Highly Thermally Conductive Polyimide Composites via Constructing 3D Networks

Alveoli-Mimetic Synergistic Liquid and Solid Thermal Conductive Interface as a Novel Strategy for Designing High-Performance Thermal Interface Materials

Thermal interface materials (TIMs) are in desperate desire with the development of the modern electronic industry. An excellent TIM needs desired comprehensive properties including but not limited to high thermal conductivity, low Yong's modulus, lightweight, as well as low price. However, as is typically the case, those properties are naturally contradictory. To tackle such dilemmas, a strategy of construction high-performance TIM inspired by alveoli is proposed. The material design includes the self-alignment of graphite into 3D interconnected thermally conductive networks by polydimethylsiloxane beads (PBs) -the alveoli; and a small amount of liquid metal (LM) - capillary networks bridging the PBs and graphite network. Through the delicate structural regulation and the synergistic effect of the LM and solid graphite filler, superb thermal conductivity (9.98 ± 0.34 W m-1 K-1) can be achieved. The light emitting diode (LED) application and their performance in the central processing unit (CPU) heat dispersion manifest the TIM developed in the work has stable thermal conductivity for long-term applications. The thermally conductive, soft, and lightweight composites are believed to be high-performance silicone bases TIMs for advanced electronics.

A Novel Hydrophobic Polyimide Film with Sag Structure Derived from Multi-Hybrid Strategy

Polyimide (PI) film with hydrophilic greatly limits their application in the field of microelectronic device packaging. A novel hydrophobic PI film with sag structure and improved mechanical properties is prepared relying on the reaction between anhydride-terminated isocyanate-based polyimide (PIY) containing a seven-membered ring structure and the amino-terminated polyamide acid (PAA) via multi-hybrid strategy, this work named it as hybrid PI film and marked it as PI-PIY-X. PI-PIY-30 showed excellent hydrophobic properties, and the water contact angle could reach to 102°, which is 20% and 55% higher than simply PI film and PIY film, respectively. The water absorption is only 1.02%, with a decrease of 49% and 53% compared with PI and PIY. Due to that the degradation of seven-membered ring and generation of carbon dioxide led to the formation of sag structure, the size of sag structures is ≈16.84 and 534.55 nm for in-plane and out-plane direction, which are observed on surface of PI-PIY-30. Meanwhile, PI-PIY-30 possessed improved mechanical properties, and the tensile strength is 109.08 MPa, with 5% and more than 56% higher than that of pure PI and PIY film, showing greatly application prospects in the field of integrated circuit.

Bio-Based Vitrimeric Silicone Materials with High-Strength, Reprocessable, Healing, and Transparent Properties

Developing reprocessable polymeric materials from earth-abundant elements and renewable biomass is attractive for dealing with fossil resource crisis and achieving sustainable development. Based on the unique reactivity of biomass-derived gluconolactone, polydimethylsiloxane (PDMS) terminated with glucosamide groups is synthesized and used for preparing a series of silicone boronic ester based vitrimers. The whole preparation process is quite straightforward without any purification required and highly efficient with water as the only byproduct. The mechanical properties of obtained vitrimers can be precisely controlled by adjusting the content of 1,4-benzenediboronic acid or the molecular weight of PDMS precursor, producing boronic ester based vitrimers ranging from soft elastomers to rigid plastics. The obtained vitrimers exhibit excellent thermal stability, robust reprocessability, and efficient healing capacity. By encapsulating green-emitting CsPbBr3 nanocrystals, these materials are fabricated into hydrophobic, transparent, and luminescent coatings, promising for applications in flexible optical devices.

Thermal Conductivity of Polyvinylidene Fluoride Films with a Multi-Scale Framework

The orientation of amorphous regions in pure polymers has been noted to be critical to the enhancement of thermal conductivity (TC), but the available reports are still rather few. Here, we propose to prepare a polyvinylidene fluoride (PVDF) film with a multi-scale framework by introducing anisotropic amorphous nanophases in the form of cross-planar alignments among the in-planar oriented extended-chain crystals (ECCs) lamellae, which show an enhanced TC of 1.99 Wm-1 K-1 in the through-plane direction (K⟂) and 4.35 Wm-1 K-1 in the in-plane direction (K∥). Structural characterization determination using scanning electron microscopy and high-resolution synchrotron X-ray scattering showed that shrinking the dimension of the amorphous nanophases can effectively reduce entanglement and lead to alignments formation. Moreover, the thermal anisotropy of the amorphous region is quantitatively discussed with the aid of the two-phase model. Superior thermal dissipation performances are intuitively displayed by means of finite element numerical analysis and heat exchanger applications. Moreover, such unique multi-scale architecture also results in significant benefit in the improvement of dimensional stability and thermal stability. This paper provides a reasonable solution for fabricating inexpensive thermal conducting polymer films from the perspective of practical applications.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

Discotic Liquid Crystal Epoxy Resins Integrating Intrinsic High Thermal Conductivity and Intrinsic Flame Retardancy

The integration of intrinsic thermal conductivity and intrinsic flame retardancy of epoxy resins shows wider application prospects in electricals and electronics. Discotic liquid crystal epoxy (D-LCE) is synthesized from pyrocatechol, 2-allyloxyethanol, and 3-chloroperoxybenzoic acid. P/Si synergistic flame-retardant co-curing agent (DOPO-POSS, DP) is synthesized from p-hydroxybenzaldehyde, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), and amino terminated polysilsesquioxane (POSS). Finally, D-LCE is cured within liquid crystal range with 4, 4'-diaminodiphenyl methane (DDM) and DP, to obtain intrinsic highly thermal conductive/flame-retardant epoxy resins (D-LCER_DP ). D-LCER_DP-10.0 (10.0 wt% DP) synchronously possesses excellent intrinsic thermal conductivity and intrinsic flame retardancy, with thermal conductivity coefficient in vertical and parallel direction (λ_⊥ and λ_∥ ) of 0.34 and 1.30 W m^-1 K^-1 , much higher than that of general bisphenol A epoxy resin (E-51, λ_⊥ of 0.19 W m^-1 K^-1 , λ_∥ of 0.65 W m^-1 K^-1 ). The limiting oxygen index (LOI) value of D-LCER_DP-10.0 reaches 31.1, also better than those of E-51 (19.8) and D-LCER (21.3).

