Phase transitions in polymers have long been a subject of intense study due to their relevance in materials science, biology, and nanotechnology. This paper investigates the conformational phase behavior of flexible and semiflexible polymers using a generalized microcanonical inflection-point analysis method. By analyzing the microcanonical entropy and its derivatives, we identify and classify phase transitions across various energy regimes. The study focuses on how bending stiffness influences the character and order of transitions between random-coil, globular, and pseudocrystalline states.
Our simulations employ a coarse-grained model where monomers interact via Lennard-Jones potentials and bonded interactions are modeled with FENE potentials. Bending stiffness is introduced through an additional angle-dependent potential, allowing us to tune the chain’s rigidity. Replica-exchange Monte Carlo simulations combined with multiple-histogram reweighting provide high-accuracy estimates of the density of states. These data enable precise calculation of microcanonical quantities essential for identifying transition points.
In the flexible polymer case (bending stiffness parameter λ = 0), two distinct transitions are observed. First, a second-order collapse transition occurs as the system evolves from extended random coils to compact globules, driven primarily by entropic forces. Second, a first-order liquid-solid transition marks the formation of a solid-like phase dominated by icosahedral symmetry. This transition is accompanied by a third-order transition, consistent with Ehrenfest classification.
When bending stiffness increases (λ = 1, 2), qualitative changes emerge. The collapse transition remains largely unchanged, indicating that entropy-driven structural compaction is robust against moderate stiffness. However, the first-order liquid-solid transition disappears entirely when bending effects dominate over attractive monomer-monomer interactions. For λ = 1, the transition weakens and becomes second-order; for λ = 2, no clear signal persists even at low energies.
Structural analysis of ground-state conformations reveals the physical origin of these changes. In the flexible case, the optimal structure is highly symmetric—icosahedral—with maximal nearest-neighbor contacts.KRT14 Antibody References As bending stiffness increases, this symmetry breaks down. For λ = 1, the structure retains partial order but shows increased strain. For λ = 2, the system adopts a disordered, entangled configuration composed of longer, slightly bent segments that avoid high curvature.CD62L Antibody site Pair distribution functions and contact maps confirm the loss of global symmetry and the emergence of local secondary structures such as hairpins and helical segments.PMID:35069852
These findings demonstrate that bending stiffness fundamentally alters the thermodynamic landscape of polymers. While short-range attractions drive collapse, long-range ordering requires flexibility to achieve symmetric packing. When bending constraints prevail, the system cannot form crystalline or quasicrystalline phases, instead favoring topologically complex, coil-like arrangements reminiscent of protein tertiary folds.
This work underscores the importance of microcanonical methods in studying finite systems, where traditional canonical approaches may fail to capture subtle transition behaviors. The generalized inflection-point analysis provides a powerful framework for classifying transitions regardless of system size, offering new insights into the role of mechanical constraints in macromolecular self-assembly. Future extensions to stronger bending regimes and more complex architectures promise deeper understanding of functional polymeric materials.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com