Dynamic Heterogeneity in Random and Gradient Copolymers: A Computational Investigation

A Numerical simulation method for analyzing 1H spin diffusion NMR for Multicomponent and multiphase polymer systems

A numerical simulation method, namely, SDNMR-WEBFIT, is reported for simulating proton spin diffusion NMR based on the Levenberg-Marquardt algorithm and a pseudo-2D diffusion model. This method is used for the precise quantification of dynamics heterogeneity of the interphase within multiphase polymer systems. The numerical simulation method provides measurements of spin-lattice relaxation time (T1), proton density (ρH), lamellar thickness (d), and spin diffusion coefficient (D) for each component. The pseudo-2D diffusion model is employed to simulate the proton spin diffusion build-up/decay curves, simultaneously calculating the lateral fraction of island-like structures (x-ratio). Such approach was successfully applied to various polymer systems, such as semi-crystalline polymer (Poly(ε-caprolactone), PCL), block copolymers (Styrene-butadiene-styrene triblock copolymer, SBS), and plasticized semi-polymers (Polvinyl alcohol, PVA).

Dynamics of polylactic acid under ultrafine nanoconfinement: The collective interface effect and the spatial gradient

Polymers under nanoconfinement can exhibit large alterations in dynamics from their bulk values due to an interface effect. However, understanding the interface effect remains a challenge, especially in the ultrafine nanoconfinement region. In this work, we prepare new geometries with ultrafine nanoconfinement ∼10nm through controlled distributions of the crystalline phases and the amorphous phases of a model semi-crystalline polymer, i.e., the polylactic acid. The broadband dielectric spectroscopy measurements show that ultrafine nanoconfinement leads to a large elevation in the glass transition temperature and a strong increment in the polymer fragility index. Moreover, new relaxation time profile analyses demonstrate a spatial gradient that can be well described by either a single-exponential decay or a double-exponential decay functional form near the middle of the film with a collective interface effect. However, the dynamics at the 1-2 nm vicinity of the interface exhibit a power-law decay that is different from the single-exponential decay or double-exponential decay functional forms as predicted by theories. Thus, these results call for further investigations of the interface effect on polymer dynamics, especially for interfaces with perturbed chain packing.

Current status and future directions of self-assembled block copolymer membranes for molecular separations

One of the most efficient and promising separation alternatives to thermal methods such as distillation is the use of polymeric membranes that separate mixtures based on molecular size or chemical affinity. Self-assembled block copolymer membranes have gained considerable attention within the membrane field due to precise control over nanoscale structure, pore size, and chemical versatility. Despite the rapid progress and excitement, a significant hurdle in using block copolymer membranes for nanometer and sub-nanometer separations such as nanofiltration and reverse osmosis is the lower limit on domain size features. Strategies such as polymer post-functionalization, self-assembly of oligomers, liquid crystals, and random copolymers, or incorporation of artificial/natural channels within block copolymer materials are future directions with the potential to overcome current limitations with respect to separation size.

Temperature dependence of the rigid amorphous fraction of poly(butylene succinate)

In this contribution the temperature evolution of the constrained or rigid amorphous fraction (RAF) of biodegradable and biocompatible poly(butylene succinate) (PBS) was quantified, after detailed thermodynamic characterization by differential scanning calorimetry and X-ray diffraction analysis. At the glass transition temperature, around -40 °C, the rigid amorphous fraction in PBS is about 0.25. It decreases with increasing temperature and becomes zero in proximity of 25 °C. Thus, at room temperature and at the human body temperature, all the amorphous fraction is mobile. This information is important for the development of PBS products for various applications, including biomedical applications, since physical properties of the rigid amorphous fraction, for example mechanical and permeability properties, are different from those of the mobile amorphous fraction.

The Martini Model in Materials Science

The Martini model, a coarse-grained force field initially developed with biomolecular simulations in mind, has found an increasing number of applications in the field of soft materials science. The model's underlying building block principle does not pose restrictions on its application beyond biomolecular systems. Here, the main applications to date of the Martini model in materials science are highlighted, and a perspective for the future developments in this field is given, particularly in light of recent developments such as the new version of the model, Martini 3.

Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement

The nature of alterations to dynamics and vitrification in the nanoscale vicinity of interfaces-commonly referred to as "nanoconfinement" effects on the glass transition-has been an open question for a quarter century. We first analyze experimental and simulation results over the last decade to construct an overall phenomenological picture. Key features include the following: after a metrology- and chemistry-dependent onset, near-interface relaxation times obey a fractional power law decoupling relation with bulk relaxation; relaxation times vary in a double-exponential manner with distance from the interface, with an intrinsic dynamical length scale appearing to saturate at low temperatures; the activation barrier and vitrification temperature T_g approach bulk behavior in a spatially exponential manner; and all these behaviors depend quantitatively on the nature of the interface. We demonstrate that the thickness dependence of film-averaged T_g for individual systems provides a poor basis for discrimination between different theories, and thus we assess their merits based on the above dynamical gradient properties. Entropy-based theories appear to exhibit significant inconsistencies with the phenomenology. Diverse free-volume-motivated theories vary in their agreement with observations, with approaches invoking cooperative motion exhibiting the most promise. The elastically cooperative nonlinear Langevin equation theory appears to capture the largest portion of the phenomenology, although important aspects remain to be addressed. A full theoretical understanding requires improved confrontation with simulations and experiments that probe spatially heterogeneous dynamics within the accessible 1-ps to 1-year time window, minimal use of adjustable parameters, and recognition of the rich quantitative dependence on chemistry and interface.

Structuring polymer gels via catalytic reactions

We use computer simulations to investigate how a catalytic reaction in a polymer sol can induce the formation of a polymer gel. To this aim we consider a solution of homopolymers in which freely-diffusing catalysts convert the originally repulsive A monomers into attractive B ones. We find that at low temperatures this reaction transforms the polymer solution into a physical gel that has a remarkably regular mesostructure in the form of a cluster phase, absent in the usual homopolymer gels obtained by a quench in temperature. We investigate how this microstructuring depends on catalyst concentration, temperature, and polymer density and show that the dynamics for its formation can be understood in a semi-quantitative manner using the interaction potentials between the particles as input. The structuring of the copolymers and the AB sequences resulting from the reactions can be discussed in the context of the phase behaviour of correlated random copolymers. The location of the spinodal line as found in our simulations is consistent with analytical predictions. Finally, we show that the observed structuring depends not only on the chemical distribution of the A and B monomers but also on the mode of formation of this distribution.

Field-theoretic simulations of random copolymers with structural rigidity

Copolymers play an important role in a range of soft-materials applications and biological phenomena. Prevalent works on block copolymer phase behavior use flexible chain models and incorporate interactions using a mean-field approximation. However, when phase separation takes place on length scales comparable to a few monomers, the structural rigidity of the monomers becomes important. In addition, concentration fluctuations become significant at short length scales, rendering the mean-field approximation invalid. In this work, we use simulation to address the role of finite monomer rigidity and concentration fluctuations in microphase segregation of random copolymers. Using a field-theoretic Monte-Carlo simulation of semiflexible polymers with random chemical sequences, we generate phase diagrams for random copolymers. We find that the melt morphology of random copolymers strongly depends on chain flexibility and chemical sequence correlation. Chemically anti-correlated copolymers undergo first-order phase transitions to local lamellar structures. With increasing degree of chemical correlation, this first-order phase transition is softened, and melts form microphases with irregular shaped domains. Our simulations in the homogeneous phase exhibit agreement with the density-density correlation from mean-field theory. However, conditions near a phase transition result in deviations between simulation and mean-field theory for the density-density correlation and the critical wavemode. Chain rigidity and sequence randomness lead to frustration in the segregated phase, introducing heterogeneity in the resulting morphologies.

Synthesis and characterisation of gradient polymeric nanoparticles

In this communication, we report the successful synthesis of gradient morphology nanoparticles composed of poly(styrene-co-methyl methacrylate) and their characterisation using X-Ray Photoelectron Spectroscopy (XPS).

Gradient and block side-chain liquid crystalline polyethers

Synthesis of gradient liquid crystalline copolymers is reported for the first time, phase structures of which on multiple length scales with composition and temperature are investigated and compared with the corresponding diblock copolymers.

Combined Dependence of Nanoconfined Tg on Interfacial Energy and Softness of Confinement

We employ molecular dynamics simulations of nanolayered polymers to systematically quantify the dependence of T_g nanoconfinement effects on interfacial energy and the "softness" of confinement. Results indicate that nanoconfined T_g depends linearly on interfacial adhesion energy, with a slope that scales exponentially with the ratio of the bulk Debye-Waller factors ⟨u²⟩ of the confined and confining materials. These trends, together with a convergence at low interfacial adhesion energy to the T_g of an equivalent freestanding film, are captured in a single functional form, with only three parameters explicitly referring to the confined state. The observed dependence on ⟨u²⟩ indicates that softness of nanoconfinement should be defined in terms of the relative high frequency shear moduli, rather than low frequency moduli or relaxation times, of the confined and confining materials.

Tailor-made compositional gradient copolymer by a many-shot RAFT emulsion polymerization method

A many-shot RAFT emulsion polymerization method to synthesize gradient copolymers with high molecular weight and a tailor-made compositional gradient.