Toughening effects of interleaved nylon veils on glass fabric/low‐styrene‐emission unsaturated polyester resin composites

The effectiveness of using interleaved nylon veils to increase the interlaminar toughness of glass fiber reinforced, low‐styrene emission unsaturated polyester resin composites has been investigated. Samples were manufactured by a hand lay‐up technique followed by compression moulding. Nylon 66 veils were used, with the veil content varying from 0% to 4% by weight. Double cantilever beam, short beam shear, and three point bend tests were performed. The increasing levels of nylon veil content improved the interlaminar toughness of the composites, which was characterized by critical strain energy release rate (G_IC). The maximum G_IC for crack propagation of a nylon interleaved composite increased by almost 170% over the baseline glass fiber reinforced composite. Dynamic Mechanical Analysis revealed an increase in the damping parameter of up to 117%. Image analysis via Digital Image Correlation and Scanning Electron Microscopy revealed increased fiber bridging between adjacent plies as a key reason for these improvements. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41462.     

Preparation and characterization of bismaleimide (N,N′‐bismaleimido‐4,4′‐diphenyl methane)–unsaturated polyester modified epoxy intercrosslinked matrices

An intercrosslinked network of unsaturated polyester–bismaleimide modified epoxy matrix systems was developed. Epoxy systems modified with 10, 20, and 30% (by weight) of unsaturated polyester were made by using epoxy resin and unsaturated polyester with benzoyl peroxide and diaminodiphenylmethane as curing agents. The reaction between unsaturated polyester and epoxy resin was confirmed by IR spectral studies. The unsaturated polyester toughened epoxy systems were further modified with 5, 10, and 15% (by weightt) of bismaleimide (BMI). The matrices, in the form of castings, were characterized for their mechanical properties. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the matrix samples were performed to determine the glass transition temperature (T_g) and thermal degradation temperature of the systems, respectively. Mechanical properties, viz: tensile strength, flexural strength, and plain strain fracture toughness of intercrosslinked epoxy systems, were studied by ASTM methods. Data obtained from mechanical and thermal studies indicated that the introduction of unsaturated polyester into epoxy resin improves toughness but with a reduction in glass transition, whereas the incorporation of bismaleimide into epoxy resin improved both mechanical strength and thermal behavior of epoxy resin. The introduction of bismaleimide into unsaturated polyester‐modified epoxy resin altered thermomechanical properties according to their percentage concentration. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2853–2861, 2002     

Effect of thermoplastic veils on interlaminar fracture toughness of a glass fiber/vinyl ester composite

The effects of two thermoplastic micro‐veils, polyamide (PA) and polyethylene terephthalate (PET) veil, on the interlaminar fracture toughness of a glass fiber/vinyl ester (GF/VE) composite were investigated. The veils incorporated into the composite as interleaving materials were first characterized via scanning electron microscopy (SEM), differential scanning calorimetry (DSC), contact angle and tensile testing in order determine the best candidate as toughening agent for the GF/VE composite. Composite laminates were manufactured by vacuum‐assisted resin infusion process. Double cantilever beam (DCB) testing was performed to investigate the Mode I type interlaminar fracture toughness of the composites, which was characterized by critical strain energy release rate (G _IC). An increased G _IC was obtained by incorporating the PA veil, but it changed negligibly by the addition of the PET veil. The analysis of the composites fracture surface via SEM revealed increased fiber bridging between adjacent plies in the case of PA veil interleaved composites which played a key role in enhancing the Mode I interlaminar fracture toughness. However, the PET veil present in the interlaminar region did not take part in any energy absorbing mechanism during the delamination, thus keeping the G _IC of the composite unaltered. POLYM. COMPOS., 38:2501–2508, 2017. © 2015 Society of Plastics Engineers     

Mode I and Mode II delamination resistance and mechanical properties of woven glass/epoxy–PU IPN composites

The effect of polyurethane on the mechanical properties and Mode I and Mode II interlaminar fracture toughness of glass/epoxy composites were studied. Polyurethanes (PU) synthesized using polyols and toluene diisocyanate were employed as modifier for epoxy resin by forming interpenetrating polymer network. The PU/Epoxy IPN was used as matrix material for GFRP. PU modified epoxy composite laminates having varying PU contents were prepared. The effect of PU content on the mechanical properties like interlaminar fracture toughness (Mode I, G_1c and Mode II, G_IIc), tensile strength, flexural strength, and Izod impact strength were studied. The morphological studies were conducted on the fractured surface of the composite specimen by scanning electron microscopy (SEM). Tensile strength, flexural strength, and impact strength of PU‐modified epoxy composite laminates were found to increase inline with interlaminar fracture toughness (G_1c and G_IIc) with increasing PU content to a certain limit and then it was found to decrease with increase in PU content. It was observed that toughening of epoxy with PU increases the Mode I and Mode II delamination toughness up to 17 and 120% higher than that of untoughened composite specimen, respectively. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Synthesis and characterization of fluorinated poly(etherimide)s toughening for carbon fiber‐reinforced epoxy composites

