Thermal Conductivity of High-Modulus Polymer Fibers

Polymers have many desirable properties for engineering systems–e.g., low mass density, chemical stability, and high strength-to-mass ratio–but applications of polymers in situations where heat transfer is critical are often limited by low thermal conductivity. Here, we leverage the enormous research and development efforts that have been invested in the production of high-modulus polymer fibers to advance understanding of the mechanisms for thermal transport in this class of materials. Time-domain thermoreflectance (TDTR) enables direct measurements of the axial thermal conductivity of a single polymer fiber over a wide temperature range, 80 < <i>T</i> < 600 K. Relaxation of thermoelastic stress in the Al film transducer has to be taken into account in the analysis of the TDTR data when the laser spot size is small because the radial modulus of the fiber is small. This stress relaxation is controlled by the velocity of the zero-order symmetric Lamb mode of a thin Al plate. We find similarly high thermal conductivities of Λ ≈ 20 W m^–1 K^–1 in crystalline polyethylene and liquid crystalline poly­(<i>p</i>-phenylene benzobisoxazole). For both fiber types, Λ­(<i>T</i>) ∝ 1/<i>T</i> near room temperature, suggesting an intrinsic limit to the thermal conductivity governed by anharmonicity, not structural disorder. Because of the high degree of elastic anisotropy, longitudinal acoustic phonons with group velocities directed along fiber axis are likely to be the dominate carriers of heat

Modeling the Effect of Charge Density in the Active Layers of Reverse Osmosis and Nanofiltration Membranes on the Rejection of Arsenic(III) and Potassium Iodide

We used an extended solution-diffusion model that incorporates Donnan electrostatic exclusion of ions and unhindered advection due to imperfections, and measurements of charge density in the polyamide active layers of reverse osmosis (RO) and nanofiltration (NF) membranes, to predict the rejection of a strong electrolyte (i.e., potassium iodide) and a weak acid (i.e., arsenious acid) as a function of the pH of the feed aqueous solution. Predictions of solute rejection were in agreement with experimental data indicating that (i) the extended solution-diffusion model taking into account Donnan exclusion and unhindered advection due to imperfections satisfactorily describes the effect of pH on solute rejection by RO/NF membranes and (ii) measurement of charge density in active layers provides a valuable characterization of RO/NF membranes. Our results and analysis also indicate that independent ions, and not ion pairs, dominate the permeation of salts

Molecular-Weight Dependence of Center-of-Mass Chain Diffusion in Polymerized Ionic Liquid Melts

Polymerized ionic liquids (PILs) with flexible polymer chains and weakly interacting ionic liquid (IL) groups have received great attention for their desirable properties in electrochemical applications such as ionic conductivity. Less is known about their dynamic properties such as center-of-mass chain diffusion and how it depends on molecular weight in the presence of IL groups. In this work, a series of acrylic PILs with imidazolium cations and bis(trifluoromethanesulfonyl)imide (TFSI) anions (TFSI-f-PILN) were synthesized via reversible addition–fragmentation chain-transfer polymerization with degrees of polymerization N ranging from 40 to 236. A fluorescent acrylic monomer with the 7-nitrobenzofurazan group was copolymerized at trace levels as a probe of chain motion, and the diffusion coefficient (D) of TFSI-f-PILN was determined by fluorescence recovery after photo bleaching at Tg + 45 K. Within the uncertainty of 3–20%, a scaling relationship of D ∼ N–2 was observed which is the same as the scaling of linear neutral polymers. Wide-angle X-ray scattering exhibited no peak at ∼5 nm–1, indicating no long-range imidazolium-TFSI ionic correlations. Our results indicate that the molecular weight dependence of center-of-mass diffusion is not affected by electrostatic interactions of IL groups. No transition from a Rouse regime (D ∼ N–1) to reptation regime (D ∼ N–2) was observed within the studied N range

Thermally Functional Liquid Crystal Networks by Magnetic Field Driven Molecular Orientation

Aligned liquid crystal networks were synthesized by photopolymerization of liquid crystal monomers in the presence of magnetic fields. Grazing incident wide-angle X-ray scattering was used to characterize the degree of molecular alignment of mesogen chains and time-domain thermoreflectance was used to measure thermal conductivity. Liquid crystal networks with mesogenic units aligned perpendicular and parallel to the substrate exhibit thermal conductivity of 0.34 W m^–1 K^–1 and 0.22 W m^–1 K^–1, respectively. The thermal conductivity and orientational order of liquid crystal networks vary as a function of temperature. The thermal conductivity of liquid crystal networks can be manipulated by a magnetic field at above the glass transition temperature (65 °C) where the reduced viscosity enables molecular reorientation on the time scale of 10 min

Thermal Conductivity, Heat Capacity, and Elastic Constants of Water-Soluble Polymers and Polymer Blends

