Session L34: Molecular Design of Polymers: Structure, Mechanics and Thermal Properties

Tunable thermal transport and reversible thermal conductivity switching in topologically networked bio-inspired materials

The dynamic control of thermal transport properties in solids must contend with the fact that phonons are inherently broadband. Thus, efforts to create reversible thermal conductivity switches have resulted in only modest on/off ratios, since only a relatively narrow portion of the phononic spectrum is impacted. Here, we report on the ability to modulate the thermal conductivity of topologically networked materials by nearly a factor of four following hydration, through manipulation of the displacement amplitude of atomic vibrations. By varying the network topology, or crosslinked structure, of squid ring teeth-based bio-polymers through tandem-repetition of DNA sequences, we show that this thermal switching ratio can be directly programmed. This on/off ratio in thermal conductivity switching is over a factor of three larger than the current state-of-the-art thermal switch, offering the possibility of engineering thermally conductive biological materials with dynamic responsivity to heat.   

A Thermal Resistance Network Model for Heat Conduction of Amorphous Polymers

Thermal conductivities (TCs) of the vast majority of amorphous polymers are in a very narrow range, 0.1 ~ 0.5 W m^-1 K^-1, although single polymer chains possess TC of orders-of-magnitude higher. Entanglement of polymer chains plays an important role in determining the TC of bulk polymers. We propose a thermal resistance network (TRN) model for TC in amorphous polymers taking into account the entanglement of molecular chains. Our model explains well the physical origin of universally low TC observed in amorphous polymers. The empirical formulae of pressure and temperature dependence of TC can be successfully reproduced from our model not only in solid polymers but also in polymer melts. We further quantitatively explain the anisotropic TC in oriented polymers.

  

Improving ductility of glassy semicrystalline polymers by pre-deformation

Having developed some elementary understanding of how glassy polymers gain ductility [1, 2], we have begun to explore molecular mechanics of semicrystalline polymers whose Tg > ambient, to which poly(L-lactic acid) (PLLA), poly(ethylene terephthalate) (PET), and syndiotactic polystyrene (sPS) belong. Our objective is to explain why crystallization can turn a ductile glassy polymer to a brittle material, as is the case for PLLA [3]. Based on PET and s-PS, the present study will show how the ductility of such polymers can be predicted to improve upon introducing predeformation effect that alters the morphology and network structure. In doing so, we are able to further advance our theoretical understanding of factors that influence polymer ductility.

[1] Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. “A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses”, J. Chem. Phys. 141 (2014), 094905.
[2] Masoud Razavi, Shiwang Cheng, Da Huang, Shufan Zhang and Shi-Qing Wang, “Crazing and yielding in glassy polymers of high molecular weight, manuscript in preparation for Polymer”.
[3] Masoud Razavi and Shi-Qing Wang, “Why Is Crystalline Poly(lactic acid) Brittle at Room Temperature?”, Macromolecules, 2019, 52, 5429.
  

Low, high, and switchable thermal conductivity in soft materials

A century of experiment and theory have produced a thorough understanding of heat conduction by phonons in simple inorganic crystals. By contrast, basic understanding of heat conduction by molecular vibrations in soft materials (amorphous and crystalline polymers, small molecule solids, biological materials) is much less mature. Complex, non-periodic structures spanning multiple length scales are difficult to characterize and model. Low thermal conductivity, fiber morphologies, poor control of defects, and anisotropy created by molecular order create daunting challenges for experiment. I will discuss our past work on the thermal conductivity and elastic constants of a wide variety of polymeric materials in the form of thin films and fibers that span a factor of 300 in thermal conductivity, 0.06 to 20 W/m-K. Time-domain thermoreflectance (TDTR) provides a common experimental platform for these studies; varying the thickness and modulation frequency changes the relative sensitivities of the TDTR measurement to thermal conductivity and heat capacity. Our recent work has employed light-activated changes in the morphology azo-polymers to switch by a factor of 3 between a low conductivity amorphous form and higher thermal conductivity crystalline form. We are developing frequency-domain probe beam-deflection and optical-fiber-based TDTR measurements to provide new capabilities for measurements of the thermal conductivity, effusivity and diffusivity of small volumes of soft materials.
  

Making transparent, super-ductile and heat-resistant semi-crystalline polymers

Based on our recent molecular picture [1], chain networking is a key factor that affords ductility for polymer glasses during tensile deformation. Therefore, it is essential, when considering the mechanics of semicrystalline polymers, that crystallization does not disrupt the chain network, which is typically not the case for semicrystalline polymers that crystallize slowly. We explore a molecular strategy to avoid depletion by crystallization of the polymer entanglement associated with the interchain uncrossability. The present study will characterize the conditions for producing such a new crystalline state and examine its mechanical behavior using class B semicrystalline polymers such as PLA and PET whose Tg is above room temperature.

