C Time/h PHEMA wt%c 11:150:1:1505 4.57

22:1100:1:15058.44

34:1200:1:150512.60

a R wt=HEMA:Mg(OH)2–Br(wt:wt).

b R

mol

=HEMA:CuBr:PMDETA(mol:mol:mol).

c PHEMA wt%=PHEMA content of the Mg(OH)2–PHEMA particles calculate

d from TGA data.

a signi?cant increase in the carbon and bromine content as well as concomitant decrease in the magnesium content.The percent-age of bromine atom for Mg(OH)2-g-PHEMA–Br particles reaches to1.75%compared to0.48%for Mg(OH)2–Br.

3.1.3.Analysis of TGA

The polymerization of PHEMA was performed on the surface of Mg(OH)2particles by ATRP in methanol at50?C.The weight feed ratio of HEMA monomer to Mg(OH)2–Br macroinitiator ranged from1:1to4:1.According to the TGA data,the grafting degree of PHEMA increased from4.57%to12.60wt%as the weight feed ratio (R wt)rose from1:1to4:1,as shown in Table2.

The preparation of poly(glycidyl methacrylate)grafted from magnesium hydroxide particles were carried out with the Mg(OH)2–Br and Mg(OH)2-g-PHEMA–Br as ATRP macroinitiator under the same condition respectively.The TGA curves of(a)bare Mg(OH)2,(b)Mg(OH)2-g-PGMA,(c)Mg(OH)2-g-PHEMA–PGMA particles are shown in Fig.4.For the bare Mg(OH)2,the weight loss between30?C and200?C was0.31wt%,which is due to the desorp-tion of physically adsorbed water.The weight loss of bare Mg(OH)2 particles was29.52wt%,which is due to the loss of the endother-mic decomposition of the crystal water from the Mg(OH)2particles. The weight loss of the grafted PGMA for both Mg(OH)2-g-PGMA and Mg(OH)2-g-PHEMA–PGMA were calculated by Eq.(1)and the obtained values were6.05wt%and52.25wt%respectively.

The

Fig.4.TGA curves of(a)bare Mg(OH)2particles,(b)Mg(OH)2-g-PGMA,(c)Mg(OH)2-g-PHEMA–PGMA.TGA was performed in nitrogen at a heating rate of10?C/min.

6132J.Liu et al./Applied Surface Science 258 (2012) 6127–

6135

Fig.5.Schematic illustration of Mg(OH)2-g-PHEMA–PGMA particle with branched brush polymeric shell.

percentage of grafting PG (%)for Mg(OH)2-g-PHEMA–PGMA par-ticles was much higher than that for Mg(OH)2-g-PGMA particles.Judging by the combination of FT-IR,XPS and TGA results,it was concluded that the density of initiator on the surface of Mg(OH)2particles is obviously increased by the method outlined in Scheme 1.Therefore,the Mg(OH)2-g-PHEMA–Br macroinitiators were selected to use in the following experiments.

3.2.Surface-initiated ATRP of GMA from initiator-functionalized Mg(OH)2particles

Surface-initiated ATRP of GMA from the surface of initiator-functionalized magnesium hydroxide particles was conducted in DMF at room temperature,using CuBr/PMDETA as the catalytic system.The schematic illustration of Mg(OH)2-g-PHEMA–PGMA particle is shown in Fig.5.

3.2.1.Analysis of FT-IR

As shown in Fig.1e,FT-IR spectra of Mg(OH)2-g-PHEMA–PGMA particles shows new epoxy peaks at 908cm ?1,847cm ?1(corre-sponding to the asymmetric vibrations of the epoxy rings)and peak at 1260cm ?1(corresponding to the symmetric

vibrations

Fig.6.SEM images of (a)bare Mg(OH)2,(b)Mg(OH)2-g-PHEMA–Br and (c)Mg(OH)2-g-PHEMA–PGMA.

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Fig.7.TGA and DTG curves of(a)bare Mg(OH)2and(b)Mg(OH)2-g-PHEMA–PGMA. of the epoxy rings),which indicated that PGMA has grafted from the surface of Mg(OH)2particles.Moreover,the peak intensity at 1730cm?1attributable to the carbonyl stretching vibration of the ester group of PGMA was signi?cantly enhanced,at the same time, the absorption peak at3700cm?1associated with O H stretching vibrations of hydroxyl group on the surface of Mg(OH)2particles was weakened.These results demonstrated that the Mg(OH)2par-ticles were encapsulated by PGMA layer.

