milleniumPreparation of High Molecular Weight Polybenzoxazine Prepolymers Containing Siloxane Unites and Properties of Their Thermots
TSUTOMU TAKEICHI,1TAKUYA KANO,1TAREK AGAG,1TAKEHIRO KAWAUCHI,1NOBUYUKI FURUKAWA2
1Department of Environmental and Life Sciences,Toyohashi University of Technology,Tempaku-cho,
Toyohashi,Aichi441-8580,Japan
2Chemical and Biological Engineering,Sabo National College of Technology,Okishin-cho,Sabo,
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Nagasaki857-1193,Japan
Received1July2010;accepted21September2010
DOI:10.1002/pola.24408
Published online8November2010in Wiley Online ).
ABSTRACT:High-molecular-weight polybenzoxazine prepolymers containing polydimethylsiloane unit i
n the main-chain have been synthesized from a,x-bis(aminopropyl)polydimethylsilox-ane(PDMS)(molecular weight¼248,850,and1622)and bisphenol-A with formaldehyde.Moreover,another type of pre-polymers was prepared using methylenedianiline(MDA)as codi-amine with PDMS.The weight average molecular weight of the obtained prepolymers was estimated from size exclusion chro-matography to be in the range of8000–11,000.The chemical structures of the prepolymers were investigated by1H NMR and IR analys.The prepolymers gave transparent free standing films by casting their dioxane solution.The prepolymer films af-ter thermally cured up to240 C gave brown colored transparent and flexible polybenzoxazine films.Tensile test of the films revealed that the elongation at break incread with increasing the molecular weight of PDMS unit.Dynamic mechanical analy-sis of the thermots showed that the T g s were as high as238–270 C.The thermots also revealed high thermal stability as evidenced by the5%weight loss temperatures in the range of 324–384 C from thermogravimetic analysis.V C2010Wiley Peri-odicals,Inc.J Polym Sci Part A:Polym Chem48:5945–5952,2010
KEYWORDS:high performance polymers;polysiloxanes;resins; ring-opening polymerization;thermots
INTRODUCTION The traditional phenolic resins posss excellent characteristics,such as good heat
and chemical re-sistance,flame retardancy,and low cost due to the inexpen-sive raw materials and fabricating process.Nevertheless, the traditional phenolic resins have some shortcomings as their brittleness,u of harsh catalyst for polymerization that corrode the equipments,prence of voids due to vola-tile byproducts,and large volumetric shrinkage upon cure. The u of aqueous phenol also results in pollution of waste water which caus environmental and health threats.Dur-ing the decade,benzoxazine-derived phenolic resins,namely polybenzoxazine,have been expected as a novel type of phe-nolic resins.1–3Chemical structures of a typical benzoxazine monomer,6,60-(1-methylethylidene)bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)(B-a),and its thermot(PB-a)are shown in Scheme 1.The polymerization process proceeds through ring-opening of benzoxazine by heat treatment with-out using harsh catalysts and without generating byproducts. Polybenzoxazine resin provides not only the advantageous properties of traditional phenolic resins but also other char-acteristics that are not prent in the traditional phenolic resins,such as excellent dimensional stability,low water absorption,and good dielectric properties.2–6Therefore,the application of polybenzoxazine as electronic materials,matrix for FRP and high performance adhesives is expected.However,the shortcoming of polybenzoxazines is lack of flexibility as is common for thermots,which limits their application.So far,some efforts that can be classified into two approaches have been done to improve the toughness of polybenzoxazines.First approach is alloyin
g with flexible polymers,such as liquid rubber,7,8polyurethane,9,10poly(e-caprolactone),11polyimide,12poly(imide-siloxane)s,13and polydimethylsiloxane(PDMS).14–16For instance,PB-a/PDMS alloy with13wt%PDMS content was achieved by combin-ing two types of reactions in situ;the ring-opening polymer-ization of B-a and the sol–gel process of diethoxydimethyl-silane.14The tensile strength and elongation at break of the obtained PB-a/PDMS alloy films were higher than the pris-tine PB-a film becau of the toughening effect of PDMS. However,at about25wt%PDMS content,macroscopic pha paration occurred and the film became brittle. Second approach is molecular design of the benzoxazine monomer,such as linear aliphatic diamine-bad benzoxa-zine17and high-molecular-weight polybenzoxazine prepoly-mers.18,19Recently,we succeeded in the preparation of high-molecular-weight polybenzoxazine prepolymers from aro-matic or aliphatic diamine and bisphenol-A with paraformal-dehyde.18The toughness of the crosslinked polybenzoxazine films from the high-molecular-weight prepolymers was greatly enhanced compared with the PB-a film.More
Correspondence to:T.Takeichi(E-mail:takeichi@ens.tut.ac.jp)
Journal of Polymer Science:Part A:Polymer Chemistry,Vol.48,5945–5952(2010)V C2010Wiley Periodicals,Inc.
