arXiv subjects

Johan Mattsson

Publications and source records attributed to Johan Mattsson.

9 recordsLinked to original sources

arXiv2025

Geometry-Dependent Adhesion in Transparent, Monodomain Liquid Crystal Elastomers

Elastomeric pressure-sensitive adhesives (PSAs) form adhesive bonds under light pressure. Liquid crystal elastomers (LCEs) are exciting PSA candidates as they can impart both anisotropy and temperature-dependence to adhesion, but the full potential of their anisotropic adhesion is unexplored. Here, identical side-chain LCEs, produced as transparent isotropic or nematic films are investigated; the latter aligned in homeotropic or planar geometries. Their room-temperature adhesion, determined through a 90-degree peel test, is consistent with theoretical predictions and strongest in a planar geometry (peeled parallel to the director) with adhesive force per unit length of 0.67 Nmm-1. In contrast, adhesion of the planar perpendicular, isotropic and homeotropic films is 62.5%, 38.5% and 23.0% lower, respectively. The surface contribution to adhesion is identical for all films, confirming that the variation in adhesion is determined solely by the bulk LCE alignment controlled during film preparation. A temperature-dependent adhesion factor is determined from 0 Celsius to 80 Celsius using dynamic mechanical analysis, and found to be in excellent agreement with the peel data at room temperature. Molecular relaxations active above the glass transition temperature are dominant in determining LCE adhesion. The results show that side-chain LCEs can function as transparent, tunable, broad-temperature smart PSAs

cond-mat.soft↗

arXiv2025

Ultra-stretchable and Self-Healable Vitrimers with Tuneable Damping and Mechanical Response

Vitrimers are a relatively new class of polymer materials with unique properties offered by cross-links that can undergo associative exchange dynamics. We here present a new class of vitrimers based on poly(methyl acrylate) with cross-links utilising dioxaboralane metathesis. These vitrimers demonstrate a combination of ultra-stretchability (up to $\sim$ 80 times their own length), mechanical toughness ($\sim$ 40 MJ/m$^3$), and thermal stability up to $T\sim$ 250 {\deg}C; moreover, the vitrimers demonstrate excellent mechanical damping characterised by a loss factor ($\tan(\delta)$) with a maximum of $\sim$ 2-3 and an effective value $>$0.3 across five decades in frequency (0.001-100 Hz), or correspondingly across a $T$-range of $\sim$ 35 {\deg}C near room temperature (for a probe frequency of 1 Hz). The vitrimers can be successfully re-processed using both a thermo-mechanical and a chemical processing route, and can for low cross-link density self-heal at room temperature, making them suitable for sustainable applications. The material properties are directly tuneable by variation of both the amount of cross-linker and by the degree of curing. Thus, this class of vitrimers are promising for applications where stretchability combined with mechanical toughness and/or a high mechanical dissipation is required.

cond-mat.soft↗

arXiv2024

A fast transferable method for predicting the glass transition temperature of polymers from chemical structure

We present a new method that successfully predicts the glass transition temperature $T_{\! \textrm{g}}$ of polymers based on their monomer structure. The model combines ideas from Group Additive Properties (GAP) and Quantitative Structure Property Relationship (QSPR) methods, where GAP (or Group Contributions) assumes that sub-monomer motifs contribute additively to $T_{\! \textrm{g}}$, and QSPR links $T_{\! \textrm{g}}$ to the physico-chemical properties of the structure through a set of molecular descriptors. This method yields fast and accurate predictions of $T_{\! \textrm{g}}$ for polymers based on chemical motifs outside the data sample, which resolves the main limitation of the GAP approach. Using a genetic algorithm, we show that only two molecular descriptors are necessary to predict $T_{\! \textrm{g}}$ for PAEK polymers. Our QSPR-GAP method is readily transferred to other physical properties, to measures of activity (QSAR), or to different classes of polymers such as conjugated or bio-polymers.