3.2.2.Analysis of SEM

The surface morphologies of(a)bare Mg(OH)2,(b)Mg(OH)2-g-PHEMA–Br and(c)Mg(OH)2-g-PHEMA–PGMA particles were studied using scanning electron microscope(SEM)as shown in Fig.6.The hexagonal platy morphology of bare Mg(OH)2parti-cles can be seen in Fig.6a,which has?at surface with clear edge. Fig.6b shows the SEM images of Mg(OH)2-g-PHEMA–Br macroini-tiator,compared to bare Mg(OH)2,it is clearly evident that the regular platy shapes of bare Mg(OH)2particles have changed to more round,which indicated that the Mg(OH)2particles were sur-rounded by a organic polymer layer.From Fig.6c,the surface of Mg(OH)2-g-PHEMA–PGMA particles seems rough and entirely cov-ered or buried by a polymer layer as well as a lot of PGMA?akes were observed on the Mg(OH)2surface.

3.2.3.Analysis of TGA

The thermal properties of bare Mg(OH)2particles and Mg(OH)2-g-PHEMA–PGMA composite particles were investigated by TGA (Fig.7).For bare Mg(OH)2,the weight loss during the heating pro-cess can be divided into three stages.The?rst weight loss from room temperature to337?C is due to the adsorption of physi-cally adsorbed water.The second weight loss in the temperature ranging337–426?C is attributed to the decomposition of mag-nesium hydroxide.The decomposition of magnesium hydroxide can release water to decrease temperature and dilute oxygen and ?ammable gases concentrated near the?ame.The?nal weight loss may be caused by the decomposition of some carbonate species added in the manufacturing process.For Mg(OH)2-g-PHEMA–PGMA composite particles,the reason for the?rst and?nal weight loss stages is identical to that of magnesium hydroxide, the weight loss from253to354?C and354to466?C are due to the decomposition of poly(glycidyl methacrylate)and magnesium hydroxide respectively.More importantly,it can be found that the initial decomposition temperature of Mg(OH)2

-g-PHEMA–PGMA Fig.8.TGA curves of(a)bare Mg(OH)2,(b)Mg(OH)2-g-PHEMA–Br,(c–g)Mg(OH)2-g-PHEMA–PGMA for polymerization time of30,60,120,180,300min. composite particles(253?C)is much lower than that of untreated magnesium hydroxide particles(337?C).

Fig.8shows the TGA curves of PGMA grafted from Mg(OH)2par-ticles surface for various polymerization time.The conversion of monomer C(%),percentage of grafting PG(%)TGA and the thickness of the polymer layer(T)on the magnesium hydroxide particles were calculated from TGA data as shown in Figs.9and10.In the process of experiments,the weight of the particles before and after grafting PGMA were weighed respectively,the PG(%)weight of grafted PGMA were calculated from these data and are also shown in Fig.8.As can be seen,the C(%)and PG(%)TGA increase linearly with increasing polymerization time and reaches to7.0%and116.6%,respectively, after a polymerization time of300min.The thickness of the poly-mer layer(T)also increases with polymerization time and reaches to197.6nm after300min.These results exhibited the characteris-tics of a controlled polymerization.What is more,it was found that the PG(%)weight results show the same trend as PG(%)TGA,which indicated that the results were reliable.However,the values of PG (%)weight are less than PG(%)TGA,which could be due to the weight loss in the operating process of

experiment.

Fig.9.The effect of polymerization time on the C(%)and PG(%).PG(%)TGA:the percentage of grafted PGMA calculated from TGA data;PG(%)weight:the percentage of grafted PGMA calculated by weighting particles before and after grafting PGMA.

6134J.Liu et al./Applied Surface Science 258 (2012) 6127–

6135

Fig.10.The thickness of PGMA layer on the Mg(OH)2surface versus polymerization time.

3.2.

4.Analysis of GPC

In this study,the molecular weights and molecular weight distri-butions of the free polymers produced in solution were determined by GPC.It was believed that these values should be close to those of the grafted polymer brushes [44,45].From Fig.11,the kinetic plot of the polymerization of GMA shows a ?rst-order reaction.Fig.12presents the molecular weights and the PDI values versus monomer conversion plots.It could be observed that the num-ber average molecular weights (M n )of the obtained polymers increase linearly with monomer conversion,the polydispersity remained fairly low (～1.4),indicating that the grafting polymer-ization should be “living”/controlled.The molecular weights (M n )are well above the predicted theoretical values (M n ,theoretical ).This might be due to poor initiator ef?ciency reducing the number of growing chains [46,47].Furthermore,three kinds of different initial monomer concentration were used in the experiments to investigate the effect of the initial monomer concentration on the molecular weight and molecular weight distribution,and the results are summarized in Table 3.The GPC results in Table 3shows that the number average molecular weights (M n )and weight average molecular weights (M w )increase with the

increasing

Fig.11.Kinetic plot for the ATRP of GMA in DMF at 30?

