Toughening of Epoxy Thermoset with Polystyrene-block-polyglycolic

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Toughening of Epoxy Thermoset with Polystyrene-block-Polyglycolic acid Star Copolymer: Nanostructure - Mechanical Property Correlation Raju Francis, and Deepa K Baby Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5025003 • Publication Date (Web): 27 Oct 2014 Downloaded from http://pubs.acs.org on November 7, 2014

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Industrial & Engineering Chemistry Research

Toughening of Epoxy Thermoset with Polystyrene-block-Polyglycolic acid Star Copolymer: Nanostructure - Mechanical Property Correlation Raju Francis#$* and Deepa K. Baby#$

#

School of Chemical Sciences Mahatma Gandhi University Kottayam - 686560, Kerala, India $

Post Graduate & Research Department of Chemistry St. Joseph’s College, Devagiri Calicut – 673008, Kerala, India

* Tel/Fax: +91 481 2731036. E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract: A novel amphiphilic polystyrene-block-polyglycolic acid three arm star copolymer (PS-b-PGA)3 synthesized was incorporated in epoxy thermoset to show reaction induced microphase separation. The diblock copolymer was prepared by Atom transfer radical polymerization, followed by chain end modification and towards the end ring opening polymerization was utilized to attach the polyglycolic acid moiety. The formation of polystyrene-block-polyglycolic acid was confirmed by using FTIR, GPC and 1HNMR. The microphase separation of block copolymer resulted in the formation of ordered nanodomains in the epoxy thermoset. It was found that the block copolymer in epoxy thermoset displayed reaction induced microphase seperation behavior as evidenced by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The tensile strength and the toughness of the thermoset were increased by the incorporation of PGA which has a high glass transition and melting temperature.

Keywords: Polystyrene-b-Polyglycolic acid star copolymer, Epoxy polymer, Polymer Blend, Micro phase separation


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1. Introduction In the past decades progress has been done to improve the mechanical properties of epoxy thermosets by using suitable modifiers. Among these, amphiphilic block copolymers play a crucial role by forming fine phase separated morphology by either self assembly or reaction induced microphase separation mechanism. According to Bates et.al these nano structures are formed by self assembly strategy.1 More recently, it was reported that ordered or disordered nanostructures in thermosets can be alternatively

accessed

via

reaction-induced

micro-phase

separation

(RIMS)

mechanism.2-6 Formation of nanostructures in thermoset can be controlled by using block copolymer composition, chain architecture and molecular weights.7-11 The formation of nanostructures in thermoset is profound for the significant improvement in mechanical properties which has been called “toughening by nanostructures.”12 In ample literature, the formation of ordered and disordered nanostructures by incorporating linear and star block copolymer in epoxy thermosets were reported. (AB type amphiphilic diblock and/or ABA type triblock copolymer).13-16 Rebizant et al. investigated the ordered nanostructures in epoxy thermosets containing ABC triblock copolymers,e.g., polystyrene-block-polybutadiene block-poly (methyl methacrylate).1718

In 2006 Meng F et. al studied the nanophase separated thermosets from Polystyrene-

PEO block copolymer.19 Serrano et al reported the formation of the ordered nanostructures in epoxy thermosets incorporating the star-shaped block copolymers (i.e.,epoxidized polystyrene-block-polybutadiene block- polystyrene).20 The blends of epoxy resin with polycaprolactone and polyethylene oxide were among the most investigated thermosetting blends and the miscibility of these polymers depends on the curing agents used. The variation in mechanical properties and the differences in miscibility for both anhydride- and amine-cross linked epoxyPolycaprolactone (PCL) blends were studied by Clark et.al.21-22 Chen and Chang reported the immiscibility of the blends when an aromatic amine was used as the curing agent.23 This effect is due to the intra chain specific interaction on the phase behavior. Miscibility and hydrogen bonding interaction between polycaprolactone and epoxy resin were reported immensely in literature.2-3 Shuying Wu et.al recently reported the 3 ACS Paragon Plus Environment


