Evaluation of force fields for molecular simulation of polyhedral oligomeric silsesquioxanes

Polyhedral oligomeric silsesquioxanes (POSS) are nanometer-size molecules suitable for the production of organic-inorganic nanocomposite materials. These organic-inorganic nano-building blocks show promise for enabling the production of polymeric materials of exceptional mechanical properties as well as novel composite materials. While the experimental studies of these materials have rapidly evolved in the past decade, their theoretical investigation is still in its infancy. Toward the validation of force fields for the molecular simulation of POSS-containing systems, we present the charge-transfer reactive (CTR) force field for the molecular simulation of polyhedral oligomeric silsesquioxane (POSS) molecules and compare the ability of this, and several force fields taken from the literature, to predict the thermophysical properties of POSS-containing systems. The literature force fields compared include the universal force field (UFF) and the COMPASS and Hybrid-COMPASS force fields. Predictions from molecular dynamics simulations of the structural parameters (unit cell vectors), melting temperature, and FT-IR spectra of crystals of POSS monomers are presented. The POSS monomers investigated are octahydride, octamethyl, and octapropyl POSS. Predicted quantities are compared to experimental results where available and provide molecular-level physical insight into several aspects of the behavior of POSS molecules. While all the force fields tested perform reasonably well, our results indicate that the Hybrid-COMPASS and CTR force fields predict structural properties that are in good agreement with experimental data.     

Density functional theory study of interactions between carbon dioxide and functionalized polyhedral oligomeric silsesquioxanes

Polyhedral Oligomeric Silsesquioxanes (POSS) are cage‐structured inorganic–organic hybrid materials which can be used in various industrial applications. It is recently discovered that POSS structures with certain functional groups can be solubilized in supercritical CO₂ allowing their applications in environmentally benign supercritical processing of materials. In this theoretical study, nature and energetics of the interactions of octatrifluoropropyl POSS, octatrifluoromethyl POSS, and octamethyl POSS with CO₂ are investigated according to the principles of density functional theory (DFT) by use of Gaussian 09 software. Simulations show that CO₂‐octamethyl POSS pair has hydrogen bonding between the O atom of CO₂ and the H atom of the methyl group, and CO₂‐octatrifluoromethyl POSS pair has interactions between the C atom of CO₂ and the F atom of the trifluoromethyl group. CO₂‐octatrifluoropropyl POSS pair is found to have both interaction types. The octamethyl, the octatrifluoromethyl and the octatrifluoropropyl POSS structures have interaction energies of ?2.18 kcal/mol, ?3.10 kcal/mol, and ?3.77 kcal/mol, respectively. This shows that the presence of Lewis acid–Lewis base interaction between C and F instead of hydrogen bonding between O and H atoms enhances the interaction of the molecule with CO₂, while the presence of both interactions between the octatrifluoropropyl POSS‐CO₂ pair makes the intermolecular interaction even stronger.     

含POSS光刻胶材料研究进展

光致抗蚀剂又称光刻胶,是微电子加工过程中的关键材料。多面体低聚倍半硅氧烷（POSS）是一种具有规则的笼型结构的聚合物增强材料,由POSS改性的聚合物实现了有机-无机纳米杂化,POSS刚性结构的引入阻碍了聚合物分子的运动,可以显著提高聚合物的玻璃化转变温度（T_g）,降低聚合物的介电常数,提高聚合物的力学性能,也提高了含POSS光致抗蚀剂的耐蚀刻性。基于这些优点,含POSS的光刻胶材料得到广泛关注。本文对含POSS光刻胶的研究进展作了简要介绍。     

Organic/inorganic hybrid star-shaped block copolymers of poly(l-lactide) and poly(N-isopropylacrylamide) with a polyhedral oligomeric silsesquioxane core: Synthesis and self-assembly

