Studies on the phase separation of polyetherimide-modified epoxy resin, 2")

Transcription

1 Macromol. Chem. Phys. 198, (1997) 3267 Studies on the phase separation of polyetherimide-modified epoxy resin, 2") Effect of molecular weight of PE on the structure formation Jun Cui, yinfeng Yu, Wenjie Chen, Shanjun Li* Department of Marcromolecular Science and the Laboratory of Macromolecular Engineering of Polymers, Fudan University, Shanghai, , China (Received: February 24, 1997; revised manuscript of June 2, 1997) SUMMARY The effect of molecular weight on the structure formation of polyetherimide modified epoxy system was studied. Using time-resolved light scattering (TRLS) and scanning electron microscopy (SEM), the process of phase separation and the morphology of the blends were investigated. As the curing reaction proceeds, the phase separation occurs first in the large molecular weight polyetherimide (PE) modified epoxy resin and the time of occurrence of phase separation is increased with the decrease of the inherent viscosity of PE. Because of the different viscosity of the blends, the blends display varied morphologies from a PE particle dispersion to co-continuous phases. Changing of the PE molecular weight may be another possible route to control the morphology of the PE modified epoxy resins. ntroduction Epoxy resin networks are currently used for many important applications such as adhesives, encapsulates, and advanced composite matrixes in aerospace industry. Besides their adhesive properties, these highly crosslinked networks possess excellent thermal and chemical stability as well as high modulus and strength. However, the further use of epoxies is limited because of their inherent brittleness. The most common method of toughening epoxy resins involves the addition of a second polymeric component that phase-separates upon curing. Rubber toughening of epoxy resins has been widely reported in the literature'-*). A two-phase morphology consisting of relatively small rubbery particles dispersed in and bonded to an epoxy network is produced by incorporating rubber into epoxy resins. However, as the crosslinking density increases and the resin therefore becomes even less ductile, rubber particles become progressively less capable of increasing the fracture resistance. Furthermore, because of the low glass transition temperature (T,) of the rubbery phase, rubber modification often lowers both the use temperature and the thermoset modulus. n recent years, the feasibility of toughening highly crosslinked networks with high modulus and high Tg thermoplastics has been investigated. Results of thermo- a) part 1: cf. ref.") , Hiithig & Wepf Verlag, Zug CCC /97/$10.OO

2 3268 J. Cui, Y. Yu, W. Chen, S. Li plastic toughening of epoxy resins, such as polysulfone (PSF)3, polyethersulfone (PES)46, poly(ether ether ketone) (PEEK).* and polyetherimide (PEp3), show that the improvement in fracture toughness is achieved without the expense of modulus at elevated temperatures. t has been revealed that a co-continuous two-phase structure is obtained in the binary system of rigid thermosetting resin and ductile thermoplastic polymer. Yamanaka and noue studied the structure development in epoxy resin modified with PES4. They found that the morphology in a multicomponent thermosetting resin can be controlled by the content of thermoplastics and the curing conditions leading to competition between phase separation and the crosslinking reaction. The polyetherimide Ultem 1000 (G. E. Co.) has been successfully used as an effective toughening modifier for epoxy resins15* 16). Li et al. have synthesized a novel polyetherimide and added to epoxy resins. The preliminary study showed modest improvement in mechanical properties compared with the corresponding unmodified epoxy resins17. The results showed that the morphologies of epoxy blends changed from PE spherical domains dispersed in the epoxy matrix to the phase-inversed epoxy domains dispersed in the PE matrix with increasing PE content. This paper concerns structure changes in mixtures of epoxy resins and PE with different molecular weights. The effect of molecular weight of PE on the phase separation of the binary system and the morphology of cured blends is discussed by combining time-resolved light scattering investigations of the phase separation and SEM observations. Experimental part The epoxy oligomer used in this study was a diglycidyl ether of bisphenol A, Epon- 828, supplied by Shanghai Resin Factory. The polyetherimide (PE) was synthesized by one step from bisphenol-a dianhydride (BSA-DA ) and 4,4 -[ 1,4-phenylenebis( 1 -methylethylidene)]bisaniline (BSP) in m-cresol at 200 C for 6 h. The structure is depicted below: With variations of the stoichimetric ratio of BSA-DA and BSP, different molecular weight PE polymers with phenyl termination were obtained. The inherent viscosity of PE polymers was obtained for the concentration of 0.5 g/dl at 30 C in 1-methyl-2-pyrrolidinone as solvent. The different polymers are designated P(rinh), where P represents PE and qinh the inherent viscosity (dug), respectively. The different PE molecules are listed in Tab. 1. The cure agent 4,4 -diaminodiphenyl sulfone (DDS) (Shanghai Third Reagent Factory) was used without further purification. An epoxy blend containing 20 phr* of PE was prepared by dissolving the PE in Epon-828 at 150 C. After a homogeneous, clear solution was obtained, 31 phr of cure *) Parts per hundred resin.