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effect of polycaprolactone as a modifier on the mechanical properties of epoxy thermoset and concluded that with increase in PCL composition, the tensile strength and the elongation at break of the modified epoxy thermoset got decreased.24 The main disadvantage of polycaprolactone is due to the plasticization effect. To overcome this problem, alternative block copolymers have been developed to modify epoxy resin. Unlike PCL, Polyglycolide (PGA) is a tough fibre forming, semicrystalline thermoplastic polymer and has a high melting point (230˚C) and the highest strength of the synthetic biopolymers.25-28 To the best of our knowledge no reports regarding the use of polyglycolic acid as the miscible phase in epoxy thermosets to improve the toughness was explored. Therefore, the modification of epoxy with block copolymer containing PGA appears to be a promising approach for obtaining high-performance engineering polymers with attractive mechanical characteristics. In view of these reports, we synthesized a novel amphiphilic star block copolymer containing polystyrene and polyglycolic acid by knowing that PGA is a tough fibre forming polymer with high glass transition and melting temperature and thereby increase the mechanical properties of the epoxy thermoset by the formation of nanostructures. The block copolymer of polystyrene and PGA was prepared by the ring opening polymerization. The PS sub-chains got phase separated to form nanodomains, while PGA sub-chains were miscible with the epoxy matrix before and after curing. The blends of epoxy resin with star block copolymers were translucent when cured with 4,4’-diaminodiphenylmethane (DDM). The TEM, SEM and AFM were used to study the nanostructures in epoxy matrix formed by reaction induced micro-phase separation. 2. EXPERIMENTAL SECTION 2.1 Materials: The synthesis of hydroxyl terminated star polymer is reported elsewhere.29 Glycolide (GA) and Stannous Octoate from Aldrich chemicals were used as such. The epoxy precursor, diglycidyl ether of bisphenol A (DGEBA) with epoxide equivalent weight of 185-210 was supplied by Atul pvt limited, Mumbai. 4,4’diaminodiphenylmethane (DDM) was also supplied by them. Solvents such as DMF, THF, methanol, toluene and dichloromethane were purified by the procedure presented in Vogel’s text book of practical organic chemistry.


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2.2 Methods 2.2.1 Measurement and Characterization 2.2.1.1Fourier transform infrared spectroscopy (FTIR): FT-IR spectra were recorded on a Bruker instrument (Alpha T) at room temperature. Spectra of both (PS-b-PGA)3 and the epoxy thermoset containing the block copolymer were recorded. The samples of thermosets were granulated and the powder was mixed with KBr pellets to press into the small flakes for measurements. 2.2.1.2 Nuclear magnetic resonance spectroscopy (NMR) The

1

H NMR measurements were carried out on a Bruker 400 MHz NMR

spectrometer. The samples were dissolved in deuterated chloroform and the solutions were measured with tetramethylsilane (TMS) as the internal reference. 2.2.1.3 Gel permeation chromatography (GPC) The molecular weights of the polymer were measured on a shimadzu gel permeation chromatography (GPC) instrument with a PL mixed-B10m column. Polystyrene was used as the standard, and THF was used as the eluent at a flow rate of 1 ml/min at 25°C. 2.2.1.4 Atomic force microscopy (AFM) A dilute chloroform solution of the sample was spread at the air/water interface and was given 30 min for solvent evaporation. This monolayer was then transferred to a freshly peeled mica sheet and was imaged using tapping mode in AFM. The epoxy thermosets were imaged after microtome cutting. The thickness of the specimen section was ca. 70nm. 2.2.1.5 Scanning electron microscope (SEM) SEM study of the prepared graft copolymer was carried out on JSM – 640 Scanning Electron Microscope, JEOL. The dried sample was sputter – coated with platinum using a microscope sputter coater and viewed through the microscope. In order to observe the phase structure of epoxy blends, the samples were fractured under cryogenic condition using liquid nitrogen. The fractured surfaces were coated with platinum of about 100 A°. 2.2.1.6 Transmission electron microscopy (TEM)