The star-shaped organic/inorganic hybrid poly(l-lactide) (PLLA) based on polyhedral oligomeric silsesquioxane (POSS) was prepared using octa(3-hydroxypropyl) polyhedral oligomeric silsesquioxane as initiator via ring-opening polymerization (ROP) of l-lactide (LLA). The molecular weight of POSS-containing star-shaped hybrid PLLA (POSSPLLA) can be well controlled by the feed ratio of LLA to initiator. The POSSPLLA was further functionalized into the macromolecular reversible addition-fragmentation transfer (RAFT) agent for the polymerization of N-isopropylacrylamide (NIPAM), leading to the POSS-containing star-shaped organic/inorganic hybrid amphiphilic block copolymers, poly(l-lactide)–block–poly(N-isopropylacrylamide) (POSS(PLLA–b–PNIPAM)). The self-assembly behavior of POSS(PLLA–b–PNIPAM) block copolymers in aqueous solution was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). DLS showed the PNIPAM block in the aggregates is temperature-responsive and its phase-transition is reversible. TEM proved that the star-shaped POSS(PLLA–b–PNIPAM) amphiphilic block copolymers can self-assemble into the vesicles in aqueous solution. The vesicular wall and coronas are composed of the hydrophobic POSS core and PLLA, and hydrophilic PNIPAM blocks, respectively. Therefore, POSSPLLA and POSS(PLLA–b–PNIPAM) block copolymers, as a class of novel organic–inorganic hybrid materials with the advantageous properties, can be potentially used in biological and medical fields.     

Temperature dependence of methanol and ethanol vapor sorption in polyhedral oligomeric silsesquioxane (POSS) containing methacrylate membranes

This study aimed to determine the solubility and temperature dependence of methanol and ethanol vapor caused by the difference in the substituents of polyhedral oligomeric silsesquioxane (POSS)-containing polymethacrylate membranes and the spacer length between the backbone and POSS backbone. Vapor sorption of methanol and ethanol was measured at 25°C, 35°C, and 45°C for three kinds of POSS-containing polymer membranes, namely, poly(methacryl isobutyl POSS), poly(methacrylate isobutyl POSS), and poly(methacryl phenyl POSS). The primary structures of the three POSS-containing polymer chains were columnar. The solubility of alcohol vapor on the POSS-containing polymer membranes followed the mechanism of solid adsorption and not the general dissolution diffusion. The sorption amount at all three temperatures was related to the surface area of the cylindrical primary structure and the solid adsorption property of the alcohol molecule of the POSS substituent. The sorption amount increased because of the large surface area and adsorption property of alcohol molecules. Although a typical glassy polymer shows exothermic mixing and a rubbery polymer displays endothermic mixing, the sample with the POSS substituent of isobutyl group exhibited an unusual behavior of endothermic mixing despite being a glassy polymer.     

Poly(ethylene imine) hybrids containing polyhedral oligomeric silsesquioxanes: Preparation, structure and properties

Both octaglycidyletherpropyl polyhedral oligomeric silsesquioxane and hepta(3,3,3-trifluoropropyl)glycidyletherpropyl polyhedral oligomeric silsesquioxane were synthesized via the hydrosilylation reactions between octahydrosilsesquioxane [and/or hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane] and allyl glycidyl ether. The polyhedral oligomeric silsesquioxane (POSS) macromers were characterized by means of Fourier transform infrared and nuclear magnetic resonance spectroscopy. The inter-component macromolecular reactions between the POSS macromers and poly(ethylene imine) (PEI) were employed to prepare the POSS-containing organic-inorganic PEI hybrids. The inclusion of octaglycidyletherpropyl POSS into PEI results in the formation of the organic-inorganic hybrid networks whereas the introducing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS to PEI affords the linear POSS-grafted PEI copolymers. Differential scanning calorimetry and thermogravimetric analysis show that the POSS-containing PEI hybrids displayed increased glass transition temperatures (T_g’s) and enhanced thermal stability compared to the plain PEI. These PEI hybrid composites can be significantly swollen with water without dissolving, suggesting the formation of hydrogels. The PEI hydrogels containing octaglycidyletherpropyl POSS is in reality the chemically-crosslinked hydrogels whereas the those containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS displayed the behavior of physical hydrogels. The formation of physical hydrogels is ascribed to the microphase-separated morphology in the hybrids. In addition, the hybrids containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS exhibited the typical amphiphilicity as evidenced by the increase in surface hydrophobilicity.     

