MODIFICATION OF POLYAMIDE 6 WITH POLYAMINOAMIDE-g-POLY(ETHYLENE GLYCOL) VIA HYDROLYTIC POLYMERIZATIO

Chinese Joumal of Polymer Science Vol. 27, No. 3, (2009), 343-350Chinese Journal ofPolymer Science02009 World ScientifieMODIFICATION OF POLYAMIDE 6 WITH POLYAMINOAMIDE-g-POLY(ETHYLENE GLYCOL) VIA HYDROLYTIC POLYMERIZATIONYao-chi Liu*b, Wei Xu", Yuan-qin Xiong", Fan Zhang" and Wei-jian Xu"'Institute of Polymer Science & Engineering, Hunan University, Changsha 410082, ChinaCollege of Urban Construction, Nanhua Universiry, Hengyang 421001, ChinaAbstract To enhance the impact strength of polyamide 6. hydrolytic polymerization modification by the polyaminoamide-g-poly(ethylene glycol) (PAAEG) derivatives with poly(ethylene glycol) (PEG) molecular weight of 400-10000 was studied.Amide groups of polyaminoamide segments were postulated to form hydrogen bonding with polyamide 6, and hydroxygroups of PAAEG units were expected to react with carboxylic acid groups of polyamide 6 forming copolymers during thepolymerization. The improved compatibility in amorphous regions of blends has been confirmed by differential scanningcalorimetry (DSC) and scanning electron microscopy (SEM) of fracture surfaces. The efects of PAAEG on the waterabsorption and notch sensitivity of blends were investigated, using water uplake measurement and mechanical testings,respectively. For comparison, pure polyamide 6 and the blend of PEG/polyamide 6 were also invetigated. The addition ofPAAEG retarded the cytallization of polyamide 6, but did not make remarkable influences on is cystalline stucture. As aconsequence of the strong interactions between the dispersed phases and polyamide 6 matrices, PAAEG was a more suitableadditive for improving the notched impact strength of polyamide 6 than PEG.Keywords: Polyamidce 6; Polyaminoamide-8-polylethye gyco); Blends; Impact strength; Compaiblity.INTRODUCTIONPolymer blends have been extensively studied because of their simplicity, and many new products have beenmanufactured to achieve improved properties generally not available in any single polymeric materiall-y.Polyamide 6 (PA6) is a kind of widely used engineering plastics because of its chemical stability and goodfatiguel4-", but its usage is limited by notch sensitivity. Generally, the low temperature impact strength isimproved by random incorporation of rubber particles into the nylon 6 matrices. Polyether is the commonly usedrubber phase with low glass transition temperature and small domain sizel8. Polymer blends of PA6 and polyetherare immiscible because of the positive enthalpy change and the small increase of entropy on mixing. Therefore,the stabilization and enhancement of adhesion between the two phases is very important.PA6 can be prepared by hydrolytic or anionic polymerization of Ecaprolactam (CL)9!. A large number ofefforts were made to incorporate poly(ethylene glycol) (PEG) into PA6 by anionic polymerization. The block orgraft copolymers formed during anionic polymerization act as compatibilizers, which reduces the size ofdispersed phase and stabilizes the dispersed phasell0-14I. For hydrolytic polymerization, Fakirov et al. investigatedmultiblock poly(ether-ester-amide)s (PEEA) based on carboxyl-terminated PA6 oligomers and PEG by a two-step polycondensation reaction'5, 16. Deschamps et al. investigated PEEA copolymers based on PEG, 1,4butanediol and a diester- diamide monomerlt, 8. These chemical routes however didn't avoid the phaseseparation between polyamide and PEG because of low esterification capacities resulting in low yields ofcopolymer. As regards the poor miscibility of polymer pairs, hydrogen bonding, dipolar interactions, phenyl'This work was financially supported by the National Natural Scienc中国煤化工7008) and ChinaPetroleum & Chemical Corporation.