C.

Fig.12.Molecular weight (M n )and polydispersity index (PDI)of PGMA as a function of monomer conversion.GMA/CuBr/PMDETA/EBiB =100/1/1/0.2.

Table 3

GPC results of the polymerization of GMA.

Sample

R m a

M n (g/mol)

M w (g/mol)

M w /M n 1100:1:1:0.2620310,110 1.632200:1:1:0.2711012,459 1.753

400:1:1:0.2

14,098

25,940

1.84

a

R m :the mole ratio of GMA/CuBr/PMDETA/EBiB;(in DMF,at 30?C for 120min).

of the initial monomer concentration,particularly,the number average molecular weights (M n )reaches to 14,098g/mol when GMA/CuBr/PMDETA/EBiB =400/1/1/0.2.However,the PDI of PGMA also increase with the increasing of the initial monomer concen-tration,this can be explained as follow,in ATRP,molecular weights increase linearly with the conversion of monomer,simultaneously,polydispersities (M w /M n )decrease with the conversion,depend-ing on the relative rate of deactivation [48].This means that at the beginning of the process,the polydispersities (M w /M n )is larger,while the monomer conversion is low in our experiments.Thus the polydispersities (M w /M n )remained high after 120min.

4.Conclusions

Controlled grafting of well-de?ned GMA polymer (PGMA)brushes could be carried out by surface-initiated ATRP of GMA on the magnesium hydroxide particles surface at ambient tem-perature.The density of initiator on the surface is signi?cantly increased,the percentage of bromine atom for Mg(OH)2-g-PHEMA–Br reached to 1.75%,compared to 0.48%for Mg(OH)2–Br.The polymerization was conducted in a controlled manner,as revealed by the linear kinetic plot,linear increase of molecular weights (M n )with monomer conversions,and narrow molecular weight distributions of PGMA chains.The percentage of graft-ing PG (%)increased with the increasing of the polymerization time and reached to 116.6%after 300min.The thickness of the grafted polymer layer increased to 197.6nm after 300min.As for the polymerization with different initial monomer concen-tration,the number average molecular weights (M n )and weight average molecular weights (M w )increased with the increasing of initial monomer concentration.TGA indicated that the initial decomposition temperature of Mg(OH)2-g-PHEMA–PGMA com-posite particles (253?C)is much lower than that of unmodi?ed magnesium hydroxide (337?C),which is very helpful to the improvement of ?ame-retardant effectiveness.

J.Liu et al./Applied Surface Science258 (2012) 6127–61356135

Acknowledgements

The authors gratefully acknowledge the?nancial support of the National Natural Science Foundation of China(no.21006125). References

[1]S.Lee,V.H.Perez-Luna,Langmuir23(2007)5097.

[2]Y.M.Mohan,K.Lee,T.Premkumar,Polymer48(2007)158.

[3]U.Hippi,J.Mattila,M.Korhonen,J.Seppala,Polymer44(2003)1193.

[4]M.Sain,S.H.Park,F.Suhara,https://www.360docs.net/doc/8d14275077.html,w,Polym.Degrad.Stabil.83(2004)363.

[5]A.B Shehata,Polym.Degrad.Stabil.85(2004)577.

[6]L.Clerc,L.Ferry,E.Leroy,J.Mari,J.M.Lopez-Cuesta,Polym.Degrad.Stabil.88

(2005)504.

[7]Z.Z.Wang,B.J.Qu,W.C.Fan,P.Huang,J.Appl.Polym.Sci.81(2001)206.

[8]J.Yin,Y.Zhang,Y.X.Zhang,J.Appl.Polym.Sci.97(2005)1922.

[9]Y.C.Qu,Z.Z.Yu,Polym.Int.37(1995)113.

[10]D.Sekutowaski,Plast.Eng.Aug.(1987)43.

[11]J.Pyun,K.Matyjaszewski,Chem.Mater.13(2001)3436.

[12]P.Mansky,Y.Liu,E.Huang,T.P.Russell,C.Hawker,Science275(1997)1458.