recently,Yagci and coworkers reported preparation of a novel high-molecular-weight prepolymer having disiloxane unit in the main-chain from a benzoxazine monomer having allyl group and1,1,3,3,-tetramethyldisiloxane by Pt-catalyzed hydrosilylation reaction.20Although the film cured up to 180 C was flexible,the flexibility was drastically reduced by further curing up to240 C that was needed for the comple-tion of the polymerization of benzoxazine.
We report here the preparation of a novel type of high-molecular-weight polybenzoxazine prepolymers having poly-siloxane unit through step-wi benzoxazine ring formation by the reaction of bisphenol-A and a,x-bis(aminopropyl)poly-dimethylsiloxane(PDMS)(Scheme2).Influences of molecu-lar-weight and content of PDMS on thermal and mechanical properties of the thermots were investigated.Furthermore, 4,40-methylenedianiline(MDA)was ud as a codiamine for high-molecular-weight prepolymers.
EXPERIMENTAL
Materials
Chloroform,1,4-dioxane and bisphenol-A were ud as received from Kishida Chemicals,Japan.Paraformaldehyde and MDA were ud as received from Merck and Aldrich, respe
ctively.a,x-Bis(aminopropyl)polydimethylsiloxane(PDMS) (molecular weight¼248,850,and1622)was kindly donated from Shinestu Chemicals,Japan.B-a was kindly supplied by Shikoku Chemicals,Japan,and ud after washing with1N aqueous sodium hydroxide solution to remove phenolic compounds.
Measurements
IR spectra were obtained with JASCO spectrophotometer model FT/IR-420.1H NMR spectra were recorded on a Var-ian Mercury300instrument(300MHz for1H).Viscosity of the prepolymer solution was measured using Ubbelohde vis-cometer at a concentration of0.5g/dL in1,4-dioxane at30 C.Size exclusion chromatography(SEC)was measured using tetrahydrofuran as the eluent.The molecular weight was calibrated with polystyrene standards.Differential scan-ning calorimetry(DSC)was conducted using Rigaku Thermo Plus2DSC8230at a heating rate of5 C/min under nitro-gen.Thermogravimetric analysis(TGA)was measured with Rigaku Thermo Plus2TG-DTA TG8120at a heating rate of 5 C/min under argon.Dynamic viscoelastic measurements were conducted on ORIENTEC automatic dynamic viscoelato-mer Rheovibron model DDV-01FP at35Hz at a heating rate of4 C/min.Tensile properties were recorded with Imada Seisaku-sho Model SV-3at a crosshead speed of0.5mm/min using film of25mm long.Tensile properties of each sample were determined from
an average of at least5tests. Synthesis of Polybenzoxazine Prepolymers Polybenzoxazine prepolymers were prepared by reacting bisphenol-A,PDMS(molecular weight¼248,850,and 1622)and paraformaldehyde(Table1).The sample codes are abbreviated in Table1using the molecular weight of PDMS such as B-PDMS(248).A typical experimental proce-dure for polybenzoxazine prepolymers containing PDMS with the PDMS molecular weight of248(B-PDMS(248))is described below:Into300-mL round flask,100mL of chloro-form,PDMS(248)(20.0mmol, 4.97g),bisphenol-A(20.0 mmol, 4.57g),and paraformaldehyde(85mmol, 2.55g) were added and mixed at room temperature for about30 min.The suspension was gradually heated and kept refluxing for5h.After the reaction time,the reaction mixture was cooled to room temperature to afford a yellow transparent solution.The solution was washed with1L of1N aqueous NaHCO3and finally two times with1L of distilled water. The chloroform solution was dried with anhydrous sodium sulfate,followed by evaporation of chloroform under vacuum to afford product(6.1g,75%).In the same way,B-PDMS(850),B-PDMS(1622),B-MDA/PDMS(850),and B-MDA/PDMS(1622)were prepared in moderate yields ($80%).
Preparation and Curing of Prepolymer Films
The prepolymers were dissolved in dioxane to prepare30 wt%solutions,and the solution was cast on
a glass plate. The solvent was removed by drying at50 C for24h to give yellow transparent prepolymer films.The prepolymer films were thermally treated at100 C for5h and120 C for2
h, SCHEME1Thermal ring-opening polymerization of
B-a.