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Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010 highresolution transmission electron microscope at an acceleration voltage of 120 kV. The samples were trimmed using a microtome machine. The specimen sections (ca. 70 nm in thickness) were placed in 200 mesh copper grids for observations. 2.2.1.7 Tensile strength measurement Tensile properties were determined according to ASTM D638 using rectangular specimens of length 100mm, width 10mm and thickness 3mm. The measurements were done using a universal testing machine (TINIUS OLSEN H50KT) at a cross head speed of 10mm/min at room temperature. Five samples were used to measure the tensile properties and the average of the five values is taken for the neat epoxy and the modified epoxy. 2.2.1.8 Differential scanning Calorimetry (DSC) The DSC measurements were performed on a Perkin-Elmer differential scanning calorimeter in a dry nitrogen atmosphere. The samples (about 9.0 mg in weight) were first heated to 250 °C and held at this temperature for 3 min to remove the thermal history, followed by quenching to 0 °C. A heating rate of 10 °C/min was used at all cases. The glass transition temperature (Tg) was taken as the midpoint of the heat capacity change. 2.3 Synthesis of Polystyrene-block-Polyglycolic acid three-arm star polymer (PS-bPGA)3 (PS-b-PGA)3 star block copolymer was synthesized by ring opening polymerization of glycolide. (PS-OH)3 macro initiator was prepared by atom transfer radical polymerization followed by chain end modification according to the literature method.2 In a typical experiment, (PS-OH)3 (1g) was added to anhydrous toluene and stirred vigorously in a 500ml round bottom flask. Glycolide (1g) and Stannous Octoate (0.001mg) as the initiator was added. The solution was transferred to an oil bath maintained at a temperature of 120˚C after purging with nitrogen to remove trace amount of oxygen30. The formed water molecules were removed continuously via Dean stark apparatus. After 48 h, a white turbid solution was formed and the solution was filtered and it was decanted to get a white crude product. The obtained powder was


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dissolved in THF, precipitated in methanol, filtered and dried. The polymer obtained was about 1.2g. The molecular weight of the block copolymer determined by means of gel permeation chromatography (GPC) is 8600 with PDI=1.6. 1H NMR (CDCl3) δ ppm: 7.3-6.3 (m, 5H, aromatic), 4.9(s, 2H,-O-CH2-C=O-). IR (KBr): ʋ=1751cm-1(ester group), 1420 cm-1(-CH2 stretch). 2.4 Preparation of Epoxy Thermosets containing (PS-b-PGA)3 star block copolymer The blends were prepared by initially dissolving (PS-b-PGA)3 in epoxy monomer by continuous stirring at 120˚C in a N2 atmosphere. The curing agent DDM was added to the above network under vigorous stirring until homogeneous solution was obtained. The ternary mixture was poured into Teflon mould and cured at 120˚C for 3h followed by 12h at 130˚C to confirm a complete curing reaction. The thermosetting blends containing (PS-b-PGA)3 up to 30 wt % were obtained by this method. 2.5 Preparation of Reference sample of Epoxy The epoxy monomer and DDM, the curing agent was stirred to made a homogenous mixture. The mixture was poured into Teflon mould and cured at 120˚C for 3h followed by 12h at 130˚C to confirm a complete curing reaction.

3. Results and discussion 3.1 Synthesis of (PS-b-PGA)3 star block copolymers The synthetic route for the preparation of star block copolymer was given in scheme .1. The macroinitiator (PS-OH)3 undergo ring opening polymerization with glycolide in the presence of stannous octoate as the catalyst. The 1H NMR spectrum of the polymer with the assignments of resonance signals is presented in Fig 1. The sharp single resonance centered at 5.1 ppm is attributed to methylene protons of PGA and the resonance signals at 7.2 are assigned to aromatic protons of PS block.



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Scheme 1. The synthetic route of (PS-b-PGA)3 star block copolymer

Figure 1. 1H NMR spectra of (PS- b- PGA)3 (a) aliphatic hydrogens, (b) aromatic hydrogens and (c) CH2 group in polyglycolic acid FTIR spectrum of (PS-b-PGA)3 diblock copolymer is presented in Fig 2. The strong absorption appears at 1751 cm-1, indicates the presence of the stretching vibration band 8 ACS Paragon Plus Environment

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of carbonyl groups in PGA block. The bands at 1187 cm-1 are due to the stretching vibrations of C-O-C and C-O bonds. These absorptions indicate the existence of ester structural units in the copolymer. Meanwhile, the relative intensity of aliphatic –CH2 group (1420 cm-1) and carbonyl group within the polymer grew large with increasing content of PGA.

