Preparation and Characterization of Chain Extended
Poly(butylene succinate)Foams
Hongfu Zhou,1Xiangdong Wang,1Zhongjie Du,2Hangquan Li,2Kejing Yu3
1School of Materials and Mechanical Engineering,Beijing Technology and Business University,Beijing100048, People0s Republic of China
2College of Materials Science and Engineering,Beijing University of Chemical Technology,Beijing100029, People0s Republic of China
3Key Laboratory of Eco-textiles,Ministry of Education,Jiangnan University,Wuxi214122,Jiangsu,People0s Republic of China
In order to improve the viscoelasticity and foamability, poly(butylene succinate)(PBS)was modified through reactive melt mixing with chain extender(CE)having multi epoxy-groups.Subquently,the prepared chain extended PBS(CEPBS)was foamed in a high pressure stainless steel autoclave using CO2as physical blowing agent.The molecular weight,thermal properties,rheological proper-ties,and foam properties of various PBS samples were characterized using gel permeation chromatography,dif-f
erential scanning calorimetry,rotational rheometer,and scanning electron microscope,respectively.With the introduction of CE,the molecular weight,the crystalliza-tion temperature,the complex viscosity and storage mod-ulus of PBS were incread and the crystallization degree of PBS was decread.At the CE content of0.75phr,the cross-linking structure was formed and the expansion vol-ume ratio incread to nearly15times,which meant the chain extension played an important role in the foaming process of PBS.POLYM.ENG.SCI.,00:000–000,2014.V C2014 Society of Plastics Engineers
INTRODUCTION
As one of the promising g materials,polymeric foam had aroud interests of scientists and rearchers becau of its low density,high impact resistance,and heat insulation[1,2]. Particularly,the rearch of biodegradable polymer foam such as poly(lactic acid)(PLA),poly(butylene succinate)(PBS), and so on,promoted the further development of polymeric foam,becau it is biocompatible and promis potential in solving rious problems of environmental pollution[3]. Unfortunately,the main disadvantages of biodegradable poly-mer were its poor viscoelasticity,low molecular weight,and narrow polydispersity index(PDI),which were adver to foam [4].
In order to modify the inferior properties of biodegradable polymer,numerous methods were employed,such as chain extension,nanocomposite,crosslinking,and so on[5–11]. Chain extension is a commonly swift and efficient method. Mihai ployed a multifunctional styrene-acrylicepoxy copolymer as chain extender to improve the molecular struc-ture and rheology property of PLA,and found that chain extended PLA exhibited obvious strain-hardening phenomenon, resulting in well foam ability[5].The catalyzed chain exten-sion of PBS was carried out with2,20-(1,4-phenylene)-bis(2-oxazoline)as a chain extender and p-toluenesulfonic acid as a catalyst by Huang et al,giving ri to a higher molecular weight and intrinsic viscosity[6].PBS was chain-extented with poly(butylene carbonate)using hexamethylene diisocya-nate as chain extender,bringing about satisfactory thermal and mechanical properties[7].Nanoparticles were also introduced into biopolymer as heterogeneous nucleating agent to improve their crystallization behavior and foaming property.Ji et al. found that the addition of nanosilica into PLA could increa the induced crystallinity,cell density,and cell structure uni-formity of PLA foam during the solid state foaming[8].Multi-walled carbon nanotube(MWCNTs)/PBS nanocomposites were investigated to study the effects of MWCNTs on physical char-acteristics of PBS nanocomposite foams.The prence of MWCNTs incread the melt viscosity of PBS so that the PBS/MWCNT nanocomposite foams could be produced with-out altering the chemical structure of PBS[9].Nanoclay could be mixe
d with PBS by both solution blending and melt mixing methods,demonstrating that the thermal degradation tempera-ture and mechanical property of PBS/organoclay nanocompo-sites incread[10].Cross-linking was employed rarely, becau it was beneficial to increa the melt strength and complex viscosity of polymer but negative to its biodegrada-tion property.Huang ported on ultraviolet-induced cross-linking of PBS in the prence of a photoinitiator and a cross-linking agent at ambient temperature.The results showed that the glass transition temperature was shifted to higher tem-peratures,and thermal stability,rigidity,as well as heat stabil-ity were improved[11].Among the abundance of modified methods,chain extension constituted a more potential one worth trying,becau it was able to change the polymer chain architecture,enhance molecular weight,and widen PDI,result-ing in excellent viscoelasticity for foaming.