cond-mat.soft↗

arXiv2024

Chain-length-dependent correlated molecular motion in polymers

We show how dynamic heterogeneities (DH), a hallmark of glass-forming materials, depend on chain flexibility and chain length in polymers. For highly flexible polymers, a relatively large number of monomers ($N_c\sim500$) undergo correlated motion at the glass transition temperature $T_g$, independent of molecular weight ($M$). In contrast, less flexible polymers show a complex $N_c(M)$ behaviour divided into three regimes, consistent with observation in both $T_g(M)$ and chain conformational structure. For short oligomers ($\lesssim$ 2 Kuhn steps), a transition from mainly $\it{inter}$molecular correlations and $N_c\sim 200$, to strongly $\it{intra}$molecular correlations and $N_c< 50$ (roughly the molecular size) is observed; for longer chains, $N_c$ increases weakly, before saturating. For poly(methyl methacrylate), a remarkable similarity is found between $N_c(M)$ and the $M$-dependent ratio of the activation barriers of the structural ($\alpha$) and secondary ($\beta$) relaxations. Our results suggest a link between the DH length-scale and the number of $\beta$ relaxation events jointly-activated to facilitate the $\alpha$ relaxation.

cond-mat.soft↗

arXiv2019

Cooperative intramolecular dynamics control the chain-length-dependent glass transition in polymers

The glass transition is a long-standing unsolved problem in materials science. For polymers, our understanding of glass-formation is particularly poor due to the added complexity of chain connectivity and flexibility; structural relaxation of polymers thus involves a complex interplay between intra- and inter-molecular cooperativity. Here we study how the glass transition temperature Tg varies with molecular weight M for different polymer chemistries and chain flexibilities. We find that Tg(M) is controlled by the average mass (or volume) per conformational degree of freedom, and that a `local' molecular relaxation (involving a few conformers) controls the larger-scale cooperative alpha relaxation responsible for Tg. We propose that dynamic facilitation where a `local' relaxation facilitates adjacent relaxations, leading to hierarchical dynamics, can explain our observations including logarithmic Tg(M) dependences. Our study provides a new understanding of molecular relaxations and the glass transition in polymers, which paves the way for predictive design of polymers based on monomer-scale metrics.

cond-mat.soft↗

arXiv2017

Mechanics from Calorimetry: A New Probe of Elasticity for Responsive Hydrogels

Temperature-sensitive hydrogels based on polymers such as poly(N-isopropylacrylamide) (PNIPAM) undergo a volume phase transition in response to changes in temperature. During this transition, distinct changes in both thermal and mechanical properties are observed. Here, we illustrate and exploit the inherent thermodynamic link between thermal and mechanical properties by showing that the compressive elastic modulus of PNIPAM hydrogels can be probed using differential scanning calorimetry. We validate our approach by using conventional osmotic compression tests. Our method could be particularly valuable for determining the mechanical response of thermosensitive submicron-sized and/or oddly shaped particles, to which standard methods are not readily applicable.

cond-mat.soft↗

arXiv2010

Dielectric secondary relaxation of water in aqueous binary glass-formers

The dielectric relaxation of water in glassy aqueous binary mixtures exhibits an Arrhenius behaviour with a nearly universal activation energy. We here demonstrate that its characteristic relaxation time follows a remarkably general functional dependence on the weight fraction of water for a wide range of molecular systems.

cond-mat.soft↗

arXiv2009

Effects of hydrogen bonding on supercooled liquid dynamics and the implications for supercooled water

The supercooled state of bulk water is largely hidden by unavoidable crystallization, which creates an experimentally inaccessible temperature regime - a 'no man's land'. We address this and circumvent the crystallization problem by systematically studying the supercooled dynamics of hydrogen bonded oligomeric liquids (glycols), where water corresponds to the chain-ends alone. This novel approach permits a 'dilution of water' by altering the hydrogen bond concentration via variations in chain length. We observe a dynamic crossover in the temperature dependence of the structural relaxation time for all glycols, consistent with the common behavior of most supercooled liquids. We find that the crossover becomes more pronounced for increasing hydrogen bond concentrations, which leads to the prediction of a marked dynamic transition for water within 'no man's land' at T~220 K. Interestingly, the predicted transition thus takes place at a temperature where a so called 'strong-fragile' transition has previously been suggested. Our results, however, imply that the dynamic transition of supercooled water is analogous to that commonly observed in supercooled liquids. Moreover, we find support also for the existence of a secondary relaxation of water with behavior analogous to that of the secondary relaxation observed for the glycols.

cond-mat.soft↗

arXiv2007

Strain-Rate Frequency Superposition (SRFS) - A rheological probe of structural relaxation in soft materials

The rheological properties of soft materials often exhibit surprisingly universal linear and non-linear features. Here we show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, Strain-Rate Frequency Superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.

cond-mat.soft↗