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Surfaces and Interphase: Synergistic Strengthening and Toughening the Interphase of Composites by Constructing Alternating "Rigid-and-Soft" Structure on Carbon Fiber Surface (Adv. Keywords: carbon fibers; interface/interphase; polymer-matrix composites; surface treatments Carbon fibers, interface/interphase, polymer-matrix composites, surface treatments. [Extracted from the article]

Interphase with nacre‐like structured multilayer is constructed by alternatively depositing two polymers, respectively "rigid" polydopamine (PDA) and "flexible" polyether amine, on carbon fiber surface via the layer‐by‐layer (LbL) approach. The optimal interfacial strength and toughness are achieved for composites with three layers of PDA/polyether amine, respectively, 39.2% and 99.8% superior to the untreated fiber composites. The outstanding mechanical properties are mainly ascribed to the synergistic interactions of covalent bond, hydrogen bonding, and π–π stacking among fiber, multilayer, and matrix by transferring stress and bridging cracks, as well as the alternating "rigid and flexible" structure of interphase by deflecting crack path and inducing plastic deformation and viscoplastic energy dissipation. This work opens a new, feasible, and easy scalable avenue for constructing integrated high‐performance fiber composites. [ABSTRACT FROM AUTHOR]

This investigation focuses on the use of sizing agent containing gradient curing agent from two directions for carbon fibers coupled with pre-heating treatment before fibers combined into composites to improve interfacial adhesion. Growth in interfacial shear strength (IFSS) up to 92.4 % was noticed for fibers functionalized with sizing agent of high-to-low curing agent from fiber-to-resin (CF H→L), relative to control fibers. This is related to the greatly enhanced interactions at both fiber/sizing and sizing/resin interfaces accompanied by increased interfacial curing degree that generate a stronger interphase. Further optimization of 4.1 % increase in IFSS was achieved for CF H→L via pre-heating treatment before being composites, while larger IFSS increment of 16.7 % after pre-heating treatment was observed for fibers coated with sizing agent of low-to-high curing agent from fiber-to-resin (CF L→H). This reveals that pre-heating is more effective for fiber that has weaker interactions with sizing agent. This work is believed to be a preliminary but crucial trial toward obtaining high-performance fiber composites by simply applying it in the already existed fiber production line. [Display omitted] [ABSTRACT FROM AUTHOR]

• Carbon fiber with 15% stoichiometric curing agent in sizing has the highest IFSS. • 150 °C on fibers with 30% stoichiometric curing agent further increases IFSS. • Variation of IFSSs for fibers without preheating positively correlates with γp/γd. • Variation of IFSSs for fibers with preheating negatively correlates with γp/γd. This paper presents an approach to increase the interfacial shear strength (IFSS) of carbon fiber/epoxy composites by regulating the quantity of chemical bonds at both interfaces of fiber/sizing and sizing/matrix through adding different contents of curing agent (respectively 0, 15%, 30% and 60% of full stoichiometry in epoxy sizing agent) and 150 °C pretreatment on sized fibers. Fibers with 15% stoichiometric curing agent have the maximum IFSS of 122.4 MPa. The reinforcing mechanisms, explored by analyzing the surface physicochemical properties of fibers, the interphase modulus, the wettability and chemical bonding between fibers and matrix, were ascribed to the formation of a gradient interphase in cross-linking density, as well as the maximal polar component of surface energy and the minimal contact angle. After 150 °C pretreatment, fibers with 30% stoichiometric curing agent could form balanced chemical bonds at both fiber/sizing and sizing/matrix interfaces, and thus improve IFSS. Interestingly, variation of IFSSs for untreated sized fibers shows a positive correlation with the ratio of polar to dispersion components, while opposite relationship is observed for 150 °C treated sized fibers. [ABSTRACT FROM AUTHOR]