SCHEME2Preparation of siloxane-containing prepolymers,B-
PDMS,and B-MDA/PDMS.
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI10.1002/POLA
followed by postcuring at160,200,and240 C for1h each to afford transparent films with color ranging brown to red-dish brown.
RESULTS AND DISCUSSION
Preparation of Polybenzoxazine Prepolymers
First of all,the preparation of polybenzoxazine prepolymers containing polydimethylsiloxane unit was attempted at 100 C for0.5h without organic solvent(solvent-less method)from bisphenol-A,PDMS(248),and paraformalde-hyde at the molar ratio of1:1:4.However,insoluble bulky solid was obtained,suggesting the difficulties of applying the solvent-less method.
The formation of typical benzoxazine monomers is usually accompanied with ring opened structures.2In the prepara-tion of prepolymers through the formation of benzoxazine ring structure as repeating units in the main-chain,the ring-opened structure may cau crosslinking and hence insolu-bility of the obtained polybenzoxazine prepolymers.21In our previous studies on polybenzoxazine prepolymers,chloro-form was efficient to afford soluble high-molecular-weight prepolymers with negligible amount of insoluble side prod-ucts.18Thus,the reaction of bisphenol-A,PDMS(248),and paraformaldehyde in chloroform was carried out under reflux(solvent method).Figure1shows the relationship between reaction time and viscosity of B-PDMS(248).As shown in Figure1,the viscosity incread with increasing the time and then started to decrea from7h.We noticed the increa of the amount of insoluble products by increas-ing the time.Therefore,in this study we have fixed the time to be refluxing in chloroform for5h.The B-PDMS prepoly-mer was purified firstly by filtration to remove any formed insoluble gel,followed by washing the prepolymer solution in chloroform by aqueous NaHCO3solution to remove any residual unreacted phenolic compounds and paraformalde-hyde.After evaporating the solvent at low temperature,pow-der was obtained having color ranging from colorless to pale yellow.The yields,viscosities,and molecular weights of B-PDMS are shown in Table2.The yield was about80%after purification.The number average molecular weight(M n)was 3000–4200and the weig
ht average molecular weight(M w)was5700–9800.This range of molecular weight was high enough to give easily free standing and flexible film by casting the prepolymer solutions.
In the same way,the B-MDA/PDMS prepolymers were pre-pared easily by using MDA as the copolymer component (runs4,5in Table2).
IR and NMR Study of Prepolymers
To confirm the chemical structure of B-PDMS,IR spectra were measured and shown in Figure2.The typical charac-teristic absorption attributed to benzoxazine structure appeared at1229cmÀ1(C A O A C,asymmetric stretching), 1323cmÀ1(CH2wagging),and at942cmÀ1(out of plane bending vibrations of C-H).The characteristic absorptions assigned to trisubstituted benzene ring at1491cmÀ1were
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TABLE1Formulation for the Preparation of Siloxane-Containing Polybenzoxazine Prepolymers
Code
PDMS
Molecular
Weight
of PDMS
Feed/
g(mmol)
Siloxane
Content(%)
MDA
Feed/
g(mmol)
Bisphenol-A
Feed/g(mmol)
B-PDMS(248)248 4.97(20.0)25– 4.57(20.0)
cantone
B-PDMS(850)850 5.00(5.9)65– 1.34(5.9)
B-PDMS(1622)1622 6.57(4.1)79–0.93(4.1)
anthropoceneB-MDA/PDMS(850)850 2.91(3.5)25 2.94(14.8) 4.16(18.3)
B-MDA/PDMS(1622)1622 2.69(1.7)25 3.22(16.2) 4.01
福州日语培训
(17.9)
FIGURE1Influence of reaction time on viscosity of the prod-
uct.The reaction of PDMS(248),bisphenol-A,and paraformal-
dehyde(1:1:4)was carried out in CHCl3under reflux.
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obrved,indicating the prence of benzoxazine-ring struc-ture in the prepolymers.Additionally,introduction of poly-siloxane was confirmed by the absorption band at 1000–1100cm À1(Si A O A Si stretching).