Novel‐fluorinated poly(etherimide)s (FPEIs) with controlled molecular weights were synthesized and characterized, which were used to toughen epoxy resins (EP/FPEI) and carbon fiber‐reinforced epoxy composites (CF/EP/FPEI). Experimental results indicated that the FPEIs possessed outstanding solubility, thermal, and mechanical properties. The thermally cured EP/FPEI resin showed obviously improved toughness with impact strength of 21.1 kJ/m² and elongation at break of 4.6%, respectively. The EP/FPEI resin also showed outstanding mechanical strength with tensile strength of 91.5 MPa and flexural strength of 141.5 MPa, respectively. The mechanical moduli and thermal property of epoxy resins were not affected by blending with FPEIs. Furthermore, CF/EP/FPEI composite exhibited significantly improved toughness with Mode I interlaminar fracture toughness (G_IC) of 899.4 J/m² and Mode II interlaminar fracture toughness (G_IIC) of 1017.8 J/m², respectively. Flexural properties and interlaminar shear strength of the composite were slightly increased after toughening. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Toughening of cyanate ester resin by N‐phenylmaleimide–N‐(p‐hydroxy)phenyl‐maleimide–styrene terpolymers and their hybrid modifiers

N‐Phenylmaleimide–N‐(p‐hydroxy)phenylmaleimide–styrene terpolymer (HPMS), carrying reactive p‐hydroxyphenyl groups, was prepared and used to improve the toughness of cyanate ester resins. Hybrid modifiers composed of N‐phenylmaleimide–styrene copolymer (PMS) and HPMS were also examined for further improvement in toughness. Balanced properties of the modified resins were obtained by using the hybrid modifiers. The morphology of the modified resins depends on HPMS structure, molecular weight and content, and hybrid modifier compositions. The most effective modification of the cyanate ester resin was attained because of the co‐continuous phase structure of the modified resin. Inclusion of the modifier composed of 10 wt% PMS (M_w 136 000 g mol^?1) and 2.5 wt% HPMS (hydroxyphenyl unit 3 mol%, M_w 15 500 g mol^?1) led to 135% increase in the fracture toughness (K_IC) for the modified resin with a slight loss of flexural strength and retention of flexural modulus and glass transition temperature, compared with the values for the unmodified resin. Furthermore, the effect of the curing conditions on the mechanical and thermal properties of the modified resins was examined. The toughening mechanism is discussed in terms of the morphological and dynamic viscoelastic behaviour of the modified cyanate ester resin system. © 2001 Society of Chemical Industry     

Epoxy‐toughened,unsaturated polyester interpenetrating networks

A series of translucent interpenetrating polymer networks (IPNs) made of a reactive elastomer [linear (D) and branched (T) with varying molecular weights] (Jeffamine?), a commercially available epoxy (D.E.R. 331), and an unsaturated polyester (15:85 wt %) were prepared. DSC data indicated complete cure after 8 h at 90°C. DMTA data showed a single glass‐transition temperature (T_g) for all elastomer‐containing IPNs, an indication of homogeneity. As expected, all IPNs showed a decrease in T_g with incorporation of elastomer, from 16 to 114°C or lower, the largest decrease being with T‐5000. Izod impact strengths were increased by 28–44%, but with no apparent pattern among structure and molecular weight variations. In several cases the standard deviation of impact data increased significantly. Flexural data were measured using a three‐point bend test. The highest flexural modulus obtained was that which incorporated linear D‐2000 with a decrease of only 22% upon incorporation of the elastomer, whereas other compositions dropped up to 55% in flexural modulus. The strongest material obtained was that using D‐2000 with a flexural strength increase of 65% upon incorporation of the elastomer. Two of the three branched elastomer components showed flexural strength increases of about 53%, but one was only equal to the base polyester resin. TGA data were recorded for all IPNs and values compared well to that of the pure polyester resin, with the exception of T‐403, which showed a 20°C decrease, and D‐2000 with a 10°C decrease. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2283–2286, 2002     