The liable interfacial delamination of carbon fiber composites is a long-lasting problem that restricts its widely application. One promising route is to hybridize fiber with nano-particles on account of their prominent mechanical properties and huge specific surface area, while trial using biobased nanoparticles are in its infancy. Mussel adhered firmly to the reef relies on its bushy byssal threads, which correlates with chelation of ferric ions with catechol-containing proteins. Inspired by this, we deposited synergistic bio-based polydopamine (PDA)-Fe complex nanospheres and silane coupling agent on carbon fiber by simply impregnation, which delivers increment of 56.4% in interfacial shear strength (IFSS) in epoxy matrix relative to untreated fiber composites. Results suggest that the improved epoxy wetting behavior that imparts full infiltration, the excellent adhesivity that eliminates weak interface layer, as well as strong but reversible PDA-Fe chelate crosslinks that advance tress transfer and energy dissipation, which are critical to IFSS growth. This work provides a new and attractive bio-nanomaterial of PDA-Fe in interfacial engineering of composites. [ABSTRACT FROM AUTHOR]

This paper offers a simple approach to increase the interfacial bonding strength of carbon fiber/epoxy composites by optimizing the degree of chemical interaction among functional groups of fiber surface, sizing agent and resin via heat treatments on carbon fibers. 130 °C treatment on CF1 and 150 °C treatment on CF2 results respectively 71.1% and 15.1% improvements on interfacial shear strength (IFSS), as compared with untreated CF1 and CF2 composites. Variations of IFSS for heat treated carbon fibers correspond well to dispersion component to polar component (γ d /γ p ) tendency of desized heat treated fibers. The work of adhesion between heat treated fibers and resin matrix only reveals close correlation with IFSS trend for CF1/epoxy composites, while not applicable for CF2/epoxy composites. With increasing heat treatment temperature, the decreasing γ d /γ p of desized fibers indicates increasing degree of chemical interaction between fibers and surface sizing, which was also proven by the results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. High degree of chemical interaction between fibers and surface sizing is adverse to the interaction between sizing and resin matrix, and thus lower the IFSS. [ABSTRACT FROM AUTHOR]

Amino-functionalized ZrO 2 was prepared and incorporated on carbon fiber surface by a simple dip-coating approach to simultaneously strengthen and toughen the interphase of carbon fiber/epoxy composite. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetry confirmed the successful functionalization of ZrO 2 by (3-Aminopropyl) triethoxysilane (APTES-ZrO 2 ). Scanning electron microscopy and atomic force microscope demonstrated that uniform coating with well dispersed particles on fiber surface was achieved at 1.0 wt% addition of APTES-ZrO 2 particles. Under this condition, the interfacial shear strength (IFSS) and fracture toughness (G ic ) revealed respective 41.3% and 257.6% augments, compared with epoxy-only coated fiber composite. The enhanced IFSS is attributed to the improved chemical bonds between fiber and resin. While the increase in G ic can be ascribed to the intermittent distribution of strong/weak bonding zones and the mobility of APTES-ZrO 2 particles in the interphase region. This work affords a simple, scalable and cost effective approach to simultaneously increase the interfacial strength and toughness of composites, which has always been sought after for structural materials. [ABSTRACT FROM AUTHOR]

In this paper, an innovative concept of alternating distributed large and small amounts of CNT/polyether amine coating is proposed to be wrapped around the carbon fiber via vacuum filtration. This coating feature is similar to "concave-convex" interval structure, which significantly distinguishes it from widely reported CNT coating with uniform encapsulation. The modified fiber reinforced epoxy composite presents a tremendous enhancement in interfacial bond strength reaching up to 81.8% relative to de-sized fiber composite. This increase comes from the synergistic effects of alternating distribution of strong-weak interactions that effectively dissipating energy, improved wettability, increased mechanical interlocking, as well as pinning and crack suppression effects of CNTs. The concept of interval distribution of large and small levels of coating has more possibilities for optimization. Thus, this unique coating structure, simple and flexible approach provides an attracting candidate for enhancing interphase performance of composites, also with the potential of practical and scalable. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Vacuum filtration is innovatively adopted to modify the circumferential carbon fiber by 2D nano-fillers for the first time. • 37.6% and 68.4% increases in IFSS are achieved for GO and PDA-modified GO wrapped fibers. • Compared to GO, smaller-sized PDA-modified GO with massive nano-protrusions are easily attached on fiber surface. • The work highlights the great possibility to implement vacuum filtration for fiber modification and interphase engineering. Interfacial delamination is an everlasting problem that exists in carbon fiber reinforced polymer composites and restricts their ability to undertake greater shear load. Herein, vacuum filtration technique is first successfully utilized to realize the functionalization of cylindrical carbon fiber by two-dimensional graphene oxide (GO) and polydopamine nano-protrusions modified GO (GO PDA) as a rapid, simple and versatile means to realize the improvement of interfacial adhesion of epoxy composites. Compared to the GO modifier (37.6% augmentation), the GO PDA addition results in higher gain in interfacial shear strength (68.4%) over that of de-sized fiber. This belongs to that the adherence of smaller-sized GO PDA and its nano-protrusion structure facilitate the high surface energy, excellent wetting, mechanical interlocking and covalent linking with epoxy resin. Our research highlights the great possibility to implement vacuum filtration to modify the traditional circumferential fiber and realize the interphase optimization of composites. [ABSTRACT FROM AUTHOR]

