In situ reinforced thermoplastic resin composites

2022-07-29
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Research progress of thermotropic liquid crystalline polymer in situ reinforced thermoplastic resin composites

1 preface

thermotropic liquid crystalline polymer (TLCP) is a new polymer material with high modulus, high strength, unique fluidity and stability in liquid crystalline state. In 1987, kiss used thermotropic liquid crystal polymer and thermoplastic resin (TP) to blend, and found that thermotropic liquid crystal polymer could form fibers in situ in thermoplastic resin during processing, which improved the mechanical properties of resin matrix, thus putting forward the concept of "in situ composite" for the first time. Compared with the traditional short fiber reinforced composites such as glass fiber and carbon fiber, this in-situ composite has the advantages of easy processing, low energy consumption, recyclable processing and obvious reinforcement effect. It has become a hot topic in the field of material research in recent years

thermotropic liquid crystal polymer in-situ composite is a kind of self reinforced micro composite material, which is formed when the main chain thermotropic liquid crystal polymer and TP blend are extruded or injection molded after melting, the dispersed phase TLCP in the system is oriented to form a microfibril structure under the action of appropriate stress, and is effectively frozen or preserved in the TP matrix. It originated from the study of rheology, microstructure and mechanical properties of tlcp/tp blends. At that time, it was found that adding a small amount of TLCP to TP could reduce the melt viscosity and make the viscosity of the system show negative deviation behavior, that is, TLCP could be used as a processing aid for thermoplastic resin, which would be very beneficial to improve the processing of difficult to process thermoplastic engineering plastics such as polyethersulfone (PES), which would have a market of tens of billions of dollars in the future. Kiss noticed that using TLCP can not only improve the processability, but also improve the mechanical properties of flexible coil polymers at a low cost, and obtain TLCP self reinforced in-situ composites. Because the TLCP microfibrils, the reinforcing phase of the composite, are formed in situ (or in situ) during processing, their diameters are between submicron and nano, which are far smaller than the diameters of macro fibers, and for the ideal in-situ composite, their length diameter ratio can reach 400 or even higher. Moreover, the interface area between the reinforcement phase and the matrix phase is very large. Therefore, we believe that the emergence of in-situ composites can not only give full play to the advantages of short fiber reinforced TP and avoid their shortcomings, but also have the advantages of easy processing, small equipment wear and low preparation cost. It can also provide a new way and method for people to obtain advanced composites with high modulus, high strength and easy processing. Since kiss put forward the concept of in-situ composites, remarkable progress has been made in the study of TLCP reinforced TP micro composites. Especially in recent years, as a new engineering material, it has attracted many scientists and engineers' strong interest in research and development, and has developed into an important research direction of composite materials []

2 tlcp/tp in-situ enhancement mechanism and fiber forming conditions

2.1 enhancement mechanism

from the structural characteristics of polymer liquid crystal, polymer liquid crystal droplets are similar to lotion in the melting process. When the static rigidity is good, the dispersed phase will maintain the shape of spherical particles; When flowing under the action of shear force and tension, the polymer liquid crystal droplets can produce maximum deformation without rupture. Therefore, fibers with large aspect ratio are formed in the polymer matrix. The diameter of this kind of fiber is one order of magnitude smaller than that of common short fibers, such as GF and CF, and has a relatively large specific surface area. It can be uniformly wrapped in the matrix to form a reinforced "skeleton", which overcomes the shortcomings of common short fibers such as uneven mixing with the resin matrix, poor compatibility, interface defects, easy delamination, etc

2.2 fiber forming conditions

the viscosity ratio of the blend is an important factor for TLCP to form microfibrils in the matrix. Generally speaking, η Dispersed phase/η When the continuous phase> 1, the dispersed phase is difficult to deform; When η Dispersed phase/η When the continuous phase is less than 1, the dispersed phase can deform greatly to form microfibrils. SEO et al. Found that when studying tlcp/pa66 composites, when the melt viscosity of TLCP is lower than that of PA66, TLCP can form a good self reinforced microfiber structure in the matrix. There are many factors that affect the viscosity of blends, such as the composition of blends, the concentration of dispersed phase, the adhesion of blends, interfacial tension, processing factors (tensile action, shear action, processing temperature, etc.) and the comprehensive effect of the above factors

3 interfacial compatibility

generally speaking, the interfacial compatibility of different TP matrices and different TLCP materials varies greatly, but they are basically incompatible with high viscosity or weak compatibility. Even though TLCP forms good microfibrils in the matrix, the poor compatibility between the two results in blocked dispersion of TLCP humidification gas, uneven dispersion of strong phase in the matrix resin and low interfacial adhesion, This is very disadvantageous to improve the mechanical properties of in-situ composites. In recent years, how to improve the compatibility between TLCP and TP matrix and improve the macroscopic properties of in-situ composites has become a research hotspot of in-situ composites

for a blend system, there are many compatibilization methods. For example, ionomer compatibilization can effectively enhance the interface interaction; Copolymers with functional groups were used as reactive compatibilizers to provide functional groups for interfacial reactions; Non reactive compatibilizer components with similar chain structure or thermodynamic compatibility with the matrix polymer are selected. This non reactive compatibilizer acts as an emulsifier in the blend system, so as to achieve the compatibility effect. Based on the recent literatures, the compatibilization technologies of in-situ composites can be divided into the following categories: (1) introducing the third component with compatibilization effect; (2) Introducing the same or similar groups as TP matrix into the main chain of dispersed liquid crystal molecules; (3) Transesterification reaction; (4) Multicomponent blending