Thermomechanical properties of alkali treated jute‐polyester/nanoclay biocomposites fabricated by VARTM process

A systematic study was carried out to investigate the effect of alkali treatment and nanoclay on thermomechanical properties of jute fabric reinforced polyester composites (JPC) fabricated by the vacuum‐assisted resin transfer molding (VARTM) process. Using mechanical mixing and sonication process, 1% and 2% by weight montmorillonite K10 nanoclay were dispersed into B‐440 premium polyester resin to fabricate jute fabric reinforced polyester nanocomposites. The average fiber volume was determined to be around 40% and void fraction was reduced due to the surface treatment as well as nanoclay infusion in these biocomposites. Dynamic mechanical analysis (DMA) revealed enhancement of dynamic elastic/plastic responses and glass transition temperature (T_g) in treated jute polyester composites (TJPC) and nanoclay infused TJPC compared with those of untreated jute polyester composites (UTJPC). Alkali treatment and nanoclay infusion also resulted in enhancement of mechanical properties of JPC. The maximum flexural, compression, and interlaminar shear strength (ILSS) properties were found in the 1 wt % nanoclay infused TJPC. Fourier transform‐infrared spectroscopy (FT‐IR) revealed strong interaction between the organoclay and polyester that resulted in enhanced thermomechanical properties in the composites. Lower water absorption was also observed due to surface treatment and nanoclay infusion in the TJPC. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Toughening of cyanate ester resin by N‐phenylmaleimide–styrene copolymers

The N‐phenylmaleimide–styrene copolymer (PMS) was prepared and used to improve the brittleness of the cyanate ester resin. PMS was an effective modifier for improving the brittleness of the resin. The morphologies of the modified resins depended on PMS molecular weight and content. The most effective modification of the cyanate ester resin was attained because of the cocontinuous phase structure of the modified resin. Inclusion of 10 wt % PMS (M_w 133,000) led to an 160% increase in the fracture toughness (K_IC) for the modified resin with a slight loss of flexural strength and retention of flexural modulus and the glass transition temperature, compared to the values for the unmodified resin. Low water absorptivity of the parent‐cured resin was not deteriorated by modification. The toughening mechanism was discussed in terms of the morphological and dynamic viscoelastic behaviors of the modified cyanate ester resin system. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2931–2939, 1999     

Effect of deposited carbon nanotubes on interlaminar properties of carbon fiber‐reinforced epoxy composites using a developed spraying processing

Carbon fiber‐reinforced epoxy composites, with incorporated carboxylic multiwall carbon nanotubes (CNTs), were prepared using vacuum‐assisted resin infusion (VARI) molding, and the in‐plane and out‐of‐plane properties, including mode‐I (G_Ic) and mode‐II (G_IIc) interlaminar fracture toughness, interlaminar shear strength (ILSS), tensile, and flexural properties were measured. A novel spraying technique, which sprays a kind of epoxy resin E20 with high viscosity after spraying the CNTs, was adopted to deposit the CNTs on the surface of carbon fiber fabric. The E20 was used to anchor CNTs on the fabric surface, avoiding that the deposited CNTs were removed by the infusing resin during VARI process. The spraying processing, including spraying amount and spraying sequence, was optimized based on the distribution of CNTs on the fibers. After that, three composite specimen groups were fabricated using different carbon fiber fabrics, including as‐received, CNT‐deposited with E20, and CNT‐deposited without E20. The effects of CNTs on the processing quality and mechanical properties of carbon fiber‐reinforced polymer composites were studied. The experimental results show that all studied laminates have uniform thickness with designed values and no obvious defects form inside the laminates. Compared with the composite without CNTs, depositing CNTs with E20 increases by 24% in the average propagation G_Ic, by 11% in the propagation G_IIc and by 12% in the ILSS, while it preserves the in‐plane mechanical properties, However, depositing CNTs without E20 reduces interlaminar fracture toughness. These phenomena are attributed to the differences in the distribution of CNTs and the fiber/matrix interfacial bonding for different spraying processing. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers     

Graft‐interpenetrating polymer networks of epoxy with polyurethanes derived from poly(ethyleneterephthalate) waste