“Corresponding author: Weijian Xu (徐伟箭), E-mail: polymer. groupT_HCNMHGReceived January 2, 2008; Revised February 23, 2008; Accepted Februaly大cwvo344Y.C. Liu et al.group coupling or ionic interactions may also contribute to a negative heat of mixing, making homogeneouspolymer blends thermodynamically stablell.In this paper, polymer blends of polyaminoamide-g-poly(ethylene glycol) (PAAEG) and PA6 wereinvestigated. Instead of using a block or graft copolymer as compatibilizer, we employed PAAEG as the blendcomponent partner directly. Amide groups of polyaminoamide segment and PA6 were postulated to formhydrogen bonding, and hydroxy groups of PAAEG were expected to react with carboxylic acid groups of PA6.Thus, the dispersed PEG phase could easily extend to the PA6 matrix. For comparison, pure PA6 and the blendof PEG2000 (Average molecular weight of PEG is 2000) and PA6 were also investigated.EXPERIMENTALMaterialsEpichlorohydrin (ECH) was purified by disillation. CL (China Petroleum & Chemical Corporation), adipic acid(AA) (Sinopharm Chemical Reagent Co., Ltd), diethylene triamine (DETA), hydroxylbenzylthioether (HBTE)and poly(ethylene glycol) [PEG, average molecular weight (M): 400, 1000, 2000, 6000 and 1000 were usedwithout further purification.Synthesis of PAAEG DerivativesPAAEG derivatives were synthesized by reacting amino terminated polyaminoamide (PAA) with PEG-ECHadduct (Fig. 1). PAA was prepared according to Ref. [20, 21] and PEG-ECH according to Ref. [22, 23]. In atypical experiment, PAA-ethanol solution (containing 0.1 mol DETA) was dropped to a flask with the PEG-ECHadduct (containing PEG400, 0.1 mol). Then, KOH (5.6 g, 0.1 mol) was added, and flask was held at 559C for5 h. The solution was purified by filtration and reduced in volume with a rotary evaporator. The product wasstored in a vacuum desiccator at 65°C for at least 2 days.HoOCM COOH+ HN~ii-Nh; HEa+Ho(^ofH170C,2.5hCal: BF;O(E)265C,4hH2~aOHAmino terminated PAAPEG-ECH adductCat: KOH ;PEGs5"C,5h。PEGmnh"YPEG'PAAEG derivativeFig, 1 Synthesis of PAAEG derivativesHydrolytic PolymerizationPolymer blends were obtained by hydrolytic polymerization of CL in the presence of PAAEG, initiated bydeionized water (DI, 3 wt%). Heat supply was provided by an electric heater and controlled by a stainlesstheostat. A stream of dry nitrogen was used to drive off oxygen and vapor in the autoclave before and at the endof polymerization, respectively. For example, in the case of the blend of 3/97 PAAEG400/PA6, PAAEG400(24.0 g, the molecular weight of PEG is 400), CL (776 g), HBTE(240) and DI 124 ml) were introduced into aGSH-2 2-L polymerization autoclave at room temperature. The中国煤化工fs K min', heldat (275土5)°C and 0.8 MPa for a polymerization time of3 h, anYHC N M H Goduct was wumnedModification of PA6 with Polyaminoamide g-Polyethylene glycol)345into threads and rapidly cooled to room temperature with water. PA6 threads were turned into fine shavings. Thelow molecular weight compounds were extracted three times by DI for 8 h. .CharacterizationFourier-transform infrared (FTIR) analysis was performed on a Nicolet Nexus 670 FTIR spectrometer between4000 cm-' and 400 cm~' in the form of KBr pellets (32 scans, resolution 1 cm").Differential scanning calorimetry (DSC) was performed on Perkin Elmer DSC-7 with sample weight 7-10mg under nitrogen atmosphere. Each sample, encapsulated in aluminum pans, was heated from 20°C to 300°C ata rate of 10 K min' before quenching to 50°C at 100 K min"'. Then, the sample was reheated to 300°C at a rateof 10 K min~' at second heaing. Crsalline melting temperature (Tm) was obtained as the maximum of meltingendotherm. Percentage crystallinity (osc) was calculated via the ratio between measured and equilibrium heat offusion (NH/OH?). The equlibrium heat of fusion (OH9) is 230 J/g for 100% crsalline PA61241.X-ray diffraction (XRD) was performed on Rigaku D/Max2500 diffractometer (Nifitered, Cu/K a radiationof wavelength 0.154 nm) in the