Host-guest co-amorphous structure revealed by the suppression of the first sharp diffraction peak in isotactic poly(4-methyl-1-pentene)

Abstract: While host-guest co-crystals are well established, and co-amorphous solids are recognized in materials science, the concept of a host-guest co-amorphous structure remains largely unexplored. A potential analogue is seen in SiO2 glass under high pressure with helium as a pressure medium; the drop in compressibility in this system is ascribed to helium atoms occupying internal voids. In this study, we investigated a semicrystalline polymer, isotactic poly(4-methyl-pentene-1) (P4MP1), which shares key characteristics with SiO2 glass, particularly regarding the first sharp diffraction peak (FSDP). The FSDP in P4MP1 is attributed to internal voids, as evidenced by its suppression under pressure and recovery upon decompression for molten P4MP1. Notably, the response to helium as a pressure medium is also known to parallel the behavior observed in SiO2 glass. Here, we analyzed two-dimensional X-ray diffraction (2D-XRD) patterns of stretched P4MP1 and found a suppression of FSDP when P4MP1 is immersed in decane. The use of stretched samples enabled the clear isolation of the amorphous FSDP from overlapping crystalline diffractions. Our findings reveal the existence of a host-guest co-amorphous system at room temperature and atmospheric pressure, in which decane molecules occupy the amorphous host matrix of P4MP1. Unlike conventional co-amorphous mixtures, this structure is defined by the specific accommodation of guests within the host's inherent voids. Intriguingly, the signature of this structure in diffraction measurements, manifested as changes in the FSDP intensity ratio, may be regarded to parallel the variations in Bragg peak intensity ratios in host-guest co-crystals. Since selective sorption and guest exchange are well-known in co-crystals, hosts capable of forming co-amorphous structures will be promising materials for molecular sieves, or more generally, liquid-phase molecular sieves.

• The paper shows that decane inclusion in P4MP1 significantly suppresses and shifts the FSDP, confirming a host-guest co-amorphous structure.
• It employs advanced XRD and SANS techniques to isolate amorphous void signatures, revealing a 3% increase in interchain distance due to guest occupation.
• The findings offer insights for designing functional molecular sieves and selective sorbents based on amorphous polymers.

Host-Guest Co-Amorphous Structure in Isotactic Poly(4-methyl-1-pentene): Suppression of the First Sharp Diffraction Peak

Introduction

The investigation assesses the existence and characterization of a host-guest co-amorphous structure in semicrystalline isotactic poly(4-methyl-1-pentene) (P4MP1). Although host-guest co-crystals and co-amorphous mixtures are well established, the concept of a co-amorphous structure where guest molecules selectively occupy voids within a disordered host matrix has not been formally demonstrated under ambient conditions. Drawing parallels to phenomena observed in SiO$_2$ glass under high-pressure helium environments, the study elucidates the nature of voids in the amorphous domains of P4MP1 and their selective occupation by guest molecules, manifesting in unique scattering signatures in X-ray and neutron experiments. The research conclusively identifies a host-guest co-amorphous structure, with implications for molecular sieving and the design of functional porous polymers.

Experimental Approach

Uniaxially stretched P4MP1 samples with a defined crystallinity (38%) were prepared, ensuring the ability to distinguish between crystalline and amorphous domains via X-ray and neutron scattering. Decane (protonated and deuterated) served as the guest molecule for occlusion studies. Wide-angle 2D-XRD at SPring-8 and SANS at J-PARC were leveraged to probe the structure, targeting the first sharp diffraction peak (FSDP) and low-Q lamellar features. Strategic azimuthal selection permitted isolation of the FSDP from overlapping crystalline reflections, while SANS contrast was optimized using deuterated guests to localize the guest phase within amorphous regions.