Figure 2. FT-IR traces of (a) (PS-OH)3; (b and c) (PS-b-PGA)3 GPC characterization of block copolymer is presented in Table 1. A clear shift in molar mass from 1843 to 8200 g/mol and the monomodal peak indicate the formation of well defined block copolymer. It can be seen that the NMR molecular weights have weak correlation with GPC molecular weights and are prominent in the case of star block copolymer. Such discrepancies and their reasons are reported in the literature.29 Table 1. Characteristics of (PS-b-PGA)3 Run Compound

Tg (PS)

Mn,NMR (g mol-1)

Mn, SEC(THF) (g mol-1)

PDI

(oC)



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1

(PS-OH)3

85

2015

1843

1.4

2

(PS-b-

77

4889

6725

1.4

75

7200

8200

1.6

PGA)3 3

(PS-bPGA)3

Figure 3. AFM image of (PS-b-PGA)3 spread on a clean mica sheet. (a) Topography and (b) phase images The topography and phase mode of AFM images are presented in Fig 3. Both images showed the phase separation in which polyglycolic acid spread at the surface due to the attraction with water and form the continuous phase and the PS being hydrophobic undergo repulsion to form aggregates and they exist in the form of protrusions.


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Figure 4. FTIR spectra of (a) neat Epoxy,(b) Epoxy+(PS-OH)3 ,(c )Epoxy+10% (PS-b-PGA)3 ,(d) Epoxy+20% (PS-b-PGA)3 , (e) Epoxy+30% (PS-b-PGA)3 cured with DDM FTIR spectra of thermosets cured with DDM (Fig 4) showed different spectral pattern. The miscibility is ascribed to the formation of the intermolecular hydrogen-bonding interactions between aromatic amine cross-linked epoxy and PGA, which is readily evidenced by Fourier transform infrared spectroscopy (FTIR). The broad bands in the region 3500cm-1 were attributed to the stretching vibration of the intermolecular hydroxyls, the width of which reflects the wide distribution of hydrogen-bonded hydroxyl stretching frequencies between epoxy and PGA. A strong shift in CO peak was also observed when the star polymer incorporation was 10%. The shift in peak 11 ACS Paragon Plus Environment


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from 1751 cm-1 to 1743cm-1 can be ascribed to inter molecular hydrogen boding interactions between the carbonyls of PGA and the hydroxyl groups of crosslinked epoxy networks. It is proposed that the formation of the intermolecular hydrogen bonding interactions between the crosslinked epoxy networks and PGA sub-chains is a decisive factor to suppress the macro-phase separation of PS blocks.

Figure 5. SEM micrograph of etched samples of (a) 10% (b) 20% (c) 30 % of (PSb-PGA)3 star block copolymer in epoxy thermoset. 3.2 Microphase Seperation: The SEM micrograph of the fracture surface of the blend containing 10, 20, 30 wt% of (PS-b-PGA)3 is presented in Fig 5. In the SEM experiment, THF was used as the solvent to rinse the block copolymer while the epoxy phase remained unaffected. The spherical holes with size less than 1 micrometer were ascribed to block copolymer whereas the continuous phase was attributed to the epoxy matrix. The spherical-rich particles were dispersed uniformly in the continuous epoxy matrix. This is a typical morphology of reaction-induced phase separation. The exact morphology of the thermoset was difficult to interpret by the SEM data but it clearly shows the formation of uniform nanodomains. It was found that on increasing the composition of block copolymer the number of spherical holes increases which in turn shows the compatibility of the block copolymer with the thermoset.


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Figure 6. HRTEM micrographs of the epoxy thermoset cured with DDM containing 10% of (PS-b- PGA)3. (a) & (b) are two different regions on the sample surface HRTEM micrograph of epoxy thermoset was made for selected samples which provided near supporting evidence for the above SEM result.

Epoxy thermoset

containing 10 % (PS-b- PGA)3 is shown in Fig 6. Due to the difference in electron density of different groups, the bright continuous phase is attributed to the cross-linked epoxy matrix and the spherical micro-domains with the size less than 100 nm in diameter were dispersed in the epoxy matrix ascribed to the PS nano domains. It should be pointed out that even without the OsO4 staining; the TEM micrograph reflects the nanostructure of the thermoset (Fig 6(b)). The compositional inhomogeneity at the nanometer scale revealed by TEM is indeed indicative of the formation of the nanostructure.