reflection mode over the range of diffraction angles (2日) from 5° to 45° at .ambient temperature. The voltage and tube current were 40 kV and 200 mA, respectively. Percentage cysallinityby XRD (XxRD) was calculated by a standard procedurel25).The morphology of copolymers was observed by scanning electron microscopy (SEM) with a Hitachi S4700microscope. The cryogenically fractured surfaces of samples were sputter coated with gold to prevent charging inthe electron beam.The samples for water absorption and notched Izod impact were made with an HD-1100 injection machineat a crosshead speed of 50 mm/min. Notched impact tests were conducted according to GB/T1043- -1993 afterconditioning the samples at 20°C and 65% relative humidity for 24 h. The data were averaged from five repeatedmeasurements. Water absorption measurements were conducted with sheet samples (50 mm x 20 mm x 3.2 mm,length x width X thickness). The dried and weighed samples were emerged in water at ambient temperature for48 h. Samples were removed, patted dry with a lint free cloth and weighed. The water absorption is expressed asincrease in weight percent.RESULTS AND DISCUSSIONChemical Bonding between PA6 and PAAEGHydrolytic polymerization modification of PA6 studied in this work was performed by PAAEG derivatives withPEG of molecular weight 400-10000. The PAAEG derivatives were synthesized by reacting PAA with PEG-ECH adduct (Fig. 1). PAA, PAAEG400, 6/94 PAAEG400/PA6 and pure PA6 were characterized by FTIR. Themajority of the PAA absorption bands (Fig. 2a) are those corresponding to the N~H, C=O, C-H and C-N.M4000 3600 3200 2800 2400 2000Wavenumber (cm^中国煤化工Fig. 2 FTIR specta of (a) PAA, (b) PAAEG400, (c) 6/9TYHCNMHG6346Y.C. Liu et al. .The N- H and C=0 stretching bands are strong, characteristic of the amide function, and appear at 3276(having a shoulder at 3477 and a weak absorption at 3072) and 1647 cmi '. The N- H deformation can be seen at1554 cm-'. The C-H stretching consisted of two main absorptions at 2935 and 2864 cmi . The band that hasbeen most widely reported for secondary amine is the C- -N vibration near 1130 cm" . Figure 2(b) shows theFTIR spectrum of PAAEG400. Compared with PAA, its absorption bands are those corresponding to the 0- -Hand C-0- C moieties. The 0- H absorption appears at 3284 and 1065 cm^，and the c- o-C antisymmtricvibration at 1126 cm . The C- N vibration of secondary amine near 1130 cmi is nearly disappeared. The aboveresults confirm the formation of PAAEG.Figure 2(C) shows the FTIR spectrum of 6/94 PAAEG400/PA6. In the hydrolytic polymerization system,hydroxy groups of PAAEG can react with carboxylic acid groups of nylon 6 to form graft copolymers, which ischaracterized by the appearance of a peak at 1758 cm~' corresponding to the ester group. This kind of graftingeffect improved the compatibility between PAAEG and nylon 6 (as confrmed in DSC analysis section). But it isworth mentioning that the esterification capacity is relatively low. Compared with pure PA6 (Fig. 2d), 6/94PAAEG400/PA6 has a strong adsorption shoulder at 3500 cm~', the characteristic 0- H stretching of hydroxygroups. Based on the above results, we would rather call the PAAECG/PA6 system a blend than a graft copolymer.Influence of PAAEG on Crystallinity of PA6An accurate investigation of both Tm and XDsc values was presented by DSC analysis for the blends and pure PA6(Table 1 and Fig. 3). Both Tm and XDsc values of PAAEG/PA6 decrease with the increase in PAAEGconcentration from 3% to 6% and the decrease in PEG length from 10000 to 400. This decreasing trend is due tothe diluent effect of PAAEG additives on the crystallizable portions of PA6 segments, i.e., the compatibility anduniformity between PAAEG and nylon 6 segments, which comes from the interactions of hydrogen bonding andreactive compatibilization. Polyaminoamide segments of PAAEG can form hydrogen bonding