Polyester polyurethanes derived from poly(ethyleneterephthalate) (PET) glycolysates were blended with epoxy to form graft‐interpenetrating networks (IPNs) with improved mechanical properties. Microwave‐assisted glycolytic depolymerization of PET was performed in the presence of polyethylene glycols of different molecular weights (600–1500). The resultant hydroxyl terminated polyester was used for synthesis of polyurethane prepolymer which was subsequently reacted with epoxy resin to generate grafted structures. The epoxy‐polyurethane blend was cured with triethylene tetramine under ambient conditions to result in graft IPNs. Blending resulted in an improvement in the mechanical properties, the extent of which was found to be dependant both on the amount as well as molecular weight of PET‐based polyurethane employed. Maximum improvement was observed in epoxy blends prepared with polyurethane (PU1000) at a loading of 10% w/w which resulted in 61% increase in tensile strength and 212% increase in impact strength. The extent of toughening was quantified by flexural studies under single edge notch bending (SENB) mode. In comparison to the unmodified epoxy, the Mode I fracture toughness (K_IC) and fracture energy (G_IC) increased by ～45% and ～184%, respectively. The underlying toughening mechanisms were identified by fractographic analysis, which generated evidence of rubber cavitation, microcracking, and crack path deflection. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40490.     

Mechanical properties of vinylester/glass and polyester/glass composites fabricated by resin transfer molding and hand lay‐up

This paper reports the comparative performance of vinylester/glass and polyester/glass laminates fabricated by resin transfer molding (RTM) and hand lay‐up. A resin transfer mold was designed and fabricated for preparing the laminates. Void content was much lower in Spartan II RTM specimens than in that of hand lay‐up. Ultimate tensile strength, Young's modulus, flexural strength, flexural modulus, impact strength, and interlaminar shear strength of RTM specimens were superior to that of the hand lay‐up for both vinylester/glass and polyester/glass. The improvements for vinylester/glass were 44%, 28%, 88%, 84%, 36%, and 78% for the respective properties. The corresponding improvements for polyester/glass were 21%, 52%, 70%, 74%, 57%, and 82%, respectively. Particle impact erosion rate was lower in RTM specimens than that of the hand lay‐up. J. VINYL ADDIT. TECHNOL. 21:166–173, 2015. © 2014 Society of Plastics Engineers     

Experimental characterization of traditional composites manufactured by vacuum‐assisted resin‐transfer molding

The primary purpose of this study is to investigate the anisotropic behavior of different glass‐fabric‐reinforced polyester composites. Two commonly used types of traditional glass fabrics, woven roving fabric and chopped strand mat, have been used. Composite laminates have been manufactured by the vacuum infusion of polyester resin into the fabrics. The effects of geometric variables on the composite structural integrity and strength are illustrated. Hence, tensile and three‐point‐bending flexural tests have been conducted at different off‐axial angles (0, 45, and 90°) with respect to the longitudinal direction. In this study, an important practical problem with fibrous composites, the interlaminar shear strength as measured in short‐beam shear tests, is discussed. The most significant result deduced from this investigation is the strong correlation between the changes in the interlaminar shear strength values and fiber orientation angle in the case of woven fabric laminates. Extensive photographs of fractured tensile specimens resulting from a variety of uniaxial loading conditions are presented. Another aim of this work is to investigate the interaction between the glass fiber and polyester matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, are interpreted in an attempt to explain the interaction between the glass fiber and polyester. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Toughening of vinylester–urethane hybrid resins through functionalized polymers

Liquid nitrile rubber, hyperbranched polyester, and core/shell rubber particles of various functionality, namely, vinyl, carboxyl, and epoxy, were added up to 20 wt % to a bisphenol‐A‐based vinylester–urethane hybrid (VEUH) resin to improve its toughness. The toughness was characterized by the fracture toughness (K_c) and energy (G_c) determined on compact tensile (CT) specimens at ambient temperature. Toughness improvement in VEUH was mostly achieved when the modifiers reacted with the secondary hydroxyl groups of the bismethacryloxy vinyl ester resin and with the isocyanate of the polyisocyanate compound, instead of participating in the free‐radical crosslinking via styrene copolymerization. Thus, incorporation of carboxyl‐terminated liquid nitrile rubber (CTBN) yielded the highest toughness upgrade with at least a 20 wt % modifier content. It was, however, accompanied by a reduction in both the stiffness and glass transition temperature (T_g) of the VEUH resin. Albeit functionalized (epoxy and vinyl, respectively) hyperbranched polymers were less efficient toughness modifiers than was CTBN, they showed no adverse effect on the stiffness and T_g. Use of core/shell modifiers did not result in toughness improvement. The above changes in the toughness response were traced to the morphology assessed by dynamic mechanical thermal analysis (DMTA) and fractographic inspection of the fracture surface of broken CT specimens. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 672–680, 2002; DOI 10.1002/app.10392     

