Polymorphism and co-crystalline phases of polymers

2010 - 2011Crystalline phases are extremely relevant for properties and applications of many polymeric materials. In fact, their amount, structure and morphology constitute the main factors controlling physical properties of fibers, films and thermoplastics and can be also relevant for properties of rubbers and gels. It is also well known that processing and physical properties of polymer-based materials are strongly affected by the occurrence of polymorphism (i.e. the possibility for a given polymer to crystallize in different crystalline forms) and mesomorphism (i.e. the occurrence of “disordered” crystalline phases, characterized by a degree of structural organization that is intermediate between those identifying crystalline and amorphous phases). Different has been the destiny of polymeric co-crystalline forms, i.e. structures were a polymeric host and a low-molecular-mass guest are co-crystallized. Systems composed of solid polymers and of low molecular mass molecules find several practical applications, including advanced applications. In several cases, additives (often improperly referred as guest molecules) are simply dispersed at molecular level in polymeric amorphous phases, although frequently, to reduce their diffusivity, the active molecules are covalently attached to the polymer backbone, either by polymerization of suitable monomeric units or by grafting the active species onto preformed polymers. A more simple alternative method to reduce diffusivity of active molecules in solid polymers and to prevent their self-aggregation consists in the formation of co-crystals with suitable polymer hosts. Polymeric co-crystalline forms are quite common for several regular and stereoregular polymers, like e.g. isotactic and syndiotactic polystyrene (s-PS), syndiotactic poly-p-methyl-styrene, syndiotactic poly-m-methyl-styrene, syndiotactic poly-p-chloro-styrene, syndiotactic poly-p-fluorostyrene, polyethyleneoxide, poly(muconic acid), polyoxacyclobutane, poly(vinylidene fluoride), syndiotactic polymethylmethacrylate. The removal of the low-molecular-mass guest molecules from co-crystals can generate nanoporous-crystalline phases. In this respect, it is worth noting that nanoporous crystalline structures can be achieved for a large variety of chemical compounds: inorganic (e.g., zeolites), metal-organic as well as organic. These materials, often referred as inorganic, metal-organic and organic “frameworks” are relevant for molecular storage, recognition and separation techniques. The removal of the low-molecular-mass guest molecules from polymer co-crystalline forms generates host chain rearrangements, generally leading to crystalline forms that, as usual for polymers, exhibit a density higher than that one of the corresponding amorphous phase. However, in few cases (to our knowledge, up to now only for s-PS), by using suitable guest removal conditions, nanoporous crystalline forms, exhibiting a density definitely lower than that of the corresponding amorphous phases are obtained. Poly-4-methyl-1-pentene isotactic (i-P4MP1) is a polymer characterized by a complex polymorphism and 4 different crystalline forms, some of which are obtainable only by crystallization with solvent, have been described in the literature. Monolithic and highly crystalline aerogels of isotactic poly(4-methyl-pentene-1) (i-P4MP1) have been prepared by sudden solvent extraction with supercritical carbon dioxide from thermoreversible gels. The cross-link junctions of i-P4MP1 gels, depending on the solvent, can be constituted by pure polymer crystalline phases (I or III or IV) or by polymer-solvent co-crystalline phases (for cyclohexane and carbon tetrachloride gels). Gels with co-crystalline phases lead to aerogels exhibiting the denser crystalline form II while all the other considered gels lead to aerogels exhibiting the thermodynamically stable form I. The effect of solvent on the aerogels pore structure and morphology has been also investigated by scanning electron microscopy and N2 sorption measurements. In all cases the areogels present highly porous interconnected structures with macropores and a large heterogeneity of mesopore size but without micro-sized pores. Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is a linear regular polymer, which as s-PS has the advantage to be a commercial thermoplastic polymer. PPO exhibits a high free volume or ultrapermeable amorphous phase and has been recognized as a membrane material with high permeation parameters. Although few papers have recognized that PPO crystalline phases can play a role in gas sorption and transport processes, no correlation between the amount or nature of the crystalline phase and guest sorption properties has been reported. This is mainly due to the scarce information available in the literature relative to the crystalline phases of PPO. Crystalline modifications, exhibiting largely different X-ray diffraction patterns, have been obtained for poly(2,6-dimethyl-1,4-phenylene)oxide (PPO), by gel desiccation procedures as well as by solvent-induced crystallization of amorphous films. The choice of the solvent allows controlling the nature of the crystalline phase. Both amorphous and semicrystalline samples of this commercial thermoplastic polymer exhibit a high uptake of large guest molecules (like, e.g., benzene or carbon tetrachloride), both from vapor phases and from diluted aqueous solutions. Surprisingly, the semicrystalline PPO samples present guest solubility much higher than fully amorphous PPO samples. These sorption experiments, as well as density measurements and classical BET experiments, clearly indicate that the obtained PPO crystalline phases are nanoporous. For these thermally stable PPO-based materials exhibiting nanoporous crystalline and amorphous phases, many applications are predictable. Finally, the preparation procedures and the thermal stability of the co-crystalline phase and FTIR and VCD analysis are presented. In particular co-crystalline phases with racemic and non-racemic guest molecules have been prepared and characterized. The experimental data indicates that the PPO/a-pinene co-crystalline form is chiral, i.e. the unit cell includes all right or left handed polymer helices and (1S-(–) or (1R)-(+) a-pinene guest molucules, respectively. [edited by author]X n.s