3 Studies on the phase separation of polyetherimide-modified epoxy resin, Tab. 1. Characteristics of the polyetherimide samples P( 0.39) P(0.58) P(0.61) P(0.67) viscosity in g/dl T,in "C agent were added while the mixture was stirred. After the cure agent was dissolved, the blend was rapidly cooled to room temperature in order to maintain the curing reaction at lower extent. The different epoxy blends are designated as E/P(qinh). The specimens for time-resolved light scattering (TRLS) were prepared by solvent casting of a film from a 1,4-dioxane solution (5 wt.-% of blend). The thin film of the blend was dried in vacuum for two days at room temperature to remove the solvent. The phase separation process during curing reaction was observed at real time and in situ on the self-made TRLS with a controllable hot chamber. The TRLS technique was described in detail elsewhere"). The change of the light scattering profiles was recorded at appropriate time intervals during isothermal curing. The morphology of the cured resins was observed with a scanning electron microscope (HTACH S-520). The samples were fractured in liquid nitrogen and some of them were etched with 1,2-dichloroethane before observation. Results and discussion Phase separation observed by light scattering The epoxype blends loaded with DDS are single-phase systems at the curing temperature and showed no appreciable light scattering in the early stages of curing. After a certain time lag a ring pattern appeared implying the development of a regular phase-separated structure. Figs. 1-3 show the changes in the light-scattering profiles with curing of different epoxype blends. The ring patterns and the characteristic changes in scattering profiles indicate that the phase separation of epoxype blends goes through a spinodal decomposition. n Figs. 1-3, the scattering profiles of blends EP(0.39) and EP(0.67) all exhibit one sharp peak and correspond to different scattering vectors, though in the early stage of phase separation another peak appears in the blend EP(0.39). n contrast, there is a broad peak in the scattering profile of blend EP(0.58). A similar situation, which was observed for blend E/P(0.61), is omitted here. t seems that the broad peak is combined with two individual peaks. One is analogous to the peak of blend EP(0.67) and the other to that of blend EP(0.39). Fig. 4 shows the time variation of maximum scattering light intensity of the three blends. t is obvious that the variation of the scattering profiles with time for the blends EP(0.67) and EP(0.39) is different. After a time lag of about 20 min, the light scattering profile of blend EP(0.67) appears at a peak vector qm = 0.55 pm-', increases quickly to the maximum and then levels off. However, the blend EP(0.39) shows the light-scattering profile not until 40 min later and a slow growth of the light profile at a vector qm = 1.50 pm-'. From the view of light-scattering theory, it seems that the phase structure development in these two blend systems is different. As assumed, the variation of the scattering profile of blend

4 3270 J. Cui, Y. Yu, W. Chen, S. Li ?? i!a.- C -$ 40 i E-P(0.39) T=15O0C 20 nm nm nin --A- 47 t& -+SO& -Ap% nm nm rrdn -0- n.5 rrdn --x rrdn q / p m-l Fig. 1. Change of light-scattering profile of blend E/ P(0.39) cured at 150 "C EP10.58) was divided into two curves. One was assigned to the variation of nrofile at a vector qm = 0.55 pm-' and the other at a vector qm sz 1.50 pm-'. nterestingly, the light-scattering profile around qm = 0.55 pm-' of blend EP(0.58) increased as quickly at the early stages of the curing reaction as the profile of blend EP(0.67) did. Subsequently, after reaching the maximum, the light profile decreases while the other light-scattering profile around qm = 1.50 pm-' appears and increases at the same time, similar to that of blend EP(0.39). Based on the above observation, one can estimate that in blend EP(0.59) a second phase separation may take place at later stages of curing and there exist kinds of phase structure in the cured blend. Morphology of cured blends The morphologies of the cured epoxype blends were examined by SEM. Although heterogeneities were observed in all cases, marked changes in morphology were noted. n order to overcome the artificial appearance of fracture surface, the fracture surface was etched with CH2C12. On the etched surface, PE is assumed to be rinsed away so that the remaining material is a crosslinked epoxy-rich phase. The etched surface of blend EP(0.39) (Fig. 5 b) consists of a continuous epoxy matrix with dispersed spherical holes which represent the etched PE particles. This

5 Studies on the phase separation of polyetherimide-modified epoxy resin, B.d 70: B.g ' 50-40: 30 - E/P(O. 5 8) T-150 'C -B- 10 nin -0-30nin -A- 37 nin nin -0-39nin -v- 395 nin -A nin -X-41.5nin ( q/pm-' El P ( ) T=150 'C 43.5 nin nin -A nin 48.5 mill *- 52 nin nin nin -x- 75 rrin -a- 85 mill -0- loo mill Fig. 2. Change of light-scattering profile of blend EP(0.58) cured at 150 C 10 0