[1] Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses. The Journal of chemical physics 2014, 141, (9), 094905
  

Estimation of mechanical properties of interfaces in polymer nanocomposites using molecular dynamics

Polymer nanocomposites are candidates for the next-generation of cable insulation and capacitor dielectrics. Nanoscale fillers, due to their high surface area, drastically increase the interfacial region which improves the dielectric permittivity and breakdown strength of the nanocomposites. Understanding the role of these interfaces in mechanical response of nanocomposites is crucial for their robust design. Modelling them at the molecular scale can capture the nanoscale features of the filler surface, the amorphous nature of the polymer and the interaction between them, while keeping the computational cost low. Using molecular dynamics, we calculate mechanical response of an ensemble of composites where polymer chains are grafted on a filler surface. We study the role of graft density and filler surface curvature on the stress and displacement field near the interface. We use an iterative finite-element-based approach to extract elastic modulus variation near the interface from the molecular dynamics stress and displacement profiles, which provides a starting point for large-scale modeling of composite nanostructures from first principles.   

Thermal transport of solid polymers and polymer blends

A grand challenge in designing polymeric materials is to tune their properties by macromolecular engineering. Here, applications of polymers are often hindered by their low thermal conductivity k. While low k values are highly desirable for thermoelectric materials, they create severe problems when used under the high temperature conditions. Going from the polymers dictated by weak Van der Waals to hydrogen-bonded interactions, k varies between 0.1-0.4 W/Km. Using molecular dynamics simulations, we study thermal transport and its links to the elastic response of polymers and polymer blends in their solid states. We find that there exists a maximum attainable stiffness, thus providing an upper bound of k for solid polymers. The specific chemical structures and the glass transition temperature play no role in controlling k, especially when the microscopic interaction is hydrogen bonded. These results are consistent with the minimum thermal conductivity model and existing experiments.

[1] D. G. Cahill, S. K. Watson, R. O. Pohl, Phys. Rev. B 46, 6131 (1992).
[2] D. Bruns, T. E. de Oliveira, J. Rottler, D. Mukherji, Macromolecules 52, 5510 (2019).
[3] C. Ruscher, J. Rottler, C. Boott, M. J. MacLachlan, D. Mukherji, arXiv:1910.04693 (2019).   

Abnormal Seebeck effect in doped polymer and two-band transport model

Employing Boltzmann transport equation coupled with Density-Functional Theory and non-equilibrium molecular dynamics simulation, we developed a combined scheme of calculating the thermoelectric property for polymeric materials. In the case of PEDOT doped with PSS, theory can give optimal carrier density for the power factor. It is found that the ideal crystalline polymer leads to a tiny thermoelectric figure of merit because of the extremely high thermal conductity. Thus, engineering of disordered structure at nanoscale is essential.
We find that the formation of polaron band upon doping is essential to understand the "abnormal Seebecj effect", namely, for the potassium doped nickel-coordinated polymer, the Seebeck coefficient first increases with temperature and then suddenly drops. We proposed a two-band transport model to explain such exotic behavior.   

Effect of polymer architecture on the gas separation performance of PIM-1 membranes

Molecular simulations are used to demonstrate the effects of chain architectures on the gas separation performance of PIM-1 membranes. Four different architectures (linear, H-shape, star, and dendritic) are considered to investigate the transport properties of four industrially relevant gases (CO₂, CH₄, O₂, and N₂). The simulations indicate that it is possible to tune the free volume morphology of PIM-1 membranes by choosing the appropriate architecture. An inverse relationship between the density and fractional free volume was observed as expected, with the highest density and lowest FFV obtained for the dendritic PIM-1. While the linear PIM-1 showed larger pores that enhanced the diffusivity of all small gases, the branched architectures (H-shape, star and dendritic) showed smaller interconnected pores with several bottle-neck morphologies. The observed modifications resulted in significant differences in the diffusivity of mid-range size gas molecules such as N₂, pushing the performance of CO₂/N₂ and O₂/N₂ separation performance beyond the Robeson’s 2008 upper bound.   

Surface Segregation of Branched Chain-ends in PDMS

It is well known that polymeric materials reorganize at their surfaces in order to minimize surface tension and free energy. This reorganization, driven by enthalpic and/or entropic forces, creates surface properties that can dramatically differ from the bulk. Enthalpically driven reorganization allows low energy groups to surface segregate and dominate the surface properties of the material. This is the case with polydimethylsiloxane (PDMS), where low surface energy pendant methyl groups accumulate at the surface. However, in some cases, higher energy groups can surface segregate when incorporated into branched or bulky polymer chain-ends. This process is entropically driven, in order to maximize polymer configurations in the bulk, despite increasing the enthalpy of the system. This research aims to study the surface segregation behavior of high energy, branched chain-ends within a PDMS network. Novel Silicone additives consisting of linear PDMS polymers, chain-end functionalized with branched polyester dendrimers containing quaternary ammonium cations (QACs) have been synthesized. The QACs provide the observable property of antimicrobial activity, which will be used to probe the surface segregation of the additive chain-ends.