[13]B.Zhao,W.J.Brittain,Prog.Polym.Sci.25(2000)677.

[14]O.Prucker,J.Ruhe,Macromolecules31(1998)592.

[15]K.Ohno,K-m.Koh,Y.Tsujii,T.Fukuda,Macromolecules35(2002)8989.

[16]R.Schmidt,T.Zhao,J.-B.Green,D.J.Dyer,Langmuir18(2002)1281.

[17]J.Hyun,A.Chilkoti,Macromolecules34(2001)5644.

[18]R.Jordan,N.West,A.Ulman,Y.-M.Chou,O.Nuyken,Macromolecules34(2001)

1606.

[19]Q.Zhou,S.Wang,X.Fan,R.Advincula,J.Mays,Langmuir18(2002)3324.

[20]T.K.Mandal,M.S.Fleming,D.R.Walt,Nano Lett.2(2002)3.

[21]H.Kawaguchi,Y.Isono,S.Tsuji,Macromol.Symp.179(2002)75.[22]R.Matsuno,K.Yamamoto,H.Otsuka,A.Takahara,Chem.Mater.15(2003)3.

[23]S.Q.Chang,T.X.Xie,G.S.Yang,J.Polym.Sci.Polym.Phys.45

(2007)2023.

[24]S.Q.Chang,T.X.Xie,G.S.Yang,J.Appl.Polym.Sci.110(2008)2139.

[25]S.Q.Chang,T.X.Xie,G.S.Yang,Polym.Degrad.Stabil.91(2006)3266.

[26]S.Q.Chang,T.X.Xie,G.S.Yang,Polym.Int.56(2007)1135.

[27]S.Q.Chang,T.X.Xie,G.S.Yang,J.Appl.Polym.Sci.110(2008)578.

[28]S.Q.Chang,T.X.Xie,G.S.Yang,J.Appl.Polym.Sci.108(2008)2488.

[29]S.Q.Chang,T.X.Xie,G.S.Yang,J.Appl.Polym.Sci.102(2006)5184.

[30]X.Xu,A.M.Lenhoff,J.Phys.Chem.B112(2008)1028.

[31]L.Sun,J.Dai,G.L.Baker,M.L.Bruening,Chem.Mater.18(2006)4033.

[32]F.J.Xu,Q.J.Cai,Y.L.Li,E.T.Kang,K.G.Neoh,Biomacromolecules6(2005)1012.

[33]F.J.Xu,J.P.Zhao,E.T.Kang,K.G.Neoh,J.Li,Langmuir23(2007)8585.

[34]J.C.Chen,M.Hu,W.D.Zhu,Y.P.Li,Appl.Surf.Sci.257(2011)6654.

[35]G.K.Raghuraman,J.Ruhe,R.Dhamodharan,J.Nanopart.Res.10(2008)415.

[36]W.Z.Yuan,J.Y.Yuan,L.L.Zhou,S.Z.Wu,X.Y.Hong,Polymer51(2010)2540.

[37]A.Carlmark,E.E.Malmstrom,Biomacromolecules4(2003)1740.

[38]U.Ojha,D.S.Feng,A.Chandekar,J.E.Whitten,R.Faust,Langmuir25(2009)

6319.

[39]Y.Luo,M.Z.Rong,M.Q.Zhang,https://www.360docs.net/doc/8d14275077.html,pos.13(2005)245.

[40]S.Edmondson,W.T.S.Huck,J.Mater.Chem.14(2004)730.

[41]W.H.Yu,E.T.Kang,K.G.Neoh,Langmuir20(2004)8294.

[42]R.Krishnan,K.S.V.Srinivasan,Macromolecules36(2003)1769.

[43]I.Luzinov,A.Voronov,S.Minko,R.Kraus,W.Wilke,A.Zhuk,J.Appl.Polym.Sci.

61(1996)1101.

[44]https://www.360docs.net/doc/8d14275077.html,ruelle,J.Parvole,J.Francois,L.Billon,Polymer45(2004)5013.

[45]E.Marutani,S.Yamamoto,T.Ninjbadgar,Y.Tsujii,T.Fukuda,M.Takano,Poly-

mer45(2004)2231.

[46]K.Min,H.Gao,K.Matyjaszewski,Macromolecules40(2007)1789.

[47]K.Matyjaszewski,H.Dong,W.Jakubowski,J.Pietrasik,A.Kusumo,Langmuir

23(2007)4528.

[48]K.Matyjaszewski,J.H.Xia,Chem.Rev.101(2001)2921.