Gas Sorption and Diffusion in Amorphous and Semicrystalline Nanoporous Poly(2,6-dimethyl-1,4-phenylene)oxide

In this contribution is presented an analysis of mass transport properties of low molecular weight compounds in amorphous PPO and in two semicrystalline PPOs obtained by treating with benzene and carbon tetrachloride the amorphous sample. It is found that semicrystalline samples are endowed with larger gas sorption capacity and diffusivity as compared to the amorphous ones: this behavior has been attributed prevalently to the nanoporous nature of the crystalline phases induced by treatment with solvents. In particular, sorption experiments, carried out at 30 °C with methane, carbon dioxide, propane and propylene, have shown that both semicrystalline PPOs display rather interesting features which make them suitable for use as membrane materials in gas separation processes, in view of the relatively high values of solubility and diffusivity. Moreover, these peculiar sorption and mass transport properties have been found to be virtually unaffected by thermal aging: in fact, sorption experiments conducted on amorphous and semicrystalline PPO after treatment at 65 °C for three months showed that sorption and transport properties of aged samples are the same as for the untreated ones. This is an important feature to ensure the stability of performances in membrane applications

Diffusion-weighted imaging and apparent diffusion coefficient mapping of head and neck lymph node metastasis: a systematic review

Aim: Head and neck squamous cell cancer (HNSCC) is the ninth most common tumor worldwide. Neck lymph node (LN) status is the major indicator of prognosis in all head and neck cancers, and the early detection of LN involvement is crucial in terms of therapy and prognosis. Diffusion-weighted imaging (DWI) is a non- invasive imaging technique used in magnetic resonance imaging (MRI) to characterize tissues based on the displacement motion of water molecules. This review aims to provide an overview of the current literature concerning quantitative diffusion imaging for LN staging in patients with HNSCC. Methods: This systematic review performed a literature search on the PubMed database (https://pubmed.ncbi.nlm.nih.gov/) for all relevant, peer-reviewed literature on the subject following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) criteria, using the keywords: DWI, MRI, head and neck, staging, lymph node. Results: After excluding reviews, meta-analyses, case reports, and bibliometric studies, 18 relevant papers out of the 567 retrieved were selected for analysis. Conclusions: DWI improves the diagnosis, treatment planning, treatment response evaluation, and overall management of patients affected by HNSCC. More robust data to clarify the role of apparent diffusion coefficient (ADC) and DWI parameters are needed to develop models for prognosis and prediction in HNSCC cancer using MRI

