•Three processing techniques were investigated to disperse CNCs in a thermoplastic matrix.•Polarized optical microscopy was used to evaluate the degree of CNC dispersion obtained.•A multi-step protocol increased CNC dispersion in a thermoplastic matrix.•Changes in polymer microstructure were observed when CNCs were well dispersed.Cellulose nanocrystals (CNCs) have garnered significant attention due in part to the remarkable properties of crystalline cellulose displayed at the nanoscale coupled with its availability from renewable resources. However, challenges exist in dispersing CNCs in thermoplastic polymers and assessing CNC dispersion. The purpose of this research was to investigate the impact different processing protocols had on CNC dispersion in two polyethylene-co-vinyl alcohol (EVOH) matrices and the resulting microstructure of the nanocomposites. Three processing strategies were proposed: a melt processing technique, a solution casting technique, and a combination of solution casting followed by melt processing. Polarized optical microscopy images of the nanocomposites showed that improved CNC dispersion and changes to the nanocomposite microstructure were observed when the multi-step protocol was implemented. Additionally, changes in microstructure as a function of comonomer ratios of EVOH suggested CNC dispersion changed as a result of differences in polymer-particle compatibility.Download high-res image (255KB)Download full-size image

Recent emphasis on the pilot scale production of cellulosic nanomaterials has increased interest in the effective use of these materials as reinforcements for polymer composites. An important, enabling step to realizing the potential of cellulosic nanomaterials in their applications is the materials processing of CNC/polymer composites through multiple routes, i.e. melt, solution, and aqueous processing methods. Therefore, the objective of this research is to characterize the viscoelastic behavior of aqueous nanocomposite suspensions containing cellulose nanocrystals (CNCs) and a water-soluble polymer, poly(vinyl alcohol) (PVA). Specifically, small amplitude oscillatory shear measurements were performed on neat PVA solutions and CNC-loaded PVA suspensions. The experimental results indicated that the methods used in this study were able to produce high-quality nanocomposite suspensions at high CNC loadings, up to 67 wt% with respect to PVA. Additionally, the structure achieved in the nanocomposite suspensions was understood through component attributes and interactions. At CNC loadings near and less than the percolation threshold, a polymer mediated CNC network was present. At loadings well above the percolation threshold, a CNC network was present, indicated by limited molecular weight dependence of the storage modulus. Overall, these results provide increased fundamental understanding of CNC/PVA suspensions that can be leveraged to develop advanced aqueous processing methods for these materials.

The objective of this research is to more fully understand the structure–property relationships in multiwalled carbon nanotube (MWNT)/high performance thermosetting polymer nanocomposites. Nanocomposites containing up to 7 wt.% MWNTs and a polyimide matrix (PETI-330) were characterized after curing and these characterization results were compared against similar nanocomposites containing carbon nanofibers (CNFs). Glass transition temperature values and thermomechanical testing indicated that the physical properties of the MWNT nanocomposites followed different trends at MWNT loadings less than and greater than the percolation threshold. In contrast, the CNF nanocomposites roughly showed a single relationship between physical property changes and CNF loading at the loadings studied here, which were all less than the percolation threshold. Overall, these results highlight the challenges in improving the properties of high performance matrices and suggest that MWNT network formation reduces reinforcement efficacy and that MWNTs can interfere with free radical curing processes.

