On the Thermal Stability of PS-b-PMMA Block and P(S-r-MMA) Random Copolymers for Nanopatterning Applications

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

Short vs. long chains competition during “grafting to” process from melt

A preferential grafting of short chains occurs during the “grafting to” reaction of hydroxy terminated P(S-st-MMA) blends consisting of short and long chains. The enrichment is enhanced when the chain length difference increases.

Quantification of molecular weight discrimination in grafting to reactions from ultrathin polymer films by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the present study, a reliable and robust method was developed to quantify the molecular weight discrimination that can occur in grafting to reactions via indirect MALDI-TOF quantification of the molecular weights of grafted chains by comparing the characteristics of the polymeric material before the grafting reaction with those of the unreacted material recovered after grafting. Two polystyrene samples with different molecular weights and narrow molecular weight distributions were employed to prepare model blends that were grafted to silicon wafers and an analytical method was developed and validated to assess and quantify the modification of the molecular weight distribution that takes place during the grafting to process. Particular attention was paid to the standardization of the sample treatment and to find the best data collection and calibration methodologies in order to have statistically significant data even in the presence of a very scarce amount of the sample. Furthermore, to evaluate the accuracy of the analytical procedure, the lack of suitable standard and certified materials required a further experiment to be carried out by comparing the new optimized MALDI-TOF method and direct measurements using TGA-GC-MS on a model blend containing deuterated and hydrogenated polystyrene samples with appropriate molecular weights and distributions. The optimized method was applied on samples obtained by a thermally induced grafting to reaction from ultrathin polymer films and, for the first time, to our knowledge, an enrichment effect occurring in the ultrathin grafted layer obtained from a melt was evidenced.

Inside the brush: partition by molecular weight in grafting to reactions from melt

A substantial partition by molecular weight takes place during the grafting to reactions.

Thermodynamics and ordering kinetics in asymmetric PS-b-PMMA block copolymer thin films

The ordering kinetics of standing cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymers (molecular weight: 39 kg mol^-1) close to the order-disorder transition is experimentally investigated following the temporal evolution of the correlation length at different annealing temperatures. The growth exponent of the grain-coarsening process is determined to be 1/2, signature of a curvature-driven ordering mechanism. The measured activation enthalpy and the resulting Meyer-Neldel temperature for this specific copolymer along with the data already known for PS-b-PMMA block copolymers in strong segregation limit allow investigation of the interplay between the ordering kinetics and the thermodynamic driving force during the grain coarsening. These findings unveil various phenomena concomitantly occurring during the thermally activated ordering kinetics at segmental, single chain, and collective levels.

Renewable Fabric Surface-Initiated ATRP Polymerizations: Towards Mixed Polymer Brushes

A totally new approach in the synthesis of mixed polymer brushes tethered on polyamide (PA) surfaces is presented herein. As a proof of concept, two types of homopolymers were synthesized in sequential surface-initiated atom transfer radical polymerization (SI-ATRP) reactions: poly(methyl methacrylate)/poly((2-dimethylamino)ethyl methacrylate) and polystyrene /poly((2-dimethylamino)ethyl methacrylate). The ATRP initiator was immobilized on the surface through PA chain-end groups in two subsequent steps, separated by homo-polymerizations. The amount of the PA chains' end groups available on the modified surface was tuned by the thermal rearrangement of the surface.

Stable polymer brushes with effectively varied grafting density synthesized from highly crosslinked random copolymer thin films

Crosslinkable epoxy copolymers enable achieving highly stable P(S-b-MMA) brushes with controlled grafting density for close examination of phase separation behaviors.