Thermodynamic and transport properties of polyhedral oligomeric sislesquioxanes in poly(dimethylsiloxane)

Polyhedral oligomeric silsesquioxane (POSS) molecules when functionalized appropriately and dispersed in polymers show promise as monodisperse organic-inorganic hybrid nanocomposite materials characterized by superior mechanical properties. We report here molecular-simulation results for POSS-POSS radial distribution functions, potentials of mean force, and self-diffusion coefficients for POSS monomers dissolved in poly(dimethylsiloxane) in the temperature range of 300-1000 K. Our results demonstrate that it is possible to modulate the effective POSS-POSS interaction by increasing the temperature or by substituting the hydrogen atoms in the POSS monomer with methyl groups. In addition, our results indicate that the motion of POSS monomers dissolved in poly(dimethylsiloxane) follows a hopping mechanism.     

含多面低聚倍半硅氧烷新型杂化聚合物的合成

基于多面低聚倍半硅氧烷（POSS）的杂化聚合物是近年发展起来的一类新型有机/无机杂化材料,性能独特,应用前景广阔。本文综述了含POSS新型杂化聚合物的合成研究进展,涉及自由基溶液聚合、可控活性聚合、开环易位聚合、缩聚和配位聚合。     

Poly(MMA-co-MBA-co-MA-POSS)三元共聚物溶液性质的反气相色谱表征

以20种溶剂作为探针分子,采用反气相色谱技术表征了甲基丙烯酸甲酯-甲基丙烯酸丁酯-甲基丙烯酰氧丙基七环戊基倍半硅氧烷三元共聚物[poly(MMA-co-MBA-co-MA-POSS)]的一系列物理化学性质,并分析了溶剂探针分子与共聚物分子间的相互作用以及共聚物在溶剂中的溶解性.结果表明:在实验温度范围内(343～393K),乙酸甲酯、乙酸乙酯、芳烃、二氯甲烷、三氯甲烷是良溶剂,乙酸丙酯、乙酸丁酯、乙酸戊酯、四氯化碳是中等溶剂,烷烃类和醇类是劣溶剂;随着共聚物中POSS含量的增大,溶剂探针分子溶解聚合物的能力增强,但引入POSS对共聚物的溶度参数无明显影响.     

Organic-inorganic telechelic molecules: solution properties from simulations

We report molecular dynamics simulations for telechelic molecules composed of two polyhedral oligomeric silsesquioxane (POSS) cages connected by one hydrocarbon backbone dissolved in liquid normal hexane. Silsesquioxanes are novel hybrid organic-inorganic molecules that are useful as building blocks for the synthesis of nanostructured materials. By including POSS molecules within a polymeric material it is possible to modify mechanical properties such as resistance to heat and glass transition temperatures. Unfortunately, the molecular mechanisms responsible for these enhancements are at present not completely understood. In an effort to elucidate the molecular phenomena responsible for these effects, we have studied the conformation of telechelic POSS molecules in solution, as well as their self-diffusion coefficients, as a function of the length of the hydrocarbon backbone. We focus on molecules in which the radius of gyration of the alkane backbone is comparable to the size of the silsesquioxane cages. Our results indicate that the backbone has a significant influence on both the equilibrium and the transport properties of dissolved telechelic hybrid molecules. These observations are useful for developing strategies to direct the self-assembly of nanostructured materials.     