Beyond N staging in colorectal cancer: Current approaches and future perspectives

Traditionally, lymph node metastases (LNM) evaluation is essential to the staging of colon cancer patients according to the TNM (tumor–node–metastasis) system. However, in recent years evidence has accumulated regarding the role of emerging pathological features, which could significantly impact the prognosis of colorectal cancer patients. Lymph Node Ratio (LNR) and Log Odds of Positive Lymph Nodes (LODDS) have been shown to predict patients’ prognosis more accurately than traditional nodal staging and it has been suggested that their implementation in existing classification could help stratify further patients with overlapping TNM stage. Tumor deposits (TD) are currently factored within the N1c category of the TNM classification in the absence of lymph node metastases. However, studies have shown that presence of TDs can affect patients’ survival regardless of LNM. Moreover, evidence suggest that presence of TDs should not be evaluated as dichotomic but rather as a quantitative variable. Extranodal extension (ENE) has been shown to correlate with presence of other adverse prognostic features and to impact survival of colorectal cancer patients. In this review we will describe current staging systems and prognostic/predictive factors in colorectal cancer and elaborate on available evidence supporting the implementation of LNR/LODDS, TDs and ENE evaluation in existing classification to improve prognosis estimation and patient selection for adjuvant treatment

Circulating hematopoietic stem cells and putative intestinal stem cells in coeliac disease

Background: The intestinal stem cells (ISC) modulation and the role of circulating hematopoietic stem cells (HSC) in coeliac disease (CD) are poorly understood. Our aim was to investigate the longitudinal modifications in peripheral blood HSC traffic and putative ISC density induced by gluten-free diet (GFD) in CD. Methods: Thirty-one CD patients and 7 controls were enrolled. Circulating CD133+ and CD34+ HSC were measured by flow cytometry, at enrolment and after 7 days and 1, 3, 6, 12, and 24 months of GFD. Endoscopy was performed at diagnosis and repeated at 6, 12, and 24 months following GFD. We used the Marsh-Oberhuber score to evaluate the histological severity of duodenal damage; immunohistochemistry was employed to measure the intraepithelial lymphoid infiltrate (IEL, CD3+ lymphoid cells) and the putative ISC compartment (CD133+ and Lgr5+ epithelial cells). Results: At enrolment, circulating HSCs were significantly increased in CD patients and they further augmented during the first week of GFD, but progressively decreased afterwards. CD patients presented with villous atrophy, abundant IEL and rare ISC residing at the crypt base. Upon GFD, IEL progressively decreased, while ISC density increased, peaking at 12 months. After 24 months of GFD, all patients were asymptomatic and their duodenal mucosa was macroscopically and histologically normal. Conclusions: In active CD patients, the ISC niche is depleted and there is an increased traffic of circulating HSC versus non-coeliac subjects. GFD induces a precocious mobilization of circulating HSC, which is followed by the expansion of the local ISC compartment, leading to mucosal healing and clinical remission

LEAP-2017 Simulation Exercise: Calibration of Constitutive Models and Simulation of the Element Tests

This paper presents a summary of the element test simulations (calibration simulations) submitted by 11 numerical simulation (prediction) teams that participated in the LEAP-2017 prediction exercise. A significant number of monotonic and cyclic triaxial (Vasko, An investigation into the behavior of Ottawa sand through monotonic and cyclic shear tests. Masters Thesis, The George Washington University, 2015; Vasko et al., LEAP-GWU-2015 Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019) and direct simple shear tests (Bastidas, Ottawa F-65 Sand Characterization. PhD Dissertation, University of California, Davis, 2016) are available for Ottawa F-65 sand. The focus of this element test simulation exercise is to assess the performance of the constitutive models used by participating team in simulating the results of undrained stress-controlled cyclic triaxial tests on Ottawa F-65 sand for three different void ratios (El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019). The simulated stress paths, stress strain responses, and liquefaction strength curves show that majority of the models used in this exercise are able to provide a reasonably good match to liquefaction strength curves for the highest void ratio (0.585) but the differences between the simulations and experiments become larger for the lower void ratios (0.542 and 0.515)