Micrometer-Scale Ordering of Silicon-Containing Block Copolymer Thin Films via High-Temperature Thermal Treatments

Block copolymer (BCP) self-assembly is expected to complement conventional optical lithography for the fabrication of next-generation microelectronic devices. In this regard, silicon-containing BCPs with a high Flory-Huggins interaction parameter (χ) are extremely appealing because they form high-resolution nanostructures with characteristic dimensions below 10 nm. However, due to their slow self-assembly kinetics and low thermal stability, these silicon-containing high-χ BCPs are usually processed by solvent vapor annealing or in solvent-rich ambient at a low annealing temperature, significantly increasing the complexity of the facilities and of the procedures. In this work, the self-assembly of cylinder-forming polystyrene-block-poly(dimethylsiloxane-random-vinylmethylsiloxane) (PS-b-P(DMS-r-VMS)) BCP on flat substrates is promoted by means of a simple thermal treatment at high temperatures. Homogeneous PS-b-P(DMS-r-VMS) thin films covering the entire sample surface are obtained without any evidence of dewetting phenomena. The BCP arranges in a single layer of cylindrical P(DMS-r-VMS) nanostructures parallel-oriented with respect to the substrate. By properly adjusting the surface functionalization, the heating rate, the annealing temperature, and the processing time, one can obtain correlation length values larger than 1 μm in a time scale fully compatible with the stringent requirements of the microelectronic industry.

Composition of ultrathin binary polymer brushes by thermogravimetry-gas chromatography-mass spectrometry

In the present paper, a reliable and rugged thermogravimetry-gas chromatography-mass spectrometry (TGA-GC-MS) method was developed to determine the composition of ultrathin films consisting of binary blends of functional polystyrene (PS) and polymethylmethacrylate (PMMA) grafted to a silicon wafer. A general methodology will be given to address the composition determination problem for binary or even multicomponent polymer brush systems using the PS/PMMA-based samples as a paradigmatic example. In this respect, several distinct tailor-made materials were developed to ensure reliable calibration and validation stages. The analytical method was tested on unknown samples to follow the composition evolution in PS/PMMA brushes during the grafting reaction. A preferential grafting of the PMMA was revealed in full agreement with its preferential interaction with the SiO2 polar surface.

Neutral wetting brush layers for block copolymer thin films using homopolymer blends processed at high temperatures

Binary homopolymer blends of two hydroxyl-terminated polystyrene (PS-OH) and polymethylmethacrylate (PMMA-OH) homopolymers (Mn ∼ 16000 g mol(-1)) were grafted on SiO2 substrates by high-temperature (T > 150 °C), short-time (t < 600 s) thermal treatments. The resulting brush layer was tested to screen preferential interactions of the SiO2 substrate with the different symmetric and asymmetric PS-b-PMMA block copolymers deposited on top of the grafted molecules. By properly adjusting the blend composition and the processing parameters, an efficient surface neutralization path was identified, enabling the formation, in the block copolymer film, of homogeneous textures of lamellae or cylinders perpendicularly oriented with respect to the substrate. A critical interplay between the phase segregation of the homopolymer blends and their grafting process on the SiO2 was observed. In fact, the polar SiO2 is preferential for the PMMA-rich phase that forms a homogeneous layer on the substrate, while the PS-rich phase is located at the polymer-air interface. During the thermal treatment, phase segregation and grafting proceed simultaneously. Complete wetting of the PS rich phase on the PMMA rich phase leads to the formation of a PS/PMMA bilayer. In this case, the progressive diffusion of PS chains toward the polymer-SiO2 interface during the thermal treatment allows tuning of the brush layer composition.

Structural Characterisation of Complex Layered Double Hydroxides and TGA-GC-MS Study on Thermal Response and Carbonate Contamination in Nitrate- and Organic-Exchanged Hydrotalcites

Layered double hydroxides (LDHs) are versatile materials used for intercalating bioactive molecules in the fields of pharmaceuticals, nutraceuticals and cosmetics, with the purpose of protecting them from degradation, enhancing their water solubility to increase bioavailability and improving their pharmacokinetic properties and formulation stability. Moreover, LDHs are used in various technological applications to improve stability and processability. The crystal chemistry of hydrotalcite-like compounds was investigated by X-ray powder diffraction (XRPD), automated electron diffraction tomography (ADT) and thermogravimetric analysis (TGA)-GC-MS to shed light on the mechanisms involved in ion exchange and absorption of contaminants, mainly carbonate anions. For the first time, ADT allowed a structural model of LDH_NO3 to be obtained from experiment, shedding light on the conformation of nitrate inside LDH and on the loss of crystallinity due to the layer morphology. The ADT analysis of a hybrid LDH sample (LDH_EUS) clearly revealed an increase in defectivity in this material. XRPD demonstrated that the presence of carbonate can influence the intercalation of organic molecules into LDH, since CO3 -contaminated samples tend to adopt d spacings that are approximate multiples of the d spacing of LDH_CO3 . TGA-GC-MS allowed intercalated and surface- adsorbed organic molecules to be distinguished and quantified, the presence and amount of carbonate to be confirmed, especially at low concentrations (<2 wt %), and the different types and strengths of adsorption to be classified with respect to the temperature of elimination.