Amine-caged ZrO 2 @GO multilayer core-shell hybrids were successfully fabricated and incorporated into epoxy matrix for the first time for enhancing the carbon fiber-matrix interfacial adhesion. Composites with the addition of this hybrid exhibit the highest interfacial shear strength, with 48.4%, 26.7% and 19.5% augmentations over those of untreated, ZrO 2 and GO reinforced composites. The strengthening mechanism is by increasing the wettability of resin to the fiber. Moreover, the outer amino-rich cage facilitates the covalent and π-π stacking interactions at interphase, thus transferring stress effectively. Mutual slippage of middle flexible GO could dissipate energy under shear force. The inner rigid ZrO 2 is apt to restrict the initiation and propagation of cracks. The three synergies work together to contribute the obvious increment in interfacial adhesion. The innovative utilization of such multilayer core-shell structured hybrids as polymer modifier may provide a design strategy for obtaining composites with ideal interfacial bonding. [ABSTRACT FROM AUTHOR]

Attempts to incorporate nanofillers onto the circumferential surface of traditional high-performance fibers via vacuum filtration toward gaining improved interfacial adhesion of corresponding composites are still in its infancy. Herein, we first achieve high-density grafting of carbon nanotube/carbon nanofiber (CNT/CNF) successfully on carbon fiber surface. The treated fibers with depositing positive and negative 5 ml CNT/CNF nanohybrid suspension shows 40.7% (85.4 MPa) gain in interfacial shear strength over that of untreated fiber (60.7 MPa). The reinforcement effect is mainly ascribed to the markedly improved wettability, which provides sufficient hydrogen bonding and pinning effect at interphase zone. Moreover, appropriate amount of CNT/CNF hybrid endows the interphase with gradual modulus transition, minimizing the stress concentration. Our research offers a promising, efficient and green approach for fully utilization of diverse nanofillers and rich interfacial interactions to realize substantial enhancement in interfacial adhesion in the future. [Display omitted] [ABSTRACT FROM AUTHOR]