with PA6, and thehydroxy groups of PAAEG can react with the carboxylic acid groups of PA6 during polymerization. The finedispersion of soft segments in the main polymer hinders the regular alignment of PA6 chains and has a strongeffect on the nascent structure of PA6. For the blends with the same composition, when PEG lengths decreasefrom 10000 to 400, the proportions of polyaminoamide and hydroxy group increase, resulting in improvedcompatibilities. The increase in PAAEG content may also induce evident dilution. Thus, we may infer thatPAAEG phase can easily extend to the PA6 matrix for the blend of 6/94 PAAEG400/PA6, so that the packing ofPA6 is relatively influenced, and the blend is characterized by low Tm and XDsc values.Table 1. Infuence of the PAAEG type and concentration on some parameters charaterizing blendsMass compositionT(C AH°(.g) Xosc(%) XxRD'(%)_ ag*(20) r(20) a (209)_PAAEG400/PA6 = 3/97218.954.623.722.823.9PAAEG1000/PA6 = 3/97219.560.626.325.120.0PAAEG200/PA6 = 3/97220.563.927.826.620.123.8PAAEG6000/PA6 = 3/9728.1PAAEG10000/PA6 = 3/97 221.468.329.728.6PAAEG400/PA6 = 6/94216.848.521.120.9PAAEG1000/PA6 = 6/94219.650.121.8PAAEG2000/PA6 = 6/94219.052.422.4PAAEG6000/PA6 = 6/9455.624.223.021.4PAAEG10000PA6=6/94 220.561.726.825.9PEG2000/PA6 = 6/94221.364.227.926.2Pure PA6222.476.833.4_"Melting temperature at second heating; Heat of fusion at second heating; 'Degree of crystallinity calculated from DSCanalysis at second heating; "Degree of cysallinity calculated from XRD analysis; Reflction of the eysalline plane (200);'Reflection of the crysaline plane (00); Reflection of the crysaline plane (002) + (202)中国煤化工MHCNMHGModification of PA6 with Polyaminoamide-g-Polyethylene glycol)347dba160 170180190200210220230240250Temperature (C)Fig. 3 DSC traces of (a) pure PA6, (6) 6/94 PEG2000/PA6, (c) 6/94 PAE6000/PA6,(d) 6/94 PAAEG2000/PA6 and (e) 6/94 PAAEG400/PA6In the PEG2000/PA6 system, addition of PEG resulted in a slight decrease in the Tm and Xosc values of PA6component. This appears to be due to the partial miscibility of PA6 and PEG. Compared with the blend of 6/94PAAEC2000PA6, 6/94 PEG200/PA6 owns higher Tm and Xosc values. Contributions determining crsalliemelting temperature and percentage crytallinity include constitution and molecular structural characteristic. For6/94 PEG2000/PA6 blend, hydrogen bonding between the dispersed phase and matrix is nonexistent. It isrelatively difcult to introduce PEG segments into PA6 matrices, and PEG segments can easily aggregate andproduce a coarsening dispersed phase because of poor compaibility (see also Fig. 5c). The relatively higherdecrease in the Tm and Xosc values of PAAEG2000/PA6 however suggests that the interaction between thedispersed phase and the matix exists sufficiently, resulting in further retardation of crystallization.For a more detailed comparison with Xosc values, XxRD values of pure PA6 and blends were evaluated by theXRD technique (Table 1). The resuts obtained by XRD have a rather good agreement with those obtained byDSC, although all XRD data are shifed to lower values. The good correspondence between the two sets of datasupports the general conclusion drawn above.Another point worth mentioning is the presence of a shoulder peak of 6/94 PAAEG6000PA6 blend(Fig. 3c). One possibility is that the PA6 components of blend crstallie in two formns, a and y23, as discussedin XRD analysis section and shown in Table 1.Polymorphism of Modified PA6Figure 4 shows some XRD intensity profiles of blends and pure PA6 as regards the polymorphism. Forcomparison, curves of 6/94 PAAEG2000/PA6 (Fig. 4d) and PEG2000/PA6 (Fig. 4c) are put together. Thecrystalline region of pure PA6 (Fig. 4a) hold the two characteristic peaks of a form at叫2θ= 20.2° and ar20 = 23.8*, corresponding to the reflections of the cysallie planes (200) and (002) + (202), respectively. Acareful observation