.



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Figure 7. AFM images of epoxy thermoset containing 10% of (PS-b-PGA)3 block copolymer (Topography) The AFM micrographs of the epoxy thermosets containing 10 wt % (PS-b-PGA)3 of block copolymer are presented in Figure 7. In the topography image, the light continuous regions represent the crosslinked epoxy networks, which were miscible with the PGA blocks of the diblock copolymer whereas the dark areas correspond to PS rich micro-domains. The thermoset containing 10 wt % of (PS-b-PGA)3 block copolymer were micro separated in the continuous epoxy matrix and the nanodomains depend on the composition of the block copolymer. The ordered nanostructures in the thermoset is due to the star shaped architecture of the block copolymer on the reaction induced phase seperation.


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Figure 8. DSC curve of (a) neat epoxy and (b) epoxy thermoset containing 10% of (PS-b-PGA)3 star block copolymer The DSC curve of neat epoxy and modified epoxy thermoset containing star block polymer is shown in Figure 8. A remarkable broadening appeared in the DSC curve of modified epoxy thermoset incorporating star block copolymer which is due to the microphase separation of polystyrene domains during the curing reaction. The glass transition temperature of neat epoxy has lowered from 156˚C to 124˚C due to the plasticization effect of PGA blocks on the epoxy networks. The miscibility of PGA in epoxy network is further confirmed by the depression in glass transition temperature of the thermoset.



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Figure 9. Photographs showing the transparency of epoxy thermoset incorporated with (a) 10% of Polystyrene three arm star block copolymer and (b) 10% of PGA homopolymer 3.3 Mechanism of Nanophase Seperation: It was necessary to know the miscibility and phase behavior of binary blends of epoxy with PS and PGA before and after curing in order to understand the formation mechanism of the morphological structures in the thermosetting blends of epoxy with (PS-b-PGA)3 diblock copolymer.31-33 It can be noted that PS has very poor miscibility with the epoxy monomer after curing (Figure 9(a)). Whereas the miscibility of PGA before and after curing reaction has been checked and it was found that it is miscible completely with the epoxy monomer (Figure 9(b)). Therefore it is anticipated that by tailoring the block lengths of both polymers results in phase separations having various dimensions. Before curing, all the mixtures of epoxy precursors (viz. DGEBA and the curing agent) and the (PS-b-PGA)3 diblock copolymer are fully miscible and homogeneous, indicating that no macro-phase separation occurred.34 The miscibility of the thermosetting blends was attributed to the formation of the intermolecular hydrogen bonding interactions between the hydroxyl groups of amine cured epoxy and oxygen atoms of PGA blocks. By the intermolecular hydrogen bonding the PGA blocks got extended to the thermoset forming the 16 ACS Paragon Plus Environment

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interconnected networks. After curing due to the cross-linking reaction between epoxy and the curing agent, PS domains got phase separated to form nanostructures. Scheme 2 limelight the formation of nanostructures in epoxy thermoset by reaction induced micro-phase separation.

Scheme 2. Formation of nanostructures in epoxy thermoset by reaction induced micro-phase separation

Figure 10. Digital Camera images showing (A) ductility and (B) transparency of Neat Epoxy and epoxy thermoset containing (a) 0, (b) 10 and (c) 20 % of (PS-bPGA)3 copolymer



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Figure 11. The stress-strain graph of (a) Epoxy thermoset (b) epoxy thermoset containing 10% of (PS-b-PGA)3 copolymer and (c) epoxy thermoset containing 20% of (PS-b-PGA)3 copolymer. 3.4 Mechanical Properties: The optical images of neat epoxy and epoxy thermoset containing block copolymer are shown in Fig 10. The images clearly show the ductility and the translucent nature of the formed thermoset. In Fig 11 the tensile measurements of the epoxy thermosets with and without block copolymer is shown. Toughening of the epoxy thermoset depends mainly on the size and shape of the modifier and also its interaction with the matrix.4, 35 Toughness increases with the size of spherical micelles in nanostructured blends.36 The epoxy miscible blocks have very low Tg value and by incorporating these blocks in epoxy matrix, due to its plasticization effect, the tensile strength of the thermoset decreased considerably.4,5,37 But the glass transition temperature (Tg) and melting temperature (Tm) of polyglycolide is considerably higher when compared to PCL.38 The advantage of using polyglycolide comes in this aspect and it is seen that the tensile strength of the thermoset increased with the incorporation of 10 and 20% of polystyrene-polyglycolide block copolymer. It is found to increase to a value approximately 52MPa and elongation at break also got increased to 51% when compared to neat epoxy (28%) (Fig 11). The stress-strain graph shows an appreciable increase in toughness of the epoxy.


