Damien Maillard's research while affiliated with Columbia University and other places

A method of forming a solid-state polymer can include grafting a graft polymer to nanoparticles to provide grafted nanoparticles, and dispersing the grafted nanoparticles in a polymer matrix to provide a specified loading of the grafted nanoparticles within the polymer matrix to form a solid-state polymer. A solid-state polymer can include grafted nanoparticles comprising a polymer graft grafted to nanoparticles, and a polymer matrix, in which the grafted nanoparticles are dispersed to form a solid-state polymer, the dispersion configured to provide a specified loading of the grafted nanoparticles within the solid-state polymer.

The compatibility between polystyrene (PS) and polybutadiene (PB) was improved by adding bare silica or PS-grafted silica nanoparticles. The grafting density varies from 0.01 chains/nm^2 to 0.10 chains/nm^2. Thin sections are obtained by cryomicrotome at -140 ^oC for TEM analysis. Without adding nanoparticles, bulk phase separation occurs for the PS-PB blend, although a few droplets of PS are found presumably due to the viscoelastic phase separation. When the grafting density is less than 0.05 chains/nm^2, the particles are found to partition between the PS-PB interface and the continuous PS phase. However, when the grafting density is greater than or equal to 0.05 chains/nm^2, the particles are found to locate only in the dispersed PS phase, and the size of the PS phases decreases with increasing grafting density. Phase inversion also occurs at 70 wt% of PS when the grafting density is fixed at 0.10 chains/nm^2.

It is commonly accepted that the addition of spherical nanoparticles (NPs) cannot simultaneously improve the elastic modulus, the yield stress, and the ductility of an amorphous glassy polymer matrix. In contrast to this conventional wisdom, we show that ductility can be substantially increased, while maintaining gains in the elastic modulus and yield stress, in glassy nanocomposite films composed of spherical silica NPs grafted with polystyrene (PS) chains in a PS matrix. The key to these improvements are (i) uniform NP spatial dispersion and (ii) strong interfacial binding between NPs and the matrix, by making the grafted chains sufficiently long relative to the matrix. Strikingly, the optimal conditions for the mechanical reinforcement of the same nanocomposite material in the melt state is completely different, requiring the presence of spatially extended NP clusters. Evidently, NP spatial dispersions that optimize material properties are crucially sensitive to the state (melt versus glass) of the polymeric material.

We have studied the surface behavior of nanoparticles, which are lightly grafted with polymer chains, when they are mixed with matrix chains of the same architecture as the grafts. We consider the particular case where the nanoparticle core and the grafted polymer chains energetically dislike each other and show that the extent of surface segregation of these "hairy" nanoparticles and their self-assembly into a variety of structures can be tuned by varying the number and the length of the grafted chains and the matrix chain length. These results unequivocally show that grafted nanoparticles in polymer matrices behave akin to block copolymers (or amphiphiles) in selective solvents, with readily controllable surface behavior.

The difference in morphology of ultrathin films of mixtures of PLLA (poly(L-lactide) and PDLA (poly(D-lactide) having different enantiomeric ratios was investigated. Ultrathin films were prepared by spin coating at a rotation speed of 3000 rpm during 20 s, following an acceleration of 4000 rpm/s. The mass ratio of the two polyenantiomers in the film was controlled by their ratio in a dichloromethane solution. The film thickness was dictated by the solution concentration were used to obtain thicknesses ranging between 5 and 100 nm. The two polyenantiomers cannot crystallize separately, and the structures observed necessarily belong to the stereocomplex. At a thickness of 50 nm, a triangular single crystal, covered by small triangular overgrowths oriented along the same direction as the mother lamella, is observed. The chiral circular distortion observed in dendritic nonequimolar crystals is unique and spectacular.

PS-grafted nanoparticles mixed in a PS matrix and annealed at a temperature above the Tg of the polymer can self assemble to form large complex structures. The morphology of these structures is ruled by the grafting density of the particles and the grafted/matrix chains molecular weight ratio, and can vary from spherical aggregates to sheets or strings. In the case of thin films, in addition to those particles-particles aggregation, a surface segregation leads to 2D structure formation. The mechanical reinforcement provided by those particles and their structures to 100 nm thick films has been studied with a homemade bulge test system. The inter-particle interaction, the structure form factor and the lateral chains alignment seem to be able to modify the Young modulus, the Yield and the fracture of the samples at the same time.

As shown previously by simulation and TEM studies in the bulk, PS grafted nanoparticles when mixed a PS matrix self-assemble into a range of superstructures. These self-assembled structures can be regrouped into a phase diagram in which the leading parameters are the particles grafting density and the molecular weight ratio of the grafted and free matrix chains. Depending on those parameters the particles can be well dispersed or aggregated in one (strings), two (interconnected sheets) or three (spherical aggregates) dimensions. Here we consider the corresponding behavior in thin films (100 nm thick) using in-situ phase contrast AFM. In addition to yielding the morphologies, this protocol allows us directly visualize the aggregation process of the particles.

Poly(L-lactide) ultrathin films of thicknesses between 10 and 80 nm were crystallized between 100 and 170 degrees C. Four different morphologies were obtained: single crystals, hedrites, dendrites, and spherulites. Dendrites were always obtained below 20 nm due to chain diffusion limitations, whereas, at larger thicknesses, the morphology heavily depends on the crystallization temperature. The thickness-temperature morphological map led to the observation of two transition temperatures, at 115 and 145 degrees C. Other parameters such as the lamellar thickness, melting temperature, and dendricity of the lamellae for 15 nm thick films show significant changes at these two temperatures. Those transitions occur at the same temperatures as Regime III/Regime II, and Regime II/Regime I transitions of the Lauritzen-Hoffman crystallization theory.

The crystallization of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) in ultrathin films (15 nm) has been followed between 125 and 160 °C using in situ atomic force microscopy. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which is related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behavior was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions, without any external nucleation. As shown by electron diffraction patterns, the crystalline unit cells of the two enantiomers are identical. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behavior. Moreover, the direction of curvature of the lamellae can be linked with the sense of twisting of the poly(lactide) lamellae in banded spherulites, and the temperature dependence of the radius of curvature can be correlated with the distance between the extinction rings. Those observations are consistent with Keith and Padden's model since the curved crystals in ultrathin films can be considered as “half-lamellae”, which give, when associated together, twisted complete lamellae.

The behaviour of ultrathin polymer films is very different from that in the bulk phase. In this work, the crystallization of poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA) was followed using in situ atomic force microscopy over a broad range of temperatures and thicknesses. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which can be related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behaviour was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions without any external nucleation. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behaviour. Moreover, the rotating direction of those lamellae can be directly linked with the sense of twisting of the poly(lactide)s lamellae in banded spherulites. Those observations can lead to a model where the curved crystals in ultrathin films can be considered as half-lamellae, which, when associated together, give twisted complete lamellae.