of the intensity profile leads to the identification of a very small percent of γform at 2θ=21.49, crresponding to the relection of the cytalline plane (001)[21 21. In the blends of PAAEG/PA6 andPEG/PA6, the PA6 components almost crysalize in the a form, assumed to be due to the high fexibilityt ofdispersed PEG facilitaing cstallization of the polyamide segments in the more perfect a-modification. It isknown that PA6 generally shows a form in the case of extended chain conformation or r form crystalinestructure in which the chains are twistedlo.Thus the addition of PAAEG does not make remarkable influences onthe cysalline structure of PA6, notwithstanding the decrease in percentage crytallinity observed by DSC.中国煤化工YHCNMHG348Y.C. Liu etal.0 520253035404520(")Fig. 4 XRD pttrms of (a) pure PA6. (b) 6/94 PAAEG400/PA6. (c) 6/94 PAEG2000/PA6,(d) 6/94 PEG2000/PA6, (e) 6/94 PAAEG600/PA6 and (I) annealed 6/94 PAAEG6000/PA6In Fig. 4(e), a relevant presence of yform is evident for 6/94 PAAEG6000/PA6, with a sharp peak, which isnonexistent for the blend of 3/97. This may be due to thermal and processing conditions'2ol. When the PAAEGconcentration is low and the PEG length is short, the strong dipole-dipole interactions between polyamidesegments will force the soft segment into the amorphous phase. As the concentration and chain length increase,the mobility of soft segment is relatively confined, especially when the samples are rapidly cooled. In order tofurther understand this variation in crystallinit, post condensation annealing experiments were carried out abovethe melting point of PA6 at 230°C under vacuum. Samples were cooled at a rate of 1 K min~'. As expected, theXRD intensity profiles of annealed samples only exhibit a form crystalline (Fig. 4f), ie., the r form almosttransforms into a form upon annealing.Morphology of the BlendSEM analysis is a simple method to clarify phase morphology of PA6 blends'" 21. The morphology of 6/94PAAEG2000/PA6 (Fig. 5b) is similar to that of the pure PA6 (Fig. 5a). The homogeneous distribution suggeststhat PAAEG was well dispersed in PA6 matnix. The discrete PEG particles observed in the SEM micrograph offracture surface of 6/94 PEG200/PA6 (Fig. 5c) however suggests that the interaction between PA6 and PEG isrelatively weak, due to the low esterification capacity of hydroxy groups reacting with carboxylic acid groups asaforementioned.5.00 um5.00 pum5.00umFig. 5 SEM micrographs of (a) pure PA6, (b) 6/94 PAE2000PA6 and (c) 6/94 PEG2000PA6According to the discussion in DSC and SEM sections, we can draw a conclusion that the compatibility ofPAAEG/PA6 blend is higher than that of PAAEG/PA6. The possible explanation is the strong interactionsconsisting of hydrogen boning and reactive compatibilizationond DAARC resulting in theenhancement of degree of mixing.中国煤化工TYHCNM HGModification of PA6 with Polyaminoamide-gPoyethylene glycol)349Impact Strength of Modifed PA6It is known that PA6 is sensitive to a notch and has a low energy of crack propagation. In this paper, We focus themechanical testing of blends on the notched impact strength measurement to investigate the effect of theimproved compatibility made by PAAEG on the notched toughness of the final blends. Notched impact strengthsof blends and pure PA6 are listed in Table 2. The crystallinity and morphology of blends discussed in theprevious sections are closely related to the notched impact strengths of blends. Commonly, an increase in thecrystallinity leads to a decrease in the notched impact strength. In the blends of PAAEG/PA6, PAAEG segmentsserve as inhibitors of crack propagation. As PAAEG content increases and PEG length decreases, the crystallinityof PAAEG/PA6 blend decreases, resulting in the increase of notched impact strength. Compared with pure PA6,more than one-fold higher value at 6/94 PAAEG400/PA6 has been measured.Table 