1
H NMR spectra of B-PDMS were also measured to confirm the chemical structure.The 1H NMR spectrum of B-PDMS(248)is shown in Figure 3.The prence of benzoxa-zine-ring structure was confirmed by the peaks at 3.9ppm due to Ar A CH 2A N (b )and at 4.8ppm due to O A CH 2A N (c )of oxazine-ring.The methyl proton of bisphenol-A structure (a )and methylene proton of propyl linkage (e )overlapped at 1.6ppm.The N-bonded and Si-bonded methylene protons
of propyl linkage were also obrved at 2.7(d )and 0.5(e )ppm,respectively.The theoretical integral ratio of (b )and (d )signals for the fully ring-clod structure is 1.However,the found ratio was 1.67/1.92,indicating that B-PDMS(248)included some ring-opened structure.Thus,the ratio of ring-clod structure was calculated to be 88%(Table 2).The ratios of other prepolymers calculated by the same way were also relatively high (75–90%).In addition,the signals at 4.39and 7.08ppm indicated by the arrows in Figure 3are assigned methylol protons and meta protons of phenol arising from the ring-opened structure,respectively.19
Ring-Opening Behavior of Prepolymer
DSC measurement was carried out to follow the ring-opening behavior of the prepolymers (Scheme 3and Fig.4).The DSC curve of B-a after drying at 60 C for 24h showed a sharp exothermic peak that started at 220 C with peak maximum at 243 C and 240J/g as exothermic heat corresponding to the ring-opening polymerization of B-a [Fig.4(a)].On the other hand,the DSC curves of B-PDMS showed broad peaks which were obrved at lower temperature than that of B-a [Fig.4(b–d)].
Here,we estimated the ratio of the ring-clod structure from the amount of exotherm by assuming that exotherm of
TABLE 2Synthesis of Siloxane-Containing Polybenzoxazine Prepolymers
Run Code Yield (%)g red a /dL (g À1)M n (M w )b
Ratio of Cyclic-and Opened-Structures
Cyclic/open (1H NMR)Cyclic/open (DSC)1B-PDMS(248)750.063,000(5,700)88/1279/212B-PDMS(850)750.084,200(9,800)86/1480/203B-PDMS(1622)790.084,200(8,900)75/2558/424B-MDA/PDMS(850)780.123,500(10,000)85/1589/115
B-MDA/PDMS(1622)
83
0.15
3,600(11,000)
90/10
92/8
a
The viscosity was measured in dioxane (0.5g/dL)at 30 C.bposition是什么意思
Determined by GPC (polystyrene
standards).
FIGURE 2IR spectra of B-a and
B-PDMS.
FIGURE 31H NMR spectrum of B-PDMS(248)measured in CDCl 3at room temperature.
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI 10.1002/POLA
one benzoxazine unit is constant,not depending on the structure.Exotherm of B-a was ud as the st
andard.In the ca of B-PDMS(248),the amount of exotherm obrved was 183J/g,which is lower than the theoretical exotherm (232J/g).The ratio of ring-clod structure was calculated to be 79%(Table 2).B-PDMS(850)and B-PDMS(1622)showed the same tendency,and the ratio of the ring-clod structure was calculated to be 80%and 58%,respectively.
The ratios of the ring-clod and the ring-opened structure calculated from both 1H NMR and DSC analys are summar-
ized in Table 2.The ratios from both methods are almost same,indicating that all prepolymers contained some ring-opened structure.It is worth noting that the DSC thermo-grams of siloxane-containing prepolymers show broad exothermic peaks started at 161–175 C that are significantly lower temperature than that of B-a (220 C).Small amount of phenolic hydroxyl group of the ring-opened structure suggested by the NMR and DSC analys described above might work as acid catalyst and lower the polymerization temperature.
Next,we investigated the progress of the ring-opening poly-merization of B-PDMS(248)after each cure stage by IR (Fig.5).The characteristic absorption bands due to C A O A C of oxazine at 1229cm À1(asymmetric stretching),CH 2at 1319cm À1,trisubstituted benzene ring at 1496cm À1(stretching)a
nd C A H at 927cm À1(out of plane bending vibrations)decread.By the end of 240 C cure,the charac-teristic absorption bands for oxazine ring and trisubstituted benzene ring disappeared,suggesting the completion of the ring-opening polymerization at the tempearature.
沈阳师范大学选课网Formation of Polybenzoxazine Prepolymer Films and Cured Filmsmho
The siloxane-containing prepolymers were dissolved in diox-ane and cast on glass plates.The free standing films were obtained by removing the solvent at 50 C for 24h.After heat treatment up to 240 C,brown transparent polybenzo-xazine films were
obtained.
SCHEME 3Thermal ring-opening polymerization of
PB-PDMS.
FIGURE 4DSC thermograms of B-a (a)and B-PDMS (b–d)measured at a heating rate of 5 C/min under
nitrogen.
FIGURE 5IR spectra of B-PDMS(248)after each cure stages.
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