Hybrid multi‐scale epoxy composites containing conventional glass microfibers and electrospun glass nanofibers with improved mechanical properties

A mechanically flexible mat consisting of structurally amorphous SiO₂ (glass) nanofibers was first prepared by electrospinning followed by pyrolysis under optimized conditions and procedures. Thereafter, two types of hybrid multi‐scale epoxy composites were fabricated via the technique of vacuum assisted resin transfer molding. For the first type of composites, six layers of conventional glass microfiber (GF) fabrics were infused with the epoxy resin containing shortened electrospun glass nanofibers (S‐EGNFs). For the second type of composites, five layers of electrospun glass nanofiber mats (EGNF‐mats) were sandwiched between six layers of conventional GF fabrics followed by the infusion of neat epoxy resin. For comparison, the (conventional) epoxy composites with six layers of GF fabrics alone were also fabricated as the control sample. Incorporation of EGNFs (i.e., S‐EGNFs and EGNF‐mats) into GF/epoxy composites led to significant improvements in mechanical properties, while the EGNF‐mats outperformed S‐EGNFs in the reinforcement of resin‐rich interlaminar regions. The composites reinforced with EGNF‐mats exhibited the highest mechanical properties overall; specifically, the impact absorption energy, interlaminar shear strength, flexural strength, flexural modulus, and work of fracture were (1097.3 ± 48.5) J/m, (42.2 ± 1.4) MPa, (387.1 ± 9.9) MPa, (12.9 ± 1.3) GPa, and (30.6 ± 1.8) kJ/m², corresponding to increases of 34.6%, 104.8%, 65.4%, 33.0%, and 56.1% compared to the control sample. This study suggests that EGNFs (particularly flexible EGNF‐mats) would be an innovative type of nanoscale reinforcement for the development of high‐performance structural composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42731.     

Simultaneous effects of silanized coal fly ash and nano/micro glass fiber on fracture toughness and mechanical properties of carbon fiber‐reinforced vinyl ester resin composites

In this study, the simultaneous effects of both silanized coal fly ash (S‐CFA) and nano/micro glass fiber (nGF) on fracture toughness and mechanical properties of vinyl ester (VE) resin filled with carbon fiber‐based composite materials were investigated. The CFA was treated with (3‐trimethoxysilyl) propyl methacrylate to introduce the methacryloxy groups into the surface of CFA, and was confirmed by using FTIR technique. The nGF and S‐CFA with different weight ratios were well mixed with VE resin by using of high‐speed mechanical stirrer, and ultrasonic technique before using as matrices for fabrication of carbon fiber‐based composite materials via sheet molding compound (SMC) method and hot curing processing. Many characteristics of both cured VE resin composites and carbon fiber‐based composite were examined such as mechanical properties, fracture toughness, and morphology. The results showed that by adding of both 0.1 wt% nGF and 1 wt% S‐CFA into VE resin the tensile strength, tensile modulus, flexural strength, K_IC, impact strength as well as the Mode I interlaminar fracture toughness (G_IC) of VE composites and carbon fiber based composites get optimum values and increased about 61.39%; 39.83%; 36.21%; 103.1%; 81.79%; 48.61%, respectively when compared with pristine materials. POLYM. ENG. SCI., 59:584–591, 2019. © 2018 Society of Plastics Engineers     

Effect of compaction treatment on laminated CFRP composites fabricated by vacuum‐assisted resin‐transfer molding

Carbon fiber‐reinforced polymer (CFRP) composites were fabricated using ordinary and compaction setups (OS and CS, respectively) in the vacuum‐assisted resin‐transfer molding (VARTM) process. The mechanical properties and acoustic emission (AE) spectra of the CFRP composites were compared among fabricated samples. The CFRP plates with sequences of [+30/−30]₆ were sectioned to make specimens for Mode I interlaminar fracture tests and three‐point bending tests. The difference between the material properties and AE characteristics of the OS and CS specimens were statistically compared using one‐way analysis of variance. The OS specimens had a thicker resin layer, a higher resin fraction, larger average fracture toughness, and AE energy corresponding to the Mode I fracture, whereas the CS specimens had more macro‐scale voids and higher bending strength. AE analysis showed that frequency bands in the interlaminar fracture tests corresponding to matrix‐related fracture were dominant for the OS specimens, whereas those corresponding to the mixed fracture mode of the fiber and matrix fracture were dominant for the CS specimens. In the bending tests, mixed fiber‐matrix fractures were dominant for the OS specimens, and fiber‐related fractures were dominant for the CS specimens. In conclusion, the compaction treatment diminished interlaminar fracture toughness, due to the enhanced formation of macro‐scale voids around the fiber bundles during the resin impregnation stage. However, the bending strength improved with an increased fiber volume fraction. POLYM. COMPOS., 38:217–226, 2017. © 2015 Society of Plastics Engineers     