Thermal stability of functional P(S-r-MMA) random copolymers for nanolithographic applications

Two strategies are envisioned to improve the thermal stability of the grafted layer and to allow the processing of the random copolymer/block copolymer (RCP/BCP) system at high temperature. From one side, a high-temperature thermal treatment of a commercial α-hydroxyl ω-2,2,6,6-tetramethylpiperidinyloxy functional RCP, namely, TR58, leads to the formation of a stabilized layer able to induce the perpendicular orientation of a symmetric BCP to temperatures higher than 310 °C. On the other side, an α-hydroxyl ω-Br functional RCP, namely, BrR58, with the same molar mass and composition of TR58, was prepared by activator regenerated by electron transfer atom transfer radical polymerization. The resulting brush layer can sustain the self-assembly of the symmetric BCP for processing temperatures as high as 330 °C. In both systems, the disruption of the BCP film, deposited on the grafted RCP layer, occurs because of the formation of bubbles, due to a low-temperature evolution of monomers from the RCP layer. The extent of the low-temperature monomer evolution is higher for TR58 than it is for BrR58 and starts at lower temperatures. For both copolymers, the thermal treatment offsets the low-temperature monomer evolution while still maintaining surface characteristics suitable to induce the perpendicular orientation of the BCPs, thus ultimately extending the range of processing temperatures of the BCP film and consequently speeding the self-organization process.

Evolution of lateral ordering in symmetric block copolymer thin films upon rapid thermal processing

This work reports experimental findings about the evolution of lateral ordering of lamellar microdomains in symmetric PS-b-PMMA thin films on featureless substrates. Phase separation and microdomain evolution are explored in a rather wide range of temperatures (190-340 °C) using a rapid thermal processing (RTP) system. The maximum processing temperature that enables the ordering of block copolymers without introducing any significant degradation of macromolecules is identified. The reported results clearly indicate that the range of accessible temperatures in the processing of these self-assembling materials is mainly limited by the thermal instability of the grafted random copolymer layer, which starts to degrade at T > 300 °C, inducing detachment of the block copolymer thin film. For T ⩽ 290 °C, clear dependence of correlation length (ξ) values on temperature is observed. The highest level of lateral order achievable in the current system in a quasi-equilibrium condition was obtained at the upper processing temperature limit after an annealing time as short as 60 s.

Fine tuning of lithographic masks through thin films of PS-b-PMMA with different molar mass by rapid thermal processing

The self-assembly of asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer based nanoporous thin films over a broad range of molar mass (Mn) between 39 kg·mol(-1) and 205 kg·mol(-1) is obtained by means of a simple thermal treatment. In the case of standard thermal treatments, the self-assembly process of block copolymers is hindered at small Mn by thermodynamic limitations and by a large kinetic barrier at high Mn. We demonstrate that a fine tuning of the annealing parameters, performed by a Rapid Thermal Processing (RTP) machine, permits us to overcome those limitations. Cylindrical features are obtained by varying Mn and properly changing the corresponding annealing temperature, while keeping constant the annealing time (900 s), the film thickness (∼30 nm), and the PS fraction (∼0.7). The morphology, the characteristic dimensions (i.e., the pore diameter d and the pore-to-pore distance L0), and the order parameter (i.e., the lattice correlation length ξ) of the samples are analyzed by scanning electron microscopy and grazing-incidence small-angle X-ray scattering, obtaining values of d ranging between 12 and 30 nm and L0 ranging between 24 and 73 nm. The dependence of L0 as a 0.67 power law of the number of segments places these systems inside the strong segregation limit regime. The experimental results evidence the capability to tailor the self-assembly processes of block copolymers over a wide range of molecular weights by a simple thermal process, fully compatible with the stringent constraints of lithographic applications and industrial manufacturing.