the introduction of the third component with compatibilization is a widely used method to improve the interfacial compatibility of blends []. For example, in liquid crystal/thermoplastic resin blends, maleic anhydride grafted elastomer is an effective compatibilizer. Polymers grafted with maleic acid mainly include EPR, EPDM, PE, PP, ABS, SEBS, etc. [12]. The anhydride group in the maleic acid complex can play a compatible role with the polar group in the liquid crystal polymer or resin through the interaction of chemical bond and van der Waals force, mainly improving the impact strength, tensile strength, penetration strength and crystallization properties of the blends

zhang et al. Synthesized psf-g-mah copolymer as the interfacial compatibilizer of psf/tlcp (VB) blend. Dsr-ftir, TGA, XSP and other analysis tests showed that psf-g-mah and TLCP had chemical interaction at the interface of the blend to form psf-vbh copolymer, which effectively improved the interfacial compatibility of the blend

When studying pc/pbt/tlcp ternary blends, chin et al. Found that the epoxy resin containing functional groups has a good interfacial compatibility effect on the interface of the above blends. It can react with the active selection of the blend components at the interface, so as to reduce the interfacial energy and achieve the compatibility effect. In addition, some epoxy compounds and polymers can also interact with some polymers such as polyacetic acid, PC, PA, PPO, etc., and can be used as interfacial compatibilizers for these polymer blends

the interfacial compatibility of blends can also be enhanced by molecular cross-linking reactions within the blends, such as vinegar exchange reaction. The results showed that the vinegar exchange reaction at the interface of the blends was closely related to the blending temperature, time, pretreatment, two-phase viscosity ratio and the selection of appropriate catalysts. Tovar studied pc/tlcp (VA) blends and found that vinegar exchange reaction occurred at the interface of PC and TLCP when the blending temperature reached the highest point. SEM results showed that the size of the dispersed phase was reduced by the vinegar exchange reaction at the interface

broadly speaking, the compatibility of polymer blends can be divided into two types: Reactive Compatibility and non reactive compatibility. Most of the examples described above belong to reactive compatibility, that is, the molecular chain of the selected compatibilizer or the third component contains active functional groups that can react with the functional groups on the thermoplastic matrix/liquid crystal molecular chain, and chemical reactions occur during the blending process to achieve the purpose of compatibility, for example, the molecular chain contains a COOH, -oh, -nh2 and other functional groups; Non reactive compatibility refers to the compatibility in the form of a/a-b/b or a/a-c/b, in which component C is a polymer miscible with a or B

4 crystallization and melting behavior

some researchers have found that [], there is an "effective nucleation" phenomenon in thermotropic liquid crystal/thermoplastic blends and blends with low content of liquid crystal, that is, a small amount of liquid crystal acts as heterogeneous nucleating agent in the crystallization process of the matrix, inducing heterogeneous crystallization of TP matrix, so that the blends have higher crystallization temperature, crystallinity and perfect crystallization morphology. When the content of TLCP is high, the thermal stability of the blends is improved. The high content of LCP prevents the cracks caused by the flow deformation of the matrix and improves the mechanical properties

a crystalline polymer, such as PA66, is blended with an amorphous polymer, such as LCP. The melting point of PA66 will decrease to a certain extent under the influence of LCP. Tiong et al. Studied the melting behavior of pa66/lcp blends. DSC results showed that the melting point of the blends moved to low temperature with the increase of LCP content, and the crystallization type also changed. The reasons for the decrease of TM are as follows: (1) the introduced amorphous polymer component has a dilution effect on the crystalline component, that is, thermodynamic reasons; (2) The introduction of amorphous components causes defects in the crystalline structure of crystalline polymers, or decreases the thickness of lamellae, which is the cause of morphology

5 processing rheology

tlcp can act as a processing aid when blending with TP matrix. Because the liquid crystal phase can be preferentially oriented along the flow direction during processing, the liquid crystal micro regions slip each other and are not easy to entangle, which can effectively reduce the melt viscosity [3]. The rheological behavior of liquid crystal/thermoplastic composites is closely related to their processing properties and macro mechanical properties, so it has attracted the attention of scholars. The flow behavior of liquid crystal polymer is complex, and it shows nonlinear viscoelasticity. During the flow process of the material, the liquid crystal as the dispersed phase breaks, and the size of the micro region becomes smaller, which makes the dispersed phase have a larger surface area

liquid crystalline polymer molecules have high order and shear thinning behavior in the molten state, which is very beneficial for the processing of tlcp/tp blends and largely depends on tensile flow

6 mechanical properties

there is no definite rule for the mechanical properties of in-situ composites, and different systems show different behaviors. Most studies have concluded that the finer the fiber, the larger the aspect ratio, the narrower the diameter distribution, the better the dispersion in the matrix, and the better the mechanical properties of its in-situ composites. Therefore, the mechanical properties of in-situ composites largely depend on the properties of TLCP 44T P matrix, TLCP concentration in the matrix, processing conditions and other comprehensive factors. In the actual processing process, injection temperature, injection speed, pressure, nozzle temperature, plasticizing time, mold temperature and other factors can affect the viscosity ratio of the blend fluid, the dispersion morphology of the dispersed phase, and ultimately affect the mechanical properties of the material. Ophir

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