Atomic Force Microscopy Nanomechanical Mapping Research Articles

Physical aging-induced embrittlement of PLA/PBAT blends probed by tensile test and AFM nanomechanical mapping

Poly(lactic acid) (PLA) is a well-known biodegradable sustainable polyester but with intrinsic brittleness. PLA can be toughened through blending with immiscible rubbery polymers like biodegradable poly(butylene adipate-co-terephthalate) (PBAT). However, little was done to examine the effectiveness of the additives after long-term physical aging. Herein, the mechanical properties of PLA/PBAT (80/20) blends were measured by combining tensile tests and atomic force microscopy (AFM) based nanomechanical mapping after ageing at 40 °C for different periods. It is found that the elongation at break of the blends decreases dramatically from 257 % after compression molding to 14 % after aging for 7 days, and an equilibrium value is not yet reached. AFM nanomechanical mapping reveals that the microscopic Young’s modulus distribution of PLA matrix broadens significantly with increasing aging time, reflecting the increase of heterogeneity and resulting in the embrittlement of the blends. This work raises the severe impact of physical aging on the mechanical properties of PLA based blends, which is usually overlooked but may limit their applications.

Nanomechanical and Chemical Mapping of the Structure and Interfacial Properties in Immiscible Ternary Polymer Systems

It is a challenge to identify each phase in a multi-component polymer system and uniquely determine the interfacial properties between the different phases. Using atomic force microscopy nanomechanical mapping (AFM-NM) and AFM-based infrared spectroscopy (AFM-IR), we identify each phase, visualize structural developments, and determine the interfacial properties in a blend of three polymers: high-density polyethylene (HDPE), polyamide (PA6) and poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS). Each phase can be identified from the Young’s modulus, along with the structural development within the phases before and after compatibilization. The interfacial widths between HDPE/PA6, HDPE/SEBS and SEBS/PA6 were determined independently in one measurement from a Young’s modulus map. The structural, mechanical property development and identity of the phases were determined by AFM-NM, while AFM-IR, providing complementary chemical information, identified interfacial reactions, showed the chemical affinity of a compatibilizer with the component phases, and mapped the distribution of the compatibilizer in the ternary polymer blends. The chemical, structural and interfacial information obtained by these measurements provide information that is essential for producing mechanically robust materials from incompatible mixtures of polymers.

Probing the structural evolution in deformed isoprene rubber by in situ synchrotron X-ray diffraction and atomic force microscopy

Developing a better understanding of the structural evolution in deformed polymers is key to designing new materials and structures that achieve superior mechanical properties. Here, we used in situ synchrotron wide-angle X-ray diffraction (WAXD) and atomic force microscopy (AFM) nanomechanical mapping (AFM-NM) to assess the strain-induced crystallization (SIC) and the associated structural evolution and mechanical properties of peroxide vulcanized isoprene rubber (IR) as a function of crosslink density (ν) and strain. The WAXD and AFM-NM results show agreement in the onset strain of SIC. Crystalline reflections appears in the WAXD while a nanofibrillar structure is found by AFM-NM. The higher ν, the smaller is the onset strain of a steep upturn in the stress-strain curves, the smaller is the onset strain of SIC, the higher is the crystallinity as evidenced in the WAXD, and the larger is the amount of nanofibrils seen by AFM-NM. Both WAXD and AFM-NM results show the SIC occurs rapidly at high strains while most chains remain in the amorphous state. The elastic modulus of the formed nanofibrils that range in diameter from several to a hundred nanometers, is two times higher than that of the amorphous regions. From the WAXD and AFM-NM results, a schematic model of structural evolution is proposed and used to illustrate the self-reinforcement mechanism in IR.

Atomic Force Microscopy Nanomechanical Mapping Visualizes Interfacial Broadening between Networks Due to Chemical Exchange Reactions.

The interfacial broadening between two different epoxy networks having different moduli was nanomechanically mapped. The interfacial broadening of the two networks produced an interfacial zone having a gradient in the concentration and, hence, properties of the original two networks. This interfacial broadening of the networks leads to the generation of a new network with a segmental composition corresponding to a mixture of the original two network segments. The intermixing of the two, by nature of the exchange reactions, was on the segmental level. By mapping the time dependence of the variation in the modulus at different temperatures, the kinetics of the exchange reaction was measured and, by varying the temperature, the activation energy of the exchange reaction was determined.

AFM nanomechanical mapping and nanothermal analysis reveal enhanced crystallization at the surface of a semicrystalline polymer

We used atomic force microscopy (AFM) nanomechanical mapping to image the surface of a semicrystalline polymer, specifically polymorphic isotactic poly (1-butene) (iPB), to determine the mechanical properties of a surface layer, and compared this to the properties in the bulk. The measured Young's modulus of the surface layer as a function of annealing time at room temperature was found to be higher than that of the bulk, indicating a more rapid transformation from form II to I polymorphs and an enhanced movement of polymer chain segments at the surface relative to the bulk for this polymer. Plate-like crystals were found at the surface and, as the annealing temperature increased from 70 to 110 °C, evidence of a surface layer was found that increased in thickness from ∼30 to ∼130 nm, respectively. After removing this layer, the morphology found in the bulk was markedly different. We also used AFM nanothermal analysis to determine the local melting point (Tm) and found, that the melting point, Tm, of the crystals at the surface was higher than that of the bulk, in keeping with the modulus measurements. The nanomechanical and nanothermal properties and the morphology at the surface suggest an enhanced movement of polymer chain segments at the surface and therefore, an enhanced crystallization at the surface.

Sequential Infiltration of Self-Assembled Block Copolymers: A Study by Atomic Force Microscopy

Sequential infiltration synthesis (SIS), when combined with novel polymeric materials capable of self-assembly, such as block copolymers (BCPs), has been shown to effectively improve the pattern transfer of nanoscale templates. Herein, we present a study of the SIS process aimed at elucidating some critical aspects such as the evolution of the BCP morphology and mechanical properties after infiltration. Atomic force microscopy nanomechanical mapping was able to measure a consistent stiffness change within the SIS-infiltrated poly(methyl methacrylate) (PMMA) blocks. Interestingly, the increase in Young’s modulus of the infiltrated blocks is small compared to the final stiffening of the same infiltrated features after a treatment with oxygen plasma.

Production of a cellular structure in carbon nanotube/natural rubber composites revealed by nanomechanical mapping

Composites based on an elastomer, natural rubber (NR), and pristine multiwalled carbon nanotubes (MWCNTs) were prepared using a two-roll mill mixer. The high shear stress produced a homogeneous dispersion of CNTs in the NR matrix. Using an atomic force microscopy nanomechanical mapping technique, the structure and nanoscale mechanical properties of the resulting composites were investigated in term of CNT loading. Topography, elastic modulus, and adhesive energy distribution maps of the materials were obtained at the same point and at the same time in a single scan. Such maps were used to identify and characterize the CNT, NR, and mechanical interfacial regions formed around the CNTs. The mechanical interfacial regions together with the CNTs divide the elastomer matrix into small “cells”. We claim that the cellular structure is produced in the CNTs/NR composites.