2. Variations of notched impact strength and percentage water absorption of blendsMass compositionE"(kJ.m2)Wb (%)PAAEG400/PA6 = 3/9712.53PAAEC100/PA6 = 3/9710.148.8PAAEG2000/PA6 = 3/979.50PAAEG6000PA6 = 3/97PAAEG10000/PA6 = 3/978.86 .7.9PAAEG400/PA6 = 6/9414.1911.4PAAEG1000/PA6 = 6/9411.0011.2PAAEG2000/PA6= 6/9410.3410.6PAAEG6000/PA6 = 6/949.4810.19.729.9PEG2000/PA6 = 6/949.86119Pure PA66.485.7"Notched impact strength; b Percentage water absorptionThe notched impact strength of 6/94 PEG2000/PA6 blend is lower than that of 6/94 PAAEG2000/PA6. Thisresult, as expected, is due to the poor misibility and small interfacial adhesion between PEG and PA6 asmentioned before. PEG segments can easily aggregate and produce a coarsening dispersed phase in the blend,which has been observed on the fracture surface of PEG2000/PA6 blend by SEM.Water Absorption of Modified PA6PA6 is semicrysalline and susceptible to water absorption. The absorbed water increases chain mobility and thuswill cause dimensional instability with property degradation. In order to study the interaction between water andblends, water absorption measurements were conducted (Table 2). On the whole, the addition of PAAEG or PEGresults in an increment of water absorption of blends. As regards the water absorption mechanism of blends, twoaspects should be taken into account: the crystallinity and the hydrophilic nature of soft segment. For pure PA6,as the crystallinity increases the amount of water absorption commonly decreases. Water molecules can onlydiffuse into the amorphous phase and displace disordered' amide-amide hydrogen bonds, but they cannotpenetrate into the crystal domain and break apart existing amide- amide bonds in this phase. The water absorptionof PAAEG/PA6 blends with the same composition also follows the above rule, e.g.. the blends with a masscomposition of 3/97, the decrease in crystalinity from 29.7 of PAAEG10000PA6 blend to 23.7 ofPAAEG400PA6 blend results in a slight increase in water absorption from 7.9% to 9.1%.Compared with the cysallinity, the hydrophilic nature of soft segment is a more important factor thataffects water absorption. As the content of PAAEG segments increases, an increase in water absorption occursobviously, e.g.. the blends of PAAEG400/PA6, the increase in PAAEG content from 3% to 6% increases thewater absorption of blends by 2.3% to 11.4%, where the crystallinity decreases slightly from 23.7% to 21.1%.Similarly, because PEG2000 has a stronger affinity for water than PAAEG2000, the blend of 6/94 PEG2000/PA6possesses higher percentage water absorption than 6/94 PAAEG2中国煤化工re resistance ofPAAEGIPA6 blends is expected to be better than that of PEG/PAYHCNMHG! display greaterdimensional sability.350Y.C. Liu et al.CONCLUSIONSPAAEG derivatives with different PEG chain lengths were introduced into PA6 during hydrolytic polymerizationof CL. The crystallinity, miscibilty, morphology, notched impact strength and water absorption of thePAAEG/PA6 blends were studied. For comparison, pure PA6 and PEG2000/PA6 blends were also investigated.The decreases in Tm and crysallinity of PA6 suggest that the addition of PAAEG retards the crystallizationof polyamide as a result of improved compatibility. The magnitude of the decrease becomes large as increasingPAAEG content and decreasing PEG length. Additional indirect evidence on the improvement of thecompatibility can also be obtained from the SEM micrographs of fracture surfaces. The XRD experimentssuggest that the addition of PAAEG does not make remarkable influences on the crystalline structure of PA6.Compared with the crystallinity, the hydrophilic nature of PAAEG is a more important factor that affects waterabsorption. Contrastive experiments suggest that PAAEG is a more suitable additive for improving the notchedimpact strength of PA6 than PEG. 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