The aim of this study was to prepare the chain extended PBS (CEPBS)foam with high blowing ratio by means of chain extender(CE)having multi epoxy-groups and investigate the effect of chain extension on the thermal properties,rheological properties,and foamability of PBS.PBS was modified through chain extension prior to being foamed.Becau of the lower
Correspondence to:Hongfu Zhou;e-mail: or Kejing Yu;e-mail:
Contract grant sponsor:the Development Project of Beijing Municipal Edu-cation Commission;contract grand number:KM201310011002;contract grant sponsor:the Open Project Program of Key Laboratory of Eco-textiles, Ministry of Education,Jiangnan University;contract grand number:No. KLET1203;contract grant sponsor:the BTBU Young Teacher Starting Foundation;contract grand number:QNJJ2011-49.
DOI10.1002/pen.23964
Published online in Wiley Online ).
V C2014Society of Plastics Engineers
POLYMER ENGINEERING AND SCIENCE—2014
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reactivity between the epoxy groups of CE and carboxyl groups of PBS,accelerant would be ud in the mixing process.
EXPERIMENTAL SECTION Materials
The biodegradable linear PBS (Bionolle 1001)ud in this experiment was purchad from Showa,J
apan.It is a kind of thermoplastic aliphatic polyester polymerized from glycols and aliphatic dicarboxylic acid.CE (ADR 4370S)was supplied by BASF Chemicals,Germany,which posss multifunctional epoxy groups bad in a styrene-acrylic oligomer with a number averaged molecular weight less than 3000.The general chemical structures of PBS and CE were reprented in Fig. 1.Anti-oxidant (Irganox1010)was provided from Ciba,Germany.The 2-ethyl-4-methylimidazole as accelerant was purchad from Sinopharm Chemical Reagent,China.To minimize the moisture effect,all chemicals were dried in a vacuum oven at 60 C for 24h before u.
Fabrication of Various PBS Samples
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PBS,CE,and/or accelerant were fed into a torque rheometer (Haake Rheodrive4,Germany)at 140 C for 15min with a rotor speed of 60rpm.The detailed experimental formula was shown in Table 1.Anti-oxygen agent was introduced into every for-mula to alleviate the degradation of PBS during processing.For purpo of comparison,PBS without CE and accelerant (1#),as well as PBS without accelerant (2#)were also procesd in the mixer with the above same procedures.
新片上映电影Foaming Processing of Various PBS Samples
The PBS foams were prepared physically using supercritical CO 2as blowing agent in a high pressur
e stainless steel auto-clave.Various PBS samples (5mm width 320mm length 32mm thickness)were dried,weighed,and placed on the sample holder in the autoclave.Typically,the autoclave was flushed with CO 2for veral conds,then pressurized with CO 2and heated to the solubilization temperature (140 C)at the desired pressure (30MPa).The solubilization time was maintained for 3h to ensure equilibrium absorption of CO 2by the samples,sub-quently the temperature was cooled down at about 5 C/min to the foaming temperature (110 C)and the pressure was reduced rapidly to atmosphere pressure within 3conds.At last,the autoclave was opened and the foamed samples were cooled nat-urally to room temperature.The foam structure was allowed to fully develop during rapid depressurization.
Characterizations
Gel Permeation Chromatography.Gel Permeation Chromatog-
raphy (GPC)(Waters Alliance HPLC systems,model 1515)equipped with microstorage columns was employed to measure the weight-average molecular weight (M w )of various PBS sam-ples at 25 C,using chloroform as the eluent at the flow rate of 1mL/min.
Gel Fraction Measurement.