Conventional carbon fiber reinforced polymer composites are in urgent need to meet the increasing demand for simultaneously strong and tough to be applied as primary force-taking structure in extreme environment. The challenge remains to solve the contradiction between strength and toughness, since an increment in one is usually at the expense of the other. The prospect of interphase structure and properties tuning may be a good solution channel. Here, by constructing alternating sporadic and dense carbon nanotubes (CNTs) around cylindrical fiber via facile, eco-friendly and highly efficient vacuum filtration approach that enables us to gain a similar concave-convex structure, which leads to rough fiber surface and significantly improved wettability of fiber with polymer matrix. The resultant composites reveal respective 69.1% and 159% increase up to 102.5 MPa and 110.6 J/m2 in interfacial shear strength and toughness, greatly exceeding other composites modified by CNTs. The original and innovative strategy of intermittent CNTs encapsulation offers an avenue for obtaining high-performance composites for structural applications. ➢ Intermittent CNTs encapsulation of carbon fiber was fabricated, which obviously differ from uniform coating reported. ➢ Vacuum filtration is innovatively adopted to modify the circumferential surface of carbon fiber for the first time. ➢ 69.1% and 159% increase in interfacial strength and toughness is achieved, greatly exceeding other composites modified by CNTs. ➢ The work offers an avenue for obtaining high-performance composites for structural applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon fiber was innovatively decorated with porous graphene oxide (prGO) for improving IFSS. • 78.64% improvements in IFSS are observed for prGO/fiber hybrid, higher than GO/fiber hybrid. • Compared with GO, prGO has more carboxyl groups and smaller lateral size. • Rich interfacial interactions, higer surface energy, improved wettability and "rigid-soft" transition zone contribute for IFSS increase. The fiber–matrix interphase governs the performance of composites. Herein, porous graphene oxide (prGO) was innovatively employed as active nanoscale reinforcement for the chemical grafting onto the carbon fiber to improve the interfacial adhesion, owning to its large surface area, anti-restacking ability and different chemical functionalities from GO. The obtained prGO/fiber hybrid reinforcement presents 110.4 MPa of interfacial shear strength, respectively 78.64 % and 22.53 % superior to those of untreated fiber (61.8 MPa) and GO/fiber reinforcement (90.1 MPa). The increase is mainly ascribed to the formation of rich interfacial interactions (covalent bonds, electrostatic adherence and hydrogen bonds) in virtue of the synergism of polyethylenimine (PEI), the improved surface energy and the wettability of prGO/fiber with epoxy resin. In addition, a "rigid-soft" transition zone formed by prGO and PEI for stress dissipation also contributes the improvement in interfacial adhesion. The advantages of prGO displayed in this paper provides more access to further gain composites with great interfacial bonding via in-depth and meticulous design of prGO. [ABSTRACT FROM AUTHOR]

Herein, innovative use of vacuum filtration approach to modify the circumferential surface of structural carbon fiber is proposed for improving the interfacial adhesion of composites by constructing a synergistic hydrogen-bond network on fiber surface via the utilization of carbon nanofibers (CNFs), carbon nanotubes (CNTs) and polyvinyl alcohol (PVA). The obtained hybrid reinforcement presents 123.18 MPa of interfacial shear strength, respectively 105.7% and 52.5% exceeding those of untreated fiber and CNF/CNT grafted fiber. Such dramatic improvement can be mainly assigned to the formation of dense hydrogen bonds among the richer hydroxyl groups of CNFs and PVA, and the active sites of CNTs, leading to larger energy consumption at interphase. Meanwhile, CNF/CNT assembled like the spider's web, which could deflect cracks and give rise to the gradual transition of interfacial modulus. This promotes the stress transfer and avoids the stress concentration. The vacuum filtration belongs to a facile, low-cost and eco-friendly strategy, which has been proven successful in obtaining composites with great interfacial adhesion, opening an advanced way for the surface modification of high-performance fibers. [ABSTRACT FROM AUTHOR]

[Display omitted] • Without PDA, longer chain length of polyether amine leads to higher IFSS. • Longer chain favors molecular interlocking and penetration into epoxy matrix. • With PDA, the effect of chain length on IFSS is less obvious. • The complementary interaction mechanisms in improving IFSS is discovered. To understand the complementary mechanisms between molecular interlocking and covalent interconnection that affects interfacial shear strength (IFSS), the adherence of polyether amine with varying chain length onto the surface of carbon fiber in the absence or presence of bio-derived adhesive polydopamine (PDA) as a bridge was subtly designed and studied. Without PDA, up to 25.6% and 34.7% increases in IFSS relative to untreated CF was achieved respectively for shorter and longer molecular chain of polyether amine. The longer chain length encourages molecular interlocking with epoxy molecules and produces a higher penetration depth into epoxy matrix, thus giving a larger increase in IFSS. In the presence of PDA, 39–42% IFSS increase by analogy to CF is observed due to the synergistic effects of hybrid interfacial interactions, the increasing rigidity of interphase and the improved wetting behavior. While, in this case, impact of chain length becomes less noticeable, which is mainly attributed to the trade-off between the enhanced chemical cross-linking points and the less molecular entanglement with resin. [ABSTRACT FROM AUTHOR]