LEAP-2017: Comparison of the Type-B Numerical Simulations with Centrifuge Test Results

This paper presents comparisons of 11 sets of Type-B numerical simulations with the results of a selected set of centrifuge tests conducted in the LEAP-2017 project. Time histories of accelerations, excess pore water pressures, and lateral displacement of the ground surface are compared to the results of nine centrifuge tests. A number of numerical simulations showed trends similar to those observed in the experiments. While achieving a close match to all measured responses (accelerations, pore pressures, and displacements) is quite challenging, the numerical simulations show promising capabilities that can be further improved with the availability of additional high-quality experimental results

Genome-wide Analyses Identify KIF5A as a Novel ALS Gene

To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.Peer reviewe

Experimental and numerical investigation of the effectiveness of some innovative techniques to mitigate liquefaction risk

Many urban areas in the world have been strongly damaged by soil liquefaction during strong earthquakes. This phenomenon is caused by a progressive increment of the excess pore pressure that leads to a significant drop of the shear stiffness and strength, resulting into large settlements and a reduction of the safety margins in terms of bearing capacity of existing structures. As a consequence, when liquefaction occurs damages are more critical than those induced by inertial seismic actions. Large economic losses induced by this phenomenon encouraged in the last decades the scientific community to spend strong efforts in the study of some techniques to reduce liquefaction risk. Conventional mitigation techniques have been used to reduce settlements and bearing capacity loss, but they require massive and expensive retrofitting interventions. So, the development of some innovative mitigation techniques characterised by lower cost and by the possibility to be used in the urban area is becoming more and more a necessity. The project called LIQUEFACT (H2020-DRS-2015), financed by the European Committee, moved in this direction. Dynamic geotechnical centrifuge experiments were performed in this project to evaluate the effectiveness of two innovative techniques: horizontal drains (HD), which are innovative mostly for their installation procedure, that allows to place them directly under existing structures, and induced partial saturation (IPS), which is innovative for the process, never used in the past to tackle liquefaction risk. Horizontal drains were studied to define their capability to dissipate the excess pore pressures that rise during the earthquake, while the induced partial saturation was adopted to reduce the bulk stiffness of the equivalent pore fluid, reducing the attitude to increase the excess pore pressure induced by distorsional deformations of the soil. Each centrifuge model, realised with a loose liquefiable sand (Ticino sand), was equipped in order to measure accelerations, pore pressures and displacements. Moreover, a simplified SDOF structure was used in some tests to evaluate the interaction between the liquefiable soil and the structure, with and without the adoption of one of the mitigation techniques. The reliability of numerical modelling of the mitigation techniques was checked by the comparison between numerical and experimental results. The presence of horizontal drains was modelled by tubes with a finite permeability, estimated on the bases of experimental tests, with an inside hydrostatic fluid pressure. Some centrifuge tests with free-field configuration were reproduced by numerical analyses, using PLAXIS 2D, to check the possibility to calibrate two constitutive models used to study soil liquefaction (UBC3D-PLM and PM4SAND) with the aim of catching the experimental results in terms of pore pressure increments and settlements of the ground surface. The induced partial saturation was implemented by a reduction of the fluid bulk stiffness with a function of degree saturation. The results indicate that the numerical analyses underestimate the settlements of the ground surface in free-field conditions but capture reasonably well liquefaction-induced building settlement in the centrifuge experiments. Some parametrical analyses of the mitigation techniques were also performed, changing the HD geometrical layouts and soil mechanical properties. Moreover, an evaluation of an optimised horizontal extension of the mitigation techniques was performed for a reference building. A simplified design method for the horizontal drains was finally proposed, for the case of three horizontal rows of drains disposed with a staggered arrangement. A simplified design method for the induced partial saturation based on the analytical procedure was proposed as well