The gel fractions of various PBS
samples were determined by the Soxhlet extraction with chloro-form until the weight of insoluble polymers did not change.Then the insoluble parts were dried at 60 C for 12h under vac-uum.The test was duplicated twice for every sample,and the average value was taken.The gel fraction was calculated by the following equation [12,13]:
Gel content %ðÞ5
W g
W 0
3100%(1)
where W 0and W g were the weights of the initial polymer and dried insoluble part of sample,
respectively.
FIG.1.General chemical structure of (a)PBS and (b)CE.Where R1–R5are H,CH3,a higher alkyl group,or combinations of them;R6is an alkyl group.
TABLE 1.Experimental formula.
Serials No.PBS/phr CE/phr Accelerant/phr
Antioxygen/phr
1#100000.12#1000.500.13#1000.50.10.14#1000.750.10.15#10010.10.16#100 1.250.10.17#
100
1.5
0.1
0.1
2POLYMER ENGINEERING AND SCIENCE—2014
DOI 10.1002/pen
Differential Scanning Calorimetry.The thermal properties of
the PBS samples were investigated using Differential Scanning Calorimetry (DSC)(Q100,TA,USA)system purged with nitro-gen.For nonisothermal crystallization,various PBS samples were heated from room temperature to 150 C rapidly,kept in the molten state for 2min to era prior thermal history and then cooled at 10 C/min down to 40 C to evaluate their ability to crystallize upon cooling.Subquently,the samples were heated back to 150 C at a rate of 10 C/min to asss the melt-ing behavior.gg是什么意思
Rotational Rheometer.
土耳其抗议The rheological properties of PBS sam-ples with various CE contents were measured in dynamic mode at 170 C using a rotational rheometer (ARES Rheometer,TA,USA)with a parallel plates (20mm in diameter with a gap of 1.0mm).The complex viscosity (g *),storage modulus (G 0),and loss modulus (G 00)were monitored at various frequencies.The frequency range was 0.1–100rad/s,and the maximum strain was fixed at 0.5%,to ensure that the analys were within the linear viscoelastic region under nitrogen.The PBS samples were dried prior to testing,to minimize oxidation and maintain a dry environment.
Blowing Ratio Measurement.The bulk densities of both pre-
foamed (q p )and postfoamed (q f )samples in g/cm 3were meas-ured by a density balance (Sartorius,Germany).The blowing ratio was calculated using the following equation:
Blowing ratio 5
q p q f
(2)
Scanning Electron Microscopy.The cross-ctional morphology
of the resultant foams was obrved using a LEICA Scanning electron microscopy (SEM)model S440.The SEM samples were prepared by cryogenically cutting the obtained foam samples and gold sputtering the fractured cross ctions.The average cell size,d (in m m),was obtained by measuring the maximum diameter of each cell perpendicular to the skin from SEM micrographs.
Cell Densities Calculation.The cell densities,N 0,in cells/cm 3,
are defined using Eq .3by assuming the cells are spherical:
N 05nM 2A
32
/(3)
where n is the number of cells in the SEM micrograph,M is the magnification factor,A is the area of the micrograph (in cm 2),and /is the volume expansion ratio of the polymer foam.
RESULTS AND DISCUSSION
Molecular Structure and Molecular Weight of Various PBS Samples
During the chain extension process,three chemical reactions maybe existed and took place;the were chain scission,chain extension,and cross-link,as illustrated in Fig.2.In order to clarify the reaction mechanism,the molecular weight,gel frac-tion,and processing torque of various PBS samples were meas-ured,as shown in Table 2and Fig. 3.When PBS (1#)was procesd without the CE and accelerant,only the chain scission occurred.Actually,the chain scission took place as well in melt mixing process of other PBS samples (2–7#),owing to the ther-mal degradation and hydrolyzation.After the introduction of CE without accelerant into PBS (2#),the chain extension took place slightly,which could be verified by the increa of M w and tor-que curve compared with the sample (1#).