Ultralow interfacial adhesion of pitch-based carbon fiber composites has always a disturbing problem in reality since the fiber surface is of high graphite degree. Herein, a novel strategy for remarkably increasing the interfacial shear strength (IFSS) of composites via incorporating silane coupling agent (APTES)-polydopamine (PDA) -polyether amine (PEA) ternary organic materials on fiber surface is reported. Due to the solid connection of pitch-based carbon fiber with APTES-PDA-PEA through previous carboxylation and hydroxylation, as well as the increased surface energy and improved wettability, a synergistic hybrid network is formed at the interphase, where various interfacial interactions, including covalent bonding, hydrogen bonding, π-π conjugated bonding and intermolecular entanglement, cooperate with each other. The IFSS of modified fiber composites exhibits an amazing 427.7% increase, much higher than publicly reported pitch-based carbon fiber composites (20.3%~235.1%). The findings in this article offer a unique insight on the design of synergistic hybrid bridging transition strategy addressing the weak interphase in pitch-based carbon fiber composites. Image 1 [ABSTRACT FROM AUTHOR]

• Smaller and decumbent GO sheets are adhered on covalent-covalent bonded fibers. • Larger and upright GO sheets are on ionic-ionic bonded fibers. • W ad of hybrid bonding acted fibers is lower than that of same bonding acted fibers. • Same bonding acted fibers have higher IFSSs than hybrid bonding acted fibers. • 48.3% and 38.3% IFSS rise are for covalent-covalent and ionic-ionic bonded fibers. Delicate design and reasonable utilization of interfacial interaction is important for improving interfacial adhesion of composites, however, how exactly the different interfacial interactions and their interaction degree affect the interphase of composites still needs further exploration. Herein, to make the impact of the interactions more prominent, two layers of combined polyether amine/graphene oxide (GO) were built on carbon fiber surface via covalent-covalent, covalent-ionic and ionic-ionic bonding. The effects of different bonding forces on surface physicochemical properties of carbon fiber and on interfacial shear strength (IFSS) of corresponding epoxy composites were compared in-depth. Compared with hybrid bonding acted fibers, same bonding acted fibers have higher IFSSs. Covalent-covalent bonded fibers show 48.3% improvement than untreated fibers due to the strong covalent bonding, more GO content and uniform coverage of smaller and decumbent GO. Ionic-ionic bonded fibers present 38.3% enhancement mainly because of the improved mechanical interlocking by upright GO sheets and abundant ionic and hydrogen bonds. While for covalent-ionic bonded fibers, the weak interface at fiber/surface coating and uneven distribution of GO lead to lower IFSS. This work provides valuable guidance for interphase design in order to obtain high performance composites with good interfacial adhesion. [ABSTRACT FROM AUTHOR]

Inspired by nacre's sophisticated "brick-and-mortar" structure, multilayered polyether amine (PEA)/graphene oxide (GO) hybrid building blocks were incorporated onto the surface of carbon fiber via Layer-by-Layer (LbL) assembly to strengthen and toughen the interphase of composites. Impressive improvements of 67.7% and 129.0% in interfacial strength and toughness over those of untreated fiber composites are achieved for composites with 9 layers of PEA/GO on fiber surface. The synergistic effects stemming from covalent bonds, hydrogen bonds and π-π interaction, as well as crack deflection caused by homogeneous GO layers, improved surface energy and wettability are major reasons for the increase in interfacial adhesion. The major toughening mechanisms are attributed to interfacial micro-cracks, viscoplastic energy dissipation of PEA and GO slippage. This work provides a promising route of bio-inspired structural composites with excellent strength and toughness. Image 1 [ABSTRACT FROM AUTHOR]

A simple anti-sandwich structure that employs strong, stiff and tough graphene oxide as the core layer, flexible polyether amine as the wrapping connection faces is constructed on carbon fiber surface via chemical grafting approach to strengthen and toughen the interphase of epoxy composites. Impressive 94.4% and 48.7% increments in interfacial toughness and interfacial shear strength are achieved in composites with anti-sandwich structure on fiber, by analogy to those of untreated fiber composites. Pertinent strengthening reasons are mainly enhanced surface energy and the hybrid molecular entanglement. Probable toughening mechanisms include crack deflection by GO core layer, appropriate chemical bonds at interphase, plastic deformation of polyether amine and interfacial friction between adjacent layers due to the build of anti-sandwich structure, as well as the localized toughening effect of the matrix. This work provides a promising thought to obtain advanced composites with excellent strength and toughness. [ABSTRACT FROM AUTHOR]