6 Cui, Y. Yu, W. Chen, S. Li 3 'El 9c *O 70.- d.g / \?? -.- EP(0.61) T=lSO C 6.7 r& rdn -+--Pnin nin -A- 2 r& nin -0-24min nin -v- 23 nm ---e nim -a- 21 mill * Fig. 3. Change of light-scattering profile of blend E/ P(0.67) cured at 150 C loo fi : qlp m-' / i' e :,+-+/+ -e- El P (0.58) at q,=0.55 pn -' -0-EP (0.58)at q,= 155 pm-' C ~ ~ i t l ~ Fig. 4. Time variation of scattering light intensities at peak vector qm during curing at 150 C

7 Studies on the phase separation of polyetherimide-modified epoxy resin, a) b) Fig. 5. SEM of cured blend EP(0.39): a) fracture surface; b) etched surface typical morphology with PE domains dispersed in epoxy-rich matrix is similar to that of rubber-toughened epoxy resins ). There is obvious co-continuous phase morphology in the blends EP(0.58) and EP(0.61) shown in Figs. 6 and 7, respectively, a) b) Fig. 6. SEM of cured blend EP(0.58): a) fracture surface; b) magnification of etched surface of the bright region of a); c) magnification of etched surface of the smooth region of a)

8 3274 J. Cui, Y. Yu, W. Chen, S. Li in which the former has a smooth surface, while the latter contains smaller particles. Although the smooth, dark region is seemingly homogeneous, after etching the dispersed PE particle holes appear in the phase with a smooth surface (Fig. 6 b), and nearly spherical epoxy particles of 2-3 pm are dispersed and surrounded by the continuous PE thread in the other phase (Fig. 6c). There are two kinds of phase structure of the PE component in the same PE modified epoxy resin system. The same morphology is also displayed in blend EF(0.61) (Fig. 7). Fig. 7. SEM of cured blend EP(0.61): a) etched surface; b) magnification of the phase containing particles of a) a) b) Fig. 8. SEM of cured blend EP(0.67): a) fracture surface; b) etched surface The blend EP(0.67) also shows the co-continuous phase morphology (Fig. 8). The only difference between the blend EP(0.58) and EP(0.67) is that the epoxyrich region is still relatively homogenous without regular PE particles dispersed in it even after etching with CH2C12.

9 Studies on the phase separation of polyetherimide-modified epoxy resin, Based on light-scattering theory, one can estimate the periodic distance in the phase-separated structure as A, = 2x/qm from the peak vector q, of the scattering profile. t is easy to correspond the actual phase structure to the results of light scattering. The variation of light-scattering intensity around qm = 0.50 pm-' indicates the growth of a co-continuous structure with a large periodic distance. Similarly, the variation of light intensity around qm = 1.50 pm-' manifests the formation of a PE dispersed structure. Discussion Thermodynamics, kinetics, and curing rates are competing effects which control the phase separation of modified epoxy system during network formation. Thermodynamic changes provide the primary impetus for the phase separation to occur. However, kinetic factors govern the extent to which the system follows the equilibrium path as the curing progresses. The rates of network formation determine whether thermodynamics or kinetics dominate the final outcome of phase separation. n our previous work2'), the effect of PE molecular weights on the curing rates of epoxype1 blend has been investigated. t was shown that the molecular weight of PE has no great influence on the rates of epoxype blends. The differences in phase separation and morphology of EPE blends are caused by changes of the viscosity of blend systems. As the curing reaction proceeds, the phase separation takes place early in the high molecular weight PE modified epoxy resin because of the poor miscibility of PE with epoxy resin. The occurrence of phase separation is delayed with decreasing inherent viscosity of the PE molecules. According to TRLS observations, the appearance of light profiles at a small scattering vector q, indicates the formation of large domains of epoxy-rich and PE-rich phases. Because the phase separation occurrs at early stages, the difference at the components of the two phases is not great. Considering the non-fixation of the network and the viscosity of the blend system, the second phase separation may take place at later stages of the curing reaction. The appearance of light intensity at large vectors in blends EP(0.58) and EP(0.61) manifests that phase separation also occurs in the former separated phases. Although the light intensity at a small vector q, appears in the EP(0.39) blend at early stages of curing, the lower viscosity of the blend causes the spinodal decomposition to go further and lead to a PE particle dispersion morphology. From this point, the kinetics of the system determines phase-separation and morphology of the blends. According to the above discussion, the phase-separation process of the EPE blends can be described as follows: First, the co-continuous phase structure with the large periodic distance is formed in the 20 phr PE modified epoxy resin. However, the phases dissolve and form the PE particle domain structure in the lower molecular weight PE modified system. n the modest molecular weight PE modified system the phase inversion takes place in the PE-rich phase and the PE particles are formed in the epoxy-rich phase. Several report^^^'^*^^) exist on changes of the morphology of PE (G. E. Ultem- OOO) modified epoxy resins as a function of blend composition. With increasing of

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