Polymers with double-decker and side-opened cage silsesquioxanes: Effects of cage structures on materials properties

Cage silsesquioxanes, also known as polyhedral oligomeric silsesquioxanes (POSS), serve as crucial building blocks in crafting precisely designed organic–inorganic hybrid materials, given that their well-defined silsesquioxane clusters can be adorned with organic substituents. While polymers with POSS in their main chains have been thoroughly examined, analyzing the correlation between cage structure and material properties in main-chain-type polymers remains challenging. This difficulty stems from the limited range of organic substituents on traditional POSS monomers, thereby precluding comparisons between polymers with unified substituents and different cage structures. In this study, we synthesized double-decker silsesquioxane (DDSQ) and side-opened POSS (SO-POSS) monomers, both featuring phenyl groups. Subsequent platinum-catalyzed hydrosilylation polymerization yielded main-chain-polymers. Both the cage and linker structures influence thermal stability and the glass transition temperature, while the hardness was primarily determined by the linker structure. This research is the first to elucidate the impact of cage structure on the material properties of main-chain-type POSS polymers.     

Ab initio analysis of the structural properties of alkyl-substituted polyhedral oligomeric silsesquioxanes

Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examined the unsubstituted POSS (SiO1.5H)8 cage as well as linear and cyclic alkyl-substituted cages where one of the terminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for the minimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclic intermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cage does influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, its effect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkyl substitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkier cyclic intermediates that result upon rotation of the Si-C-C-C dihedral angles. The interatomic potentials for these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate sterically hindered substitutents on the cage. Our results suggest that combining an atomistic force field independently developed to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbon chains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids the need to develop specific force fields for each substituted POSS cage studied and opens up the possibility of using molecular simulation to probe the thermodynamic and structural properties of these unique nanoscale building blocks.     

POSS聚合物及其新进展

POSS 作为一种特殊的有机/无机杂化纳米构件备受关注。本文总结了近几年有关探索POSS聚合物结构与性能之间关系的研究进展,并重点综述了POSS在有机半导体材料领域的应用。     

正十六烷聚脲微胶囊化相变材料

用界面聚合法，合成了直径大约2.5 μm可用于热能储存含相变材料的聚脲包覆微胶囊.在含乳化剂的水溶液中，将溶有芯材正十六烷的有机相乳化成微米级油性液滴，随后加入的水溶性单体二胺与甲苯2,4-二异氰酸酯在胶束界面相互反应形成囊壁.分别用乙烯二胺，1,6-己二胺和它们的混合物作为水溶性单体进行了研究.并用红外光谱和热分析分别考察了不同胺类对微胶囊化学结构和热性质的影响.红外谱图显示合成了聚脲微胶囊，热重曲线表明含正十六烷的聚脲微胶囊能够耐受大约300 ℃高温，差示扫描量热测试表明所有样品均具有合适的相转变热，冷热循环实验揭示微胶囊能够维持储热容量不衰减.研究表明微胶囊化的正十六烷作为相变储热材料具有良好的应用前景.     

Tunable effective interactions between dendritic macromolecules

We employ extensive Monte Carlo and molecular-dynamics simulations to investigate the effective interactions between the centers of mass of dendritic macromolecules of variable flexibility and generation number. Two different models for the connectivity and steric interactions between the monomers are employed, the first one being purely entropic in nature and the second explicitly involving energetic interactions. We find that the effective potentials have a generic Gaussian shape, whose range and strength can be tuned via modifications in the generation number and flexibility of the spacers. We supplement our simulation analysis by a density-functional approach in which the connectivity between the monomers is approximated by an external confining potential that holds the monomer beads together. Using a simple density functional for the interactions between the monomers, we find semiquantitative agreement between theory and simulation. The implications of our findings for the interpretation of scattering data from concentrated dendrimer solutions are also discussed.     

Polymers and cyclic compounds based on a side‐opening type cage silsesquioxane

Polymers having polyhedral oligomeric silsesquioxane (POSS) in the main chains are an important class of organic–inorganic hybrid materials. Despite the increasing attention to the POSS polymers, variation of the monomers is still limited. Herein, we have proposed side‐opening POSS (SO‐POSS) monomers. Platinum‐catalyzed hydrosilylation polymerization proceeded to produce polysiloxanes having SO‐POSS in the main chains. The obtained polysiloxanes showed good solubility, high thermal stability, high transparency, and tunable reflective index. In addition, cyclic compounds were obtained during the investigation of the polymerization, and were synthesized with high selectivity under the slightly diluted conditions. The obtained cyclic compounds showed high thermal stability due to the silsesquioxane backbone, and the high dispersibility as a filler in poly(methyl methacrylate) was demonstrated. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2243–2250     