Rubber modification of unsaturated polyester resin with core–shell rubber particles: Effect of shell composition

Poly(butyl acrylate)/poly(vinyl acetate‐co‐methyl methacrylate) PBA/P(VAc‐co‐MMA) core–shell rubber particles with various shell compositions, i.e., VAc/MMA weight ratios, were used to toughen unsaturated polyester. The morphology and surface‐free energy of the rubber particles were determined by transmission electron microscopy (TEM) and contact angle measurements, respectively. The effect of shell structure on the dispersion state of rubber particles inside the unsaturated polyester resin was studied by scanning electron microscopy and TEM. Increasing MMA units in the shell changed the particle dispersion state from small agglomerates or globally well‐dispersed particles to large aggregates in the cured‐resin matrix. For the blends that contain 5 wt% rubber, the highest un‐notched impact toughness, stress‐intensity factor (K_IC), and fracture energy (G_IC) were observed for the blend containing PVAc shell particles. The results showed that by increasing the particle level from 5 to 10 wt%, the highest K_IC and G_IC values were obtained for the blend containing rubber particles with VAc/MMA (80/20 wt/wt) copolymer shell. The crack‐tip damage zone in the neat and rubber‐modified unsaturated polyester resins was observed by means of transmission optical microscopy. In addition, using PVAc shell particles exhibited a minimum reduction in the volume shrinkage and tensile properties of the rubber‐modified resin. POLYM. ENG. SCI., 52:1928–1937, 2012. © 2012 Society of Plastics Engineers     

Mechanical Behavior and Fracture Toughness of Poly(L‐lactic acid)‐Natural Fiber Composites Modified with Hyperbranched Polymers

Summary: The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L ‐lactic acid) (PLLA)‐flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E′) both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan δ) indicated a lowering of the glass transition temperature (T_g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T_g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G_IC) was significantly influenced by HBP. At 10% v/v HBP, G_IC was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber‐matrix interface during crack propagation.

ESEM photograph of propagation region of the interlaminar fracture toughness specimens with 30% v/v of HBP.     

Enhanced fracture toughness of epoxy resins with novel amine‐terminated poly(arylene ether sulfone)–carboxylic‐terminated butadiene‐acrylonitrile–poly(arylene ether sulfone) triblock copolymers

Amine‐terminated poly(arylene ether sulfone)–carboxylic‐terminated butadiene‐acrylonitrile–poly(arylene ether sulfone) (PES‐CTBN‐PES) triblock copolymers with controlled molecular weights of 15,000 (15K) or 20,000 (20K) g/mol were synthesized from amine‐terminated PES oligomer and commercial CTBN rubber (CTBN 1300x13). The copolymers were utilized to modify a diglycidyl ether of bisphenol A epoxy resin by varying the loading from 5 to 40 wt %. The epoxy resins were cured with 4,4′‐diaminodiphenylsulfone and subjected to tests for thermal properties, plane strain fracture toughness (K_IC), flexural properties, and solvent resistance measurements. The fracture surfaces were analyzed with SEM to elucidate the toughening mechanism. The properties of copolymer‐toughened epoxy resins were compared to those of samples modified by PES/CTBN blends, PES oligomer, or CTBN. The PES‐CTBN‐PES copolymer (20K) showed a K_IC of 2.33 MPa m^0.5 at 40 wt % loading while maintaining good flexural properties and chemical resistance. However, the epoxy resin modified with a CTBN/8K PES blend (2:1) exhibited lower K_IC (1.82 MPa m^0.5), lower flexural properties, and poorer thermal properties and solvent resistance compared to the 20K PES‐CTBN‐PES copolymer‐toughened samples. The high fracture toughness with the PES‐CTBN‐PES copolymer is believed to be due to the ductile fracture of the continuous PES‐rich phases, as well as the cavitation of the rubber‐rich phases. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1556–1565, 2002; DOI 10.1002/app.10390