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The chemically different nature of the blocks in the star polymer is responsible for the nanodomain formation. There are two effects operating in our case to raise the toughness of the star-epoxy network. They are; high tensile strength which arises from PGA blocks and uniform nanodomain formation by PS. The PS acts as a plasticizer because of its uniform size and distribution which helps to achieve ~52% strain. The multiple anchoring effects given by the star PGA blocks simultaneously raise the tensile strength and toughness of the epoxy network. It should be noted that plasticizing effect of PCL is comparatively more than PGA by substantially compromising the hydrophilic interaction with the epoxy chains.38,39 Thus with the addition of PS-PGA star block copolymer the mechanical properties of the thermoset is enhanced due to the presence of uniform nanostructures formed by the reaction induced micro-phase separation and high PGA miscibility.

Conclusion The (PS-b-PGA)3 star copolymer showed reaction induced microphase separation when blended with epoxy precursor. This blend gave a translucent film. The SEM, AFM and TEM studies confirmed the formation of nanodomains of PS phase which was formed by hydrophobic aggregation. The PGA block which is highly miscible with the epoxy precursor was completely dispersed in the epoxy matrix after curing. The tensile strength of the formed epoxy thermoset was found to be superior compared to the conventional thermoset. The high toughness of this epoxy thermoset was achieved by the formation of the PS nanodomains in a sea of epoxy-PGA matrix.

Acknowledgements DKB thank CSIR, SERB-DST Govt. of India for project fellowship. The authors very much thank SERB-DST, FIST, UGC and State Government for general funding and facilities. Atul Pvt Lmt, Mumbai, India is also gratefully acknowledged for epoxy precursor.



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References (1) Lipic, P. M.; Bates, F. S.; Hillmyer, M. A. Nano structured Thermosets from SelfAssembled Amphiphilic Block Copolymer Epoxy Resin Mixtures. J. Am. Chem. Soc. 1998, 120, 8963-8970. (2) Williams, R. J. J.; Rozenberg, B. A.; Pascault, J. P. Reaction induced Phase Separation in Modified Thermosetting Polymers. Adv. Polym. Sci. 1997, 128, 95156. (3) Meng, F.; Zheng, S.; Zhang, W.; Li, H.; Liang, Q. Nanostructured Thermosetting Blends of Epoxy Resin and Amphiphilic Poly(ε-caprolactone)-block-polybutadieneblock-poly(ε-caprolactone) Triblock Copolymer. Macromolecules 2006, 39, 711.719 (4) Mijovic, J.; Shen, M.; Sy, J.W.; Mondragon, I. Dynamics and Morphology in Nanostructured Thermoset Network/Block Copolymer Blends during Network Formation. Macromolecules. 2000, 33, 5235-5244.

(5) Grubbs, R. B.; Dean, J. M.; Broz, M. E.; Bates, F. S. Reactive Block Copolymers for Modification of Thermosetting Epoxy. Macromolecules. 2000, 33, 9522-9534. (6) Hernan Garate.; Inaki Mondragon.; Norma B. D’Accorso.; Silvia Goyanes. Exploring Microphase Separation Behavior of Epoxidized Poly(styrene-b isopreneb-styrene) Block Copolymer Inside Thin Epoxy Coatings. Macromolecules 2013, 46, 2182-2187. (7) Ritzenthaler, S.; Court, F.; Girard-Reydet, E.; Leibler, L.; Pascault, J. P. ABC Triblock Copolymers/Epoxy−Diamine Blends. 2. Parameters Controlling the Morphologies and Properties. Macromolecules 2003, 36, 118-121. (8) Dean, J. M.; Verghese, N. E.; Pham, H. Q.; Bates, F. S. Nanostructure Toughened Epoxy Resins Macromolecules 2003, 36, 9267-9270. (9) Zucchi, I. A.; Galante, M. J.; Williams, R. J. J. Comparison of morphologies and mechanical properties of crosslinked epoxies modified by polystyrene and poly(methyl methacrylate)) or by the corresponding block copolymer polystyreneb- poly methyl methacrylate. Polymer 2005, 46, 2603-2609. 20 ACS Paragon Plus Environment