We use time-domain thermoreflectance (TDTR), and the generation and detection of longitudinal and surface acoustic waves, to study the thermal conductivity, heat capacity, and elastic properties of thin films of poly­(vinyl alcohol) (PVA), poly­(acrylic acid) (PAA), polyacrylamide (PAM), poly­(vinyl­pyrrolidone) (PVP), methyl cellulose (MC), poly­(4-styrene­sulfonic acid) (PSS), poly­(<i>N</i>-acryloyl­piperidine) (PAP), poly­(methyl methacrylate) (PMMA), and a polymer blend of PVA/PAA. The thermal conductivity of six water-soluble polymers in the dry state varies by a factor of ≈2, from 0.21 to 0.38 W m^–1 K^–1, where the largest values appear among polymers with a high concentration of hydrogen bonding (PAA, PAM, PSS). The longitudinal elastic constants range from 7.4 to 24.5 GPa and scale linearly with the shear elastic constants, suggesting a narrow distribution of Possion’s ratio 0.35 < ν < 0.40. The thermal conductivity increases with the average sound velocity, as expected based on the model of the minimum thermal conductivity. The thermal conductivity of polymer blends of PVA (0.31 W m^–1 K^–1) and PAA (0.37 W m^–1 K^–1) is in agreement with a simple rule of mixtures

Role of Remote Interfacial Phonon (RIP) Scattering in Heat Transport Across Graphene/SiO₂ Interfaces

Heat transfer across interfaces of graphene and polar dielectrics (e.g., SiO₂) could be mediated by direct phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand the relative contribution of each component, we develop a new pump–probe technique called voltage-modulated thermoreflectance (VMTR) to accurately measure the change of interfacial thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-effect transistors and find that the thermal conductance of SiO₂/graphene/SiO₂ interfaces increases by up to Δ<i>G</i> ≈ 0.8 MW m^–2 K^–1 under electrostatic fields of <0.2 V nm^–1. We propose two possible explanations for the small observed Δ<i>G</i>. First, because the applied electrostatic field induces charge carriers in graphene, our VMTR measurements could originate from heat transfer between the charge carriers in graphene and RIPs in SiO₂. Second, the increase in heat conduction could be caused by better conformity of graphene interfaces under electrostatic pressure exerted by the induced charge carriers. Regardless of the origins of the observed Δ<i>G</i>, our VMTR measurements establish an upper limit for heat transfer from unbiased graphene to SiO₂ substrates via RIP scattering; for example, only <2% of the interfacial heat transport is facilitated by RIP scattering even at a carrier concentration of ∼4 × 10¹² cm^–2

Anisotropic Thermal Transport in Thermoelectric Composites of Conjugated Polyelectrolytes/Single-Walled Carbon Nanotubes

We report a method to determine the thermal conductivities of polymer composites with single-walled carbon nanotubes (SWNTs) using time-domain thermoreflectance. Both through-plane and in-plane thermal conductivities were determined. Two types of CPEs used in these studies are of the same conjugated backbone but with either cationic (CPE-PyrBIm₄) or anionic (CPE-Na) pendant functionalities. The CPE-Na/SWNT composites are p-type conductors, whereas the CPE-PyrBIm₄/SWNT counterparts exhibit n-type charge transport. The CPE/SWNT films were prepared through a filtration method that preferentially aligns the SWNTs in the in-plane direction. Attaching the composites onto glass substrates with a precoated heat transducer allows one to measure the through-plane thermal conductivity of materials with rough surfaces. The in-plane thermal conductivity can be measured by embedding thick samples into epoxy followed by microtoming to expose the relatively smooth cross sections. The thermal conductivity along the in-plane direction is found to be higher than that along the through-plane direction. Indeed, the anisotropy factor of thermal conductivity in these composites is approximately an order of magnitude, favoring in-plane direction

Development and Performance Characterization of a Polyamide Nanofiltration Membrane Modified with Covalently Bonded Aramide Dendrimers

A first generation of amine terminated aramide dendrimers (G1-NH₂) was covalently attached to the polyamide (PA) active layer of a commercially available nanofiltration (NF) membrane. Amide bonds between G1-NH₂ and PA free carboxylic groups were formed by activation of the carboxylic groups with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) or 2-chloro-1-methylpyridinium iodide (CMPI), followed by aminolysis. Dendrimer attachment was assessed by indirectly measuring the concentration of carboxylic groups and amine groups before and after membrane modification with RBS using yttrium and tungstate ions (Y³⁺ and WO₄^2‑) as ion probes. RBS analyses showed a decrease in the concentration of carboxylic groups and an increase in amine groups on the membrane active layer, consistent with dendrimers attaching covalently to the active layer. Permeation experiments with Rhodamine WT (R-WT) revealed that the water and solutes permeability decreased after modification with dendrimer G1-NH₂. Water permeability of G1-NH₂ modified membrane decreased by 16–19% using EDC combined with sulfo-<i>N</i>-hydroxysuccinimide (s-NHS), and by 17–33% using CMPI. The permeability of the electrolyte BaCl₂ decreased by 54% after G1-NH₂ modification using EDC/s-NHS and only by 20% using CMPI, the latter consistent with a weaker Donnan exclusion effect. The permeability of the larger solute R-WT decreased by 82% in modified G1-NH₂ membranes when using EDC/s-NHS, and 64% for cross-linking reagent CMPI. Thus, the use of EDC/s-NHS was more favorable because it resulted in higher gains in solute rejection with lower losses in water permeability