With the increasing content of CE (the accelerant content about 0.1phr),the M w of CEPBS incread,as shown in Table 2.When the content of CE reached to 1phr,an important phe-nomenon was obrved,that is the gel generated becau of the cross-linking of PBS and CE.Thus,the GPC results of the cross-linking samples were insignificance.Further rearching the gel fraction of CEPBS,there existed a sharp increa of gel fraction (from 2.3%to 39.4%)when the content of CE incread from 0.75to 1.5phr,which was also proved by the obvious increa of torque curve (5#).Cr
oss-linking was helpful to improve the viscoelasticity of PBS but adver to the increa of M w (e Table 2)and the degradation of PBS.Moreover,PDI of PBS samples became wide (from 2.26to 8.36)with the con-tent of CE (within 0.75phr),which endowed fine melt elasticity and good processibility [14].The changes of PDI of PBS may be resulted from only a portion of PLA molecular chains reacted with epoxy-bad CE.CEPBS samples were the mixture of lin-ear PS and chain branched PBS.
Crystallization and Melting Behavior of Various PBS Samples
DSC was utilized to evaluate the crystallization and melting properties of various PBS samples.The cooling and heating curves are shown in Figs.4and 5,and the corresponding results are summarized in Table 3,respectively.In Fig.4,with the con-tent of CE,the crystallization temperature of PBS samples incread from 85.80 C to 92.16 C,which may be becau of the branched structure of CEPBS.The branch point of CEPBS acting as the heterogeneous nucleating sites was beneficial
to
FIG.2.Illustration of possible chemical reactions during the chain exten-sion of PBS.
TABLE 2.M w ,MWD,and gel fraction of various PBS samples.Sample No.1#2#3#4#5#6#7#M w (105g/mol) 1.94 2.15 3.41 4.83–––PDI
2.26 2.37 5.318.36–––Gel fraction (%)
2.3
8.6
20.9
39.4
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—20143
the increa of both nucleation and crystallization temperatures [15].The obvious increa of crystallization temperature of PBS samples were very helpful for the growth and cooling stability of PBS foam in the subquent foaming process,becau the melt strength of PBS could be incread rapidly through decreasing foaming temperature to around crystallization tem-perature.However,the crystallinity of CEPBS samples (2–7#)decread with the content of CE.The possible reason is that t
he chain branching and cross-linking structures of PBS may affect the structural regularity of PBS chains [16].
Melting point (T m )value of PBS is another important param-eter,which significantly affects its application.Double melting peaks were obrved in the melting curves (as shown in Fig.5)after the PBS was procesd.The lower temperature melting peak was at around 107 C and the higher one was at around 112 C.Generally,it was common to obrve double melting peaks in PBS,which might be attributed to veral reasons reported in some rearch papers [17–19].Yasuniwa et al.found that PBS showed double melting endotherms,ascribed to the recrystallization and melting mechanism [17].Im et al.also found that PBS exhibited multiple melting behaviors,and there was no crystal modification change for PBS crystallizing at dif-
ferent temperatures according to the wide angle X-ray diffrac-tion measurement [18].Therefore,it could be concluded that the multiple melting behavior of PBS formed thermally did not originate from the different crystal modification.In this study,the double melting endotherms behavior of linear PBS and CEPBS obrved might be explained reasonably by the melting and recrystallization model [19].With the increasing CE con-tent,the double melting peaks became weak and combined together gradually when the cross-linking occurred.
Rheological Property of Various PBS Samples
It is very important to posss sufficient melt strength for the foaming process of polymers to be successful.The melt strength of a polymer generally depends on the viscosity,dynamic stor-age modulus,and loss modulus.High molecular weight and complex polymer architecture could impart high melt strength for a given polymer.In general,the melt strength of PBS was relatively low.In order to overcome this limitation,in this study,CE having multi epoxy-groups was employed to increa the melt strength of PBS by branching and/or cross-linking to achieve foaming successfully.
Rheological measurements were employed to evaluate the viscoelastic properties of various PBS samples,as shown in Figs.6–8.Figure 6showed the changes in the complex viscosity of PBS at 170 C as a function of the angular frequency.For all samples,the complex viscosity decread with an increasing fre-quency.The complex viscosity of the modified PBS incread,as the amount of the CE having multi epoxy-groups incread.The enhancement of the complex viscosity may be attributed to the chain extension and cross-linking,which made
PBS
FIG.3.Torque curves of various PBS
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samples.