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(10) Maiez-Tribut, S.; Pascault, J.P.; Soule, E.R.; Borrajo, J.; Williams, R.J.J. Nanostructured Epoxies Based on the Self Assembly of Block Copolymers: A New Miscible Block That Can Be Tailored to Different Epoxy Formulations. Macromolecules 2007, 40, 1268-1273. (11) Raita Goseki.; Akira Hirao.; Masa-aki Kakimoto.; Teruaki Hayakawa. Cylindrical Nanostructure of Rigid- Rod POSS-Containing Polymethacrylate from a StarBranched Block Copolymer. ACS Macro Lett. 2013, 2, 625-629. (12) Ruiz-Perez, L.; Royston, G. J.; Fairclough, J. P.; Ryan, A. J. Toughening by nanostructure. Polymer 2008, 49, 4475-4488. (13) Lei Wang.; Chongyin Zhang.; Houluo Cong.; Lei Li.; Sixun Zheng. Nanostructures and Surface Hydrophobicity of Self-Assembled Thermosets Involving Epoxy Resin and Poly(2,2,2-trifluoroethyl acrylate)-block-Poly(ethylene oxide) Amphiphilic Diblock Copolymer. J. Phys. Chem. B. 2013, 117, 8256-8268. (14) Rentong, Yu.; Sixun Zheng.; Xiuhong, Li.; Jie Wang. Reaction-Induced Microphase Separation in Epoxy Thermosets Containing Block Copolymers Composed of Polystyrene and Poly(ε-caprolactone): Influence of Copolymer Architectures on Formation of Nanophases. Macromolecules 2012, 45, 9155-9168. (15) Di Hu.; Chongyin Zhang.; Rentong Yu.; Lei Wang.; Sixun Zheng. Self-organized thermosets involving epoxy and poly(ε-caprolactone)-block-poly(ethylene-coethylethylene)-block-poly(ε-caprolactone)

amphiphilic

triblock

copolymer.

Polymer 2010, 51, 6047-6057. (16) Lodge, T. P. Block Copolymers: Past Successes and Future Challenges. Macromol. Chem. Phys. 2003, 204, 265-273. (17) Rebizant, V.; Venet, A. S.; Tournillhac, F.; Girard-Reydet E.; Navarro, C.; Pascault, J. P.; Leibler, L. Chemistry and Mechanical Properties of Epoxy-Based Thermosets Reinforced by Reactive and Nonreactive SBMX Block Copolymers. Macromolecules 2004, 37, 8017-8027. (18) Rebizant, V.; Abetz, V.; Tournihac, T.; Court, F.; Leibler. L. Reactive Tetrablock Copolymers Containing Glycidyl Methacrylate. Synthesis and Morphology Control in Epoxy−Amine Networks. Macromolecules 2003, 36, 9889-9896.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

(19) Meng, F.; Zheng, S.; Li, H.; Liang, Q.; Liu, T. Formation of Ordered Nanostructures in Epoxy Thermosets: A Mechanism of Reaction-Induced Microphase Separation. Macromolecules 2006, 39, 5072-5080. (20) Serrano, E.; Larranaga, M.; Remiro, P. M.; Mondragon, I. Nanostructured Thermosetting Systems from Epoxidized Styrene Butadiene Block Copolymers Macromol. Rapid Commun. 2005, 26, 982-985. (21) Clark, J. N.; Daly, J. H.; Garton, A. Hydrogen bonding in epoxy resin/poly(εcaprolactone) blends. J Appl Polym Sci. 1984, 29, 3381-3390. (22) Hameed, N.; Guo, Q. Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(ε-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid). Polymer 2008, 49, 5268-5275. (23) Chen, J. L.; Chang, F. C. Phase separation process in poly(ε-caprolactone)-epoxy Blends. Macromolecules 1999, 32, 5348-5356. (24) Shuying- Wu.; Qipeng- Guo.; Shuhua- Peng.; Nishar- Hameed.; Martin- Kraska.; Bernd-Stuhn.; Yiu- Wing Mai. Toughening epoxy thermosets with block ionomer complexes: a nanostructure-mechanical property correlation. Macromolecules 2012, 45, 3829-3840. (25) Israel Engelberg.; Joachim Kohn. Physico-mechanical properties of degradable polymers used in medical applications: A comparative study. Biomaterials 1991, 1991 12 , 292. (26) Park K. E.; Kang H. K.; Lee S. J.; Min B. M.; Park W. H. Biomimetic nanofibrous scaffolds: Preparation and characterization of PGA/chitin blend nanofibers.