Enhancing thermal conductivity of polyimide composite film by electrostatic self-assembly and two-step synergism of Al2O3 microspheres and BN nanosheets

To improve the perfection of a three-dimensional thermally conductive network in polyimide (PI) composite film and with respect to the economy and simplicity of processing, a strategy of the two-step synergism of Al2O3 microspheres and hexagonal boron nitride (BN) nanosheets was proposed. First, BN nanosheet-coated Al2O3 microspheres (Al2O3@BN) were prepared by electrostatic self-assembly method for the first step of the synergism. Then, the Al2O3@BN&BN/PI composite film containing Al2O3@BN and BN was fabricated by a two-step method for the second step of the synergism, and was systematically characterized. With an optimized mass ratio of 2 : 1 of Al2O3@BN to BN, the thermal conductivity of the 35 wt% Al2O3@BN&BN/PI composite film reached 3.35 W m-1 K-1, and was increased by 1664% compared to that of pure PI. The synergism of the Al2O3 and BN was the most significant in the Al2O3@BN&BN/PI composite film with the thermal conductivity, which was 36.6%, 23% and 22% higher than that of the Al2O3/PI, BN/PI and Al2O3@BN/PI composite films, respectively. The enhancement mechanism of heat conduction was clearly demonstrated. The BN coated on the surface of Al2O3 mainly played a bridging role between the Al2O3 and the BN network, which improved the perfection of the thermally conductive network. The Al2O3@BN segregated the PI matrix to construct the BN network with the typical segregated structure in the composite film, resulting in an efficient thermally conductive network. This work provided a novel strategy for the preparation of conductive polymer composites.

Polydopamine-Modified Al2O3/Polyurethane Composites with Largely Improved Thermal and Mechanical Properties

Alumina/polyurethane composites were prepared via in situ polymerization and used as thermal interface materials (TIMs). The surface of alumina particles was modified using polydopamine (PDA) and then evaluated via Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and Raman spectroscopy (Raman). Scanning electron microscope (SEM) images showed that PDA-Al2O3 has better dispersion in a polyurethane (PU) matrix than Al2O3. Compared with pure PU, the 30 wt% PDA-Al2O3/PU had 95% more Young's modulus, 128% more tensile strength, and 76% more elongation at break than the pure PU. Dynamic mechanical analysis (DMA) results showed that the storage modulus of the 30 wt% PDA-Al2O3/PU composite improved, and the glass transition temperature (Tg) shifted to higher temperatures. The thermal conductivity of the 30 wt% PDA-Al2O3/PU composite increased by 138%. Therefore, the results showed that the prepared PDA-coated alumina can simultaneously improve both the mechanical properties and thermal conductivity of PU.

In situ synthesis of MWCNT-graft-polyimides: thermal stability, mechanical property and thermal conductivity

Herein, MWCNT-graft-polyimides (MWCNT-g-PIs) were prepared by the in situ grafting method. Strengthening the interfacial interaction between MWCNTs and polyimide chains decreased their interfacial thermal resistance (R_C). In contrast to the R_C of 10% MWCNT/PIs, the R_C of 10% MWCNT-g-PI decreased by 16.7%. Hence, MWCNT-g-PIs possessed higher thermal conductivity than MWCNT/polyimides (MWCNT/PIs). Meanwhile, the T_g values of all the samples (MWCNT/PIs and MWCNT-g-PIs) were greater than 399 °C (by DMA). Compared with MWCNT/PIs, 5% and 10% MWCNT-g-PIs showed enhancement in thermal stability in air. The storage modulus retentions were greater than 63% at 200 °C and 45% at 300 °C. Also, 5% and 10% MWCNT-g-PIs maintained the high tensile strength of pure PI, and the tensile modulus increased up to 2.59 GPa on increasing the loading amount of MWCNTs. This study sheds light on improving the thermal conductivity of polyimides effectively at relatively low loadings.

In situ synthesis of MWCNT-graft-polyimides enhanced thermal conductivity at a relatively low loading.

Novolac/Phenol-Containing Phthalonitrile Blends: Curing Characteristics and Composite Mechanical Properties

The phenol-containing phthalonitrile resin is a kind of self-curing phthalonitrile resin with high-temperature resistance and excellent properties. However, the onefold phthalonitrile resin is unattainable to cured completely, and the brittleness of the cured product is non-negligible. This paper focuses on solving the above problems by blending novolac resin into phenol-containing phthalonitrile. Under the action of abundant hydroxyl group, the initial curing temperature and gelation time at 170 °C decrease by 88 °C and 2820 s, respectively, monitored by DSC and rheological analysis. FT-IR spectra of copolymers showed that the addition of novolac increased the conversion rate of nitrile. When the novolac mass fraction is 10%, the peak of nitrile group disappears, which means the complete reaction. The mechanical test of blends composites shows that the maximum fracture strain of 10 wt% novolac addition is 122% higher than those of neat phthalonitrile composites on account of the introduction of flexible novolac chain segments. The mechanical properties are sensitive to elevated post-cured temperature; this is consistent with the result of morphological investigation using SEM. Finally, the dynamic mechanical analysis indicated that the glass transition temperature heightened with the increase of novolac content and post-curing temperature.