The objective of this research is to develop a nanoparticle synthesis scheme that controls nanoparticle shape and surface chemistry concomitantly. Specifically, a method to synthesize hydroxyapatite nanoparticles using a dispersed block copolymer template is explored, which produces spherical and needle-shaped nanoparticles, and at the end of the synthesis, the block copolymer is retained as a surface coating on the nanoparticles. This strategy has been used previously with double-hydrophilic block copolymers (DHBCs) as the dispersed template; however, in this work, an alternative block copolymer chemistry is explored in an effort to extend this method to synthesis in organic solvents, producing nanoparticles that are organophilic instead of hydrophilic. The hydroxyapatite nanoparticles were synthesized using poly(methyl methacrylate)-b-poly(methacrylic acid) (PMMA-b-PMAA) as the dispersed template and tetrahydrofuran as the solvent. The synthesis proceeds following the ionization of the PMAA block of the copolymer and association between this ionized group and the calcium precursor ions. To investigate the degree of shape control available, the concentration of block copolymer solution and the amount of precursor were systematically varied, and the synthesized HAp nanoparticles were characterized. SEM images showed that needle and spherical HAp nanoparticles could be synthesized by changing the block copolymer concentration. TGA, FT-IR, and XRD results indicated that the block copolymer used for synthesis remained on the HAp particle surface. Overall, these results indicate that the shape of the nanoparticles produced by this method was related to the Ca2+/COO– mole ratio used during synthesis, similar to results obtained with DHBC template synthesis. The qualitative agreement between the shape control mechanisms in the two synthesis schemes suggests that this relationship could be general to the overall synthesis scheme and provide a mechanism for controlling nanoparticle shape with many block copolymer chemistries.

In this research, a nanoparticle synthesis method was explored in order to produce nanoparticles with different shapes and polymer-coated surfaces. Specifically, a poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) block copolymer was employed as a dispersed template for the controlled synthesis of hydroxyapatite (HAp) nanoparticles in water. Six different synthesis conditions were used to produce different nanoparticle shapes, and characterization of these nanoparticles was performed to understand the level of morphological control available with this synthesis method. SEM images showed that needle and spherical nanoparticles could be produced by changing the block copolymer concentration. The shape of the nanoparticles produced by this synthesis method was shown to be related to the relative concentrations of the calcium precursor and the block copolymer used. Significant quantities of the block copolymer used for synthesis remained on the particle surface, constituting a functionalized moiety and providing an opportunity to control component interactions in composites.

Among the physical and chemical attributes of the nanocomposite components and their interactions that contribute to the ultimate material properties, nanoparticle arrangement in the matrix is a key contributing factor that has been targeted through materials choices and processing strategies in numerous previous studies. Often, the desired nanocomposite morphology contains individually dispersed and distributed nanoparticles. In this research, a phase-segregated morphology containing nanoparticle networks was studied. A model nanocomposite system composed of calcium phosphate nanoparticles and a poly(3-hydroxybutyrate) matrix was produced to understand how polymer crystallization and crystal structure can facilitate the formation of a phase-segregated morphology containing nanoparticle networks. Two chemically similar calcium phosphate nanoparticle systems with different shapes, near-spherical and nanofiber, were synthesized for use in the nanocomposites. The different shapes were used independently in nanocomposites in an attempt to understand the effect of the nanoparticle shapes on crystallization-mediated nanoparticle network formation. The resulting nanocomposites were characterized to establish the effects of component interactions on the polymer structure. Additionally from the viscoelastic properties, structure–property relationships in these materials can be defined as a function of nanoparticle shape and concentration. The results of this research suggest that when the nanocomposite components are not strongly interacting, polymer crystallization may be used as a forced assembly method for nanoparticle networks. Such a methodology has applications to the design of functional polymer nanocomposites such as biomedical implant materials and organic photovoltaic materials where judicious choice of nanoparticle–polymer pairs and control of polymer crystal nucleation and growth processes could be used to control the length scale of phase segregation.Keywords: calcium phosphate; dynamic mechanical properties; nanocomposite; nanoparticles; phase segregation; polymer crystallinity;

