mechanically gradient polymers

Reinforcing fillers such as carbon nanotubes that have high mechanical moduli have been used commonly to create polymer composites with high strength and toughness.Andrews, R.; Weisenberger, M. C., Carbon nanotube polymer composites. Curr Opin Solid St M 2004, 8 (1), 31-37. Since the modulus and filler amount are linked, by varying the amount of filler across the polymer the modulus will similarly change.Fox, J. D.; Capadona, J. R.; Marasco, P. D.; Rowan, S. J., Bioinspired Water-Enhanced Mechanical Gradient Nanocomposite Films That Mimic the Architecture and Properties of the Squid Beak. Journal of the American Chemical Society 2013, 135 (13), 5167-5174. Additionally, since long nanofillers create anisotropic moduli, if the direction of the nanofiller could be modified along the length of the polymer, the modulus gradient could also be tuned in this manner.

A common approach to increasing the mechanical strength of polymers includes changing the crosslinking density of the polymer. Crosslinks connect the polymer chains creating a web that resists deformation. Therefore, increasing the crosslinking density in a section of a polymer will increase the modulus in this location. This can be used to create a mechanical gradient if the crosslinking density changes across the polymer. A common approach to achieving this is using a photopolymerization process which with changes in UV exposure you can change the degree of crosslinking or polymerization in the area exposed.Sunyer, R.; Jin, A. J.; Nossal, R.; Sackett, D. L., Fabrication of Hydrogels with Steep Stiffness Gradients for Studying Cell Mechanical Response. PLOS One 2012, 7 (10). In a similar manner, the amount of initiator or crosslinker could be varied across the sample creating a similar effect.

Porosity gradients in polymers result in spatial variations in pore morphology and density, similar to structures observed in foams and natural materials like bone.{{Cite journal |last=Claussen |first=Kai U. |last2=Scheibel |first2=Thomas |last3=Schmidt |first3=Hans-Werner |last4=Giesa |first4=Reiner |date=2012 |title=Polymer Gradient Materials: Can Nature Teach Us New Tricks? |url= |journal=Macromolecular Materials and Engineering |language=en |volume=297 |issue=10 |pages=938–957 |doi= |issn=}} These gradients introduce mechanical variation across the material by altering local stiffness and viscosity, primarily through a reduction in modulus with increasing porosity.{{Cite journal |last=Miao |first=Xigeng |last2=Sun |first2=Dan |date=2009-12-25 |title=Graded/Gradient Porous Biomaterials |url= |journal=Materials |language=en |volume=3 |issue=1 |pages=26–47 |doi= |issn= |pmc=}} A notable example is asymmetric polymer membranes created via solvent-induced phase separation, which exhibit a graded structure suited for liquid–liquid separation.{{Cite journal |last=Guo |first=Miao |last2=Wang |first2=Shuhao |last3=Gu |first3=Kaifeng |last4=Song |first4=Xiaoxiao |last5=Zhou |first5=Yong |last6=Gao |first6=Congjie |date=2019-01-01 |title=Gradient cross-linked structure: Towards superior PVA nanofiltration membrane performance |url= |journal=Journal of Membrane Science |volume=569 |pages=83–90 |doi= |issn=}} Such membranes have widespread applications across industries, including water treatment, biopharmaceuticals, and food processing.{{Cite journal |last=Dalwadi |first=M. P. |last2=Griffiths |first2=I. M. |last3=Bruna |first3=M. |date=2015-10-08 |title=Understanding how porosity gradients can make a better filter using homogenization theory |url= |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=471 |issue=2182 |pages=20150464 |doi=}}

Gradient polymers in additive manufacturing enable the creation of parts with spatially tunable properties, expanding the potential for advanced functional components. Beyond geometric design freedom, this approach allows precise control over material composition within complex architectures, enabling functionally graded structures with enhanced adaptability, performance, and application-specific functionality. It is a simple, scalable, and cost-effective method compatible with a wide range of polymer systems. Notable examples include the 3D printing of biocompatible porous PDMS scaffolds shaped into organ-like structures for biomedical use,{{Cite journal |last=Montazerian |first=H. |last2=Mohamed |first2=M. G. A. |last3=Montazeri |first3=M. Mohaghegh |last4=Kheiri |first4=S. |last5=Milani |first5=A. S. |last6=Kim |first6=K. |last7=Hoorfar |first7=M. |date=2019-09-15 |title=Permeability and mechanical properties of gradient porous PDMS scaffolds fabricated by 3D-printed sacrificial templates designed with minimal surfaces |url= |journal=Acta Biomaterialia |volume=96 |pages=149–160 |doi= |issn=}} and gradient interpenetrating polymer networks (IPNs) embedded within elastomeric lattices to significantly improve and tailor energy absorption in protective applications. {{Cite journal |last=Sampson |first=Kathleen L. |last2=Li |first2=Hao |last3=Laqua |first3=Kurtis |last4=Aranguren van Egmond |first4=Derek |last5=Dickson |first5=Laura E. |last6=Barroeta Robles |first6=Julieta |last7=Lamouche |first7=Justin |last8=Guthrie |first8=Aria |last9=Ashrafi |first9=Behnam |last10=Zou |first10=Shan |last11=Chen |first11=Maohui |last12=Bell |first12=Joshua |last13=Paquet |first13=Chantal |date=2024 |title=Gradient-Interpenetrating Polymer Networks in 3D Printed Lattices for Tunable and Enhanced Energy Absorption |url= |journal=Advanced Materials Technologies |language=en |volume=9 |issue=22 |pages=2400403 |doi= |issn=}}

There are many examples in nature where soft tissue and hard surfaces are connected by a mechanical gradient to improve the fracture and impact resistance. Examples include mussels that connect to hard rocks by the mussel byssus which connects back to the soft muscle of the foot.Harrington, M. J.; Waite, J. H., How Nature Modulates a Fiber's Mechanical Properties: Mechanically Distinct Fibers Drawn from Natural Mesogenic Block Copolymer Variants. Adv Mater 2009, 21 (4), 440-+. A more extreme example is the squid beak which has an extremely hard tip required to kill and dismember its prey which connects back to the soft flesh of the body of the squid. Without the mechanically gradient in the beak, the squid would be unable to withstand the high impacts despite its hardness since it would break off from the body at the junction between the materials.Tan, Y. P.; Hoon, S.; Guerette, P. A.; Wei, W.; Ghadban, A.; Hao, C.; Miserez, A.; Waite, J. H., Infiltration of chitin by protein coacervates defines the squid beak mechanical gradient. Nat Chem Biol 2015, 11 (7), 488-+.

As mentioned above, many systems in nature incorporate mechanical gradients, and similarly for biomedical implants these gradients can be useful. Many implants are stiff and can cause damage to the surrounding tissues due to this difference in stiffness. This is a problem for instance in microelectrodes implanted into the brain which is extremely soft. The damage caused can create a buildup of fibrous tissue which can then interfere with the signal between the electrode and the brain.Tate, M. L. K.; Detamore, M.; Capadona, J. R.; Woolley, A.; Knothe, U., Engineering and commercialization of human-device interfaces, from bone to brain. Biomaterials 2016, 95, 35-46. Similarly, in knee and hip implants, there is a need for high integration between the strong bone and the cartilage and tissue. Otherwise problems such as stress shielding can occur where the bone degenerates due to the implant having too strong of a modulus.Huiskes, R.; Weinans, H.; Vanrietbergen, B., The Relationship between Stress Shielding and Bone-Resorption around Total Hip Stems and the Effects of Flexible Materials. Clin Orthop Relat R 1992, (274), 124-134.

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Category:Polymers