Biomacromolecules 2006, 2006 7, 635-643. (27) Boland E. D.; Wnek G. E.; Simpson D. G.; Pawlowski K. J.; Bowlin G. L. Tailoring tissue engineering scaffolds using electrostatic processing techniques: a study of poly(glycolic acid) electrospinning. J. Macromol. Sci. 2001, 2001 38, 1231-1243. (28) Aghdam R. M.; Najarian S.; Shakhesi S.; Khanlari S.; Shaaban K.; Sharifi S. Investigating the effect of PGA on physical and mechanical properties of electrospun PCL/PGA blend nanofibers. J. Appl. Polym. Sci. 2012, 2012 124, 123129. 22 ACS Paragon Plus Environment

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Francis, R.; Taton, D.; Logan, J. L.; Masse, P; Gnanou, Y.; Duran, R. S. Synthesis and Surface Properties of Amphiphilic Star-Shaped and Dendrimer-like Copolymers Based on

Polystyrene Core and Poly(ethylene oxide) Corona

Macromolecules 2003, 36, 8253-8259. (30) Odile Dechy-Cabaret.; Blanca Martin-Vaca.; Didier Bourissou. Controlled RingOpening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 61476176. (31) Luo, X.; Zheng, S.; Zhang, N.; Ma, D. Miscibility of epoxy resins/poly (ethylene oxide)blends cured with phthalic anhydride. Polymer 1994, 35, 2619-2623. (32) Ni, Y.; Zheng, S. Influence of intramolecular specific interactions on phase behavior of epoxy resin and poly(ε-caprolactone) blends cured with aromatic amines. Polymer. 2005, 46, 5828-5839. (33) Yin, M.; Zheng, S. Ternary Thermosetting Blends of Epoxy Resin, Poly(ethylene oxide) and Poly(ε-caprolactone). Macromol Chem Phys. 2005, 206, 929-937. (34) Horng, T. J.; Woo, E. M. Effects of Network Segment Structure on the Phase Homogeneity of Crosslinked Poly(ethylene oxide)/Epoxy Networks. Polymer 1998, 39, 4115-4122. (35) Yonathan S. Thio.; Junxian Wu.; Frank S. Bates. Epoxy Toughening Using Low Molecular

Weight

Poly(hexylene

oxide)-Poly(ethylene

oxide)

Diblock

Copolymers. Macromolecules. 2006, 39, 7187-7189. (36) Thio, Y. S.; Wu, J. X.; Bates, F. S. The Role of inclusion size in toughening of epoxy resins by spherical micelles. J. Polym. Sci., Part B: Polym.Phys. 2009, 47, 1125-1129. (37) Nishar Hameed.; Qipeng Guo.; Zhiguang Xu.; Tracey L Hanley.; Yiu-Wing Mai. Reactive block copolymer modified thermosets: highly ordered nanostructures and improved properties. SoftMatter 2010, 6, 6119-6129. (38) Shayla S. Spearman.; Iris V. Rivero.; Noureddine Abidi. Influence of Polycaprolactone/Polyglycolide Blended Electrospun Fibers on the Morphology and Mechanical Properties of Polycaprolactone, J. Appl. Polym. Sci. 2014, 131, 40224-40227.



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(39) Moorea T.; Adhikaria, R.; Gunatillake. Chemosynthesis of bioresorbable poly( gbutyrolactone) by ring-opening polymerisation: a review, Biomaterials 2005, 26, 3771-3782.


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Toughening of Epoxy Thermoset with Polystyrene-block-polyglycolic

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