In this research, hydroxyapatite nanoparticles sparsely coated with tethered poly(ethylene oxide) (PEO) chains were added to a bulk PEO matrix of greater molecular weight. The nanoparticles were found to be dispersed homogeneously when the particle concentration was high. Two different low-aspect ratio shaped particles were used, spherical and needle, and had similar dispersion behavior in this matrix. Characterization results indicated that the homogeneous dispersion achieved here at higher particle loadings was a kinetically trapped condition and an interfacial zone had developed at the grafted polymer/matrix polymer interface. Additionally at a critical nanoparticle loading related to specific values of the nanoparticle surface area and interparticle spacing, the crystallite size and amount of crystallinity in the PEO matrix began to decrease as opposed to the level of nanoparticle dispersion. Overall, these results provide a strategy for incorporating relatively high loadings of nanoparticles into these matrices without large scale phase segregation.Graphical abstract

In previous published research, network formation has been used to understand morphology and properties in polymer nanocomposites containing carbon nanotubes (CNTs) through measurements of rheological and electrical percolation thresholds, largely in thermoplastic matrices. In this research, these tools are explored as a means to understand network transport mechanisms and changes in CNT dispersion during curing in a thermosetting matrix. Specifically, rheological and electrical measurements were performed on the uncured nanocomposites, and electrical measurements were performed on the cured nanocomposites. The resulting data were applied to a percolation model. The results showed that the uncured resin played a limited role in mediating rheological transport and that little CNT aggregation occurred during curing. The results of this initial work suggest that such a combination of techniques is applicable to understanding dispersion changes resulting from curing and provides complementary insight to that provided by electron microscopy imaging of the same phenomenon.

In order to understand more fully how polymer matrix attributes influence polymer nanocomposite properties, nanocomposites containing hydroxyapatite nanoparticles and a poly(3-hydroxybutyrate) matrix were prepared and compared to results for a chemically-similar nanocomposite system with a lesser degree of matrix crystallinity. Experimental results indicated that the higher degree of matrix crystallinity hinders nanoparticle dispersion at loadings above 0.5 wt.% and together these structural factors, high matrix crystallinity and nanoparticle aggregation, produced different mechanical reinforcement behavior below and above the glass transition temperature than has been seen previously in amorphous matrices or matrices with moderate crystallinity levels. Overall, these results suggested that the amorphous character of the polymer does not govern the properties at all crystallinity levels in polymer nanocomposite matrices.

•Tensegrity principles were applied to the design of nanofilled polymers.•The tensegrity-inspired microstructure was accomplished with a semi-crystalline matrix.•Storage modulus was improved below Tg with the tensegrity-inspired microstructure.Investigating the design space available in nanofilled polymers is expected to produce materials with unique structure-property relationships. In this work, the concept of tensegrity is used as inspiration for microstructural design. Tensegrity is an elegant and efficient structural design, relying upon interactions between elements, specifically elements in compression which are connected through a tensioned web. In this embodiment, the nanoparticles are considered the compressive elements, and the polymer matrix, when suitably processed, is considered the tensioned web. Nanocomposite materials containing polymer-decorated hydroxyapatite nanoparticles were used to evaluate the tensegrity-inspired approach. Nanocomposite systems with a semi-crystalline matrix (polyethylene oxide) and an amorphous matrix (polymethyl methacrylate) were prepared and characterized with compatible polymer-decorated nanoparticles. The results showed that the nanocomposite with a semi-crystalline matrix was more promising as a candidate for producing the tensegrity-inspired microstructure and that the concept could produce improved thermomechanical properties in the glassy temperature regime.

In this research, a nanoparticle synthesis method was explored in order to produce nanoparticles with different shapes and polymer-coated surfaces. Specifically, a poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) block copolymer was employed as a dispersed template for the controlled synthesis of hydroxyapatite (HAp) nanoparticles in water. Six different synthesis conditions were used to produce different nanoparticle shapes, and characterization of these nanoparticles was performed to understand the level of morphological control available with this synthesis method. SEM images showed that needle and spherical nanoparticles could be produced by changing the block copolymer concentration. The shape of the nanoparticles produced by this synthesis method was shown to be related to the relative concentrations of the calcium precursor and the block copolymer used. Significant quantities of the block copolymer used for synthesis remained on the particle surface, constituting a functionalized moiety and providing an opportunity to control component interactions in composites.