Poly(N-isopropylacrylamide) nanocrosslinked by polyhedral oligomeric silsesquioxane: temperature-responsive behavior of hydrogels

Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) was used as a nanocrosslinking agent to prepare the crosslinked poly(N-isopropylacrylamide) (PNIPAM) networks with POSS content up to 50 wt%. The inter-component crosslinking was achieved via the reaction between NH moieties in amide group of PNIPAM and epoxide groups of OpePOSS. When the organic-inorganic nanocomposites were swollen in water the POSS-crosslinked PNIPAM exhibited the characteristics of hydrogels. With the moderate contents of POSS, the POSS-containing hybrid hydrogels displayed much faster response rates in swelling, deswelling and reswelling experiments than the PNIPAM hydrogels prepared via the free radical copolymerization of N-isopropylacrylamide (NIPAM) and N,N(')-methylenebisacrylamide (viz. the conventional crosslinker). The improved hydrogel properties have been interpreted on the basis of the formation of the nanosized hydrophobic microdomains around the POSS moieties (i.e., the nanocrosslinking sites).     

Synthesis and characterization of a novel arylacetylene oligomer containing POSS units in main chains

A novel arylacetylene oligomer containing octamethyl POSS units in main chains was prepared from difluoride octamethyl POSS (diexo-(CH₃)₈Si₈O₁₁F₂) and diethynylbenzene (DEB) by Grignard reaction and characterized by FT-IR, NMR, WAXD, GPC, DSC, and TGA. The curing reaction kinetic of the oligomer was studied by Kissinger and Ozawa methods and the kinetic parameters were obtained. The cured polymer had good thermal and thermal-oxidative properties. TGA analyses demonstrated that the thermal decomposition temperature (T_d5) of the cured polymer in nitrogen and air were 503 and 479 °C, respectively.     

Tunable Multiple Morphological Transformation of Supramolecular Hyperbranched Polymers Based on an A2B6-type POSS Monomer

The growing demands of supramolecular hyperbranched polymers integrating noncovalent interaction and unique topological structure merits had received considerable interest in the fabrication of novel materials for advanced applications.Herein,we prepared A2B6-type POSS-containing supramolecular hyperbranched polymers with multiple morphologies including lamellar-like,branched,hollow,core-shell and porous spherical structures through regulating self-assembling monomer concentrations and solvent polarities.The incorporation of appropriate emulative guest molecules would further trigger morphological transformations (such as vesicles and spherical micelles) by synergistic effects of unique POSS aggregation ability,supramolecular complexations and hydrophilic-hydrophobic interactions.Thus,this facile and universal strategy may enable a modular nanofabrication of supramolecular hyperbranched polymers with diversiform topological structure and sophisticated multifunctionality for their potential applications.     

Synthesis of silsesquioxane nanocomposites

In order to investigate the effect of presence of well defined nano-sized inorganic particles on the molecular mobility a conformation statistics of polymer chains, well defined polystyrene (PS) and poly(methyl methacrylate) (PMMA) macromolecules containing polyhedral oligomeric silsesquioxanes nanoparticles (POSS) were synthesized by copper-mediated atom transfer radical polymerization (ATRP). Two approaches were used for the synthesis — the first involves POSS as the initiator of ATRP; the second way considers an addition of POSS to the polymer (prepared by ATRP) with an appropriate functional group. Kinetics of polymerization was determined using common analytical methods and it was compared to the polymerizations initiated by low-molecular weight initiators, regarding the polymerization rate, initiation efficiency and polydispersity of the polymer. Efficiency of the initiation with POSS-containing initiators was low, causing remnants of inseparable free POSS in polymer. The second approach bypassed these disadvantages —POSS is connected to the polymer through a pending allyl group using the very efficient hydrosilylation reaction. Presented at the 1st Bratislava Young Polymer Scientists Workshop, Bratislava, 20–23 August 2007.