Results

FSDP Suppression and Structural Signature

XRD patterns of dry, stretched P4MP1 exhibit a pronounced, isotropic FSDP at $Q \approx 0.67$ Å$^{-1}$—a direct analog to the FSDP observed in oxide glasses. Upon decane occlusion, this FSDP undergoes a significant decrease in intensity as well as a shift to lower Q ($\sim$0.65 Å$^{-1}$), equivalent to a $\sim$3% increase in the average interchain distance within the amorphous region. These observations are consistent with guest molecules occupying intrinsic voids, reducing the electron density contrast responsible for FSDP formation. Critically, the accompanying crystalline Bragg reflections experience only minor shifts ($<$0.3%), reaffirming that the guest penetrates primarily amorphous domains.

SANS Confirmation of Amorphous Occlusion

SANS measurements employing deuterated decane display the emergence of a distinct lamellar peak at $Q \sim 0.016$ Å$^{-1}$ along the stretching (meridional) direction upon solvent sorption. In dry samples, the lack of sufficient scattering contrast (owing to identical densities of P4MP1’s crystalline and amorphous regions) suppresses this feature. The appearance of the lamellar peak post-occlusion quantitatively verifies that the solvent partitioning is confined to the amorphous layers, corroborating the XRD findings. The presence of isotropic Å-scale voids in a macroscopically oriented sample is further substantiated by the persistence of ring-like FSDP in XRD.

Discussion

Structural Nature and Analogy to Glasses

The suppression of the FSDP in P4MP1 upon guest sorption reflects a parallel to the high-pressure behavior of SiO$_2$ glasses with helium: the FSDP (and thus the intermediate-range order associated with voids) is sensitive to guest occupation. However, unlike co-crystals, the host polymer framework retains its amorphous structure, and guest inclusion modifies the S(Q) intensity ratio. This effect, distinct from general co-amorphous mixing, depends on the molecular architecture of the host, specifically P4MP1’s bulky side chains that stabilize persistent intrinsic voids.

Notably, the magnitude of voids in P4MP1 exceeds those in typical oxide or chalcogenide glasses, enabling accommodation of large hydrocarbons such as decane rather than just small gases (e.g., He). The tetrahedral local packing motif (due to sp$Q \approx 0.67$0 carbon hybridization) configures void networks analogous to inorganic glasses, despite the polymer’s organic composition.

Implications for Functional Materials

The identification of host-guest co-amorphous systems at ambient conditions has direct implications for the design of advanced molecular sieves and selective sorbents. Unlike crystalline sieves (zeolites, MOFs), the amorphous host here allows for tunable guest uptake and selectivity, exemplified by P4MP1’s preference for longer alkanes. The robustness of the host’s void topology—even in liquid states—further points toward applications in porous liquids and soft-matter separation processes.

From a theoretical standpoint, the work provides a platform for future studies on the thermodynamics of guest inclusion, polyamorphism, and the dynamic interplay between host flexibility and guest selectivity. It also poses challenges for modeling the structure factor S(Q) in amorphous host-guest systems, especially regarding the impact of guest size, shape, and interaction on the FSDP and related features.

Future Perspectives

Key research directions include:

• Quantifying guest-induced expansion and void accessibility: Systematic studies with varying guest size, polarity, and concentration to delineate the limits of host adaptability.
• Molecular dynamics and modeling: Microstructural simulations to resolve void networks and their correlation with experimental S(Q).
• Functional testing: Assessment of host-guest co-amorphous polymers as molecular sieves in gas, vapor, and liquid separations; exploration in responsive or switchable sieve systems.
• Dynamic studies: Time-resolved X-ray and neutron scattering to investigate diffusion, exchange kinetics, and stability of guest binding, especially under varying stress or external field conditions.

Conclusion

The study rigorously establishes the existence and characterization of a host-guest co-amorphous structure in P4MP1, verified through suppression and shifting of the FSDP upon decane sorption. The combined XRD and SANS evidence signifies the selective occupation of amorphous voids by guest molecules, with minimal perturbation to crystalline order. This structural paradigm, not previously formalized at ambient conditions in polymers, holds considerable promise for the development of amorphous-phase molecular sieves and void-engineered functional materials. The findings bridge the gap between classical host-guest crystallography and the flexible topology of amorphous polymers, extending both the conceptual and application horizons in materials science.