FIG.4.Crystallization behaviors of various PBSnapier
swallow是什么意思samples.
FIG.5.Melting behaviors of various PBS samples.
TABLE 3.Thermal properties of various PBS samples.
Serials No.1#2#3#4#5#6#7#T c ( C)85.886.386.686.687.788.792.2H c (J Ág 21)64.772.570.769.366.667.359.7T m ( C)
112.7113.4112.7112.7111.8111.7112.4107.5106.7105.7----X c (%)
63.5
71.1
69.3
71.0
65.3
65.9
58.5
4POLYMER ENGINEERING AND SCIENCE—2014DOI 10.1002/pen
interchain slippage difficult.Similar phenomenon was obrved in other study [20].
In Figs.7and 8,the changes in the storage modulus (G 0)and the loss modulus (G 00)of PBS are shown with the angular fre-quency.Obviously,both G 0and G 00of various PBS samples incread with angular frequency.The G 0–x curves of CEPBS with higher CE content were higher than that of the procesd PBS (1#),which implied that the melt elasticity of CEPBS was enhanced [21].This phenomenon indicated that CEPBS had a longer relaxing process,which also testified that the branching structure of CEPBS improved melt elasticity of PBS.The G 00–x curves of various PBS samples had similar trend with the G 0–x curves.However,the amplification or changes of G 00was smaller than that of G 0,indicating that increment of elastic com-ponent of PBS melt was greater than that of the viscous compo-nent of PBS melt resulted from the formation of branching structure of PBS.
Cellular Morphology of Various PBS Foams
Generally,cellular morphology of polymeric foam mainly contains cell shape,cell size,and cell size distribution,which depend on the viscoelastic properties of polymer melt.Owing to
its poor viscoelastic properties,the cell size of procesd PBS foam (1#in Fig.9a)decread to 131l m,and the cells are hav-ing open-cell structure.Its cell density incread (in Table 4)and cell wall became thin,resulting from the decrea of com-plex melt viscosity beneficial to growth of cell but likely to merge and rupture.After the CE was introduced into PBS (2#in Fig.9b),the phenomenon of open cell and rupture alleviated.As can be en in the micrographs (Figs.9a and b),the majority of the cells were open,becau of its poor melt strength.It was indicated that the growing cells were not effectively stabilized,and the PBS melt could not support the expansion of cell walls becau of its low extensional viscosity when draining.Thereby,the cell walls became thin,ruptured,and merged,leading to the formation of open cell.In Figs.9c–g,it could be obrved that the cell of CEPBS has fine shape (pyritohedron),clod-cell,and uniform cell distribution,which should be attributed to the chain extension and cross-linking effects.As the cross-linking effect would restrict the growth of cell,the cell size of CEPBS (4–7#)decread from 243to 146l m.
Expansion volume ratio is another parameter to characterize foamability of polymer,which mainly depends on the cell den-sity,cell size,and so on.Seen from Table 4,it could be found that chain exten
sion was in favor of the increment of expansion volume ratio and cross-linking restricted cell growth leading to low expansion volume ratio.For example,the expansion volume ratio of the sample (4#)as high as 14.3was obtained,which was more than 2.5times that of procesd PBS (1#)and higher than that of cross-linking CEPBS (5–7#).
CONCLUSIONS
A facile and efficient approach to modify PBS through chain extension has been developed.With the content of CE,the molecular weight of PBS incread from 1.943105to 4.833105g/mol,and the PDI of PBS widened from 2.26to 8.36.The DSC heating scans showed that the crystallization temperature of PBS was incread from 85.8 C to 92.2 C,with the content of CE increasing.Becau of the changes of molecular structure,both complex viscosity and storage modulus of PBS were incread by about an order of magnitudes at the low shear rate.This was of great potential for proposing a model for
the
sweptFIG.6.Relationships between complex viscosity and shear rate for various PBS
yingyusijisamples.FIG.7.Relationships between storage modulus (G 0)and shear rate for vari-ous PBS
samples.
FIG.8.Relationships between loss modulus (G 00)and shear rate for various PBS samples.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—20145
